SV: [ RadSafe ] questions regarding LSC Direct DPM Assay

[Olsson Mattias :MSO]

Hi Carol,

First of all, I suspect you use a fairly recent Tri-Carb counter of some sort. An interesting thing about the conditions for the reporting of Direct DPM values can be seen if you compare a recent Tri-Carb manual with an old one. In the new manual, it says like you wrote below about SIS limits. In an older version of the manual, it's not the SIS number but the end point of the spectrum that is mentioned (with the same numerical limits, though!). That I find rather odd.

In any case. Could it be that this particular sample of yours has a very low activity content? If the sample is near the background level, I suppose it would not be so appropriate to use the Direct DPM method which requires the determination of a SIS value, which in turn requires fair statistics. I guess that a low activity content could screw up the analysis with this method.

I set up some methods for a Tri-Carb instrument a while ago, making quench curves and all that, and having no detailed previous experience with the different evaluation methods, I didn't really feel that the Direct DPM method was very appealing at all. As often as I could I used the single DPM method with tSIE as a quench indicator. That means that I minimize dual label calculations and that counting statistics are not important for the determination of the quench parameter.

Hope some of that helped. There are one or two LSC gods on this forum. Maybe you will get more detailed replies.

Regards,

Mattias Olsson
Forsmark NPP, Sweden

-----Ursprungligt meddelande-----
Från: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] För Wen, Xiaoqian (wenxa)
Skickat: den 15 oktober 2008 23:46
Till: Radsafe
Ämne: [ RadSafe ] questions regarding LSC Direct DPM Assay


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
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