1217
Anal. Chem. 1092, 64, 1217-1220
Trace Determination of Benzene and Aromatic Molecules in Hexane by Laser Two-Photon Ionization Teiichiro Ogawa,' Manabu Kise, To-oru Yasuda, Hirofumi Kawazumi,+and Sunao Yamadat Department of Molecular Science and Technology, Kyushu University, Kasuga-shi, Fukuoka 816,Japan
Laur two-photon lonlzatlon of several aromatlc molecules In hexane has been carrkd out wlth the fourth hannonlcr (266 nm) of a NCYAQ laser as an excltatlon source. The photolonlzatlon current k quadratlcally proportional to the laser pulw onorgy, lndlcatlng two-photon lonlzatlon. There Is a large Mank dgnaJ due to tho rolvmt (hexane) lonltatlon, and lt has boon canceled wlth a dlfferentlaltechnlque. The effect of laser p u b energy fluctuation ha8 been corrected by an on-llno nonnallzatlon procedure. The photolonlzatlon current ako depends on the molar abmptlvlty of the molecule, and tho flrst step to lonhatlon should be excltatlon at 266 nm above the ground date. The detectlon llmlt ranges from 3.8 ppb for benzeneto 61 ppt for pyrone. Analytlcalcurves were linear for at lead 3 orders of magnltude above the detection Ilmns.
Figure 1. The schematlc diagram of the experimentalsystem. Laser,
the 4th harmonics (288 nm) of a W Y A G laser; L, kns; BS, beam spiHter; PD, pyro detector; W, pulse generator;HV, hlgh-vottage power supply unl; DA, dlfferentiai amplifler; A X , AID converter: CPU, microcomputer.
INTRODUCTION Laser two-photon ionization combined with a conductivity measurement is a sensitive technique1 for detection of conjugated organic molecules such as polynuclear aromatic hydrocarbona,2-9drugs: quinones,SJand vitamins6 in solution. The sequential two-photon ionization via a resonant state is much more probable and useful for analytical purposes than the simultaneous ionization via a virtual state. The first step of sequential photoionization is the electronic excitation by the f i i t photon, and the efficiency of photoionization depends on molar absorptivity at the laser wavelength.5~9Simultaneous action of both 355 and 1064 nm or of both 355 and 532 nm from a Nd-YAG laser enhanced the detection sensitivity; the detectionlimit of anthracene is as low as 1.7ppt.lOJ1 However, this technique has not been successful for such a molecule as benzene and its derivative, which has absorption maximum below 300 nm, because unwanted blank signals due to photoionization of solvent were large at an excitation wavelength below 300 nm and the signal-to-noiseratio was poor in these measurements.
* Author to whom correspondence should be addressed.
+ Faculty of Literature, Kitakyushu University, Kitakata, KokuraMinami, Kitakyuehu 802,Japan. College of General Education, Kyushu University, Ropponmatau, Chuo-ku, Fukuoka 810, Japan. (1)Yamada, S.; Ogawa, T. B o g . Anal. Spectrosc. 1986, 9,429-453. (2)Voigtman, E.;Jurgensen, A.; Winefordner,J. D. Anal. Chem. 1981, 53, 1921-1923. (3)Yamada, S.;Kano, K.; Ogawa, T. Bunseki Kagaku 1982,31,E247-
*
E250.
(4)Voigtman, E.; Winefordner, J. D.Anal. Chem. 1982,54, 18341839. (5)Yamada, S.;Hino, A.; Kano, K.; Ogawa, T. Anal. Chem. 1983,55, 1914-1917. (6)Fujiwara, K.; Voigtman, E.;Winefordner, J. D. Spectrosc. Lett. 1984,17,9-20. (7)Yamada, S.; Ogawa, T.; Zhana, P. H. Anal. Chim. Acta 1986,183, 251-256. (8)Sato, N.; Yamada, S.; Ogawa, T. Anal. Sci. 1987,3,109-111. (9)Yamada,S.;Sato,N.; Kawazumi, H.;Ogawa,T. Anal. Chem. 1987, 59,2719-2721. (10)Yamada, S.Anal. Chem. 1988,60,1975-1977;1989,61,612-615. (11)Yamada, S.Anal. Sci. 1991,7,223-227. 0003-2700/92/0364-1217$03.00/0
-
1 ,
~
slow
,
,
I
0
l
l
I
100 T h e I ms
I
,
I
200
Figure 2. The time profile of the photolonlzatlon current. 1 X M anthracene In hexane.
