Simple Method for Accurately Determining Retention Times Ronald F. Skinner State Toxicology Laboratory, School of Medicine, University of Washington, Seattle, Wash. 98105
ACCURATEDETERMINATION of retention times is essential for the identification of compounds in gas chromatography, This has been attempted in many ways, such as setting the recorder pen on a major division of the chart paper and starting the chart drive at a known speed while making the injection, placing a push button near the injection port to generate an event mark on the recording or start a timer, or equipping the syringe or injection port with a microswitch to key a mark or start a timer. These methods are all dependent to some extent on the analyst’s injection technique for their accuracy and precision and often require the analyst to do two things at once, increasing the chance for error. The need for a switch of any kind in most applications can be eliminated by taking advantage of the fact that the commonly used 10-p1 syringe can be used as a microswitch. In all 10-pl syringes examined, Precision Sampling Corporation Series CG, Hamilton Model 701 RN, and Glenco Model No. 19913-10, there is a metal to metal contact between the plunger and the head of the needle when the plunger is fully depressed. The resistance across this contact is very low, on the order of 1 ohm, making the syringe an ideal switch to key a marking circuit or start a timer. The mark generating circuit used can be easily and inexpensively constructed with readily obtainable materials. The circuit diagram is given in Figure 1. If a larger or smaller spike is desired, this can be effected by changing the value of the capacitor. The clip to the needle of the syringe can be eliminated if the needle is grounded in the injection port and the circuit will generate a spike by giving the recorder a voltage with respect to ground. This system has been successfully used on several gas chromatographs of different manufacture with no apparent difficulties. It has the added advantage of not interfering with the syringe handling and filling as the wires are attached to the syringe only while an injection is being made.
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Recorder terminals CI ip
‘I-
lop1 syringe
P
I
or microswitch
I
.5 megohm
Figure 1. Circuit for generating event mark
The syringe can also be used as a switch to initiate the start function on integrators, such as the Hewlett-Packard Model 3370A. In this application, the plunger of the syringe is connected to the start pin of the integrator. The integrator starts when the start pin is grounded. A mixture of alcohols was injected on a Porapak Q column 10 times, with the integrator set to time units of 0.01 minute, and the retention times were exactly the same in each analysis. In this particular application the needle was grounded in the injection port so only one wire to the syringe was necessary. If the needle is not grounded in the injection port, it would be necessary to attach a ground wire to the hub of the needle. In summary, this report describes a very simple and inexpensive method for providing a start signal which aids in determining retention times with high precision and accuracy by either keying an event marker or an integrator with a 10-pl syringe. RECEIVED for review April 1,1971. Accepted June 2,1971.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
