2451
Anal. Chem. 1983, 55, 2451-2453
56
fillor _
.
8: I
Approx 100 W a l l s Powarid By OC Sourca
*I6
IOK
__
141 + 4
.... .. FPT.100
=
e
CHART RECORDER
-15
/
To Pump
I mM/L PO4
IOK +I5
PhotoT1an8~61or
Fiboropllc
3 mM/L PO4 2mM/L PO4
4 m M / L PO4
OFFSET -15 IOK
0
+ 0 -I
0 13
TIME (SECONDS)
Figure 2. Response curves for phosphate. Details in text. Reagent ReI e IY 0 I,
Figure 1. Schematic of nanocolorlmeter. Details in text.
metal chelator. The phosphate reagent was a combination of ammonium paramolybdate and a catalyst in sulfuric acid added to a reducing reagent containing sodium acetate trihydrate, acetic acid, sodium metabisulfite, p-methylammoniumphenolsulfate, and a surfactant. Test solutions were placed in microcaps filled with paraffin oil which had been equilibrated with water and 100% C02 This was done to prevent the loss of calcium, magnesium, and phosphate (3). Pipettes and capillaries used in the analysis were siliconized with Prosil-28 (PCR, Gainesville, FL). An 8-10 nL pipette was used to transfer the test solution from the capillaries to the injection port of the colorimeter. The outside of the pipette. was rinsed with chloroform and deionized water. Before the transfer of test solution, the pipette wm placed in a holder which was affixed to a Brinkman micromanipulator (Brinkman Instruments, Toronto, Ontario). The transfer procedure was carried out under a stereomicroscope with a 30X magnification.
RESULTS AND DISCUSSION When calcium, magnesium, or phosphate was deleted from artificial tubule fluid, there was no significant effect from the other tubule fluid components. Calcium and phosphate were analyzed a t a flow rate of 20 nL/s with a 630-nm filter. Magnesium was analyzed at 18 n L / s with a 530-nm filter. The curves that were generated on the colorimeter were analyzed by weighing them on a Mettler balance (Mettler Instrument Corp., Hightstown, NJ). In order to maximize the area of the curves, various flow rates were tested. Variations in light intensity were also tested until the optimum combination of light and flow rate was established. Figure 2 demonstrates the sensitivity of the nanocolorimeter. These curves were produced by injecting 10 nL volumes of phosphate standard, ranging in concentration from 1.0 to 4.0 mM/L, into the cuvette. Calibration curves were generated by injecting 10-nL aliquots of calcium, magnesium, and phosphate into the flow-through cuvette. Each point on these curves was the mean of five measurements. The straight line plots were
obtained by linear regression analysis. The correlation coefficient for each line was 0.99. The introduction of bubbles into the reagent stream has been a major problem with this system. The cause of the problem has been the negative pressure produced by the withdrawal pump on tubing connections in the system. Bubbles produce noise peaks and interrupt the reagent flow. Maintaining a constant flow rate is essential for the accurate measurement of standard solutions and tubule fluid samples. By fabricating the injection port in the right arm of the cuvette and placing the right side of the cuvette directly into the reagent reservoir, we have achieved several benefits. Three tubing connections were eliminated tubing to cuvette, cuvette to injection port, and injection port to reagent reservoir. These were primary sites of bubble formation. The distance from the injection port to the end of the cuvette light path has been reduced to approximately 1.2 cm. This represents a substantial decrease in the dilution of the analyte as its development proceeded through the flow-through system. The change from a light emitting diode to a fiber optic system provided a significant increase in the sensitivity of the nanocolorimeter. This modification also gives the operator the flexibility of ion measurements at wavelengths for which there are no light emitting diodes. The fiber optic light sowce, along with the bubble-free flow-through cuvette, represents a significant improvement in the accuracy, sensitivity, and the ease of operation of the nanocolorimeter. Registry No. Calcium, 7440-70-2; magnesium, 7439-95-4.
LITERATURE CITED (1) Vurek, 0. G. Anal. Blochem. 1981, 114, 288-293. (2) Vurek, 0. Q.; Knepper, M. A. Kidney h f . 1982, 21, 656-658. (3) Muhlert, M.; Julita, M.; Quamme, G. Am. J . Physiol. 1982, 242, F202-F206.
RECEIVED for review June 17,1983. Accepted August 8,1983. This work was supported by National Institutes of Health Grant AH649.
