Anal. Chem. 1986, 58,3242-3244
3242
SAMPLE SOLUTION
-
I
I
OSMOTIC CONCENTRATION CELL
t '
SOLUTION SALT
I
RESERVOIR
,GT, j INTERFACE
SAMPLE
~
I
~
RESERVOIR
3.
NEBULIZER DELIVERY PUMP
~
ICP
1
Figure 2. Schematic diagram of osmotic concentrating assembly.
Table IV. Enrichment Factors for Osmotic Preconcentration"
LITERATURE CITED
concn, pg/L % enrichment element* initial measd calcd disagreement factor Mn
100
839
Zn Cd
100
826
91.6
722
848 848 179
1.1 2.6 7.3
the system the flexibility needed to interface it with a nebulizer of an ICP. The present interface was designed to eliminate the buildup of pressure in the sample channel. The delivery of the sample from the interface to the nebulizer was set a t 0.7 mL/min. The input of the sample into the concentrating cell was adjusted so that the sample output matched the flow rate to the nebulizer. Enrichment factors for the preconcentrator are illustrated in Table N. "he concentration factor for water removal by osmosis was 9.0; however, the enrichment factors were slightly lower due to the incomplete analyte recovery (93.7-94.7%). Enrichment factors for Mn and zinc (Zn) are in good agreement with the predicted values while Cd shows marginal agreement. One limitation of the technique a t this time is the back permeation of NaCl from the saturated salt solution. Even though rejection of NaCl by the membrane was greater than 99.98%, the sample solution after concentration contained about 3600 pg/mL Na. This salt concentration can cause problems in the measurement of analyte emission intensity. For example, we noted a deterioration in precision after prolonged nebulization into the ICP unit, presumably from the nebulizer fouling. Preliminary tests on other salts such as sodium hydrogen phosphate, sodium citrate, sodium tartrate, and magnesium chloride have shown a reduced back permeation of the salt with only a modest loss in the sample concentration rate. These investigations are continuing. Registry No. Cd, 7440-43-9; Cu, 7440-50-8;Mn, 7439-96-5; Ni, 7440-02-0; Zn, 7440-66-6.
8.39
8.26 7.88
"Sample input = 6.30 mL/min, sample output = 0.70 mL/min, concentration rate = 5.60 mL/min, concentration Factor = 9.0. *Sampleschelated with EDTA. not yet been identified. Analyte recoveries (chelated) for three different cuts of membrane also show good agreement having only a 2% range a t the 200 pg/L level. This agreement indicates that the pore size of the membrane is consistent throughout the entire sheet that was purchased. The percent recoveries for analyte concentrations (chelated) ranging from 20 gg/L to 20 pg/mL are in good agreement, indicating that analyte recovery is independent of the sample concentration. The ability to vary the rate of sample output from the concentrating cell by varying the rate of sample input gives
(1) Minczewski, J.; Chwastowska, J.; Dybczynski, C. Separation and R e concentration Methods in Inorganic Trace Analysis, Williamson: New York, 1982. (2) Mizuike, A. Enrichment Techniques for Inorganic Trace Analysis ; Springer-Verlag: Berlin, Heildelberg, New York, 1983. (3) Matsuura, T.; Pageau, L.; Souirajan, S. J . Appi. Polym. Sci. 1975, 19. 179-198.
R. J. Stec S. R. Koirtyohann* Department of Chemistry University of Missouri Columbia, Missouri 65211
H. E. Taylor U S . Geological Survey Box 25046, MS 407 Denver Federal Center Denver, Colorado 80225 RECEIVED for review March 4,1986. Resubmitted August 20, 1986. Accepted August 20, 1986. Use of trade names does not imply endorsement by U.S. Geological Survey.
Capillary Gas Chromatograph Determination of Aniline Derivatives by Supersonic Jet Resonance Multiphoton Ionization Mass Spectrometry Sir: Supersonic jet expansion cools a sample molecule to several kelvin and it greatly simplifies the spectrum. Therefore, this spectrometric technique is advantageous for identification of molecular species. This method has already been used for analytical purposes (1,2). Polycyclic aromatic hydrocarbons such as benzo[a]pyrene are determined by fluorometry with the excitation sources of dye lasers (3-5) and xenon arc lamps (6-9). Multiphoton ionization mass spectrometry is also used for determinations of aromatic molecules
(10-16). More recently, a supercritical fluid sample introduction technique has been developed for measurements of nonvolatile substances (17, 18). Since the real samples, such as airborne particulates, contain more than 100 organic species, a sample separation procedure is necessary before the determination of these compounds. Gas chromatography is successfully used for this purpose because of good separation resolution and high sensitivity. Hayes et al. demonstrated the determination of naphthalene
0003-2700/86/0358-3242$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
3243
Gas Chromatograph
Signal
Liquid Nitrogen Trap Raman Shifter Dye Laser Rotary Pump
Excimer Laser
Flgure 1. Capillary gas chromatograph equipped with detector based on supersonic jet multiphoton ionization mass spectrometry.
