Number of injections per standard / test solution

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Peter K.
Peter K.'s picture
Number of injections per standard / test solution

Hello Forum,

i established the USP 467 analytics in 2009 for both water-soluble and water insoluble substances procedure A and C in my lab; but i could not find any information about the number of consecutive injections for each of the standard solutions (class 1, class 2a, standard solution, test solution, spiked test solution).

Is there any kind of recommendation for the number of injections per standard or test solutions?

Dr. Analytical
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The USP method does not

The USP method does not require multiple injections of the standard.  Only single injections are required. 

Most analysts would agree that this is not a good approach, especially since this is a headspace method.  But USP does things the way they want to, even if it is not the best approach (and it usually isn't, in my opinion).

The method is essentially a "limit test," in that the peak area is either above or below the standard, which is set at the regulatory limit.  So, it either passes or fails, and the actual amounts are not important.

There are some procedural problems with the method as written, as I suspect you have already discovered.  Standard preparation and technique are important, and make sure the headspace vials are properly sealed.

Peter K.
Peter K.'s picture
Thank you for your response;

Thank you for your response; I agree that injecting headspace samples only once is not a good approach.
Actually I inject every solution prepared four times as a compromise between result quality and time consumption of the method, as a total run time of about 75 minutes is very long.

I think I would contact USP directly to get an official response which I can use for my documentation of analysis conduction. I will reply when I get information of USP.

What problems do you mean with the standard preparation and technique? I have implemented the analysis 100% as written in USP, the only thing is that the peak response due to CCl4 is pretty low. But using N2 as makeup gas for the FID gives better response for CCl4.

Dr. Analytical
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Peter:

Peter:
The main concern most people will have is the stability of samples in headspace vials when stored for long times.  During development I did notice that the response would decrease by the end of the day, and the final method had a time limit for waiting before analysis.

Certainly, you could prepare them one at a time and analyze immediately, but this is a very inefficient use of resources.

I have not checked lately, but the original method (and USP standards) had cyclohexane and methylcyclohexane at concentrations that were above the water solubility limit of these compounds.  So, they would sometimes form a separate layer, or adhere to the container.  Peak areas were not reproducible, unless you were very careful about mixing and handling.  To minimize this problem, we used a diluent of 10% DMSO/water for all solutions, which worked much better.