Automated Optical Emission Spectrochemical Bulk Analysis of Silicate Rocks with Microwave Plasma Excitation Kuppusami Govindaraju, * Guy Mevelle, and Charles Chouard Centre de Recherches Petrographiques et Geochimiques (C.N.R.S.), 54500 Vandoeuvre-les-Nancy, France
The development and routine application of a fully automated (from the initial sample weighlngs up to the printout of finished analytical reports) spectrochemicalscheme for bulk analysls (SiO2, A1203, Fe2O3, MnO, MgO, CaO, Na20, K20, Ti02) of silicate rocks and minerals are described. Acid-dissolved, lithium borate fusion melts of silicate samples are analyzed using a direct-readlng optlcal emission spectrometer equlpped with a microwave plasma excitation source. The analyzed solutions are buffered with Sr for ellminatlng matrix influences; over 30 internationalgeochemical standards have been analyzed to verify the accuracy of the results. The computerassisted, microwave-direct reader system permits rock analysis at low cost but with high precision multielement information; over 5000 silicate samples of varying chemical composition have been analyzed during the past 18 months. The summation values of bulk analyses fall normally in the 99 to 100% range.
Reports of international collaborative studies on the chemical composition of geochemical rock standards reflect, in general, the state.of-the-art of silicate analysis the world over. In such recent reports (1-5), optical emission spectrometry (OES) is conspicuously absent as an analytical tool for bulk analysis, whereas atomic absorption (AAS) and x-ray fluorescence spectrometry (XFS) account for nearly half of the contributed data. By bulk analysis, we mean the determination of nine major and minor elements which normally make up 99% of silicate rock samples: Si02, A1203, Fe2O3, MnO, MgO, CaO, Na20, KzO, and TiO2. I t is evident that geochemists, in general, have been paying less and less attention to OES during the past decade. However, their interest is now being revived because of recent developments in plasma sources capable of exciting lines of a large number of elements. In fact, plasma-OES systems for solution analysis present possibilities of simultaneous multielement analysis which responds directly to the growing needs of geochemists. Mainly, three types of plasma sources are proposed. They are double-electrode dc arc plasma jets, single-electrode capacitatively coupled microwave plasma (CMP), and electrodeless inductively coupled plasma (ICP). Review papers have appeared on plasma jets (6)and on microwave plasmas ( 7 ) . More dispersed is the literature on ICP (8-15). Two common features of all proposed plasma sources are their high temperature, usually more than 5000 “C and their low detection limits. With ICP systems, detection limits of the order of less than 1ng/ml are easily attained for many elements in aqueous solutions, whereas with CMP and plasma jets detection limits lag behind by l to 3 orders of magnitude (13). The high temperature of plasmas leads in general to minimal interelement effects. These qualities render the plasma excitation sources promising ones for simultaneous multielement solution analysis using OES. Reconnaissance studies have been carried out in order to exploit these possibilities in analytical geochemistry. With a plasma jet as an excitation medium, some major and minor elements have been determined in rock samples (16), in coal ashes (17), and in phosphate rocks (18). With CMP, a few major elements in carbonate
rocks (19) and in glasses (20) have been determined. With ICP, the determination of some trace elements in soil samples (21) has been reported. Although the literature available on the geochemical applications using plasma-OES systems is still scant, the consensus seems to be in favor of ICP-OES systems because of multielement capabilities a t ultratrace levels and of relative freedom from interelement effects ( 10-1 4). The singular absence of OES during recent international cooperative studies on new geochemical standards (1-5) is not strictly true because of one exception: a set of OES results contributed by our laboratory is to be found in all these first reports. In fact, during the past 15 years, our laboratory has been much concerned with the development of a series of OES procedures for silicate analysis (22-24). These procedures were all based on initial lithium borate fusion of rock samples; after grinding the fusion bead, major and minor elements (Si02, A1203, Fe2O3, MnO, MgO, CaO, TiOz) were determined with a direct reading optical emission spectrometer; and after acid dissolution of the ground fusion product, NazO and K2O were determined by flame photometry and, later on, by AAS. The latest of these procedures (24) enabled us to determine simultaneously by OES, 6 major (A1203,Fe203, MnO, MgO, CaO, Ti021 and 7 trace elements (Ba, Co, Cr, Cu, Ni, Sr, V) after ion exchange dissolution of borate fusion product; after acid dissolution of fusion product, we determined Si02, Na2O and K2O by AAS. The acquisition during 1974 of a new direct reader (OES) equipped with a microwave plasma excitation unit (CMP), after Kessler (25),for solution analysis led us to develop yet another but fully automated scheme of silicate analysis, from the initial sample weighings up to the printout of ready-forrelease analytical reports. The new scheme is indeed real progress in silicate analysis inasmuch as all the nine major and minor constituents needed for a “total” bulk analysis are determined simultaneously on a single solution (nitric acid solution of lithium borate fusion product) without further need for dilution. Prior to its routine application, it has been tested for all possible interelement effects. With CMP, matrix effects are encountered but can be eliminated by suitable buffering (Li Sr) of the solutions analyzed. Almost all available geochemical standards were analyzed for ascertaining the extent of matrix effects. Fully automated, our CMP-OES scheme of silicate analysis is production-oriented. More than 4000 rock samples of varying chemical composition have been analyzed during the past one year of its routine application.
