5
ATOMIC SPECTROSCOPY
5.1 Introduction
Atomic species include neutral atoms, as well as elemental ions. They are not bonded or associated
with any other species, and thus are termed "free". Only the valence (outer-shell) electrons orbiting
the nucleus are capable of absorbing energy. In atomic species, the energy levels are few and far
between. As a consequence, the atomic spectrum, whether absorption or emission, will consist of a
series of single wavelengths, known as a line spectrum, as you have already seen in Chapter 1.
Both atomic emission and absorption techniques are generally limited to metallic elements,
since one of the characteristics of metals is that they have valence electrons that are “easily” moved
from their normal orbital. Remember, when a metal atom ionises, it loses its electron completely. So
the absorption of energy to move the electron to a higher energy level can be considered as simply the
first step towards ionisation.
Non-metal atoms, such as oxygen and chlorine, hold their valence electrons tightly in the lowest
orbitals, since non-metals generally are electronegative, and "search" for more electrons rather than
losing them. Therefore, it is much less likely that non-metal valence electrons would be capable of
absorbing energy to move to a higher orbital. The same applies to ionised metals, such as Na +, which
have a full outer shell of electrons.
The critical aspect of atomic spectroscopy – absorption or emission - is that the analyte atom
must be in the form of free and neutral atoms, known as an atomic vapour, a gaseous cloud of atoms.
Obviously, the production of a such a state requires considerable energy in itself. Over the years,
various means of achieving this aim have been developed, among them flames, electrical arcs and
heating, plasmas and lasers.
In this chapter we will cover the following techniques:

flame atomic absorption spectrophotometry (AAS)

flame emission photometry

inductively coupled plasma (ICP) emission spectroscopy
5.2 Flame generation of the atomic vapour
Sample Requirements
The principal sample requirement for each technique examined in this chapter is that it must be in a
liquid state, so that it can be drawn into (aspirated) the flame (or plasma). Thus, solid or gaseous
samples must be dissolved in a suitable solvent (which can be organic, but not a highly flammable
one) for analysis.
The liquid sample needs to have the following characteristics:

non-viscous (otherwise it will not aspirate properly and will most likely clog up the sample
transport system)

free of suspended solids (which will block up the nozzle)

free of dissolved gases (which will disrupt the even combustion of the flame gases)
CLASS EXERCISE 5.1
How do you ensure your solution will meet the requirements above?
Viscosity
Suspended solids
Dissolved gases
5. Atomic Spectroscopy
Sample atomisation
The most common atomisation device consists of a nebuliser, which produces a fine spray from the
liquid sample, and an energy source – for example, a flame - which evaporates the solvent and
decomposes any compounds into the atomic state. Figure 5.1 shows the sequence of processes
undergone before any radiation is absorbed or emitted.
Solution
nebulisation – formation of fine droplets;
large droplets removed to waste
NEBULISER
Aerosol
removal of solvent by evaporation
Solid
particles
FLAME/
PLASMA
molecules melt and boil
Gaseous
molecules
ionic molecules are not stable in gaseous
state, so they fall apart into atoms
Atomic
vapour
FIGURE 5.1 Atomisation processes
CLASS EXERCISE 5.2
For a solution of sodium chloride, write a series of chemical equations that describe what is
happening.
Nebulisation
NaCl (aq) 
Loss of solvent
Vaporisation
Atomisation
Energy sources
As mentioned above, a number of energy sources have been employed in atomic spectroscopy. In this
section and later sections, we will concentrate on the flame.
The flame is generated by a combination of gases, supplied under high pressure. These gases
can be divided into two types, the flame requiring one of each: fuels and support/oxidants. The most
common fuels used for flame production are ethyne (acetylene) and natural gas (propane). The
common oxidants are air, oxygen and nitrous oxide (N2O). Figure 5.2 shows the temperature ranges
obtained from the common gas combinations.
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5.2
5. Atomic Spectroscopy
Natural gas/air
1800
Acetylene/air
2100
Acetylene/N2O
2400
2700
3000
3300ºC
FIGURE 5.2 Flame temperatures
5.3 Atomic Absorption Spectrophotometry
The technique of atomic absorption spectrophotometry (AAS) has become one of the most widely
used analytical tools for the determination of metal concentrations. AAS is used for the analysis of
over 60 metals and metalloids, and works well for concentrations typically ranging from 1-100 mg/L.
It was developed by the Melbourne laboratories of the CSIRO in the late 1950s. Unfortunately,
like numerous other Australian inventions, no one was prepared to invest the time and money required
to take the idea through to the final product. Thus, the rights to the idea were sold to Varian
Corporation in the USA, who have reaped considerable profits in the last 25-30 years selling the
instruments around the world. Only recently has an Australian company bought 50% of the rights to
the technology. However, it is considered that the main reason for Varian loosening its grip on the
rights is because AAS will be eventually superseded by a new technology, known as ICP (inductively
coupled plasma), which has several advantages over AAS.
Instrumentation
The basic components of an AA spectrometer is shown in Figure 5.3, and in general is similar to other
absorption instruments. However, two of the components are substantially different in their detail to
those previously described for UV-visible and IR instruments. The major differences are:

