1975
Anal. Chem. 1988, 6 0 , 1975-1977
the ringer solution. These interferences include peaks B and F, and the negative deflections found at ca. 5.1,7.7,8.2, and 13 min. Schemes are being developed to eliminate these interferences. Peaks A, C, D, E, and G appear to be "real" peaks. Peak G is presumably an anionic compound, possibly a metabolite of dopamine, and has been seen in larger quantities in other samples. Future work will involve determination of the identity of solutes detected in these experiments.
ACKNOWLEDGMENT The authors wish to thank Jennifer Chien for her assistance with the snail dissections. Registry NO.5-HT, 50-67-9; NE,51-41-2;IP, 7683-59-2;HVA, 306-08-1; DOPAC, 102-32-9;AA, 50-81-7.
LITERATURE CITED (1) Virtanen, R. Acta Polym. Scand. 1974, 723,1-67. (2) Mlkkers, F. E. P.; Everaerts, F. M.; Verheggen, Th. P. E. M. J. Chromatogr. 1979, 769,11-20, (3) Jorgenson, J. W.; Lukacs, K. D. Anal. Chem. 1981, 53,1298-1302. (4) Jorgenson, J. W.; Lukacs, K. D. Science (Washington, D . C . ) 1983, 222, 266-272. (5) Tsuda, T.; Nomura, K.; Nakagawa, G. J. Chromatogr. 1983, 2 6 4 , 385-392. (6) Gebauer, P.; Deml, M.; Bocek, P.; Janak, J. J. Chromatogr. 1983, 267, 455-457. (7) Hjerten, S. J. Chromatogr. 1983, 270, 1-6. (8) Green, J. S.;Jorgenson, J. W. HRC C C , J. H/gh Resolut. Chmmatogr. Chromatogr. Commun. 1984, 7 , 529-531. (9) Demi, M.; Foret, F.; Bocek, P. J. Chromatogr. 1985, 320,159-185. (10) Gassmann, E.; Kuo, J. E.; Zare, R. N. Science (Washington, D . C . ) 1985, 30,813-814. (11) Lauer, H. H.; McManlgill, D. Anal. Chem. 1986, 58, 165-170. (12) Walbroaehl, Y.; Jorgenson, J. W. Anal. Chem. 1988, 58,479-481.
(13) Gozel, P.; Gassmann, E.; Michelson, H.; Zare, R. N. Anal. Chem. 1987, 59,44-49. (14) Wallingford, R. A.; Ewing, A. E. Anal. Chem. 1987, 59, 678-681. (15) Cohen, A. S.; Terabe, S.; Smith, J. A.; Karger, B. L. Anal. Chem. 1987, 59, 1021-1027. (16) Ollvares, J. A.; Nguyen, N. T.; Yonker, C. R.; Smith, R. D. Anal. Chem. 1987, 59, 1230-1232. (17) Wallingford, R. A.; Ewing, A. E. Anal. Chem. 1987, 59, 1782-1766. (18) Huang, X.; Pang, T. K.; Gordon, M. J.; Zare, R. N. Anal. Chem. 1987, 59. 2747-2749. (19) Wa'llingford, R. A,; Ewing, A. E. Anal. Chem. 1988, 6 0 , 258-263. (20) Knecht, L. A.; Guthrie, E. J.; Jorgenson, J. W. Anal. Chem. 1984, 56, 479-482. (21) St.Claire, R. L., 111; Jorgenson, J. W. J. Chromatogr. Sci. 1985, 2 3 , 186-191. (22) Marsden, C.; Kerkut, G. A. Comp. Gen. Pharmacol. 1970, 7 , 101-1 16. (23) Berry, M. S.; Cottrell, G. A. J. Physiol. (London) 1975, 2 4 4 , 589-612. (24) Powell, B.; Cottrell, G. A. J. Neurochem. 1974, 22, 605-806. (25) Berry, M. S.; Cottrell, G. A.; Pentreath, V. W.; Powell, B. Physiol. SOC. (London) 1973, 237, 19P-20P. (26) Chien, J. 8.; Saraceno, R. A.; Ewing, A. G. Proc. Electrochem. SOC. in Dress.
To whom correspondence should be addressed.
