Characterization of the Voltage Fluctuation in the Direct Current Arc for Spectrochemical Analysis JAMES W. MELLICHAMP
U. S.
Army Electronics Command, Fort Monmouth,
Recordings of the voltage fluctuation in the d.c. arc give direct evidence of events that are taking place and can b e used for the development of analytical procedures. Recordings show sample consumption time, arc stability, reproducibility, and irregularities that are not readily discernible from spectrograms alone. A study has been made of the voltage fluctuation for different materials, including 67 elements in the elemental state, to correlate the recorded voltage patterns with the properties of the material. The patterns are found to b e a function of both the analytical conditions under which they are made-e.g., arc current, electrode design, arc gap, etc.-and the properties of the sample material such as boiling point, density, ionization potential, and reactivity with the electrode material or the surrounding atmosphere. Under standardized analytical conditions, voltage fluctuation patterns are different for each material and are characteristic of that material.
T
Institute for Exploratory Research,
I
analysis by the d.c. arc, the voltage across the analytical gap varies from a high when atoms or molecules originating from the electrode material and atmosphere are the only conducting medium to a low with the maximum prevalence of sample material. A previous study ( 6 ) shows that linear relationships exist between ionization potential of the arc gas and arc temperature, and between voltage drop and arc temperature. An increase in atoms with lower ionization potentials causes a drop in both voltage and arc temperature. The voltage remains constant only when the arc is stable and fluctuates with the dynamics of the atomic or molecular population as well as arc gap variations, current changes, and other nondetermined deviations. Direct evidence of events taking place in the arc, many of which can be observed visually, is obtained by voltage fluctuation recordings that can serve as permanent records for the development of analytical procedures. I n fact, the ability t o repeat a voltage fluctuation pattern requires that the analytical conditions be the same. I n two prior publications (1, 5 ) a change in voltage is h’ SPECTROCHEMICAL
N. J .
’
4
7
-
o 25~n
+- 0 . 5 - tIOrnv.
--
-RECORDER
ARC GAP
Figure 1. Circuit diagram of voltage divider that bridges arc gap with voltage recorder
used to indicate completion of sample consumption. Vallee and Thiers ( 7 ) show the relationship between the anode temperature and gap-voltage records for alkali and alkaline earth chlorides in the d.c. arc in helium and argon. .A study has been niade of the voltage fluctuation in the d.c. arc during the consumption of different materials, including 67 elements in the elemental or metallic state, to see what correlation could be found between the recorded voltage pattern and the properties of the material. “Characteristic-voltagefluctuation” (CVF) recordings give patterns that are repeatable and, although of secondary interest, can be used for identification of the elements. At present, patterns of mixtures of elements or the differentiation of compounds of the same element are either too complex or not sufficiently repeatable to permit much interpretation.
tional Carbon Co., Grade AGKSP), cored, and packed with a 1 to 2 mixture of BaC03 and graphite powder (Kational Carbon Co., Grade SP-1). With a cored cathode the arc is more stable, and the observed voltage fluctuation is attributable mostly t o effects originating with the anode (3). The cored cathode is necessary to obtain meaningful patterns. The excitation source is a Jarrell-Ash Varisource. The excitation stand and dual grating spectrograph used are made by Bausch and Lomb. RESULTS
X voltage fluctuation recording (Figure 3) made under the analytical conditions shown in Table I, but without the sample material, illustrates the relationship between the voltage drop and electrode separation. Both voltage and current vary linearly between 2 and 10 mm. The current is read and the
EXPERIMENTAL
Figure 1 shows the circuit of the voltage divider that bridges the arc gap and the recorder. The recorder scale is set, usually at 50 volts, by the use of a voltmeter and a dry cell battery with a variable resistor. The recorder traces the voltage fluctuation in the operating arc when switched into the system. With the divider in the circuit the arc must be drawn rather than struck with a spark because of damage by the applied high voltage. This system is accurate to within =t1 volt at 50 volts. Table I is a summary of the conditions under which t h e recordings are made. For simplicity, only the elemental state is studied. When practical, the sample is selected as a single, IO-mg. piece rather than as a powder or turnings. T h e cathodes (Figure 2 ) are cut from 0.18-inch-diameter graphite rods (Xa-
Table 1. Analytical Conditions for Study of Voltage Fluctuation in the D.C. Arc
