system currently under test, shows promise of correcting this problem.
termination of the best conditions should be made for each specific application.
CONCLUSIONS
LITERATURE C I T E D
Advantage can be taken of the non-homogeneity of the optical emission from laser-induced plasmas to optimize analytical sensitivity. Greater than twofold increases in response, relative to the on-axis optical coupling conditions, were obtained from gelatin and liver sections sampled by a single-spike Q-spoiled laser delivering 35 mJ, by employing spatial differentiation. A single optimal position could be found common to all elements tested. Since time-differentiation was employed in all measurements, these increases are probably conservative relative to a non-time-differentiated system, although measurements with the latter were not made. The optical noise in the emission signal of the plasma constitutes primarily a continuum radiation. Both qualitative and quantitative variations in the optical emission of the plasma have been found to be the principal factors affecting signal response and detection limits. As a result, even though the photomultiplier tubes were about 1000 times more sensitive than the photographic film used, the response recorded photoelectrically was less than 60 times that of the photographic-densitometric method. This increase resulted mostly from use of the time-differentiated photoelectric recording. Optimal effective emission from specific regions in the plasma will vary with laser energy, atmospheric conditions, and probably the nature of the sample. Consequently, de-
(1) K. W. Marich, P. W. Carr, W. J. Treytl, and D. Glick. Anal. Chem., 42, 1775 (1970). (2) W. J. Treytl, K. W. Marich, J. B. Orenberg, P. W. Carr. D. C. Miller, and D. Glick, Anal. Chem., 43, 1452(1971). (3) W. J. Treytl, J. B. Orenberg, K. W. Marich, and D. Glick, Appl. Spectrosc., 25, 376 (1971). (4) W. J. Treytl, J. 6. Orenberg, K. W. Marich, A. J. Saffir, and D. Glick, Anal. Chem., 44, 1903 (1972). (5) A. J. Saffir, K. W. Marich, J. B. Orenberg, and W. J. Treytl. Appi. Spectrosc., 26, 469 (1972). (6) A. W. Ehlen, J. Appi. Phys., 37, 4962 (1966). (7) A. J. Alcock, C. Demichelis, K. Hamal, and B. Tozer. Phys. Rev. Lett., 20, 1095 (1968). (8) N. G. Basov, 0.N/ Krokhin, and G. V. Sklizkov, Appi. Opt., 6, 1814 (1967). (9) E. Archbald, D. W. Harper, and T. P. Hughes, Brit. J. Appi. Phys., 15, 1321 (1964). (10) D. D. Burgess, B. C. Fawcett, N. J. Peacock, Proc. Phys. Soc., 92, 805 (1967). (11) E. H. Piepmeir and H. V. Malmstadt, Anal. Chem., 41, 700 (1969). (12) E. H. Piepmeir and D. E. Osten, Appi. Spectrosc., 25, 642 (1971). (13) C. D. Allemand, Spectrochim. Acta, Part E, 27, 185 (1972). (14) R. H. Scott, and A. Strasheim, Spectrochim. Acta, Part E, 25, 311 (1970). (15) K. W. Marich. W. J. Treytl, J. G. Hawley, N. A. Peppers, R. E. Myers, and D. Glick, J. Phys. E,7, 830 (1974). (16) E. W. Sucov, J. L. Pack, A. V. Phelps, and A. G. Engelhardt, Phys. Fluids, I O , 2035 (1967).
RECEIVEDfor review February 4, 1974. Accepted April 14, 1975. Supported by Research Grant GM 16181 and Research Career Award 5K6AM18,513 (to D.G.) from the National Institutes of Health, U S . Public Health Service and the Stanford Research Institute.
