[ RadSafe ] questions regarding LSC Direct DPM Assay

[Wen, Xiaoqian (wenxa)]

Hi, All,

I have some questions regarding the LSC Direct DPM Assay.

After I finished counting some samples with the Direct DPM assay, I realized I did not have the BKG subtracted for the final DPM results. So I changed the counting condition to subtract the BKG from the first vial. I recounted my samples with the new counting condition. However one vial was reported as “INDETRM” and had no DPM result reported.

I found the following information from the Reference Manual “A sample is flagged as indeterminate (INDETRM), if its SIS is less than 40 and the nuclide is determined NOT to be H-3”.

First, the nuclide in that sample is H-3.

Second, the SIS values changed significantly from 74.769 (without BKG subtracted) to 5.940 (with the BKG subtracted from the first vial).

So does anyone know why the SIS values changed so significantly after I changed the counting condition? Should I keep the original counting condition (without BKG subtraction) for Direct DPM assay and use the DPM results or any other suggestions?

Thanks for your time and attention!

Carol