The Determination of 226Ra in Nontypical Soil Samples by ICP-MS

This article describes a way to measure ²²⁶Ra using inductively coupled plasma–mass spectrometry (ICP-MS) rather than the conventional method of gamma spectroscopy, taking into account an undocumented interference (²⁰⁷Pb¹⁹F) that is caused by the requirement to use hydrofluoric acid during sample preparation. The unusually high Pb levels observed in a number of soil samples caused significant interferences at the very low concentrations of ²²⁶Ra that were measured. The expected ²⁰⁸Pb¹⁸O interference was insignificant under optimized instrument conditions.

As part of a remediation project, soils were analyzed for a list of elements of interest, plus a number of specific isotopes. Inductively coupled plasma–mass spectrometry (ICP-MS) has seen increasing use for the determination of long-lived isotopes in preference to traditional counting techniques because of various efficiencies. In this study, ²³⁰ Th and ²²⁶Ra were of specific interest. Typically, ²²⁶Ra has been determined using gamma spectroscopy. However, determination by gamma spectroscopy requires equilibration of the sample before final determination can be made, and full equilibration can require up to 30 days. Despite this, the initial phases of the remediation work did not allow for such a long delay period, hence ICP-MS was considered as a more rapid evaluation tool. Without this equilibration, there is danger of producing a significant low bias if gamma counting is used, which is a major factor if a reduction in equilibration times is considered. In matters of health and safety, it is always prudent to choose a method with a potential positive bias rather than one with a negative bias. Hence, it was decided that an ICP-MS method should be developed to speed up the determination of ²²⁶Ra in soil samples instead of simply reducing the equilibration period before gamma counting.

It became apparent during the analysis of these samples that they were not typical environmental soils, but more like mine tailings because of the mining and smelter activities over the last several decades in the area they were taken from. Besides having concentrations of many elements that exceeded environmental regulations in Ontario, the lead content in the soils caused an interference on the ²²⁶Ra determination by ICP-MS. The known Pb oxide (²⁰⁸ Pb¹⁸ O) interference on ²²⁶Ra was minimized by optimizing instrument conditions so as to have low oxide formation (<3%). The overall impact was therefore found to be insignificant in all of the samples examined.

However, an undocumented interference was caused by the requirement to use hydrofluoric acid (HF) in the sample preparation to get a complete digestion. The interference caused by ²⁰⁷ Pb¹⁹F, although much greater than that due to oxide formation, is not significant for normal soils. The unusually high Pb levels observed in a number of soil samples caused significant interferences at the very low concentrations of ²²⁶Ra being measured. This problem led to the present study.

Experimental

Sample Preparation

Soil samples were dried in ovens at 40 °C for approximately 4–6 h to remove any moisture from the soil without a significant loss of volatiles that were of interest for other measurements. The drying time varied because of the fact that some of the soil samples were extremely wet and contained clay-like material that took a longer time to dry than the other soils. After they were dried, the samples were pulverized and homogenized before being passed through 50 mesh sieves. A representative 0.5-g portion of the dried homogenized samples was digested using a CEM Mars 5 microwave (CEM Corporation, Matthews, North Carolina) using 10 mL of a mixture of ultrapure HNO₃, HCl, and HF (8:2:1, respectively). After they were digested and cooled, the samples were quantitatively transferred to clean Environmental Express (Charleston, South Carolina) disposable digestion tubes and bulked to 50 g using >18-MΩ/cm² water. This represented the "base" working concentration for samples. Appropriate quality control samples were included in the digestion, including acid blanks, sample duplicates, and certified reference materials (contaminated soil SS-2 from SCP Science [Baie D'Urfé, Quebec, Canada], NIST SRM 1645 river sediment, or NIST SRM 1646).

Analysis

Samples were analyzed for ²³² Th, As, Sb, Co, Cu, Ni, U, Pb, Ba, Be, B, Cd, Hg, Mo, Se, Ag, V, and Zn using the PerkinElmer (Waltham, Massachusetts) Sciex Elan 6100 DRC ICP-MS system. All samples were diluted a further 10 times from the "base" concentration with >18-MΩ/cm² water before analysis to minimize both the acid content and the total dissolved solids present in the samples introduced to the ICP-MS. Where possible, multiple isotopes of an element were analyzed to ensure that interferences were not present. The ICP-MS system was calibrated every analytical run using second source standards for quality control standards and internal standardization (with 100 ppb rhodium added using a second channel on the peristaltic pump) to compensate for instrument drift. All standards were matrix-matched to the acid content of the digested and diluted samples.

Samples were also analyzed for ²³⁰ Th, and ²²⁶Ra using a Varian (Palo Alto, California) 820-MS ICP-MS system. All samples were diluted four times from the "base" concentration with >18-MΩ/cm² water before analysis to meet the required detection limits for these isotopes. Although this was not an ideal situation, the physical stability of the instrument because of its cone design and 90° reflecting ion optics system (1) allowed for smaller dilution to be used. The ICP-MS system was calibrated every analytical run using second source standards for quality control standards. All standards were matrix-matched to the acid content of the prepared samples. A matrix-matched acid blank was analyzed after every sample to rinse out the system, and the instrument was recalibrated after the analysis of every five samples.

Both ICP-MS instruments were optimized daily before sample analysis to ensure that instrument conditions achieved maximum sensitivity, low background, low oxides (<3%), and low doubly charged species (<3%).

²²⁶Ra and ²⁰⁷Pb¹⁹F Study

Standards Preparation

Standards of varying concentrations of lead and HF were prepared to determine the effect of the ²⁰⁷Pb¹⁹F interference on ²²⁶Ra. These standards were made using a commercially available single-element lead standard with natural abundance isotope distribution from Inorganic Ventures (Christiansburg, Virginia). The standard was first diluted to produce a 100 ppm intermediate lead standard and finally to the desired working concentrations required for the study.

