Anal. Chem. 1988, 58,3095-3100
pairs, and solving the three equations for the three k m k ,we find that k l = 83.73, kz = -2,263, and 123 = 30.48. Applying these results to a new solute-solvent pair, cis-3-heptene and HDI, the predicted retention index is Icis-%heptene,HDI =
(83*73)(7*1)(1)+ (-2.263)(0.356) (1.83)
+ (30.48) (21) (0.17)
= 704.7 r.i. units (experimental value = 698) The target combination model predicts the datum fairly near experimental error. Carrying out a similar calculation for 18 representative solute-solvent pairs, the average error in prediction is 9.8 r.i. units. The complete target FA model is a useful one, a satisfying conclusion considering the considerable complexity of this solute-solvent interaction problem.
ACKNOWLEDGMENT We are indebted to the Computer Center of The City University of New York for the use of its facilities. Registry No. n-Pentane, 109-66-0; n-hexane, 110-54-3; nheptane, 142-82-5;1-pentene, 109-67-1;1-hexene, 592-41-6; 1heptene, 592-76-7; 1-chloropropane, 540-54-5; 1-chlorobutane, 109-69-3; 1-chloropentane, 543-59-9; bromoethane, 74-96-4; 1bromopropane, 106-94-5;1-bromobutane,109-65-9;iodoethane, 75-03-6; 1-iodopropane, 107-08-4; trans-2-pentene, 646-04-8; cis-2-pentene,627-20-3; trans-2-hexene, 4050-45-7;cis-2-hexene, 7688-21-3;trans-2-heptene, 14686-13-6;cis-2-heptene,6443-92-1; trans-3-heptene, 14686-14-7; cis+heptene, 7642-10-6; 2methylbutane, 78-78-4; 2-methylpentane, 107-83-5; 3-methylpentane, 96-14-0;2,2-dimethylbutane,75-83-2;2,3-dimethylbutane, 79-29-8; 2-methylhexane, 591-76-4; 3-methylhexane, 589-34-4; 3-ethylpentane, 617-78-7; 2,2-dimethylpentane, 590-35-2; 2,3-
3095
dimethylpentane, 565-59-3; 2,4-dimethylpentane, 108-08-7;3,3dimethylpentane, 562-49-2; 2,2,3-trimethylbutane,464-06-2; 4methyl-1-pentene, 691-37-2; 2-methyl-2-pentene, 625-27-4; 3,3dimethyl-1-butene, 558-37-2; 2-methyl-l-hexene, 6094-02-6; 5methyl-1-hexene, 3524-73-0; 4,4-dimethyl-l-pentene, 762-62-9; 2-chluropropane, 75-29-6; 2-chlorobutane, 78-86-4; l-chloro-2methylpropane, 513-36-0; 2-chloro-2-methylpropane, 507-20-0; 2-bromopropane, 75-26-3; 2-bromobutane, 78-76-2; l-bromo-2methylpropane, 78-77-3;2-bromo-2-methylpropane, 507-19-7.
LITERATURE CITED (1) Malinowski, E. R.; Howery, D. G. Factor Analysis in Chemistry: Wllev: New York, 1980. (2) Dahlman, G.; Koser, H. J. K.; Oelert, H. H. J . Chromatogr. Sci. 1979, 17 .. , 307-313 - - . - .- . (3) Buydens, L.: Massert, D. L.; Geerlings, F. Anal. Chem. 1983, 5 5 , 738-744. (4) Gemperline, P. T. J . Chem. I n f . Comput. Sci. 1984, 24, 206-212. (5) Ramos, L. L.; Burger, J. E.; Kowalski, B. R. Anal. Chem. 1985, 5 7 , 2620-2625. (6) Weiner, P. H.; Howery, D. G. Anal. Chem. 1972, 4 4 , 1189-1194. (7) Weiner, P. H.; Liao, H. L.; Karger. 8. L. Anal. Chem. 1974, 4 6 , 2182-2190. (8) Kindsvater, J. H.; Weiner, P. H.; Klingen, T. J. Anal. Chem. 1974, 4 6 , 982-988. (9) Zielinskl, W. L.; Martire, D. E. Anal. Chem. 1976, 4 8 , 1111-1116. (10) Malinowski, E. R.; Howery, D. G.; Weiner, P. H.; Soroka, J. M.; Funke, P. T.; Selzer, R. B.; Levinstone, A. "FACTANAL", Program 320, Quantum Chemistry Program Exchange; Indiana University: Bloomington, IN, 1976. (11) Mallnowski, E. R. Anal. Chem. 1977, 4 9 , 612-617. (12) Weiner, P. H.; Malinowski, E. R.; Levinstone, A. R. J . Phys. Chem. 1970. 74. 4537-4542. (13) Weiner, 6. -H.; MalGowski, E. R. J . Phys. Chem 1971, 75, 3160-3163
RECEIVED for review September 1, 1983. Resubmitted August 7, 1986. Accepted August 28, 1986.
