88
Anal. Chem. 1983, 55, 88-91
(18) Haverkamp, J.; Eshuls, W.; Boerboom, A. J. H.; GuinBe, P. A. M. “Advances In Mass Spectrometry”; Heyden: London, 1980; Vol. Ea, pp 983-989. (19) Haverkamp. J.; Meuzelaar, H. L. C.; Beuvery, E. C.; Boonekamp, P. M.; Tiesjema, R. H. Anal. Blochem. 1980, 104, 407-418. (20) Meuzelaar, H. L. C.; Kistemaker, P. G.; Schutgens, R. B. H.; Veder, H. A.; Cardinal, J. R.; Bowers, J. H.; Antoshechkln, A. I n “Current Developments In the Clinical Applications of HPLC, GC and MS“; Lawson, A. M., Lim, C. K., Richmond, w., Eds.; Academic press: London, 1980; pp 209-23 1.
(21) Rummel, R. J. “Applied Factor Analysls”; Northwestern University Press: Evanston, IL, 1970.
RECEIVED for review Januray 18, 1982. Accepted September 20, 1982, This investigation was supported by the Royal Netherlands Academy of Arts and Sciences (KNAW) and by the Foundation for Fundamental Research on Matter (FOM).
Ionization Spectra of Neodymium and Samarium by Resonance Ionization Mass Spectrometry J. P. Young* and D. L. Donohue Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830
Ionlzatlon spectra of the elements neodymium and samarium have been obtained over the wavelength range of 423-463 nm by uslng the technique of resonance lonlzatlon mass spectrometry (RIMS). These studies have been performed to determine the wavelengths at whlch lonlzation occurs under RIMS condltlons. The observed wavelengths have been correlated where possible with allowed transitions between known electronic energy levels. RIMS has previously been applled to the measurement of Isotope ratlos of these rare earth elements uslng a slngie wavelength of excltatlon for each element. The fact that there are a number of effectlve wavelengths available should be of Interest to other workers in the RIMS fleld.
Resonance ionization spectrometry (RIS) is a photoionization process in which atoms in a gas phase are ionized by the absorption of photons that energetically match transitions between quantum states of these atoms (1). General application of RIS to various analytical problems has also been discussed (2). There are several RIS schemes that have been developed (1,2) by using two or more photons of one or several colors. Although RIS is normally used to detect atoms either by the electron or the ion generated in the photoionization process, the detection of either of these species can also be used to study the absorption spectra of amenable atoms. Worden et al. (3) have used a several-color resonant photoionization process to identify upper Rydberg states and determine ionization potentials of lanthanides and actinides. Their study made use of an atomic beam; one or two lasers were tuned to generate a particular excited state, and with a tunable laser, the autoionizing and ionizing energies of the species could be identified. Recently, we have applied a one-color RIS scheme to the selective photoionization of either Sm or Nd for mass analysis in a mass spectrometer ( 4 ) . By means of this combination technique, resonance ionization mass spectrometry (RIMS), the removal of the isobaric interference of either element on the other has been demonstrated. By observing the presence and intensity of a particular mass signal for either of these rare earth ions as the wavelength of the lasing is changed, one can obtain a spectrum of each element. Knowledge of such a spectrum is useful not only to identify analytically useful RIMS wavelengths but also to evaluate particular subsets of atomic transitions that in normal emission spectral studies
are combined with many other subsets of possible atomic transitions. The present study was initiated in order to find suitable wavelengths for the isotopic analysis of neodymium and samarium by RIMS. In the analysis of a mixture of elements, it is necessary to know which wavelength (transition) to use to avoid overlap with interfering species. It was also found in preliminary studies that many more transitions were observed than originally expected. This complexity is a mixed blessing in that the probability of overlap with adjacent elements is increased for a particular transition but a larger choice of transitions is available for choosing a suitable RIMS wavelength. A systematic study was therefore performed to catalog the observed wavelengths a t which ionization occurs for the two elements neodymium and samarium. Due