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EXPERIMENTAL Equipment. A brief account of the spectrochemical equipment and the working conditions are presented in Table I. Sample Preparation. The preparation scheme involves fusion of rock sample with a mixture of boric acid and lithium carbonate (Prolabo, Paris); the fused sample is then acid dissolved. All steps in the scheme, including the tedious weighing operations, have been automated. Weighing. Seven weighing operations are necessary for each sample of which six are handled by an automatic weighing system (AWS). The AWS consists of an electronic balance (Mettler HE10, precision: 0.1 mg) coupled to a programmable calculator (Hewlett-Packard, model ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
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Table I. SDectrochemical Equipment a n d Working Conditions Spectrometer Applied Research Laboratories (ARL) 31000, multichannel optical emission spectrometer; 1-meter Paschen-Runge mounting; grating, 1440 rulings/mm; dispersion, 6.95 A/mm; spectral range from 2560 to 6100 8. Spectral lines Major and minor elements: Si, 2516; Al, 3961; Fe, 3720; Mn, 4030; Mg, 5173; Ca, 4227; Na, (8) 5890; Ti, 3653; K, 7664, with a separate adjustable Jobin Yvon H 2 0 monochromater. Trace elements (A): Ba, 4554; Co, 3453; Cr 4254; Cu, 3247; Ni, 3414; Sr, 4607; V, 4379. Excitation unit ARL Microwave plasma excitation source, after Kessler (25); nitrogen (support gas) flow rate, 3 l./min; frequency, 2400 MHz; power, 600 W; anode current, 200 mA; copper or silver electrode. Sample feeding Laboratory constructed programmable circular turntable as auto sampler; Electronest pneumatic nebulizer; sample feed rate, 7 ml/ min; nebulization efficiency, 2%. Timings Preintegration time, 20 s (without intermediate water washes); integration time, 8 s (fixed, without internal standard). Data processing Minicomputer PDP 11/05; core memory, 8K; software, program “TEMPS”, written in PAL11 and PDP-11 single-user (paper tape) BASIG; data printout teletype Model 33-
(ASR). 9810). The calculator itself is interfaced to a vibrating powder-distributor for controlled flow of powder samples. With AWS, the weighing step has become less tiresome and more foolproof; in addition, the gain in weighing time is at least threefold. The three different types of weighing jobs handled are briefly mentioned in the following three main steps of the preparation scheme. The seventh weighing operation is that of the rock sample itself. This weighing is done manually, without AWS, in order to avoid unwanted segregation of heavy and light minerals of rock samples by the AWS vibrating powder-distributor. Loss o n Ignition. This step consists of measuring the total loss of volatile constituents (COz,H20) of rock samples. About 1g of sample, accurately weighed into a preweighed silica crucible, is ignited for 15 h at 1000 OC using an electric muffle furnace. The loss on ignition step is carried out by sets of 30 samples. All the three weighings (empty crucible, crucible sample, crucible ignited sample) for each sample are done with AWS. The first two weighings and relevant information on sample and crucible identification are stored on magnetic cards. The calculator is also programmed to print, just after the recording of the third weighing, the loss on ignition in percent vs. the sample number. Fusion and Lamination. Small graphite crucibles (Carbon Lorraine, diameter, 20 mm; height, 17 mm) are used for fusingthe samples and the fusion is done with the help of an automatic tunnel furnace. The fusion charge in each crucible is a mixture of 400 mg of ignited 1 9). In rock sample and two fluxing agents (Li&03,500 mg; practice, a set of 200 crucibles, each one containing the two fluxing agents (weighed by AWS), is kept ready for further use. The tunnel furnace is coupled with a laminating system;after the fusion, the molten content of each crucible is laminated to produce 80-p thick glass sheets suitable for the next acid dissolution step. The tunnel furnace and the laminating system have been described elsewhere (24). One of the advantages of the tunnel-fusion procedure is that it assures identical heat treatment for all the samples, as the crucibles are obliged to pass through a hot tunnel at a slow but regular speed (1h for crossingthrough the tunnel). After a dead time of 1h, during which no personal supervision is necessary, 120 samples can be fused and laminated in 2 h. The ribbons obtained after lamination are ground for a minute in an agate mortar and stocked in polystyrene plastic vials (diameter, 20 mm; height, 35 mm), closed with polyethylene caps. Acid Dissolution. For each sample, 200 mg of ground fusion product is weighed into a 250-ml Pyrex beaker containing a magnetic stirring rod.
