Anal. Chem. 1999, 71, 162-166
Procedure for Analysis of Radium in Freshwaters by Adsorption on Basic Lead Rhodizonate M. T. Valentini Ganzerli,* L. Maggi, and V. Crespi Caramella
Dipartimento di Chimica Generale e Centro di Radiochimica e Analisi per attivazione del C.N.R., Universita´ di Pavia, viale Taramelli 12, 27100 Pavia, Italy
Radium analysis is carried out by batch adsorption from natural waters on basic lead rhodizonate supported on charcoal, LERHO, starting from 2-L samples. 133Ba is added to allow the measurement of the overall chemical yield by γ counting. Radium is recovered with a few milliliters of 1.5 M HCl, and lead is removed by a chromatographic column filled with Dowex 2 × 8. Finally 50 µg of barium carrier is added, and the radium is coprecipitated as sulfate on a preformed bed of barium sulfate, to prepare a sample suitable for r and γ counting. The detection limit of the proposed method is 0.002 Bq/L 226Ra. This value is far beyond the radium activity admissible for drinking waters. Due to lack of appropriate samples, the procedure was tested using mineral waters spiked with 226Ra and two commercially available mineral waters with very low radium contents. Radium is the most dangerous natural radioisotope, because as it decays radon and its daughters are formed. Moreover, the chemical and biological behavior of radium is similar to that of other alkaline earth metals and so it is easily incorporated into the bones of mammals. The International Commission of Radiology Protection recently reduced the maximum admissible concentration of radium in water and in substances related to human life. The known chemical procedures for the determination of 226Ra in different matrixes are usually time-consuming processes. They are based on many chemical steps, which become tedious and difficult to apply routinely, so many radiochemists prefer the analysis of radon and/or radon daughters to evaluate the radioactivity of natural samples.1-6 Radon activity, however, is seldom related to the presence of radium, and radium analysis becomes imperative for those matrixes involved in nutrition, especially the infant diet. Two methods,7,8 published recently, are based on the (1) Jiang, H.; Holtzman, R. B. Health Phys. 1989, 57, 167-168. (2) Yamamoto, M., Komura, K, Ueno, K. Radiochim. Acta 1989, 46, 137-142. (3) Crespo, M. T.; Gascon, J. L.; Acena, M. L. Appl. Radiat. Isot. 1992, 43, 19-28. (4) Flores-Mendoza, J.; Iturbe, J. L.; Jimenez-Reyes, M. J. Radioanal. Nucl. Chem. 1992, 162, 131-138. (5) Bickel, M.; Mobius, S.; Kilian, F.; Becker, H. Radiochim. Acta 1992, 57, 141-151. (6) Hodge V. F.; Laing, G. A. Radiochim. Acta 1994, 64, 221-215. (7) Orlandini, K. A.; Gaffney, J. S..; Marley, N A. Radiochim. Acta 1989, 55, 205-207. (8) Aupiaus, J.; Fayolee, C.; Gilbert, P.; Dacheux, N. Anal. Chem. 1998, 70, 2353-2359.
