Anal. Chem. 1981, 53, 2212-2216
2212
for a 1% error in CA(obsd,.Any k'greater than this has the potential of introducing an error larger than 1% and any k' less than this will indicate that the interferent has an insignificant effect on the sensor for analyte A. When k'is calculated as in eq 11 and compared with the GSAM k normalized, many of the GSAM k normalized are indeed found to be insignificant. For example, the effect of Cu on Zn has a GSAM kcu,k = therefore, if Cu and Zn are a t equal concentrations, then a khu,& = will give a 1% error in C k ( o ~ However, ! when Cu is lo2larger in concentration then Zn (as in the instrument characterization study), then a khU@ = lo4 is needed for a 1% error in CZn(obsd). But the GSAM kcu,%= and hence there will be greater than 1% error in C k ( o M )if no correction is made for the interference of Cu. As another example the effect of Co on Cu has a GSAM kG,cu = 2.3227 X Since Co is present a t a lo2 greater concentration than Cu during the addition of Co for the characterization of the sensors to Co the empirical k'needed would be Since the GSAM determined k is less than this empirical k' there is not a significant interference of Co on the Cu sensor. The condition number for AN was 1.078 denoting little error amplification in the estimated K matrix. The condition number for K was 10.73 resulting from the wide range of primary sensitivities (kmd*). Thus GSAM offers a real advantage of characterizing the instrument in a short period of time and allows the estimation of sensitivity and interference values that are nonsample related.
CONCLUSION The GSAM can detect, characterize, and correct for matrix effects and interference effects common to ICP-AES analysis. It also allows the analyst to use the most sensitive wavelengths for all elements of interest at all times. The GSAM is the only general method that can simultaneously correct for interference and matrix effects. The method can be applied using as little as one spike (addition) per analyte/interference pair and need not represent an inordinate amount of additional labor. The real potential will be realized when the standard
additions are automated, a subject under study in our laboratory. Finally it was shown that the GSAM is able to characterize a multicomponent instrument. With the GSAM, maximum information on the characterization of the instrument is obtained with minimal effort.
ACKNOWLEDGMENT The authors are grateful to C. Jochum, B. Vandeginste, and Maynarhs da Koven for their helpful discussions and F. Bulman of Jarrell-Ash for assistance with instrument operation.
LITERATURE CITED (1) Larson, G. R.; Fassei, V. A.; Winge, R. K.; Kniseley, R. N. Appl. SpectrOSC. 1978, 30, 384-391. (2) Larson, G. F.; Fassel, V. A. Appl. Spectrosc. 1979, 33, 592-599. (3) Barnes, R. M. CRC Crit. Rev. Anal. Chem. 1978, 7 , 203-296. (4) Ediger, R. D.; Fernadez, R. J. At. Spectrosc. 1980, 1 , 1-7. (5) Edlger, R. D.; Hoult, D. W. At. Spectrosc. 1980, 1 , 41-47. (6) Marciello, L.; Ward, A. F. Jarrell-Ash Plasma News/. 1978, 1 , 12-13. (7) Larson, G. F.; Fassel, V. A.; Scott, R. H.; Kniseley, R. N. Anal. Chem. 1975, 47, 238-243. (8) Kawaguchi, H.; Ito, T.; Ota, K.; Mizuike, A. Spectrochlm. Acta, Part 8. 1980, 358, 199-206. (9) McQuaker, N. R.; Kiockner, P. D.; Chang, G. N. Anal. Chem. 1979, 51, 888-895. (10) Greenfield, S.; McGeachin, H. McD.; Smith, P. B. Anal. Chlm. Acta 1978, 84, 67-78. (11) Dahlquist, R. L.; Knoll, J. W. Appl. Spectrosc. 1978, 32, 1-30. (12) Saxberg, Bo E. H.; Kowalski, B. R. Anal. Chem. 1979, 51, 1031-1038. (13) Jochum, C.; Jochum, P.; Kowaiski, B. R. Anal. Chem. 1981, 53, 85-92. (14) Neter, J.; Wasserman, W. "Applied Linear Statistical Models"; Richard D. Irwin Inc.: 1974; Chapter 6. (15) Dahlquist, G.; Bjorck, A,; Anderson, N. "Numerical Methods"; Prentice Hall: Englewood Cliffs, NJ, 1974; Chapter 5. (16) Naylor, T. H.; Balintfy, J. L.; Burldick, D. S.; Chu, K. "Computer Slmulation Techniques"; Wiley: New York, 1966; Chapter 4. (17) Salin, E. D.; Horlick, G. Anal. Chem. 1980, 52, 1578-1582. (18) Garden, J. S.; Mitchell, D. G.; Wills, W. N. Anal. Chem. 1980, 52, 23 10-231 5.
