2770
Anal. Chem. 1084, 56,2770-2775
Systematics of Multielement Determination with Resonance Ionization Mass Spectrometry and Thermal Atomization L. J. Moore, J. D. Fassett,* and J. C. Travis
Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
The systematics for multielement determlnatlon uslng resonance lonlzatlon mass spectrometry and thermal atomization Is developed. The aspects of atomlzatlon, lonlzatlon, and detectlon are dlscussed and resonance lonlzatlon Is demonstrated for 19 elements. The selectlve, sequentlal lonlzatlon of seven elements from a single sample Is also demonstrated. A one-wavelength, two-photon lonlzatlon scheme generally Is used In which the flrst photon excites a bound transhion In the near-ultraviolet region and second photon promotes the electron Into the ionlzatlon continuum. The wavelength-dependent Ion formation from the thermally produced atom reservoirs Is demonstrated for these elements by scannlng a Nd:YAG-pumped dye laser across Its tunable wavelength range. The observed wavelengths where lonlzatlon occurs have been correlated where posslble with allowed transitlons between known electronic energy levels. The elements accessible by using four common dyes are tabulated. More than 20 elements are accesslble withln the wavelength range of each dye.
Resonance ionization mass spectrometry (RIMS) is a recently developed technique that promises to provide a measurement sophistication in elemental analysis that is much greater than the simple sum of laser spectroscopy and mgss spectroscopy, its bases. Ion formation from optical excitation of thermally produced atom reservoirs been demonstrated now for several elements (1-6). In most of these cases the elemental selectivity provided by resonance ionization is used to remove isobaric interferences that can occur in mass spectrometry using alternative nonselective ionization processes. As a result of this work, the processes involved in resonant ionization from thermal atom reservoirs are rapidly unfolding. For the purpose of studying RIMS, a tunable ultraviolet laser system has been combined with a magnetic sector mass spectrometer containing a modified thermal source. Although several ionization schemes are possible with the single frequency-doubled laser (7), a single wavelength, two-photon ionization scheme using ultraviolet light is applicable to nearly 50 elements (1). RIMS of Mo, Re, V, Fe, and Ni previously has been reported using this system (1,2). In this paper RIMS is demonstrated for 19 elements. The importance of wavelength dependence studies is again illustrated, with broad spectral scans for each element. In Figure 1are summarized the 19 elements for which RIMS has been demonstrated in this paper, as well as the other elements potentially amenable to the simple two photon ionization scheme. The facility and generality of the technique as demonstrated here cannot be overemphasized. The ability to similarly prepare single element samples and control thermal atomization for resonance ionization strongly suggests that RIMS can be extended to multielement mixtures and multielement chemical analysis. Multielement mass spectrometric analysis using isotope dilution is done on a limited scale in thermal ionization mass spectrometry (8, 9) where elements with differing vaporization temperatures are
sequentially ionized and in spark source mass spectrometry (10) where photoplate detection is used for simultaneous multielement measurement. RIMS has been used to certify Fe in two Standard Reference Materials by isotope dilution analysis (11) and there are no reasons to prohibit its application to multielement analysis. The systematics of sequential multielement analysis using RIMS and thermal atomization is explicated here. In contrast to single or dual element experiments, to which the RIMS investigations have been limited to date, the multielement application of RIMS more broadly exploits its elementally selective and sensitive measurement capabilities. The components of multielement RIMS include the atomization and ionization processes and the mass and optical spectroscopies. We demonstrate how a fundamental understanding of each of these components is applied in multielement determination by means of an experiment in which seven elements are sequentially measured from a single sample. A survey of the elemental wavelength tables (12) indicates that greater than 23 elements are accessible by each of four common dyes. We have demonstrated the RIMS of 18 elements within the wavelength range of a single dye. Thus, the basis for sensitive and selective multielement RIMS analysis has been established.