Detection and Spectroscopic Study of Zinc by Laser-Enhanced Ionization Spectrometry George J. Havrilla* and Kee-Ju Choi* Standard Oil Research a n d Development, 4440 Warrensville Center Road, Cleveland, Ohio 44128
Detection of rlnc by laser-enhanced lonlration spectrometry is demonstrated for both single and stepwlse excltatlon schemes Involving seven rlnc transttlons, lncludlng resonance llnes at 213.8 and 307.6 nm. Although rlnc has an lonlratlon potential of 9.4 eV, It exhlblts hlgh sensltlvlty for one-photon, hlgh oscillator strength transttlons and stepwise, low osclllator strength transltlons. Depending upon the excltatlon wavelengths utlllzed, detectlon limlts range from 10 pg/mL to 1 ng/mL. Comparison between excltatlon schemes Is given. Observation of spectral background at both steps of a stepwlse scheme Is reported.
Laser-enhanced ionization (LEI) spectrometry is a method that is based upon a two-step mechanism for the production of the ionization signal of the analyte. This two-step mechanism involves laser photoexcitation followed by collisional thermal ionization. Since the ionization step is dependent upon the thermal energy of the flame to move the atom from the excited state populated by the laser excitation, the closer the excited state is to the ionization potential the easier the ionization step will be. Until now, detection by LEI has been limited to elements with ionization potentials less than 9.2 0003-2700/86/035S-3095$0 1.50/0
eV (gold) and has used excitation wavelengths longer than 228 nm ( I ) . The present work describes the LEI detection of zinc, which has an ionization potential of 9.4 eV. In addition, we demonstrate the use of 213.8 nm for LEI spectrometry. In this report, zinc is detected directly by a laser-based method utilizing the resonance transition at 213 nm. A previous effort (2) reported zinc detection using two-photon laser-induced fluorescence. Since the 213-nm line is the strongest line in the zinc spectrum, it is the line used for most analytical emission and absorption spectrometric determinations. Several stepwise transitions have been found that have the same lower energy level as the excited level of the 213-nm transition. Comparisons will be made among the single-step and stepwise excitation schemes concerning sensitivity and selectivity for zinc determination. Of particular interest is the comparison with the second zinc resonance line at 307 nm. The transition at 307 nm has an emission intensity that is 2 orders of magnitude less than the 213-nm transition. This is expected since the transition is a spin-forbidden singlet to triplet. The sensitivity of this line should be fairly poor for LEI detection as well, since the excited state is less than half (4.1 eV) of the ionization potential. An attractive feature of the 307-nm line is that it is easier to generate; the laser energy is much higher and is less 0 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986 Nd-VAG LASER
Zn I GENERATOR
3Pp
' 0 , 'D, 1 1
75.8
'D, 'Pp
I
I
I
'5, I
I
t
70 -
60
PDP-11/23 COMPUTER
2ol
STEPPER AtoD CONTL'R CONV. CAMAC
10
Figure 1. Schematic diagram of experimental setup.
likely to have as much spectral interference as the 213-nm line. Stepwise excitation should have a significant enhancement on the sensitivity of the zinc detection. Only one transition, at 472 nm, was found that can couple with the excited state of the 307-nm line. This stepwise scheme puts the final excited level a t 6.7 eV above the ground state. This is a full electronvolt above the single-step excitation by 213 nm. T h e analytical capability of the 213-nm laser wavelength is demonstrated through the determination of zinc in a National Bureau of Standards Standard Reference Material (NBS SRM). In addition multiphoton ionization spectra of nitric oxide will be presented.
