Anal. Chem. 1986, 58,31-35
31
Determination of Boron Isotope Ratios by Thermal Ionization Mass Spectrometry of the Dicesium Metaborate Cation Arthur J. Spivack* and John M. Edmond Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 A method for the high-preclslon determinatlon of the lsotoplc composttlon of boron by thermal ionization mass spectrometry of Cs2B,0, b descrlbed. The method Is capable of producing Isotope ratlo analysis wlth a standard deviation of 0.12%0. Procedures for the separation and chemical isoiatlon of boron from silicates and aqueous solutions are also descrlbed. Careful attention has been pald to the analysis of blanks and other procedural artifacts.
Large variations (-50%) in the isotopic composition of boron occur in nature (1-3). These variations can be used as a diagnostic geochemical tool and tracer. However, the development of boron isotope geochemistry has been severely limited by the relatively poor precision of analytical methods used to determine the isotope ratio. By modification of existing techniques a reproducible, high-precision, solid-source thermal ionization technique has been developed and used to analyze aqueous and solid-phase sample8 of geological interest. Though a variety of techniques have been used to determine the isotopic composition of boron (4-14), the NazB4O7thermal ionization mass spectrometric method (10,12) has been almost exclusively used in boron isotope geochemistry. Uncertainties ( 2 0 ) of 2-3%0 in the l1B/l0B ratio have routinely been reported (1, 2, 10, 12). The method described here, based on the thermal ionization mass spectrometry of Cs2B407,is capable of producing data with an uncertainty of 0.25% ( 2 0 ) . The mass spectrometric technique and extraction procedures for isolating boron from aqueous solutions and silicates are described in this paper. Another paper describing the thermal ionization mass spectrometry of Cs2B407has recently been published (15). In general, the experimental techniques and results are consistent with the work reported here. Both illustrate the advantages of this method compared to standard techniques. However, the work reported here contains a more detailed investigation of analytical reproducibility and of sample preparation techniques.
EXPERIMENTAL SECTION Instrumentation. Mass Spectrometry. All isotopic determinations were performed on a first-order,direction-focusingmass spectrometer that has a 60", 12 in. radius magnetic sector and operates at a nominal resolving power of -430. A Faraday cup collector and a 9.2 X 1O1O Q resistor were used. The accelerating voltage was 3900 V. Single-filament beads (Cathodeon, Cambridge, England) with tantalum filaments (0.025 x 0.50 mm) were used. Ion Chromatography. A Wescan Model 262 ion chromatograph was used with a 25-cm Anion/R column utilizing an eluant of 0.1 mM potassium hydrogen phthalate adjusted to pH 11.5 with sodium hydroxide. Pyrohydrolysis. The complete pyrohydrolysis apparatus is shown in Figure 1. The furnace is a Lindbergh Model 54233 with a Model 54545 controller. The combustion tube is 99.8% alumina manufactured by McDanel Refractories (Beaver Falls, PA) with an outer diameter of 3/4 in. and an inner diameter of in. The connectors are Cajon Ultratorr made of 316 stainless steel, which
