Vaporization of Radium and Other Alkaline Earth Elements in Electrothermal Vaporization Inductively Coupled Plasma Mass Spectrometry ROY ST. C. MCINTYREa , D. CONRAD GRE� GOIREb and CHUNI L. CHAKRABARTIa aOttawa-Carleton Chemistry Institute, Department of Chemistry, Carleton University, Ottawa, Ontario, Canada K1S 5B6 bGeological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8 Reported are the mechanism of vaporization and optimum have shown promise, no in-depth study on the vaporization of Ra or study of optimum experimental conditions for ETV experimental conditions for the determination of Ra and other alkaline earth elements (Be, Mg, Ca, Sr and Ba) by determinations has been reported.Alpha spectrometry1,2 is the most commonly used technique electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS). Calculated and published data for the determination of 226Ra. Sample sizes range from 1–50 ml of solution or 1–5 g of solid.The limit of detection along with new experimental results suggest that these elements are vaporized from the surface of the graphite tube obtained reported for alpha spectrometry1 was 1.8×10-4 Bq (0.1 fg ml-1) for a 50 ml sample, pre-concentrated and counted as oxides. These oxides are then transported to the argon plasma where dissociation and ionization take place. for 1000 min. Using TIMS,3,4 sample sizes can be as small as 1 g and yield detection limits of about 10 fg ml-1.Techniques Appearance temperatures and maximum pyrolysis temperatures obtained experimentally generally agree with based on radon emanation5 involve the collection of 222Rn, a decay product of 226Ra. Large sample sizes (litres) are required values obtained using graphite furnace atomic absorption spectrometry (GFAAS). For Ra, the optimum pyrolysis and and ingrowth of radon takes from several days to weeks before sucient quantities of Rn are produced for an accurate analy- vaporization temperatures were 1400 and 2500 °C, respectively.Diluted (15500) seawater, used as a physical sis. Cerenkov counting,6 based on b-particle detection using a liquid scintillator, also requires a relatively large sample and carrier, was eective in improving sensitivity when used in small quantities, but caused significant suppression of the Ra involves a relatively long sample preparation time giving a limit of detection of 0.035 Bq l-1 (0.95 fg ml-1). Although all signal when the analyte was co-vaporized with quantities of salt in excess of 40 mg.An absolute limit of detection of 1.7 fg of the techniques discussed above generally oer low limits of detection for Ra, this advantage is somewhat oset by long was obtained corresponding to 34 fg ml-1 in a 50 ml sample. preparation and analysis times. Preparation of liquid samples Keywords: Alkaline earth elements; electrothermal usually involves an ion-exchange separation of Ra followed by vaporization ; inductively coupled plasma mass spectrometry; an electrodeposition preconcentration step.Each step necessar- radium ily leads to a greater possibility of contamination and/or analyte loss. Studies on the determination of 226Ra by ICP-MS using Of the four naturally occurring isotopes of Ra, 226Ra is the most abundant having a half-life of approximately 1600 years. solution nebulization7 and ETV8,9 sample introduction gave The other isotopes are of lesser importance since they are not limits of detection of 0.2 pg ml-1 for solution nebulization7 as persistent with half-lives of 6.7 years for 228Ra, 11.7 days and 0.27 fg ml-1 for ETV when 50 ml of sample solution were for 223Ra and 3.64 days for 224Ra.No stable isotope of Ra used. These methods used either or both preconcentration by exists. Radium found in nature is derived from both natural ion exchange and evaporation techniques to improve sensiand anthropogenic sources and is highly toxic.The element tivity. Analysis by ETV-ICP-MS8 was done by drying successreplaces Ca in bone structure and can result in bone degra- ive samples without vaporization (multiple deposition) to dation and cancer. Radium (226Ra) is also the natural precursor increase sensitivity. This