Low Recovery in Benzene and Retention Time Shifts

[rochelle]

I am having a strange problem with benzene detection on my PID detector . The reponse for a 30.0 ppb standard within a run will fluctuate between ~10 and ~20.0. Here is the strange 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. We replace the j-trap(carbopack) that were using and after changing the trap the benzene peak is now visible and we can get a good recovery in benzene but after a few weeks the problem is still occuring. Has anybody had this problem or do you have any suggestions. I really don't think that we have to change the trap again... Changing trap every two weeks is quite strange... Do you think we need to change the moisture trap in GC? Does it have effect?

And another thing, everytime we change our carrier gas and Air, we experience retention time shifts. I know that this will happen and we need to check the flowrate but our problem is how we are going to know the RT of our gasoline range(C6-C9) if the RT shifts... Does interpolation applicable?

Thanks for your answer...

Rochelle...

[Bruce Hamilton]

Provided you have a good pressure regulation system on your gases, changing cylinders should not affect retection time.

If you are changing your carrier gas ( eg from Helium to Hydrogen ), you can still get consistent retentions for each gas if the pressures for each carrier gases remain the same as when last used.

You need to ensure that the system primary regulators ( on or near the cylinder ) are set and not changed, even when swapping cylinders. The most common technique is to put that regulator on the wall, and have a flexible high pressure hose from cylinder to the regulator. You can include a purge and simple shut-off valve so that the pressure controls are not touched.

Once you primary supply is set at sufficiently high pressure to ensure that the instrument pressure controllers can work as designed, there should be very little retention time drift for the carrier gas you use.

Bruce Hamilton

[AICMM]

Rochelle,

1) Is your toluene problem gone?

2) Retention times shift only when you change gases or more often? I have seen problems with the 6 port valve on LSC's leaking which causes RT shifts. Might be worthwhile to tighten that up a bit. Changing air should have nothing to do with RT's (sensitivity maybe, but RT's no.)

3) Is the FID parallel to the PID or in series to the PID?

4) If memory serves me correctly, GRO integration range is set by RT's of benzene to napthalene (which I thought was C6 to C10 hydrocarbons.)

5) No clue on the benzene problem, need more information.

Have you thought about headspace?

Best regards.

[JI2002]

Based on the fact that FID works well with benzene, I wouldn't suspect anything else other than PID itself. You can check/clean the lamp or replace it with a new one to see if it makes a difference.

[chromatographer1]

I suspect the change in response of the benzene is caused by a coelution of another non-FID responsive peak which affects (absorbs part of the light) the amount of light reaching the benzene.

My guess: water

The water probably comes from the trap (which normally does not retain water). Either something in your samples is retained on the trap which then it helps to retain water or it is retained on the surfaces of your sample path.

As noted by others, poor pressure regulation can affect the head pressure of your GC which directly affects the flow rate through your columns and thus the retention times.

A change in retention time means a change in flow, a change in temperature, or a change in the phase in your column.

I hope you can find the causes of your problems.

best wishes,

Rod

[JI2002]

I thought about water too, but usually water and benzene are well resolved in a non-polar like dimethylpolysiloxane column.

[chromatographer1]

Yes, you are quite right, water elutes with C5 or less on a PDMS column. But Rochelle did not say she was using one. If she is using a PEG column water and benzene are not far apart around (C9-10) depending upon temperature Plus the water may elute from the trap/lines continuously for a very broad plug. Since I don't know the details I can only guess and surmise.

But something may be eluting with the benzene which inhibits its detection inconsistently by PID....... and it might not take much of it whatever it is.

best wishes,

Rod

[rochelle]

Good Day!

Thanks for you're reply...

1. As of now we don't have toluene contam, we change the source of our purge water, bake the LSC and column and run a calibration standard. We always experience toluene contam. Thanks AICMM
2. We are using dimethyl polysiloxane column.
3. This last week of May we will install a shut-off valve for hydrogen because when we do a Preventive maintenace in Gc the engineer noticed that even if we turn off the Hydrogen Valve, the Hydrogen still flow and install a pressure regulator for Air and Hydrogen.
4. As of today we still get low recovery of benzene.. We wanted to change the J-trap but we dont have stock and we just change it two weeks from now... I really dont know what to do...
5.About the lamp, do you know the life span of of it? because im not sure but we already replaced the lamp last year... mid-month i guess...
6.We calculate the retention time for GRO using GRO standard... and Get the +/-3sd... Maybe if we install a pressure regulator in Air and Hydrogen and shut-off valve, the RT will improve.

Thanks really for all you're reply..