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General Description
If you are having trouble with a purge and trap analysis, this systematic approach can help you determine the source of the problem.
- Make a direct injection of your standard onto the GC column, using the volume you normally would spike into your 5mL or 25mL water sample. If the chromatogram is unsatisfactory, the problem is in the GC system. If the chromatogram is satisfactory, go to 2.
- Disconnect the purge trap at the top fitting and inject your standard directly onto the top of the adsorbent bed in the trap. Reconnect the trap and switch the purge and trap unit to desorb (do not dry purge). If the chromatogram is unsatisfactory, the problem likely is in the transfer line and/or the 6-port valve. If the chromatogram is satisfactory, go to 3.
- Change the trap and run a standard through the purge and trap as you would normally. If the chromatogram is satisfactory, the trap is the problem. If the chromatogram is unsatisfactory, the problem is between the purge vessel and the top of the trap. If you have an autosampler, go to 4.
- Purge the standard on the front mount of your concentrator (if possible). This will help eliminate the autosampler and/or its transfer line as the source of the problem.
General Recommendations
Confirm that your chemical standards were made correctly and have not degraded. Make your standards as concentrated as possible, so minimal methanol is added to the samples. Problems usually occur when high levels of methanol and/or water are adsorbed on the trap. Use water and hydrocarbon traps to ensure that your purge gas is dry and free of contaminants. Change the traps regularly, just as you change those on your GC carrier gas line. |
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