Anal. Chem. 1987, 59, 2924-2927
2924
assures a precision that is often adequate for routine analyses. The present method can be applied to many systems. In a high molar absorptivity and highly selective system such as the one using chromium(V1) and 1,5-diphenylcarbazide, considerable trace levels of the target element can be determined in natural water (11). These systems will be discussed in detail in later papers (11, 12). Registry No. Cu, 7440-50-8. -
LITERATURE CITED Yoshimura, K.; Waki, H.; Ohashi, S. Talanta 1976, 2 3 , 449-454. Yoshimura, K.;Toshlmitsu, Y.; Ohashi, Ta&n& 1980, 2 7 , 693-697. Yoshimura. K.; Nlgo, S.; Tarutani, T. Talanta 1082, 29, 173-176. Waki, H.; Korklsch, J. Talanta 1983, 30. 95-100. Ishii, H. Fresenlus' Z . Anal. Chem. 1984, 319, 23-28. Capkan, F.; Valencia, M. C.; Capitan-Vailvey, L. F. Mkrochim. Acta 1984, III, 303-311.
s.
(7) Shrkdah, M. M. A.; Ohzeki, K. Analyst (London) 1985, 7 70, 677-879. (8) Yoshlmura, K.: Ishii, M.; Tarutani, T. Anal. Chem. 1986, 58, 591-594. (9) Yoshimura, K.; Waki. H. Telenta 1985. 32, 345-352. (10) Yoshimura, K.;Waki, H. Taknta 1987, 3 4 , 239-242. (11) Yoshimura, K. Proceedings of International Symposium on New Sensors and Methods for Environmental Characterization, S1-02, Kyoto, 1986; submitted for publication in Analyst (London). (12) Yoshlmura, K. Bunseki Kagaku, in press.
Kazuhisa Yoshimura Chemistry Laboratory College of General Education Kyushu University Ropponmatsu, Chuo-ku, Fukuoka, 810 Japan RECEIVED for review
November
39
1986. Accepted
139
1987.
Characterization of Multilayer Thin Films by Laser- Induced Thermal Desorption Mass Spectrometry Sir: Identifying the molecular species adsorbed on a surface is a difficult analytical problem that is important in studies of heterogeneous catalysis, corrosion, and surface contamination. Most surface analytical methods reveal very little about the molecular identity of an adsorbate. For example, low-energy electron diffraction (LEED) is most sensitive to the structure and ordering of a surface, and Auger electron spectroscopy (AES) only reveals the elemental composition. X-ray photoelectron spectroscopy (XPS) can yield both the elemental composition and the oxidation state of species present on the surface. Surface spectroscopic techniques such as electron energy loss spectroscopy (EELS), Raman spectroscopy, and infrared spectroscopy have been used to identify surface molecular species. However, these techniques lack the sensitivity and specificity that are needed to identify complex molecular adsorbates, particularly if there is a mixture of several species present. A method that shows great promise for identifying molecular adsorbates is laser desorption coupled with mass spectrometry (1,2). Laser desorption experiments fall into two major categories: high laser power experiments in which ions are formed directly by the laser pulse (&5), and low laser power experiments in which only neutrals are desorbed (6-11). The first method is used by commercial laser mass spectrometers such as the LAMMA, which is manufactured by Leybold-Hereaus. The second method is often referred to as laser-induced thermal desorption (LITD) because the surface is subjected to a rapid temperature jump but is not ablated. In this paper we report on the use of laser-induced thermal desorption and postionization by an electron beam to characterize molecular species absorbed in the amorphous carbon layer which covers the surface of a computer magnetic hard disk platter. Adsorption of molecular species in and on hard overcoat layers has been found to be of great importance to the performance characteristics of the magnetic disk (12). Since the samples we have examined have a carbon overcoat layer, identifying carbon-containing molecules on the surface presents a particularly difficult analysis problem. Figure 1 is a schematic drawing of a typical laser-induced thermal desorption experiment. A pulsed laser beam is focused onto the surface to rapidly heat a small part of the sample. At low laser power, only neutral species are vaporized from the surface. A few centimeters away from the surface, an electron beam ionizes the desorbed molecules, and the ions
thus formed are detected by a Fourier transform (FT) mass spectrometer. Previous work in our laboratory has shown that this method can be used to identify a wide variety of organic molecules adsorbed on a single-crystal platinum substrate (9-11). In most cases, the electron ionization fragmentation patterns were indicative of the original surface adsorbates. Submonolayer sensitivity is possible with the FT mass spectrometer, and a complete mass spectrum can be obtained for each laser shot.
