288
Anal. Chem. 1902, 6 4 , 268-274
(3) Rubinstein, I.; Martin, C. R.; Bard, A. J. Anal. Chem. 1983, 55, 1580-1582. (4) Noffsinger. J. E.; Danielson, N. D. Anal. Chem. 1987, 59, 865-868. (5) Noffsinger, J. B.; Danielson, N. D. J. Chromatogr. 1987, 387,
520-524. (6) Danielson, N. D.; He. L.; Noffsinger, J. E.; Treiii, L. J. Pharm. Homed Anal. 1989, 7 , 1281-1285. (7) He, L.; Cox, K. A.; Danielson, N. D. Anal. Left. 1990, 23 (2),195-210. (8) Brune, S. N.; Bobbttt, D. R. Talanta 1991, 38, 419-424. (9) . . Eae. D.: Becker. W. G.: Bard, A. J. Anal. Chem. 1984, 56. 2; 13-2417. (10) Leland, J. K.; Poweii, M. J. J. Electrochem. 1990, 737, 3127-3131. (11) Rubinstein, I.; Bard, A. J. J. Am. Chem. SOC. 1980, 702,
(17) Laitinen. H. A.; Harris. W. E. Chemlcal Ana&&: McGraw Hili: New York, 1975;p 14. (18) VanHouten, J.; Watts, R. J. J. Am. Chem. SOC. 1978, 98, 4853-4858 .- - - .- - -.
(19) Wallace, W. L.; Bard, A. J. J. Phys. Chem. 1979, 83 (lo), 1350-1357. (20) Targrove, M. A.; Danielson, N. D. J. Chromatog. Scl. 1990, 28, 505-509. (21) Vlning, W. J.; Meyer, T. J. J. Electfanel. Chem. 1987, 237, 191-208 and references thereln.
6641-6842 - - . . - - .-.
(12) Rubinstein, I.; Bard, A. J. J. Am. Chem. SOC. 1981, 703, 5007-5013. (13) Rieto, N. E.; Martin, C. R. J. E/ectroch8m.Soc. 1984, 737,751-755. (14) Hercules, D. M.; Lytle, F. E. J . Am. Chem. SOC. 1988, 88, 4745-4746. (15) Rubinstein. 1.; Bard, A. J. J. Am. Chem. SOC. 1981, 703, 512-516.
RECEIVED for review June 18, 1991. Accepted October 25, 1991. This research was supported by grants from NIH (PHS l-RO3-RR03205-01Al)and from the Biotechnology Research and Development Corp.
Selective Detection in Pump-Probe Laser Photolytic-Fragmentation Fluorescence Spectrometry of Nitriles, Amines, and Alkenes Sang C. Lee,l Bobby J. Stanton, Brent A. Eldridge, and E. L. Wehry*
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996
SeiectlvHy In laser photoiytlc-fragmentation fluorescence spectrometry can be achieved by a “pumpandprobe” technique, wherein nonfluorescent parent molecules are photolyzed in the gas phase uslng 193-nm radiation and fragment fluorescence Is then excited uslng a tunable “probe” laser. The pumpandprobe photofragmentatlonfluorescence spectrometry of 11 nonfluorescent organic compounds (three lsomerlc nltrlles C4HSN,four ailphatic amines, and four lsomeric alkenes C,H,,) Is described. CN or C2 fragments (both of whlch have spin-allowed excHed states less than 10 000 cm-l above the ground state) are detected vla dye laser-induced fluorescence. Substantial differences In the pumpandprobe fragment fluorescence spectra of structuraiiy-slmllar parent molecules, earlier attributed to differences In the electronlc state distributions of fragments formed by 193-nm photolysis, are shown to have a more complex orlgln. The dependence of fragnent fluorescence lntensltles on the probe laser fluence implies that the probe laser alters the electronic state distributions and/or Induces further photofragmentatlon of specles formed by the “photoiysls” laser.
Laser photolytic-fragmentation fluorescence spectrometry (LP-FFS) extends the applicability of molecular fluorescence spectrometry to the many organic and organometallic molecules that do not exhibit appreciable fluorescence quantum efficiencies (1-6). In LP-FFS, a sample in the gas phase is illuminated with a pulsed laser (usually an excimer laser operating a t 193 nm (2-12)) to convert nonfluorescent parent molecules to emissive fragments, such as CH, CN, NH, and
CZ.
LP-FFS can be performed via “one-laser” or “two-laser” procedures. In one-laser LP-FFS, fluorescence is observed ‘Present address: Ames Laboratory, Iowa State University, Ames, IA 50011.