162 Analytical Chemistry, Vol. 71, No. 1, January 1, 1999
direct measurement of 226Ra activity after isolation by a simple chemical procedure, such as electrodeposition or solvent extraction. Both methods are limited by having to use a small sample volume. Other research has been directed toward the use of inorganic exchangers for isolating radium from different matrixes, including natural waters, in order to set up a simple analysis procedure.9,10 For this purpose lead rhodizonate, which results from the experimental preparation conditions as a partially basic lead rhodizonate, showed an ability to adsorb radium from basic solutions. The preparation and characterization of the adsorber has already been described.9,10 As its mechanical stability is poor, it was supported on charcoal and named as LERHO. It was formerly used in chromatographic columns, but the time required to percolate large sample volumes was excessive, so batch adsorption was preferred. A first attempt to use this adsorber was made in the analysis of mineral waters spiked with 0.1 Bq of 226Ra.11 The results obtained were so encouraging that we believed that it was worthwhile making a more thorough study of the possibility of setting up a reliable procedure that could be applied to different kinds of samples and that could detect very low levels of radium. Considerable attention was devoted to improving the working conditions. The resulting method, which is directly connected to the particular properties of LERHO, supplies a simple, tested procedure suitable for routine use in the analysis of freshwaters. EXPERIMENTAL SECTION Reagents. All chemicals used were reagent grade. Sodium rhodizonate from Aldrich and charcoal-activated powder from Riedel-de Haen were used as starting reagents to prepare the adsorber, LERHO. Diluted starting solutions of 133Ba and 226Ra were employed after delivery from a stock solution from Amersham International Ltd. The barium solution used contained 10 µg/mL barium as barium chloride, spiked with 800 Bq of 133Ba/mL; it was 0.1 M in HCl. Two diluted 226Ra solutions, namely 1 and 0.1 Bq/mL, were prepared in 0.1 M HCl solution containing 1 mg/mL barium as (9) Valentini-Ganzerli, M. T.; Maggi, L.; Crespi Caramella, V. J. Radioanal. Nucl. Chem. 1997, 221, 105-110. (10) Valentini-Ganzerli, M. T.; Maggi, L.; Crespi Caramella, V. J. Radioanal. Nucl. Chem. 1997, 221, 109-113. (11) Valentini-Ganzerli, M. T.; Maggi, L.; Crespi Caramella, V. J. Radioanal. Nucl. Chem. 1998, 229, 39-41. 10.1021/ac980673o CCC: $18.00
© 1998 American Chemical Society Published on Web 12/01/1998
Table 1. Adsorption Coefficients, Kd, for Barium as a Function of the LERHO Composition sodium rhodizonatea (g)
av % LERHO
name
Kd (×103)
1 2 3 4
18.2 31.6 41.0 48.1
LERHO 18 LERHO 32 LERHO 41 LERHO 48
2.16 3.82 3.21 3.95
a
Starting amount of sodium rhodizonate added to 10 g of charcoal.
barium nitrate and were used in the preparation of samples for counting. Apparatus. γ-Spectra were recorded with a high-purity germanium detector (EG&G Ortec γ coaxial Ge detector; relative efficiency 25%; resolution 1.70 keV at 1332 keV of 60Co) connected to a multichannel analyzer. R Spectra were recorded with an Alpha King spectrometer (EG&G Ortec) coupled to a computer-assisted multichannel analyzer. Two chimney glass funnel apparatuses, with sintered glass filter support, were employed for the filters, which were a 0.45-µm membrane filter, 47-mm diameter, from Millipore S. A. Molheim, and a 0.11-µm membrane filter, 25-mm diameter, from Gelman Science, Inc. (Ann Arbor, MI). Preparation of LERHO. The different LERHO batches were prepared using the following procedure: solutions deaerated by nitrogen bubbling for at least 30 min were used in all cases to avoid oxidation of the rhodizonate ion into croconate ion. A 3-mL aliquot of glacial acetic acid and from 2.2 to 4.8 g of lead nitrate were added to 200 mL of distilled water; and then 10 g of charcoal was suspended under vigorous stirring. Separately, the amount (from 1 to 4 g) of sodium rhodizonate needed to achieve the ratio 2.2 g of lead