EXPERIMENTAL SECTION A schematic diagram of the instrumentation used in these experiments is given in Figure 1. Laser excitation is provided by dye laser 1 (Quanta Ray PDL-1, Mountain View, CA) and dye laser 2 (homemade) pumped by the harmonics of a NdYAG laser (Quanta Ray DCR-1). The far-UV wavelengths (around 214 nm) were generated by frequency mixing in a KDP crystal. The type-I phase-matching conditions for a KDP crystal are such that if two input beam wavelengths are widely separated, it is possible to generate shorter wavelengths. The shortest wavelength that can be generated using the Nd:YAG fundamental of 1064 nm is 217 nm. In order to generate 214 nm, wavelengths around 1100 nm are required. Wavelengths as low as 190 nm should be possible by carefully selecting the input wavelengths, since the absorption of the KDP crystal starts below 190 nm. An infrared dye, DNXTPC (Exciton, Dayton, OH), pumped by the fundamental of the Nd:YAG laser generated high-energy output (20-80 mJ) in the region 1090-1300 nm. This dye laser is a very convenient source for frequency mixing. It should be noted that the toxicology of the dye is unknown, and coupled with the fact that it is dissolved in Me,SO, the d37e solution should be handled with caution. The MezSO solvent must be deoxygenated and the dye solution must be maintained under an oxygen-free atmosphere within the dye laser. The dye remains stable for up to 40 h under these conditions. The near-IR dye output is frequency-mixed with the 266-nm fourth harmonic of the Nd:YAG. The type-I KDP crystal is cut at 82'. Since the frequency mixing process is not a threshold process, it generates very stable far-UV light. The output energy after the prism separator is about 30 WJat 214 nm. The low conversion efficiency is due t o a mismatch of the spatial profile of the two input beams. Further improvement was not necessary since this energy level was sufficient for these experiments. The laser bandwidth of both the UV and the visible beams was -0.02 nm, and pulse duration was -5 ns. The DCM dye was pumped by the second harmonic of the Nd:YAG laser (533 nm) and frequency doubled with type-I KDP to generate 307 nm. The UV energy is about 1 mJ at the 307-nm line. Coumarin 480 dye was pumped by the third harmonic (355 nm) of the Nd:YAG fundamental. The dye laser output at the 472-nm line was around 1.5 mJ. In order to generate the 397-nm wavelength, the output of the DCM dye laser was mixed with the fundamental of the Nd:YAG laser. Direct generation of 397 nm was not possible because the fourth harmonic was used to generate the 213-nm wavelength. This prevents the use of the third harmonic (355 nm) to pump a dye and generate the 397-nm wavelength directly.
I I
307.590
34'041 I
2s
9.4 - 9
213.856
I
I
Figure 2. Partial zinc Grotrian energy level diagram of LEI-detected transitions.
Table I. Observed LEI Zinc Transitions and Limits of Detection (LOD) transition
1st step A, nm
'So
213.86
-+
'PI
transition
'Pi
-
'P, 'PI
307.59
LOD,
ng/mL 3
'Po
'Si
2nd step A, nm
+
--
+
-
3P1
'So 3D, 3D, 'D,
396.55 623.79 623.96 636.24 472.22
1
8
16 2 10 000
15
The flame was acetylene-air supported on a premix burner (Perkin Elmer 0057-0988, Norwalk, CT) equipped with impact bead, flow spoiler, and 5-cm slot burner head. The pressure and flow rates of the respective gases were as follows: oxidant 30 psi, 3.5 L/min; auxiliary oxidant 30 psi, 11-13 L/min; and fuel 8 psi, 2.5 L/min. Aqueous solutions of zinc were prepared by serial dilution of a 1000 clg/mL aqueous zinc standard solution (SPEX Industries, Inc., Edison, NJ). The solutions were aspirated at about 3 mL/min. The ionization signals were detected with a water-cooled cathode shaped from 1/4-in.-o.d.stainless-steel tubing and placed 15 mm above the burner head (3). The 2-mm-diameter laser beam was centered about 3 mm below the edge of the cathode. A -1.5-kV potential was applied to the cathode from a photomultiplier power supply (EM1 Gencom, Inc., 3000R, Plainview, NY). The burner head served as the anode and was connected to a homemade charge-sensitive preamplifier ( 4 ) . The output from the preamplifier was further amplified and filtered with a differential amplifier (Tektronix 7834, Beaverton, OR) and digitized with a gated analogto-digital converter (Kinetic Systems, 1500 Camac Crate, Lockport, IL). The data were obtained by averaging 10 laser shots/point. All of the data acquisition, processing, and control of the stepper motors for dye lasers and harmonic crystals were performed with a computer (Digital Equipment Corp. PDP-11/23, Maynard, MA). Laser power measurements were made with either a Scientech power meter (Series 36, Boulder, CO) or a pyroelectric energy detector (Molectron, J3-05DW, Sunnyvale, CA).
