Adjustable-waveform high-voltage spark source for optical emission

the analytical spark gap. The discharge circuit uses only a single capacitor, two inductors, and a high- voltage solid-state diode for generation and ...
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an Adjustable-Waveform High-Voltage Spark Source for Optical Emission Spectrometry J. P. Walters Department of Chemistry, University of Wisconsin, Madison, Wis. 53706

A high-voltage spark source is described that is capable of producing high-current discharges ranging in wave form from fully oscillatory at frequencies in the submegacycle range to fully unidirectional in the conventional overdamped mode. Essentially continuous adjustment between the extreme modes is possible by simply fine-tuning a single inductor in series with the analytical spark gap. The discharge circuit uses only a single capacitor, two inductors, and a high. voltage solid-state diode for generation and adjustment of a wide range of wave forms. Triggering devices and coupling are simplified by the high voltage aspect of the discharge circuit, and may be selected independently of the wave form chosen. The source is capable of producing a train of subsequent high- and low-current sinusoidal pulses separated in time by a few microseconds and suitable for separation in time and space of sampling and excitation processes in the sparkgap. A circuit analysis and typical time-resolved spectra are given.

THESPARK SOURCE described in this paper is essentially a simple oscillatory tank circuit modified for current shunting the analytical spark gap with a high-voltage diode. The simplicity of the circuit, and the relative ease with which its parameters may be adjusted, is sharply in contrast to the wide range of discharge types that may be generated, ranging from a fully oscillatory high current spark to an overdamped unidirectional spark at the extremes and including a large number of intermediate wave forms previously unavailable in spectrochemical excitation sources. The source unit was designed to fulfill needs in experimentally controlling the types of spectra observed from electrode material injected into the spark gap. The basic excitation patterns observed in a high voltage oscillatory spark have been presented ( I ) . Basically they correspond to high excitation and ionization of freshly-sampled electrode material as it leaves the electrode surface and penetrates the electrode surface space charge. This trend is likely caused by stepwise excitation, is exaggerated by increasing the discharge current, is sustained while the spark gap is filled with electrode material, and is generally incompatible with high sensitivity from a spectrochemical viewpoint. When the discharge current is decreased, radiation begins from parent ions, cascades from high to low levels within a single stage of ionization, and is generally diluted for any particular transition by the large numbers of competing radiative decay routes. The radiation flux from a particular state is further decreased in a volume sense by the radial expansion ( 2 ) of sampled material as it crosses the spark gap, compounding what is commonly detected in a time-integrated experiment as the low sensitivity associated with a spark discharge. The unique incompatibility of high current and high signal in low-energy neutral-atom radiation observed in our discharges, first re(1) J. P. Walters, ANAL.CHEM., 40, 1540 (1968). (2) C. M. Cundall and J. D. Craggs, Spectrochim. Acta, 7, 149

(1955). 1672

ANALYTICAL CHEMISTRY

ported for aluminum (3) and previously for aluminum and copper ( I ) , was instrumental in establishing the need for more versatility in short-time current control in the spark. As pointed out in the previous study of current-radiation patterns for aluminum and copper ( I ) , specific ionic and neutral atom transitions may show unique correlations to the instantaneous magnitude of the discharge current. While it is desirable from a routine spectrochemical aspect to choose source parameters providing blanket excitation response for wide classes of elements and lines, such goals may be impossible when radiation arises primarily through parent-ion recombination and level cascading. In this case, the current must be chosen to provide a collision (or excitation) rate that can just compete with the radiative decay rate of the state producing the line desired, a choice that will be unique for each element, and likely be specific for particular electron configurations within a particular stage of ionization for the element. Such currents may be readily determined from timeresolved spectra, and the basic information acquired is in accord with time-integrated spectra obtained with conventional discharges. A clear example of the unique current sensitivity of selected transitions that is apparent on a time integrated scale is shown in Figures 1 and 2, where the total integrated charge passed in a unidirectional overdamped discharge (4) was increased for constant peak current and constant pulse width using a mild low-alloy steel sample sparked in argon. The Cu 3247 line intensity is related to the discharge current through ionic parent species ( I ) . The intensity of the iron line reflects the increased amount of sample in the spark with increased current or pulse length. However, the carbon 1930 line is similar to the aluminum resonance lines cited previously (3) in being relatively insensitive to instantaneous discharge current because it is radiatively populated through parent ion recombination as a prerequisite to radiation. Increasing the discharge peak current or pulse duration exerts an inconsequential effect on its integrated intensity in spite of the increased sample introduced into the gap. Clearly, specific excitation conditions will be required for such transitions. The discharge should provide direct population of the upper level of the transitions desired, as opposed to excitation to higher levels or stages of ionization followed by inter-level cascading radiation or recombination and cascading radiation. This requirement must be compromised with the high current densities that are necessary to cause heavy electrode erosion if an accurate spectro-chemical representation of bulk sample composition is to be seen. The latter requirement is generally incompatible with direct low-level excitation ( I ) and suggests spatial and temporal separation of the two processes. A further basic need for a source similar to that described here is demonstrated by the propagation behavior of electrode material as it fills the spark gap. Examples have been given (3) J. P. Walters and H. V. Malmstadt, ANAL.CHEM.,37, 1484

