Direct-Current Arc and 

Alternating-Current Spark 

Emission Spectrographic 

Field Methods for the 

Semiquantitative Analysis 

of Geologic Materials 

Direct-Current Arc and 

Alternating-Current Spark 

Emission Spectrographic 

Field Methods for the 

Semiquantitative Analysis 

of Geologic Materials 

By D. J. Grimes and A. P. Marranzino 

G E 0 L 0 G I C A L S U R V E Y C I R C U L A R 591 

Wasltington 1968 
United States Department of the Interior 

STEWART L. UDALL, Secretary 

Geological Survey 

William T. Pecora, Director 

Free on opplicotion fo the U.S. Geoloficol Survey, Washington, D.C. 20242 
CONTENTS 

Page 

J\bstract - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Introduction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Equipment - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Direct-current arc method - - - - - - - - - - - - - - - - - - - - - - - 1 

Preparation of matrix for standards - - - - - - - - - - - - - - - - - 1 

Preparation of field standards - - - - - - - - - - - - - - - - - - - - 2 

Operating conditions- - - - - - - - - - - - - - - - - - - - - - - - - - 3 

J\nalytical procedure - - - - - - - - - - - - - - - - - - - - - - - - - 3 

J\lternating-current spark method- - - - - - - - - - - - - - - - - - - - 4 

Preparation of matrix solution for standards- - - - - - - - - - - - - 4 

Preparation of standard solutions- - - - - - - - - - - - - - - - - - - 5 

Operating conditions- - - - - - - - - - - - - - - - - - - - - - - - - - 5 

J\nalytical procedure - - - - - - - - - - - - - - - - - - - - - - - - - 5 

Conclusions - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 6 

References - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 6 

ILLUSTRJ\ TION 

Page 

Figure 1. Drawing of funnel and venting l'od - - - - - - - - - - - - - - 4 

TJ\BLES 

Page 

Table 1. Matrix used in direct-current arc method - - - - - - - - - - 2 

2. Concentrations of elements in Field Standard I - - - - - - - - 2 

3. Dilution chart-three-step standards - - - - - - - - - - - - - 3 

4. Concentrations of elements in Field Standard II - - - - - - - 3 

5. Limits of determination-direct-current arc method- - - - - 4 

6. Matrix solution-alternating-current spark method- - - - - - 5 

7. Standard solutions-alternating-current spark method- - - - 5 

8. Limits of determination-alternating-current spark 

method- - - - - - - - - - - - - - - - - - - - - - - - - - - - 5 

III 

Direct-Current Arc and Alternating-Current Spark Emission 

Spectrographic Field Methods for the Semiquantitative 

Analysis of Geologic Materials 

By D. J. Grimes and A. P. Marranzino 

Abstract 

Two spectrographic methOds are used in mobile field laboratories of the U. S. Geological Survey. In the direct-current arc 

method, the ground sample is mixed with graphite powder, packed 

into an electrode crater, and burned to completion. Thirty 

elements are determined. In the spark method, the sample, ground 

to pass a 150-mesh screen, is digested in h y r o fluor i c acid 

followed by evaporation to dryness and dissolution in aqua regia. 

The solution is fed into the spark gap by means of a rotating-disk 

electrode arrangement and is excited with an alternating-current 

spark discharge. Fourteen elements are determ1ned. In both techniques, light is recorded on Spectrum Analysis No. 1, 35-millimeter film, and the spectra are compared visually with those of 

standard films. 

INTRODUCTION 

Data on the trace element 1 content of geologic materials has increasi,ngly greater application in geochemical r e conn a i s san c e and in studies of oreweathering processes. Semi quantitative spectrographic methods provide a rapid and economic means 

for acquiring such information. 

In the early 1950's the U.S. Geological Survey designed and tested a mobile spectrographic laboratory 

and established the feasibility of using rapid semiquantitative methods in the field (Canney and others, 

1957). Initially, analytical procedures were similar 

to those described by Myers, Canney, and Dun ton 

( 1956 ). Subsequently, the techniques were improved 

as described by Oda, Myers, and Cooley ( 1958). With 

the implementation of the Heavy Metals program, it 

became necessary to increase the number of elements 

determined and to modify existing procedures so that 

lower concentrations of elements such as silver, gold, 

bismuth, cadmium, cobalt, copper, palladium, platinum, and antimony could be measured. 

