Strange observation

[ozric]

Hi All

Can anyone throw any light on this problem I have...
I am running headspace injections of mixed solvents. What has been happening recently is that the retention time of the components in the first injection varies greatly from the subsequent injections...In the subesquent injections the components have retention times within +/- 0.002 mins.....

Any suggestions??

[Peter Apps]

Let me ask the usual questions:

what instrument are you using ?

what column are you using (phase, length, diameter)?

automated or manual sampling, how do you transfer sample from headspacer to column (e.g. split or splitless)

what solvents in what matrix ?

temperature conditions for sample equilibrium and separation ?

are all peaks affected the same ?

how big is the difference ?

how do you expect anyone to troubleshoot your problem without your providing any information about operating conditions ?!

Peter

[ozric]

Hi Peter...


I am using a varian 3900 with a combi Pal
10 microlitre sample heated at 80 degrees for 3 mins...

Column CP-Wax 52 CB 30 x .25

It happens with all samples run containing mixtures of up to 15 different solents...Aslo happens with injections of individual solvents..

Retention time difference between first and subsequent injection varies.
The retention times from the 2nd injection onwards are consistent...

I didn't include the operating conditions as I assumed people would think that operating conditions were ok given the consisteny of 2nd injection onwards...

[pi3832]

Retention time difference between first and subsequent injection varies.
The retention times from the 2nd injection onwards are consistent...

Are the retention times for the first injection consistently earlier than the subsequent ones, or later? Or are they random?

Also, how long is it between the last consistent run and the next "first" run?

[Peter Apps]

Hi Ozric

Thanks for all the details.

This might be a problem of sequence synchronisation between the combipal and the GC. Depending which part of the system is triggering the run, if the GC is in standby before a batch is started it does not wake up until the inejction is actually being made. There are then some changes in gas pressure, and maybe oven temperature etc just as the sample starts to run. As a quick and dirty solution, do one dummy run at the beginning of the batch.

It is also possible that you have water vapour in your carrier gas that is building up on the column during standby and making the phase more polar. You will then see more change in retention for the more polar solvents. I assume that your sample is just solvents, no water. If the sample has water in it you might be seeing the opposite process - the phase dries out during standby and then the first and subsequent samples make it more polar.

With 10 microliters of solvent at 80 degC it sounds as if you are completley evaporating your sample. It would be more straightforward to do a liquid injection, especially since you have a Combipal that can do both headspace and liquids.

Check that the start temperature of your programme is not below the minimum for your wax phase. At too low a temp the phase solidifies ( more so as time goes on). You will then see distorted peaks as well as retention shifts.

When you say "greatly different", what is this in terms of actual retention times in minutes and seconds ?

Regards Peter

[ozric]

Pi and Peter

Thanks for your interest....

The difference in retention time varies but is between 0.1 and 0.3 mins but yesterday (and this melted my brain and prompted this post )

I ran a mixture of Isopropyl acetate and methanol..first injection gave nice separation. Second injection gave 1 peak with a shoulder. The retention time had increased by approx 1.8 mins...

The temp. programme is from 35 to 140.
Does the colunmn need more time to "stabilise" between injections??

BTW Peter sync between Cpal and GC is Ok.

Have a good weekend!!

O

[Peter Apps]

Hi Ozric

Depending which make of PEG column you are using 35 degC may be too low (check on the column box or the manufacturer's web site).

What happens if you increase the start temp to 50 deg C ?

What volume of headspace are you injecting ? What is the split ratio (if any) ?. What is the temperature of the syringe, how long are you purging the syringe between injections ? and how many times do you pump the syringe to fill it.

There might some other things going on, it would be a big help if you could post actual chromatograms.

Peter

[ozric]

Hi again Peter

I have been away for a few days..

The minimum temp of the column is 20 degrees however the manufacturer says it begins to solidify below 50 degrees.
Im gonna up the temps and see what happens...

regarding the syringe etc i dont think its a problem as I purge for three mins, temp is 100 degrees.

Thanks again....