Reproducibility with manual injections

Chromatography Forum: GC Archives: Reproducibility with manual injections
Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Wednesday, May 16, 2001 - 01:52 pm:

I am analysing five and six ring PAHs produced in hexane soot with the VarianSaturn 2000 GC/MS. For the five ring PAHs,I am getting large amounts of variability within samples as well as within the same sample..that is the first problem. We are unsure if it is GC related or sample related. Any suggestions on how to test this?

For my manual injections, I am drawing up 1 uL of sample and then drawing up 1 uL of air prior to injecting the sample. The current analysis involves hitting the start button before injecting the sample. We went to this because of the poor reproducibility in peak areas from injections. Is there a better way to inject sample? Any suggestions would be most appreciated.

Thanks.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By HW Mueller on Thursday, May 17, 2001 - 01:02 am:

Sounds like a splitting problem. Do you have access to splitless, cold... injection where you could test your samples? Did you change splitting ratios, injector geometry, or injector deposits (clean the lining...)?


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Ron on Thursday, May 17, 2001 - 07:00 am:

Are you injecting in splitless mode? If so, what is the timing of the switching of the split vent? I have seen methods set up so the GC is supposed to switch to splitless mode at the same time as the injection is made, then switch to split 15 or 20 seconds later. This doesn't work well at all. The GC should be in splitless mode before the injection, with the split vent opened 45 to 60 seconds later. Too large a volume or too small a liner can also cause problems in splitless injection.

I am not very familiar with the design of the Varian injection port. One thing I have seen in the past on older HP systems without a true splitless injection design is operators turning the total flow way down, not realizing that due to the port design at least 30 ml/min was required out the split vent to prevent loss of sample during injection. Too high a septum purge flow on systems with adjustable flow can also cause problem.

In splitless injections I usually leave the syringe in the inlet for 15 to 20 seconds after injection. In many cases this can help ensure a more consistent injection volume.

Finally, I have assumed that you are doing splitless injections because with the experiment you are doing I would expect the concentration to be small. If you are doing split injections be sure that you are in an appropriate total flow range for the inlet system. Again lacking familiarity with the Varian inlet, I would guess that total flow during a split injection of less than 15 to 20 ml could cause reproducibility problems.

Good luck.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, May 17, 2001 - 10:26 am:

Thank you for your comments.

Yes, I am injecting in splitless mode. The switching of the vent was at 1s and then reopened at around 1min. How could too small a liner affect precision?

I'll try leaving in the syringe longer and see if that helps.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Ron on Thursday, May 17, 2001 - 11:43 am:

If the volume of the liner is too small, then backflash can occur when the solvent vaporizes to a larger volume than the internal volume of the liner. Some of the solvent vapor and analytes flow backwards into the carrier in line, where part of the analytes may be lost. In addition, some of the analytes may be lost out the septum purge. Since this is splitting of the sample by accident, the splitting may not be reproducible from run to run.


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