Behavior of Trace Refractory Mlnerals In the Lithlum Metaborate Fusion-Acid Dlssolution Procedure Cyrus Feldman Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 In 1966, Suhr and Ingamells (1) suggested lithium metaborate (LiB02)as a fusion medium for most siliceous rocks. This salt, like other lithium borates, can be used to prepare samples for X-ray fluorescence. The authors also showed that the LBOz melt can easily be converted to a stable acid solution of the components without precipitating silica by quenching and dissolving the melt in 3% "OB. This solution can then be used for determining any component, including silicon, by emission spectrometry ( I ) or any other technique. Other authors have used various other mineral acids for quenching 0003-2700/83/0355-2451$01.50/0
and dissolution. In order to retard or prevent the precipitation of silica in a Li~C03-H3B03-SrC03-Co304fusion of rock powders, Govindaraju ( 2 , 3 )used dilute citric acid. Neither author was concerned with determining trace constituents. An excellent discussion of the use of lithium borate fusion in rock analysis is included in a review by Abbey, Aslin, and Lachance (4). It was desired to use this technique to determine trace impurities in rocks and other siliceous materials, using small disposable graphite crucibles. Preliminary experiments were 0 1983 American Chemical Society
2452
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983
performed on 100-mg samples of a variety of rocks and ores. In many cases, clear acid solutions were obtained; these solutions remained clear for a t least 6 months. In other cases, most of the quenched melt dissolved, but a residue remained or a precipitate formed. These residues were usually small enough to be ignored in analyzing for major components. In trace determinations, however, these residues or precipitates might well contain most or all of the element sought. Isolation and dissolution of such residues would at best be difficult and tedious and would sacrifice the desired advantage of speed. Some other way of dealing with the problem was therefore sought. Residues might be produced in one or more of several ways: 1. Incomplete Attack. Trace elements often exist in rocks in the form of a few small discrete grains of a particular mineral. These grains might not be completely dissolved by the fusion procedure used. 2. Conversion to Carbide. Several elements (e.g., Cr, Ti, W, Nb, Ta, Hf, Zr) may react with the walls of a graphite crucible to form very stable carbides. These carbides might not be dissolved by the acid solution used to dissolve the melt. 3. Hydrolysis. In the 3% HN03 solution suggested by Suhr and Ingamells for dissolving the melt, several elements (e.g., Sn, Ti, W, Nb, Ta) can produce insoluble hydrolysis products. 4. Intercomponent Reactions. Phosphate, present in many rocks, can react with many of the above mentioned metals, as well as others, to produce compounds insoluble in 3% HNO,. 5. Suspension of Graphite Particles. After dissolution of the melt, the solution sometimes contains graphite particles. However, it is never possible to be sure that all of the black particles seen in a particular solution are graphite. Since the quantities of these minerals existing in natural rock samples are often of the order of only micrograms, the types of behavior mentioned above would be difficult to identify experimentally, especially if they occurred in combination. In order to make this behavior more visible, a separate fusion was carried out on a relatively large quantity of each of several minerals which might cause difficulties of the types mentioned. In order to reveal any possible interactions between each mineral and a natural rock, a separate fusion was also carried out on a mixture of 20 mg of each mineral with 100 mg of Cody shale.
EXPERIMENTAL SECTION Materials. High-purity lithium metaborate (LiB02)was obtained from Spex Industries, Metuchen, NJ (Catalog No. LI70454). The graphite crucibles (12 mm i.d. X 14 mm high; wall 1 mm) were obtained from Leco Corp., St. Joseph, MI (CatalogNO. 767-2778), The small studs on the bases of these crucibles were removed with a belt sander. Specimens of individual minerals were kindly made available by 0. C. Kopp, University of Tennessee, Knoxville, TN. Corundum, beryl, and ore specimenswere obtained from Ward's Natural Science Establishment, Rochester, NY. All other chemical reagents used were analytical reagent grade. Procedures. Each mineral was ground to a particle size 545 pm. Fusion mixtures of one or more of the types listed in Table I were prepared, thoroughly mixed, and ignited 15-20 min at 950 "C. Each crucible was then removed separately from the muffle, and the molten ball poured as quickly as possible into a beaker containing 50 mL of a quenching liquid (see Table I). The quenched melt was quite brittle and was easily fragmented with a 3/8 in. diameter flat-ended glass rod. Dissolution was hastened by immersing the beaker in an ultrasonic bath (AutomaticCleaner Model HS-20, Heat Systems Ultrasonics, Inc., Melville, NY). Eight 50-mL beakers could be processed simultaneously in this instrument. The quenched melt was usually completelydissolved within 15-20 min.
