Making sense of Limits of Detection (with respect to Tritium analysis) James Thomson Meridian Biotechnologies Ltd.

Transcription

1 Making sense of Limits of Detection (with respect to Tritium analysis) James Thomson Meridian Biotechnologies Ltd.

2 The dilemma Customer commented that the background of Gold Star had increased and was now outside their contracted level. Customer needed to measure 1 Bq/Lt Tritium in various media. Analytical conditions:- Instrument: Quantulus Windows: channels Count temperature: 15 C Count time: 12 hours (one continuous count) Vials: 20 ml plastic Sample: 10.0ml Gold Star Data: Background increased from 0.5 to 0.7 cpm

3 The investigation Gold Star was never designed to be used in low level counting and is not routinely tested for such applications. There is every chance that some lots will be suitable but we do not guarantee suitability through quality control. Irrespective of the above statement I commenced an investigation to determine the background levels in Gold Star and to try and reduce these levels as much as possible.

4 The investigation Gold Star Samples tested (all in duplicate):- # # # # # once treated # twice treated Analytical conditions Instrument: Tri-Carb 2550 TR in LLCM Windows: 0-4.5; & keV Count Temperature: 12 C Count time: 6 x 60 minutes Temp & light adaptation: 6 12 C Vials: 20 ml plastic

5 The results All above are averages of duplicate counts. A = once treated AA = twice treated

6 The results To facilitate the 6 hour count delay I counted an empty 20 ml plastic vial and the results are shown below. (Average of two empty vials) 0-4.5keV keV keV Empty vial 1.65 cpm 1.64 cpm 1.63 cpm This was something of a surprise as I did not expect such a contribution from an empty plastic vial.

7 The results The background reduction shown by the once and twice treated Gold Star (#100302) is most probably due to reduction of peroxide content. Peroxides are know to induce chemiluminescence. To confirm this I tested samples of each for peroxide content and the results are:- Peroxide % (ppm) # % (122 ppm) #100302A % (24 ppm) #100302AA % (16 ppm)

8 The enigma Gold Star has between 2.4 to 2.5 cpm background. Empty plastic vial shows about 1.6 cpm background. So where is this background coming from and how is it affected? Cosmic contribution Potassium (cocktail & PMT) Plastic vial Instrument noise Temperature

9 The enigma Cosmic contribution Definitely some contribution from neutrinos etc, but probably quite small due to lead shielding. Potassium (cocktail & PMT) Ethoxylates in the cocktail contain a small amount of potassium that will certainly contribute. Glass faces of the PMT s also contain potassium. Instrument noise Difficult to quantitate but estimated at about 30% of the contribution. Plastic vial Manufactured from virgin PE (oil derived) and should be very low in C-14 and 3 H. Temperature Lowering temperature lowers background.

10 The enigma Instrument noise To measure I tricked the instrument into counting an empty chamber (Average of 6 x 60 minutes). Result = 1.64 cpm ( keV) This suggests that instrument contribution is about 66% Plastic vial To measure I cut up a complete plastic vial and placed the pieces in another plastic vial which had already been counted (Average of 6 x 60 minutes). Result = 1.64 cpm ( keV)

11 The enigma Temperature I monitored the temperature of the instrument (TriCarb 2550 TR fitted with a chill pack) over a period of 7 days and arranged for some low level counting during this period.

12 The enigma Does this indicate that there is only ~ 0.8 cpm contribution from the LSC cocktail? It would appear to be but apparently only with respect to my particular LSC counter. Other factors which can and may affect counting: Count time single count vs multiple counts. Type of vial used - plastic = lower background vs. Glass. Static - can be present especially on plastic. Weather High pressure system gives higher Background.

13 What is Limit of Detection LOD (limit of detection), is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) within a stated confidence limit (generally 95.5%).

14 What is Limit of Detection Suppose you are at an airport with lots of noise from jets taking off. If the person next to you speaks softly, you will probably not hear them. Their voice is less than the LOD. If they speak a bit louder, you may hear them but it is not possible to be certain of what they are saying and there is still a good chance you may not hear them. Their voice is >LOD but <LOQ. If they speak even louder, then you can understand them and take action on what they are saying and there is little chance you will not hear them. Their voice is then >LOD and >LOQ. Likewise, their voice may stay at the same loudness, but the noise from jets may be reduced allowing their voice to become >LOD. Detection limits are dependent on both the signal intensity (voice) and the noise (jet noise).

15 How do we measure LOD in LSC? LOD = Limit of Detection = Lower Limit of Detection MDA = Minimum Detectable Activity There are many methods, but using the standard Currie:- Calculation of LLD & MDA LLD = 3 x (Counts) ½ Time cpm MDA* = LLD x 100 Bq/Lt Efficiency x 60 * Based on a 10.0ml sample size

16 LLD and MDA results (Based on obtained results) keV LLD MDA # cpm 0.23 cpm 0.77 Bq/Lt # cpm 0.24 cpm 0.78 Bq/Lt # cpm 0.28 cpm 0.91 Bq/Lt # cpm 0.29 cpm 0.98 Bq/Lt #100302A 2.81 cpm 0.26 cpm 0.88 Bq/Lt #100302AA 2.53 cpm 0.25 cpm 0.84 Bq/Lt Empty vial 1.64 cpm

17 1. Be careful when quoting LLD or MDA. 2. Check your instrument blank. 3. Statistics are great but only in the right context. 4. Think before you report!