Determination of boron by methyl ester formation and flame emission

Jul 1, 1982 - ... in milligram-sized tissue samples by inductively coupled plasma-atomic emission spectroscopy. D.A. Johnson , D.D. Siemer , W.F Bauer...
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Anal. Chem. 1982, 5 4 , 1321-1324

vary with temperature in the same way that the macroviscosity does so that less viscouii solutions can be prepared at room temperature and cooled in the NMR probe for spectral accumulation. Also the combination of solvent viscosity optimization and paramagnetic reagent addition as an approach to obtaining efficient relaxation without involving extreme solution conditions of either viscosity or relaxation reagent concentration is being studied. One obvious limitation of this method is that it cannot be used in cases where a specific solvent is required for some reason. This limitation may, however, be overcome to some extent by operation at temperatures near the freezing point of the solvent. Studies of this sort are in progress.

(8) Martin, G. J.; Martin, M. L.; Gouesnard, J. In “NMR Basic Principles (9) (10) (11) (12) (13) (14) (15) (16)

ACKN 0WLEDGMENT We thank C. McCormick of the University of Southern Mississippi for donation of the water-soluble polymers.

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LITERATURE CITED

(19) (20)

Randall, E. W. I n “Nitrogen N.M.R.”; Witanowskl, W., Webb, G. A., Eds.; Plenum Press: London, 1973; pp 47-49. Gust, D.; Moon, R. B.; Roberts, J. D. f r o c . Nafl. Acad. Scl. U . S . A . 1975, 4698-4700. Becker, E. D.; Ferretti, $J. A,; Gambhir, P. R. Anal. Chem. 1979, 57, 14 13- 1420. Levy, G. C.; Lichter, R. I.. “Nltrogen -15 Nuclear Magnetic Resonance Spectroscopy”; Wiley: New York, 1979; Vol. 19. Lew. G. C.: Edlund, V.: Hexem, J. G. J . Maan. Reson. 1975, 19, 259-262. LaMar, G. N. Chem. f h y s . Lett. 1971, IO, 230-232. Freeman, R.; Pachler, H:. G. R.; LaMar, G. N. J . Chem. fhys. 1971, 55, 4566-4593.

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and Progress“; Riehl, P.; Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: Berlin, 1981; Vol. 18, pp 28-35. Bertrand, R. D.; Moniz, W. B.; Garroway, A. N.; Chingas, G. C. J. Magn.Reson. 1978, 32, 465-467. Walborsky, H. M.; Murarl, M. P. J . A m . Chem. SOC. 1980, 102, 428-429. Balch, A. L.; Yow, J. R. J . Am. Chem. SOC. 1980, 702, 1449-1450. Levy, G. C.; Lichter, R. L. ”Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy”; Wiley: New York, 1979; Chapter 5. Bloembergen, R.; Purcell, E. M.; Pound, R. V. fhys. Rev. 1948, 7 3 , 679-712. Witanowski, M.; Stefanick, L.; Webb, G. A. In “Annual 9eport on NMR Spectroscopy”; Webb, G. A., Ed.; Academic Press, London, 1977; Vol. 7, 144-148. Williamson, M. P.; Wllllams, D. H. J . Chem. SOC.,Chem. Commun. 1981. 165-166. Panekon; G.-D.; Lindsey, C. P.; Alms, G. R. Macromolecules 1978, 1 7 . 1242-1244. Lyerla, J. R., Jr.; Levy, G. C. I n “Topics in Carbon-13 NMR Spectroscopy”; Levy, G. C., Ed.; Wiley: New York, 1974; pp 120-1 21. Lippmaa, E.; Saluvere, T.; Lalsaar, S. Chem. fhys. Lett. 1971, 1 7 , 120-123. Schweitzer, D.; Spless, H. W. J . Magn. Reson. 1974, 16, 243-251. Cargioll, J. D. In “Annual Report on NMR Spectroscopy”; Webb, G. A., Eds., Academic Press: New York, 1979; p 228.

