Direct-current arc and alternating-current spark emission spectrographic field methods for the semiquantitative analysis

Geological Survey

William T. Pecora, Director J\bstract - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Introduction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Equipment - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Direct-current arc method - - - - - - - - - - - - - - - - - - - - - - - Preparation of matrix for standards - - - - - - - - - - - - - - - - - Preparation of field standards - - - - - - - - - - - - - - - - - - - - Operating conditions- - - - - - - - - - - - - - - - - - - - - - - - - - J\nalytical procedure - - - - - - - - - - - - - - - - - - - - - - - - - J\lternating-current spark method- - - - - - - - - - - - - - - - - - - - Preparation of matrix solution for standards- - - - - - - - - - - - - Preparation of standard solutions- - - - - - - - - - - - - - - - - - - Operating conditions- - - - - - - - - - - - - - - - - - - - - - - - - - J\nalytical procedure - - - - - - - - - - - - - - - - - - - - - - - - - Conclusions - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - References - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

2. Concentrations of elements in Field Standard I - - - - - - - -
3. Dilution chart-three-step standards - - - - - - - - - - - - -
4. Concentrations of elements in Field Standard II - - - - - - -
5. Limits of determination-direct-current arc method- - - - -
6. Matrix solution-alternating-current spark method- - - - - -
7. Standard solutions-alternating-current spark method- - - -
8. Limits of determination-alternating-current spark method- - - - - - - - - - - - - - - - - - - - - - - - - - - - CONTENTS

Data on the trace element 1content 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.

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.

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

EQUIPMENT 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 , which is obtained from natural quartz pure except Si0 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 Compound used

Sio (quartz crystals)------------- 71 Fe 2o3 ------------------ 10 Al 2o3 ------------------ 7 Caco3------------------ Na2co 3----------------- 4

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

for 100 g of matrix (in grams)

Ba, Mn, Sr Equivalent (percent)

Si(33.1) Fe(7.0) Al(3.7) Ca(2.0) Na(l.6) K(l.l) Mg (. 6)

B, Co, Mo 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:

Ba-------- BaC0 Sr-------- SrC0 Mn-------- Mn

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. Compound Concentration used desired

o (ppm)

B, Co, Mo, Y Amount of compound for 10 g of base mixture (in grams)

0.0667 .0782 .0644 Table 3.--Dilution chart--three-step standards

Dilution No.

1 (base mixture)---Xg of metal oxide

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

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

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

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

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.

Dilution No.

1-(base mixture)- 2-: ______________

11---------------- 12~(base mixture)- Mixture

=4 g

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

(percent)

0.05 Resulting standard (ppm)

Ca (percent)

.5 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 Si0
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 Si0 Generally, geologic material that is high in mag-

nesium 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.

Electrode gap spacing: 4 millimeters. Exposure: 140 seconds. Current: Initial excitation of 3 amperes increased to 11 amperes after 5 seconds.

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

Fe (percent) matrix.

.OS (ppm)

5,000 2,000 1,,000 Nb (ppm)

2,000 1,000 matrix.

Au (ppm) (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.

l

w--------- 2946.9 Y--------- 3327.8

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

Figure 1.--Funnel and venting rod. Approximate scale 1:2.

o.s

s

soo s s soo

s

so s

so

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

Element Concentration fpercent)

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 Al o3 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.

sb---------

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.

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.

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--alterna-

Bi-------- 3067.7 Cd-------- 3261.1

!/From Meggers, Corliss, and Scribner

~/Numbers in parentheses are alternate wavelengths used.

Wavelength!/ Lower limit of determination

so

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. 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.

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