Table 11. Hydrogen Determinations in wppm for Uranium and Uranium Alloy Specimens. Samoleb Specimen 1 2 3 4 Uranium-le 1.6 2.3 1.5 ... Uranium-2e 2.5 2.5 2.2 ... Uranium Alloy-ld 33 31 30 33 Uranium Alloy-2d 32 28 28 27 Uranium Alloy-3d 2.3 2.0 2.4 2.0 Uranium Alloy-4d 3.0 2.9 2.4 2.2 a Extraction temperature IO00 “C. * Each sample was cut from an adjacent position along a lia-inch by lis-inch rod. Uranium specimens were from different billets of high-purity derby uranium. Uranium alloy specimens were from different batches of similar materials except uranium alloys 3 and 4 had been vacuumheat treated. Some hydrogen determinations for titanium, uranium, and uranium alloys were performed. Tables I and I1 demonsirate the precision of the instrument when used as a routine analytical device. During these analyses, the samples were de-
greased in’acetone, and the samples were allowed to accumulate in the furnace chamber until all eight samples were analyzed. Samples from a given specimen were analyzed on two different days in order to demonstrate the stability of the instrument. CONCLUSIONS
The instrument described in this article has been satisfactory for the determination of hydrogen in metals over the concentration range from 0.01 wppm to 100 wppm with a precision of 10%. Diffusion parameters, surface reactions, and the distribution of hydrogen in the sample can be evaluated using the hydrogen evolution rate data and the thermal history of the sample after it is dropped into the furnace. Micromole quantities of other gases can also be determined; thus, the instrument can be used to study a great variety of thermally activated gas evolution processes.
RECEIVED for review March 16, 1971. Accepted June 7, 1971. Work performed at Oak Ridge Y-12 Plant under contract W-7405-eng-26 with the U. S. Atomic Energy Commission.
Effect of Atmosphere on Spectral Emission from Plasmas Generated by the Laser Microprobe William J. Treytl, Kenneth W . Marich, James B. Orenberg, Peter W. Cam,’ D. Craig Miller, and David Glick Division of Histochemistry, Department of Pathology, Stanford University School of Medicine, Stanford, Calg. 94305 The effects of various atmospheres on laser-induced optical emission of plasmas from solid samples have been investigated. Atmos heres of argon, air, oxygen, nitrogen, helium, andalso vacuum, were used. Samples of iron in steel and in iron oxide-coated tape, and of magnesium in aluminum foil, human serum, and liver were employed. Laser energies of approximately 1.2, 3.6, and 8.0 mJ were used. Signal-to-background ratios were not found to vary systematically with atmosphere and laser energy, but significantly larger values occurred in vacuum at 1.6-mJ laser energy. Signal intensities were greater in denser atmospheres at 3.6 and 8 mJ, but approximately the same at 1.2 mJ. Signal intensities varied directly with these laser energies, except in vacuum where the signal was independent of laser energy. Metallic and nonmetallic targets behaved similarly. SINCETHE INNOVATIONS in laser microprobe instrumentation reported by Peppers et al. (I), efforts have been directed to optimization of conditions for analysis of elements. The effect of composition of the sample on the laser-induced emission signal has been studied (2). The present investigation concerns the effects of various atmospheres on signal intensity from laser-generated plasmas. Effects on emission spectra of atmospheres in which plasmas Present address, Department of Chemistry, University of Georgia, Athens, Ga. 30601 ( I ) N. A. Peppers, E. J. Scribner, L. E. Alterton, R. C. Honey, E. S. Beatrice, I. Harding-Barlow, R. C. Rosan, and D. Glick, ANAL.CHEM., 40, 1178 (1968), (2) K. W. Marich, P. W. Carr, W. J. Treytl, and D. Glick, ibid.,
42, 1775 (1970). 1452