EXPERIMENTAL SECTION The instrumental RIMS configuration is illustrated schematically in Figure 2. It consists of three basic components: a laser system capable of producing pulsed tunable UV radiation, a magnetic sector mass spectrometer with a suitably modified thermal source, and an electron multiplier detection system capable of measuring pulsed ion currents. This instrument has been described in detail previously (1, 2). Ribbons (0.025 X 0.762 mm) of rhenium, hafnium, and tantalum were used for direct sublimation of these elements. All other atom reservoirs were produced by one of two generic approaches. The first involved the reduction in hydrogen atmosphere of the element from a dried salt deposit on a rhenium filament (13). The second was effected using a rhenium ribbon filament fashioned into the form of a “boat”. A drop of graphite slurry in acetone was added to this filament and the solvent was evaporated under an infrared lamp before adding 5 WL of sample soIution which was similarly dried. Sample size for both methods was 0.5-1.0 pg. Ionization was achieved by slowly heating the filament in the mass spectrometer and adjusting the laser wavelength to a preselected optical transition of the element while focusing the mass spectrometer on the major isotope. Since the major isotope of Ni, 58Ni,could be potentially interfered with by 5sFe,the data for nickel were acquired while monitoring @“i to ensure that the Ni resonance ionization optical spectrum was not compromised by iron spectral lines. Most of these elements produced a stable resonance ionization signal with the microgram size sample. Two dyes were used in this study. Rhodamine B was used to produce the data for 18 elements in the wavelength range 292-304 nm. The dye Rhodamine 6G was used for the multielement experiment where the wavelength range 282-286.5 nm was examined. With operation of the frequency doubled Nd:YAG at 100 mL/pulse of 532 nm radiation with 20-3070 conversion efficiencies by the dye laser and 10% conversion frequency in frequency-doubling, the typical ultraviolet energy was 1-3 mJ/ pulse. The laser system operated at 10 Hz with 4-115 pulses and 0.3 cm-l bandwidth. The 2 mm diameter laser beam was posi-
This article not subject to U.S. Copyright. Published 1984 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
H-l
r-rm--M N 0 FNe
8 C
L i Be
lvlvl
' * I
I
I
I
I
Table I. Summary of Atomization Conditions ionization ele-
ment
co DY Er Fe Hf In
Lu
F r Ra Rc Ku Ha
Mo Ni Np Pu Rm Cm Bk C f Es Fm Md No L r Flgure 1. Elements suitable for resonance ionization by two-photon ionization scheme with wavelengths between 260 and 355 nm (starred)
and elements for which resonance ionization has been reported in this paper (crossed).
2771
Re Sr Ta Th Ti U V
Y Zr
loading method"
obsd temp, K
potential, cm-I
H H H
1195 1470 1560 1230 2190 1160 1760 1180 1230 2290 1300 2000 1970 1800 1930 1830 1740 2030
63 430 47 810 49 210 63 700 53600 46 670 43 762 57 260 61 579 63 530 45 932 63 600 49 000 55 138 48 800 54 361 52 000 56 077
H C
H H H H C C C C C C
SahaLangmuir factor 6X 6X 2x 1x 4x 2x 2x 8x 7x 4x 4x 2x 2x 6X 2x 3x 2x
10-4 10-12 10-8 10-4 10-2 10-10
10-12 10-7
10-3 10-10
10-3 lo4 10-3 10-6 10-5
8X
H, hydrogen reduction; C, carbon reduction. Tracker Frequency Photodiode Trigger
I I
I I
Tunable
rcI
Figure 2. Block diagrams of R I M S system: M.S., magnetic sector; Cal., calorimeter; DVM, digital voltmeter; Comp., computer.
tioned parallel to and about 5 mm from the filament. Data were collected by stepping the laser across the wavelength range (292-304 or 282-286.5 nm) in 5-pm increments and measuring the mass selected major isotope ion current. The output of the electron multiplier, operating at 10' gain, was amplified and input to a boxcar averager which exponentially integrated the signal for 30 laser pulses. The boxcar averager was used with a 500-11s gate width, a 1-ps time constant on the processor module, and a 100-ps time constant on the instrument mainframe. During wavelength scanning the ion signal was logged and displayed by a Hewlett-Packard 98451' computer. These data were used to produce the spectral information reported here. No corrections were made for the varying laser power in the recorded data. A multielement mixture was prepared by mixing solutions of After a drop salts of metals dissolved in dilute HCl or "Os. of graphite slurry was dried in a rhenium boat filament, a drop of the multielement mixture was added and dried under an infrared lamp to provide approximately 1-2 pg each of V, Fe, Ni, Co, Lu, Pb, and U. The temperature of the filament was slowly increased in the mass spectrometer until the appropriate vaporization temperature was reached for each element. As with the single element samples, wavelength-dependent ionization spectra were taken for each element, except the dye Rhodamine 6G was used and the wavelength range 282-286.5 nm was examined. The data for V, Fe, Ni, Co, Lu, and U complement the single element data acquired in the wavelength range 292-304 nm. Pb did not have a resonant transition in the 292-304 nm range.