nitrate/g of sodium rhodizonate was dissolved in ∼200 mL of water and gently warmed under stirring at ∼60 °C until a clear orange solution was obtained. This solution was mixed with the above one under stirring. The lead rhodizonate formed, adsorbed on the charcoal, gave rise to a black suspension, while the supernatant solution became colorless. The slurry was warmed at ∼60 °C for at least 1 h and then cooled. The black product was filtered in a Buchner funnel with qualitatively suitable filter paper and repeatedly washed with 0.01 M acetic acid to eliminate the excess lead. It was then washed with ∼100 mL of distilled water and dried overnight in air. Afterward, it was left in an oven at 110 °C for 12 h. This step was necessary to improve the mechanical stability. Finally the obtained black solid was powdered for use. All prepared products were analyzed by a previously reported procedure.8 This showed that the molar ratio between lead and rhodizonate ions was 1.5:1, so the partially basic lead rhodizonate supported on charcoal had the composition (PbOH)2C6O6‚PbC6O6. It was named LERHO followed by a number that gives the basic lead rhodizonate content in the mixture with charcoal, depending on the initial amount of lead nitrate used, as reported in Table 1. Kd Measurements. The adsorption behaviors of barium and radium were studied as a function of temperature or of the lead rhodizonate/charcoal ratio. Owing to similarity between radium and barium, the experiments were carried out in the presence of 133Ba tracer, whose activity can be evaluated easily by γ spectrometry. About 200 mg of LERHO, of a given composition, were
equilibrated batchwise with 10 mL of the appropriate solution containing 400 Bq of 133Ba and 5 µg of barium by shaking for at least 1 h at the desired temperature. The pH was adjusted to the desired value with ammonia or hydrochloric acid, and the ionic strength was adjusted to 0.1 by adding KCl. For each batch of LERHO, Kd was measured at pH 9.5. After equilibration an aliquot of 5 mL of solution filtered through a 0.45-µm membrane filter was submitted to γ spectrometry. Comparison between activities before and after adsorption allowed the calculation of the distribution coefficients, Kd, from the usual relationship:
Kd ) cpm (or millimoles) of the adsorbed element per gram of LERHO at equilibrium/cpm (or millimoles) of the element per milliliter of the solution at equilibrium Chemical Procedure. The adsorption of radium was achieved in the presence of 133Ba to evaluate the chemical yield by γ counting. The following procedure was set up. To 1 or 2 L of the water under investigation, 1 mL of concentrated HCl was added to eliminate carbon dioxide arising from hydrogen carbonate under nitrogen bubbling, which was carried out for at least 20 min, and then 0.5 mL of the 133Ba solution (400 Bq, 5 µg of barium) was added. Afterward, 2 g of LERHO and 1 mL of 10 M NaOH followed by 3 mL of concentrated ammonia were poured into the sample. The pH during boiling automatically fell to ∼9.5, owing to the presence of the ammonia buffer. The suspension obtained was heated under magnetic stirring at about the boiling temperature for at least 1 h, then cooled at room temperature, and filtered onto the funnel with the 0.45-µm membrane filter. The beaker and the collected LERHO were washed twice with 0.01 M ammonia and then with deionized water; the filtrates were discarded. The beaker was then rinsed with small portions of 1.5 M HCl, which were then poured onto the filter, and the adsorbed barium and radium eluted under a low suction. The final volume collected was ∼40 mL. The lead was partially dissolved by the hydrochloric acid. The recovered solution was percolated through a column filled with Dowex 2 × 8, 100-200 mesh (1-cm i.d. and 15-cm height), preconditioned with 30 mL of 1.5 M HCl. The column was washed with ∼30 mL of 1.5 M HCl in order to recover the barium and radium completely. Finally, to remove the lead that was retained by the resins the column was washed with 50 mL of deionized water. To the mixed eluates, 50 µg of