are generated by a dc arc have been reported (3-7). Vallee (3, 4 ) demonstrated that an argon-atmosphere enhanced spectral line emission without significantly increasing the background. In contrast, increased signal-to-noise ratio was achieved with helium by suppression of the background (3). Undesirable cyanogen bands were reduced by use of noble gases. Rates of volatilization of elements in a sample varied with the composition of the atmosphere. Vallee (3) concluded that, for particular metals, certain atmospheres could be used to enhance the sensitivity of analysis. The use of inert gases with the Stallwood jet increased signals, diminished background, reduced selective volatilization, and produced a more stable dc arc (8). Stabilization of the ac arc was also achieved by Sukhnevich (9) with inert gases. More recently, use of controlled atmospheres in spark excitation showed improved precision and accuracy, higher sensitivity, and reduced inter-element effects (IO, I I ) . (3) B. L. Vallee, C. B. Reirner, and J . R. Loofbourow, J . Opt. SOC.Amer., 40, 751 (1950). (4) B. L. Vallee and S. J. Adelstein, /bid., 42,295 (1952). (5) B. L. Vallee and S. J. Adelstein, Specrrochini. Acta, 6 , 134 (1954). ( 6 ) Z. L. Szabo and I. Toth, Mugy. Kem. Foly., 74, 394 (1968). (7) C. K. Matocha and J. Petit, Appl. Sprcrrosc., 22, 562 (1968). (8) A. J. Mitteldorf, “Trace Analysis: Physical Methods,” G. H. Morrison, Ed., Interscience Publishers, New York, N. Y., 1965, p 200. (9) V. S. Sukhnevich, Z. Prikl. Spektrosk., 9 , 199 (1968). (IO) R. R. Boyd and A. Goldblatt, Appl. SpPcrrosc., 19, 22 (1965). (11) J. Kashirna and M. Kubota, Rep. G r s r . Res. Lrrb., W m 4 Utiio., 1967, No. 18, pp 9-19.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
Buravlev (12) observed a strong dependence of sampling rate on the arc discharge atmosphere, and greater reproducibility of the effect of sample matrix in noble gas atmospheres. Laser-induced plasmas and the effects of ambient conditions on them have been studied extensively. Minck (13) measured gas breakdown thresholds in air, argon, helium, hydrogen, and neon at various pressures. Browne (14) explained this breakdown as a n inverse bremsstrahlung process triggered by an unspecified multiphoton interaction. Daibler and Winans (15) studied the laser-induced emission spectrum of nitrogen and argon, and Litvak and Edwards (16) ob,served emission from hydrogen in the vicinity of the 6563 A Balmer line. Both groups observed intense continuous emission from the central portion of the plasma and considerable Stark broadening of the lines. The nature of the interaction of focused laser radiation with solid materials and their resulting vapor plasmas have also been investigated. These studies, however, have been primarily concerned with the physics of rapid absorption of laser energy in condensed media (17-20), thermal distribution and equilibration at the surface (19,21-24), and the equations of state of the gas dynamics which describe the evolution, expansion, and properties of the plasmas generated by the ejection of atoms, ions, and electrons from the target (17-19, 21,25). While the formation of plasmas by focused laser beams has been studied, the effects of ambient atmospheres on the optical emission properties of these plasmas have not been investigated from an analytical standpoint. In the present work the effects of atmospheres of argon, helium, nitrogen, oxygen, and air, and partial vacuum on optical emission from laser-induced plasmas, as a function of three laser energy densities were investigated. Metallic, nonmetallic, and biological samples were employed. EXPERIMENTAL
