I use HP GC 5890 II and FID detector for the analysis of valatile Fatty Acids in water samples. I use Restek Stabiwax 30 m, 0.53 mm, and 0.5 film thickness column. Because I need to get detection limit of 0.1 ppm, I use split ratio of 2. While the sensitivity and separation are very good, carry over from the previous sample to the next sample make it necessary to put a water blank in between two samples. I checked all GC troubleshotting guide and couldn't find any answer. Does anybody ever encounter this problem before? Thank you for your response.

First, when using water as the solvent, inlet maintenance becomes much more critical. Replacing the septa,liner,o-ring and gold seal might even have to be done daily. Cleaning the split line and changing the chemical trap also ways to minimize carryover since non-volatile material collects and builds in these areas, especially when introduced in water. Second, a split ratio of 2:1 indicates low split vent flow. Total flows (column + split+ purge) below 20 ml/min can cause problems because most systems have a flow restrictor in the pneumatics that operate best between 40 -400 ml/min. If you switch to splitless with a purge time around .75 min and purge flow (split vent flow) of 50 ml/min, carryover molecules will focus more on the inlet item easy to replace (liner,oring,gold seal etc).

Regarding my original message -- I forgot to say the carry over peaks were not the late peaks which can be eluted with longer running time or higher temp. I have also used more washing after injection and more rinsing before injection, but these washing and rinsing did not help.

Hi Mike:

Sounds to me like you need better syringe cleaning between runs. Am I correct about this? Or are you getting carryover even when you do a blank run without any injection?

Steve has addressed a lot of things that could help with instrument carryover. A total injector flow below 40 mL/min would usually make it impossible for any solvent's vapor to be contained in the liner without overflowing beyond the septum, potentially going much further upstream toward the GC's carrier inlet than you would imagine. The run could bake out and appear normal, but the vapors could come back from upstream and appear as peaks on the next temperature-programmed run.

Also are you going for 0.1 ppm for each isomer, or total VFA, that could make it much harder. What is your injection size, and injection temperature? Are you using an autoinjector? For my difficult samples I don't know of an autoinjector that could clean the syringe as good as I could manually. Also, what kind of syringes? For real trace work I like the ordinary 701N, since any teflon tips or seals can have more places for prevoius sample to escape flushing or be absorbed/desorbed by polymers. I use the Hamilton heated cleaner, but I connect to 60 psi of nitrogen instead of vacuum. Before blowing out the syringe with hot nitrogen though, I thoroughly rinse by removing the plunger and insert it into the pinhole of the methanol squeeze bottle & rinse, then follow with the DI water. Then the syringe body is inserted into the squeeze bottles also, needle first and a few drops of solvent slowly purged out the plunger end. There is just no way to expect a syringe from a positive sample to be cleaned enough to get a negative result from a blank sample if the only rinsing is by inserting into a wash vial and plunging up & down, except with easy samples, where a few dozen to a hundred plungings may suffice. Also depends on your injection volume.

Another possible instrument carryover mechanism is when a relatively high positive sample is injected, especially if the analytes are somewhat nonvolatile. Even if the needle is well wiped, a detectable amount of analyte can be retained in the septum hole, only to be introduced with the next sample injected through the same hole. This is when septa should be changed after a high level sample, and that may mean more than once a day.

Hope this helps

Best Regards,
Mike Setzer
Quality Analytical Services, Inc.
QualityAna@aol.com

Mike -

If you're still having problems - is your inlet liner packed with glass wool? Even silanized glass wool will have "hot spots" where you may have interaction with your sample. You might try making an injection without glass wool in the liner, to see if the problem goes away. I've had similar problems with analyses, and this sometimes helps.

Also, it may be time for a service call on your GC. We were having "carryover" problems that turned out to be caused by a clogged split vent line. We also use relatively low split ratios for some of our analyses, and this was causing servere problems for us. Over the course of several years, with little or no maintenance, we managed to clog the split vent line with analytes and glass wool. Changing the split vent line may help.

Steve, Mike and Kdunn:
Thank you for your answers to my carry over problem. I'll try those suggestions you gave to me one by one and will let you know the progress later.

How much sample are you injecting? What is the temperature, head pressure, and the volume of your liner? Water with it's huge vapor expansion characteristics expands and fills the injector, then overflows to places it should not go, this can cause carry over. HP has a pressure/flow calculator you can download from it's website. We recently looked at a water injection we were doing and found that we we filling the liner well over what we should have been.

Its not something like sterols being left behind is it ??