Problem with phosphine by headspace GC/NPD

[sara.morsiani]

Hello,
I’m dealing with the analysis of phosphine residue in foodstuff by HSS/GC/NPD and I encountered the following problem: a repeatable interference peak very close (hardly resolvable) to phosphine peak and exceeding 30% of my Limit of Quantification.

I use as reagent solvent sulfuric acid 5 % in water (heated at 80 °C in the HSS oven) and I observed the interference even in the pure water only (and even in different types of water i.e. normal pure water, HPLC pure water and IC pure water as well) but it wasn’t present in injections of air (empty vial).

I’ve already done the same analysis two years ago and this problem did not appeared but since then:
a) the fused silica transfer line of the HSS was replaced with a more handling proof steel one;
b) the ceramic bead of the NPD was replaced because exhausted;
c) the GC column for gas analysis (Super-Q Plot) was not used anymore up to this moment.

Does anyone have suggestions to let me understand (and overcome) the cause of this interference?
Thank you,

Sara

[AICMM]

Ms. Morsiani,

Since no one else is commenting, my two cents worth.

1) Your compound nominally has to be halogen, nitrogen or phosphorous containing.
2) Q's (as far as I know) don't have the above mentioned materials (unless trace as initiators.)
3) The water appears to be stripping the compound off.
4) Even water without sulfuric, right?
5) Does your GC still have an inlet on it or is it direct Xfer line to column? If with inlet, have you tried injection of water HS through inlet? Would eliminate the HSS and xfer line.
6) I can offer an alternative detector for the phosphine (argon ionization detector) but it would not necessarily solve this particular problem. It would solve the problem of spend bead though.

Best regards,

AICMM

[Don_Hilton]

A couple of ideas:

Stainles steel catches my eye. Have you tried running empty headspace vials (no water or acid in them)? And have you tried runnign a vial with water with acid multiple times to see if the interference peak drops between runs? Metal surfaces have can interestign adsorbtion characteristics and intersting surface activity.

If ammonia were to be present in the headspace, would it elute with the phospene or at a different spot in the chromatogam? If the same, is the sulfuric acid solution freshly made from a freshly opened bottle of concentrated sulfuric acid?