Exploding Wires as an Intense Ultraviolet Continuum Excitation Source with Preliminary Application to Atomic Fluorescence Spectrometry D. W. Brinkman and R. D. Sacks’ Department of Chemistry, University of Michigan, Ann Arbor, MI 48 104
Preliminary studies are presented which show that thin metallic wires exploded by capacitive discharge can produce very intense continuum radiation extending far into the ultraviolet spectral region. A dense, high dielectric strength ambient atmosphere yields the most intense and reproducible continuum emission. The irradiance of this emission increases linearly with the energy initially stored on the capacitor bank. Although the wire material seems to have little effect, decreasing the wire diameter from 0.25 mm to 0.08 mm yields increasing irradiance. The radiation from a 54-mm long, 0.08-mm diameter Chromel-A wire exploded with 720 J in argon at 1 atm is both intense and reproducible, with a YO relative standard deviation from shot to shot of 5.4%. At 220 nm, the peak irradiance of this continuum is more than five orders of magnitude greater than that obtained from a 1600 W Xe arc lamp. This controlled, intense continuum radiation source then is used in conjunction with a premixed nitrous oxide-acetylene flame, using a watercooled, capillary tube burner, to demonstrate the potential application of this system for atomic fluorescence spec-
’ Author to whom correspondence should be directed.
trometry. Analytical curves are presented for Mn, Co, and Cd, along with detection limits for these elements in the low ppm range.
Since the first analytical applications of atomic fluorescence were proposed ( I ) and demonstrated ( 2 ) barely over a decade ago, the interest shown by the considerable amount of work in the area is an indication of the method’s potential. With the introduction of high intensity thermostated electrodeless discharge tubes and tunable dye lasers, very low detection limits have been obtained for most elements normally analyzed by atomic spectrometry ( 3 ) . However, various deficiencies have proved a hindrance to any general application and practical use of the method. Both of the above sources are relatively expensive and rather complicated to operate, while applicable to only one element a t a time. In order to do multi-element analyses simply, an intense continuum source is needed whose emission remains high far into the ultraviolet spectral region. Xenon arc lamps are useable down to about 250 nm, although they are not always intense enough for optimum application, and their intensity falls off rapidly below this region ( 4 ) . ANALYTICALCHEMISTRY, VOL. 47, NO. 8 , JULY 1975
1279
I t has been reported that thin wires exploded by a capacitive discharge, under appropriate conditions, emit intense continuum radiation extending well into the ultraviolet region (5-10). Up to this time, research in exploding conductors has been the nearly exclusive property of physicists and engineers. The first known study using exploding wires was reported by Nairne in 1773 ( 1 1 ) and little additional work was reported until J. A. Anderson and S. Smith’s studies in the 1920’s (12-14). Since 1959, four conferences have been held on the phenomenon, resulting in four books which essentially delineate the state of the art a t this time (15-18). Applications t o flash photolysis (5, 19) and chemical synthesis (20) are among the limited applications in chemistry to date. The specific objective of this research was the development and characterization of a convenient, low cost, very high intensity ultraviolet continuum excitation source for analytical applications such as atomic fluorescence and condensed-phase molecular luminescence spectrometry.
THEORY The explosion, and resulting radiation, is the product of rapid energy dissipation in a thin wire. The energy is stored on large capacitors until an air gap in series with the capacitors and the wire is broken down with a spark, delivering the energy into the system. By making the wire the major source of resistance in the circuit, most of the energy will be dissipated a t this point. Depending on many parameters, the spectral emission will be predominantly either a line spectrum or a continuum. A detailed discussion of the theory and mechanisms of wire explosions has been presented recently by Sacks and Holcombe (21). Line emission with low continuum background is observed with explosions conducted in low density surrounding atmospheres of low dielectric strength. This is seen as a transfer of current conduction from the vaporizing wire to the surrounding gas and is referred to as a peripheral restrike. The line emission with low background from peripheral explosions has been shown to be a viable analytical technique by Holcombe and Sacks (22), using a process in which the sample was electroplated onto a silver wire before the wire was exploded. Working in low pressure helium, they were able to emphasize surface vaporization and line emission of the analyte, using the silver emission as an internal standard. The mechanism found to produce an intense continuum is referred to as an axial restrike. When the wire material vaporizes, temporarily becoming a dielectric, current conduction is transferred to the metal vapor near the inner surface of the expanding cylinder of vapor. This mechanism, which produces a highly excited, high density metal vapor plasma, occurs a t ambient pressures of around 1 atm and above and is most pronounced in gases having high dielectric strength.
