Chemical Geology - Elsevier Publishing Company, Amsterdam Printed in The Netherlands

ATOMIC ABSORPTION ANALYSES OF 18 ELEMENTS FROM A SINGLE DECOMPOSITION OF ALUMINOSILICATE1

D.E. BUCKLEY and R.E. CRANSTON

Marine Geology, Atlantic Oceanographic Laboratory, Bedford Institute, Dartmouth, Nova Scotia (Canada)

(Received August 18, 1970)

ABSTRACT

Buckley, D.E. and Cranston, R.E., 1971. Atomic absorption analyses of 18 elements from a single decomposition. Chern. Geol., 7: 273-284.

The atomic absorption determination of 18 elements in aluminosilicate minerals and marine sediments is accomplished after a single decomposition method using hydrofluoric acid and aqua regia. The digested sample is combined with boric acid to give a 100 ml solution from which all 18 elements including Si, Ti, V, and Al can be determined directly by means of atomic absorption spectroscopy. Inter-element interferences and ionization effects are fully compensated by using combined stand­ards having a matrix similar to that of the samples. Precision and accuracy for most determinations are comparable to the other more complex analytical techniques while the speed of analyses is considerably faster than existing methods.

INTRODUCTION

In the field of marine inorganic geochemistry an ever increasing need exists for good quality chemical analyses of large numbers of samples of natural sediments. These analyses commonly require determinations of both major and minor elements from a wide variety of materials. These range from nearly pure calcium carbonate to clay sediments, the latter consisting of an assemblage of 4 or 5 principal aluminosilicate minerals. This paper describes the procedure developed at our laboratories to facili­tate the rapid and reasonably precise analyses of 18 elements in clay minerals by means of atomic absorption spectroscopy.

The objective was to analyse small samples (100 mg) of suspended clay sized sediments. These analyses were required to obtain data on the major cation constituents of the silicates to assist in mineralogical identifi­cation and classification. In addition minor element analyses were required to obtain data on the concentration, absorption and exchange of trace­transition elements from the marine environment. To carry out these analyses a single decomposition technique was adapted from that described by Bernas (1968). Samples were digested in aqua regia and HF in a sealed teflon bomb. This technique offered several distinct advantages over open acid digestion methods or fusion methods. Because the sample was decom­posed in a closed container volatilization losses were eliminated thus allow­ing the analyses of Si, AI, V, and Ti from the same solution as all other

1 n.1. Contribution 222 Chern. Geol., 7 (1971) 273-284 273

elements. Because no fluxes were used, all alkali and alkaline earth elements could be analysed. The entire sample preparation is simple requiring no special skills and is easily adaptable to mass production procedures. The total decomposition and dilution procedure could be completed in less than one hour.

Effective reduction of interelement interference was achieved over a range of concentrations by matching the matrix of standards and samples. The presence of fluoboric acid formed by the addition of boric acid to the digested sample had no apparent detrimental effect on the analyses, and dissolved any insoluble fluoride salts which may have precipitated in the digestion bomb.

Instrumental settings were selected in such a way that a wide variety of aluminosilicates could be analysed with the single solution preparation technique. No dilution beyond the initial 1:1,000 was necessary for comple­tion of the 18 element analysis.

TECHNIQUE

Reagents and standards

All reagents used were certified reagent grade chemicals. Standard cation solutions are prepared from high purity metals or salts as noted in Table I. Water used as solvent or dilutant is prepared by micro-filtration and ion exchange to obtain a resistivity of 18 MQ-cm (Super Q water -Millipore Corp.).

A one litre stock standard of each element is prepared to give a cation concentration of 1,000 J.l g/ml. From these stock standards a combined standard "A" is prepared by adding the proportionate quantity of stock solu­tion to a one litre polypropylene bottle thus giving the final concentr;ltions of each element as shown in Table I. These combined quantities represent an upper limit of the concentration to be expected from digestion and 1:1,000 dilution of most marine sediments (Chester, 1965, pp.68-77). Three additional standards are prepared from combined standard "A" by 1:2, 1:10, and 1:20 dilutions. All four standards are prepared in such a way that all contain 5 % v Iv HF, 5.6% w Iv H3 B03, and 1% v Iv aqua regia in order that the standards match the sample solution obtained by the decomposition method described below.