barium as barium nitrate was added as a carrier, necessary in the following step of precipitation of the sulfates. The Dowex column was restored by eluting lead with 60 mL of water and reconditioned with 1.5 M HCl. In some experiments, carried out to set up the method, different amounts of LERHO were used. The method was also run in the presence of different 226Ra activities, ranging from 0.027 to 1 Bq, obtained by adding different volumes of the radium solutions. Preparation of Samples for Counting. We used a modification of the procedure reported in the literature12 to prepare samples suitable for R and γ counting, taking into account that it Analytical Chemistry, Vol. 71, No. 1, January 1, 1999
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is imperative that a very thin film of R emitters is formed in order to avoid undesirable degradation of the R spectra. For this purpose, radium (barium) sulfates were adsorbed on a bed formed by inactive barium sulfate. The bed of barium sulfate was prepared according to the following procedure: 4 mL of the barium solution was added to 50 mL of a solution of ∼0.1 M HNO3 or HCl (the choice is irrelevant); then 1 mL of concentrated sulfuric acid was added, and the solution was heated to boiling for a few minutes and then left to cool at room temperature. The barium sulfate formed was then filtered through a 0.1-µm membrane filter (22-mm diameter). Fortunately, the quality of the R spectrum in the case of sulfates seems not to be affected by the use of a glass support, so a commercial filtration apparatus could be used. To obtain a smooth surface on the substrate and have the substrate itself drawn into a tightly packed layer, 10-mL portions were poured each time onto the filter and full suction was achieved in ∼10-15 s after filtration of the solution. Separately, 50 µg of barium carrier followed by 1 mL of concentrated sulfuric acid was added to the solution of radium, in the presence of 133Ba, resulting from the above chemical procedure. The solution was heated to boiling temperature, then cooled and filtered on the preformed substrate without interrupting the suction, and washed with a solution of 1% sulfuric acid and with 20 mL of deionized water. The filter was dried under vacuum, dismantled, dried for 5 min under an infrared lamp, and mounted on the support of the R spectrometer. Before counting, the samples were stored under vacuum, as moisture can cause damage to R-spectrum apparatus. The primary reference standard for γ counting was prepared by pouring a small aliquot of the 133Ba solution corresponding to 400 Bq onto the barium sulfate bed, prepared as described above. The area of the peak at 356 keV was evaluated after a counting time of 10 min. The reference standards, used for the R-calibration curve, were prepared, as for the samples, starting from 50 mL of 1.5 M HCl solutions containing 400 Bq of 133Ba and different amounts of 226Ra. The γ activity of these standards was compared with the primary standard, to be sure that the barium and radium sulfates were all precipitated on the barium sulfate bed. Before R counting, the 133Ba activity of the samples under investigation was measured and compared with a standard to evaluate the overall chemical yield. The activity of 226Ra was measured by evaluating the intensity of the R-ray peak at 4784 keV. The counting time lasted from 5.0 × 104 to 7.2 × 104 s according to the activity to be measured. Natural water samples were all counted for 7.2 × 104 s. The counting efficiency in the selected conditions was checked to be ∼22%. Safety Considerations. Lead soluble salts and organolead complexes are harmful and irritants and should be handled accordingly, avoiding contact with skin. Care should be taken to dispose of lead waste solutions. By their nature, all radioactive materials can be hazardous if they are not handled correctly. Precautions should be taken during the physical handling, by using disposable latex gloves and operating in a hood, equipped with an absolute filter. (12) Sill C. W.; Williams, R. Anal. Chem. 1981, 53, 412-415.
