I am having a strange problem with benzene detection on my PID detector (O.I. analytical). The reponse for a 2.0 ppb standard within a run will fluctuate between ~1.3 and ~2.0. Here is the interesting part - the FID which is in line with the PID has the correct response and the other component, toluene, ethylbenzene and xylenes are correct on both detectors. This precludes anything wrong with the standards, the purge and trap, and the column. I tried a new PID lamp (different manufacturer), and increasing the lamp intensity to 4. Problem is still occuring. Has anybody had this problem or do you have any suggestions.

Are you doing a purge & trap analysis? If so, you may have some water coeluting with your benzene and causing the response fluctuation. Try doing some direct injections of low concentration standards to see if the response still fluctuates. If it is water interference, you can try to reduce the amount of water carried over to the GC by a number of things: decreasing the desorb time, increasing or adding a dry purge time, using a P&T system with water management, purging your sample for less time (if possible). The easiest one to change is the length of desorb time -- if you're desorbing for 4 minutes, try some runs at 2 minutes. This will have quite a dramatic effect on the amount of water introduced to the column.

Jason Ellis
Applications Chemist
J&W Scientific, Inc.
e-mail: jasone@jandw.com

It is almost certain you have water being transferred. Jason is correct. In addition you may want to change traps to one that does not contain silica gel. Carbon based BTEX traps and Tenax usually work well. Good luck.