Study Notes: Sample Preparation Background for Spectroscopic Analysis

Sample preparation includes all of those procedures the analyst performs on the sample to make it ready for the actual ‘measurement’ stage of analysis.

We shall now explore general aspects of sample preparation procedures used in spectroscopic analysis. In essence, the procedures aim is to convert the sample into a form that is suitable to be measured by the instrument. This aspect is as important as knowing how to operate the instrument. That test samples are representative and homogeneous is assumed in this discussion.

In considering sample preparation there are a number of general factors that need to be addressed.

• Is the sample in liquid form?
• Does the sample contain compounds that would interfere with the reading given by the instrument?
• Is the sample too concentrated?
• Is the sample too dilute?

There are various considerations that impact these factors as follows.

• Analyte properties, organic or inorganic
• Required detection limit
• Sample matrix
• The particular requirements for the instrument.
  

The primary criterion for most spectroscopic techniques is that the sample be presented to the instrument as a liquid, often an aqueous solution. A liquid sample may be suitable for measurement ‘as is’ but further pre-treatment may be involved even if it is only dilution into the measurement range of the instrument.

Solids need to be converted into a soluble form:

• All that may be necessary is to dissolve the sample in water.
• Where the sample is insoluble an alternative solvent might be sought, such as hexane to dissolve non-polar organic compounds.
• The matrix of a water-insoluble sample can be chemically broken down or dissolved in order to release an analyte into solution.

Once in solution, further processing of the sample may still be required.

‘Matrix’ refers to the overall chemical composition as well as the physical properties of the sample in which the analyte resides. Bread, for example, has a soft organic matrix consisting predominantly of a mixture of carbohydrates, protein and fat. Steel has a solid metallic matrix of atoms of iron mostly. To overcome interferences presented by/in a matrix, the analyte may need to be separated from its matrix - such processes are known as ‘sample clean up’. Interferences may also be chemically masked, or in other words ‘blocked’, to negate their impact.

Detection limit (or sensitivity) concerns the concentration level down to which an analysis must be able to measure for the purpose of the analysis. A detection limit could be 0.1 µg/L or only as low as 1 mg/L.

Improvements to analytical methods that result in lower detection limits and shorter analysis times often arise from improvements to sample preparation. To illustrate this, imagine a biological toxin that has been discovered and is active at concentrations lower than normal. A new approach to sample preparation might be developed to include steps that chemically alter the toxin by attachment of an organic reagent. The choice of an appropriate reagent can significantly enhance sensitivity (ie lower the detection limit) and selectivity (ie reduce the impact on the analyte from interfering compounds). ‘Chemical derivatisation’ is the name of this type of procedure.

Sample preparation can be tedious and time consuming. Using acid to break down (digest) a sample of rock could take some hours compared to seconds to acquire the output from an instrument.

The more steps involved in sample preparation, the greater the potential for loss of sample with consequent negative effects on accuracy and precision. For quantitative analysis, the number of steps needs to be optimal and the analyst needs to perform the work with due care.

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