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Evaluation of Methods for Analysis of Sr-89 & Sr-90 in Environ Samples.
	ML18086B013
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Site:	Salem
Issue date:	10/09/1981
From:	; RADIATION MANAGEMENT CORP. (RMC)
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I EVALUATION OF METHODS FOR THE ANALYSIS OF Sr-89 AND Sr-90 IN ENVIRONMENTAL SAMPLES Prepared by Radiation Management Corporation 3508 Market Street Philadelphia, PA 19104 r1s3:1i:1il1li11/oirol:;7;c;5~

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EVALUATION OF METHODS FOR THE ANALYSIS OF Sr-89 AND Sr-90 IN ENVIRONMENTAL SAMPLES I. INTRODUCTION As a continuing effort to improve its services, Radiation Management Corporation has evaluated its procedure for Sr-89 and -90 on environmental samples. Three areas have been looked into specifically. These areas are: *

1. Methods to increase the sensitivity of the analysis for certain sample types *

. 1. Methods to better-differentiate, di~crete Sr-89 and Sr-90 contributions to the total radio-strontium activity, and

3. Methods to improve the chemical yields for preserved milk samples.

A new analytical method is proposed to achieve the above objectives.

II. EVALUATION OF EXISTING PROCEDURE Radi~tion Management Corporation's methodology for Sr-89/90 determination in environmental samples utilizes classic wet chemistry means to purify strontium from competing elements. A final strontium carbonate precipitate is prepared for beta counting. A second count of the prepared samples is done 7-14 days later. The Sr.89 and -90 activities are calculated from equations using Sr-89 decay and Y-90 ingrowth terms in a solution of 2 simultaneous equations with two unknowns. Ari evaluation was initiated to review the various aspects of the analytical method for Sr-89/90. The chemistry procedures used for Sr-89/90 analysis for various ~edia were internally reviewed .. In addition, an independent evaluation was conducted by our radiochemistry consultant, Dr. K.K.S. Pillay. As a result of these reviews, it was concluded that the radiochemistry methods w~re scientifically sound in chemical theory. Dr; Pillay was *then.requested to observe and evaluate the RMC technician in performance of the analyses, and also present a training seminar on the radiochemistry of strontium.

From his technological review, Dr. Pillay found no outstandin9 technique problems~ but he was able to provide some sup~estions to handle certain specific conditions.

II I. RESULTS OF QUALITATIVE AND QUANTITATIVE TE~llNG In order to evaluate some of the proposed improvements to the analytical method, testing was performed to accomplish this goal.

TEST 1 The first testing to be performed was to make a more accurate determination I of the chemical yield by means of a radioactive tracer Sr-85. A standard solution of Sr-85 was obtained to perform this test. Radioactive Sr-85, which is not a B- emitter would be added alona with stable strontium carrier so that the Sr.BS would act as a tracer for the chemical yield of I

a the prepared samples. By knowing the a~ount of Sr-85 added, the chemical yield could be determined from a gamma count of each prenared sample to ouantitate the Sr-85 remaining in the sample. The ratio of measured and

~ctual Sr-85 amounts would be the chemical yield of the analysis. The chemical yield could thus be determined almost imriediately for a prepared

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sample. Befor~ per.forming any actual tests for use of Sr~85 as a radioactive tracer, an aliq0ot of the Sr-85 standard was counted on the gas-flow proportional counter to detennine the counting efficiency of the Sr-85 photons in the proportional counter. Since Sr-85 was going to be added to both samples and blanks, the Sr-85 should not be efficiently counted in the gas-flow proportional counter since the blank is used in the LLD calculation. One additional important concern for this initial test was an evaluation of the potential for the Sr-85 to become a contamination problem and thus have an adverse affect on other samples being prepared and counted. An activity of approximately 50 nCi was used and resulted in a beta counting efficiency of approximately 0.84~{ (890 cpm/

105700 dpm). Since a quantity of about 10 nCi was to be added to each sample and blank to provide a suitable quick determination on the finished planchet, the activity of the sample would be masked and a hi9her LLD result. Approximately 100 cpm additional would result from the use of this .amount of activity if a yield of approximately 60~~ was achieved. The use of Sr-85 was thus not feasible as a determination of yield for low-level detennination of Sr-89 and -90. The more convential gravimetric or atomic absorption spectrophotometric methods will be utilized for chemical yield determinations for the strontium procedures.

TEST 2 The next test performed was an evaluation of the Sr-90 results for some previously analyzed samples by means of a separate detennination for the Y-90 daughter activities of these samples. Since a condition of secular equilibrium exists in these samples, the Sr-90 activity can be inferred from the Y-90 detennination. This test was- not intended to make auantitative comparisons to the initial Sr-90 measurement, but was done to confirm the Sr-90 results. The following data in Table 1 show results from previous Sr-90 analyses (with some duplicates) and the Y-90 results.

