成都理工大学 2010-1-23 授课单位：成都理工大学材料与化学化工学院...

成都理工大学

2010-1-23

授课单位：成都理工大学材料与化学化工学院主讲教师：郎春燕 2009-09

双语课程《环境分析化学》

成都理工大学

http://www.cmcc.cdut.edu.cn/

Part 1 Introduction

Definition and characteristics of environmental analytical chemistry

1.1 The relationship between environmental science and analytical chemistry

1.2 General principles and terms in analytical chemistry

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1.3 Overall analytical processes

1. Definition of the aims; 2. Selection of an analytical method; 3. Sampling, sample handling and pretreatment; 4. Separations; 5. Final measurements; 6. Method validation; 7. Assessment and interpretation of results; 8. Safety.

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1.3.1 Defining the aims of an analytical programme

This stage is clearly the fundamental one in designing an overall analytical procedure.

difficulty Sometimes it is straightforward, as in the case of monitoring

a system to comply with specified legal requirements. Indeed, it is increasingly common to find a standard overall procedure also specified.

In other circumstances the problem can be much more difficult. This is especially so in investigatory or research situations. It may not even be obvious what measurements are needed, let alone the levels of sensitivity, precision and accuracy required.

suggestion To define the analytical requirement, unquestionably

requires full consultation between the environmental scientists and the analytical chemists.

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1.3.2 Selection of an analytical method

A whole range of factors needs to be taken into account

1. What is the analyte? 2. Detection limits; 3. Precision; 4. Accuracy; 5. Turn-round time; 6. Are analyses needed ‘on-site’? 7. Number of samples and any consequent automation; 8. Cost. It is as important to avoid ‘over analysing’ as it is to avoid producing data which are inadequate in quality or quantity.

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1.3.3 Sampling, sample handing and pretreatment

Samples must be collected according to a plan which ensures that they are representative of the original aims. Throughout subsequent handling and treatment they must remain so.

It must be borne in mind that the overall variability of the results and the precision of the analysis can be significantly influenced by variations in the compositions of replicate samples.

whatever care is taken such variations can never be entirely eliminated. These variations must be assessed and allowed for.

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Hypothesis guided sampling

It employs the combined technical expertise of the environmental scientists and the analyst.

Judgements are made about likely analyte distribution in an attempt to focus the programme efficiently on to the established objectives.

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Random sampling It can be usefully applied if no obvious patterns of

analyte exist.

A practical form of random sampling for a surface, is to collect samples along the lines of a ‘W’ pattern across the sampling area.

This approach is useful in initial surveys, and provides a reasonable chance of obtaining information on the general distribution of an analyte while limiting the total number of samples.


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Systematic sampling

It involves collecting samples according to a rigid, predetermined plan with respect to e.g. location, time, depth. It assumes no previous knowledge with regard to analyte distribution.

The results should be free from bias but

typically, large numbers of samples are generated, and this may lead to a programme that is expensive to implement.

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Stratified sampling

It may be employed in circumstances where the system to be studied contains a number of regions that are distinctly different.

For example in a study involving depth, the strata are selected on basis of depth and random sampling carried out within each stratum.

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根系土

母岩

母质层

淀积层

淋溶层

Stratified sampling in vertical profile of soil

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Stratified sampling, examples

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Water sample

P9 Explanation

Seasonal overturn, which occurs in many lakes. In this process, thermal effects acting on the water bring about mixing of the upper and lower strata twice yearly, in spring and autumn.

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1.3.4 Analytical measurements

The making of an analytical measurement is often the simplest and most straightforward stage in the overall process. This is especially so in modern laboratories where instrumental and automated procedures now dominate.

It is timely to recall here that unless the earlier stages in the analytical process have been properly executed, the results obtained will be meaningless or, perhaps worse, misleading.

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1.3.5 Method validation and quality assurance

It is axiomatic that the experimental results obtained

must give a good indication of the true result. Ways (1) The calibrations carried out in setting up the method

go part of the way to ensuring this. (2) Analysis of validated standards, often known as

standard reference materials (SRMs), to check the performance of a method is almost always essential.

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1.3.5 Method validation and quality assurance

Standards used should have a composition, which is closely similar to the sample, both in term of its analyte concentration and matrix.

Some SRMs accepted nationally and internationally can be obtained from bodies like the National Institute of Standards and Testing (NIST), in the United States, The Bureau of Certified Reference Materials (BCR) in Europe, or the Laboratory of the Government Chemist (LGC) in the UK.

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quality assurance

An initial validation of a procedure is not sufficient to guarantee the continuing quality of the data generated over an extended period. In order to do this, an ongoing quality assurance (QA) programme must be employed.

A QA programme must be comprehensive and inclusive of the whole analytical operation. It has two main elements.

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quality assurance

The first is the development, validation and recording

of the overall procedure and ensuring that as far as possible samples are all handled in exact compliance with the defined procedure. This procedure must ensure the traceability of the processing of each individual sample and measurement.

The second requires regular performance monitoring. Routine analysis of reference samples and maintenance of quality control charts, are important.

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1.3.6 Data assessment and interpretation

The data yielded by an analytical programme are used to try to fulfil the original aims defined at the beginning. A two-stage process required for this. The reliability of the data must first be assessed. As discussed in chapter 2, errors are implicit in all measurements and correct procedures must be used to ensure that data with excessive errors which may endanger the validity of the conclusions, are not used. It is then possible to attempt to interpret the data in the light of the defined aims of the measurement. A blend of environmental, analytical and statistical expertise will be required to do this

soundly. Careful thought should also be given to data presentation, so that it is readily digested and conclusions are clearly demonstrated.

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1.3.7 Safety

A safety assessment for the programme and the activities within it must be routinely made.

It is important to bear in mind, that the initial interest in environmental analytes often derivers from their hazardous properties, and that special precautions may well be needed when dealing with some analytes.

The main problem will often arise not so much with the samples themselves, as with the preparation and handling of standards which utilize higher concentrations of the analyte.

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Self-study exercises

1. Which stages are included in an overall analytical process?

2. What needs to be taken into account when selecting an analytical method?

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成都理工大学材料与化学化工学院 2009.9

成都理工大学 2010-1-23 授课单位：成都理工大学材料与化学化工学院...

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