The ionization threshold of neat liquid hexane was reported to be 8.8 eV,l2 and laser photolysis of aliphatic hydrocarbon solvents has been investigated.13-16 Since the two-photon energy of the 266-nm light is 9.32 eV, any effect due to the solventphotoionizationhas to be suppressed for an analytical application of photoionization at 266 nm. Picosecond 266nm two-photon laser photolysis of aromatic hydrocarbons in aliphatic hydrocarbon solvents disclosed the contribution of the ionized state for the formation of triplet states.16 It was found that a double-beam technique is useful to subtract the unwanted signals and to increase the signal-tonoise ratio." The effect of the solvent blank signal may be overcome with the use of a similar technique. A differential measurement has been undertaken for detection of benzene derivatives using the fourth harmonics (266 nm) of a NdYAG laser as an excitation source. The sensitivity of the new technique has been found to be superior to conventionalspectrophotometry. (12)Schwarz, F.P.;Smith, D.; Lias, S.G.; Aueloos, P. J. Chem. Phys. 1981,75,3800-3808. (13)Miyasaka, H.; Matada, N. Chem. Phys. Lett. 1986,126,219-224; 1987,134,-48&484. (14)Miyasaka, H.;Masuhara, H.; Mataga, N. J.Phys. Chem. 1990,94, 3577-3582. (15)Mivasaka.. H.:. Masuhara.. H.:. Matana. . . - . N. J. Phvs. Chem. 1981.89, i63i-i636: (16)Scott, T. W.; Braun, C. L. Chem. Phys. Lett. 1986,127,501-504. (17)Yamada, S.;Hino, A.; Ogawa, T. Bunseki Kagaku 1984,33,E37E40. 0 1992 Amrlcan Cbmlcal Soclety
1216
ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992
1000
6
1000
(all
.
2 100
- 300
h h
a
Y
- 200 .o iij a - 100 0.01
0.1
1
Laser Power / mJ
10
0.01
4
@ .
10
0
v CI
K
g
1;
2
0.1
0
s0
0.1
Laser Power I mJ
Laser Power / mJ
Flguro 3, Photocurrent dependence on laser power of (a) anthracene In hexane, (b) 2amlnoanthracene In hexane, and (c) anthracene In carbon tetrachloride: (0)photocurrent for 1 X 10" M anthracene or 2amlnoanthracene; (0)photocurrent for pure hexane or carbon tetrachlorlde; (A) ratio (A = (0 O)/O),rlght ordlnate should be used. Calculations of the ratlo (A)were made by Interpolation of photocurrents with a smooth line.
-
EXPERIMENTAL SECTION The schematic diagram of the experimental system used for analytical measurements is shown in Figure 1. The fourth harmonics of a Nd-YAG laser (Spectra Physics GCR-11; 30 mJ and 4-5 ns at 266 nm) was operated at a repetition rate of 2 Hz and was focused softly into a sample cell (cell B) and a reference cell (cell A). The two cells were constructed identically; each of them was made of a fluorescence cell (4 X 1 X 1cm), and a pair of copper electrodes (3.5 X 1 cm) was set inside the cell. The electrodespacing was 8 mm. One of the electrodeswas connected to a high-voltage power supply unit (Ikegami HD1.5KM) via a current-limiting resistor of 10 MQ;the typical voltage applied between the electrodes was 1 kV. The other electrode was connected to an NF Circuit P-61 differential amplifier through a 2-MQvariable potentiometer, and the difference of the current signal from the two cells was obtained. Two potentiometerswere adjusted to balance signals from the two cells. The laser pulse energy was simultaneously monitored with a Molectron P1-13H pyroelectric detector in order to calibrate laser pulse energy fluctuation. The photoionization signals and the laser pulse energy were measured with a CanopusAnalog-ProA/D converter (12 bits/( ~s resolution) and the data were analyzed with an NEC PC9801RX microcomputer. For physicochemical measurements only one of the two cells was used without differential amplification. The molar absorptivity was determined with a JASCO UVIDEC-505 spectrophotometer, and the detection limit by fluorometry was obtained with a Hitachi F-4010 fluorescence spectrophotometer. The hexane (research grade) was obtained from Wako Chemicals; the solventwas used after distillationand dryingwith CaC12. Benzene and other samples were used as supplied.
