Plasma Arc Solution Spectrochemistry Evaluation and Optimization of Operating Parameters EDWARD H. SlROlS Major Appliance Division Laboratories, General Electric Co., Louisville, K y .
b The parameters employed during operation of a spectrographic plasma jet have been quantitatively studied and interrelated. Criteria for selecting optimum conditions are specified. Items included are the open discharge distance and cathode position, helium rate, plasma flow, effective arc resistance, operating current and voltage, response vs. position of view within the open gap, and sample size and intensity as a function of plasma composition. The effect of each parameter on response in a high current arc is discussed. An axial, uncooled carbon cathode was employed in this work with an analytical Spex industries-type plasma jet operating as a gas stabilized arc device.
T
HE ANALYTICAL PLASMA JET represents a significant advance in spectrochemical excitation. Because of the fundamental basis of plasma jet operation, plasma arc spectrometry provides the analytical chemist for the first time with all the factors necessary to achieve low background excitation independently of matrix effects. The plasma state is the source of highest, continuously controllable temperatures available today. No solids can exist in nature a t atmospheric pressure under these conditions. Because of the high stability and sensitivity of high enthalpy plasma excitation, analytical curves for specific elements can be achieved under noninterinfluence conditions. For example, a single direct calibration for manganese in solutions of high chromium stainless steel alloys, zinc base die castings, wrought aluminum, nickel steel, silicon bronze, cast iron, and other material has been achieved (4). Since the introduction of the plasma excitation source to spectrochemistry by the National Bureau of Standards (1 , b), various commercial devices have become available. Despite the excellence of some of these units, some workers have had difficulty in obtaining results with the plasma jet which reflect the true worth of plasma excitation above that of conventional approaches. This publication is intended to provide assistance to spectrochemists
by making available an integrated evaluation of the key operating parameters. I t is also an attempt to bring about better understanding of the properties of this source. Ry providing procedures for optimizing the operating parameters it is hoped that others will be aided in defining operating conditions for their particular equipment and analytical requirements. EXPERIMENTAL
The direct current arc employed in this work is struck for each separate exposure. A fresh cathode rod is used for each ignition. The axial cathode is lowered to contact the anode during helium flow and then withdrawn to proper height for normal viewing. The shutter mechanism. controlled by the exposure timer, is kept off until after the glowing cathode has reached proper height and is masked from the area to be viewed by the spectrograph. The exposure time interval begins an instant before the aspirator gas flow of preset rate is snapped on. I t terminates a t the instant the prenieasured volume of liquid sample is exhausted. Sample completion is signaled by both a sharp change in the arc current and the operating voltage, terminating both the interval timer and shutter. Apparatus and Equipment. -\ Gaertner L254-15 large quartz a n d i o r glass prism spectrograph; S S L Spec. reader densitometer, with custom recording and a Sational Spec. Labs. multiple power source, 1Iodel C-127 were employed with a Spex Industries Model 9010 arc stand. A Spex Industries KO.9030 plasma jet with individually calibrated large bore atomizer caidlary was used. ;\n S S L Model PD-101 processor was used with Eastman Kodak S.1 S o . 1 plates, 4 x 14 X 0.040 inches. Teflon conical bottom sample cups with a maximum capacity of approximately 5.0 ml. and 7il8-inch maximum height were employed as sample holders. Filtered (5-micron filter pore siw) hirco helium and high purity I h r d e t t argon were fed through Oswell Type R-503 He control regulators and toggle-type valves. The Grey Co. I-niversal llodel 168 timer used (73,'8-inch diameter face) was wired into the shutter and electrical system. Operating voltage was monitored by external metering. Direct current was monitored with a DuMont Type 274-A oscillograph. Three-step neutral optical wedge filters
