Anal. Chem. 1990, 62, 1893-1895 100
s C
Z 50
h X
F0
I
0.1
1 10 conc. of HCI,
100 mM
Figwe 2. Extraction of NO,- and F- as a function of HCI concentration in aqueous phases: organic phase, dichloromethane saturated with water; aqueous phase, 0.1 M PEG 1000 with HCI; phase ratio, 1:l.
above, since this interaction was not detected in aqueous media, low permittivity circumstances may be required. The column used in this work is almost completely endcapped. However, there still remain a few silanol sites. H F is thought to interact with silanol groups. In fact, HF was retained on a silica gel column, and its retention became stronger with the acidity of mobile phases. Other anions and their acids (including NO< and HNO2) were not retained on the silica gel column. Thus, the retention of HF on the POE-coated ODS column is possibly due to the interaction with residual silanol groups. This interaction was observed only for the POE-coated ODS column but not for the bare column. This will be explained by the following inference. In the bare column, octadecyl groups prevent the contact of H F with silanol groups because wetting of such a hydrophobic surface with water does not occur. Adsorption of POEs lowers the hydrophobicity of the surface of the stationary phase and permits contact with aqueous mobile phase. Thus, the interaction of residual silanol groups with H F appeared. The retention of acidic analytes on the bare ODS column should be investigated by using a purely aqueous mobile phase to verify the effect of residual silanol groups. However, anions accompanied by H+ as a countercation were not eluted from the bare column regardless of the nature of anions. This phenomenon is probably due to the hydrogen bonding to H+ in the column packing material under the low ionic strength
1893
condition. A counteranion is trapped by the electrostatic attraction to H+. Solvent extraction of nitrite and fluoride was attempted to ensure the interaction chromatographically observed. Dichloromethane was selected as a solvent because it was successfully used for the extraction of metal cations with crown ethers and was not emulsified even in the presence of POEs (8). Poly(ethy1ene glycol) (PEG) lo00 was chosen as a POE. Choice of a countercation was also important. Some metal cations (K+, Rb+, Hg2+,Ba2+,etc) are possibly extracted to an organic phase by forming PEG complexes. In such cases, the extraction of a cation is accompanied by the phase transfer of desolvated anions. To avoid this ion-pair formation, Na+, the complexation of which was known to be rather weak, was chosen as a countercation (6,8).Although Cl-, Br-, NO3-, and I- were not extracted, fluoride and nitrite were extracted from acidic aqueous phases to the organic pase as the neutral species as shown in Figure 2. In particular, the extraction ratio of HNOz is very high. HNOz is extractable as N203 to some extent even in the absence of POEs (9). However, the extraction ratio of Nz03 was much lower than the results shown in Figure 2. Thus, the results presented here strongly support the interaction of POE with HNOz in low permittivity media, though the nature of the reaction has not been elucidated. Spectrometric investigation of this interaction is now in progress in our laboratory. Registry No. POE, 25322-68-3;HN02,7782-77-6;F,1698448-8. LITERATURE CITED (1) Izatt, R. M.; Bradshaw, J. S.; Nlelsen, S. A.; Lamb, J. D.; Christensen, J. J. Chem. Rev. 1085. 65, 271. (2) Izatt, R. M.; Christensen, J. J. Synthesis ofhcrocyc/es-7h Design of Selective Complexing Agent; Wlley: New York, 1987. (3) Kimura, K.; Hayala, E.; Shono, T. J . Chem. Soc., Chem. Commun. 1984, 271. (4) Graf, E.; Lehn, J. M. J . Am. Chem. SOC. 1078, 97, 6403. (5) Wuthler, U.; M m , H. V.; Zund, R.; Welti, D.; Funck, R. J. J.; Bexegh, A.; Ammann, D.; Retsch, E.; Simon, W. Anal. Chem. 1084, 56, 535. (6) Cross, J. Nonionic Surfactants; Dekker: New York, 1987. (7) Okada, T. Anal. Chim. Acta 1000, 230, 9. (8) Yanagida, S.; Takahashl, K.; Okahara. M. But. Chem. Soc. Jpn. 1077. 50, 1386. (9) Beattle, I . R. Progress in Inorganic Chemistry; Cotton, F. A., Ed.; Wiley-Interscience: New York, 1963; Voi. 5, pp 1-26.