we machined to a constant inside diameter just slightly larger than 3/4 in. The condenser and boiling flask are made of fused quartz. Ultraviolet Photooxidizing Apparatus. The UV photooxidizing apparatus consists of a vertical 30-cm, 1250-W,UV lamp, radially surrounded by 150-mL, 3 cm diameter fused quartz sample tubes at a distance of 7.5 cm. Beakers. All beakers were made of either FEP or TFE Teflon. Small-volume (1 mL) concave-bottom beakers were made by drilling out a 5/8-in.rod made of Teflon. Evaporation Apparatus. Aqueous solutions were evaporated under filtered air on a hot plate. An infrared lamp was used to prevent condensation on the surface of the evaporator. The filters consisted of two sodium hydroxide impregnated quartz fiber filters in series prepared according to Fogg (16) followed by a 0.45-pm Millipore filter. Subboiling Still. A subboiling still utilizing filtered forced air was used. The boiling flask and condenser are made of fused quartz. The filters used are the same as described for the evaporation apparatus. Trimethyl Borate Apparatus. A 250-mL quartz round-bottom flask was connected to an 8-in. Graham condenser. A sample injection tube fitted with a stopcock made of Teflon extended to 1 cm from the flasks bottom. Reagents. The boron standard used for concentration and isotope determinations is National Bureau of Standards standard reference material (NBS SRM) 951 boric acid, certified for total boron, *OBand IIB. Cesium hydroxide was obtained by ion exchange of spectroscopically analyzed (>99.9%) cesium chloride using Bio-Rad AGl-x8 in the hydroxide form. Water was distilled at 65 "C following conventional deionizing by ion exchange and distillation. The subboiling distillation apparatus described earlier was used. A mixed bed resin composed of a 1:1v/v mixture of Bio-Rad AG3-X4 and AG50-x8 in the hydroxide and proton forms, respectively, was used in a 4.5-cm polypropylene column with a porous polyethylene frit. Anhydrous, reagent grade methanol and sulfuric acid were used in the trimethyl borate isolation. Procedure. Mass Spectrometry. Prior to sample loading the tantalum filaments were outgassed for 20 min with a current of 2.7 A ac at lo4 torr, allowed to cool, and heated again at 2.7 A ac for 10 s. The filaments were then allowed to oxidize in the ambient atmosphere (protected from contamination in closed boxes) for at least 3 days prior to use. Cesium hydroxide solution was added to the boron-containing solution to give a B/Cs mole ratio of 2:l. In samples, boron concentrations were first determined by ion chromatography. In the standard procedure a drop of -2 pL containing between 1.2 X and 5.0 X mol of boron was placed directly on the filament from a capillary tube made of Teflon in a laminar flow clean box. The drop was dried with a current of 0.7 A ac for 3 min, heated for 10 s at 2.0 A ac, and then held for 30 s at 1.7 A ac. The boron species analyzed were the molecular ions Cs;OBO2+ and Cs2"B02+ at m/e 308 and 309, respectively. Because Cs ionizes at a lower filament temperature it was used for focusing the ion beam. The Cs+ ion current was slowly increased over 15 min to 3 X A at which point the Cs2'lB02+ion current was A. The filament current was increased in steps typically 5 X of 0.0025 A over a period of 30 min until the Cs2"B02+ion current was approximately 1.2 X A. The ion current for a 5 X mol sample could be maintained at this intensity for approximately 4 h before it was exhausted. The temperature of the filament could not be determined accurately by an optical pyrometer as the temperature was relatively low. The filament was just perceptively dull red.
32
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
B
I
Flgure 1. Pyrohydrolysis apparatus: A, quartz boiling flask: B, Cajon Ukratorr fitting: C, furnace; D, quartz condenser; E, alumina combustion tube.