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
The weighing (200 & 0.2 mg) is done with AWS. As the sample to be weighed is different each time, the vibrating system is fitted with a metallic spring for housing horizontally one plastic vial. Through a 3-mm round hole made at the lowest extremity of the cap of the plastic vial, powder contained in the vial is made to fall by controlled vibration. With this weighing procedure, about 60 samples can be weighed in 60 min. The dissolution is carried out with an automatic dissolution bench provided with places for sixty 250-ml beakers and with 60 individual magnetic stirring mechanisms. The addition of acid or water to the beakers is carried out with a travelling fountain head which is programmable for unattended dissolution of 1to 60 samples. Beginning with the first beaker, the fountain head delivers 20 ml of dilute nitric acid (2.8 N) containing 50 g of Sr(N0& per liter. Just before the addition of the acid, the magnetic stirring rod inside the beaker begins to rotate. After a minute’s interval, the fountain head moves to the next beaker for delivering dilute nitric acid and so on, up to the last beaker. Then the fountain head retraces its way back to the first beaker and begins the dilution step by adding 180 ml of distilled water to each beaker. It takes about 2 h for the dissolution of 60 samples. DEVELOPMENT Nebulizer. While first analyzing sample solutions using microwave plasma excitation, it was soon realized that the original ARL pneumatic nebulizer, direct injection type, was chiefly responsible for lack of reproducibility in results during routine analysis. We substituted the ARL nebulizer with one (with spray chamber) recovered from a flame photometer (Electronest, model STA-58)with which it became possible to record reproducible results. The borosilicate spray chamber in which a cloud of aerosol is produced is voluminous enough to let very fine droplets escape towards the plasma electrode while heavy aerosol particles are drained off. P r o g r a m m a b l e Turntable. Even when using a suitable nebulizer, several minutes were needed before we could get reproducible results for silica if the plasma torch had been left without any injection of sample solution for some time. Better results could be recorded if care was taken to feed the plasma with identically buffered (in our case, Li Sr) rock sample solutions at regular intervals with very little loss in time during switch over from one sample t o another. In order to satisfy these conditions, we constructed a programmable turntable (PTT) with a circular sample tray. According t o the program put on the tape reader of the PTT, the circular sample tray can rotate horizontally, in either direction, t o bring sample solutions in any desired order below the fixed capillary t u b e of the nebulizer. In addition, the PTT has a mounting mechanism for maintaining the capillary tube at a constant depth inside the solution. T h e need for dipping the fixed nebulizer capillary tube at a constant level inside the solution became apparent when i t was observed that for the same sample solution, results for silica were systematically different according t o the level at which the sample solution was aspirated. This “level effect” is not usually observable for elements present in low amounts but is noticed for major elements like silica. For instance, when the distance between the tip of the nebulizer capillary tube and the bottom of the vial containing solution was kept constant, we have observed a value of 76% for Si02 with a granite sample solution when it was analyzed in the vial filled to contain 200 ml (height of solution: 7 cm); the same solution when it was filled to contain only 60 ml (height of solution: 2 cm) in the same vial gave 73.5% for SiOz. The observed variation on SiOz, under these two extreme conditions of nebulization, is due to the effective change in the nebulization efficiency, less solution being nebulized when the vial is filled to contain only 60 ml. Instrumental Drift. In all physical methods of analysis, instrumental drift is an inevitable phenomenon. T h e determination of silica, an element easily affected by small changes in analytical working conditions, shows that the drift is
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P
I
i
4
d
3000c
’“I/ CONCENTRATION
Figure 2.
Calibration curve for SiOn(YO). A and B curves, with Li as Sr) as buffer elements
buffer element. C curve, with (Li
Calibration curve for Al2O3 (%). A and B curves, with Li as buffer element. C curve, with (Li Sr)as buffer elements Flgure 1.
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practically negligible during short periods of time (5 min) but it slowly becomes important when the plasma torch is to be run continuously for 1to 2 h for purposes of routine analysis. In order to control effectively the instrumental drift during routine analysis, it was decided to separate in real-time the events happening during analysis from those occuring after analysis, the main idea being to reduce as far as possible the time spent during analysis. In addition, the direct reader was recalibrated during analysis a t regular intervals of time (4 min). These ideas led to the development of a computer program (TEMPS) written in PAL-11 and BASIC for the minicomputer PDP-11. Standards-Calibration Curves. Only geochemical standards (5,26,27)are used for the construction of calibration curves. With only seven standard samples, it is possible to cover the field of common silicate rocks and silicates (for instance, Figures 1and 2). When dealing with particular types of geological materials, one or two appropriate standards are appended to this list. The calibration curves are stored in the computer core memory as second-degree polynomials with 3 regression coefficients for each curve. A computer program specially developed for “automatic” regression allows the calculation of the coefficients and their subsequent transfer to core memory without manual keyboard input for intensities and concentrations. Auto-calibration. Once a direct reader is calibrated for a desired analytical program, it is customary to check at regular intervals the eventual consequences of instrumental drift by running two or three calibration standards which represent the high and low concentration levels of the calibration curves. If any significant change is manifest, depress and sensitivity potentiometers of each of the affected elements are readjusted to reflect the new instrumental state. This manual recalibration step can be done automatically (auto-calibration) by computer software logic. Our program TEMPS requires four standards for auto-calibration: 1)one acid rock-granite GH; 2) one basic rock-basalt BR; 3) one intermediate rock-