(1965). (4) M. F. Hasler and H. Dietert, J . Opt. SOC.Am.,33, 4 (1942).

6-

w

e

3

-

-

(3

DISCHARGE PEAK CURRENT HELD CONSTANT A T Ca. 20 AMPERES

5-

----4-----

0

K

0

4-

2 a

-

0 K

w--

%

3I

-

BACKGROUND 4 0 8 8 CARBON 1930

WIDTH OF THE CURRENT PULSE AT 1 / 3 HEIGHT IN MICROSECONDS

Figure 1. Integrated current response of selected lines to changes in excitation pulse width 151413w 12-

/

WIDTH OF THE CURRENT PULSE AT 113 P E A K H E I G H T HELD CONSTANT AT 2 3 0 MICROSECONDS

(3

I5

II-

-I

0

> 10K

g

9-

,RON 24,7

I

PEAK CURRENT

(RELATIVE UNITS)

Figure 2. Integrated current response of selected lines to changes in excitation pulse height (2, 5, 6) clearly showing that such introduction is pulse-like and predominantly from the cathode, propagation velocity being largely independent of the discharge current. A volume of sampled material will fill a typical 3-mm spark gap in a few microseconds, and then continue out of the spark gap. Clearly, if intensity readings are integrated on such propagating vapor, and a high discharge current is selected to cause continued sampling and propagation, the basic incompatibility of high current and total signal from low ~

(5) C. M. Cundall and J. D. Craggs, Spectrochim. Acta, 9, 68 (1957). (6) N. K. Sukhodrev, “On Spectral Excitation in a Spark Discharge,” Trans. P. N. Lebedey Physics Inst., XV, Consultants Bureau, New York, 1962.

energy lines will prevent desirably-high signal-to-noise values, While lowering the current may produce some intensity enhancement in the low energy lines, it will not alter the migration velocity--i.e., the rate at which material leaves as well as fills the gap-and will reduce the sampling at the cathode, Thus integrating the recombination radiation occurring during the tailing portion of a high peak current unidirectional discharge would not be expected to increase significantly the total signal observed from low-energy neutral-atom lines in the gap, simply because of the combined effects of the basic inefficiency of recombination radiation, the short and currentindependent residence times of sample material in the spectrometer field of view, and the decreasing sampling with decreasing current. Such arguments have been experimen. VOL. 40, NO. 11, SEPTEMBER 1968