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Table 2. Adsorption Coefficients, Kd’s, on LERHO 32 and Corresponding Rhodizonate Ion Concentration as a Function of Temperature temp (°C)
Kd rhodizonate ion concn (M)
25
50
90
3.82 × 103 1.1 × 10-5
6.58 × 103 2.5 × 10-4
3.20 × 104 8.1 × 10-4
RESULTS AND DISCUSSION Preliminary Experiments. The Kd’s measured at pH 9.5 for LERHO samples are reported in Table 1. Values are quite similar for different batches. As previously reported,10 the adsorption occurs through a mechanism of precipitation of the insoluble rhodizonates, rather than through an ionic exchange reaction. The adsorption occurs as a precipitation of barium and radium salts over the solid surface. It is, therefore, related to the rhodizonate ion concentration in solution, i.e., to the lead rhodizonate solubility. In all cases, the rhodizonate ion concentration was similar and near to 10-5 mol/L, so the smaller Kd for LERHO 18 can probably be ascribed to the size of the active surface. In this case, the lower amount of the adsorber might not have been able to cover the whole surface of the charcoal. As most rhodizonates, once formed, are colloidal, they are likely to coagulate on sites already formed from other insoluble rhodizonates, in this case lead rhodizonates, so to prevent their dispersion there should be as many of these sites as possible. On the basis of these results, LERHO 32 was chosen to be used as it meets the goal of employing a small amount of the active adsorber as well as obtaining maximum adsorption. The effect of temperature on the adsorption was also investigated by measuring the Kd for LERHO 32 in the presence of 400 Bq of 133Ba and 5 µg of barium at 20, 60, and at 90 °C (near boiling temperature). The results are reported in Table 2. As previously pointed out,9 heating causes the transformation of partially basic lead rhodizonate into hydroxylead rhodizonate, [PbOH]2C6O6, and the excess rhodizonate ions are released in solution. In turn, the increase of the rhodizonate ion concentration causes lowering of the barium and radium concentrations in solution, so adsorption is enhanced. For this reason the adsorption of barium and radium from the real samples was carried out at boiling temperature. Finally a commercially available mineral water, from Pasubio Valley (northern Italy), named as water R, was used in some experiments. The water was chosen because it does not contain measurable radium activity. A 0.1-Bq aliquot of 226Ra and 400 Bq of 133Ba with 5 µg of barium were added to 1 L of real sample and then the adsorption procedure was run in the presence of 4, 2, 1, or 0.5 g of LERHO 32, to find the minimum amount of LERHO necessary. The activity measurements were carried out as described above. Both barium and radium were completely adsorbed and recovered in the final sample, prepared for counting, with at least 1 g of LERHO/L of mineral water. The chemical yield was greater than 95%. Using 0.5 g of LERHO/L, an adsorption yield of ∼80% was found for both barium and radium. This may be due to the lower concentration of rhodizonate ions released. Thus, the amount of 1 g of LERHO 32/L of water was chosen for the standard procedure.
Figure 2. Calibration curve in the range 0.01-0.3 Bq of 226Ra. The descriptors of the obtained straight line are R ) 0.995, a ) 15 316, and b ) 16.2. Table 3. Recovery Yield According to the Described Procedure Applied to 2 L of Water R in the Presence of 400 Bq of 133Baa
added 226Ra activity, Bq/2 L 2 0.5 0.1 a
Figure 1. The R spectrum obtained for 0.01 Bq of 4 days after the preparation. The peaks of radium and its daughters can be clearly recognized.
rec yields at room temp (%)
rec yields at boiling temp (%)
133Ba
226Ra
133Ba
226Ra
62 ( 5
59 ( 5
85 ( 9 93 ( 8 88 ( 12
88 ( 7 98 ( 2 90 ( 7
At least two replicate measures.
226Ra
r Spectrometry. Aliquots of 0.025 Bq of 226Ra counted in the presence of 400 Bq of 133Ba were used as usual to measure the overall yield, and in its absence, gave the same counting rate, confirming that barium activity does not interfere with the R measurements. The calibration curve was set starting from different aliquots of 226Ra activity, in the presence of 50 µg of barium carrier and of 400 Bq of 133Ba: its linearity was checked from 0.01 to 2 Bq. Figure 1 shows an R spectrum of a sample of 0.01 Bq of 226Ra, recorded 4 days after the separation step. The spectrum clearly reveals the presence of 226Ra and of its daughters, 222Rn and 218 Po. The peak area is narrow (∼62 keV full width) and the resolution appears very good (∼20 keV full width at halfmaximum, fwhm). Decreasing the activity, the peak becomes wider and the resolution smaller. Figure 2 reports the areas of the peak at 4784 keV, for a counting time of 7.2 × 104 s, vs the activity (range 0.01-0.3 Bq). The linearity is followed very closely below 0.009 Bq, which corresponds to an area of 155 counts. The limit of 0.009 Bq results from statistic treatment of the curve13 and may be assumed as the limit of quantification (LOQ), whereas the limit of detection (LOD) is calculated to be 0.003 Bq. The full linearity (for activities greater than 0.009 Bq) of the calibration curve indicates that the method used to prepare the samples for counting is appropriate. Testing of the Method. The commercial mineral water (water R) from the springs of Pasubio has no detectable activity. It was (13) Miller, J. C.; Miller, J. N. Statistics for Analytical Chemistry, 2nd ed.; Hellis Horwood: New York, 1992; Chapter 5.
