What’s in my sample? Qualitative Analysis

How do I obtain a sample for testing?

One of the lots if then sent to the laboratory for testing - this is the laboratory sample.

Once the laboratory sample has been divided into smaller portions these are called the test samples.

The test sample may be further divided into test portions. This is the portion that the analyst tests.

The content of each test portion will be measured.

Laboratory sample

Test sample

Test portion

Measurement

Sampling is important because…...

• It is not cost effective for a laboratory to test all of the material in a batch (e.g. every bottle of shampoo).

• The test portion that is measured in the laboratory must be representative of the batch.

• Poor sampling can lead to inaccurate results and so waste time and money.

Example: The analyst wants to confirm that the colours listed on the shampoo label are present in the product.

Looking at this shampoo label the colours are the analyte (highlighted in yellow boxes) and all other ingredients are the matrix.

Sampling

• Sampling is the process of selecting a portion of material to represent or provide information about a larger body of material.

• Consideration needs to be given to whether the sample is part of something larger, how it should be obtained, and how it should be stored and treated before measurement.

• The procedure by which the sample is obtained is called sampling.

• The sample will contain both the analyte and the matrix.

• The analyte is the chemical being identified (qualitative analysis) or measured (quantitative analysis).

• The matrix is all other components of the sample, other than the analyte.

Total amount of shampoo produced at the same time is the batch.

An amount of material taken at random from the batch, is a bulk sample.

The bulk sample is then divided into lots.

Batch of shampoo

Bulk sample(bottles taken at random)

Sample lots (taken at random)

Lot 1 Lot 2 Lot 3

The sampling process

Subject to Crown licence, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording or any retrieval system, without the written permission of the copyright holder. © LGC Limited, 2005. All rights reserved. 1304/CB/1005

What’s in my sample? Qualitative AnalysisWhat’s in my sample?

50

40

30

20

10

0

Which metals are present in these salts?

Flame colour Cation

Intense yellow Sodium Brick-red Calcium Apple green Barium Blueish green Copper

How much calcium is present in a sample of water?

In volumetric analysis, the concentration of a solution (e.g. the amount of calcium in a sample of water), is determined by measuring the volume of a standard solution (e.g. EDTA) that will react with a known volume of a solution (e.g. water sample).

The procedure of adding one solution to another, in a measured way, until the reaction is complete is called a titration.

If the titration is performed correctly the final results can be very accurate.

Volumetric analysis is commonly used to obtain accurate results in industrial and research laboratories.

Substances arrive at an analytical laboratory that need to be identified (qualitative analysis) and on some occasions the amount of a substance needs to be determined (quantitative analysis).

Platinum wire

Calcium salt

Qualitative analysis

• Qualitative analysis is the process of determining if any of a particular substance or class of substances is present in a sample.

• The elements and compounds in the sample are identified. • Some examples of types of qualitative analysis are:

- Simple chemical tests (e.g. flame tests) - Mass spectrometry - Infrared spectroscopy. • Qualitative analysis can tell you what chemicals are present in a sample but not how much is present.

Quantitative analysis

• Quantitative analysis usually provides more information than qualitative analysis of a sample.

• It determines the absolute or relative amounts of elements and compounds in a sample (e.g. their concentration).

• Some examples of types of quantitative analysis are: - Volumetric analysis: • Acid - base titrations • Redox titrations • Complexometric titrations - Colorimetry - High performance liquid chromatography (HPLC) - Gas chromatography (GC).

Subject to Crown licence, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording or any retrieval system, without the written permission of the copyright holder. © LGC Limited, 2005. All rights reserved. 1304/CB/1005

How reliable is my experiment?Precision, Bias and Accuracy

How reliable is my experiment?Precision, Bias and Accuracy

• Precision and bias are two terms which

are commonly used to describe the

performance of an analytical method.

• Precision is a measure of the spread

of results. “If I make repeated

measurements on the same sample,

by how much can I expect my results

to change?”

• Bias is a measure of the difference

between the average of repeated

measurements and the true value.

“How close are my results to the

right answer?”

• Results that are precise and unbiased

are described as accurate.

• An archer is practising shooting some arrows, the aim is to get all the arrows as close together and as near the centre of the target as possible.

• The results that the archer wants to achieve are precise with insignificant bias as shown on this target.

• The results of some of the archer's attempts are shown below on targets A-D.

Poor precision with bias Good precision with bias

Poor precision with no bias Precise and unbiased ‘Accurate’

Subject to Crown licence, no part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording or any retrieval system, without the written permission of the copyright holder. © LGC Limited, 2005. All rights reserved. 1304/CB/1005

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