Variations in detector respons

[karlssa2]

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.

[skunked_once]

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

[karlssa2]

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.

[Peter Apps]

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

[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

[Peter Apps]

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

[karlssa2]

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%.