only from those fragments that are formed in electronically excited states in the fragmentation event or are excited by the tail of the laser pulse (2-4, 6,8, 12, 13). In “two-laser” or “pump-probe” LP-FFS, fragments produced by the first (“photolysis”)laser are excited into an emissive excited state by a second (“probe”) laser (I, 7, 9, 10, 11). Although pump-probe laser LP-FFS experiments obviously require more complex instrumentation than one-laser measurements, they offer two advantages. First, significant improvements both in limits of detection and analytical precision may result from use of a pump-and-probe procedure (7). Second, pump-and-probe methods offer the possibility of selective detection of individual constituents in multicomponent samples by LP-FFS. (One-laser LP-FFS spectra of structurally-similar compounds often are virtually indistinguishable (8).) The most straightforward approach to selectivity in pump-probe LP-FFS is expansion of the sample into a supersonic molecular beam followed by photolysis a t a wavelength a t which the analyte absorbs but other sample constituents do not. This approach, demonstrated successfully for multiphoton ionization mass spectrometry by Lubman and co-workers (14, 15), generally requires use of a dye laser as the photolysis source. Dye laser photolysis of parent molecules has two potential shortcomings. First, fragment fluorescence signals in LP-FFS virtually always increase as the photolysis laser fluence is increased (5-8); thus, replacing an excimer laser with a photolysis source (such as a dye laser) that produces lower fluences is expected generally to result in decreased fragment fluorescence signals. Second, dye lasers produce photons of energies lower than those available from an excimer laser. Photodissociation of a parent molecule to produce the fragment(s) of interest often requires absorption of two or more photons, even when 193-nm photolysis radiation is used (8, 16, 17). To supply the total energy required to achieve fragmentation may require absorption of more dye laser photons by the parent than would be needed if more energetic
0003-2700/92/0384-0268$03.00/00 1992 Amerlcan Chemical Society
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POWER SUPPLY Figure 1.
Schematic diagram of pumpand-probe LP-FFS spectrometer.
photons (e.g., from an excimer laser) were used. For these reasons, approaches to selective detection in LP-FFS that do not require use of a tunable photolysis laser are potentially attractive. It is possible to achieve selectivity in pumpprobe LP-FFS by photolyzing all (or nearly all) constituents of a multicomponent sample at 193 nm and relying on the probe laser to produce differences in fragment spectral patterns for the various sample constituents. We recently reported a study of the isomeric compounds acrolein and propargyl alcohol, for which probe laser excitation of fluorescence of the Cz fragment formed in the 193-nm photolysis of both compounds produced significantly different fluorescence spectra that could be used to distinguish the two compounds and to quantify one in the presence of the other (11). In that work, substantial differences in distributions of electronic states of Cz produced via 193-nm photofragmentation of the parent molecules were presumed to be responsible for the observed spectral selectivity. In the present report, these considerations are amplified, and results are presented for three additional classes of compounds: isomeric nitriles, C4H5N;isomeric alkenes, C5H10; several aliphatic amines. The fragments of principal interest are Cz and CN, both of which exhibit low-lying spin-allowed excited states.
EXPERIMENTAL SECTION Spectroscopic Apparatus and Procedures. Figure 1 is a schematic diag-ram of the LP-FFS spectrometer. Radiation from the photolysis laser (Questek 2210 operating with ArF, 193 nm, 10-Hz repetition rate, 10-ns pulse duration, 40-120-mJ pulse energy) is focused by a 15-cm focal-length plano-convex lens into a rectangular vacuum chamber of 2.5-L volume. The chamber is evacuated to a base preasure of ca.10-7Torr by a turbomolecular pump. Probe laser pulses are derived from either a NdYAGpumped dye laser (Quanta-RayDCR-2A10 and PDL2 with WEX) or an excimer-pumped dye laser (Questek 2210, operating with XeC1, and Lambda-Physik FL-2000). Both lasers produce ca. 1O-ns pulses. For either dye laser, the unfocused beam enters the chamber at an angle of 180° with respect to the photolysis beam. Photolysis and probe laser beams are carefully overlapped in space to produce mnnimAl fragment fluorescence signals. At the position where the laser beams overlap, the cross-sectional area of the photolysis beam is 1mm2,while that of the dye laser beam is 7 mm2. The probe beam is temporally delayed, relative to the photolysis beam, by a variable delay circuit. A sample (usually a liquid or solid at room temperature) is inserted as a vapor into a stagnationreservoir via a heated transfer
PSV
= pulsed supersonic valve.
