Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

84 EXPERIMENTAL METHODS FOR SLOW REACTIONSamplifier. The detector is relatively temperature-insensitive and therefore ideal forPTGC. However, it is also insensitive to a number of substances, notably the inertgases, CO, CO, and formic acid. The sensitivity of this detector is such that10-2-10-3 pmole may be estimated with a precision of 1 % under optimum conditions.(c) Gas density balance (Fig. 55)Column effluent enters the detector at A and splits at B and B', and leaves atC and C . Pure carrier gas splits at F and F' and also leaves at C and C'. Forbalance there is zero flow across the detector element H. Addition of vapour at Achanges the density of the gas mixture, causing an unbalance and a flow across H.In commercial instruments, the detector elements are usually matched thermistorsor hot wires which can be incorporated into a Wheatstone bridge network. Thedetector is unique in that it measures an accurately defined property of the elutedvapour. The peak area, A,, in the chromatogram, is related to the molecularweights of the carrier gas M, and the unknown M, by equation (M'). The carrierA, = M,/M,- M, (M')gas is usually nitrogen, unless x is of the same mol. wt. as N,, i.e. CO, C2H, etc.,AJ-310'-1Ic---L-G'C'I LJcTI11-Fig. 55. Schematic view of the gas density balance detector. See J. H. P~rnell*'~ p. 271.