I am running an analysis that has six compounds that are evaluated for precision, linearity and drift. I have the unusual problem that drift for 5 of the compounds after analysis is ~ 20% higher, but drift for one compound, TOTM, is doubled, with a drift of 100% or more. I am using Acenaphthene-d10 as the internal standard, and response of this compound is really steady, but especially after samples are injected, the response of TOTM increases significantly. I've been working on this project for about a month, and if I look back at older data, the response of TOTM has been slowly increasing through each analysis, while the response of the internal standard has remained relatively constant. (TOTM initial response was ~ 100,000 - but now it is close to 2 million).
Some of our thoughts for the source of this problem are:
That there are active sites (or something) in the column that is causing TOTM to hang up- running the sample injections pushes that out so that more TOTM elutes in later injections.
That there is a matrix effect from the sample that increases the response from TOTM.
That the inlet is discriminating against TOTM, but then less later on? (I'm running the same method the whole time.)
Other typical players in GCMS methods are the source, the inlet, and the column, but it's still hard to see how any of those would lead to increased response from mostly just one compound.
Any thoughts? Thanks!
Hi.
I second Noel's suggestion and would like to ask about the ionisation technique. Are you using EI or CI? Also, have you tried to do a "blank" run? I mean, without injecting anything, just starting the program and doing data acquisition? If so, what happens?
Cheers!
I'm using a splitless injection with a low pressure drop liner, and EI source (agilent GCMS). I've done several blank injections of methylene chloride- checking for carryover from one injection to the next. There are low level carry over peaks, but they are at ~ 1000 counts, not enough to throw % drift to 150% for the next injection.
I'm curious what your ion source temperature is, especially in light of Noel's comment that TOTM's boiling point is high. We once used a relatively low ion source temperature (150 deg C) with CI to enhance the abundance of certain diagnostic ions (in accordance with published literature), and saw something similar. The source became dirty, even while injecting standards, and we wondered if our analyte, but not our internal standard, was selectively sorbing/condensing on the clean ion source, and sorbing less as the ion source became dirtier. Like you, all blanks were clean, so there was never carryover. We didn't solve it, but I would at least consider raising your ion source temperature, especially if your source is getting dirty with low matrix standards.
Thanks Robert- the source temperatures are the normal 150 and 230- that's an interesting idea. I haven't taken out the source to see how dirty it looks, but after I injected the stock standard on the column, the drift seems to be settling down. We'll see after today's experiment.
I think the answer lies on the boiling point of TOTM. In my opinion, incomplete volatilization of the compound is happening, and your sample matrix acts as a better solvent than pure methylene chloride. I've seen this before with some pesticides and I had to modify the injection mode (from splitless to PTV) and/or using analyte protectants or doing matrix-matched calibration. Here are some of my suggestions:
-Try a split method with a ratio of 10:1; as you know, splitless injections tend to get the inlet dirty and your analyte is very likely behaving as a heavy matrix component. This will also enable you to set a higher inlet temperature in order to enhance analyte volatilization. If you think your samples are dirty and you can get enough sensitivity, I'd change to this injection mode.
-Test some other solvent like DMSO, to see if this solvent can clean the inlet more effectively. If you use DMSO as your injection solvent, be sure to use an adequate solvent delay time, since that solvent can give a very broad peak. Also, make sure your early eluting analytes aren't lost. Be careful of its expansion volume, make sure your liner volume can hold the sample gas volume; because of this, again, a split injection can be helpful, PTV can work too.
-Setting a higher ion source temperature would help you maintaining the ion source clean, but this parameter surely isn't the cause of your increased response. 150 C is indeed a very low temperature, I wouldn't work with such value unless I was interested in some specific ions, but then the samples should be very clean. It is a good idea though, to increase the ion source temperature, due to the high boiling point of your analyte and if you think your samples are dirty. You can also increase the transfer line temperature for a more stable response due to improvements in peak shape, specifically for late eluting compounds.
Thanks Victor- that may be helpful. I injected some of the stock solution on the column (with MS off) and added two blanks before the drift- and it is much better. a split injection will be my next thing to try. And I checked again, my source is at 230. Thanks!
I've seen similar problems that were solved by adding another syringe rinse step following injection. Using a solvent that your compound is very soluble in. For instance methylene chloride, followed by toluene etc. It might be sticking to the glass in the syringe, rinsing with 2 solvents could help. Also GC liner type, injection volume, and purge flow. Silanized liner? injection volume < 2uL? purge flow valve on after ~ 2min for remainder of run to purge inlet?
Not sure if this will help - good luck!
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