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Quantitative spectrographic analysis by spark excitation of metallic oxides
(From Research Department, Imperial Chemical Industries Ltd., Metals Division, Witton, Birmingham.)
Qaantitatlve Spectrographic Analysis by Spark Excitation of Metallic Oxides. BY Maurice Milbourn a’nd H. E. 11. Hartley. With 2 Figures. (Received:
Xarch
3, 1948.)
lutroductiou. Spectrographic analysis of a powder, such as a mixture of metallic: oxides, has hitherto been carried ant l)y supporting the mixture on a conducting rod and vaporizing it by means of an arc discha8rge. Many workers have adopted fhis procedure, including Llrrtkyd in his analysis of metallic oxides, h’cott nnd NitcheR in their in\-rstigations of plants and soils, and HasZer and Haruey2 for the i~~~~aly& of zinc. While the arc is satisfactory for detecting traces of impurities, it does not generally supply results of a high degree of accuracy, and care is necessary to obtain reliable values, particularly when the constituents being estimated are present in quantities greater t,hn,n traces. It is well known that, spa’rk sources, including any discontinuous discharge initiated primarily by a high voltage, supply more accurate figures than an arc, when solid electrodes are used, but, their disruptive nature ejects loose powder from the sphere of action of t,he spark, aga#in leading to erratic resnlt’s. By employing t,he technique herein described, it has been found possible to control the eje&ion so that good reproducibility is obtained. The powder is ground into a fine state of division, and introduced into a ca,vity drilled in a graphite rod forming the lower electrode. A spaSrk is passed between this rod and a pointed graphite c,ounter-electrode, and, although the discharge does not, pass directly to the oxide, the powder is ejected over a considerable period of time into the spark column, where it, is excited to radiation by the therm:11 and electrical conditions which it encounters. 1. Preparation
of Samples
aud Electrodes.
While this technique could doubt,less be used for the analysis of powders in general, it has, in fact, been employed mainly for investigating problems in metallurgical analysis. The sample, generally in the form of drillings, is dissolved in an appropriate acid, and the so-
halyeis
by
Spark
Excitation
of Metallic
Oxidos.
321
lution is evaporated nearly to dryness and ignited to oxide, which is then thoroughly ground to a fine state of division. The grinding serves not only to reduce the powder to particles of a size which are ejected readily, but also to mix the components thoroughly, particularly those which are liable to be thrown out of solution. Standard mixtures of known composition for comparison purposes may be prepared from dry powders or solutions of reagents, or by converting chemically analysed metallic samples to oxide. Samples of known and unknown composition should preferably be in the same chemical state when they are being compared, and it is convenient to make them both oxides. The lower electrode is prepared by drilling an H. S. quality graphite rod of 3/gN diameter to a depth of llq”. For light, or easily ejected oxides (e.g. aluminium) a 3/1B” diameter hole is used, and for heavy oxides (e.g. copper) the diameter is increased to lip”. The crater is loosely filled with oxide, any excess above the flat end of the electrode being levelled off. The upper electrode is also a 3/8” diameter graphite rod with a sharp point having an sngle of about 80°, which is set concentrically above the crater. 2. Conditions of Exposure. The spark is generated by a standard Hilger spark circuit, with a I/& kW transformer, set to supply 10,000 volts, and having a capacity of 0.005 p_CF in parallel with the gap and an inductance of 0.075 mH in series with it. The exposure is started at the same time as the spark, and its duration is adjusted to suit the matrix and the elements being estimated, as well as the spectrographic and electrical conditions. Thus, with a slit width of 0.025 mm on a Hilger E 498 spectrograph, times of 20 set and 60 set have been found satisfactory for the estimation of added constituents in matrices of aluminium and copper, respectively, and these times are increased by 50-100’70 for the estimation of impurities. It has been customary to make exposures in duplicate on Kodak B 10 plates. The spark passes between the rim of the crater and the point of the upper electrode, and the powder is ejected upwards so that it passes through the gas column, where it is vaporized. Some powder should remain in the crater at the end of the exposure.
