Faculty of Applied Science
Introduction to Atomic Spectroscopy
Code/s:CHA3021 / CHA3001
Compiled by: Dr G Rubidge
February 2023
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Table of Contents
Topic
Introduction
Classification of Analyses
Electromagnetic Radiation
Polarization
Wave-particle duality
Electromagnetic Spectrum
Electromagnetic Phenomena Used in Analysis
The Beer-Lambert Law
Spectrometers - General Instrumental Components
Limitations of the Beer-Lambert law
Solvatochromism
Fluorimetry
Infrared(IR)and Raman Spectroscopy
Mass Spectroscopy
Atomic Spectroscopy
Spectrometers
Monochromator parameters
The Atomic Spectrum
Instrumental Components
Graphite furnace
Light Detection
Sample Introduction
Interferences
Atomic Emission Spectroscopy (AES)
Instrumentation for AES
Flame Temperatures
Direct-Current Plasma
Spark and Arc Emission Sources
Inductively-Coupled Plasma (ICP)
Plasmas versus Flames
Inductively Coupled Plasma – Mass Spectroscopy (ICP-MS)
Laser-Ablation (LIBS)
Atomic Absorption Measurements
Optimization of AA settings
Background Correction
Detection Limit and Sensitivity
Applications of Atomic Spectroscopy
Sample preparation
Organic solvents
A comparison of atomic emission and atomic absorption
Methods of Quantification
Quality Control and Validation
Analytical calculations
Video links to mass spectroscopy, nuclear magnetic resonance and polarimetry
Tutorial Questions
Study Guide
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ATOMIC SPECTROSCOPY
Introduction
The following notes are an introduction to spectroscopy with a focus mostly on atomic
spectroscopy, there is also brief coverage of related analytical topics. The notes cover the topic in
sufficient detail to permit the reader to have an understanding of the principles and their
applications. A more detailed description of these techniques is found in the advanced atomic
spectroscopy course(Adv. Diploma) along with selected specialized texts.
These notes are to be used in conjunction with the recommended textbook: “Skoog and
West’s Fundamentals of Analytical Chemistry” 9th or 10th edition. The textbook is very
useful to analytical chemists and all students should have a copy.
Video links: In these notes there are hyperlinks that can be followed to videos or PDF documents
that explore certain topics further. You will be supplied with an electronic copy and a hard copy of
these notes. Use the electronic copy to easily follow hyperlinks to further information.
Classification of Analyses
Below is a table showing various classes of analytical instruments. All make use of instruments,
some simple while others can be very complex.
Physical properties With electromagnetic radiation By an electric charge (or current)
mass
absorption
electrochemistry
refractive
index emission
mass spectrometry
thermal conductivity
scattering
Physical methods are general and instruments are simple and not very costly but these methods
often but lack sensitivity compared to instrumental methods that use spectroscopy(light) or electric
charge detection. The more sensitive instruments can become very costly(Millions of rands).
Electromagnetic Radiation
Electromagnetic radiation is a transverse wave that is composed of an oscillating electric field
component, E, and an oscillating magnetic field component, M. The electric and magnetic fields
run at right angles to one another.
A wave is characterized by its wavelength, , which is the physical length of one complete
oscillation, and its frequency, , which is the number of oscillations per second.
Exercise: Sketch a radiation source and show how the electromagnetic components of
light are visualized.
Velocity of light
Electromagnetic waves travel through a vacuum at a constant velocity of 2.99792x108 m/s, which
is known as the speed of light, c. This is almost
300 000 km per second.
Here is a thought – Johannesburg is 1000 km from Port Elizabeth. The flight takes about 1 hour 20
min in a jet. Light could travel from PE to Jhb and back about 150 times in one second!
3
The relationship between speed frequency and wavelength is:
c=
When light passes through other media, its speed decreases. Matter thus interferes with the
passage of light – this should make sense to you as the electromagnetic fields in radiation will
interact with the charged particles of matter(electrons and protons). This decrease in velocity is the
reflected refractive index, n.
n=c/v
Since the velocity of light is lower in other media than in a
vacuum, n is always a number greater than one. The table lists
the refractive index of several compounds and mixtures.
Refractive index can be used to monitor purity or the
concentration of a solute in a solution. You will apply this
principle in your practicals this year by using a refractometer.
Note how small the difference between nair and nvacuum is.
medium
n
air
1.0003
water
1.333
50% sucrose
water
in
1.420
carbon disulfide
1.628
crystalline quartz
1.544 (no)
1.553 (ne)
diamond
2.417
Exercise: If you wished to analyze syrup for its sugar content could you use refractive
index(i.e. refractometer)? Explain.
Exploration topic: Light takes just over 8 minutes to travel from the sun to earth. Imagine that
you are a photon emitted from the center of the sun. What does the sun consist of? How do we
know what it consists of? How long would it take to reach the surface of the sun? What particles
would be encountered? What is the temperarture in the center of the sun? Surface temp? How
does this temperature influence the particles. Comment on the pressure.
Polarization
A light source, such as the beam emitted from a filament in your torch consists of waves with their
electric-field vector oriented in all directions. The resulting light emission is called unpolarized light.
Polarized light oscillates in one plane only - your sunglasses may perform this function.
Polarization devices are occasionally used in analytical chemistry to gather information from
samples that may not be observed with unpolarized radiation. One such example is in forensic
testing.
Exercise: Can you give an example of two more such emitters?
Plane polarized light is radiation in which the electric-field vector is oscillating in only one direction.
Linearly polarized light is produced by isolating one orientation of the electric field with a polarizer.
Polarized sunglasses do just this and reduce the amount of light reaching your eyes.
Polarization is also used in some analytical instrumentation(Zeeman background corrector) to
correct for errors.
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Wave-particle duality
Electromagnetic radiation exhibits both particle and wave characteristics. Einstein theorized that
the energy of radiation is quantized i.e. radiation is composed of little energy packets or photons.
The energy, E, of a photon depends on its wavelength:
E=h
= hc /
Where:
h is Planck's constant (6.62618x10-34 Js),
is the frequency of the radiation,
c is the speed of light, and
is wavelength.
Exercise: How is frequency related to wavelength?
de Broglie equation
Particles; such as electrons, protons and neutrons possess momentum and the particle nature is
related to wave properties as follows:
=h/p
where is wavelength,
h is Planck's constant, and
p is the momentum of the particle.
Beams of particles show wave effects such as interference.
Exercise: Take some time to compare wind generated waves in the ocean and a light beam
directed at you. You may need to sit by the seaside and study the waves. Consider the
following aspects in your comparison: Particles involved, orientation of waves, speed of
waves
Electromagnetic Spectrum
Electromagnetic radiation has been divided into different spectral regions depending on how such
radiation is perceived or interacts with matter. Our eyes can only detect radiation that is in the
visible region of the spectrum which is both transmitted by the lens of the human eye and
absorbed by the photoreceptors in the retina. There is no fundamental difference in the nature of
electromagnetic radiation of 450 nm versus 425 nm, other than we can see the different colours.
Visible Spectrum
The visible region of the electromagnetic radiation is 400 to 750 nm. The short wavelength
cutoff(400nm) is due to absorption by the lens of the eye and the long wavelength cutoff(750 nm)
is due to the decrease in sensitivity of the photoreceptors in the retina for longer wavelengths.
Light at wavelengths longer than 750 nm can only be seen if the light source is very intense.
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The various categories of light are tabulated below along with their frequencies, wavelength region
and how they interact with matter. It is these interactions with matter that scientists have applied to
create spectroscopy – analysis of matter using light – we will spend much time on this topic and its
applications. The boldfaced regions are most used in analytical work.
In this course the chief focus is on UV, visible and IR radiation with an introduction to X-Ray
application
Classes of Radiation in the Electromagnetic Spectrum
Type of Radiation
Frequency
Range (Hz)
Wavelength
Range
Type of Transition
gamma-rays
1020-1024
<1 pm
nuclear
X-rays
1017-1020
1 nm-1 pm
inner electron
ultraviolet
1015-1017
400 nm-1 nm
outer electron
visible
4-7.5x1014
750nm-400 nm
outer electron
near-infrared
1x1014-4x1014 2.5 µm-750 nm
infrared
1013-1014
25 µm-2.5 µm
molecular vibrations
microwaves
3x1011-1013
1 mm-25 µm
molecular rotations, electron spin flips
radio waves
<3x1011
>1 mm
nuclear spin flips
outer electron molecular vibrations
Spectroscopy – Definition
Spectroscopy is a very important tool to the analytical chemist. You will find it as a self-contained
instrument that measures light or it may be used in a detector in a more complex analytical
system. The definition below is comprehensive in that it covers all applications of spectroscopy in
analytical chemistry.
