Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

82 EXPERIMENTAL METHODS FOR SLOW REACTIONSimpurities since these are likely to be retained on the cold column and eluted asflat peaks when it is hot. With katharometer detectors, it is necessary to use adual-column technique because they are so sensitive to flow-rate fluctuations. Theionisation and flame detectors are more satisfactory when only a single columnis required. PTGC has a considerable application in preparative gas chromatographyon a small scale.4.3.5 DetectorsOf the many detectors that are discussed in the literature, we shall considerthree which are the most important for gas-phase kinetic studies. These are thekatharometer, the hydrogen flame ionisation detector (FID), and the gas densitybalance.(a) KatharometersKatharometers or thermal conductivity detectors are usually incorporated in abridge circuit as matched pairs, such that the flow over the reference element isalways pure carrier gas. There are two types of elements, the thermistor (low temperatures)and metal filament elements (high temperatures). The metal filamentsare constructed from platinum or tungsten. The sensing device depends upon thechange in thermal conductivity of the carrier gas stream due to the presence of theunknown gas. Table 16 lists the values of thermal conductivity coefficients forseveral gases at 0" C. Clearly, hydrogen and helium are the best choice for carriergases, of which the former is the most practicable in the United Kingdom. Noticethat H, may be estimated with a reasonable sensitivity relative to He or N2 butTABLE 16THERMAL CONDUCTIVITY COEFFICIENTS (K,) OF GASES AT 0" c(cal. cm-l. sec-l. deg-l x 108)276X Kx (KN~ - Kx) RelativesensitivityNa 58 0 0HI 416 -358 -25.5He 348 -290 - 20.7CH4 12 ' -14 -1.00C*HI 44 14 1 .ooC& 36 22 1.57C4HIO 32 26 1.85CHCI, 16 42 3.00CH,OH 34 24 1.71(CHdKO 24 34 2.43358 2900 - 6868 0344 216372 304380 312384 316400 332382 314392 324