NPD Signal Increase

[cbingham23]

Hi everyone,

I am experiencing an interesting problem, and I was hoping someone might be able to help. I am currently using an Agilent 7890B, with an NPD detector. My baseline is relatively flat and clean, and does not seem to increase much over the course of a 10 sample bracket. However, my signal does increase. I run a 3-point calibration curve, bracket it for 10 samples. The final standards are significantly higher than the initial standards, which has been leading to inconsistent values for our check standard that runs in each bracket. I have cleaned the instrument well, replaced all consumables for the inlet and detector, replaced the NPD electrometer, and have even replaced the S/SL inlet pressure manifold. I'm at a loss as to why I am still seeing signal increase. I even run 4-8 injections immediately preceding the start of a sequence, to help stabilize the baseline and performance. Any suggestions? Thank you for any help. Have a great day.

Chris

[AICMM]

cbingham23,

What is your solvent? NPD's will become sensitized with chlorinated solvents for example. Are you turning off the flows when the solvent elutes?

Might I suggest you contact Paul Patterson at DET to ask him since he is THE NPD wizard?

Best regards,

AICMM

[cbingham23]

We are using ethyl acetate for our solvent. We do not turn off the flows when the solvent elutes, but this hasn't been an issue before. It has only been recently that we have been seeing this trouble.

Do you have the contact information for Paul Patterson?

Thanks again for your help.

Chris

[GasMan]

Sometimes detectors tell you the truth, this is what you are giving to me. Do the increasing values plateau out and then remain constant for the rest of the day only to repeat the cycle next day when the GC is left overnight in standby? You could have active sites in the column or any where else in the sample path which absorb some of your sample and when all sites have been "deactivated" you get consistent results. Overnight the sample desorbs, and the cycle repeats itself. This is common problem in sulfur analysis, but can occur with other compounds.

Gasman

[Peter Apps]

What element are you looking for; nitrogen or phosphurus ?. Organophosphate pesticides are unpleasantly sticky in chromatographic systems and I expect that you are seeing tailing peaks which are a symptom of the active sates that Gasman was talking about.

You have changed almost everything except the second most likely source of adsorption; the column (the most likely is the inlet). A new column would seem like a sensible next step.

Peter

[cbingham23]

Hi Peter,

I guess I forgot to mention that I did replace the column as well. That was one of our first thoughts. We are looking at organonitrogen fungicides, and since we cleaned everything, our peak shape is looking great. There is little to no tailing. Also, we do not see it really stabilizing either. Sometimes it appears as though the signal just continues to increase, with no obvious shift in the baseline, and other times it is just being very consistent, meaning it looks as though it is not injecting a consistent amount. We also tried putting a different ALS tower, and are still seeing the same inconsistent results.

Chris

[Peter Apps]

Are the calibration standards at the end of the sequence injected from the same vials as the standards at the beginning ? How long does a sequence last - I'm guessing 8- 10 hours ?

Peter

[cbingham23]

Yes, the standards are injected from the same vial. However, each bracket in our sequence is roughly an hour, as we have short run times, due to only looking for a few analytes. Also, it has only been the last few weeks that this has been an issue, so it doesn't seem likely to me that it would be due to solvent evaporation in the vials.

Chris

[Peter Apps]

Yes, the standards are injected from the same vial. However, each bracket in our sequence is roughly an hour, as we have short run times, due to only looking for a few analytes. Also, it has only been the last few weeks that this has been an issue, so it doesn't seem likely to me that it would be due to solvent evaporation in the vials.

Chris

Solvent evaporation was what I was thinking. It is at least as likely as any other explanation until you have tested it by putting fresh vials at the end of the sequence.

Peter