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Anal. C M . 1982, 54. 1647-1650
lETLrnHlWIIYE..l"
Fbun 2. Comparison of the FID chromatograms obtained Wlth (A) and wlthout (6) Injection pwt septum of an alkane mixture dissolved In n-haptane. From left to right the main peaks are solvent peaks. nundecane. ndodecane, n-mecane, n-tebadecane. nhexadecane. n-wtadecane, n-eicosane, and ndocosane.
O
L
-
ncE.. 1. Schermtic drawings (exploded view and vwtkal cut) of the sepbn*as h pat. A stahless steel bsk (E) wlth an exammcaly positioned hole of a slightly greater diameter than Um outer diameter of the 1n)ecUon madb (A) used is enclosed between two TeRon disks (D). At injection time the disk (E) is rotated about 45' from the closed psmon to a ked open pmM. The two halves (C and F)of the brass metal bk& are hdd io@w by spfbq (I) loaded screws (J) and ~ealed against the gas chomatogaph (H) wlm an aluminum dm (G). A piece of septum (E) pressed against the metal block by Um syrlnw prevents
leakage during the injection. regulated), hut comparahle resulta are obtained if the carrier gas flow is presaure regulated all the time to give a flow rate of 3 mL/min at 300 "C. The temperature program was started 30 s after the flash heated, hot needle, solvent flush (2), splitless 'quasi on column" injection. The injection volume waa 0.2 i.L containing about 50 ng of each component and the flushing solvent (ca. 0.7 NL)was n-pentane. The column was a 28 m, 0.5 mm i.d Pyrex column statically coated (except for the first meter of the column) with crosslinked SE-54 principally according to Grob and Grob (3) to yield a film thickness of 0.15 fim. The chromatographic conditions given are those currently used in our laboratory hut are not critical for the performance of our injection port, as long as an effective trapping of the
sample onto the column is ensured to prevent hack-flushing when withdrawing the syringe. The injection port is gastight even at mum temperature and can thus he used for cold on column injection as well. Recently we have started to use our injection port for hot needle, flash heated 'quasi on column" injection in conjunction with a modified autosampler (Model 8o00, Varian, Palo Alto, CA) with very satisfying results. One of the reviewers directed our attention to a device marketed by Perkin-Elmer ("septum swinger") which looks similar to our device hut cannot be modified to he used in the way we use our device.
ACKNOWLEDGMENT We thank Nils Ekendahl for the skillful electronic modification of the autosampler. LITERATURE CITED (1) u.ob. K.; Orob. K.. Jr. J . chromamg. i878. f51, 311-320. (2) @ob. K.. Jr.; Rennhard. S. HSC CC J . H&h Resolut. Chromatogr. Uvomemg. C o r n . 1880. 3. 627-633. (3) Orob, K.: Upb, 0. J . Chromsfogr. 198i. 213. 211-221.
RFCEIVEDfor review February 8, 1982. Accepted April 12, 1982. This work was supported hy a grant (13X-5853 from the Swedish Medical Research Council and a grant (80/1385) from the Swedish Council for Planning and Coordination of Research.
Determination of Boric Oxide by Ion Chromatography and Ion Chromatography Exclusion John P. Wllshlre' and Wlllla A. Brown U.S. Bwax Research Cmpcintiw?,412 Crescent Way, Anaheim. Callrornia 92801-6794 Ion chromatography is a relatively new technique (I),which is rapidly gaining stature as a method for trace anion and
cation analysis (2). Although used principally in the determination and quantitation of simple anions and cations in
o o o ~ z ~ o o 1 8 ~ 1 0 3 5 c 1 ~ ~ 5 0 1 . 205 /1982 o ~mericanC h a m b l Soclew
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
aqueous solutions, ion chromatography is of utility in many fields ( 3 , 4 ) . The technique has proven to be of considerable value in geochemical analysis (5) and in production/quality control. While ion chromatography can be utilized to monitor chloride, sulfate, sodium, and calcium concentrations in geochemical samples, the question arises whether ion chromatographic techniques could be employed in the direct determination of boric oxide, The techniques commonly employed in the analysis of borates are titration with the base in the presence of excess mannitol to a phenolphthalein end point and colorimetric procedures employing indicators such as carminic acid, turmeric acid, or curcumin (6-10). Of these, the former method is suitable only for samples containing greater than 0.001% Bz03and is susceptible to interference from silicates and phosphates. The latter technique yields enhanced detection limits (typically 0.01 ppm with curcumin); however the range of concentration is severely limited (-30 ppm maximum). Furthermore, colorimetric analyses are severely affected by interfering elements and solution pH. It is thus beneficial to develop a method of analysis which has a greater analytical range and is independent of interelement interferences. This has been accomplished employing ion chromatography and ion chromatographic exclusion techniques. A variety of samples have been analyzed for boric oxide content ranging from parts per billion to percent. The technique of ion chromatographic exclusion, using a 0.1 M mannito1/0.001 M HCl eluent gives the best results. EXPERIMENTAL SECTION Apparatus. A Dionex Model 16 ion chromatograph was used throughout the study. IC Columns. Standard 3 X 150 mm, 3 X 250 mm, and 3 X 500 mm anion separator columns and standard 6 X 250 anion suppressor columns were used throughout the study. All columns were obtained from the Dionex Corp. Ion Chromatographic Exclusion (ICE) Columns. A separator, suppressor, and postsuppressor column were used in the study. All columns