1989
Anal. Chem. 1986. 58. 1989-1993
software, PUSHSUB, is a improvement over PEAK PICKER alone on spectra with nonlinear base lines. In addition, PAIRSPLUS was able to lower the qualitative limit of detection of the PAIRS interpretation in specific mixtures where spectral similarity effects could be subtracted. To understand PAWMI's ability to identify major and minor components in infrared spectra of mixtures, an array of mixtures were prepared, both transmission and internal reflection spectra were generated, and the spectra were interpreted. The qualitative limit of detection of minor components in mixtures ranged from 0.4% to 15%, depending on the components of the mixture. This was not only dependent on the concentration of the component of interest in the mixture but also on the concentration of other spectrally similar components in the mixture. In the array of mixtures interpreted during this study, internal reflection spectra where correctly interpreted at a slightly higher percentage than transmission spectra. The reasons were: intensity differences between transmission and internal reflection spectra causing additional peaks to be selected by PUSHSUB, resulting in higher goodness value assigned for minor components in the internal reflection spectra; greater peak shifting in transmission mixture spectra, causing lower total goodness values being reported by PAIRS and the minor components not being identified by PAIRSPLUS. False positives represented 13% of the total positive results reported by PAWMI. This occurred when the spectral similarity of a mixture to the false positive component was greater than the additive spectral similarity of each component of the mixture. Many of these false positives could be eliminated by placing additional constraints on the PAIRSPLUS processing routine. However, a number of true positives would
also be lost. Reducing the number of false positives is an area of future research.
ACKNOWLEDGMENT The authors wish to thank Arthur Palmer for his help with the programming and Greg Kinnes for his help in preparing the mixtures.
LITERATURE CITED Puskar, M. A.; Levine, S. P.; Turpin, R. I n Protecting Personnel at Hazardous Waste Sites; Levine, S. P., Martin, W. F., Eds.; ButterworthslAnn Arbor: Woburn, MA, 1985; Chapter 6. Gurka, D. F. Project Summary: Interlaboratory Comparison Study: Methods for Volatile and Sernlvolatlle Compounds ; Environmental Monitoring Systems Laboratory: Las Vegas, NV, June 1984; EPA600/S4-84-027. Hallstedt, P. A.; Levine, S. P.; Puskar, M. A. J. Hazard. Waste Haza d . Mater., in press. Eckel, W. P.; Trees, D. P.; Kovell, S. P. "Distribution and Concentration of Chemicals and Toxic Materials Found at Hazardous Waste Dump Sites"; Proceedings of the National Conference on Hazardous Waste and Environmental Emergencies, May 1985. Woodruff, H. B.; Munk, M. E. J . Org. Chem. 1977, 42, 1761-1767. Woodruff, H. B.; Munk, M. E. Anal. Chim. Acta 1977. 95, 13-23. Woodruff, H. 0.; Smith, G. M. Anal. Chem. 1980, 52, 2321-2327. Woodruff, H. 0.; Smith, G. M. Anal. Chim. Acta 1981, 733, 545-553. Tomellini, S. A.; Saperstein, D. D.; Stevenson, J. M.; Smith, G. M.; Woodruff, H. B. Anal. Chem. 1981. 53. 2367-2369. Tomellini, S. A.; Stevenson, J. M.; Woodruff, H. B. Anal. Chem. 1984, 56, 67-70. Tomelllni, S . A.; Hartwick, R. A.; Stevenson, J. M.; Woodruff, H. B. Anal. Chim. Acta 1984, 162. 227-240. Puskar, M. A.; Levlne, S. P.; Lowry, S. R. Anal. Chem. 1988, 58, 1156-1162. Harrick, N. J. Internal Reflection Spectroscopy; Wiley: New York, 1967; 2nd Printing, Harrlck Scientific Corp.: Ossining. NY, 1979.
RECEIVED for review January 6,1986. Accepted April 18,1986. This work was supported by Grant 1-R01-OH02066-01from the National Institute for Occupational Safety and Health of Centers for Disease Control.
Spatial and Temporal Distributions of Particulates Formed from Metallic Surfaces by Laser Vaporization Carmen W. Huie and Edward S. Yeung* Ames Laboratory-USDOE
and Department of Chemistry, Iowa State University, Ames, Iowa 50011
By observation of scattered radiation at 90' relative to a probe laser beam, the particulate matter that is formed above metallic surfaces foHowing laser vaporization can be monitored. An acoustooptic deflector scans the direction of the probe beam 80 that spatially and temporalty resolved distributions can be recorded from a single transient event. Distinct scattering maps are obtained as a function of laser power, surface property, absorption characteristics, and material volatility. The resuits can be used to understand the laser-surface interactions, 80 that atom formation can be better controlled for analytical applications.
