Volatilization Rates of Elements in the Helium Direct Current Arc Utilization of a Matrix for Biological Work BERT L. VALLEE AND RUTH W. PEATTIE Massachusetts Institute of Technology, Cambridge, .?lass. The volatilization rates of 31 elements were studied a t 6 and 15 amperes for neutral atom lines and lines of the first ionization by the use of the moving plate technique. The volatilization characteristics of some elements differ in air and helium, all other parameters being constant. This is an important phenomenon when quantitative analytical spectrography is to be performed with the direct current arc using the internal standard principle, as the volatilization characteristics of the elements are an important criterion for the selection of internal standards. When helium is the environment of the direct current arc, it favors the excitation of ion lines and may depress the signal of neutril atom lines. The relative sensitivity of spectrochemical deter-
minations in helium using neutral atom lines is nevertheless often better than in air because of the low background. This feature is especially favorable when lines are in the cyanogen region of the spectrum where, otherwise, they could not have been determined at all. The volatility of many elements exhibited a qualitative resemblance related to their chemical grouping. Arcing at 15 amperes reduced sensitivity markedly and diminished differences in volatilization time and contour. Thus the choice of helium as an environment of the direct current arc brings about decisive changes in volatility and line intensities. The resultant improvement in sensitivity for the condition stated should be a material aid to the analyst.
A
Composition of the matrix: cations, anions, and graphite or carbon mixed with the sample Electrode material and shape Electrode separation and voltage drop Amperage Source of excitation
PRETTOUS communication (6) reported the use of helium as an environment for the direct current arc and a resulting increased ratio of signal to noise for the detection of the lines measured. This technique has been used with good results for quantitative spectrochemical analysis of many elements in biological material. In the pursuit of this work it became apparent that the volatilization characteristics of some elements in helium and air differed, all other parameters being constant. Similar observations, when argon was the gaseous environment of the arc, have already been reported briefly (6). These findings are of more than academic interest when quantitative analyses are to be performed in helium or argon utilizing the internal standard principle. The selection of an ideal internal standard aims at matching the element or elements to be determined with others, the chemical and physical behavior of which resemble one another in a flame, arc, or spark discharge. The intensity ratios of the “unknown” and internal standard elements are used as criteria of concentrational variation, and the internal standard element minimizes the effects of volatility, physical composition, source fluctuation, timing of the exposure, and the photographic process. Among tho criteria governing the selection of internal standards for the direct current arc, the similarity in volatilization rates of the internal standard element and the analysis element is an important one, as is the similarity in excitation potentials of the lines chosen ( 1 ) . While internal standards are often selected empirically, optimal results are best obtained when such factors are taken into consideration. Under especially favorable circumstances a very high degree of precision can thus be obtained with the direct current arc as exemplified by work on rare earths (3). The highly desirable sensitivity of the direct current arc, required for specinl purposes, may thus be retained, while precision is much improved. The volatilization of elements in an arc is generally a function of their vapor pressure. However, many other factors have a profound influence on this phenomenon. They may be listed as follows:
Any system used in spectrochemical analysis must be rigidly controlled if reproducible volatilization, intensity ratios, and consequent precision are to be obtained. Any data obtained will hold only if all experimental conditions are rigidly reproduced. Extrapolations from these data to other matrix and compositional conditions are only a first approximation. The experiments described were performed to m e s s thetvolatility of elements by a study of neutral atom and-Rrhere possible-lines of the first ionization a t different amperages and in different gaseous environments for 31 elements by use of the moving plate technique. One group of elements chosen for this work has been found present in biological materials: lithium, rubidium, magnesium, calcium, strontium, barium, titanium, zirconium, vanadium, chromium, molybdenum, manganese, iron, cobalt, nickel, copper, silver, tin, lead, and aluminum. Another group was evaluated as potential internal standards on the basis of their absence from biological samples thus far examined: beryllium, yttrium, scandium, columbium, rhenium, ruthenium, palladium, gallium, indium, germanium, and bismuth. Where possible, a neutral atom line and a line of the first ionization of the same element were evaluated for reproducibility and sensitivity in analytical procedures. METHODS AND RIATERIALS
