Anal. Chem. 1990, 62, 1345-1349
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Application of Reversal Electron Attachment for Ultrasensitive Detection of Thermal Electron-Attaching Molecules: CCI, and C6H5N02 Mark T. Bernius' and Ara Chutjian* J e t Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109
A standard gas dllutlon method was used to determine the selective response of the reversal electron attachment detector (READ) to carbon tetrachlorlde concentrations In nitrogen. Data are provlded that determine the lowest concentratlon of sample detectable wlth the present lnsirunental conflguratlon as being below 1.0 pptrv (part per trillion by volume). The Incorporation of a 90' electrostatic deflector with the quadrupole mass spectrometer Is shown to be vital, and with It, negatlve-Ion quadrupole mass spectrometry Is used to characteke the lonlratlon process. Observatbns are also made of the reversal electron attachment response to nltrobenrene. The analytical potential of reversal electron capture negative-Ion mass spectrometry Is examined, and areas for future development are discussed.
New techniques for ultrasensitive analysis of vapors by molecular electron capture have burgeoned over recent years through advances in the field of electron-molecule and ionmolecule scattering (1). Detection levels at or below partsper-trillion by volume (pptrv) are now possible in the laboratory either through direct detection of positive or negative analyte ions or through secondary detection processes involving chromatographic separation and electron attenuation measurements. As detection levels approach (1part per 1014 by volume), such ultrasensitive methods become increasingly attractive in biological investigations of aromatic organics, atmospheric and environmental monitoring using taggants, and public safety where vapor detection of toxic compounds, explosives materials, and narcotics is important. Among the most intriguing methods of analysis are those based on dissociative and nondissociative electron capture, followed by mass detection of the negative ion species. Although the positive-ion spectrum produced by energetic electron dissociation-ionization is relatively easy to obtain, the resultant mass spectrum is replete with fragment ions, making analyte identification in multicomponent samples difficult. In contrast, the negative ion spectrum obtained with low-energy electrons can have fewer dissociation channels available. In the case of nondissociative electron capture with the analyte molecule, species identification is readily accomplished by detection of the parent ion mass peak. Coupled with a deeper experimental-theoretical understanding of the basic attachment phenomenon, negative ion mass spectrometry is rapidly becoming a mature technique for solving structural and analytical problems. Modern instrumental methods for negative-ion production and analysis include electron capture detection (ECD) (2), atmospheric pressure ionization (API) (3),and negative-ion chemical ionization (NICI) mass spectrometry ( 4 ) which em-
ploys a buffer gas to collisionally stabilize the negative ion with respect to autoionization. API operates at atmospheric pressure, while the ECD and NICI techniques operate typically between atmospheric pressure and Torr. The electron capture detector is commonly used with vapor analysis by gas chromatography. Its popularity results primarily from the fact that thermal electron attachment cross sections of several classes of molecules are quite high. These classes include chlorohalocarbon compounds, perfluorinated compounds, "superacids", metal carbonyls, and explosives molecules. Low-pressure (single-collision),low-energy negative-ion mass spectrometry is not an area of intense exploration, very likely because of experimental difficulties involving the transport and focusing of reasonable densities of ultralow-energy electrons (electron energy E I eV) and difficulties in sample introduction from atmospheric pressure to high vacuum. In this paper, we explore a new type of electron capture detector which retains the objectives of the standard ECD, in part by continuing the trend toward more "gentle" means of ionization for molecules that can easily fragment. Specifically,extensive examination of the physical processes involved in molecular-capture reactions has demonstrated a large capture probability for ultralow-energy electrons by the several classes of molecules through the s-wave capture phenomenon at threshold (56).Such large electron-capture cross sections make the area of ultralow-energy electron capture quite attractive to the analyst. However, operating pressures in the collision region must be kept low to permit operation of the electron emitter and ion detector and to ensure single-collision events. In an effort to exploit this large ultralow-energy electron capture cross section, we have developed a variant on the ECD which operates under high vacuum Torr) for single-collision capture events. The technique of attachment by electron reversal utilizes the large s-wave attachment cross section by concentrating the electron density in a small energy band near E = 0 where attachment is most efficient. Unlike the ECD, API, and NICI techniques, negative-ion generation by reversal electron attachment is also able to access resonances at E > 0, beyond the range of thermalized electron energies. This is accomplished by shifting the location of the lowest energy electrons with respect to the target beam (see below). We refer to the present technique as vapor analysis by reversal electron attachment and give below details of its initial analytical characterization.
