Journal of Radioanalytical and Nuclear Chemistry, Articles, VoL 110, No. 1 (1987) 47-50

DETECTION LIMITS AND PRECISIONS IN VARIOUS IRRADIATION AND COUNTING REGIMES

A. EGAN

Process Analysis Services, Radiochemical Company, 413 March Road, P.O. Box 13500, Kanata, Ontario, K2K 1• (Canada)

(Received January 3, 1987)

The widespread introduction of rapid pneumatic sample transfer systems has enabled instru- mental neutron activation analysis to be based on an increasing number of very short-lived acti- vities. Furthermore, these transfer systems have been interfaced to computer-based MCA's so that the experimenter has complete control over irradiation, decay and counting times, as well as being able to arrange the automatic transfer of numbers of Samples between the various stations. Thus the analyst now has a series of options available to him to make the best use of time and facilities. Based on the requirements of detection limits and preeisions, he will choose between various irradiation and counting r6gimes (a) single i.e. conventional (b) cyclic and (e) repeated; or he may choose to replicate the sample a number of times. This paper examines how detection limits and precisions are affected by the above options. By considering a specific isotope, being detected in backgrounds of different half-lives, it is possible to calculate "signal-to-noise ratios" in each of these cases, and hence compare these r6gimes from this aspect. Based on calculations for the isotope 77rnse (17.5 s), which is now being widely accepted as the basis for selenium analysis, it is shown that, if a low detection limit is the prime consideration, then replicating samples is the procedure of choice; however, if commercial considerations of sample throughput are important then a "pseudocyclic" r6glme would provide the best compromise.

In t roduct ion

The technique of cyclic activation analysis (CAA), particularly. using reactor neutrons, is now widely used, and was the subject of a review by Spyrou (1), having developed considerably since its introduction by Caldwell et al (2). Its chief application has been to improve quantitative aspects of analyses based on activities having half-lives 1 minute, and enabling the experimenter to optimise his use of the total experiment time (TT). To a large extent, in a research environment, TT is an arbitrary choice, and the number of samples subjected to CAA is likely to be small; furthermore there is some pressure on the experimenter to improve on ex i s t i ng detect ion l i m i t s , w i th a view to subsequent pub l i ca t i on . However, Neutron Ac t i va t i on Analysis is now commercially ava i lab le , as a service to geolog is ts , engineers, b i o l o g i s t s , env i ronmenta l is ts and the l i k e , and the vendor of such services is motivated d i f f e r e n t l y from the researcher; he must optimise his time so as to maximize p r o f i t , and to do so he must consider the number of samples he can analyse in a given time, at pr ices, and to detect ion l i m i t s , which are

Elsevier Sequoia S. A., Lausanne Akad6miai Kiado, Budapest

A. EGAN: DETECTION LIMITS AND PRECISIONS

acceptable to his customers. The commercial world of elemental analysis is a competitive one, and providers of NAA services must compete, not only with each other, but also with laboratories using more classical techniques.

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Discussion

Guinn(3) has pointed out that cumulative, or repl icate, analysis of several al iquots of the same sample should improve detection l im i ts considerably, over single sample analysis, even though this l a t t e r may use C.A.A. Parry(4) used this cumulative method to obtain lower detection l imi ts for precious metals, with improvement factQrs essent ia l ly in agreement with those calculated from Guinn's(3) work.

Considering the calculat ion of detection l imi ts and precisions, the standard equation used involves the signal (photopeak) counts, S, and the background continuum, B, where

S = (S + B) - B

and 2 =(~2 + 2 O-S S+B CrB

Now, in the case of short- l ived ac t i v i t i es producing the signal S, the counting time (tc) in any scheme wi l l be comparable with, (and probably greater than,) the relevant h a l f - l i f e sTl/2, and hence S wi l l not be subject to Poisson s ta t i s t i cs . Exactly what s ta t i s t i cs do apply in these circumstances has been the subject of some discussion(5) , but in the l im i t when t c > 7 sTl/2, there is no uncertainty associated with S i t s e l f , and we have then:

S ~ r

(assuming that BTl /2>>tc) .