EXPERIMENTAL Apparatus. A schematic of the instrumentation is shown in Figure 1. The single sweep of the Tektronix 547 oscilloscope activates the SCR trigger which then breaks down the air gap, discharging the capacitor bank through the wire. The radiation from the explosions then passes through the window W and is collimated by lens L1. The folding mirror M is not in place a t this time. The collimated light is focused by plano-convex lens L2 onto the adjustable slit aperture assembly S, and the image of S is directed by plano-convex lens L3 onto the flame so that the entire width of the flame is illuminated. This lens-aperture-lens arrangement eliminates many of the aberrations and, in conjunction with considerable baffling, reduces the scattered light reaching the detection system by reducing the radiation incident on the burner body. The resulting fluorescence radiation is collected by plano-convex lens L4 and passed into the monochromator (GCA McPherson, Model EU1280
ANALYTICAL CHEMISTRY, VOL. 47, NO.
8. JULY 1975
I
: m
I
Figure 1. Apparatus block diagram (L) Plano-convex lenses, (M) Folding mirror, (S) Adjustable slit aperture, and (W) Quartz disk window. The folding mirror is in place only when the Xe arc is
in operation
700). The exit slit of the monochromator is fitted with an RCA 1P28 multiplier phototube biased a t -900 V. The output across a 1-MR load resistor is monitored by a Tektronix Type 1A7A differential plug-in unit on the oscilloscope, using photographic recording of the trace. Both peak height and peak area are measured. A modification of the instrumentation shown in Figure 1 was used to allow direct observation of the explosion. Lenses Lz and L3 along with aperture S were removed, and the folding mirror M was placed in the burner’s position to reflect radiation directly into the monochromator. In some of these studies, a medium quartz prism spectrograph (Adam Hilger, Ltd.) was used with Kodak SA1 photographic plates. These plates were developed in Kodak D-19 for five minutes and microdensitometer traces taken using a JoyceLoebl Mark IIIB microdensitometer. The densities obtained were converted to relative intensities using calibration and density-tointensity computer programs reported previously ( 2 5 ) . This arrangement allowed detailed study of the qualitative as well as quantitative aspects of the wire radiation. Reference Sources. The xenon arc shown in Figure 1 is a 1600 W source (Christie Corp., Model CXL1600) used for fluorescence optimization in relation to burner parameters. The radiation from the arc was collimated by lens Lj, mechanically chopped, and the resulting fluorescence monitored with a lock-in amplifier (Princeton Applied Research, Model 121). The reference signal for the lock-in was obtained by placing a photodiode (EG&G, 040A) just to the right of the flame in Figure 1. A tungsten-iodine lamp (General Electric, 6.6A/T4/1CL) was used as a standard to determine quantitatively the spectral distribution and irradiance of the exploding wire emission. The lamp was operated a t 6.5 A as in Stair et al. (26) and was placed as closely as possible to the position occupied normally by the wire. This source was used with the same optical arrangement as described for the parameter study. The multiplier phototube output was monitored across a 1-MQload resistor with a stripchart recorder. Electronic Circuitry. The basic circuit for the exploding wire apparatus is shown in Figure 2 . The main power supply is a luminous tube transformer T2 (General Electric, 9T61X21, 15 kV, 30 mA) whose output is rectified through a diode bridge. The output The main discharge voltage is regulated by an autotransformer TI. capacitor shown in series with the wire is actually a bank of three 12.5-kV, 7.5-pF General Electric pyranol capacitors which are charged to the appropriate voltage. The explosion is initiated by a pulse delivered to the 2N4443 thyristor either from the 25-V gate of the oscilloscope using the single-sweep trigger ST or from a separate pulse generating circuit shown in the lower portion of the figure. This pulse produces a current surge through the TR-69 (EG&G) pulse transformer by discharging the LMFcapacitor. The resultant high-voltage pulse in the secondary causes a spark a t the three-electrode spark-gap switch, breaking down the air gap and dumping the stored energy through the wire. The residual resistance and inductance of the discharge circuit itself are denoted as R, and L,, respectively. The solenoid S4 is a safety shorting switch which can rapidly discharge the capacitors. The ammeter in series with the 109-R resistor monitors the voltage on the capacitors, while the second ammeter displays the charging current. Explosion Chamber. The chamber in which the explosion occurs is shown in Figure 3. The spark-gap switch shown to the right of the chamber consists of two 1.6-cm diameter hemispherical brass electrodes. A coaxial tickler electrode, by which the trigger pulse is delivered, is placed through the grounded electrode. The gap size is dependent on the required voltage and is 4 mm for a voltage of 8 kV.