Sample weight, mg. Anode design Cathode design
10 ( + l o mg. graphite powder) Ultra Carbon Co., preformed l 0 l L Special cored cathode (see Figure 2) 14 (short)
Arc current, amperes Analytical gap, 5 (manually mainmm. tained) Impressed line 300 voltage, volts Time of burn Total consumption of sample Electrode Not artificially cooled holders Voltage divider (see Figure 1) Recorder Bristol Dynamaster, 0.5- to 10-mv. range
VOL. 37, NO. 10, SEPTEMBER 1965
121 1
.042"4
(# 58 DRILL)
CORED CATHODE
TIP
Figure 2. Dimensions of cored cathode for arc stabilization for voltage fluctuation recordings Core packed with 1 to 2 mixture O F BoCOg and graphite powder suitable for pellets
voltage recorded while electrodes are separated in 1-mm. steps a t 10-second intervals. A 1-mm. change gives rise to about a 3-volt fluctuation. Figures 4 and 5 are CVF patterns of 42 of the 67 elements tested. The patterns are a function of both the analytical conditions and the properties of the element, such as boiling point, density, ionization potential, and reactivity with the electrode material or surrounding atmosphere. Since the conditions are standardized, differences are attributable to the specific properties of the element that determine the arc temperature and the rate of evaporation. Of lesser significance but detectable in the patterns are the sample form (whether powder, turnings, or a single piece) , how the sample lies in the anode cup, and the presence or absence of graphite as a buffer. Because the voltage is sensitive to factors not completely under control, the over-all pattern characteristic of the element varies
Table II. Consumption Time of 10-rng. Indium Samples with Changes in Arc Current, and Electrode Design and Grade as Obtained b y Voltage Fluctuation Recordings
Electrode design" and effective cross-
sectional area, sq. mm.b 115 (1.16) 116 (1.77) 117 ( 3 . 5 2 ) 118 (4.94)l 0 l L (7.00) ClOlL (7.05) 115 ( 1 . 1 6 ) 116 ( 1 . 7 7 ) 118 (4.94) lOlL (7.05) 117 ( 3 . 5 2 ) 117 (3.52) 117 (3.52) 117 (3.52)
Sample conLowest sumpvoltion Amperes tage t'ime, (short) drop seconds 14 14 14 14 14 14 8
10.5 17 21 8 10.5 17 21
23 22 22 22 22 23 27 24 22 20 27 24 22 20
12 20 35 52 96 75 55
28 52 86
io 55 30 20
a Preformed, Ultra Carbon Co., Grade U2, except ClOlL which is U5 carbon. b Effective cross-sectional area of anode cup = A (outside) - a (inside).
1212
ANALYTICAL CHEMISTRY
slightly with each repeat burn. Ten repeat burns of Si and Fe show that the patterns are sufficiently similar to distinguish one element from the other. Variations between patterns of the same element are due to factors that limit reproducibility of d.c.-arc procedures in general and are based on accuracy of repeat weighing, sample loading, variations in electrode dimensions, the arc stability, and anomalous reasons not subject to control. Thus far, no way for assigning numerical values to the patterns has been devised. The use of voltage patterns as a method for the study of the stability and reproducibility of the d.c. arc is to be discussed elsewhere ( 3 ) . Recordings show that the voltage is highest, 60 volts, when no sample material is in the arc column. The voltage drops with the entry of the element from the anode cup and reaches a minimum at the maximum sample evaporation rate. Upon total consumption the voltage returns to its original high value. The arc temperature, approximately 7000" C. maximum, is assumed to follow a similar pattern, provided that a linear relationship does exist with voltage drop. Initially the arc is unstable until the electrodes become incandescent. Inadvertent Eakes of graphite or sample on the electrode tips also give rise to irregularities. The warm-up period necessary for an element to reach boiling point, which is determined by its density-depth in cup-as well as the electrode thermal properties, appears in the CVF pattern as a relatively straight line near 60 volts. The alkali and alkaline earth metals ignite with little or no warm-up period. Total consumption time varies from under 10 seconds for Hg, P, ils, S, Se, and Te to as much as 5 minutes for Nb. Some of the U, Th, Ir, refractory elements-e.g., and 110-are not totally consumed and give erratic patterns with few discernible characteristics. Fluctuations are also caused by possible reactions of the sample with the electrode material or the atmosphere, or by uneven sample distillation. Other fluctuations are caused by arc wandering due to sample condensation on the cooler portions of the cathode. The slope of a CVF pattern shows the changes in the evaporation rate of a n element and can be steep-e.g., Al, Mn, Pd, Cr, Zr-or gradual-e.g., Ga, T1, Gel Pb, Xg, Cu. Other elements-e.g., the rare earths-do not attain a constant evaporation rate under the selected analytical conditions but pulsate throughout consumption. Voltage drop is determined by the ionization potentials of the atomic or molecular population in the arc column, the component having the lowest potential being the most effective. Voltage drop
4
2
0 yl
e
nk!