RESULTS AND DISCUSSION The transitions of zinc detected by LEI are illustrated in the partial Grotrian energy level diagram in Figure 2. The analytical utility of seven transitions for zinc has been investigated in the acetylene-air flame and limits of detection are listed in Table I. There are two known resonance transitions of zinc in the UV spectral region. A spin-forbidden 'S 3P transition is at 307.590 nm, and a strong 'S 'P transition is at 213.856 nm. 'S 3PTransition. Excitation of the 307.59-nm line gives rise to a transition from the 4s' 'So ground state to the 4s14p' excited level at 32 501 cm-'. The spin-forbidden nature of this line is reflected by the transition probability of 3.29
-
-
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986 000
I-
Z W
lY
I 470
47 1
472
473
WAVELENGTH
700
474
475
(nm)
c
465
470
475
WAVELENGTH
the presence of the zinc in the flame. Most of the lines observed are unknown in the literature. The situation here appears similar to that observed by Turk and Watters (7) and Hart et al. (8)of two-photon excitation. Since the stepwise excitation of 307 nm plus 472 nm is still well below the ionization limit (2.7 eV), the possibility exists of two-photon 472-nm excitation into the closely spaced energy levels near the ionization limit. All of the lines observed are due to the presence of the zinc solution in the flame. Excitation from the J = 0 and J = 2 level populations are observed in the lines at 468.0 and 481.1 nm, indicated by the X's in Figure 3b. These levels are populated by collisional processes from the directly populated J = 1 level. However, the J = 1 level is at least 10 times stronger because the system is far from equilibrium within the time scale of the 6-ns laser pulse. Of the remaining lines observed, some have been tentatively identified, using a modified Rydberg formula, as two-photon transitions from the J = 1 level to high-lying Rydberg states of zinc (np 3P, where n = 10-25). The progression is indicated by the triangles in the lower portion of Figure 3b. There does not appear to be any two-photon excitation from the J = 2 or J = 0 levels. This is probably due to the low population in these levels. At the two-photon photoionization limit, 462.1 nm, there is a slight increase in the base line. Many of the lines observed are unassignable to known zinc transitions. Low-level impurities are an unlikely source of these lines. 'S 'P Transition. Due to a strong transition moment and smaller energy gap between the photoexcited and ionization levels, 213.8-nm ex. iiation to the 4s14p' 'P, state gives a much higher sensitivity. However, the far-UV excitation presents other problems because more flame species and other elements are likely to have absorption bands in the same wavelength region. This is a general problem when a far-UV wavelength is used. An example of this is given below, which demonstrates how this problem can be solved by the high selectivity offered by laser spectroscopy. Single-Step Excitation: NO Interference. A wavelength scan of the 213-nm region is shown in Figure 4a for deionized water. This spectrum is due to flame species and is a laserinduced ionization signal. The lines observed have been assigned to the NO y(1,O) band (9). A zinc signal can be observed by aspirating a 50 ng/mL solution of zinc as shown in Figure 4b. Background subtraction of a from b results in Figure 4c, the LEI signal for 50 ng/mL of zinc. The detection limit for this single-step excitation scheme is around 3 ng/mL. This sensitivity should be compared with 307-nm excitation where the detection limit is higher even with stepwise excitation. The excited state is still 3.6 eV below the ionization potential, even with the high-energy excitation of the 213-nm wavelength. In addition the background ionization of the NO molecule interferes with the zinc detection. To avoid the NO interference, stepwise excitation was attempted. Other transitions that have the same lower energy level as the 'P, excited state of the 213-nm transition were found (10). Stepwise Excitation to 8s 'So. It was expected that the 396.545-nm transition coupled with the 213-nm wavelength would increase the sensitivity considerably, since the upper level would be within 0.5 eV of the ionization potential. The important benefit in this case of stepwise excitation was the enhancement of selectivity not in sensitivity. The enhancement of the LEI signal was only a factor of -3. This could be due to a number of factors. The laser energy at the 397-nm wavelength was -0.2 mJ. The spatial overlap of the two beams was difficult due to the poor spatial quality of the 397-nm beam. Finally, the transition probability for this line may be low. A visible wavelength scan of the 397-nm transition with 213-nm wavelength fixed is shown in Figure 5.
