The atomizer orifice conditions are maintained in the proper relationship in two respects: sample and atomizer gas openings concentric in exit plane, and sample tube axis and arc direction in line on the vertical axis. Attention to details is essential. All factors must be under control for each exposure, since the basic tenet of the plasma arc as an excitation source is continuously controllable excitation of dilute plasmas a t very high levels of thermal equilibrium. There is no optimum value for a single
variable except in relation to all of the others. The most significant direct variable in plasma arc spectrometry is the rate of flow of the sheathing gas (with arc current held constant). I t must be optimized by each investigator using conditions best suited to the geometry and electrical limitations of his particular experimental apparatus.
Arc Source,” 9th Colloquium Spectroscopicum Internationale, Lyon, France, June 6, 1961. (2) Scribner, B. F., hlargoshes, AI.> 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 solutions 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
2394
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
1 .o
added element. I t is evident now that if we wish to measure the concentration of aluminum in a solution of a zinc alloy or in a solution of a magnesium alloy with a linear and uninfluenced analytical curve, we must not dissolve more than 6.8 mg.;’ml. of zinc from a zinc alloy or 2.4 mg./ml. of magnesium from a magnesium alloy relative to about 2.7 nig./ml. of aluminum, Since the aluminum analytical curves in this case are act,ually found to be linear and essentially unaffected up to -10 mg. -il/ml., we can also safely measure aluminum using solutions of other than aluminum alloys in which the solid state matrix element concentration in sohtion is not more than 1.5 mg. atoms of that element per milliliter. When the amount by weight of an added element contributes more atomic species than the element already present (in this case aluminum), the total plasma composition is significantly altered. The added species have then exceeded an equivalent atomic concentration level which no longer represents a negligible portion of the total plasma system. I n any plasma arc, to achieve conditions under which elements will not enhance the emission of others, no species can exceed a concentration above which the emission for the given line of the element of interest is affected. Experience shows that interinfluence conditions such as shown in Figure 1 are absent in sufficiently dilute systems. an example for which the alloy solute element no longer reflects its origin and previous metallurgical history, Figure 3 s h o w an analytical curve for
NONLINEAR, ENHANCED EMISSION AAl AT 4 rngJml. CONSTANT AI CONC.
+ .* ou 03 03
K
9 v)
m .6
7 UJ
U
Z
3
5m
.4
m
u .3 10
20
mg./ml. CONC. OF ELEMENT
30 ADDED
-
Figure 1. Plasma solute elements actively enhancing the intensity of the element of interest
dissolved and the absorbance of an aluminum line is plotted us. the concentration of the zinc, a plot of continuously increasing slope is obtained if the zinc concentration is increased while the aluminum concentration is held constant. When the zinc concentration is low and the plasma is sufficiently dilute in respect to zinc, a linear plot of t,he absorbance of the aluminum line 11s. zinc concentration is obtained. As the zinc concentration is increased further, the absorbance of t,he aluininuni line increases a t a greater rate indicating that at higher concentrations of the former matrix element (zinc) in the total plasma, there is an enhancement effect on the elements of lesser concentration. This situation is shown in Figure 1. If an aluminum alloy is dissolved so that the concentration of aluminum in the solution is 4 ing.;ml., it is found that the absorbance of aluminum line 2567.8 -1.under the specified conditions is approximately 0.3 unit as in Figure 1. Metallic zinc is added to the solution by dissolving it without changing the volume. The absorbance of the aluminum line is plotted 21s. the concentration of the added zinc. I t is found that the rising absorbance of the aluminum line is no longer a linear function concentration of the element added or of its own concentration. The same situation is .shown in the case of disolving magnesium instead of zinc or of adding a compound containing boron atonis. In each case the plot of the absorbance of aluminum t’s. t,he concentration of the added
element intercepts the z axis a t a different value. This intercept value indicates that each different added element has a different threshold effect in enhancing the aluminum absorbance. T o interpret this effect, the atomic weight of the element added is plotted us. the milligram per milliliter concentration intercept and is shown as Figure 2 . The intercept values in Figure 2 represent equal numbers of atonis of the
INTERCEPT VALUES
100
AT
REPRESENT EQUAL NUMBERS OF ATOMS
80
0
M I 60 C W E 40
I G
H 20 T 1 mgJml.
2
3
4
CONCENTRATION
5
6
7
INTERCEPT
Figure 2. Enhancement of aluminum emission interpreted in terms of atomic weight of added element VOL. 36, NO. 13, DECEMBER 1964
2395
aluminum, zinc spelter or zinc based dye casting alloys, and stainless steels such as 18 chromium, 8 nickel, etc. In each of these solutions the former solid state alloy matrix element (namely iron, nickel, coplier, zinc, aluminum, or chromium) has an atomic weight of a fairly high order and thus contributes few atoms of the Imrticular element (at the 5 mg./'ml. concentration level). In no case is the concentration uf the former alloy matrix element high enough to influence or enhance the emission of this manganese line at these manganese Concentration levels. These solutions were prepared by dissolving Sational Bureau of Standard certified alloys. I t was assumed in each case that the concentration value for manganese wai correct. I t was predicted that the absolute concentrations of manganese a t other apl)rol)riate dilutions should, therefore, all fall on a similar curve. This is the case as shown in Figure 3. The wet chemical preparation of these alloy solutions requires only that the
2396
ANALYTICAL CHEMISTRY
20
-5. ._
'\
-
'\
3
-
60
2
\
-
z
2 4 0
N I C K E L 3414.765
3 9 15 17
I
-
TYPE
K E_ Y _ NB _ _S
'
0.2
161 126a lOlb 160
Ni/Fe Fe/Ni Fe/Cr/Ni Fe/Cr/Ni/Mo I
1 .o
I
2 .o
\-
centration levels within which the alloy elements are significant portions of the mass of the plasma, enhancement, nonlinear calibrations, and poor reproduction of response is obtained. Since elements that are normally solutes to the alloy matrix element are now subsolutes to the plasma, analytical curves based on the energy emitted as light in these dilute systems are quite
linear and very reproducible. The dependent relationship of alloy solute elements to the principal alloy element is eliminated. Elements in solution become reasonably independently emitting species in the total plasma. The author recognizes the necessity for considerably more work in substantiating the universality of the example demonstrated here with
manganese. It is hoped that spectrochemists will try this general approach on their individual problems, after first optimizing their plasma arcs, and thus provide additional evidence. RECEIVED for review June 1, 1964. Accepted September 28, 1964. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1964.