line. Argon is used as reservoir gas; the typical reservoir pressure is 780 Torr. The diluted sample vapor is expanded through an 0.5" circular orifice into the vacuum chamber via a pulsed valve (R. M. Jordan C-211SS) that produces 200-p~gas pulses at 10 Hz. The control unit for the pulsed valve triggers both the pulsed valve and the firing of the photolysis and probe lasers. Fragment fluorescence is collected at 90° to both laser beam paths and 90° to the sample expansion, collimated by a 12.7-cm focal-lengthplano-convex lens, and focused by a 15.2-cm focallength cylindrical lens onto the entrance slit of a 1-m monochromator (J-Y HR-1000), typically operated at a spectral band-pass of 0.1 nm. The detector is a photomultiplier (Burle 8850); the signal is processed by a Stanford Research Systems SR-250 boxcar averager. Data collection is controlled by a laboratory computer. Chemicals. The following compounds, obtained from commercial sources, were used as received: methacrylonitrile, crotononitrile, allyl cyanide, allylamine, propargylamine, Nmethylallylamine, NJV-dimethylallylamine, 2-methyl-2-butene, 2-methyl-l-butene, 2-pentene, 1-pentene, and acrolein. Each compound was subjected to a freeze-thaw cycle, to remove dissolved oxygen, before introduction into the jet expansion. Many of these compounds are reported by their suppliers to be highly toxic (e.g., methacrylonitrile, allylamine, propargylamine, allyl cyanide) and/or extremely corrosive (e.g., crotonitrile, Nmethylallylamine, NJV-dimethylallylamine); appropriate precautions should be exercised in their use. RESULTS AND DISCUSSION Energy-Level Diagrams for Cz and CN. It is useful fust to summarize the electronic spectxoscopy of the two fragments of principal interest (C, and CN). Energy-level diagrams for Cz and CN (showing only the excited states detected in this work) appear in Figure 2. Both fragments exhibit low-lying spin-allowed excited states; the 0-0 band in the X A 'II, absorption of Cz appears at 1192 nm, and the 0 band in the X 2C A 211 absorption of CN appears at 1082 nm (18). Photofragmentation of a parent organic molecule may generate sizable quantities of Cz in the low-lying A spin-allowed excited state, and (for nitrogen-containing parents) significant amounts of CN in the low-lying spin-allowed A 211excited state may also be produced. If the proportions of Cz and/or CN fragments, in their ground and low-lying spin-allowed excited states, differ significantly for different parent molecules, then a basis exists for selectivity via pumpprobe LP-FFS of those parent molecules. For example, in pump-probe experiments based on C,, fragments produced in the A state can be detected by probe
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992 A: Methacrylonitrile
c2
B: Crotononitrile
m
C: Allyl c y a n i d e 193
F@m 2. Energy-bei diapms for CN and C2,showing the electronic states accessed in this work. The arrows Indicate probe laser wavelengths used is this work. Adapted from data compiled by Huber and Herzberg ( 78). A: Methacrylonitrile B: Crotononitrile C: Allyl Cyanide
361
379
+
360 n m
397
416
Wavelength ( n m )
-
-
Fbwr 4. Pump-and-probe LP-FFS spectra of C,H5N isomers, using a probe wavelength of 360 nm (A C transition in C2 and X B transition in CN). The feature at 386 nm comprises overlapping CN and C2 bands. condltlons: photolysis laser pulse energy, 30 ml; probe laser pulse energy, 2.5 ml; delay between photolysis and probe pulses, 700 ns.
A: Methacrylonitrile
B: Crotononitrile
CN
C: Allyl c y a n i d e
-+ -
193
+
I
597 n m
4
38 1
412
443
475
Wavelength ( n m )
Flgure 3. Onelaser LP-FFS spectra of C4H5N isomers produced by 193-nm photolysis. The photolysis laser pulse energy was 60 mJ.
--
laser excitation in the 360-400-nm region (A 'II, C 'II, transition; cf. Figure 2), following which the C A (Deslandres-d'hambuja band system) fluorescence can be observed. C2fragments formed in the ground state, or any other excited state, will not be detected in such a measurement. Similarly, CN in the low-lying A excited state can be excited to the emissive B 2Cstate by a probe laser at ca. 580 nm, following which B A or B X fluorescence can be observed. Likewise, for both C2and CN, ground-state fragmenta formed by photolysis of a parent molecule can be excited into emissive states by proper choice of probe wavelength (e.g., ca. 278 nm for the X C transition in C2and ca. 360 nm for the X B transition in CN. As previously described (II),'anti-Stokea" fluoreacencemay be observed in pump-probe LP-FFSexperiments. For example, if CN is formed in the A excited state by 193-nm photolysis of a nitrogen-containing compound, use of a probe beam at ca. 580 nm (A B transition) may result in observation of B X fluorescence at ca. 388 nm. In such a situation, background due to laser scatter or fluorescence of adventitious impurities is expected to be minimal. The manner in which the fragment fluorescence pattern changes as the probe wavelength is altered is expected to be highly sensitive to variations in the electronic state distributions of a particular fragment generated by 193-nm photolysis of different parent molecules. However, as noted below, the actual role of the probe laser in these experimenta may be considerably more complex than that implied above.
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-
-
-
379
385
391
397
Wavelength ( n m )
-
Flgwe 5. Pump-and-probe LP-FFS spectra of C,H,N isomers, using a probe wavelength of 597 nm (A B transition in CN). Condltions: phobiy& laser pulse energy,40 ml; probe laser pulse energy, 15 mJ; delay between photolysis and probe pulses, 400 ns.