to the limitations imposed by the dye laser system, the spectra were limited to a certain range of wavelengths which is smaller than the range over which ionization OCCUTS. However, useful transitions have been predicted to lie within the wavelength range reported here. Future work will seek to extend the wavelength range for the elements Nd and Sm, as well as to report results for other rare-earth elements. EXPERIMENTAL SECTION Laser System. The laser system and optical components used for this investigationare the same as used in the previous RIMS study (4) except that the tuning micrometer of the NRG 0.03 dye laser module (National Research Group, Inc., Madison, WI) was equipped with a stepping motor so that the wavelength range of
a particular dye could be scanned under computer control. The details of this scanning are in a later part of the Experimental Section. Three different dyes, S-420, C-440, and C-460 (available from Exciton Chemical Co., Dayton, OH), were used to cover the wavelength range of 423 to 463 nm that was used in this study. The relative intensity of these dyes as a function of wavelength is given in Figure 1. Under our experimentalconditions, the dye S-420 at a wavelength of 425.8 nm yielded an energy of approximately 300 pJ. Although the relative energy correlation from dye to dye cannot be considered to be highly precise, a qualitative assessment of dye intensity vs. wavelength can be made from the data in Figure 1. The tunable laser beam was brought to a focus ( 4 )just in front of and in the vertical center of the first slit of the ion source of the mass spectrometer, described below. The spot size of the beam at the focal plane was approximately 1 mm, but any ions generated by RIS were accepted over the entire length of the first slit, 2 cm. It was necessary to calibrate the wavelengths of the respective dyes vs. micrometer dial reading of the dye laser module. This
0003-2700/83/0355-0088$01.50/00 1982 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983
Table I. RIMS Spectrum of Neodynium obsd peak initial in cm-' in nm intens transition (air) (vat 1 M 423.2 23 623
0423
429
431
1
435
8
,
~
442 441
447
1
453
,
~
460
W4VELENGTH l n m l
Flgure 1. Dye laser output spectra for dye 1 = S-420; dye 2 = C-440; dye 3 = C-480.
was done by recording the wavelength of five or more settings of the micrometer dial of the laser by means of a 1/2-mmonochromator. The wavelength accuracy of the monochromator was verified by the Hg emission line at 435.8 nm. From this series of wavelength vs. laser dial readings, a calibration factor was computed which converted a given dial reading to wavelength. Mass Spectrometer, The mass spectrometer has been previously described ( 4 ) and consisted of a single 90' magnetic sector of 30 cm radius. Ion detection was by means of a 14-stage electron multiplier (RCA Corp. type 6810A, Lancaster, PA) operated at 4000 V. Laser-generated ions, upon striking this detector, produced a signal which was processed and digitized by a 13-bit analog-to-digital converter (ADC). These digital data were accepted by a PDP-11/34 computer (Digital Equipment Corp., Maynard, MA) and stored in an array of 256 channels. Each scan of the spectrum covered 512 steps of the stepping motor driving the micrometer dial of the dye laser. The wavelength range covered was 20.8 nm. Therefore, each channel of the final data array represented a resolution element of 0.08 nm composed of the average of signals from 10 laser pulses. The estimated number of ions detected per laser pulse was 5 to 20. Samples of Nd and Sm were prepared from the pure oxides in 1 N HN03solution to give a concentration of 100 pg/wL. One microliter of solution was loaded onto a Re filament, and a drop of colloidal graphite suspension (Aquadag) was added to enhance the production of neutr,el Nd or Sm atoms in the ion source in preference to oxide species. Isotopes chosen for study (142Nd, 14Nd, 14%m)were free from isobaric overlap with other elements present. A check of this fact was made by comparing spectra of two Nd isotopes; there were no significant differences due to an interfering species. The samples were maintained at a temperature of approximately 1500 "C for this study. The individual scans of a given wavelength range took about 8.5 min to complete. For these studies sufficient sample was loaded on the filament that the Nd RIMS signals degraded only slowly with time, approximately 10% reduction in ion intensity per hour. RESULTS AND DISCUSSION Nd Spectrum. Figure 2 shows the composite spectrum obtained for Nd over the wavelength range 423-463 nm. The
