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Anal. Chem. 1983, 55, 1420-1422
I I
I
I I I I I I
A
Figure 1. Schematic diagram of experimental apparatus.
5 M HzS04solutions,4 mL of 1.15 X M NZH4*H$O4solutikm, about 50 mL of distilled water. The sample solution was heated to 100 "C during 10 min and filled to 100 mL.
RESULTS AND DISCUSSION The absorption maximum of molybdenum blue species is located at 824 nm, so that the wavelength coincides with the emission wavelengths of a semiconductor laser (818 nm, 98% of the maximum absorption intensity) and a solid-state emitter (814 nm, 97%). It is noted that the use of a He-Ne laser (633 nm, 33%) would have produced considerably less signal intensity. The PA signal of the prepared sample solution was measured with a semiconductorlaser at the modulation frequency of 200 Hz. The analytical curve was straight from 0 to 100 ppb, and the background signal corresponded to 50 ppb of phosphorus. The tentative detection limit in this system was 10 ppb. The signal amplitude for the 100 ppb sample solution (A = 0.87) was 0.13 pV when the background signal (0.07 pV) was subtracted. A similar analytical curve was obtained with a near-infrared emitting diode, where the modulation frequency was increased up to 710 Hz in order to improve the SIN ratio of the signal. In this case the signal intensity decreased to 0.1 HV, while the background signal increased to 0.12 pV which corresponded to 120 ppb of phosphorus. The solid-state emitter provides an incoherent beam and has a large illumination area (160 pm), so that an emission beam could not be collimated. Most of unwanted emission was removed by the diaphragms, but residual stray light might hit a surface of the PZT. Sawada et al. have reported that the intensity of the P A signal is 0.9 p V for the sample of A = 5.5 X using an argon ion laser (300 mW, 185 Hz) (2). The detector sensitivity of the present instrument (0.043 pV/(A mW)) is poorer than the reported sensitivity (0.55 @VI@mW)). The determination sensitivity achieved here is far from state of the art. For the determination of yet lower concentrations, it is essential to reduce the background signal originating from direct light absorption by the PZT surface. Moreover, 60-Hz noise from a power line should be reduced by increasing the modulation frequency and using a band-pass filter in the lock-in amplifier system. It is noted that the S I N ratio of the PA signal has been improved 40 times by increasing the
modulation frequency from 0.2 kHz to 6 kHz (3). Unfortunately the modulation frequency in this study was limited to 200 Hz due to the limited performance of the driving electronic circuit, but the semiconductor laser could be essentially operated up to 200 MHz. Very recently a semiconductor laser with 20 mW average power in the continuous wave (CW) mode has become commercially available and can be operated at 40 mW under the 50% duty cycle, which corresponds to an 80 mW CW laser modulated by a beam chopper (e.g., RCA Electro Optics and Devices, Model C86030E-CDH-LOC Laser). A 500-fold improvement in the detection limit (0.02 ppb) could be readily achieved by improving the cell design and the electronic circuit and using the recently developed laser. This estimation is consistent with the previous result was obtained that the detection sensitivity of A = 2.4 X with a 300-mW argon ion laser (2). It is noted that the semiconductor laser whose power exceeds 400 mW has been already developed in the laboratory (4). The concentration of phosphorus in environment lake water and seawater is on the order of 0.1-8 ppb (5). The present estimation encourages us to develop this semiconductor laser photoacoustic spectrometer for detection of phosphorus in the environment, We would like to emphasize that the present spectrometer can be constructed very compactly and requires a small power supply. Therefore, it may be quite useful for field research and environmental monitoring. The semiconductor laser performs very well with respect to the output power, line width, beam coherence, and capability of generating ultrashort pulses. Furthermore, these performances will be greatly improved in a few years. As a result, the semiconductor laser may be quite promising not only for photoacoustic spectrometry but also for fluorimetry and thermal lens spectrophotometry. However, the emission wavelength is located in the near-infraredregion and therefore it is essential to investigate and develop near-infrared colorimetric and fluorimetric reagents for their practical use. Registry No. Phosphorus, 7723-14-0.
LITERATURE CITED (1) Oda, S.; Sawada, T.; Kamada, H. Anal. Chem. 1978, 5 0 , 885-867. (2) Oda, S.; Sawada, T.; Nomura, M.; Kamada, H. Anal. Chem. 1879, 51, 686-688. (3) Oda, S.; Sawada, T. Anal. Chem. 1981, 53, 471-474. (4) Scifres et al. Topical Meeting on Optical Flber Communication, PD4-1, Apr., 1982; cited from Nikkei Electronics 1982, 5-24, 193. (5) Fujlwara, K.; Lei, W.; Uchiki, H.; Shimokoshi, F.; Fuwa, K.; Kobayashi, T. Anal. Chem. 1982, 5 4 , 2026-2029.
