, was also invented by James and Martin but, for some reason it seems, was never formally reported in the literature. Its description, however, did appear in a review by A. T. James [5] and a detailed explanation of the function described by Munday and Primavesi [6]. The Gas Density Balance The gas density balance was a very complicated and ingenious device and, incidentally, the modern 'so–called' gas density bridge bears little or no resemblance to the original design. A diagram of the gas density balance is shown in figure 7. The detector consisted of a compact Wheatstone network of capillary tubes, drilled out of a high conductivity copper block. The reference flow of mobile phase and the eluent from the column entered at two opposing junctions of the bridge arms (the center of tube (C)) such that the eluent was contained in one vertical arm (C) and the pure mobile phase in a parallel vertical arms (A) and (B).     Figure 7  The Martin Gas Density Bridge

The Simple Gas Density Balance The original gas density balance has already been described. It was complicated, difficult to fabricate and its manufacture was notacommercialsuccess. Intheearlydays of chromatography GOW-MAC developed some elegantly designed filaments for use in the construction of katharometers, which, in due course, were used in many other manufacturer's katharometer products. These sensing filaments were rugged and highly reliable and were used by GOW–MAC to emulate Martin's density balance in a simple form. A diagram of the GOW-MAC gas density balance is shown in figure 15. The sensor consists of a pneumatic bridge of tubes containing three vertical tubes all connected by horizontal tubes at the top and the bottom. Pure carrier gas enters the center of the right hand vertical tube and splits into two streams one passing along the lower horizontal tube and the other along the upper horizontal tube. The eluent from the column enters the center of the middle tube and

The gas density balance, was the first detector with a truly catholic response that was linearly related to the vapor density of the solute and consequently its molecular weight. The gas density balance had a maximum sensitivity (minimum detectable concentration) of about 10-6 g/ml at a signal to noise ratio of two. This detector inspired the invention of a wide range of detectors over the next decade providing both higher sensitivity and selective response. The Modern Gas Chromatograph The modern gas chromatograph is a fairly complex instrument mostly computer controlled. The samples are mechanically injected, the analytical results are automatically calculated and the results printed out, together with the pertinent operating conditions in a standard format. However, the instrument has evolved over many years although the majority of the added devices and techniques were suggested or describe in the first three international symposia on gas

in a vapor jacket. Initially, the detector was situated at the base of the column and consisted of the automatic titrating device, the separation was presented as a chromatogram in the form of a series of steps, the height of each step being proportional to the mass of solute eluted. The apparatus was successfully used to separate some fatty acids, but the limited capability of the device to sense only ionic material motivated Martin to develop a more versatile detector, the Gas Density Balance

a differential flow through the horizontal tubes with a consequent change in the output from the sensors. As the differential flow will be proportional to the pressure difference between the right-hand column of pure carrier gas and the center column full of vapor, the output from the sensor filaments will be proportional to the vapor density of the solute and consequently be related to the molecular weight. In fact with a second detector that measured the concentration of the solute, the gas density balance can be used to determine molecular weight of an eluted solute. This device has about the same sensitivity and linearity as the katharometer but, unfortunately, is no longer commercially available. It was one of the very few simple and inexpensive methods available for  measuring the molecular weight of an eluted solute.   The Flame Ionization detector Without doubt, the Flame Ionization Detector (FID) is the most useful GC detector available and by far that most

Courtesy of GOW-MAC   Figure 15. The GOW-MAC Gas Density Balance