TABLE 1 Sr-90 CONFIRMATION BY Y-90 COUNTING SAMPLE # Sr-90 RESULT(s) Y-90 RESULT 1 <2.36 x 10_4 -4 <2.17 x 10- 44 2 <2.75 x lQ4 <2.20 x 10-3 . <3.8 x 10_4 3.5 ~

4 <4. l x 10 <3.54 x 10-*

5 14.9 10. 7 6 <.572/<.53 <.449 7 <.62 <. 51 8 <.51/<.43 <.46 9 <. 50/<.43 <.52 10 <.67 <.58 11 2.06 2. 34 12 4.2 <l .32 The testing showed that the initial Sr-90 results were in general agreement with the Y-90 result. The Sr-90 results was not verified in a few of the tests, but the method of analysis of Sr-90 by Y-90 {which R~~c has not previously been doing} shows promise. To further test this the Y-90 analysis, a series of Y-90 known spikes were analyzed. Results of this testing appear in Table 2. Results show that the spike recoveries were in decent agreement with the known activities. We expect improved agree~ent

with future refinements in the technique.

TABLE 2 Y-90 SPIKE RECOVERIES SP I KE ACTIVITY RECOVERED ACTIVITY SAMPLE # pCi/1 pCi/l % DIFFERENCE 1 lb.2 l 0. 9 + 6.9 2 l 0. 2 12.0 + 17. 7 3 10.2 12. 7 + 24.5 4 10.2 l l. 1 + 8.8 5 20.4 j 23.2 + 13. 7 6 20.4 22.6 + 10.8 7 20.4 22.8 + 11.8 8 20.4 22.8 + 11. 8 AVERAGE + 13.3%

TEST 3 One additional series of tests that was conducted was detennination of anv upward or downward trends of a repeated second count on prepared samples.'.

A recount of the second count was done on approximately 57 samples. The count was done after Sr-90/Y-90 e(!uil ibrium was: reached, but before any significant Sr-89 decay. A review of this data showed that the ori~inal second count and the repeat second count were not significantly different.

This test was thus inconclusive to further improve the analysis or identify any particular problems in the present analysis.

TEST A Tests of a method to improve the chemical yield for preserved milk samples is currently being conducted. This test involves denaturation of a fluid milk sample with Trichloroacetic Acid {TCA) prior to per'forrning the remainder of the analysis rather than performing the analysis directly on an ashed milk sample.* Results of this test will be presented upon its completion.

IV. PROPOSED NEW METHOD FOR ANALYSIS OF Sr-89 AND -Sr-90 Radiation Management Corporation's present procedure for analyzin~ Sr-89 and Sr-90 has been previously discussed in section II of this report. One major advantage of this method was to eliminate the need for a.second che~ical separation for Yttrium. One apparent disadvantage is that the method yields higher calculated LLD values than a two separation method since the LLDs are calculated from the two sequential counts rather than individually from single counts performed f~r each isotope.

Radiation Management Corporation thus investigated alternative calculational methods for the detennination of Sr-89 and -90 usinq a more classic two separation chemical method rather than the simultaneous equation method currently in use. Two principle methods for detennining the quantitative Sr-89 and -90 contributions to the total radiostrontium activity in a sa~ple are in popular use. The Sr-89 activity can be determined by counting a freshly prepared strontium sample through an absorber to shield the Sr-90 soft beta and then determing the Sr-90 by Y-90 separation. A second Method is to determine the Sr-89 activity by subtraction from the total radiostrontium

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activity by first measuring the Sr-90 (by Y-90). The prepared method for the Sr-89/90 analyses will utilize a technique based upon the absorbance of the soft Sr-90 beta to detennine Sr-89 activity and a separate Y-90 detennination to infer Sr-90 activity.

The Beckmen Wide Beta fI gas-flow proportional counters can be equioped with absorbers of different thicknesses for beta counting. An absorber of 90.6 mg/cm2 thickness was chosen since it reduced the Sr-90 counting efficiency to <1% (44 cpm/4575 dpm) and allowed a Sr-89 counting efficiency of approximately 12.1% (201 cpm/1665 dpm). The use of an absorber should thus be suitable for the measurements to be done.

V. ACHIEVABLE SENSITIVITIES FOR SELECTED SAMPLE TYPES Typical sensitivities achievable by thi~ method have been calculated for selected sample types. The equation for LLD as described in Regulatory hUide 4.8 has been used with suitable additional terms to correct for Y-90 ingrowth and the second chemical* separation required from the Sr-90 analysis.