A matrix simulating biological ash was prepared having the following composition:
This matrix had previously been found adequate when a null. method of analysis for biological samples was employed. Follow-
Nature of the chemical compound in which the analysis element is present
434
V O L U M E 2 4 , NO. 3, M A R C H 1 9 5 2 ing extensive trials with various other salts, calcium carbonate was chosen as a spectroscopic buffer. Calcium carbonate was found to enhance the lines of most elements measured, whereas sodium chloride, potassium chloride, sodium dihydrogen phosphate, sodium monohydrogen phosphate, sodium phosphate, potassium monohydrogen phosphate, potassium dihydrogen phosphate, potassium phosphate, sodium sulfate, potassium sulfate, and calcium phosphate depressed line intensities. Alumina was comparable to calcium carbonate, but had no additional advantages. The addition of carbon further increased sensitivity. One part of matrix plus one part of calcium carbonate plus one part of graphite was found satisfactory. Calcium carbonate acted as buffer throughout the arcing period, and background was not unduly increased upon the addition of graphite. Further augmentation of calcium carbonate or graphite concentration gave no additional improvement but decreased sensitivity as a result of dilution or increased background. Spectroscopically pure salts of the elements were incorporated into the matrix for volatilization. Contamination is a severe hazard when minute concentrations of elements must be measured in biological material. Because the availability of spectroscopically pure salts is severely restricted, their use in these exploratory investigations appeared mandatory if the data were to be applicable to subsequent analytical work based on them. A report of the Atomic Energy Commission ( 2 )served as a guide for selection. Most salts were products of the Johnson-Matthey Co., Ltd., London, obtained through the Jarrell-Ash Co., Boston, Mass. Wherever possible, the oxide was chosen t o keep conditions comparable for all elements. Salts of other anions had to be used in some instances where spectroscopically pure oxides were not available. The following 28 salts were employed (calcium, magnesium and iron were present in the matrix): LikOs, RbCI, BeS04.4Ha0, SrCO,, SCZO~ BaCI2.2Hz0, YzO~, TiOz, ZrOz, VZOS,Cb205, CrzO3, hfOO3, Mna& NHaRe04,CoaO,, XiO, (“4)zRuCI~,.(NH,)IPdCl~,CuO, AgzO, GanOa, Inz03, GeOz, SnOz, Pb(NO&, Biz03, 8 1 ~ 0 3 . These salts were weighed to obtain 2 mg. of the elements, the volatilization of which was to be studied. Cadmium and zinc were added in 4-mg. concentrations to obtain volatilization data, Iridium, tantalum, gold, and cesium could not be detected satisfactorilv even a t these concentrational levels. All were added togethe? to 500 mg. each of biological matrix, calcium carbonate, and spectroscopically pure graphite, and ground for 1 hour in an agate mortar, giving an approximate final concentration of 1330 p.p.m. This high concentration was chosen to give measurable blackening for successive 5-second exposures. Self-absorption of some lines was anticipated and observed a t some stages of volatilization but never seriously interfered with the analysis of the data. The material was packed into “special graphite spectroscopic electrodes” (National Carbon Go.) of 0.181-inch outside diameter which were drilled to 0.131-inch inside diameter and 0.078-inch depth. All samples were stored in a desiccator until arced, to prevent the adsorption of moisture which might cause the sudden ejection of the sample from the arc. When helium was employed, the conditions described in a previous communication (6) were reproduced with the addition of a 40-mesh wire screen in the beam of the emitted radiation; the arc gap was approximately 5 mm. A 15,000 line per inch 35foot Wadsworth grating spectro raph was employed using three 4 x 10 inch 103-0 Eastman KoJak plates; the slit width was 30 mp. The plate factor is 3.46 A. per mm. in the first order, and one exposure in this order photographs the region of 2500 to 5000 A. Selection of lines for.measurement was confined to this region. -4fter the arc was struck, the plate holder was racked manually at &second intervals, timed by a stopwatch. The interval b e tween subsequent positions was held close to 1 second, though the nature of the operation resulted in minute differences in timing. Each spectrum exposure was made 3 mm. high by means of a fishtail. Racking was continued as long as plate space permitted. The sample concentration and size were chosen to allow almost complete volatilization and recording on one set of plates. Plates were developed for 3 minutes in D-19 developer, immersed for 30 seconds in an acid stop bath, and‘fixed in acid hypo (Eastman Kodak). Plates were calibrated as previously de-