of 2O%, 5%, loo'%, 2 mm. each, were used simultaneously a t the position of vertical focus. A three-lens system and mask were used with the arc imaged inversely a t the mask between the cylindrical lens and the source. Operating Conditions. T h e conditions specified were selected for t h e tem employed in this study. I t is expected t h a t other invevtigators will employ conditions best nd optimum for their tems. An excitation current nominally 24.0 + 0.2 amperes, and 240-volt d.c. open circuit were used. A spectral region of 2180 to 3600 .\. was viewed with a 20-micron slit width and 8.0-mm. analytical gap. The anode to cathode distance was -17 mm. A 60-second nominal exposure on S-4No. 1 emulsion was developed with Kodak D-19 for 3.0 minutes a t 70.0' F. .?, . large bore Spex-type atomizer, centered axially and concentrically, was used. The cont'rol gas used was helium a t 40 liters per minute nominal, 38.8 ideal. -4rgon was used for the aspirator gas a t pressures required for the sample rate selected. 'The He/argon ratio range was 6/1 minimum, 12/1 maximum, 10/1 nominal. Sample solutions of 40y0 by volume of concentrated HCl, 10% by volume of concentrated HKOa, and distilled water or other formulas were used as required a t a liquid flow rate of 1.0 ml. per minute. .4 ',Ir-inch diameter, 1.92 density, carbon cathode rod was used axially and uncooled with the tip tapered a t -60' to blunt point N 1 / , 6 inch diameter. The hole in the neutral gra1)hite control ring is 0.01 inch larger than ':d inch to accommodate the cathode rod. The anode ring is 'i4-inch i d . a t the top and is tapered to inch a t the bottom. Contact is made by the cathode rod near the top of the stepped anode ring. The aspirator does not impinge on thf anode ring steps during proper operation. DISCUSSION A N D RESULTS
'The total available vielv between the top of the neutral orifice ring and any arhitrary Iiosition of the cathode is composed of zones of lighter, darker, and variously colored regions of discrete yet changing thermodynamic conditions. .In understanding of these zones in relation to the numerous operating parameters of the total plasma arc is essential for consistent and successful quantitative alililications VOL. 36, NO. 13, DECEMBER 1964
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I Figure 1 .
Geometry of the neutral orifice, cathode, and open g a p plasma areas
The work to be discussed here is based solely upon the inert, inorganic plasma. However, a comparison of the unreactive inorganic plasma and flame plasma arcs reveals the magnitude of the magnetohydrodynamic pinch. The absence of the outer color zone in the inorganic helium plasma corresponds to the outer color zone in the flame plasma running on such solvents as acetone. The diameter of the arc column is pinched to approximately 60% of the diameter of the orifice. A representation of these areas is shown in Figure 1. The brighter zone in the center is surprisingly transparent. There is evidence of a darker leading edge on the cathode. h right to left curl in the bright vertical zone indicates the effect of the horizont'al plasma vector of the tangential helium stream used in forming the inert sheath. The arc colunin seems to precess during operation. The actual geometry employed in the plasma aspirator, orifice, and cathode areas is also shown in schematic form by Figure 1. .In angle of 30" is employed on a fresh cathode. The anode, which when new is stepped down to a diameter less than that of the cathode rod, is shown here after being naturally worn by long use to approxiniately 60". The neutral orifice does not wear during operation with proper adjustment of the operating
2390
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ANALYTICAL CHEMISTRY
conditions. The major portion of anode wear occurs during ignition of the arc. Its wear is greatly accelerated if the arc is struck after an anode has become moistened when aspiration of
sample is allowed to continue after the arc is extinguished electrically in a previous run. The anode is easily replaced and should be replaced when it begins to show loss of concentricity. In plasma arc spectrometry, the final optimum conditions overall are initially related to the dynamic arc resistance calculated from Ohm's law. The desired situation is an uninhibited arc in which the electrical resistance within the specifically viewed portion of the total electrical gap is a linear function of the distance of the cathode from the anode (Figure 2). The extrapolated linear resistance approaches zero a t the anode if only the plasma and not the external source circuit contributes to the resistance. For plasma arc work