In article <[email protected]>, [email protected] says... > > Hi Priscila, > > I ran into the same problem a few years ago -- had a really old > scintillation counter but no information on efficiency and no > standards. If you use the old standards, I think there will always > be caveats to the measurement. For example, I don't think the > difference in scintillation liquid or vial type should be a problem, > but I don't know for sure. If it's really important, you might be > better off buying new standards. > > When I asked our radiation safety office about this, they basically > told me that they don't bother calculating dpm and just go by cpm. > So my conclusion at the time was that what's good enough for the > radiation safety people is good enough for me, and I did all my > calculations with cpm. The dpm calculation is just for comparing data > between instruments, right? > > Sorry I can't be of more help. > > Good luck! > Irit > > > On Jul 8, 2010, at 5:16 AM, Priscila Peña Diaz wrote: > > My question is regarding the calculation of dpms. As far as I know > > the > > efficiency of the apparatus to measure the isotope is what defines the > > relationship between cpms and dpms. I managed to find some 3H > > unquenched > > standards from 1966. The date should not be a problem as the decay > > can be > > easily calculated.
In principle yes, but that is about 4 half-lifes, so there is only 1/16 of the original activity left. There may have been 100,000 dpm in there originally, so now there are 6200 dpm left. At a counting efficiency of 30% that's 1900 cpm for the "unquenched" samples and less for the quenched ones. Forget it. > > The problem is the scintillation liquid they > > are in. I > > have no idea what the cocktail might be, although its must be a mix of > > toluene, which is what was used in the old days. We use a > > biodegradable-scintillation liquid. If I use these standards in this > > solution, can they be compared to the counts I obtain using the other > > scintillation liquid? Should I purchase a standard in the same > > liquid as I > > use? Also the vials are different. Why not make your own standards. Whatever radiochemical you are using is calibrated for so and so many MBq/ml. 1 Bq is 1 dps, so multiply by 60 to get dpm. If you are good at pipetting, you can get below 2% error, the calibration error of your chemical is probably in the same magnitude. Note also that the absolute error is usually not so important, as it cancels. Important is that you pipett the same amount of radioactivity into all the vials for the quench series. Count the vials with scintillator only, to make sure. Sort out any vials that are "off". Chloroform is a very good quencher, so add varying amount of it to the vials. Count again. The counter has a special program for calibration, use this. You will be asked what the total dpm in the samples is. Comparing the countrate before and after addition of CHCl3 gives you the quench. If the counter does not have a calibration program, plot quench against the channel ratio (SCR) to get a calibration curve. You can then do the conversion manually. If the Americium external standard of your counter is still good, you can also use the quench identification parameter (QIP) in a similar fashion. A similar protocoll is also used to calibrate the counter for isotopes that were not on the market when the counter was build (e.g., 33-P). However, at least with single isotope counting, as long as all your sample quench about the same (check with QIP and SCR), you can use the countrate directly to convert cpm into amount or effect, with a standard sample that you need anyway. Btw, even if your cocktail is labeled "biodegradable", it is still nasty, just not quite as nasty as the solvent-based cocktails of yesteryear. _______________________________________________ Methods mailing list [email protected] http://www.bio.net/biomail/listinfo/methods
