Chromatography Forum

I have searched the Forum posts for information on analytes that remain adhered to the GC capillary column long after the run is over and the next sample inject has begun. We often see small peaks that match the Tr of an analyte in our blank methanol injects which are interspersed between sample/calibrator injections. The peaks are small but they are "there" when we don't expect them to be there. This begs the Question: are their peak areas contributing, in a systematic way to the peak areas of the analytes? Obviously the answer is "yes", but we have tried cutting 10 inches off the column and baking it but to no avail they persist. Our syringe rinse is set for 3, but this does not seem to help remove the small peaks, our reagents are clean.

Have you tried inlet maintenance, cleaning out the inlet and replacing the split vent filter? We had a similar problem once and it turned out to be a dirty inlet.

If you are seeing a sharp peak at the analyte retention time then it is most likely not coming from the column, probably it is in the inlet, but it also needs more than three rinses to clean a syringe - try dissolving an intensely colours substance e.g. paprika oleoresin or iodoine in solvent, charge a syringe with it, and see how many rinses it takes to get it clean. You will be unpleasantly surprised.

Peter

A couple of suggestions:

1) Rather than making an injection, push the start button on the GC and record the run. If the peak is coming from the inlet - it will show up. If it is coming from the syringe, it will show up only after an injection from the syringe. I have an analysis where I make a blank run of the instrument - no injection to be sure that the instrument is clean before starting other runs. (My syringe is indirectly checked on the next injection when I shoot a sensitivity standard.)

2) You may need to use two solvents in cleaning the syringe. Some compounds wash out too slowly with your injection solvent. I have seen a case where I suspect that some marginally soluble materials were being left behind in a syringe and were carrying analytes over from one injection to another. (Standards in a clean syringe were fine, but after samples with matrix issues, the analytes would carry over, even after solvent blanks.) In this case washing with the injection solvent (non-polar) and a moderately polar second solvent eliminated the carryover problems.

A bit curiosity here: Are there autosamplers out there which have sample contact the syringe?
Concerning manual injection, I found that the most certain way to stop syringe carryover is to take the piston out, wash it, and draw solvent through the syringe body via vacuum.

The autosamplers for GC that I know (Agilent, Thermo, CTS based systems) use a syringe that can be used for manual injections - and for microliter injections the sample is drawn into the syringe - just like for manual injections. It seems to me that I've seen an autompler that takes a special syringe with a port high up on the barrel. The plunger is withdrawn past the port, but not out of the barrel and then solvent is drawn through the syringe.

Varian autosamplers have a wash port in the side of the barrel, and you can select one or both of two solvents to flush from top to bottom.

Peter

jumpshooter,

If it is on-column carry over, then the peak width of the contamination should be much broader than the neighboring peaks. If it is carryover from sampling or inlet, then the peaks should be about the same peak width as their neighbors since you are introducing them "fresh."

Best regards.

Just trying to think about mechanisms by which analyte left on a column can produce a peak in a subsequent run.
If one uses two columns in series (Deans system which I used on FA analyses) one may overload the first column, not clean it properly during analysis on the main column, and get an analyte peak on the next cycle, even when no sample is injected.
The other thing that comes to mind is a very broad peak, somewhere, when a temperature program is used, and some "stuck" analyte comes off at the higher temp. end. This would seem to indicte that a serious mistake or a serious failure is at play, cusing some analyte to "stick".

I am grateful for all of the well-considered suggestions for removing the analyte that is stuck on the column. To wit, I will run a "Temperature Blank" of the instrument, then observe the real-time plot. If peak is not there, then I will make a methanol injection and inspect the r-t plot. If peak is not there then I will assume the temp bake off has solved the problem. I will post my results anon.

Hello Hans

I believe that sharp ghost peaks (as opposed to broadones) can be generated when there is a spot of extra retention near the inlet end of the column - say some heavy contaminants, a tiny fragment of septum, or a pool pf dispalced stationary phase. This soaks up analyte, and then releases a pulse of it as the solvent vapour from the injection sweepes past, or the temperature programme reaches a high enough temperature. If this is themechanisn then the ghosat peak disappears as soon as the dirty spot is removed by trimming the column.

I would expect to see some peak tailing with this kind of column contamination, unless the programme ramp is very steep.

Peter

Well, yes I must concede that if something is getting stuck on the column it is indicative of a larger problem---as it could happen on each and every run we do.

I don't mean to be "talking to myself" here on the message boards--but I am at the mercy of a method that was "imposed" on me from above. So, I cannot change it---if I could I would add an extension of 10 minutes at a higher temp to bake off the offending peaks. What have others done to cope with this issue?

Hi Jumpshooter

How can we help you if you are not allowed to change anything ?

Is there anything that you may and can change ?

Peter

Another bit of curiosity: Are there now automatic systems installed in laboratories which strike an operator with lightning as soon as he attempts to change a parameter on an instrument?