RECEIVED for review May 11,1981. Accepted September 10, 1981. This work was supported in part by the Department of Energy.
Determination of Radium-224, -226, and -228 by Coincidence Spectrometry David E. McCurdy" and Russell A. Mellor Yankee Atomic Electric Company, Environmental Laboratory, 167 1 Worcester Road, Framingham, Massachusetts 0 170 1
A radlochemical technique for the quantitatlve determination of 224Ra, 22BRa,and 228Ra which affords high sensitlvlty measurements and high sample throughput with a mlnimum number of chemical steps Is described. The basis of the technique Is the coincidence measurement of the characteristic particle-photon emissions of the isotopes: the a-y colncidence emissions of 224Raand 22BRaand the 6-7 colncldence emlssions of 2 2 8 A ~the , first shod-lived decay product of 228 Ra. The radiochemical procedure for a water medium and the nuclear measurements technlque and calibration are described. Under normal operating conditions, minimum detectable concentrations of 0.2, 0.3, and 0.5 pCi/L can be attalned for 224Ra,226 Ra, and 228Ra,respectively, based on a 1-L sample and 200-min count.
Current methodologies for the determination of 226Raand
2zsRarequire separate radiochemical procedures due to the inherent differences in the nuclear measurement techniques employed. For 226Ra,the chemical techniques range from a minimum of steps involving inert degassing of the sample following 222Rnsecular equilibrium (1,2) to various sequential procedures to isolate and purify the radium by utilizing classical coprecipitation (3, 4 ) , complexing (3, 5-9), or ion exchange (10-12) which are dependent on the measurement technique to be employed and the sample media being processed. The 226Rameasurement techniques employed vary from a simple 22zRnmeasurement by scintillation chamber (1-3, 5 , 7, 9, 12-15) to more sophisticated instrumentation involving a spectrometry (10, I I ) , mathematical procedures related to gross a counting of coprecipitated fiial sample forms (8), and liquid scintillation techniques (16). Since there is great difficulty in measuring the very low energy 6 particle (Ern== 0.055 MeV) emitted by zzsRain the presence of the short-lived, 0-emitting decay products of the
0003-2700/81/0353-2212$01.25/00 1981 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
other radium isotopes, most radiochemical and nuclear measurements procedures (1, 3,5-7,10,11,17, 18) for this isotope aire developed for the isolation, purification, and the first decay product of zzsRa. Some measurement of 228A~, procedures (2,19) for 22'8Rado measure 228Thsubsequent to a long ingrowth time after radium purification. Most referenced methods utilize the same initial 22aRachemical purification steps for zzsRa but provide isolation and final eoprecipitation of zzsAcon various substrates following an ingrowth time. The z2sAc is normally measured on a GeigerMueller or gas proportional counter very quickly after sample precipitate mounting in order to gain the maximum sensitivity for this 6.1 h half-life nuclide. The procedure presented herein simplifies and/or reduces the analytical steps of the previous references as well as the labor involved in the processes without sacrificing the requirements for precision, accuracy, and limits of detection. This technique is predicated on the separate measurement of the coincident a-y transitions of z26Raand 224Raand the coincident P-y transitions of zzsAc. The approach taken is somewhat unique in that 226Rais measured directly and 228Ac is being supported by 32sRa,thus eliminating the restriction placed on the detection sensitivity by the measurement of a short-lived nuclide.