3. Experimental Results. Many analyses employing this technique have been made to investigate 1) its sensitivity and reproducibility, 2) the effect of one
322
M. Milbourn
end H. E. R. Hartley:
Quantitative
Spectrographic
constituent upon the estimation of another, and 3) its use in cases where other analytical methods could not readily be employed. Sensitivity has been studied by testing synthetic mixtures, with a matrix of copper, containing traces of elements such as tin, lead, iron, nickel, silver, antimony, bismuth and arsenic, limits of detection being detailed in Table I. It has been found difficult to distinguish between spectra obtained from a TABLE I. conventional arc between metallic Sensitivity of Detection electrodes, and spectra from the of Impurities in Copper. powder spark technique, which inLimit dicates that the latter is predomiElement of Detection O/ nantly arolike in character. Consequently, it has a sensitivity com0.002 Ti I1 parable with, but rather less than, 0.003 J,eail that of an arc discharge. There is, IrOll 0.001 0.002 Sirliel however, a considerable and easily Silver 0.003 recognizable difference between .1ntimony 0.001 spectra from arc and spark sources l
ie
ib 24
26
Fig.
28
10
1. Estimation
12
I4
16
/8
log%Zn
of Zinc in Aluminium.
sensitivity is comparable with that of an arc, and sufficient for the estimation of these elements when present as impurities in commercial materials. As an illustration, a graph for the estimation of
Analysis
by Spark
Excitation
of Metallic
Oxides.
323
zinc in aluminium over the range 0.6% to 0.03% zinc is shown in Fig. 1. For assessing reproducibility, additions of manganese, magnesium and iron were made to an aluminium matrix, and of lead to a mixture representing pure brass. Several preparations were made, and each was tested many times, details being recorded in Table II, TABLE
II.
Results of Reproducibility Alloy and Preparation
Tests.
Composition y.
Aluminium Alloy Six preparations, each tested in sextuplicate Brass Six preparations, each tested in quintuplicate Three preparations, each tested in sextuplicate
Mg 0.61 Mn 0.55 Fe 0.66
4.7 5.0 6.2
Pb
2.35
4.2
Pb
1.5
3.1
together with the standard deviations of the results about the mean. The indication is that the reproducibility of the method can be maintained at an acceptable level, particularly when we also bear in mind
i4-.
iz_L
I3
i4 Fig.
is
76
2. Estimation
i7
ia
i9
of Magnesium
0
01
0.2 log%Mg
in Aluminium.
its sensitivity. Its accuracy has been tested by comparing synthetic mixtures with powders prepared from chemically analysed metallic specimens, a typical example being illustrated in Fig. 2 for magne-
324
M. Milbourn
and H. E. R. Hartley:
Quantitative
Spectrographic
sium in alumiuium, over the range 0.2 o/oto 2.0 o/oof magnesium. Since all results are represented by one calibration curve, figures obtained from synthetic samples are consistent with those supplied by chemical analysis. Two cases have been encountered in which the powder spark technique eliminates or greatly reduces the influence of one constituent on the estimation of another, an influence that is observed with conventional spectrographic tests on solid electrodes. Thus, the presence of 5% zinc in an aluminium alloy does not influence the relative densities of iron, silicon, manganese, and magnesium lines, whereas it has a considerable effect when present in metallic electrodes. Similarly, in the estimation of nickel in copper-chromium alloys by conventional spectro~ap~c methods, it was found that the presence of chromium reduced the density of nickel lines, compared with samples containing no chromium, but spark tests on powders supplied similar results from both types of material. The powder spark technique has been very useful for the estimation of impurities in cases where established chemical methods can be used only with caution. For instance, boron has been est~ated in copper-nickel-manganese alloys in the range 0.005% to 0.05 %, and aluminium in tin-bronzes in the range 0.002% to 0.02%.
4. Mechanism of the Powder Spark. In studying the mechanism by which ejection is effected, observations have shown that it happens more quickly or to a greater extent with less dense matrices, wide craters, higher voltages or without inductance. The spark passes a short distance down the inside of the crater, and does not play directly on to the surface of the powder, and more sample is ejected in the first few seconds while the electrode is cold, than during later stages when it is warm. There does not appear to be any selective distillation of certain constituents nor a “ sparking-off )’ effect, and it has been established that the absolute densities and the relative differences in density between lines remain sensibly constant for considerable periods of time during any one test. With a unidirectional discharge, powder is ejected at a greater rate when it is on the positive electrode than when it is on the negative, and the level of the residual powder is lowest at the point where the discharge was last striking. It has also been found that powder is ejected from two electrodes fitted side by side in a conducting clamp, when a spark is passing to only one of them.
Analysis
by Spark
Excitation
of Metallic
Oxides.
