May Be Column contaminated

Ajay, I have checked with keeper solvents, and spiked solvents. I run spiked solvents to check on instrument and get good recoveries.

It is not column. But thanks.

Dear Calvin

I do not have experience with PAH, but the first thing I try working with standards (if there are some problems) is repeated injections (4-6 times) from the same vial within the same run conditions. Did you try this? Are the results consistent?

Have you tried a standard addition calibration?

Hi Albert, thank you for the response. I have not tried this, thanks. I have however ran standards in duplicate. The results were consistent. I will run with 6 repeats and see what happens. It is so random. Will let you know.

Jerzy - I have not tried standard addition. I will also try this.

Hi,

Maybe, clarifying the subject with a couple of questions would be more beneficial.

Are you using deuterium standards for internal standardization?

If you are using them, are you observing also the same area shifting at deuteriums?

What are your sample matrix and is it possible to get some accumulation (fouling) based on matrix componenets at your liner/inlet ? At this point, post sample pre-treatment spiking with a known amount of analytical standarts mixture just before injection and comparing the results should show something.

If it is a kind of reproducibility problem then we may consider about liner type (inertness-glass wool(packed) interactions - vaporization efficiency etc.) and temperature for vaporization that you applied. Higher inlet temperatures are typical at PAH applications and ultrainert-carbofrit liners would be usefull at this point.

Since working with GC-MS instrument, we need to keep in mind that alternative ion responses. Is this situation available at all fragments...?

Simply change your quantifty ions and recalculate them. If one of the fragment mass which representing your molecule has interference from your matrix, using different ions for quantifying purposes is the practical way of online eliminating the impurity. Just use them for qualifying if this occurs.

Thanks.

Overestimating analyte concenration as you describe, may be signs of solvent evaporation (and hence analyte concentration) prior to injection or simply matrix effect. As I understand it, you quantify higher with increasing boiling point/retention time, correct? So the first eluted compound is somewhat overestimated and the last is significantly overestimated (here 6.5-fold).

In my experience, the former is a much more significant issue with volatiles, but in my experience with PAHs, this shouldn't be such a big issue, as they are SVOCs and and should, as such, not evaporate so differently at ambient conditions, as you describe.

So that leads my attention to matrix effects. But I need to get clarity on a couple of things firstly. Are your deuturated stadards just dissolved in the extraction solvent and injected directly or do you spike it into a representative matrix and then extract? Do you use a deuturated standard for each compound or just a selection for internal/quantification/recovery standard and then calibrate with the non-deuturated standard? Please outline the the protocol to get a better idea of where the issue may arise from.

There are conditions under which you can exchange the deuterium for hydrogen in the injector port. I have had it happen to me with Toluene-d8 in certain matrices. Have you compared the areas of your deuterated IS in good and bad runs? If your areas are consistent then you don't have the exchange occurring, but if your IS areas vary then you have to take that into account, especially with the major impact that you get in the quantification using the deuterated for comparison. Obviously you get analyte, not deuterated analog, when you have H-D exchange, so your internal standard signal goes down and your analyte signal goes up; a bad combination for quantification.

Mark Krause Thank you for explaining that. Yes - the area for the deuterated compounds show variation. What solutions did you find assist you with this? Changing temp of injector port? Thanks again.

Simon Gregersen Echers Thanks for the message. So the deuterated compounds are spiked into a soil or sediment matrix and run through a extraction and clean up process. Following this, the samples are reduced and the samples are run on GC-MS where isooctane is used as the solvent for the extract. I use a mix of five deuterated compounds which cover 2, 3, 4 and 5-6 ring compounds. In the one std mixture added to matrices there are 5 deuterated compounds at the same concentration. Calibration is undertaken with standards which contain the five deuterated compounds, 16 PAH compounds and 2-flurobiphenyl as an internal standard for instrument check.

Look for oxidizing conditions. You might have a leak at the injector port, for example, or a worn septum. We first noticed it when we ran purge and trap on a chlorinated sample; we then knew what to look for and found that injector leaks or bad tanks of helium could cause it. Also, as a general rule, use the lowest injector temperature you can get by with. For your case, with PAHs, that's probably around 250C depending upon your GC and configuration.

Hi Mark. It all makes sense now. Thanks again. There was a major issue with a leak in the instrument. This was corrected and samples after this have been displaying atypical results. Its so refreshing to have something explained so clearly. Have a great day.

Es necesario trabajar con química analítica de procesos los errores experimentales de los métodos.

Another potential source of the variation is the differences between deuterated compounds and non-deuterated compounds. Although it is commonly stated that deuterated and non-deuterated behave identically, at one point I was using D8-toluene to quantify Toluene, and there was baseline separation between the two species in the GC chromatogram.

Given that you are using iso-octane, which is a pretty weak solvent, with respect to dissolving adsorbed species from polar surfaces, it is possible that the iso-octane is not a sufficiently strong solvent to displace the PAH's from the surface of the sample, or perhaps that the PAH's are depositing on you glassware.

Deposition of PAH's onto polar surfaces has been noted in a number of studies.

Adding a small amount of a miscible polar solvent to the extraction, for instance iso-propyl alcohol, may to help decrease issues with recovery of the PAH's.

Alternatively, going to a stronger slovent for the extraction and analysis may aide in resolvign your issues.

Juan José Lozada Castro Yes, you always need to have a way to determine recovery and quality of extraction.

Thanks John Canham . I am looking at other solvents, however had proved the method was providing high recovery prior to moving the instrument, getting it back online and isolating an air leak. Column bleed is occurring, and replacement of gas trap is occuring this week. Apart from all of the technical aspects of the GC-MS, I have tried many combinations of solvents and deuterated compounds in these. Now it appears - that after running the extraction and clean-up, the deuterated compound concentrations in comparision to standards or simply spiked solvent, still have highly atypical results. The atypical results are in all replicates (3) and then in standards the deuterated compounds are the concentration they should be. I have just washed all equipment including solvent washing and am trying this today to see if there is potential contamination coming in from equipment. I am not confident about this - as the results show such good repeatability through the process. Inflated results which are only occuring in the extracts deuterated compounds is a new challenge for me.

Thanks all for your interest and assistance. Still have not resolved this issue.

Any further suggestions would be most welcomed, as I have been through many references, GCMS based trouble shooting documents and asking other technicians.

It is probably due to the evaporation of solvent and concentration of analyte.

Thanks Zdeněk Košnář

Dear @ Calvin D Leech

Thanks a lot for your interest to work with GC-MS. I have some experience to work with GC-MS for the quantification of pesticides residues. I think, this variations related to the preparation of standard and the sample as well, it is not related to the instrument. I suggest you to prepare the standard properly, and I advice you , please do not prepare the standard following serial dilutions, prepare the standard from one working standards solution. The another important issue is matrix. If you are using matrix matched standard, then you have to be very careful to mix with the matrix properly, otherwise a large variations may come. Thank you for your interest.

Dear @ Stephan C. Mann

Thank you so much for your appreciation.