the radiation source is specific for each element, and

the sample cell is a flame
Hollow
cathode
lamp
Monochromator
Detector &
readout
Sample
aspirator
FIGURE 5.3 Schematic diagram of an atomic absorption spectrometer
RADIATION SOURCE
Atomic absorption lines are very, very narrow: the peaks are approximately 0.005 nm wide. For this
reason, a continuous spectrum radiation source, such as those in the UV/Vis and IR instruments would
not work. The tiny “sliver” of radiation absorbed would be undetectable: like trying to count how
many grains of sand are missing from the beach!
So to make atomic absorption measurable, we must get rid of all the wavelengths other than the
exact right one for a particular element. Monochromators can’t do this because they can only cut
down the range to 0.1 nm, which is 20 times too wide.
The development of a special radiation source for each element – known as a hollow cathode
lamp – was the major invention by the CSIRO which led to the development of the AAS instrument.
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5.3
5. Atomic Spectroscopy
FLAME GASES
As described in an earlier section, the purpose of the flame in atomic spectroscopy is to create an
atomic vapour of the analyte. The flame needs to be hot enough to vaporise and atomise the sample
without being too hot, because other problems can then arise. Three gas combinations are commonly
used in AAS: natural gas/air, acetylene/air (the most common) and acetylene-nitrous oxide. Table 5.1
summarises the characteristics and applications of the three gas combinations.
TABLE 5.1 Flame gas temperatures and applications
Gas combination
Natural gas/air
Max. Temp. (K)
1800
Elements
Group IA (eg Li, Na, K)
Acetylene/air
2250
Transition metals (eg Fe, Cu, Zn)/
lanthanides (eg Ce) /actinides (eg U)
Acetylene/N2O
3000
Groups 2A (Ca, Mg, Ba), 3A (Al)
The fuel:oxidant ratio, known as the flame stoichiometry, is an important variable in flame
spectroscopy. The fuel and oxidant are generally combined in approximately their standard reaction
ratio - a stoichiometric flame). However, in some circumstances, a reducing flame, which contains an
excess of fuel, is used, even at the cost of some heat. A hotter flame is obtained by maximising the
combustion of the fuel by ensuring that the oxidant is in excess - an oxidising flame.
Quantitative Analysis
Beer's Law applies equally well to atomic absorption measurements, and the same principles apply as
described in Chapter 2: calibration standards are prepared to cover the absorbance range of 0.1 to 1.0
(or thereabouts) and samples measured under the same conditions.
However, matrix interference is much more of a problem in AAS than it is in UV/VIS
absorption, for example. Matrix interference is the change in measured response of the analyte due to
substances in the analyte. Basically, what it means that a sample containing, for example 10 mg/L of
the analyte, gives a different reading to a standard with the same concentration. Looked at another
way, if a sample gave the same reading as a standard, you would assume they have the concentration,
but if matrix interference is occurring then an error arises.
To get around this problem, it is necessary to duplicate the matrix in the sample as closely as
possible in the standards. This is known as matrix matching and is not particularly easy, for while the
major matrix components are generally known, minor ones may vary from sample to sample, and it
may be these which cause the interference.
STANDARD ADDITION
Where matrix matching is not possible, a method known as standard addition can be used to correct
for matrix interference. In this method, a known amount of analyte is added to the sample (this
solution becomes the “standard”), and its absorbance compared with that of the original sample. In
this way, the matrix of the standard is exactly that of the sample, since the sample is being used as the
"solvent". You are not required to undertake standard addition calculations in this subject.
5.4 Flame Emission Spectroscopy
Historically, the oldest spectroscopic technique, flame emission of metals have been observed since
the eighteenth century, when various experimenters observed the colours introduced into candle
flames by different metals. It was shown by the inventor of the Bunsen burner that the source of the
metal (be it pure element or any salt) did not change the colour or the wavelengths of the emission
spectrum.
