Difference EI to CI in GCMS

[es-rosenharz]

Hi everybody

I am a little bit confused concerning EI and CI. I'm doing quantitative
measurements with malondialdehyde + PFPH for derivatisation.
For EI my mother ion is 234 m/z and easy to detect.

Since I want to measure things now on cellular levels, I decided to have
a closer look on CI, especially on negative CI. It was reported in literature
that this would be 100 fold more sensitive for my compound.
If I do now a CI with Methane, signal to noise is worse
I did it for SIM 233 but as well of 234m/z because I was so confused.

Any ideas?

[Don_Hilton]

1) Have you run some other compound in NCI mode to confirm that the instrument conditions are reasonable? Something that does not have to be derivatized and is known to work well in NIC would help confirm the instrument as being configured properly.

2) For your application of NCI to the derivatized material, you are starting with the same derivatization - conditions, concentrations, and all as you used in the EI runs? (My thought, if you have started with derivitizing lower concentrations, the derivitization could be part of the problem.)

[es-rosenharz]

Hi
1) No, I agree this would be a very easy to confirm whether things work well in theory. Do you have any proposition ?

2) Conditions, concentration - everything is the same. I wanted first to confirm the results. So I just run my samples in EI and afterwards in CI.

Just to add: We have a Gocus GC + DSQII both from Thermo.

[Don_Hilton]

I know that PAHs and PDBEs are sometimes done with NCI. I don't have details handy, but standards are easy to come by and with a little digging you should be able to come up with some information out of the literature. Or someone else reading these posts may be a bit more helpful than I can be at the minute.

[es-rosenharz]

Hi thanks

I just wanted to add, that I can detect the compound. So standardcurve and measurements result in the the values (since the area for the internal standard is lower as well).
This is as well confirmed by the retention times. The only problem is, the lower SN in CI
I am constantly wondering whether I am looking for the correct mass.

EI: 234 m/z

for positive CI: 234 +1 m/z
but for negative CI: 234 - 1m/z ???

[jh1]

Hi thanks

I just wanted to add, that I can detect the compound. So standardcurve and measurements result in the the values (since the area for the internal standard is lower as well).
This is as well confirmed by the retention times. The only problem is, the lower SN in CI
I am constantly wondering whether I am looking for the correct mass.

EI: 234 m/z

for positive CI: 234 +1 m/z
but for negative CI: 234 - 1m/z ???

Negative CI mode typically gives fragment masses like 234- 15m/z or infrequently the molecular weight ion so the mass spectra is frequently quite different from an EI spectra. This is due to the increased sensitivity coming from electron capture which only effects the m/z when the molecule fragments.

I'd recommend running a high standard in scan mode to see what your primary ion is.

[es-rosenharz]

Thanks a lot, I will try that