THEORY Radium-226 decays by two possible a transitions, 94.5% by Q emission to the ground state of z22Rnand the remaining 5.5% by a emission to a metastable state of zzzRnwhich promptly releases energy by isomeric transition (20). Further analysis of the decay scheme indicates that of the zzzRn isomeric l,ransitnons, 601% =e associated with the emission of a 186-keVy-ray and 40% are by conversion electron emission. Therefore, the a-y coiincidence abundance for 2z6Rais 0.033 per transformation. No other a-y coincident emissions are prevalent, in the 226Raseries. In the zzsRa decay chain (20) only z24Radecays by an CY emission followed by H prompt isomeric transition (zzoRr$ Although this decay mode occurs 5.2% of the time, the a-y coincidence abundance (4.1%) associated with the 241-keV y-ray is of similar magnitude to that of zzsRa. The energy differentid of the and %Ra y-rays is sufficient to permit sodium iodide detectors3 of average resolution to completely separate the photopeaks of 224Raand 2z6Rawithout much difficulty. The presence of mRa in the sample will create some Compton interference in the photopeak area of '=Ra but the magnitudle of the interference is dependent on the concentration ratio of the two isotopes at the time of measurement and the photopeak area selected for zz6Ra. The y-ray lines to be used in the identification and quantification of zzsAcmust be selected judiciously so that other nuclides in either of the zz6Raand 22sRadecay chains which exhibit P-7 transitions do not cause interference. Fortunately, the radiochemical purification steps within the procedure remove period 6 and 7 decay products of 224Ra,226Ra,and zzsRa, In addition, the zzsThdecay product of 2 2 8 Ais~ long lived (1.9 year) and is an a emitter. Consequently, in the absence of any initial amount of 2aRa, the short-lived decay products of 2281'h will not reach any significant degree of equilibrium in the propored time frame for analysis. Although the purification steps remove the short-lived progeny of 226Ra and 224Ra,within 4 days after separation these decay products would a t h i n about 50% equilibrium. Evaluation of the 226Fta decay chain reveals that '14Pb is a 0-y emitter and its decay produces about the same lower energy y-ray emissions as does 228Ac.For this reason, the lower energy y-rays are not ideal for '"Ac 8 7 analysis. The only other 0-y transitions of high abundance which me free of major n6Ra progeny interferences are those associated with the 909 and 967 keV y-ray emissions.
2213
-PM HOUSING
DELRIN SAMPLE HOUSI""--
LOCK RING
c '
LIGHT PIPE
(125mrn)
STOP,
*--
Figure 1.
a-0
SPEX SAMPLE MOUNT
particle detection assembly.
A slight interference of the photopeak area for the 909-967 keV photopeaks will be caused by the Compton scattering of higher energy y-rays (principally 1765 keV) from 214Bidecay. However, the degree of interference is small for normal situations and is a function of the quantity of 2z6Rapresent in the final sample matrix, the degree of '%Ra progeny ingrowth permitted, and the size of the y-ray detector. Similarly, but to a much lesser degree, the 2611-keV y-ray from the decay of the mRa decay product q 1 will contribute some Comptm in the 909-967 keV photopeak region. From a measurement point of view, the interference by this high energy y for a typical 10 cm X 10 cm NaI(T1) detector appears to be insignificant.
EXPERIMENTAL SECTION Instrumentation. The detection of both a and 0 particle emissions is accomplished by the same detection assembly. Zinc sulfide, incorporated with the final 2 cm2 precipitate matrix in a nearly 4a configuration, is utilized for a particle measurements. The 0detector is a 0.1 cm thick, 2.54 cm diameter disk of Pilot B or NE102 plastic scintillator coupled to an EM1 type 9824B photomultiplier (PM) tube by a 1.25 cm thick plastic light pipe. The plastic p scintillator appears to be transparent to the light output of the ZnS and therefore does not interfere with 01 particle measurements. The entire a-p detection assembly fits into a PVC or Delrin sleeve utilized as a sample mount retainer as well as a light tight enclosure as shown in Figure 1. The enclosed a-P detection system is inserted into a 3.8 cm diameter by 7.1 cm deep well of a 10 X 10 cm NaI(T1) y-ray detector. With the exception of the top area, the entire detection system is enclosed in 