Dear all,

Is there any rule of thumb in optimizing conditions in GC-NCIMS analysis? I have recently installed a GC-NCIMS from Agilent. Does anyone have any valuable experience that could share with me?

Thanks in advance.

That's a very broad question. Can you be a little more specific in the information you need?

I have run NCI on Agilent equiment for about 9 years. Here's what I do in a nutshell: I use a roughly 3%N2O2 in CH4 mixture (i have a reference, let me know if you want it). This yields a 17 OH- radical. I adjust the gas flow/source pressure until 17 abundance is optimized. I verify the accuracy of the mass axis by putting just a touch of PFTBA into the source (too much and you'll be seeing it in your background for an hour or more, NCI is VERY sensitive). I look at masses 452; 595; 633 and make sure that the axis is set correctly. This gas is very good at making negative ions of hydroxy's.

I'll check back in a day or two to see if you have any questions.

Thank you for your replies. With regard to the optimization of NCI-MS operation, I have the following queries:-

1)For Agilent GCMSD and according to the manual, does the emission current affect the sensitivity of individual analyte? Agilent GCMSD optimises the emission current to 50 uA in the NCI tune.
2) Could you tell me how to mix a gas mixture of N2O2 in CH4?
3) What parameters do you adjust in optimizing the 17 abundance of OH-? For optimizing, does it mean to obtain maximum 17 abundance?
4) In NCIMS operation, it usually has rare fragmentation pattern. How can I obtain a fair fragmentation in NCI-MS? Some journal papers mentioned about increasing the ion source temperature. Is it true? Does it do any harm to the ion source, such as increasing ease of breaking the filament etc.

Thanks in advance.

1. I optimze the elctron voltage to give the highest aboundance of 17. EI is always at 70kev, I scan the voltage at 17 and set to the apex, ususually around 200-230. The emission is set to 300, I have not changed that. I will check into that setting.
2. I used to but the mixture from Scott, but that was a total rip off. I have two big cylinders of N2O2 and CH4. I bleed some N2O2 into a mixing vessel and then supply it with straight CH4 whihc flows to the instrument. The composition varies, the sensitivity varies alittle as the gas composition changes, but it works well for qualitative analysis.
3. I adjust the CI gas flow. Ususally a pressure of about 1.4torr works well.
4. Rare fragmentation pattern? Most cmpds. (especially alcohols) produce a base peak of M-H that makes mw determination very easy. Sometimes there are other major fragments, but I usually depend on NCI for MW. I run LC-MS with a particle beam interface (as well as GC_MS)and typically run the source at 300C with no loss of filament life.

I am not sure that I understand your comment about the fragmentation pattern. The fragmentation patterns in NCI are very different from the fragmentation in EI. NCI is designed to be a soft ionization, and one of the goals is to preserve the structure and molecular weight information. The goal of NCI is electron capture, which may be followed by some fragmentation to produce a more stable species. If you want library matching in NCI, the best way to do this is to run pure standards of the compounds of interest and create a user library for matching. There are not libraries of mass spectra for CI and NCI like there are for EI because the fragmentation and thus the mass spectra are very dependent on conditions in the source, reagent gas used, etc.

The electron voltage and the emission current will both affect fragmentation. Find a set of values that works for the analyte set you are interested in, then set that as your standard. If you turn the electron voltage down fragmentation is reduced, as is senstivity. Turning emission current up too far can shorten filament lifetime.

NCI takes more work to optimize than EI. If your goal is to identify unknowns, then EI and library matching is the place most users start with. CI and NCI can add valuable information, but the interpretation can take a little practice.