325
Such observations show that ejection does not arise from thermal effects, such as vaporization or convection. It is more probably the result of electrostatic repulsion between neighbouring particles, or of a pressure effect associated with the regions of the discharge in the immedia,te vicinity of the electrodes. 5. Discussion. While the method described would not be suitable for purely routine analyses, it is of value in dealing with samples which call for special treatment, by reason of their form or metallurgical state or of the nature of t,he estimation needed. Almost all samples for which spectrographic methods can be used should fall within its scope, and its application for the estimation both of impurities and of added constituents is a favourable feature. Conventional arc and spark discharges consume very little material, a fact which may be turned to good account, for instance, in analysing segregates; but when it is necessary to obtain a representative analysis of a bulk of material, the powder spark method may be used advantageously. The sensitivity and reproducibility are both of an acceptable order, and the increase of the former over that of a spark discharge between metallic electrodes may be ascribed to the greater amount of material passing into the discharge, thereby making it more arclike in character. By way of illustration, it may be noted that for zinc in an aluminium matrix, the line Zn 3345 A has the same density at 0.02% Zn in a powder test as it has at 0.66 y0 Zn for a conventional spark. Some evidence has been obtained indicating that the operation of the powder spark is influenced by the acid radicals present, and it seems preferable to use it only when each constituent of all samples is present as oxide. The preparation and particularly the grinding of the powder are important, and the most concordant results are obtained when samples and standards are prepared at the same time and tested together. The danger of contaminating samples in the course of preparation is always present, but little trouble has been experienced. Silicon, for instance, has been estimated successfully in spite of the fact that ignition was carried out in silica crucibles. The authors wish to express their thanks to Dr. Maurice Cook for his helpful advice and interest in the work.
326
31. Milbourn
and H. E. R. Hartley:
Analyeis of Metallic Oxides.
A new spectrographic technique is described for analysing a finely ground powder, such as a mixture of metallic oxides, which is placed in a crater in the lower electrode of a spark discharge. The powder is ejected continuously into the vapour column, and a source combining good sensitivity and reproducibility is obtained. This technique has been used successfully for many metallurgical analyses, more especially those which are not of a routine nature. It appears that ejection is brought about by electrostatic repulsion rather than by thermal or convective effects, and that t’he large amount of material passing into the discharge makes it arclike in character. R&urn& Une nouvelle technique est d&rite consistant S spectrographier dans 1’6tincelle une poudre finement broyde, comme un melange d’oxydes mdtalliques, apr&s l’avoir introduite dans un cratbre creusb dans l’dlectrode infdrieure. La poudre est continuellement Bjectbe dans la colonne de vapeur et l’on obtient une bonne sensibilitt! et une bonne reproductibilit& La m&hode a 4x5 employBe avec succ&s pour de nombreuses analyses m&allurgiques, plus spdcialement celles non routinibres. 11 semble que la projection soit causde par repulsion Blectrostatique plut6t que par effet thermique et que la grande quantit,d de substance apportbe dans la ddcharge donne & celle-ci un caractbre d’art.
Es wird ein neues spektrochemisches Verfahren zur Analyse von fein gepulverten Proben, wie z. B. Metalloxyden, beschrieben. Die auf eine gebohrte Graphitelektrode gebrachte Probe wird in einem $!! kW Funken (10000 V, 0.005 ?F, 0.075 mH) angeregt. Da das Pulver St&dig in die Entladungsbahn getrieben wird - anscheinend mehr durch elektrostatische ah durch thermische oder mechanische IZr&fte - so erhtilt man eine bogenghnliche Anregung von grosser Empfindlichkeit bei gleichzeitig guter Reproduzierbarkeit. Das Verfahren hat sich gut bewlhrt bei vielen metallurgischen Analysen, besonders wenn es sich nicht urn blosse Routinearbeiten handelte.
Bibliography. 1 Breckpot, R.: Dosage de quelques constituants secondaires dans le cuivre par voie spectrographique. Ann. Sac. Sci. Bruxelles 63,219-247 (1933). - 2 Ha&r, M. I?., and 0. E. Harvey: Quantitative Spectrochemical Method for Zinc Die Casting Analysis. Ind. Eng. Chem., Anal. Ed., 13, 540-544 (1941). - 3 Bcott, R. O.? and R. IL. Mitchell: Concentration Methods in Spectrographic Analysis. I. J. Sot. Chem. Ind. 62, 4-8 (1943).