10 cm of lead molded to the shape of the NaI-PM tube configuration. A 10 cm thick Pb disk, pivoting on a hinge, shields the top of the detection system and facilitates access for the sample changes. The electronic components of the coincidence system as illustrated in the block diagram of Figure 2 are commercially available Nuclear Instrument Module (NIM) units. The timing and pulse shape parameters will vary according to the design of the individual electronic units produced by different manufacturers. Any small multichannel analyzer (MCA) which permits analog to digital conversion (ADC) gating can be utilized satisfactorily for this application. The signal pulses from both detector PM tubes are coupled to spectroscopy amplifiers through charge sensitive scintillation preamplifiers. The amplifiers provide internal pulse shaping (0.5 p shaping) as well as unipolar and bipolar outputs. The unipolar pulse is delayed 2 ws with respect to the bipolar pulse in order to operate the MCA in the coincidence gate mode. Under normal operation, the bipolar outputs from both amplifiers are connected to their respective timing single channel analyzers (TSCA)which are operated in the crossover-base line discriminator mode. The base line discriminator of the particle counting TSCA is set at 0.1 V for both 01 and 0 counting measurements. The discriminator setting for the y TSCA is also 0.1 V corresponding to a y-ray energy of about 30 keV. If the bipolar input signal satisfies the discriminator requirement for the respective TSCA, positive and negative logic output pulses are generated in each of the TSCAs at the time the input bipolar pulse crosses zero potential. Due to the basic differences in the fluorescent decay constants of the a , p, and y scintillating
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
2214
SUPPLY I
0.1 V DISC
wt-
Q - P DETECTION
-+ HV SUPPLY
Delay Period 200 ns for P y 0 ns for Q Y
ASSEMBLY
DELRIN SLEEVE
N a I (TI)
0.45~~ I
‘ ADC
425 ns for ay UNIPOLAR SIGNAL
Flgure 2. Block diagram of the electronic components of the coincidence counting system.
detectors and the timing and shaping characteristics of their respective bipolar pulses, the logic pulses of the particle and y TSCA must be adjusted in terms of timing in order to obtain “true” electronic coincident events. In the case of a-y coincidence (226Ra,224Ra),the logic pulse from the y TSCA must be delayed approximately 425 ns. For P+-y coincidence (zz8Ra)the logic pulse from the particle TSCA must be delayed by 200 ns. If the positive logic pulses from the TSCAs arrive at the coincidence analyzer within 450 ns of each other, a positive logic pulse from the coincidence analyzer, denoting a P-y or a-y event, is sent to the gate of the MCA to permit processing of the unipolar signal from the y amplifier. The application of different time delay parameters for the P and a TSCA pulses is the basis for differentiating between a-y and P+-y coincidence events. With the timing parameters utilized, an a-y event cannot be electronically registered as a P-y event and P-y events cannot be recorded as a-y events. Once a particle-? analysis system has been established a simple change in delay times for the particle and y-ray TSCAs will transform the unit from an a-y to a P-y coincidence counter. While setting up the system, care must be taken not to over amplify the signals from the particle detector so that flattopping (analog signals greater than 10 V) occurs. Typically, the PM tube high voltage (nominally a t 600 V) and amplifier gain are set t o give a noise level of 50 mV maximum. With these settings, the resultant a ZnS pulses are between 3 and 10 V, whereas the resultant P plastic scintillator pulses range from 100 mV to 2 V. Over amplification will distort the voltage pulses with subsequent loss of timing characteristics. If this technique is to be employed for ‘%a measurements only, the electronics can be simplified somewhat by replacing the MCA with a scaler-timer NIM unit. In this mode of operation, the y TSCA must be set for the single channel analyzer mode and its window adjusted for the 909-967 keV photopeak area. During the initial setup, a MCA incorporated in the system as depicted can assist in determining the TSCA window settings. Reagents. Deionized water and analytical reagent grade chemicals were used throughout the procedure.
