Chromatography Forum

Hi everyone
I've just started my work with GC.
Does anyone have experience in phosphate analysis by GC-FID?
As I noticed phosphate/phosphite sample can cause unrepeatability and probably reacts with the film? Does split help to reduce it?? Someone said that after each analysis she has to clean detector. Is that needed so? Does anyone know some special column for this application?
Thank you in advance
Monika

Hi Monika,
The compounds are very polar and will 'stick' to the GC liner and column, so people generally derivative them
to make them volatile.

The usual derivatising reagent is BSFTA, though MTBSTFA is better, it is more expensive
http://www.registech.com/Library/gcderrev.pdf.

The advantage of using these silylating reagents is that the HF produced cleans the detector
http://www.sigmaaldrich.com/etc/mediali ... /bstfa.pdf

The other advantage for using a TBDMS reagent over a TMS , is the increase in FID response, due to the increase in the number of carbon atoms, as such the quanitity needed to be injected can be reduced, thus increasing the number of analyses that can be performed between GC maintenance.

Also if, you SOP requires you to inject a solvent blank between each sample to demonstrate no carry over, you need to use the derivatising reagent as the solvent blank, in case there is any unreacted Phosphonates in the system, that eg DCM, will not flush out.

Alex

Thank you Alex for your quick respons.
I have main problem with diethyl phosphite as I need to determined the purity of it.
In our Quality Control Laboratory they inject the sample with no derivatization and they use no split mode. I told them to use split, that it me be effective to reduce problem with detector and that will make analysis more repeatable but they ansered split mode is not responsible for that. Why is that so, if you inject to much sample. Too much sample leads to greater intereaction on the column and make every analizysis diffrent. Am i right?
I heared there are some columns that reduces that impact. Has anyone know some?
At the moment in QC uses AT-1 (50 m).
M

Hi Monica,
You didn't mention that it as an 00-Alkyl Phosphite, as derivatising is aimed more for reactive functional groups like O-H, S-H etc...

IMHO, If they are not using split, then detector sensitivity/linearity is likely to be the issue that they are addressing, unless of course if you are looking at the trace and the top of the peak has plateaued because they have saturated the detector.

I am assuming that the reason why they have to clean the detector is because of the phosphate coating, I don't know was sate these coating is in, but I am thinking that if it is the phosphate, it may derivatise, if you inject eg BSTFA, thus cleaning the detector, and reducing the need for cleaning, but the phosphate has probably glued itself in place so it may not work.
Alex

Thanks again for your help
I've read your notes carefully and noticed in BSTFA product specification that there are SPB-1701 and SP-2250 columns from Supelco recommend to analysis of P,N,S atoms and groups. Are they better for phosphite/phosphate analysis? I'm looking for some suitable column.

Monika

Hi Monika,
IMHO, I do not think that the diethyl phosphite will silylate, as there is too much steric hindrance around the phosphorous atom (in the five oxidation state I am assuming?, as the three oxidation state will silylate).

AT-1 is 100% DimethylPolysilioxane, the BSTFA will not affect it, if anything it would silylate any active sites in the liner and column.

If peak tailing is not an issue (integration), then don't worry about the column. Are they using an internal standard to minimise reproducibility errors (eg varying injection volume), or, if they inject the same sample 20 times and plot the area, does the area integrated over subsequent injections drop. As you can't just calculated %RSD and state that there is a problem. As it could well be that there is an issue with the injection port, and injecting twice the volume with a 1:1 split will increase the linear flow and deposit your analyte onto the column faster, so that you can open the split vent sooner to flush the liner, and reduce peak tailing.

The only reason why I mentioned injecting BSTFA to clean the detector is purely speculative, and if I was
to have a bet, then I would think that any phosphate stuck to a detector @ 250'C will need mechanical removal as it has baked itself into that crystalline state, and the probability of enough of the BSTFA that would survive the combustion process in the FID AND find its self to the Phosphite is minimal IMHO, the greater effect would potentially come from the HF that gets generated because of the combustion, and as the HF is generated after the column, if doesn't destroy the column. (of course the instrument should already have any combustion products being ventilated away properly).

You also need to ask what part of the FID is being cleaned, as I'm guessing it would be the jet & not the collector.

Alex

Hi Alex
I'll tell them your suggestion, thank you very much for that.
As I know it is a problem with liner and they have peak tailing problem as well. They don't use internal standard, but it may be needed. They've been still developing this method. On Monday I will find out more about the method and what had been done.
Have a nice weekend
Monika