2228
Anal. Chem. 1984,56,2228-2233
Registry No. HCB, 118-74-1;TCP, 95-95-4.
(8) Weiss, M.; Karnofsky, J.; Hass, J. R.; Harvan, D. J. 27th Annual Conference on Mass Spectrometry and Allied Topics, Seattle, WA, June
LITERATURE CITED (1) Bristol, D. W.; Crist, H. L.; Lewis, R. G.; MacLeod, K. E.; Sovocool, G. W. J. Anal. Toxicol. 1982,6 , 289-275. (2) Lakings, D.; Subra, W.; Going, J. Task 12 Final Report, July 29, 1980, EPA 560113-80-030. (3) Edgerton, T. R.; Mozeman, R. F.; Linder, R. E.; Wright, L. H. J. Chromatogr. 1970, 170, 331-342. (4) Wright, L. H.; Edgerton, T. R.; Arbes, S.J., Jr.; Lores, E. M. Biomed. Mass Spectrom. 1081, 8 , 475. (5) Brotherton, H. 0.; Yost, R. A. Anal. Chem. 1983, 55, 549-553. (8) Shabanowitz, J. S.;Hunt, D. F. J. Chromatogr. 1083, 271,93-105. (7) Harvan, D. J.; Hass, J. R.; Wood, D. Anal. Chem. 1982, 5.4, 332.
1979. (9) Hass, J. R.; Friesen, M. D.; Harvan, D. J.; Parker, C. E. Anal. Chem. 1078, 5 0 , 1474-1479. (IO) Hass, J. R.; Harvan, D. J., NIEHS, unpublished data, 1983.
Received for review ~ ~2, 1984. ~ ~~~~~~~d i l May 29, 1984. The University of Florida gratefully acknowledges a research assistantship provided by Occidental Chemical Corp. and support from the National Science Foundation for this research.
Determination of Iron in Serum and Water by Resonance Ionization Isotope Dilution Mass Spectrometry J. D. Fassett,* L. J. Powell, and L. J. Moore Center for Analytical Chemistry, National Measurement Laboratory, National Bureau of Standards, Gaithersburg, Maryland 20899
Resonance lonlzatlon mass spectrometry has been used in conjunctlon with Isotope dilution to determine the iron content of SRM 909 (Human Serum) and SRM 1643b (Trace Eiements in Water). Iron was thermally vaporlzed from a fliament at 1250 K. A one-wavelength, two-photon ionlzation scheme was employed utilizing UV light at 283.6 nm provlded by a NdYAG pumped dye laser with frequency doubling. The iinearlty of the detectlon system was verified by the determination of the 57Fe/56Feratios in a set of gravimetrically prepared Isotopic calibration mixes. The precision and accuracy of the measurements were typically 2-3 % The maw spectrometrlc loading blank is presently the ilmiting source of error. The concentratlon and uncertainty (95 % confldence limits) of Fe in SRM 909 are 1.94 f 0.29 pg/g; in SRM 1643b they are 99.2 f 8.3 ng/g.
.
Resonance ionization mass spectrometry (RIMS) is a relatively new technique that combines optical spectroscopy with mass spectrometry (1-3). The technique exploits lasers to selectively and efficiently ionize gas phase atomic species. When the laser is tuned to a discrete resonant electronic transition of an element, the ionization probability increases many-fold due to the stepwise absorption of photons. Most of the initial applications of RIMS have been made with modified thermal ionization mass spectrometers (TIMS). RIMS can be applied to TIMS measurement problems where lack of sensitivity (failure to produce ions) and/or nonselectivity (isobaric interferences) results in fundamental limitations. The RIMS potential is realized here by its application to the measurement of iron, an element for which thermal ionization is particularly insensitive. TIMS is characteristically a highly accurate and precise method of making isotope ratio measurements which, when combined with isotope dilution (IDMS), results in highly accurate and precise analytical measurements. Because of its inherent accuracy, IDMS has been used to certify the concentrations of a broad range of elements from ultratrace to macro levels in a variety of matrices ( 4 ) . Making accurate and precise isotopic measurements with RIMS is complicated by two factors relative to TIMS. The
first factor results from the pulsed nature of the laser, which produces a pulsed ion current to be quantitatively measured. As is done in spark source mass spectrometry (5) and laser microprobe mass analysis (6),the pulsed ion current is measured by an electron multiplier operating as a current amplifier. This mode of detection is limited in dynamic range and is less precise than continuous methods of detection. In this respect RIMS systems based on continuous wave (CW) lasers (7)should have a measurement advantage relative to pulsed systems. However, as is demonstrated, pulsed RIMS systems possess the ability to discriminate against isobaric interferences from thermally ionized species, an important advantage. The second factor complicating accurate and precise isotope ratio measurements using RIMS is isotopic splitting of energy levels used in the RIMS process. Isotopic splitting results in different ionization efficiencies for the isotopes of an element at a given wavelength setting of the laser. The extent of this effect will be dependent upon the element, laser, and transition, but must be considered in any quantitative measurement. Donohue et al. have published the only RIMS isotopic ratio measurements made to date (B), demonstrating the increased selectivity of resonance ionization over thermal ionization for Nd and Sm. We have calibrated our detection system by measuring gravimetrically prepared iron isotopic calibration mixes. The possibility of an isotope effect has