RESULTS AND DISCUSSION Atomization. Thermal vaporization, as used in thermal ionization mass spectrometry (TIMS) (14-1 7), provides the basis for the development and utilization of resistively heated filament sources as atom reservoirs for resonance ionization (1). Here, the simplest mechanism of atom reservoir formation, direct elemental evaporation from a single filament, has been employed experimentally. Elemental vaporization can be expected to predominate from elemental deposits. Given the fact that more than 80% of the elements in the periodic table have a metallic character, the formation of elemental deposits on filaments should provide for efficient atomization of these elements. The direct elemental vaporization of atomic species from a filament requires that the sample be in a reduced state prior to vaporization. This reduction can be achieved before loading or in situ during vaporization in the vacuum system of the mass spectrometer. We used two techniques here that illustrate both these sample loading approaches: (1)reduction in H2 outside the mass spectrometer, and (2) mixing the sample with graphite allowing chemical reduction to occur on the filament in the mass spectrometer. The sample loading method is given in Table I for the elements studied. The atom reservoirs for Re, Hf, and T a were produced from filaments made of these elements. Ideally, the vaporization process is a simple physical process in which the surface and sample do not chemically interact. In fact, sample and matrix associated with it, the filament, and residual gases in the mass spectrometer can chemically interact, changing the efficiency of the atomization process. In TIMS the variability of behavior among the elements and matrices is typically controlled by separating the element from the matrix and reproducibly loading the same chemical form of the analyte on the filament for every sample. The graphite loading procedure, as used here and in TIMS (It?), illustrates a second philosophy. It is designed to supply a matrix that dilutes the sample and prevents oxidation from occurring in the mass spectrometer. Vanadium provides an example of the importance of the sample loading procedure. In previous work (1) the H2 reduction procedure was used and the resonance ionization signal was immersed in a large off-resonance background. This background may result from the dissociation of VO which was observed in excess of the V signal in the experiment. By loading the V sample in graphite, the oxide component was
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
2772
Table 11. Summary of Individual RIMS Scans: 292-304 nm
(a) I ,
Ti V
Fe co
u
0 J
-
-
I I
I
I ,
I
II
I I
1
NI
Sr 292
296 298 WRVELENGTH,
294
300 nm
Y
302
r
> c
Zr
t
I Mo
Ln
z
I
w
c
t, W
,
I I
I
I
,II.I
,
I ,
,
0 J
I,
ESO" 3 0 1 17 8 0 0 0 8 54 0 18
Y
4 4 2 4 5 1 1 2 3 2 1 2 2 4 2 2 5
Zr
6
co DY Er Fe Hf In Lu Mo Ni Re Sr
Ta Th Ti U
I
I
DY
Er Lu Hf
WRVELENGTH. n m
E
( C )
Ta
~
w
F
z
H
Re
u
0
-.
IIII
294
,I
296
Th
298
300
302
6
9 2 11 7 6
unassigned* 1 7 2 0 0 0 0
1 18 3 0
1 28 2 180 1 1 2
intensity 30 0.7 6 140 16 0.8 0.4 47 11 1400 1.5 137 85 27 3.5 3.7
40 3.8
" GSO, ground state originating; ESO, excited state originating. *Not tabulated in ref 12 and not assignable to transitions between known levels (32). For most intense transition, arbitrary units, uncorrected for laser power.
In
I
292
GSO"
v
Y
LJ
element
u
WRVELENGTH, n m
Figure 3. Ion signal vs. wavelength for elements loaded individually on thermal filaments. Ion intensities are arbitrary among the elements.