Figure 2. Circuit diagram of wire exploding system (A) Ammeter, (L) Indicator lamps, (S) Switch, (ST)Oscilloscope single-sweep trigger, and (T) Transformer.The main discharge circuit is the 22.5 pF capacitor in series with the wire, the spark-gap switch, the residual lead resistance R, and the residual lead inductance L. LUCITE ENDPLATES,
I
'CAPACITOR' TRIGGER
Figure 3. Schematic diagram of explosion chamber The glass chamber and the attached spark-gap switch are mounted directly between the terminals of the discharge capacitor to minimize lead inductance and resistance The chamber has been designed to allow careful control of all parameters. I t consists of a 12.7-cm long, 12.7-cm outside diameter, 6.35-mm wall glass tube epoxied to the right methyl methacrylate (acrylic plastic) endplate. This assembly slides on two aluminum rods to allow access to the interior. The wire is mounted in a cassette made of two conical brass electrodes held 5.4 cm apart by a U-shaped piece of polycarbonate plastic. The electrodes are each made in two halves with thumbscrews holding the wire in place between the sections of each electrode. When the loaded wire cassette is resting on the main 1.27-cm diameter copper rod electrodes, the chamber is slid against an O-ring groove in the left 1.27-cm thick methyl methacrylate endpiece and secured with wing nuts on threaded rods. The space around the wire now can be filled with any gas a t any pressure u p to several atmospheres. The window assembly is a 40-mm i.d. glass tube with an O-ring groove at the end. It is fitted with a 6.35-mm thick, 7.0-cm diameter quartz disk (Suprasil 2, Esco Products). The disk is held against the O-ring by an aluminum collar assembly. A plano-convex lens in a threaded sleeve is also part of this assembly as shown in Figure 1 and is adjustable t o allow focusing or collimation. The entire assembly is positioned between the two terminals on top of one of the capacitors, with the remaining two capacitors mounted directly above. This eliminates almost all leads, reducing circuit inductance and resistance to a minimal 0.49 /IH and 0.03 n, respectively. This maximizes both the rate and efficiency of energy delivery, and the entire discharge circuit functions as an underdamped tank circuit having a ringing frequency of 55.5 kHz. Burner. The sample cell is an argon-sheathed, premixed nitrous oxide-acetylene flame using the stainless steel burner shown in detail in Figure 4.The inner core is made of thirty-eight 2.5-in. long, 0.041-in. i.d. capillary tubes individually sealed into the top and
From Mixing Chamber
Figure 4. Cut-away schematic diagram of the argon-sheathed nitrous oxide-acetylene burner This shows the water-cooled inner core capillary tubes which carry the flame gases and sample. This is surrounded by an outer chamber of tubes which carries the argon sheath gas
bottom of the inner chamber. This allows for water cooling of the inside, as well as the outside, of the burner which provides relative thermal stability throughout. The sheathing gas passes through a cluster of 88 tubes fitted between the inner core and the outer ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
1281
100 TORR
740 TORR
25
-
O D " ' 400 362
'
320
300
280
260
240
WAVELENQTH , nm
Figure 5. Microdensitometer traces of exploding wire spectra under different ambient pressures for 0.08-mm diameter copper wire using a stored energy of 720 J and an argon atmosphere
burner wall in such a way that the argon flows both through and between the individual tubes. The gas flow rates were monitored by rotameters and were adjusted to 3 l./mjn for acetylene, 6.1 1./ min for nitrous oxide, and 11.5 l./min for argon. These rates produced a separated, laminar, 35-cm high flame with a large analytically useful zone of low background. The nebulizer and mixing chamber are modifications of previous designs (23, 24). Because of the danger of flashback, the mixing chamber was made of aluminum instead of glass and was fitted with an aluminum foil window which acted as a safety valve. With the above flow rates, aqueous samples are aspirated at 5.5 mlimin, and flashback never was experienced. Sample Preparation. All samples were volumetric dilutions of 1000-ppm stock solutions made from reagent grade salts. New dilute solutions were prepared each day.