Once spectroscopes were developed sufficiently, the individual wavelengths characteristic of
each metal were examined, and found to be constant for a particular metal, and characteristic of that
metal. Thus, qualitative identification of metal mixtures became relatively easy, and indeed a number
Sci Inst Analysis (Spectro/Chrom)
5.4
5. Atomic Spectroscopy
of elements, including caesium and rubidium, were discovered through the presence of previously
unknown spectral emission lines.
The first experiment in quantitative flame analysis dates back to 1873, where a basic visual
comparison system was used for sodium-containing samples. The first instrumental device for the
detection of emission intensity was developed in the 1930s, and employed a monochromator, a lightsensitive cell as a detector and an ammeter. The replacement of the monochromator with a far
simpler, smaller and cheaper filter system came about soon after, and forms the basis for a range of
modern flame emission instruments, known as flame photometers. The basic construction of a such a
device is shown in Figure 5.4.
Filter
Detector &
readout
Sample
aspirator
FIGURE 5.4 Schematic representation of a flame photometer
Flame photometers are built as an economical and simple-to-operate alternative to atomic absorption
spectrophotometers for the analysis of certain elements. As mentioned, the absence of a
monochromator makes the instrument much cheaper and smaller, and also less sensitive to outside
influences, such as vibration. The flame photometer is therefore much more rugged and less prone to
breakdown. Modern instruments for the analysis of Na/K/Li cost about $2,000.
A filter system would not work if the flame produced too many emission lines other than the
analyte’s. So the other difference to AAS is the use of a propane/air mixture for flame gases. This
produces a lower temperature and reduces the number of emission lines down to only those very easily
excited elements: Na, K, Ca and Li. It also makes the burner simpler and cheaper! The most intense
lines emitted by these elements - those used for quantitative analysis - are listed below in Table 5.2.
TABLE 5.2 Emission lines used for quantitative analysis
Element
Li
Wavelength (nm)
670.8
Na
589.0, 589.6
K
765.5, 769.9
Filters
The radiation emitted by a sample will be a combination of the characteristic wavelengths of each
element in the sample. Even those elements not suitable for flame photometric analysis still give a
weak emission spectrum in the flame. For example, copper gives a pale green colour to even a Bunsen
flame. Therefore, a filter which blocks out all radiation other than that of the analyte is needed.
As can be seen in Table 5.1, the characteristic lines of the three common analytes are well
spaced, and filters that selectively allow only the analyte radiation are easily chosen. Generally, the
manufacturer builds a filter system into the instrument, and the technician needs only to change the
element selector, rather than swapping filters. This does mean that analysis of elements not originally
designed for the instrument is impossible, since there is no easy way of including a filter for the new
analyte.
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5.5
5. Atomic Spectroscopy
Calibration
Emission is measured by the intensity of radiation, and there are no “rules” about intensity ranges, like
there are for absorption measurements. The actual intensity number has no meaning by itself, only by
comparison with other values measured at the same time. The normal methods for calibration of an
emission instrument is:
1. Select the correct filter setting for the analyte.
2. Zero the instrument by aspirating a blank solution, which may contain all other species used in the
standards, other than the analyte.
3. Using the highest concentration standard, adjust emission intensity to full scale or at least 100*,
depending on the style of readout.
* there is nothing magical about the number 100 – it can be your favourite number, as long as it is a
large number, so that the calibration graph will show a wide range of emission intensities, ensuring
better precision.
Quantitative analysis
There is no equivalent to Beer’s Law that defines the relationship between emission intensity and
concentration of analyte. However, the intensity does increase with concentration, but is not perfectly
linear. Table 5.3 shows the typical concentration ranges for the common elements.