completely suppressed, the off-resonance background disappeared, and a clean resonance ionization spectrum was observed. The experimental temperatures used are also listed in Table I. These temperatures were measured with an optical pyrometer with corrections made for the emissivity of the Re or carburized Re filament substrate. At these temperatures reasonably constant atom reservoirs were maintained for most of the elements examined. Only the Th became significantly depleted in the measurement process, a result evident from the disappearance of lines above 296.5 nm, particularly the resonance line at 296.98 nm, as illustrated by its spectrum in Figure 3. The gas phase atom reservoir, or the number density distribution of free atoms in the plume adjacent to the filament, can be described by a simple model based on the following
assumptions (19): (1)the flux of atoms leaving the filament is derived from vapor pressure data for the pure element (20) and (2) the angular distribution of atoms leaving each infinitesimal area element of the sample is given by the cosine law of thermal vaporization theory (21). By extrapolating vapor pressure data for Ta, for instance, in the vicinity of 3000 K (20) to the 2000 K temperature used here, and numerically integrating over the finite filament size and the laser volume, we predict an average of about 100 atoms in the active volume. For an illustrative ionization efficiency of I O % , 10 ions per pulse amplified by lo6 in the multiplier and by a lo5 V/A, 1-MHz bandwidth preamplifier, would yield a 1 - k ~pulse of 10 mV amplitude, in reasonable agreement with experiment. The ratio of ions to atoms produced in the vaporization of elemental species can be described by the Saha-Langmuir equation (22). This ratio can be very small for high ionization potential elements,thus stressing the improvement in absolute ionization efficiencies and sensitivities possible using resonance ionization. Included in Table I are the ionization potentials of the elements studied here, as well as the ratio of ions to neutrals calculated by using the experimental temperatures and a work function of 5.0 eV for Re. The Saha-Langmuir relationship suggests that the neutral atom production could be enhanced relative to ion production for elements with lower ionization potentials by choosing filament substrates with lower work functions. A large number of materials, such as LaB6, ZrC, or Hf (23),could satisfy this requirement and are likely candidates for investigation. Spectroscopy. Histogram plots of the wavelength scan data are summarized in Figure 3 for the 18 elements investigated in the wavelength range 292-304 nm. Although the data are subject to a number of internal measurement variations that would affect the relative intensities among elements, the qualitative spectral information is valuable in assessing both spectroscopic selectivity and sensitivity. The spectral information for these elements are summarized in Table 11. The intensity of the most intense peak is included for each element to provide a rough comparison of the signal-to-noise achieved in the wavelength scanning. These intensities are somewhat arbitrary since the signal could have been enhanced in many cases by a temperature increase. The rhenium signal is much larger than the other elements because the amount of sample available from the substrate material is virtually infinite. Background ion currents were essentially
A!rlAL\ITICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table 111. Boltzmann-Distributed Electronic State Populations POP.
element co
DY Er Fe Hf In Lu Mo Ni Re Sr Ta Th Ti U V
Y Zr
ground
pop next
state, %
higher, %
67.6 98.3 98.9 53.3 69.9 88.6 76.2
20.2, 7.4, 3.1 1.5 0.66, 0.14, 0.13 25.4, 12.6, 6.3 20.8, 6.3, 1.7 11.4 22.4, 1.3, 0.17
100
48.3 99.7 100 67.8 71.2 27.9 56.8 14.8 50.8 33.6
no of excited states, pop.
> 0.1% 6 1
3 4 7 1
3 0
29.6, 9.6, 7.9 0.10
21.9, 6.6, 2.3 12.2, 4.8, 3.4 36.9, 34.1, 0.25 30.3, 3.9, 2.3 23.9, 22.9, 19.9 49.1 31.5, 25.3, 1.8
6 1 0
5 15 6 17
8 1 12
zero for all the elements investigated. The low background has allowed resolution of peaks produced by signal levels as small as 0.1 ion per second or 1ion per 10 laser pulses, when signal averaged scanning was used with Re (2). Although the RIMS study has been based upon a onewavelength, two-photon scheme using ultraviolet photons, a relatively large number of the elements addressed by this scheme could also be ionized by our system with a twowavelength, two-photon process using frequency-doubled UV radiation and the fundamental visible radiation from which it is produced. This scheme is applicable to elements in which an appropriate resonance level is more than two-thirds of the way to the ionization continuum. The advantage of this scheme is that the higher power of the visible radiation is used to compensate for the lower absorption cross section of the boundlfree ionization step. Consequently, the resonance transition need not be as severely optically saturated in order to achieve 100% ionization as for the single-color scheme. The Sr and In spectra were produced by using this scheme. The elements