THEORY In the bombardment of many classes of molecules with eV) electrons, two principal modes ultralow-energy (E 5 of electron capture are possible e-
+ AB
e-
+ AB
and
* Author to whom corres ondence should be directed.
Present address: CaltecR Division of Geological and Planetary Sciences, Pasadena, CA 91125. 0003-2700/90/0362-1345$02.50/0
-
A
+ B-
AB-
(dissociative)
(la)
(nondissociative)
(lb)
If a dissociative channel is energetically open, as in the case 0 1990 American Chemical Society
1346
ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
-
for CCl,, then a characteristic fragmented anion will be detected, viz. e- ( E = 0) + CCl, CC13 + C1-. If the parent negative ion formed is stable with respect to dissociation and autoionization (lifetime T 2 1 ps), then the parent anion will be detected, viz. e- ( E == 0) + SF6 SFs-. This type of parent-formation reaction is of interest as it enables unambiguous determination of the analyte species. The total negative ion yield, or net signal S, is a product of several parameters: the analyte concentration C (normalized to 1 for loo%), molecular target density N in the electron-molecule interaction region, the volume V in which the interaction takes place, the electron density n available in this volume, the electron velocity u,, the energy-dependent electron attachment cross section UA (E),and the overall extraction-transmission-detection efficiency k . Specifically
-
S(HZ) = ~ C N V ~ U(~E U ) A
, , , , , ,, , , , , REVERSAL ELECTRON ATTACHMENT DETECTOR
ia)
+---
'
ELECTRON FOCUSING
V5
VI
VZ
~
V3
ELEC T R 0N SOURCE
c1
V ,6
ION EXTRACTION
V8
V7
V4
REVERSAL REGON AND TARGET INLET
. 4
ibl
(2)
Increasing any of these parameters raises the available signal, but constraints on n and V due to charged-particle space charge and instrument size limit what is practically achievable. The value of uA ( E )at E = 0.01 eV for CC14is (5)4.68 x cm2,while that for nitrobenzene is estimated at about 1 X cm2. The cross section describing the thermal electron capture can be extremely large. Experiment (5) and theory (6)show that the cross section is dominated by electron's s-wave component (zero angular momentum) as the electron's energy approaches zero. This process in molecules is the direct analogue of the s-wave phenomenon in nuclear physics governing thermal neutron capture by light nuclei (7).Both are described by a cross section varying as UA ( E )= where E is the electron or neutron energy. The physical phenomenon in molecules can be described as being due to a pile-up of the electron's wave function a t the molecular boundary, in the absence of any angular momentum centrifugal barrier [1(1+ 1)h2]. The large resultant component of the squared modulus of the electron's wave function in the vicinity of the molecule increases the likelihood of capture. The instrumental method described herein incorporates a means of delivering a high electron density (large n ) a t the lowest experimentally achievable energy [ E 0, hence large UA ( E ) ] to a molecular sample. By utilizing the divergent electron capture cross section [which typically manifests itself at E 5 eV (5)], one will be able to detect higher signals, or lower concentrations of molecules that follow the s-wave threshold law.