Table 4. Replicate Measures by Tracing 2 L of the Mineral Water R with 0.027 Bq of 226Ra and 400 Bq of 133Ba, on LERHO 32a expt no.
chemical yield (%)
R peak area
found activ (Bq)
1 2 3
98 88 100
366 ( 64 417 ( 64 448 ( 55
0.023 0.029 0.027
a
The R counting refers to a count time of 7.2 × 104 s.
chosen because its total ion concentration is intermediate from among Italian mineral waters. Different 226Ra activities, ranging from 0.1 to 2 Bq, were added to 2 L of water R and the whole described procedure with LERHO 32 was performed. The experiments were carried out both at room temperature and at boiling temperature. The recovery yields for barium and radium are presented in Table 3. These results clearly indicate that working at about boiling temperature is better. The recovery of barium is ∼89% and of radium ∼92%, confirming, within the experimental errors, that barium and radium behave similarly. The recovery yield appears satisfactory to ensure a good sensitivity. To evaluate the reliability of the method, the same natural water was traced with 0.027 Bq of 226Ra, added to 2 L of water. Three replicate experiments carried out on LERHO 32 are reported in Table 4. The radium activity added is similar to that which may be found in some natural waters. The reported chemical yields are calculated on the recovered 133Ba activity. Thus, an average activity of 0.026 ( 0.04 Bq is obtained. The experimental error is ∼15%, confirming the good features of the presented method. Analytical Chemistry, Vol. 71, No. 1, January 1, 1999
165
Table 5. Overall Chemical Yields Starting from 400 Bq of 133Ba and 0.1 Bq of 226Ra in the Presence of Sulfates or Phosphates on LERHO 32 overall yields (%)
molarity of added ion
carrier added before the adsorption
133Ba
226Ra
133Ba
226Ra
5 × 10-3 M sulfate 5 × 10-5 M phosphate
yes not yes not
30 ( 5 52 ( 7 15 ( 5 30 (5
28 ( 5 55 ( 6
88 ( 9 85 ( 10 75 (9 74 (7
92 ( 7 90 ( 8 82 ( 10 67 ( 10
at room temp
near boiling temp
According to R-spectrometry data and yield behavior, the detection limit of the proposed method can be evaluated to be 0.002 Bq/L 226Ra. So the choice of the volume on which to carry out the radium analysis is closely related to the activity of the sample under investigation. A volume of 2 L seems to be appropriate for mineral and flowing waters. Interference. Of the possible interfering ions, only sulfates and phosphate anions were considered as some spring waters are sulfate-rich and traces of phosphates may be found in some water from country wells or rivers. To 1 L of water R, sulfate ions were added to reach 5 × 10-3 mol/L to simulate a natural sulfate-rich water. Then 400 Bq of 133Ba was added, and the usual procedure was run on the LERHO 32, again carrying out the adsorption either at room temperature or at about boiling temperature, adding 50 µg of barium as carrier before or after the adsorption step, just before the precipitation of sulfates. In some experiments, 0.5 Bq of 226Ra was also added to confirm that radium and barium do behave similarly. Another experiment was run by adding 5 × 10-5 mol of phosphate ions to water R instead of sulfates. The adsorption yields were acceptable for those experiments carried out at boiling point, while they were too low at room temperature. The results are summarized in Table 5. These results show that if the adsorption is carried out at near boiling point, neither sulfate nor phosphate ions interfere to a large extent. In contrast, if the adsorption is carried out at room temperature, barium and radium sulfates could precipitate and would not be reconverted to rhodizonates. The sulfates may be finely dispersed so that they could be filtered off with the original solution. These results support the delay in adding the barium carrier until immediately prior to precipitation for mounting. As far as concerns cations, alkaline and alkaline earth cations do not