Dear all, I hope that you can help me with this problem:
Im trying to validate a TOC method for traces of productos for cleaning validation. To do this I spike some surface with a known quantity of the product, let it dry and then rinse it to determinate recuperation level.
However after the first test, while the samples got an expected value for TOC, the “Blank” sample (prepared with the same water used to rinse the surface) had about 2 or 3 times higher TOC. Thinking it was a procedural error on my part I tryed adding more “Blanks” from the same water, to discard contamination on the vial, and one more “Blank” made form low TOC water (from Shimadzu), to discard some cotamination on my part. The test got the same results: the “Blanks” made from the rinsing water had higher TOC than the “Spiked” samples, while both the samples and the Shimadzu Blank had expected values.
This seems so illogical to me since there is no reason for the blanks to be more “contaminated” than spiked samples, and even in the case that the water used for rinsing were dirty, the samples would be with higher or around the range TOC levels. Also the periodic analysis for water quality for the water I am using for rinsing in the tests always give good values (0.200-0.300 or so ppm).
So what I am doing wrong? This are the conditions of the experiment:
Water used: Inyectable grade (0.350 ppm TOC mean)
Method used: NPOC by Combustion and NDIR
150 mcl inyected
1.5 % acid
1.5 min sparging
Product concentration (after rinse recovery): around 0.150 ppm (50% of it being carbon).
So while the samples get me TOCs of around 0.350 to 0.450, the blanks are around 1.000 to 1.500 ppm
Thanks for your help