steady-state value, respectively. On falling toward background, flushing volumes of about 1 and 2 ml. are required for the response to drop to 10 and 1% of the original steady-state value, respectively. These flushing volumes are independent of flow rate in the range 0.5 to 4 ml. per minute. The effect of flow rate on diffusion current is not great. At 4 ml. per minute, the diffusion current of 0.001-41 CdC12 in 0.2111 NaCl is only about 57,
different from the static value. For precise work, the flow rate should not be allowed to vary greatly. For monitoring of the f l o rate, ~ it is convenient to replace stopcock F with a threen a y stopcock, one branch of which voids directly and the other branch of n hich voids-through a flowmeter. When flow of the sample solution through the cell is stopped, the cell will fill slonly with air-saturated reference electrode solution, and the
background current will rise. Holyever, the rise is very slow (0.04 pa. per hour). It is therefore easily possible to stop the flow, if desired, and to run a current-voltage scan on the solution held in the cell. ACKNOWLEDGMENT
Financial support in the form of a research grant from the U. s. Atomic Energy Commission is gratefully acknowledged.
A Simple Method of Temperature Programming for Gas Chromatography Harmon Borfitz, Norwich Pharmacal Co., Norwich, N. Y. N
a search of recent literature, a
1temperature programming procedure
for gas chromatography in which the temperature could be increased while maintaining a constant base line vithout auxiliary and expensire equipment was not found. However, a simple method of temperature programming w s found in these laboratories n-hich has been entirely suitable. The problem consisted of analyzing a mixture of acetic acid, water, methanol, and methyl acetate. Using a PerkinElmer Model l 5 4 D Vapor Fract'ometer, helium gas, and a column of polyethylene glycol (Carbowas 1500) on powdered Teflon, this mixture n-as chromatographed a t 150" C. a t a pressure of 28 p s i . and a gas flow rate of 80 cc. per minute at' 21" C. The methyl acetate and methanol were eluted quickly, both within 20 seconds of each other, but could not be resolved. The water was eluted in the relatively short period of 2.5 minutes with an excellent peak, while the acetic acid had a retention time of 19.5 minutes Ivith a useable peak.
Figure 1.
Attempts to separate the methyl acetate and methanol peaks bj- decreasing the temperature resulted in serious tailing of the acetic, acid peak. However, a t 100" C. the methyl acetate, methanol, and water peaks could all be resolved, with only an insignificant overlapping of the methanol xnd methyl acetate peaks. A known sample n-as then injected into the chromatograph wit'h the initial conditions of 100" C., 28 p.s.i., ant1 a gas flow rate of 116 cc. per minute a t 21" C. Following the elution of the methyl acetate, methanol, and water (total elapsed time of 12 minutes), the oven temperature was quickly brought to 1 3 " C. for the elution of the aretic acid. The 21-minute interral between the elution of the water and the acetic acid proved to be too short a period for the column temperature to equilibrate. Largc changes occurred ill the base line, RS illustrated in Figure 1; nhich in turn c a m d severe tailing of the arctic acid peak.
Effect of increasing temperature with (left) and without (right) interrupting gas flow
A.
Methyl acetate
5. Methanol
1632
Measurement of this peak xould have been extremely difficult. The problem \Ta3 solved 11) simply shutting off the hcliuni supply before the column inlet, in tlik gas sampling valve, after t h the water peak and nllnn-ing about 30 minutes for thc column tctiii)c~r3ta!'c~ to equilibrate (Figure 2 ) . l'he pwnution of shutting off the detector xhilc the gas flo\\- \vas int,crrupted I\-L> not obstarved, hut no difficultics ncw because of this. The nn:il! completed by turning the lielium flow on again, and in the time remaining before t'he elution of the acetic acid (about 15 minutes), the base line n'ns adjusted. The esact tiriie required to elute the acetic acid after restarting the f l o ~was not rwordcd. but it is approximately 27 minutra. This is a simple method of temperature programming which would be applicable to many gas clironiatographp problems without the need of added accessories to maintain :I ronstunt base line.
ANALYTICAL CHEMISTRY
C. D.
Water Acetic acid