-
EXPERIMENTAL SECTION Instrumentation. The prototype reversal electron attachment detector (hereafter referred to as READ) used in this study has been described in detail elsewhere (8). A schematic diagram is shown in Figure 1. Briefly, the READ consists of an electron gun column, an electrostatic mirror, and ion-extraction optics. The electron gun column consists of an electron source coupled to a projector lens that collimates and focuses a paraxial, nearmonochromatic beam of electrons into an electrostatic mirror. The mirror was designed to decelerate a monochromatic beam to zero longitudinal and radial velocity at the reversal plane. The position of this reversal plane is electrostatically movable to the right and left of the midpoint of lens elements V5 and V6. Movement to the right results in higher energy electrons traversing the target beam ( E > 0). In fact, these electrons traverse the target twice: once during deceleration and once during acceleration (8). Lens elements V5-V7 in the reversal region are pulsed with a near 50% duty cycle (square-wave modulation), so an ensemble of electron reversals in the vicinity of the reversal region will exist in half of the cycle. The ensemble of planes is due to the chromaticity of the electron beam in the vicinity of the reversal region over a small length of the optical axis centered about the calibrated collision center at R (see Figure 1). The second half of the chopping cycle is for ion extraction, with the primary electron
==.a
VOLTAGE
QMS
Figure 1. (a) Schematic diagram of the READ device, showing the electron gun, electron reversal, and ion extraction lens systems. The indirectly heated cathode is denoted by F, the reversal region and collision center by R, the exit window by W, and the deflectors by D. (Reprinted with permission from ref 8. Copyright 1989, American Institute of Physics.) (b) Complete experimental arrangement, showing the READ coupled to a 90' spherical-plate deflector (ESA) and quadrupole mass spectrometer (QMS). Note the offset channel electron multiplier (CEM) housed in the quadrupole. The READ shows schematicaly the lens elements which cycle (by switches S) and those that are statically run.
beam being deflected off. An adjustable delay is introduced between cycle halves to allow for dispersal of residual electrons from the ion-extractionregion prior to extraction of the negative ions. The electrons will disperse in the first 50 ns due to their higher velocity in the presence of applied fields. The heavier, more sluggish ions will move away from the collision volume consistent with the selected time delay and then be extracted. After extraction, product ions are focused into a spherical 90° electrostatic analyzer (ESA) with a 30° acceptance window. No provisions are made for limiting the transmitted energy band-pass (with slits) beyond the variable plate voltage selection. The beam transmitted by the ESA is focused into an Extrel quadrupole mass spectrometer capable of unit mass resolution to 500 amu. The commercially supplied quadrupole detection system was mechanically altered to eliminate stray, energetic neutral- and secondary-electron detection. The channel electron multiplier was brought out of the line-of-sightpath through the quadrupole, and its entrance direction made perpendicular to the optical axis. The negative-ion repeller plate was removed. It was found during the course of this study that this plate was partly responsible for the generation of secondary electrons by transmitted energetic neutral or positive charges. These secondaries were subsequently detected as mass-selected negative ions. Additionally,we found that the quadrupole, tuned for negative ions with an appropriately biased channel electron multiplier, could readily detect positive ions and register them in the negative ion spectrum. To see how this superposition can occur, we refer to Figure 2, which shows an electrodepotentials diagram for selected READ lens elements in both the electron-on (ion-off')and ion-on (electron-off) cycles. Positive ions can be formed by the higher-energy electrons available to the left of the reversal region as
ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990
+
ELECTRON ON CYCLE
\
-1
u
F z w
I
4
h
v-
k
2
V5
V6
0
V7
I
r-
*z
V8ANDQUAD
-I
+I
ION ON CYCLE
z w c
2
I
-I
V6
V7
Table I. Operating Voltages for the READ
READ element
0
V8ANDQUAD
v5
Flgure 2. Potential energy diagrams for the READ'S cycled lens elements, V5-V7, coupled to the last lens element V8 and the quadrupole (which is floated at the same potential as V8). The solid line depicts the relative voltages at which the elements are held, while the dashed line represents the potentlal in free space found Inside the READ bore (obtained by potentlal relaxation). The READ was designed to extract negative ions during the ionon cycle (lower), but note the unfortunate extraction of positive ions dlving the electronon (ionoff)cycle (upper). The positive ions are formed In a region of higher electron-interactlon energy and easily enter the quadrupole mass spectrometer without the ESA in place.