-
....... . . . . 460
x
480
.
485
(nm)
Figure 3. (a) Detailed visible scan of 472-nm transition with 307-nm excitation fixed for 1 Fg/mL zinc solution. Ion current is in arbitrary units. (b) Visible scan of 10 Fg/mL zinc solution, with 307-nm excitation fixed. The triangles indicate Rydberg formula calculated twophoton transitions for ,/ = 1.
x lo4 s-l (5). Due to the energy gap between the excited state and ionization potential, the detection limit at this wavelength is -10 pg/mL. Since the excited state of the 307-nm transition is less than halfway to the ionization potential of 75 768 cm-', stepwise excitation (6) was used to improve the sensitivity of the zinc detection. A line at 472.216 nm was found that had a common energy level with the 307-nm line. This transition is from the 4p1 3P1upper level of the 307-nm line (32 501 cm-l) to the 5s2 3S1level at 53 672 cm-l. The result was enhancement of the zinc LEI signal by a factor of 650 over the single-step excitation. The stepwise excitation reduces the energy gap between the excited state and the ionization potential from 5.4 eV to 2.7 eV. The detection limit is -15 ng/mL for this excitation scheme. A detailed scan of the 472-nm transition a t 1 hg/mL is shown in Figure 3a. Since the transition is from 3P to 3S,a doublet would be expected. The appearance of the triplet remains unexplained at this time, although it could be due to a resonance excitation into a Rydberg level close to the ionization potential. A wavemeter would be useful for analytical work to ensure the laser wavelength positions of both excitation steps and avoid off-peak on-resonance measurements. Stepwise excitation has pushed the detection limits of zinc into the nanogram per milliliter range. In Figure 3b, the visible laser is scanned over a larger region with the UV laser fixed a t 307 nm. The LEI stepwise signal of zinc at 472 nm saturates the preamplifier and goes off scale. This spectrum was recorded with a 10 wg/mL zinc solution. Wavelength scans with only deionized water and with the UV beam blocked indicate that the signals observed are due to
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
625
-
z W
506
-
3
375
-
250
-
t-
a a
0
z 0
1251
‘ 92
WAVELENGTH (nm)
t-
z
1
213.8
213.9
W
1
U a 3 0
214.0
WAVELENGTH (nm)
Flgure 4. (a) Wavelength scan of deionized water in acetylene-air flame. The lines are due to multiphoton ionization of NO y band region. Ion current is in arbitrary units. (b) Wavelength scan of 50 nglmL zinc solution. (c) Background-subtracted signal of 50 ng/mL zinc solution (b - a). -‘-I
1
l i
h
z
z!
396.4
396.55
3%.7
3%.85
397
WAVELENGTH (nm) Flgure 6. (a) Wavelength scan of NBS S R M 1643a diluted 1:3: (top) raw LEI signal and (bottom)background-subtracted and laser energy corrected zinc signal. Ion current is in arbitrary units. (b) Stepwise LEI signal of NBS SRM 1643a.
