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nitrogen transferred to a glass sample tube as well. Since all traps are now free of gas, the line is prepared for the next sample analysis. Isotopic measurements were performed on a 6-in. radius of curvature, dual collector isotope ratio mass spectrometer. Background, tailing, and leak rate corrections were performed as previously described ( 4 ) . Isotope ratios are reported in the conventional delta notation
where the standard for oxygen is Standard Mean Ocean Water and for nitrogen is air. The 6 1 7 0 notation represents the 170/160 ratio and 615Nthe 15N/14Nratio. By convention, the standard for nitrogen is atmospheric air nitrogen.
RESULTS AND DISCUSSION The results for the separations are shown in Table I. The nitrogen results show that, in each case, they are isotopically identical with the atmospheric ratio (615N= 0) within the mass spectrometric error. Since a calibrated volume sample tube (5.5284 cm3) was used at measured temperatures and pressure, the expected amounts of N2 and O2 were known. Recovery in all cases was better than 99.5%, as determined by comparison of expected yield with the measured quantities of both Nz and 02.The recovery variation in Table I is strictly due to temperature and pressure variations. For oxygen, the previously reported value for oxygen is 6l8O = 23.50% with a standard deviation ( l a of 0.3%) and a range extending from 23.18 to 23.79% (5). All of our values have an average of +23.51, the same as that previously reported (5). When performing the isotopic measurements, a background for other components was always taken. When measuring oxygen, the nitrogen amounts were less than the mass spectrometer background. The Nz/02ratio is less than 0.04%. For nitrogen
isotopic measurements, the Oz/Nzratio was always less than 0.02%. We have run sample blanks by performing the entire separation procedure with an evacuated sample tube as a mock sample. There was no observable (C0.06 pmol) N2 or 0 2 in six blank runs. It may also be seen in Table I that the 6170 is, within error, identical with that expected from the 6 l 8 0 (6170 = 0.526180). It is interesting to note that run 13 is clearly different from the mean. This was the only sample collected during a Santa Ana when the winds were easterly rather than westerly. No definitive statement may be made from only one measurement, but it does suggest that it is worthwhile to measure samples over an extended period to look for deviations in the oxygen isotopic composition.
ACKNOWLEDGMENT We wish to thank Andy Gaynor and Maria Hozbor for their participation in this project. John Heidenreich I11 offered numerous suggestions as well. Registry No. leg,7782-44-7;170,13968-484;oxygen, 7782-44-7; nitrogen, 7727-37-9. LITERATURE CITED (1) Thiemens, M. H.; Heidenreich, J. E. H., I11 Science 1983 219, 1073-1075. ( 2 ) Heidenreich, J. E. H., 111; Thiemens, M. H. J. Chem. Phys. 1983 78, 892-895. (3) Cicerone, R. J.; McCrumb, J. L.; Geophys. Res. Left. 1980, 7 , 4, 251-254. (4) Craig, H. Geochim. Cosmochim. Acta 1957 12, 133-159. (5) Kroopnick, P.;Craig, H. Science 1972, 175, 54-55. (6) Epstein, S. Lunar Planet. Sc;. 1980, 1 7 , 259-261.
RECEIVED for review June 24, 1983. Accepted October 20, 1983. The research described in this paper was provided by the Research Corporation (Grant No. 8315) and the IGPP-UC program (Grant No. 82101).
Pulsed Thermal Atom Source for Resonance Ionization Mass Spectrometry J. D. Fassett,* L. J. Moore, R. W. Shideler, and J. C. Travis Center for Analytical Chemistry, National Measurement Laboratory, National Bureau of Standards, Washington, D.C. 20234
A pulsed thermal atom source has been developed for use with a resonance lonlratlon mass spectrometer system based on a lowduty cycle-pulsed laser. The nature of the thermal atom pulse has been evaluated by temporal scannlng of the atomlratlon pulse relative to the laser lonlratlon pulse. Changes In the deslgn of the atomizing fllament are required to achieve a sharp atomlzatlon pulse. The system has been tested by uslng the element Iron. A 30-fold Improvement In sample utlllzatlon eff lclency was demonstrated for the pulsed thermal source relative to a continuous thermal source.