Lu, Er, Dy, Th, and U could also have been ionized by using this process. We have found it desirable, especially with the single-color scheme, to compromise the goal of total ionization in favor of reducing the laser power density, which reduces the nonresonant multiphoton ionization background ( I ) and the spectral distortions which accompany severe optical saturation of bound/bound transitions. All of t h e elements surveyed had readily observed ground-state-originating transitions in the wavelength region studied. Also generally observed were transitions that arose from low-level, long-lived excited states. A detailed study of excited-state-originating transitions has been done for the Fe, Ni, Re, and Mo data (2). Not surprisingly, the conclusion is that atomic species evaporate from thermal sources in thermodynamic equilibrium with the emitting surface. For species studied to date, no excited states which are predictably short-lived are significantly thermally populated, and therefore no population redistribution is expected in the mass spectrometer source. Table I1 summarizes the observed ground-state-originating and excited-state-originating lines for the elements surveyed here. The observed data were compared with the Boltzmann distributions predicted for the given experimental temperatures. The calculated ground-state populations, the most populated excited states, and the total number of excited states with a fractional population greater than 0.1% are summarized in Table 111. In general, those elements with
2773
Table IV. Atomic Transitions in These Dye Laser Bandwidths (Frequency Doubled)"
eledye eledye ment R6G RB RlOl DCM ment R6G RB RlOl DCM
A1 X Ba X Bi X C o M X X M Dy X X Er X Eu X X X G d M M X Ga X M H f X X X In X X X F e X X X Pb X Li L u X X X Mg x Mn X Mo x x
X M X X M
X M
T a X X X T h X X X T m X X X S n X M M Ti X W X M M
u
x
x
x
X X X M X M
X
V Y
M
Zr
X X X
X M
X
X X X
M
X
X X X X X M
x
x
Ni M X O s X M R e X X R h M M Ru X X
x
sc Sr
X
x
X
x
x
Rhodamine 6G, 278-290 nm; Rhodamine B, 292-304 nm; Rhodamine 101, 302-314 nm; DCM, 310-325 nm. X, ground-state originating; M, excited state originating; excited state population > 1%.
lowest excited states greater than 8000 cm-' ( 1eV) tend to vaporize in the ground state a t the analytical temperatures indicated. Since a single tunable dye laser system is able to ionize only a state population a t a time, the maximum resonance ionization efficiency is proportional to the fractional population of the most populated state. Thus, the Boltzmann distribution for certain elements provides a fundamental sensitivity limit, fractionally worse than the 100% ionization ideal, for this type of atomization source. To the extent that atomic lines overlap in mixtures of the elements, the ionization (wavelength) selectivity will also be limited by the proliferation of metastable states. However, the multiplicity of metastable states also provides a greater variety of analytical lines for potential use in multielement determinations. The spectra illustrated in Figure 3 and summarized in Table I1 possess a number of unassigned lines. It is unclear where these lines originate, although for elements such as U and T h it is known that existing spectral tables are incomplete (24, 25). In general, the unassigned lines were weak. Evidence exists (26) for spectral features originating from selectively populated high-lying energy states and it has been speculated that this phenomenon may be widespread. Details of the spectroscopy are available from the authors. Multielement Systematics. The extrapolation of single element RIMS measurements to multielement mixtures requires relatively rapid access to a number of wavelengths, production of a mixed atom reservoir, and relatively rapid mass spectral measurement. Wavelength access is reflected in the data of Table I1 and Figure 3, which were all obtained by successive single element analyses, but within the same tunable dye range of 292 nm to 304 nm. By examination of the elemental wavelength tables (22) over the experimental wavelength range, ground state (and appropriate metastable) transitions can be culled that are available for ionization, via the two-photon process, of a number of elements. By use of this approach, the groups of elements in Table IV were derived for adjacent tunable wavelength ranges of commonly used dyes. The use of metastable lines provides an added measure of selectivity to determine an atomic species for which an interference with another element may occur at the groundstate-originating wavelength. Wavelengths within a given dye range can be reproducibly set within seconds by using comN
2774
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
Table V. Summary of Multielement RIMS Scans: 282-286.5 nm
element Wavelength SeIe~IIvlly h
GSO"
ESO"
unassigned*
intensityC
Pb
1
Fe Ni
2
0 6
0 0 0 0 0 1 65
3.2 16.6 5.9
co V Lu U
0 0 3 1 1
2 5 5 0 0
18.4 8.3 4.2 51.1
GSO, ground state originating; ESO, excited state originating. *Nottabulated in ref 12 and not assignable to transitions between known levels (32). For most intense transition, arbitrary units, uncorrected for laser Dower.