Nitriles,C4H$. Shown in Figure 3 are one-her LP-FFS spectra for the isomeric compounds methacrylonitrile (HzC=C(CHJCN), allyl cyanide (H2C=CHCH2CN),and crotononitrile (CH3CH=CHCN);the spectra (dominated by CN B X emission) exhibit virtually identical spectral features and intensities. However, pump-probe spectra for the compounds exhibit significant intensity differences. Figure 4 shows pumpprobe spectra for equal quantities of the three compounds using 360 nm as the probe wavelength. At 360 nm, both CN (X B) and Cz(A C) absorptions occur; fluorescence signals from both Cz (the C A or Deslandres-d'hambuja bands) and CN are observed. Figure 5 compares the pump-probe LP-FFS spectra of the three nitriles when the probe laser wavelength is 597 nm, a wavelength at which C2is transparent but CN absorbs (excitation of the low-lying A excited state to the emissive B state). Figure 6 shows fluorescence spectra for the three isomers using 481 nm as the probe wavelength; at this wavelength, Cz absorbs (a 3nu d 3&); the fluorescence is the Swan system (d a) of Cz. At all probe wavelengths investigated, Cz and/or CN fluorescence signals decrease in the order methacrylonitrile > crotononitde > allyl cyanide. Relative fluorescence signals for the three compounds at the various probe and fluorescence
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Table I. Relative Fragment Fluorescence Signals for C4H5NIsomers probe X,nm
fragment detected
481 360 360 589
C2 C2 CN CN
abs transition in fragment
- --
a db A(u” = 0) X(u” = 2) A(u” = 1)
C(u‘ = 1) B(u’ = 3) B(u’ = 1)
fluor transition
--
d(u’ = 0) C(u’ = 1) B(u’ = 3) B(u’ = 0)
--
a(u” = 3) A(u” = 2) X(u” = 4) X(u” = 0)
fluor A, nm
photolysis pulse energy, mJ
probe pulse energy, mJ
rel” intens
467.5 406.5 414.3 388.0
45 30 30 40
20 2.5 2.5 15
1:0.12:0.02 1:0.16:0.08 1:0.11:0.001 1:0.5:0.3
-
Relative fragment fluorescence intensities, at the indicated wavelength, for methacrylonitrile (normalized to unity):crotononitrile:allyl cyanide. * A t this probe wavelength, several vibronic transitions within the a d electronic transition are excited.
Table 11. Dependence of Fragment Fluorescence Signals on Laser Fluence parent
laser A, nm”
methacrylonitrile
193 + 385 193 + 385
+
193 359 193 + 359 193 + 359 193 + 359 193 + 359 193 359 193 + 382 193 + 382 193 + 382 193 382 193 + 385 193 + 385 193 + 360 193 + 360 193 + 360 193 360 193 + 359 193 359 193 359 193 + 359 193 + 382 193 382 193 382 193 + 382
+ +
allylamine
2-methyl-2-butene acrolein
+
+ + + +
fragment fluor A, nm; transition
--
CN[388; B(u’ = 0) CN[388; B(u’ = 0) C2[360; C(U’= 1) C2[355; C(U’= 3) C2[386; C(U’= 0) C2[360; C(U’= 1) C2[355; C(U’= 3) C2[386; C(U’= 0) C2[385; C(U’= 0) C2[4O6; C(U’= 1) C2[385; C(U’ 0) C2[406;C(u’ = 1) CN[388; B(u’ = 0) CN[388; B(u’ = 0) c2[407; C(U’= 1) C2[4O7; C(U’= 1) C2[382; C(U’= 1) C2[382; C(U’= 1) C2[355; C(U’= 3) C2[360; C(U’= 1) C2[355; C(U’= 3) C2[360; C(U’= 1) C2[385; C(U’= 0) c2[406; C(U’= 1) C2[385; C(U’= 0) C2[406;C(u’ = 1)
X(u” = O)] X(u” = O)] A(u” = O)] A(u” = 2)] A(u” = O)] A(u” = O)] A(u” = 2)] A(u” = O)]
---+
+
+
+
+
--
+
-+
--
A(u” = O)] A(u” = 2)] A(u” = O)] A(u” = 2)] X(u” = O)] X(u” = O)] A(u” = 2)] A(u” = 2)] A(u” = l)] A(u” = l)] A(u” = 211 A(u“ = O)] A(u” = 2)] A(u” = O)] A(u” = O)] A(u” = 2)] A(u” = O)] A(u” = 2)]
fluence dependenceb
fluence range: 101sphotonscm-2
0.63 f 0.02 1.56 f 0.12 0.95 f 0.04 1.30 f 0.05 0.76 f 0.04 2.02 f 0.18 2.03 f 0.14 2.02 f 0.15 0.61 f 0.04 0.88 f 0.05 2.10 i 0.30 2.20 f 0.10 0.96 f 0.05 1.26 f 0.08 1.29 f 0.05 2.75 f 0.04 1.16 f 0.08 2.70 f 0.21 0.83 f 0.03 0.70 f 0.02 1.14 f 0.22 1.21 f 0.16 1.23 f 0.04 1.26 f 0.06 3.51 f 0.36 3.67 f 0.49
2.92-7.80 0.030-0.056 2.92-7.80 2.92-7.80 2.92-7.80 0.010.028 0.010.028 0.010-0.023 2.92-7.80 2.92-7.80 0.018-0.030 0.018-0.030 2.72-8.87 0.030.056 3.90-9.75 0.028-0.038 2.92-9.75 0.028-0.038 2.92-7.80 2.92-7.80 0.005-0.021 0.005-0.021 2.92-7.80 2.92-7.80 0.025-0.041 0.025-0.046
Photolysis X first; probe X second. Underlined X denotes the laser whose influence was varied. bSlope of linear plot of log(fragment fluorescence signal) vs-log(fluence). Based upon 1- and 7-mm2beam areas of pump and probe laser beams, respectively.