Tetsuo Okada
Faculty of Liberal Arts Shizuoka University Shizuoka 422, Japan RECEIVED for review December 18,1989. Accepted May 16, 1990.
Multiple Mode Semiconductor Diode Laser as a Spectral Line Source for Graphite Furnace Atomic Absorption Spectroscopy Sir: Since the report of atomic absorption spectroscopy
(AAS)by Walsh in 1955 (I),AAS is a principal technique for trace element analysis. An important component of an AA spectrometer is the radiation source. Glow discharges including hollow cathode lamps (HCL) and electrodeless discharge lamps have been used as the line sources (21, and continuum sources also have been employed (3). HCLs give stable, sharp spectral characteristics, high sensitivity, and ease 0003-2700/90/0362-1893$02.50/0
of operation and are the preferred line sources used in virtially all commercial instruments. L'vov in 1961 (4) introduced the technique of graphite furnace (GF) AAS, which proved to provide superior sensitivities and detection limits over the flame approach. The electrically heated graphite furnace, however, requires reproducible operation for getting an acceptable signal precision. This reproducible operating includes the heating temperatures, the position of the sample in the 0 1990 American Chemical Society
1894
ANALYTICAL CHEMISTRY, VOL. 62, NO. 17, SEPTEMBER 1. 1990
Table I. Instrumentation and Manufacturer
instrument component
manufacturer
1. laser diode
diode (NDL-3200) temperature controller (LDT-5910) current supply (LDX-3620) 2. flame burner, 10-cm slot 4. graphite furnace (CTF Atomizer 555) 3. predisperser monochromator, Jobin-Yvon H10 (0.1 m, 1200 grooves/mm grating, linear dispersion 8 nm/mm), 2-mm slits 3. Diode-array spectrometer diode array (OSMA Model IRY-1024) monochromator, Jobin-Yvon HRlOOO (1 m, 2400 grooves/ mm grating, linear dispersion 0.5 nm/mm) 4. data acquisition system PC computer software (St-120)
m
NEC Electronics, Inc., Mountain View, CA ILX Lightwave Corp., Bozeman, MT ILX Lightwave Corp., Bozeman, MT Perkin-Elmer, Norwalk, CT Instrumentation Lab., Inc., Lexington, MA
0
FLAME
Instruments SA, Inc., Metuchen. NJ Flgure 1. Instrumentationof the multimode diode laser graphite furnace atomic absorption system.
Princeton Instruments, Princteon, NJ Instruments SA, Inc., Metuchen, NJ
I
13
IBM Princeton Instruments. Princeton, NJ
GF, and the furnace deterioration, which can affect the atomization efficiency and scattering effects of the source radiation. In real world sample analysis, GFAAS requires use of a background correction technique to account for large non-atomic absorption and scattering effects (5). Continuum sources (6),Smith-Hieftje (SH) (3, and Zeeman effects (8) have served in this regard. The SH and Zeeman background correction techniques are superior to the deuterium source method because of molecular absorption profiles/lines close to the atomic absorption line are corrected for. The Zeeman approach is favored by most GFAAS instrument manufacturers. The Zeeman effect involves splitting the source radiation or the absorption profile, into three (or more) wavelengths (a resonance wavelength and shorter and longer wavelengths). Background correction is achieved by comparison of absorption intensities for both the unsplit and split situations. Since HCLs are somewhat unstable upon the application of a magnetic field, most instruments apply the Zeeman effect on the analyte in the graphite furnace. Recently, low power continuous wave (CW) diode lasers have been used as line sources in AAS (9). These lasers are inexpensive, compact, and tunable. The use of coherent laser radiation has minimized the need of focusing optics, and the single-mode output of a known wavelength may eliminate the need of a monochromator for isolation of the desired radiation. We have previously reported the use of a single-mode laser diode (10) and a multiple-mode laser diode (MMLD) (11) as line sources in flame AAS. With the multiple mode laser, a single MMLD supplies several nearby wavelengths with spectral bandwidths narrower than that of atomic absorption profile. One of these wavelengths can be tuned to an analyte line position, and an adjacent line can be used for peak ratio measurement, thus improving signal precision and providing a simultaneous background correction capability. These characteristics resemble some of those of Zeeman splitting and appear ideal for the GFAAS application. This paper reports the use of the MMLD as the line source for GFAAS. Lithium is the analyte used in this demonstration. EXPERIMENTAL SECTION Instrumentation. The instrumentation and setup are shown in Figure 1; the component suppliers are listed in Table I. The optical components inside the dashed line of Figure 1 are actually unnecessary for this system, but due to the arrangement of other
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Flgure 2. Diode array spectrometric display of the multimode diode laser emission lines around the 670-nm region. A is the lihium 670.780-nm resonance line; B is the reference llne used for peak ratio measurement.