435 scribed (7). Densitometry was performed with a Jarrell-Ash microdensitometer; the slit width was 10 mp. Wherever possible, a neutral atom and an ion line of an element were measured. This was done to evaluate the influence of atmospheres on line excitation and any ossible relationship of observed differences to excitation potentiat’ Lines were measuredin air and helium a t 6 and 15 amperes. Background was determined about 0.4 mm. from the line. All readings obtained in per cent transmittance were expressed in r e l a tive intensities using an “H & D” curve, and the relative intensity of the lines was corrected for background. The corrected relative line intensity (signal) was then plotted against time. The data presented were the results of one volatilization experiment. Reproducibility was ascertained by subsequent experiments not reported here. R E LATIP E INTENSITY
‘WE
ION LINE- Be 3131@72V2 E Ve*132eu
6 AMPERES
15 AMPERES
1 II
I
\
NEUTRAL AT@M LIN,E- Be 3321.343U2 IOOC
6 AMPERES
15 AMPERES
t
‘OF
c 1
HI
TlME IN
\
SECONDS
Figure 1. Volatilization Rates of Beryllium
- - - InIn
helium air
The migratory behavior of the direct current arc is notorious. Minor fluctuations in the release of elements from the sample are always encountered, are not reproducible, and cannot be considered significant. The curves were idealized allowing for a 10% fluctuation. A method of standard usage in graphic practice was applied. Table I shows uncorrected data as obtained for Cu 3247.540 (VI) (symbols as used in 4 ) . The line intensity (signal) rather than the signal-noise ratio has been plotted. The area under each curve is an index of total radiation received by the plate but not of absolute sensitivity, which is a function of the “signal noise” ratio. The “noise” is always less in helium, however, than in air under comparable conditions (7). Therefore, when areas under a curve are larger in helium than in air, this always indicates greater relative sensitivity in helium as compared with air. The converse, however, is not necessarily true. The terms “volatilization” or “volatility” as used in this communication refer to the time period during which a line or different lines of a given element could be detected on the photographic plate. The detectability of a line has served as an index of volatilization of elements into the arc column, not as an absolute measure of the number of atoms released from the anode per unit time. T o avoid cumbersome repetition, the phenomenon has been described in the text in some instances as “the volatilization
436
ANALYTICAL CHEMISTRY
of a line” rather than by the more precise formulation “the volatilization of an element as evidenced by the appearance and disappearance of a given line.” RESULTS AND DISCUSSION
Beryllium, Magnesium, Calcium, Strontium, and Barium of Group 11. The idealized volatilization cukes for beryllium, magnesium, calcium, strontium, and barium are shown in Figures 1 to 5. Volatilization in helium is prolonged over that taking place in air, save for the neutral atom line of strontium a t 6 amperes, and all barium lines. Cyanogen bands interfere with Mg 3829.350 (UJ(4)in air, but the lines can be read easily in helium. No comparison between air and helium for this line can be made, of course.
serves special emphasis. There is a sudden terminal release of the element for both neutral atom and ion lines in air. This is obliterated in helium, where the volatilization is smooth and uniform. Similar behavior has been found for other elements. Calcium, the spectroscopic buffer, is present throughout the period of volatilization. The characteristics of the arc can be assumed to be conditioned by its presence throughout the period of observation. Aluminum, Scandium, and Yttrium of Group 111. The volatilization curves for aluminum, scandium, and yttrium are shown in Figures 6, 7 , and 8. 1701atilizationsin helium and air
RELATIVE IkTEhSITY
ION LINE-Mq 2795.53 VI
looF F
6 AMPERES I5 AMPERES
F
Table I.
Densitometer Data for Cu 3H7.W (VI) Plate W-125 Emulsion 103-0, Lot 448, 052 Initial amperage, 6 Atmosphere, helium
Time, Seconds 5 10 15 20 25 3I)
35 40 45
50 55 60 ~. 65 70 75
80
85
90 95 100 105 110 115 120 125 130 4
b 6
%Tea 4.8
5.6 6.2 10.6 11.8 4.9 5.3 3.7 19.3 37.7 18.1 29.4 37.1 36.8 33.6 45.2 40.2 40.4 64.7 88.0 93.3 90.6 83.0
% Tb Bkg. 95 100 96 99 98 98 98 98 98 100 108 99 99 99 99 99 99 98 100 100 98 98 98
I=
Line 3.20 2.90 2.70 1.94 1.. 78 3.15 ‘3.00 3.78 1.33 0.85 1.38 1.03 0.86 0.88 0.93 0.74 0.81 0.81 0.49 0.28 0.23 0.25 0.32
Id
Bkg. 0.21 0.10 0.19 0.14 0.16 0.16 0.16 0.16 0.16 0.10 0.10 0.14 0.14 0.14 0.14 0.14 0.14 0.16 0.10 0.10 0.16 0.16 0.16
IL
IO
I-
t
t
- IB’
2.99 2.80 2.51 1.80 1.62 2.99 2.84 3.62 1.17 0.76 1.28 0.89 0.72 0.74 0.79 0.80 0.67 0.65 0.39 0.18 0.07 0.09 0.10
0.1
NEUTRAL ATOM L I N E - Mg 3829.350 U q 6 AMPEClES I 5 AMPERES
IO
-u “0
30
60
90
120
150 TIME
No Line
Figure 2.
yo transmittance of line. % ’ transmittance of background.
30
60
90
I20
IN SECOhlDS
Volatilization Rates of Magnesium -In helium --In air
-
Relative line intenaity.
d Relative bmkground intensity.
* Col. 4 minus 001. 6.