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diorite DR-N; 4)one ultrabasic rock-serpentine UB-N. The run of four (unknown) samples followed immediately by the four calibration standards is defined as one unit of routine analysis. Automated Analytical Procedure. The ARL direct reader together with the minicomputer is already an advanced system for simultaneous multielement analysis. By enriching this system with a programmable turntable and with a specially developed computer program (TEMPS) for spectrochemical‘analysis of solutions, the analytical procedure has been entirely automated from the feeding of solutions up to the output of analytical reports. During Analysis. The nebulization time for each sample is fixed, with 20 s of preintegration followed by 8 s of integration for analysis. As soon as one analysis is terminated, the values of the integrated spectral intensities are transferred quickly to the computer core memory, in less than 3 s for 10 intensities. The over-all “during analysis” time for one sample is about 30 s; the program TEMPS allows the stacking of over 1000 intensity values, each value being stored as one PDPword (16 bits). In practice the “during analysis” automated procedure consists of: 1)arranging the samples and calibration standards on the sample tray of the PTT; 2) arming the tape-reader of the PTT with a n appropriate tape program according to the number of samples to be analyzed; 3) pressing the “start” button of the direct reader. From that instant on, the analytical procedure becomes entirely automatic. T o begin with, the PTT presents the first of four samples in the first unit of routine analysis, followed by the three others and the four auto-calibration standards. Let us recall that after each analysis, intensity values are stacked in computer core memory. The analyses are continued u p to the last unit of analysis. Without a break, a maximum of eight units of analysis is possible during routine analysis. After Analysis. As soon as the last sample has been analyzed, the processing of the stacked spectral intensities begins, starting with the last (Nth) unit analyzed with the principle, last in, first out: 1)Auto-calibration with the four calibration standards of the N t h unit of analysis. 2) Conversion of the ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
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11t
0
100
cao
I
0
I
5
I
10 N q O CONCENTRATION
(X)
I
I
15
20
Figure 3. Effect of Na20 on , 3 0 2 , AI2O3, MgO,and CaO (concentration in % with reference to real rock samples present in solution)
intensities into concentrations using the latest auto-calibrated curves. 3) Print out of analytical reports successively for the four samples. 4)Auto-calibration of (N- 1)th unit of routine analysis and so on up to the first unit. The time necessary for processing spectral intensities and printing the analytical reports of one unit of routine analysis is about 8 min. RESULTS AND DISCUSSION Matrix Effects. Kessler (25) has studied interferences due to ionization and has recommended the addition of a spectroscopic buffer (Na) while dealing with samples of different chemical composition. Greenfield and co-workers (7) in their recent review on microwave plasma excitation sources have warned against the matrix effects to be expected due to the presence of refractory radicals when using low power microwave plasmas. During a comparative study, Boumans and co-workers (28) have shown that the Kessler-type CMP suffers more from ionization interferences than ICP. Our first trials to apply microwave plasma excitation to rock analysis were naturally with the solutions which were prepared almost daily in our laboratory during 1974 for atomic absorption determination of NazO, KzO, and Si02 ( 2 4 ) .These solutions were prepared in the same way as indicated in the Experimental section except that they did not contain Sr. T o begin with, analytical curves were constructed with a set of geochemical rock standards (granites GA, GH, GS-N; Diorite DR-N; Basalt BR; Serpentine UB-N). The minor (Mn, Ti) and alkali (Na, K) elements gave straight line analytical curves. For three of the major elements (FezO3, MgO, CaO) less satisfactory working curves were obtained with one or two points a little astray. However, two calibration curves were evident for A1203(Figure l),one (B) for the basic samples and another (A) for the three granite samples. A similar tendancy was also visible for Si02 (curves A and B, Figure 2). Before these manifestations of matrix effects, a detailed study was undertaken on this subject. Spectroscopic Buffers. It is common practice to add an alkali element in excess to diminish ionization interferences. The two alkali elements, Na and K, being in our list of elements to be determined, we tried first of all, to augment the amount of Li already present at a level of 80 ppm in the sample 1328
ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