EXPERIMENTAL SECTION Our aim in these studies was to identify the molecular species adsorbed on the surface of magnetic disk structures with carbon overcoats. The samples used are multilayer structures typical of commercial metal film hard disks (12), consisting of an aluminum base, a nickel and phosphorus layer, and a magnetic layer which is a metal film of Co/Cr/Ni. An overcoat layer of carbon is sputter deposited on top of the magnetic layer to act as a protective film and lubricant. Auger electron and X-ray photoelectron spectroscopy experiments were carried out by using a VG ESCALABMK II surface analysis instnunent. Auger electron spectroscopydepth p r o f h g experimentsshowed that the carbon films were approximately 30 nm thick. The Fourier transform mass spectrometer utilized for these experiments has been described previously (9, 10). Basically, it consists of a large electromagnet, an ultrahigh vacuum chamber, and a data system manufactured by the IonSpec Corp., Irvine, CA. The magnetic field was normally set at 0.6 T, but it was lowered to 0.05 T to detect Hzat m/z 2 and was raised t o 1.2 T to obtain high-resolution mass spectra. The vacuum chamber is pumped to a base pressure of 6 X lo-" Torr by both an ion pump and a turbomolecular pump. The sample to be studied (typically 1 cm2) is positioned in front of a hole in one of the electrodes of the FTMS analyzer cell by a Varian sample manipulator. A focused laser beam (Lambda Physik Model EMG 103 MSC excimer laser, wavelength 248 nm, pulse width 20 ns) enters the cell through a hole opposite the sample and strikes the sample at near normal incidence causing heating of a 0.2 mm2 area. The reflected laser beam exits the system by retracing the incoming path. Molecules desorbed from the sample expand out to fi the volume of the chamber and some of them pass through a pulsed 10-fiAelectron beam, which traverses the cell perpendicular to the path of the laser beam and parallel to the magnetic field. Ions formed in the electron beam are trapped by the magnetic and electric fields and are mass analyzed by measuring their cyclotron resonance frequencies. Since all the desorbed species are detected simultaneously, the relative abundance5 of desorbed species can be compared directly, without concern for
0003-2700/87/0359-2924$01.50/00 1987 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59. NO. 24. DECEMBER 15, 1987
' pulsed
'
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'
2925
I
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.
,
,
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,
800 600 400 BINDING ENERGY lev1
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FIgure 3. Broadscan X-ray photoelectron spectrum of the sample surface: Incident radiation Mg Ka (1253.6 eV), analyzer pass energy held constant at 50 eV.
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I
Figure 1. Schematic drawing of a laser-induced thermal desorption experiment. Sample moiecub%vaporized by Uw laser are ionized by an electron beam and detected with a FT mass spectrometer.
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Flpure 2. Auger electron spectrum of the sample surface prior to
depth profile analysis. Incident electron energy is 3 keV. the shot-to-shot stability of the laser heam. In our previous LITD studies of molecular adsorbates on a platinum surface, the temperaturejump induced by the laser pulse could be estimated since the reflectivity and thermal properties of platinum me well known. However, since the detailed structure and optical and thermal properties of the multilayer structures used in magnetic hard disks are poorly understood at present, we have not attempted m estimate the temperature jump induced hy the laser pulse. All laser power densities reported here are incident power densities.