Data were acquired by cycling the magnetic field through three steps: focusing at m/e 308,309, and a base l i e of 306.5. A settling time of 2.5 s was used with two 1-s integrations on each m/e. Typically 150 ratios were collected in blocks of 6. All the data from a block was discarded if the mean deviation of the calculated ratios in the block relative to the block average was greater than 1.5%0.Peak overlap was examined by comparing the zero corrected signal at m/e 308.5 to that at m/e 308. This ratio was less than 2.5 X Chemical Separation and Purification. The chemical procedures were intended to extract and isolate boron quantitatively in an aqueous solution with a minimum of other dissolved species. The procedures used depended on the bulk composition of the sample. Three separate techniques were developed: one for silicates, one for aqueous solutions with low dissolved silica and organic carbon concentrations, and one for solutions with high dissolved silica and organic carbon concentrations. The steps that were used for aqueous solutions containing low dissolved organic carbon and silica concentrations (Si/B < 0.02) comprise part of the procedure used for all samples and are described first. The additional procedures for each of the other types of samples follow. The mixed bed resin was used for separating boron from dissolved ions. A &fold or greater excess of resin was used. The resin bed was rinsed with at least 40 bed volumes of the subboiling water at a rate of 0.5 mL/min. The resins had previously been treated with solutions of 1 N hydrochloric acid and 1N sodium hydroxide and then rinsed repeatedly in bulk with conventional distilled water to remove fines. The sample was eluted at a rate of 0.5 mL/min with 15 bed volumes of subboiling water followed by photooxidation for 12 h. This solution was then evaporated at 70 "C in the evaporation apparatus to 0.5 mL. It was then transferred to the 1-mL concave-bottom beaker and evaporated to dryness. The sample was redissolved in 15 fiL of subboiling water, evaporated to 2 fiL, and then loaded on the tantulum filament. Solutions containing high concentrations of dissolved silica (Si/B > 0.02) or organic carbon required an additional isolation step. A modified version of the classical trimethyl borate s pa ration was carried out. The 150 mL of anhydrous methanof, 12 mL of anhydrous sulfuric acid was added slowly in the roundbottom flask. This mixture was boiled in the open flask for 30 minutes at a rate of 0.75 mL/min in order to reduce the blank. At that point the injector/connector, which had been rinsed in distilled water and completely dried, was placed in the flask. After 15 min the condenser was connected. The distillate was collected in 50 mL of subboiling water in a 100-mL beaker made of Teflon. Up to 3 mL of sample was rapidly added through the injector into the flask. The distillate was collected for 45 min at a rate of 0.75 mL/min. Higher rates should be avoided. When higher rates were used involatile organic material was collected along with the tr imet hyl borate. This solution was evaporated in the evaporation apparatus at 50 O C until it was reduced to 1 mL. The previously described ion-exchange ,did photooxidation procedures were then followed.
Boron was extracted from silicates by pyrohydrolysis, using the apparatus shown in Figure 1. Most samples, either powders or fragments, were placed in a platinum boat and pushed into the furnace at 500 "C. Basaltic silicates were loaded into a 25 cm long, 10 X 12 mm fused quartz glass tube, which was open on both ends. With steam passing through the system (water distillation rate of 20 mL/h), the furnace temperature was raised t o 1400 "C. The condensate was collected in a beaker made of Teflon. The time required for extraction depended on the type and size of sample. Fractions were collected hourly, and concentrations of boron were determined by ion chromatography until the boron concentration of the condensate was less than 1% of the integrated total. At 1400 "C, 1 h was required to extract >98% of the boron from 0.10 g of a synthetic glass of tholeiitic basalt composition while 8 h were needed for 7.0 g of an oceanic tholeiite basalt. Collected fractions were combined, and the boron concentration was determined by ion chromatography. For isotopic determination the collected solution was evaporated to 3 mL in the evaporating apparatus. The previously described procedures for high organic carbon and silica samples were then followed. Between analyses, the alumina tube was baked at 1490 "C for 6 h with a water distillation rate of 3 mL/min. New tubes required baking out for 1 day to 2 weeks to reduce the boron blank.