Figure 4. Dimensions of selectivity in muitielement resonance ionization. puter driven wavelength scanning. A mixed atom reservoir can be produced by reduction of the mixed element solution, as described above. While the elements of Figure 3 and Table I were measured from elemental deposits produced from solutions of high purity metals, there was no observable evidence that polyatomic vaporization occurred. The vaporization of mixed metal deposits is relatively unexplored and the potential for interelement matrix effects is difficult to assess. We predicted that the procedure where a dilute elemental mixture was combined with an excess of graphite upon a filament should suppress major interelement matrix effects by the introduction of a dominant carbon/element matrix effect. This procedure did prove effective for the element mix examined here. Since each element has a demonstrated optimum vaporization temperature, multielemental vaporization can be partitioned according to temperature to sequentially measure each element of a mixture. The element with the lowest vaporization temperature is measured first and the element with the highest vaporization temperature is measured last. To the extent that there is overlap in the vaporization behavior of the elements of interest, this procedure provides an optimum utilization of the sample in the measurement of a multielement mix. If matrix effects are controlled, the vaporization temperatures can be predicted from the single element data. With the ability to change laser wavelengths rapidly and the ability to form mixed atom reservoirs and to control multielement atomization, the mass spectrometry requirement becomes one of being able to make isotope ratio measurements. A magnetic sector instrument was used in this study, but a quadrupole of time-of-flight mass spectrometer could have been used with appropriate trade-offs in sensitivity, resolution, and accuracy (27). To illustrate the principle and potential efficacy of multielement analysis using RIMS, such a system is conceptualized in Figure 4. In principle, the wavelength selectivity of the laser (for the Nd:YAG system used here the bandwidth is 0.3 cm-l a t 560 nm, and order of magnitude higher resolutions are easily achievable), provides a basis for elemental ionization selectivity, and the mass spectrometer reduces the interferences to ones between isobars. By combining the elemental ground and metastable absorption wavelengths of Figure 3 onto a series of element-specific axes (the ordinates of Figure 4) and plotting vs. m / z , we form a three-dimensional matrix that defines d(X)-cl(rn/z)-d(e1ement) resolution segments that can be unambiguously addressed by the proper
combination of wavelength and mass settings, where d(X) and d ( m / z )represent small but finite resolution segments of the laser and mass spectrometer, respectively, and d(e1ement) represents a discrete element dimension for illustrative purposes. Ideally, laser bandwidth should be infinitesimal to permit the resolution of potential atomic spectral overlaps. The reduction of the laser bandwidth results in a new complication that is due to the existence of isotopic energy level shifts. These shifts are typically very small relative to the present laser bandwidth of 0.3 cm-l except for the very light, very heavy, and rare earth elements (28). The existence of an isotope effect would be addressed in quantitative isotope ratio measurement by simultaneously optimizing both the mass and wavelength settings of the RIMS system. It has been proposed that the isotope effect could be utilized analytically by the selective ionization of rare isotopes (29). There are two fundamental limitations that potentially preclude the realization of highest wavelength resolution: Dopper atomic line broadening caused by random thermal velocities in the atomization source and the finite laser bandwidth. Both these limitations can be experimentally addressed (30). The sensitivity of a multilelement RIMS system will be limited by the fact that mass, wavelength, and temperature settings will be different for each element. To the extent that atomic vaporization for different elements is partitioned in time and temperature, the multielement sensitivity should be no worse than that achieved in single element RIMS. If elements cannot be segregated in time, there certainly exists the potential for many-color resonance ionization with multichannel mass detection to achieve simultaneous elemental measurement. Nevertheless, the high ionization efficiency and selectivity (low noise) of resonance ionization should allow the RIMS system to operate for a considerable time without sacrificing a significant amount of sample. That is, quality measurements can be made on only a small fraction of a sample. The present limitation to "ultimate" sensitivity remains the low duty cycle of the RIMS system. A recently developed pulsed thermal atom source has demonstrated a 30-fold improvement in the ionization/sample utilization efficiency (31), and this pulsed source could be used with multielement samples. Further advances with pulsed atom sources and laser repetition rates appear achievable. Multielement Experiment. In order to illustrate the potential multielement capabilities of RIMS with thermal atomization, a sample was prepared which contained approximately 1 l g of each of the following elements: V, Fe, Co, Ni, Lu, Pb, and U. The elements were sequentially measured in the following order at the temperatures indicated P b (950 K), Fe (1400 K), Ni (1550 K), Co (1550 K), V (1750 K), Lu (1900 K), and U (1950 K). For each element the wavelength vs. ion signal was determined between 282 and 286.5 nm. The spectra are shown in Figure 5 and the data
ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
r U
>. +
Lu
Y
ffl Z
II
,
I
W
I
v
t-
Z H
I
I
c7 0
CO
J
Ni Fe
282
283
284
285
286
Pb
WRVELENGTH, nrn
Figure 5. Ion signal vs. wavelength for elements loaded together on a single thermal filament. Ion intensities are arbitrary among the elements.