Apparatus. The laser microprobe assembly, the spectrograph, and the photoelectric detection system employed in this study were described in detail in previous publications ( I , 2 , 26). The widths of the entrance and exit slits on the spectrograph were 50 microns. Oscilloscope tracings of the photomultiplier anode signals were photographed with a Polaroid camera mounted on a Tektronix (No. 454) oscilloscope. The 3020 and 2802 A lines of iron and magnesium, respectively, were chosen for the present work because of (12) Yu M. Buravlev, Zlr. Prikl. Spektrosk., 6, 583 (1967). ( 1 3) R. W. Minck, J . Appl. Pliys., 35, 252 (1964). (14) P. F. Browne, Proc. P h y s . Soc., 86, 1323 (1965). (15) 3. W. Daibler and J . G. Winans, J . Opt. SOC.Amer., 58, 76 (1968). (16) M. M. Litvak and D. F. Edwards. IEEEJ. Oircrtrtrm Elecrroti.. ' QE-2,486 (1966). (17) J. F. Ready, J . ADD/.Plrvs., 36, 462 (1965). (18) H. Klocke; S,,c.crroc/iirii..Acta, 24B, 263 (1969). (19)A. Caruso, B. Bertotti, and P. Guipponi, Nuoco C'imetiro, 45B, 176 (1966). (20) W. G. Griffin and J. Schluter,flrys. Lett., 26A, 241 (1968). (21) Yu V. Afans'ev and 0. N. Krokhin, JETP, 25, 639 (1967); 52, 966 (1967). (22) J . M. Baldwin, Appl. Spccrrosr., 24, 429 (1970). (23: V. P. Veiko, Ya. A. Imas, A. N. Kokora, and M. N. Libenson, Sor. f l i y s . Tocli. Pltys., 12, 1410(1968). (24) S. Namba, P. H. Kim, S. Nakayama, and 1. Ida, Jcrp. J . Appl. Plrys., 4, 153 (1965). (25) N. G. Basov, V. A. Boiko, V. A. Dement'ev., 0. N. Krohkin, and G . V. Sklizko, JETP, 24, 659 (1967); 51, 989 (1966). (26) E. S. Beatrice and D. Glick, Appl. Specrro.w., 23, 260 (1969).
Figure 1. Evacuable glass cuvette (12.5 X 12.5 X 45 mm) with ( a ) open end facing spectrograph, (6) gas inlet tube, and (c) plastic support slide their relatively high intensities and lack of interference from other lines in the emission spectra from laser-induced plasmas. Iron and magnesium hollow cathode lamps (Perkin-Elmer) were used for spectral alignment. The atmosphere chamber consisted of a removable quartz window on an evacuable glass cuvette (Flow Cell No. 480, Scientific Cell Co., Forest Hills, Long Island, N . Y . )fastened to a plastic slide with expoxy cement (Figure 1). Highvacuum silicone grease was applied to the ground edges of the cuvette at its open end and the quartz window was firmly sealed to the opening by application of vacuum (-5 Torr) Materials. The argon, helium, nitrogen, and oxygen gases used were USP grade. The gas was passed through a column of Drierite before entering the chamber. The iron samples used were from a block of NBS (No. 462) standard steel and Memorex (MRX-3) magnetic tape. The tape was composed of a ferric oxide coating, 10 microns thick, deposited on a polyester backing, 38 microns thick. Heavy duty aluminum foil, human serum and liver were used for the magnesium samples. Procedure. Alignment of spectral lines was performed by centering lilies from the hollow cathode lamps in the slits in front of the photomultiplier tubes by rotation of the spectrograph grating. The sample was placed in the cuvette near the opening and then the quartz window was mounted. The chamber was evacuated, checked for leaks with a manometer, and then gas was introduced. After each laser discharge, the chamber was evacuated before it was refilled with a fresh atmosphere to prevent the previous atmosphere from influencing the spectral emission of the succeeding sample. Light from the laser-induced plasma was focused on the slit of the spectrograph by a quartz lens as previously described ( I ) . Samples of 40 nl of serum were deposited o n plastic cover slips with a precision pipetting device (Oliver Instrument Company, Sunnyvale, Calif.) employing a 1+I Hamilton syringe and air-dried as reported earlier (2). Five-micron thick sections of formalin-fixed, paraffin-embedded, deparaffinized liver tissue were mounted on plastic slides.