no significant resistance external to the arc itself is necessary or desirable because the external circuit resistance will effectively be additive to the operating arc value. Since the geometry of the arc assembly is fixed-i.e., the distance of anode to top of neutral orifice ringone can determine the range of the limiting system resistance from the plot of arc resistance us. cathode position. The nominal value is the ordinate intercept over the center of the viewed gap. The term limiting resistance was used above because at fixed availaflle the nlaximum possible current and, therefore, the most intense arc conditions are dictated by this resist'ance as determined under dynamic conditions. From a plot such as Figure 2 one can evaluate the suitability of his particular I
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Figure 4. Response increasing with arc current for operation a t corresponding optimum helium rates
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Figure 3. Optimum g a p as cathode distance yielding linear function of resistance and flow rate
source for delivering the high current desirable in plasma arc work. It has been discovered that some sources, because of their circuit design, do not permit sustained high current delivery. For example, to appreciate the resistance effect, suppose that the plasma resistance of Figure 2 is 2.6 ohms-Le., i o actual operating volts divided by 27 operating amperes. If the source circuit effectively adds even 1 ohm of resistance to the arc, only 70/3.6 or 19.5 amperes are available for useful excitation. In the example shown, the superposition of 4 liters of argon per minute on a plasma of 40 liters of helium per minute lowers the effective arc resistance. The addition of 1.0 ml. of water per minute to the 10/1 ratio of the helium/argon plasma produces no drastic change in arc resistance. However, the solution volume does have a thermodynamic effect on intensities. The lower resistance of the argondiluted arc is probably caused by argon ionization and other ionization which can be observed in the spectrum. On pure helium the arc shows helium I spectra composed of eight lines in the 2600- to 3200-A. region. These do not always appear in the diluted plasma with the argon present. One of the most potent direct variables in plasma arc spectrometry is the rate of flow of helium (the inert swirl gas) which must be optimized by each investigator using his own particular experimental apparatus and orifice conditions (Figure 3). Optimum helium flow, since this is a heliuni-controlled plasma, is necessarily that value which produces a maximum in a plot
of the intensity of emission of an element us. helium rate (at fixed gap and other conditions). Selection of the optimum gap should precede the selection of a helium rate. The optimum gap can be defined as the cathode distance representing a linear function of arc resistance us. helium flow rate. The optimum value shown for this plot in Figure 3 is 8.0 mm. from the top of the neutral orifice ring to the cathode. The resistance function for other gaps shows the complexity of interaction effects from other factors. Through this point in this discussion we have related both cathode position (Figure 2) and helium rate (Figure 3) to the plasma arc resistance. The physical significance of the measurements expressed in terms of simple resistance relates to the kinetics and interaction of physical forces in the arc. I n contrast, the current carried by the plasma in the arc gap appropriately expresses the magnetomotive force and energy of the system. Since the applied potential is fixed and the resistance and current are related by Ohm’s law, we can in our thinking translate to current the significance of the physical forces acting on the arc resistance and anticipate the following. First, each fixed position of the cathode will have an accompanying optimum operating current, desirable for maximum excitation. Second, each fixed helium rate will have an associated maximum attainable current at fixed cathode position. Third, if we suppose that the cathode position chosen is one point on the line for which the helium rate is a linear function of resistance, it follows that helium rate
