Paraffins in petroleum wax according ASTM D5442

[svetlinahristova]

Dear ALL,

I've involved in GC-FID (manual injection) analysis of petroleum wax samples for determination of paraffins. So far the laboratory has followed ASTM standard D 5442 and they have great problmes with data interpretations. We are looking for the appropriate way to make all calculations, but I think that they are really very complicated in this standard as there is an requirement for the Internal standard (IS) use. I'm not aware what is the role of IS here, because in our case the response factor is equal to 1 (+-0.05), and all calculations have to be done between responses into chromatogram (so if we have smaller or bigger injected quantity it will reflect to all components) and I believe that the simpliest way is to use response (area) % as report data.
The softwer we use is Clarity.
Please advise me how to proceed with this issue.

Thank you in advance

[Consumer Products Guy]

Typically when using manual injection (people still do this for other than early method development?), using internal standard is a good thing.

[rb6banjo]

I am not familiar with the details of this ASTM standard but generally, the internal standard is used to compensate for errors in injection or recoveries in extraction procedures. So, it's usually a good thing to have it. The way it is used is for the analyte:

Ai = k1*Ci

where Ai is the peak area, Ci is the concentration, k1 is proportionality constant the translates the response to concentration. Same holds for the internal standard:

Ais = k2*Cis

with the same designations for the internal standard. The ratio of these equations gives:

Ai/Ais = (k1/k2)*(Ci/Cis)

The slope of the line of a plot of Ai/Ais vs. Ci/Cis for a given set of concentrations (to include the range you expect for your samples), the slope of the line is k1/k2 which is the response factor (RF).

Rearranging gives:

Ai*Cis/(Ais*RF) = Ci

Now you can add a known amount of internal standard to your sample, and if you measure Ai and Ais, using the known response factor you can calculate the Ci (concentration of the unknown).

In the ASTM standard you're using, they might have cast the description differently but the fundamental principle under it all is (or should be) what I have described.

Good luck.

[lynoguchi]

According to ASTM, to prepare the calibration standard of n-paraffins, is not necessary to include in standard all the n-paraffin from C16 to C44. Is only necessary that you have the C16, C44 and every fourth compound (C16, C20, C24, C28, C32, C36, C40 and C44).
You can prepare the n-paraffins as an external standard from C16 to C44, but you need to have a good precision to weigh each compound and consider the dilution factor. If you use the ISTD you need only the exact mass of C16, because the n-paraffins calibration standard will be used only like a qualitative standard and you don't need to weigh the compounds with a good precision, you need only to have approximately the same mass.
You can identify the RT for all the n-paraffins in your sample based in your standard with C16, C20, C24, C28, C32, C36, C40 and C44. And the concentration for the other compounds that are not in calibration standard can be determined through the ISTD response factor.

[Peter Apps]

I am not familiar with the details of this ASTM standard but generally, the internal standard is used to compensate for errors in injection or recoveries in extraction procedures. So, it's usually a good thing to have it. The way it is used is for the analyte:

Ai = k1*Ci

where Ai is the peak area, Ci is the concentration, k1 is proportionality constant the translates the response to concentration. Same holds for the internal standard:

Ais = k2*Cis

with the same designations for the internal standard. The ratio of these equations gives:

Ai/Ais = (k1/k2)*(Ci/Cis)

The slope of the line of a plot of Ai/Ais vs. Ci/Cis for a given set of concentrations (to include the range you expect for your samples), the slope of the line is k1/k2 which is the response factor (RF). Relative response factor ?? k1 and k2 are the response factors ??

Rearranging gives:

Ai*Cis/(Ais*RF) = Ci

Now you can add a known amount of internal standard to your sample, and if you measure Ai and Ais, using the known response factor you can calculate the Ci (concentration of the unknown).

In the ASTM standard you're using, they might have cast the description differently but the fundamental principle under it all is (or should be) what I have described.

Good luck.

[rb6banjo]

I would argue that it IS the response factor as the problem was cast as a "relative response factor" situation. Semantics........................

[svetlinahristova]

Many thanks for your discussion,
But I'm a little bit confused and can't put in order all the steps for data processing, described into ASTM D 5442.

So, in the case we use only this standard C16,C20,C24, C28, C32, C36, C40 and C44 how can calculate ISTD response factors for the remain hydrocarbons as we need their amounts and their areas, I mean the equation:

IRF sc=(AREA is x AMOUNT sc)/(AMOUNT is x AREA sc), where "sc" is a specific compound.

lynoguchi explanaitions are connected with 7.5. (D 5442) "Linearity Standard"

but there is also "Standards for Calibration and Identification (7.3. ASTM D 5442)" and I wonder what are they, because they aren't mentioned further in the article.

So I have to clarify what steps I need in data processing, using more capabilities of Clarity software and avoid using other external calculations, which are not so representative.

Awaiting your discussion.

[lynoguchi]

svetlinahristova

The "Standards for Calibration and Identification (7.3. ASTM D 5442)" are the same from "Linearity Standard".

I'm work with Agilent hardware and a specific software for SimDis calculation, from AC Analytical Controls. All this calculation are made automatically by this third part software.
I never worked with the Clarity software, only with the ChemStation. But I think the steps are the same.

The first thing, you need to identify all of your n-paraffins, and the rest integrate like non-normal-paraffin for each carbon number . As described in 11.4 ASTM section.
Prepare the ISTD with C16 as described on 7.6.1 section and put in your calibration table the area and concentration of ISTD to found the RF. For all the other compounds you can use area and concentration as 1 in your calibration table.
After that, in your calibration table you will set the C16 as your ISTD for all the other ones.

When you run a sample, weigh the ISTD solution added to sample, multiply the ISTD weight by his concentration and punt the found mass in ISTD amount. The sample mass, you will put in the amount box.

Adjust the report to give results by ISTD.
If all the compounds are identified and linked with your ISTD in calibration table, the results should be given automatically in the report.

Regards.

[svetlinahristova]

Appreciate your kind support, lynoguchi!

Now, Hope I'll put in order all my considerations

Regards

[Peter Apps]

Won't there be a problem if the sample already contains n-C16 ? Or are waxes all heavier than this ?

Peter

[lynoguchi]

This method cover the range from C17 to C44 of n-paraffins.

Regards.

[Peter Apps]

This method cover the range from C17 to C44 of n-paraffins.

Regards.

But the sample still has to be free of C16 or the IS peak will be a sum of the IS and what is already in the sample - giving a bigger peak and a bias to the low side on the results.

Peter

[lynoguchi]

Yes it can be a problem if your samples have the C16. But you can run your samples with and without ISTD and compares the results.
If you following strictly the ASTM method, you could only integrate the compounds from C17 and normalize the results.

[Peter Apps]

Yes it can be a problem if your samples have the C16. But you can run your samples with and without ISTD and compares the results.
If you following strictly the ASTM method, you could only integrate the compounds from C17 and normalize the results.

Since the large saturated hydrocarbons that are the target analytes have FID signal:mass responses that are equal within a margin that is smaller than injection repeatability and inlet bias due to different vapour pressure, simply working from peak areas makes more sense to me than using an internal standard (which, because it us lighter than any of the target analytes, cannot correct for inlet bias).

Peter

[lynoguchi]

Yes, this make sense.
But in this case if you don't use the internal standard, you cannot report your result as: According to ASTM D5442. To report a result like this, you must use the ISTD in your sample analysis.
In petroleum industry, if you don't follow the standard methods, your analysis is not valid to specify a product.