Qaantitatlve Spectrographic Analysis by Spark Excitation of Metallic Oxides. BY Maurice Milbourn a’nd H. E. 11. Hartley. With 2 Figures. (Received:
Xarch
3, 1948.)
lutroductiou. Spectrographic analysis of a powder, such as a mixture of metallic: oxides, has hitherto been carried ant l)y supporting the mixture on a conducting rod and vaporizing it by means of an arc discha8rge. Many workers have adopted fhis procedure, including Llrrtkyd in his analysis of metallic oxides, h’cott nnd NitcheR in their in\-rstigations of plants and soils, and HasZer and Haruey2 for the i~~~~aly& of zinc. While the arc is satisfactory for detecting traces of impurities, it does not generally supply results of a high degree of accuracy, and care is necessary to obtain reliable values, particularly when the constituents being estimated are present in quantities greater t,hn,n traces. It is well known that, spa’rk sources, including any discontinuous discharge initiated primarily by a high voltage, supply more accurate figures than an arc, when solid electrodes are used, but, their disruptive nature ejects loose powder from the sphere of action of t,he spark, aga#in leading to erratic resnlt’s. By employing t,he technique herein described, it has been found possible to control the eje&ion so that good reproducibility is obtained. The powder is ground into a fine state of division, and introduced into a ca,vity drilled in a graphite rod forming the lower electrode. A spaSrk is passed between this rod and a pointed graphite c,ounter-electrode, and, although the discharge does not, pass directly to the oxide, the powder is ejected over a considerable period of time into the spark column, where it, is excited to radiation by the therm:11 and electrical conditions which it encounters. 1. Preparation
of Samples
aud Electrodes.
While this technique could doubt,less be used for the analysis of powders in general, it has, in fact, been employed mainly for investigating problems in metallurgical analysis. The sample, generally in the form of drillings, is dissolved in an appropriate acid, and the so-
halyeis
by
Spark
Excitation
of Metallic
Oxidos.
321
lution is evaporated nearly to dryness and ignited to oxide, which is then thoroughly ground to a fine state of division. The grinding serves not only to reduce the powder to particles of a size which are ejected readily, but also to mix the components thoroughly, particularly those which are liable to be thrown out of solution. Standard mixtures of known composition for comparison purposes may be prepared from dry powders or solutions of reagents, or by converting chemically analysed metallic samples to oxide. Samples of known and unknown composition should preferably be in the same chemical state when they are being compared, and it is convenient to make them both oxides. The lower electrode is prepared by drilling an H. S. quality graphite rod of 3/gN diameter to a depth of llq”. For light, or easily ejected oxides (e.g. aluminium) a 3/1B” diameter hole is used, and for heavy oxides (e.g. copper) the diameter is increased to lip”. The crater is loosely filled with oxide, any excess above the flat end of the electrode being levelled off. The upper electrode is also a 3/8” diameter graphite rod with a sharp point having an sngle of about 80°, which is set concentrically above the crater. 2. Conditions of Exposure. The spark is generated by a standard Hilger spark circuit, with a I/& kW transformer, set to supply 10,000 volts, and having a capacity of 0.005 p_CF in parallel with the gap and an inductance of 0.075 mH in series with it. The exposure is started at the same time as the spark, and its duration is adjusted to suit the matrix and the elements being estimated, as well as the spectrographic and electrical conditions. Thus, with a slit width of 0.025 mm on a Hilger E 498 spectrograph, times of 20 set and 60 set have been found satisfactory for the estimation of added constituents in matrices of aluminium and copper, respectively, and these times are increased by 50-100’70 for the estimation of impurities. It has been customary to make exposures in duplicate on Kodak B 10 plates. The spark passes between the rim of the crater and the point of the upper electrode, and the powder is ejected upwards so that it passes through the gas column, where it is vaporized. Some powder should remain in the crater at the end of the exposure.