424.3 424.8 425.9 426.4 426.8 427.2 428.1 428.7 429.3 430.5 430.8 431.1 431.7 433.0 433.8 434.4 437.1 437.4 438.2 439.7 440.1 440.9 441.8 442.3 442.7 443.6 444.3 444.7 445.3 446.8 447.8 449.7 451.0 451.6 452.7 454.0 455.3 455.8 456.1 456.5 461.8 463.1
~
23 561 23 533 23 473 23 445 23 423 23 401 23 352 23 320 23 287 23 222 23 206 23 190 23 158 23 088 23 045 23 016 22 871 22 856 22 814 22 736 22 715 22 674 22 618 22 603 22 582 22 536 22 501 22 481 22 450 22 375 22 325 22 233 22 166 22 137 22 083 22 020 21 957 21 933 21 919 21 899 21 648 21 587
S S S
w
S W M M W W S M ~
w
430
435
440
2
51,-5~,
51,-5~,
51,-5~, 51,-5~,
~I,-~H,
1
14 -7 11
-4 -2 8
M M S
51,-5~,
5 2
M M
514-5H4,5 51,-5~,
1 1
w w
w w w M w W S
w w S S
M
51,-5~, 51,-5~,
51,-5~, 51,-5~,
51,-5~,
1
4 4 3
51,-5~,
-6 -10 -1 0
51,-5~,
-7
51,-5~, 51,-5~,
5
I,-~M,
51,-5~,
12 -4
W
w w w S S
51,-7~,
W
51,-7~,
w W w
51,-7~,
6 4 5
horizontal axis consists of 430 channels each representing the sum of 10 laser pulses. Amplitude information is only approximate due to the variations in RIMS signal for each laser pulse. Reproducibility of peak heights in replicate spectra was estimated to be *30%. A summary of the Nd RIMS spectrum shown in Figure 2 is given in Table I. As noted in the table, both the wavelength in air and the wavenumber under vacuum of the peaks are
IONIZATION LIMIT 4 4 8 . 8
425
error cm-
51,-6~,
51,-5~,
89
445 WAVELENGTH i n m i
Figure 2. Resonance ionization spectrum of neodymium obtained by monltorlng 14*Nd.
450
455
460
~
90
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983 9,
I
I
I
' 1
I
2
1
1 -I
-
I
445 125
430
435
WAVELENGTH I n m
Figure 3.
2
450
455
440
460
I
Resonance ionization spectrum of samarium obtained by monitoring I4'Sm.
given. The accuracy of the wavelength assignments was based on the repeatability of five separate scans which were summed and on the accuracy of calibration of the dye laser micrometer dial. The main peaks in Figure 2 are labeled and listed in Table I with their wavelength of maximum intensity to an accuracy of f O . l nm. The intensities of the various peaks are given only as strong (S),medium (M), and weak (W) because of the semiquantitative nature of the data gathering process as discussed in the Experimental Section. On the basis of assignments of energy levels for Nd ( 5 ) ,some of the initial transitions involved in the RIS process can be identified. It is apparent from the table that ionization is possible with photoenergies of less than half of the ionization potential, 22280 cm-'. An RIS scheme (1) (1,2) requires that twice the energy of the exciting photon is equal to or greater than the ionizing potential of the atom; this, of course, assumes that the atom is in its atomic ground state. The fact that RIMS spectral peaks were observed at energies less than this limit implies that the Nd atoms generated from the thermal source were not all a t the lowest lying ground state, 514.As seen in the table, several of the lower energy peaks can be assigned to transitions from higher states. Of the 43 peaks given in Table I, 23 have been tentatively correlated to particular transitions; the errors in these assignments are given in the last column. The assignments were made from published spectral data (5-7). It is apparent that no assignments seem possible for almost 50% of the RIMS peaks observed. Since the sample taken for these studies was Nd and the mass detected was 142, it is probable that these peaks arise from Nd and that the photoionization proceeded by some, unidentified, allowed process. This points up a unique feature of RIMS spectral data. Such data can be useful in identifying various subsets of spectral transitions that originate from low lying levels that might be difficult to recognize in a complex emission spectrum. Sm Spectrum. Figure 3 shows the ionization spectrum obtained for Sm, covering the same wavelength range as in Figure 2. Measurement conditions were kept as similar as possible so that the precision of peak heights (areas) and the accuracy of wavelength assignments are the same as those in Figure 2. The relative intensities of peaks in Figure 2 are not directly comparable to those in Figure 3, because there was no convenient means of normalizing the RIMS signals obtained for each element. A summary of the Sm RIMS spectrum shown in Figure 3 is given in Table 11. The presentation of these data is similar to that in Table I in that wavelength, wavenumber, intensity, transition, and error of tentative assignments are given. Contrary to the case of Nd in Table I, all peaks listed can be identified with known transitions in the Sm spectrum. In fact, several assignments are possible for some of the peaks, and it may well be that the peak is made up from contributions