U 6
4
2
3
ARC G A P Lmml
Figure 3. Recording of voltage change with electrode separation Amperes Volts C. Recorder tracing of voltage Current read while voltage is recorded and electrodes separated in 1-mm. steps at 10second intervals
a.
b.
along with the degree of arc domination by the sample element varies with the ionization potential, arc current, and the rate of evaporation. Ii'ith the carbon arc the sample element may never be totally dominant,.as can be seen by the presence of CK, Cp, etc., bands or spectral lines in the emitted spectra; however, volatile elements with low ionization potentials-e.g., the alkali metals-approach domination with the lowest recorded voltage of around 20 volts. The highest is above 40 volts for elements such as Au, Pt, T a , and W. The lowest voltage drop for an element is reached only when both electrodes are constructed of that element. Indium is selected because of its good burning quality and ease of handling to illustrate further the use of CVF patterns for procedural development. The experiments are based on prior work (4) in which it is shown that electrode burning rates are dependent on the arc current, as !Tell as cross-sectional area and thermal properties of the electrodes. Sample consumption is, in turn, dependent on the electrode burning rate. Figure 6 shows differences in sample consumption time with anodes of varying capacity. Figure 7 shows that the time is reduced from 96 to 7 5 seconds simply by the substitution of a carbon anode for a graphite anode of the same design. Figure 7 also shows that when graphite powder is added as a buffer the arc is a little more stable a t the start and the burn is smoother. However, the added graphite causes a "rise and fall" in voltage near the end of the burn when the bulk of the powder is swept into the arc. Table I1 is a summary of experiments made by vary-
60
40 50
AI 40
-ti6
30
3060
40 30 20 -
50
50
Na
40
30t 30
30
-
60
cs
50
60
a
40
40
30 20 301
I
Ge sc
50k
cu
60
I
40 50
40
40
-
30 I
,
60
50 40
+IO-
30 60
60%
50
50
40
40-
30
30-
120
90
60
30
mm
301-
0
-
120
so
'
40 30 90
60
30
0
120
I
I
I
90
60
SO
11
0
SECONDS Figure 4.
Characteristic-voltage-fluctuationpatterns of 2 1 elements
For purpose of charting portions of several patterns removed as indicated
VOL. 37, NO. 10, SEPTEMBER 1965
1213
:D ::b
Ce
50
40
-
30
-
4
30
30
PI
30 -
40
40
30
i:
Nd
40
60
-
50
-
40
-
:,::
v
40
-+70
30
30
60
60-
50
50
30
-
VI
5
I r i
40
40
0
' 30
30
--
60
60
-
30
ti20
7.7 i':
50
40
40
30
30
--+35
30
60 50
40
40
:D 30
60-
40
40t 1 -
30 120
60
90
30
0
180
I
IJO
I
120
I
I
90
60
I
30
4.
SECONDS Figure 5.