also been investigated. The iron content in serum (SRM 909) and water (SRM 1643b) has been determined by spiking with 57Feand making isotope dilution measurements using the 57Fe/66Fe ratio. The previously certified SRM 498 Copper V was also analyzed to verify the accuracy of the Fe RIMS measurements. The ability to measure Fe isotopic ratios has an important application in nutritional research. Iron is an essential trace element in man, and thus, its bioavailabiilty is of critical interest. Bioavailability studies have been carried out using the radioactive tracer 69Fe (9). However, the use of this radiotracer and others in nutrition research is not desirable in many human population groups, such as infants and pregnant women, where the most significant problems occur. Thus, the nutrition field has been increasingly turning to the use of stable isotopes (10). Human studies have been conducted
This article not subject to U S . Copyright. Published 1984 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
utilizing neutron activation analysis to detect %Fe (11). Mass spectrometric measurements have been made on iron using a chelation-gas chromatography-organic mass spectrometry method (12). The precisions of the chelation methods of trace element detection have never approached those routinely achieved in TIMS. This imprecision translates into an increased requirement for ingestion of the stable isotope because of the need to significantly alter the natural isotopic composition. Stable isotopes are becoming increasingly expensive and difficult to obtain (13). A sensitive, precise, and accurate method of determining Fe isotopic ratios would be of obvious importance. The elemental selectivity of RIMS would add a further dimension to stable isotope nutritional studies. Potential interferences exist for two of the lower abundance isobars of iron: 54Feby "Cr (2.36% abundant) and 5sFe by 5sNi (68.3% abundant). Since these isotopes could be used in addition to 57Fein a nutritional isotope dilution study, their inherently interference-free measurement by RIMS would be an advantage. Thermal ionization of Fe has been done by Garner and Dunstan (Powell) ( 1 4 ) using a triple filament procedure and Faraday cup detection. The amount of iron required for measurement was 20-30 pg. The analytical blank and mass spectrometric loading blank were major problems in their work. Stukas and Wong (15) have reported a single filament, silica gel, thermal ionization procedure. Although nanogram detection limits for iron are reported, the blank for iron determined from a kilogram of seawater was 40 ng, and isobaric interferences at masses 56 and 57 hindered measurements. The amount of iron loaded on the thermal filament in this RIMS study was 2-3 pg with the loading blank proving to be a serious problem. In contrast to TIMS, the sensitivity of the RIMS did allow the loading blank to be measured with confidence. Significant improvement in the magnitude of the loading blank was achieved in this experiment. Mass spectrometric methods which couple high ionization efficiency with blank control have demonstrated extraordinary precision and accuracies in ultratrace measurements (16). Thus, RIMS isotope dilution has the potential for revolutionizing the ultratrace measurement of iron.
EXPERIMENTAL SECTION Reagents. High-purity reagents, produced by subboiling distillation ( 1 7 ) at NBS and stored in Teflon bottles, were used in this study. Instrument. The instrument consists of three basic components: a laser system capable of producing tunable UV radiation; a magnetic sector mass spectrometer with a suitably modified source; and a detector system capable of measuring the pulsed ion currents produced in the experiment. The instrument is outlined in Figure 1and has been described in detail previously ( 1 , 18). The laser consists of a Molectron Corp. MY-34 Nd:YAG laser with second and third harmonic generation, capable of producing 350-mJ pulses at 532 nm. This laser is used to pump a Molectron Corp. DL-18 tunable dye laser and operates at 10 Hz. Rhodamine 6G dye (R6G) was used in this study to access the wavelength region 558-578 nm, frequency doubled to 279-289 nm with a typical pulse energy of 3 mJ. The laser is automated to provide wavelength scanning with a step size of 0.5 pm in the UV. The mass spectrometer is a 60°,15-cm magnetic sector thermal ionization instrument whose source was modified to allow for the entry and exit of laser radiation, parallel to the thermal filament. No focusing of the laser light was done in this work. The central axis of the laser beam, with a nominal diameter of 2.5 mm, passed within 5 mm of the vaporizing filament. The ions were detected with an electron multiplier operated at a gain of IO5. For the majority of isotope ratio measurements made, the output of the electron multipier was directed into a vibrating reed electrometer. The potential developed across a 1O1O D resistor was measured by an integrating voltmeter (Hewlett-Packard
2229
Auto Tracker
4
I
I
I
Tunable Laser
TI
-
Laser
Figure 1. Box diagram of RIMS system: M.S., magnetic sector; Cal., calorimeter; DVM, digital voltmeter; VRE, vibrating reed electrometer; Comp., computer.