RECEIVED for review November 30, 1981. Accepted March 26, 1982. Financial support from the National Science Foundation for the purchase of the FT NMR through Grant CHE 78-02081 is acknowledged. Support from a Merck Go. foundation faculty development grant (R.F.E.) is also acknowledged. Presented in part at joint SE/SW regional meeting, ACS, New Orleans, LA., Dec 1980.

Determination of Boron by Methyl Ester Formation and Flame Emission Spectrometry Darryl D. Siemer Exxon Nuclear Idaho Co., Idaho Fails, Idaho 83401

Soluble boron in aqueous nuclear fuel reprocessing plant streams resulting from the Rover process is determined by compiexing any fluoride present with aluminum chloride solution and then adding, first, sulfuric acid and, finally, methanol to form the volatile trimetthoxyboron ester. The gaseous ester is aspirated into a conve~ntlonalAAS air-acetylene slot burner through the sample pick-up tube normally used for soiutlon analysis. The resulting transient green boron oxide band emission signal at 548 nm is integrated. The detection limit of the method Is 0.2 pg and the precision Is on the order of 1-2% relative standard deviation. No matrix effects from any of the usual concomitants were observed.

The flame photometric determination of boron has an extensive history of both development and practical application work. Gilbert’s treathe on the flame spectroscopic determination of nonmetals gives a fascinating and comprehensive description of this work up to 1970 (1). In practice today, the analysis is usually done with the same solution nebulizerpremixed flame sourcei3 primarily designed for atomic absorption analyses. A t this installation, flame photometric boron determinations ate routinely performed on the tetran-butylammonium fluoborate ion pair extracted with methyl isobutyl ketone (MIBK) from sample solutions to which an excess of hydrofluoric acid has been added (2). The primary 0003-2700/82/0354-1321$01.25/0

drawback to this approach is that a number of transition elements also extract to some degree and give strong positive spectral interference from metal oxide band and incandescent particulate continuum emission. Another practical disadvantage is that the presence of nitrate in the sample requires an additional lengthy sample preparation step. The distillation of the methyl or ethyl borate ester from sample solutions to which alcohol and an acid have been added has long been used for the separation of boron from most sample concomitants (3). This paper describes a rapid analytical method combining the excellent separation inherent in the volatile ester formation process with the sensitivity and procedural simplicity inherent in determinations done with typical modern AAS/AES flame spectrometers. The conditions of the procedure were designed to rapidly form the volatile ester, quantitatively strip it from the sample solution, and then introduce it while still in the gaseous form to a flame spectrometer.

EXPERIMENTAL SECTION Written descriptions of the boron separation procedure involving the methyl borate distillation process typically call for large volumes of reagents and distillation times of as much as an hour. It was obvious that the classical “still”designs recommended for that purpose are totally impractical for rapidly transferring the boron in a small sample aliquot to a flame for a rapid analysis. Therefore, a reaction vessel with small heat capacity and minimum dead space and which is used with reagent volumes chosen t o 0 1982 Amerlcan Chemlcal Society

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Table I. Effect of Water“ total water, /.kL 5 10

25 55 105 155

peak integralb

peak height

3.370 3.405 3.440 3.373 2.997 2.595

0.231 0.223 0.218 0.190 0.153 0.110

a 25 p L of saturated AICl,, 300 p L of H,SO,, and 1 mL of methanol. A 1 min integration period was used.