Alkaline Ethylenediaminetetraacetic Acid (EDTA), 0.25 M. With stirring and slight heating, dissolve 93 g of disodium ethylenediaminetetraacetic acid dihydrate in 900 mL of water. Filter and add concentrated ammonium hydroxide to bring the pH to 10.0. Dilute the solution to 1 L. Barium Carrier, 10 mg/mL of Barium. Dissolve 1.90 g of barium nitrate in a minimum of water. Add 0.5 mL of concentrated nitric acid and dilute the solution to 100 mL. Filter and store in a glass container. Citric Acid, 1 M. Dissolve 19.2 g of citric acid in water and dilute to 100 mL. Zinc Sulfide. Available as a silver activated, nickel poisoned powder from Nuclear Enterprises, San Carlos, CA. Procedure. The preliminary radiochemical steps of the EPA sequential method for zzsRaand 226Rain drinking water (7) have been modified. Filtration of the original sample through a 1-km filter is recommended to remove any significant amount of undissolved solids which might affect the accurate prediction of gravimetric recoveries or discolor the final precipitate matrix. If desired, these solids can be chemically treated (3) in order to quantitate the radium isotopes. Since the counting scheme employed does not require substantial decontamination from other radiochemical impurities, the yttrium and lead carriers have been eliminated to reduce the background of the final precipitate. Citric acid is used as a masking agent (21). Field Preparation of the Sample. For each liter of sample collected add 20 mL of concentrated nitric acid. Label each sample container noting the collection location, preservation, and time and date of collection. Coprecipitation of Radium. Filter the sample through a 47 mm-l krn filter housed in a filtration apparatus of 300 mL capacity equipped with a magnetic chimney hold down ring (Catalog No. 4200 Gelman Sciences). Weigh approximately 1.0 kg (to the nearest gram) of the filtered sample into a 2-L beaker. In an adequate fume hood, add 5 mL of 1M citric acid and adjust the pH of the solution to 9.5 with concentrated ammonium hydroxide. Add 1mL of previously standardized barium carrier and stir on a magnetic stirrer for 5 min. Add 1mL of 50% ammonium sulfate
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
and adjust the p1-I of the aolution to 2.5 by the dropwise addition of 18 N sulfuric acid. Stir the sample on a magnetic stirrer for 30 min. A barium sulfate precipitate should appear within 10 min. Purification and Mounting of the Barium Sulfate. Filter the soluticln through a 0.4 pm, 47 mm polycarbonate filter (Catalog No. 111107 Nuclepore, Pleasanton, CA) using the Gelman filtration apparatus. Carefully fold the filter containing the precipitate and place in a IOO-mL centrifuge tube. Add 20 mL of 0.25 M alkaline EDTA. Heat the solution in a hot water bath until all the precipitate has dissolved (22). The addition of 2-3 drops of 10 M sodium hydroxide may aid in dissolving the precipitate. Remove the filter and wash with approximately 5 mL of 0.25 M alkaline EDTA followed by approximately 5 mL of the deionized water. Combine all washes with the original EDTA solution. Cool the solution to room temperature; add 1 mL of 50% ammonium sulfate and reprecipitate the barium sulfate by adjusting the solution plH to 4.6 with the dropwise addition of 17.4 N acetic acid. Allow the precipitate to stand, with occasional mixing, for 5-10 min. Note the time of precipitation. Preweigh a 0.4 pm (26 mm) (Nuclepore Catalog No. 110607) filter and mount consisting of a nylon cup with bottom removed, snap ring (Catalog Nos. 3515 and 3519, SPEX Industries, Metuchen, NJ), microporous film (Chemplex Industries Inc., East Chester, NY), and 0.013 mm Mylar film (23). Add approximately 44 mg of ZnS (nearest tenth of a milligram)to the centrifuge tube. This will result in a final ZnS/BaS04 ratio of 2.4. Vortex mix and fiiter the mixture through the preweighed polycarbonate filter mounted in a Millipore suction filtration apparatus (Catalog No. 1002500 Millipore Co., Bedford, MA) modified to hold 150 mL of liquid. Rinse the filter and precipitate with approximately 20 mL of deionized water and place the filter, precipitate side up, on top of the piece of microporous film. The piece of 0.013 mm Mylar is placed on top of the precipitate and this “sandwich’ is locked firmly intlo place using the sample cup and snap ring. Dry the sample to constant weight by vacuum desiccation. Determine the weight of the precipitate and gravimetric recovery based on the total predicted weight of ZnS and BaS04. Insert a 0.13 mm thick tin disk (Catalog No. T-131,Fisher Scientific) with cellulose plug (23)behind the microporous film as a backscatter material.