Sample preparation

Decomposition of all samples as well as the Si02 and Ti02 standards is carried out in a teflon-lined bomb similar to that described by Bernas (1968). A 100 mg sample is placed in the bomb and wetted with 1 ml of aqua regia. 6 ml of- concentrated HF is added and the bomb sealed tightly. The bomb is heated at 1000 C for 30 min for most samples, but has required 60 min for some clay samples. After cooling, the sample solution is washed into a 125 ml polypropylene bottle containing 5.6 g of H3B03 and 20 ml of water. The insoluble metal fluorides, present in the digested sample after treatment with the HF, are dissolved in the boric acid system (Bernas,

274 Chern. Geol., 7 (I971) 273-284

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1968, pp.1685-1686). Shaking the sample completes the dissolution more quickly. The sample solution is transferred to a glass volumetric flask to bring the volume to 100 ml and is then returned to the polypropylene bottle for storage until analysed. Blank solutions are prepared in the bomb in the same way as the sample. Bernas (1968) tested the effect of the HBF4 -H3 B03 system on glass for short periods of time and found no contamination. We confirmed this finding from use of glass volumetrics for periods of a few minutes to an hour.

Instrumental analyses

An atomic absorption spectrophotometer (Perkin-Elmer Model 303) was used with a model 165 recorder. Intensitron lamps (trade mark: Perkin­Elmer Corp.) were used for all elements except Si, Sr, Pb, Fe, and Ti in which cases standard hollow cathode lamps were used. Except as noted in the text and tables of this paper, all instrumental settings were as recommended in the Perkin-Elmer Analytical Methods Book.

A baseline for each element was determined continuously by aspirating an aqueous solution containing HF, H3B03 and aqua regia in the same con-

TABLE II

Analytical conditions for atomic absorption spectroscopy

Major elements

Upper limit elemental conc. in rock Upper limit oxide cone. in rock Sensitivity (ug/ml/1 %abs) Burner type1 Burner position2 Recorder expo

Minor elements

Lower limit cone. in rock (p.p.m.) Sensitivity (ug/ml/1 %abs) Burner type1 BUrner position2 Recorder expo

Al

15%

28%

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20

0.15 B II X 30

Ca

10%

14%

1.3 N + Xl

Cr

20

0.20 B II X 30

Fe K Mg

10% 5% 5%

14% 6% 8%

4.0 1.5 0.6 5 cm 5 em N + + + x3 X3 Xl

Cu Li Ni

10

0.07 B II X 30

10

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20

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0.3%

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0.06 B II x3

Pb

50

0.50 B II X 30

Na Si

4% 35%

5% 75%

0.5 4.0 5 em N + II X 1 xl

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100

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Ti

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5%

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Zn

20

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15 em = short path air-acetylene burner; N = nitrous oxide-acetylene burner; and B = Boling (3 slot) burner. 2 I I = burner parallel to light path; and + = burner perpendicular to light path.

276 Chern. Geol., 7 (1971) 273-284

centrations as in the sample and standards. Burner type, burner orientation, fuel selection and recorder expansion were all chosen to allow the direct determination of the element from the 100 ml sample solution without any concentration or dilution steps. The specific instrumental conditions are given in Table II and the range of concentrations of the elements or oxides in the undigested sample for which these conditions are appropriate are also given. Only the lower limits for the common minor elements are given since a 1,000-fold dilution of these elements is always below the upper limit of the instrument. Sensitivity is defined as the concentration of metal in solution that gives a 1% absorption signal for the instrumental conditions designated in Table ll.

Calculations

Calibration for each set of standards was accomplished by computing the derivative between each pair of standards. Thus, rather than an integrated curve, a segmented curve was calculated in which small portions of the curve were bracketed by two standards. The sample concentrations were interpolated from within the small ranges. The use of the computer to carry out this standard-sample comparison has resulted in the removal of much of the human error and bias that usually influences the final result. The computer was also used for error analyses and data compilation.