Table I. Acronyms for Fusion Mixtures and Quenching Liquids Used quenching liquids fusion mixtures ( 5 0 mL) MB: 20 or 100 mg sample, 500 mg N : 3 vol % concd of LIBO, HNO, MBP: 20 or 100 mg sample, 500 mg NT: 2.5% tartaric of LiBO,, 50 mg of NaB0,.4H20 acid dissolved in N S-MB: MB + 100 mg of shale NTH: 5 vol % of S-MBP: MBP + 100 mg of shale 30%H,O, mixed with NT If any residue was detected, appropriate changes were made in the fusion or dissolution procedures. When the procedure for a given mineral had been optimized,the solution obtained, which sometimes contained black particles, was filtered through a Millipore filter (Type HA, pore size 0.45 rm). The filter was washed thoroughly with the quenching liquid and slowly ignited in a platinum dish. If the black particles disappeared in the ignition, they were presumed to have been graphite. In a time trial, 12 samples of Cody shale were processed in 150 min (12.5 min/sample). This included all steps from weighing the sample to bottling the solution.
RESULTS AND DISCUSSION Minerals. Chromite (FeCr20,). A 20-mg sample of powdered chromite was fused by the MB procedure and the melt quenched in N T (see Table I). Most of the glass dissolved, but substantial amounts of a dense black powder did not. Isolation and ignition of the latter material showed that it was not graphite. A second specimen of the dense black material was subjected to X-ray fluorescence and then to X-ray diffraction analysis. Cr was the only main constituent detectable by XRF. The XRD pattern was well defined but did not resemble the pattern of any listed carbide (or boride) of chromium. An arc emission spectrographic analysis showed only 0.02% B, indicating that the material was not a boride. It had to be assumed that the material was a carbide or oxide of chromium, since no other possibilities fit the evidence. In order for the chromite to be converted to a carbide, the mineral (FeCr204)first would have had to dissolve in the LiB02, so that the Cr probably existed in solution in the melt before it reacted with carbon. This implies that the Cr from any rock or mineral that is fused with LiBO, may also form a carbide, and not go into solution when the melt is dissolved. To check this point, a 20-mg sample of reagent grade Cr203 was subjected to the MB fusion. As expected, a black, insoluble residue was also obtained in this case. A possible way to remedy this situation seemed to be to add an oxidizer to the LiB02. The most compatible candidate available was sodium perborate (NaB03.4H20). Repetition of the above experiments with 50 mg of NaB03.4H20added to the fusion mixture (MBP fusion) converted the Cr to a completely soluble form; the slight residue that was filtered out of the solution burned off completely and was assumed to have been graphite. The fusion was equally successful when 100 mg of Cody shale was also present (S-MPB fusion). Ilmenite (FeTi03).Three 20-mg samples of ilmenite (also a potential carbide former) were subjected to the MB fusion. One was quenched in N and one in NT. A slight turbidity appeared after dissolution in both cases, but the turbidity slowly vanished after the addition of 2 mL of 30% H202. The third MB fusion and an S-MBP fusion, quenched in NTH, showed no turbidity. Rutile (230,). Twenty-milligram samples of powdered rutile were subjected to both the MBP and S-MBP fusions and quenched in NTH. Clear stable solutions were obtained in both cases. Similar samples quenched in N and N T gave considerable amounts of a hydrolytic precipitate.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983
Cassiterite (Sn02).Twenty-milligram samples of powdered cassiterite were subjected to both the MBP and the S-MBP fusions, with N, NT, or NTH used as the quenching liquid. In both types of fusion, a portion of the melt adhered to the graphite crucible when the melt was poured into the quenching liquid. The crucible was therefore reheated to 950 "C and dropped with its contents into the quenching liquid. After the usual dissolution by sonic agitation, the N and N T solutions showed noticeable hydrolytic precipitation and were discarded. The NTH solution was filtered; the crucible was crushed and combined with the residue. The entire residue was washed thoroughly with NTH and ignited in a Pt dish. No visible residue remained; it thus appears that all of the cassiterite was dissolved in the fusion in this case too. Wolframite ((Fe,Mn)W04).Subjected to the same fusions as previous minerals, 20-mg and 100-mg specimens of wolframite showed ideal behavior. There was no evidence of carbide formation nor was there any hydrolytic precipitation of tungsten, iron, or manganese. Separate experiments in which the quenching liquid was NT showed that the presence of hydrogen peroxide was not necessary to prevent precipitation of the tungsten under these conditions. Molybdenite (MoS2). It was expected that this mineral might form carbides in the fusion