Yuji Kawabata Teiichiro Kamikubo Totaro Imasaka Nobuhiko Ishibashi* Faculty of Engineering Kyushu University Hakozaki, Fukuoka 812, Japan
RECEIVED for review January 20,1983. Accepted April I, 1983. The research is partially supported by a Grant in Aid for Scientific Research from the Ministry of Education of Japan and by a Kajima Foundation Research Grant.
Microdroplet Mixing for Rapid Reaction Kinetics with Raman Spectrometric Detection Sir: Since the first rapid kinetics work by flow methods by Hartridge and Roughton more than a half century ago ( I ) ,
many schemes have been developed for extending the time range of mixing experiments to below 1 ms (2,3).A few of
0003-2700/83/0355-1420$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55. NO. 8. JULY 1983
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Flgure 1. Photograph of two streams of droplets coalescing to form a third mixed stream. The distance scale is shown in the photo. The frequency of generation of droplets was 40 kHz, and the velocity of the impinging streams was 15 mls. The diameter of the droplets was approximately 130 pm, and the angle of intersection of the streams was 51'.
these experiments are capable of providing kinetics data on second-order reactions with rate constants as large as lo9 M-' s-l ( 4 , 5 ) and, in a t least one case, a mixing time of ahout 5 ps is reported (6).T h e detection methods used have in general been nonselective, and furthermore, no structural information is obtained hy using these techniques. In this paper we present a genuinely novel rapid mixing technique which utilizes Raman spectroscopic detection. We describe preliminary experiments that clearly demonstrate the potential of the method for studying the kinetics of rapid reactions and for detecting and prohing the structure of short-lived intermediates in solution. In these experiments, two high velocity streams of droplets of ea. 100 pm diameter impinge upon one another a t a small angle as shown in the photograph of Figure 1. The resulting stream of coalesced droplets passes into the beam of an AI+ laser, the scattered radiation is focused by the collection optics onto the entrance slit of a double monochromator, and the intensity is determined by photon counting. T h e microdroplet generator is mounted on a micrometer stage 80 that the mixed stream of droplets can be moved linearly with respect to the laser beam. In this way it is possible to observe the products of a given reaction at precise distances, and thus precise times, after mixing. The droplets are generated by an oscillating capillary through which reagent solutions are forced by gas pressure. The apparatus for droplet generation, which was developed for study of charged droplets (7-10), has been utilized for several years by Hieftje and eo-workers (11-23) for injecting analyk solutions into various flames for atomic spectroscopic studies. Droplet velocities are calculated from the product of the measured droplet spacing and the frequency of the generated droplets (7).The spacing is determined either by microscopic observation of strohoscopically illuminated droplets or by measurements on photographs such as that presented in Figure 1. The droplet sizes may be similarly measured, or they may be determined with relatively high precision by computation from the weight of a known number of droplets (8). Such results are in good agreement with the theory of droplet production (8). Initial experiments were undertaken to determine the feasibility of obtaining Raman spectra from streams of microdroplets with conventional spectroscopic equipment. For these experiments we chose to investigate the resonance-enhanced Raman spectrum of the well-studied tris(1,lOphenanthroline) complex of ferrous ion (24). A stream of microdroplets containing 1X lW3 M Fe(phen),2+ was passed a t a velocity of approximately 15 m/s through the focused beam of the laser which was tuned to the 514.5-nm line. The resulting spectrum (Figure 2A), which is comparable in all
Flgue 2. (a)Raman specbum of a single stream of 100 pm diameter droplets of 1 X IO3 M Fe(phenh*. (b) Raman spectrum ofcoalesced droplet pairs. One droplet stream was 2 X lo3 M in Few, the other stream was 1.2 X M in phen. and the resuning stream was 1 X lW3 in Fe(phen);+. The spectrum was obtained 650 ps after coa-