Definition: Spectroscopy is the use of the absorption, emission, or scattering of electromagnetic
radiation by atoms or molecules (or atomic or molecular ions) to qualitatively or quantitatively
study the atoms or molecules, or to study physical phenomena.
The interaction of radiation with matter can cause redirection of the radiation and/or transitions
between the energy levels of the atoms or molecules. A transition from a lower energy level to a
higher level with transfer of energy to the atom or molecule is called absorption. A transition from
a higher level to a lower level is called emission if energy is emitted as light (usually the
wavelength of that light depends on the nature of the emitter). Redirection of light due to its
interaction with matter is called scattering, and may or may not occur with transfer of energy, i.e.,
the scattered radiation has a slightly different or the same wavelength.
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Electromagnetic Phenomena Used in Analysis
Absorption
When atoms or molecules absorb light, the incoming energy excites a quantized structure to a
higher energy level. The type of excitation depends on the wavelength of the light. Electrons are
promoted to higher orbitals by ultraviolet or visible light, vibrations are excited by infrared light, and
rotations are excited by microwaves.
Exercise: How does a microwave warm your food?
An absorption spectrum is the absorption of light as a function of wavelength. Absorbance is
normally plotted on the y axis and wavelength on the x axis. You will make extensive use of
absorption spectra in analytical chemistry.
Emission
Atoms or molecules that are excited to high energy levels are not stable at these excited states
and will revert to their “ground” state by emitting radiation (emission). For molecules it is called
fluorescence if the transition is between states of the same spin and phosphorescence if the
transition occurs between states of different spin. Emission is shortlived while phosphorescence
lasts as long as 30 seconds or more.
The emission intensity of an emitting substance is linearly proportional to analyte concentration at
low concentrations, and is useful for quantification.
Exercise:If a sodium atom emits radiation at 589 nm in a flame would a sodium ion also
emit at the same wavelength? Explain.
Scattering
As light passes through matter, most of it continues in its original direction but a small portion is
scattered in other directions. Light scattered at the same wavelength as the incoming light is called
Rayleigh scattering.
Other forms of scattering involve shifts in wavelength.
Light that is scattered in transparent solids due to vibrations is called Brillouin scattering.
Light that is scattered due to vibrations in molecules is called Raman scattering. There is an
instrumental technique that makes use of this principle – Raman Spectrometers are useful in fields
such as industry and forensic chemistry.
Scattering is applied as an analytical technique but is also a source of interference
when unwanted particles scatter light that we wish to measure in an instrument.
Exercise: If you mix silver and chloride ions a white precipitate is produced. Can you use
this precipitate to scatter light and thus quantify an analyte?
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Reflectance is a mode of radiation transmission. Short videos https://youtu.be/6p7Q6ppY7r4
(9min) https://youtu.be/C3WT34Muldk (7 min) https://youtu.be/snZPN4qDChY (5 min)
https://youtu.be/snZPN4qDChY (12 min)
Fluorescence
Absorption and re-emission at a less energetic wavelength occurs when radiation
excites an atom or molecule (the molecule is usually conjugated i.e. alternating double
and single bonds). The excited molecule/atom releases the energy as radiation of
lesser energy than that initially used to excite the species.
Exercise: Tonic water glows under a UV light but the glow terminates immediately when
the UV light is switched off. Explain how this occurs. Can we quantify the ingredient that
causes the glow?
The structures of quinine and fluorescein
Demonstration: Your lecturer will demonstrate fluorescence using tonic water and a
UV lamp.
The Beer-Lambert Law
The Beer-Lambert law (or Beer's law) is the linear relationship between absorbance and
concentration of an absorbing species.
A=exbxc
where A is the measured absorbance,
E is a wavelength-dependent absorptivity coefficient,
b is the path length,
and c is the analyte concentration.
Using molar concentrations, the Beer-Lambert law is written as:
A = E* b * c
where E is the wavelength-dependent molar absorptivity coefficient
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units: M-1 cm-1.
Measuring of Absorbance
Experimental measurements are usually made in terms of transmittance (T), which is defined as:
T = P / Po
where P is the light power after it passes through the sample and Po is the initial light power.
A = -log T = - log (P / Po)
Absorption instruments can usually display the data as either transmittance, %-transmittance, or
absorbance. An unknown concentration of an analyte can be determined by measuring the
amount of light that a sample absorbs and applying Beer's law. If the absorptivity coefficient is not
known, the unknown concentration can be determined using a calibration curve of absorbance vs.
concentration derived from standard solutions.
JoVE video link on uv-viz spectroscopy (9 min) https://www.jove.com/scienceeducation/10204/ultraviolet-visible-uv-vis-spectroscopy
JoVE video link on calibration curves (9 min) https://www.jove.com/scienceeducation/10188/calibration-curves
Spectrometers - General Instrumental Components
It is very important to know how instruments are assembles and the purpose of they key
components. All too often the modern instrument is a black box somewhat like the modern cell
phone. For an enhanced knowledge as a spectroscopist you should know what component make
up an instrument, their function and how they are arranged. Solving problems is far easier when
the analyst or technician is empowered with knowledge of the instrument components and their
function.
Sketch the basic instrumental components of photometer in the space below:
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Sketch the basic instrumental components of spectrophotometer in the space below:
Limitations of the Beer-Lambert law
The linearity of the Beer-Lambert law is limited by chemical and instrumental factors. Causes of
non-linearity include:
1.
2.
3.
4.
5.
6.
7.
deviations in absorptivity coefficients e or E at high analyte concentrations (>0.01M) due to
electrostatic interactions between molecules in close proximity
scattering of light due to particulates in the sample Tyndall
fluorescence or phosphorescence of the sample
changes in refractive index at high analyte/solute concentration
shifts in chemical equilibria as a function of analyte concentration
non-monochromatic radiation, deviations can be minimized by using a relatively flat part
of the absorption spectrum such as the maximum of an absorption band
stray radiation
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Solvatochromism is when a molecule changes colour due to interactions with solvents of
different polarity. Merocyanine: https://youtu.be/RGG2GNvpwA4 (22 minutes)
The structural forms of Brooker’s merocyanine is given below and colour variations (source
Wikipedia)
Colors of Brookers merocyanine solutions in as a function of solvent polarity
Solvent
Color
λ(max, nm)
Relative solvent polarity
Water
Yellow
442
1
Methanol
Red-orange 509
0.762
Ethanol
Red
510
0.654
2-Propanol
Violet
545
0.546
DMSO
Blue-violet 572
0.444
Acetone
Blue-violet 577
0.355
Pyridine
Blue
603
0.302
Chloroform Blue
618
0.259
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See Reichardt's dye below – it also exhibits solvatochromism.
FLUORIMETRY
Introduction
Fluorimetry is the application of fluorescence in analysis. Light emission from atoms or molecules
can be used to quantify the emitting substance in samples e.g. quinine in tonic water.
Fluorescence is measured at right angles to the source radiation. The relationship between
fluorescence intensity and analyte concentration is:
F = k x QE x Po x (1-10[-E*b*c])
where F is the fluorescence intensity, k is a geometric instrumental factor, QE is the quantum
efficiency (photons emitted/photons absorbed), Po is the radiant power of the excitation source, E
is the wavelength-dependent molar absorptivity coefficient, b is the path length, and c is the
analyte concentration (E, b, and c are the same as used in the Beer Lambert law).
Expanding the above equation in a series and dropping higher terms gives:
F = k x QE x Po x (2.303 x E x b x c)
This relationship applies at low concentrations (<10-5 M) and shows that fluorescence intensity is
linearly proportional to analyte concentration.
These terms: k, QE, Po, 2.303, E and b remain constant so k x QE x Po x (2.303 x E x b) is
equated to k’ then,
F = k’c
Since F is directly proportional to c we may generate calibration curves and use them to quantify
analytes that fluoresce.
Limitations of Fluorimetry
The limitations of the Beer Lambert law also affect quantitative fluorimetry. Fluorescence
measurements are also susceptible to inner-filter effects. These effects include excessive
absorption of the excitation radiation (pre-filter effect) and self-absorption of atomic resonance
fluorescence (post-filter effect).
