relative to 10 Torr strongly suggest the use of Ar at 50 Torr for analytical work. Table I1 compares absolute detection limits for 18 elements in Ar a t 50 Torr with previously reported values obtained in He at 100 Torr ( I ) . Since these previous values were obtained using 282 J a t 22.5 pF, new values were obtained using these same conditions to provide a more direct comparison. These detection limits are defined as the minimum amount of the element required to produce a line intensity equal to three times the intensity equivalent of the root-mean-square noise on the microdensitometer trace in a nearby wavelength region of continuum background. Of these 18 elements, only Bi obtained a poorer detection limit in Ar. The improvement for the other 17 elements ranged from a factor of 1.5 for Sn to a factor of 13 for Mn and Mg.
CONCLUSIONS Since the pressure range which produced the lowest detection limits also is the range where the pressure dependencies of the A1 foil matrix line radiation and of the radiation from the Ni and Cd samples are quite different, the use of the A1 foil as a built-in internal reference is not suggested despite its convenience. This is consistent with reproducibility data presented in Ref. 1where the use of the A1 foil as an internal reference provided no significant improvement in precision; while an added internal reference was, in most cases, very effective. The detection limits presented here are not the lowest attainable values but reflect realistic values obtained on a slow photographic emulsion a t an ambient gas pressure and explosion energy which resulted in reproducibility consistent with previously reported values (I).Suggested operating parameters include Ar support gas a t 50 Torr and a 5-kV discharge a t 22.5 KF. Since the exploding foil plasma must be very heterogeneous in both time and space, time- and spacial-resolution techniques should obtain additional improvements in analysis line to background ratios. This approach has been quite successful with laser microprobe excitation (I1,12).Studies of this type are in progress. Based on quantum efficiency alone, photoelectric detection
should result in a considerable reduction in detection limits. An increase in precision also should be realized with photoelectric detection. However, the shot-to-shot fluctuation in continuum background intensity will require a background correction for each explosion. A recently described dualchannel monochromator attachment (13) in conjunction with a gated dual-channel integrator will be used in future work to facilitate this background correction. The greater sensitivity of photoelectric detection also will permit studies of analysis line to background intensity ratios a t very low ambient pressures and with very thin foils where continuum background intensity is below detection from single explosions record on photographic emulsions ( I ) . It is anticipated that the lower detection limits and greater speed and convenience which should be obtained with photoelectric detection coupled with the simplicity and low cost of exploding foil excitation may provide a useful analytical system for the determination of trace metals in micro solution samples.
LITERATURE CITED (1) C. S. Ling and R. D. Sacks, Anal. Chern., 47, 2074 (1975). (2) R. D. Sacks and J. A. Holcombe, Appl. Spectrosc., 28, 518 (1974). (3) J. A. Holcornbe, D. W. Brinkman, and R. D.Sacks, Anal. Chern., 47,441 (1975). (4) J. A. Holcombe, Ph.D. Thesis, Department of Chemistry, University of Michigan, 1974. (5) W. G. Chace, R. L. Morgan, and K. R. Saari, in "Exploding Wires", W. G. Chace and H. K. Moore, Ed., Plenum, New York, 1959, Vol. 1, p 59. 16\ J. P. Walters and H.V. Malmstadt. Anal. Chem.. 37. 1484 (1965). i7j D. Cobine, "Gaseous Conductors", Dover, New York, 1958. (8) F. D. Bennett, Phys. Fluids, 1, 515 (1958). (9) F. D. Bennett, in "Exploding Wires", W. G. Chace and H. K. Moore, Ed., Plenum, New York, 1959, Vol. 1, p 211. (IO) J. A. Holcornbe and R. D. Sacks, Spectrochirn. Acta, Part B, 28, 451 (1973). (1 1) W. J. Treytl, J. B. Orenberg, K. W. Marich, and D. Glick, Appl. Spectrosc., 25,376 (1971). (12) W. J. Treytl K. W. Marich, and D. Glick, Anal. Chern., 47, 1275 (1975). (13) D. W. Brinkrnan and R. D. Sacks, Anal. Chern., 47, 1723 (1975).