Characteristic-voltage-fluctuationpatterns of 2 1 elements
For purpose of charting portions of several patterns removed
1214
ANALYTICAL CHEMISTRY
a5
indicated
'I1
0
40 30
0
ing the electrode 'grade and cross-sectional area, and changes in arc current. As seen in Table 11, the voltage is a function of the current and not of the consumption time. Moving plate studies of indium in conjunction with voltage fluctuation patterns (Figure 8) show that the voltage drops beIow 50 with sample emission and that the maximum emission and minimum voltages are in the last seconds of the burn. The background emission, represented in Figure 8 by a segment of the C N band system, is the reverse in intensity to the sample emission. A break in the final voltage
c
-n
-e 5
-
In
5
0
2
-
s
-
I20
4
so
-
100
80
60
40
20
SECONDS
Figure 6. Voltage-fluctuation recordings of consumption of 10-mg. indium samples with different anode designs Ultra Carbon Co., preformed designs (a) 1 1 5, (b) 1 16, (c) 1 17,(d) 1 18,(e) 1 0 1 1 Initial dip i s start of evaporation, "upsweep" Is completion. Slight break in "upsweep" i s coincident with impurity emission
' O t
120
\ d too
eo
60
40
20
o
c
SECONDS
Figure 8. Voltage fluctuation recordings of IO-rng. indium samples, with segments of background and sample emission Ultra Carbon Co., design.
TO O R l M SHUT
PIIIL-APERTURE
CAM
Figure 9. Diagram shutter to increase ground" ratio
of
automatic "line-to-back-
Shutter opens below 50 volts and closes above 50 volts durlng consumption of sample material
in a 10-mg. sample, and their reactivity with the electrode material and atmosphere. These differences are reflected in their CVF patterns. Since the ionization potential of the cation is in common, the lowest voltage drop is about the same for all. An example is the CVF patterns of M g and MgO, as shown in Figure 10. They are almost mirror images of each other with the same low value of 27 volts. Eventually it may be possible to differentiate and identify compounds of the same element by their CVF patterns alone. Patterns of mixtures of elements are complex as compared with a single element and little interpretation is practical. For example, Nichrome combines the characteristics of both elements with no separation in volatilization. Another example is CaWO4, where some separation in the volatilization period of the Ca and W can be observed in the pattern. I n general, a pure compound has less voltage fluctuation than
Top 1 1 8, bottom
1011 Top spectrum in each recording is portion of C N band system representing background, bottom spectrum is indium spectral line.
c
SECONDS
Figure 7. Voltage fluctuation recordings of the consumption of IO-mg. indium samples with different electrode grades and the presence or absence of graphite powder Ultra Carbon Co., preformed designs ( 0 ) Carbon lOIL with IO-mg. indium and added graphite powder, (b) Graphite 1011, with IO-mg. indium and no added graphite powder (less smooth burn), (c) Graphite lOlL, with IO-mg. indium and graphite powder
upsweep is coincident with a flash from impurity emission. To take advantage of these points and increase the line-tobackground ratio, a device has been constructed (Figure 9) to open the spectrographic shutter automatically when the arc voltage is below 50 and close i t above 50. A battery-operated magnetic shutter, mounted on the preaperture of the spectrograph, is actuated with a microswitch triggered by a cam on the recorder drive shaft. Modifications of this device are possible for operation with any d.c.-arc procedure. However, a necessary precaution is that a prior time study be made to obtain the correct voltage of matrix and impurity emission. The compounds of a n element differ from the element and each other in boiling point, density, available cations
lZp
100
BO
60
40
20
0
c
SECONDS
Figure 1 0. Characteristic-voltagefluctuation patterns of M g and M g O Patterns are almost mirror images of each other and both have same lowest voltage value of 27 volts
VOL. 37, NO. 10, SEPTEMBER 1965
1215
an impure sample of the same compound. Although the use of patterns of mixtures is somewhat limited because of their complexity, such patterns are of value for procedural purposes. DISCUSSION
Voltage recordings are a useful supplement to laboratories that analyze a variety of materials. They, in conjunction with the spectrograms, are direct evidence of the responses of sample materials to analytical conditions and are indicators for the improvement of these conditions. The recordings show not only consumption time but also arc stability and irregularities due t o sample ejection, incomplete consumption, etc., which are not readily discernible with spectrograms alone, Voltage recordings in conjunction with moving plate studies are particularly useful for the interpretation of events that are taking place in the arc and are somewhat similar to a motion picture of
the arc. With experience a preliminary identification of a material can be made prior to the development of the spectrogram. Since recordings are dependent on the analytical conditions and the properties of the analyzed material and not on the method of recording, it is possible to use these patterns as a reference standard between laboratories. For example, in publications of d.c.-arc procedures a pattern would contribute to the comprehension and duplication in other laboratories. Voltage fluctuation recordings are a source of data for the understanding of the mechanism of the arc under analytical conditions. While information on the physics of the arc is available in the literature-e.g., (2)-much does not apply to the spectroscopist confronted with a large number of analyses. This investigation is an application of known technology in another field for the solution of problems in analytical spectroscopy.