RELATIVE HITENSITY ICQC
There i s a marked lag period for beryllium for both the neutral atom and ion lines. Beryllium was added as beryllium sulfate tetrahydrate, as no spectroscopically pure oxide was available. This lag L believed to be due to a conversion at arc temperatures of beryllium sulfate to another compound, which then volatilizes readily. Supplementary experiments ( 5 ) with other chemically but not spectroscopically pure beryllium salts appear to confirm this explanation. The ion linea of beryllium, magnesium, calcium, and strontium are enhanced in helium, beryllium markedly so. The increased relative sensitivity as judged by plotting the signal is about 100fold for beryllium and is reproducible in analytical work ( 5 ) . Arcing at 15 amperes shortens the volatilization time, more in air than in helium except for Sr 4832.075 (Uz), Be 3131.072 (V‘2), and Ba 4934.0G8 ( V I ) . This prolongation of burning time is almost tripled for the ion lines of beryllium and calcium. The total area under all curves at 15 amperes is decreased, suggesting decreased relative sensitivity compared with 6 amperes. The contour of the volatilization profiles of all alkali earths is very similar when identical experimental conditions are considered. The coincidence of terminal fluctuations in the magnesium, calcium, and strontium ion lines in helium should be noted. One feature of the beryllium volatilization a t 6 amperes de-
ION LINE -Go 3(79.332 II E V, = 13.I e.v.
6 AMPERES
15 AMPERES
NEUTRAL ATOM
LINE
- Ca 4425.441
U4
Ve‘4.7e.u.
6 AMPERES
15 AMPERES
TIM€
Figure 3:
IN SECONOS
Volatilization Rates of Calcium -In ---Inhelium air
V O L U M E 24, NO. 3, M A R C H 1 9 5 2 RELATIVE INTENSITY
437
The aluminum ion line does not appear at all in helium or air and was only weakly recorded in helium a t 15 amperes, and its appearance was delayed. The areas under the 15-ampere curves are less than a t 6 amperes, constituting a loss in relative sensitivity, aii was observed for the alkali earths. Comparison with the data for the alkali earths shows a similarity in contours of the neutral atom lines of aluminum, scandium, and yttrium to those of beryllium, while the ion lines in helium of scandium and yttrium are analogous to those of magnwium, cal-
ION LINE- S r 4 0 7 7 , 7 1 4 VI
6 AMPERES
NEUTRAL ATOM LINE 6 AMPERES
-
S r 4 8 3 2 . 0 7 5 Ue Ve = 4 . 3 e v. 15 A M P E R E S
Ve = 10.6
A I 2609.'66 V I RELATIVE INTENSITY
ION
LINE
6 AMPirlES
i
NO L I N E 6 A M P S AIR NO L I N E 6 A M P S H E L I U M
Figure 4. Volatilization Rates of Strontium
- - -InInhelium air
' O r , ,
'0.1
I
,
,
I
,
In,, ,
,
,
,
I 5 AMPERES
m,,;,,
R E L A 1 11 E INTENSITY
ION L I N E - BO4 9 3 4 068 VI
IOOE
6 AMPERES
I
IO
,
N E U T R A L ATOM L I N E V, = 4 . 0 e . v A I 3 0 8 2 I 5 5 Uq 6 AYPERES
IO
NO L I N E 15 P M P S A I R
I 5 AMPERES
'.__--
, ,
TIME
IN
SECONDS
Figure 6. Volatilization Rates of Aluminum
\
- - - In helium
I
In
ni
NEUTRAL ATCM L I N E - Bo 3071 591 Us 6 AMPERES
air
V e = 4 01e5 vA M P E R E S RELATIVE INTENSITY
t
Figure 5.
Volatilization Rates of Barium In helium -- - I n air
NE:-RAL .CCE
ATC',l
6 ANPER-CS
E
L ' N E - Sc 3 9 3 7 4 7 6 U 2 L'e:3 i e v 15 A M P E R E S
are comparable except for Sc 4246.829 (11) a t 15 amperes, for which volatilization is delayed. The volatilization of all three elements is very similar, as judged by their neutral atom lines. Differences are demonstrated by the general contours of the curves. As was observed for beryllium, a terniinal release of the elements occurs a t 6 amperes in air, while the volatilization in helium proceeds smoothly throughout. This phenomenon is also observed for the ion lines of scandium and yttrium. The area under the helium curves is decreased a t both 6 and 15 amperes. The ion lines of scandium and yttrium are enhanced in helium.
Figure 7.
Volatilization Rates of Scandium
- - -InInhelium air
I
,
ANALYTICAL CHEMISTRY
438
cium, strontium, and barium-and, if the initial lag in beryllium volatility w disregarded, beryllium as well. Figure 9 shows the volatili~ationprofiles of beryllium, magnesium, calcium, strontium, barium, scandium, and yttrium ion lines at 6 amperes in helium. Similarities and dissimi1;trities in the volatiliiation of these elements are immediately discernible. Scandium and yttrium behave almost identically, 8s do magnesium and calcium; less so beryllium and strontium. There is poor correlation with barium. Ion lines of scandium and yttrium might serve as good internal atandads for those of magnmium, calcium, and strontium under these conditions, BS judged by these criteria. Iron, Cobalt, Nickel, Palladium, and Ruthenium of the Transi-
RELATIVE INTEN5,T"
(ON LINE-Fe 2628.29ZII 15 AMPERES
6 &MPERES
'.al,A NO LlNE iN
AlR
,
, ,
01
,
,
, , , ,
NEUTRAL ATOM LINE- Fe 3719.935 U, v, 3.3e.v 6 AMPERES i
15 m4PERES
ION LINE-Y 3710.290 V,
Figure 10. Volatilization Ratea of Irnn helium
-In --.In NEUTRAL ATOtrl LIWE- Y 4643.695Up Ue'2.7e.v 6 aIMPERES 15 AMPERES
NiO RELbTIVE INTENSIT"
air
Ni 3 4 9 2 . 9 5 6 U z NEUTRAL
Ve
~
ATOM
3 . 6 e.".