RESULTS AND DISCUSSION The integrated optical emission data from the steel and iron oxide samples are given in Tables I and 11. In general, the signal intensities showed a marked dependence on incident laser energy and atmosphere conditions, the strongest signals being produced in argon at high laser energies. Signal-tobackground ratios (S/B) did not exhibit systematic trends. The exception occurred in vacuum where the S/B varied inversely with laser energy, an approximately fourfold increase being obtained at 1.2 mJ, although the signal intensity did not vary regularly. A small laser energy independent increase in SIB was found for steel in argon. These observa-
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
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Table 1. Effect of Atmosphere and Laser Energy on Laser-Induced Optical Emission from Iron in Steep (Integrated Photoelectric Data) Atmosphere Nb Laser energy, mJ Signalc SIBd Vacuum 14 1.05 f 0.06 5.99 it 0.56 18.5 f 3.9 10 3.63 f 0.26 10.3 it 0.4 8.07 it 1.14 10 8.09 f 0.86 6.83 f 0.33 4.79 f 0.64 Helium 11 1.27 0.06 11.1 f 1.3 5.77 it 1.13 45.7 i 4.4 4.67 f 0.80 12 4.38 f 0.47 102 f 9 4.19 f 0.53 10 9.09 it 0.56 Nitrogen 11 1.19 f 0.06 15.4 f 1 . 6 5.60 f 0.77 12 4.33 f 0.42 103 it 14 6.69 it 1.49 10 8.48 f 0.68 230 f 40 6.89 f 0.74 Air 10 1.12 f 0.06 16.2 f 1.9 5.39 f 1.06 10 3.96 f 0.20 137 i 19 7.92 f 1.32 10 8.86 f 0.46 182 i 25 7 . 1 2 3 ~1.10 Oxygen 13 1.57 0.05 24.9 f 3.4 5.41 it 1.00 109 16 5.79 f 0.98 11 4.82 f 0.16 12 7.86 f 0.72 244 40 7.25 it 1.54 Argon 11 1.15 f 0.07 57.6 f 7.7 11.5 f 2.1 276 f 48 9.7 f 3.5 13 4.01 f 0.20 10.8 1.4 11 7.63 f 0.61 608 f49 National Bureau of Standards (NBS No. 462). * Number of laser shots. e Background-corrected, arbitrary units, std dev. d Signal-to-background ratio, f std dev.
*
*
* *
*
0
*
Table 11. Effect of Atmosphere and Laser Energy on Laser-Induced Optical Emission from Iron in Magnetic Tapea (Integrated Photoelectric Data) SignaF SlBd Atmosphere N* Laser energy, mJ Vacuum 12 1.39 f 0.10 5.22* 0.31 20.5 it 4.4 12 3.89 f 0.33 9.30 f 0.69 9.20 f 1.36 9.70 it 1.01 5.34 f 1.20 13 8.40 f 1.44 Helium 12 1.28 0.08 8.12 f 1.07 4.99 f 0.94 11 3.84 it 0.25 3.1 4.36 i 1.81 35.8 67.9 it 6.8 4.85 it 2.95 12 8.17 f 1.17 Nitrogen 12 1.13 it 0.09 3.04 it 0.77 3.69 i 1.28 28.5 f 2.2 3.43 0.63 11 3.75 f 0.22 11 7.86 f 0.31 76.1 it 9.0 4.07 f 0.66 5.05 i 1.05 3.53 & 1.05 Air 11 1.16f0.09 10 3.89 f 0.24 42.7 8.3 4.20 f 1.10 10 7.47 it 0.34 86.6 it 12.2 4.39 f 0.84 Oxygen 11 1.06 f 0.05 4.45 + 0.89 3.17 f 1.06 43.3 f 6.2 4.14 zt 0.71 10 3.62 i 0.21 10 7.78 f 0.28 96.8 it 7 . 3 4.40 i 0.52 Argon 10 1 .21 f 0.09 12.9 f 1.9 4.45 f 0.85 10 3.93 f 0.19 105 f 12 4.72 i 0.62 10 7.94 it 0.15 229 f 23 4.42 zt 0.54 a Memorex (MRX-3) (Memorex Corp., Santa Clara, Calif.). * Number of laser shots. c Background-corrected,arbitrary units, i std dev. d Signal-to-background ratio, std dev.