Table 11. RIMS Spectra of Samarium obsd peak in nm in cm-' (air) intens (vat ) 23 579 M 424.0 M 23 434 426.6 23 336 428.4 S
error cm-' -3 -2 1
430.0
23 249
M
431.3
23 179
M
3 -5 4 -5
432.0 433.0 433.9
23 142 23 088 23 040
w
3
435.6 436.5
22 950 22 903
S
438.0 439.7 441.1 441.9 442.9 443.4 444.2
22 824 22736 22 664 22 623 22 572 22 546 22 506
w M w S
446.0 447.1
22 415 22 360
-1 1
S S
4 3
449.9 452.4
22 221 22 098
M
1 -1
453.3
22 054
M
457.1
21 871
W
4 7 2 -4 7
S
M M
M S
w
w
1
0
-5 0 0
-5
11 -2
-2 -3 -2 -3 5
of both. As was the case with Nd, many Sm peaks are observed at energies less than half the ionization potential of Sm, 22760 cm-'. In the data shown in Table 11, it is obvious that transitions from the Sm ground state, 7Fo,account for few, if any, of the RIMS peaks. On the basis of the transition assignments of the Sm spectrum, it appears that the 'F2 and 7F3 levels are the most populated states. From emission spectral results (8),it has been reported that 7F1and 7F2states are most populated a t 1270 K. The temperature of our samples was not continually monitored, but the sample temperature was the order of 1750 K. Note the assignment of the 441.1-nm and 441.9-nm peaks as arising from a 'F3 and a 'F1 level, respectively. As a check of this assignment, the relative intensities of these peaks were observed at three temperatures, 1773, 2073, and 2273 K. With increasing temperature, the population of the higher energy ground state increased relative to the lower state as expected from Maxwell-Boltzmann considerations.
Anal. Chem. 1983, 55, 91-94
It is interesting to note that there is no correlation between published data for relative oscillator strengths of Sm lines (9) and the relative intensities of the Sm RIMS peaks. Because of the power level of our laser system and the optical arrangement necessary to transmit the laser beam into the mass spectrometer, optical saturation of the ionization was not realized, and one might assume that the RIMS signals would be some function of oscillator strength of the respective transitions involved. There are certain deficiencies in our intensity monitoring procedure. With time, the concentration of the thermal sample changes. Our evaluation of laser dye intensity over the three dye ranges is only qualitative. Even within a given dye range there is, perhaps, 30% variation possible in signal strength measurement caused mainly by statistical variation in the small number of RIMS ions generated. As a result of the above reasons, or because of presently undefined reasons, there is little if any correlation of relative oscillator strength and RIMS signal. Compare, for example the strong intensity of a RIMS peak at 435.6 nm and a weak RIMS peak at 438.0 nm with reported relative oscillator strengths of 28.8 and 330, respectively (9). This observation is interesting, but until more precise control of the
91
experimental parameters is possible, no real conclusions can be drawn. LITERATURE CITED Hurst, G. S.; Payne, M. G.; Kramer, S. D.; Young, J. P. Rev. Mod. Phys. 1979, 5 1 , 767. Youno. J. P.: Hurst. G. S.: Kramer. S. D.: Pavne. M. G. Anal. Chem. 1979y51, 1050A. Worden, E. F.; Conway, J. G. ACS Symp. Ser. 1980, No. 131, 381-425, and references therein. Donohue, D. L.; Young, J. P.; Smith, D. H. I n t . J. Mass Spectrom. Phys. 1982, 4 3 , 293. Martin, W. C.; Zalubas, R.; Hagen, L. Natl. Stand. Ref. Data Ser. ( U . S . , Natl. Bur. Stand.) 1978, NSRDS-NBS 60. Meggers, W. F.; Corllss, C. H.; Scribner, B. F. NBS Monogr ( U . S . ) 1975, No. 145. Blaise, J.; Wyard, J. F.; Hockstrd, R.; Kruiver, P. J. G. J. Opt. SOC. Am. 1971, 61, 1335. Parr, A. C.; Inghram, M. G. J. Opt. SOC.Am. 1975, 6 5 , 613. Kamarovskli, V. A,; Perkin, N. P.; Nikiforova, G. P. Opt. Spectrosc. 1970, 2 9 , 116.