wavelengths used are compiled in Table I. Clearly, the intensity ratios depend strongly on the probe wavelength. The highest selectivity is observed for measurement of the CN B(u’ = 3) X (u” = 4) fluorescence at 414.3 nm, using a probe wavelength of 360 nm; under these conditions, the ‘discrimination fador” (16)for detection of methacrylonitrile in the presence of crotonitrile is 9, and that for detection of methacrylonitrile in the presence of crotononitrile is lo3. By comparison, the discrimination factor for any isomer in the presence of either of the other two is less than 2 for one-laser LP-FFS. For methacrylonitrile, the dependence of the Cz and CN fluorescence signals upon the pulse energy of the photolysis and probe lasers has been examined at several probe and fluorescence wavelengths. Whenever the photolysis laser fluence was varied, the probe laser fluence was held constant, and vice versa. The data are compiled in Table 11. (Nonintegral fluence dependences are presumed to indicate partial saturation of one or more of the successive absorption transitions in the “ladder-climbing”process that ultimately forms and/or excites the fragment (4,8,19).)Second- or higherorder dependences of fragment fluorescence signals on the photolysis fluence are common (8,17), because the absorption of two or more 193-nm photons is often required to fragment a molecule. An unanticipated, but very significant,observation
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is that, for both CN and Cz fluorescence from methacrylonitrile, the dependence of the observed fluorescence signal on the probe laser fluence is at least second-order. That fragment fluorescence signals may exhibit nonlinear dependences on both the photolysis and probe laser fluences has important practical implications. If the fluence dependence of a given fragment fluorescence signal is not the same for different compounds in a multicomponent sample, the relative fragment fluorescence signals obtained for those compounds will vary with changes in the probe and photolysis laser fluences. In such a case, numerical comparisons of fragment fluorescence signals observed for different compounds (e.g., the “relative intensity” column in Table I) are valid only for specific values of the photolysis and probe laser fluences. Thus, within limits, the degree of selectivity that can be achieved by pumpprobe LP-FFS may be altered both by changing the probe wavelength and the probe fluence. For reproducible analytical measurements to be feasible, it is essential to reproduce the probe and photolysis pulse energies. In practice, using commercial excimer and dye lasers, reproducible pulse energies can be achieved without difficulty if each laser is operated signifcantly below ita maximum rated pulse energy. If one or both lasers are operated at or near the maximum output, irreproducibility in the laser pulse energy may lead to inadequate quantitative precision.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992
Table 111. Relative Fragment Fluorescence Signals for Aliphatic Amines probe
fragment detected
A, nm
360.5 360.5
A(u” = 0) X(u” = 0) X(u” = 0) X(u” = 4) A(u” = 0)
C2
CN CN CN C2
360.5 384.5 384.5
abs transition in fragment
--+
fluor
fluor transition
C(u’ = 1) B(u’ = 1) B(u’ = 1) B(u’ = 4) C(u’ = 0)
---
C(u’ = 1) B(u’ = 13) B(u’ = 2) B(u’ = 0) C(u’ = 0)
+
A, nm
A(u” = 2) X(u” = 11) X(u” = 2) X(u” = 0) A(u” = 1)
photolysis pulse energy, mJ
probe pulse energy, mJ
50 50 50 40 40
2.6 2.6 2.6 2.1 2.1
406.5 352.2 386.2 388.1 409.3
rePb intens
l:nd:nd:0.44 l:nd:5.3:nd l:nd:nd0.83 1:0.06:0.23:1.2
1:nd0.26:0.54
Relative fragment fluorescence intensities, at the indicated wavelength, for allylamine:propargylamine:N-methylallylamine:N,N-dimethvlallvlamine. * nd: no fraement fluorescence sienal detected. A: Propargj-lamine
A: Methacrylonitrile
C : Allyl Cyanide 193
+
B: N , S - D i m e t h y l a l l y l a m i n e
c2
B: Crotononitrile 1
n
1
481.4 n m
C : K-hlethylallylamine
.3
--
y.
C C C
.m n .-
-
w
450
457
465
4i2
480
350
Wavelength ( n m )
-
370
410
390
Wavelength ( n m )
-
-
Flgure 6. Pumpand-probe LP-FFS spectra of C4H,N isomers, using a probe wavelength of 481.4 nm (a d transition in C2). Conditions: photolysis laser pulse energy, 45 ml; probe laser pulse energy, 20 mJ; delay between photolysis and probe pulses, 400 ns. The broad feature near 460 nm is due to several overlapping vibronic transitions in the Swan band system for C,.
Flgure 7. Pump-and-probeLP-FFS spectra of aliphatic amines, uslng a probe wavelength of 360.5 nm (A C transition in C2 and X B transition in CN). Conditions: photolysis laser pulse energy, 50 mJ; probe laser pulse energy, 2.6 ml; delay between photolysis and probe pulses, 800 ns.
