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- Posts: 56
- Joined: Fri Feb 04, 2005 10:05 pm
Hello,
We are having problems with severe run-to-run variation in our fatty acid methyl ester (FAME) analysis technique (GC-FID).
First, to give our run conditions: The FAMEs are dissolved in hexane, transferred to tapered vials, crimped with teflon-lined caps and placed on the autosampler prior to injection. A 2uL aliquot is injected into the injection port, which contains a 5mm diameter straight deactivated glass liner held at 250 degrees C. Carrier gas is hydrogen, running at a head pressure of 145psi and a 50:1 split ratio. Separation is through a 60m polar capillary column, with detection by FID.
We have successfully used this technique for several years with thousands of samples. However, we have recently noticed that one out of every five to ten samples is failing to give a full-scale chromatogram - i.e. all the peaks are there but severely reduced in height. The solvent peak is also much reduced in height and width, which indicates that this is not a sample prep problem, but rather an injection/loading problem. We routinely use 10uL removeable needle syringes in the autosampler, with 70mm cone-tipped needles. Observations during injections show that sample pick-up is OK. Leak checks indicate that the system is leak-free, and split flow is stable. We assume the detector is OK as when there is a problem run, all peaks are down in that run and then the next run is fine (I would expect random decreases in response during a run id the detector was playing up).
We are at a loss to explain this problem, especially given that the same method and equipment has served us faithfully for years without a problem. Our assumption is that sample is not getting onto the column in the faulty runs, and that this problem has to lie in either sample transfer from the syringe into the injection port, or a faulty split path. Can anyone shed any light on what the problem might be or how to diagnose it?
Thanks
Tony
We are having problems with severe run-to-run variation in our fatty acid methyl ester (FAME) analysis technique (GC-FID).
First, to give our run conditions: The FAMEs are dissolved in hexane, transferred to tapered vials, crimped with teflon-lined caps and placed on the autosampler prior to injection. A 2uL aliquot is injected into the injection port, which contains a 5mm diameter straight deactivated glass liner held at 250 degrees C. Carrier gas is hydrogen, running at a head pressure of 145psi and a 50:1 split ratio. Separation is through a 60m polar capillary column, with detection by FID.
We have successfully used this technique for several years with thousands of samples. However, we have recently noticed that one out of every five to ten samples is failing to give a full-scale chromatogram - i.e. all the peaks are there but severely reduced in height. The solvent peak is also much reduced in height and width, which indicates that this is not a sample prep problem, but rather an injection/loading problem. We routinely use 10uL removeable needle syringes in the autosampler, with 70mm cone-tipped needles. Observations during injections show that sample pick-up is OK. Leak checks indicate that the system is leak-free, and split flow is stable. We assume the detector is OK as when there is a problem run, all peaks are down in that run and then the next run is fine (I would expect random decreases in response during a run id the detector was playing up).
We are at a loss to explain this problem, especially given that the same method and equipment has served us faithfully for years without a problem. Our assumption is that sample is not getting onto the column in the faulty runs, and that this problem has to lie in either sample transfer from the syringe into the injection port, or a faulty split path. Can anyone shed any light on what the problem might be or how to diagnose it?
Thanks
Tony