TABLE 5.3 Typical concentration ranges
Element
Na
Concentration Range (mg/L)
1-10
K
1-50
Li
3-100
If the calibration standards do not produce a straight line graph between intensity and concentration,
use of base 10 logarithms can help: a plot of log10(intensity) vs log10(concentration) should be tried.
Excel can do this automatically for you with a scatter graph, where an logarithmic scale option in
Format Axis is selected.
EXAMPLE 5.1
Potassium standards were prepared over a range of 25-100 mg/L and their emission
intensities measured as shown below. The data was plotted directly and by the log-log
method to find the better straight line.
Concentration (mg/L)
25
50
75
100
Sci Inst Analysis (Spectro/Chrom)
Emission Intensity
45
68
85
99
5.6
5. Atomic Spectroscopy
110
100
100
long (intensity)
Intensity
90
80
70
60
50
40
10
25
45
65
Conc.
85
10
100
log (conc)
Plotting this data in the different ways clearly shows that the log graph produced the better
straight line calibration graph.
CLASS EXERCISE 5.3
Using graph paper, plot a log-log graph for the data above, and determine the concentration
of a sample with an intensity of 74.
Hint – what do you have do with sample intensity before using the graph? What do you do
with the result from the graph to convert it to concentration?
Interferences
Similar chemical interferences occur in flame photometry as those in AAS, but because the technique
is mainly used for the Group I elements, ionisation effects are the most important. Sodium and
potassium are normally both present in any real sample and their presence increases the emission
intensity of each other. For example, the addition of 200 mg/L of sodium to a pure 20 mg/L potassium
solution almost doubles the emission intensity of potassium.
The simplest method of coping with this is to prepare matrix-matched standards which contain
not only the analyte at the desired concentrations but also the other major species present in the
sample at equivalent levels. Standard addition is also a possibility.
However, the emission intensity can change for another reason, totally unrelated to the matrix:
flame conditions. It is extremely difficult to maintain consistent flame conditions over the course of an
analysis run. Fluctuations in gas flow from the cylinders and other variations in flame temperature
will cause changes in intensity from the same sample. Matrix matching and standard addition are of
no help here. The method of internal standards described below is very commonly used in flame
emission to combat this problem.
Internal Standards
An internal standard is a substance added to all solutions, blanks, standards and samples at the same
concentration in all. Its emission intensity is measured at the same time as that of the analyte for each
solution. Make a note of this! You do NOT measure the analyte intensity of all the solutions, then go
back and measure the internal standard intensity afterwards!
The idea behind the technique is that if analyte emission is affected by certain flame variations,
the internal standard will be likewise affected.
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5.7
5. Atomic Spectroscopy
EXAMPLE 5.2
A sodium standard with lithium internal standard gives a sodium intensity of 100 and lithium
10. Thus the ratio is 10. When run again, the flame temperature has decreased through loss
of oxidant pressure, causing a decrease of 20% in emission intensity for both species.
Sodium will drop down to 80 and lithium 8, but the ratio of Na/Li intensity will remain
unchanged at 10.
You are not required to carry out these calculations in this subject.
5.5 ICP Emission Spectroscopy
AAS dominated metals analysis for 40 years, but in the 1980s, a new technique, based on emission,
was developed: ICP emission spectroscopy. ICP stands for inductively coupled plasma: only the last
word of these three need interest you. A plasma is a cloud of ions and electrons, which occurs at high
or very high temperatures. The sun is a plasma at a few million degrees. Plasmas can be generated
safely in the laboratory at around 7000-10000 K. It is this energy source that is used in the new
instruments instead of a flame. The rest of the instrument is more or less the same: monochromator,
detector etc.
The high temperatures and plasma conditions remove many of the problems associated with
flames. In comparison to AAS and flame emission, ICP emission spectroscopy offers the following
advantages:

