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Chapter 1: Introduction

- Analytical Chemistry
- Qualitative analysis
- Quantitative analysis
- Classification of Analytical Methods
- Classical methods (wet chemical methods)
- 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)

Selection of An Instrumental Method

- Defining the problem
- Performance characteristics

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

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

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

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

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

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

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

Analyte

Concentration Region of reliability

< 3σ Region of Questionable detection

(unacceptable)

3σ Detection limit

3σ-10σ Region of less certain quantitation

10σ Limit of quantitation

> 10σ Region of quantitation

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

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

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

Sanalyte/Sinternal

Concentration

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