Resonance ionization mass spectrometry (RIMS) is a recent technique combining mass spectrometry with laser photoionization to produce ions selectively and efficiently from
a gas-phase reservoir of atoms (1-3). The technique exploits the limited bandwidth and intense photon flux inherent in laser radiation. By tuning the laser to discrete, resonant electronic transitions of an element, we can increase the probability of multiphoton ionization many-fold. The overall elemental and isotopic selectivity is a combination of the selectivity of the ionization and mass selection steps. RIMS requires free atom reservoirs; most of the initial demonstrations of RIMS have been made with modified thermal ionization mass spectrometers (TIMS) in which the source regions were made accessible to laser radiation. Thermal sources can be used to produce neutral atomic beams for a wide range of elements ( 4 ) . RIMS has the potential to be applied to problems where thermal ionization measurement fails because of limited sensitivity (inability to produce ions) and/or selectivity (isobaric interferences). Demonstrations
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of the selectivity of RIMS have been made for mixtures of Lu/Yb (2) and Nd/Sm (5) where isobaric interferences exist, The demonstration of single atom detection by Hurst et al. (6)using resonance ionization spectrometry (without mass selectivity) illustrates the potential ultimate sensitivity. However, almost all of the early RIMS systems have coupled pulsed lasers, which have inherently low-duty cycles, with continuous thermal atom sources and, thus, even with 100% ionization efficiency, are limited to the duty cycle of the combined atomization/ionization process. This duty cycle is principally determined by the diffusion rate of the sample in the laser interaction volume and will be on the order of Even with this duty cycle, RIMS is more efficient for certain elements than conventional thermal ionization ( 4 ) . Pulsed laser RIMS systems have been established because they provide maximum flexibility to measure the majority of elements in the periodic table. A Nd:YAG pumped tunable dye laser, such as used in this study, can generate intense, highly coherent light to a wavelength as low as 210 nm, which can efficiently ionize a wide range of elements. Presently available high repetition rate or CW lasers capable of significantly improving the duty cycle of the atomization/ionization process have neither the peak power nor wavelength stability required for broad analytical utility. The alternative method of improving the duty cycle of the atomization/ionization process is to pulse the atomization source to match the pulsed laser. For instance, pulsed sources are used in molecular spectroscopy (7,8) in order to maximize sample utilization with pulsed lasers. However, the developmental impetus in molecular spectroscopy has been the reduction in the gas load produced by continuous molecular beams in the high-vacuum instrumentation. The principal element in a pulsed molecular beam source is a fast gas valve acting between a high-pressure reservoir and a vacuum system. Limited sample material is not generally an issue in molecular spectroscopy as is commonly the case in analytical mass spectrometry. Established methods of producing pulsed ion beams in analytical mass spectrometry can be used to produce pulsed atom beams. These methods are pulsed ion or atom sputtering, as used in secondary ion mass spectrometry or fast atom bombardment, or pulsed laser ablation, as used in laser microprobes. The use of a pulsed ion sputtering source for RIMS has been demonstrated (9). Although the matching of these pulsed atomization techniques to pulsed RIMS appears attractive, questions concerning the energetics of these processes have been raised (10) with the possibility that the atoms produced will exist in a myriad of metastable excited states, thus reducing both spectroscopic selectivity and elemental sensitivity. In this study a pulsed thermal source has been developed and investigated. The pulsed thermal atom source logically evolves from the success of RIMS with continuous thermal sources and the impetus to achieve the ultimate sensitivity. Thermal sources are relatively low in energy and the distribution of energy in atomic excited states can be modeled by a simple Boltzmann relationship with the temperature defied by the filament (10). Thus, the spectroscopy is highly predictable. The time-dependent characteristics of the thermal atom reservoir were determined in this study by varying the time between thermal and laser pulses. The element chosen to demonstrate the pulsed source was iron. EXPERIMENTAL SECTION Figure 1 schematically illustratesthe instrument which consists of three basic components: a laser system capable of producing tunable UV radiation, a magnetic sector mass spectrometer with a suitably modified thermal source, and a detection system capable of measuring pulsed ion currents. The laser has a repetition rate of 10 Hz and a pulse length of 3 ns. This instrument has been
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Tlme (ms) Figure 2. Characteristics of pulsed thermal source: AI, current pulse; At, width of current pulse; t,, time between thermal and laser pulses. described in detail previously (3, 10). The pulsed thermal source was made by modifying the solidstate, constant-current filament supply that has been developed and used in this laboratory for thermal ionization (11). This supply provides up to 10 A of current. In the pulsed mode of operation a 10-A current pulse is applied to the filament for a selected period of time. The 10-A pulses can be applied on top of an established base current level. The thermal pulse can be triggered in conjunction with the laser or a delay can be incorporated such that the laser pulse is triggered at the time of maximum atom density. The details of the electronic circuitry are available from the authors. Figure 2 summarizes the thermal pulse parameters. Iron samples were used with RIMS to evaluate the source. Samples were prepared by the reduction, in a hydrogen atmosphere, of the nitrate salt of iron which had been dried on the rhenium filament substrate. Typical sample size was 500 ng. The wavelength used to make the measurements was determined by scanning the frequency doubled range of the dye, Rhodamine 6G, 279-289 nm at a constant filament temperature of 1230 K. There are four ground-state-originating transitions of iron in this range and the most efficient multiphoton channel at 283.544 nm was chosen. A stable and constant RIMS signal could be generated and maintained for many hours at a constant temperature indicating that sample depletion would not occur under controlled atomization conditions.
