Chromatography Forum

I have an old (I guess that's by definition) 5972, and a 6890 GC.

A few questions:
1. From longtime users of this instrument, what do you consider to be the high end of the concentration range you feel comfortable working in (say in ng of analyte put on the column, and in peak intensity, in scan mode)?

2. I find that sometimes after I run the autotune for standard spectra, the background goes up, sometimes to over 10,000. Clearly it's PFTBA. I've thought it seems like a faulty calibration valve, but it seems to happen sometimes and not others.

3. I have been asked to run some samples which are extracts from bacterial cultures growing on certain PAHs as sole carbon source. They want to quantify the parent compound and try to identify metabolites. The problem is that the concentration of the parent is huge (hundreds of ppm), but the metabolites are much lower. I can dilute them down and use a high split injection ratio, but then I don't think I'll be able to characterize any of the metabolites. I'm wondering if it would work to program the MS to turn off during the time the big parent peak comes off the column (like a solvent delay). These PAHs have 3-4 rings (so 175-200 MW), and the extracts are reasonably clean.

Thanks!
Harry

We normally top out at around 100ppm with 1ul injection, so that is about 100ng injected, either splittless or with 1:5 split ratio. Over that usually wont top out the detector but becomes non linear with the later eluters.

5971 and 5972 were always bad about taking a while to evacuate the tune gas after tuning. We normally didn't open the tune valve unless we had to. Checking the tune daily with DFTPP per EPA 8270 criteria was always much easier on the instrument than autotuning every day.

You can program it to cut off the filament at different times during the run, depending on software version. The instrument will also cut off the EM when a single mass exceeds its limits, so you can still get data for compounds with other masses when you have one that is very high.

Can you assume that the metabolites are more polar than the parent PAH?

James_Ball - thank you very much, that information is very helpful. I see that the background does drop back down after tuning but very slowly--over several hours.

I spoke with someone at Agilent who confirmed that putting that much analyte on the column shouldn't be a concern as long as it's sufficiently volatile--which is true for these PAHs.

The metabolites we expect to see without doing derivatization would be single or double hydroxy- substitutions, possibly epoxides, esters or ketones so yes, more polar; but they should be at much lower concentrations.

I was wondering if it would be ultimately easier (or more robust) to separate the metabolites from the parent first with column chromatography rather than trying to find a way to get the GC to do it.

Steve, I agree that would have been a better approach; the person whose project this is unfortunately can't spend any more time on it.