Figure 1. Esterification reaction apparatus: (A) thistle tube with Vigreux “fingers” and “knitted” baffle, (B) quartz test tube, (C) 21 gauge syringe needle, (D) sample pickup tube from the nebulizer.

produce the heat necessary to volatilize the borate ester was developed to permit the necessary reaction speed. The borate ester formed is flushed from the system with air drawn through the reaction vessel by the partial vacuum present at the nebulizer sample pickup tube. Figure 1dipicts this simple apparatus. It consists of a standard 21 gauge stainless steel hypodermic syringe needle, a quartz thistle tube, a shortened no. 1 rubber stopper, and a number of quartz test tubes (12 nm id., 10 cm long). The thistle tube has a number of internal “fingers”to prevent expulsion of methanol when the alcohol fist encounters the sulfuric acid and the resultant vigorous reaction ensues. A loose baffle made of quartz “knitted” onto the tube about 7 cm from the bottom reduces the amount of spray striking the stopper and thereby the amount possibly being aspirated. The needle is inserted point-upward through the bottom of the stopper and then into the plastic nebulizer sample pick-up tube. The bottom of the needle is bent upward into a “hook”. Analyses are performed by stoppering a test tube containing a sample aliquot which has been treated with aluminum chloride and sulfuric acid with this device and then simultaneouslypushing the READ button on the AES spectrometer and pipetting an aliquot of methanol into the thistle tube. An unmodified Instrumentation Laboratories IL 951 AAS/AES spectrometer with a standard 10-cm air-acetylene slot burner was used for the project. For development work, the CRT record of the transient signal responses displayed with that instrument is advantageous, but for routine applications,any stable instrument with a signal integration capability of at least 30 s is satisfactory. A wide range, multialkalai, photomultiplier tube (Hammatsu R 955) was used with the maximum permissible instrumental band-pass (2 nm) centered on the 548-nm BOz emission band. Plastic tipped micropipets from several manuacturers were used for transferring reagent and sample solutions. In the convenient 2 and 5 pL sizes a micropipet design (Oxford) utilizing a length of plastic capillary tubing for the tip instead of the more common conical plastic tip configuration gave superior precision. In larger sizes all of the various pipet designs served equally well. Reagent grade salts, acid, and methanol were used throughout the study. The saturated aluminum chloride was prepared by equilibrating 14 g of AlCI3.6H,O with 10 mL of water and then separating the supernate by centrifugation.

RESULTS AND DISCUSSION The sample solutions this method was developed to analyze may contain high concentrations of ionic matrix concomitants (from RSD) of determinations made with a single sample aliquot volume of 10 pL. There should be enough acid to effectively dehydrate the sample and enough methanol to ensure rapid ester formation. The highly exothermic reaction between the methanol and the acid generates enough heat to raise the temperature of the reaction mixture to1 within a few degrees of the boiling point of the borate ester (65 “C) if enough of the two reagents is used. Integrated emission signals for a fixed amount of boron in 10 pL of water varied less than 10% in response to variations of total reagent volumes (sulfuric acid plus methanol) from 400 to 1300 plL as long as the approximate 1/3 ratio of acid to methanol volumes was adhered to. In routine analytical practice at this facility, 300 pL of acid and 1000 pL of methanol are used for sample aliquots of from 5 to 25 pL. Flame stoichiometry and the choice of observation zone are important experimental parameters. Unfortunately, flame background continuum emission constitutes a significant fraction of the total signal observed by the spectrometer. Background emission from lean flames tends to increase when the methanol is introduced while the background from overly rich flames tends to decrease. To approximate ideal flame stoichiometry the following procedure is followed. First, the nebulizer uptake rate 11sset a t about 4 mL/min (for water) and the total air flow is adjusted to a value which gives a ”stiff” flame with the 10-cm d o t burner head (approximately 21 SCFH for the IL 951 system). Then the acetylene flow is adjusted so that no change in background emission is noted when the nebulizer sam,ple pickup tibe is alternately inserted into and removed from n beaker of methanol. Following this, the nebulizer needle is readjusted to give the maximum possible uptake rate. The BO2 band emission signal decreases less rapidly as the observation zone above the burner is increased than does the flame background signal. However, flame “flicker noise” increases as the observation zone is raised. A good overall compromise is an observation zone 15 mm above the burner slot with a “stiff” (i.e., high combined fuel and oxidant flowrates) flame to reduce “flicker noise”. With indicated flows of 4.5 SCFH of acetylene and 21 SCFH or air into the IL951 burner, typical water uptake rate through the nebulizer sample pickup tube is 7 mL/min. This rate corresponds to air flow through the reaction tube of on the order of 450 mL/min ([an estimate based on the relative viscosities of the two fluids). The actual air flow is reduced from this figure somewhat by the additional resistance of the thistle tube and the pressure head imposed by the solution in the reaction tube. The choice of the olptimal analytical wavelength to use depends on the photomultiplier tube used in the spectrometer and upon the blaze angle of the grating. With the wide-range tube used in the IL951, the 548-nm band is somewhat “brighter” than the 518-nm band chosen by some of those who have done work previously on flame photometric boron determinations (4). A typical analytical signal is shown in Figure 2. A 10-pL aliquot of sample containing 25 pg of boron was treated with

