Decrease in responses over a HS-GC run

[Mac2]

Hi.
I've noticed that the peak areas of my TOL standard are decreasing over the course of running an assay. My first area could be about 700 and my last area (20 vials later) has an area of 400.
It is a relatively clean water based ophthalmic solution that I'm testing.
Thanks.

[chromatographer1]

You are losing the toluene over time. I am guessing that the series of runs is longer than a few hours.

if not then your vial septa are absorbing the toluene or toluene is reacting chemically in your solution.

best wishes,

Rod

[Peter Apps]

If you make up a fresh vial and add it to the end of the sequence, does the area go back up to 700 ?

Peter

[Mac2]

Hi Rod, the length of run is a bit over 24 hours. However, the same assay was run a month ago with no issues of decreasing peak area. The same vial caps are used and crimping seems to be fine. Also I noticed the same problem using screw cap vials. Is it possible to prevent the TOL reacting with the sample?

Hi Peter, I've just added a vial to the end of the run to see how area looks. I would be betting that it will increase.

[chromatographer1]

Not knowing what you sample is, it is extremely difficult to answer your question.

I would guess it is possible to block a reaction of toluene, but that is a completely uninformed guess.

I would suspect your loss of sample might be elsewhere in your system.

The fresh vial test is a must.

Peter always has good suggestions.

Let us know what happens.

Rod

[Mac2]

That's what I was thinking, somewhere else in the system but parameters haven't changed from the last time. That is what's really throwing me!

[chromatographer1]

Contaminated lines streak, absorb, toluene.

Valve rotors can leak.

Crimpers can lose adjustment.

Connections can leak.

So many causes, so little time.

Rod