Gas flow

[jenm]

Can someone explain to me how to optimize the gas flow in GCMS system? I adjust parameters to get the system to work but there must be a better way to optimize. I really do not understand the ideals behind gas flow, linear velocity etc and now I have changed my column from a 25m to 50m and I am struggling with perfecting the method as i am trying things to make it work without any logical reason. I would appreciate some insight (into something I know I should already understand)

[Peter Apps]

Depending on which GC-MS system you have "optimising" gas flows varies from simply setting the linear velocity that you want (on Agilents and probably some others that I have not used) to setting a volume flow and letting the software tell you what linear velocity it generates, and then changing the volume accordingly.

What the optimum flow is has been a subject for interesting discussion on this forum (we have an expert on the subject posting as lmb), but it is safe to say that if you are running helium (which is usual in a GC-MS) that the mean linear velocity should not be less than 35 cm/s.

I should also add that unless you are way off the optimum, changing carrier gas flow is the least useful of all the changes that you can make in method development.

NB that I am assuming that you have a 320 or 250 micron diameter column so that volume flow to the MS is not an issue for the vacuum pumps.

When you changed the column did you change the column settings on the GC or in the software to reflect the new column length ?

Peter

[jenm]

Sorry for lack of details. It is the Saturn 2200 Varian now aka Agilent. I did change the length and id in the GC setup before starting method development. Column is a 200 micron and flow is 1.0ml/min. Linear velocity 28cm/sec, set pressure 28.5, total flow is 15.7. Helium regulator @ 80psi from tank.

When I calibrated flows with meter, the system would not come to pressure, I had to play with column head pressure to get system to come to pressure. Which is my inital reason for posting today- I dont understand the concept of system flows to solve my problem. This is the first time I have used a 50m column and I didnt think the change over would be that difficult but I guess when you dont understand the variables you end up wasting a lot of time in the trial and error phase. So if you have any additional insight with the new info, I would appreciate the help. Thanks Jennifer

[Peter Apps]

Hi Jennifer

When you say "calibrated flows" what exactly did you do ? - measure split and septum purge flows with a flow meter ?, or hold up time of the column with an iinjection of somethig that is not retained ?

You say that the system does not come up to pressure. Are you setting pressure or flow ? Do you see any flow out of the split while the pressure is below the set point ?

Have you leak checked with a leak seeker ?

Peter

[Yama001]

The total flow at 15.7 ml/min may be a little low for the inlet to reach the 28.5 psi pressure, particularly if there is a small leak.

[jenm]

Yes, I measured the septum purge with a flow meter and inputted the headpressure (system calculated) and septum purge flow and pressed the "purge calibrate" option. I am only running a 10 split so the total flow is not too far off (i dont believe).

This morning I reduced the column flow to 0.5mL/min and recalibrated flows. This reduction in allowed the system to come to pressure. So I am guessing this means that I was asking too much of the system at 1.0 mL/min. If this is the case- is there a way to figure this out on paper instead of trial and error at the instrument?

I was also thinking that maybe the system could not come to pressure because the vacuum was too great and pulling the flow through?? Can this senario occur? I really apoligize for my ignorance about a concept that I hould already have grasped.

[Peter Apps]

Hi Jennifer

Before you worry about calculating optimum flows you need to be sure that your GC plumbing is working.

With a back pressure controlled inlet (which is what you have) a fixed flow of gas is fed into the inlet sufficient to supply the septum purge, the column carrier flow and the split flow. The pressure in the inlet, which determines the flow through the column is controlled by a back-pressure regulator on the line that goes to the split outlet.

The GC flow control system feeds enough gas to the inlet to provide the flows that you have set, and the inlet pressure will reach the set value. If there is a leak the gas going into the inlet is less than the gas coming out, and so the pressure cannot reach its set point.

You need to leak check the connection to the column and the septum.

Does the split flow rate correspond to the split ratio that you have set, or alternatively to the total flow minus septum purge minus carrier flow. If it is less there is a leak.

On a 50 m 0.2 mm column the MS vacuum has relatively little effect, but you may need to tell the GC that the column outlet is at vacuum.

Peter