RESULTS AND DISCUSSION P a r a m e t e r Study. A number of parametric investigations were conducted to determine the conditions favoring high ultraviolet irradiance with good shot-to-shot reproducibility. In Figure 5 , representative microdensitometer traces of spectra obtained on the prism instrument with a slit width of 10 pm during a pressure study show the progression from primarily line emission to continuum emis1282
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
sion as the ambient pressure increases. These spectra are from 0.08-mm diapeter copper wires exploded with 720 J in argon. The line spectra are of copper, as predicted by the peripheral restrike mechanism. As the pressure is increased, the plasma generated becomes more confined and extreme line broadening can be observed. An almost structureless continuum is produced a t 740 Torr, with self-reversals seen a t the Cu(I) 324.8- and 327.4-nm resonance lines, as would be expected from an axial restrike, since this is really a continuum emitter surrounded by an expanding cylinder of relatively cool metal vapor. The decrease in optical density toward the low wavelengths in the figure is a result of the emulsion spectral sensitivity and, as will be seen, does not reflect a decrease in irradiance. The change in irradiance with increasing pressure in He, air, nitrogen, and argon was monitored a t various wavelengths and produced plots similar to that shown in Figure 6 for argon a t 250 nm for a 0.08-mm Chromel-A wire exploded with 720 J. In this example, argon exhibits a leveling off of the linear intensity rise a t around 900 Torr. This pressure independence was observed above about 500 Torr in helium and above 1 atm in air and nitrogen. Argon gave the highest irradiance, which was about 1.5 times that of helium, which yielded the lowest radiation level. Room air, dry air, nitrogen, and COL,were tried also and gave similar, intermediate results. Both the material and size of the wire were varied. Silver, copper, tungsten, and Chromel-A wires of 0.08-mm diameter were exploded under identical conditions. Surprisingly, the material had no experimentally significant effect on the irradiance when the available energy was a t least several times that needed t o vaporize reversibly the wire. However, the alloy Chromel-A (80% Ni, 20% Cr) did not show the self-reversals observed with the silver, tungsten, and copper wires. Wire diameter, conversely, was a significant variable. Copper wires of 0.08, 0.13, 0.18, and 0.25-mm diameter were exploded, with the 0.08-mm wire yielding the highest irradiance. The irradiance decreased in the ratios of 62 to 33 to 13 to 1 for these four wire sizes, respectively. Similar results were obtained with other wire materials. The increase in continuum intensity with increasing pressure with axial restrike probably is the result of greater confinement of the plasma. The increased mass and current densities result in greater line broadening and collision rates which in turn would increase the continuum associated with free-free and free-bound electron transitions. A high dielectric strength gas promotes axial restrike by inhibiting dielectric breakdown through the gas peripheral t o the expanding cylinder of metal vapor during the early stages of the explosion. If dielectric breakdown occurs in the gas, the current and, hence, current density would be reduced in the metal vapor.
1
i
I i
02
01
,'
#
08 0
200
1; 400
STORED ENERGY.
00
00
240
1
2m
320
360
I
400
W A V E L E N G T U . nm
€00
1
800
boules
Figure 9. Absolute spectral irradiance as .a function of wavelength for 0.08-mm diameter Chromel-A wire exploded in 740 Torr argon with 720 J Values of irradiance were obtained by comparison with a quartz-iodine standard lamp
Figure 7. Intensity-energy plot for 0.08-mm diameter Chromel-A wire exploded in argon at 740 Torr and observed at 250 nm
Table I. Reproducibility Study Peak irradiance Explosion KO.