the electrons are decelerated, then accelerated on their return flight. The positive ions are extracted and transmitted by the quadrupole. As they leave the vicinity of the quadrupole, they will be deflected away from the channel electron multiplier by the few kilovolts on the multiplier and will strike some ubiquitous metallic structure, such as an exit aperture. Secondary electrons from the metal will then be accelerated toward the multiplier and be detected as negative ions at the positive ion mass number! We have performed extensive work in the laboratory to confirm this phenomenon. The transmitted positive-ion spectrum will be superimposed upon, and presented as, the product negativeion spectrum for the reaction under study. The inability of the quadrupole bias to distinguish ion charge state indicates that an additional charge selector is needed. Two ways to distinguish between the positive and negative ions are by (a) noting whether the extracted ions are in or out of phase with the appropriate extraction cycle and (b) incorporating a charge-state selector before the entrance to the quadrupole mass analyzer. We elected to incorporate the latter method in the form of a 90° ESA with a variable energy band-pass. This is shown to be vital to our laboratory setup, especially when it is desired to study the dissociative fragment ion pattern of heretofore uncharacterized molecules. The READ, ESA and quadrupole mass analyzer are housed in a stainless-steel UHV chamber lined with 1.5 mm-thick mumetal to shield external magnetic fields to less than 10 mG over the experimental region. The chamber is pumped by a 1500 L/s (air) Edwards oil-diffusion pump with a liquid-nitrogen trap. When the READ was operated as a residual gas analyzer, no backstreaming of pump oil was detected in over 1 year of operation. Operating pressures with gas load are between 1.0 X and 5.0 X Torr, with a base pressure of 5.0 X lo4 Torr, as measured in the UHV chamber close to the READ apparatus. The ion signal detected by the channel multiplier is amplified in a lab-built preamplifier which produces 5-V TTL pulses corresponding to counts above a preset 50-mV signal threshold. Signal intensity (in hertz) of each mass peak is read on an Ortec counter operating as a single-channel scaler. Straightforward
1347
v1 v2 v3 v4 v5 v5 V6 V6 v7 v7 V8
(t)" (I)* (7) (1)
(t, (1)
voltage, V 0 500 30 450-550 40 -200 -10 0
-10 400-800 60
First half cycle. * Second half cycle. Note: 90' ESA mean energy = potential on V8. modification can be made for multichannel scaling to allow for detection of weaker negative-ion signals. Reagents. Samples of high-purity gases and liquids were obtained from the Matheson Gas Co. and Aldrich Chemical Co. No further purification of the gases was carried out. The liquids involved were housed in special Pyrex ampules and were subjected to eight freeze-thaw cycles to remove trapped air. Procedure. The experimental runs reported herein were conducted with a lens element chopping frequency of 8.3 kHz (60-ps pulse widths in the square wave driver) to accommodate the time-of-flight of the heavier nitrobenzene (mass = 123). Operating voltages for the READ are presented in Table I. The electron current was 4 x lo-"A, and the pressure at the gas inlet Torr. to the reversal region was estimated to be about 2 X With the method of standard dilutions (9), mixtures were prepared in an all-stainless-steel vacuum system heated to 350 K (IO). Reagent grade CC14and 99.99% pure N2 were mixed in a concentration ratio C (volume-to-volume) = 1 (pure CC4) to 5 x lo-". Concentrations below lo-'* were not used for the quantitative study. Present engineering limitationsin our sample preparation vacuum system compounded the error in sample preparation below this value. Between measurements, the main vacuum chamber was flushed with dry nitrogen at 5 X lo-' Torr and baked at 350 K for approximately 90 min to remove any residual CC14from the inlet lines and chamber walls. Care was taken to ensure that the gas mixture was stable; i.e., that there was no depletion of the CC14 through wall adsorption or chemical reaction. This stability was checked at C = 10" by monitoring the C1- signal over a period of 2 h. The signal was constant at the 5% level. In addition, "blank" runs were performed with N2 No background ions were detected in the mass ranges 30-40 or 120-130 amu.
