Principles of Instrumental Analysis

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Dwain Holt
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1 Principles of Instrumental Analysis Chapter 1. Introduction 1A. Classification of Analytical Methods Analytical Chemistry: Qualitative method versus quantitative method Qualitative method: yields information about the identity of atomic or molecular species or the functional groups in the sample. Quantitative method: provides numerical information as to the relative amount of one or more of these components. Classical methods: Separating analytes ( 分析物 ) by precipitation, extraction, or distillation. Instrumental methods: 1) Physical and chemical properties of analytes. 2) Separation: chromatographic and electrophoretic techniques. 1

Instruments for Analysis An instrument for chemical analysis converts information about the physical or chemical

2 1B. Types of Instrumental Methods TABLE 1-1 Chemical and Physical Properties Used in Instrumental Methods require a source of energy to stimulate a measureable response from the analyte. P.2 1C. Instruments for Analysis An instrument for chemical analysis converts information about the physical or chemical characteristics of the analyte to information that can be manipulated and interpreted by human. FIGURE 1-1 Block diagram showing the overall process of an instrumental measurement. P.3 2

The bottom half is made up of electrical

3 TABLE 1-2 Some Examples of Instrument Components P.4 FIGURE 1-2 Data-domain map. The upper (shaded) half of the map consists of nonelectrical domains. The bottom half is made up of electrical domains. Note that the digital domain spans both electrical and nonelectrical domains. P.4 3

4 Measurement process can be represented as a series of interdomain conversion. (input transducer) (output transducer) FIGURE 1-3 A block diagram of a fluorometer showing (a) a general diagram of the instrument, (b) a diagrammatic representation of the flow of information through various data domains in the instrument, and (c) the rules governing the datadomain transformations during the measurement process. P.5 Detectors( 檢測器 ): a mechanical, electrical, or chemical device that identifies, records, or indicates a change in one of the variables in the environment, such as pressure, temperature, electrical charge, electromagnetic radiation, nuclear radiation, particulates, or molecules. Transducers( 變換器 ): devices that convert information in nonelectrical domains to information in electrical domains and the converse. (photodiodes, photomultipliers, ) Sensors( 傳感器 ): analytical devices that are capable of monitoring specific chemical species continuously and reversibly. P.8 4

5 FIGURE 1-7 Chemical sensor. The sensor consists of a molecular recognition element and a transducer. A wide variety of recognition elements are possible. Shown here are some fairly selective recognition elements particularly useful with biosensors. The recognition phase converts the information of interest into a detectable characteristic, such as another chemical, mass, light, or heat. The transducer converts the characteristic into an electrical signal that can be measured. P.10 1D. Calibration of Instrumental Methods Calibration ( 校正 ) determines the relationship between the analytical response and the analyte concentration. Usually this is determined by the use of chemical standards. 1) Comparison with Standards (direct comparison, titrations) 2) External-Standard Calibration (Figure 1-8) ideal blank: identical to the sample but without the analyte. (matrix effects: 基質效應 ) 3) Standard-Addition Methods (when matrix effects are substantial) adding one or more increments of a standard solution to sample aliquots containing identical volumes (spiking). 4) The Internal-Standard Method: an internal standard is a substance that is added in a constant amount to all samples, blanks, and calibration standards in an analysis. 5

6 Appendix a1d Method of Least Squares FIGURE 1-8 Calibration curve for the determination of isooctane in a hydrocarbon mixture. The residual is the difference between an experimental data point y i and that calculated from the regression model, mx i + b, as shown in the insert. P.12 c x = bc s /mv x c x = -(V s ) 0 c s /V x FIGURE 1-10 Linear calibration plot for the method of standard additions. The concentration of the unknown solution may be calculated from the slope m and the intercept b, or it may be determined by extrapolation, as explained in the text. P.15 6

7 FIGURE 1-11 Spreadsheet for standard-addition Example 1-1. P.16 FIGURE 1-12 Spreadsheet to illustrate the internal-standard method for the flame spectrometric determination of sodium. P.18 7

8 1E. Selecting An Analytical Method 1) Defining the Problem 2) Performance Characteristics of Instruments * Precision: random error (appendix a1a) (Table 1-5) * Bias: systematic error ( : population mean, true value) * Sensitivity calibration sensitivity (m): S = mc + S bl analytical sensitivity: = m/s S (s S : standard deviation) * Detection limit: c m = (S m -S bl )/m * Dynamic range: between limit of quantitation (LOQ) and limit of linearity (LOL). * Selectivity: S = m A c A + m B c B + m C c C + S bl selectivity coefficient: k B,A = m B /m A ; k C,A = m C /m A Figure of Merit ( 品質因素 ): A quantity used to characterize the performance of a device, system or method, relative to its alternatives. In engineering, figures of merit are often defined for particular materials or devices in order to determine their relative utility for an application. In commerce, such figures are often used as a marketing tool to convince consumers to choose a particular brand. Examples Calorie per serving Contrast ratio of an LCD Frequency response of a speaker Battery life of a notebook computer 8

9 TABLE 1-3 Numerical Criteria for Selecting Analytical Methods P.19 TABLE 1-5 Figures of Merit for Precision of Analytical Methods P.19 9

10 TABLE 1-4 Other Characteristics to Be Considered in Method Choice P.19 FIGURE 1-13 Useful range of an analytical method. LOQ = limit of quantitative measurement; LOL=limit of linear response. P.21 10

CEM 333 Instrumental Analysis

CEM 333 Instrumental Analysis Simon J. Garrett Room: CEM 234 Phone: 355 9715 ext 208 E-mail: garrett@cem.msu.edu Lectures: Tuesday, Thursday 9:00-9:50 am Room 136 Office Hours: Tuesdays 10:00-11:00 am