K
3 a
i
390
400
in order to reduce the spectral and ionization interferences from the alkali and alkaline earth elements present in the 320c SRM. The total undiluted alkali and alkaline earth element 0 concentration is 50 Fg/mL. The one-photon wavelength , 240 f scan of the diluted SRM is shown in the top of Figure 6a. The background interference results in poor signal-to-noise in the 160 [ I I (5.8 eV) light can directly phoraw signal. The 213.76-nm 395.8 396.2 396.6 397 397 4 toionize sodium (5.1 eV) and potassium (4.3 eV), which would WAVELENGTH (nm) give rise to wavelength-independent ionization. The increasing base line is simply a result of the increasing laser energy. The Flgure 5. Visible wavelength scan of 50 ng/mL of zinc, with 213-nm dye tuning range begins just below 213.76 nm. The signal was wavelength fixed. Ion current is in arbitrary units. extracted from the spectrum by correcting for the dye laser power and by using background subtraction resulting in the This illustrates the high degree of selectivity obtained relative trace shown in the bottom of Figure 6a. Calibration with to the one-photon excitation in Figure 4. The signal offset aqueous zinc standards gave a value of 120 ng/mL zinc. This between the zinc sample and the deionized water blank is due is probably the result of nonoptimum background subtraction. to the zinc ionization by the 213.8-nm single-step excitation. The selectivity of the stepwise excitation scheme is illustrated However, the scan is devoid of any indication of flame species in Figure 6b, which is a wavelength scan of the same dilute ionization. The stepwise excitation has effectively removed sample, with the 213-nm wavelength fixed and the visiblethe NO spectral interference on the zinc. Thus, analytical wavelength region scanned. In this case the zinc signal is easily determinations could be conducted by measuring the peak observed and gives a calculated zinc value of 96 ng/mL. The and base line of both the water blank and the sample. Dehigh zinc value could be due in part to the enhancement of tection limits below a nanogram per milliliter level would be the zinc LEI signal by the easily ionized species present ( 11 , expected with longer integration times. 12). Although these results do not agree with the certified SRM Analysis. In order to demonstrate the utility of the value, they demonstrate the capability of this wavelength 213.8-nm wavelength for analytical determination of zinc, we range for analytical use and the high selectivity gained through analyzed a standard reference material from the National stepwise excitation. These results could be improved with Bureau of Standards. The sample was SRM 1643a, trace an ion exchange pretreatment to remove the interfering eleelements in water, with a certified value of 73 ng/mL zinc. ments or matrix match the calibration standards. However, The sample was diluted by a factor of 3 with deionized water 0
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986 630,
a
i
h
I
z w a
5
540
b
I
-
4501
l
360-
C
634
637
640
643
E
WAVELENGTH (nm) Figure 7. (a) Visible-wavelength scan of 25 ng/mL zinc, with 213-nm wavelength fixed. Ion current is in arbitrary units. (b) Same-wavelength scan with deionized water. (c) Backgroundsubtracted zinc signal (a - b).
matrix matching requires an additional analysis to determine the level of concomitants. Other Energy Levels. Two transitions a t 623.79 and 623.92 nm were observed. These transitions are coupled with the 4p 'P, (46745 cm-') level of the 213-nm line to the 4d 3D2 (62 772 cm-') and 4d 3D1 (62 769 cm-') levels, respectively. Although both lines have very weak transition moments, again spin-forbidden singlet to triplet, a clean spectrum was observable due to the high sensitivity of this method. There was no significant enhancement of the LEI signal from the stepwise excitation of these lines. The last transition that was observed occurred a t 636.23 nm; this is from 46 745 to 62 459 cm-l (4p lP 4d 'D). This transition is listed in the NBS tables as hazy and shaded to the red. With the 213-nm wavelength fixed, the visible scan in Figure 7a was obtained for 25 ng/mL zinc. The spectrum has a strong peak a t the 636-nm line; however, a number of additional lines were also observed. When deionized water was aspirated into the flame the spectrum in Figure 7b was the result. This spectrum is due to the ionization of flame species. Although line assignments were not made, it is suspected that these lines are due to excited-state ionization of NO (13). The 213-nm radiation is exciting the NO into the A2Z+state, and the 636-nm light is exciting it into the E2Z+ state resulting in the ionization signal observed with the water blank. If the water blank in Figure 7b is subtracted from Figure 7a, the stepwise excitation spectrum of zinc is obtained and is shown in Figure 7c. The background signal is due to excitation from the 213-nm wavelength, since it disappears when the W beam is blocked and only the visible light is used. Although the enhancement of the LEI signal is quite significant with the 636-nm wavelength, a factor of several hundred, the NO background in both the first step and the second step of excitation presents some problems in using this line for
-
3099
analytical purposes. Both accurate background subtraction and accurate laser positioning would be necessary to utilize this stepwise scheme for analytical work. There are no previous reports in the literature of a spectral background at both excitation wavelengths in stepwise LEI. The ionization background could be reduced by decreasing the visible laser energy below the 10-mJ level used in these experiments. However, a fine balance would have to be maintained between the threshold level of the background ionization signal and the required energy level for the LEI signal.