1673

D4

LI

b

(b.1

-60 VOLT

Figure 3. Electrical schematic of the spark source

(d. 1

p 1 -

Tl = Variac, 208 V, 15.A. T2 = High-voltage transformer, 23 kV rms, 700 mA maximum output current D1-D3 = RCA CR-110 high-voltage rectifiers 6+ 6R1-R, = 20 mR,5 W, 35 kV test Rq = 1500 Q , 200 W C1 = 0.0625 pF, 20 kV Figure 4. Basic diode shunting action Rs = 20 mR, 5 W, 35 kV test R6 = 20 mR, 5 W,35 kV test Ll = Self-inductance, 0-300 pH that can synthesize current wave trains similar to those shown, L2 = Self-inductance, 0-16 FH the combined triggering ease, the circuit simplicity, and the G2 = Analytical spark gap ease of continuously adjusting the repetitively-pulsed disR, = 1 ma, 5 W, 35 kV test G1-G2 = Control spark gaps charge current intervals, all accomplished in a predictable V = RCA 5563-A manner with commercially available components, offer a Rs A 50 kQ, 100 W unique excitation source for both basic and applied spectroRs = 47 kQ, 1 W metric studies. Although the source is intended for timeCz = 1 p F , 600 V AC resolved (11) or time-gated (7) radiation detection, the present D4= Westinghouse +1-18M-lH/441B-D high-voltage rectifier stack state-of-the-art and accessibility to components and systems for such detection should not cause significant experimental tally verified (7). It is necessary that periods of low discharge complications in its use with existing spectrometric or spectrocurrent essential to low-energy radiation be followed within graphic apparatus. a few microseconds by similar high current pulses to refill the gap for renewed excitation and integration. BASIC WAVEFORM GENERATION The previous comments can only introduce the needs for A schematic diagram of the complete source unit is shown a discharge of rapidly changing and controllable current in Figure 3. The circuit should be considered in three parts. magnitudes. If basic spectrometric studies are t o be carried The first part concerns the components used for charging the out o n oscillatory discharges, currents should be adjustable capacitor. In this source, high-voltage, low-power solid independently of discharge frequency and repetition rates for state diodes D1 through D Sare connected in series for peakthe separation of those variables genuinely associated with inverse voltage protection (12). No attempts are made to excitation on a propagating sample vapor. Examples have achieve nonresonant charging, a single resistor R qbeing used been presented (1). If unidirectional discharges are to be only to limit the diode peak surge current. The remaining studied or chosen to exaggerate recombination radiation, charging components are conventional. Second, the trigthose discharges should be sequentially synthesized from gering devices used, thyratron V and gaps GI and Gs, are conparent discharges allowing pulsed recombination and reventional and their dynamic switching characteristics have newed unidirectional sampling and excitation on a time scale been fully described previously (13). Electronic triggering comparable to sample propagation times and volume radiahas been used t o allow synchronization to existing (11) tive ,relaxation times. These considerations dominated the time-resolved spectrometric apparatus. Other nonsynchrodesign of the source unit described here. To be of ultimate nized triggering schemes ( 4 , 14) are also generally acceptable. spectrochemical use, the source should be relatively simple The final aspect of the source relates to diode D 4and inductors and reliable. While other devices have appeared (8-10) L1and L2which are used for waveform generation.

(7) K. Laqua and W. D. Hagenah, Spectrochim. Acta, 18, 183 (1962). (8) G. N. Glascoe and J. V. Lebacqz, Eds., “Pulse Generators,” Dover, New York, 1965. (9) F. B. A. Friingel, “High Speed Pulse Technology,” Vol. I., Academic Press, New York, 1965. (10) W. D. Hagenah and K. Laqua, Colloq. Spectroscopicum Internationale, VIII, 204 (1959). 1674

ANALYTICAL CHEMISTRY

-

~~

(11) J. P. Walters, ANAL.CHEM., 39, 770(1967). (12) P. Mlynar, “High-Voltage Silicon Rectifier Designers Handbook,” Westinghouse Electric Corp., Youngwood, Pa., 1963. (13) J. P. Walters and H. V. Malmstadt, Appl. Spectrosc., 20, 80 (1966). (14) J. H. E m s and R. A. Wolfe, “Symposium on Spectroscopic Light Sources,” A S T M Spec. T e c h . Pub., 76, p 12 (1948).

