Chromatography Forum

Hi. When I inject same standard solution (fatty alcohols, c10- c16 i acetone), from the same vial, 12 times after each other I get big variations in respons area, RSD of about 20%. I have done a number of attempts to reduce the variation without any success, like

- new injection syringe
- new liner
- checked at various inlet temps
- changed FID jet

The variations can be reduced to an acceptable level (<8%) in practies by the use of internal standard, but still it is not good.

I would be happy to get some ideas about this problem.

Please supply us with complete information on your analysis in order to help us figure out your problem.

Yes, of cause.

Firstly I run on an Agilent 7890 GC-FID system with auto-injector. I inject 1 uL sample into a split/ splitless inlet in splitless mode (1 min) kept at 260 degrees. An ordinary spliltess liner, tapered in column end. I run a Innowax column, 30 dm, id. 0.25mm, 0,50um. No retention gap. Oven program from 110degreeC/ 1min; 20degrees/minute until 240 degreeC and kept for 3 min. FID at 240degreeC.

The componets I analys is tert-butyl-cyclohexanol (cis and trans), tetradecanol (IS) and hexadecanol dissolved in aceton, approx 100 ug/g.

The chromatograms and peak shapes looks very nice so the basic chromatography is okey.

There is nothing obviously wrong with the method.

Have you tried other solvents ?

Is there a trend in areas during the series of 12, or is the variation random ? Are there results that are very far from the others ?

Repeated sampling from one vial can cause a partial vacuum in the vial, and then you get poor uptake into the syringe. Try loosening the vial cap between injections.

What it your syringe wash programme between injections ?

Peter

Although I am not a common user of the splitless technique, I was brought up that the initial oven temperature should be below the boiling point of the solvent. If acetone is the solvent, then a starting temp of 110°C is way above the boiling point of acetone.

Gasman

Although I am not a common user of the splitless technique, I was brought up that the initial oven temperature should be below the boiling point of the solvent. If acetone is the solvent, then a starting temp of 110°C is way above the boiling point of acetone.

Gasman

Hi Gasman

That would certainly be the usual way of doing it, but in this case I think that the peak focussing that you get by loading the stationary phase with solvent is not required because there is a very steep programme ramp - I am presuming that the analytes elute somewhere near the end.

Peter

At last I seems to have found a solution to the problem with the respons variations. I tried another liner, a split liner with glass wool, instead of the splitless liner and get probably a more efficient evaporation. I reduced the RSD from 20 to 5%, and increased the sensitivity by almost 100%.