*
*
*
*
*
tions are quite different from those reported in dc arc studies (3, 4). Under the conditions giving the highest S/B values (vacuum, 1.2 mJ laser energy), the actual signals were reduced by a factor of 100 (steel) and 50 (iron oxide) from the largest signals observed (argon, 8 mJ). A typical oscillograph of direct photomultiplier anode pulses is shown in Figure 2. In the signal channel trace, the rapid continuum portion (prepulse), and the longer discrete signal pulse are clearly discernible. The background channel trace shows only the prepulse. Measurements from oscillographs for the steel and iron oxide tape samples are shown in Tables 111 and IV, respectively. Inherent imprecision of measuring the parameters from the oscillographs gives semiquantitative results. 1454
The most striking feature of the oscillographic data was the enhanced amplitude of signal pulses obtained in helium atmospheres, approximately twice those of corresponding pulses in other atmospheres. No systematic differences were observed in signal pulse duration for air, oxygen, and nitrogen, but argon generally supported longer pulses and helium shorter ones. Plasmas produced in vacuum were both shorter and less intense, except at the lowest laser energy. Tables I-IV reveal that signal intensities at 1.2 mJ were less sensitive to changes in atmosphere than at higher laser energies. The enhanced signal and S/B values obtained in argon from iron in steel, relative to iron in iron oxide, indicate a possible effect of metallic us. nonmetallic matrices. The emission of
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
Table III. ElTed of Atmosphere and Laser Energy on Laser-Induced Optical Emission from Iron in NBS Steel Signal height, mA 43.3 59.2 23.3 41.7 37.5 41.7 49.3 176 76.0 67.7 79.0 117 64.2 304 132 132 132 146
Laser energy”, mJ Atmosphere 1.21 f 0.15 Vacuum Helium Nitrogen Air Oxygen Argon 4.00 f 0.35 Vacuum Helium Nitrogen Air Oxygen Argon 8.14 f 0.49 Vacuum Helium Nitrogen Air Oxygen Argon a Means for 12 samples f std dev. . * Band-width at half-height. c Product of signal height and ‘/z BW. Ratio of signal area to prepulse area.
coscillographic Data) Signal Repulse Repulse BW, psecb height, mA ‘/z BW, p s e 9 0.23 17.2 0.03 0.34 52.5 0.03 0.34 24.2 0.03 0.40 60.4 0.05 0.35 43.1 0.03 0.58 60.4 0.03 0.30 58.3 0.04 0.44 243 0.05 0.75 145 0.04 0.90 257 0.06 0.70 182 0.03 1.10 260 0.04 0.28 106 0.05 0.58 425 0.06 0.75 425 0.03 0.90 700 0.05 1.00 480 0.03 1.35 840 0.05
magnesium, present as an impurity in aluminum foil, was measured in different atmospheres (Table V) and exhibited the same trends as iron in steel. The degree of sample vaporization is largely dependent on the amount of laser energy incident to the sample. A plasma generated by a focused laser beam (13-16, 25, 27-30) can absorb laser light strongly; e.g. 80% absorption of incident light has been reported (31). Acceleration of plasma expansion rates due to additional energy absorption has also been observed (25, 27,32). Because of the triggering effect of electrons and ions ejected by a solid target (13,28,29,33),atmosphere breakdown occurs at substantially lower laser energy densities than with omission of the target. Large prepulses should indicate strong “shadowing” of the target by the plasma, and therefore reduced laser