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-Flgure 2. Typical analytical signal shape.

Table 11. Matrix Concomitantsa,b concomitant U(VI) Mo( V)

Fe( 11) Sn( IV)

cr( 111) WV)

Cd( 11)

Gd(1III

zr(v1j Al(111) HF HNO, HCl

concn 400 g/L 1 0 g/L 1 0 g/L 1 0 g/L 1 0 g/L lOg/L 10 g/L 1a l L IO~/L 1 0 g/L 7M 8M 6M

form nitrate ammonium molybdate sulfate chloride chloride in 5 M H F nitrate nitrate i n 4 ~ ~ ~ chloride

a The solutions analyzed consisted of 2 5 pg of boron in 5 p L of water to which 5-pL aliquots of the concomi300 p L of sulfuric acid, tant(s) solutions were added. 100 p L of methanol, and 2 5 p L of the aluminum chloride

solution were used for this study.

20 pL of saturated AlCl,, 300 pL of H2S04,and 1 mL of methanol. The signal rises in approximately 2 s and then decays exponentially with a decay constant ( l / e ) of approximately 10 s. On the same scale as in Figure 2, the steady flame background base line signal would be approximately one-third of the magnitude of the maximum BOz emission signal. A 45-5 integration period is adequate to “catch” over 95% of the total peak area and reduces the deleterious effect of base line drift encountered with longer (60 s) integration periods. If a large variation in sample volumes is anticipated, a longer (60 s) integration period may be desirable providing that the instrument’s base line is sufficiently stable. A matrix study indicated that no cationic or anionic species encountered in either extant or anticipated uranium reprocessing plant solution streams interfere with the determination. Table I1 lists the solutions used to investigate possible interferences. Various random combinations of any five of these concomitants added along with the aliquot of 5 g/L boron solution caused no noticeable (greater than 2% change in integrated emission signal) signal perturbation either. This is not unexpected in view of boric acid’s limited chemical reactivity to most inorganic species (except fluoride) in acid solutions. An apparent signal enhancement initially observed with the 400 g/L uranium concomitant disappeared when the lower end of the hypodermic needle (see Figure 1)was bent into a hook. In its initial straight-down orientation, the needle would aspirate any solution droplets which first condensed out onto the stopper and then ran down the needle. There is some uranium carried upward (perhaps as spray or as a relatively volatile uranyl sulfate-methanol adduct) by the air bubbled through the hot reaction mixture. The precision of the method is typically on the order of 1-2% RSD (at total boron levels from 15 to 50 pg) with the instrument used for this project. Using the integral as opposed to the peak height signal as a measure of analytical response is definitely desirable for most analytical applications because

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Table 111. Comparison of This Procedure with ICP/AES samplea