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Spectrofluorimteter
Because of the fact that not all molecules will fluoresce fluorimetry is rather limited but in some
cases the analyte may be altered chemically so that it may fluoresce. This alteration process is
called derivitization. A fluorescent molecule can be selectively bonded onto an analyte that does
not fluoresce, thus converting it into a fluorescent molecule. On the other hand it is a very specific
technique with few interferences.
An example of derivitization is the determination of the free amino acids in chamomile flowers
using reverse-phase high-performance column chromatography preceded by pre-column
derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) structure and
reaction shown below.
Application:
https://www.knauer.net/Application/application_notes/vbs0011n_uhplc-pdafld_determination_aqc_amino_acids_bluespher.pdf
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The main focus will now be spectroscopy of atoms at high temperatures.
Infrared(IR)and Raman Spectroscopy
Molecules absorb and scatter radiation in the infrared region. In the textbook the coverage of IR is
week and Raman spectroscopy is not covered. Both these techniques are excellent for qualitative
analysis and good for quantitive since most compounds absorb or scatter radiation in this range.
Review the JoVE videos below for an introduction.
IR spectroscopy (8min): https://www.jove.com/science-education/10351/infrared-spectroscopy
Raman Spectroscopy (9 min): https://www.jove.com/science-education/5701/raman-spectroscopyfor-chemical-analysis
Mass Spectroscopy
Another technique that is very widely applied for qualitative and quantitative analysis is mass
spectroscopy. Basically the sample is fragmented in a vacuum and the mass to charge ratio of the
fragments is the focus of this method. Due to unique structures and masses of molecules ions and
atoms, produce different pattern in a mass spectrometer.
JoVE
video
on
mass
spectroscopy
(13
min):
https://www.jove.com/scienceeducation/5634/introduction-to-mass-spectrometry
Atomic Spectroscopy
spectroscopy requires that the sample be converted into free atoms rather than molecules so we
need considerable energy to free the atoms. Such energy is readily available in the form of flames,
electrical discharges or plasmas.
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Some Theoretical Concepts
Elemental analyses involve the conversion of the sample into a free atomic vapour i.e. the vapour
consists of free atoms with some ions and electrons but not molecules as we find at room
temperature. The key component to atomic spectroscopy is the vapor generation system this may
be accomplished by any of the following vapor generation methods: flame, plasma, arc (DC),
spark (AC), lasers, electrothermal heating, and microwave plasmas.
The purpose of the ideal vapor source is multifold, namely:








To convert all samples (solids liquids, and gases) into an atomic vapor
To do so for all elements at any concentrations
To have no matrix interferences
To have the analytical signal be a simple function of concentration
To allow accurate and precise analyses
To be reliable and simple to operate
To have low capital cost and running & maintenance costs
To be readily understood by all operators
Three atomic spectroscopic configurations are commonly used; emission in which the atomic
vapor is the source, absorbance in which a separate radiation source is directed through the
atomic vapor which absorbs radiation, and fluorescence in which a separate source shines
excitation radiation into the vapor thus creating emission of a fluorescent nature.
Flames have been and still are commonly used to create atomic vapors for atomic spectroscopic
measurements. A variety of combinations are used due to their varying chemical environments
and temperatures. Higher temperatures are favorable in that they promote atomization of potential
molecular interferences (refractory(stable) compounds such as calcium phosphate) that may
reduce the quality of analytical measurements. High flame temperatures also extend the scope of
the analytical technique by making it more sensitive and allowing one to analyze elements that are
less readily vaporized and exited by thermal means – examples are tungsten, zirconium and
titanium.
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Spectrometers
A spectrometer is an optical system that transmits a specific band of radiation. Dispersion of
different wavelengths is accomplished using a diffraction grating. Typical applications are isolation
of a narrow band of radiation from a continuum light source for absorption measurements, or
analysis of the emission from excited atoms or molecules. Monochromators or filters are used to
select the specific band of radiation that is desired. Polychromators select a number of
wavelengths at the same time.
Question: Why do we require a specific band of radiation?
Mono1chromator Designs
Monochromator is derived from Greek: Mono – single and Chromos –colour. It consists of the
diffraction grating, slits, and spherical mirrors all encased in a dark housing. Scanning is
accomplished by rotating the grating thus shifting the diffracted wavelengths to the exit slit.
Sketch a typical monochromator in the space below:
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In some cases a grating may remain fixed and a detector and exit slit are moved along the focal
plain to change the wavelength that the instrument measures.
Exercise: Go online and look up “grating”.
What device do you common/y used that exhibits diffraction of white light into colours?
You
most
likely
answered
some
form
of
compact
disc.
See
https://en.wikipedia.org/wiki/Compact_disc for detail especially on how the disc is read and the
difference between CD, DVD. HD DVD and Blue Ray
Monochromator parameters
Monochromators select the desired wavelength. The are a number of optic designs resulting in
different quality monochromators. In your CHA3011 practical you will make a homemade
monochromator and test it qualitatively.
Bandpass The wavelength range that the monochromator transmits.
Dispersion The wavelength dispersing power, usually given as spectral range / slit width
(nm/mm). Dispersion depends on the focal length, grating resolving power, and the grating order.
Resolution The minimum bandpass of the spectrometer, usually determined by the aberrations of
the optical system.
Spectrographs are spectrometers that record a wide bandpass with a photographic plate or an
array detector. The spectrometer requires a flat image field. Such instruments are used in
metallurgical analysis.
Polychromators are spectrometers with multiple detectors for simultaneous detection of multiple
analytes. This capability allows rapid multielement analysis.
The Atomic Spectrum
The tremendously narrow (typically 10-5nm) lines produced by atomic excitation permit numerous
lines to exist in a narrow wavelength range. Spectral line shapes are altered by various factors.
Assume that an atomic line is 0.003 nm wide, how many lines can fit in side by side into the visible
spectrum ranging from 350 nm to 750 nm. Compare this result to the number a molecular bands
that could fit into the visible spectrum assuming each band is 25 nm wide. You should find that
there are over 8000 times as many atomic line that can fit in compared to molecular bands. This
quality make atomic spectroscopy much more specific that molecular spectroscopy. Atomic
spectroscopy may permit the analysis for 20 elements easily while molecular spectroscopy may
seldom permit more than three substance to be determined simultaneously, and even then the
molecular spectroscopy requires corrections.
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Doppler broadening and shift – atoms moving rapidly toward the detector emit shorter
6wavelengths than those moving away from the detector; and vice versa. The line width is a
function of the square root of the temperature. Typical degrees of broadening are 0.001-0.005 nm.
Collisional broadening – pressure dependant broadening resulting in 0.01 – 0.02 nm effects on
lines. Collisions among similar (van Der Waals broadening) or dissimilar atoms (Lorentz
broadening) cause a variation in the excited state energies. The use of wider slit widths allows
more light to the detector thus improving signal/noise ratios. Stray radiation may more easily enter
the detector when using a wide slit and thus worsen the signal/noise ratios.
Instrumental broadening – broadening increases with increasing numbers of optics and poor optic
quality. All the optics will never be perfect and some dispersion of the radiation will always occur
though in many well-manufactured instruments such dispersion is minimal.
Stark broadening – occurs in the presence of an electrical or magnetic field whereby the source
line is split into several lines of lesser intensity. This splitting is the basis of Zeeman background
correction.
Natural broadening – due to the finite lifetime of the exited state not being the same for all
radiating atoms. A negligible degree of broadening occurs.
Self absorption – as the concentration of free analyte atoms in the excitation medium increases so
the radiation from an atom in the center of the medium encounters more of its own kind of atoms
that may absorb the radiation emitted by the inner atom. Calibrations curves are linear at lower
concentrations, but the proportionality between analyte concentration and emission intensity
reduces as analyte concentration increases. The absorption curve bends toward the concentration
axis. Absorption at the line center is greater that at the edges resulting in a saddle shaped
curvature – known as self-reversal.
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Field splitting - Magnetic or electric fields split energy levels in simple or complex manners thus
alter the line by broadening or even multiplying them
19
Instrumental Components
Most spectrometers contain similar components, it is just their arrangements and capabilities vary
according to the applications intended.
Light source: The light source for absorbance and fluorescence is usually a hollow cathode lamp
(HCL) of the element that is being measured. Lasers are also used in selected research
instruments. Since lasers are intense enough to excite atoms to higher energy levels, they allow
atomic absorption (AA) and atomic fluorescence spectroscopy (AFS) measurements in a single
instrument. The disadvantages of these narrow-band light sources are that only one element is
measurable at a time, and it is expensive to purchase a great number of such units. A hollow
cathode lamp costs on the order of R3000 per unit. Multielement HCLs are available.