The use of lasers to vaporize, dissociate, excite, or ionize species on solid surfaces has the potential of becoming a powerful analytical tool. Since laser beams can be focused to very small sizes, local concentrationsof materials on surfaces can be probed. The material vaporized is usually quite well confined, 80 that analytical sensitivity is enhanced. In general, either optical spectroscopy (neutral species) (1) or mass spectrometry (ions) (2,3)can be used to probe the laser-va0003-2700/86/0358-1989$01.50/0
porized material. The main problem is the lack of reproducibility in the generation of atoms or ions, in part due to the pulse-to-pulse fluctuations in the laser intensity, and in part due to variations in the surface properties of the sample. To turn the semiquantitative results obtained so far into reliable quantitative results, one must try to obtain a better understanding of the laser-surface interaction. Progress has been made by obtaining the spatial and temporal distribution of atoms (4), molecules (5), translational temperature (6),and vibrational temperature (7) in the laser-generated plumes. It may then be possible to use thermodynamic information to model the vaporization-dissociation-excitation-ionization process for various laser beam and surface properties. The one type of data that has so far been neglected is that of particle formation from the laser vaporization process. This is the first step in the chain for atom production. One must be able to volatilize these particles completely before even worrying about dissociation. Current successes in atomic spectroscopy in flames, inductively coupled plasma, and graphite furnaces have benefited greatly from earlier studies of nebulization processes and the design of sample introduction techniques to optimize nebulization (8). The same 0 1986 American Chemical Society
1990
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST
1986
type of insight is needed for developing the laser microprobe into a quantitative analytical tool. The ability to measure spatially resolved concentrations of molecular and atomic species in real time is also of great importance to understanding the dynamics of many physical systems (9). Because of the typically nonintrusive nature of optical methods, they are most often employed to make these measurements. To this end, techniques such as atomic and molecular emission (10, II), absorption (12),fluorescence (13), and scattering (14) have been applied to study a wide range of systems and problems. Nearly all published spatial information is obtained one point at a time by some sort of rastering mechanism. Aside from the tedious nature of this approach, there are problems with reproducibility since the system must maintain its integrity over the entire duration of the measurements. Furthermore, dynamic systems are in general not reproducible from one trial to the next, so that the "average" system behavior may not be meaningful a t all. These measurements become even more difficult when temporal information is desired. Recently, we have reported two distinct types of spatial probes for transient events. First, a continuous laser, properly gated by an electronic shutter, is beam-expanded to cover the entire cross section of interest. The spatial information is preserved due to the collimated nature of the laser beam and can be recorded on a vidicon camera (TV camera) ( 4 ) . Since the camera is an integrating device, the time resolution is provided by the electronic shutter. A continuous laser is chosen as the probe to provide stability in the spatial intensity distribution and to provide high spectral resolution. The main limitation of the laser-vidicon spectrometer is that the scan speed of the vidicon camera allows the recording of only a single event for a laser-generated plume, since it takes */so s for each video frame to be read by the camera. So, even though the temporal resolution is in the microsecond range, as dictated by the optical shutter, spatially resolved information on plume growth and decay can only be obtained by taking a time-lapse sequence from plumes generated from different laser shots. However, the variations in plume concentrations and contours make it difficult to correlate information from different time-lapse events. In a second type of spatial probe, the continuous laser is directed by an acoustooptic beam deflector to scan through the spatial region of interest a t high sweep rates (15). The laser beam is focused a t the probe region to provide spatial resolution, but it subsequently imaged onto a single photodiode. Spatial information is thus transformed into temporal information at the photodiode output and can be digitized and analyzed by a wave form digitizer. Acoustooptic beam deflectors can have scan rates of 50 kHz or larger, so that the transient event is essentially "frozen" in time at each sweep. Continuous sweeping can then provide temporal information about the development of the transient event. In this paper, we report the first study of the spatial and temporal distributions of particles formed by laser vaporization of metallic surfaces. An acoustooptic deflector scans a probe laser beam above the metallic surface repeatedly after the vaporization event. Light scattering a t 90' to the probe beam is monitored to map the particulate matter. At the vaporization conditions used here, relatively large particles are formed, and an estimate of particle size at the spatial resolution of the laser beam is possible. EXPERIMENTAL SECTION When an acoustic wave is present in a medium, light can be diffracted by it according to the Bragg angle. These acoustooptic deflectors for laser beams have made substantial technical advances in recent years because of laser printers, laser disks, and laser machining applications. With change of the frequency of the acoustic wave, the deflection angle changes, and with change
I
Flgure 1. Experimental arrangement for the determination of scattdng profiles of particulates in laser-generated plumes: BS, beam splitter; CA, current amplifier: L,, 1 m focal length spherical lens; Lz, 10 cm focal length cylindrical lens; L,, 10 cm focal length spherical lens; L,, 6 cm focal length cylindrical lens; M, prism; OA, Bragg cell; PD, photodiode; RF, Bragg cell driver: TC, timing circuit; WA, wave form analyzer: WG, wave form generator: YAG, vaporization laser: LASER, Ar ion laser; C, vaporization cell; LF, line filter; PMT, photomultiplier tube.