approach, however, is not suitable to 222Rn, which is retained in the lungs in the form of 210Pb when sample solutions contain large quantities of dissolved and 210Po.salts. Alvarado and Mitchell9 used Freon-23 as a chemical There is a substantial body of literature reporting on GFAAS modifier and obtained a limit of detection of 0.6 fg ml-1 for a studies on the mechanism of atomization of all of the alkaline 25 ml sample aliquot. A limit of detection of 1 fg ml-1 was earth elements with the exception of Ra. As will be shown obtained without the use of Freon-23. below, much of this information is useful when applied to The use of ETV sample introduction provides an alternative ETV-ICP-MS studies.Since little published information exists means of sample introduction for ICP-MS, which serves to on Ra, the vaporization of 226Ra compounds and its extend the range of application of the technique. The main determination by ETV-ICP-MS is the focus of this paper. advantages provided by ETV sample introduction include the Radium, as 226Ra, is currently determined largely by such use of microlitre or microgram sample sizes, the removal of techniques as alpha spectrometry,1,2 thermal ionization mass matrix interferences by thermal pretreatment of the sample spectrometry (TIMS),3,4 radon emanation5 and Cerenkov prior to vaporization and the ability to analyze solids, slurries counting.6 Recently, 226Ra has been successfully determined by and organic materials directly.Electrothermal vaporization inductively coupled plasma mass spectrometry (ICP-MS) using sample introduction provides for rapid analysis in minutes solution nebulization sample introduction7 and electrothermal compared with days or even longer for counting techniques.5,6 vaporization (ETV) sample introduction.8,9 While these studies As an additional advantage, many samples do not require preconcentration owing to the inherent high sensitivity of ICP-MS.In fact, in a recent paper by Smith et al.,10 it was GSC Publication No. 1996264. Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 (547–551) 547shown that it was advantageous to use ETV-ICP-MS rather modifier solutions were 10 ml in volume.The 226Ra solution (provided by the Atomic Energy Commission of Canada, than counting techniques for the determination of all radionuclides whose half-lives were greater than about 570 years. Pinawa, Manitoba) contained 5.2 Bq g-1 of the element. The concentrated Ra stock solution was calibrated against NIST The purpose of this study is to elucidate the mechanism of vaporization of Ra and the other alkaline earth elements in SRM 4966 (Radium-226) which was certified to contain 300 Bq g-1 of 226Ra. NASS-3 Open Ocean Seawater reference ETV-ICP-MS and to determine the optimum measurement conditions for these elements.This work is to serve as the material was obtained from the National Research Council of Canada and diluted 500-fold with deionized water prior to use basis for the development of methodology for the direct determination of Ra in solids, sampled as slurries, using as a chemical modifier or simulated sample matrix.ETV-ICP-MS. RESULTS AND DISCUSSION EXPERIMENTAL Mechanism of Vaporization of Be, Mg, Ca, Sr and Ba A Perkin-Elmer SCIEX Elan 5000a ICP mass spectrometer The mechanisms of atomization for a number of elements have equipped with an HGA-600 MS electrothermal vaporizer and been studied using GFAAS.11–13 Sturgeon et al.11 showed that a Model AS-60 autosampler was used. Pyrolytic graphite three processes occurred leading to the production of free coated tubes were used throughout.The experimental conatoms: (1) carbon reduction of the analyte oxide followed by ditions for both the Elan 5000a and the HGA-600MS are sublimation of the metal; (2) thermal dissociation of the oxide given in Table 1. on the graphite surface or in the gas phase; and (3) thermal A PTFE tube of 80 cm and 6 mm id was used to connect dissociation of the metal chloride. Thermogravimetric14 and the HG600MS to the plasma torch.Optimization of the atomic absorptiondata11–13 show that the oxides of the alkaline plasma and mass