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assembly. The temporal nature of the thermal pulse was determined by delaying the pulser relative to the laser. A photodiode detector of the laser pulse was used to trigger both an oscilloscope and a Hewlett-Packard 8007B pulse generator. The pulse generator which provided a pulse delay (available range, nanoseconds to milliseconds) was used to trigger the thermal pulser. The oscilloscope was used to monitor the position of the thermal pulse relative to the laser pulse. Thus, the n - 1laser pulse was used to trigger the thermal pulse which was interrogated by the nth laser pulse. The ion signal vs. the time delay was determined by manually scanning the relative position of the two pulses in the 100 ms region between pulses. The ion signal was integrated at each time position in the scan for 20 s or 200 laser pulses.
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RESULTS AND DISCUSSION Previous experiments using RIMS (3) demonstrated that the ionization signal, when measured as a function of filament temperature, exhibits a log I vs. 1/T linear behavior as predicted by the Clausius-Clapeyron equation for all elements tested. This behavior was verified for iron. The exponential signal vs. temperature behavior implies that an experimental temperature pulse applied to a sample filament will result in a sharper signal pulse. Furthermore, this relationship suggests that a filament can be heated to a threshold temperature below the onset of vaporization and a smaller thermal pulse can be used to produce the vaporization. The advantage of this technique is increased temperature response of the filament. In order to determine the most favorable operating conditions using a pulsed thermal source, the temperature vs. time relationship for a filament must be known for a given thermal pulse. In practice this relationship is a complex function of the physical character of the filament, its heat capacity, resistance, mass, and geometry, as well as its support structure, which will act as a heat sink. It is comparatively easy, however, to determine the temperature vs. time relationship experimentally by measuring the ionization signal vs. time relationship for the characteristic thermal pulse and then deconvoluting this curve with the experimentally determined log Z vs. T relationship as measured with a nonpulsed thermal source and the same filament geometry. The assumption is made in this procedure that the sample is positioned on the fiament at one isothermal spot, that is, all parts of the sample see the same temperature at the same time. The first experiment was completed with a standard Re filament, which is a ribbon 25 pm thick, 750 pm wide, and 12 mm long, shaped as shown in Figure 3a. The filament is spot-welded to stainless steel posh. Initially, a usable current pulse width was established empirically. The pulse width was increased until an ionization signal in the midrange of the detector system was reached. The width of the current pulse required was 7.5 ms and the thermal pulse shape, atom concentration vs. time, is illustrated in Figure 4a. The experimental curve of Figure 4a illustrates the limitations of the standard filament assembly. It takes a relatively long time to heat up the filament, and an even longer time for the filament to cool. The half height of the atom popu-
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lation produced by the thermal pulse was 12.5 ms; the decay was exponential. In this case the signal never reached zero before the next thermal/laser pulse sequence. The temperature vs. time curve determined for this filament is illustrated in Figure 4b. The filament temperature reached 1270 K in 7.5 ms and only cooled to 1190 K between thermal pulses. Thus, a significant base line temperature was established in the process. Sample utilization can be defined as the ratio of the number of atoms that are detected divided by the total number of atoms leaving the filament. An improvement factor can be defined for the pulsed vs. continuous mode of atomization as the ratio of the total number of atoms produced by a continuous atom source in the pulse period to atoms produced in the pulse. This improvement factor is the ratio of the product of the maximum signal and pulse period to the integrated area beneath the experimental time vs. signal curve which delineates the atom pulse. The measured improvement using the standard filament was 3.8. Since it appeared that the mass of the standard filament limited its temperature response and therefore sample utilization, a second filament was prepared whose length and width were reduced, Figure 3b. For this filament, a 4.5-ms current pulse produced an ionization signal comparable to the
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Figure 6. RIMS signal vs. time for experimental arrangement of Figure 5b and maximum ionization signal. Each point represents 20-s integration; error bar is standard deviation of the mean. Time (ms) Figure 5. Profile of thermal pulse with miniaturized filament assembly of Figure 3b, R I M S signal vs. time after thermal pulse initiation: (a) no current offset and 4.5-ms pulse width: (b) 0.9-A offset and 3.0-ms pulse width.