2.
RECEIVEDfor review February 20,1976. Accepted May 19, 1976. The authors acknowledge support of this study by the National Science Foundation through grant number MPS72-05099.
Laser Microprobe Spectrometry of Single-Crystal Metals and Alloys R. Kirchheim," U. Magorny, K. Maser, and G.T61g Max-Planck-lnstitut fur Metallforschung, Labor fur Reinststoffe a m Insthut fur Werkstoffwissenschafn, Stuttgart, Germany
Single-crystal metals and alloys of high purity and different orlentation were used as specimens for laser microprobe emission spectrometry, in order to study the matrix effects of this analytical tool. Low-Index planes were prepared from the fcc metals Au, Cu and AI; the bcc metals Fe and Nb; and the hcp metal CO. Density of characteristic lines In photographically recorded spectrograms and reproducibility of lines revealed a strong dependence on crystal orientation; as a rule, density of lines increases with the density of atoms within a crystal plane. Physical reasons for this anisotropic effect and its Influence on quantitative analysis are discussed. Theoretical conslderations were checked by measuring momentum transfer durlng the evaporation process, and by performing quantitative analysis of single-crystal Cu-AI alloys. The an-
isotropic effect is canceled in these alloys by using a matrix line as an internal standard. Over an AI concentration range from the detection limit of about 300 ppm up to 7 wt %, !he calibration curve is a straight line, and the AI content of the samples could be determined with a precision better than 10%.
The laser microprobe has been used for sampling and sometimes also for excitation, in optical emission spectroscopy and mass spectroscopy (1-3). Ruby or neodymium laserlight can be focused down to a spot size of about 5-10 w, which is the lower limit of lateral resolution in local analysis (2, 4 ) . By varying the laser energy, the amount of evaporated material,
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ruby
Q-switch
r
al/,sh
lamp oulput mirror
energy- meler
Figure 1. Schematic dlagram of the apparatus
and therefore the depth of crater formed, can be varied reproducibly with a resolution of about 5 p ( 5 , 6 ) .Though these values of depth resolution and lateral resolution are somewhat less satisfactory, than these obtained with an electron or ion microprobe (7-10),laser sampling is still valuable, especially in emission spectroscopy,because of the following advantages. No special preparation of the specimen is necessary, as all materials in various shapes can be evaporated by the high power densities delivered within the focal spot. With few exceptions (11--13), analyses were conducted in air a t normal pressure. In this study, single-oriented crystals of AI, Au, Co, Cu, Fe, Nb, and of Cu-A1alloys were prepared, and the effect of orientation on the evaporation process and on emission intensities is studied. All parameters of the Q-switched ruby laser and of the auxiliary spark excitation were kept constant in order to determine only the influence of crystal orientation. This investigation is of basic importance to laser excitation of metallic samples, because such samples will react a s single crystals if the focal spot is smaller than the grain size of the polycrystalline material; and for smaller grains the metallic samples usually have a texture, where special orientations are preferred. In this way, analytical results may depend on the grain size, as has been found for Zr and U alloys (14). The advantages of using multiple pulse laser operation in reducing the effect of differing sample reflectivities is discussed, and the influence of reflectivity on crater volumes and emission intensities is considered.
