Chromatography Forum

Hi to all, i'm working with an Agilent GC-MS and i've started to change gas carrier from helium to hydrogen.
Today i've done some tests in Scan and SIM + Scan modes converting also my helium method in H2 with agilent Method translator.

My problem is that in Scan i see only 3 peaks instead of 10 and in SIM its impossible to detect them because i dont know the retention time of my compounds with hydrogen.

Someone has any suggestions to find out my peaks?

Maybe i can try to make 10 vials (1 for each compound with high concentration) in order to see in Scan Mode only 1 peak.

thanks in advance

I've never had to utilize anything but He as a carrier gas, and I think someone should come along soon with a better suggestion but if it were me, based on the information you've given, I'd see if I could determine the kovat's retention indices of my compounds or get them from the literature, utilize retention index standards to do the conversion to retention indices from retention time and hunt them down that way, hopefully looking for ions that distinguish themselves from whatever your matrix is.

Hi to all, i'm working with an Agilent GC-MS and i've started to change gas carrier from helium to hydrogen.
Today i've done some tests in Scan and SIM + Scan modes converting also my helium method in H2 with agilent Method translator.

My problem is that in Scan i see only 3 peaks instead of 10 and in SIM its impossible to detect them because i dont know the retention time of my compounds with hydrogen.

Someone has any suggestions to find out my peaks?

Maybe i can try to make 10 vials (1 for each compound with high concentration) in order to see in Scan Mode only 1 peak.

thanks in advance

Have you tried making a SIM method with all the ions of interest in a single segment so it looks at all the ions for the full run?

If there are too many for one segment, then try making an individual method for each SIM analyte and run ten runs with different methods until you identify the new retention times. Then just delete the extra methods.

I would either run full scan (SIM off) and try and detect where the peaks were. If you can't detect them because the concentration is too low, run SIM only but set only one SIM window the length of the run and with only one ion, the quant ion of the peak you're looking for. Dwell perhaps 50 ms, you should find the peak. If not, I would think there's something else going on.

Thanks to all,
I've prepared 10 vials, each for 1 of my compound (in high concentration) and i finally found the Retention time of my compounds.
My problem now is that i have high background noise in my chromatogram and I dont see acrylamide and phenol that are in low concentrations.

How can i decrease that background?

thanks

I use H2 in a Agilent 5977 MS and though I'm a relatively new GC-MS user but it seems to me it has is plus points and drawbacks.

Assuming your flow rate int0 the MS is low the background problem is most likely solved by time, its takes much longer for a hydrogen system to settle than a helium system. You also need to clean the source more often, though the frequency is obviously due to what you throw down it.

On the plus side your run times should be a bit shorter and peak shape/width should be slightly better. And gas costs far, far cheaper with a more reliable source,

If you can't see it in SIM and you are sure you are sensitive enough, consider the fact that hydrogenation might have occurred, so your ions might be a bit off.

Thanks to all,
I've prepared 10 vials, each for 1 of my compound (in high concentration) and i finally found the Retention time of my compounds.
My problem now is that i have high background noise in my chromatogram and I dont see acrylamide and phenol that are in low concentrations.

How can i decrease that background?

thanks

When switching a system that was run with helium for a long time to hydrogen, you will see a very high background of what looks like hydrocarbons for weeks. Hydrogen scrubs any residual hydrocarbons from the gas lines and controller valves, inlet and column. It will eventually get better but never completely go away, you can try running a high flow and high temperatures in a bake out mode for a few days and see if that helps.

When running samples I drop the flow and gain a lot of sensitivity. You can't really take advantage of the much shorter run times when using MS as the MS can't pump out the hydrogen fast enough so you have to reduce the column flow to the low end of what is considered the optimal linear velocity or even below it, so is better if you can switch to a 0.18mm column if possible.

Thanks to all, with 30m column i used 2.1 ml/min as flow.
For increase my sensitivity i want to change my column with 20 m and 0.3 film, change drawoutplate and lens insulator from 3 mm to 6 or9 mm.

I will say to you if the noise and background will decrease.

Thanks

Nicolò

Thanks to all, with 30m column i used 2.1 ml/min as flow.
For increase my sensitivity i want to change my column with 20 m and 0.3 film, change drawoutplate and lens insulator from 3 mm to 6 or9 mm.

I will say to you if the noise and background will decrease.

Thanks

Nicolò

If the column is 0.25mm ID and 30m length and oven temperature is 40C then the inlet pressure with Hydrogen is 6.75psi, If the column is 0.25mm ID and 20m length and oven temperature is 40C then the inlet pressure is only 2.81psi. You will have to be careful not to back flash solvent when injecting with such low inlet pressures, also the vacuum system would have trouble pumping 2.1ml/min hydrogen so you will lose sensitivity because of higher analyzer pressure.

If you use a column that is 0.18mm ID and 20m length and oven temperature 40C then at 2.1ml/min the inlet pressure is 19psi. But you can lower the flow to 1.2ml/min and still have an inlet pressure of 10.9psi but much lower analyzer vacuum for better sensitivity. A 0.25mm ID 20m length column will not go below 1.69 ml/min with hydrogen without requiring less than 1psi inlet pressure at 40C.

If you increase the analyzer pressure you lose sensitivity, so better to go with lower flows with hydrogen if possible, but that also requires lower inlet pressures or smaller diameter columns.