energy density and direct sampling efficiency. In competition with shadowing, the hot plasma itself can produce more sampling (34,and material vaporized into the plasma will share in its energy by elastic and inelastic collisions. Since plasma expansion is sensitive to pressure and density (29),expansion against a dense atmosphere could confine the sample vapor and prolong the emission. Helium has a 21.3 eV metastable state while the other atmospheres have metastable states of -12 eV. Ion kinetic energies are -10 eV (25, 35, 36). This energetic helium level
(27) E. Archbald, D.W. Harper, and T. P. Hughes, Brit. J. Appl. Phys., 15, 1321 (1964). (28) V. V. Korobkin, S. L. Mandel’shtam, P. P. Poshkin, A. V. Prokhindeev, A. M. Prokhovrov, N. K. Sukhodrev, and N. Ya Shchlev, JETP, 26.78 (1968). (29) Yu V. Afans’ev.and‘0. N. Krokhin, ibid., 25, 639 (1967). 41,700(1969). (30) E.Piepmeier and H. Malmstadt, ANAL.CHEM., (31) P. Nelson, P. Veyrie, M. Berry, and Y. Durand, Phys. Lert., 13, 226 (1964). (32) H.Weichel and P . V. Avizonis, Appl. Phys. Lett., 9,334 (1966). (33) E. Bernal G., J. F. Ready, and L. P. Levine, ZEEEJ. Quanrum Electron., QE-2, 480 (1966). (34) A. W. Ehler, J. Appl. Phys., 31,4962(1966). (35) E. W.Sucov, J. L. Pack, A. V. Phelps, and A. G.Engelhardt, Phys. Fluids, 10, 2035 (1967). (36) D. W.Koopman, ibid., p 2091.
Signal areac 9.97 20.1 7.93 16.7 13.1 24.2 14.8 77.4 57.0 79.0 55.3 129 18.0 176 98.7 118 131 197
Prepulse areac 0.70 2.25 1.20 2.37 1.67 1.33 1.73 8.27 6.59 11.7 4.85 6.73 4.71 19.6 17.1 31.0 14.1 28.6
SIBd 14.2 8.92 6.61 7.04 7.87 18.2 8.54 9.36 8.65 6.73 11.4 19.1 3.82 8.96 5.76 3.82 9.36 6.91
Figure 2. Typical photomultiplier anode pulses Upper trace is signal channel at 3020-A and lower trace is background channel at 3015 A. Horizontal scale 1 psec/div, vertical scale 4 mA/div. Conditions: iron in NFS steel sample, air atmosphere, 4.0 mJ laser energy should result in increased sample excitation (37). The higher ionization potential could also permit more efficient sampling by the laser beam. The lower density and mass of helium would lead to more rapid plasma expansion. The net result would be a signal with a larger amplitude but shorter duration as shown by the iron-in-helium data Tables I11 and IV. The inverse applies to the iron-in-argon data while the other atmospheres give intermediate results. Although it is difficult to explain the relative invariance of signal intensity with changing laser energy in vacuum (Tables I-IV), a rapid plasma expansion against the vacuum could be responsible. The interactions in laser-generated plasmas are complex, and the data are not sufficient to permit exhaustive or rigorous theoretical interpretation. Magnesium emission from human serum and liver tissue
(37) H. E. Taylor, J. H. Gibson, and R. K. Skogerboe, ANAL. CHEM., 42, 1569 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
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~
~~