are summarized in Table V. Each scan took 45 min. This experiment illustrates the facility of making multielement measurements and the long-term control over atomization which is achievable. Furthermore, neither Ni nor Co have ground-state-originating transitions in this spectral range, therefore demonstrating the analytical utility of the metastable excited states.
CONCLUSIONS The unfocused laser bean used in this work greatly simplified the spectroscopy described in the initially published work ( I ) . The selectivity has increased manyfold because of the reduction in the width of the major spectral features and the absence of wavelength-independent photoionization. Although the ionization process was not saturated, the increased laser/mass spectrometer interaction volume with the unfocused beam resulted in only a minor loss of absolute signal intensity. The absence of wavelength-independent ionization background, presumably produced by dissociation/ionization of photoexcited polyatomic species under intense laser flux, is a significant analytical gain. Atomic line overlaps among elements may provide a limitation to the breadth of elemental applicability or preclude application to certain mixtures or groupings of elements; again, however, the presence of mass selectivity limits this problem to one of interfering isobars. The evolution of an atomic RIMS line library and computerized sorting routine should minimize the line overlap problem by providing access to interference-free analytical lines. Metastable atom populations, although diluting the ultimate sensitivity of the technique, provide an added dimension of flexibility in line assignments and elimination of line overlaps. The ultimate goal for the RIMS multielement system described herein is to provide a hybridized laser-mass spectrometry approach to analysis that is broadly applicable, highly sensitive, and extremely selective. Such a system could be applied to a variety of analytical problems, such as certification of Standard Reference Materials, the ultratrace analysis of environmental, biological, and high-purity materials, or essential trace element bioavailability studies in nutrition.
2775
Registry No. Co, 7440-48-4; Dy, 7429-91-6;Er, 7440-52-0; Fe, 7439-89-6; Hf, 7440-58-6; In, 7440-74-6; Lu, 7439-94-3; Mo, 7439-98-7; Ni, 7440-02-0; Re, 7440-15-5; Sr, 7440-24-6; Ta, 744025-7; Th, 7440-29-1;Ti, 7440-32-6;U, 7440-61-1;V, 7440-62-2;Y, 7440-65-5;Zr, 7440-67-7;Pb, 7439-92-1.
LITERATURE CITED (1) Fassett, J. D.; Travis, J. C.; Moore, L. J.; Lytle, F. E. Anal. Cbdm. 1983, 55,765-770. (2) Fassett, J. D.; Moore, L. J.; Travis, J. C.; Lytle, F. E. I n t . J. Mass Spectrom. I o n Proc. 1983, 54,201-216. (3) Donohue, D. L.; Young, J. P.; Smith, D. H. I n t . J . Mass. Spectrom. I o n Phys. 1982,4 3 , 293-307. (4) Young, J. P.; Donohue, D. L. Anal. Cbem. 1983, 55,88. (5) Miller, C. M.; Nogar, N. S.;Gancarz, A. J.; Shields, W. R. Anal. Chem. 1982, 5 4 , 2377-2378. (6) Miller, C. M.; Nogar, N. S. Anal. Chem. 1983, 55. 1606-1608. (7) Hurst, G. S.;Payne, M. G.; Kramer, S. D.; Young, J. P. Rev. M d . Phys. 1979, 57,767. (8) Rosrnan, K. J. R.; de Laeter, J. R.; Chegwidden, A. Talanta 1982,2 9 , 279-283. (9) Walker, R. L.; Bertram, L. K.; Musick, W. R.; Smith, D. H. USDOE Rep.
ORNLITM-6808: Oak Ridge Natlonal Laboratory: 1979.
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RECEIVED for review April 16, 1984. Accepted July 31, 1984. The authors gratefully acknowledge that a portion of this research was supported by the Department of Energy through Contract DE-AI22-83PCG1275. Certain commercial e q u i p ment, 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.
