Chromatography Forum

In a LC/MS, you can use He, N2 or Ar as collision gas. But in a GC/MS you can use He only. At least the carrier gas used in GC/MS can not be N2.
So what is difference?

The problem with N2 as a carrier gas in GC/MS is that it will provide a strong mass at m/z 28. This gets in the way of instrument optimization steps like a leak check.

With that out of the way, the next problem will be one similar to that seen with hydrogen. Ionization of the carrier gas can result in the carrier gas affecting the ionization processes in the ion source. Hydrogen is commonly used in GC and is occasionally used in GC/MS, and some effects are seen on sensitivity and spectra. I would take some caution moving away from helium to any other carrier gas for this reason.

Remember that a carrier gas is not intended to change the fragmentation process - a collision gas is.

Also, there is the issue that the optimum linear velocity becomes quite low when using N2 in a GC column, so you will have to slow down your chromatography to maintain the quality of a chromatogram. (And this chromatographic issue may be the biggest reason that folks don't go toward nitrogen as a carrier gas.)

I have heard of GC/MS being done with nitrogen carrier. But there are some steps in instrument optimization that would require modification. And then, it depends on the options available for that particular instrument. Also, the instrument design will place some limitations. The efficiency of pumping changes depending on the particular gas used. And the design of the instrument may or may not allow you make this change.

If you have an application that is pushing you toward N2 as a carrier gas, talk with your instrument manufacturer. They may be able to help you out.

In most triple quadrupole (QqQ) GC/MS instruments, Ar is used as the collision gas. I think you are referring to GC/MS instruments based on the 3D quadrupole ion trap (3D QIT). These instruments use He as the collision gas because it is there acting as a buffer (to cool the ions) at a pressure of about 10^-1 Pa, which is the normal pressure of a collision gas. When the 3D QIT is used as an LC/MS and MS/MS is done, the collision gas is He. All commercial 3Q QIT instruments use He as both a buffer gas and a collision gas, except for the Shimadzu ESI-QT-TOF instrument which uses Ar for both purposes.

thohry,

One thing that has not been addressed yet is pumping capacity. Typical single quads for EI have relatively small pumping capacity. CI source quads have bigger pumps and LC/MS systems have bigger pumping capacities still. If you are running a typical single quad, you barely have enough pump to pull away the carrier, much less a reaction gas.....

Best regards.

I should also think it's due to the capacity of the vacuum pump system. But there's another point. Do you think N2 can not be used as carrier gas in GC/MS due to the mass of N vs He as well?. Nitrogen is too big/heavy to be contaminated inside the MS manifold.

By the way, thank you all for the posts.
Regards