RESULTS AND DISCUSSION A typical time profile of the photoionization current signal is shown in Figure 2; this profile was measured with a single cell. This profile was obtained with 1X M anthracene in hexane excited a t 266 nm, and the resolution time of the system was about 30 ws. The profile has two components as in the case of 337-nm excitation, and the fast component can be assigned to electrons and the slow one to i0ns.15 The sharp fast signal is intense and more useful for highly sensitive detection; thus, ita peak intensity is used for the following measurements. The current signal in hexane depended on the laser pulse energy quadratically a t lower pulse energies as shown in Figure 3 (parte a and b). This finding indicates that the photoionization is a two-photon process. The current signals of anthracene and 2-aminoanthracene leveled off at higher laser (18)Yamada, S.;Yoshida, S.; Kawazumi, H.; Nagamura, T.; Ogawa, T.Chem. Phys. Lett. 1986,122,391-394.
"
0.5
1
1.5
Normalized Intensity Flgure 4. Histogram of the laser photolonlzation current slgnal for 1200 laser shots. (a) Current slgnal before correction. The standard devlatlon Is 0.17 due to the fluctuation of the laser pulse energy. (b) Fluctuation of the laser pulse energy. The standard deviation Is 0.080. (c) Current signal after correctlon. It was set as: corrected signal = current slgnal/(pulse energy)2. The standard deviation Is 0.035.
pulse energies due to saturation. But the blank signal of the solvent also increased at these pulse energies; however, it increased less than quadratically because part of the blank signal was due to laser light scattering, which should increase linearly. Thus, there was an optimal laser pulse energy; the ratio of signal-to-blank [(solution - solvent)/solvent] had a maximum at about 1.0 mJ/pulse for anthracene and at about 0.3 mJ/pulse for 2-aminoanthracene in hexane. The blank signal due to the solvent (hexane) is large enough to disturb a highly sensitive detection, although hexane is one of the solvents which offer the lowest dark current.' The quadratic part of the blank signal is probably due to twophoton ionization of hexane, because the two photon energy (9.32 eV) at 266 nm is sufficient for its photoionization (8.8 eV12). The ionization of hexane should be a simultaneous process because it has no optical absorption at 266 nm. The fluctuation of the blank signal is the largest source of experimental errors because the signal-to-noiseratio is defined as the ratio of the photoionization signal versus the standard deviation of the blank signal. The photoionization was also carried out using carbon tetrachloride as solvent, as shown in Figure 3c. It has a higher ionization potential than hexane has and ita ionization may
ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992
Table I. Detection Limits and Molar Absorptivities of Aromatic Molecules Excited at 266 nm in Hexane detection limit, Mg/L molar absorptivity photoionization absorptiono compound at 266 nm, cm-l M-l 266 nm 337 nmc at 266 nm 20 3.8 2000 benzene 1.2 260 180 toluene p-xylene
1.3 3.3 2.7 0.064 0.72 0.061
520 170 260 4750 730 18600
chlorobenzene nitrobenzene naphthalene anthracene PFene
fluorescence* at 266 nm 94
100 330 240 13 120 5.5
0.01 0.006
1219
900
0.016
Detection limits by absorption were estimated as a concentration at absorbance = 0.0005. * Detection limits by fluorescence were obtained at the excitation wavelength of 266 nm and at the fluorescence maximum wavelength. Reference 5.