RECEIVED for review July 2, 1982. Accepted September 27, 1982. Research sponsored by the U.S. Department of Energy, Division of Chemical Sciences and the Office of Health & Environmental Research, under Contract W-7405-eng-26 with Union Carbide Corp.
Determination of Organic-Bound Chlorine and Bromine in Human Body Fluids by Neutron Activation Analysis dames D. McKinney" National Institute of Environmental Health Sciences, Research Trlangle Park, North Carolina 27709
Adel Abusamra and John H. Reed Science Applications, Inc., 4030 Sorrento Valley Blvd., San Diego, California 9 2 12 1
The levels of organic-hound chlorine and bromlne In human milk and serum are determlned by neutron activation analysis. Desalted milk and seruim fractions are Irradiated wlth neutrons In a nuclear reactor and the resultlng y-rays of 38CIand "Br are measured. The dosaltlng procedure, achieved by using Blo-Gel molecular sleves, virtually removes all lonlc chlorlde and bromides from milk and serum. Radloactlve tracer studles wlth polychlorinated blphenyl-14CIndicate a recovery of 90% through the Blo-Gel column. The total organlc chlorlne In 2,2-(4-chlorophenyl)-l,l-dlchloroethene spiked milk and heptachlor spiked milk, determlned after being desalted and lrradlated accordlng to thls procedure, substantlates a good recovery of the added spike. The lower limits of detection of organlc-bound chlorine and bromine In milk or serum are 50 and 5 parts per bllllon (ppb), respectlvely.
The level of organic-bound chlorine (TOCl) and bromine (TOBr) compounds has been rising in the environment due to the growing commercial use of large quantities of halogenated hydrocarbons and the previously unregulated dumping of organic w a i k Halogenated hydrocarbon residues in breast milk or serum are normally measured by a combination of solvent extraction and gas chromatography with electron capture detection (GC/EC) (2). Identification and measurement of the full spectrum of halogenated organic 0003-2~00/83/0355-0091$01.50/0
compounds become a difficult task. A value for TOCl and TOBr can be produced by using neutron activation analysis on desalted milk and serum fractions. This information is valuable in answering questions regarding the total amount of such compounds present in human milk or serum. For determination of TOCl and TOBr in milk or serum, ionic species such as sodium chloride (NaC1) and sodium bromide (NaBr) must be removed completely by a desalting process before neutron activation is performed. Neal and Florini used Sephadex G-25 as a molecular sieve to remove NaCl from serum, utilizing batch extraction with a centrifuge (2). Uziel and Cohen used gel filtration for desalting certain nucleotides (3). Ludkowitz and Heurtebise determined protein-bound iodine in serum, through neutron activation, after desalting with ion exchange resins ( 4 ) . Fritz and Robertson determined protein-bound trace metals in serum by using neutron activation after gel filtration with Bio-Gel P-6 ( 5 ) . The nuclear activation process does not differentiate between inorganic chlorides and chlorine bound to organic molecules. Also neutron capture can recoil organically bound chlorine into free atoms through the Szilard-Chalmers reaction (6). Preirradiation separations of ionic chlorides and bromides from organic-bound chloride and bromine species are therefore necessary. The separation of inorganic sodium chloride and other ionic species in milk and serum from the organic-bound halogens associated with the protein and lipid fractions can be accom0 1982 American Chemical Society
