ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
design, construction, and operation of any microprocessorbased real-time signal processing system is without pitfalls.
LITERATURE CITED (1) Philbrick Researches, Inc., "Applications Manual for Computing Amplifiers", Nimrod Press, N.Y., 1966, p 75. (2) H. V. Maims&@, C. G. Enke, and S. R. crouch, ''Electronic Measurements for Scientists", Benjamin, Menlo Park, Calif., 1974, p 822.
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(3) Gary Horlick, Anal. Chem., 47, 352 (1975).
RECEIVED August 26, 1977. Accepted January 12, 1978. Presented in part a t the 28th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 2, 1977.
Metaborate Digestion Procedure for Inductively Coupled Plasma-Optical Emission Spectrometry Jan-Ola Burman, Christer Pontkr, and Kurt Bostrom" Department of Economic Geology, University of Lulea, 951 87 Lulea, Sweden
Microwave plasmas (MWP) and Inductively coupled plasmas (ICP) have proved to be good-to-excellent excitation sources in optical emission spectrometry for the analysis of geological materials (Govindaraju et al. ( I ) ;Burman et al. ( 2 ) ) . The analysis of silica rich geological materials requires awkward digestion procedures involving, for instance, lithium metaborate or sodium carbonate fusion as a critical step. The complete dissolution of silicate rocks usually demands a large excess of fusing agent, commonly in the proportions 1 O : l to 7:1, but even proportions as low as 4:l have been successfully used (Suhr et al. (3);Ingamells ( 4 ) ;Govindaraju et al. ( I ) ; Joensuu (5)). In our MWP spectroscopic work, the preferred ratio has been 7 parts of LiB02 (350 mg) to 1 part sample (50 mg). Our preparation of the metaborate is essentially identical to that in references (3,4 ) . In the fall of 1977, the MWP source in our sequence reading emission spectrometer (ARL 33 000) was replaced by an ICP unit, see Table I. Samples dissolved in HF-HC104can easily be analyzed by means of this setup, but metaborate fused samples always cause a continuous drift in the readings, and usually the nebulizer clogs and becomes inoperable within 15-20 min after it is turned on. Inspection of the tip of a clogged nebulizer reveals that a deposit is the culprit. This deposit changes the gas flows to a fatal extent. including the sample flow. This clogging can be avoided by additional dilution of the solutions, but this is objectionable, since it decreases the sensitivity of the procedure. We have therefore experimented with less metaborate in the fusing step.
EXPERIMENTAL The instrumental system is described in Table I. The selected wavelengths are those listed in (2). Various observation heights are selected to optimize the signal-to-noiseratio for each element. This observation height is 26 mm for all elements, except for Mg, Ba, and Si which are observed at a height of 18 mm over the induction coil. It should be pointed out, however, that future studies in our laboratory may lead to better experimental conditions than those reported in Table I. All chemicals used are of reagent or analytical reagent grade.