EXPERIMENTAL Apparatus. The Jenoptic model LMA 1laser probe was used with a Q-switched ruby rod, an auxiliary spark source, and a Q 24 spectrograph (2). Figure 1shows a schematicdiagram of the LMA 1with an additional control unit for the laser energy. By deflecting a fraction of the laser beam with a quartz plate to a calorimeter, or to a photodiode connected to a Tektronix 585 A oscilloscope, the number of spikes or total energy of one laser shot could be measured. Maximum output energy was about 200 m J and its reproducibility could be reduced to &6%by improving the cooling system of the laser housing. For spark excitation, the electrodes were spaced 1 mm apart and centered 2 mm above the sampling position. All subsequent experiments were done with the spark circuit adjusted for 3000 V, 60 fiH, and 2 fiF. The slit width of the Q 24 spectrograph was 15 y and the slit length 2 mm. Kodak 103-0photoplates were used for the photographic detection of the spectral intensities, where relative intensity values were obtained from density measurements with a Steinheil Optonic microphotometer and a density vs. intensity calibration curve. Sample Preparation. Single crystals were prepared from the high-purity metals (99.999%Au, Al, Cu, Nb; 99.99%Fe; 99.9% Co) by the zone melting or by the Bridgman technique and low-index planes were cut out of these crystals by electroerosion. Then the surface of the specimen was electrolytically polished. Cu-A1alloys were melted and homogeneody mixed in an induction furnace under high vacuum conditions. The aluminum content was smaller than 7 wt % (-15 at. %) and, according to the phase diagram, aluminum atoms are in solid 1506
0
craler deplh Cpm?
Figure 2. Typical plot of the cross section of a crater vs. its depth
solution in the single-crystal copper matrix. Comparison of the quantity of A1 before melting with the A1 concentration of the final specimen, determined by atomic absorption, reveals slight evaporation losses of the more volatile aluminum at higher concentrations. These differences are, however, less than the analytical error introduced by the laser microprobe. Crater Volume. The crater volume produced by a laser shot can be determined approximately by measuring crater diameter and depth, and computing the volume on the assumption of a conical shape. When more precise determination was necessary, definite layers of the surrounding material were ground away, and thus the area of the crater cross section was obtained as a function of depth, and a graphical integration of this function yields the crater volume. A typical result is shown in Figure 2 for a copper crystal. A similar plot of the mean radius calculated from the area reveals the nearly conical shape of the crater. Torsion Balance. A thin quartz-thread torsion balance was constructed, one of the scale-beams carrying a small sample (200 mg). The deflection caused by the ejected material after a laser shot was about 5 mm and could be observed by a microscope. From the momentum transfer and the crater volume, an average velocity of the ejected material was calculated.
RESULTS AND DISCUSSION Pure Metals. Table I is a compilation of experimental data for all the pure metals, including crystal structure, spectral line used to determine relative intensities, and Miller indices of crystal planes chosen as sample surfaces. Ten laser shots were fired on different places on one crystal plane, from which the average value of crater volume and photographic density, with corresponding standard deviations, were calculated. As stated above, reproducibility of the output energy is about 6%, while crater volumes fluctuate considerably. In a different set of experiments, crater volumes were measured for copper as described in Figure 2 and compared with the volumes calculated from their diameter and their depth assuming a conical shape; the actual crater volume is about 40% smaller than the calculated one because the actual shape is not exactly that of a cone. Standard deviations are better for the exact volumes, as experimental errors in determination of diameter and depth through a microscope were reduced. Nevertheless, mean values of volume for different crystal orientations are the same for each metal within experimental errors. Thus, differences in the average emission intensities cannot be attributed to different amounts of evaporated material; it must be the spatial distribution of atoms and their velocities in the vapor plume, which is different for each crystal plane and which gives rise to different excitation conditions during the spark discharge. In this way,
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
Table I. Relative Intensities of Characteristic Lines and Crater Volumes for Various Single Crystal Metals with Different Orientation Metal (line)
A1 (fcc) 2669.1 A
Au (fcc) 2688.2 A c u (fcc) 2824 A Fe (bcc) 3059.1 A
Nb (bcc) 2661.8 A
co (hcp) 2569.7 A
Relative line intensity
Crater volume
Itel std dev, % 1 0 5 prn3
Table 11. Deflection of the Torsional Balance for Cu with Different Crystal Orientations Plane
Itel std dev, V,
...
16 17 18 16 0.73 0.77
11
...