less matrix interferences

greater sensitivity

greater linear response

more elements analysable
As times goes by, the currently more expensive ICP technique will become the standard method for
elemental analysis at concentrations from 50 ug/L upwards.
What You Need To Be Able To Do
 describe how atomic absorption and emission spectra occur
 describe what an atomic vapour is and why it is necessary in atomic spectroscopy
 describe the atomisation process
 indicate what gases are commonly used in AAS and flame photometry
 indicate what the role of each gas is
 indicate what elements are commonly analysed by AAS and flame photometry
 using schematic diagrams, show how a basic flame photometer and AAS are constructed
 describe the various interferences that occur in flame spectroscopy
Terms And Definitions
Match the term with the definition.
1.
3.
5.
7.
line spectrum
oxidising flame
nebuliser
internal standard
A.
B.
C.
D.
cloud of ions and electrons
cloud of atoms
analytical technique designed to compensate for matrix interference
flame with an excess of fuel
Sci Inst Analysis (Spectro/Chrom)
2.
4.
6.
8.
atomic vapour
reducing flame
standard addition
plasma
5.8
5. Atomic Spectroscopy
E.
F.
G.
H.
spectrum of narrow absorption or emission peaks generated by atoms
device used to convert solutions into mist
flame with an excess of oxidant
analytical technique designed to compensate for flame variations
Review Questions
1.
Why can't organic molecules be analysed by flame emission or absorption spectroscopy?
2.
Describe the processes leading to the absorption of radiation, when a sample containing sodium
is introduced into a flame.
3.
Why is a filter system used in flame photometry?
4.
Why are Na, K and Li often the only elements available for analysis with commercial flame
photometers? What changes to such an instrument would be necessary to analyse elements such
as copper and iron?
5.
An acetylene/nitrous oxide gas mixture is used for production of the flame in the analysis of
calcium and magnesium by AAS.
(i) What does this gas mixture provide?
(ii) Why is it needed for these elements?
(iii) Potassium is often added to samples containing calcium before AAS analysis. Explain
why.
6.
Why does self-absorption of radiation occur in solutions of high concentration only?
7.
Why is a propane/air mixture, and not acetylene/air, used for Na and K analysis in both flame
photometry and AAS?
8.
Why is standard addition used in flame spectroscopy?
9.
For the following sets of flame photometry data, determine a linear or logarithmic plot is better,
and use it to determine the sample concentration.
SET I
Conc. (mg/L)
Intensity
1
19
2
40
3
59
5
103
Sample
51
10.
SET II
Conc. (mg/L)
20
40
60
80
Sample
Intensity
38
65
83
100
71
0.1032 g of a sample of potato chips were mixed with water until all the salt had dissolved, the
liquid filtered and made up to 100 mL. A 25 mL aliquot of this solution was diluted to 200 mL,
and analysed by flame photometry, using a calibration graph obtained from the following data.
Determine the % Na in the sample.
Conc. (mg/L)
10
20
30
40
Sample
Sci Inst Analysis (Spectro/Chrom)
Intensity
27
51
79
105
60
5.9
5. Atomic Spectroscopy
11.
A 12.841 g sample of ground peanuts was continuously extracted with water to dissolve all
ionic material. The filtered extract was then made up to 100 mL. To analyse the potassium
content in the sample, a 10 mL aliquot was diluted to 250 mL. In each of the potassium
standards, enough sodium chloride was added to give a concentration of 200 mg/L Na.
Potassium emission values for each solution were as follows.
Conc. K (mg/L)
10
20
40
60
Sample
Intensity K
17
31
69
100
64
(i) Calculate the mg K/kg peanut in the sample.
(ii) Explain why 200 mg/L sodium was included in each standard.
Sci Inst Analysis (Spectro/Chrom)
5.10