instruments and space consideration in this laboratory, they were included. The laser beam was collimated and shaped into a round beam by a lens system. The flame was used to supply lithium emission and to optimize the laser diode wavelength for atomic absorption signals. Procedure. The diode-array spectrometer was operated at second order of the 670.780-nm lithium line. The analyte (Li) emission from the flame was used to set the spectrometer to the resonance line position, with the following procedure (the laser and furnace were off): (1) a 1000 pg/mL (ppm) Li solution (Standard, Inorganic Ventures, Inc., Brick, NJ) was aspirated into the air-acetylene flame; (2) the scattered Li emission was monitored with a slit width of 100 pm; and (3) the lithium emission was observed (appeared as a peak) on the monitor and its position was set by using the cursor. The laser radiation was then turned on ad monitored with a slit width of 40 pm. The laser diode was tuned so that one of the output wavelengths was centered at the curser (the Li resonance/absorption line position). The resonance line position was optimized by maximizing the absorption with the 5 and 10 ppm solutions aspirating into the flame. The laser diode was operated at 17.1 "C and at 109.07-mA current. These laser operation conditions were maintained throughout the experiment. The flame was then turned off and the graphite furnace was used for signal measurements. The furnace operating condition was as follows: drying with a 30-s ramp to 90 "C; drying with a 5-s ramp to 120 "C; ashing with a 5-s ramp to 500 "C; ashing with a 5-27, ramp to 700 "C; atomizing with a 5-s step to 2200 "C; and cleaning with a 5-5 step to 2800 "C. Ten microliter volume samples were used. The signals were acquired for 15 s with the computer data-acquisition system, during the second ashingatomization-cleaning stages; 15 spectra were obtained, and each spectrum represented the average of 1-s signals. The spectrum that gave the highest absorption was used for the absorbance measurement. The analyte emission was obtained by blocking
Anal. Chem. 1990, 62, 1895-1899
Table 11. Comparison of Sensitivities and Detection Limits for Lithium
sensitivity (1% abs), pg
system0 MMLD-GFAA HCL-GFAA HCL-GFAA
detection limit, pg
(ng/mL)*
(ng/rnL)*
ref
4 (0.4) 10 (1)
6 (0.6) 5 (0.5) 1 (0.1)
this system 12 13
4 (0.4)
O Key GFAA, graphite furnace atomic absorption; HCL, hollow cathode lamp; MMLD, multiple mode laser diode. *Based on 10LLLsamples and 2a of blank.
1.20
,
I I
/
1895
source. Since the background and reference lines are from the same source and are close to the analyte line, accurate correction and measurement are possible. With a solution equal to or greater than 0.2 pg/mL Li, the entire resonance line was absorbed, indicating the laser line width is narrower than the width of the absorption profile. I t has been shown that peak ratio measurement gave superior signal precision with the MMLD-flame AA system (IO);this method of measurement was used in this system. Table I1 shows the detection limit and the sensitivity obtained with this system, for lithium, and the limit and sensitivity are compared with those of conventional HCL-GFAA systems. Similar values are seen (Table 11). Figure 3 shows the calibration curve. The linear dynamic range is approximately 2 orders of magnitude. If several laser diodes are used, simultaneous multielement AA measurement is feasible; this implementation is much simpler using coherent laser radiation than using incoherent hollow cathode lamp radiation. The limitation of the present system is that laser diodes are currently only available with wavelengths suitable for a few elements. Shorter wavelength laser diodes, however, will soon be available. LITERATURE CITED
/
/
Walsh, A. Spectrochim. Acta 1955, 7, 108. Sullivan, J. V. Prog. Anal. At. Spectrosc. 1981, 4 , 311. Jones, B. T.; Smith, B. W.; Winefordner, J. D. Anal. Chem. 1989, 67, 1670.