and/or hydrolyze in solution, but neither thing happened: the mineral remained unchanged by either the MBP or the S-MBP fusion process. The flaky nature of molybdenite had prevented its reduction to 45-pm particles, but particle size probably was not a critical factor in this mineral's failure to be attacked by the MBP and SMBP fusions. Magnetite (Fe304).Powdered 20-mg samples were fused readily by the MBP and S-MBP procedures; the quenched melt dissolved easily in NT. No carbide formation or hydrolytic precipitation was observed. Zircon ((ZrfiflSiO,).Natural zircon usually contains 1-7'31 HfSi04 in solid solution. Both Zr and Hf can form very stable carbides, but MBP and S-MBP fusions, with NT quenching, gave clear solutions in all cases. There was no evidence of carbide formation or hydrolysis by Zr or Hf. Columbite-Tantalite [(3'e,Mn)(Nb,Ta),O6)].Both the Nb and the Ta in this mineral posed potential dangers with regard to the fusion procedure. Both can form refractory carbides, and if instead both elements dissolve in the melt, both can hydrolyze and precipitate in N. Fusion of 20-100 mg samples of the powdered mineral by the MB procedure showed that carbide formation did not occur but that copious precipitation occurred when N was used as the quenching liquid. When N T was used for quenching, however, no precipitation occurred. Beryl (Be3Al2Si6OI8). Fused readily by the MP, MBP, and S-MBP procedures and gave stable solutions when quenched in NT. Corundum (A1203).Fused readily by the MB, MBP, and S-MBP procedures and gave stable fused solutions when quenched in NT. Monazite (rare earth-thorium phosphate). This mineral fused readily by the MB procedure. In view of the nature of its components, it was not expected that N, NT, or NTH would be able to dissolve the melt and produce a clear solution, and indeed they did not. Nitric acid in concentrations as high
2453
as 10 N also failed to give complete solubility. Quenching the melt in 6 N HCl, however, produced a clear, residue-free solution with monazite samples as large as 100 mg. This solution could be diluted to 0.3 N in HC1 without causing precipitation. On the other hand, when a melt of this type was quenched directly in 0.3 N HC1, the melt dissolved completely, but a precipitate soon appeared in the solution. The metals in a 2 N HCl solution can be separated from the phosphate by depositing the metals on a highly acidic ion exchange column such as Dowex-50 or depositingthe phosphate on acid-washed alumina (5) or on an anion exchange resin. Experiments also showed that phosphorus can be removed completely from a 6 N HC1 solution of a monazite melt by treating the solution with excess Zr02+.
CONCLUSIONS If present in a rock in microgram quantities, most of the chemically resistant minerals investigated could be expected to be converted to soluble form by fusing with lithium metaborate and dissolving the melt in 3% HN03 containing tartaric acid and, in some cases, hydrogen peroxide. To be sure that chromite is converted,however, N&O3-4H20 should be added to the fusion mixture. The melt obtained with monazite requires 6 N HC1 for dissolution. Molybdenum may escape detection by subsequent solution procedures if originally present as molybdenite. The present experiments show that if trace elements in a material are to be determined by LiBOz fusion, dissolution of the melt and analysis of the solution, it is advisable to establish whether refractory minerals of these trace elements exist and to study the behavior of large (20-100 mg) specimens of such minerals by the procedure outlined above.
ACKNOWLEDGMENT The author is grateful to 0. C. Kopp, University of Tennessee, Knoxville, TN, for furnishing most of the mineral specimens used. He also wishes to thank J. H. Stewart, Jr., of ORNL, for suggesting the use of a sonic agitator for dissolving the melts, H. W. Dunn, of ORNL, for performing X-ray fluorescence and diffraction analyses, and S. A. MacIntyre, D. R. Heine, and J. C. Price for performing optical emission spectrochemical analyses. Registry NO.LiB02,13453-69-5;N&03, 7632-04-4;cassiterite, 1317-45-9;ilmenite, 12168-52-4;rutile, 1317-80-2;wolframite, 1332-08-7;magnetite, 1309-38-2;zircon, 14940-68-2;corundum, 1302-74-5; beryl, 1302-52-9; columbite, 1306-08-7; tantalite, 1217848-2;chromite, 130&31-2;monazite, 1306-41-8;molybdenite, 1309-56-4. LITERATURE CITED Suhr, N. H.; Ingamells, C. 0. Anal. Chem. 1966, 38, 730-734. Govindaraju, K. Appl. Specfrosc. 1966, 20, 302-304. Govindaraju, K. Appl. Specfrosc. 1970, 24, 81-85. Abbey, S.; A s h G. E. M.; Lachance, G. R. Rev. Anal. Chem. 1977. 3 (3-4), 181-248. (5) Jangida, B. L.; Krishnamachari, N.; Varde, M. S.; Venkatasubramian, V. Anal. Chlm. Acta 1965, 32, 91-94.
(1) (2) (3) (4)
RECEIVED for review July 11, 1983. Accepted September 9, 1983. Research sponsored by the Office of Energy Research, U.S. Department of Energy under Contract W-7405-eng-26 with the Union Carbide Corp.
