Notes

Outline

Instrumental Analysis Instructors: Upali Siriwardane, CTH 311, Phone: 257-4941)  Frank Ji, CTH 343/IfM 218, Phone: 257-4066/5125 Dale L. Snow, Office: CTH 331,Phone: 257-4403 Jim Palmer,  BH 5 /IfM 121, Phone: 572885/5126 Bill Elmore, BH 222 /IfM 115 Phone: 257-2902/5143 Marilyn B. Cox,  Office:  CTH 337, Phone: 257-4631 REQUIRED TEXT: Principles of Instrumental Analysis, 5th Edition, Douglas A. Skoog F. James Holler and Timothy A. Nieman. .

Content Atomic Absorption & Fluorescence Spectroscopy (Upali)       6. An Introduction to Spectrometric Methods. 9. Atomic Absorption and Atomic Fluorescence Spectrometry. Ultraviolet/Visible Spectroscopy(Snow)       13. An Introduction to Ultraviolet/Visible Molecular Absorption Spectrometry. 14. Applications of Ultraviolet/Visible Molecular Absorption Spectrometry. Infrared Spectrometry( Frank Ji) 16. An Introduction to Infrared Spectrometry. 17. Applications of Infrared Spectrometry. Nuclear Magnetic Resonance Spectroscopy) (Upali) 19. Nuclear Magnetic Resonance Spectroscopy. Mass Spectrometry (Palmer/Upali/Cox 20. Molecular Mass Spectrometry.  Gas Chromatography ( JimPalmer)       26. An Introduction to Chromatographic Separations. 27. Gas Chromatography.  High-Performance Liquid Chromatography (Bill Elmore) 28. High-Performance Liquid Chromatography. Special Topics (Upali) 12. Atomic X-Ray Spectrometry. 31. Thermal Methods

Analytical Chemistry art of recognizing different substances & determining their constituents, takes a prominent position among the applications of science, since the questions it enables us to answer arise wherever chemical processes are present. 1894 Wilhelm Ostwald

You don’t need a course to tell you how to run an instrument They are all different and change Most of you won’t be analysts We will talk about experimental design Learn about the choices available and the basics of techniques

Questions to ask??? Why?  Is sample representative What is host matrix? Impurities to be measured and approximate concentrations Range of quantities expected Precision & accuracy required

Off flavor cake mix (10%) Send it off for analysis Do simple extractions Separation and identification by GC/MS Over 100 peaks but problem was in a valley between peaks (compare) Iodocresol at ppt Eliminate iodized salt that reacted with food coloring (creosol=methyl phenol)

I.  Significant Figures • The digits in a measured quantity that are known exactly plus one uncertain digit.

I.  Significant Figures Rule 1: To determine the number of  significant figures in a measurement, read the number from left to right and count all digits, starting with the first non-zero digit.

There is a difference - you

II.  Precision & Accuracy A. Precision – the reproducibility of a series of measurements.

"B." B. Accuracy – How close a measured value is to the known or accepted value.

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

III. Types of Error Ea = Es + Er Absolute error in a measurement arises from the sum of systematic (determinate) error and random (indeterminate) error.

"B." B. Random Error - Uncertainty in a measurement arising from an unknown and uncontrollable source . (Also commonly referred to as Noise.)

IV.  Statistical Treatment of Random Error (Er) A large number of replicate measurements result in a distribution of values which are symmetrically distributed about the mean value.

A.  Normal Error Curve

A.  Normal Error Curve 68.3% of measurements will fall within ± s of the mean.

Gaussian Distribution Random fluctuations Bell shaped curve Mean and standard deviation 1sigma 68.3%, 2sigma 95.5%, 3sigma 99.7% Absolute Vs Relative standard deviation Accuracy and its relationship to the measured mean

B.  Statistics Population Mean (m)

B.  Statistics 2. Population Standard Deviation (s)

B.  Statistics 3. Population Variance (s2)

B.  Statistics 4. Sample Mean (x)

B.  Statistics 5. Sample Standard Deviation (s)

B.  Statistics 6. Sample Variance -  s2

B.  Statistics Population Mean (m)

B.  Statistics 2. Population Standard Deviation (s)

B.  Statistics 3. Population Variance (s2)

B.  Statistics 4. Sample Mean (x)

B.  Statistics 5. Sample Standard Deviation (s)

B.  Statistics 6. Sample Variance -  s2

Comparing Methods Detection limits Dynamic range Interferences Generality Simplicity

Analytical Instruments

Data Domains Data Domains: way of encoding analytical response in electrical or  non-electrical signals. Interdomain conversions transform information from one domain to another. Detector (general): device that indicates change in environment Transducer (specific): device that converts non-electrical to electrical data Sensor (specific): device that converts chemical to electrical data

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h(t) = a cos 2 pi freq. x time sum = cos(2pi((f1+f2)/2)t beat or difference = cos(2pi((f1-f2)/2)t 5104-sine-wa

Definitions Analyte - the substance being identified or quantified. Sample - the mixture containing the analyte. Also known as the matrix. Qualitative analysis - identification of the analyte. Quantitative analysis - measurement of the amount or concentration of the analyte in the sample. Signal - the output of the instrument (usually a voltage or a readout). Blank Signal - the measured signal for a sample containing no analyte (the sample should be similar to a sample containing the analyte)

Definitions A standard (a.k.a. control) is a sample with known conc. of analyte which is otherwise similar to composition of unknown samples. A blank is one type of a standard without the analyte. A calibration curve - a plot of signal vs conc. for a set of standards. The linear part of the plot is the dynamic range.lin Linear regression (method of least squares) is used to find the best straight line through experimental data points.

Liner Plots                         S = mC + Sbl where C = conc. of analyte; S = signal of instrument; m = sensitivity; Sbl = blank signal. The units of m depend on the instrument, but include reciprocal concentration.

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Signal-to-noise (S/N)

Noise Reduction Avoid (cool, shield, etc.) Electronically filter Average Mathematical smoothing Fourier transform

Limit of detection signal - output measured as difference between sample and blank (averages) noise - std dev of the fluctuations of the instrument output with a blank S/N = 3 for limit of detection S/N = 10 for limit of quantitation

The limit of detection (LOD) The limit of detection (LOD) is the conc. at which one is 95% confident the analyte is present in the sample. The LOD is affected by the precision of the measurements and by the magnitude of the blanks. From multiple measurements of blanks, determine the standard deviation of the blank signal sbl Then LOD = 3sbl /m where m is the sensitivity.

The limit of quantitation LOQ The limit of quantitation LOQ is the smallest conc. at which a reasonable precision can be obtained (as expressed by s). The LOQ is obtained by substituting 10 for 3 in the above equation;                i.e., LOQ = 10sbl /m. Ex. In the earlier example of absorption spectroscopy, the standard deviation of the blank absorbance for 10 measurements was 0.0079. What is the LOD and LOQ? sbl = 0.0079; m = 0.0665 ppm-1 ; LOD = 3(0.0079)/(0.0665 ppm-1 ) = 0.36 ppm LOQ = 10(0.0079)/(0.0665 ppm-1 ) = 1.2 ppm