Exercise: A HCL will be passed around the class – identify the anode, cathode, quartz window
and note what filler gas is used.
Sketch and label a typical HCL below:
Atomizer: AA spectroscopy requires that the analyte atoms be in the gas phase. Ions or atoms in
a sample must undergo desolvation and vaporization in a high-temperature source such as a
flame or graphite furnace. Flame AA can only analyze solutions, while graphite furnace AA can
accept solutions, slurries, or solid samples.
Flame AA uses a slot type burner to increase the path length(b), and therefore to increase the total
absorbance thus making the instrument more sensitive. Sample solutions are usually aspirated
with the oxidizer gas(air or nitrous oxide) flowing into a nebulizing/mixing chamber to form small
droplets before entering the flame.
The graphite furnace has several advantages over a flame. It is a much more efficient atomizer
than a flame and it can directly accept very small absolute quantities of sample. The
improved efficiency is due to longer residence times and the whole sample is atomized. The
improvement in sensitivity, relative to flame spectroscopy varies from a few hundred fold to
about four thousand fold across the periodic table. It also provides a reducing environment for
easily oxidized elements. Samples are placed directly in the graphite furnace and the furnace is
electrically heated in three steps to (1)dry the sample, (2)ash organic matter, and (3)vaporize
the analyte atoms.
Sketch a typical AA burner system as well as a graphite furnace below – label each sketches.
Graphite furnace system
20
AA Burner
Pictures of instrument GF Flame
Light Detection
AA spectrometers use monochromators and detectors for uv and visible light. The main purpose
of the monochromator is to isolate the absorption line from background light due to interferences.
Simple dedicated AA instruments often replace the monochromator with an interference filter.
Photomultiplier tubes were the most common detectors for AA spectroscopy but lately semic
conductor electronic detector systems are taking over – these are the CCD and CMOS detectors
that are found in cell phones, cameras and other digital recording systems.
Photomultiplier Tube (PMT)
Photomultiplier tubes (PMTs) convert photons to an electrical signal. They have a strong
amplification and are very sensitive detectors.
A PMT consists of a photo emissive cathode and a series of dynodes in an evacuated glass
vessel fitted with a quartz window to let radiation in. The dynodes are sequentially more positive
than the cathode so as to attract ejected electrons and amplify the signal as much as 10 million
times. When a photon of sufficient energy strikes the photo-emissive cathode, it ejects an electron.
The cathode material is usually a mixture of alkali metals, which make it sensitive to photons
throughout the visible region. The cathode is at a high negative voltage, typically -500 to -1500
volts. The emitted electron is accelerated towards a series of additional electrodes called dynodes.
Additional electrons are generated at each dynode. Amplification depends on the number of
dynodes and the accelerating voltage – this voltage is adjusted to alter the sensitivity.
21
Diagram of a PMT
Phototubes are similar to PMTs, but one photocathode and an anode, hence they find use in less
sensitive applications such as absorption spectrometers.
Semiconductor Detectors:
During the nineties of last century rapid advances have been made in semiconductor application in
most fields including analytical chemistry optics. Capability normally doubles every 18 months.
Most cellphones have very powerful electronic devices that can take pictures or video clips. These
devices and those found in digital cameras are very similar to those in an analytical instrument.
Normally a CCD or charge coupled device is used. Though they were invented back in 1969 they
only found extensive use later. Below is a typical CCD found in the earlier 2 megapixel cameras.
There are many types of CCD that have been developed. Another type of detector is the active
pixel sensor(APS), which due to its lower power consumption has take over cellphone digital
recorders.
A CCD sensor
A CCD is an integrated-circuit chip that contains an array of capacitors that store charge when
light creates e-hole pairs. The charge accumulates and is read in a fixed time interval. The whole
spectrum can thus be observed simultaneously and the intensity of individual lines can be
determined at specific positions on the CCD chip – the stronger the light falling onto the chip the
more charge accumulates. Overload (over exposure) can occur as is seen on a CCD screen when
22
a strong light source shines onto it – e.g. a reflection. The CCD is very sensitive for
measurement of low light levels.
Diagram of a CCD
Photodiode Array Detectors (PDA): A photodiode array is a linear array of discrete photodiodes
on an integrated circuit (IC) chip. For spectroscopy it is placed at the image plane of a
polychromator to allow a range of wavelengths to be detected simultaneously. The PDA
essentially records an electronic version of what photographic film would. Array detectors are
especially useful for recording the full ultraviolet and visible spectra of samples that are rapidly
passing through a sample flow cell, such as in an liquid chromatographic detector. The massive
spectral capability has led to the PDA’s replacing PMTs in polychromators. Advantages are
reduced size, and cost, greater speed as well as greater resolution – i.e. more lines are able to be
processed than with PMT polychromators. PDAs work on the same principle as simple photocells
– only there are many separate units in a very small space.
23
Sketch of a typical PDA:
Light creates electron-hole pairs and the electrons migrate to the nearest PIN junction. After a
fixed integration time the charge at each element is sequentially read with solid-state circuitry to
generate the detector response as a function of linear distance along the array. PDAs are
available with 512, 1024, or 2048 elements with typical dimensions of ~ 25 µm wide and 1-2 mm
high.
A low cost device is a light dependent resistor – LDR. In your practical you will use an LDR in
measuring intensity of light reaching it. To do so you will use a multi meter.
Sample Introduction
The sample has to be introduced consistently and reproducibly into the flame (or other excitation
technique) to produce accurate and precise analytical results.
The most common approach is to convert it into a liquid then to nebulizer it as a fine mist that is
carried into the flame by the oxidant. The whole nebulization process may be depicted as follows:
Demonstration: Your lecturer will demonstrate atomic emission using electrical or flame
excitation. Note how specific atoms can colour the flame or electrical discharge. The apparatus
described below will be used.
24
A liquid sample is converted to atomic vapour as shown below:
AIR
FLAME
H2O VAPOR
&
Sample
SALT PARTICLES
3000oC
FREE ATOMS
A flow diagram showing how a liquid sample is converted into free atoms in a flame
Sketch a laminar flow nebulizer and burner in the space below showing the components:
The liquid sample is drawn into the capillary tube by the ventury action generated by the jet of
oxidant passing over the capillary outlet. Here the sample stream gets torn apart by the jet of
oxidant then dashed against the impact bead. Large droplets are drained. The fuel mixes with the
25
oxidant and nebulized sample and flows through flow spoilers or baffles the permit the passage of
only the finest droplets, which now only constitute about 2% of the entire sample entering the
nebulizer – the remainder drains to waste. As the gaseous mixture rapidly flushes out of the
burner slot it encounters the flame and burns up thus atomizing the sample. Should the pressure
of the gas mixture within the nebulizer drop then the flame could creep back into the nebulizer
creating a flashback. The blow out plug is s pressure release safety mechanism that prevents part
of the nebulizer attaining sufficient kinetic energy to injure unsuspecting bystanders.
A total consumption burner(also called a turbulent flow burner) transports the sample solution
directly into the flame. All desolvation, atomization, and excitation occurs in the flame. Atomicabsorption instruments almost always use a nebulizer and long(5-15 cm) slot burner to increase
the path length(b) for the sample absorption.
Sketch a total consumption burner in the space below:
Laminar flow burners are advantageous in that they are quiet and produce very small droplets of
sample that enter the flame. They are, however, prone to back-flash in which the flame moves into
the spray-chamber through the burner slot if the flow of gas is insufficient resulting in a loud report
and possible stress on operator/spectator nerves. Laminar flow burners are also tremendously
inefficient in that of the sample entering them only about 2-5 % of this sample actually enters the
flame while the remainder is wasted. Long path-lengths improve sensitivity.
Turbulent flow burners cannot result in explosion as the fuel and oxidant only mix in the flame
zone. The sample is totally consumed as no wastage occurs. These burners tend to have narrow
flames thus the pathlength (b) is reduced which reduces the sensitivity.
26
Interferences
Both absorption and emission spectroscopy depend on the analyte existing as free atoms in the
gas phase. There are two common types of interferences that reduce the concentration of free
gas-phase atoms: ionization and the formation of molecular species.
Note that the distribution of gas-phase atoms between the ground and excited states is a physical
property that depends on the temperature of the environment. This distribution will affect the
analytical signal, but it is not a chemical interference. Basically, the hotter the environment, the
greater the excited portion of the population. The excited portion of atoms depends on the
electronegativity within the periodic table. Sodium is easily excited, while zinc is much harder to
excite, and chlorine is not even able to be excited using a flame.