Ross A. Wallingford Andrew G. Ewing* Department of Chemistry Penn State University University Park, Pennsylvania 16802 RECEIVEDfor review March 16,1988. Accepted May 2,1988. This material is based upon work supported by the National Institutes of Health under Grant No. 1 R 0 2 GM37621-02. A.G.E. is the recipient of a Presidential Young Investigator Award (CHE 8657193).
I nf rared-Enhanced Multiphoton Ionization Detection of Aromatic Molecules in Solution Sir: Laser two-photon ionization spectrometry in solution is relatively new and is a sensitive and versatile technique for the detection of organic molecules that absorb photons (1-8). The photocurrent signal correlates with the one-photon absorption spectrum, but the blank signal substantially increases with decreasing excitation wavelength below about 400 nm (7, 8). The detection sensitivity of this technique depends on laser power (8)) excitation wavelength (7, 8)) molar absorptivity of a molecule (4,6-8),and some other experimental conditions (9). The use of an intense laser at a longer wavelength with a larger molar absorptivity will improve the detection sensitivity (8),but the improvement will be limited by saturation effects (10). The photocurrent signal is large for a high quantum yield of charge carriers. Multiphoton ionization spectrometry in the gas phase is extremely sensitive, because the recombination of initial cation-electron pairs is insignificant and amplification of the charge carriers is efficient with the use of a proportional counter (11). This is not the case in solution, since most of the geminate cation-electron pairs recombine quickly due to solvent interaction, especially in nonpolar solvents (12);thus the quantum yield of charge carriers is substantially low in nonpolar solvents. Also, no techniques for amplifying the charge carriers in solution have been developed. In recent multiphoton ionization studies in nonpolar solvents, the cooperative action of both an ultraviolet (UV) and a visible (vis) laser (13,14)or both a UV and an infrared (IR) laser (15-19) enhanced the quantum yield of the charge carriers in comparison to that when the UV laser acted alone. This paper demonstrates a significant improvement in the
photoionization detection sensitivity in n-heptane by simultaneous irradiation at 355 and 1064 nm.
EXPERIMENTAL SECTION Figure 1shows the experimental apparatus. A Nd:YAG laser (Quantel YG580A, pulse duration 10 ns) was used as an excitation light source and was operated at a repetition rate of 5 Hz. A 1064-nm (fundamental) laser beam was emitted from exit A (Figure l),while a 355-nm (third harmonic) laser beam was emitted from exit B. A KDP crystal was placed in front of exit B to generate a 266-nm (fourth harmonic) laser beam by frequency doubling of a 532-nm (second harmonic) laser beam as it passed from exit B. Both IR (1064 nm) and UV (355, 266 nm) lasers passed through slits of 3.5-mm diameter and were combined collinearly by a dichroic mirror. They irradiated the solution simultaneously. The photocurrent signal was converted to voltage by a homemade current-to-voltage(I/V) converter (gain lo9V/A, time constant 10 ms) and was amplified by an amplifier (NF P61). Its output was fed into a low-pass filter (NF P82) with a cutoff frequency of 1kHz. The output signal was monitored by an oscilloscope (Iwatsu SS-6122) and was averaged by a boxcar integrator (NF BX-530A: integrating time constant 30 ms, gate duration 10 ms). The boxcar output was recorded by a strip chart recorder (RDK R-02). The laser pulse energy was monitored by a calorimeter (Photon Control Model 25V-VIS). A photoionization cell has a pair of brass disk electrodes (diameter 0.5 cm) supported in a quartz cuvette (4 X 1 X 1cm). The electrode spacing was 0.2 cm. A stabilized direct current power supply (Ikegami HD 2.5K-M) gave a voltage gradient across the electrodes. Laser beams that were focused by a quartz lens (focal length 6 cm) irradiated the solution between the electrodes. The cell was placed at a defocused position of the laser beam in order to avoid damage at the quartz/liquid interface and cavitation due
-
0003-2700/88/0360-1975$01.50/00 1988 American Chemical Society
1976
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
~o'"''scope
I 4 1
0 4
4 1
Recorder
I
I 0 0 0
A A
9
0
9 c
Log[FhAnl
Figure 1. Schematic diagram of the experimental apparatus. 1064
1064
355
T
c
lo
Figure 3. Calibration curves of 9-phenylanthracene with (0)and without ( 0 )the additional irradiation at 1064 nm in n-heptane and the ratio (A) of PI(355 + 1064)to PI(355). Table I. Photoionization Detection Limits (S/N= 3) of Some Aromatic Molecules in Nonpolar Solvents