RESULTS

Over a period of several months a record of 20 replicate analyses of diabase W-1 (U.S. Geological Survey Standard Rock) and 16 analyses of Illite 35 (American Petroleum Institute Clay Minerals Standard Project 49, Fithian Illinois) was maintained in our laboratory. A summary of these data is given in Table III. Our average determination for W-1 is compared willi the amount present as reported in the summary by Fleischer and Stevens (1962). The average relative error for the 9 major elements in W -1 is 1. 9 %. The chemical composition of illite minerals appears to be variable (Grim, 1968, p.580), although no comparative studiesbydifferinganalyticaltech­niques are known. For this reason only the determinations by our laboratory are reported.

The coefficients of variation differ considerably from element to element for the trace metals. These coefficients can be labelled as the "noise" of the system since this variation is directly dependent on sensitivity of the element as shown by vanadium, and inversely dependent on low metal concentration, such as is the case for lithium. The sensitivity / concentration ratio for each element has been plotted with the coefficients of variation (Fig.1) for W-1 and Illite 35 replicate analyses (Table III).

Table IV is a compilation of data from a single analysis of each of five U.S.G.S. rock standards. The elemental concentration found was determined by means of the computer interpolation method described earlier. The values tabulated as (a) were calculated on the basis of standard curves, derived from the four combined standards, plus solutions of G-2 and W-1 treated as known standards containing the amount of metal quoted from

Chern. Geol., 7 (1971) 273-284 277

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Fig.1. Each pair of points represents the variation of the trace element statistics for two types of silicates. The positive slope of the line indicates that decreasing concentration results in higher coefficients of variation.

Fleischer and Stevens (1962). The values tabulated as (b) were calculated on the basis of the four combined standards alone and treating G-2 and W-1 as unknowns. In all cases the data calculated from the six-standard curves have a lower relative error than those from the four-standard curves. Calcium values determined in this particular set of analyses have higher relative errors than are normally obtained. No apparent reason could be found for these high values since such anomalies did not occur in other replicate analyses of the same standard rocks. Nickel and chromium values for GSP-1 roay be high because of contamination from the stainless steel casing surrounding the teflon bomb.

The data in Table V represent part of the results obtained from analyses of suspended clay-sized sediments collected from a glacial fiord in southeast Alaska (Buckley and Loder, 1968). These data are from analyses of sediments from two sample stations with fractionation of the clay-sized material into three size groups having average diameters less than O.5J.1., O.5-2J.1. and 2-4J.1.. The mineralogy determined by X-ray diffractometry consists of biotite, chlorite, hornblende, plagioclase feldspar, and quartz. The variability of the mineral assemblage with the variation in size is reflected in the major element data. The increase in calcium and sodium with increasing grain size correlates with a relative increase in the quantity of plagioclase feldspar. Similarly increasing silicon reflects higher per­centages of quartz as well as feldspar. The lesser percentages of biotite and chlorite in the coarser sizes are indicated by decreasing iron and

Chern. Geo!., 7 (1971) 273-284 279

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TABLE V

Analyses of glacial marine suspended sediment

Major elements (%) Al Ca Fe K Mg Mn Na Si Ti

ND-2 surface 0.5Jl 8.78 2.64 9.30 3.75 4.18 0.146 1.53 18.2 0.96 0.5-2Jl 8.53 3.22 8.25 3.31 3.67 0.130 2.01 17.7 0.80

2-4Jl 8.65 3.76 7.19 2.81 3.16 0.122 2.41 22.0 0.75

ND-4F 1 meter 0.5Jl 8.53 2.15 9.72 3.42 3.34 0.155 1.25 18.4 1.12 0.5-2Jl 8.65 2.68 9.24 3.40 4.24 0.152 1.59 18.8 1.01