If the temperature continues to increase the degree of analyte ionization also increases and
converts the analyte into an ion which alters the absorption and emission energies which in turn
alters the wavelength.
Concentration measurements are usually determined from a calibration curve after calibrating the
instrument with standards of known concentration. To prevent any bias due to differences between
the standards and the samples, any reagents that are added to reduce chemical interferences
should be added to the standards as well as the sample solution.
A number of solid sample introduction techniques exist and include electrical erosion by arcs, laser
erosion or ablation, powder sample introduction and slurry sample introduction. In the advanced
diploma/ BScHonours course more detail is covered.
1. Apparent analyte loss due to ionization of the analyte
Since samples are usually liquids or solids, the sample must be vaporized and atomized in a hightemperature source such as a flame, graphite furnace, or plasma. This high-temperature
environment can also lead to ionization of the analyte atoms. Analyte ionization can be
suppressed by adding a source of electrons, which, according to Le Chateliers’ principle, shifts the
equilibrium of the analyte from the ionic to the atomic form:
Analyte
Analyte+ + e-
Cesium and potassium are common ionization suppressors that are added to analyte solutions
since they are easily ionized and produce a high concentration of free electrons in the flame.
2. Apparent analyte loss due to refractory compound formation
Some elements can form refractory compounds(thermally very stable) that are not atomized in
flames. An example is the presence of phosphates, which interferes with calcium measurements
due to formation of refractory calcium phosphate:
3 CaCl2 (aq) + 2 PO43- (aq)
Ca3(PO4)2 (s) + 6 Cl- (aq)
27
Formation of refractory compounds can be prevented or reduced by adding a releasing agent or a
protecting agent. For calcium measurements, adding lanthanum to the sample (and standard)
solutions binds the phosphate as LaPO4. LaPO4 has a very high formation constant Kf and
effectively ties up the phosphate interferent thus freeing the calcium.
Multidentate ligands such as EDTA or APDC(ammonium pyrolidine dithiocarbamate) may be
added in excess to complex with the analyte thus functioning as a protecting agent preventing the
phosphate from attacking the analyte.
3. Apparent analyte increase due to refractory compound formation
The oxides of titanium, tungsten and zirconium are stable in flames and if their particles are larger
than the wavelength of interest they may scatter radiation causing a loss during absorbance
measurements that appears as an increase in analyte concentration. These compounds are not
readily decomposed by flames but plasmas, arcs or sparks are thermally energetic enough to
atomize such samples.
The solution is to use a background correction system, or to switch to a system sufficiently hot to
decompose the oxide such as plasma rather than flame. The interferent can also be added in
excess to all samples and standards to attempt cancel out its effect. A quantitative method used
when sample matrix interferences are expected is the standard addition method permits this - to
be covered later. (Matrix refers to everything in the sample except the analyte)
Atomic Emission Spectroscopy (AES)
Atomic-emission spectroscopy (also called optical emission spectroscopy - OES) is used to
analyze elements in samples. Common excitation sources include flames (using propane and
acetylene as fuels with air and nitrous oxide as fuels) and plasmas, with flames being most
suitable for easily excited atoms such as alkali and alkaline earth metals.
Exercise: Write out a balanced reaction for the reaction between acetylene and nitrous
oxide in which the products are water, carbon dioxide and nitrogen.
In atomic emission spectrometers analyte atoms in solution are aspirated into the excitation region
where they are desolvated, vaporized, and atomized by a flame or plasma. These hightemperature atomization sources provide sufficient energy to promote the valence electrons of
analyte atoms into high energy levels. The electrons decay back to lower levels by emitting light of
a characteristic wavelength. The spectra of multielement samples contain many peaks, and a
high-resolution spectrometer is required to differentiate between the emission lines.
Sketch a simplified diagram depicting AES(Atomic emission spectroscopy):
28
Exercise: Which analytical technique can analyze faster when determining multi-element
samples, atomic absorption or atomic emission? Explain.
Instrumentation for AES
As in AA spectroscopy, the sample must be converted to free atoms by means of a flame or
plasma. Liquid samples are nebulized(converted to a mist) and carried into the flame or plasma by
a flowing gas. Solid samples can be introduced as a slurry or by other excitation means such as
arc, laser or thermal vaporization.
Flame Temperatures
A flame provides a high-temperature source for atomizing samples. The table below lists typical
temperatures that can be achieved in some commonly used flames.
Temperatures of some common flames
Fuel Oxidant
Temperature (K)
H2
Air
2000-2100
C2H2
Air
2100-2400
H2
O2
2600-2700
C2H2
N2O
2600-2800
29
C2H2
O2
(CN)2
O2
2900-3100
4500
Solids undergo a number of changes as they are heated, namely
SOLID → LIQUID → GAS → PLASMA
At 2000 K most molecules are converted to atoms, at 4000 K ionization begins to occur with even
the most stable molecules dissociating. No molecules exist beyond 6000 K and few neutral atoms
exist (they ionize), at 8000 K and above, no neutral atoms remain and only ionized gas exists
containing such species as N+, N2+(10000K), N3+(12000K).
Direct-Current Plasma (DCP) A direct-current plasma is created by a continuous electrical
discharge between two electrodes. DCP instruments make use of a cathode and two anodes – the
sample is injected between the two anodes. In DCP a plasma support gas is necessary, and Ar is
commonly used. Samples can either be deposited on one of the electrodes, or if samples are
conductors they can be shaped into an electrode. Liquids are aspirated into the plasma from a
nebulizer that converts the sample into a mist.
\
30
Sketch a DCP system:
Other electrical methods of excitation:
Electricity has been used as a source of excitation in various way. Some early experimenters even
tried pulsing a huge current through a very small wire that had been dipped into the sample – the
technique was called exploding wire emission. The high current swiftly vapourizes the wire and the
sample on it.
Spark and Arc Emission Sources
Spark and arc excitation sources use a current pulse (spark) or a continuous electrical discharge
(arc) between two electrodes to vaporize and excite analyte atoms. The electrodes are either
metal or graphite. If the sample to be analyzed is a metal, it can be used as one electrode. Nonconducting samples are ground with graphite (or pure, non-analyte metal powder) powder and
placed into a cup-shaped lower electrode. Arc and spark sources can be used to excite atoms for
AES. Arc and spark excitation sources have been replaced in many applications with plasmas, but
are still widely used in the metals industry.
Spark excitation is a discontinuous electrical discharge and is commonly found in spectrographs –
sparks may reach temperatures of up to 40 000 K. Electrical methods are best suited to
conductive samples such as metals and find extensive use in the steel industry.
31
Sketch the electrodes of a typical arc or spark system below and indicate where the sample is
placed:
A liquid sample introducing electrode called the rotrode consists of a trough with a rotating
graphite wheel that turns in the liquid analyte and an arc atomizes a portion on the top of the
rotating electrode.
Sketch a rotrode in the space below
32
Inductively-Coupled Plasma (ICP)
Inductively coupled plasma is a very high temperature (7000-8000K) excitation source that
efficiently desolvates, vaporizes, excites, and ionizes atoms. Molecular interferences are greatly
reduced with this excitation source but are not eliminated completely. ICP sources are used to
excite atoms for AES and to ionize atoms for mass spectrometry in ICP-MS instruments that are
extremely sensitive and can permit isotope determinations.
Instrumentation
The plasma torch consists of three concentric quartz tubes, with the inner tube containing the
sample aerosol and Ar support gas and the outer tubes containing an Ar gas flow to form the
plasma(auxillary), and to cool the tubes to prevent them from melting. A radiofrequency (RF)
generator (typically 1-5 kW @ 27 MHz or 41 MHz) produces an oscillating current in a 3-turn,
water-cooled copper induction coil that wraps around the tubes. The induction coil creates a
fluctuating magnetic field. The fluctuations mean that N and S poles swap over at 27 or 41 MHz (1
MHz = million cycles per second). A Tesla coil is used to initiate the ionization of the auxiliary
argon stream. The magnetic field in turn sets up an oscillating current in the ions(Ar+) and
electrons of the support gas. These ions and electrons transfer energy to other atoms in the
support gas by collisions to create a very high temperature plasma (6000 –10000deg C).
ICP has a number of advantages over other sources – the advantages are compared with flames
as sources below:
Use the space below to sketch diagrams showing how the plasma is sustained:
Exercise: Take some time to Google “Nikola Tesla” after whom the Tesla coil was named.
Find out who he was and what he was famous for. How did he contribute to you daily
lifestyle?