CONCLUSIONS The nanogram-per-milliliter sensitivity of the stepwise 307and 472-nm transition is expected if one considers both the lifetimes and the transition probabilities of the transitions. The 307-nm transition probability is 3.29 X lo4s-l while that of the 213-nm transition is 7.09 X lo8 s-l ( 5 ) . This explains the sensitivity of the single-photon transition at 213 nm. The trade-off comes in the lifetimes of the excited states (14). The 213-nm excited-state lifetime is 1.45 ns while the 307-nm lifetime is 13 ns. This difference in excited-state lifetimes partly accounts for the low enhancement of the 213-nm and 397-nm stepwise excitation. Clearly the longer an atom remains in the excited state, the greater the probability the second photon has of exciting the atom closer to the ionization potential. The lifetime of the final state reached by the stepwise excitation also affects the enhancement factor because the thermal ionization probability increases as the excited state lifetime increases. The latter point is exemplified by the large enhancement factor of the 213- and 636-nm stepwise excitation. In this case, the short lifetime of the 213-nm excited level is compensated by the 20.5-11s lifetime of the 636-nm excited level. This work has demonstrated that the far-UV laser wavelengths coupled with stepwise excitation provide both high sensitivity and selectivity to analytical atomic spectroscopy. The detection limits by LEI are comparable to ICP detection limits and could be improved by increasing the integration time of the data acquisition. In addition, better beam quality and higher visible energy would aid in improving the sensitivity for stepwise excitation. Although the limits of detection reported here are higher than those of other elements by LEI, the analytical potential lies not only in the sensitivity but also in the selectivity of stepwise excitation. This is evident in the SRM determination. A more amenable sample could have been chosen; however, we wanted to demonstrate the potential of this wavelength range in a hostile matrix to illustrate some of the hazards that can be encountered. Other methods of analysis can be used for zinc determinations with comparable sensitivity, but the strength of LEI is in the added selectivity that cannot be provided by methods such as ICP or AA. Thus, in cases where conventional methods of analysis are incapable of performing the determination, the complexity of the LEI method is justified for routine analysis. ACKNOWLEDGMENT We thank J. G. Pruett for assistance with the homemade dye laser and R. L. Swofford, J. C. Travis, and G. C. Turk for their helpful discussions. Registry No. NO, 10102-43-9;Zn, 7440-66-6. LITERATURE CITED (1) Turk, G. C.; Travis, J. C.; DeVoe, J. R. J . Phys. Colloq. 1983, C 7 , 301-311. (2) Fraser. L. M.; Wlnefordner, J. D. Anal. Chem. 1972, 4 4 , 1444-1451. (3) Turk, G. C. Anal. Chem. 1981, 5 3 , 1187-1190. (4) Havrilla, G. J.; Green, R. B. Chem. Biochem. Environ. Instrum. 1981, 1 7 , 273-260. (5) Handbook of Chemistry and Physics, 63rd ed.: CRC: Boca Raton. FL. 1982. (6) Turk, G. C.; DeVoe, J. R.; Travis, J. C. Anal. Chem. 1982, 5 4 , 643-645.
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Anal. Chem. 1986, 58,3100-3103
(7) Turk, G.C.;Watters, R. L. Anal. Chem. 1985,5 7 , 1979-1983, (8) Hart, L. P.; Smlth, E. W.; Omenetto, N. Spectrochim. Acta, Pari 6 1985, 4 0 8 , 1637-1649. (9) Breckenridge, W. H.;Bllckensderfer, R. P.; Fitzpatrick, J.; Oba, D. J . Chem. Phys. 1979, 7 0 , 4751-4760. (IO) Natl. Bur. Stand. Circ. ( U S . ) 1952, N o . 467. (11) Green, R . E.; Havrilla, G. J.; Trask, T. 0. Appl, Spectrosc. 1980, 3 4 , 561-569. (12) Havrilla, G. J.; Green, R. 6. Anal. Chem. 1980,5 2 , 2376-2383.
(13) Huber. K. P.; Herzberg, G. Molecular Spectra and Molecular Structure I V . Constants Of Diatomic Molecules; Van Nostrand Reinhold Co.: New York, 1979. (14) Andersen, T.; Sorensen, G. J . Quant. Spectrosc. Radiat. Transfer 1973, 73, 369-376.
RECEIVED for review May 12, 1986. Accepted July 25, 1986.
