In my previous messages, my pb was that in a mixture of free fatty acids with heptanoic as IS(direct injection, no derivatization of free fatty acids), the lactic acid peak did not appear anymore while before it did.
We've sylilated the liner and syringe and now lactic acid is there.But at a concentration of 0.5% it sometimes appears and sometimes not while at 1% is always present in all chromatograms.
Is someone has a clue of what is happening. Do we have to focus on the liner, in other words is the quality of the liner can influence a separation????I am thinking of put some glass wool in the liner.We change the septum regularly.
By David McCalley on Tuesday, June 29, 2004 - 03:50 am:
In my opinion, lactic acid is on the borderline of being determinable underivatized by gas chromatography. It will stick to any active surface such as the liner in a capillary GC. It is possible that a constant amount of your sample is irreversibly adsorbed on acitive sites, and when you inject a larger sample a proportion of the sample is lost but some gets through to the detector. I think the best way to get round some of the problems is to inject the sample directly on to the column. We had success doing this with a packed column based on a carbon support (available from Supelco). On column injection in capillary GC is also a possibility, but there may be problems if you inject aqueous solutions. I think if you put glass wool in your liner you will create even more potential adsorption sites. If you must use a liner, I believe there are some liners around which are deactivated by other ways rather than simple silylation, which may not be stable indefinitely to injections of water/acids. Maybe these might work better.
By Consumer Products Guy on Tuesday, June 29, 2004 - 08:26 am:
I've used this for lactic acid as trimethylsilyl derivative: "Simultaneous Determination of Alpha and Beta Hydroxy Acids in Personal Care Products by Capillary Gas Chromatography" J. cosmet. Sci. 53 (121-126) March/April 2002
By Ron on Wednesday, June 30, 2004 - 09:17 am:
I have also seen some problems with active compounds when the column phase is not thick enough. I have seen better results on 1 micron phases than on 0.25 micron phases. There are also special acid deactivated columns that may help. The liner may indeed need a different deactivation also to get best results.
As both of the above posts have suggested it would probably be a good idea to use a derivitzing reagent on your samples.