IMPROVEMENTS IN OR RELATING TO GAS CHROMATOGRAPHY

1,240,121. Gas chromatography apparatus. PYE Ltd. 24 Sept, 1968 [30 Sept., 1967; 25 July, 1968], Nos. 44629/67 and 35661/68. Heading B1L. [Also in Division G3]. Separate traps in trap unit TU are presented to the outlet of gas chromatograph column C to isolate desired fractions, the trap changing mechanism being controlled by means of a predetermined punched tape programme in conjunction with means for comparing the output signal of detector D with a reference level, preselected for low, medium or high settings by means of microswitches MS1-3 in pen recorder PR. Trap index controller TC comprises a number of banks of switches (S1A to S6C), any one of which may be selected in sequence by solenoid actuator SA which is advanced by closing selector advance microswitch MS5; when a sample is injected on to the column, sample inject microswitch MS6 is operated and solenoid actuator SA is returned to the first bank of switches. Switches SID to S6D select the operative bank of switches S1A to S6C, so that when (as shown) S3D is selected as well as one of the other switches S3A-C in bank 3, and solenoid actuator SA also selects bank 3, the upper pole of S3D opens to disconnect power supply from the tape drive motor M while its lower pole closes. As a result, solenoid actuator SA advances to the next bank, and motor M restarts, but only when the pen of recorder unit PR passes the low, medium or high level which has been selected, effluent from the column C meanwhile passing into a stationary trap. The trap is switched when solenoid actuator SA is energised by pen recorder PR and passes to the next selected bank. In this way, collection of desired fractions of a chromatogram may be isolated in separate traps, undesired fractions being allowed to pass to waste, and it is also feasible to synchronise the tape and chromatogram at the leading or trailing edge of an unambiguous peak. In the latter case, one may thereby isolate in higher concentration a minor fraction whose elution overlaps with that of a major fraction (see Fig. 2, not shown). In an alternative embodiment (see Fig. 3, not shown), trap index controller TC is replaced by a comparator circuit in which the signal from the detector D is compared with the signal level selected from any one from a group of potentiometers, a relay being operated when the selected level is reached which operates the trap changing mechanism.