33
Plasmas versus Flames
Plasmas have higher temperatures (than flames) ranging from 4000 to 8000 K and this property
leads to the following advantages:
- Less interference as the high temperatures decompose molecular interferent species such as
the refractory oxides of Zr, Ti, and W; Ca phosphates, and alkali metal halides.
- Greater sensitivity due to the increased emission (lower limit of detection - LOD)
The Inert environment of Ar permits a lower UV cut-off thus allowing extended analytical
capabilities (from 190 – 160 nm) and no possibility of the formation of refractory oxides since
oxygen is essentially absent as a gas (W, Zr, Ti).
Precision of 0.2 –0.5 % is typical for both flame and plasma techniques.
ICP has fewer interferences leading to less operator skill requirements, or easier automation.
There are no possibilities for explosions using plasmas as compared to acetylene and gas
flames.
ICP has extended linear dynamic ranges due to optical thinness (the power is transferred to the
outer edges of the plasma where the temperature is greater.
ICP is more expensive than flame spectroscopy – radio-frequency generators, and coolant units
tend to add to the overall cost.
ICP plasmas may also be expensive to run due to the high consumption of argon – typically 1115 L /minute.
Both ICP and flame techniques suffer from physical matrix problems such as viscosity
differences between samples and standards.
34
ICP-Mass Spectroscopy (ICP-MS)
ICP instruments can be used as a source of free atoms that can be further analyzed using a mass
spectrometer which determine the m/z mass/charge ratio of ions produced. This is particularly
useful and allows isotope analysis which cannot be performed with ICP or AA analysis. The
ICP_MS technique is extremely sensitive and can measure concentrations down to low ppb levels.
A typical maximum concentration calibration standard could be 1 ppm as compared to flame AA
and ICP where it can be on the order of 10-100 times higher. Instruments are much more costly
than flame AA and ICP systems are of intermediate cost. For an easy introductory description of
ICP-MS see: http://www.perkinelmer.com/PDFs/Downloads/tch_icpmsthirtyminuteguide.pdf
Laser-Ablation (LIBS)
Lasers produce high energy that can cut metals and be specifically focused. Solids can be
vaporized (ablation) using laser beams. When a high-energy laser pulse is focused into a gas or
liquid, or onto a solid surface, it can cause vaporization and thus create a hot plasma. The
specificity of lasers allow micro analysis of sections of samples – e.g. a blemish on a piece of
steel. Laser systems are very costly(R1.2 million)!
Recently both spark and LIBS portable hand held instruments have appeared on the market.
These instruments are well suited to field analysis and are largely non-destructive.
See the Sciaps product video for LIBS (6 min): https://youtu.be/kJhUGRQcw2E
Question: If you had only a small amount of a very precious sample such as a wedding ring or a
small amount of residue in a forensic case, which instrument would be best suited to the analysis?
Explain.
ATOMIC ABSORPTION MEASUREMENTS
The Beer –Lambert law describes the ideal absorbance of radiation within a flame.
Pt= Po(10-ABC)
Where
Pt is the transmitted intensity of the source radiation,
Po is the initial intensity of the source radiation,
A is the absorptivity coefficient,
B is the pathlength of the flame, and
C is the concentration of the atoms in the flame
A number of sources of error may readily be found in theory and practice, namely:




The source may be spectrally impure thus a component of radiation that may not be able to
be
absorbed will exist, Pimp
The analyte atoms in the flame may emit radiation at the exact same wavelength of the
monochromator setting, Pe
Matrix interferences may cause background absorbance’s, Pba
Matrix particulates may result in scattering of source radiation, Ps
35
The corrected Beer-Lambert law now becomes:
Pt= Po(10-ABC) + Pimp + Pe - Pba – Ps
The above equation theoretically solves our problem, but how do we achieve this equation in
practice?
Pimp is readily solved by using hollow cathode lamps as the radiation source. The choice of a
narrow operating slitwidth and the precise wavelength will result in clear isolation of the atomic line
from the hollow cathode lamp (HCL). If too wide a slitwidth is chosen stray radiation may cause
poor sensitivity.
Pe is eliminated by source modulation. Source modulation may be implemented by mechanical
chopping of the light beam, or by electronically pulsing the HCL source intensity. The signals may
be monitored and demodulated electronically.
Pba and Ps are handled by a number of techniques. These terms can lead to higher absorbances
than are really actually occurring since the scattering and background absorbances will reduce the
transmission of radiation. The flame or species within it beside the sample and its’ matrix may also
contribute background absorbances. The latter problem is solved by running a blank and zeroing
the absorbance, or by using the standard addition method.
Scattered radiation or backgrounds absorbances may be corrected for by one of the following
methods:
Optimization of Flame Atomic Absorption Settings
Optimizations are performed prior to and after flame ignition:
Prior to flame ignition:
 lamp position (the lamp’s beam must pass through the flame to the detector)
 wavelength( the wavelength optimization is most critical because if there the wavelength
does not match the energy of the electronic transition there will be no absorption, and
 slitwidth are adjusted to obtain the maximum transmittance signal.
After the flame has been ignited:
 the burner position is adjusted rotationally, horizontally and vertically to maximize the
absorbance signal produced when a standard solution is aspirated.
 the position of the impact bead and,
 the flame composition are also adjusted. The reason for these adjustments is to maximize
the signal produced by a sample, thereby enhancing the possibility of trace analyses.
Question: Why do analysts perform optimizations of the signal produced by a solution?
36
Background Correction
Assume you are looking at a flame and add a little strontium, the flame turns red due to strontium
emission. Now if you added sodium(yellow) potassium (lilac) and lithium (also red) you would not
be sure if there is strontium emission and your uncertainty increases. Though strontium radiation
is still being emitted is obscured by the other light and the flame may even emit the same colour
as the element. Analysts need to have methods of separating radiation from other sources even if
it is at the same wavelength of that analyte. This is called background correction.
1. Deuterium lamp (secondary continuum) correction
Two beams of radiation, one from the HCL and the other from the D2 lamp traverse very similar
paths through the flame. They are synchronized with the detector and a beam-chopping device.
Alternately, the beams pass through the flame and the HCL beam will experience both
absorbance by the analyte and interference due to scattering or background absorbance. The D2
beam, covering a much wider wavelength range experiences a negligible absorbance due to the
analyte but a significant background absorbance. The corrected absorbance is obtained by
subtraction of the D2 lamp’s signal from that of the HCL.
Sketch a D2 Background correction system below:
37
2. Near line method
The HCL may produce another line close to that of the analyte line of interest. The line may be
due to the filler gas emission, or another element included in the hollow cathode. The absorbance
of the secondary line is assumed to be solely due to background and is then subtracted from that
of the analyte at its wavelength. Disadvantages are that the near line may experience a
background absorbance different to that at the analytes wavelength thus introducing an error and
there may not even be a near line. It is essential that the ground state atoms in the flame do not
absorb any of the radiation of the near line – only scattering or brad band background absorption
should attenuate the near lines intensity.
3. Hhollow cathode pulse method
The HCL’s line may be broadened by pulsing the current. This broadening results from the extra
energy supplied to the lamp causing Doppler and collisional broadening. The broadened lines are
now similar to the continuum produced by the D2 lamp. The correction is also based on subtraction
of the pulsed absorbance from the un-pulsed one.
An advantage of this approach is that it does not require an extra lamp or major modifications to
the instrument.
38
Detection Limit and Sensitivity
Exercise: Recall an eye test you performed.
Initially you read the “E” easily, as it got small it became harder to read. Eventually you began to
guess. Our eyes are detectors and they have their limits. Instruments also “look” for analytes and
at some point they cannot “see” the analyte anymore. At or below a certain concentration the
results are guesswork and larger errors can be made. All analytical systems have certain levels
below which they become inaccurate.
Detection limit is defined as the concentration of the element that will produce a signal that is 2-3
times the average noise level of the baseline. The detection limit varies from on instrument to the
next – the reason for this variation is that each instrument is differently manufactured and may
have various dark currents due to stray radiation. Manufacturers may also try to boast the best
detection limit by stating the lowest detection limit that they have been able to attain. Analyzing a
blank 20 times can give a good estimate of the noise level – the std deviation of the analytical
signal is the y component(absorbance, emission of fluorescence). This std deviation should be
converted to the x component – typically concentration using the regression equation of the
calibration curve.