is also a direct expression of current. The preceding presumptions are shown to be correct as determined experimentally. They are represented in Figure 4, which shows the effect of arc current and helium flow rate on the absorbance of a manganese line relative to an almost insignificant background. The emission from manganese line 2576.1 A . at the 1 wg. per ml. constant concentration level increases in intensity with increasing arc current (Figure 4). The discrete ordinate values shown were obtained for each constant current optimization of the helium rate (and at constant liquid aspiration rate). The advantages of olierating a t the highest current, lowest resistance, arc situation are evident. Increasing the direct current in ordinary arcs usually increases excitation. In a plasma arc additional physical forces are present, making it necessary to discover to what extent the helium flow and the additional MHD factor-namely the hydrodynamic and magnetic pinch associated with the flow of the concentrated stream of ions and helium atoms-can, by adjustment,, yield changes in the degree of excitation of a given element, For example, the manganese response shown as line-to-background ratio increases sixfold-i.e., 18 + 3-from operation a t 12 amperes and i o liters per minute of helium to 25 amperes and 40 liters per minute of helium. .It 12 amperes all flow rates less than 70 liters per minute are less than optimum. At 18 amperes lower absorbances are obtained a t helium rates above or below 58 liters per minute. The optimum helium rate should be used with each fixed current condition to yield maximum response ( 3 ) and best precision in quantitative plasma spectrochemistry. Further inspection of Figure 4 shows that the third supposition made earlier is also correct. The optimum helium rate is a direct expression of associated VOL. 36, NO. 13, DECEMBER 1964
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current and vice versa, as judged by the straight line obtained by connecting companion points in the XS: plane of Figure 4 (at constant value of absorbance on the y axis). For example, the pointsas (X, I', Z) for (12, 0, 70), (15, 0 , 64), (18, 0, 58), and (21.1, 0, 52.5) form a straight line in the S Z plane. The portion of the open gap discharge + 2 nim. from the optical center of view as shown in Figure 1 was selected for the consistency of its emissive characteristics as disclosed in Figure 5 . Line absorbance values for carbon, manganese, and aluminum solutions shown in Figure 5 represent linear functions of the gal) within the first half of the open gap distance between the neutral ring and cathode. d width of view to be seen by the spectrograph should normally be chosen from this
Figure 7. Current optimizing on cathode side of center of gap; response of ion line increasing with increasing arc current
2392
ANALYTICAL CHEMISTRY
area. Intensities increasing toward the cathode rise inordinately as the view encompasses the leading edge of a cathode layer (dark space visible to the eye with 1)roper filtering). Kearer the cathode, ionization increases as do line intensities. Pnless extreme sensitivity for detection is required this region is wisely avoided to obtain the beat quantitative analyses and linear c,alibrations. Figure 6 s h o w that the arc current is highest at, lowest operating voltage or maximum voltage drop. Relatively little change in current is associated with the -2- to f6-mni. or 8-mm. normal or optimum operating gap. Slight wearing of the cathode tip has only a minor effect on current at high current value.. 1Iasimum cathode erosion occurs 6 nim. above the tip of the cathode. At the lower currents the optimum helium rate for a given gap indicates that if the arc were operated a t a lower optimum helium flow rate, the line intensities would tend to optimize on the cathode side of the center of the opening. To further evaluate this indication, Figure 7 shows the absorbance of some chromium ion lines a t wider gap and indicates the higher intensity with higher current. I t indicates optimization of response on the cathode side of center at about two thirds of the orifice to cathode distance. ;It a current of 15 amperes and a t a distance of 8 mm. from the top of the neutral orifice to the cathode, the ordinate difference shown in Figure 7 increases from 1.0 at 15 ampere3 to 1.5 A. a t 20 amperes to 2.4 A. a t 25 amperes. The lower ordinate values for cathode distances other than 8 mm. at constant current show higher
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Figure 6. Changes in arc current, smaller for higher currents and shorter gaps, shown with accompanying . - arc resistances and voltages