RESULTS AND DISCUSSION Figure 2 shows an Auger electron spectrum of a typical sample obtained prior to depth profiling. The dominant peak is due to carbon (KLL) emission at 272 eV, and additional peaks are seen for silicon (LMM),chlorine (LMM),and oxygen (KLL) transitions. The oxygen (KLL) intensity at 510 eV is most likely due to compound formation at the sample surface by reaction with atmospheric oxygen as the sample was exposed ta atmosphere for several weeks before analysis. The double-peak structure at 85 and 68 eV indicates the presence of a partially oxidized silicon species (13). Auger electron spectroscopy depth profiling experiments showed a carbon film approximately 30 nm thick that was supported on a Cot Cr/Ni magnetic suhstrate. Auger spectra
taken after 1min of AI+ sputtering (approximately 3 nm of the surface removed) showed that a vast majority of the Si, CI, and 0 speeieshad been removed. Subsequent depth profde spectra showed no species other than carbon and argon (from the ion beam) until the carbon-Co/Cr/Ni interface was exposed. Thus, the Si, C1, and 0 species were present as near surface adsorbates and were not incorporated into the bulk of the carbon layer. X-ray photoelectron spectroscopic analysis of the sample was also performed to obtain elemental and chemical state information for the species on the surface. Figure 3 shows a typical broad scan spectrum. The strong C 1s emission occurs a t 284.5 eV, which is the correct energy for emission from graphitic-like carbon (14). Since the C 1s emission is dominated hy the 30-nm-thick carbon film, it is not possible to learn anything about trace molecular carbon-containing species on the surface from the C 1s emission. Significant intensity is also observed from 0 1s emission a t 532.5 eV. Figure 3 also shows the presence of N, Na, C1, F, and Si on the surface. A trace amount of potassium was detected in a narrow scan, but it is not apparent in Figure 3 because of its extremely low intensity. High-resolution spectra of the Si 2p region revealed a Si 2p binding energy of 102.9 eV, which is indicative of an oxidized Si species (14). The relatively high binding energy of 689.5 eV for the F 1s peak points toward the presence of a CF,-type species on the surface (14). Also, most C12p intensity appears a t 200.0 eV, which suggests a C-C1 type species (14). Identification of the exact surface species exhibiting these CI 2p and F 1s binding energies is not possible from these data. Figures 4 and 5 show typical mam spectra obtained for the same samples by using laser-induced thermal desorption followed by 70-eV electron ionization. The effect of different laser power densities was investigated systematically. The sample was mounted on a manipulator and inserted into the vacuum chamber of the FT mass spectrometer. Figure 4a shows that a t the lowest laser power densities utilized (1 MW/un2), only CO, and water are seen in the mass spectrum. Comparison of Figure 4a with a background mass spectrum obtained with the laser beam blocked showed that most of the water signal a t m/r 18 (approximately 90%) is from the chamber background. However, CO, is only a minor background constituent, and the background contribution to the m/z 44 peak in Figure 4a is only about 20% of the total peak height. At slightly higher incident laser power densities (between 2 and 20 MW/cm2), a multitude of peaks are observed in the mass spectrum up to 341 u. Figure 4b is a typical mass spectrum that was taken a t a power density of 5 MW/cm2. At these low laser power densities, ions are not produced
2026
ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
Table I. Structural Assignments for the Peaks Observed in the Medium-Powder Laser-Induced Thermal Desorption Mass Spectra of the Surface of a Magnetic Hard Disk
I iI
Possible ion structure
/rq
Measured m h
Calculated m/z
43.979
43.972
57.065
57.01 6
73.043
73.047
147.07
147.07
207.04
207.07
221.10
221 .08
266.99
267.07
281.05
281.09
326.97
327.08
340.96
341.09
(b)
50
150
100
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200
250
300
350
400
Figure 4. Laser-induced thermal desorption FT mass spectra of the surface of a computer magnetlc hard disk at different incident laser power densities: (a) 1 MW/cm2; (b) 5 MW/cm2. The peaks with asterisks are identified in Table 1. The insert in part b shows a region of a broad-band spectrum obtained at a magnetic field strength of 1.2 T. These data were zoom transformed to obtain accurate peak positions.
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Figure 5. Laser-induced thermal desorption FT mass spectra of the surface at a laser power density of about 25 MW/cm2: (a) mass spectrum taken following the second laser shot at a particular spot; (b) mass spectrum taken foilowlng the fifth shot at the same spot.
