Chromatography Forum

The GC running in this lab has a thermal conductivity detector and HayeSep D column that is used to seperate natural gases at room temperature. However, it can be used at high temperatures and we use it at about 80 degrees celcius. My question is can we leave the GC on at this temperature without the carrier gas(He) running through as it takes two hours for the GC to heat up and it would save us alot of time if we could do this. However, we will not do it at the expense of the column. If anyone is able to answer this question, I would appreciate it very much.

Why bother turning the carrier gas off ?. Calculate the cost of the gas - depending on where you are it will come to less than 0.05 US cents per minute.

Peter

Peter is quite correct, the cost of the gas is so small why bother.

It sounds like a supervisor wants to write a report how is he saving the company money by doing everything possible.

In any case if you have a steel column not glass you could disconnect the column from the detector and put a cap on the end of the column while keeping the pressure low on the column to reduce useage,

or simply lower the gas flow down to 1 or 2 cc/min.

The cost of the cap should be returned in 1 or 2 years and the other alternative is very inexpensive as it only costs the labor involved to set the flow and then reset the flow, but what would the actual savings be?

If labor is even 20 cents a minute, is it worth waiting 10 or 20 minutes for the flow to equilibrate if you have a sample to run?

best wishes,

Rod

Why bother turning the carrier gas off ?. Calculate the cost of the gas - depending on where you are it will come to less than 0.05 US cents per minute.

Also, you might consider a cheaper grade of helium. Unless you're looking for concentrations less than 10 ppm (which would be unusual in natural gas analysis) a "Zero" or 4.8 grade would be sufficient, and cheaper than the "UHP" or 5.0 grade you're probably using.

Or is the issue how often you have to change out the carrier gas cylinder?

The column will degrade without the carrier gas at 80C, as it's a porous polymer, and the equilibrium time will be even longer once carrier is restarted. You will still have to recondition the column, and column life will decrease from years to months.

Because, unfortunately, bean counter laboratory managers are not allowed to be killed by their staff, one alternative solution is to add a bypass to the instrument front end carrier gas line.

The system usually consisting of an on-off toggle valve and a restriction that reduces the flow to 1-2 ml/min that is plumbed across the usual carrier on-off valve. You open the bypass, and close the normal valve. It can all be combined in a multi-port valve, if the savings are justified.

Why not go the whole hog, and suggest a $2,000+ time-controlled, auto-switching, multiport valve to save $100 of gas per year?. It should be worth some sort of performance bonus .

I'd also endorse using the most appropriate grade of carrier gas for TCD and FID. I used welding grade argon and technical grade hydrogen and helium on packed and capillary column GCs with gas purifiers that were regenerated once a year. In general, most carrier gas disappears through leaks or high split vents when instruments aren't running, and using cheap gases meant instruments were always ready to go.

You need ultrapure gases if you are looking for trace impurities or have oxygen-sensitive columns or detectors. If you are buying ultra-pure gases, invest in a leak detector and some simple bypass valve setups.

Bruce Hamilton

Like chromatographer1 stated, I'd also just lower the helium flow. Keep it simple.

Another way of looking at it is (somebody - please check my calculations!)

A standard 1.5m (5ft) cylinder of helium holds 9 cu.m
=9000000mls
at 30ml/min =300000mins=5000hrs=200 days approx. (and assuming that nobody in the lab is talking in Donald Duck)

Hope that helps

Ralph

Another way of looking at it is (somebody - please check my calculations!)

A standard 1.5m (5ft) cylinder of helium holds 9 cu.m
=9000000mls at 30ml/min =300000mins=5000hrs=200 days approx.

A+ for maths . That's US$500 here for ultrapure grade, much less than the 200 days of depreciation for many GCs. Helium is a non-renewable resource, and should be conserved.

One "advantage" of using a bypass system, for both capillary and packed column GCs, is that instrument settings are not changed, consequently retention times are almost identical day to day.

That "advantage" may no longer exist with modern electronic pressure or flow controls, but is very valuable for older and simpler GCs ( eg HP5890 ).

Please keep having fun,

Bruce Hamilton

Thanks Bruce. Recently, when I expressed concern about the amount of helium that we were losing through leaks in the piping (due to the numerous couplings used in the supply to the lab from a remote location) my boss replied "Can't we use a helium generator?" !!! Imagine the sunburn!

Regards,

Ralph

That boss sounds like Dilbert's boss (US comic strip about the work world).

I liked this one, it's posted in our office:

Dilbert's boss - What's this MFU2 on your timeline?
Dilbert - That's management foul up number 2 - it usually happens around the third week.
Dilbert's boss - We don't anticipate any management mistakes.
Dilbert - That's MFU1.