RESULTS AND DISCUSSION The precision and accuracy of a mass spectrometric technique utilized in the analysis of samples of complicated compositions are limited in a general sense by two things: artifacts introduced during sample preparation and the precision and accuracy of the mass spectrometric technique itself. Careful attention has been paid to each step of the analytical procedure to determine the level of contamination and isotopic fractionation introduced. Notation and Oxygen Isotope Corrections. NBS SRM 951 boric acid, a boron isotope standard, was used in all experimental development work except where specifically stated. In this paper delta ( P B ) notation is used to express isotope ratios. It is defined as a per mil deviation in the l1B:loB ratio from SRM 951
where R(st) is the I1B:loBratio of SRM 951 and R is the l1B:loB ratio of a sample. The actual P B values are calculated as
611B(%o) =
[ '$ ] -1
X
lo3 - 0.19
(2)
where R(309) is the measured mass 309/308 ratio of a sample. The subtraction of 0.19 is to correct for the distribution of oxygen isotopes. The oxygen isotope correction was derived following the work of Craig on carbon isotope measurements (17). In this section the symbol R is used for both isotope ratios and for mass ratios measured by the mass spectrometer. The oxygen isotope ratios are always molecular. Two sets of subscripts are used denoting the particular ratio and the material to which the ratio refers. The terms are
Rll = I1B/l0B
(3)
(4)
+
Csz11B160160+ Csz10B160170+ '309
=
Csz10B160160+
(5)
ANALYTICAL CHEMISTRY, VOL. 58,
NO. 1,
JANUARY 1986
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Table I. Measured NBS SRM 951 I1B:l0BRatio, Oxygen Isotope Corrected
The subscript (x) denotes any sample analyzed while the subscript (st) denotes NBS SRM 951. After rearrangement the following relation is derived:
First, consider the magnitude of the second term relative to the 20 statistical uncertainty of the measurement, which typically is -0.2%0. Assuming that R17(st)is approximately the ratio expected from the natural abundance of "0 &(st) = 7.4 X W4),R1,(x) would have to differ from it by 1000%0 to make the whole term of the same magnitude as the statistical measurement uncertainty. While this seems like an extraordinarily large and unlikely isotope variation, the variation of the oxygen isotope composition of the measured Cs2BO2+still must be considered. Both samples and standards are dissolved and loaded onto the tantalum filament in aqueous solution. Isotope-exchange experiments have shown that borates rapidly exchange oxygen with water (18). Since the mass of oxygen in the water greatly exceeds that in the borate species, the oxygen isotope composition of the borates will reflect that of the water. Thus the difference in R1,(x) and R17(st) depends on the difference in the oxygen isotopic compositions of the water in which the sample and standard are loaded. If the same water is used in processing both samples and standards the whole term will cancel. In this study, since all the water used was from a single batch, this term was completely neglected. However, there will be differences in the oxygen isotope ratio in water used in different labs. If we take 5% as the extreme variation likely to occur between labs for the 170/160 ratio an approximately 0.02%0 error is introduced, which is small relative to the measurement uncertainty. The first term, after rearranging, gives
Based on the measurement of the Cs211B1s0170+/ Csz10B160160+ ratio using boron enriched in "B, R17(~t)= 0.000 78; thus, the correction factor is 0 . 1 9 % ~ Mass Spectrometry. In the mass spectrometric analysis a constant stoichiometric B:Cs ratio was used as it has been shown that the measured boron isotopic composition of sodium borates determined by thermal ionization is influenced by the B:Na ratio (19). To examine the possibility of artifacts due to interfering ions, the m / e range from 20 to 350 was scanned while the was just below detection and while it was C S ~ ~ B ion O ~signal + 5.2 X 10-l' A. Besides the Cs+ and CsZBO2+ion beams, the only other detectable signal was at m / e 285 in samples evaporated in beakers made of Teflon, most likely due to Cs2F+. Standards not evaporated in beakers made of Teflon did not exhibit this peak. The measured ratios at 5.2 X A and at 2.1 X 10-l1A were indistinguishable to within 0.2%0. The extent of isotopic fractionation during the course of the mass spectrometric analysis was a major concern. The relative mass difference of the two isotopes is large and there are no other boron isotopes to use for normalization. This problem was examined by looking for drift in the measured isotope ratio during an analysis. Of the approximately 100 analyses of standards and samples, only five exhibited in-run isotopic fractionation that could be resolved from the ana-
analysis no.
cor. l1B:loB measd ratio
1 2 3 4 5 6 7
4.045 76 4.045 47 4.045 43 4.045 68 4.046 66 4.045 98 4.04565
analysis no.
cor. l1B:loB measd ratio
10 11 12
4.045 98 4.044 80 4.045 21 4.045 35 4.045 04
av.