CONVENTIONAL OSCILLATORY TANK CIRCUIT

DIODE SHUNTED TANK CIRCUIT PRIOR TO DIODE CONDUCTION

DIODE SHUNTED

TANK CIRCUIT AFTER DIODE CONDUCTION

Figure 5. Potential distributions controlling diode bias

A basic damped oscillatory current waveform is used to synthesize the discharge current in the analytical spark gap Gp. This choice is made on the basis of the experimental ease with which repetitive high-voltage, high-current, shortduration pulses (current half-cycles) may be generated in the oscillatory capacitor-discharge mode. Wave shaping o n the oscillatory parent discharge is based o n current splitting or shunting around Gp via the high-voltage solid state diode D4. The source thus operates on the principle of generating excess power through the discharge of Cl and controlling the fraction delivered to Gp through current shunting. The magnitudes of C, and the series combination of L1and Lz determine the basic oscillatory frequency and peak currents in a conventional manner ( 1 5 ) . The relative impedances of L2 and Gpin series, control the shunting current. paralleled with that of Dq, A qualitative description of the waveshaping action of L,, Lp, and D4 is shown in Figure 3. In Figure 4a the discharge has been triggered and the first quarter-cycle of discharge current is as shown in L1,L2, and R, the spark gap Gl dynamic impedance. Diode D4is reversed biased. During this time interval, the first peak current of the parent oscillatory discharge appears in spark gap R. Diode D qwill sustain a reverse bias until the current peaks, as the potential drop it experiences is determined by Ls in series with R. I n Figure 4b the current has peaked, and the capacitor voltage is just beginning to reverse. At approximately this time, D 4will go into a forward-biased condition establishing the current shunt path. The point in time with respect t o the main oscillatory current at which D 4sustains a forward bias depends on the relative phase shift across Lz and R compared to the total potential drop across L1,L2, and R. This relationship is shown in Figure 5. Here, inductor L corresponds to the series connection of L1 and Lpin Figure 3. A conventional oscillatory tank circuit is included for reference. Potential E3 corresponds to the capacitor voltage E,:

Where V ois the zero-time capacitor voltage. If the spark gap is noninductive, the potential El will correspond to a pure ZR drop:

The potential El, which controls the bias of D4prior to its conduction, will be at a phase relationship t o E3 according to the differences between Equations 1 and 2, corresponding to (15) C. H. Corliss, Spectrochirn. Actn, 5, 378 (1953).

Figure 6. Spark current modes (100 amps and 5 psecicm) ( A ) LI = 150 pH,LZ = 0 (residual) ( B ) Li = 150 HH,Lt = 0.8 pH (C) LI 150 pH,La = 2.4 p H (D)LI = 150 pH,Lt = 5.6 pH ( E ) L1 150 pH, Lz 8.8 pH (F)LI =_150 pH,Lz = 12.0 pH (G)LI = 150 pH, L? = 16.0 pH ( H ) L = 16 p H , Lz = 16.0 /IH

the cosine term in Equation 1 and expressed by the phase angle: 2TfL tan 6 = -

R

(3)

where f is the oscillatory discharge frequency. Using Equation 3, the turn-on point of D4 ( E 2 ) may be estimated with respect to El. However, assigning L1a typical value of 130 p H and R a typical value (13) of 2 a,E? will lead EBby approximately 70 degrees as soon as L2 shows an impedance of about 6 Q , corresponding to value of L2 of 16 p H for a discharge around 60 Kc. Generally R is significantly inductive, because ignition transients are preserved in this source through coaxial coupling of the spark gap Gl to the low end of Lz and the high end of G1. Thus, in practice, El is found to always lead E3, usually by more than 50" and typically closer to 90" for moderate values of L,. After D qis in a forward-biased condition, the bias is current-controlled (16) and Figure 5 indicates the diode voltage drop will now be in phase with the current in the diode, as long as that current is not in a direction capable of causing reverse bias. Thus, there are two distinct biasing regions for D4with respect to the main discharge current. The first controls its turn-on, and will assume a phase angle 6 with respect t o the discharge current according to Equations I through 3. The second controls the diode turn-off, and will not occur (16) R. A. Greiner, "Semiconductor Devices and Applications," McCraw-Hill, New York, 1961, p 130. VOL. 40, NO. 1 1 , SEPTEMBER 1968

1675

until the forward-bias current in the diode reverses direction. The time required for this reversal is controlled by the magnitude of Lz and the diode forward-bias resistance rf, and is responsible for the generation of unidirectional discharges. As shown in Figure 4b, the onset of diode conduction leads to a loop current in R, Lz, and De. This current is caused through the voltage induced in LB, and is the same direction as the initial current in Lz. Its circulation will add t o the decreasing discharge current at the junction of D 4and R, sustaining the initial current in R . During this interval, and prior to current reversal in the main oscillatory discharge, the only current in 0 4 will be that associated with the field relaxation in L2, as the main discharge current is in a direction to cause D 4to enter reverse bias. Following current reversal in the main oscillatory parent discharge (Figure 4c), there will be two currents in D4,one associated with the relaxation of'LZ and the other a fraction of the main discharge current as determined by the relative impedances of the two loop paths. This situation will continue during the entire next half cycle of the discharge, as D 4 will sustain forward-bias as long as a forward current is present. Thus, D 4will turn-on slightly after the peak of the first current half cycle of the parent oscillatory discharge, and stay on until completion of the entire first current cycle. The current in G2,the analytical spark gap, will be reduced by the difference of the loop current associated with L2 relaxation and the main oscillatory component shunted through R. The reduction will occur only during the second current half cycle of the main oscillatory discharge, because only then will D4 carry a component of the main discharge current. The result of the combined relaxation of L2 and resistive shunting of D 4on the current in R is to produce a variableamplitude current half cycle during the time D 4is forwardbiased after the main discharge current has reversed direction. This amplitude adjustment is shown for a series of values of LBin Figure 6. The full oscillatory discharge is indicated in Figure 6A, having a peak current of approximately 300 amperes. In Figure 6B through 6G, L1 remained fixed and L2 was varied. No other parameter changes were made until 6H, when L1 was decreased to approximately 10 p H , a magnitude comparable to the 16 pH for L2. Here, the discharge is essentially purely unidirectional. It is to be stressed that all intermediate cases between Figure 6B and 6G are available by fine-tuning L2. For example, a 5-amp interval of approximately 1 pecond duration an be generated from the wave form in Figure 6 E by setting L2 down 0.4 FH.