IIISCUSSION In the counting system described, a MCA is utilized to collect the a--y and P-fy spectra from 226Raand 22aRa. This approach is somewhat more sophisticated than the use of scalers coupled with TSCAs having energy windows set for the full-width-tenth maximum of the identifying y photopeak that was described in an earlier description of the technique (24). This newer total spectral approach was prompted by a recent study (25) of bottled mineral waters which showed that %Ra may be present in significant amounts in the original sample. Although the extent to which z24Raexists in some form of equilibrium with zz8Rain other media is unknown, this newer approach would eliminate any bias in the calculation of the zz6Raconcentration due to the compton interferences from 22412a.For any given detection system employed, the Compton scattering associated with the 241-keV y-ray of z24Rain coincidence with the 5.447-MeV a that is observed in the 186-keV photopeak region for the zzsRa y-ray is a constant fraction of the 241-keV photopeak of z24Ra.For the system described, this fraction is approximately 0.21. However, the presence of 22‘lRahas only been observed in waters that appear to be a minerally enhanced medium which may be atypical compared to common surface and ground water supplies and in 22aRastandards that are fairly old with respect to the half-life of 228Th. Since thorium is not coprecipitated with BaS04 during the chemical purification steps, any z24Ra in the final precipitate is not supported by its parent and decays according to its 3.64-day half-life. Similarly, the presencla of z23Rain the final barium sulfate matrix will cause an interference in the determination of z24Ra and ‘%a due to the Compton scattering associated with the
2215
a-y coincidence events of the 5.71-MeV a particle and 270keV y-ray. The magnitude of this interference is negligible in samples which are not artificially enriched in z23Rabut may be determined in a manner analogous to that of z24Ra. The amount of Compton interference from the 1765-keV photopeak of 214Biof the zzsRa decay chain in the 909-967 keV region of n8Ac was evaluated by analyzing a 226Ra-BaS04 precipitate in the P-y coincidence mode. Of course, the degree of interference will depend upon the physical parameters of the detection system used, but for the system described it was found that the interference amounted to 0.017 (counts/ min)/(disintegration/min) of 226Rain the final counting precipitate a t full equilibrium with its decay products. For normal processing times, the interference for a sample counted within 2 days should be no greater than 0.005 (counts/ min)/(disintegrations/min) of zz6Rain the final mount or a 25% increase in the typical P-y background of O“02 counts/min for each disintegration/min of zzsRa. While attempting to optimize the a-y counting efficiency for zz6Raduring the early stages of procedural development, it became evident that the total response varied with the amount of ZnS added to the final precipitate. Previous investigators (26,27) employing ZnS powder mixed with precipitates or ZnS mounts for a counting have reported minimum thicknesses for optimum counting efficiency. For ZnS mounts (27),a thickness of 4 mg/cm2 was found to maximize the efficiency whereas Rosholt (26) used a ZnS powder mixed with the final BaS04 precipitate to yield a ZnS/BaS04 ratio of 4.4 a t a mounting thickness of 60 mg/cm2. The variability of the a-y counting efficiency of the detection system was evaluated as a function of the ZnS added, as well as the final weight of the total amount of ZnS plus BaS04. The effect of the amount of ZnS in the precipitate on the counting efficiency was measured in two different ways. The first experiment involved adding various amounts of ZnS to 34 mg of BaS04 in suspension followed by mounting and counting. The results of this investigation indicate that a near constant a-y coincidence efficiency was attained for ZnS/ BaS04 ratios between 1.9 and 3 and over a wide range of final precipitate weights of 31 to 63 mg/cm2. On the basis of this experiment, an optimum ZnS/BaS04 ratio of 2.4 was selected for the routine processing of samples. This ratio was a compromise between the amount of final precipitate weight (about 60 mg) which did not exhibit breakup during handling of the mounts and the possible error in determining the gravimetric recovery of the BaS04when amounts of initial barium carrier less than 10 mg are added to the sample. Once these parameters were defined, a second experiment was conducted in which the final precipitate weight of 60 mg chosen for the technique was held within *lo% and the amounts of ZnS arid BaS04 varied to achieve a range of ZnS to BaS04 ratios between 0.5 and 5. This experiment showed that the a-y efficiency is essentially constant a t 2.4% for ratios above 1.5 and final precipitates having variable BaS04 weights ranging from 54 to 58 mg. The effect of the final precipitate weight on the 0-7coincidence efficiency for 228Rawas also evaluated. This evaluation was deemed very important in that the technique would be greatly simplified if no 6-y self-absorption curve would be required over the anticipated weight range, Le., 30 mg/cm2 minus 10% for maximum possible losses. Although previous studies (23, 28) using P-y coincidence counting of 1311 precipitated as either PdIz or CUI have shown that the counting efficiency did vary with precipitate weight due to the low (3 energy (Emm= 0.61 MeV), it was anticipated that the /3 efficiency for the 1.026- and 1.124-MeV fls of zz6Acwould not be as severely affected.