(arb, units)
1.7 2 .o 1.8 1.9 1.9
1.9
u
r
~
TIME, y s e c
Flgure 8. Typical intensity-time and current-time plots for 0.08-mm diameter Chromel-A wire exploded in 740 Torr argon with 720 J
The reason for the decrease in intensity with increasing wire diameter is not completely clear but probably is due, in part, to the greater amount of energy required for reversible vaporization, the greater amount of energy carried away from the wire as hydrodynamic energy of shock waves, and the less efficient coupling of the wire to the tank circuit because of the lower resistance of the larger wires. A plot of relative irradiance vs. stored energy for 0.08mm diameter Chromel-A wires exploded in argon at 1 atm is shown in Figure 7. The curved portion at low initial capacitor energy is a result of several factors. Although the energy needed to vaporize reversibly a 5.4-cm long, 0.08mm diameter Chromel-A wire is only about 16 J, considerable energy is dissipated in the generation of cylindrical shock waves (27, 28). The magnetic pinch generated by the extremely high current density during the initial current half-cycle (29), estimated a t about lo6 to lo7 A/cm2(30), has the potential of raising the boiling point of the metal significantly. In addition, other parts of the discharge circuit necessarily dissipate some of the available energy. This is particularly true early in time, before joule heating and subsequent melting has increased the resistance of the wire to the point where highly efficient energy transfer occurs. Thus, a rather large excess of stored energy is needed to attain sufficient excitation after vaporization of the wire. The
Av 1.9 Re1 std dev: 5.4%
upper limit of the linear portion was dictated by the limitations of the apparatus itself and corresponds to 720 J (22.5 gF charged to 8 kV). The remainder of the work reported here utilized a stored energy of 720 J to explode a 0.08-mm diameter Chromel-A wire in argon a t 1 atm. A pressure of 1 atm was chosen because of the ease of control while attaining nearly maximum irradiance. Using these conditions, a sample of the output waveform from the multiplier phototube and a current waveform, using a calibrated current shunt described previously ( 3 1 ) ,are shown in Figure 8. Most of the radiation is emitted in the first 20 wsec following initial breakdown of the air gap, with the peak irradiance corresponding to the first current peak. Shot-to-shot reproducibility studies produced a relative standard deviation of 5.4% as shown in Table I. Absolute Irradiance. A comparison with the known radiation intensity of the tungsten-iodine lamp yielded the spectral irradiance plotted vs. wavelength in Figure 9. No explanation for the pattern has been postulated at this point. However, if blackbody radiation is the primary contributor, the irradiance peak near 240 nm would correspond to a blackbody color temperature of about 1.2 x IO4 K. The absolute peak irradiances are given as W/cm2-nm as measured a t 43 cm from the source (26).As can be seen, the irradiance actually increases into the ultraviolet. In Table I1 this high ultraviolet irradiance is demonstrated again in a comparison of the photocurrent of the wire with that of the xenon arc. It is significant to note here that, a t 220 nm, the peak irradiance of the exploding wire is over five orders of magnitude greater than that obtained from ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
1283
r
t z c
z W
5 Q W
J I ,
I
1
Table 11. Photocurrent Comparison of 1600 W Xenon Arc with Exploding Wire Photocurrent, A Wavelength,
Ratio,
nm
Arc
400 360 320 280 240 220
4.4 x 10-7 4.3 3.6 1.6 0.24 0.029
\Vire
4.7 x 10-4
5.7 5.7 5.7 4.9 3.7
Wire/ Arc
1.1 x
io3
1.3 1.6 3.6 20 .o 130 .O
Table 111.Atomic Fluorescence Detection Limits Wavelength,
Detection l i m i t ,
Element
nm
PPm
Cd co Mn
228.8 240.7 279.9
10
7 2
the arc lamp. This suggests a number of potential applications in both atomic and condensed-phase luminescence spectrometry in the ultraviolet region. Analytical Results. Analytical application of the exploding wire source for atomic fluorescence was attempted with Mn, Co, and Cd. The analytical curves are shown in Figure 10, and the detection limits are presented, along with the wavelengths used, in Table 111. Each point in the figure is the average of a t least five determinations, having a % relative standard deviation of around 25%. A slit height of 12 mm was used in all cases with slit widths of 500 pm for Mn and Cd and 750 pm for Co. These slit widths correspond to band widths of 1 nm and 1.5 nm, respectively. The detection limits were taken as that concentration yielding a signal-to-noise ratio of two. In addition to the fact that these results are of a preliminary nature, it is important to note that the noise level used to calculate the detection limits included shot-to-shot statistical variations in the explosions, in the electronics, in the amount of sample and number of particles in the flame during the few microseconds of observation, and the flame flicker. While the best 1284