X-ray Photoelectron Spectroscopy of Potential Technetium-Based Organ Imaging Agents Michael Thompson*
Department of Chemistry, University of 'Toronto, 80 S t . George Street, Toronto, Ontario M5S I A I , Canada Adrian D. Nunn and Elizabeth N. Treher
The Squibb Institute for Medical Research, P.O. Box 191, New Brunswick, New Jersey 08903
Technethrm-99 3dbindlng energies were measured for a set of 12 compounds whkh included model specles and several potential radiopharmaceutlcais. The range of compounds from the free metal to the technetlum(VI1) valence state gave a span of 4.9 eV. Anlonlc and coordinated halogen was distlnguished in the chlorine 2p and bromine 3d spectra of a number of phosphine complexes. Similar spectra of two dloxbne complexeg indicated signtfkantly more electron density on the halogen than is the case for a coordinated species. The result is Indicative of weakening of the metal-chlorine bond. Analogous spectra were obtained when bromine was substituted for chlorlne. The boron 1s Mnding energy for boronic acid present In the dioxlme cornpiexes was In agreement wlth the proposed oxygen population on the boron atom.
Technetium-99m radiopharmaceuticals are used extensively in nuclear medicine as in vivo diagnostic agents in part because of the favorable decay characteristics of this radionuclide ( I ) . Selective imaging of the organs is heavily dependent on the pharmacokinetic properties of complexes as influenced by the nature of the particular ligand employed. The latter is varied comprehensively to obtain structure-in vivo distribution correlations, which in turn allow radiopharmaceutical chemists to target labeled material to specific tissue. Of significant interest in this area has been imaging of the heart and the more difficult problem of imaging the brain, which involves passage of the technetium complex across the intact bloodbrain barrier (2, 3). A potentially important factor in the in vivo behavior of metal complexes is the overall charge on the complex and charge distribution on the central metal and ligand atoms. X-ray photoelectron spectroscopy (XPS) can yield relative information on the charge distribution and metal valence state through an examination of binding energy shifts compared to values from model compounds. The potential identification of metal oxidation states by XPS has attracted continued interest with mixed results. For example, qualitative correlation of metal binding energies with variable valence state in arrays of palladium (41, vanadium
Table I. Technetium 3dSIzBinding Energies and Metal Environment in Chelates and Model Compounds" compd
I
I1 I11 IV V VI VI1 VI11 IX X XI XI1
formula
Tc oxidn state n
NH~TcO~ [ ( ~ - C ~ H ~ ) , N ] T C O C ~5~ 4 [(CHJ~NITCC~G 4 (NH4)2TcBr6 [T~(dmpe)~Cl~]Cl 3 3 [Tc(d~pe),Cl~lCl [T~(dmpe)~Br~]Br 3 [Tc(diars),Brz]Br 3 TcC1[(dmg),bub] 3 TcBr[(dmg),bub] 3 TcCl[( ~ d o ) ~ m b ] 3 Tc 0
Tc
3d5jz
f0.2 eV
258.8 257.5 256.9 256.1 254.8 255.0 254.6 254.9 254.9 254.9 255.0 253.9
q:
1.6 1.3 0.9 0.7 0.3 0.3 0.2 0.3 0.3 0.3 0.3 0
"dmpe, 1,2-bis(dimethylphosphine)ethane;dppe, 1,2-bis(dipheny1phosphine)ethane; diars, o-phenylenebis(dimethy1arsine); dmg, dimethylglyoxime; cdo, cyclohexyldioxime; bub, n-butylboronic acid; mb, methylboronic acid. *Values for chelates V-XI obtained from plot of calculated qp vs. binding energy shift with respect to Tc metal from this work and ref 14.
( 5 ) ,silver (6),antimony (7), rhenium (8), molybdenum (9), tin (lo),ruthenium ( l l ) and , osmium (12) complexes has been attempted. In a semiquantitative approach Larsson et al. (13) obtained a good correlation of estimate of partial charge on the metal atom for 2p3jzbinding energies of copper in a series of analogous compounds. Charge calibration was achieved by theoretical calculations for a standard compound. The studies have shown that there is often a rough correlation of XPS chemical shift with metal oxidation, but ligand polarizability and T- and metal-metal bonding invariably provide influences that obscure such relationships. Furthermore, it is difficult to distinguish different oxidation states in the heavier metals where the range of binding energies is small or where low oxidation states as a group are involved. A dearth of binding energy data is evident in the case of compounds of technetium. Gerasimov et al. (14) measured Tc 3d,,z values for a set of simple inorganic compounds, and a good correlation of binding energy with estimated Pauling partial charge was obtained. In addition, the internal conversion electron technique has been used to examine the va-
62 1986 American Chemical Society 0003-2700/86/0358-3100$01.50/0
