Following the burst of innovation that provided the detectors in common use today attention was turned to the other parts of the gas chromatograph that had an impact on quantitative accuracy. Quantitative LC lagged behind that of GC and had first to await the development of sensitive detectors, then, columns with adequate efficiency, then, high pressure sampling systems and finally, mobile phase supply systems that could provide accurate solvent flow rates at the necessary high pressures. Sampling at high pressures was solved by the use of internal and external loop valves. These comprised of rotary valves consisting of two discs having finely machined contact faces pressed against each other by very strong springs. The valve could be rotated to allow the different ports in the upper and lower disk to be matched with out leaking. Columns with the required high efficiencies were produced by using very small particles (initially 10 mm in diameter) and operating them at very high pressures. Slurry packing procedures were also developed to obtain stable, permeable packed beds. The first high pressure pumps were designed as single stroke reciprocating piston pumps made from a stainless steel cylinder and a sapphire piston fitted, at the inlet and outlet, with non return valves consisting of sapphire balls and seats. Later, dual pumps were devised which had specially designed driving cams to reduce pump pulsation.
The LC detectors are many orders of magnitude less sensitive than their GC counterparts and have significantly less linear dynamic ranges. The first practical detector that could be easily constructed and used for quantitative work was the refractive index detector developed by Tiselius and D. Claesson [7] that monitored the change in the refractive index of the column eluent. Today, the simple refractive index detector, although having limited sensitivity, a restricted linear dynamic range and other serious disadvantages, has survived and is the fourth most popular detector, largely on account of its universal response. The most popular detector used in quantitative LC is the UV detector, first described by Horvath and Lipsky [8]. UV absorption detectors respond to substances that absorb light between 180 to 350 nm. Many substances absorb light in this wavelength range, including substances having one or more double bonds (p electrons) and substances having unshared electrons, e.g. all olefins, all aromatics and compounds such as those containing >C=O, >C=S, –N=N– groups. In practice the UV light passes through a cell carrying the column eluent and the transmitted light strikes a photo–electric cell (or an array of diodes) the output of which, after appropriate electronic modification, is monitored by a recorder or fed to a data acquisition system. This detector is the work horse of quantitative LC analysis, it is reasonably sensitive and has a linear dynamic range of about three orders of magnitude.