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
reported literature values for these elements using cw continuum sources (32) are considerably better than those reported here, it should be noted that they were obtained with the aid of a phase sensitive lock-in amplifier which can not be used with nonrepetitive, transient systems and with a special multiplier phototube. However, considerably better detection limits should result from dual-channel operation. By having a simultaneous background measurement a t a wavelength near the analyte line, many of the unpredictable variations could be compensated for. An attempt is presently being made in this laboratory to convert the monochromator to dual-channel operation, and the results will be published in a subsequent article. A final factor to be considered is the sample cell itself, which gave good emission detection limits in preliminary testing, but which may not be as efficient for fluorescence as those used in the reference work. CONCLUSIONS Although the results presented here are only from preliminary studies, they clearly demonstrate the potential of exploding wires as a very intense continuum ultraviolet excitation source for analytical applications such as atomic fluorescence spectrometry. In contrast to other sources now in use, the output far into the ultraviolet is remarkably high. No tuning or expensive replacement parts are necessary, and the instrumentation is simple and relatively inexpensive to build and operate. While the nonrepetitive, transient nature of the phenomenon precludes the use of conventional noise reduction techniques, simultaneous background correction through dual-channel operation should produce results much more in line with those of other sources. ACKNOWLEDGMENT The authors express their appreciation to N. Johnston and W. Wolfe for their help in the design and construction of the apparatus and to J. A. Holcombe for his many helpful comments and suggestions. LITERATURE CITED (1) C. T. J. Alkemade, "Proceedings of the Xth Colloquium Spectroscopicum Internationaie". E. R. Lippincott and M. Margoshes. Ed., Spartan Books, Washington, DC, 1963. (2) J. D. Winefordner and T. J. Vickers, Anal. Chem., 36, 161 (1964). (3) R. F. Browner, Analyst (London), 99, 617 (1974). (4) J. D. Winefordner and R. C. Elser, Anal. Chem., 43 (4), 24A (1971). (5) G. K. Oster and R. A. Marcus, J. Chem. Phys., 27, 189 (1957). (6) K. B Abramova and B. P. Peregud, Sov. Phys.-Tech. Phys., 16, 1758 (1972). (7) B. Stenerhag, S. K. Handel, and A. Dejke, J. Appl. Phys., 41, 831 (1970). (8) P. Gorlich, J. Karres, G. Kotitz, and R. Lehmann, Phys. Status Solidi, 8, 385 (1965). (9) E. C. Cassidy, Naturwissenschaften, 55, 126 (1968). (10) S. K. Handel and B. Stenerhag, "Exploding Wires", Vol. 4, W. G. Chace and H. K. Moore, Ed., Plenum Press, New York, 1968, p 161. (11) E. Nairne. Proc. R. SOC.London, Dec. (1773). (12) J. A. Anderson, Astrophys. J., 51, 37 (1920). (13) J. A. Anderson and S. Smith, Astrophys. J., 64, 295 (1926). (14) S. Smith, Astrophys. J., 61, 186 (1925). (15) W. G. Chace and H. K. Moore, Ed., "Exploding Wires", Vol. 1. Plenum Press, New York. 1959. (16) lbid.,Voi. 2(1962). (17) /bid.,Vol. 3 (1964). (18) lbid., Voi. 4 (1968). (19) R. A . Marcus, "Exploding Wires", Vol. 1, W. G. Chace and H. K. Moore, Ed.. Plenum Press, New York, 1959, p 307. (20) M. J. Joncich and D. G. Reu, "Exploding Wires", Voi. 3, W. G. Chace and H. K. Moore, Ed., Plenum Press, New York, 1964, p 353. (21) R . D. Sacks and J. A. Hoicombe, Appl. Spectrosc., 28, 518 (1974). (22) J. A. Holcombe and R. D. Sacks, Spectrochim. Acta, Part B, 28, 451 (1973). (23) R. Hermann, "Flame Emission and Atomic Absorption Spectrometry", J. A. Dean and T. C. Rains, Ed., Voi. 2, Marcel Dekker, New York, 1971. (24) C. Veillon and M. Margoshes, Spectrochim. Acta, Part B, 23, 553 (1968). (25) J. A. Holcombe, D. W. Brinkman, and R. D. Sacks, Anal. Chem., 47, 441 (1975). (26) R. Stair, W. E. Schneider, and J. K. Jackson, Appl. Opt., 2, 1151 (1963).
