MS SENSITIVITY????

[oscarBAL]

Hello!!
I have to admit that after so many year working in CG and HPLC is just recently that I have a GC/MS on my hands!!
I am trying to analize one sample that looks to have DDT, I see the signal on nthe GC/ECD but the shape of the peak is a little difererent, when I inJect the sample on the GC/MS I can not idetify any symilar compounds that could give a signal on the ECD, and I am usin the same type of column, well I injected a standar DDT 1ppm and I can not see any thing as yet, I am wondering if I really can determinate DDT to 1ppm inthat equipment???? I don't think so.

I am working with a Clarus 500 MS from PE, I am scannig from 50-450uma, Using a column 5%diphenyl/95%dimethyl siloxane, My multiplier is on 400 mv.
Any one can say me what is the aproximate limit of detection for that kind of compund??

Thanks a lot!

Oscar.

[james little]

Would depend on how much you are injecting and how well focusing upon injection.

Sometimes we find it better on our GCMS to inject 3-5 ul's with a low split (5:1) vs 1-2 ul's in Grob mode.

Might want to reproduce the sensitivity specification on the instrument then compare to what you are trying to analyze.

Could try selected ion monitoring which will increase the sensitivity or possible negative ion Chemical ionization using methane as the CI gas.

[oscarBAL]

Thank you very much James.

My GC/MS is realy new, so I; and I am not worry about my sample, I am worry about my STD, I don't know why if I inyect DDT 1 ppm I can not see anything, I wonder if this is normal for a GS/MS, I would spect to see sometihng I am usssing your same split ratio 5:1, but I am injecting 1ul. I don't want inject more than that becaues I am workin with a 0.25mm DI capyllar column, I afraid to inject more volumen and saturate my column, is this possible with that split ratio?

I don't want to use ion monitoring because we belive that could be DDT jus because our Rt coincide with DDT std in the ECD, any way if that is not DDT I would like to know what could be, but I can not be sure because I neither can not see my STD and I want to know if this is normal.

Thanks Again.

Oscar.

[james little]

It has been a while since I did any sensitivity checks on our instruments and my memory is a little stretched. However, will make some crude calculations.

As I remember, our VG70 could see about 30 pg full scan on column and our old Finnigan TSQ could see about 200 pg on column full scan (35-650 in 0.5 to 1 sec per scan) for many classes of compound. However, the detection limit could vary significantly for different classes of compounds and also depended on the cleanliness of the system.

Thus IF your instrument had similar sensitivity (not familiar with specifications) and you injected splitless (Grob, assume none loss through septum purge, etc.), then 1 ul injection (1 mg) would be 1 x 10-6 x 1 mg x 1000 ug/mg x 1000 ng/ug= 1 ng.

If I haven't slipped a zero somewhere in the calculations, seems like it would be reasonable to see this concentration.

If I were you, I would still check to see what the EI GC/MS specifications are for the machine and see if I could reproduce them. Lots of things can go wrong with a GC/MS system including column, source, electronics, interface, injection port, injector, etc. It always surprises me when it works. I usually have a personal specification check on the instruments I use for a compound of significance to our company. Always good to check your assumptions.

Good luck.

[CIL-BAHA]

(Removed)

[Mark]

oscarBAL,

If I might suggest, if your GC-MS has CI capability and is capable of negative CI mode this would be a good choice if this is really DDT. This is sort of the equilvalent of electron capture but you get mass information also. NCI should be more sensitive for electron capture compunds like chlorinates so 1 ppm should give a good response.

Regards,
Mark