RESULTS AND DISCUSSION Using the ESA a t the entrance to the quadrupole mass analyzer allowed the unambiguous selection of ion charge state. It also limited the energy band-pass of the transmitted ions and, hence, improved mass resolution (full width at half maximum, fwhm). Similar improvements have been noted in the past (II), particularly in the field of secondary ion mass spectrometry where the transmitted ions to the quadrupole have a broad energy distribution. With the ESA in place and the experimental apparatus properly tuned, sensitivity studies for the detection of CC14 in N2 at various concentrations C were performed by using the method of standard dilutions. The results at low C are shown in Figure 3. The errors represent the quadrature sum of the statistical counting error, and the error in reading the two pressure gauges used to make up the concentration C. There is a uniform decrease in signal over the range examined down to S = 900 Hz a t C = 5.0 X lo-". Quantitative measurement was stopped at this lower level of concentration because of the decreasing precision in preparing a fraction C using our sample measuring and delivery system. The linearity of the READ, defined as the slope of the detector response curve on a log-log scale, is determined to
1348
ANALYTICAL CHEMISTRY, VOL. 62, NO. 13, JULY 1, 1990 NITROBENZENE I
I
I
I
I
I
[C,H,
NO,)
I
I
I
I
1021 I 10.1 1
1 1 1 1
1
10-10
1 1 1 1
10-9
1
I
10-8
1 / 1 1
10-7
1
1 1 1 1
I
10-6
I ' I I
1
1
10-5
CC14 FRACTIONAL CONCENTRATION
Figure 3. READ sensltivity evaluation using the method of standard dilutions for various concentrations C of CCI, in N1. The soid line represents a least-squares fit to the data.
be 0.08 from the sensitivity curve shown (correlation coefficient = 0.994). We attribute the sensitivity curve's deviation from a slope of 1.00 to several factors, all of which stem from the presence of partially neutralized space-charge effects unaccounted for in the READ design. We note that the size of the interaction volume V (eq 2) was calculated in the absence of positive ions. These ions, shown experimentally to be present, serve to offset the space-charge spreading of the electron beam by partially neutralizing the charge in the interaction volume. This volume is in turn related to the product of the cross-sectional area of the reversal region and the linear distance over which the ensemble of reversal planes exists. Computer trajectory simulations of the electron reversal region reveal that positive ions can alter its cross-sectional area by up to 2 orders of magnitude. The exact change depends on the localized positive-ion density, which in turn is a function of the analyte concentration C. Further, negative-ion space-charge effects in the ion-extraction optics, which were neglected in the present design, could also affect the extraction and transmission efficiency of the product anions for large values of C. We therefore conclude that the sensitivity curve should enter a region of slope 1.00 at sufficiently low values of C where space-charge-related forces no longer dominate and that this region must lie below C = The average peak-to-peak value of the background "noise" is observed to be 20 Hz. Therefore the minimum detectable quantity, defined as that concentration which gives a detector response equal to twice the noise level, would give a peakminus-background count rate of 40 Hz.We cannot reasonably extrapolate the sensitivity curve to this value. As quantitative measurement was stopped at C = 5 X lo-", the inference of a lower detectable limit assumes a continued linearity down to that level. Qualitative studies were performed below C = lo-'* that were not included in the sensitivity curve. These lead us to state that the lower limit of detectability is conservatively below 1.0 pptrv, already 5 orders of magnitude better than that previously reported (8). The improvements were realized through increased duty cycle in the electron and ion chopping and in the use of higher electron currents. The primary motivation for developing this technique has been the measurement of explosives-class molecules, without the need for sample preconcentration or derivitization, and using more gentle means of ionization to enable the identification of the parent analyte molecule. To this end, the READ has been applied to the detection of nitrobenzene, an explosives simulant. These results are shown in Figure 4. Here, the parent molecular anion C6H&Oz- (maas 123) and the fragment anion NO2- (mass 46) characteristic of this class of molecules are clearly present. It should be noted that
40
1
1
1
50
60
70
I
I
I
80 90 100 MASS ( a m )
I
I
110
120
I
1. i
130
Figure 4. Negative-ion spectrum obtained by reversal electron attachment to nitrobenzene, an explosives simuiant. Note the exlstence of the parent ion peak, C,H,NOc at mass 123 and the fragment NO; at mass 46. Observation of NO2- is one characteristic of explosives-molecule detection.
without the ESA interface between the READ and the quadrupole mass analyzer, a pronounced CsH5 peak dominated the negative ion spectrum. The ESA, acting as a charge-state selector, revealed this peak to be due to the positive ion CsH5+that had not been adequately filtered by the quadrupole operating in the negatiue-ion mode. Such superposition would clearly lead to erroneous conclusions from the negative-ion spectrum for this and other molecules were the charge-state selector not incorporated in the negative-ion quadrupole mass spectrometry.
