Chromatography Forum

Hello,
I used GC-MSD (6850-5975c) for aliphatic and aromatic hydrocarbons (petroleum hydrocarbons) analysis. When I want to Split the n-C17 and Pristane or n-C18 and Phytane it is not work or not splitted. It is not split in SIM mode nor SCAN mode. I threw all my samples and now I have just MS and SIM data of samples.
Normally in Chemstation I can split but in MSD Chemstation can't. What can I do? What is your suggestion?
Thanks.

I don't even understand if your issue that you cannot inject in the "split" mode, or if your trouble is splitting (resolving) two peaks....

If you only have SIM data then you probably can't do anything with it. If you had scanned data then you may be able to extract ion chroms carefully and separate them that way. Look at the mass spectra and your data, maybe some peaks show up on two or three masses but others only on one.

I don't even understand if your issue that you cannot inject in the "split" mode, or if your trouble is splitting (resolving) two peaks....

hello CPG,
my problem is resolving, n-C17 and Pristane peaks are not sptlitting

Thanks

If you only have SIM data then you probably can't do anything with it. If you had scanned data then you may be able to extract ion chroms carefully and separate them that way. Look at the mass spectra and your data, maybe some peaks show up on two or three masses but others only on one.

Hello Johnny Rod,

I have Scan and SIM data.
When I wanted to calculate n-C17 and Pristane peaks, MSD Chemstation analyzed them as a single peak. I could not seperate them in chromatograms because of that the results of them (after calculation) are not trustable.
I hope explain my problem.
If you have MSD Chemstation I can upload my peak as a MSD Chemstation file.(or who wants to see the peaks I can send via pm)

Thanks

Enis

I'd try a different GC column, approach as a GC issue more than a GCMS issue, especially if this is an assay you'd need to run routinely.

If it's a one-time thing, force integration parameters; but like Johnny said, the MSD does enable one to key in on ions not found in the other co-eluting component.

Hmmm - We can easliy resolve these peaks on a GC-FID using 30m x .32 mm x .25 um RTX-5MS column. On the other hand, when we try this on a GC-MSD using a 60m x .25 mm x .25 um RTX-5MS, we have a lot more trouble - it appears the vacuum at the outlet makes a lot of difference. I do not recall how we got around it - just that it was a bit counter-intuitive to have more trouble on a longer, narrower column.

Hmmm - We can easliy resolve these peaks on a GC-FID using 30m x .32 mm x .25 um RTX-5MS column. On the other hand, when we try this on a GC-MSD using a 60m x .25 mm x .25 um RTX-5MS, we have a lot more trouble - it appears the vacuum at the outlet makes a lot of difference. I do not recall how we got around it - just that it was a bit counter-intuitive to have more trouble on a longer, narrower column.

1. Match up the linear velocity of the carrier flow on the column inside the GCMS; Agilent flow control software (free) has a drop-down box to choose whether the column outlet is at atmospheric pressure (like GC-FID) or under vacuum (like GCMS)

2. Your GCMS column is narrower, has less capacity. So inject less (more-dilute and/or smaller injection volume) and/or increase split ratio and see if the separation gets better. Maybe try this choice first.