2-4Jl 8.81 2.97 8.05 3.06 3.86 0.140 2.03 19.0 0.92

Minor elements Co Cr Cu Li Ni Pb Sr V Zn

ND-2 surface 0.5Jl 59 125 138 61 91 716 252 145 412 0.5-2Jl 50 263 93 49 100 519 300 155 282

2-4Jl 40 230 66 38 131 317 352 144 208

ND-4F 1 meter 0.5Jl 58 216 130 63 68 68 215 175 284 0.5-2Jl 66 136 100 56 72 73 340 175 229

2-4Jl 55 119 75 43 71 104 316 155 220

potassium. The minor element data generally illustrate the tendency of transition and heavy metals to be concentrated with layered silicates and minerals having the smallest grain size (largest specific surface area) because of increased adsorption capacity (Krauskopf, 1967, pp. 592-593).

DISCUSSION

Advantages of the preparation technique

In the preparation of samples, our objective was to choose a decompo­sition technique that would be applicable to the widest possible variety of naturally occurring aluminosilicates and that this decomposition would yield a single sample solution in which all elements could be determined directly. Langmyhr and Paus (1968a) described several methods for decomposition of silicon containing materials and a modification of their method number 2 was chosen by Bernas (1968) and by ourselves because of several adv;p1tages 'Fhe HF digestion technique contributes little contamination to the sample solution if carried out in plastic containers, and eliminates the necessity of using fluxes which contain high concentrations of alkali salts as well as trace element contaminants. The addition of 5.6g/100 ml boric acid to dissolve the fluorides appeared to be an effective means of completing the decomposition of the aluminosilicates and allowed the precise volumetric handling of the sample. Although Langmyhr and Paus (1968b) encountered

Chem. Geol., 7 (1971) 273-284 281

difficulty in dissolving the precipitated fluorides formed in the digestion of clay samples we found no such difficulty. Perhaps this discrepancy is clue to differences in the concentration of H3B03. We found that certain samples containing high carbon concentrations yield an insoluble residue after treat­ment with HF, but that this residue is probably unoxidized carbon and represents less than 1% of the total weight of the sample. In a few cases we had difficulty in digesting clay samples in the 30 mi~ period at 100° C usually required for other silicates. In these cases we found that longer periods of time for the digestion is more effective than heating at higher temperatures. The over-all preparation technique including decomposition of 20 samples and preparation of a complete set of standards can easily be accomplished by one technician in a day.

Advantages of the analytical technique

In addition to the normal instrumental analyses advantages that atomic absorption spectroscopy offers over more cumbersome classical methods of silicate analyses there are some specific advantages to the technique described here. The principal advantage is that all 18 elements may be determined from a single sample solution and with the use of a set of only four or six combined standards. This achievement is based on two factors as follows: (1) all interelement and reagent interferences are fully compen­sated over a range of chemical compositions by matching'the matrix of samples and standards; and (2) the sensitivity of the instrument can be ad­justed so that major and trace constituents can be analyzed from the same sample solution.

Van Loon and Parissis (1969) carried out a fairly extensive study of inter-element interferences in silicate analysis by atomic absorption. Their results indicated that Fe, Si, AI, Na and Mn could be analyzed at the relative concentration levels found in most silicates without mutual interference. They found that interference of potassium by sodium could be overcome by the addition of sodium to the standards. The interferences of Si and Al with Mg and Ca determinations was reduced by addition of 10,000 p.p.m. La to samples and standards, and an unspecified quantity of lithium to the stand­ards. In the case of the calcium analyses the Si02 level had to be maintained below 150 p.p.m. Langmyhr and Paus (1968b) state that no interferences are encountered for Co, Cu, Ni, Pb or Cr analyses.

It should be obvious that if all cation standards are combined in a single solution in the same proportion as that present in the sample then all mutual interferences will be compensated and no need exists for addition of excessive salts or acids to standards or sample solutions. Our data indicate that this is indeed the case even for aluminosilicate samples having matri­ces considerably different from those of the combined standard. It should be noted that no effort is made to match anion matrix of the combined standards with that of the samples although care is taken to match precisely the concentration of the reagents in all solutions including the aspiration fluid.