In a study of the reproducibility of the CN fluorescence signal from methacrylonitrile (probe wavelength 360 nm; fluorescence wavelength 388.4 nm; quantity of methacrylonitrile 50 ng; 10 replicate measurements; photolysis pulse energy 60 mJ; probe pulse energy 2.5 mJ), a relative standard deviation of 3.8% was observed. This value is comparable to the precision commonly observed in conventional linear fluorometry and indicates that the nonlinear dependences of fragment fluorescence signals on photolysis and/or probe pulse energies do not cause unacceptable degradation of precision, provided that proper care is taken to secure reproducible laser pulse energies. The lowest limit of detection (LOD) for methacrylonitrile was observed using a probe laser wavelength of 360 nm and measuring the C2C A fluorescenceat 382 nm; the LOD (the quantity of analyte required to produce a fluorescence signal at the wavelength of maximum fluorescence equal to twice the standard deviation of the “blank” observed for the empty sample chamber) is at a partial pressure of 5 X Torr which, for a 2.5L chamber, corresponds to a LOD of 4 pg. For this LOD, and other LOD’s reported herein, the assumption is made that the parent molecule is distributed uniformly within the photolysis cell. Actually, because a pulsed nozzle system is used to introduce analyte into the chamber, the number density of analyte in the interrogation region is probably significantly larger than the number density of analyte molecules averaged over the entire photolysis chamber. To the extent that the parent molecules are distributed nonuniformly within the photolysis chamber, the detection limits estimated here are conservative (i.e., the actual amount of analyte present in the chamber is lower than the value calculated here).
Amines. We have examined the LP-FFS of four aliphatic amines: propargylamine (HC=CCH2NH2), allylamine (H2C=CHCH2NH2), N-methylallylamine, and NJV-dimethylallylamine. In one-laser LP-FFS, propargylamine and allylamine exhibit fluorescence from CN, CH, C2, and NH; the one-laser LP-FFS spectra of N-methylallylamine and NJVdimethylallylamine are very similar except that NH features are absent. The pump-probe LP-FFS spectra for the four compounds a t probe wavelengths of 360.5 and 384.5 nm are shown in Figures 7 and 8, respectively. At 360.5 nm, allylamine exhibits CN (B X) and Cz (C A) fluorescence; N-methylallylamine exhibits these CN and Cz fluorescence features (albeit at lower intensities) and also exhibits CN visible tail bands at 341.7,345.5, 352.2, and 353.8 nm. Propargylamine and NJV-dimethylallylamine exhibit no detectable fragment fluorescenceat a probe wavelength of 360.5 nm. At a probe wavelength of 384.5 nm, all four compounds exhibit CN (B X) and Cz (C A) fluorescence (Figure 8),but at different intensities. In Table I11 are compiled the relative fluorescence intensities for CN and Cz exhibited by the four compounds at the probe wavelengths and fluences used. As in the case of the nitriles described above, by proper choice of probe wavelength and fragment fluorescence feature, high selectivity is achieved. The limit of detection for allylamine, using a probe wavelength of 384.5 nm and measuring CN B X fluorescence a t 388 nm, was determined to be a partial pressure of 1.5 X Torr, corresponding to a LOD of 1 ng. For allylamine, the dependence of the Cz and CN fluorescence signals upon the photolysis and probe laser fluences have been measured; the results are listed in Table 11. The fluorescence of both fragments exhibits an essentially first-
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992
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Table IV. Relative Fragment Fluorescence Signals for C5HloIsomers probe X,nm
fragment detected
382 382 382 360 360
C2 C2 C2 C2 C2
abs transition in fragment A(u” = 1) A(u” = 1) A(u” = 1) A(u” = 2) A(u” = 2)
---
fluor transition
C(u’ = C(u’ = C(u’ = C(u’ =
1) 1) 1) 1) C(u’ = 1)
C(u’ = 0) C(v’ = 1) C(u’ = 1) C(u’ = 2) C(u’ = 3)
---
A(u” = 0) A(u” = 2) A(u” = 0) A(u” = 1) A(u” = 2)
X, nm
fluor
photolysis pulse energy, mJ
probe pulse energy, mJ
rela intens
384.7 406.5 360.5 357.6 356.2
50 50 50 40 40
2.5 2.5 2.5 3.0 3.0
1:0.16:0.200.14 1:0.140.15:0.13 1:0.160.21:0.16 1:0.83:0.700.44 1:0.640.62:0.39
Relative fragment fluorescence intensities, at the indicated wavelength, for 2-methyl-2-butene (normalized to unity):2-methyl-1-butene:2-pentene:l-pentene. A: B: C: D:
Propargylamine N,N-Dimethylallylamine N-Methylallylamine Allylamine
Mechanistic Considerations. As earlier reported (11), when pump-probe measurements are performed (193-nm photolysis + 382-385 nm probe, corresponding to excitation of various vibronic bands in the A C transition of CJ,much * I larger Cz fluorescence signals are observed for acrolein than for propargyl alcohol. We inferred (11)that the function of the probe laser was to effect straightforward single-photon excitation, to the C state, of C2 fragments formed in the low-lying A excited state following absorption of two or more 193-nm photons by acrolein. With that assumption, the data indicated that the efficiency with which C2 fragments are formed in the low-lying A excited state by 193-nm photolysis is greater, by a factor of at least lo4, for acrolein than for propargyl alcohol (11). However, the laser pulse energy deCN pendences of acrolein C2 C A fluorescence indicate this picture to be oversimplified. As noted in Table II,the C2signal exhibits virtually firsborder