4.045 58 +Z 0.000 33"
8 9
95% confidence limit.
lytical uncertainty. In all other analyses, including runs where the isotope ratio was measured until the ion current was exhausted, no fractionation could be resolved. This observation contrasts with the thermal ionization mass spectrometry of NazB407,for which an in-run drift of 1.5%0was typically observed during the collection of 300 ratios. It thus appears that the isotopic fractionation observed during the thermal ionization of alkali borates is dependent on the alkali and most probably on its mass. In addition to reducing fractionation, a second advantage of using CsZB407instead of NazB407is that it avoids the interference at n / e 88, which is isobaric with
Wr+. SRM 951 was repeatedly analyzed to determine the precision and reproducibility of the mass spectrometrictechnique. The 2a standard deviation based on 12 separate analyses was 0.24%0, which was just slightly larger than the average ~ U / M uncertainty of each analysis, 0.19%0,where each analysis consisted of 150 ratios (Table I). The oxygen isotope corrected average measured llB1% ratio of SRM 951 was 4.045 58 f 0.000 33 (95% confidence limit). The stated composition on the certificate of analysis supplied with SRM 951 is 4.044 f 0.003 where the confidence limit includes terms for inhomogeneities in the material as well as analytical error. However, in a published NBS report, which contains a detailed description of methods and data, the absolute ratio is reported as 4.04362 f 0.00137 (95% confidence limit) (18). The overall limit of error of this more precise number is the sum of the 95% confidence limits for the ratio determination, correction factor, and chemical analysis. Our measured ratio corresponds to a 0.48 f 0.08 offset from the NBS absolute ratio. This difference is within the error limits that are stated on the certificate of analysis, f0.8%0, but is outside of the error limits stated in the NBS report, fO.34%0(18). Thus, even though we have determined the ratio precisely there does appear to be a systematic offset from the absolute ratio. Evaporation Yields. To determine recovery yields and possible isotopic fractionations during drying and redissolution, 1-mL solutions were evaporated to dryness at 65 "C and redissolved in 1 mL of distilled water. Recovery yields were 100 f 1% with no resolvable isotopic fractionation. Evaporation of samples was carried out under filtered air as it has been shown that there is significant gaseous and particleassociated boron in the ambient atmosphere (16). The filters that were used have been shown to be effective in removing atmospheric boron (16). In our experiments less than 1X mol of boron was picked up as a blank during the evaporation of 50 mL of subboiling water. During the evaporation of 100 mL of an aqueous solution containing 5 X mol of boron to 1 mL, 97 h 1% of the boron was recovered. Though there was a small loss of boron, no isotopic fractionation was resolvable. Ion Exchange. There is a tremendous potential for isotopic fractionation during ion exchange. In fact, ion-exchange techniques are used to produce enriched loB and llB. It has
/~
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
Table 11. Measured P B of Seawater analysis no.
PB 39.45 39.34 39.30 39.70 39.68 39.32 39.48 39.80 39.60
av
39.52 f 0.13"
"95% confidence limit. been shown that the single-stage separation factor for boron using a secondary amine resin similar to AG3-x4 is approximately 1.02 (20). Boron was eluted by water as a broad peak with 99 f 1% of the boron eluted by 35 mL when 5 mL of the previously described mixed bed resin was used. To ensure complete recovery and avoid fractionation, twice this volume of water was used during sample preparation. When smaller quantities of resin were used the volume of eluant was scaled proportionally with resin volume. No resolvable isotopic fractionation was introduced by this procedure. Water Distillation. Ion exchange and pyrolysis require large volumes of water. Thus water with a reproducibly low blank must be available. Water that was passed through a mixed bed resin followed by distillation in a vycor still exhibited high blanks, between 50 and 100 nmol/L. This was not surprising since vycor glass is 3% Bz03by weight. Distillation in a fused quartz still was also unsatisfactory. When water was distilled to half its initial volume the concentration of boron in the distillate the approximately one-third of the initial concentration, the boron being volatilized during boiling. A relatively low-temperature, high-rate still was assembled from fused quartz. I t was operated a t 65 OC as recovery experiments showed boron to be only slightly volatile a t that temperature. Heated air was forced through the still to increase the distillation rate. The air was stripped of boron by passing through the previously described filters. Water with a boron concentration of