lescence. respects to that ohtained using conventional sampling devices (24),clearly demonstrates that high quality spectra can he ohtained from droplet streams. In order to determine whether or not spectra could be obtained from coalesced streams of two reagents, solutions of Fe2+(aq)and phen(aq), each 0.5 M in were mixed and the Raman spectrum of the resulting stream was ohtained as before at 650 ps after mixing. This spectrum, shown in Figure 2B, exhibits somewhat lower quality than that of a simple droplet spectrum, largely because of occasional instahilities in the coalesced droplet stream. These instabilities have been minimized to a great extent and should continue to decrease as our knowledge of the technology of droplet production improves. Finally, in order to demonstrate the potential of the droplet technique for the study of fast reactions, it is important to determine the mixing time of the pairs of droplets. The chemical system chosen for this study was the reaction of HSO; (0.5 M H2S04)with OH- (1.1 M NaOH), which bas a second-order diffusion controlled rate constant of 10" M" 8' (25). By monitoring the HSO; and SO,*- bands (1053 cmd and 982 cm-', respectively) and computing the ratio Ilos3/19a2, a measure of the extent of mixing may be obtained. The results of such an experiment show the l(t90% mixing time (20 psi driving pressure; 100 pm droplets; 57O angle between streams; final stream velocity -15 m/s) to he ca. 200 ps. This value is approximately an order of magnitude shorter than values obtained by other mixing techniques such as stopped-flow (1-2 ms) hut is longer by a factor of 40 than that reported by Davidovits and Chao (6). The time resolution of the technique may be estimated by calculating the time required for a single droplet to traverse the focused laser beam. Since the beam diameter at this point is approximately 100-200 pm and the diameter of the droplets is less than 100 pm, we estimate a time resolution of 10 ps a t a stream velocity of 15 m/s. It should be emphasized that these results were
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Anal. Chem. 1083, 55, 1422-1424
obtained under rather conservative conditions and that as mixing conditions are optimized and droplet velocities are increased, the mixing time is expected to decrease dramatically. A much more sophisticated system for generating and mixing droplet streams and obtaining Raman spectra of the product (reactant) molecules is under development in our laboratory. A careful, systematic study of the experimental variables is expected to enable considerably faster mixing times and thus permit study of a wider range of kinetic systems.
ACKNOWLEDGMENT The authors are indebted to A. S. Behrman for financial support during the early experiments and to P. C. Eklund and M. D. Barkley for the use of some equipment.
LITERATURE CITED (1) Hattrldge, H.; Roughton. F. J. W. Proc. R. SOC.London, Ser. A 1923, 704, 376. (2) Change, 8.; Elsenhardt, R. H.; Gibson, Q. H.; Lonberg-Holm, K. "Rapid Mixing and Sampllng Techniques in Biochemlstry"; Academic Press: New York, 1964. (3) Berger, R. L. 6lophys. J . 1978, 24,2. (4) Owens, G. D.; Taylor, R. W.; Rldley, T. Y.; Margerum, D. W. Anal. Chem, 1980, 52, 130. (5) Gerischer, H.; Helm, W. 2. 2.Phys. Chem. (Wiesbaden) 1965, 46, 345. (6)Davidovits. P.; Chao, S. Anal. Chem. 1980, 52,2435. (7) Schneider, J. M.; Hendricks, C. D. Rev. Sci. Instrum. 1964, 35, 1349.
(8) Llndblad, N. R.; Schneider, J. M. J . Sci. Instrum. 1965, 42,635. (9) Schneider, J. M.; Llndblad, N. R.; Hendrlcks, C. D. J. Collold Scl. 1965, 20, 610. (10) Lindblad, N. R.; Schneider, J. M. Rev. Sci. Instrum. 1967, 38,325. (11) HleftJe, G. M.; Malmstadt, H. V. Anal. Chem. 1968, 4 0 , 1860. (12) Hieftje, G. M.; Malmstadt, H. V. Anal. Chem. 1969. 4 1 , 1735. (13) Clampitt, N. C.; Hieftje, G. M. Anal. Chem. 1972, 4 4 , 1211. (14) Clampitt, N. C.; Hleftje, G. M. Anal. Chem. 1974, 4 6 , 382. (15) Bastlaans, G. J.; Hleftje, G. M. Anal. Chem. 1974, 4 6 , 901. (16) Boss, C. 8.; Hieftje, G. M. Anal. Chem. 1977, 49, 2112. (17) Boss, C. B.; Hieftje, G. M. Appl. Spectrosc. 1978, 32,377. (18) Boss, C. B.; Hieftje, G. M. Anal. Chem. 1979, 57,895. (19) Russo, R. E.; Hieftje, G. M. Anal. Chlm. Acta 1980, 178, 293. (20) Boss, C. 8.; Hieftje, G. M. Anal. Chem. 1979, 51, 1897. (21) Russo, R. E.; Hieftje, G. M. Spectrochim. Acta, Part 6 1981, 366, 231. (22) Bieasdeli, B. D.; Wittig, E. P.; Hieftje, G. M. Spectrochim. Acta, Part 6 1981, 366. 205. (23) Russo, R. E.; Withneii, R.; Hleftje, 0. M. Appl. Spectrosc. 1981, 35, 531. (24) Clark, R. J. H.; Turtle, P.; Strommen, B.; Streusand, B.; Kincald, J.; Nakamoto, K. Inorg. Chem. 1977, 16, 84. (25) Eigen, M.; Kurze, 0.; Tamm, K. 2. Elektrochem., Ber. Bunsenges. Phys. Chem. 1963, 57, 103.