100 PA I
/ 1064
I
excitation
1064
355
t.1
(a)
Figure 2. Typical photocurrent signals of S-phenyhnthracene and bhnk signal in nheptane induced by simultaneous irradiation at 355 and 1064 nm and by irradiation at 355 nm alone. Concentrations of 9-phenylanthracene are as follows: (a) 0 M, (b) 1 X lo-'' M, (c) 5 X lo-'' M. Laser pulse energies are 2 mJ at 355 nm and 25 mJ at 1064 nm. Arrows show laser on (t) and off (l)at 355 and/or 1064 nm. to a breakdown phenomenon near a focal point; the distance between the lens and center of the electrode was about 5 cm. All chemicals (reagent grade) were used as received. Sample M). The solutions were freshly prepared from stock solutions photocurrent signal of the analyte was obtained by subtracting the blank signal from the observed signal, and the detection limit ( S I N = 3) was determined as before (7, 8).
RESULTS AND DISCUSSION The detection sensitivity of the laser two-photon ionization technique depends on the excitation wavelength (7,8). Under the identical photon flux (2.7 X 1015photons/pulse) of the laser pulses, the photocurrent signal at 355 nm was comparable with that a t 266 nm, while the blank signal a t 355 nm was more than 2 orders of magnitude smaller than that at 266 nm, probably due to solvent ionization at 266 nm (20). The detection limit of the analyte was more than 2 orders of magnitude lower than that at 266 nm. Therefore, the 355-nm light was used for photoionization of an analyte in this study. Figure 2 shows typical photocurrent signals of 9-phenylanthracene and the blank signal in n-heptane induced by simultaneous irradiation at 355 and 1064 nm (PI(355 + 1064)) and by irradiation at 355 nm alone (PI(355)). The additional irradiation a t 1064 nm considerably enhanced the photocurrent signal of 9-phenylanthracene generated by the irradiation at 355 nm. However, the blank signal (that is, its standard deviation) increased only slightly by the additional irradiation a t 1064 nm (Figure 2a). Neither the solvent nor the sample solution showed appreciable photocurrent signals when only the 1064-nm laser was used.
compd
solvent
wavelength, nm
9-phenylanthracene n-heptane 355 n-heptane 355 + 1064 pyrene n-hexane 337 n-hexane 337 n-heptane 355 n-heptane 355 + 1064 perylene n-hexane 337 n-hexane 337 n-hexane 383 n-heptane 355 n-heptane 355 + 1064
detection limit, pg/mL
ref
23
a
4.5
a
9 30
4
140
a a
28 300 200 50 58 10
5 8 5
8 a
a
"This work. log-log calibration plots of PI(355 + 1064) and PI(355) versus the concentrations of 9-phenylanthracene were roughly linear in the 10-10-10-8 M range, as shown in Figure 3; ratios of PI(355 1064) to PI(355) were 541. Detection limits (SIN = 3) with and without the additional irradiation a t 1064 nm were 1.8 X lo-" M (4.5 pg/mL) and 9.0 X lo-" M (23 pg/mL), respectively. Other compounds showed similar results; detection limits are summarized in Table I. The additional irradiation a t 1064 nm enhanced the detection sensitivity by 5- to 6-fold as compared with that when the 355-nm laser acted alone. The detection limit of 4.5 pg/mL for 9-phenylanthracene obtained in this study is the lowest value in the multiphoton ionization detection in solution. Since the detection sensitivity profoundly depends on the excitation wavelength and the molar absorptivity of a molecule (8),the combination of a tunable (UV-vis) dye laser and an IR laser will further improve the detection sensitivity, by tuning the wavelength of the dye laser at the highest SIN ratio. The cooperative action of 30-11s laser pulses a t 347 and 694 nm (delay time -7 ns) a t relatively low temperatures (- 250-180 K) ( 1 3 , 1 4 ) or picosecond laser pulses at 351 and 1053 nm or at 355 and 1064 nm (delay time 0-- 1 ns) (15-18) enhanced the photocurrent signal of a solute in hexane as compared with that when the UV laser acted alone. Such enhancement was attributed to an increase in the quantum yield of charge carriers caused by the photoexcitation of electrons of the geminate cation-electron pairs. In this study, however, 10-ns laser pulses a t 355 and 1064 nm irradiated a
+
Anal. Chem. 1988, 6 0 , 1977-1979
solute simultaneously. Thus, it is suggested that the IR light acts through the excited neutral parent solute as well as through its geminate cation-electron pair. In conclusion, this study shows that a considerable improvement in the photoionization detection sensitivity is possible by the simultaneous action of a UV and an IR nanosecond laser at room temperature. Further physicochemical and analytical studies are now in progress.