and its derivatives in crude oil by the gas chromatograph equipped with a laser fluorometric detector (19). In our previous study, we clarified advantages of the gas chromatograph detector based on multiphoton ionizationftotal ion detection (20). More recently, we demonstrated capillary gas chromatography based on fluorometry using a high-temperature pulsed nozzle (21). These methods are apparently superior to the conventional detectors with respect to selectivity. However, they have limited performances in identification of the sample species, since assignment is performed only from the retention time of the chromatograph peak. In this study we construct a capillary gas chromatograph equipped with a supersonic jet multiphoton ionization timeof-flight mass spectrometer and demonstrate selective determination of aniline derivatives. EXPERIMENTAL SECTION A block diagram of the experimental apparatus is shown in Figure 1. The sample is separated by a capillary gas chromatograph (Shimadzu, GC-8A, CBP-1-425,0.2mm i.d., 20 m). The stream is introduced into the high-temperature pulsed nozzle, which is maintained at 200 "C (21). It is expanded into vacuum, and the sample molecule passes through a skimmer, which is succeedingly multiphoton-ionized. The ionization chamber is maintained below 5 x IO+ torr (20, 21). The excimer-laserpumped dye laser (Lambda Physik, EMGlOBMSC, FL2002) is converted to the first anti-Stokes emission by a homemade Raman shifter (22) and used for sample ionization. The induced ions are introduced into a flight tube (42.5 cm) by a repulsion potential (210 V). The flight tube is evacuated by a turbomolecular pump (Osaka Vacuum, ST520) and is maintained below 3 X torr. The ions are detected by assembled microchannel plates (Hamamatsu Photonics, F1094-32s). The time-of-flight signal terminated by a 5 0 4 resistor is 50 times amplified and is recorded by a digital memory (Iwatsu, DM901). The observed signal is converted to a 100 times slower analog signal, and it is recorded again by an autodigitizer (Autonics,S-210)combined with a signal averager (Autonics, F-610). A multiphoton ionization spectrum and a chromatogram are recorded by detecting a parent ion with a boxcar integrator (NF Circuit Design Block, BX-530A). RESULTS AND DISCUSSION Figure 2 shows multiphoton ionization spectra for aniline derivatives, which were measured by replacing the separation column with a sample reservoir containing the solid sample, The spectra are composed of sharp line structures as reported (10). The line width of the original transition for aniline (293.77 nm) was measured to be 5.3 cm-l, corresponding to a jet temperature of 10 K. These results show that the sample molecules are sufficiently cooled in the supersonic jet. Figure 3 shows gas chromatograms for a mixture of aniline derivatives. The resolution of the chromatograph is not sufficient at present to resolve isomers of N-methylaniline and p-toluidine. However, a specified molecule can be selectively
Wavelengthhn)
Figure 2. Multiphoton Ionization spectra for aniline derivatives. N-methylaniline + p-toluidine
I
2
I
N-methylaniline + p-toluidine
I
N-methylaniline
(a)
1
31
I
I
I
I
0
2
4
6
Retention Time ( m i d
I
I
8
I
I
I
0
2
__
4
6
8
Retention Time (min)
Flgure 3. Chromatograms for aniline derivatives. (a) The wavelength of the exciting source is adjusted to 302.2 nm, which is optimized to p-toluidine. (b) The wavelength of the exciting source is adjusted to 300.3 nm, which is optimized to N-methylaniline. The 1 FL of 2 % sample solution dissolved in cyclohexane is injected into the capillary gas chromatograph. 1
10
50
100
200 300 Mw
(b)
I
10
50
101
Fllght Time
1
200 ; .(003
ols)
Fllght Tlms (rra)
Figure 4. Mass spectra for mixture of N-methylaniline and p-toluidine. The measurements are carried out at the Chromatograph peaks shown in Figure 3. The wavelength is optimized to (a) N-methylaniline and (b) p-toluidine.