EQUIPMENT 

Spectrograph: 1.5-meter Wadsworth-mounted, having 

a dispersion of 5.45 A (angstrom units) per millimeter over the spectral range from 2,100 A to 4,800 

A in the second order. 

Source: 

Direct-current arc-220 volts with maximum arc 

current of 15 amperes; control of arc current is 

maintained by resistances in series. 

I 

Alternating-current spark-18,000 rms (root mean 

square) volts; output of 10 amperes RF (radio frequency) modulated with a 0. 005 microfarad condenser at 310 microhenrys of inductance andresidual resistance. 

Slit width: 10 microns. 

Comparator: X 20 magnification. 

Upper electrode: Preformed graphite 1/8 inch in diameter with an 8° taper. 

Lower electrode: 

Direct-current arc-preformed graphite 1/4 inch in 

diameter with a crater wall thickness of 0.04 inch, 

crater diameter of 0.144 inch, and crater depth of 

0.313 inch. 

Alternating-current spark-graphite rotating disk 

0.492 inch in diameter and 0. 2 inch thick. 

Film: 35-millimeter Spectrum Analysis Nc. 1 emulsion, processed with D-19 developer for 2 minutes 

at 20°C, fixed for 5 minutes, rinsed in cold water, and 

air dried. 

DIRECT -CURRENT ARC METHOD 

Preparation of Matrix for Standards 

To reduce one of the rna jor problems in the spectrographic analysis of geological materials, namely the 

matrix effect, the composition of the matr:'x powder 

for the field standards should approximate that of the 

sample (Ward and others, 1963). Ideally, a matrix of 

each rock type analyzed is needed, such as a gossan 

matrix, a silicate matrix, or a c~rbonate matrix. Preparation of a matrix for each rock type necessitates 

a knowledge of the composition of the samrle before 

it can be analyzed, and the procedure is expensive 

and time consuming. 

For reconnaissance geochemical studies, I a r g e 

numbers of samples are scanned for groE's differences in concentrations, and a simple matrix is desirable. With only a slight loss of accuracy, a single, 

average matrix can be used (table 1). All compounds 

used in preparing the matrix are spectrographically 

pure except Si02, which is obtained from natural quartz 

crystals. The quartz is crushed, pulverized to pass a 

200-mesh screen, and washed free of impurities using 

the following procedure: 
1. Soak in hot 6 N HCl tor 1 day, decant, and repeat. 

2. Rinse with demineralized water, alternately washing and decanting six times. 

3. Check for impurities. 

4. Repeat complete procedure if necessary to remove 

all impurities. 

Table 1.--Matrix used in direct-current arc method 

Compound used 

Amount of 

compound used 

for 100 g 

of matrix 

(in grams) 

Sio2 (quartz 

crystals)------------- 71 

Fe 2o3 ------------------ 10 

Al 2o3 ------------------ 7 

Caco3------------------ 5 

Na2co3----------------- 4 

K2co 3 ------------------ 2 

MgO-------------------- 1 

Equivalent 

(percent) 

Si(33.1) 

Fe(7.0) 

Al(3.7) 

Ca(2.0) 

Na(l.6) 

K(l.l) 

Mg (. 6) 

Preparation of Field Standards 

Two standards are prepared for use in the field. 