solution. The immediate result was a depressing effect on Si and an enhancement effect on the other major elements. As we did not wish to sacrifice the available sensitivity for Si, we tried the addition of Sr, a buffer commonly employed in our laboratory for atomic absorption determination of Ca and Mg using the same sample solutions (29). The first results were really beyond our expectations. The double curves previously observed for A1203 and Si02 merged into single straightline ones (curve C, Figures 1and 2). Along with this spectacular redressing, there were distinct improvements (better slope, no points astray) for the other major constituents; the minor elements were not affected and the new curves were exactly the same as those without Sr buffer. In view of the widely varying chemical composition of geological materials, it was deemed necessary to undertake further studies on interelement effects with Sr as buffer. Interelement Interferences. Because of the enormous complexity of geological materials, it was decided to use only geological sample solutions as test materials rather than pure aqueous solutions containing two or three elements of interest. We looked therefore for some geological materials in which some of the interfering elements would be virtually absent. One such sample available in our laboratory is a silicate mineral (Tremolite) in which NazO, K20, and A1203 are present in trace amounts. By adding, for instance, varying amounts of Na, K, and A1 to aliquots of Tremolite solutions, it would be possible to study the interfering action of these elements on the other major elements present in the sample (SiOz: 57%; CaO: 13%;MgO: 24%; Fe2O3: 3%);it would also be possible to study the interaction between interfering elements, for instance K2O and Na2O. Further examples of geological base materials which we have used are: 1) ultrabasic rocks (dunite, serpentine) for studying the interference of Ca, Na, or K on Si02 and MgO as analytes; 2) a Kyanite sample for testing the influence of Ca, Mg, Na, or K on Si02 and A1203 as analytes. Furthermore, these test solutions after proper additives were analyzed as unknown samples using the “automated analytical procedure” in order to establish quantitatively the extent of interelement effects under routine conditions of silicate analysis. Elements Insensitive to Matrix Changes. One of the first results of our studies on interelement effects is that at least three elements, Fe, Mn, and Ti, are free from matrix interferences. Kessler has shown ( 2 5 )that the presence of Na, K, Cs, Ca, Ba, or S r had no influence on Fe; he attributed this behavior to the high ionization potential of Fe (7.86 eV). We have already mentioned that the analytical curves for Mn and T i were insensitive to the buffering action of Sr. Ionization Interferences. In common silicate rocks and minerals, the total alkali content (Na2O KzO) may vary from trace quantities up to 15%.Therefore, the effect of alkali elements was studied in detail. It may be seen from Figure 3 that Na has a depressing action on Si and an enhancing effect on Al, Ca, and Mg. The interfering action of Na being equal to that of K, if not slightly stronger, the results in Figure 3 obtained with Na are also valid for K or (Na K). I t is therefore observed that the interfering action of the total alkali elements is negligible up to about 10 to 12% of their content. In this context, it is worthwhile recalling that the majority of geological materials contain less than 12% of alkali elements, two notable exceptions being Na and K feldspars. Three available feldspar geochemical standards were analyzed and no significant differences were observed between our values and those recommended. For studying the mutual effect of Na and K, 16 test solutions were prepared starting with the Tremolite sample: keeping Na2O a t a constant level, 0% to begin with, four test solutions were prepared to have four different concentration levels of K2O (0, 2.5, 5.0, 10%). The Na2O level was shifted
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Table 11. Mutual Effect Alkali Elements. Determination of NazO (K20) in the Presence of KzO (NazO)
Theoretical concentration (NazO or KzO, %) Fixed alkali element,,%
0
KzO
2.5
5.0
10.0%
NazO (Found)
0 2.5 5 10
0 0 0 0
4.94 4.98 5.01 4.96
2.40 2.46 2.43 2.53
9.97
L
10.10 10.11
10.06
A A CURVE
. 1
KzO (Found)
NazO 0
0 0
2.5 5
0 0
10
4.95 4.90 4.96 4.99
2.50 2.53 2.53 2.55
0
10.01 9.90 9.95 10.04
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+
+
+ +
1
. I
io0
10 MOLAR
RATIO
(AIICA)
Flgure 4. Effect of AI on Causing pure aqueous solutions. CMP curve, results (emission) with microwave exicted plasma. AA curve, results with atomic absorption spectrometry
successively to 2.5,5, and 10%to obtain a t each level four more solutions with four different concentrations of K20. These sixteen solutions were analyzed and the results on NazO and K20 presented in Table I1 do not reveal the presence of any mutual interferences. The effect of Ca and Mg on A1 is an interesting case to examine before switching on to the inverse case (A1 on Ca and Mg). Kessler ( 2 5 )has studied the individual effect of Na, K, Cs, Mg, Ca, Sr, and Ba on A1 present in aqueous solutions with CMP; he noticed that only Mg did not exert any influence on A1 whereas all the other elements had enhancing effects on Al. While developing the analysis of carbonate rocks with CMP, Kemler and Gebhardt (19) observed that the slope of the calibration curve for A1203(constructed with four standards ranging from 0.06 to 0:9%) depended heavily on the concentration of CaO in the four standards: the slope values varied from 0.2 to 0.9 for changes in concentration of CaO from 3 to 56%. The slope variations were attributed to the ionization effect of Ca. In this connection, we observe that the two calibration curves obtained for A1203 (A and B, Figure 1)with only Li as buffer element have similar slopes; one point which lies in between the two curves is due to an intermediate rock sample (Diorite DR-N) with global chemical composition intermediate between “acid” granites, such as granite GH and “basic” basalts, such as basalt BR. Chemically, one of the main differences between the acid and basic rock samples is their total alkaline earth content (MgO CaO), Sr and Ba being trace elements in both cases. The three granites lying on the acid curve contain less than 5% of (CaO MgO). The