2ooL I80

w

-I

INDUCTANCE (LE) IN M I C R O H E N R Y S

Figure 7. Shunted half-cycle current dependency on L.2 for no polarity reversal is mainly of value in choosing values of L and C to give a desired peak current and frequency combination. When D 4goes into conduction, a voltage V , is induced by L2 to produce a loop current in L2,R , and D 4according to the relation: (5)

If this point is considered to coincide with the peak current as given by Equation 4, and is then set to correspond to zero time relative to diode shunting, Il may be written as I,

-

2 vo 4 4 L I C - R Ze

-3[sin ( 4 4 L E - R2)*]

ANALYTICAL CHEMISTRY

(6)

v

dF -

(7)

because 6 is approximately 90" in most operating cases and R 2 is typically small compared to 4L/C. Thus:

V i = wL210sin

wt

(8)

The voltage drop across L2and D 4( VL--D) will decay according to the L2RTtime constant of the loop, where the total loop resistance RT is the sum of the diode forward resistance rl and the gap resistance r g :

VLvD= (4)

Where R is the total circuit resistance, Vothe initial capacitor voltage, and L the sum of L1 and LZ. The minus sign indicates current direction corresponding to Figure 6A. The limitations of this relation, and appropriate approximations, have been previously discussed (13, 15). Equation 4 1676

Io cos w t

Z o E -

The quantitative description of the discharge cases shown in Figure 6 is compounded by the time discontinuities in the role of D4. Prior to its forward bias, it is essentially out of the circuit. After De conduction, two time intervals must be considered, one prior to current reversal in the main oscillatory discharge and the other during current reversal. Prior to D4conduction, the spark current is described by the main capacitor discharge current : =

=

neglecting discharge damping and where

WAVEFORM ANALYSIS

I1

\

2oi

v,e--(RTILz)t

(9)

thus :

VL-n = wLnZosin w t e-@TIL2)' The induced current ZZ in the diode loop is thus:

(10)

400

DISCHARGE CURRENT

4001 300

200

4001

Figure 8. Constant-frequency, variable-current modes for diode shunting The total current in the spark gap ( I 3 ) will be the sum of the induced current in the diode loop and the main discharge current during the time prior to the discharge current reverses: I3

= I1

+

I2

Upon completion of the first current half cycle, the main discharge current will divide between the diode path and the spark gap Gz in series with L2 according to the relation: 13

Xdiode

= xgsp

f

Xdiode

+

I1

xL1

(13)

The reactance of the diode and spark gap may be considered resistive, such that Equation 12 may be written for the second current half cycle of the discharge as:

Recall that Il is negative during this time interval. Equation 14 will describe the current in the spark gap G2 during the time the diode is forward-biased for the main discharge current. As such it is useful for showing the effects of varying LBand LOin producing a particular waveform. For example, Equation 14 predicts a decrease in the magnitude of the current in the spark gap because of the partial