The sample media addressed include water, ~ilk and air particulates. Real data has been used for the variables such as chemical vield and backaround count rates. Prepared Y-90 samples will be counted within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to minimize its decay tenn. Prepared strontium samples will also be counted within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to minimize Y-90 ingrowth.

Where: 8 = Background counts = 1 cpm x 200 min = 200 t = Counting time = 200 min V =*sample volume= 1.5 liters Ys = Strontium chemical yield = .33 Yy = Yttrium chemical yield= .85 Ey = Counter efficiency for Y-90 = .45 2.22* = dpm/pCi (constant)

IF = Y-90 in~ro~th fact~rt{time from iRitial Sr separation to Y separation) = 1 -e}.. *= .974 for 14 days DF = Y-90tdecay factor (time from Y separation to countin~) =

e -}. = .878 for 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months ;_

LLD90 = 4. 66 v' 200 (200) (l.5) (.33) (.85) (.45) (2.22) (.974) (.878)

= 0.92 pCi/liter Sr-89 Achievable Sensitivity (LLD): 5 pCi/l

4. 66 v' B t . Es

Ys

V

2. 22 ~ DF

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Where: B = Background counts = 1 cpm x 200 min = 200 t = Counting time = 200 minutes Ys = Strontium chemical yield = .33 2.22 = Constant V =Volume= 1.5 liters Es= Counter efficiency for Sr-89 = .121 DF = Sr-89 decay factor (assumes aR~roximately 30 days between collection and counting) = e- = .674 4.66/ 200 (200) ( .121) ( .33) (1.5} (2.22) ( .674)

- 3.7 pCi/liter Water Sr-90 Achievable Sensitivity -'(LLD): l pCi/l LLD 90 = 4. 66/ B t

V

Vs

Vy

Ey

2.22

IF

OF Where: B = Background counts = 1 cpm x 100 min = 100 counts t = Counting time = 100 minutes V =Sample volume= 2 liters Vs = Strontium chemical yield = .80 .

and all other terms are same as in the Sr-90 milk determination.

LLD 90 = 4.66/ 100 (100) (2.0) (.80) (.85) {.45) (2.22) (.974) (.878)

- 0.40 pCi/liter Sr-89 Achievable Sensitivity (LLD): l pCi/liter

4. 66 v7 B t

Es

Ys

V

2.22

OF Where: Vs = Strontium chemical yield= .80 V =Sample volume= 2.0 liters _

DF = Sr-89 decay factor for 14 days = .832 and all other terms are the same as in the Sr-89 in milk determination

4. 661200 (200) (.121) (.80) (2.0) (2.22) (.832)

- 0.92 pCi/l "Air Particulates Sr-90 Achievable Sensitivity (LLD): l x 10- 4 pCi/m3 LLD 90 = 4. 66 I B t

V

Vs

Vy

Ey

2.22

IF

DF

~

6 *.

.., ~* ,..

r-Where: B*

t

= (.7

= 750 cpm) (750 min) = 525 ~cunts minutes V = 2500 m3 and all other terms are the same as Sr-90 analysis in water

Low Background Counter Used

4. 66 y' 525 (750) (2500) (.8) (.85) (.45) (2.22) (.974) (.878)

0.98 x 10- 4 pCi/m 3 .

Activity calculations for Sr-89 and -90 would be as follows:

Sr-90 (pCi/unit volume) = G- B Activity V

Ys

Yy,

Ey

2. 22

IF

OF Where.: !he terms are identi_cal ,to those in the Sr-90 LLD equations and G = Gross count rate B = Background count rate Sr-89 (pCi/unit volume) =

G - B - Bs - Bv V

Ys

Ey

2.22

DF Where: the terms are identical to those in the Sr-89 LLD equations and G = Gross count rate B = Background count rate (instrument)

Bs = Additional backaround count rate from fractional Sr-90 countin9 By= Additional background count rate.from Y-90 ingrowth VI.. CONCLUSIONS

1. RMC has eva 1 uated its current chemistry procedures and have determined them to be scientifically sound.

2. RMC proposes to chan9e the analytical method in the following manner:

a. A two separation chemical procedure will be performed in lieu of the current single separation method.

b. An absorber will be utilized in the determination of Sr-89

c. A Y-90 separation will be performed for the determination of Sr-90

3. Achievable sensitivities will meet client requirements.

Sr-90 Sr-89 Milk 0.92 pCi/l 3.7 pCi/l Water N 0.57 pCi/1 0.92 pCi/l Air Particulates 0.98 x lo-4 pCi/m3 I

4. Although the proposed methods will achieve the above sensitivities, more testing (e.g. TCA usage in the milk procedure to improve the chemical yield) is currently being conducted and will be reported upon completion.

I t

p Radiation Management Corporation will conduct some additional testing as necessary to make further refinements .in its analytical procedures.

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ML18086B013

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