Retention Index Alkane ladder dilution?

[Arne]

Hi All

I am introducing retention index to our lab to reduce the fraction of false ids. I got 1000 ug/mL C7-C30 alkanes in hexane from Sigma.

My plan is to dilute 1 in 100 in n-hexane to have 10 ppm of each analyte. Do you agree with this approach. I am planning to stockpile GC vials with this solution and use them for the construction of RI calibrations on our GC-FID, GC-TCD, and GC-MS systems.

A few concerns:

1. hexane will elute when the MS is already on. I do not really want to have lab techs make changes to the solvent delay time. How bad is it really for the source to analyze the solvent peak?

2. Would you use DCM or pentane instead for dilution?

3. Should we spike with dodecane to 100ppm or something to get another "anchor" landmark in the chromatograms in order to avoid "one-off" errors?

4. Is 10 ppm an appropriate concentration for GC-FID and GC-TCD systems? I know it is plenty for GC/MS?

Thanks!

[Consumer Products Guy]

1. hexane will elute when the MS is already on. I do not really want to have lab techs make changes to the solvent delay time. How bad is it really for the source to analyze the solvent peak?

Your hexane solvent will act as "fuel" to your MS detector, don't want that to elute after the MS switches on.

Should we spike with dodecane to 100ppm or something to get another "anchor" landmark in the chromatograms in order to avoid "one-off" errors?

I think something like this is a good idea.

[Gizmo]

We prefer pentane instead of hexane for this purpose.

[Arne]

Thanks guys!

Do you think 10ppm is enough for GC-TCD for the ladder, though?

[AICMM]

First, no 10 ppm probably not enough for TCD.

Second, why not 1000 ppm at 20:1 split for the FID and 1:1 split for the TCD?
You do put all your eggs in the 1000ppm basket so people need to replace the septum as soon as it is shot but....

Third, adding dodecane is a good idea in my opinion.

Best regards,

aicmm

[Arne]

So I went with 40ppm (1mL in 25mL DCM). At a 1:20 split, we get very good S/N on the GC-FID. On an old GCMS it is also acceptable at a 1:50 split.

We spiked hexadecimal 120ppm, which as you had predicted, is a great idea in case heptane is in the solvent delay or C30 is not eluted.