Our experience with the analyses of the U.S.G.S. rock standards PCC-l and DTS-l has shown that our standard matrix based on the average compo­sition of W-l cannot be applied to silicates having low concentrations of Al

282 Chern. Geol., 7 (1971) 273-284

and high concentrations of Mg. Similar experience in the analyses of carbo­nates has indicated that the matrix of the combined standard must match within certain limits. These limits are determined by the nature of the major element composition. The disadvantage of determining this matrix and its limits in a completely unknown substance is obvious.

Analyses of major constituents in silicate rocks are usually achieved by diluting the sample solution to a level where the element concentration is within the optimum range of the instrument at its maximum sensitivity. This dilution places the concentration of the minor elements below the detection limit and therefore makes it impossible to analyse both minor and major elements tn the same solution. We found that a series of dilution steps may introduce random and systematic errors that are difficult to evaluate. As a result we chose a single dilution technique which yields a 1:1,000 dilution of the original sample concentration. This leaves the con­centration of AI, Ca, Fe, K, Mg, Na, and Si above the optimum range for standard conditions of the instrument. To establish a convenient working range we found that altering the name path-length by turning the burner perpendicular to the light source (see Table II) reduced the sensitivity by as much as 95% and still allowed the element to be determined with suffi­cient precision.

The precision and accuracy of the minor element analyses were limited by the sensitivity of the particular element, and by the concentration level. The signal noise of the absorption from the recorder output may be such that the analyst may have difficulty in determining the precise peak height. In the analyses of U.S.G.S. rock standard G-2 the relative detection limit (Slavin, 1968) was exceeded for the elements Co, Cr, Ni, Pb, and V. These low concentrations rarely occur in layered silicates or marine sediments.

One of the improvements in the analytical procedure has been the incorporation of the computer in the calculation stages. The method of cal­culating the slope of the standard curve between standards has eliminated random errors caused by manual plotting and interpolation. In addition non­linearity of the curve as a function of ionization effects has been calculated more precisely than could be graphically plotted. An example of this phenomenon was encountered in the analyses of potassium in which case the standard curve had a steeper slope at the intermediate concentration levels than at low concentration levels. This phenomenon may have been caused by the ionization of relatively more potassium atoms at low concen­trations than at high concentrations. By calculating the slopes exactly as the combined standards indicated, all interferences were taken into consid­eration and the interpolated values for the samples should be accurate provided the sample matrix and standard matrix were matched.

Future extensions of the technique

Work has already begun on analyses of Ba in the U.S.G.S. standards and we have found that our results are within the range of recommended values in all cases, without the addition of excess quantities of alkali metal to suppress ionization. Other trace metals such as Hg, Cd, Mo, and Sn will be added to future analyses of marine samples after preliminary standard calibration work is completed.

Chern. Geol., 7 (1971) 273-284 283

ACKNOWLEDGEMENT

The authors wish to thank Drs. H~ Baadsgaard, Department of Chemis­try of the University of Alberta; D.W. Spencer of Woods Hole Oceanographic Institution; and J.M. Bewers and B.R. Pelletier of the Atlantic Oceano­graphic Laboratory, Bedford Institute, for critically reading the manuscript and offering helpful suggestions in the presentation of the data.

REFERENCES

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Buckley, D.E. and Loder, T.C., 1968. Particulate organiC-inorganic geochemistry of a glacial fiord. In: D.C. Burrell and D.W. Hood (Editors), Clay-Inorganic and Organic-Inorganic Associations in Aquatic Environments, I. University of Alaska, Institute of Marine Science, College, Alaska, pp.1-53.

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Slavin, W., 1968. Atomic Absorption Spectroscopy. Interscience, New York, N.Y., 307 pp.

Van Loon, J.C. and Parissis, C.M., 1969. Scheme of silicate analysis based on the lithium metaborate fusion followed by atomic absorption spectrophotometry. Analyst, 94: 1057-1062.

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