Problems with recovery studies

Dear all

Im trying to validate an analitical method for traces detection in rinsing samples by NPOC in a TOC-VCSH Shimadzu. Overall, the method seems to be good, since it had already shown adequability and linearity.

But, when performing the residues recovery studies there is a problem, the “blank” samples always show an higher NPOC result than the spiked samples themselves. I can even imagine how can this be posible, since the blank isnt even prepared in a diferent way than the samples, and the rinse water is practically the same, and Im using inyectable grade water. I ve tried to do a lot of experiments to discart bad sampling methods on my part, or dirty glassware, but nothing like that seems to be the reason.

I think that maybe the problem is the product. Im using very dilute soltions of an ophtalmic that contains polimixine, neomicin and gramidicine. Its any of that component hard to read in TOC?.

I also think that it can be the water I use for the rinsing, since the NPOC present in those samples is always higher than the NPOC in the samples taken by the quality assurance department, being the only diference is that they purge the point by a lot more time than me, but even the NPOC results are around 0.2 ppm TOC for me, against 0.01 ppm for the quality assurance samples. Also, I know that theyre using the NPOC method becuase they had “unsolved problems” with a TC-IC method.

So, what do you think, the results are this way becuase the product, the water or me.

Thanks in advance

[quote=AiC]Dear all

Im trying to validate an analitical method for traces detection in rinsing samples by NPOC in a TOC-VCSH Shimadzu. Overall, the method seems to be good, since it had already shown adequability and linearity.

But, when performing the residues recovery studies there is a problem, the “blank” samples always show an higher NPOC result than the spiked samples themselves. I can even imagine how can this be posible, since the blank isnt even prepared in a diferent way than the samples, and the rinse water is practically the same, and Im using inyectable grade water. I ve tried to do a lot of experiments to discart bad sampling methods on my part, or dirty glassware, but nothing like that seems to be the reason.

I think that maybe the problem is the product. Im using very dilute soltions of an ophtalmic that contains polimixine, neomicin and gramidicine. Its any of that component hard to read in TOC?.

I also think that it can be the water I use for the rinsing, since the NPOC present in those samples is always higher than the NPOC in the samples taken by the quality assurance department, being the only diference is that they purge the point by a lot more time than me, but even the NPOC results are around 0.2 ppm TOC for me, against 0.01 ppm for the quality assurance samples. Also, I know that theyre using the NPOC method becuase they had “unsolved problems” with a TC-IC method.

So, what do you think, the results are this way becuase the product, the water or me.

Thanks in advance[/quote]

Dear AiC,

as said in a response to your previous post, try ruling out water issues using certified water for TOC analysis. It is expensive, but hopefully it is once in a lifetime… Too, clean all the materials with a strong oxidising agent (percarbonate or persulfate) to eliminate contamination.

Although all three components of the product should be readily oxidised, next step would be ruling out product issues by spiking a product sample directly into the water used for sampling and determining TOC levels on that solution, compared to the bias TOC level of the same water sample.

Hope this helps a little.

Best regards

Alfred

Dear Alfred:

Thanks for your advice.

I tried using certified water for TOC in some analysis, and the issue didnt appear. I concluded that the water system has too high TOC concentration in some points of the system due to “dead legs” problems. Hopefully This will be resolved with a new water system that is going to be implemented.

Anyway I increased the spiked concentrations (still around the levels of the contamination levels), and that resolved the problem, but increased the LOQ and LOD levels.

I’m going to continue with a validation of a swap recovery method, and I fear this issue will continue to appear. So, to eliminate as possible any kind of contamination during the method validation, Im thinking in using Ammonium persulfate to clean the coupons that I’m going to use(stainless steel 304), but, ammonium persulfate is the correct reagent to use?, if yes, at what concentration?

Best regards, and thanks again for your advice.