4 7 9 4 6 23
0.233 0.597 0.522 0.557 0.449 0.545 0.621 0.579 0.608 0.444 0.521 0.576 0.452 1.120 0.849
25 4 7 11 9 4 4 4 6 7 2 5 4 6 8
0.87 3.0 3.3 3.7 4.0 2.3 2.7 2.3 1.7 3.3 2.7 2.7 3.3 1.4 1 .o
29 16 12 32 20 33 22 31 21 28 27 30 30 8 22
0.750 0.672 0.686 0.764 0.602 0.291
...
16 20 19 19 28
(111) (110)
(100) (112)
Deflection, 9
Energy, E(J)
PIE
15.8 11.6 16.0 10.6
0.25 0.27 0.27 0.27
63 43 59 39
...
a plume with a slender shape will supply more atoms in the center of the spark, increasing the emission intensity. The arguments above are based on equal crater volumes, which were measured after the experiments. There are also theoretical arguments demonstrating that the evaporated mass from a cubic metal should be the same for all orientations. This is because all physical quantities governing the evaporation process (15-1 7)-such as heat of sublimation, reflectivity, and thermal conductivity-are independent of crystal orientation. As is proved in solid state physics, these quantities are scalars or tensors of second rank, which are isotropic in cubic crystals. The elasticity coefficients of a cubic metal, however, are anisotropic, and they influence the velocity of the evaporating atoms both in direction and magnitude, thus explaining different shapes of vapor plumes. Influence of the corresponding kinetic energies on the total energy balance is negligible a t applied laser power densities of los W/cm2 (17) and, therefore, the energy delivered for evaporating material is determined only by the isotropic quantities specified above. The failure to explain the density results of Table I by different crater volumes is discussed in some length, because it is the expected correlation. On a second look a t Table I, one can discover a relationship between the highest photographic density values, and the crystal plane (1 11) for fcc metals, which has the highest packing density olatoms. This relation still holds for the bcc metals and for the hcp metal Co, where the atoms are closer packed in the (110), (loo),and the (0001) planes. Co may be an exception, because here a correlation exists between crater volume and line intensity and, for this hexagonal metal, the reflectivity and the thermal conductivity are no longer isotropic. Table I also shows that a correlation exists between the reproducibilities of the crater volumes and those of the photographic densities, where as a rule the close-packed planes give the lowest standard deviations for both quantities. During crater formation, liquid droplets or solid particles are ejected. Their influence on momentum transfer was not considered in the foregoing discussion, because calculations of the energy balance and measurements of crater volume (15, 17) revealed that most.of the material is removed by evaporation. On the other hand, little is known
b
a
n C
Figure 3. Time-integrated photographs of laser plumes from (111) planes of (a) AI, (b)Au, and (c) Nb showing different shapes of plasmas caused by the different atomic masses
about the parameters governing the ejection of material in liquid or solid state. Momentum Transfer. In order to check the assumption' of different vapor plumes, measurements of the momentum transfer were made with the torsion balance. Time-integrated photographs of the ejected and expanding plasma had not been able to reveal any remarkable differences between the crystal orientations. In Table 11, the average deflection of the balance is shown, calculated from 20 values for each crystal orientation of copper. The deflection rp is proportional to the momentum mo, where ii is the component of the velocity perpendicular to the surface, averaged over all evaporating aloms, and m is the mass corresponding to the crater formed. With the reasonable assumption that m is proportional to the laser output energy (15),the following proportional relation is valid P
Eao High values of p/f< or 0, respectively, are caused by slender plumes with preferred velocity directions perpendicular to the surface and, according to Table 11,they correlate well with the high photographic densities and high packing densities of the crystal planes, providing good agreement with the interpretations given above. The relative standard deviation ofthe PIE values in Table I1 is about 20% and, on applying a t-test, only the results for the (111) and (100)-plane are proved to be statistically different from those ofthe (110) and (112)-plane. With the sensitivity of the balance, a mean velocity of about 3.10,' cm/s was calculated, which is in the order of magnitude by other investigators a t comparable power densities (18,19). From the results of laser microprobe analysis of pure metals, it can be concluded that the crystallographic orientation has an efl'ect on velocity of the evaporating atoms or ions, where velocities perpendicular to the surface of close-packed crystal planes are preferred. During collisions with the molecules of the air, the atoms will lose their memory of the initial velocities, this depending more or less upon the mass of the atoms. 'rhis may be the reason that for the heavier Au atoms differences in the emission intensities at; the spark dischar,ne are more pronounced than for the A1 atoms. This is also in agreement with the shapes of the plumes shown in Figure 3, where the velocity direction perpendicular to the surface is more pronounced for Au and Nb than for Al, though this may