spectrometer was accomplished using soluearth elements (Mg, Ca, Sr and Ba) are common intermediates, tion nebulization, prior to switching to ETV mode. No further resulting from the heating (prior to vaporization) of either the optimization of the ICP mass spectrometer was required with chloride or nitrate form of the element. the exception of small (±50 ml min-1) variations in the carrier In GFAAS, Mg oxide (MgO) and Ca oxide (CaO) vaporize Ar flow rate.The operation of the HGA-600 MS was comfrom the graphite surface as oxides and in the vapour phase pletely computer controlled. During the drying and pyrolysis dissociate giving atoms.15 Hutton et al.16 showed by molecular steps of the temperature program, opposing flows of argon emission measurements in a carbon furnace that the gaseous (300 ml min-1) originating from both ends of the graphite tube oxide species for Mg, Ca and Sr exist, suggesting dissociation removed water and other vapours through the dosing hole of into the elements in the vapour phase.Sturgeon et al.11 the graphite tube. During the high-temperature or vaporization suggested that the vapour phase composition was dependent step, the dosing hole was sealed by a pneumatically activated upon the sample size; large samples produced gaseous elements graphite probe. Once the graphite tube was sealed, a valve directly, with thermal dissociation occurring on the surface, located at one end of the HGA workhead directed the carrier and small samples produced gaseous oxides before dissociating argon gas flow, originating from the far end of the graphite into gaseous atoms.Kantor et al.17 supported this interpret- tube, directly to the argon plasma at a flow rate of ation and Prell et al.13 used mass spectrometry coupled with 800 ml min-1. a graphite furnace to show that the oxide directly precedes the production of the analyte atoms.Standards and Reagents There is some uncertainty surrounding the vaporization of Sr oxide. It was suggested by Moore et al.18 that SrO(s) High purity argon gas (99.995%, Matheson Gas Products, vaporizes to SrO(g). However, mass spectrometric studies by Ottawa, Ontario, Canada) was used. A solution of mixed Porter et al.19 suggested direct formation of the gaseous alkaline earth elements was prepared by dilution of SPEX elements from the solid oxide.Nagdaev and Bukreev20 (using standard 1000 or 10000 mg ml-1 stock solutions (SPEX a graphite rod atomizer) proposed that the most probable Industries, Edison, NJ, USA) using de-ionized water (Millipore, mechanism for free Sr and Ba atom formation is sublimation Mississauga, Ontario, Canada). Aliquots of all samples and of the oxide with dissociation in the gas phase. Hutton et al.16 obtained similar results using molecular emission measure- Table 1 Instrumental operating conditions and data acquisition ments of SrO and Prell et al.13 also showed that the oxide parameters directly preceded the appearance of free Sr atoms.For Ba, the oxide is thought to exist predominately in the ICP mass spectrometer— vapour phase as a gas.15 Jasim and Barbooti21 suggested that Rf power/W 1000 Coolant argon gas flow/l min-1 15.0 free Ba is formed from the gas phase thermal dissociation of Auxiliary argon gas flow/ml min-1 900 the oxide as did Nagdaev and Bukreev.20 Frech et al.22 used Carrier argon gas flow/ml min-1 800 thermodynamic calculations to show that the hydroxide (gaseous and liquid) and the gaseous oxide were the precursors to HGA-600MS electrothermal vaporizer— gaseous Ba atom formation.Byrne et al.23 added oxygen to Sample volume/ml 10–20 the purge gas to produce signal shifts giving evidence in Dry step 10 s ramp 110 °C for 30 s support of the gaseous oxide dissociation hypothesis.Prell Pyrolysis step 1 s ramp et al.13 showed BaO as the precursor to Ba atom formation. 