interfere at all with the assay. Most rhodizonates are soluble, including uranium rhodizonate, because complex ions in solution are formed, so the presence of cations does not affect the separation. Adsorption of barium and radium was also measured from another type of mineral water, water II (see below), to which 1 mg of uranium, in the form of uranyl nitrate, had been added. The final measurement of the radium activity was not affected at all by the presence of the uranyl ion, nor were 238U traces detected. Application of the Method. Owing to the lack of standard natural water samples, the method was applied, using LERHO 32, to two samples of commercially available mineral waters which had been shown to have moderate gross R activity. One was a mineral water from the Lurisia springs (northern Italy), denoted water I, the other a mineral water from the springs of Camonica Valley (northern Italy), denoted water II. Some properties of these 166 Analytical Chemistry, Vol. 71, No. 1, January 1, 1999
Table 6. Replicate Analyses for Two Mineral Waters with LERHO 32a water sample II, run 1 II, run 2 II, run 3 II, run 4
chem yield (%)
overall counts
85.3 98.5 97.2 82.2
207 ( 31 304 ( 43 241 ( 43 190 ( 30
0.007 0.010 0.008 0.008
87.5 97.0 100 74.7
182 ( 35 214 ( 32 209 ( 40 168 ( 37
0.006 0.007 0.006 0.007
0.008 ( 0.001
II av activity I, run 1 I, run 2 I, run 3 I, run 4 I, av activity
activity (Bq/L)
0.007 ( 0.001
a Samples volume, 2 L; counting time, 7.2 × 104; 133Ba added activity, 400Bq. b Mineral water composition: Ca2+, 3.5 (water I) and 124 mg/L (water II); Mg2+, 0.4 (water I) and 41 mg/L (water II); SO42- 0.0 (water I) and 235 mg/L (water II); HCO3-, 15 (water I) and 305 mg/L (water II); total residue, dried at 180 °C, 36 (water I) and 597 mg/L (water II).
mineral waters are reported in the footnote to Table 6. Water II is rich in sulfates and carbonates, while mineral water I is poor. Table 6 reports the 226Ra activities, from four replicate measurements, found in waters I and II. Total counts refer to a counting time of 7.2 × 104 s and the method was applied to a 2 L of sample on LERHO 32; 400 Bq of 133Ba was added, as usual. From the results obtained, the method setup appears to be reliable. The reproducibility of the values gives a precision of ∼14%, which is acceptable, taking into account the very low activity to be measured. The sensitivity also appears to be satisfactory, if compared with the limits permitted in natural waters. The proposed procedure does not require difficult or tedious chemical steps and can be easily accomplished in 2 h. The preparation of samples for R counting bypass the electroplating step, to which more care could be paid. Many samples can be processed at the same time, so the method could be routinely used by people concerned with environmental radiochemical analysis and in those laboratories assigned to monitoring the public health. Due to lack of interference, the method can be applied to a large variety of waters and dissolved biological samples. By comparing this method with past works,7,8 based on simple 226Ra isolation steps, the detection limit of 0.002 Bq/L appears to be lower. Our detection sensitivity related to the R spectrometry is lower than that obtained by Orlandini and co-workers7 because the efficiency of the R camera we used is lower. Our method is not limited by the volume of the sample under investigation and leads to isolation of 226Ra of very high radiochemical purity, without reduction of the initial volume, which could give losses. For these reasons, this method may be applied to samples with a wide range of 226Ra activity. Moreover the sensitivity could be improved by using a more efficient R camera. ACKNOWLEDGMENT The authors thank Prof. S. Meloni for his helpful discussion and suggestions about the manuscript. Received for review June 19, 1998. Accepted October 12, 1998. AC980673O