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
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Table 111. Analytical Results and Laser Parameters for Single Crystal Cu-A1 Alloys A1 concentra t i o n [w/o A l l Plane
7.0 3.4 1.67 0.84 0.55 0.20 0.07
(100 (110 (111 (100 (110 (111 (100 (110 (111 (100 (110 (111 (100 (110 (111 (100 (110 (111 (100 (110 (111
Crater vol-
No. of
Std dev
Energy J
ume,
AY
1osp3
spikes
1.364 1.229 1.308 1.062 0.997 1.073 0.862 0.824 0.827 0.562 0.516 0.600 0.453 0.423 0.415 0.156 0.107 0.114 -0.301 -0.185 -0.219
0.041 0.068 0.030 0.028 0.018 0.034 0.028 0.051 0.051 0.037 0.041 0.020 0.036 0.021 0.029 0.036 0.018 0.043 0.038 0.069 0.039
0.31 0.20 0.33 0.23 0.20 0.35 0.18 0.11 0.29 0.25 0.22 0.31 0.82 0.15 0.32 0.69 0.30 0.30 0.33 0.44 0.35
2.1 1.6 2.5 3.2 1.5 2.0 1.8 1.0 1.1 3.7 1.4 1.2 2.0 1.2 1.0 2.4 0.9 1.0 1.7 2.0 1.9
10-1 4 15-18 20-22 30 16-17 20-22 25 13-14 9-11 30 18 12-14
...
13-16 11-14 18-25 15 11-14 14 14-16 16
be also due to an easier excitation of Au and Nb atoms by the subsequent laser spikes. Cu-AI Alloys. The anisotropic effect of the laser microprobe, described here for the first time for metals, is wellknown with other analytical tools like Secondary Ion Mass Spectroscopy, ion microprobe, or in a lesser way with the electron microprobe. In quantitative analysis, it will usually be overcome by relating all signals to a matrix signal. The ratios of these signals are less dependent on, or independent of, crystal orientation. For Cu-A1 alloys, the intensity ratio of the A1 I 3082 A line to the Cu I 3073 8,line was chosen and the logarithm of this intensity ratio AY is shown in Table I11 for seven different compositions c and three orientations. As can be seen, the anisotropic effect vanishes or, strictly speaking, is smaller than the experimental error. If these results are plotted in an analytical curve, a straight line is obtained over the concentration range of two orders of magnitude. Though the chosen A1 line is a sensitive line with the property of selfabsorption, no significant deviations from the straight line occur at higher A1 concentrations and no self-reversal of the line was discovered. In order to calculate the detection limit of A1 in a copper matrix, the standard deviation of the density produced by background fluctuations was measured and enlarged by the factor 3 d , giving a value which corresponds to the lowest detectable density of the line (20). Using the density vs. intensity curve and the analytical curve, this value was converted to the lowest amount of A1 detectable. The result for this detection limit is 3.10-9 g Al, which corresponds to a concentration of 300 to 600 ppm, depending on the crater volume. This value appeared to be high, because one of the most sensitive A1 lines was selected, but it agrees excellently with values given in the literature for copper and steel matrices
1508
(21, 22). However, it could be decreased remarkably by superimposing the intensity of 5 to 10 laser shots on one spectrogram. The relative standard deviation of the A1 determination is about 1096, where different conditions during the spark discharge and errors in densitometric measurements mostly contribute to this deviation. It can be shown that the isotropic behavior of the intensity ratio in Cu-A1 alloys is consistent with the explanation given for the anisotropy of the pure metals. If one assumes that the composition of the solid alloy is maintained in the vapor plume, then the intensity ratio of an A1 to a Cu line is independent of the total number of excited atoms. Thus, the different shapes of the laser plume obtained from differently oriented surfaces do not influence the intensity ratio, but do give rise to a change in the intensities of the A1 and Cu line by the same factor. Changes of the Reflectivity. For analytical purposes, single-pulse operation of the Q-switched laser has some advantages over multiple-pulse operation (5,23).One drawback, however, is the strong influence of the reflectivity or the surface conditions upon the crater volume (6,15,17).In multiple-pulse operation, the first spike evaporates or melts a surface layer and the following spikes are no longer influenced by the initial conditions. This was shown with one sample of the Cu-A1 alloys which was covered with carbon layers of various thickness, produced by discharging the spark source several times before the laser shot was fired.. Neither the spectral intensities nor the crater volume depend within experimental error upon the number of sparks or the thickness of the carbon layer, respectively, though the reflectivity changed drastically by varying the number of sparks from 0 to 32.