3. Experimental Results. Many analyses employing this technique have been made to investigate 1) its sensitivity and reproducibility, 2) the effect of one
322
M. Milbourn
end H. E. R. Hartley:
Quantitative
Spectrographic
constituent upon the estimation of another, and 3) its use in cases where other analytical methods could not readily be employed. Sensitivity has been studied by testing synthetic mixtures, with a matrix of copper, containing traces of elements such as tin, lead, iron, nickel, silver, antimony, bismuth and arsenic, limits of detection being detailed in Table I. It has been found difficult to distinguish between spectra obtained from a TABLE I. conventional arc between metallic Sensitivity of Detection electrodes, and spectra from the of Impurities in Copper. powder spark technique, which inLimit dicates that the latter is predomiElement of Detection O/ nantly arolike in character. Consequently, it has a sensitivity com0.002 Ti I1 parable with, but rather less than, 0.003 J,eail that of an arc discharge. There is, IrOll 0.001 0.002 Sirliel however, a considerable and easily Silver 0.003 recognizable difference between .1ntimony 0.001 spectra from arc and spark sources l
ie
ib 24
26
Fig.
28
10
1. Estimation
12
I4
16
/8
log%Zn
of Zinc in Aluminium.
sensitivity is comparable with that of an arc, and sufficient for the estimation of these elements when present as impurities in commercial materials. As an illustration, a graph for the estimation of
Analysis
by Spark
Excitation
of Metallic
Oxides.
323
zinc in aluminium over the range 0.6% to 0.03% zinc is shown in Fig. 1. For assessing reproducibility, additions of manganese, magnesium and iron were made to an aluminium matrix, and of lead to a mixture representing pure brass. Several preparations were made, and each was tested many times, details being recorded in Table II, TABLE
II.
Results of Reproducibility Alloy and Preparation
Tests.
Composition y.
Aluminium Alloy Six preparations, each tested in sextuplicate Brass Six preparations, each tested in quintuplicate Three preparations, each tested in sextuplicate
Mg 0.61 Mn 0.55 Fe 0.66
4.7 5.0 6.2
Pb
2.35
4.2
Pb
1.5
3.1
together with the standard deviations of the results about the mean. The indication is that the reproducibility of the method can be maintained at an acceptable level, particularly when we also bear in mind
i4-.
iz_L
I3
i4 Fig.
is
76
2. Estimation
i7
ia
i9
of Magnesium
0
01
0.2 log%Mg
in Aluminium.
its sensitivity. Its accuracy has been tested by comparing synthetic mixtures with powders prepared from chemically analysed metallic specimens, a typical example being illustrated in Fig. 2 for magne-
324
M. Milbourn
and H. E. R. Hartley:
Quantitative
Spectrographic
sium in alumiuium, over the range 0.2 o/oto 2.0 o/oof magnesium. Since all results are represented by one calibration curve, figures obtained from synthetic samples are consistent with those supplied by chemical analysis. Two cases have been encountered in which the powder spark technique eliminates or greatly reduces the influence of one constituent on the estimation of another, an influence that is observed with conventional spectrographic tests on solid electrodes. Thus, the presence of 5% zinc in an aluminium alloy does not influence the relative densities of iron, silicon, manganese, and magnesium lines, whereas it has a considerable effect when present in metallic electrodes. Similarly, in the estimation of nickel in copper-chromium alloys by conventional spectro~ap~c methods, it was found that the presence of chromium reduced the density of nickel lines, compared with samples containing no chromium, but spark tests on powders supplied similar results from both types of material. The powder spark technique has been very useful for the estimation of impurities in cases where established chemical methods can be used only with caution. For instance, boron has been est~ated in copper-nickel-manganese alloys in the range 0.005% to 0.05 %, and aluminium in tin-bronzes in the range 0.002% to 0.02%.
4. Mechanism of the Powder Spark. In studying the mechanism by which ejection is effected, observations have shown that it happens more quickly or to a greater extent with less dense matrices, wide craters, higher voltages or without inductance. The spark passes a short distance down the inside of the crater, and does not play directly on to the surface of the powder, and more sample is ejected in the first few seconds while the electrode is cold, than during later stages when it is warm. There does not appear to be any selective distillation of certain constituents nor a “ sparking-off )’ effect, and it has been established that the absolute densities and the relative differences in density between lines remain sensibly constant for considerable periods of time during any one test. With a unidirectional discharge, powder is ejected at a greater rate when it is on the positive electrode than when it is on the negative, and the level of the residual powder is lowest at the point where the discharge was last striking. It has also been found that powder is ejected from two electrodes fitted side by side in a conducting clamp, when a spark is passing to only one of them.
Analysis
by Spark
Excitation
of Metallic
Oxides.