shunting of the main oscillatory current, with a corresponding increase in the LP relaxation current It. Such behavior is evident in Figure 6 A through H, with Figure 6H showing a complete domination of Equation 11. The exponential relaxation is evident. The combined effect of decreasing the first term of Equation 14 while increasing the second term because of an increase in Lp will produce a n approximately linear decrease in the peak spark current in GPduring the second and subsequent odd current half cycles of the main discharge current. Such behavior is shown in Figure 7. This response represents a significant operating mode of the source, because by fine-tuning L2 it is possible to reduce the spark current to arc discharge levels in a continuous fashion while exerting only a negligible effect on the current in the first current half cycle. The absolute changes in the diode current I, compared t o the spark current laand discharge current Il during this adjustment are shown in Figure 8. It is apparent, in agreement with Equation 14, that as the spark current is reduced in the second current half cycle, eventually entering a pulsed unidirectional mode, the diode current increases and the peak spark current during the first half cycle of the discharge remains essentially constant. Equations 4, 12, and 14 covering the three temporal operating regions of diode switching action predict a linear dependence of the spark current during the second current half cycle on the peak first half cycle discharge current, without regard to the direction of the spark current. Such reVOL. 40, NO. 1 1 , SEPTEMBER 1968

e

1677

sponse is shown quantitatively in Figure 9 for lo values up to 400 amperes. Again, the absolute changes in Zz and Z3 relative to Zl with changes in lo(essentially caused by changes in LJ are shown in Figure 10. It is evident that as the spark current becomes unidirectional due to increasing lo,the diode current increases. In each case shown in Figure 10, the spark current during the second and subsequent even current half cycles may be driven from a pseudo-oscillatory mode to a pulsed unidirectional mode by fine-tuning LZ as shown in Figure 9. The maximum oscillatory spark current that can be synthesized from the fully oscillatory parent current is limited by the residual inductance associated with Gz,which is estimated to be only a few microhenries in this source. Figure 10 illustrates an operating mode where both the initial first half cycle peak current and main oscillatory frequency have been changed. If the spark source capacitance is adjusted simultaneously, the first half cycle peak current may be adjusted with only minor changes in the discharge current frequency, and with provision for independent change of the spark current on even discharge current half cycles through changes in Lz according to Equation 14. The exact analytical description and duplication of the experimental spark current according to Equation 14 is difficult because it is necessary to know within a few per cent all residual inductances in the diode loop, as well as the forward bias diode resistance and the dynamic resistance of spark gap Gz. The latter two quantities are not independent of the current (13, 16), compounding the difficulty of their estimation. However, Equation 14 clearly describes the functional dependency of the spark current on LI and Lz. In practice,

1401

/

I

Figure 9. Shunted half-cycle current dependency on lo with polarity reversal

'Ol

200

300J

3001

100

500

400-

300200-

n

n 20

LI. 74pH L p i 8.8pH

400-

500SOOJ

600-1

Figure 10. Variable-frequency, variable-current modes for diode shunting

1678

ANALYTICAL CHEMISTRY

the source has been constructed to concentrate the majority of the stray inductance in the Lz-G2 connection in the coaxial cables to GP,aiding in the rapid and precise breakdown of this gap (13). For flowing atmosphere work, where GDmay be rapidly quenched, a mercury germicidal lamp is used t o illuminate GB and ensure its immediate breakdown in sequence with gap GI. SAMPLE SPECTRA Because of the range of waveforms available with this source, several types of spectra may be observed over the analytical spark gap Gz. The ultimate utility of the source from a spectrochemiczl viewpoint will depend upon the control that may be exerted on the intensities of lines observed with respect t o interfering lines and spectral background. A simple example is given here to illustrate only one form of spectral control that could be of use in an analysis. Other examples will be reported in future articles. It has been previously noted for pure copper and aluminum electrodes (1) that at distances removed from the parent cathode the nonintegrated intensities of several neutral atom lines are independent of the discharge current when the current exceeds approximately 60 amperes. Simple direct observation of the time-resolved spectra in discharges similar to those used previously (3) establishes that the discharge background radiation is strongly related, if not directly proportional, to the instantaneous current. Thus it should be possible to sustain neutral-atom emission for copper as a n impurity in aluminum, as a minimum t o the same degree observed in a conventional oscillatory or unidirectional discharge, while simultaneously tuning the current t o a level unsuitable for background production. By using repetitive high current pulses only for renewed sampling to compensate for sample material propagating either out of the spectrometer field of view or out of the spark gap, it should be possible to sustain the signal emission at the expense of background. If sample can be introduced at a rate competitive with its radiative decay due t o volume excitation relaxation, radial diffusion, and counter-electrode deposition, then a net increase in line intensity should be observed in the vicinity of the anode as additional sampling-relaxation half-cycles of the discharge occur. Such behavior is illustrated here as part of the transition from a n oscillatory to a rapidly-pulsed unidirectional discharge. I n interpreting the intensities shown, it must be recognized that the measurements are dynamic and were made o n a flowing sample with a spatial integration in the direction of parent sample propagation of only 0.05 mm of a 3.0-mm spark gap, for a time-integrated emulsion exposure of only 250 kseconds, using a spectrometer with a reciprocal dispersion of only 16.4 A/mm, and with a slitwidth-limited spectral resolution of only 2 A. Specific operating parameters are given in Table I. Sample spectra are shown in Figure 1 1 . The lines marked are identified in Table 11, and the index for each inset corresponds to the current waveforms shown in Figures 12 and 13. Controlled current shunting has been accomplished on the second and subsequent even current half-cycles of the parent oscillatory discharge. Spectra are not shown for a fully oscillatory case because it shows behavior similar t o that in Figure 11A. Here, the presence of significant background and nitrogen-ion line spectral contamination is evident during most discharge current half-cycles. A qualitative indication of the removal of the background as the spark current is decreased during the even half-cycles may be seen through visual comparison of the A1 I line densities and widths

