Molecular peaks at EI GC-MS of solvents

[Vivid_Grey]

Good day !

Making EI analyses of mixtures of solvents on ITS40 ( it still works ) I do not see molecular peaks - usually present +1 or sometimes -1 from molecular wt. For example, in EI MS of Acetone, Ethyl and Butyl Acetates must be 58, 88 and 116 peaks respecivelly, but in fact 59, 89 and 117 are observed. Sample load variations leads to change in relative intensity of peaks, but general picture remails all the same. It makes difficulties with identification for more complicated compounds.

0.25mm ID column, 99.9995 helium as a carrier, two stage vacuum pump EVP DRV16 ( 16 cub.m. per h ), ionisation current 25uA, 1280V multiplier voltage, all samples tested contains 0.1 - 2 % of water. FC-43 calibration shows lower intensity of 414 and 502 peaks then expected ( 2 - 3 % of 69 ).

Any idea about the reason ? Thanks for your help !

[JMB]

You are seeing the [M + H]+ signals that are characteristic of chemical ionization, in this case so-called self-ionization;
this is where an ionized molecule reacts with a neutral molecule, which suggests that there is too high an amount of the analyte in the ion source.

What molecular ion region do you see if you record the spectrum ar either the leading or trailing edge of the GC peak and close to the baseline?

Can you plot the m/z for M+. and [M+H]+ for each analyte?

[Vivid_Grey]

Sorry I have missed your reply.
Thanks for your idea, I agree that load is somewhat high. I am injecting 50 ul of heapspace ( lab air ) containing 4 ug of solvent, and if Acetone content in mixture was 20%, we inject 0.8 ug, after 1/60 split - peak load is 12 ng or so.
I have attached AMDIS plots in SIM - intensity of m=58 almost zero even at the end of tail.
[img][img]https://i.ibb.co/VJJWxSm/646-solvent-SIM-58-59-res.jpg[/img][/img]
Second picture shows comparison of library spectra of Acetone and taken from the tail of Acetone peak - really, there is no 59 mass anymore, but anyway 58 is more then ten times less then it should be...
[img][img]https://i.ibb.co/bHRDngj/646-solvent-5ug.jpg[/img][/img]
Is it possible, that HS water acts as a CI agent here ?

[JMB]

Having looked at your spectra, it seems to me that you have nothing to worry about!! However, I would like t osee the entire plots of m/z 58 vs m/z 59 across the acetone peak.
The molecular ion at m/z 58 loses the CH3 radical very easily to give m/z 43 as the base peak and only a very small amount of the M+. at 58 remains.

Note that you SHOULD see m/z 59 peak, because this is formed mostly from the 13C isotope; acetone has C3H6O1, so intensity of m/z 59 should be at about 3 x 1.1% =3.3% of the intensity of m/z 58. It looks to be about 80-90% in your spectrum, so there is probably some background m/z 59 signal in the mass chromatogram.

In your second chromatogram, the mass spectrum of acetone (from the library??) is not a good one because it does not show ANY m/z 59; I suspect that there has been too much background subtraction applied.

[Vivid_Grey]

Having looked at your spectra, it seems to me that you have nothing to worry about!!

Thank you for your suppot !

However, I would like t osee the entire plots of m/z 58 vs m/z 59 across the acetone peak.

Here is entire peak plots for Acetone.



Also I have attached same for EthylAcetate



and ButylAcetate.



If we look at Acetone spectra, yes, 58 shifts to 43 after CH3 is eliminated and it leads to increase of 43 relative to 58. But I can't find explanation of absense of molecular peaks in case of EtAc and ButAc, may be as I am almost newbie in MG/MS
.

In your second chromatogram, the mass spectrum of acetone (from the library??) is not a good one because it does not show ANY m/z 59

This library spectra is taken from NIST library integrated into AMDIS, ver 2.73
2017. Time to time use Varian 2000 WS library - itseems good enough for my needs, but here also absence of molecular peaks creates problems with identification.

Have tried to decrease load by 10 times, but for mixtures of solvents I just loosing minor components, M+1 peaks decreases, but molecular peaks do not appear anyway....

[JMB]

It is really very difficult to diagnose your problem from a distance of ca.4000 miles, especially because I am not familiar with the ITS40 instrument.

Is it an ion trap system?

To observe a greater intensity of the molecular ion you may need to reduce the energy that is deposited.

1) reduce the electron energy (eV) that ionizes?

2) reduce temperature of ion source?

3) manipulate the trap parameters to change residence time of ions in the trap?

Regards
JMB

[Vivid_Grey]

It is really very difficult to diagnose your problem from a distance of ca.4000 miles, especially because I am not familiar with the ITS40 instrument.

ITS40 is really very old device, my is manufactured in 1989. At lest you are tryine to help and I thank you very much for that !

Yes, it is ion trap. One word you suspect overionization. Have not thought about that. What can be varied - multiplier voltage, ionization current or ionization time. Temperature of detector now is 220 deg C. May be really is too much for solvents. I will try these manipulations. Hope this helps !