Stanley F. Simpson James R. Kincaid* F. James Holler* Department of Chemistry University of Kentucky Lexington, Kentucky 40506-0055 RECEIVED for review February 28,1983. Accepted March 16, 1983.
Chemical Reactivity of Glycerol as a Mass Spectrometric Matrix Sir: Because of its good solvent properties (I) and its low vapor pressure (3 X lod4torr at 25 "C)(2), glycerol has been the matrix of choice for several recently developed mass spectrometric techniques including fast atom bombardment mass spectrometry (FABMS) (3, 4),"liquid" secondary ion mass spectrometry (SIMS) (5), and electrohydrodynamic ionization mass spectrometry (EHMS) (6-8). Despite the growing use of these techniques (especially FABMS), the chemical reactivity of glycerol as a mass spectrometric matrix has not been carefully examined, although Field has recently discussed the effect of atom bombardment on the spectrum of the glycerol matrix (9). Products of reactions between the glycerol matrix and analytes can complicate the mass spectra obtained. For example, in an earlier EHMS study of transition metal complexes, it was found that glycerol (or glycerate anion) participated in ligand exchange reactions (10). This note reports another example of undesirable glycerol solution chemistry in EHMS, now with the specific aim of alerting potential users of this matrix material to possible complications. EXPERIMENTAL SECTION Mass spectra were obtained with a double-focusing mass spectrometer (AEI MS902) equipped with an EH ion source described elsewhere (6, 7). Source emitter potential waa about A8.5 kV, extractor potential was r1.5 kV, and the collector was fixed at ground potential. For all spectra, exact emitter potential and spectrometer electric sector potential were empirically matched; thus only ions that had not undergone any metastable evaporativeloss of solvating glycerol molecules prior to the electric sector were detected (11). Spectrometer resolution of about 600 was employed. Typical ion emission current was to lo4 A. The gain of the electron multiplier was roughly los. Infrared spectra were obtained with a Perkin-Elmer 137 spectrometer using NaCl cells.
Scheme I 0
0
II
HO-C-CHZ
,CH2-
II
C-OH
\
HO-C-CHz
/N-
+XtS
H4Y (MW.292)
8
HOR (MWz92)
8
?
FH2C-oR \ /N-CH2CH2-N c ~ ~ ~ f i - 0 ~
HO- C - CHz \ HO-C-CH2
H+(Fe3+) __C
CH2CHz- 'CH2-C-OH
8
:
OH OH OH I I I CH2-CH-CH2
+H2YR2 +HYR3 +YR4
(hW=4401 (MWz514I (MWs5881
0 H3YR (MW=366)
Analytical reagent grade glycerol (Fisher), Fez(S04)3-xHz0 (Mallinckrodt), and HzSO4 (Baker); 99% ethylenediaminetetraacetic acid disodium salt, NazHzY.1.5Hz0(Sigma),and practical grade Na(FeY) (G.F. Smith) were all used aa received. Glycerol solutions of analytes were prepared by dissolving the solutes with heat (-65 O C ) and vigorous stirring while degassing (to I 1X torr) (6-8). Positive and negative ion spectra were obtained.
RESULTS AND DISCUSSION The negative ion EHMS spectrum of a glycerol solution containing 230 mg of Fez(SOk)3.xHz0per mL of glycerol (about 3 mol %, assuming x = 9) and 3.0 mol % Na2HzY.1.5H20 showed only [G, - HI-and [G, HSOJ where G = glycerol and n = 1-5. Despite its large formation constant (Kf> in water (12),no Fey- was detected. The positive ion spectrum of this solution (Figure 1) was dominated by ions of massto-charge ratio ( m l z ) 367, 441, 463, 515, 537, 589, and 611. These ions can be assigned as the cationized (sodium or proton attachment) products of the acid catalyzed esterification re-
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