0

1 2 3 4 5 6 7 8 9

Figure 11. Sample time-resolved spectra Inserts correspond to Figures 12 and 13 Lines identified in Table I1

Table I. Spectrometer (11) and Source Operating Parameters C = 0.0625 p F L1 = 200 /.AH Lz = variable as noteda Vo 15,000 volts No. of sparks = 3600 superimposed/spectrum Sparking frequency = 60 discharges/second Spark frequency = as noteda Emulsion exposure = 0.07 psec/0.05-mm plate distance/discharge Spectrograph X slit = 120 p (2A) Spectrograph time slit = 0.05 mm (0.07 psec) Upper electrode = anodic first current half-cycle, 60" pure silver pin, 0.250 diameter Lower electrode = cathodic first current half-cycle, 60" aluminum 2011 alloy pin, 0.250 diameter, '/*-inch flat diameter sparking surface, Cu concentration nominally 4% Electrode separation = 3.0 mm Observation zone = 0.05 mm at 1.0-mm cathode displacement Sparking atmosphere = free air Emulsion = Type 103-0 a Notations in Figures 12 and 13.

Table 11. Identification of Lines in Figure 11 Line Wavelength Species 1 3995.0 N I1 3961.5 2 A1 I 3944.0 3 AI I 3749.5 4 0 I1 5 3723.3 0 I1 3712.7 6 0 I1 3612.3 7 AI I11 3601.6 8 A1 I11 3586.9 9 AI I1 multiplet

VOL. 40, NO. 1 1 , SEPTEMBER 1968

1679

0.2 0.1

k

I

I I 8

8

I

il 8

ii

2 . 0 7 0

0.3 0.2

o'll 0

300 tn 200 IO0

0

IO0 200

300

I

,

,

,

25

#

I

,

,

,

iI

30

,

,

#

I

35

#

,

,

,;,,,. , , ( ,1 410 I

5:

50

TIME IN MICROSECONDS

Figure 12. Line and background intensities for 4 x copper impurity in aluminum 2011 alloy with a shunted oscillatory discharge

1680

ANALYTICAL CHEMISTRY

0

2 .o

:I

.

Ag3280.7 X

cu

I

8

0.2 0.1

n

2 .o

n

0.3

-> Iv)

z w

0.1

I-

r w

f -I w

-wla

J

w

I

a

m I

0.3

I 8

0.2

I

0.1

8

z

I I,

0-l

200-

TIME I N MICROSECONDS

Figure 13. Line and background intensities for a 4

copper impurity in aluminum 2011 alloy with a shunted unidirectional discharge

VOL. 40, NO. 11, SEPTEMBER 1968

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with the apparent background in Figure 11B through D. Insets E and F correspond to the synthesis of a fully unidirectional discharge, emphasizing that the continual sampling resulting from sustained or increased unidirectional current does not cause a significant improvement in signal at the expense of background, except for the A1 I1 lines which are intermediate in the production of A1 I radiation from A1 I11 parent species ( I , 3). A more quantitative interpretation of the spectra in Figure 11 may be seen in Figures 12 and 13, where transmission measurements have been made on the resonance lines of the copper impurity in the aluminum alloy. It is clear that the maximum net line-to-background ratio for these lines during the entire series of discharge half-cycles monitored occurs when the shunted current is reduced to a level below 50 amperes but without polarity change. Here, the background intensity has been essentially reduced to scattered light and fog levels, and the signal may be integrated over the shunted current half-cycles for purposes of enhancement. The above spectra are offered only as the most obvious examples of the control that may be exerted over the spectra

observed in a free-air discharge. Their major significance from a spectrochemical viewpoint is that sample has been replenished in the observation zone between observation intervals and it was possible to sustain low-energy neutralatom emission at the expense of background radiation, a situation not encountered in either a fully oscillatory or fully unidirectional system. The value of the source for basic studies has been previously illustrated ( I ) . ACKNOWLEDGMENT

