Chromatography Forum

hi,

currently using a method that runs in 5psi constant pressure mode on a DB-5 column giving me a flow of 5.6ml/min.

I separate heptane and toluene and they come out at 5.2min (heptane) and 8.5min (toluene). never had a problem but now I have changed to a different GC and now the peaks are coming out at 3.9min and 6.9min. the assay is now therefore failing for resolution. remember this is the exact same column and exact same method parameters so it has to be the system right? checked column dimensions and stuff and there fine. the column has never caused issues so far.

now when I check total flow on the system its ~ 18.8ml/min. this cant be right? the column flow is 5.6ml/min and septum purge is ~2ml/min. The total flow on the system I usually use is ~8.5ml/min which seems more in line.

what can cause a significant change in RT when only thing that changes is the system your using?

any help would be great.

Hello

Is it Split or Splitless?
I think that 5psi is too low - check GC column calculator but I think that 5 psi will not give you 5.6 flow (of course I dn't know what are column dimensions and temperature)

Check inlet for leaks - perhaps you have leak somwhere.

Total flow = column flow + split vent flow + spetum purge Check if you have correct values

Regards

Tomasz Kubowicz

thanks for your reply,

5psi may be too low but it doesn't change the fact that running at the same pressure still results in a significant change of RT for the peaks. you would imagine the same column and same flow will result in the same RT even when running on different systems?

the method is splitless, inlet 250C, purge on 1.5min at 25ml/min.
column is 40M x 530um x 1um ID.

in the meantime I will check for leaks at the inlet

thanks

What carrier gas were you using and what are you using now ? What carrier was selected on the previous GC and what is selected now ? NB the what you are using and what is selected may not be the same !

How did you check total flow ? Have you measured any flows on either machine with an independent flow meter, not with the onboard gas controls of the GCs ?

Peter

Are column dimensions properly set in GC ?

yes it was helium on the old system and helium on the new.

im not entirely sure how to use the electronic flow meter we have. whats the best way to measure flow from the inlet?

yes it was helium on the old system and helium on the new.

im not entirely sure how to use the electronic flow meter we have. whats the best way to measure flow from the inlet?

Then measure flows at outlets. Read the GC manual for details. Column flow is essential. Check it at detector outlet.

To measure flow through the column turn off the detector gasses and connect the flow meter of the chimney of the FID. You can connect straight to the outlets of split and septum purge.

What make and models of GC are involved ?

Peter

Thanks peter

Is it considered a bit inaccurate measuring low flow rates with an electronic meter?

By the way it's the 6890N.

If the new retention time is earlier, then the leak would have to be in the old system not the new.

Electronic flow meters should be accurate down to 0.5ml/min or less. On a 6890 FID the tubing of the flow meter will probably fit snugly into the outlet of the FID enough to measure the flows. Use only the column flow, no air or fuel gas.

Another way is to remove the column from the detector and slide it into the tubing on the flow meter and pinch it down to make a seal while measuring flow. Definitely measure the flow from both instruments if possible to compare them. Once you know the actual flow on the old instrument, adjust the flow on the new instrument until the outlet flow matches. Also look in the column setting window to make sure the "constant flow" is not set, as this will cause the GC to increase inlet pressure as oven temperature increases to maintain constant flow rate through the column. If the old instrument only maintained 5psi head pressure and the new one is varying pressure to maintain constant flow, you will have peaks the elute earlier on the new one.

Thanks peter

Is it considered a bit inaccurate measuring low flow rates with an electronic meter?

By the way it's the 6890N.

Your lowest flow is the 5 ml/min or so through the column, and that is well within the range of a flow meter.

Just to be sure; both instruments are 6890Ns ?

I agree with James that you need to double check all the settings to make sure that they are indeed the same on the two instruments, and keep in mind that the fault might have been in the one you were using previously.

Peter

I read this post and a couple of things bother me.

1. You state that your column flow is roughly 5 mL/min of He. For the column dimensions and head pressure that you give you would have to hold the column at 250 C. This you can calculate with any of the readily-available flow calculators. At 250 C hexane will be unretained on a DB-5 column, so this is in direct conflict with your stated retention time for hexane.

2. Since you haven't made any external flow measurements, and since your retention times and flow rates are inconsistent, I must draw the conclusion that your on-board flow measurements are incorrect.

3. If you don't actually know what your flows are, then you absolutely do NOT have the same method on both instruments. The only way to know is to have external flow measurements.

Two different methods will most definitely lead to two different retentions times for the same analyte.

I read this post and a couple of things bother me.

1. You state that your column flow is roughly 5 mL/min of He. For the column dimensions and head pressure that you give you would have to hold the column at 250 C. This you can calculate with any of the readily-available flow calculators. At 250 C hexane will be unretained on a DB-5 column, so this is in direct conflict with your stated retention time for hexane.
...

You assumed outlet pressure 0 psi.
FID was mentioned several times here and poster didn't deny it so we may assume an outlet pressure 14,7 psi. Parameters are roughly correct for oven temp. 40 degC.

i have measured the flows of the two GC instruments. the first one (the one i didnt have any problems with) was giving me a flow of around 6.5ml/min with the electronic flow meter even though the front of the GC panel was saying 5.8ml/min. the second system (the one im having problems with) is giving a flow from the detector outlet of 24ml/min! so im assuming this is a major miscalculation of the GC.

i do not know how to interpret this issue.its clearly very wrong on the second system but why? will i measure flows from anywhere else or where else is best to proceed with this?

just to be clear, the retention times are earlier on the system giving the 24ml/min flow using the flow meter.

again my apologies if the info given is limited. im still very much a novice in this area.

... the second system (the one im having problems with) is giving a flow from the detector outlet of 24ml/min! so im assuming this is a major miscalculation of the GC.

i do not know how to interpret this issue.its clearly very wrong on the second system but why? will i measure flows from anywhere else or where else is best to proceed with this?

just to be clear, the retention times are earlier on the system giving the 24ml/min flow using the flow meter.

again my apologies if the info given is limited. im still very much a novice in this area.

You have to zeroe and calibrate carrier gas flow in the instrument in question. Details ? - RTFM.