LINE 15 bIMPLRES
0 AMPERES
Figure 8. Volatilization Ratea of Yttrium -10 helium ---In nir
Go,%
Co.3453.505 Ut
Ve = 4.0 8.". 45 M P E R E S
6 bMPEAES
I
I I
160
'
.
90
'
Ti0
'
:kel and Cobalt
Fig -In
helium
- - - In " i l
Figure 9. Volatilization Profiles of Beryllium, Magnesium, Calcium, Strontium, Barium of Group 11, and Scandium and Yttrium of Gmup 111 Ion linea 6 ampersr, helium
tional Group of Elements. Volatilization curves me shown in Figures 10 to 13. In the region 250 ta 5000 A,, ion lines could be found for only iron and palladium. The behavior of Fe 2628.292 (11) and Pd 2763.092 (11) will, therefore, be discussed first. At 6 amperes Fe 2628.292 does not appear a t all in air, and the line in helium is delayed for 10 seconds and in weak. At 15 amperes the line in helium is enhanced. Pd 2763.092 behaves almost identically, BS does Fe 2628.292, though i t does appear weakly a t 6 amperes in air, The volatility of the neutral atom lines of iron, palladium, nickel, cobaltlt,aud ruthenium are not changed remarkably in
V O L U M E 24, N O . 3, M A R C H 1 9 5 2
439
helium and air. The signal in helium is deprmed in all instances. The profiles in air are analogous, and in helium only ruthenium deviate, being more refractory than the other elements of this group. The data a t 15 amperes illustrate the facility with which Fe 3719.935 (VI) and Ru 3498.942 (UL) are discernible in helium, while they are interfered with in air by cyanogen h e structure. All lines in air at 15 amperes are enhanced over comparable lines in helium, Pd 3634.695 so much so that two exposures were too dense to he read. Figure 14 represents volatilization profiles of neutral atom lines oi iron, cobalt, nickel, palladium, and rutheninm a t 6 amperes in
air. The similarity of volatilization characteristics is apparent. Palladium and ruthenium suggest themselves 88 internal standards for iron, nickel, and cobalt and have, indeed, been found of great value in analytical work.
ION LINE-Pd2763.092 U 15 AMPERES
6 AMPERES
IO
tL -
,,-.
, ,
0 ' .1 Om;,
6
h,, ,;
Figure 14. Volatilization Profiles o f Iron, Cobalt, Nickel, R u t h e n i u m , a n d Palladium of Transitional Gmup
,
Neutral atom line. 6 "mpercr, air
UEUTRAL 4TOM LINE-Pd3634.695U3 '5UAMPEnES ve=4.2e.". ,5
t
1
ION
$$,
6 AMPERES
I
\
Figure 12. Volatilization Rates of Palladium .
.
0.1
-In
helium ---In air
C
l
L
R
,
, ,
LINE-Mn 2543.729,$ v, = 12 2 e v I 5 AMPERES
,
I
b,
, , , , ,
,.J
' NEUTRAL ATOM LlNE-Mn4030.755U, 6 AMPERES
j ; ~ 3 , ' e ~ ' " , 15 AMPERES ,
,
I I I
I
_i
120
150
0
M
60
90
120d
I , M E IN SECONDS
Figure 15. "4Re04
Re3460.47 Vi
b AMPERES
Y e . 3 . 6 e.". I 5 AMPERES
Figure 13. Volatilization Rates of R u t h e n i u m Rhenium '
-1nheFum In e,*
--
~
and
Manganese and Rhenium. The vohtilizatioY y1 manganese and rhenium are indicated in Figures 15 and 13. The is slightly depressed a6 both neutral atom line Mn 4030.755 ( UI) 6 and 15 amperes in air. The ion line Mn 2543.729 (V,)on the other hand after a short delay is enhanced a t both 6 and 15 amperes in helium. The charscteristica of the volatility of manganese are very similar to the transitional group of elements. The ion lines of chromium and maaanese do not amcar " _. in air a t 6 anmeres (see iron and palladium). Re 3460.47 I U,) at 15 amDeres in helium. -.is markedly. denresaed . as is manganese. At 6 amperes the line in helium is very faint and is delayed. The element is very refractory in air,s.a is apparent by its incomplete volatilization even a t 15 amperes.
.