5328A). The 1-s integrated voltages were transferred via an IEEE-488 interface to a programmable calculator equipped with standard isotope ratio software. A second measurement pathway based on the use of a boxcar averager is also available (1). The output of the electron multiplier can be directed to a preamplifier and the boxcar averager. The boxcar averger integrates the voltage in a preset time window which is made to correspond to the width of the ion pulse. The position of the time window is determined by the ion time-of-flight through the mass spectrometer. The output of the boxcar averager is also connected to an integrating voltmeter, similarly interfaced to a programmable calculator. Sample Preparation. The freeze-dried serum was reconstituted with 10 mL of diluent according to the protocol dictated by the SRM 909 certificate. One-gram samples were weighed into 30-mL quartz beakers, spiked with 57Fe,and digested with 2 g of 1mol/L HCIOl and 3 g of 16 mol/L HN03 The solutions were evaporated to dryness, the residues were redissolved in 5 g of 6 mol/L HC1, and each solution was loaded onto 1 cm3of AG 1x8 anion exchange resin. The column was eluted with 6 mol/L HCl until a flame test indicated that all the sodium had been eliminated. The purified iron was then eluted with 10 g of 0.2 mol/L HC1 and the solution was evaporated to dryness. Thirty grams of each water sample was weighed into poly(methylpentene) beakers, spiked with 57Fe,and evaporated to dryness. The residue was converted to the chloride form by repeated evaporations with HC1. The residue was redissolved in 5 g of 6 mol/L HCl and loaded onto 0.5 cm3 of AG 1x8 anion exchange resin. The other elements were eluted with 20 g of 6 mol/L HC1 and the iron was eluted with 10 g of 0.2 mol/L HCl. All chemically separated iron samples were prepared the same for mass spectrometric measurement. After treatment with HN03 to help destroy any organic residues, the iron was converted to FeC13and diluted to a concentration of 100 gg/g with 19 (v/v) HN03. A 1 0 - ~ L aliquot containing 1 gg of Fe was dried with a heat lamp on the Re filament in air. The dried sample was then reduced by heating in a hydrogen atmosphere for 1 min at 750 "C.
RESULTS AND DISCUSSION Atomization. The systematics of the resonance ionization process for iron and a comparison with thermal ionization have been articulated previously (19). It is estimated that RIMS is lo7 more sensitive than thermal ionization at 1250 K from the Saha-Langmuir equation, the Boltzmann equation, and the duty cycle of the ionization/vaporization process. This improvement in sensitivity is dominated by the Saha-Langmuir factor which predicts that iron vaporizes primarily as a neutral species because of its high ionization potential, 7.9 eV. The n'/no ratio is calculated from the Saha-Langmuir
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
Table I. Resonance Ionization Transitions of Iron, 279-289 nm
wavelength, nm
Wavelength, nm
Figure 2. Relative "Fe' signal vs. wavelength, 279-289 nm; laser energy nominally 3 mJ.
