Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

4ANALYSIS 87Il a2t51 L1 2 h 3Fig. 59. Push-pull gas-sampling valve. The connections are: 1, vacuum or sample; 2 and 3,carrier gas; 4, sample volume; 5, connecting bypass 2 to 3; 6, sample volume. (a) evacuation andintroduction of sample; (b) isolation of sample; (c) injection of sample on to column; (d) bypassconnection. From ref. 283.is withdrawn from the RV and injected through a series of columns such that thoseused for the slower eluting materials are placed first, when the faster are eluted as asingle peak but are subsequently analysed in detail by subsequent columns. Suitableswitching of the transfer valves allows secondary injection and the sweepingout from the system of the slower eluting materials. Alternatively, the faster eluentsmay be held in the transfer valve sample volume for later analysis. The use ofPTGC with adsorption columns, poisoned to prevent both tailing and allow theanalysis of high mol. wt. compounds, may reduce the necessity for this techniquez8’.A successful sampling valve is that due to Pratt and Purnell (Fig. 58)282. Adead-space sample is first withdrawn, followed by a reacted sample. Cundall etUI.’~~ claim a better valve is the ‘‘push-pull’’ type (Fig. 59), a variation of thevalve used on some commercial GLC instruments. For sample injections after fractionation,a constant volume and manometer is employed for gases, and precisionmicrolitre syringes for liquids.References pp. 104-Ill