of the amplitude of the acoustic wave, the deflection efficiency can be controlled. The nature of the Bragg deflection depends on the frequency of the acoustic wave. For a given beam size at the crystal, there is an aperture time T that limits temporal response. 7 is given by the beam size divided by the acoustic velocity in the medium. The number of spots that can be resolved, N , is then given approximately by N=rAf
(1)
where Af is the range of acoustic frequencies used. In our studies, Af = 80 MHz and T = 0.25 HS. So, there are only 20 spatially resolvable points in one sweep. One will always have to make a compromise between the time response (per point) and the spatial resolution (totalpoints)N . There are commercial products available that yield 1000 to 2000 resolvable spots at scan rates of 10 spots/ps. For two-dimensional information, two Bragg cells can be used in an orthogonal arrangement. The number of resolvable spots is then p. The schematic for pulsed diagnostic studies based on beam deflection is shown in Figure 1. The vaporization cell used is identical with the one described in ref. 4. In order to provide optimum confinement of the vapor plume, 500 torr of argon was introduced into the cell. A 6 cm focal length cylindrical lens was used to focus the vaporization laser beam to a line of 1 cm X 0.4 mm parallel to the probe laser. Even though some variation in the cross-sectional distribution is still expected along the length of the plume, the geometry is good enough to study local distributions without mathematical deconvolution. The samples were mounted on a brass rod so that a new surface can be exposed for each measurement. The vaporization laser used is the 1060-nm output of a Nd:YAG laser (Quantel, Santa Clara, CA, Model VG480) operated at 1 Hz. Typical vaporization laser pulse energy was about 140 mJ in a 10-ns pulse. An argon ion laser (Control Laser, Orlando, FL, Model 554A) provides a monochromatic light beam at 488 nm with a typical output of 1 W. This beam was directed toward the aperture of an acoustooptic beam deflector (Isomet, Springfield, VA, Model 1205C-2) driven by a deflector driver (Isomet, Springfield, VA, Model D322B). The driver accepts a tuning voltage between +2 and +18 V and provides a tuned rf output to the deflector that follows the input voltage. The acoustic tuning range is from 57 to 103 MHz. The output from a wave form generator (Wavetek corp., San Diego, CA, Model 184) was used to provide the tuning voltage. Since the maximum output voltage of the Wavetek is about 12 V, an external 6-V battery was used to bias the tuning voltage to between +2 and +18 V. A symmetrical triangular wave form with a 200-r~cycle was used to deflect the laser beam up and down at the first order. It is necessary to focus the first-order beam so that the maximum number of resolvable spots can be obtained at an imaged area immediately above the sample surface. The P A f product of the 1205C device is specified by the manufacturer to have a value of 22 obtained with a laser beam diameter of 2.0 mm. According to eq 1,the resolution of this device will start to degrade at a scan rate of 50 kHz or higher. This means that if one wants to deflect the first-order beam over a distance of 1 cm and retain the op-
ANALYTICAL CHEMISTRY, VOL. 58. NO. 9, AUGUST 1986
1991
I
Flgun 3. Spatial and temporal Ustrlbuhon 01 particulates hom a singb laser vaporization event on CU Power = 10 MW/cm'
Figure 2. Photcdiie sgnal (intensity shown as downward defleaion) showing the image 01 a transparent grid. Two consecutive up and down scans are displayed. timum resolution, the laser beam has to be deflected at a scan rate below 50 kHz and the beam has to he focused to approxicm a t the region of observation. This mately a diameter of 'Iz2 diameter defines the spatial resolution of the system. Higher spatial resolution is possible if the height of the observation region is reduced proportionately. In these experiments, the optical system consisted of a 1 m focal length spherical lens and a 10 cm focal length cylindrical lens placed at a distance of 4 em and 125 cm from the existing aperture of the deflector, respectively. The vaporization cell was placed at a distance of 10 an away from the cylindrical lens. The dimensionsof the deflectingfirst-order beam a t the cell were about 12 mm X 0.6 mm. After the vaporization cell, the first-order beam was reflected by the first surface of a 5% beam splitter and then focused by a 10 cm focal length spherical lens onto a 10 x 10 mm2photodiode (Hamamatsu, Middlesex. NJ, Model 51790-01). This signal is used to establish the diffraction efficiency and the resolution of the system and to relate the scattering signal to the proper space and time mordinates. The current from the photodiode was fed into a current amplifier operating with a gain of 40 d 8 (Hewlett-Packard. Palo Alto, CA, Model 4618). The output from the amplifier was then fed into a transient wave