Sensitivity is the concentration of analyte that yields and absorbance of 0.0044 (or 1%
absorbance). The lower the concentration that yields 1% absorbance, the more sensitive the
instrument is considered to be. (An alternate definition for absorbance is defined as the slope of
the analytical curve abs vs. conc. in the linear region)
Questions:
1. What is the difference between detection limit and sensitivity?
2. Can you determine the detection limit and sensitivity for three of your analyses from the
CHA3011 practical course?
3. Could you determine a detection limit for a manual acid base titration?
4. Is it possible to alter a method to reduce the detection limit without changing the instrument?
Consider the analysis of copper in water.
Applications of Atomic Spectroscopy
The key strength of any analytical tool is its ability to solve the problems of industry, science,
government, and society. The popularity of atomic spectroscopy is an indicator of the diversity of
its application in the aforementioned fields. More than 70 elements may be determined using this
technique. Some of the latest technological advances allow the quantitative determination of 10
000 atomic lines in a sample in 10 seconds! A metallurgist may walk up to a large metal sample
hold gun-like probe against it for 20 seconds and receive a digital readout of the micro and macro
39
components of the sample in less than ½ a minute! Such instrument power combined with robotics
may well question the future of the human analyst in affluent and highly developed nations.
Sample Preparation
Flame based atomic spectroscopy usually requires that liquid samples be presented to the
instrument. (Numerous solid sample introduction devices are on the market but generally reduce
precision and raise capital costs) Solid samples require digestion in acids that are free of
interferent or contain an exactly known amount. An exact mass of sample has to be digested and
made to a precise volume.
Preconcentration is required when low levels of analyte are detected in samples. Electrodeposition, solvent extraction, ion exchange, and hydride generation are commonly used methods
of preconcentrating trace level analytes.
Jove video link: (9 min) https://www.jove.com/science-education/10205/sample-preparation-foranalytical-characterization
Generally sample ranging from ppm levels to greater than 1 % can be determined using atomic
spectroscopy. Highly concentrated sample should be diluted to less than 2%. A wide variety of
samples may be determined: cement and its raw materials (PPC), ferrous metals and ores
(ISCOR), agricultural and veterinary samples (Grootfontein), Catalytic converter industry(Umicore)
glass and raw materials (Shatterproof), petroleum products (Shell), medicinal products (Lennon,
Fresenius Kabi), bio-medical (hospitals), water (municipality), chemical manufacturing (Sasol).
Organic solvents
Occasionally a chemist will make use of organic solvents in atomic spectroscopy. Standards and
samples will both be in the organic solvent, which is then aspirated into the instrument. Not all
instruments can be effectively used with organic solvents since the flame composition will require
adjustment to suit the analysis.
The overall atomization efficiency in a flame is increased when an organic solvent is used instead
of water. This occurs because of:

Faster aspiration – lower viscosity of the organic solvent

Finer droplets – lower surface tension of the organic solvent

More efficient evaporation – weaker bonds between atoms

Combustible solvents enhance the flame by their own combustion
Solvent extraction is used to preconcentrate the sample (by extracting into a smaller volume) and
it improves the sensitivity due to the above-mentioned reasons. An additional advantage of solvent
extraction is that the matrix is left behind.
The introduction of an organic solvent into the flame results in fuel excess (orange flame that may
smoke), thus the flame composition has to be made lean to compensate. An oxidizing flame is
blue in color and tends to be smaller than a reducing flame under the same conditions.
40
A commonly used extractant is ammonium 1- pyrroidinecarbodithionate (APDC) dissolved in
methyl isobutyl ketone (MIBK)
A comparison of atomic emission and atomic absorption






Emission instruments do not require a separate lamp as a source, while absorption
instruments make use of a HCL.
Emission instruments require higher quality monochromators than absorption instruments
do because the HCL emits the exact wavelength required.
Emission methods require greater operator skill than absorption methods.
Interferences are similar for both techniques.
Background correction is more easily carried out for emission systems.
Atomic absorption is more forgiving in terms of precision than emission for unskilled
operators, but 0.5 -1% precisions are attainable for both techniques.
Methods of Quantification
Methods of quantification are the processes that are used to determine how much of an analyte
exists in a sample. Each method is different and has specific requirements. There are specific
advantages and disadvantages to each method and in selecting one method one must consider
factors such as cost, time available, equipment and the nature of the sample and its matrix(the
matrix is sample without the analyte). You should be able to recognize the method being used as
you read over a written description of an analysis.
Direct calibration involves preparing a series of standards in the same solvent as the sample as
well as preparing a blank to simulate the sample matrix. All the solutions are analyzed and a plot
of the analytical signal vs. the concentration or mass of analyte is used to determine the slope of
the analytical curve (or the equation describing it). From this curve or equation the unknown may
be determined after correcting for the blank.
Standard addition involves preparation of numerous solutions from a sample, each containing
exactly the same amount of sample. No standard analyte is added to one sample and increasing,
exactly known amounts of analyte are added to the remainder of the mixtures. (alternately a
sample may be read and increasing additions of standard may be made to the same sample and
the solution is read after each addition. From the readings taken a calibration graph may be
determined. The advantage of this method is that the matrix is present in all the standards and its
41
effect on the analytical signal is cancelled out by being equal in each solution – thus no blank is
needed. This process can become time consuming if many samples are to be analyzed.
Jove link to video on standard addition
education/10201/method-of-standard-addition
(11
minutes)
https://www.jove.com/science-
Internal Standard requires that a molecule ion or element which is not present in the sample be
added to the sample and the standard in exactly known amounts, preferably the exact same
amount of this substance must be added to the standards and the sample/s to simplify
calculations. The samples and standards are analyzed and the ratios of the sample signal to the
internal standard are used to calculate the unknowns concentration. Often this method involves a
plot of the ratio of analyte analytical signal to internal standard signal(the internal standard should
produce an essentially constant signal) against the analyte concentration. Note that such a plot will
exist for each analyte if there are more than one in a mixture. The advantage of this method is that
if the amount of sample entering the instrument varies from one sample to the next the final result
still depends on a ratio that remains constant despite variation of sample input. This advantage is
unique to the standard addition method.
Jove link to video on internal standard method (9 min) https://www.jove.com/scienceeducation/10225/internal-standards
Exercise: Draw flow diagrams showing each method of quantification
External standard method:
Standard addition method:
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Internal Standard Method:
Quality Control and Validation
Quality control often involves analyzing a sample and comparing the results with known for that
sample. Certified reference materials are a stable solutions of elements of exactly known
concentration. To ensure your method is giving the desired result you analyze the certified
standard and compare the result you obtain to the concentration of the standard. Useful statistics
include the average, std deviation, confidence limits(95% or p = 0.05), variance and the T –Test.
Instrument signals may drift i.e change slowly with time thus corrupting a series of results. To
detect such drift one should analyse a standard and a blank after every 3-5 analyses. Any
continuous upward or downward change indicated drift. Significant drift can cause untold problems
if not detected early in analyses.
Validation of analytical methods is the process of doing certain experiments to confirm the
correctness of results produce by an analytical method. The method involves the collection of the
sample right through to the calculation of the final result.
Important aspects of method validation include:
1. Representative sampling – Does the sample you collected represent the bulk?
2. Sample preparation – many factors come to play here – analyst skill, reagent and equipment
quality, chemical interactions etc.
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3. Analyte concentration range over which is suitable
4. Linearity of analytical signal vs concentration (if linearity deviates we can make errors using a
simple linear equation to determine concentration.
5. Detection limit (lowest concentration of analyte that can be determined)
6. Sensitivity – How steep the calibration curve is
7. Reproducibility – How similar are the results of analyzing the same solution repeatedly?
8. Ruggedness – What effect does a change of analyst have on the results?
9. Control charting – tracking the result of analysis of a certified reference material regularly over
an extended period e.g. 6 months
Analytical method validation is a large field and is dealt with in detail dedicated modules. In your
practicals you have covered some aspects: calibration, detection limit sensitivity and % error. Eight
useful statistics in analytical method development and validation are:
1. Q test – outlier rejection,
2. Standard deviations for calculating other statistic or rejection of outliers,
3. F test for comparing precision,
4. T test for comparing means,
5. Confidence limits for reporting uncertainty of your result or comparing means,
6. Analysis of variance(ANOVA) for studying effects of changes in procedures or variability of
results
7. Control charting for tracking consistency
8. Chi Squared test for studying the frequency of occurrence
Consider an analysis you have done. How many of these validation steps were included in
your method? How can you improve on your former method?