Table IV. Effect of Atmosphere and Laser Energy on Laser-Induced Optical Emission from Iron in Memorex Magnetic Tape (OdOgraPhlC Data) Signal Signal Repulse Prepulse Signal Repulse Laser energy," ml Atmosphere height, mA '12 BW, PW' height, mA '/z BW,pwb areac areac SIBd 1.21 f 0.15 Vacuum 37.5 0.22 4.97 8.25 0.75 11.0 0.04 Helium 60.6 0.23 35.0 0.03 11 .o 13.9 1.27 Nitrogen 29.2 0.20 43.1 1.12 5.19 5.83 0.03 Air 29.2 0.20 31.1 1.03 5.65 5.83 0.04 Oxygen 41.7 0.30 34.5 12.5 0.03 1.67 7.50 Argon 29.2 0.28 25.9 8.17 0.04 1.08 7.54 4.00 f 0.35 Vacuum 56.3 0.26 37.9 0.04 14.6 2.25 6.50 Helium 199 0.44 87.7 164 0.04 4.22 20.8 Nitrogen 91.4 0.50 195 0.03 10.4 45.7 4.39 Air 102 0.56 242 0.04 57.3 6.43 8.91 Oxygen 102 0.60 274 61.4 8.80 6.98 0.02 Argon 102 0.56 153 0.06 6.43 8.91 57.3 8.14 f 0.49 Vacuum 64.2 0.20 57.6 0.04 12.8 3.25 3.95 Helium 235 0.54 231 0.05 127 8.80 14.4 Nitrogen 133 0.70 623 0.03 93.0 4.98 18.7 Air 150 0.70 276 0.03 5.87 105 17.8 Oxygen 99.7 85.7 0.86 467 0.04 15.0 5.70 Argon 188 1 .oo 188 27.1 6.94 646 0.04 Means for 12 samples f std dev. ' b Band-width at half-height. Product of signal height and BW. Ratio of sig.il area to prepulse area, Table V. Effect of Atmosphere on Laser-Induced. Optical Emission from Magnesium in Aluminum Foils and Correlation with Iron Samples (Integrated Photoelectric h t a ) Atmosphere Signalc SIBd Rl' R*' Vacuum 51 f 12 1.1 f 0.3 7.3 f 2.2 8 . 4 f 2.6 Air 720 f 140 1.3 f 0.3 6.1 f 1.7 3.2 f 1.0 Argon 1070 f 180 1.3 f 0.3 7.5 f 3.0 3.6 f 0.9 Laser energy = 3.6 f 0.2 mJ. * Alcoa Wrap, Aluminum Co. of America, New Kensington, Pa. c Mean for 10 samples f std dev. d Signal-to-background ratio, f std dev. e R1 = (SIB) Fe Steel/(S/B) Mg, i std dev. f RZ= (S/B) Fe Tape/(S/B)Mg, f std dev. Table VI. Effect of Atmosphere on Laser-Inducedo Optical Emission from Magnesium in Serum and Liver (Integrated Photoelectric Data) Signal SIBd Atmosphere Serumb Liverc Serum Liver Vacuum 4.5 f 1.3 1.6& 0.3 2.3 f 1.0 5.7 f 1.9 Air 20.33~ 2.8 10.5 f 1.6 1.8 i 0.5 3.7f 1.0 Argon 34.1f7.6 36.2f7.9 1.1zk0.4 4 . 2 f 1 . 1 Laser energy = 3.6 i 0.2ml. b Human blood serum, air dried a n 1 samples, (approx. 400 p dia.) mean for 10 samples f std dev. c Formalin-fixed, paraffin-embedded, and deparafinized tissue sections, 5 ,u thick. d Signal-to-background ratio, f std dev. 0
1456
samples (Table VI) was measured for comparison with magnesium in a metallic matrix, aluminum foil. The data show that in the different atmospheres, the serum and aluminum foil exhibited a similar trend of S/B, while liver and aluminum matrices did not. A possible matrix effect (2) due to differences in sample structure and composition between liver and serum may be indicated by the significantly lower background intensities observed in liver. At 1.2-mJ laser energy, significant enhancement of S/Bwas obtained in a 5-Torr vacuum with little loss of signal intensity. At higher laser energies, S/B values behaved irregularly with atmosphere variation. Argon gave appreciably higher signals while vacuum conditions yielded reduced signals. The results suggest that it may be advantageous in certain cases to select a n appropriate atmosphere in order to optimize S/B or signal intensity, but in general the atmosphere composition did not appear to affect the S/B sufficiently to warrant changing to atmospheres other than air in time-integrated laser microprobe emission spectrometry.
RECEIVED for review March 29, 1971. Accepted June 7, 1971. Studies in Histochemistry, No. CXIV. Supported by Grants GM 16181, GM 09227, HE 06716, and 5K6AM18513 (to D. G,), from the National Institutes of Health, U. S. Public Health Service.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971