For the cumulative signal obtained by adding the spectra of r repl icates

r(Z~ s --~z/2 r B

and hence the precision is improved by the factor r l /2 as deduced by Guinn(3). Similarly the detection l im i t (DL) w i l l improve by the same factor, since

DL ~ 0"- B

1),, the'case of CAA for n ident ical cycles, improvements are also obtained in both precision and detection l im i t compared with the single i r rad ia t ion procedure. ~he improvements obtainable depend on n, but, more c r i t i c a l l y , on the re la t ive ha l f - l i ves sT1/2 and BTI/2. In the worst case, when

A. EGAN: DETECTION LIMITS AND PRECISIONS

Table 1

Signal-to-noise ratios (S/x/~), arbitrary units, for various background half-lives (BT1/2). Signal halfolife (ST1/2) = 17.5 s. Total experiment time 2 hours, throughput 25 samples, transfer time 5 s

IRRADIATION AND COUNTING REGIME

CYCLIC BTI/2{S) SINGLE IST CYCLE CUMULATIVE (n) PSEUDO-CYCLIC(n) REPLICATE

i0 I 20 i 50 I

i00 i 500 1

1,000 1 I0,000 i

0 72 0 76 1 1 15 2 1 21 2 2

1.3 (3 1.7 (4 2.7 (5 3.2 (5 3.7 (5 3.8 (5 3.8 (5

1.8 {5) 5.O 2.4 {7) 5.0 4.2 (10) 5.0 {5.5) 5.8 ( i i ) 5.0 (7.5) 6.4 (9) 5.0 (10.5) 5.6 (9) 5.0 (10.5) 4.0 (8) 5.0 (11.0)

B71/2,6~> ~ sTI/2 and TT>>STI/2, i t can be shownt I , that the improvement obtained when n > c ~ o i s only a factor of ~ although, in fac t , th is is approached very rapid ly . When BT I / 2~sT1 /2 the improvement is Qrea~er. Table I below gives calculated "signal to noise ra t ios " iS/W~), in a rb i t r a l 7 values, for various i r r a d i a t i o n and counting r~gimes, for some f ixed experimental condit ions. The signal isotope is 77MSe, with T1/2 = 17.5 seconds, and 25 samples or rep l i ca tes are to be processed in 2 hours (,~5 minutes each); a system t rans fer time of 5 seconds was used in the calculat ions. "Pseudo-cyclic" is the term used for i r r a d i a t i n g and counting each sample n times in sequence, ( i . e . the sample returns to the back of the queue at the end of the counting, not, as in CAA, to the i r r a d i a t i o n pos i t ion) , with opt imisat ion of the tota l time for each sample.

Clearly the Pseudo-cyclic procedure w i l l give bet ter detection l im i t s than a sinQle or CAA r~gime, but not as good as the repl icate procedure. Moreover the rep l i ca te procedure could improve yet fu r ther , in some cases, from a reduction in the time per sample, to that of a single cycle of a CAA scheme; these fur ther improvements are shown in brackets. A l t e r n a t i v e l y , the improvement fac tor of 5.0 could be achieved with a much shorter sample time. 7he improvement shown in (cumulative) CAA is greater than that estimated by Guinn, because l a t e r cycles provide more signal counts than e a r l i e r ones, since the former bu i ld upon the residual a c t i v i t y of the l a t t e r . Thus i f the signal counts from cycle n are Sn then

Sn > S n - I > . . . . S 1

and i f the cumulat ive signal is CS

where cS = n ~ s i i=1

then cS > n S 1

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A. EGAN: DETECTION LIMITS AND PRECISIONS

In real examples

and then

Sn~Sn_ I when n~6

cS._.~+1.3 nS

which relationship leads to the better precisions. Chatt(7) has demonstrated these improvements for 46MSc (T1/2 = 18.7 S). In the world of commercial activation analysis the improved statistics obtainable by replicate analysis must be set against the increased cost, not only of machine time, but also of sample preparation time (which, in the example given, would, even in the least complicated case, be ~TT). I t may well be that use of a pseudo-cyclic procedure, on suitably-sized batches of samples, would yield data of an acceptable quality at an acceptable price. The procedure will be more complicated than CAA, but with a computer-based M.C.A., the collection and summation of the various spectra can be made routine; the inclusion of real-time correction of counting losses(8) can overcome the problems associated with high and varying dead-times. There will not be the confidence in homogeneity that replicate analysis can offer, but i t is generally the case that only well-homogenised samples are submitted for NAA.

Now that Neutron Activation Analysis has become both available and acceptable as a method of routine commercial elemental analysis, i t is important that providers of such services remain competitive in both the business and scientific senses. To do this i t is necessary to examine developments in the technique cr i t ica l ly , and adapt them as necessary to a production environment.

References

1. N. M. SPYROU, J. Radioanal. Chem., 61 (1981) 211. 2. R. L CALDWELL, et al., Science, 152 (1966) 457. 3. V. P. GUINN, Radiochem. Radioanal. Imtt., 44 (1980) 133. 4. S. PARRY, J. Radioanal. Chem., 75 (1982) 253. 5. N. M. SPYROU, et al., J. Radioanal. Chem., 61 (1981) 121. 6. A. EGAN, Pit. D. Thesis, University of Surrey, Guildford, England, 1977. 7. A. CHATT, et al., Can. J. Chem., 59 (1981) 1660. 8. G. P~ WESTPHAL, J. Radioanal. Chem., 70 (1982) 387.

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