325
Such observations show that ejection does not arise from thermal effects, such as vaporization or convection. It is more probably the result of electrostatic repulsion between neighbouring particles, or of a pressure effect associated with the regions of the discharge in the immedia,te vicinity of the electrodes. 5. Discussion. While the method described would not be suitable for purely routine analyses, it is of value in dealing with samples which call for special treatment, by reason of their form or metallurgical state or of the nature of t,he estimation needed. Almost all samples for which spectrographic methods can be used should fall within its scope, and its application for the estimation both of impurities and of added constituents is a favourable feature. Conventional arc and spark discharges consume very little material, a fact which may be turned to good account, for instance, in analysing segregates; but when it is necessary to obtain a representative analysis of a bulk of material, the powder spark method may be used advantageously. The sensitivity and reproducibility are both of an acceptable order, and the increase of the former over that of a spark discharge between metallic electrodes may be ascribed to the greater amount of material passing into the discharge, thereby making it more arclike in character. By way of illustration, it may be noted that for zinc in an aluminium matrix, the line Zn 3345 A has the same density at 0.02% Zn in a powder test as it has at 0.66 y0 Zn for a conventional spark. Some evidence has been obtained indicating that the operation of the powder spark is influenced by the acid radicals present, and it seems preferable to use it only when each constituent of all samples is present as oxide. The preparation and particularly the grinding of the powder are important, and the most concordant results are obtained when samples and standards are prepared at the same time and tested together. The danger of contaminating samples in the course of preparation is always present, but little trouble has been experienced. Silicon, for instance, has been estimated successfully in spite of the fact that ignition was carried out in silica crucibles. The authors wish to express their thanks to Dr. Maurice Cook for his helpful advice and interest in the work.
326
31. Milbourn
and H. E. R. Hartley:
Analyeis of Metallic Oxides.
A new spectrographic technique is described for analysing a finely ground powder, such as a mixture of metallic oxides, which is placed in a crater in the lower electrode of a spark discharge. The powder is ejected continuously into the vapour column, and a source combining good sensitivity and reproducibility is obtained. This technique has been used successfully for many metallurgical analyses, more especially those which are not of a routine nature. It appears that ejection is brought about by electrostatic repulsion rather than by thermal or convective effects, and that t’he large amount of material passing into the discharge makes it arclike in character. R&urn& Une nouvelle technique est d&rite consistant S spectrographier dans 1’6tincelle une poudre finement broyde, comme un melange d’oxydes mdtalliques, apr&s l’avoir introduite dans un cratbre creusb dans l’dlectrode infdrieure. La poudre est continuellement Bjectbe dans la colonne de vapeur et l’on obtient une bonne sensibilitt! et une bonne reproductibilit& La m&hode a 4x5 employBe avec succ&s pour de nombreuses analyses m&allurgiques, plus spdcialement celles non routinibres. 11 semble que la projection soit causde par repulsion Blectrostatique plut6t que par effet thermique et que la grande quantit,d de substance apportbe dans la ddcharge donne & celle-ci un caractbre d’art.
Es wird ein neues spektrochemisches Verfahren zur Analyse von fein gepulverten Proben, wie z. B. Metalloxyden, beschrieben. Die auf eine gebohrte Graphitelektrode gebrachte Probe wird in einem $!! kW Funken (10000 V, 0.005 ?F, 0.075 mH) angeregt. Da das Pulver St&dig in die Entladungsbahn getrieben wird - anscheinend mehr durch elektrostatische ah durch thermische oder mechanische IZr&fte - so erhtilt man eine bogenghnliche Anregung von grosser Empfindlichkeit bei gleichzeitig guter Reproduzierbarkeit. Das Verfahren hat sich gut bewlhrt bei vielen metallurgischen Analysen, besonders wenn es sich nicht urn blosse Routinearbeiten handelte.
Bibliography. 1 Breckpot, R.: Dosage de quelques constituants secondaires dans le cuivre par voie spectrographique. Ann. Sac. Sci. Bruxelles 63,219-247 (1933). - 2 Ha&r, M. I?., and 0. E. Harvey: Quantitative Spectrochemical Method for Zinc Die Casting Analysis. Ind. Eng. Chem., Anal. Ed., 13, 540-544 (1941). - 3 Bcott, R. O.? and R. IL. Mitchell: Concentration Methods in Spectrographic Analysis. I. J. Sot. Chem. Ind. 62, 4-8 (1943).