equation to be 2 X at 1250 K. As predicted, no thermal iron ions are detected at this temperature, although the detector system becomes saturated by the RIMS signal. The fact that iron has a number of low-energy electronic levels results in a fractional reduction of atoms leaving the filament in the ground state. The Boltzmann equation predicts that 53.3% of the iron atoms are in the ground state when vaporization occurs at 1250 K. There are four low-energy levels of iron with fractional populations greater than 1% . Transitions from these levels are readily observed in the resonance ionization spectra (18). For the experimental RIMS system used here the laser repetition rate is a significant limitation to the sensitivity. This duty cycle is determined by the diffusion time of the atoms, the size of the laser focal volume, and the repetition rate of the laser, and is estimated to be 4.1 x 10" (19). The sample utilization efficiency can be increased in RIMS by better matching the ionization and vaporization processes. this increase can be achieved by either using higher repetition rate lasers or pulsed vaporization (20). Resonance Ionization. The RIMS system in this laboratory was developed around an ionization scheme in which two photons of equal wavelength are absorbed. This scheme requires that the resonant level be more than halfway to the ionization continuum and it is applicable for the ionization of roughly 50 elements with resonant transitions in the range 260-355 nm (21). Iron has five ground-state-originating transitions in the frequency-doubled spectral range of the dye used, R6G. The resonance ionization spectrum of iron was determined by scanning the wavelength vs. the ion signal. The spectrum is illustrated in Figure 2, and the peaks are identified in Table I. In addition to the five ground-state-originating transitions, there were transitions arising from the low-energy electronic levels. The most intense wavelength a t 283.6 nm was used for all isotope measurements. Our calculations indicate that the present laser system with 2-3 mJ/pulse would be capable of saturating the ionization process; that is, at the intersection of the atom and laser beams, within the acceptance volume of the mass spectrometer, all atoms resonantly excited should be ionized. The limiting factor in ionization wing this scheme is the probability of absorption of the second photon since photoionization cross sections are typically 3 orders of magnitude less than the cross sections of bound-bound transitions. The relationship between laser power and ionization signal was experimentally checked for the resonant transition at 283.6 nm with and without a 30-cm focusing lens. Saturation was observed with the lens in position and was not observed without the lens. The effect of the differing geometrical overlap of the laser and atom beams with and without the lens in position was also demonstrated. Although saturation did not occur without the lens, the ionization signal was approximately twice an intense as the ionization signal obtained
279.50 280.31 280.72 282.08 282.57 282.60 282.79 283.55 284.04 284.21 284.39 286.39 286.93 287.42
initial state energy, cm-l
final state energy, cm-'
0 416 0 416 0 704 416 0 416
35 768 36 079 35 612 35 856 35 379 36 079 35 768 35 257 35 612
a
a
704 704 416 0
35 856 35612 35 257 34 782
intens 2.50 1.17 2.28 1.33 8.99 1.11 3.81 10.32 3.79 0.12 1.56 .21 2.48 5.26
'Undetermined transition. with the lens. This increase is explained by an increased geometrical overlap of the laser and atom beams without the lens. An estimate of the absolute efficiency of the ionization process would require a mapping of the plume emitted from the filament and a knowledge of the overlap of the laser and this plume. These experiments suggest that the majority of the atoms in the atom plume are ionized and that by carefully engineering the geometry of the system, 100% ionization per laser pulse of a thermally produced atom reservoir is feasible. Saturation of the ionization process is an extremely desirable attribute in precise and accurate isotope ratio measurement, since pulse-to-pulse variations in laser power would not be reflected in the pulse-to-pulse variation in the ion signal. Isotope effects must be considered in any RIMS measurement process. Energy level shifts caused by isotope effects are small except for the very light, rare-earth, and actinide elements (22). Iron was not expected to exhibit an isotope effect since it is at a minimum of the curve of isotope shifts vs. atomic number and the bandwidth of the laser (specified as 0.3 cm-I at 560 nm) is large relative to this minimum. However, the possibility of an isotope effect between 57Feand 56Fewas checked by scanning the laser wavelength over the resonance line with the highest step resolution for both masses. When the results were normalized and compared, no measurable shift in the wavelength position could be detected. The ratio of 57Fe/56Fevs. wavelength was constant across the resonance line, whose half-height bandwidth was 0.02 nm (2.4 cm-l). Measurement System. Two distinct methods of ion measurement were investigated. The first method utilized a boxcar averager and time discrimination of the ion current. The second method used an electrometer with no time discrimination of the pulsed RIMS ion signal. The use of the boxcar averager to quantitatively measure the pulsed ion currents presents a number of difficulties. The ion pulses from the electron multiplier are directed into a preamplifier which stretches the ion pulses to 600 ns (half height). These pulses are asymmetric and possess a negative tail. The positions of the 57 and 56 mass peaks differ by 80 ns because of the differences in the time-of-flight of the isotopes. These facts describing the ion pulses, combined with the fact that the boxcar is not designed to make this type of measurement and does not possess a strictly linear response in its time-gated window (23),strongly suggest that the boxcar be carefully evaluated before routine use. The boxcar window could be made larger than the ion pulse widths, encompassing both isotopes, but the nonlinear response of the boxcar and
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
2231
Table 11. RIMS Measurement of Isotopic Calibration Mixes 57/56 iron ratio mix
calcd
found
RSD"
error
1 2 3 4 5
14.83 4.697 0.8778 0.05938 0.0231
14.661 4.528 0.8997 0.05874 0.02323
1.1 1.2
-1.1 -3.6 +2.5 -1.1 +0.6
1.7 0.5 2.2
Relative standard deviation of measured ratio sets.
