Laser Light Scattering Detectors

On-line coupling of high performance liquid chromatography (HPLC) to capillary gas chromatography (GC) means that LC fractions comprising one or several peaks are directly transferred to the gas chromatograph, mostly in a fully automated mode. The Rrst coupled system, developed by R. Majors in 1980, involved splitless injection by means of a GC autosampler andmerely transferred a small part of an LC peak. The Rrst transfer of complete LC fractions was described in 1984. Full transfer is essential, Rrst because of sensitivity (the sample capacity of LC is limited), and second, to obtain reliable quantitative results. The main obstacle to overcome was the introduction of 100}1000 L volumes of LC eluent into a gas chromatograph. LC-GC seldom corresponds to ordinary LC with GC added as a detector. Usually GC performs the main analysis and LC is specially designed for a kind of pre-separation or clean-up. The technique presupposes that the compounds to be analysed are amenable to GC, i.e. that they are of limited polarity and rather low molecular mass. For most applications, only normal-phase LC is suitable, either because the matrix material to be injected is not soluble in reversed-phase eluents (e.g. mineral or edible oils), the components are derivatized prior to LC, or the sample consists of an extract from an aqueous phase. LC columns are kept small (mostly of 2 mm i.d.) in order to keep the fraction volumes below 1000 L. For the isolation of wider fractions of compounds, solid-phase extraction-type cartridges of lower separation efRciency, yielding smaller fractions, may be more suitable. In contrast to clean-up by solid-phase extraction (SPE) cartridges, HPLC columns are used over long periods of time. During GC analysis, they are reconditioned, commonly by backSush with a stronger solvent.