Gas Chromatographic Measurement of Halothane

Sir,—MacDonald and Mackenzie (1976) reported an alinear relationship between peak height and halothane concentration when measuring halothane (0.2—4.0%) with a gas chromatograph equipped with a flame ionization detector. Alinearity persisted when the sample loop size was reduced from 2 ml to 0.1 ml, and a linear response was obtained only when the flame ionization detector was replaced with a katharometer. We wish to report that use of a low deadspace value with a 20-fJitre gas sampling loop (No. 2015, Carle Instruments, Inc., 1141 E. Ash Avenue, Fullerton, CA 92631) also provides a linear response without requiring replacement of the detector. Halothane standards were prepared by delivering microlitre quantities of liquid halothane into a glass gas sampling flask (Nahrwold, 1976). The volume of liquid halothane added for each concentration was calculated using the following formula: (273/r) (K/22.4) (MWjSG) (0.01C) d °f liquid halothane to be added, pressure (mmHg), T = temperature V — gas sampling flask volume (litre), weight of halothane, SG = specific gravity of halothane and C = desired halothane vapour concentration (per cent of a standard atmosphere). Standards were analysed in triplicate using a Gow-Mac model 750 gas chromatograph with a flame ionization detector (Gow-Mac Instrument Co., 100 Kings Road, Madison, NJ . 07940). The stainless steel column was 1.83 m x 0.3 cm and packed with 80-100 mesh 20% SE30 on chromabsorb W. Gas flows were 30 ml/min helium, 450 ml/min air and 30 ml/min hydrogen. Column and detector temperatures were 90 °C and 150 "C, respectively. Figure 1 shows calibration curves constructed by plotting peak height as a function of halothane concentration, using either the 2-ml sample loop supplied with the chromatograph or the 20-(ilitre loop. Regression analysis demonstrated that the data for the 2-ml loop were best represented by a parabola. However, inspection of the data suggested a linear relationship at lower concentrations, followed by a where Kanaesth = P = atmospheric (degrees absolute), MW = molecular E E