three points lying on the basic curve are due to basalt BR (CaO MgO: 28%), Serpentine UB-N (CaO MgO: 41%) and a composite sample obtained from equal amounts of basalt BR and diorite DR-N (CaO MgO: 20%), While the composite sample (DR-N BR) is on the basic curve, the intermediate rock sample (DR-N, CaO MgO: 12%) gets itself ejected from the two families of curves (A and B). Now, curve C (Figure 1) obtained with (Li + Sr) as buffer elements is very closely similar to that of the basic curve B with only Li as buffer element. The spectacular merging of the basic and acid curves into a straight-line one (curve C, Figure 1)seems therefore due to the enhancement effect of Sr on A1 in the case of the acid rocks poor in alkaline earth elements. The presence of a minimum amount of alkaline earth elements in the solutions analyzed appears therefore to be necessary to bring the enhancement effect of these elements to a constant level so that acid and basicrocks are not differentiated; in other words, (Li + Sr) seem to be efficient ionization buffers. No doubt because
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Flgure 5. Effect of A1203 on MgO and CaO (concentration in reference to real rock samples present in solution)
YO with
of this efficient buffering action, we did not notice any significant influence of Ca and Mg on A1 during our interelement study using real-life rock sample solutions. Solute Vaporization Interferences. In flame methods of analysis, one important source of analytical inaccuracies stems from the formation of refractory compounds, a classical example of such an interference being the depressing effect of A1 on Ca due to the formation of calcium aluminate. Larson and co-workers (30) have studied the system Ca-A1 using aqueous solutions with ICP; they report freedom from solute vaporization interferences by this system. We have studied the same system using aqueous solutions of the same concentration levels with CMP (Figure 4, CMP curve); for further comparison, these solutions were also analyzed by AAS using an air-acetylene flame (Figure 4, AAS curve). It may be seen from Figure 4 that CMP appears also to be free from solute vaporization interferences by the system Ca-AI. With Al/Ca molar ratios greater than 10, Ca emission is enhanced, probably due to the high salt content of the solutions. This enhancement corresponds to the plateau region of the AAS curve where Ca absorption is practically null with an excess of Al. Using real-life solutions (with Sr as buffer), the effect of A1 on Ca and Mg was also studied (Figure 5). The interfering effect of A1 is seen to be virtually negligible up t o a limit of about 20% of A1203 in the rock samples analyzed. The depressing effect of A1 is more important for CaO (10.0 to 9.4%) than for MgO (16.4 to 16.2%).The system Ca-A1 is one of the rare cases where our analytical procedure leads to small inaccuracies in the determination of CaO and MgO, whenever the A1203exceeds the 20% dead level and when the alkaline earth elements are present in high concentrations. Such cases are limited to only one group of silicate minerals (plagioclases). ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
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Table 111. Precision Data on Two Geochemical Rock Standards Granite GS-N
Si02 A1203
Fez03 (TI MnO MgO CaO NazO K2O Ti02 LOIe
f a
sb
65.97 14.76 3.69 0.06 2.20 2.46 3.77 4.60 0.72 1.33
0.25 0.09 0.03
RVC
65.80 14.67 3.75 0.003 0.056 0.05 2.30 0.02 2.50 0.04 3.77 0.04 4.63 0.09 0.68
Gabbro MRG-1 fa
sb
RVd
39.10 8.55 18.02 0.17 13.42 14.57 0.75 0.16 3.43 1.38
0.19 0.06
39.17
0.11 0.01
17.91 0.17 13.47 14.66 0.72 0.18 3.69
0.12 0.10 0.02 0.03 0.08
8.50
a Average (%) of 28 analyses carried out on two different days using 7 different sample solutions. Standard deviation. Recommended values from Ref. 5. Recommended values from Ref. 4. e Loss on ignition.
Nebulization. During the course of our studies on interelement effects, we were much struck by the fact that the nebulization of the solutions is probably one of the first subjects to be studied in detail before proceeding to any study on matrix effects. In fact, it was found that under certain conditions of nebulization, interelement effects were not discernible whereas under other conditions they were flagrant. I t was observed that when the nebulization leads to a thick mist inside the spray chamber, matrix effects were negligible. The mist formation can be modified by slowly pushing in or pulling out the metallic nebulizer which is partly inside the glass spray chamber. Accuracy. Our campaign of studies on interelement effects has permitted us to delimit the rare cases where matrix effects would lead to inaccuracies in analysis. In addition to these studies, about 30 international geochemical rock and mineral standards were analyzed during routine analysis and our results were found to compare favorably with the values recommended for the standards. Unfortunately, no geochemical standards are still available for some of the rare cases of interelement effects that we have encountered. For such cases, a correction procedure has been developed. After some first trials, it seems possible to eliminate, for instance, the effect of A1 on Ca by further additions of Sr or by changing the buffer element to Ba. However, this approach has not yet been fully studied. Routine Analysis. In instrumental silicate analysis, the nine major and minor elements (Si02, Fe203, MnO, MgO, CaO, TiOz, NazO, K2O) plus loss on ignition should normally make up a “total” analysis with summation values falling in the range 99 to 100%.During routine analysis, any significant deviation from this range may mean the presence of either analytical errors (higher summation values) or of elements other than the nine determined (lower summation values). Analytical errors detected by higher summation values may be due to accidental or systematic errors. Systematic errors are normally controlled by appropriate steps in the analytical scheme whereas accidental errors, or more generally manipulation errors, are more difficult to control, particularly during routine analysis. I t is for getting rid of manipulation errors that we were led, first of all, to automate all the steps in our analytical procedure. One of the first visible consequences of our “automated analytical procedure” is the high production level with more than 90% of the analyzed samples giving satisfactory summation values after one single analysis. Such a high performance during routine analysis reveals two salient 1330
ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
features: high reproducibility (Table 111) and freedom from matrix influences. Incidently the use of automated high production-level multielement analytical schemes lowers considerably the cost of rock analysis. Lower summation values are more troublesome to take care of, as some elements (P, Ba, Sr, Cr, Ni) which are usually present as trace constituents may, in particular cases, turn out to be in higher concentration levels. In such circumstances which are inevitable during routine analysis of geological materials, our analytical program provides quantitative data on six “trace” elements (Ba, Co, Cr, Cu, Ni, V) whenever their concentration exceeds the limit of 200 ppm. Because of lack of sensitivity, it has not been possible to include P205 in this check list. For obvious reasons of its use as buffer element, we have to drop out S r also. Both the two multielement approaches to silicate analysis, XFS and OES, are invariably short of “total” analyses for the reason that at least one major element happens to be missing, Na for XFS (31-34) and Na and K for OES (24,35,36).That a total bulk analysis is realized, at a stretch, with a single solution and without further dilution is one of the salient features of our CMP-OES scheme. The fact that only a single solution is necessary for total analysis has greatly facilitated our task for automation from the uptake of sample solutions up to the print out of finished reports in due form. The sample preparation scheme is in no way a bottleneck, as it has also been automated from the initial weighings up to the final preparation of solutions ready for analysis. The analytical possibilities of plasma-OES systems have been in the limelight during the very recent years. Our experience derived from the analysis of over 5000 samples demonstrates indeed that such possibilities are real and well grounded, even during routine analysis of such complex materials as geological rocks and minerals. Very recently our spectrochemical bulk analysis scheme for nine major and minor elements has been extended to include seven trace elements (Ba, Co, Cr, Cu, Ni, Sr, V) during routine analysis. As such, the routine application of our CMP-OES scheme for 16 elements has created an unprecedented situation in our rock analysis laboratory by reversing the respective roles of OES and AAS: OES, traditionally a tool for analysis on solid samples is being routinely employed for solutions whereas AAS, eminently a tool for solution analysis, is no longer applied for solutions but for solid sampling determination of elements such as Li, Rb, Cs, and P b (37). To sum up, the net result of these transformations and automations is a n accelerated production of rock analyses at low cost but with high precision multielement information for geochemical purposes. LITERATURE CITED F. J. Flanagan, Geochim. Cosrilochim. Acta, 33, 81 (1969). A. Ando, H. Kurasawa, T. Ohmori, and E. Takeda, Geochem. J., 5, 151 (1971). B. G. Russell, R. G. Goudvis, G. Domel, and J. Levin, Nat. inst. (Johannesburg) Metall. Res. Rep., 1351 (1972). S. Abbey, A. H. Gillieson. and G. Perrault, Can. CertifiedRef.Mater. Proiect, MRP/MSL 75-132 (1975). H. de la Roche and K. Govindaiaju. Assoc. Nat. Rech. Tech., Rep., 9699 (1975). S. Greenfield, H. McD. McGeochin, and P. B. Smith, Taianta, 22, 1 (1975). S. Greenfield, H. McD. McGeochin, and P. B. Smith, Taianfa, 22,553 (1975). S. Greenfield, I. L. Jones, and C. T. Berry, Ana/yst(London), 89, 713 (1964). R. H. Wendt and V. A. Fassel, Anal. Chem., 37, 920 (1965). V. A Fassel and R. N. Kniseiey, Anal. Chem., 46, 11 10A (1974). S. Greenfield, I. L. Jones, H. McD. McGeochin,and P. B. Smith, Anal. Chim. Acta, 74, 225 (1975). P. W. J. M. Boumans and F. J. de Boer, Specfrochim.Acta, Part B, 27,391 (1973). P. W. J. M. Boumans and F. J. de Boer, Spectrochim. Acta, Part B, 30,309 (1975). J. C. Souilliart and J. P. Robin, Analusis, 1, 427 (1972). L. R. Layman and G. M. Hieftje, Anal. Chem., 47, 194 (1975). D. W. Golightly and J. L. Harris, Appl. Spectrosc., 29, 233 (1975). S.S.Karacki and F. L. Corcoran, Appi. Spectrosc., 27, 41 (1973)
F. L. Corcoran, P. N. Keliher, and C. C. Wohlers, lnt. Lab., 52, (1972). W. Kessler and F. Gebhardt, Glastech. Ber., 40, 194 (1967). F. Gebhardt and H. Horn, Glastech. Ber., 44, 483 (1971). R. H. Scott and M. L. Kokot, Anal. Chim. Acta, 75, 257 (1975). K. Govindaraju, Pub/.Group. Av. Methodes Spectrogr., 221 (1960). K. Govindaraju, Bull. SOC.Fr. CBram., 67, 25 (1965). K. Govindaraju, Analusis, 2, 367 (1973). W. Kessler, Glastech. Ber., 44, 479 (1971). M. Roubault, H. de ia Roche and K. Govindaraju, Sci. Terre, 15, 351 (1970). H. de la Roche and K. Govindaraiu, Bull. SOC. Fr. CBram., 100, 49 (1973). (28) P. W. J. M. Boumans, F. J. Dahmen, J. W. de Boer, H. Hoelzel, and A. Meier, Spectrochim. Acta, Part B, 30, 449 (1975). (29) K. Govindaraju. Colloq. C.N.R.S.,No 923 (Nancy), 269 (1970).
(30) G. F. Larson, V. A. Fassel, R. H. Scott, and R. N. Kniseley, Anal. Chem., 47, 238 (1975). (31) K. Norrish and J. T. Hutton, Geochim. Cosmochim. Acta, 33, 431 (1969). (32) R. Tertian and R. aninasca, X-Ray Spectrom., 1, 83 (1972). (33) E. P. FabbiandL. F. Espos, U.S.,&I. SUN. Prof.Pap., 8008, B 147(1972). (34) P. K. Harvey, D. M. Taylor, R. 0. Hendry, and F. Bancroft, %Ray Spectrom., 2, 33 (1973). (35) N. H. Suhr and C. 0. Ingameiis, Anal. Chem., 38, 730 (1966). (36) Y. Besnus and R. Rouault, Analysis, 2, 111 (1973). (37) K. Govindaraju, G. Mevelle, and C. Chouard, Am/. Chem., 46, 1672 (1974).