LITERATURE CITED (1) G. H. Morrison, “Trace Analysis”, Wiley-lnterscience Publishers, New York,
1965.p 470. (2)H. Moenke and L. Moenke-Blankenburg, “Laser Micro-Spectrochemical Analysis”, translated by R. Auerbach, Adam Hilger, London, 1973. (3)M. Margoshes and B. F. Scribner, Anal. Chem., 40, 223 R (1968). (4)M. D. Adams and S. C. Tong, Anal. Chem., 40, 1762 (1968). (5) S.D.Rasberry, B. F. Scribner, and M. Margoshes, Appl. Opt, 6, 87 (1967). (6)C.D. Allemand, Spectrochim. Acta, Parts, 27, 185 (1972). (7)L. S.Birks, “Electron Probe Microanalysis”, 26 ed., Wiley-lnterscience Publishers, New York, 1971. (8) H. Liebl, Anal. Chem., 46, 22A (1974). (9) C. A. Anderson, ”Microprobe Analysis”, Wiley-lnterscience Publishers, New York, 1973. (IO) S. J. B. Reed, “Electron Microprobe Analysis”, Cambridge University Press, Cambridge, 1975. (11) E. H. Piepmeier and D. E. Osten, Appi. Spectrosc., 25, 642 (1971). (12)W. J. Treytl, K. W. Marich, J. B. Orenberg, P. W. Carr, D. C. Miller, and D. Glick, Anal. Chem., 43, 1452 (1971). (13)H. J. Stupp and Th. Overhoff, Spectrochim.Acta, Parts, 30, 77 (1975). (14)E. Cerrai and R. Trucco, Energ. Nucl. (Milan), 15, 581 (1968). (15) H. Klocke, Spectrochim.Acta, Part 6, 24, 263 (1969). (16)J. F. Ready, d. Appl. Phys., 36, 462 (1965). (17)M. K. Chun and K. Rose, J. Appl. fhys., 41, 614 (1970). (18)S.Asimov, A. M. Bonch-Bruevich, M. A. El’yashevich, Ya. A. Emas, N. A. Parlenko, and G. S. Romanov, Sov. fhys.-Tech. fhys., 11, 945 (1967). (19)R. H. Scottand A. Strasheim, Spectrochim. Acta Parts, 25, 311 (1970) (20)H. Kaiser, 2.Anal. Chem., 216,80 (1964). (21)L. Moenke-Blankenburg, J. Mohr, and W. Quillfeldt, Mikrochim. Acta, Suppl.,
4, 229 (1970). (22)K. G.Snetsinger and K. Keil, Am. Mineral., 52, 1842 (1967). (23)E. H. Piepmeier and H. V. Malmstadt, Anal. Chem., 41, 700 (1969).
RECEIVEDfor review March 9,1976. Accepted May 11,1976.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