Field Standard I (table 2) is divided into two 

parts....,a high-concentration standard and a lowconcentration, or trace element, standard. For each 

part, a base mixture of the elements sought is prepared by adding a specified amount of a compound of 

each element, usually the oxide or carbon ate, to a 

predetermined amount of matrix to give the desired 

concentration of each element; the following are 

examples of amounts of various compounds used: 

Element 

sought 

Compound 

used 

Ba-------- BaC03 Sr-------- SrC0

Mn-------- 3 Mn3o 4 

Concentration 

desired 

(ppm) 

5,000 

5,000 

5,000 

Amount of 

compound 

for 10 g 

of base 

mixture 

(in grams) 

0.0667 

.0782 

.0644 

After the base mixture is prepared, stardards containing successively lower amounts of th~ elements 

are made using the reciprocal of the cub~ root of 10 

(0.464) as a dilution factor. For example, if the base 

mixture contains an element at a concentration of 100 

ppm (parts per million), the succeedinr- standards 

would contain the same element at concentrations of 

46.4 ppm, 21.5 ppm, 10 ppm, 4.64 ppm, End so forth 

(table 3). When the standard is used in a semiquantitative procedure, these numbers are rounded off to 

50, 20, 10, and 5. 

After each dilution, the standard is ground in an 

agate mortar for 15 minutes, then transferred to a 

clean glass vial containing several glass or plastic 

beads and shaken or mixed mechanically for 1 hour. 

Table 2.---Concentrations (in parts per million) of elements in Field Stan1ard I 

Dilution No. 

1 (base 

mixture)--

2------------- 3------------- 4-------------

Dilution No. 

l (base 

mixture)--

2 ------------ 3 ------------ 4 ------------ 5 ------------ 6 ------------ 7 ------------ 8 ------------ 9 ------------ 10 ------------

V10,000 

5,000 

2,000 

1,000 

Ti, Zn 

10,000 

5,000 

2,000 

1,000 

500 

200 

100 

50 

20 

10 

Elements in high-concentration standard 

Cr, Cu, 

Ni, Pb 

5,000 

2,000 

1,000 

500 

B, Co, Mo 

2,000 

1,000 

500 

200 

Ag, Be, 

Bi, Sn 

1,000 

500 

200 

100 

Elements in low-concentration (trace) standard 

Ba, Mn, Sr La, V, Zr 

5,000 1,000 

2,000 500 

1,000 200 

500 100 

200 50 

100 20 

50 10 

20 5 

10 2 

5 1 

2 

Cd, Cr, Cu, 

Ni, Pb 

500 

200 

100 

50 

20 

10 

5 

2 

1 

• 5 

B, Co, 

Mo, Y 

200 

100 

50 

20 

10 

5 

2 

1 

• 5 

• 2 

Ag, Be, Bi, 

Sc, Sn 

100 

50 

20 

10 

5 

2 

1 

• 5 

• 2 

.1 
Table 3.--Dilution chart--three-step standards 

Dilution No. Mixture 

1 (base 

mixture)---Xg of metal oxide 

+Yg of matrix 

=4 g 

2--------------1.857 g of 100 ppm 

+2.143 g matrix 

=4 g 

3--------------1.857 g of 46.4 ppm 

+2.143 g matrix = 4 g 

4--------------1.857 g of 21.5 ppm 

+2.143 g matrix 

=4 g 

5--------------1.857 g of 10 ppm 

+2.143 g m<1;trix 

=4 g 

Resulting 

standard 

(ppm) 

100 

46.4 

21.5 

10 

4.64 

The importance of this step must be stressed, for 

unless thorough mixing is achieved, the succeeding 

lower concentrations will be inaccurate. The quantity 

of each standard should be kept low (2 to 4 grams) to 

help insure a good mix. Extreme care must also be 

taken to prevent contamination. 

Field Standard II consists of two base mixtures, one 

containing the carbonates of calcium and magnesium 

and the other, the oxides of iron, arsenic, antimony, 

tungsten, and niobium. Gold is also added to the second 

base mixture. The high malleability of gold metal and 

the hygroscopicity of its available compounds make it 

impossible to obtain a reliable gold standard by adding 

gold directly to the mixture. To achieve an accurate 

and uniform distribution of go I d in the standard, a 

solution of gold metal in aqua regia is evaporated onto 

a weighed amount of pure Si02 matrix. 

