Sulphur compunds in light petroleum-HELP!!!

[bublle]

Hi!
When quantifying S compounds in gasoline with an internal standard, how much of an internal standard should be added to the sample of unknown sulphur concetration in order to get good results?

how to estimate the ratio between sample volume (mass) and i.s.volume (mass)?
Concentration of i.s. standard solution is known, but S compound concentration in sample is unknown!
Ratio between sample and i.s. plays the main roll because it inflences the peak size of both.
GC results are much different everytime from the one from XRayWD.
(one time lower and the other two times higher!?)

Help,please!!!

Thank you

[yangz00g]

you need understand that the IS is not the major culprit for your problem. While XRF measures the total elemental S in the samples, GC determines indivaudal molecules containing S assuming that you uses a S-specific detector, you need add them up to get the total.

Homogenouseness (spell?) of the sample significantly affects XRF results.

[larkl]

Not only homogeneity, but also sample thickness (path) in XRF is critical. Hopefully, the XRF is calibrated with a known volume or weight of standard and the same volume/weight is used for all samples. Would be better to run at infinite thickness, but not always possible.

I too would be more worried about the 'totality' of the XRF measurement vs. the 'specificity' of the GC method. If you aren't measuring all the S components on your GC method, the results aren't going to match the XRF. I'm not sure even if you did have them all that they would match that well, frankly. I'll go out on a limb and state that I've heard that S measurements by GC can be pretty tough to get right. I'd consider using the XRF result for the total, the individual peak identifications could be used for troubleshooting, process improvements etc.

[bublle]

Some details: PFPD is detector used for that measurement.
Yes, I'm trying to quantify all sulphur compounds being present in the sample and use formula for quantifying sulphur compund as sulphur.
When summing all components determined on that way-results aren't close. Not even near o be accepable, in fact.
If we would neglect Xray results, what would you recomend when quantifying S compounds in sample? Something for sample/i.s. ratio?
How to get that measurement as precise as possible?

Thnx

[CE Instruments]

If you want your answer in a quantity/concentration of Sulphur in Gasoline then you need to identify each of the peaks and both calculate the concentration of each peak and from that the quantity of Sulphur, then sum all the peaks This also assumes that you are not losing any compounds in the hot injector etc.. This is why if you just need a total Sulphur most people run a Total Sulphur system. XRF which is very simple (and more prone to interferences ) or Combustion and either UV-F or Coulometric detection and for very low levels Lead Acetate tape.

[bublle]

My procedure is simillar to that.
I quantify a group peak by one peak of i.s. which concentration is known(by knowing the mass of i.s. added). I have few peak groups, determine concentration of every group and quantify it againts i.s. peak(of known concetration and peak area).
GC is more sensitive than Xray so I asume that fot the same concetration I would get a bit higher results than from Xray.
but that's not the case every time!
What to conclude?
That It's not possible to get the same result for concetration(ok.not the same but simillar)for the same sample?