ionized by adjusting the exciting wavelength. The chromatograph resolution is limited by both the dead volume of the nozzle and the performance of the capillary column. In Figure 4 is shown the mass spectra measured a t the chromatograph
Anal. Chem. 1986, 58,3244-3248
3244
1
IO
50
200 300 Mw
100
8
0
10
20
30
40
50
Flight Time ols) Figure 5. Mass spectrum for mixture of aniline, N-methylaniline, and p-toluidine. The wavelength is optimized to aniline. The mass spectrum is measured at the chromtograph peak for aniline. The 1 pL of 2% sample solution dissolved in toluene is injected into the capillary gas chromatograph. peaks in Figure 3. Only the parent molecules are detected, and soft ionization is performed. The measurement was also carried out a t the point where background noise appeared. The molecular weight observed was identical to that of the sample species. Then, this background noise was tentatively concluded to be coming from the sample remaining in the nozzle or the gas chromatograph system. The detection limit was 5 pg for these samples, at present. In this study we use a split-injection method for sample introduction, in which only several percent of the sample is introduced into the capillary column. A more updated technique, such as a cold-on-column method, allows 100% sample introduction so that the detection limit might be improved to the order of 100 ng by using this method. Further improvement in detectability may readily be achieved by reducing background and by increasing the output energy of the laser. In fact, the multiphoton ionization signal is proportional to the square of the output energy at present power levels. Thus, use of a nonlinear crystal instead of a Raman shifter for more efficient frequency conversion might greatly improve the detection sensitivity. We expect that sub-nanogram detection of aniline is possible by straightforward extension of the present approach. Figure 5 shows the mass spectrum for a mixture of aniline derivatives in which the exciting wavelength is optimized to aniline. A larger peak corresponding to a molecular weight (MW) of 93 is originating from aniline. A smaller peak corresponding to MW = 107 is consider to be originating from N-methylaniline, since it gives a slightly congested multiphoton ionization spectrum a t this wavelength, as shown in Figure 2. This result shows that selective determination of
these samples can readily be achieved by mass spectrometry. The resolution of the mass spectrometer constructed was found to be AM = 0.8 a t MW = 97. In biochemistry, slight modification of the molecular structure of a sample greatly changes its bioactivity. These substances are currently determined by gas chromatography/mass spectrometry, but this method sometimes suffers from its poor selectivity especially in determinations of the isomers. The present approach may have a great performance for assignment of the sample species because of very high selectivity given by capillary gas chromatography and supersonic jet multiphoton ionization mass spectrometry. Thus, we expect that the present method is advantageous for selective determination of vitamins, metabolites, and medicines. Registry No. Aniline, 62-53-3; rn-toluidine, 108-44-1; p toluidine, 106-49-0;N-methylaniline, 100-61-8. LITERATURE CITED Hayes, J. M.; Small, G. J. Anal. Chem. 1983,55, 565A. Johnston, M. V. Trends Anal. Chem. 1984, 3 , 5 8 . Warren, J. A.; Hayes, J. M.: Small, G. J. Anal. Chem. 1982,54,138. Amirav, A.; Even. U.; Jortner, J. Anal. Chem. 1982,54, 1666. Imasaka, T.; Fukuoka, H.; Hayashi, T.; Ishibashi, N. Anal. Chirn. Acta 1984, 756,111. Imasaka, T.; Hirata, K.; Ishibashi, N. Anal. Chem. 1985, 57, 59. Yamada, S.;Winefordner, J. D. Spectrosc. Lett. 1985, 18, 139. Yamada, S.;Smith, B. W.; Voightman, E.; Winefordner. J. D. Analyst (London) 1985, 110, 407. Yamada, S.;Smith, B. W.; Voigtman, E.; Winefordner, J. D. Appl. Spectrosc. 1985, 3 9 , 513. Lubman, D. M.; Kronick, M. N. Anal. Chem. 1982, 54,660. Tembreull, R.; Lubman, D. M. Anal. Chem. 1984, 56, 1962. Sin, C. H.; Tembreull, R.; Lubman, D. M. Anal. Chem. 1984, 56, 2776. Lubman, D. M.; Ternbreull, R.; Sin, C. H. Anal. Chern. 1985, 57, 1084. Tembreull, R.; Sin, C. H.; Li, P.; Pang, H. M.; Lubman, D. M. Anal. Chem. 1985,57, 1186. Ternbreull, R.; Sin, C. H.; Pang, H. M.; Lubman, D. M. Anal. Chern. 1985, 57,2911. Opsal, R. B.; Owens, K . G.; Reiliy. J. Anal. Chern. 1985, 57, 1884. Fukuoka, H.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1986,58,375. Sin, C. H.; Pang, H. M.; Lubrnan, D. M.; Zorn, J. Anal. Chem. 1988, 58. .., 487. Hayes, J. M.; Small, G. J. Anal. Cbem. 1982, 54, 1204. Imasaka. T.; Shigezumi, T.; Ishibashi, N. Analyst (London) 1984, 109, 277. Imasaka, T.; Okamura. T.; Ishibashi, N. Anal. Chern. 1986, 58, 2152. Kawasaki, S.;Imasaka, T.; Ishibashi, N., submitted for publication in Anal. Chern. ~
Totaro Imasaka Kouji Tashiro Nobuhiko Ishibashi* Faculty of Engineering Kyushu University Hakozaki, Fukuoka 812, Japan RECEIVED for review May 21, 1985. Resubmitted May 19, 1986. Accepted July 17,1986. This research is supported by a Grant-in-Aid for Scientific Research from the Ministry of Education of Japan and by a Nissan Science Foundation.
Atomic Emission Spectrometry with a Reduced-Pressure Afterglow Extracted from an Inductively Coupled Plasma Sir: The inductively coupled plasma (ICP) has proven to be an excellent excitation source for elemental analysis of solutions by atomic emission spectrometry (AES) (1,2). One reason for the success of the ICP is that volatilization and atomization interferences are minimal because the analyte is efficiently atomized in the high-temperature, atmosphericpressure environment. It seems that such an environment is
essential for proper dissociation of analytes from sample particles such as those generated by solution nebulization. In a conventional ICP the analyte atoms then continue through the axial channel where they are excited and ionized a t atmospheric pressure. In some ways, the observation of ICP emission at reduced pressure might offer potential advantages in that line widths should be sharper than from an atmos-
Q 1986 American Chemical Society 0003-2700/86/0358-3244$01.50/0
