Chromatography Forum

I'm running EPA Method 8270 on an HP 5890 GC/5972 MSD, and I keep getting a negative concentration value for a couple of my compounds. I've tried manually integrating the peak in QEdit, which hasn't helped. I believe everything in the Edit Compounds form is correct for these compounds, but I really don't understand enough about how the calculations work to know what might be wrong. If anyone has any ideas, I would be grateful!
Here's the ion peak:
And the response for Isophorone:

Hello

Possible reasons:
1.Your calculated concentration (negative) is lower than your first concentration on calibration curve - perhaps you're working out of the calibration curve value
2.Check your calibration curve settings - is it forced to go through (0,0)?

Regards

Tomasz Kubowicz

Ok, this makes sense. Thank you! I also have a question about manual integration and the QEdit function. Sometimes my ion ratios are out of range, and I can manually integrate to get them in range, but I know this isn't allowed. Any ideas what I can do differently? I'm starting up this program at my company and neither I nor anyone else here has any previous experience with GC/MS or the methods (EPA 8260/8270). I'm having such a hard time! Also, sometimes the computer tells me I have 0% of an ion, but I can clearly see the ion peak. Is there something I can do about that?

My company has asked me to find someone to come in and train me on little things like this--I've figured out all the basic instrumentation/methodology, but the analysis is really tricky. Do you know where I could find someone?

I know this is all random, but I need so much help! Thanks so much!

The parameters for integrate can be adjusted to properly integrate peaks. The RTE integrator doesn't have many parameters to set so it may be worth your while to try changing the expected peak widths and minimum peak areas to understand how they affect your results. If your peaks are small they may be excluded because they are less than the minimum peak area, that is something you can change, either globally or just for one compound.
It sounds like you need someone to train you on how to do this and what the requirements are for the data you produce. There is nothing in 8260/8270/8000 that says you can't or even shouldn't manually integrate peaks when necessary, but you should know what is required to document what you did and why you did it. These requirements are based on the final use for the data and if you are a regulated environment.

I had instances where there was negative peak areas. I checked those and found that on rare occasions where two peaks are not resolved, during integration ChemStation set baseline a weird way and that is why calculation yield a negative result. Possibly it is your case too.