directly by the laser and postionization by the electron beam is required. Peaks labeled by a star are listed in Table I along with some possible ion structures. Good agreement is seen between the experimental data and the expected masses for poly(dimethylsi1oxane) compounds, which are common to many silicon-based oils. The insert in Figure 4b is an expanded plot of the data in the region of 221 and 222 u. The structure shown in Table I for the 221 u,which contains seven carbons and three silicons, would produce an M 1 isotope peak of 21.7% relative abundance, which is in reasonable agreement with our results. At this power level, essentially the same pattern of peaks is observed after as many as 600 laser shots at the same spot. This indicates that the laser beam is not significantly decomposing these surface species and is not ablating the carbon film. Other samples, manufactured under different conditions, showed completely different mass spectra. In particular, peaks due to the poly(dimethylsi1oxanes) were absent in many of the samples, and this indicates that these peaks do not come from residual pump oil in the vacuum systems of the FT mass
+
spectrometeror the ESCALAB. Typically, the FTMS vacuum chamber was not given a bakeout before the laser desorption experiments were begun. In fact, Figure 4 was obtained after the sample had been under vacuum for 48 h. Comparison of spectra obtained after 2 h with those obtained after 10 days under vacuum showed no substantive differences except lower background signals for water and C02. Figure 5 shows typical mass spectra obtained with laser power densities greater than 20 MW/cm2. Under these conditions, the outer carbon layer and the underlying metal layers are ablated, as shown by the spectra in Figure 5 obtained by using 25 MW/cm2. Figure 5a is an example of the first shot at this power and shows that K is the major peak in the mass spectrum with traces of Co and Cr from the magnetic layer. Potasaium is most likely present in the carbon layer as a trace impurity, but because of its low ionization potential, it dominates the spectrum of ions produced in the desorbed plasma. Similar effects are seen in laser microprobe experimentswhich use high laser power densities (5). The F T mass spectrum obtained after the fiith high-power laser pulse at this spot is shown in Figure 5b. Cobalt is the dominant peak in the mass spectrum and the signal for potassium has decreased significantly. Preliminary attempts to use the laser to depth profile the samples were hampered by the widely different ablation thresholds for the different layers. We observed that laser power densities, which were useful for etching the metal layers, caused rapid ablation of the carbon layer. On the other hand, lower power densities did not appear to etch the carbon layer at a significant rate. The ablation thresholds appear to be very sharp. In a new apparatus we are constructing, an ion beam will be used to etch the surface and LITD at low power densities will be used to detect the surface species and construct a depth profile.
C0NCLU SIO N S One of the major conclusions of this study is that LITD can provide valuable information about the molecular species on
Anal. Chem. 1087, 59,2927-2930
a complex surface. In our XPS experiments, the measured 0 1s binding energy a t 532 eV is consistent with oxidized graphite (15) and supports the LITD results which showed large amounts of COP Both the AES and XF’S spectra showed evidence for Si, yet it is clear that the LITD results are far more useful for identifying the molecular nature of the Si species. These experiments do not permit us to estimate the detection sensitivity of LITD, but prior work in our laboratory has shown submonolayer sensitivity for a variety of organic species on platinum (9-11). One of the advantages of LITD over conventional rapid heating techniques is that the vaporized species are removed in microseconds rather than the several tens of seconds that are required to resistively heat the sample. Thus,even if there is a very small amount of material on the surface, it can be detected by LITD because the short laser pulse produces a large flux of desorbed material. Conceptually, this can be thought of as a type of high-resolution chromatography because the sample is compressed into a narrow pulse of high flux density. An essential requirement, of course, is a detector that can capture the complete mass spectrum, and this makes FT mass spectrometry well suited for these experiments.
ACKNOWLEDGMENT We thank F. J. Feher for helpful discussion of the LITD
2927
Cotter, R. J.; Van Breemen, R.; Yergey, J.; Heller, D. Int. J . Mass Spectrom. Ion. Fhys. 1983, 46, 395-398. Van der Peg, G.J. Q.; Van der Zande, W. J.; Kistemaker, P. G. Int. J . Mass Spectfom. Ion Processes 1984, 62, 51-71. Hall, R. 6.; DeSantolo, A. M.; Bares, S. J. Surf. Sci. 1985, 161, L533-L542. Brand, J. L.; George, S. M. Surf. Sci. 1988, 167, 341-362. Burgess, D. R., Jr.; Hussla, I.; Stair, P. C.; Viswanathan, R.; Weitz, E. Rev. Sci. Instrum. 1984, 5 5 , 1771-1776. Sherman, M. G.;Kingsley, J. R.; Dahigren, D. A,; Hemminger, J. C.; McIver, R. T., Jr. Surf. Scl. 1985, 148, L25-L32. Sherman, M. 0.; Kingsley, J. C.; Hemminger, J. C.; McIver, R. T., Jr. Anal. Chlm. Acta 1985, 178, 79-89. Sherman, M. G.; Land, D. P.; Hemminger, J. C; McIver, R. T., Jr. Chem. Phys. Len. 1987, 137, 298-300. Sato, I . I€€€ Trans/. J . Magn. Jpn. 1987, 2 , 4. Translated from Sato, I.J . Magn. SOC.Jpn. 1988, 10, 6. Handbook of Auger Nectron Spectroscopy, 2nd ed.; Physical Electronics Division, Perkin-Elmer Corp.; Eden Prairie, MN. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics Division, Perkin-Elmar Corp.; Eden Prairie, MN. Barber, M.; Evans, E. L.; Thomas, J. M. Chem. Phys. Len. 1973, 18, 423.