dependence on the photolysis laser 350 370 390 410 pulse energy, but a greater than fit-order dependence on the Wavelength ( n m ) probe laser pulse energy. Similar results for other compounds Flgure 8. Pump-an&probeLP-FFS spectra of aliphatic amines, using appear in Table 11. a probe wavelength of 384.5 nm (A C transition in C2). Conditions: Nonlinear dependences of fragment fluorescence signals on photolysis laser pulse energy, 40 mJ; probe laser pulse energy, 2.1 the probe laser fluence are incompatible with the view that mJ; delay between photolysis and probe pulses, 800 ns. the only function of the probe laser is to effect single-photon excitation of fragments formed in ground or low-lying excited power dependence on the photolysis laser fluence, but a states by the photolysis laser. At the very least, the probe nonlinear dependence on the probe laser fluence. laser appears to have the effect of redistributing (rather than Alkenes, C5HI0.The four molecules studied to date that merely “probing”) fragment state populations formed by produce the most intense C2fragment fluorescence signals in 193-nm photolysis. It is conceivable that at least a part of pumpprobe LP-FFS are acrolein (H,C=CHCHO) (11), the fragment fluorescence detected in these LP-FFS expermethacrylonitrile, allylamine, and Nfl-dimethylallylamine, iments results from probe laser photolysis of highly excited all of which have a terminal carbon-carbon double bond. parent molecules, and/or relatively large intermediate moAccordingly, we have examined the o n e and twelaser LP-FFS lecular fragment($, formed via absorption of one or more spectra of four isomeric alkenes, two of which (1-pentene and 193-nm photons by the parent molecule. 2-methyl-1-butene) have a terminal carbon-carbon double The evident complexity of the sequence(s) of events in these bond and two of which (2-pentene and 2-methyl-2-butene)do pumpand-probe LP-FFS systems makes it difficult at present not. As in other sets of isomeric compounds, the one-laser to predict the nature of the fragment fluorescence spectrum LP-FFS spectra of these isomers are virtually indistinguishthat might be generated from a particular parent molecule able. Appreciable, but not dramatic, differences are observed in such an experiment. If the probe laser acts to photofragin pumpprobe LP-FFS spectra. For example, using a probe ment intermediate species formed by 193-nm photolysis of wavelength of 360 nm (which excites the A ( u f f = 0) C (u’ the parent molecule, the ultimate possibilities for analytical = 1)transition in Cz,the C2fluorescencesignal is most intense selectivity in these experiments may be greater than earlier for 2-methyl-2-butene(which doea not contain a terminal C-C anticipated or currently demonstrated. At the same time, double bond) and least intense for 1-pentene; the intensity however, the need for careful control of both laser pulse endata are compiled in Table IV. ergies in analytical applications of pump-probe LP-FFS is The limit of detection for 2-methyl-2-butene,using a probe strongly underscored by the data in Table 11. wavelength of 360 nm and measuring Czfluorescence a t 405 CONCLUSIONS nm, was at 3 x Torr, corresponding to a LOD of 1ng of Selective detection in mixtures of structurally-similar the parent compound. compounds by pump-probe LP-FFS, earlier reported for the For a-methyl-Zbutene, the dependence of the Cz fluoresisomeric compounds acrolein and propargyl alcohol (II), is cence signal on the photolysis and probe lasers has been observed for other compound classes (nitriles, amines, and measured, using a probe laser wavelength of 360 nm. The alkenes). These observations imply that the pump-probe results are listed in Table 11. The fluence dependence is approach to selectivity in LP-FFS is quite general. The hynonintegral for both lasers; one infers that the process(es) pothesis used to rationalize previous results (11) (viz., that induced by each laser entail absorption of at least two photons. 193-nm photolysis of different parent molecules produces As for the other compounds investigated, the fragment significant differences in the electronic-state distributions of fluorescence signal exhibits a nonlinear dependence on the fragments, such as C2 and CN, that have low-lying spin-alprobe laser fluence. 193
+
384.5 nm
-
-
-
-
274
Anal. Chem. 1992, 6 4 , 274-2133
lowed excited states) is rendered dubious by indications that the number of probe laser photons absorbed may be 22. This observation suggests that the role of the probe laser in these experiments is-complex. Because of the possibility that a fragment fluorescence signal may exhibit nonlinear dependences both on the photolysis and probe laser pulse energies, care must be exercised to control and reproduce the operating parameters of both lasers in analytical applications of pump-probe LP-FFS.
ACKNOWLEDGMENT We thank PriscillaJ. Gannicott for assistance in performing the experimental work. REFERENCES (1) Rodgers, M. 0.;Asai. K.; Davis, D. D. 1980, 3597-3605. (2) Donnelly, V. M.; Karlicek, R. F. J. Appl. Phys. 1982, 5 3 , 6399-8407. (3) Halpem. J. B.; Koker, E. B.; Jackson, W. M. Anal. Chem. 1983, 5 5 , 2000-2002. (4) SauS& R. c.; Alfano, A. J.; Miziolek, A. Appl. Opt. 1087, 2 6 , 3588-3592. ( 5 ) Wehry, E. L.; Hohmann. R.; Gates. J. K.; Gullbault, L. F.; Johnson, p. M.; Schendel, J. s.;Radsplnner. D. A. Appl. Opt. 1087, 2 6 , 3559-3565. (6) Schendel, J.; Hohmann, R.; Wehry, E. L. ~ p p i spectrmc. . ig87,41, 840-844.