first experiment with a standard filament. The experimental signal vs. time curve is illustrated in Figure 5a. The signal decayed to half-height for this filament in 3.5 ms and to 1% of the signal maximum in 30 ms. In contrast to the standard filament the base line was reached between thermal pulses. A sample utilization improvement factor of 12 is calculated from this curve. Since this second miniaturized filament did reach base line between thermal pulses, the experiment in which the current pulse is applied atop a constant current base level was attempted. A base level of 0.9 A was established and a midlevel ionization signal achieved with a 3-ms pulse. The experimental signal vs. time curve is illustrated in Figure 5b. In this experiment it took less than 1 ms to decay to half-height. However, it still required 30 ms to reach 1% of the maximum signal. The best observed enhancement in sample utilization efficiency, a factor of 31, was achieved in this case. Figure 6 illustrates the stability and reproducibility of the signal produced in the thermal pulsing with a 10-min scan consisting of 30 204 points. The variation in signal level results from irreproducibility not only in the thermal pulse but also from the laser pulse, since the RIMS signal was not saturated. The laser specification is *lo% pulse to pulse reproducibility. The pulse-to-pulse imprecision in the ion signal as measured by the relative standard deviation is 5%. Further improvement in sample utilization was attempted by further reduction in the size of the filament (400 pm X 1200 pm X 25 pm). However, a reproducible signal vs. time curve could not be maintained with this microminiature filament. It is obvious that reducing the size of the filament achieves the highest heating response. This gain in response must be balanced against the ability to physically load the sample upon the filament. Ideally, the sample should exist on the filament as a point source to experience an isothermal heating pattern.
As the sample is dispersed on the filament, it will experience a more complex time, position, and temperature response. Improvements in sample loading will be required to address this difficulty. A point source loading of a sample on the filament, such as approximated in the resin bead technique (12),is expected to be most successful. Geometrical constraints thus appear to limit the ultimate utility of the technique when the filament is microminiaturized. The 30-fold improvement of sample utilization demonstrated with this pulsed source is not trivial. The temporal efficiency of the pulsed atomization-pulsed ionization RIMS system now approaches and this system is applicable to at least half of the elements in the periodic table (3).
LITERATURE CITED (1) Donohue, D. L.; Young, J. P.; Smlth, D. H. Int. J . Mass Spectrom. Ion Phys . 1982,4 3 , 293-307. (2) Miller, C. M.; Nogar, N. S.; Gancarz, A. J.; ShieMs, W. R. Anal. Chem. 1982. 54. 2377-2378. (3) Fassett, J. D.; Moore, L. J.; Travis, J. C.; Lytle, F. E. Anal. Chem. lS83, 55,785-770. (4) Moore, L. C.; Fassett, J. D.; Travls, J. C., unpublished work, NBS, 1983. (5) Young, J. P.; Donohue, D. L. Anal. Chem. 1983,55, 88. (5) Hurst, G. S.;Nayfeh, M. H.; Young, J. P. Appl. Phys. Lett. 1977, 30, 229. (7) Gentry, W. R.; Glese, C. F. Rev. Scl. Instrum. 1978,49, 595-500. (8) Bassi, D.; Iannotta, S.; Nlccollnl, S. Rev. Scl. Instrum. 1981, 52, 8-11. (9) Winograd, N.; Baxter J. P.; Kimock, F. M. Chem. Phys. Lett. 1982, 88, 581-584. (10) Fassett, J. D.; Moore, L. J.; Travis, J. C.; Lytle, F. E. Int. J . Mass Spectrom. Ion Proc. 1983, 5 4 , 201-215. (1 1) Shldeler, R. W.; Barnes, I . L., unpublished work, NBS, 1983. (12) Smlth, D. H.; Walker, R. L.; Carter, J. A. Anal. Chem. 1982, 5 4 , 827A.
RECEIVED for review September 7, 1983. Accepted October 20, 1983. 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.
