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Coupled techniques

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  400-401

2026-11-08

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Coupled techniques

Where large numbers of samples must be analysed quickly, the use of automated instruments becomes viable. One path is to use the automated sample handling in the flow injection analysis (FIA) technique.

FIA is an example of a continuous flow system: the sample becomes part of a flowing stream in which the unit operations of the analysis take place as the sample is carried from the injection point to a flow-through measuring device (such as a photometer) and finally to waste. The intensity of the radiation that reaches the detector is recorded continuously; when an absorbing species is passing through, a sharp peak is generated, with the height of the peak proportional to the analyte concentration. Sample sizes injected are mostly in the range 10–30 μL. Instruments are available with the ability to carry out manipulations such as in-line heating, in-line distillation, in-line UV-digestion or in line extraction, so that UV and/or visible spectroscopy can be used for a wide variety of compounds. For complex sample matrices, the limited specificity of spectroscopic methods can be an important drawback and a restriction for analysis. This restriction can be overcome by coupling liquid chromatography with a suitable detector, thereby enabling separation and detection of a wide variety of substances including macromolecules and ionic species in complex mixtures. Detection systems used depend upon the nature of the component of interest, but the most widely used detectors in liquid chromatography are based upon UV or visible radiation. Photometers often make use of the 254 nm and 280 nm lines from a mercury source, because many organic functional groups absorb in this region. However, to achieve adequate specificity, often diode arrays are used, which are able to display an entire spectrum as an analyte exits the column (liquid chromatography with diode array detector (LC-DAD)). Compared to single wavelength detection, which provides no information about peak purity, the diode array’s full spectral comparison provides results with a far greater confidence level . Generally, the LC-DAD technique is applied for toxicological screening. Substances are identified on the basis of both retention time and UV spectrum (Bogusz and Wu 1991; Lambert et al. 1995; Tracqui et al. 1995; Elliott and Hale 1998). The diode array can also be connected to a mass spectrometer (LC-DAD-MS), where MS increases the sensitivity and gives an extra confirmation of the component’s identity.