intensities on the cathode side of the 6-mni. center position. This effect is less pronounced a t the higher currents for anv given cathode nosition. Throughout the work previously discussed, a single sample size of 1 .O nil. was employed. The term sample size can h a w several meanings of some significance. I t is used hcre to denote total volume of liquid solution introduced into the arc. A\bsolute control of this quantity is required if 1)recision better than ordinary arc work is to be achieved. Figure 8 shows that when the amount of liquid sample introduced into the arc is increaqed, the time required to achieve a constant response for ion lines also increases and at a greater rate. Changes between 1.5 and 2.0 ml. per minute are greater than between 0.5 and 1.0 nil. per minute for constant gas plasma flow rates. This indicates that since the number of photons escaping from the discharge is decreased. "
Figure 5. Line intensities increasing linearly within the viewed orifice to cathode distance
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Figure 8. Sample sizes showing nonlinear effect on intensity-expressed as exposure time required to yield constant intensity; longer exposure times required at lower than optimum helium-to-argon ratios
the temperature of the arc is lowered 4gnificantly by the higher quantity of heat required. Xlthough the temperature inay drop only 100" in a 10,000' arc, the ion lines of chromium used for recording the intensities may be sharply affected. h change in ionization may take place. The example also shows that for lowvrr samlile sizes a higher helium rate is ncwl(d to shorten reslmnse time. For an a i y n rate of 4 liters ~ i e rminute, this 1)lot (*oversthe helium rate range Iielon. 40 liters of helium per minute. I n t h c w cq)rriinents argon is used as thc ahpii,ating gas while helium, a t thew loa.cr ratch. is us:cd as the tai ti:d $\vir1 ant1 control gab. -1 relat \waker hytlrodyiiamic~])inch on the arc cwlumn as it leaves the orifice probably iwult.: in lower temlierature, lower witha11)y, arid decrcased emission. PARAMETER OPTIMIZATION
The olwn gal) p1:tsma should be tl rarefully for smooth operation. ' l hoxit it ion of the cathode should be axially on center. Evidence of droplet buildul) on the top edge of the neutral oriLw indicates either an improperly adjusted as1:irator or a solution overloaded with additives. 'The condition of the neutral orifice should be .miooth, ~iarticularlyon the IOWCI. i n 4 r lip! to avoid unusual pattwnb in the horizontal curl of the arc cohinin. l'hc arc and circuit re. be evaluated as a funct ~iosition. h simple calculation using Ohm's law. as expressed by the actual olmating voltage measured across the m o d ( , and cathode and the simultaneous ar(8 current, yields the total circuit rwistance. If the total Iilasma is furirtioning as the sole significant re>i>tancc in the circuit, a smooth, noninfl(dtig extrallolation of the calculated rt4stance ix. cathode position will be ol)tained. If additional resistancc (or other c,lectrical circuitry acting to yicld resistance) is present, thc cwrve will intersect the resistance ordinate at the clcntr'r of the viewed area a t a higher ~.alue. The plot will then tend to yic.lti an inflection 1)oint when the ( w v c is turned to intersect the ' 'a at the Iiosition of the anode Iinonlcdge of the arc resistance is also essential to the >election of an 01)timuin cathode, I)o>ition. The oi)timuin gal) is selected as a iiinrtion of arc resistance and helium rate. I'ositionirig of the cathode at a distance that yields a linear function of arr resistance 1,s. helium flow rate produces an arc best suited to the geometrjof the aliliaratus used. It also permits one to select the optimum helium rate at any given current by linear exl'ression. T o measure the optimum gap, an arc
is operated on the swirl gas, in this case helium, and operating voltage and simultaneous current are measured for a series of gap settings. The plot shown as Figure 3 will reveal the most desirable gap. Selection of the optimum helium rate as a function of response at given current is a simple process, T o proceed, one selwts an arbitrary olierating (burion line of a,n elentrrestetl and then operates the Iilasma arc at the fixed gap he has previously determined. X sample of fixed concentration is fed to the arc a t a steady, fixed flow rat Spectrochzm. Acta. 15, 138-45 (1959). (3) Serin, P. A , , Eldorado LIining and Manufacturing Ltd., Port Hope, Ontario, ,private communication, 1963. (4) Sirois, E. H., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Paper 197, March 6, 1964.
LITERATURE CITED
RECEIVEDfor review June 1, 1964. Accepted August 24, 1964. Mid-America Symposium on Spectroscopy, Chicago, Ill., June 1964.