Infrared Emission Spectra of Solid Surfaces M. J. D. LOW and H. INOUE School of Chemistry, Rutgers, The State University, New Brunswick, Infrared emission spectra were obtained of oleic acid on the surfaces of aluminum plates, as well as of silicone lubricant, paint, paper, rubber, and polyethylene. The exploratory experiments indicate that emission methods could b e applied in special cases for the examination of the surfaces of opaque bulk solids.
S
OMF YEARS AGO
Eischens and Pliskin
( 1 ) described several experiments
debigned to explore the possibility of using infrared emission spectroscopy to study the structure of molecules chemisorbed on the surfaces of bulk metal specimens. The very poor resolution of the spectra obtained and the failure of some experiments cast doubt on the scope of application of the method. However, the potential of the emission method is great, as it could provide a means of studying the surfaces of opaque materials. IVe have conaequently made a series of experiments to explore the application of this method.
N. J .
cause of poor thermal contact between foil and plate. For experiments with oleic acid, the emitter plates were polished with grade 000 sandpaper and heated in air a t 400' C. for about 4 hours prior to each experiment to obtain stable surfaces, except where otherwise noted. A plate or pair of identical plates was then installed and brought to constant temperature, and a background spectrum was obtained. A thin layer of oleic acid was then quickly spread on a plate with a filter paper or glass-wool brush, excess oleic acid was wiped off with filter paper so that the plate surface appeared to be dry, and the emission spectrum was recorded. Samples of materials other than oleic acid were pressed against a n emitter plate. The emitting surfaces were placed about 5 cm. from the PE 112 slits and about 2
EXPERIMENTAL
A11 the emission spectra shown below were obtained with a Perkin-Elmer ?*Iode1 521 spectrophotometer. Satisfactory .spectra could also be obtained with a Perkin-Elmer Model 112 spectrometer using NaC1 optics. I n each case, the original thermocouple detector was replaced by one of higher sensitivity purchased from C. 11. Reeder Co. Spectra were rerorded a t normal ami)lification-i.e., a t 1 x scale expansion excellt where noted. [-nits of emission and absorption ordinates are arbitrary . Figure 1 shows the emission devices. Plain sheets of metal were used, rather than the rods described by Eischens and Pliskin. The emitter plates could be easily replaced or removed for cleaning. Some experiments were made with plates coyered with household foil, but were abandoned because it was very difficult to obtain reproducible background emissions, probably be-
s
Figure 1 .
S
Emission devices
The devices were m a d e o f '/ls-inch aluminum sheet. Dimensions a r e not critical. Device A, used far single-beom operation, could b e positioned b y sliding the plate, P , into the sample holder of the instrument. Device 8 wos used for double-beam operation. A heater, H , directly behind the emitter, S, was m a d e o f insulated Nichrome wire. Temperature was measured b y means of a thermocouple behind S
em. from the PE 521 slits. Variations in sample position did not bring about significant changes in the detected emission. The calculated spectral slit width a t 1600 em.-' with 290- and 500micron slits was 3 and 5 cm.-l, respectively. RESULTS
Most of the work was done with oleic acid on aluminum plates. Representative emission spectra as w-ell as an absorption spectrum of oleic acid are shown in Figure 2. h'oticeable in all spectra are bands near 5.82 microns, the carbonyl C=O stretching mode that is characteristic of C 4 associated with the acid hydroxyl groups. This is the pronounced feature of the oleic acid spectrum. Ionization of the carboxyl group removes the 5.82-micron band and brings about' bands at' higher frequencies, spectra of oleates showing strong bands near 6.4 microns as well as weak ones in the region of 6.8 to 7.0 microns, corresponding to antisymmetrical and symmetrical vibration of the ionized carboxyl group, respectively. The emission spectra also show bands near 6.4 microns. Or, the emission spectra show the pressure of both undissociated and dissociated oleic acid, implying that an interact,ion had occurred between the aluminum emitter plate and the acid. The C-H stretching frequencies of CH3 and CH2 groups of both acid and oleate occur near 3.37 microns with a separate CH, band at 3.52 microns. There are additional bands for oleic acid in the 3- to 4-micron region corresl)onding to OH stretching bands of strongly bonded acid dimers. Broad emission bands centering near 3.4 microns could be detected from plates treated with oleic acid, indicating that the C-H stretching frequencies were observed. Resolution was relatively poor, however, separate bands a t 3.37 and 3.52 microns not being clearly distinguishable, Reproducible spectra could be obVOL. 36, NO. 13, DECEMBER 1964
* 2397