400–2650 °C for 30 s Sucient vapour pressure (a few Torr) for all oxides15 exists at Vaporization step 1 s ramp their appearance temperatures to produce a signal in ETV- 900–2700 °C for 6 s ICP-MS. Beryllium has an atomization mechanism similar to the Data Acquisition— Dwell time/ms 20 other alkaline earth elements12 with one significant dierence. Scan mode peak hopping For the alkaline earth elements (Mg, Ca, Sr and Ba), the Points/spectral peak (m/z) 1 species preceding the appearance of the free element has been Signal measurement Integrated counts the simple oxide, MO.Beryllium, on the other hand, forms a Resolution 0.7 u at 10% peak-height polymeric oxide, (BeO)n and releaseof the free element proceeds 548 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12as follows: and as high as 1230 °C. Thermochemical properties of the alkaline earth elements follow well defined trends26 and much (BeO)n(ad)�(BeO)n-1(g)+.. .+BeO(g) of the information available on Ra is currently derived from these trends. Few experimental measurements of the thermo- BeO(ad)�Be(g)+O(g) dynamic properties of RaO have been completed owing to its There are, however, some dierences in the mode of oper- high reactivity.25 Radium oxide (RaO) is a highly aggressive ation of ETV-ICP-MS that may aect the usefulness of data substance that readily attacks crucible and calorimeter mate- obtained by GFAAS studies.For example, in GFAAS, during rials and, to date, no definite material has been conclusively the signal measurement step (high-temperature step) no argon identified for which the formula RaO can be assigned.25 is flowing through the graphite tube. Production of atoms for In ETV-ICP-MS, it is often observed that for many sub- atomic absorption can proceed by the vaporization of metal stances there is sucient vapour pressure (several Torr) at the from the graphite surface to the gas phase and/or by the melting point to give rise to a signal.Radium oxide was thermal dissociation of molecular species in the gas phase. calculated to melt at 1615 °C.27 This temperature is (within For ETV-ICP-MS, there is a constant Ar flow of about error) the same as the appearance temperature (1600 °C) for 0.8–1 l min-1 through the graphite tube, which serves to Ra obtained from the vaporization of both the nitrate and remove any vaporized material (atomic or molecular) and chloride form.This supports the possibility that the oxide is carry it to the argon plasma where atomization and ionization the most likely form of Ra vaporized at the appearance takes place. For the alkaline earth elements discussed above temperature. and for both GFAAS and ETV-ICP-MS, analyte oxide is As shown above, all the alkaline earth elements most prob- vaporized into the gas phase.This does not mean, however, ably volatilize as oxides. The chemical and thermodynamic that analyte signal will be observed at the same vaporization similarities between Ra and the other alkaline earth elements, temperature for both techniques. As will be shown below, this the appearance temperature for Ra in ETV-ICP-MS and its is because, for GFAAS, the production of the analyte signal is similarity to the melting point of Ra oxide suggest a likely dependent on a high enough gas phase temperature to eect mechanism of vaporization.The evidence provided above is thermal dissociation of the oxide, whereas, in ETV-ICP-MS, incomplete and perhaps circumstantial, but, in the absence of the analyte signal will result at whatever temperature the oxide more quantitative data, a reasonable mechanism for the pro- is vaporized. This means that, in general, appearance tempera- duction of the Ra signal at the appearance temperature in the tures (defined as the lowest temperature at which the analyte ETV is vaporization of RaO(s) to RaO(g) which is then signal can be detected above baseline noise) for ETV-ICP-MS transported to the argon plasma where atomization and will always be lower or equal to appearance temperatures ionization take place.measured using GFAAS. Appearance temperatures for the alkaline earth elements measured using ETV-ICP-MS are compared in Table 2 to literature values for appearance Optimization of Experimental Conditions temperatures determined using GFAAS.Within experimental A typical ETV heating program contains at least three steps. error (±100 °C), appearance temperatures agree except for Ba A low temperature step ranging from 