10'
1o3 lo4 Molar Absorptivity
lo2
lo5
-
Figure 5. Photocurrent dependence on molar absorptlvtty of various aromatlcmolecules at 1 X 10-5M: (0)measured at laser pulse energy of 1 mJ/pulse; (0)measured at 0.3 mJ/pulse and corrected for 1 mJ/pulse. 1, benzene; 2, chlorobenzene; 3,bromobenzene; 4, nltrobenzene; 5, toluene; 6, &xylene; 7, pxylene; 8, fluorobenzene; 9, aniline; 10, anthracene; 11, l-chloroanthracene; 12, 9-bromoanthracene; 13, naphthalene; 14, 2-methylanthracene; 15, pyrene; 16, P-aminoanthracene. be smaller, but it can scavenge electrons produced in the photoionization process. Thus, it gave a smaller photoionization current of the solute, and the signal-to-blank ratio was poorer. In order to overcomethe difficulty due to the solvent signal, a pair of cells, which were similar to the double-beam cell for an HPLC detector,12were constructed. Only the difference in the current signal of the two cells was amplified in order to gain the dynamic reserve and to suppress the effect of the signal fluctuation due to the solvent photoionization. The fluctuation of the pulse energy of the fourth harmonics of a Nd-YAG laser is fairly large and induces a sizable fluctuation of the photocurrent because of its quadratic dependence on the laser pulse energy. Such fluctuation should be corrected for a highly sensitive determination. The correction was carried out by dividing each current signal pulse-by-pulse by the square of the corresponding laser pulse energy, as shown in Figure 4. The standard deviation was improved by a factor of 5. In addition, averaging of such data for 100laser pulses can reduce the standard deviation to 0.02, as compared to 0.17 for the uncorrected signal. The current signal depended on the molar absorptivity of each molecule as shown in Figure 5. This dependence was measured a t a laser pulse energy of 1 mJ, and the data for pyrene and 2-aminoanthracene were corrected for saturation. The correction was made by measuring the current signals both at 0.3 and 1 mJ for naphthalene, pyrene, and 2-aminoanthracene and by calibrating data of the latter two molecules to that of naphthalene because naphthalene showed no noticeable saturation a t 1mJ. This finding indicates that the first step of the photoionization is an excitation into an excited singlet state at 4.7 eV (266 nm) above the ground
' """"
10-l1 10'
' """" ' """"
lo3
Molar Absorptivity
io5
Figure 7. Detection limit and molar absorptivity: (a)excitation at 266 nm (4thharmonics of Nd-YAG laser); (0)excitationat 337 nm (nitrogen laser) (ref 5). The laser pulse energy was 1 mJ/pulse. Be, benzene; Tol, toluene; pX, pxylene; Na, naphthalene; An, anthracene; CIBe, chlorobenzene; NOPBe, nitrobenzene; Py, pyrene; Bph, benzophenone; Chr, chrycene;CIAn, chlaroanthracene;MeAn, mthylanthracene.
state. However, molecules with a substituent such as C1 and Br show a smaller current signal than the others with a similar molar absorptivity, indicating a quenching of the excited state, a molecule with a halogen atom usually has a shorter fluorescence lifetime. Analytical curves of many aromatic compounds were linear over a concentration region of about 3 orders of magnitude above detection limits, as shown in Figure 6. The detection limits were obtained as a concentration where S/N = 3 and are summarized in Table I; the blank level at detection limits was about 3 PA. Those obtained at 337 nmsand by the optical absorption and fluorescencemethods a t 266 nm are also shown for comparison. The detection limits by absorptiometry were estimated as a concentration at which absorbance is O.OOO6 Abs. The dependence of detection limits on molar absorptivity is shown in Figure 7. For those molecules which have a large molar absorptivity a t 337 nm, detection limits by 266nm photoionization were worse than those by 337-nm photoionization. This is due to a larger blank signal in hexane
1220
ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992
at 266 nm. A 266-nm photon excites the larger molecule, which has an absorption band at 337 nm or longer, to the higher excited states such as Sz and SB,and the lifetime of the higher excited states is usually shorter due to fast relaxation; this would contribute a smaller signal for excitation at 266 nm. However, this technique offers a sensitive method for those molecules whose absorption maxima lie below 300 nm, especially for a weakly fluorescent or nonfluorescent molecule.
RECEIVED for review November 26, 1991. Accepted February 21, 1992. Registry No. Be, 71-43-2; Tol, 108-88-3; pX, 106-42-3;Na, 91-20-3; A ~120-12-7; , ClBe, 108-90-7; 98-95-3; py, 12900-0;Bph, 119-61-9; Chr, 4985.70-0; MeAn, 613-12-7; CeHaBr, 108-86-1;Me-o-C6HaMe, 95-47-6;CsH5F,462-0643;CeHsNH2,6253-3; 9-bromoanthracene, 1564-64-3; 2-aminoanthracene, 61313-8.