ANALYTICAL CHEMISTRY
440
Titanium and Zirconium. The volatilization curves of titanium and zirconium are shown in Figures 16 and 17. For Ti 3653.496 ( U 2 )the volatilization time is similar a t 6 amperes in air and helium. For Zr 3547.682 (U,) it is shortened in helium. RELATIVE INTENSITY
T i 3685.195n
LINE
ION
6 AMPERES
I S AMPERES
'OOE-
t .
At 15 amperes the volatilization time in air is prolonged, markedly 80 for zirconium. Vanadium, Columbium, and Tantalum. The volatilization curves of vanadium and columbium are shown in Figures 18 and 19. The volatilization times of the neutral atom lines a t 6 amperes is shortened in helium, markedly SO for Cb 4058.938 (VI). Both Cb 4058.938 (VI) and V 4379.238 (VI) are depressed in helium, The ion lines of both elements on the other hand are enhanced in helium. Both are delayed, Cb 3094.183 (VI) for 75 seconds. The contours of T i 3685.195 (11), Zr 3391.975 (VI), and V 3102.299 (V,) in helium show a definite resemblance. For V
D E L AT! V E INTENSITY
ION L I N E - V 3102.299Ve I5 A M P E R E S
6 AMPERES
Ti 3653.496 Ue NEUTRAL
Ve
:
3.4 e,v
ATOM L I N E 15 A M P E R E S
6 AMPERES
NEUTRAL ATOM L I N E
-
6 AMPERES
100,
E
4379.238U1 V,:3lev 15 A M P E R E S
p-\
t TIME
-V
F
IN S E C O N D S
Figure 16. Volatilization Rates of Titanium
- - - In helium air -In
IO
RELATIVE INTENSITY
\ c n
TI?,IE
IU
SECONDS
Figure 18. Volatilization Rates of Vanadium
- - -In helium air In
RFLAITIVE lhTENSITY I 00=
F
NEUTSA- A W L ' lO0E
E
- LINE-
6 AMPERES
Zr 3547682U2 Ve=35ev 15 A M P E R E S
ION L I h E - C b 3 0 9 4 1 8 3 V 1
E
6 AMPERES
le ' 8 0 e v
15 A M P E R E S
t
'0:
E
12p 01
L I N E - C b 4 0 5 8 9 3 8 UI V e = 3 2ev
hE'JTRSIL A?": 6 AMPERES
1
I5 AMPERES
h;,, I\
I2
Figure 17. Volatilization Rates of Zirconium
--In - - In helium air Volatilization in air, as judged by the ion line, is not completed during the period of arcing. There are delay and enhancement of the ion lines and a depression of the neutral atom lines for both elements. In both instances there is a terminal increase in volatility for the ion line in air which does not occur in helium. This is also observed for the neutral atom line of titanium.
'\
, , ,
,
\
0
30
60
93
IN S E C O N D S
Figure 19. Volatilization R a t a of Columbium
-- -InIn helium air
123
V O L U M E 24, NO. 3, M A R C H 1 9 5 2 RELCTIVE INTENSITY
ION L I N E - M O Z ~ 1I ~5 4 ~ ~ "e ' ' I 9 e v-
6 AMPERES
IO;
44 1
15 CMPERES
t
c
i:
A
NO LINE IN C I R
- LINE -
N E U T R A L ATON
6 AVPERES
Mo3170347 I V e =3 9 e Y 15 C M P E R E S
i
L E
Figure 20. Volatilization Rates of Molybdenum
- - - Inhelium air
-In
6 AMPERES
NEUTRAL
15 A M P E R E S
ATOM
LINE
.
NO L l k E 15 AMPS ClR
.
r r
t ,
V,/' I:, ,
I
0 1-
P+EUTSAL
ATOM c
6 IVFERES
TIME
,
, ,
,
IN
SECONDS
Figure 22. Volatilization Rates of Rubidium and Lithium
---
LIVE- Cr4254346U Ve:2 9 e v
-In
15 CNPERES
helium In air
Ve = 3.0 e.v.
In 4101.773 Uz NEUTRAL
INTENSITY
ATOM
LINE
6 AMPERES
TIVE
1'4
SEZOYDS
Figure 21. Volatilization Rates of Chromium
---
In helium In air Go 4 172.056 Ut
Go203
3102.299 in air a terminal increase in volatility is noticed, previously seen for beryllium, scandium, yttrium, titanium, and zirconium. The contour of Cb 3094.183 (VI) in air is similar to that of Zr 3391.975. Tantalum was refractory; no spectrum could be recorded even when the amount of the metal in the sample was doubled. Molybdenum and Chromium. The volatilization of these two elements shows little resemblance; this is evident in Figures 20 and 21. Mo 3170.347 (I) is markedly suppressed a t 6 and 15 amperes in helium. This was confirmed by measurement of other neutral atom lines of molybdenum located in the cyanogen region not recorded here.
6 AMPERCS
NEUTRAL
ATOM
Ve
* 3.1
[
e.v.
