The most common form of high performance liquid chromatography uses ultraviolet absorption as its method of detection. From your knowledge of liquid chromatography, what alternative detection techniques may find use in environmental analysis?
Conductivity detection can be used for ionic species (see Section 3.4.3 above). Fluorescence detection has an extremely high sensitivity and selectivity to specific groups of compounds and may find use for such species.
Conductivity detection has found widespread use for inorganic ions. Low-molecular-mass carboxylic acids (e.g. formic and acetic acids) have very similar physical properties to the inorganic acids and ion chromatography provides a convenient alternative to gas chromatography for these acids.
One group for which fluorescence detection has high sensitivity are polynuclear aromatic hydrocarbons (PAHs). Some examples of these are shown in Figure 4.9. They are highly carcinogenic compounds which are produced in trace quantities whenever fossil fuels are burnt. Typical water extracts could include up to 70 PAHs with a total concentration of around 1 ^g l-1. In order to monitor these low concentrations, sample preconcentration is needed. Solid-phase extraction, using an octadecylsilane (ODS) column, or a combination of ODS and amino-type columns, has been used. Sensitivity can be maximized if the detector is capable of changing the excitation and detection wavelengths throughout the chromato-graphic run, since each component has different optimum settings. The range of wavelengths used is 270-300 nm for excitation and 330-500 nm for detection.
Fluorescent derivatives can be made from non-fluorescent or weakly fluorescent compounds. Phenols and N-methylcarbamate pesticides (Figure 4.10) are often analysed in this way. The procedure for N-methylcarbamates uses post-column derivatization. The HPLC eluent is hydrolysed with sodium hydroxide at 95°C, thus producing methylamine. The latter is then reacted with o-phthalaldehyde and 2-mercaptoethanol to produce the fluorescent derivative. The fluorescent excitation wavelength is 230 nm and detection is > 418 nm, giving a limit of detection of approximately 1 ^l g-1 per component for a 400 ^l sample, injected without preconcentration.
Figure 4.9 Some typical PAHs found in environmental samples.
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Figure 4.10 Some examples of pesticides that can be analysed by using liquid chro-matography.
HPLC with ultraviolet detection is sometimes used for these and similar species, e.g. N-methylcarbamate, urea and triazine pesticides can be analysed by this method. These are 'second-generation' pesticides which have been developed to replace organic halogen compounds. The sensitivity with UV detection is lower than that achieved by fluorescence measurements and preconcentration (solvent extraction or solid-phase extraction) has to be used prior to injection. This form
of detection is also less specific than fluorescence and there is a greater possibility of chromatographic interference from other components in the sample. As with the case of phenols, the development of liquid chromatographic methods often stems from the difficulties encountered with analyses using gas chromatographic techniques. In many cases, this may be attributed to the polarities of the molecules (e.g. phenols and N-methylcarbamates), or their thermal labilities (e.g. N-methylcarbamates and phenylureas).
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