To prepare Field Standard II, both base rrixtures 

are diluted with a pure Si02 matrix using tl ~ same 

factor as for Field Standard I. After several dilutions, 

a weighed amount of one base mixture is added to a 

weighed amount of the other and both are miJ·ed with 

the Si02 matrix to give the desired concen<:rations 

(table 4), as shown in the following examples: 

1. To make dilution 4 on table 4, add 0.5 g (g:ram) of 

dilution 1 to 0.05 g of dilution 10 and mix b".lth with 

4.45 g of Si02 matrix. 

2. To make dilution 5, add 0.5 g of dilution~ to 0.05 

g of dilution 11 and mix with 4.45 g of Si02 matrix. 

Generally, geologic material that is high in magnesium and calcium is low in iron, and the naterial 

that is high in iron is low in magnesium and calcium. 

The standards listed in table 4 are set up to take account of this general rule. 

Operating Conditions 

Electrode gap spacing: 4 millimeters. 

Exposure: 140 seconds. 

Current: Initial excitation of 3 amperes increased to 

11 amperes after 5 seconds. 

Analytical Procedure 

Sample preparation 

Rocks: Crush the sample in a jaw crusher to 1/4-

inch-size particles or less, split with an al:tminum 

Jones Splitter, and pulverize in ceramic ball mills 

(Myers and Wood, 1960) to pass a 150-mesh screen. 

Soils and stream sediments: Air dry the sample, 

sieve to pass an 80-mesh screen, split, and pulverize. 

Analysis 

Weigh 10 milligrams of the prepared sample, add20 

milligrams of pure graphite powder, andmixir an aluminum weighing pan with a disposable wooden toothpick. Transfer the mixture into the cavity of a preformed electrode with the aid of an aluminun funnel 

Table 4.---Concentrations of elements in Field Standard II 

Ca Mg Fe As, Sb, w Nb Au 

Dilution No. (percent) (percent) (percent) (ppm) (ppm) (ppm) 

1-(base 2-: ______________ mixture)- 20 10,000 2,000 500 

10 5,000 1,000 200 3---------------- 5 2,000 500 100 

4---------------- 0.05 0.02 2 1,,000 200 50 

5---------------- .01 .05 1 500 100 20 

6---------------- . 2 .1 • 5 200 50 10 

7---------------- .5 • 2 • 2 100 20 5 8---------------- 1 .5 .1 50 10 2 

9---------------- 2 1 .OS 20 5 1 

10---------------- 5 2 

11---------------- 10 5 12~(base mixture)- 20 10 

3 
~"r 

:-. \ 

7"r 

32 

~"r 

\ I 

\ I 

\ I 

~===:t, 

I I 

I I 

I I 

I I 

l I I 

Aluminum funnel 

t_"r 

' t"r 

t"r 

I" -'4 

I 3 " :{ 

1 ~" 

Aluminum venting rod 

~" "' 'f .-.. 1 " f ~ y' 

2 

5 " 7 l 16 

i 

21.14" 

Figure 1.--Funnel and venting rod. Approximate 

scale 1:2. 

4 

(fig. 1) and pack tightly with an aluminum venting rod 

(fig. 1 ). The resulting hole in the sample-graphite mixture affords a vent for extruding gas resulting from 

initial excitation. If a vent is not used, the sample and 

graphite should be preheated under an infrared lamp 

prior to excitation. Clamp the sample·~carrying electrode and counter electrodes in the arc stand, initiate 

a current of 3 amperes by touching the electrodes together, open the arc gap, and burn the sample to completion at 12 amperes. Process the filll. and compare 

it visually with standard films using a X 20 comparator. The limits of determination and the wavelengths 

read are given in table 5. 