2218
e
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
The ZnS/BaS04 ratio was held constant at 2.4 and the final precipitate weight varied from 28 to 96 mg by adding different amounts of barium carrier and ZnS. The mounted samples were then counted in the @-y coincidence mode following full ingrowth of zzsAc. If the z2sRastandard utilized for the calibration contained substantial amounts of 224Ra(>30% equilibrium with s2sRa), counting was delayed 10 days to permit decay of the z24Raand remove any @-y coincidence bias due to the high energy Compton interactions from 224Ra decay products. The results of this experiment showed that the coincidence efficiency decreased exponentially as a function of precipitate weight according to the fitted equation: efficiency = exp(1.724 - 8.96 X 10-3W(mg/cm2)). For routine analyses and typical recoveries, the efficiency of detection is 4.3% Since the slope of the equation is small, any uncertainty in the final weight of the precipitate produces only a slight error in the counting efficiency; a 5% uncertainty in recovery for typical mounts will only result in a 1.4% error in the efficiency factor. The 0 7 coincidence technique for zzsRawas also evaluated for those laboratories which may wish to use the technique for 228Raonly and continue to employ their ongoing 226Ra methodology. In this case, there is no need to incorporate ZnS in the final precipitate and the final BaSO, weight directly reflects the gravimetric recovery. However, since the decay products of z26Ramay interfere with the quantitative measurement, it is essential that the total activity of 2z6Rain the precipitate be known and the appropriate correction factor applied as mentioned previously. The coincidence counting unit should be calibrated using a known 228Rasource strength over a range of BaS04 precipitate weights. Experiments have been conducted which show that efficiency factors within 4-5% of one another will be generated whether ZnS is or is not incorporated as long as the final precipitate weight is the same. Apparently the ZnS is slightly sensitive to the highenergy 2 2 8 A@~ and provides an additional 4-5 % increase in efficiency over the pure BaS04 efficiency. The ZnS @ response was noted when the zzsRacalibration standards with the ZnS incorporated were covered with aluminized Mylar to prevent the light emitted from the ZnS from reaching the P M tube of the B detector assembly. The observed P-7 coincidence efficiency for each source decreased by approximately 4-5%. In addition, the ZnS response to 0 interaction was also verified by removing the @ scintillator and plastic light pipe from the P M tube and counting the uncovered standard in the @-y coincidence mode. Evaluation of the data corroborated the response factor of 4-5% of the total coincidence efficiency. Therefore, the efficiency for the system for Ba(Ra)S04without ZnS should be approximately the same as the efficiency for BaSQ4 with ZnS. The a--y efficiency of 224Rawas evaluated by counting the ZnS incorporated z28Rastandards in the a-y detection mode following an ingrowth period of approximately 1.6 years. The coincidence efficiency at a ZnS/BaS04 ratio of 2.4 was measured to be 3.3% over the weight range of 2Et40 mg/cm2. This value is within 10% of the predicted coincidence efficiency based on the coincidenceefficiency of 226Raand the differences in the CY-ybranching ratios of these two isotopes. Although the observed detection efficiencies appear to be low in comparison to other a and @ measurement techniques, the background count rates for the system are extremely low and, therefore, the figure of merit ( E 2 / B )values for the technique are extremely high. With 10 cm of lead shielding and a 10 cm by 10 cm NaI detector, the background for the 226Raa 7 photopeak is about lo4 counts/min. For the same configuration, the background for the zzaRaP-y coincidence photopeak region is about 0.018 counts/min. Reducing the I
y shielding to a 2.54 lead annulus surrounding the cylindrical coincisurfaces of the XaI detector only, increases the CY-? dence background to 0.002 count/min and the @- coincidence background to 0.08 count/min. This factor of 10 increase in the background with less shielding is probably due to high energy photons from the natural uranium and thorium decay chains and/or cosmic ray interactions that simultaneously occur in both scintillators employed. The figure of merit for 2z6Raunder typical counting conditions with 10 cm of lead shielding is about 58 000; a value which compares favorably with the figure of merit for a z22Rn scintillation chamber detection system. For the same shielding and detection system, the figure of merit for 22sRa is approximately 1030. This value could be improved if the size of the NaI detector were increased to improve the y detection efficiency. The minimum detectable concentration (MDC) (29, 30) defines a concentration at which a given probability (for this case 95%) of repeated measurements would be statistically (set at the 95% confidence level) different than background, i.e., 5% risks of false detection and false nondetection of true activity. For a 200-min counting period, a 1-L sample and the typical efficiencies and backgrounds mentioned in the text, the MDC values for z2sRaand zzsRaare 0.3 and 0.5 pCi/L, respectively, assuming the presence of one isotope only. The MDC for 224Raunder similar circumstances would be approximately 0.2 pCi/L.
