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lecture 1 introduction to instrumental analysis

Some notes and figures in this course have been taken from Skoog, Holler and Neiman, Principles of Instrumental Analysis, 5th Edition, Saunders College Publishing.

INTRODUCTION

OUTLINE: CLASSIFICATION OF ANALYTICAL METHODS TYPES OF INSTRUMENTAL METHODS INSTRUMENTS FOR ANALYSIS SELECTING AN ANALYTICAL METHOD CALIBRATION OF AN INSTRUMENTAL METHOD

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PROBLEM SET # 1

1-4, 1-9, 1-10, 1-11

Due Tues., April 19.

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

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INTRODUCTION

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.

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TYPES OF INSTRUMENTAL METHODS

PROPERTY EXAMPLE METHOD Radiation Emission Emission spectroscopy - fluorescence,

phosphorescence, luminescence Radiation Absorption Absorption spectroscopy -

spectrophotometry, photometry, nuclear magnetic resonance, electron spin resonance

Radiation Scaterring Turbidity, Raman Radiation Refraction Refractometry, interferometry Radiation Diffraction X-ray, electron Radiation Rotation Polarimetry, circular dichroism

TYPES OF INSTRUMENTAL METHODS

PROPERTY EXAMPLE METHOD Electrical Potential Potentiometry Electrical Charge Coulometry

Electric Current Voltammetry - amperometry, polarography Electrical Resistance Conductometry Mass Gravimetry Mass-to-charge Ratio Mass spectrometry Rate of Reaction Stopped flow, flow injection analysis Thermal Characteristics Thermal gravimetry, calorimetry Radioactivity Activation, isotope dilution

Often combined with chromatographic or electrophoretic methods

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BASIC INSTRUMENT COMPONENTS Source: produces some form of energy or mass that is relevant to the

measurement at hand

Sample Holder or “Cell”: contains the sample with your analyte of interest

Discriminator: selects the desired signal from the source or the sample

Input Transducer: detects the signal from the sample, source or discriminator. (the detector)

Processor: manipulates the signal electronically or mechanically to produce some useful value

Readout: displays the signal in some useful form.

INSTRUMENTS FOR ANALYSIS

Example: spectrophotometry Instrument: spectrophotometer Stimulus: monochromatic light energy Analytical response: light absorption Transducer: photocell Data: electrical current Data processor: current meter Readout: meter scale

Block diagram for the overall process of instrumental measurement.

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INSTRUMENTS FOR ANALYSIS

DATA DOMAINS: way of encoding analytical response in electrical or non-electrical signals. Interdomain conversions transform information from one domain to another. 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: DOMAINS

NON-ELECTRICAL DOMAINS ELECTRICAL DOMAINS Physical (light intensity, color) Current (Analog) Chemical (pH) Voltage (Analog) Scale Position (length) Charge (Analog) Number (objects) Frequecy (Time)

Pulse width (Time) Phase (Time) Count (Digital) Serial (Digital) Parallel (Digital)

Time: vary with time (frequency, phase, pulse width) Analog: continuously variable magnitude (current, voltage, charge) Digital: discrete values (count, serial, parallel, number*)

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INSTRUMENTS MEASURE 1 OF 2 THINGS. VOLTAGE (V), volts, electrical potential across two electrodes.

Current (A), amperes, the flow of electrons across some point.

V = IR R= resistance in Ohms

THEY DO NOT MEASURE ABSORBANCE, FREQUENCY, MASS, CONCENTRATION OR ANY OTHER CONVENIENT UNIT. THEY MEASURE CURRENTS OR VOLTAGES, WHICH ARE THEN CONVERTED TO THESE CONVENIENCE UNITS!

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The most important single step in an analysis is collecting the sample of the material to be analyzed.

Real materials are usually not homogeneous, so the sample must be chosen carefully to be representative of the real material!

A representative sample is one that reflects the true value and distribution of the analyte in the original material.

If the sample is not taken properly, no matter how excellent the analytical method or how expert the analyst, the result obtained will not provide a reliable characterization of the material.

SAMPLING

Gas Samples. Gas samples are generally considered homogeneous, but gas mixtures may separate into layers of differing density.

