internal standard issue

[wrmb041170]

I am running VOC analysis on an HP 6890N GC with a 5973 inert MSD and Encon Evolution concentrator. Each analysis contains three internal standards and four surrogates standards, all at 5 ppb. The problem is that in a calibration curve of all method analytes, the response for each internal and surrogate standard increases with each successive level. Theoretically they should remain constant. Also, the responses for IS/Surr in a lab blank containing no method analytes are lower than they are in any calibration level. Looking for a suggestion on how to solve this problem, thanks.

Column: Restek 502.2, 60M x 25um x 1.4um
Flow = 1.0 mL/min
20:1 split
inj port temp = 200

[James_Ball]

I ran a topic on this a while back with interesting conclusions.

viewtopic.php?f=3&t=23167&hilit=internal+standard+area

It is a common issue in purge and trap. One way to help the problem is to spike the internal standard/surrogates near the upper end of the curve instead of in the middle to lower end of the curve. The amount of recovery seems to be related to the total mass of analytes on the trap, so spiking with high internal concentrations gives less total mass change across the range of the calibration than spiking at the low end.

[wrmb041170]

James, thanks for the reply. Yes, the issue at hand is contribution to the IS area counts from the almost co-eluting non-isotope forms of the compound. I see your point on how at low IS levels, the adverse effect of this contribution is more pronounced.

I was in fact spiking with IS at a concentration (5 ppb) toward the lower end of the curve. This was to comply with what I interpreted as EPA method 524.2 requirement. But I will review the method to see if it allows for a higher final IS concentration.

Walter

[James_Ball]

We always spike 524.2 internal standards at 25ppb. We have never had a problem with NELAC or State drinking water auditors.(your auditors may vary)

Of course we have to calibrate up to 200ppb for some of the THM samples we have so we don't have to dilute the samples. We always have a few each month where Chloroform exceeds 100ppb.