Donald P. Land Tsong-Lin Tai John M. Lindquist John C. Hemminger* Robert T. McIver. Jr.* Department of Chemistry University of California Irvine, California 92717
results. Registry No. 02,7782-44-7;Si, 7440-21-3;Clz, 7782-50-5;Nz, 7727-37-9;Na, 7440-23-5;Fz, 7782-41-4;K, 7440-09-7;COz, 12438-9; HzO,7732-18-5;Co, 7440-48-4; Cr, 7440-47-3.
LITERATURE CITED (1) Hall, R. 6. J . Phys. Chem. 1987, 9 1 , 1007-1015. (2) Novak, F. P.; Balasanmugam, K.; Viswanadham, K.; Parker, C. D.; Wilk, 2. A.; Mattern, D.; Hercules, D. M. Int. J . Mass Spectrom. Ion P h y ~ 1983, . 5 3 , 135-149. (3) Balasanmugam, K.; Viswanadham, S. K.; Hercules, D. M. Anal. Chem. 1988, 5 8 , 1102-1108.
RECEIVED for review April 27, 1987. Accepted September 1, 1987. Support for this research was provided by the National Science Foundation under Grant CHE8511999 and the donors of the Petroleum Research Fund, administered by the American Chemical Society. J.C.H. wishes to acknowledge support from the Alfred P. Sloan Foundation in the form of an Alfred P. Sloan Research Fellowship. J.M.L. and D.P.L. wish to acknowledge support in the form of IBM Research Fellowships.
AIDS FOR ANALYTICAL CHEMISTS Detection of Aerosol Formation in the Effluent of a Supercritical Fluid Chromatograph Steven R. Goates,* Norman A. Zabriskie, John K. Simons, and Bahram Khoobehi’
Department of Chemistry, Brigham Young University, Provo, Utah 84602 Supercritical fluid techniques continue to grow in importance in analytical chemistry, especially supercritical fluid chromatography (1). Accompanying this growth is the development of postcolumn detection methods, including flame ionization detedion (2,3),inductively coupled plasma emission ( 4 ) ,mass spectrometry (5,6),and supersonicjet spectroscopy (7-9). In each of these detection methods, the behavior of the fluid at the point of decompression, especially in regard to nucleation processes, can severely affect the performance of the detector. Giddings (10) pointed out the “fogging” problem in supercritical fluid expansions almost 2 decades ago, and the problem has been examined recently by others, including Smith et al. (11). The problem is not widely appreciated, however, and statements such as “when a supercritical fluid is introduced to the vacuum it immediately converts to a molecular beam” are not uncommon. Current address: Department of Ophthalmology, Eye and Ear Infirmary, University of Illinois College of Medicine at Chicago.
For convenience in discussion, we have categorized aggregate-forming processes into three primary types: (1) the formation of liquid droplets of the carrier solvent at the point of expansion, which we have termed aerosol formation, (2) microscopic clustering of carrier molecules in weakly bound van der Waals complexes with solute molecules, and (3) precipitation of solute molecules into relatively stable particulates. However, whether the first two categories really represent distinct processes is open to question. We have discussed the problem of solute precipitation elsewhere (7). It occurs when the sample approaches saturation in the carrier (7,10, 12) as can occur in a long restrictor where the change in pressure is significant and places important constraints on nozzle design for superson’ic jet detection. There has been some confusion in the analytical chemistry literature about the different types of clusters represented by categories 2 and 3 and the effect of the Mach disk (13) in a supersonic expansion; weakly bound van der Waals clusters may form in the jet but will be broken up at the Mach disk, whereas sample
0003-2700/87/0359-2927$01.50/0 0 1987 American Chemical Society
