Historical Introduction

Originally liquid chromatography was largely used as a preparative technique, quantitative evaluations being carried out off-line using separate analytical methods. Actual quantitative assays made directly by monitoring the column eluent commenced with gas chromatography (GC). A. J. P. Martin (the inventor of gas chromatography) developed the first in-line detector, the Density Balance [1], which was fairly sensitive, had a linear response, but was an extremely complicated and difficult to make. The detector was linear over about 3 orders of magnitude of concentration and had a sensitivity (minimum detectable concentration) of about 5 x 10-7 g/ml (n-heptane). The detector provided a differential output that displayed solute peaks in the conventional manner that could be assessed quantitatively using peak areas or peak heights (methods for which will be discussed later).

Sensitivity, however, was very important and a number of new detectors were rapidly developed. The next GC detector to be described was the katharometer detector [2] in 1954 (now also known as the thermal conductivity detector or the hot wire detector). This was another, relatively low sensitivity detector (about the same as the density balance) but was simple to construct and was quickly followed by another simple detector of similar sensitivity, the flame thermocouple detector developed by Scott in 1956 [3]. The flame thermocouple detector was the forerunner of the flame ionization detector. The last of the early detectors having limited sensitivity to be reported was also the first of the ionization detectors to be invented, and that was the cross section detector described by Boer in 1956 [4]. Subsequent to 1956, the age of the high sensitivity GC detectors began and the first to make its appearance was the ubiquitous flame ionization detector (FID) described by McWilliams [5] in 1958. This was to become the workhorse of all GC analyses having an extremely high sensitivity and a linear dynamic range exceeding five orders of magnitude. Finally, the exciting family of argon ionization detectors was described by Lovelock [6] in 1960. Correctly designed and operated, the argon detectors could provide sensitivities at least one order of magnitude greater than the FID and the electron capture detector nearly two orders of magnitude greater than the FID.