Chapter 1: Classification of Analytical Methods

Naaimat Muhammed

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Introduction:

Analytical Chemistry deals with methods for determining the chemical composition of samples of matter. A qualitative method yields information about the identity of atomic or molecular species or the functional groups in the sample; a quantitative method, in contrast, provides numerical information as to the relative amount of one or more of these components.

Analytical methods are often classified as being either classical or instrumental. This classification is largely historical with classical methods, sometimes called wet-chemical methods, preceding instrumental methods by a century or more.

Classical Methods:

Separation of analytes by precipitation, extraction, or distillation.

Qualitative analysis by reaction of analytes with reagents that yielded products that could be recognized by their colors, boiling or melting points, solubilities, optical activities, or refractive indexes.

Quantitative analysis by gravimetric or by titrimetric techniques.

In the early years of chemistry, most analyses were carried out by separating components of interest in a sample by precipitation, extraction, or distillation. For quantitative analyses, the separated components were then treated with reagents that yielded products that could be recognized by their colors, boiling points or melting points, their solubility in a series of solvents, their odors, their optical activities, or their refractive indexes. For quantitative analyses, the amount of analyte was determined by gravimetric or by titrimetric measurement.

Gravimetric Methods – the mass of the analyte or some compound produced from the analyte was determined.

Titrimetric Methods – the volume or mass of a standard reagent required to react completely with the analyte was measured.

Instrumental Methods:

Measurements of physical properties of analytes, such as conductivity, electrode potential, light absorption, or emission, mass to charge ratio, and fluorescence, began to be used for quantitative analysis of a variety of inorganic, organic, and biochemical analyte. Highly efficient chromatographic and electrophoretic techniques began to replace distillation, extraction, and precipitation for the separation of components of complex mixtures prior to their qualitative or quantitative determination. These newer methods for separating and determining chemical species are known collectively as instrumental methods of analysis.

Instrumentation can be divided into two categories: detection and quantitation.

Measurement of physical properties of analytes - such as conductivity, electrode potential, light absorption or emission, mass-to-charge ratio, and fluorescence-began to be employed for quantitative analysis of inorganic, organic, and biochemical analytes.

Efficient chromatographic separation techniques are used for the separation of components of complex mixtures.

Instrumental Methods of analysis (collective name for newer methods for separation and determination of chemical species.)

Define The Problem

In order to select an analytical method intelligently, it is essential to define clearly the nature of the analytical problem. Such a definition requires answers to the following questions.

1. What accuracy and precision are required?2. How many samples are available?3. What is the concentration range of the analyte?4. What components of the sample will cause interference? /What else is present?5. Where is the origin of the sample?6. What are the physical and chemical properties of the sample matrix?7. When was it released or discharged?8. How/ under what circumstances will certain reactions occur?

Instrumentation is necessary to decipher these values. The challenge for the instrumental scientist is to mimic the 5 senses. Substances have physical and chemical fingerprints with unique thresholds. The object is to detect a chemical substance within a matrix and selectively perturb the substance of interest. Signals must be readable (in a voltage or electrical signal).

Signals Employed in Instrumental MethodsTable 1-1

The table below lists the names of instrumental methods that are based upon various analytical signals.

Signal Instrumental MethodsEmission of radiation Emission spectroscopy (X-ray, UV,

visible,electron, Auger); fluorescence,phosphorescence, and

luminescence

(X-ray, UV, and visible)Absorption of radiation Spectrophotometry and

photometry (X-ray, UV, visible, IR); photoacoustic spectroscopy; nuclear magnetic resonance and electron spin resonance spectroscopy

Scattering of radiation Turbidimetry; nephelometry; Raman spectroscopy

Refraction of radiation Refractometry; interferometryDiffraction of radiation X-Ray and electron diffraction

methodsRotation of radiation Polarimetry; optical rotary

dispersion; circular dichroismElectrical potential Potentiometry;

chronopotentiometryElectrical charge CoulometryElectrical current Polarography; amperometry

Electrical resistance ConductometryMass-to-charge ratio Mass spectrometry

Rate of reaction Kinetic methods

Thermal properties Thermal conductivity and enthalpyRadioactivity Activation and isotope dilution

methods

Analytical SignalsData Domain – information

encoded

Non-electrical Domains

(scale, number, chemical)

Electrical Domains – (volts,

current, charge)

Analog Domains – continuous

quantities (volts, current)

Time Domains– (pulses,

slopes)

Digital Domains – (Off/On or

Hi/Lo)

Selecting an Analytical MethodRequired AccuracyAmount of sampleConcentration range(s) of analyte(s)Possible interferencesChemical and physical properties of matrixNumber of samples

