Chapter 1: Introduction. Analytical Chemistry Qualitative analysis Quantitative analysis Classification of Analytical Methods Classical methods (wet chemical methods) Instrumental methods. Comparisons of Wet Chemical and Instrumental Methods. Types of Instrumental Methods

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Chapter 1: Introduction

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Comparisons of Wet Chemical and Instrumental Methods

Types of Instrumental Methods

Separation techniques

Chromatographic

Gas

Liquid

…….

Electrophoresis

Many types

Detection techniques

Optical spectroscopy

Absorption

Emission

Fluorescence

Mass spectroscopy

Atomic

Molecular

Instruments for Analysis

Signal

Generator

Sample

Analytical

Signal

Input

Transducer

(Detector)

Signal

Processor

Output

Transducer

(Readout)

Instruments for Analysis

Selection of An Instrumental Method

Defining the problem

Performance characteristics

Figure of Merits: Quantitative performance criteria of instruments that are expressed in numerical terms

Precision

A measure of the random or indeterminate error

Relative Standard Deviation

Bias

A measure of the systematic or determinate error

Bias = µ - Xt

µ - the population mean for the concentration of an analyte

Xt – true concentration

Standard reference materials (SRM)

Signal

Signal

x

x

x

x

x

x

x

x

Sbl

Sbl

Concentration

Sensitivity

A measure of the ability of an analytical method to discriminate between small differences in analyte concentration

Calibration sensitivity (S = mc + Sbl)

Concentration

Signal

Concentration

Sensitivity

Analytical sensitivity ( = m/sS)

m – slope; sS – standard deviation of the measurement

Relative insensitive to amplification factors

Increase the gain of the instrument by a

Factor of two

Sm = Sbl + ksbl

Standard deviation of

the Blank signal

Analytical Signal

Blank signal

Detection limit (Limit of detection, LOD)

The analyte concentration giving a signal equal the blank signal, Sbl, plus three time standards deviation of the blank, sbl

Convert signal response, Sm, to analyte concentration

Sm = Sbl + ksbl

Standard deviation of

the Blank signal

Analytical Signal

Blank signal

Dynamic Range

Lowest concentration (LOQ) to the concentration where the calibration curve departure from linearity (limit of linearity, LOL)

Convert signal response, Sm, to analyte concentration

Dynamic Range

Guideline for Reporting Data (Recommended by ACS Committee of Environmental Improvement)

Analyte

ConcentrationRegion of reliability

< 3σRegion of Questionable detection

(unacceptable)

3σDetection limit

3σ-10σRegion of less certain quantitation

10σLimit of quantitation

> 10σRegion of quantitation

99.7%

Question: When the value of 10 sbl (standard deviation of the blank) is used as LOQ, what is the relative standard deviation of the measurement at this point at the 99.7% confidential level, and why?

Selectivity

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

S = mAcA + mBcB + mCcC + Sbl

The selectivitycoefficient for B with respect to A

kB,A = mB/mA

……………………………………

Note: this is the selectivity of an analytical detection technique

Calibration Methods

Analytical response

Comparison with Standard

Direct comparison

Colorimetric

Titration

Analyte Concentration

Signal

Concentration

Calibration Methods

External Calibration Curve

Normally use the method of least squares

X

X

X

X

Calibration Methods

External Calibration Curve

Two Assumptions:

1. Linear relationship

2. deviation of the individual point from the straight line arises from the error in the measurement

Signal

Without sample matrix effect

With sample matrix effect (signal suppressed)

Concentration

Calibration Methods

Standard Addition

C1, C2, and C3 are the concentrations of analyte after spiked with stnard, with counting the amount of analyte in the original sample solution.

C0 is the concentrations of analyte in sample without spiking

Standard Addition

C0

C1

C2

C3

Signal

x

S = mC + Sbl

Sample signal

Sbl

x

x

C0 =Sbl/m

x

Conc. of analyte C0

Concentration

Internal Standard

Add a substance in a constant amount to all samples and, blanks, and calibration standard in an analysis