80 to 110 °C is used to for which the ETV-ICP-MS temperature is significantly lower. remove solvent and volatiles not containing the analyte. A This lower temperature may be attributable to the higher second step, generally called the pyrolysis step, is used to sensitivity of ICP-MS compared with GFAAS.There is no removeselected matrix components and/or to activate chemical reported value for the appearance temperature for Ra using modifiers. The third step is the high temperature or vaporiz- GFAAS. ation step which is used to vaporize the analyte. It is during this step that ICP-MS data are collected. The temperature and the rate of heating of the drying step are normally determined Mechanism of Vaporization of Ra experimentally and are usually dependent on the nature of the Radium is the heaviest of the alkaline earth elements and sample matrix.The maximum pyrolysis temperature that can shares many chemical properties with the rest of the group, be used is the temperature at which the analyte is lost in particularly Ba. When the alkaline earth elements are discussed significant quantities. In order to measure the maximum as a group, Be and Mg are usually kept separate because they allowable pyrolysis temperature for the alkaline earth elements, exhibit dierent chemistries than the rest of the group.24 a curve was constructed of the analyte signal obtained at Calcium, Sr and Ba are usually classed together, with Ra often dierent temperatures for a 30 s (1 s ramp time) pyrolysis step.paired to Ba. In each case, the same vaporization temperature (2500 °C) Radium compounds follow the general solubility trends for was used throughout. Increasing the hold time for the pyrolysis the alkaline earths (sulfate solubility decreases and hydroxide step generally decreased the temperature at which analyte solubility increases as atomic number increases, etc.) except for losses were detected.The 30 s hold time selected for this study the nitrate, which is slightly more soluble than the Ba com- is reasonable based on typical heating programs used for the pound. Thermogravimetric analysis of radium nitrate and analysis of real samples.The pyrolysis and vaporization curves carbonate25 have revealed the existence of several phases of an for Ra are shown in Fig. 1. These data show that losses begin oxide, believed to be RaO, at temperatures as low as 300 °C to occur at a pyrolysis temperature of around 1400 °C for Ra. Fig. 2 shows pyrolysis curves for the other alkaline earth elements. The pyrolysis temperatures obtained for these Table 2 Appearance temperatures for the alkaline earth elements elements (Table 3) are generally lower by several hundred degrees than those reported in GFAAS studies.This may be Appearance temperature/°C due to the much smaller quantities of analyte vaporized in Element GFAAS ETV-ICP-MS Reference ETV-ICP-MS (fg–pg) compared with GFAAS (ng–mg).32 As shown in Fig. 2, Be losses occurred at relatively low tempera- Be 1227 1200 11 Mg 1237 1200 10 tures. This may result from the low temperature vaporization Ca 1577 1500 10 of Be compounds such as Be(NO3)2 (bp 142 °C) or BeCl2 (bp Sr 1367 1300 13 520 °C) depending upon the acid used in solution.Maessen Ba 1727 1500 21 et al.33 reported that approximately 10% of the Be is lost from Ra — 1600 this work the furnace due to diusional losses before the free atom signal Journal of Analytical Atomic Spectrometry, May 1997, Vol. 12 549Analytical Figures of Merit Analytical figures of merit for the determination of 226Ra by ETV-ICP-MS are given in Table 4. The limit of detection for Ra was calculated as the mass of Ra equivalent to a signal equal to three times the standard deviation of the blank.An absolute limit of detection of 1.7 fg was obtained, which corresponds to a relative limit of detection of 34 fg ml-1 in a 50 ml sample. The analyte signal could be measured with a precision of 3.9% and 5.3% for integrated and peak-height signal measurements, respectively. Eect of Physical Carrier and Signal Suppression Due to Matrix Components The eect of using physical carriers to improve analyte transport