ACKNOWLEDGMENT I thank Professors T. Ogawa and I. Shinno of Kyushu University for their encouragement and for the use of their electronic instruments and optical components. Registry No. 9-Phenylanthracene,602-55-1; pyrene, 129-00-0; perylene, 198-55-0.
(8) Yamada, S.;Ogawa, T.; Zhang, P.-H. Anal. Chim. Acta 1986, 783, 251-256. (7) Sato, N.; Yamada, S.; Ogawa, T. Anal. Sci. 1987, 3 , 109-111. (8) Yamada, S.;Sato, N.; Kawazumi, H.; Ogawa, T. Anal. Chem. 1987, 59, 2719-2721. (9) Yamada, S.; Ogawa, T. frog. Anal. Spectrosc. 1986, 9 , 429-453. (IO) Speiser, S.; Jorlner, J. Chem. fhys. Lett. 1976, 4 4 , 399-403. (11) Letokhov, V. S. Laser fhotoionlzation Spectroscopy; Academic: Orlando, FL, 1986; Chapters 4 and 5. (12) Yakoviev, B. S.; Lukin, L. V. I n Photodissociation and photoionization; Lowley, K. P., Ed.; Why: New York, 1985; pp 99-160. (13) Lukin, L. V.; Toimachev, A. V.; Yakovlev, B. S. Chem. fhys. Lett. 1981, 87, 595-598. (14) Lukin, L. V.; Tolmachev, A. V.; Yakovlev, B. S. Chem. fhys. Lett. 1983, 99, 16-21. (15) Braun, C. L.; Scott, T. W. J. fhys. Chem. 1983, 8 7 , 4776-4778. (16) Braun, C. L.: Scott, T. W. J. fhys. Chem. 1987, 97, 4436-4438. (17) Scott, T. W.; Braun, C. L. Can. J. Chem. 1985, 63, 228-231. (18) Scott, T. W.; Braun, C. L. Chem. fhys. Lett. 1986, 727, 501-504. (19) Scott, T. W.; Braun, C. L. J. fhys. Chem. 1986, 90, 1739-1741. (20) Miyasaka, H.; Mataga, N. Chem. fhys. Lett. 1986, 126, 219-224.
LITERATURE CITED (1) Voigtman, E.; Jurgensen, A,; Winefordner, J. D. Anal. Chem. 1981, 53, 1921-1923. (2) Yamada, S.; Kano, K.; Ogawa, T. Bunsekl Kagaku 1982, 37. E247E250. (3) Voigtman, E.; Wlnefordner, J. D. Anal. Chem. 1982, 5 4 , 1834-1839. (4) Yamada, S.;Hino, A.; Kano, K.; Ogawa, T. Anal. Chem. 1983, 55, 1914- 19 17. (5) Fujiwara, K.; Voigtman, E.: Winefordner, J. D. Spechosc. Lett. 1984, 77, 9-20.
1977
Sunao Yamada Laboratory of Chemistry College of General Education Kyushu University Ropponmatsu, Fukuoka 810, Japan RECEIVED for review February 2,1988. Accepted May 6,1988.