15 A M P E R E S
LINE
Figure 23. Volatilization Rates of Indium and Gallium
-- - In helium air
-In
ANALYTICAL CHEMISTRY
442
could still not be recorded beyond the first 5 seconds, during which blackening was too intense for densitometry. Sodium and potassium were not studied. The contrast of the volatility of these elements to others described is manifest, as shown in Figure Ge 3269.494 U j RELATIVE INTENSITY
GO2
NEUTRAL
Ve
a
ATOM
4.7
e.”.
LINE
6 AMPERES
1
15 AMPERES
The rubidium and iron lines can be resolved in helium but not in air because of the high background in this region. Gallium and Indium. Indium is depressed a t 6 and 15 amperes in helium. While the gallium profile appears similarly depressed in helium, this is compensated for by a prolongation in volatilization time, aa shown in Figure 23. Germanium, Tin, Lead, Bismuth, and Arsenic. Figures 24 and 25 show the volatilization curvea of germanium, tin, lead, and bismuth. Ge 3269.494 (Val) and Sn 2839.989 (VI) are cu 3273.962~3 RELATIVE ,NTENSITY
NEUTRAL
ve = 3.8
ATOM
e.v
LINE
6 AMPERES
SnO2
Sn 2839,989Ut 6 AMPERES
V e = 4.8 e,v.
15 A M P E R E S
L
NEUTRAL ATOM LINE Cu 3 2 4 7 . 5 4 0 UI V, * 3 8 e
too
TIME
Y
6 ClMPERES
15 AMPERES
SECONDS
IN
Figure 21. Volatilization Rates of Germanium and Tin -In
helium
- - - In air
Ve
81 3067.716 UI RELATIVE INTENSITY
81203
NEUTRAL
IO
ATOM
:
\
4 0 e.v. TINE
LlNE 15 AMPERES
6 AMPERES
0 30 IN SECONDS
60
90
Figure 26. Volatilization Rates of Copper -In helium ---Inair
A q 3382 891 U p R E LATl V E
NEUTRAL
INTENSITY
6 AMPERES
Ve
:
3 7 e v.
LlNE
ATOM
i
NEUTRAL ATOM L I N E Ag 3 2 8 0 . 6 8 3 U t V e 3 3.8
TIME
IN
I 5 AMPERES
C.V.
SECONDS
Figure 25. Volatilization Rates of Bismuth and Lead heliuni --In - - In air
22. The volatilization of Li 3232.61 ( U 2 ) is completed in 30 seconds in helium at 6 amperes and is so rapid at 15 amperes in air that no line is recorded. The line is delayed a t 6 amperes in air; but once volatilization has begun, it is completed rapidly. The data for rubidium are incomplete because of the restricted cheice of rubidium lines which can be measured between 2500 and IO00 A. Rb 4201.851 (U3) is interfered with by Fe 4201.58.
Figure 27.
Volatilization Rates of Silver -In
- - - Inhelium air
V O L U M E 24, NO. 3, M A R C H 1 9 5 2
443
markedly depressed at 6 and at 15amperes in helium. The volatilbation of germanium and tin is very similar in hoth envimnments. In helium the volatilisation occurs in irregular spurts. Cd 3261.057 I
NEUTRAL
INTENSITII
Ve
,3.8 e.".
ATOM
LLNE
6 &MPLRES
ZnO
Zn3343.020 Uz
I5 AMPERES
Ve = 7.8 e.". 15 AMPERES
6 &.MPEREB
t
t
Figure 28. Vohtihation Rates of Cadmium and Zinc
..- Inhelium air
-In
The volatiliiation times for lead and hismuth me prolonged a t 6 mnperes in air; the lines in helium are depressed slightly st hoth 6 and 15 amperes. The vol~tilieationtimes a t 15 amperes are shortened markedly. The characteristics of the rmrves of both elements are very similar. Arsenic and its salts were presumably so volatile that the element could not he recorded under the conditions of this experiment. Copper, Silver, and Gold. Two neutrd atom lines of copper and silver are shown in Figures 26 aud 27. I n hoth instances reproducibility of the volatilization phenomena is demonstrated when different lines of the =me element have comparable excitation potential. The copper lines are all deprressed in helium, while the differenceshetween helium and air for silver are minute. The volatilization time for copper in air is prolonged over that in helium, while the gaseous environments do not change this parameter significantly for silver. Gold could not be followed for an adequate period of time to allow of significant conclusions concerning ita volatilization rate. Cadmium and Zinc. Both elements volatilize rapidly. At 6 amperes Zn 3345.020 volatilizes more slowly in helium than in air. The similarities hetween cadmium and zinc are apparent, hut the choice of helium a8 an arc environment does not significantly alter the contour of either volatility cuwe, as shown in Figure 28. Figure 29 relates volatilization profiles of neutral atom and ion lines of titanium, zirconium, vauadium, and columbium a t 6 amperes to each other in helium and air. The relative intensity of neutral atom lines is depressed in helium, while ion lines are enhanced. Moreover, the contours are altered similarly, as were those of beryllium, aluminum,
Heliu
.