(27)S-C. Lin. J. Appl. Pbys., 25, 54 (1954). (28)G. L. Clark, J. J. Hickey. R. J. Kingsley, and R. F. Wuerker, Exploding Wires”, Vol. 2,W. G. Chace and H. K. Moore, Ed.. Plenum Press, New York. 1959.I) 175. I’
(31)J. H. Park, J. Res. NaN. Bur. Std. ( U S . ) ,39, 191 (1947). (32)D.J. Johnson, F. W. Plankey, and J. D. Winefordner, Anal. Chern., 46, 1898 (1974).
Automatic Identification of Chemical Spectra. A Goodness of Fit Measure Derived from Hypothesis Testing S. L. Grotch Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 9 1 103
The heart of a file searching procedure is the algorlthm used to measure “goodness of fit.” In previous work, the weights applied to each channel have generally been obtained in an ad hoc manner. A maximum likelihood approach is used here to pre-select these weights to maximize the separation between spectra which are the same and those which are different. With this method, the effects of errors on the weights is expressed analytically. Using binary encoded mass spectra, this new weighting procedure invariably produces results which are superior to other weighting procedures.
Because of the increasingly widespread use of computers in analytical chemistry, the automated identification of spectra continues to be a very active research field. In only the past five years, over 100 papers have appeared in this area. These have been most recently reviewed in Refs. 1-3. This study addresses the problem of file searching from a somewhat novel viewpoint, first suggested to this author by Herman Chernoff of MIT. This new view derives the “goodness of fit” measure which is the heart of any file searching procedure from hypothesis testing. It will be shown how the weighting factors in a goodness of fit metric can be calculated, a priori, using a hypothesis testing concept. Furthermore, as a direct consequence of these procedures, it will be seen how errors in measurement or encoding will affect the choice of these weighting factors. Finally, it will be seen, using binary mass spectral patterns, that the weighting factors predicted from this theory yield results which are, in virtually all cases, superior to the best weighting procedure previously used. The extension of this technique to the more general non-binary situation will also be described.
also apply to a wide variety of other types of chemical spectra. In binary coding, a spectrum is encoded as a string of I‘ 0 I s and “1’s’’ where the “1’s” typically represent the presence of a line or a band and the “0’s’’ the absence of such a feature. Generally, a large variety of encoding procedures is possible with any type of spectrum. For example, with mass spectra, the mass positions of the M most significant peaks could be denoted by 1’s in a binary string in which each bit represents a nominal mass. The mass position of the single most intense (or two most intense) peaks per 7 amu ( 1 6 ) or 14 amu (9) could similarly be encoded. The spectrum could also be encoded by thresholding; e.g., only the masses of peaks with intensities above a transition level could be encoded as 1’s (12). Clearly, many similar procedures could be employed with other forms of spectra. In the actual file searching procedure, an unknown code is identified. by comparison against a larger known file which has been previously encoded using the same coding algorithm as the unknown. When an unknown code is compared against a specific library member, a “goodness of fit” is calculated for each comparison. At the conclusion of the search, those K spectra in the file “closest” to the unknown code are displayed and the nearest neighbor(s) is (are) assumed to be the correct answer. Typically, this goodness of fit, or distance, can be considered as the summation, over the N channels of interest, of values taken from a “truth table” (See Table I). The values of the unknown code and the library code are compared, channel-by-channel. Depending upon the simultaneous appearance of a “00”, “ O l ” , “IO”, “11” in channel n, one c y n ( l O ) , or c y , ( l l ) is added to value; either c y n ( O O ) , (~~(011, yield a distance or goodness of fit, d: 9)
N
d =
BINARY ENCODED SPECTRA Because of simplicity, speed of searching, and the extreme amount of data compression which can be achieved using a binary encoded spectrum, a number of workers have investigated these methods in the file searching of chemical spectra (2-15). Most of this work has focused on mass spectra, but infrared, NMR, and emission spectra have all been considered. It should be realized that most techniques in this field are generally applicable to different types of spectra since the same mathematical principles underlie the identification of binary codes. Thus, although this paper will focus on mass spectra, the same techniques
ol,(ij) n=l
where: cy,(ij) = value in the truth table for the n t h channel ~~
~~
~
Table I . T r u t h Table for Binary Codes
ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
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