RESULTS AND UiSCUSSION In our experiments we found that as little as 50 mg of LiB02 can fuse 50 mg of silicate rock in a graphite crucible a t 1000 "C under 30 min. As was noticed previously ( 2 ) there is no tendency for the formed borate beads to stick to the crucible walls, but they can be dissolved in toto in dilute nitric acid (8% v/v). The total dissolution time is 3-4 h, and after this the solutions are diluted to 100 mL before the analysis. Solid residues were never observed after this step. This observation is well supported by our analyses for Cr. Cr is present to a large extent as refractory oxides (e.g., FeCr204)in our rock 0003-2700/78/0350-0679$01 .OO/O
Table I. Instrumental System The ARL 33000 CA sequential rending spectrometer is described in ( 2 ) Plasma conditions: RF-Generator model Henry; 3 kW; 27.12 MHz, Crystal controlled. Forward R F power: 1200 W Argon coolant gas flow rate: 1 0 L/min Argon plasma gas flow rate: 0.8 L/min Argon sample transport gas flow rate: 1.0 Limin Nebulizer: type J. E. Meinhard concentric glass nebulizer. Sample uptake rate: 0.8 mL/min Table 11. Standard Deviations and Relative Errors for Certified Rock Analyses" A B C D E F SiO, (in % ) Al,O, (in %) TiO, (in % ) CaO (in %)
MgO (in %) Na,O (in %) Fe,O, (in %) MnO (in ppm) Cr (in ppm) Ba (in ppm) Zr (in ppm) Cu (in ppm)
0-76 38-76 0-1 8 0-2.6 0-14 0-2.5 0-35 0-4.5 0-3.9 2.9-3.9 0-13 0-2100 0-420 0-850 0-240 0-1000
0.45 1.2 . .. . . . 0.54 0.95 0.75 1.4 0.19 2.1 1.5 4.4 0.032 2.5 1 0 22 8.1 3.6 0,090 1.3 0.043 3.4 ... ... 6.9 0.103 0.60 3.6 0.031 1.4 . . . . .. 0.055 2.8 . . 6.6 0.048 1.4 3.6 ... 0.046 0.70 3.1 6.8 32 3.1 13 17 15 7.1 ... ... 22 5.2 10 35 28 23 ... ... 7.7 1.5 . .. .. .
A. Analyzed component and concentration units. B. Studied concentration interval. Units as in A. C. Stanard error, Sy3x (this work). This represents the standard error of estimate of y on x in a linear regression fit for the data, where y = concentration and x = intensity of readings. All results based on one set of readings. D. Relative error (in % ) for the ICP analyses reported in column C, defined as 100 S y . x , divided by the mean concentration for the studied concentration interval. E. Relative errors (in %), obtained by MWP-analyses ( 2 ) . Defined as for D. All results based on one set of readings, except the data for SiO,, which is based on four readings of each sample. F. Average relative errors (in 7%) reported in (6-8). NOTE: Ba and Cr have been studied only for the concentration interval near the detection limit; higher concentrations cause no problems in the analytical work. Artificial solutions have been used for Cu, since the range of Cu values in the standard rocks is very limited, 12-70 ppm. samples. Such oxides are not completely decomposed by HF, but are easily put into solution by our procedure. Clogging and drift were not noticed during the analyses. The fusion procedure described here has now (Dec. 1977) been in routine 1978 American Chemical Society
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use in our laboratory for 3 months without complications. During this period we have made more than 100 rock analyses with the reproducibilities indicated in Table 11, columns C and D. The usefulness and reliability of the method was tested on the standard rocks BR, GA, GH, GS-N, DR-N, and UB-N; the composition of these rocks has been discussed by de la Roche et al. (6, 7)and Roubault et al. (8). The results of this test are shown in Table 11. These standard rocks are extensively used in our analytical work, fresh standard solutions being prepared every week. The results demonstrate that the borate fusion advocated here is excellent for the analysis by ICP spectrometry of silicate rocks. The fusion and dissolution brings all material in solution, and the ICP-excitation leads to a marked improvement in the reproducibility and to lower detection limits than are possible with MWP excitation (2). As to the success of our digestion procedure, we can only speculate, but possibly the use of only a little metaborate leads to an incomplete destruction of the silicates. The silica and alumina may therefore exist as dissolved large complexes in the final acid solutions. These complexes cannot be broken down in a cold atomic absorption flame (3, 4 ) , but are completely decomposed in a hot plasma. The data for Si02 in Table I1 superficially suggest that the data here are somewhat inferior to the results by MWP spectrometry. However, the MWP-data in Table I1 for SiOz were obtained by four repeated readings in order to reduce the drift in the MWP system; this drift is particularly annoying for SiOz (2). The ICP data in Table I1 for SiOa on the other hand are based on only one set of readings. The spread in the data for Ti02, MgO, Fe203,MnO, and BaO is considerably smaller than that reported in the literature for certified rocks, see last column in Table 11. The geochemical literature is rich in rock analyses that have poor Ti, Ba, and Mn determinations. The method presented here therefore represents a considerable improvement in rock analytical procedure. Our results demonstrate that several minor and trace elements can be determined in a satisfactory way by the
procedure outlined here. Studies in progress suggest that this conclusion is correct also for many trace constituents not reported here, such as Ni, Co, Sr, La, and Y. For traces present in low concentrations, however, (at the 1 ppm level or less) one will have to use HF-HC104 dissolution of larger quantities of sample to be able to measure low element concentrations. Other advantages with the present method are the small amounts of contaminants that are added by reagents, the reduced costs for high purity (or purification of) LiB02, and the reduced work in preparing it. The method is well suited for the analysis of very small geological samples (15-50 mg), which frequently are the only ones available, for instance when thin layers from sediment cores are studied. Comparisons of our MWP results (2) with the present ICP results suggest that the MWP method is very sensitive to interelement effects; compare for instance the spread in Ti02 and Ba in this work and in (2). These observations will be discussed elsewhere.