ACKNOWLEDGMENT
The author expresses his appreciation to B. F. Frowner, F. M. Plock, and J. J. Finnegan for their helpful assistance throughout the course of this study. LITERATURE CITED
(1) Ahrens, L. H., Taylor, S. R., “Spectrochemical Analysis,” p. 186, AddisonWesley, Reading, Mass., 1961. (2) Loeb, L. B., “Fundamental Processes
of Electrical Discharges in Gases,” Wiley,.New York, 1939. (3) Mellichamp, J. W., unpublished data, November 1964. (4) Mellichamp, J. W., Buder, R. K.,
Appl. Speclros. 17, 57 (1963). ( 5 ) Oertel, A. C., “Spectrochemical Analysis of Mineral Powders,” p. 63, Div. of
Soils, C.S.I.R.O., Adelaide, Australia,
1961. (6) Semenova, 0. P., Compt. Rend. Acad. Sca. URSS 51. 683 (1946). ( 7 ) Vallee, B. L., Thiers, ’R. E., J . Opt. SOC.Am. 46,83 (1956).
RECEIVEDfor review April 21, 1965. Accepted June 23, 1965.
Calculation of Concentration Corresponding to the Point of Intersection of High and Low Concentration Segments of Analytical Curves in Atomic Emission Flame Spectrometry JAMES WINEFORDNER, THOMAS VICKERS,l and LLOYD REMINGTON2 Department o f Chemistry, University o f Florida, Gainesville, Fla.
b Analytical curves in atomic emission flame spectrometry plotted on logarithm coordinates are generally characterized by two distinct regions. In such cases, the slope of the linear portion of the high concentration segment is about one half of the slope of the low concentration segment. By means of simple theory, the concentration corresponding to the point of intersection between the extrapolations of the linear segments of the high and low concentration portions of an analytical curve can be calculated. The case in which a multiplet is not resolved by the experimental system is also considered. Experimental data is in good agreement with calculated results for several spectral lines of several elements in several flame types.
A
CURVES in atomic emission flame spectrometry are generally characterized by two distinct regions (1, 2, 7 ) . The slope of the analytical curve plotted on logarithmic coordinates in the high concentration region is, in many rases, approximately NALYTICAL
1216
ANALYTICAL CHEMISTRY
one-half of the slope for the low concentration region, as one would predict from astrophysical growth curves (8). Deviation from this ratio (1, 2, 7) may occur a t very low sample concentrations because ionization may then vary significantly with concentration, and a t very high sample concentration because compound formation may vary significantly with concentration and because of instrumental reasons such as saturation of the photodetector. For analytical purposes, working in the concentration region of maximum slope of analytical curves is obviously advantageous. Two points on analytical curves in atomic emission flame spectrometry are therefore quite important when characterizing spectral lines of atoms for analytical purposes. These two points are the limiting detectable concentration and the concentration corresponding to the point of intersection between the extrapolations of the linear segments of the low and high concentration portions of an analytical curve. The limiting detectable concentration can be calculated according to the
method of Winefordner and Vickers (12). The concentration corresponding to the point of intersection between the extrapolations of the linear segments of the high and low concentration portions of an analytical curve can be simply calculated by the equations given in the following section. THEORY
The flame source will be assumed to be in thermal and chemical equilibrium (6). This assumption is approximately valid for the central region of the outer cone of most analytical flames. The integrated intensity, 11, of a single, isolated spectral line for an optically thin medium-i.e., for a dilute atomic vapor of the emitting species-in units of watts cm.+ ster.-’, is given (12) by the well known equation Present address, U. S. Army Missile Command, Directorate of Research and Development, Physical Sciences Laboratory, Redstone Arsenal, Ala. Present address, Asheville-Biltmore College, Asheville, N. C.