Mass
Figure 3. Resonance ionization mass spectrum of an iron sample, spiked with 100 ng of 57Fe(laser wavelength 283.6 nm; magnetic field scanning, 1 s per point and 500 s total scan times): (a) signal measured using electrometer and no time discrimination of ion signal; (b) same si nal using a boxcar averager with window of 500 ns centered
on 5 B 9~. e
the negative deviations in the ion pulse shapes imply that the position and width of the boxcar window are critical. The extent of this problem was estimated by defining the ion pulse shapes and positions of the 56 and 57 masses as determined by scanning the boxcar window across the respective ion pulses. For a boxcar window of 500 ns, centered between the times-of-flight of 56 and 57, there is no error in the resulting ratio measurement. A 40-11s error in the boxcar position (gained by optimizing the signal at a particular mass position) results in a 2.0% systematic error in ratio measurement; a 100-ns error results in a 5% systematic error in ratio measurement. The boxcar was not used here because of the difficulty in optimally setting and maintained the position of the boxcar window, as the RIMS instrument is now configured. This decision, however, does not mean that a boxcar averager or other critically designed time-gated detection system cannot be used to make quantitative ratio measurements. In fact, time discrimination provides a valuable measurement advantage in the discrimination of potential thermal ion interferences. This fact is demonstrated in Figure 3 where the iron mass spectral region is scanned: in Figure 3a the signal was directed to the electrometer where no time discrimination occurs; in Figure 3b the signal was directed to the boxcar with a window of 500 ns. The sample was a natural iron sample spiked with an excess of 57Fe. The mass 58 peak is a hydrocarbon peak that is efficiently thermally ionized (24). Less intense hydrocarbon components are unresolved and are seen as shoulders on the high mass sides of the 57Feand =Fe peaks as well. The boxcar provides a 5 X lo* discrimination factor against thermally produced ions. The large 58 hydrocarbon interference is calculated to be more than 2 orders of magnitude below the 58Fe signal in Figure 3b. Thus, time discrimination is required for the accurate measurement of 58Fe in the above example. The 57Fe/56Feion ratios were quantitatively measured by using an electrometer combined with magnetic field switching to alternatively bring the two iron isotopes to the detector. This type of measurement system is successfully utilized in spark source mass spectrometry (5). Although the duty cycle of the laser source is less than the spark source, the instan-
taneous ion currents are similar. The determination of the base line correction is simplified in the RIMS measurement process. The background is determined at the same magnetic setting as the measurement by turning off the laser. The correction for any isobaric interferences due to thermally ionized hydrocarbon species is also made in this process. Hydrocarbon isobars consistently appeared at the 57 and 56 mass positions. No measurements were made if their magnitudes were greater than 1% of the RIMS signals. These hydrocarbons would slowly disappear during the course of the measurement. The RIMS iron signal, in contrast, would typically rise over the course of measurement and could be maintained for many hours. Both the linearity and dynamic range of the RIMS ratio measurement system were investigated. Calibration standards were prepared gravimetrically from a solution of 57Fethat was previously calibrated and a freshly prepared standard solution of natural iron. The 57Fe/56Feratios measured ranged from the spike ratio of 14.83 to the natural ratio of 0.0231, a dynamic range of 50 in relative ion intensity and 650 in isotope ratio measurement. The results are summarized in Table 11. The standard deviation between ratio sets (internal precision), ranged from 0.5 to 2.2%; the accuracy relative to the Calibration values ranged from -3.6% to +2.5%. Furthermore, these ratios were measured at differing absolute ion intensities to check for any potential nonlinearity in the measurement system. There was no measurable change in isotope ratios for the range of intensities and electrometer scales checked. The signal intensity was controlled by changing the temperature of the filament in the range 1250-1400 K. The maximum signal was arbitrarily limited to the 10 V scale and 1O1O flresistor of the electrometer, which roughly corresponds to 10000 ions per second or 1000 ions per pulse. Isotope Dilution Experiments. The isotope dilution experimental program consisted of blank measurements, spike calibration measurements, and sample measurements. The analytical blank is often the limiting factor in trace determinations and thus must be both controlled and characterized (25). This statement is particularly true for iron, a pervasive environmental contaminant. Both loading blanks and chemical blanks were measured in this study. The loading blank consists of iron intrinsic to the vaporizing filament and iron contamination added in the mass spectrometric loading process. The chemical blank is contamination incurred during the dissolution and separation of the samples. It was originally hypothesized that the relatively low temperature of the atomizing process would help to eliminate the loading blank, one of the major problems faced when determining iron by thermal ionization which requires a hightemperature ionizing filament (14). This hypothesis proved false, and the loading blank was a major limiting factor in the measurement process. The loading blank was determined by drying 100 ng of spike directly on filaments representative of those used for measurement of samples. Loading blanks were 10-20 ng for regular rhenium material, normally degassed at 4.0 A for 30 min. Loading blanks were 1-2 ng for the same
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
Table 111. Chemical Blanks, ng
SRM 1643b Water"
av
correction, %
Table IV. Results of Analysis, Iron Concentration
SRM 909 Serumb
SRM 458 Copperb
40.1 175.4 54.1
62.6 1.7 5.8
745 846 805
90 2.7
23 0.84
799 8.0
'Regular rhenium filament, degassed 30 min at 4.0 A. Regular rhenium filament, degassed 60 min at 5.0 A.