(7) Schendel, J.; Wehry. E. L. A M I . Chem. 1988, 60, 1759-1782. (8) Jlnklns, J. G.; Wehry. E. L. Appl. Specbosc. 1980, 4 3 , 861-885. (9) Rcdgers, M. 0.; Davis, D. D. Mviron. Scl. Techno/. 1989, 2 3 , 1 106-11 12. (10) Schendel, J. S.; Stickel, R. E.; van Dljk, C. A,; Sandholm, S. T.; Davls, D. D.; Bradshaw. J. D. Appl. Opt. 1990, 2 9 , 4924-4937. (11) Lee, S. C.; Stanton, B. J.; Wehry, E. L. Anal. Chem. 1991, 63, 744-748. (12) Papenbrock, T.; Stuhl, F. At". €nvkon. 1091, 25A, 2223-2228. (13) Oldenborg, R. C.; Baughcum, S . L. Anal. Cbem. 1986, 58, 1430-1436. (14) Lubman. D. M. Anal. Chem. 1987, 5 9 , 31A-40A. (15) Sln, C. H.; Tembruell, R.; Lubman, D. M. Anal. Chem. 1984, 5 6 , 2776-2781. (16) Gedenken, A.; Robin, M. B.; Kuebler, N. A. J . mys. Chem. 1982, 86, 4096-4101. (17) Colson, S. D. Nucl. Instrum. Methods 1987, 8 2 7 , 130-135. (18) Hubw, K. P.; Henberg, 0. Cawtents of B e t o m ( c Mokmrles; Molecular Specfra and Molecular Structure; Van Nostrand Reinhold: New York, 1978: Vol. IV. Deshmukh. S.; Brum, J. L.; Koplk, B. Cl" mys Lett. 1991, 176. 198-202.
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RECEIVED for review July 8,19B1. Accepted October 31,1991. This work was supported in part by the National science Foundation under Grant CHE-8822722. The YAG-pumped dye laser was obtained via funds provided by the Science Alliance, a State of T"3see Center of Excellence at the University of Tennessee-Knoxville.
Inductively Coupled Plasma Mass Spectrometry Signal Fluctuations Due to Individual Aerosol Droplets and Vaporizing Particles Steven E. Hobbs' and John W . Olesik*J
Department of Chemistry, Venable and Kenan Laboratories, University of North Carolina, Chapel Hill, North Carolina 27599-3290
ICPMS slgnal fluctuatlons, In some cases larger than 1 order of magnltude, are observed on a tens of mlcrosecond time scale. The analyte ICPMS slgnal fluctuations are larger than those characterlstlc of analyte Ion emlsslon lntensltles observed from the slde d the plasma. The analyte, solvent and plasma gas ICPMS m a l Ructuatkns correlate wlth atom and Ion emlsslon lntensltles monltored 1.3 mm In front of the 88mplhg cone. Analyte and 0' ICPMS slgnals are depressed near Incompletely desolvated droplets and enhanced near vaporlrlng analyte partlcles. ICPMS molecular Ion slgnals lndudlng ArH', 02', H,O', and H,O+ are enhanced near both Incompletely desolvated droplets and vaporlzlng partlcles. I n contrast to other spedes, Ar' ICPMS slgnals are depressed near a vaporlzlng particle. The nature of the ICPMS slgnal fluctuatlons (splkes or dlps) observed at a fixed sampling posMon ( w n g depth) Is crtllcalty dependent on the center (nebulizer) gas flow rate.
INTRODUCTION Investigation of the time dependence signals in ICP optical emission spectrometry (OES) or mass spectrometry (MS) is
* To whom correspondence should be addressed.
Current address of the authors: Laboratory for Plasma Spectrochemistry, Laser Spectroscopy and Mass S ctrometry, De art ment of Geological Sciences, The Ohio State X v e m i t y , 275A;cot!. Hall, 1090 Carmack Rd, Columbus, OH 43210.
important from both practical and fundamental pinta of view. Signal fluctuations can limit analysis precision. In general, precision in ICPMS is poorer than in ICPOES. Time-resolved studies of emission or mass spectrometric signals can provide insight into the sources of noise as well as fundamental processes occurring in the plasma. The sample introduction process is typically the main source of flicker noise in ICP spectrometry. Optical emission intensity fluctuations occurring on a tens of microsecond time scale have been investigated by Olesik et al. (I-3), Cicerone and Farnsworth (4), and Horlick et al. (5). Antanavichyus et al. (6-8) have attempted to explain species-dependentnoise power spedra from 0 to 10 kHz in terms of individual particles introduced into the plasma. Winge et al. (9) monitored emission using high-speed photography of a plasma used for ICPMS. However, no correlation was made between a time-resolved ICPMS signal and droplets or particles. Noise power spectra for ICPOES and ICPMS have been measured and compared ( 1 0 , I I ) . Winge et al. (9) correlated the periodic expansion and contraction of the axial channel of the ICP measured via high-speed photography with the 250-550-Hz frequency component of ICPMS noise power spectra. However, no attempt was made to determine if ICPMS signal fluctuations occurred on a tens of microsecond scale or if individual aerosol droplets or analyte particles were a source of noise. Olesik et al. determined the origin of emission intensity fluctuations through experiments that probed the correlation between atom and ion emission intensities (I, 3), the corre-
0003-2700/92/0364-0274$03.00/0 0 1992 American Chemlcal Society