The donation of apparatus used in the spark source construction from General Motors Research Laboratories, Warren, Mich., and Baird-Atomic Corp., Cambridge, Mass., is appreciated. The data for Figures 1 and 2 were obtained by the author at Baird-Atomic through the courtesy of William G . Langton and John A. Norris. RECEIVEDfor review March 13, 1968. Accepted May 21, 1968. Work supported by the National Science Foundation through Grants GP-5073 and GP-7796.

A New Method for Decomposition and Comprehensive Analysis of SiIicates by Atomic Absorption Spectrometry Bedrich Bernasl National Academy of Sciences, National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, Md. Rapid decomposition of silicates i s achieved in a specially designed vessel made of Teflon (Du Pont) without volatilization losses by hydrofluoric acid at 110 OC. A fluoboric-boric acids system was found to provide a favorable decomposition medium and a suitable salt-free single matrix system. Conditions were developed for sufficient inhibition of the hydrolytic decomposition rate of fluoboric acid. This matrix was found to diminish significantly or to eliminate entirely the chemical, ionization, matrix, and instrumental interferences for atomic absorption measurements. The system permits contamination-free sample handling in glass equipment, ensures sample solution stability, eliminates the need for closely matching the matrix constituents of the sample and standard solutions, contributes to signal stability, and to date provides an interference-free environment for the rapid and reliable atomic absorption spectrometric determinations of silicon, aluminum, titanium, and vanadium using a nitrous oxide-acetylene flame and for iron, calcium, magnesium, sodium, and potassium using an air-acetylene flame.

METHODS conventionally used t o decompose igneous rocks are based on the use of acids o r fusion approaches. The decomposition step using either of these procedures for atomic absorption measurements is known t o present difficulties (1). Several authors successfully decomposed a number of silicates and refractory minerals by hydrofluoric acid in Teflon vessels but volatilization losses could not be entirely prevented be1 Permanent affiliation, Israel Mining Industries, Institute for Research and Development, Haifa, Israel.

(1) J. A. Bowman and J. B. Willis, ANAL.CHEM.,39, 1210 (1967).

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cause of Teflon’s dimensional instability (2-5). The Teflon decomposition vessels described in the literature use stoppers, machine-tapered sealing areas, and bolts and screws for sealing (.?,4). However, the stoppers tend to become loose after repeated use and cause low results (6). To prevent volatilization of the sample material being decomposed the tapered areas need frequent remachining or trimming off of extruding Teflon edges. Fusion, in metal crucibles with fluxes such as carbonates, borates of sodium, potassium, or lithium may lead t o incomplete attack of the sample as well as to the limited solubility of some metal ions in the particular flux environment and t o losses caused by reduction and alloying. Such fusion agents furthermore preclude the determination of the particular cation used in the flux (7). I n addition, fluxes are usually used in a ten to twenty fold excess over the sample weight. These undesirably high salt concentrations cause solution instability as well as high and fluctuating instrument background readings. Light scattering and/or molecular absorption phenomena which are known t o exist in such environments are detrimental features in any analytical scheme which should (2) F. J. Langmyhr and S. Sveen, Anal. Chim. Acta, 32, 1 (1965). (3) F. J. Langmyhr and P. R. Graff, “A Contribution to the Analytical Chemistry of Silicate Rocks: A Scheme of Analysis for Eleven Main Constituents Based on Decomposition by Hydrofluoric Acid,” Universitetsforlaget, Oslo, 1965. (4) J. Ito, Bull. Chem. SOC.,Jap., 35, 225, (1962). (5) I. May and J. J. Rowe, Anal. Chim. Acta, 33,648 (1965). (6) J. P. Riley and H. P. Williams, Microchim. Acta, 4, 516 (1959). (7) A. Katz, Amer. Mineralogist, 53, 283 (1968).