The first step was to fix the amount of HF at 1% by weight while varying the concentration of total lead in the standards. The lead standards used were 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, and 20 ppm. The lead standards were made gravimetrically, using the appropriate weight of 100 ppm intermediate standard, adding 1% high-purity HF, and bulking to the appropriate final weight using >18-MΩ/cm² water.

The second step was to fix the amount of lead present at 20 ppm, while varying the concentration of HF in the standard. The concentrations of HF used (all in weight %) were 0.25, 0.5, 1, 2, and 3. Again, the standards were made gravimetrically using the appropriate weight of intermediate Pb standard, adding the appropriate amount of high-purity HF to reach the desired acid concentration, and bulking to the appropriate final weight using >18-MΩ/cm² water.

Analysis of ²⁰⁷Pb¹⁹F Standards

Samples were analyzed for ²²⁶Ra and ²⁰⁷Pb using the Varian 820-MS ICP-MS system. The system was calibrated before analysis using second source standards for quality control standards. All standards were once again matrix-matched to the acid content of the digested samples that would be analyzed using this method.

Table sI: Effect of varying concentrations of lead while fixing the concentration of HF at 1% on 226Ra

The ICP-MS instrument was optimized daily before sample analysis to ensure that instrument conditions achieved maximum sensitivity, low background, low oxides (<3%), and low doubly charged species (<3%).

Results and Discussion

Table sI (available online) and Figure 1 show the results of the first step of the study, looking at varying concentrations of lead while fixing the concentration of HF at 1%. As seen in these results, the higher the concentration of Pb, the greater the ²⁰⁷Pb¹⁹F interference on ²²⁶Ra. This effect had been noted when comparing the ICP-MS data to that found by gamma spectroscopy. The second step of the study looked at varying the concentration of HF while keeping the concentration of lead constant. These results are shown in Table sII (available online) and Figure 2. These data show that there is not a direct linear relationship between the concentration of lead and HF in the sample. This is as expected for a moderate equilibrium (equilibrium constant has a moderate value).

Figure 1: Effect of varying concentrations of Pb while fixing the concentrations of HF at 1% on 226Ra.

Table sII: Effect of varying concentrations of HF while fixing the concentration of Pb at 20 ppm on 226Ra

Table sIII (available online) and Figure 3 show the ²²⁶Ra comparison results between analysis by gamma spectroscopy and ICP-MS (both with and without the correction factor for lead concentration). It is important to note that the gamma data were determined on samples upon arrival at the laboratory, based on ²¹⁴Pb and ²¹⁵Bi analysis, and will be biased low. ²²⁶Ra concentrations are expected to increase during the 30-day in-growth period required for this analysis by gamma spectroscopy. This is clearly seen in samples containing higher concentrations of ²²⁶Ra, in which the initial gamma data are significantly lower than those found by ICP-MS using the determined correction factor. However, there is a good correlation between the two techniques when the correction factor is used, as illustrated.

Figure 2: Effect of varying concentrations of HF while fixing the concentration of Pb at 20 ppm on 226Ra.

Because a constant concentration of HF is generally used for sample digestion in a given study, a simple correction factor can be experimentally determined for any given level of HF rather than using a more complex correction equation. A complex correction equation also could be determined by a more extensive study of the Pb–F equilibrium under spectroscopic conditions. This factor was determined for the HF concentration that was used for the digestion of the soils for this remediation work: The more tightly experimental conditions are controlled (particularly the acid digestion), the more accurate the results of the correction factor. ICP-MS was the method of choice for the determination of ²²⁶Ra because of the required turnaround times for this project (24–48 h from date of receipt at the laboratory). As mentioned above, ²²⁶Ra determination by gamma spectroscopy requires an equilibration time of up to 30 days for the sample before final determination to avoid significant low bias. An extended commentary on radiochemical methods is required to explain why no viable alternatives existed using the gamma measurement method. In matters of health and safety, it is always prudent to choose a method with a potential positive bias rather than one with a negative bias. Therefore, to meet the required timelines, ICP-MS was used for ²²⁶Ra.

Table sIII: 226Ra comparison data

To further test the accuracy of the data produced using the ICP-MS method for the determination of ²²⁶Ra, the Kinectrics Inc. laboratory (Toronto, Ontario, Canada) participated in an interlaboratory study from Environmental Research Associates (ERA) in Arvada, Colorado (Study RAD-84). The value determined by the laboratory was 7.81 pCi/L ²²⁶Ra. The preliminary assigned value was reported as 8.26 pCi/L ²²⁶Ra; therefore the value determined using this method was well within the preliminary acceptance limits of 6.21–9.71 pCi/L ²²⁶Ra (2).

Figure 3: 226Ra (Bq/g) comparison.

Conclusion

The determination of ²²⁶Ra in soil samples in the presence of high concentrations of Pb was not a straightforward analysis. However, a correction factor was determined that could be applied to the ²²⁶Ra data to account for this interference. This correction factor worked well, as seen when ²²⁶Ra found by gamma spectroscopy was compared to the ICP-MS data, showing a very good correlation between the two independent techniques. Any additional method improvements would require chemical separations for sample cleanup before analysis or the use of a sector-field ICP-MS system to improve the distinction between species (hence smaller correction factor).

Teresa Switzer, Otto Herrmann, and Darko Ilic are with Kinectrics Inc., Toronto, Ontario, Canada. Direct correspondence to: Teresa.SWITZER@kinectrics.com.

References

(1) Varian 810-MS and 820-MS ICP Mass Spectrometers Specification Sheet, SI-0184 10/05.

(2) Environmental Research Associates Study RAD-84, February 2011.