ACKNOWLEDGMENT We thank K. Govindaraju, who cordially sent us the standard rocks GA, GH, UB-N, BR, DR-N, and GS-N. LITERATURE C I T E D (1) K. GovinQraju. G. Mevelle, and C. Cbuard, Anal. Chem., 48, 1325-1331 (1976). (2) J. 0. Burman, B. Bostrom, and K. Bostrom, Geol. Foeren. Stockholm Foehr., 99, 102-110 (1977). (3) N. H. Suhr, and C. 0. Ingamells, Anal. Chem., 38, 730-734 (1966). (4) C. 0. Ingamells, Anal. Chem., 38, 1228-1234 (1966). (5) 0. Joensuu, Rosenstiel School, University of Miami, Coral Gables, Fla., personal communication, 1976. (6) H. de la Roche, and K. Govindaraju, Rapport (1972) sur quatre standards gBochimiques (DR-N, URN, BX-N and DT-N), Bull. Soc. Fr. Gram., 100, 49-75 (1973). (7) H. de la Roche, and K. Govindaraju. Rapport pr6liminaire (1974) sur deux nouveaux standards g6ochimiques de 1'A.N.R.T. (GS-N, FK-H), (1975). ( 8 ) M. Roubauk, H. de la Roche, and K. Govindaraju, Etat actuel (1970) des Btudes coopBratives g6ochimiques, Sci. Terre, 15, 351-393 (1970).
RECEIVED for review November 4, 1977. Accepted December 27, 1977. This work was supported by the Swedish Board of Technical Development (STU) under a grant to K. Bostrom.
Size, Shape, and Position of a Spectrophotometer Light Beam Stephen D. Rains Bausch & Lomb Incorporated, Analytical Systems Division, 820 Linden Avenue, Rochester, New York 14625
Among the users of spectrophotometers, there is a great interest in testing instrument performance. Determining the size, shape, and position of the light beam as it passes through the cuvette location is a useful part of such testing, since it informs the user of the size and position requirements for the optical free aperture of the cuvette. This information may be photographically recorded by trimming a small piece of light sensitive material comprising a diazonium compound (for example, blueprint paper) t o fit in the cuvette holder just ahead of the cuvette itself. That is, the incident light should, for the purpose of the test, strike the photosensitive surface of the blueprint paper rather than the optical window of the cuvette. After an exposure a t approximately 425 nm of from 1 to 24 hours (depending on the light level used in the particular 0003-2700/78/0350-06~0501 .OO/O
instrument), a photographic image will be obtained of the size, shape and position of the cross-section of the light beam as it enters that location. The chief advantages of using blueprint paper are that it can be cut and mounted in room light if this is done without undue delay, and the development process (exposure to ammonia) is fast, simple, and dry. Although they may not be as convenient as blueprint paper, other photosensitive materials such as photographic emulsions could be used to test a t other wavelengths or to decrease the exposure time needed. All of the above variations share the advantages of creating a lasting record and of being usable in locations that are not accessible for direct visual observation. 1978 American Chemical Society