CONCLUSIONS The technique of low-pressure (single-collision) molecular-electron attachment by electron beam reversal is a new facet to conventional electron capture techniques familiar to chromotographers. It offers (a) higher electron densities in the region of maximum capture cross section over other electron capture techniques because of its space-charge compensating optics, (b) an ability to access attachment resonances at electron energies E > 0 beyond the range of thermalized energies, and (c) higher electron delivery rates to the active volume offering an improved linear dynamic response range to the sample. As such, the READ as a new analytical tool provides information and sensitivity for reactions involving smaller attachment cross sections, for E I 0, than could be previously measured. Several avenues of improvement are possible with the present experimental embodiment, but we have elected to first concentrate on the sample delivery system. Quantitative experimental data are unavailable to determine the detection limit for CCl, in Nz below C = due to concentration errors using the vacuum gauges available on our present system. Also, several classes of large molecules are found to be quite "sticky", and even with a carrier gas are inadequately transported into the READ with our delivery system. For the complete development of a viable analytical tool, and its continuing improvement, the area of sample handling and delivery has to be addressed. This represents the next experimental thrust in our laboratory. ACKNOWLEDGMENT We thank T. Tombrello (Caltech), J. Hobbs (U.S.Department of Transportation) and 0. Orient (JPL)for helpful discussions. Registry No. CC4,56-23-5; C6H5NO2,98-95-3; Nz, 7727-37-9.
Anal. Chem. 1990, 62, 1349-1352
LITERATURE CITED (1) W .Am. Phys. Soc. 1988, 33, 045. (2) LovokXk, J. E. A M . At. CbIklon Phys. 1982, 5 , 1-30. ( 3 ) -1, M. W.; W e o w n , M. C. J . ~ ~ t o g1976, v . 722, 397-413. Farwen. S. 0.; Rasmussen, R. A. J . Chrometugr. Scl. 1976, 74, 224-234. (4) SzUbko, J. E.; Howe, I.; Beynon, J. H.;Schiunegger, U. P. &g. Mass spscbwn.1980, 15, 263-267. Deugherty. R. C. Anal. chem.1981, 5 3 , 625A-836A. Laramee, J. A.; Arbogast, B. C.; Delnzer, M. L. Anal. C h m . 1986, 56, 2907-2912. (5) Chutjlan, A.; Alajajian. S. H. Phys. Rev. A : Gen. Phys. 1985, 37, 2885-2892. Orient, 0. J.; Chutjlan, A.; Crompton, R. W.; Cheung, B. mys. Rev. A : Gen.Phys. 1989, 39, 4494-4501. (6) Qauyacq, J. P.; Heclenbarg, A. J . Phy~.8 : At. Md. Phys. 1984, 17, 1155-1171. Domcke, W. J . Phys. 8 : At. Mol. Phys. 1981, 1 4 , 4889-4922. (7) Wlgner, E. P. Phys. Rev. 1978, 73. 1002-1009. (8) Bernius, M. T.; Chutjlan, A. J . Appl. Phys. 1989, 66,2783-2788.
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Orlent. 0. J.; Chutjian, A.; Alajajlan S. H. Rev. Sci. Instrum. 1985, 5 6 , 69-72. (9) Willard, Hobart H.;Merrltt, Lynn L., Jr.; Dean, John A.; Settle, Frank A.. Jr. Instrumental Methods of Analysis, 6th ed.; Van Nostrand: New York, 1981; Chapter 29. ( I O ) Orient, 0. J.; Chutjian, A.; Leung, K. N. J . Appl. Phys. 1987, 62, 397-40 1. (11) Shubert, R.; Tracy J. C. Rev. Sci. Instrum. 1973, 44, 487-491. Shubert. R.; Tracy, J. C. Rev. Sci. Instrum. 1974, 45, 457-458.
RECEIVEDfor review March 5,1990. Accepted March 5,1990. This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, and was supported by the Department of Transportation through agreement with the National Aeronautics and Space Administration.
