Analytical Chemistry 2
Chapter 1
Measurement Principles and
Electronics
Instructor: Dr. Nguyen Thao Trang
Outlines
CLASSIFICATION OF ANALYTICAL METHODS
TYPES OF INSTRUMENTAL METHODS
INSTRUMENTS FOR ANALYSIS
SELECTING AN ANALYTICAL METHOD
CALIBRATION OF AN INSTRUMENTAL METHOD
Classification of analytical methods
Classical vs Instrumental
Qualitative instrumental analysis is that measured property
that indicates presence of analyte in matrix.
Quantitative instrumental analysis is that magnitude of
measured property that is proportional to concentration of
analyte in matrix.
Species of interest
All constituents including analyte.
Often need pretreatment - chemical extraction, distillation,
separation, precipitation.
Classification of analytical methods
CLASSICAL:
Qualitative - identification by color, indicators, boiling
points, odors
Quantitative - mass or volume (e.g. gravimetric,
volumetric)
INSTRUMENTAL:
Qualitative - chromatography, electrophoresis and
identification by measuring physical property (e.g. spectroscopy,
electrode potential)
Quantitative - measuring property and determining
relationship to concentration (e.g. spectrophotometry, mass
spectrometry). Often, same instrumental method used for
qualitative and quantitative analysis.
Types of instrumental methods
PROPERTY
Radiation Emission
EXAMPLE METHOD
Emission spectroscopy - fluorescence,
phosphorescence, luminescence
Radiation Absorption Absorption spectroscopy spectrophotometry, photometry, nuclear
magnetic resonance, electron spin
resonance
Radiation Scattering Turbidity, Raman
Radiation Refraction Refractometry, interferometry
Radiation Diffraction X-ray, electron
Radiation Rotation
Polarimetry, circular dichroism
Types of instrumental methods
PROPERTY
Electrical Potential
Electrical Charge
Electric Current
EXAMPLE METHOD
Potentiometry
Coulometry
Voltammetry - amperometry,
polarography
Conductometry
Gravimetry
Mass spectrometry
Stopped flow, flow injection analysis
Thermal gravimetry, calorimetry
Electrical Resistance
Mass
Mass-to-charge Ratio
Rate of Reaction
Thermal
Characteristics
TYPES OF INSTRUMENTAL METHODS
Radioactivity
Activation, isotope dilution
Instruments for analysis
Block diagram for the overall process of instrumental measurement.
Example: spectrophotometry
Instrument:
spectrophotometer
Stimulus:
monochromatic light energy
Analytical response: light absorption
Transducer:
photocell
Data:
electrical current
Data processor:
current meter
Readout:
meter scale
Instruments for analysis
DATA DOMAINS: way of encoding analytical response in
electrical or non-electrical signals.
Interdomain conversions
domain to another.
transform information from one
Detector :
device that indicates change in environment
Transducer : device that converts non-electrical to electrical
data
Sensor :
device that converts chemical to electrical data
Instruments for analysis: domain
NON-ELECTRICAL DOMAINS
Physical (light intensity, color)
Chemical (pH)
Scale Position (length)
Number (objects)
Time:
Analog:
Digital:
ELECTRICAL DOMAINS
Current (Analog)
Voltage (Analog)
Charge (Analog)
Frequency (Time)
Pulse width (Time)
Phase (Time)
Count (Digital)
Serial (Digital)
Parallel (Digital)
vary with time (frequency, phase, pulse width)
continuously variable magnitude (current, voltage, charge)
discrete values (count, serial, parallel, number)
Defining the problem
What accuracy is required?
How much sample is available?
What is the concentration range of the analyte?
What components of the sample will cause interference?
What are the physical and chemical properties of the sample
matrix?
How many samples are to be analyzed?
Selecting an analytical method
Performance Characteristics: Figures of Merit
• How to choose an analytical method? How good is
measurement?
• How reproducible? - Precision
• How close to true value? - Accuracy/Bias
• How small a difference can be measured? - Sensitivity
• What range of amounts? - Dynamic Range
• How much interference? - Selectivity
Figures of merit: precision
INDETERMINATE OR RANDOM ERRORS
Figures of merit: accuracy, sensitivity
DETERMINATE ERRORS (operator, method, instrumental)
(larger slope of calibration curve m, more sensitive measurement)
Figures of merit: detection limit
Signal must be bigger than random noise of blank
Figures of merit: dynamic range
At detection limit we can say confidently analyte is
present but cannot perform reliable quantitation
Level of quantitation (LOQ): k=10
Limit of linearity (LOL): when signal is no longer
proportional to concentration
Figures of merit: selectivity
No analytical method is completely free from
interference by concomitants. Best method is more
sensitive to analyte than interfering species
(interferent).
k's vary between 0 (no selectivity) and large number
(very selective).
Calibration methods
Basis of quantitative analysis is magnitude of measured
property is proportional to concentration of analyte
Calibration curves
Sample problem
Sample problem
Calibration of instrumental methods
Calibration Curves: several standards containing exactly known
concentrations of the analytes are introduced into the
instrument and the response is recorded.
• Standard addition method
– Known quantities of analyte are added to the unknown.
From the increase in the signal, how much analyte can be
deduced.
Calibration of instrumental methods
• Internal standard method
- An internal standard is added in constant amount to samples,
blanks, and calibration.
- The analyte to internal standard signal as a function of the
analyte concentration is determined.
- The ratio for the samples is then used to obtain their analyte
concentrations from a working curve.
-
Internal standard addition is applicable when a sample loss
occurs during preparation prior to analysis.