Analytical chemistry

There are two major types of analysis:

• Qualitative analysis, which seeks to establish the existence of a given element or compound.
• Quantitative analysis, which seeks to establish the amount of a given element or compound.

Most modern analytical chemistry is quantitative, however quantitative measurement can be further split into different areas of study. A simple example of quantitative analysis is the measurement of a sample's percentage of a given element. However, this gives no indication of the molecules that this element may be contained in (also referred to as species). More detailed analysis methods investigate what species a given element might be contained within. This is of particular interest in biological systems; the molecules of life contain carbon, hydrogen, oxygen, nitrogen, and others, in complex structures. The analyst may perform studies that indicate the presence and abundance of any of thousands of known compounds.

Techniques

There are a bewildering array of techniques available to separate, detect and measure chemical compounds. The oldest methods required painstaking separation of substances in order to measure the weight or volume of a final product. Many modern, sensitive and accurate devices rely on the principles of 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.

Methods

Analytical methods rely on scrupulous attention to cleanliness, sample preparation, precision, and accuracy. 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 concentration curve. The experimenter will create a series of standards across the range of concentrations that are of interest. They must take care that these concentrations are in the detection range of the technique (instrumentation) they are using. These standards will have a precisely known concentration of the element or compound under study. Running each of these standards several times using the chosen technique will produce a series of readings, each set indicative of one of the known concentrations. By plotting these points (reading vs concentration) on a graph, it is possible to plot a line of concentration vs reading across the detection range of that technique. Thus, when the sample is run and a reading obtained, the experimenter can simply refer to the graph to read off the concentration.

If the concentration of element or compound in the 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.