RECEIVEDfor review March 2,1976. Accepted April 27,1976.
Determination of Sulfate by Flame Emission Inhibition Titration J. R. Sand’ and C. 0. Huber* Department of Chemistry, University of Wisconsin-Milwaukee,
Milwaukee, Wis. 5320 1
Flame emission photometric titrations exploiting the signalinhibiting effects of sulfate, provide accurate, rapid, and convenient determinations for real samples at concentrations down to 0.2 ppm. An Improved detection system was built around an interference filter, a photomultiplier tube, and an elementary dc amplifier. Detection and determination limits are, at present, limited by the level of flame noise encountered in the system. Collection and analysis of the refractory particulates formed during the course of these titrations yielded data on the compositionalchanges which give rise to the unusual shapes seen for anion inhibition titration curves. A mechanism involving rate-regulated stolchlometric changes in the refractory inhibition products Is compatible with the data.
onstrate superior performance with relatively simple spectroscopic equipment. During the course of the work, it was also possible to elucidate and apply a newly discovered inhibition signal. An improved method for the collection and analysis of the refractory particles formed in flames used in analytical flame spectroscopic procedures is described. Refractory particulates corresponding to various points during the course of these inhibition titrations were collected and analyzed. The results of these analyses indicate changes in the composition of the particles collected from different regions of the titration curve. These compositional changes are correlated with observed signals to describe a mechanism for the titration process.
EXPERIMENTAL Titration methods based on inhibition effects of anions on flame photometric signals of metals were introduced by Torok ( 1 ) and have since been applied to several determinations (2-4). A method for the simultaneous determination of silicate, phosphate, and sulfate utilizing magnesium atomic absorption has also been reported (5). T h e inhibition titration experiment involves addition of a metal titrant solution to a stirred solution of anions from which metal cations have been removed. A flame photometer (see Figure I ) , tuned to respond to titrant metal, is used to sample the titration mixture and provide the analytical signal. The type of titration curve obtained is dependent on the flame conditions and the type of anions in the solution to be analyzed. The unusual shapes of these “titration curves” result from complicated processes occurring in the evaporating droplets of sample formed by the burner nebulization process. Thus, the method is a “titration” as to procedure, but there ordinarily is not a stoichiometric reaction in the “titration” vessel. Direct correlations are made between features of the titration curves and sample anion concentrations. Calcium flame emission spectroscopy was examined in order to extend and enhance flame photometric titration techniques by incorporating the instrumental advantages inherent in flame emission. The shape of the analytical signals is such that spectral purity is not a major concern in these analyses. The instrument was therefore redesigned for increased optical throughput and efficiency by sacrificing spectral resolution. In this manner, it was possible to demPresent address, The Trane Company, Lacrosse, Wis. 54601.
Reagents and Solutions. All aqueous solutions were made up in distilled water which had been further purified by passage through a mixed-bed ion-exchange column. Titrant solutions of calcium were prepared by quantitative dilution of a 2000-ppm Ca solution which was made up by dissolving reagent grade CaCO3 in a minimum amount of concentrated hydrochloric acid and diluting to volume. Stock sulfate solutions were prepared by dilutions of reagent grade sulfuric acid and standardization vs. primary standard sodium carbonate. Stock solutions of 1000 ppm Si02 were prepared by fusing the appropriate amount of dried chromatographic grade Si02 with an excess of anhydrous, reagent grade sodium carbonate a t 1000 “C for 30 min and dissolving the fusion products to volume. Silicate solutions were deionized with a cation-exchange resin on the same day they were used to prevent silicic acid precipitation. Stock solutions of the cations used for interference studies were made up from the chloride salts, while the sodium or potassium salts were used to make up stock solutions of potential anionic interferences. Interfering cations were removed from sample solutions by treatment in a “batch” process with the hydrogen form of Rexyn 101,16-50 mesh cation-exchange resin. Apparatus. All glassware and polyethylene apparatus used for these determinations were acid-hardened by soaking in 3 N hydrochloric acid overnight. Initial flame emission measurements were obtained on a Jarrell-Ash model 82-516 spectrometer equipped with a JA82-374 laminar flow, slot burner using hydrogen and air. This instrument was used with the 100-Wm entrance slit and 15O-Wm exit slit provided, or with 1-mm slits made from I/lG-in. aluminum stock. The 620-nm interference filter used was 51 X 51 mm with a 10-nm nominal band pass (Optikel Corp., Lexington, Mass.). A solid-state dc amplifier (6) was designed and built for use with the interference filter detection system using integrated circuit operational amplifiers. T h e flame temperature was approximately 1910 “C. An R 106 photomultiplier tube was used a t the shorter wavelengths and a 1P28A photomultiplier tube was used in conjunction with the interference filter. A commercial electrostatic room air cleaner was used prior to and during sub-ppm sulfate determinations. A Harvard Apparatus, Inc. infusion pump and 50-ml polyethylene ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
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