rhenium material which was heated at 5 A for 1 h. A zonerefined rhenium material, which had been leached in HCl and used for thermal ionization iron measurements, was tested as well. This material proved less clean than the regular rhenium material. A major investigative effort to find filament material free of iron will be required to exploit the high inherent sensitivity of the RIMS technique for iron. The loading blank is significant relative to the sizes of both the chemical blank and sample and thus directly affects the isotope dilution measurements. The measured chemical blank includes the contamination that results in the loading blank, and, in principle, the chemical blank correction should account for the loading blank contribution. However, if the yield of the chemical blank is not 100% and its loading upon the filament is not complete, the actual loading blank is magnified in the determined chemical blank. Thus, the chemical blanks must be and were completely loaded upon the filaments. Class 100 clean air environments were used for isotopic spiking and chemical processing of all samples. Triplicate determinations of the chemical blanks were made for each of the materials analyzed. The results are summarized in Table 111. Both the serum and water blanks are dominated by the contribution of the loading blank the decreased serum blanks relative to the water can be attributed to the more rigorous cleaning of the filaments that was done for these samples. The variability that is seen in these two sets of blanks can also be attributed to the probable nonhomogeneous distribution of the iron in the filament material. In contrast to the water and serum blanks, the larger copper blanks can be attributed to the more extensive dissolution and separation procedures required for this material. The corrections required for these blank levels are also listed in Table 111. The concentration of the 57Fespike solution was determined by using gravimetrically prepared natural iron solutions. The concentration determined was 0.3542 f 0.0095 pmol/g (2.7% RSD) which agreed with the value previously determined by thermal ionization, 0.3598 pmollg. The difference was -1.6%. The more precisely determined thermal ionization values for both the spike isotopic composition and concentration were utilized in the calculations of all results. The results of the analyses of the water, serum, and copper samples are listed in Table IV. As opposed to thermal ionization where the precision of measurement is better, optimum spiking of the isotope dilution samples is more critical for these resonance ionization measurements. The error magnification factor used for estimating the minimum error introduced in measuring mixes of natural and spike isotopes in isotope dilution was calculated (26) and the samples were spiked appropriately. The optimum spiking ratio in this case is (0.0231 X 14.83)0.5,or 0.58. The sample ratios ranged from 0.56 to 1.3, well within the minimum of the curve, spiking ratio vs. error magnification factor. The accuracy of the procedure was addressed by the determination of iron in SRM 498 Copper V which has a certified value of 11and uncertainty limits of f 2 kg/g. This material required significantly more chemical processing in its disso-
av SD RSD, %
SRM 1643b Water,
SRM 909 Serum,
ng/g
rg/g
97.4 97.4 97.4 100.1 103.8 98.9
1.96 2.08 2.05 1.88 1.80 1.89
9.78 9.73 8.93 9.46
99.2 2.5 2.5
1.94 0.11 5.5
9.45 0.38 4.0
SRM 458 Copper, rglg
lution and in the separation of the iron than did either the serum or water. As a result, the chemical blank was significantly greater. The agreement between the measured concentration and certified value provides assurance of the quality of the mass spectrometric measurements of the spike, unknown samples, and blanks (27). The sources of uncertainty in this procedure are the spike calibration, isotope ratio measurement, blank correction, and fractionation correction. The uncertainty in the previously done spike calibration is estimated at 0.25%. There is no way to assess a fractionation correction: the imprecision of measurement is much larger than any anticipated isotopic fractionation effect and there are no isotopic reference standards available for iron. From past experience an upper limit of 0.25% can be estimated. Isotopic fractionation, a phenomenon readily observed in thermal ionization, is caused by the vaporization process and therefore should be applicable to these thermal vaporization resonance ionization measurements. However, the fractionation effect is self-correctingin an isotope dilution measurement program where the same fractionation occurs in the determination of the spike concentration, spike and natural isotopic compositions, and sample concentration. The two major sources of uncertainty are presently caused by blank and isotope ratio measurements. The blank uncertainty is dominated by iron in the vaporizing filament. Finding filament material relatively free of iron is not considered a major problem. Rhenium has been used as a vaporizing filament in TIMS because of its high work function, refractory nature, and nonreactivity. Resonance ionization, however, does not require a vaporizing filament with either a high work function or a refractory nature and therefore many different substrate materials are viable candidates. Once the loading blank is reduced to an insignificant level, the other sources of iron blank can readily be investigated and controlled. The isotope ratio uncertainty is dominated by the variability in the pulsed ionization/measurement process. This variability is reduced when the ionization process is saturated, that is, decoupled from variations in the laser power. Implementation of advanced detection methods has the potential to further improve the precision of pulsed ion current measurement. For instance, quantitative time-of-flight isotope ratio measurements have only begun to be explored (6). The concentrations and uncertainties at the 95% confidence limit for iron in the materials analyzed here are as follows: SRM 1643b, 99.2 f 8.3 ng/g; SRM 909,1.94 f 0.29 pg/g; SRM 498, 9.4 f 1.2 pg Jg. Registry No. Fe, 7439-89-6 SBFe,14093-02-8; 67Fe,14762-69-7; water, 7732-18-5.