Table S.--Limits of determination--directcurrent arc method 

Lower limit of 

Element Wavelength (A)!/ determination 

(ppm) 

Ag-------- 3280.7 (3382. 9)~/ o.s 

As-------- 2860.4 (2780.2) 200 

Au-------- 267S.9 10 

B--------- 2497.7 10 

Ba-------- 4SS4.0 (233S.3) s 

Be-_------- 3131.1 1 

Bi-------- 3067.7 10 

Ca-------- 31S8.8 soo 

Cd-------- 3261.1 20 

Co-------- 34S3.S s 

Cr-------- 42S4.3 s 

Cu-------- 3273.9 2 

Fe-------- 3100.6 soo 

La-------- 3337.S 20 

Mg-------- 2781.4 200 

Mn-------- 2949.2 10 

Mo-------- 3170.3 (3193 0 9) s 

Ni-------- 3414.7 2 

Nb-------- 3163.4 10 

Pb-------- 2833.0 10 

Sb-------- 2877.9 100 

Sr-------- 4607.3 (3464.4) so 

Sn-------- 317S.O (2839.9) 10 

Sc-------- 33S3.7 s 

Ti-------- 3168.S 10 

v--------- 3102.3 (3183.9) 10 

w--------- 2946.9 so 

Y--------- 3327.8 10 

Zn-------- 334S.O (3302.6) 200 

Zr-------- 3279.2 20 

!/From Meggers, Corliss, and Scribner 

(1961) 0 

~/Numbers in parentheses are alternate 

wavelengths used. 

ALTERNATING-CURRENT SPARK METHOD 

Preparation of Matrix Solution for Standards 

The matrix solution for the standards is obtained by 

dissolving . the nitrates of the five eleir~nts listed in 

table 6 in a 1:1 nitric acid solution. The concentration 
Table 6.--Matrix solution--alternating-current spark method 

Element Concentration fpercent) 

Fe----------------- 5.0 

Ca----------------- 1.0 

K ----------------- 1.0 

Na----------------- 1.0 

Mg----------------- .2 

of these elements in the solution is approximately equal 

to their average concentration in rocks and soils. Although Si02 and Al203 constitute about 75 percent of 

all geologic material, they are not added to the matrix 

solution because both are present only in minute concentrations in the sample solutions; during sample 

preparation, Si02 is removed by hydrofluoric acid 

treatment, and Al2o3 is very insoluble in the acid used 

in the leaching step. 

Preparation of Standard Solutions 

Two separate standard solutions are used (table 7). 

For standard solution 1, the nitrates of silver, cadmium, copper, nickel, and zinc areweighedtogive the 

desired concentrations and then are dissolved in the 

matrix solution. Standard solution 2 contains arsenic, 

gold, bismuth, cobalt, molybdenum, palladium, platinum, antimony, and tin; the gold, molybdenum, and 

antimony are put in solution by dissolving the metals 

in aqua regia, and this solution is added to the matrix 

solution containing the rest of the elements. By subsequent dilutions of these two standards with the 

matrix, three-step standards are obtained with concentrations ranging from 5,000 to 0.2 ppm. 

Table 7.--Standard solutions--alternatingcurren~ spark method 

Element Compound used 

Standard solution 

Cu--------- Cu(N03)2•2H20 

Zn--------- Zn(N03)2•6H20 

Ag--------- AgN03 

Ni--------- Ni(N03)2•6H20 

Cd--------- Cd(N0 3 )2•4H20 

Standard solution 

As--------- K3As04 

sb--------- Sb(metal) 

Au--------- Au(metal) 

Pd--------- (NH4 ) 2PdC14 

Mo--------- Mo(metal) 

Pt--------- H2PtCls•6H20 

Bi--------- BiCN03) 3 • 5H20 

Co--------- Co(N03)2•6H20 

Sn--------- SnC12•2H20. 

ConcentratJ.on 

range (ppm) 

1 

5,000-2 

5,000-20 

500-0.2 

500-2 

500-20 

2 

5,000-20 

5,000-50 

500-Q. 2 

500-1 

500-5 

500-5 

500-10 

500-10 

500-10 

Operating Conditions 

Electrode gap spacing: 2 millimeters. 

Exposure: 105 seconds. 

Inductance: 40 microhenrys. 

Capacitance: 0.005 microfarad. 

Resistance: Residual. 

Current: 5 amperes. 

Break per half cycles: Four. 

Disk speed: 10 revolutions per minute. 