LITERATURE CITED (1) MacKenzle, A. B.; Baxter, M. S.;McKlnley, 1. G.; Swan, D. S.:Jack, W. Radloanal. Chem. 1979, 48, 29. (2) Michel, J.; Moore, W. Health Phys. 1980, 38,663. (3) Percival, D. R.; Martln, D. B. Anal. Chem. 1974, 46, 1742. (4) Tomza, 1. International Conference on Low-Radloactivity Measurements and Appllcations, Czechoslovakia, Oct 1975. (5) Johns, F. 8.; Hahn, P. B.; Thome, D. G.; Bretthauer, E. W. Report EMSL-LV-0539-17; USEPA: Las Vegas, NV, 1979. Kahlos, H.; Asikalnen, M. Report SFL-A-19; Institute of Radlatlon PhysIcs: Helsinki, Finland, 1973. Krieger, H. L. Report EPA-600/4-75-008; USEPA: Clncinnatl, OH, 1976; pp 24-28. Krleger, H. L. Report EPA-600/4-75-008; USEPA: Clncinnatl, OH, 1976; pp 13-15. Harley, H. L., Ed. Procedure E-Ra-03, HASL 300; Health and Safety Laboratory, ERDA, 1972. Kiode, M.; Bruland, K. W.. Anal. Chim. Acta 1975, 1. Humphrey, H. W.; Adkins, E. L.; Templin, G. G. Report NLCO-1084; USAEC, 1975. Kobal, I.; Shahin, M.; Kristan, J.; Senegavik, M. Health Phys. 1974, 27, 381. Blanchard, R. L. Publlcation 999-RH-9; Public Health Service, 1964. Lucas, H. F. Rev. Sci. Instrum. 1957, 28, 680. Mastlnu, G. G. Health Phys. 1975, 28,97. Prichard, H. M.; Meyer, C. R.; Gesell , T. F. Presented at the 24th Annual Meeting of the Health Physics Society, Philadelphia, PA, July 1979. Baratta, E. J.; Feldman, M. H. Topical Report WINI-123; USAEC, 1961. Sakanoue, M.; Nomura, T.; Imai, T.; Uzuyama, H. I n Proceedings of Symposium and Hydrogeochemistry and Blogeochemistry, Tokoyo, Sept 1970. Kaufman, A.; Trier, R. M.; Broecker, W. S.; Feeley, H. W. JGR, J . Geophys. Res. 1973, 78(36), 8827. Martin, M. J.; Blichert-Toft, P. H. "Nuclear Data Tables"; Academic Press: New York, 1970, 125-143. Goldin, A. S.Anal. Chem. 1981, 33,406. Sill, C. W.; Williams, R . L. Anal. Chem. 1969, 4 1 , 1624. McCurdy, D. E.; Mellor, R. A,; Lambdin, R. W.; McLain, M. E., Jr. Health Phys. 1980, 38,203-213. McCurdy, D. E.: Mellor, R. A. Presented at the 24th Annual Meeting of Health Physics Society, Philadelphia, PA, 1979. McCurdy, D. E.; Mellor, R. A. Health Phys. 1981, 4 0 , 250-253. Rosholt, J. N. Anal. Chem. 1957, 29, 1396. Hallden, N. A.; Harley, J. H. Anal. Chem. 1960, 32, 1861. Paperiello, C. J.; Matuszek, J. M. IEEE Transactlons on Nuclear science, Vol. NS-22, Feb 1975. Altshuier, B.; Pasternack, B. Health Phys. 1983, 9 ,293. Currie, L. A. Anal. Chem. 1968, 48, 586.
RECEIVED for review March 30, 1981. Accepted August 20, 1981.