LITERATURE CITED (1) Fassett, J. D.; Travis, J. C.; Moore, L. J.; Lytle, F. E. Anal. Chem. 1983, 55, 765-770. (2) Miller, C. M.; Nogar, N. S.; Gancarz, A. J.; Shlelds, W. R. Anal. Chem. l9%2,54 2377-2370. (3) Young, J. P.: Donohue, D. L. Anal. Chem 1983, 5 5 , 88. ~
Anal. Chem. 1904, 56,2233-2237 (4) Barnes, I.L.; Murphy, T. J.; Michlels, E. J. Assoc. Off. Anal. Chem. 1982, 65, 953. (5) Paulsen, P. J.; Burke, R. W.; Maienthal, E. J.; Lambert, G. M. ASTM Spec. Tech. Pub/. 1981, STP 747, 113-120. (6) Simons, D. S.Int. J . Mass Spectrom. Ion Phys. 1983, 55, 15-30. (7) Miller, C. M.; Nogar, N. S. Anal. Chem. 1983, 55, 1606-1608. (8) Donohue, D. L.; Young, J. P.; Smith, D. H. Int. J . Mass Spectrom. Ion Phys. 1982, 43, 293-307. (9) Saylor, L.; Finch, C. S.Am. J. Physlol. 1953, 172, 372. (10) Janghorbanl, M.; Young, V. R. I n “Advances in Nutritional Research, Volume 3”; Draper, H. H.. Ed.; Plenum Press: New York, 1980; pp 127-150. (11) Janghorbanl, M.; Ting, B. T. G.; Young, V. R Clin. Chim. Acta 1980, 708, 9-24. (12) Mlller, D. D.; Van Campen, D. Am. J. Clin. Nutr. 1979, 32, 2354. (13) Proceedings of Workshop on Stable Isotopes and Derived Radioisotopes; National Academy of Sclences, Washington, DC, Feb 3-4, 1982. (14) Garner, E. L.; Dunstan, L. P. Adv. Mass Spectrom. 1978, 7A, 481-485. (15) Stukas, V. J.; Wong, C. S. I n “Trace Metals in Sea Water”; Wong, k y l e , Burton, Goldberg, Eds.; Plenum: New York, 1983; pp 513-536. (16) Kelly, W. R.; Fassett, J. D. Anal. Chem. 1983, 55, 1040-1044. (17) Kuehner, E. C.; Alvarez, R.; Paulsen, P. J.; Murphy, T. J. Anal. Chem. 1972, 4 4 , 2050-2056. (18) Fassett, J. D.; Moore, L. J.; Travis, J. C.; Lytle, F. E. Int. J. Mass Spectrom. Ion Phys. 1983, 54, 201-216. (19) Fassett, J. D.; Moore, L. J.; Travis, J. C. I n “Proceedings of the 25th Conference on Analytical Chemlstry in Energy Technology”; Lyon, W., Ed.; Knoxville, TN, Oct 1983.
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(20) Fassett, J. D.; Moore, L. J.; Shideler, R. W.; Travis, J. C. Anal. Chem. 1984, 56, 203-206. (21) Moore, L. J.; Fassett, J. D.; Travis, J. C. Anal. Chem. in press. (22) Letokhov, V. S. Annu. Rev. Phys. Chem. 1977, 2 8 , 133-159. (23) Turk, G., NBS, personal communication, 1983. (24) Cotter, R. J.; Yergey, A. L. J. Am. Chem. SOC. 1981, 103, 1596-1598. (25) Murphy, T. J. I n “Accuracy In Trace Analysis: U.S. Department of Commerce, National Bureau of Standards: washington, DC, 1975; NBS Spec. Publ. 422, pp 509-539. (26) Jamieson, R. T.; Schrelner, G. D. L. I n “Electromagnetically Enriched Isotopes and Mass Spectrometry”; Smith, M. L., Ed.; Butterworths: London, 1956; pp 169-176. (27) Call, J. P.; Mears, T. W.; Michaelis, R. E.; Reed, W. P.; Seward, R. W.; Stanley C. L.; Yoiken, H. T.; Ku, H. H. “The Role of Standard Reference Materials in Measurement Systems”; U.S.Department of Commerce, National Bureau of Standards: Washington, DC, 1973; NBS Monograph 148.