Gradient Elution of Anions in Single Column Ion Chromatography Sir: Traditionally, gradient elution has been considered difficult or impossible to accomplish with liquid chromatographic systems utilizing bulk property detectors ( I , 2). Changes in composition of mobile phases required during the development of a gradient were thought to cause under all circumstances too large a change of the response by bulk property detectors. Relatively small deflections due to the zones of analytes passing through the detector cell were expected to remain undetected at the crude sensitivity setting imposed by the simultaneously occurring change in the bulk concentration of the eluent. In recent years there have been several reports (3-6) describing a successful utilization of conductivity detection in conjunction with the gradient elution of ions. Initial steps toward gradient elution followed by refractive index detection have also been reported (7). In all published work so far,conductivity detection is made compatible with gradient elution by the employment of suppressors connected between the separator column and the conductivity detector. The main function of suppressors is the conversion of the high conductivity signal produced by the eluent into a low level background reading (8). In the same fashion a pronounced change in conductivity-as observed during the gradient elution-can be reduced considerably. A computer-aided base-line subtraction of prerecorded blank gradients is then usually employed to improve the appearance of chromatographic recordings and to enable a reliable quantitation. In a new technique which we call isoconductive gradient, the background conductivity changes can be minimized by a judicious choice of countercations in the mobile phases employed for gradient elution of anions. This approach enables gradient separations under the conditions of single column (nonsuppressed) ion chromatography.
EXPERIMENTAL SECTION Preparation of Eluents. All chemicals were utilized as ob-
tained from commercial sources. Milli-&water (Millipore Corp.) was used for all aqueous solutions mentioned in this report. Unadjusted Eluents (“Conventional Gradient”). A1 (background conductivity ca. 321 &): 11 mM boric acid, 1.48 mM 0003-2700/88/0360-1977$01.50/0
gluconic acid, 3.49 mM potassium hydroxide, 0.65 mM glycerin, and 12% acetonitrile. B1 (backgroundconductivity ca. 395 wS): 13.75 mM boric acid, 1.85 mM gluconic acid, 4.36 mM potassium hydroxide, 0.81 mM glycerin, and 12% acetonitrile. Increased cohcentration of glycerin in B1 did not contribute to the eluting strength. This increase was derived from the fact that 25 mL of a 50-fold concentrate of A1 (without acetonitrile) was used to prepare B1. Eluents Adjusted t o Equal Conductivity (“Zsoconductive Gradient”). A2: Eluent A2 was identical with Al. B2 (background conductivity ca. 322 WS). Lithium hydroxide monohydrate (5.13 mM), concentrations of boric acid, gluconic acid, glycerin, and acetonitrile were the same as in B1. A3 (background conductivity ca. 344 &. 8.25 mM boric acid, 1.11mM gluconic acid, 3.08 mM cesium hydroxide, 0.48 mM glycerin, and 12% acetonitrile. B3 (background conductivity ca. 341 &): 12.65 mM boric acid, 1.70 mM gluconic acid, 4.72 mM lithium hydroxide, 0.75 mM glycerin, and 12% acetonitrile. Lithium hydroxide monohydrate (99%),potassium hydroxide (86.2%),cesium hydroxide monohydrate (99%),boric acid (99% A.C.S. reagent), and D-gluconic acid (50% wt in water) were supplied by Aldrich Chemical Co. Glycerin (U.S.P.-F.C.C.) and acetonitrile (HPLC grade) were obtained from J. T. Baker Chemical Co. Close match of conductances is an overriding concern in the preparation of the isoconductive eluents. Generally it was found to be feasible to match a pair of gradient eluents within 3 p S simply by weighing in and by pipetting the amounts and volumes specified for A2,B2 or A3,B3 into a 1000-mL volumetric flask. Because of the fluctuating purity of hydroxides due to the changes occurring during their storage, small variations in the actual molar ratios of alkaline hydroxide to other more stable components of isoconductive eluents had to be expected. Corrections were carried out by adding small volumes of water to the eluent of higher conductivity. Such f i e adjustments could be calculated by assuming a linear relationship between the background conductance and the volume of an eluent. Conductivity mismatch of 10-15 p S could still be successfully compensated by the employed base-line subtraction routine (Waters M 840 Expert Software). However, the precision data presented in Table I were obtained with eluents A2,B2 differing by no more than 1 fiS. Preparation of Standards. All standard solutions of inorganic 0 1988 American Chemical Society