from the graphite tube to the argon plasma was shown Fig. 1 Pyrolysis (&) and vaporization (%) curves for 0.25 pg Ra. by Hughes et al.35 In this study, the use of NASS-3 (reference seawater diluted 500 times) was shown to be an eective physical carrier for many elements resulting in signal enhancements of up to a factor of ten. The eects of NASS-3 seawater on the Ra signal (Table 5) was studied by adding 10 ml of solutions containing increasing concentrations of NASS-3 to a 10 ml aliquot of sample solution containing 0.1 ng ml-1 Ra.Pyrolysis and vaporization temperatures of 700 and 2500 °C, respectively, were used. A seawater matrix can also be used to determine the eect of added matrix components on possible signal suppression. When small quantities of salt were added, the Ra signal was enhanced by up to 30%. Upon adding greater quantities of salt, the signal was suppressed by as much as 75% when 360 mg of salt were added.Radium is present in seawater at 8.9×10-11 mg ml-1 (ref. 36) corresponding to 8.9×10-4 fg in a standard 10 ml sample. This quantity of 226Ra is well below the observed ETV-ICP-MS limit of detection for Ra (1.7 fg) and will not aect the observed signal in experiments using Fig. 2 Pyrolysis curves of the alkalineearth elements in ETV-ICP-MS (Vaporization 2600°C): Be (&), Mg (1), Ca (+), Sr (%), and Ba ($). seawater as a modifier. As was reported by Hughes et al.35 the initial enhancement eect is probably due to an increased transport eciency, Table 3 Pyrolysis temperatures for the alkaline earth elements while, at higher added salt masses, signal suppression occurs.The modest enhancement observed for Ra when small quantit- Pyrolysis temperature/°C Element GFAAS ETV-ICP-MS Reference Table 4 Analytical figures of merit for Ra Be 900 500 28 Blank— Integrated Signal Peak height Mg 900 900 29 Ca 1200 700 30 226Ra 53 94 Sr 1500 800 31 s, n=10 8 16 Ba 1500 1500 30 RSD (%) 15 17 Ra — 1400 this work 3s 24 49 Radium— 226Ra (1.4 pg) 19 600 36 737 s, n=5 760 1959 is observed in GFAAS. Vanhoe et al.28 reported loss of Be at RSD (%) 3.9 5.3 900 °C in GFAAS when no modifier was used.Meah34 also LOD (abs) fg 1.7 1.7 recorded Be losses at very low temperatures when vaporized LOD (rel) (50 ml)/fgml-1 34 34 in the presence of acids and salt matrices. Thus if all alkaline LOD/Bq g-1 1.3×10-3 1.3×10-3 earth elements were to be determined as a group by ETVICP- MS, a pyrolysis temperature of 700 °C or less should be used to prevent losses of the more volatile Be.However, with Table 5 Eect of added salt on Ra signal the use of magnesium nitrate as a chemical modifier, Be can Amount of salt/mg Enhancement* be stabilized to a temperature of 1500 °C. A second set of experiments exploring the relationship 0 1.00 between the vaporization temperature and the integrated signal 0.72 1.32 3.6 1.29 for the alkaline earth elements showed that the maximum 7.2 1.12 signal was obtained at a vaporization temperature of 2500 °C 36 0.78 or greater when a 500 °C pyrolysis step was used.As an 72 0.61 example, the vaporization curve for Ra is given in Fig. 1. Using 360 0.25 these vaporization conditions completely removes the analyte from the surface of the graphite tube with no memory eects * Defined as the ratio of the signal obtained when Ra is vaporized with salt divided by the Ra signal obtained when vaporized alone.observed. 550 Journal of Analytical Atomic Spectrometry, May 1997, Vol. 1211 Sturgeon, R. E., Chakrabarti, C. L., and Langford, C. H., Anal. Chem., 1976, 48, 1792. 12 Styris, D. L., and Redfield, D. A., Anal. Chem., 1987, 59, 2897. 13 Prell, L. J., Styris, D. L., and Redfield, D. A., J. Anal. At. Spectrom., 1991, 6, 25. 14 Duval, C., Inorganic T hermogravimetric Analysis, 2nd edn., Elsevier Publishing Co., New York, 1963. 15 Margrave, J. L., T he Characterization of High-T emperature Vapors, John Wiley & Sons, USA, 1967, pp. 555. 16 Hutton, R. C., Ottaway, J. M., Epstein, M. S., and Rains, T. C., Analyst, 1977, 102, 658. 17 Ka�ntor, T., Bezu�r, L., Pungor, E., and Winefordner, J. D., Spectrochim. 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