.
.-.
.L Neu
I o n lines in A i r
Figure29.
'
Vc xtilization Profilesof Titanium, Zirconium, Vanadium, and Columbium 6 -pa
444
ANALYTICAL CHEMISTRY
In Helium
In Air
Figure 30. Volatilization Profiles of Gallium, IndiumL. Lead, Silver, Copper, B i s m u t h , Tin, and G e r m a n i u m Neutral atom line.
yttrium, and scandium: The terminal release of elements in air is smoothed out and obliterated in helium. The graph emphasiaes the differential volatilization of neutral atom and ion lines in the two gaseous environments. The change brought about by helium is particularly noticeable for the ion line of columbium. The volatilization profiles of neutral atom lines of copper, silver, gallium, indium, lead, tin, and germanium tLt 6 amperes are contrasted in helium and air, as shown in Figure 30. Relative intensity is depressed in helium-strikingly 60 for tin and galliumaccompanied by an alteration of contours. This graph summarises information presented in Figures 23 to 27. No simple relationship between magnitude of excitation potential and relative h e suppression and burning t,ime in helium and air could be derived. The neutral atom lines of different elements were not suppressed in direct, simple proportion to their excitation potential, nor was the enhancement of ion lines a direct simple function of their ionieation potential when helium was used. Genwalized background has been thought to be due to incandescent radiation from csrhon and sample particles when the carbon or graphite arc is used. Background is suppressed in helium even more than n6utrel atom lines are. A suppression of most neutral atom line intensities measured and of background and an enhancement of ion line intensities which were measured with a marked increase of signal to noise for the latter were the over-all result of the volatilkation experiments in helium. The ratio of signal to noise of neutra.1 atom lines is a. function of the degree of suppression of the line. The theoretical basis of the phenomena observed is in doubt, though certain inferential conclusions appear permissible and are being studied. A method for the simultaneous analysis of elements in biological materials has been perfected, utilizing the data here presented (6). CONCLUSION AND SUMMARY
The data presented permit evaluations of optimal analyticnl conditions for 31 elements on the basis of their volatilization characteristics in air and helium and ION of selection of internal standards. The use of the measured ion lines for analytical purposes, when helium is the environment of the direct current arc, brings about an increase in absolute and relative Sensitivity. Helium favors the excitation of ion lines and may depress the signal of those neutral atom lines which were studied, The relative sensitivity of spectrochemical determinations in helium is nevertheless often
better than in air because of the low background. This feitture is particularly favorable wben lines are in the cyanogen region of the spectrum, where othermse they could not have heen determined a t 511. The volatility of many elements presented in this communication exhibited a qualitative resemblance related to their ohemical grouping. Similarities in volatility, however, are noted also hetween elements with consecutive atomic numbers In some instances volatility seems to bear no relstionship to chemical grouping. Various elements were not added as the oxide, and their volatilieation profiles may be contingent upon the anion. Arcing a t 15 amperes reduces sensitivity markedly and diminishes differencesin volatilization time and contour. The choice of helium as an environment for the direct current arc brings about decisive changes in volatility and relative l i e intensities. These changes cannot be disregarded when the direct current is used for quuitntitative spectrochemical analysis. ACKNOWLEDGMENT
It is a real pleasure to acknowledge the suggestions and constructive criticisms of James E. Archer, Department of Physics, and S. A. Coons, Graphics Department, Massachusetts Institute of Technology. LITERATURE CITED (1) Ahrens,
L. H., “Spectrochemical Analyais,” Cambridge, Msss.,
Addison-Wesley Press. Ino.. 1950.
(2) Covey, E. H., “Aveilability and Souraea of the 96 Elements in
High Purity Forms,” Techniosl Informstion Division, AEC, Oak Ridge, Tenn., Domienl AECU-222 (UCRL-266). (31 Tassel. V. A,. J . Oolical Sod. Am..39. 187 (1949).
..
Wiley &Sons; 1939. ( 5 ) Vsllee, B. L., unpuhlished data. (6) Vellee. B. L.,and Adelstein. 9. J., T. Optical Soe. Am., 41. 869 (1951). (7) Vallee, B. L.,Reimer. C. B.,and Loofbomow, J. R., I&L, 40, 7 5 1 4 (1950). R&CDIVDD far revielv .MY 8, 1951. Accepted October 14, 1951. Prb sented before Section 2. Analytiaal Chemietry, at the XIIth Intarnational Congress of Pure and Applied Chemistry, Nsa York. N. Y., September 10 to 13, 1951. Work supported by gmntainaid from the Charles F. Kettering Pounds*tion,Dayton. Ohio.and the John Lee Pratt Fund of the sloan-Kettsrinp Institute for Cancer Researoh, New York. N. Y. B. L. Vallee is senior research fellow of the NstionalResearoh Council Committee on Growth, supported b y the American Cancer Society.