(1).Scribner, B. F., Margoshes, lI.$ “Excitation of Solutions in a Gas-Stabilized
Element Calibrations by Plasma Jet Spectrochemistry EDWARD H. SlROlS Major Appliance Division laboratories; General Electric Co., Louisville, Ky
b Quantitative analysis has been achieved by plasma spectrochemistry under noninterinfluence conditions in multielement environments. A single, widely applicable concentration calibration for manganese in acid s o b tions of different NBS alloy types is demonstrated. Application to systems such as iron, nickel, copper, aluminum, and zinc alloys is illustrated. The advantages of dilute solution preparation and plasma excitation are combined to yield reproducible results from accurate, linear analytical curves derived from solutions of NBS alloys. The basic approach used appears to be widely applicable.
S
have long been seeking the conditions necessary to produce a single, widely applicable analytical curve for an excited element. To achieve this desirable situation, it has been necessary to wait for a means of excitation capable of operation in a continuously controllable fashion at thermal equilihriurn. The plasma arc satisfies these requirements and also provides an operating temperature at atmospheric pressure well over the minimum value above which no solids can exist in nature. Furthermore, matter in this fourth state, the plasma state, behaves according to a unique combination of magnetic and hydrodynamic properties. 13y combining the advantages of having a liquid solution of alloys to Ilrovide the ideal sample with those of the plasma arc as an excitation source, it is now possible to establish a single analytical curve for a given element in a variety of metals and alloys. T h e econoniics of having separate PECTROCHEMISTS
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ANALYTICAL CHEMISTRY
quantitative calibrations for limitless combinations of elements in endless numbers of slightly dissimilar alloys is both painful in cost and staggering in labor. In these days of advanced automation one is still limited to using a special calibration for a particular element in a particular alloy. The work described here is an attempt to reduce this number and to approach the goal of a single analytical curve for the same element in any alloy, regardless of the original alloy composition. EXPERIMENTAL
The results reported here were undertaken in an analytical research effort after evaluation of the operating parameters of the plasma arc as discussed in a companion article in this issue. An external, uncooled, axial cathode was used in this work to transfer the direct current arc from the anode to a normal cathode height of 8 mm. above the neutral orifice ring. The cathode, composed of nominal l,/c-inch diameter carbon rod of 1.92 bulk density, was lowered to ignite the arc by contact a t 240 volts open circuit. Helium, used as the sheathing gas, was always turned on prior to ignition. .\rgon was used as the sample aspirator gas. Each liquid sample, of 1.0-nil. volume, was introduced a t full, y e s e t argon pressure and a t a precalibrated liquid flow rate of 1.0 ml.jminute. .ispiration was begun only after dynamic operating conditions had been equilibrated on helium. The exposure internal was begun instantaneously a t the start of both argon and liquid Sample flow. I t was terminated a t the end of liquid flow as signaled by abrupt changes in the arc current and operating voltage. The time interval eniployed was held constant. The amounts of acidic liquid solution samples used
were premeasured volumetrically in conically bottomed Teflon sample cups. They were then aspirated into the arc under constant,, previously evaluated conditions. The apparatus, equipment, and other operating parameters used in this invest,igation are the same as those previously reported by the author. (see page 2389). RESULTS
The operating parameters have been evaluated and interrelated. Since the sample size of 1 ml. of liquid solution has been fixed for injection into the arc at the const’ant rate of 1 ml./minute, there remains only one further quantity to specify when preparing t’he solution for quantitative analysis. This quantity is the amount by weight of total alloy remaining dissolved aft,er dilution of the original specimen in the acid formula. The task of specifying this quantity can be undertaken by first considering the total plasma within the arc per unit time as a fluid or a total solution of mixed ionic and atomic species. In contrast with ordinary “burning” arc environments, when an element is energized in the plasma arc the most, numerous species present are those of helium. Since helium is used as the sheathing gas and niay comprise over 90% of the mass of the plasma per unit time, one has essentially a dilute helium fluid in which all other elements are as solutes. To demonstrate the effect of the solute element (formerly representing the matrix element in t’he solid state) one niay measure and plot the absorbance of a line of any other element in the mixture against the concentration of t’he former matrix element. For example, if a zinc alloy containing aluminum is