Analytical Procedure 

Transfer 1.5 g of ground sample to a 30-ml (milliliter) teflon beaker containing 3 ml of concentrated 

hydrofluoric acid. Swirl to mix and let stand ir a hood 

for at least 18 hours. If an excess of hydrofluC'ric remains, heat gently to dryness, cool, add 4 ml aqua 

regia, place on a vibrating hotplate, and heat r1: 100°C 

for 1 hour. Cool, transfer to a 10 X 75-millime .. er culture tube, wash with a minimum amountofderrineralized water, centrifuge, decant to a graduated test tube, 

and dilute to 3 ml with water. Mix and transfer to a 

No. 00 porcelain boat. Place on rotating disk a~.,~embly 

stage and excite. 

A good exhaust system for the arc stand is necessary 

to remove fumes from the vaporization of the acidic 

solution on the disk. The platform, the e 1 e c trod e 

holders, and the interior of the arc stand sh>uld be 

carefully cleaned to prevent corrosion. 

5 

After excitation of the sample solutions is co'llplete, 

the film is processed and compared to the s1:andard 

films in a comparator with a X 20 magnificati"ln. The 

limits of determination and the wavelengths read are 

given in table 8. 

Table 8.--LiEits of determination--alternating-current spark method 

Element Wavelength!/ 

Ag-------- 3280.7 (3382.9)~/ As-------- 2860.4 

,Au-------- 2675.9 (2427.9) 

Bi-------- 3067.7 

Cd-------- 3261.1 

Co-------- 3453.5 

Cu-------- 3273.9 

Mo-------- 2871.5 (2848.2) 

I~~======== ~~~~:~ ~~~~~:~~ Pt-------- 2659.4 (3064.7) 

Sb-------- 2877.9 Sn-----~-- 3034.1 (3175.0) 

Zn-------- 3302.6 (3345.0) 

Lower limit of 

determination 

(ppm) 

0.2 

20 

10 

20 

10 

• 2 

• 2 

5 

2 

1 

5 

so 

10 

20 

!/From Meggers, Corliss, and Scribner 

(1961). 

~/Numbers in parentheses are alternate 

wavelengths used. 
CONCLUSIONS 

The direct-current arc method allows a great number of determinations over a wide range of geologic 

samples. It is especially useful as a reconnaissance 

tool. With this method a team of four men can log, 

prepare, weigh, excite, read, and report at least 100 

rock or soil samples per day. 

The alternating-current spark method is more cumbersome and is better suited to fixed laboratory operations, but it alsomaybeperformedin the field. A team 

of two men can analyze 30 to 40 samples per day. 

The question of which method to use depends on the 

nature of the geologic material, the elements sought, 

the limits of determination, the number of samples to 

be analyzed, and the amount of,time available." 

REFERENCES 

Canney, F. C., Myers, A. T., and Ward, F. N., 1957, 

A truck-mounted spectrographic laboratory for use 

in geochemical exploration: Econ. Geology, v. 52, 

no. 3, p. 289-306. 

6 

Meggers, W. F., Corliss, C. H., and Scribner, B. F., 

1961, Tables of spectral-line intensities: Natl. Bur. 

Standards Mon. 32, pt. 1, 473 p. 

Myers, A. T., Canney, F. C., and Dunton, P. J., 1956, 

Semiquantitative spectrographic ar al y sis in a 

truck-mounted laboratory for g eo c hem i c a I exploration-a preliminary report [ abs.]: Spectrochim. Acta, v. 8, no. 2, p. 110. 

Myers, A. T., and Wood, W. H., 1960, Ceramic mills 

in a paint mixer for preparation of multiple rock 

samples: Appl. Spectroscopy, v. 14. p. 136-137. 

Oda, Uteana, Myers, A. T., and Cooley, E. F., 1958, 

A field method of spectrographic an'l'lysis for use 

in geochemical exp1oration-animproved technique 

lab: Anal. Chemistry andAppl. SpectJ·oscopyConf., 

Pittsburgh, Program, p. 46. 

Ward, F. N., Lakin, H. W., Canney, F. C., and others, 

1963, Analytical methods used in geochemical exploration by the U.S. Geological Surv~y: U.S. Geol. 

Survey Bull. 1152, 100 p.