00
20.0
40 0
60
0
80 0
L'vov, B. V. Spectrochim. Acta 1961, 77, 761. Slavin, W.; Manning, D. C. Prog. Anal. At. Spectrosc. 1982, 5 , 243. Kahn, H. L. At. Absorpt. Newsl. 1968, 7, 40. Smith, S. 8.;Hieftje, G. M. Appl. Spectrosc. 1983, 37, 419. De Loos-Vollebregt, M. T. C.; de Galan, L. Prog. Anal. At. Spectrosc.
100 0
LI c o n c e n t r a t i o n (ng/rnl)
1985, 8 , 47.
Hergenrcder, R.; Niemax, K. Spectrochim. Acta 1988, 438, 1443. Ng, K. C.; Ali, A. H.; Barber, T. E.; Winefordner, J. D. Unpubllshed work. Ng, K. C.; Ali, A. H.; Barber, T. E.; Winefordner, J. D. Appl. Spectrosc., in press. Ingle, J. D.; Crouch, S. R. Spectrochemical Analysis; Prentice-Hall, Inc.: Englewocd Cliffs, NJ, 1988; p 300. Analytical Values in Flame AA , Furnace AA and ICP; Thermcd Jarrell Ash, 1987.
Flgure 3. Lithium (10 pL sample volume) analytical calibration curve
for the diode laser graphite furnace atomic absorption system. the laser radiation and subtracted to measure the net absorption. RESULTS AND DISCUSSION Figure 2 shows the spectral lines of the MMLD radiation, with one of the lines at the resonance wavelength of lithium (670.780nm). The lines were positioned by tuning the laser diode temperature and/or the current. The intensity of each line was wavelength dependent; the most intense line (mode) could not be tuned to the desired wavelength position, nor could the lines (modes) be tuned to obtain equivalent intensity. However, the spacing (-0.3 nm) between lines did not appear to change with tuning. The lines adjacent to the lithium line can be realized for simultaneous background correction and for peak ratio measurement. The real-time simultaneous background correction is beneficial since the GF background and/or atomic absorptions may change with time, and the simultaneous peak ratio measurement can improve signal precision by compensating for the fluctuation in the radiation
* Author to whom correspondence should be sent. On leave from Department of Chemistry, California State University at Fresno, Fresno, CA 93740-0070.
'
Kin C. Ng' Abdalla H.Ali Tye E. B a r b e r J a m e s D. Winefordner* Department of Chemistry University of Florida Gainesville. Florida 32611
RECEIVED for review March 5,1990. Accepted May 18,1990. This research was supported by NIH-5-R01-GM38434-03.
Surface Acoustic Wave Detector for Screening Molecular Recognition by Gas Chromatography Sir: The generation of interfacial structures capable of molecular recognition at the surfaces of chemical and biological sensors is a crucial element in the fabrication of these devices. For biosensors, the approach to achieving the desired selectivity has been almost exclusively through the use of particular species such as antibodies, enzymes, and molecular receptors extracted from biological milieu (1). For chemical sensors designed to detect organic molecular species there has to date been a heavy reliance placed on both conventional adsorption 0003-2700/90/0362-1895$02.50/0
at solid interfaces and partitioning of the analyte into stationary-phase-like films (2). We are interested in the study of selective molecular recognition at interfaces through specific functional group interactions between gas-phase analyte molecules and chemically modified surfaces. The aim of this approach is to achieve the rational design of chemical sensing devices through a better understanding of the molecular recognitive process combined with computational methods for structure comparison, host-guest docking, and molecular 0 1990 American Chemical Society