Analytical calculations
An inability to solve analytical problems is a severe weakness and results in an incompetent
analyst. Even having done the lab work brilliantly cannot make up for calculation errors that
produce the wrong result.
Use the following steps in problem solving:
1. Sketch out the problem – i.e. convert the text to diagrams that clearly show all steps – this
will enable you to consider all the steps and get a clear understanding of the problem.
2. Determine the method of quantification – i.e. is it standard addition internal standard or
direct calibration?
3. Note the analytical signal and calculate the sample concentration from the data given. This
may involve plotting a graph or simply calculations.
4. Once you have the sample concentration go back to the sketch and take into account any
dilutions to determine the sample concentration in the original sample.
5. Check the problem for errors if time permits.
Additional analytical techniques of interest include X-Ray methods, Nuclear Magnetic Resonance
and Poparimetry. Below are JoVE videos that serve to introduce the topics.
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X-Rays in Analysis
X-ray fluorescence (XRF)
JoVE link to video with a PDF transcription: https://www.jove.com/science-education/5498/x-rayfluorescence-xrf
Nuclear Magnetic Resonance(NMR)
JoVE video link NMR (10 min)
https://www.jove.com/science-education/5680/nuclear-magnetic-resonance-nmr-spectroscopy
Polarimetry
JoVE video on polarimetry (7 min) https://www.jove.com/science-education/10348/polarimeter
Atomic Spectroscopy Tutorial
1.
2.
3.
4.
4.1
4.2
4.3
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Which regions of the electromagnetic region are used for atomic spectroscopy. 2
Define spectroscopy.2
Mention three electromagnetic phenomena used in atomic spectroscopy and briefly describe each.6
Beer-Lambert law:
Write it down and define all symbols. 3
How is absorbance determined? 3
Discuss the limitations of the Beer-Lambert Law. 7
Use an equation to define fluorimetry. Define all terms. 4
Mention seven high temperature excitation sources used for atomic spectroscopy. Briefly describe each. 7
Sketch a monochromator with a movable detector and describe how it works.5
Atomic lines are inherently narrow but in practice they are widened considerably. Explain. 7
Compare flame atomization to electrothermal atomization. 4
Compare PMT and PDA detectors. 8
What is a photocell and how does it work? 3
Mention and compare two types of nebulizer. 10
Discuss interferences in flame atomic spectroscopy. 6
What is ICP, discuss in detail. 8
Compare plasmas and flames. 8
Discuss electrode options for arc or spark spectroscopy.6
Adapt the Beer-Lambert law for interferences and sources of error. 5
What gases are commonly used in flame AA or AE? Specifcy which is the oxidizer and fuel. 4
Define detection limit and sensitivity.
21 What is the use of knowing the detection limit of an instrument.
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22 Write down 10 applications for atomic spectroscopy, in each case mention how you would prepare the
sample.
(20)
23 Write down and briefly describe five methods of preconcentration.
24 Discuss the use of organic solvents in flame spectroscopy. 4
25 Discuss optimization of flame AA settings. 6
26 Compare AA and AE
27 Compare three methods of quantification in detail, use diagrams in your answer. 15
28 Suggest five steps to be used in solving an analytical problem.5
29 You determine the manganese content of an ore. 4.8876 g of ore is dissolved in acid and made up to 500 mL. 5.00
ml is added to a 100 mL flask and is made up to volume. To each of four 100 mL flasks a 5.00 mL aliquot of sample is
added. Increasing aliquots of a 100 ppm Mn2+ solution are added to each flask (1.00ml, 2.00mL, 4.00 mL and 8.00 mL,
respectively). The solutions are read on and AA spectrometer.
The diluted sample reads 0.089 while the remaining solutions read 0.129, 0.170, 0.249, and 0.409.
29.1 What method of quantification is this? 1
29.2 Determine the manganese content of the ore. 7
29.3 Why would you use this method of quantification.1
30 A sample of contaminated flour is analyzed on an ETAA unit for its Pb content. A sample of flour is analyzed and
then a lead salt is added to separate flour samples in increasing amounts. The results of the experiment are tabulated
below:
SAMLPE ID
SAMPLE
Std 1 Std2
Std3
Mass Pb added mg/100g
none
0.20
0.30
0.40
Pb absorbance
0.104
0.280 0.418
0.554
30.1 Mention how you present this sample to the instrument. i.e. what form would it take? Justify your answer. 2
30.2 What method of quantification is being used and comment on its suitability.3
30.3 Graphically determine the Pb content of the flour. 4
31. You are requested to analyze a sample of rock for chromium. Preliminary analysis with an
XRF reveals that there is about 5% Cr and no copper. A 4.6722g of ore is dissolved in nitric acid
and is made to 200 mL. An aliquot of 25 mL is further diluted to 500 mL. Then an aliquot of 10 mL
is taken from the 500 mL flask and is added to a 100 mL flask along with 5 mL of Copper soln to
make it 2.5 ppm Cu when made to volume. A A 5.00 mL aliquot of copper standard is added to
each flask to obtain a copper concentration of 2.5 ppm Cu when each flask has been made to
volume. Chromium is added, in increasing amounts to each standard, except the first one which is
made directly up to volume. The following data are generated:
SAMLPE ID
Copper Absorbance
Cr absorbance
[Cr] microgram per
milliliter
SAMPLE
0.291
0.144
Std 1
0.345
0.080
2.02
Std2
0.341
0.131
4.04
Std3
0.346
0.180
6.06
Std4
0.343
0.229
8.08
Std5
0.345
0.277
10.10
31.1 Determine the Cr content using the external std methods’ data. 7
31.2 Determine the Cr content using the internal std methods’ data. 8
31.3 Why do the results differ? Suggest a reason for the difference. 2
32. Discuss the use of blanks, reference standards, recovery testing, control charts, Q Test, TTest, and F-Test in the field of quality control in atomic spectroscopy. 12
33. How can you apply ANOVA and confidence limits in spectroscopy? 6
34. Discuss qualitative and quantitative analysis using atomic spectroscopy. (4)
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Study Guide:
SPECTROSCOPY
Subsection: Analytical Chemistry 3a / Analytical Spectroscopy
(Code/s: CHA3021 / CHA3001)
Objectives:
To teach the student basic atomic spectroscopic theory covering flame atomic absorption and
emission and introductory electrothermal atomization and Inductively coupled plasma. The student
should know the theory and be able to apply it with the goal of accurate analysis of samples using
these techniques.
Outcomes:
A student possessing:
Knowledge of the basics of atomic spectroscopy and molecular spectroscopy.
Knowledge of flame spectroscopy, introductory ICP & electrothermal analysis, and applications of
these techniques. Molecular spectroscopy includes knowledge of the theory and practice of
infrared, uv and visible spectroscopy for the analysis of molecules and ions. The student will also
know what technique is best suited for a given analytical problem.
The ability to sketch diagrams describing depicting the above techniques and corresponding
sample introduction methods.
Knowledge of the advantages and disadvantages of atomic spectroscopy (including flame AA,
AE, ICP, and electrothermal analysis) and molecular spectroscopy (IR, and UV/Viz spectroscopy).
The ability to solve practical analytical problems i.e. to be able to determine the concentration of
elements in solid, liquid or gaseous samples from given data.
Syllabus:
Electromagnetic Radiation, Electromagnetic Spectrum, Absorption, Emission, Scattering, BeerLambert Law, Interferences, Atomic Emission Spectroscopy (AES) , Flame Excitation Source,
Inductively-Coupled Plasma (ICP) Excitation Source, Plasmas versus Flames, Laser-Induced
Breakdown Excitation Source(LIBS), Spark and Arc Emission Sources, Detectors, Sample
Background Correction, Applications of atomic spectroscopy, Sample preparation, A comparison
of atomic emission and atomic absorption, Methods of quantification, Direct calibration , Standard
addition, Internal standard method, Organic solvents and flame spectroscopy.
Infrared spectroscopy, IR instrumentation – Fourier transform vs. conventional dispersive
technology, sample requirements and preparation for IR, UV/VIZ instrumentation single and
double beam systems, sample holders. Spectroscopic titrations and direct spectrometric
measurements. Elementary knowledge of X Ray fluorescence.
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Evaluation:
CHA3021/CHA3001 students will be evaluated continuously, so tests and assignments will
contribute to the final mark. Due to covid 19 the evaluation mode may be online or written in the
classroom, we will only know as we progress through the year. A retest on selected portions of the
syllabus will be given for students whose marks are between 45 and 50%, and then a maximum
mark of 50% may be awarded.
Course Period
Feb to May
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