ICP/AES,b g/L

this procedure, g/L

1 2 3 4 5

3.98 7.11 8.57 6.87 5.20 4.37 6.16

4.02 7.03 8.50 6.72 5.19 4.43 6.15

6 7

a These samples represent a cross section of uranium reprocessing plant process streams and contain widely varying concentrations of uranium and dissolved cladding The ICP/AES instrument consists of a material. Jarrell-Ash torch, power supply, and impedance matching box, a 1-m Spex monochromator, and a mechanical chopper, lock-in amplifier, photomultiplier light detection system.

variations in the rate of methanol addition, solution volumes, gas flows, etc., do not affect the integrals as much as they do the peak height signals. The detection limit (that mass of boron giving a signal equal to three times the standard deviation of blanks) is on the order of 0.2 pg and the analytical response is linear from the detection limit to well over 100 pg of boron. The most stringent instrumental criterion for this method is base line stability over the relatively long integration periods. The instrument’s gas regulation equipment must be capable of maintaining a flame of constant background emissivity and the electronics must be free of dc drift. Another new “state of the art” AAS/AES spectrometer recently purchased for use in our routine analytical service laboratory was capable of only 5-7% precision at the 25 pg boron level and permitted a detection limit more than 10-fold greater than did the spectrometer used for this development work. The service laboratory’s instrument also exhibits marginal precision in the routine AAS determination of aluminum (but not for elements which are easily atomized). This indicates that the probable cause of both problems is inadequate control of flame stoichiometry by the automatic “gas box”. With a flame photometer capable of simultaneously monitoring two wavelengths, the problem of flame background drift would become far less important (4). The flame background emission signal differs little between wavelengths on the “peaks” (e.g., 548 nm) or in the “valleys” (e.g., 535 nm) of the BOz emission bands. Equally important, the intensities

of these background signals vary in the same manner and to a similar degree in response to changes in flame stoichiometry. Therefore, the difference in signals observed at a peak and at a valley is essentially independent of moderate changes in flame conditions. An instrument based on this principle utilizing interference filters for isolating the desired spectral regions is presently being constructed for use in the routine analytical service laboratory. The results of a comparative study of data gathered using this method and those obtained by ICP AES are shown in Table 111. The sample solutions required a prior T B P (tributyl phosphate) in kerosene, extraction-back-extraction “cleanup” procedure before the ICP AES boron determination could be run because of serious spectral interference by the uranium and other metals present. Potentially far better detection limits should be obtainable by interfacing the esterification reaction tube system with an ICP or DC plasma AES source in order to excite the extremely strong atomic lines of boron instead of the rather weak BO2 bands emitted by flames. However, the sudden influx of methanol (or other organic products of the reaction of methanol and sulfuric acid) into such AES sources may prove to be a problem in actual practice. This procedure is far better suited to the determination of high boron levels in these complex samples than is the old extraction/solution nebulization flame photometric method. It is much more rapid, consumes less sample, requires less expensive reagents, and, most important, is not matrix sensitive. Boron at much lower concentration levels can be determined if an evaporation step to remove the bulk of the water precedes the actual determination. Aqueous solutions should be made basic prior to evaporation, however. Organic samples could presumably be run too if they were first gently dry ashed with magnesium nitrate (rendered basic with a little magnesium oxide) in the same quartz test tubes to be used for the final analyses.

LITERATURE CITED (1) Gilbert, P. T. “Analytical Flame Spectroscopy”; Mavrodineanu, R. M.,

Ed.; McMlllan: London, 1970; Chapter 5, pp 181-377. (2) Maeck, W. J.; Kussy, M. C.; Glnther, 8. E.; Wheeler, G. V.; Rein, J. E. Anal. Chem. 1963, 35, 63-65. (3) Nemodruk, A. A.; Karaiova, Z. K. “Analytical Chemistry of Boron”; Ann Arbor-Humphrey Science Publishers: Ann Arbor-London, 1969; pp 120-126. (4) Dean, J. A.; Thompson, C. Anal. Chem. 1955, 2 7 , 42-46.

RECEIVED for review January 18,1982. Accepted April 1,1982.