Desirable Characteristics for anAnalytical MethodSpeedEase and ConvenienceSkill required of operatorCost and availability of equipmentPer-samples cost

Numerical Criteria forSelecting an Analytical MethodPrecisionAbsolute standard deviationRelative standard deviationCoefficient of variationVarianceBiasAbsolute systematic errorRelative systematic errorSensitivityCalibrationAnalyticalDetection LimitBlank plus three times Std.Dev. of blankConcentration RangeLimit of Quantitation(LOQ)Limit of Linearity (LOL)

SelectivityEffects of interferencesCoefficient of Selectivity

Examples of Instrumental Components

Table 1-2

The table below lists most of the analytical signals that are currently used for instrumental analysis.

Numerical Criteria for selecting Analytical Methods

The table below lists quantitative performance criteria of instruments, criteria that can be used to decide whether or not a given instrumental method is suitable for attacking an analytical problem. These characteristics are expressed in numerical terms that are called figures of merit. Figures of merit permit the chemist to narrow the choice of instruments for a given analytical problem to a relatively few. Selection among these few can then be based upon qualitative performance criteria such as speed, ease of convenience, skill required of operator, cost and availability of equiptment, per sample cost.

Signal Analytical Input TransducedSignal Readout

Instrument Generator Signal Transducer Signal

Photometer Tungsten lamp, Attenuated light Photocell ElectricalNone Current

glass filter, beam current metersample

Atomic emission Flame, mono- UV or visible Photomultiplier Electrical Amplifier,Chart

spectrometer chromator, radiation tube potential demodulator recorderchopper, sample

Coulometer DC source, Cell current Electrodes Electrical Amplifier Chartsample current recorder

pH meter Sample Hydrogen ion Glass-calomel Electrical Amplifier, Digital unit

activity electrodes potential digitizer

X-Ray powder X-Ray tube, Diffracted Photographic Latent Chemicaldiffractometersample radiation film image developer images

on film

Color Sunlight, Color Eye Optic nerve Brain Visualcomparator sample signal color re-

sponse

Criteria Figure of Merit1. Precision Absolute standard

deviation,relative standard deviation,coefficient of variation,variance

2. Bias Absolute systematic error,relative systematic error

3. Sensitivity Calibration sensitivity, analytical sensitivity

4. Detection limit Blank plus three times standard deviation of a blank

5. Concentration Range Concentration limit of quan-titation (LOQ) to concentra-tion limit of linearity (LOL)

6. Selectivity Coefficient of selectivity

Precision

Precision of analytical data is the degree of mutual agreement among data that

have been obtained in the same way. Precision provides a measure of the random, or indeterminate,, error of an analysis. Figures of merit for precision include: absolute standard deviation, relative standard deviation, standard deviation of the mean, coefficient of variation, and variance.

Bias

Bias provides a measure of the systematic, or determinate, error of an analytical method.

bias = g – xt

Where: g is the population mean for the concentration of analyte in a sample that has a true concentration of xt

Sensitivity

Ability to discriminate between small differences is analyte concentration.

calibration sensitivity S = MC + Sbl

S = measured signal c =

concentration of analyte Sbi = instrument signal for a blank M = slope of

the straight line

Analytical Sensitivity

= m / Ss

= analytical sensitivitym = slope of straight line Ss = standard deviation of the signals

Detection Limit

The minimum concentration or weight of analyte that can be detected at a known confidence level.

The useful range of an analytical method ranges from the lowest concentration at which quantitative measurements can be made (LOQ) to the concentration at which the calibration curve departs from linearity (LOL).

Cm =

Cm = detection limitSm = minimum distinquishable analytical signalSbl = mean signal of blankM = slope of calibration curve

Selectivity

The degree to which it is free from interference by other species contained in the sample matrix.

Other Characteristics to Be Considered in Method Choice

1. Speed2. Ease and convenience3. Skill required of operator4. Cost and availability of equipment5. Per-sample cost

Applicable Concentration Range

The figure below illustrates the definition of the useful range of an analytical method, which is from the lowest concentration at which the quantitative measurements can be made (limit of quantitation LOQ) to the concentration at which the calibration curve departs from linearity (limit of linearity LOL)

Useful Websites Dealing With Instrumental Analysis

American Chemical Society:

http://www.acs.org

Chemical Abstracts Service:

http://www.cas.org

Chemical Center Home Page: (maintained by ACS)

http://www.chemcenter/org

Science Magazine:

http://www.sciencemag.org

Journal of Chemistry and Spectroscopy:

http://www.kerouac.pharm.uky.edu/asrg/wave/wavehp.html