form analyzer (Data Precision, Danvers, MA, Model 6000/620). The sampling period of the wave form analyzer was set at 2 ps so that 50 data points were recorded fnr each up and down scan. The total number of data p i n k collected was about 1500, which means that the total number of scans recorded for each laser-generated plume was about 30. To ohserve scattering, a photomultiplier tuhe (Hamamatsu, Middlesex. NJ. R-928)is placed at 90-to the probe beam and the vaporization laser. A laser line filter at 488 nm O r i o n , Hollistor, MA, 10-4880-1) is placed in front of the phototube to reduce rwm light and light from the vaporization laser. The low particle densities here are not expected to affect spatial information via absorption and thermal deflection. Two types of data files were recorded. The background file contained the scattering signals due to cell walls, windows, and other materials. This file can be subtracted from the data file that contained scattering signals due to particles ejecting from the sample surfaces. To make sure that samples were positioned at the same relative location relative to the first order beam and the detertor. one could observe the backgrnund signal on the wave form analyzer, which was relatively flat, until a peak appeared. This peak indicated that the first-order beam was striking part of the sample surface and provides a convenient reference location. The two files were comhined in a computer (Digital Equipment, Maynard, MA, PDP 11/10) to prnduce scattering files. The rmulk are then plotted on a graphics display (Visual Terhnology Inc., Tewksbury. MA, MndelS.50) and printed on a dot-matrix printer (Micm Peripherals, Salt Lake City, UT. Model MPISOG).
RESULTS AND DISCUSSION To see how spatial and temporal information are encoded, one can refer to Figure 2, which is the photodiode output
4. Spatial and tempual distrfbution of particulates hcm a single laser vapwization event on CU. Power = 250 MW/cm2.
obtained by putting a transparent grid at the location of the laser-generated plume with the cell removed in Figure 1. Figure 2 shows two consecutive u p d o w n scans of the beam deflector. Zero output is obtained a t the upper and the lower limits of the deflected beam, alternately, when no light reaches the photodiode because of the limiting apertures. As the beam crosses the transparent regions of the grid, intensity is recorded as a negative voltage. Fifty data points are digitized for each up or down scan,and these discrete locations are clearly visible in Figure 2. The trace is symmetrical about the center, since the spatial information should be identical during both the up and the down scans for this static system. Careful inspection shows that the laser intensity is nnt constant during a single scan. This is expected due to the change in diffraction efficiency of the device at different acoustic frequencies. These intensity envelopes are however constant once the system is warmed up, and corrections to the measured scatterng intensities can be easily made. The range displayed in Figure 2 is only that of the entire memory buffer. So, for each up or down scan, one can consider the transient event to be essentially 'frozen" in time. One-dimensional spatial information in subsequent experiments is then decoded from Figure 2 using the calibration provided by the transparent grid. When the entire buffer memory is taken together, one obtains a time-lapse sequence of 30 consecutive spatial maps for the single transient event. A three-dimensional plot of the scattering intensity, height above the surface and time delay after vaporization can be constructed from the digitized data. The data points for the up and down scans are simply inverted in the indexing to correspond to each common spatial location. The computer algorithm is nothing more than an indexing routine that keeps track of the start positions and the start times of each scan. Figures 3-8 are all plotted on the same scale. However, the scattering intensities can nnly he cnnsiderd apprnximate since the optical collectinn efficiencies are nut absolutely identical when samples are changed. Figure 3 is a scattering map for a 1 mm thick piece of copper. A fresh surface is expnsed fur each data set. Althnugh
1992
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
t
Flgure 5. Spatial and temporal diswibution of particulates from a single laser vaporization event on polished AI. Power = 250 MW/cm2.
Qure 6. Spatial and temporal distributionof particulates from a single laser vaporization event on nonpolished AI. Power = 250 MW/cm2.
Flgure 7. Spatial and temporal distribution of particulates from a single
laser vaporization event on Mo. Power = 250 MW/cm2. pulse-to-pulse variations in the vaporization laser cause each transient vapor plume to be slightly different, the general features are always the same under the same experimental conditions. One can see that the particles are formed very quickly (
