Can someone give me a explanation why i can't get a consistent ratio between cetyl alcohol, and stearyl alcohol. 2 microlitre injection,

i need an average ratio of 5 injections, without any single injection deviating more then +/- 1.5% of average ratio

without seeing the chromatograms, it might be hard to tell. What kinds of peak shapes do you have, Is there a lot of tailing? this could make the integration problematical. In addition, there may be other, physical issues, such as sample carry-over of one of the analyted between injections, etc.

Maybe someone with experience with these particular alcohols can be of assistance.

Good luck

Our lab has never had any issues with these. We assayed these by nonpolar capillary after dissolution in N,N-dimethylformamide and amking mixing with BSTFA to make trimethylsilyl derivatives. Inlet temperature was 300 C (split) but we have also done 0.53mm capillaries without splitting.

So many answers are possible to this question.
The obvious is 'something is wrong'.
Short list of possible problems:
wrong injection liner
worn syringe
septum cored
poor injection technique [nah, can't be that :-) ]
wrong injection technique
failing or cracked column
leaking connections
too low injection temperature
too high injection temperature
integrator problem (wrong paramters)
decomposition products hidden under peaks of interest

Murphy is alive and kicking. Good luck. Hey, if chromatograhy was easy, anybody could do it.

Rodney George

it is probable that 1.5% of variation is the limit of instrument.

I forgot to add that a metal syringe under the right conditions can react with the higher alcohols and dehydrate the alcohol, especially if residency time of sample contacting the metal is too long and large amounts of solvent are injected (which you are doing ).

Are you backflushing sample upstream from your injection port into cooler areas of the carrier inlet line (wrong injection liner, too fast of an injection, and/or inlet temperature are all factors here) ?

Rodney George

There are additional factors yet, others from the forum may add to the list.

I tend to believe that it is an injection problem, although with polar compounds like alcohols it is quite easy to have integration issues with tailing peaks if a high polarity column is not used. I would expect tailing and integration to be more of a problem with lighter alcohols, and I am also making an assumption that something more polar than a 5% phenyl column is being used.

As Rodney George has pointed out, 2 uL is a relative large injection, especially in splitless mode. I would guess that one or more of the inlet parameters are set incorrectly, temperature, sampling time, split flow, wrong liner, solvent with too large an expansion volume, injection technique, inconsistent injection times if manually injecting. It is also possible that there is a detector problem, but this is less likely than an injector problem

If you will provide more information about the method it would be easier to identify possible solutions to the problem. As you can see from the postings above there are many possible causes, so more information will help pinpoint the most likely problems.

Hey it's tim again,

Parameters--N2 1.50kg/cm2, Air 0.70,hydrogen 0.70
injector temp 230, column 195 degrees, column-- SE30 packed column( 2-m X 3mm, packed with 10% phase G2 on support S1 A) splitless injector. FID detector.

i reduced my injection volume to 1 microlitre--which helped with overloading of useable range,

these parameters helped me achieve required results--but it was all just trial and error--any way of really knowing parameters to use

I am a HPLC man, relatively new at using GC kinda got thrown onto it.

thanks for the help.