Analytical chemistry can be split into two main types:
- Qualitative inorganic analysis seeks to establish the presence of a given element or inorganic compound in a sample.
- Qualitative organic analysis seeks to establish the presence of a given functional group or organic compound in a sample.
- Quantitative analysis seeks to establish the amount of a given element or in a sample.
Most modern analytical chemistry is quantitative. Quantitative analysis can be further split into different areas of study. The material can be analyzed for the amount of an element or for the amount of an element in a specific chemical species. The latter is of particular interest in biological systems; the molecules of life contain carbon, hydrogen, oxygen, nitrogen, and others, in many complex structures.
There are a bewildering array of techniques available to separate, detect and measure chemical compounds.
- Separation of chemicals in order to measure the weight or volume of a final product. This is an older process and can be quite painstaking.
- Analysis of substances with devices using spectroscopy. Measuring the absorption of light by a solution or gas, we can calculate the amounts of several species, often without separation. Newer methods include atomic absorption spectroscopy (AAS), nuclear magnetic resonance (NMR) and neutron activation analysis (NAA).
- Many techniques combine two or more analytical methods. Examples of this include ICP-MS(Inductively-Coupled Plasma - Mass Spectrometry), where volatilisation of a sample occurs in the first step, and measuring of the concentration occurs in the second. The first step may also involve a separation technique, such as chromatography, and the second a detection / measuring device.
- Techniques that involve volatilisation aim to produce free atoms of the elements making up the sample, which can then be measured in concentration by the degree to which they absorb or emit at a characteristic spectral frequency. These methods have the disadvantage of completely destroying the sample, and any species contained within it. These techniques include atomic absorption spectroscopy and ICP-MS / ICP-AES. These techniques can still be used to study speciation, however by the incorporation of a separation stage before volatilisation.
Analytical methods rely on scrupulous attention to cleanliness, sample preparation, accuracy and precision.
Many practitioners will keep all their glassware in acid to prevent contamination, samples will be re-run many times over, and equipment will be washed in specially pure solvents.
A standard method for analysis of concentration involves the creation of a calibration curve.
If the concentration of element or compound in a sample is too high for the detection range of the technique, it can simply be diluted in a pure solvent. If the amount in the sample is below an instrument's range of measurement, the method of addition can be used. In this method a known quantity of the element or compound under study is added, and the difference between the concentration added, and the concentration observed is the amount actually in the sample.
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