RECEIVED for review March 7,1984. Accepted June 11, 1984. Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
Applications of Microwave Oven Sample Dissolution in Analysis R. A. N a d k a r n i Analytical Research Laboratory, Exxon Research and Engineering Company, P.O. Box 4255, Baytown, Texas 77522
Heating In a mlcrowave oven In the presence of acid mixtures dlssolves the metals from powdered coal, fly ash, 011 shales, rocks, sediments, and blological materlals. The dlssolutlon Is complete wlthln 3 min. Nearly 25 elements (AI, As, Ba, Be, Ca, Co, Cr, Cu, Fe, K, LI, Mg, Mn, Na, NI, P, Pb, SI, Sr, TI, V, and Zn) from the dlssolved samples are determlned by Inductlvely coupled plasma emisdon spectrometry. The method has been tested oh a varlety of standard reference materlals, with reproducible and accurate results.
Recently we published a multitechnique analytical scheme for the multielemental analysis of coal and fly ash samples (1). A majority of elements in that study were determined following Parr acid bomb dissolutions. This procedure needs prior ashing of the coal samples in a muffle furnace (8 h) or in a low-temperature rf asher (3 days). Subsequently, the ash is dissolved in an acid mixture in a Parr bomb in about 2 h of heating. Even though Parr bombs have been extensively used in dissolution in our laboratory and elsewhere, with prolonged usage, some of the defects apparent in them are occasional contamination of the sample solution from the metallic parts of the bomb and increasing difficulty of fitting the inner Teflon vessel into the outer metal casing as the bombs are repeatedly used. When several hundred samples a month are rountinely to be analyzed in this fashion, the cleaning and the maintenance of these bombs become quite a chore. Most other wet digestion techniques involve constant supervision and prolonged time for complete dissolution. There are also the possibilities of trace elements’ loss and contamination during these steps. In the search for an alternative wet digestion technique, use of a microwave oven for rapid sample dissolution seemed to be an attractive procedure. Previous reports of analytical applications of microwave oven include those for biological
and mineral-metal samples dissolution by Abu-Samra et al. (2),Barrett et al. (3),and Matthes et al. (4). Major and trace elements were determined by atomic absorption spectrometry and neutron activation analysis in these studies. We have systematically investigated the use of microwave oven dissolutions in a variety of complex matrices and very satisfactory results have been obtained for about 25 elements measured by inductively coupled plasma emission spectrometry. EXPERIMENTAL SECTION Microwave Oven. A commercial domestic microwave oven-the Sears Kenmore-was purchased in a local store. The oven has a variable timing cycle from 5 s to 60 min and a variable heating cycle based on power settings from ”warm”through “high” which are equivalent to 90 through 625 W of power output. The microwave frequency is 24.5 MHz. The oven’s capacity is 1.3 ft3. Earlier reports on the use of microwave ovens for the dissolution of biological materials (2,3)indicate that the acid fumes generated during the dissolution rapidly attack the electronics of the oven and damage the magnetron. The above workers used Pyrex and Plexiglas boxes inside the oven and evacuated the acid fumes with different degrees of success. It has been found that it is not really necessary to evacuate the acid fumes, so long as they are contained inside the desiccator and later released into a fume hood. At present we use a Pyrex vacuum desiccator in which samples are loaded. A partial vacuum is created, and the desiccator is placed in the microwave oven. After the dissolution is completed, the desiccator is removed to a fume hood and vented, leaving the NO, and HC1 fumes in the hood. Normally no acid fumes are seen or smelled inside the microwave oven after this operation. Parr Bombs. Parr Teflon acid bombs were obtained from Parr Instrument Co., Moline, IL. The procedure for using this bomb has been described earlier (1). Inductively Coupled Plasma Emission Spectrometry (ICPES). All the analytical measurements were made on a Jarrell-Ash Plasma AtomComp 90-975. Details of our instrumentation are described by Botto (5). Reagents. All inorganic acids used were of “Ultrex” quality from J. T. Baker Chemical Co. Other chemicals were of analytical
0003-2700/84/0356-2233$01.50/00 1984 American Chemical Society