CORRESPONDENCE Direct Characterization of Phthalic Acid Isomers in Mixtures Using Surface-Enhanced Raman Scattering Sir: Raman spectroscopy is an important tool for analytical chemists because of its specificity for chemical group identification, the fact #at it avoids time-consuming tasks of wet chemistry, and its complimentary aspect to infrared spectroscopy. A major limitation of conventional Raman spectroscopy, however, is its low sensitivity due to the small Raman scattering cross section. Thus for many years, most analysts have refrained from using Raman spectroscopy for routine chemical analysis. However, the discovery by Fleischmann e t al. (1)and Van Duyne et al. (2) of a strongly enhanced Raman scattering from pyridine molecules adsorbed on roughened silver electrodes has generated renewed interests in the field of Raman spectroscopy over the past decade. Many efforts have been devoted to determining the sources of enhancement for the surface-enhanced Raman scattering (SERS) effect, and two models have been proposed as a result of these studies. In the chemical model, signal enhancement arises from adsorption-induced modification of the molecular polarizability of the analyte. In this model, chemisorption and charge transfer are clearly important features ( 3 , 4 ) . The second model attributes the SERS effect to an increased local electric field a t the metal surface due to electromagnetic field concentration at sharp protrusions (“lightning rod” effect) and by excitation of the collective oscillation of the conduction electrons (surface plasmons) of the metal substrate (5,6). In practice all of these effects are significant. Recently, the analytical usefulness of SERS as a new spectrochemical tool has been actively investigated by several research groups. The general applicability of SERS as an analytical technique has been reported for a variety of compounds on various SERS-active media and substrates, such as silver electrodes (7, 81,silver sols (9, IO), quartz posts (11-13),silver island films (14), and silver-coated microparticles (15-19). In addition, a SERS postcolumn detection system for high-performance liquid chromatography (20)and a system incorporating the SERS technique to flow injection analysis (21)have been developed. Because vibrational spectra provide abundant information on the structure of molecules, SERS would be an excellent tool for the characterization and identification of structurally similar compounds. In spite of these features, the SERS 0003-2700/90/0362-1349$02.50/0
technique has not been widely used for mixture analysis. In this work, the SERS technique that we recently developed for liquid solutions (19) is evaluated for the characterization of binary and ternary synthetic mixtures of isomers of phthalic acid. Without prior separation of the three isomers into individual components, it would be very difficult to identify and quantify these isomers in a mixture by using other spectroscopic techniques such as luminescence or UV-vis absorption techniques, since the isomers essentially give similar absorption and fluorescence spectra.
EXPERIMENTAL SECTION Instrumentation. SERS measurements were conducted with a SPEX Model 1403 double-grating spectrometer (SPEX Industries) equipped with a water-cooled gallium arsenide photomultiplier tube (RCA, Model C31034), operated in the singlephoton counting mode. Data storage and processing were handled with a SPEX Datamate computer. The monochromator bandpass was 2 cm-l. The 647.1-nm line of a krypton ion laser (Innova 70, Coherent) was used for excitation, and the laser power was set at 80 mW for all measurements. A right-angle geometry of the laser excitation source and the scattered radiation was employed. Chemicals. The agglomerate-free alumina (0.1-pm nominal particle diameter) used to prepare the SERS substrates was provided by Baikowski International Corp. and used as received. Terephthalic acid (Pfaltz and Bauer), phthalic acid (Aldrich), and isophthalic acid (Pfaltz and Bauer) were purchased at their highest purity and used without further purification. The alumina was suspended in high-performance liquid chromatographic grade water (Burdick and Jackson), and sample solutions were prepared from spectroscopic-grade ethanol (Warner-Graham Co.). Procedure. SERS subqrates were prepared in the following manner. Three drops of a 5% aqueous suspension of the alumina were deposited and then spread evenly on the surface of a precleaned rectangular glass strip (2.5 cm X 1.25 cm; 1 mm thick) cut from a microscope slide. The alumina-covered glass slide was then placed on a spinning device and spun to spread the alumina uniformly on the glass support. Next, a 75-nm layer of silver was thermally evaporated onto the alumina-coated glass strip at a pressure of 2 X 10“ Torr. Approximately 1 mL of the sample solution was pipetted into a standard fluorescence cuvette. Then, the silver-coated alumina SERS substrate was inserted directly into the cuvette via forceps so that the long edges of the glass strip were supported by diagonal 0 I990 American Chemical Society