Gas samples can be taken at a single point in time (called a grab sample) or can be collected over a period of time or from different locations to provide an average or composite sample

Liquid samples can also be collected as grab samples or as composite samples.

Solid samples are often the most difficult to sample because they are usually less homogeneous than gases or liquids.

SAMPLING

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In order to design the correct experiment to answer the analytical question being asked, statistics is needed to select

the size of the sample required, the number of samples, and

the number of measurements that must be performed to obtain the needed accuracy and precision in the results generated by the experiment.

Statistics is also used to express

the uncertainty in measured values, so that the users of the data understand the limitations associated with results.

BASIC STATISTICS AND DATA HANDLING

BASIC STATISTICS AND DATA HANDLING

Significant Figures All measurements have uncertainty in them, so the results should be reported as a number that tells us about the uncertainty In enumerating data, report all significant figures, such that only the

last figure is uncertain.

Reject all other figures, rounding off in the process.

In reporting results obtained by addition and subtraction, the figures in each number are significant only as far as the first uncertain figure of any one of the numbers to be added or subtracted.

For multiplication and division, the number of significant figures in the answer should be no greater than that of the term with the least number of significant figures.

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Accuracy is a measure of how close a measured analytical result is to the true answer.

Precision is a measure of how close replicate results on the same sample are to each other.

ACCURACY AND PRECISION

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TYPES OF ERRORS

DETERMINATE ERRORS are caused by faults in the analytical procedure or the instruments used in the analysis. The name determinate error implies that the cause of this type of error may be found out and then either avoided or corrected. Determinate errors are systematic errors; that is, they are not random. Systematic error is under the control of the analyst. It is the analyst’s responsibility to recognize and correct for these systematic errors that cause results to be biased, that is, offset in the average measured value from the true value

TYPES OF ERRORS

DETERMINATE ERRORS can arise from uncalibrated balances, improperly calibrated volumetric flasks or pipettes, malfunctioning instrumentation, impure chemicals, incorrect analytical procedures or techniques, and analyst error.

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TYPES OF ERRORS

Random Errors

TYPES OF ERRORS

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The aim of all sample preparation is to provide the analyte of interest in the physical form required by the instrument, free of interfering substances, and in the concentration range required by the instrument.

SAMPLE PREPARATION

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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?

DEFINING THE ANALYTICAL PROBLEM

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

SELECTING AN ANALYTICAL METHOD

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INDETERMINATE OR RANDOM ERRORS

FIGURES OF MERIT: PRECISION

FIGURES OF MERIT: ACCURACY

DETERMINATE ERRORS (operator, method, instrumental)

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Sensitivity of both instrument and method is the ability to distinguish between small differences in analyte concentations.

The sensitivity of an analytical method can be defined as the slope of the calibration curve, that is, as the ratio of change in the instrument response with a change in the analyte concentration.

FIGURES OF MERIT: SENSITIVITY

(larger slope of calibration curve m, more sensitive measurement)

Sensitivity of both instrument and method is the ability to distinguish between small differences in analyte concentations.

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Signal must be bigger than random noise of blank

FIGURES OF MERIT: DETECTION LIMIT

The limit of detection (LOD) is the lowest concentration of analyte in a sample that can be detected at a known confidence level.

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: DYNAMIC RANGE

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No analytical method is completely free from interference . Best method is more sensitive to analyte than interfering species (interferent).

k's vary between 0 (no selectivity) and large number (very selective).

FIGURES OF MERIT: SELECTIVITY

Basis of quantitative analysis is magnitude of measured property is proportional to concentration of analyte

CALIBRATION METHODS

Three most common calibration methods: a) The use of calibration curves b) Standard addition method c) Internal standard method

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CALIBRATION CURVES (WORKING or ANALYTICAL)

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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 METHOD

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SAMPLE PROBLEM:

SAMPLE PROBLEM

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SAMPLE PROBLEM

Calibration Curves:

several standards containing exactly known concentrations of the analytes are introduced into the instrument and the response is recorded.

CALIBRATION OF INSTRUMENTAL METHODS

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STANDARD ADDITION METHOD

Adding one or more increments of a standard solution to sample aliquots of the same size.

STANDARD ADDITION METHOD