were obtained from the Dionex Corp. The ion chromatographic exclusion columns have been described in detail previously (11,12).A brief description is presented here. The ion chromatographic exclusion separator column contains ion exchange resins in a form more polar than in the conventional IC separator, and with less cross-linkage. As a result, strongly ionic anions are repelled by the center of the resin beads by Donnan exclusion. Consequently, the strong ions tend to circumvent the resin beads and elute rapidly. Less polar anions are less affected by the polar resin, and elute in typical fashion. Thus strongly ionic anions are effectively isolated from weakly ionic species, which aids in observation and quantitation of the latter. The upper portion of the ICE suppressor contains a silver impregnated ion exchange resin, which removes halides from the sample and eluent by precipitation. The removal of halides allows the use of dilute hydrochloric acid as an eluent, since the chloride background is removed before reaching the conductivity cell. The lower portion of the ICE suppressor column contains the standard Dionex IC resin in the protonated form. The postsuppressor column also contains the standard IC suppressor resin in the protonated form. Reagents and Standards. All reagents were of analytical grade. Water used throughout the study was pursed with a Milli Q system (Millipore Corp.). Preparation of Samples. Ore Samples (a) Kernite (NapB&*4H@). A 15.0-g sample is placed in 250 mL of 1.3 N hydrochloric acid and refluxed for 2 h. The resultant mixture is filtered, in the presence of 5 g of Celite (filtering aid), through a Gooch crucible. The filtrate is then diluted to 500 mL with water. From this solution aliquots are obtained for analysis by ion chromatography. (b) Borax (NazB407-10Hz0).A 15.0-g sample is placed in a 500-mL beaker, with approximately 10 g of Celite and 7.5 g of sodium chloride. To this is added 350 mL of boiling water, and the mixture is stirred vigorously for 1 min. The solution is then
filtered through a Gooch crucible and the filtrate diluted to 500 mL. fiquots may then be used for ion chromatographic analysis. The filter residue is placed in a flask, to which is added 15.0 mL of concentrated HCl and 250 mL of water. The mixture is then refluxed for 2 h, and fitered through a G m h crucible. The filtrate is again diluted to 500 mL, from which aliquots are obtained for ion chromatographic analysis. The results of the analyses from the filtrate and residue solutions are combined to yield the total BzO3 content. (c) Howlite (CU~S~B~O~(OH)~). A 1.0-gsample is combined with 4.0 g of sodium carbonate and placed in a platinum crucible. The mixture is then heated until molten over a Meeker burner. 'The fusion product is cooled and digested with a minimum volume of 6 N HCl. The solution is fiitered through Whatman No. 2 filter paper, and the filtrate diluted to 100 mL with water. Aliquots of this solution are then analyzed for boric oxide content. Borate Products. A 5.0-g portion of a product sample (boric acid, borax pentahydrate, borax decahydrate, or ammonium pentaborate) is dissolved in water to a final volume of 100 mL. From this solution, dilute samples can be obtained to determine the boric oxide content of the product. Procedures. Two different procedures were employed in the analyses. The first involved conversion of the borates to the tetrafluoroborate ion and its subsequent detection and analysis using standard ion chromatographic techniques. The second involved the direct injection of borate-containing samples into the ion chromatograph, employing the ion chromatographic exclusion columns. (a) Tetrufluoroborate Procedure. The formation and quantitation of B as the tetrafluoroborate ion is a modification of a previously published procedure (13). A 5-mL aliquot of water as a blank, 5 mL of howlite solution, 5 mL of a urine specimen, and a 0.5-mL aliquot of a blood sample were each placed in a 100 mL "Nalgene" plastic volumetric flask, with 1 mL each of 12 N hydrochloric acid and 28 N hydrofluoric acid. Similarly,20-mL, 10-mL, 5-mL, and 1-mL portions of the 100 ppm Bp03solution were placed in volumetric flasks with acid, as above. The flasks were then stoppered loosely and allowed to heat on an air bath at 30-35 "C, overnight. After 12 h, the standards and samples were removed from the air bath and diluted to 100 mL with water. Silver oxide powder (0.5 g) was added to each flask and the flash were stoppered and shaken vigorously for 5 min. The resultant silver chloride and the excess silver oxide were allowed to precipitate before the solutions were decanted into plastic beakers. The solutions were then injected into the ion chromatograph. A 100-rL sample injection loop was employed in the instrument, with a 3 X 150 mm anion precolumn, 3 X 250 anion separator, a 40% flow rate, and a 0.003 M NaHC03/0.0024 M Na2C03eluent. A peak due to BF4- was observed after 11 min. ( b ) Ion Chromatographic Exclusion Procedure. Calibration standards containing 50,20,10, 5.0, 2.0,1.0, and 0.5 ppm Bz03, respectively, were prepared by dilution of the 100 ppm standard solution. From the above solutions, a standard curve was obtained. Samples of kernite, howlite, and silt were analyzed. A 5-mL aliquot of each sample solution was employed, diluted to 100 mL with water. The standards and dilute sample solutions were then injected into the Dionex ion chromatograph. Again a 100-pL sample injection loop was employed. With an ion chromatographic exclusion separator, suppressor and postsuppressor columns, a 12% flow rate, and a 0.1 M mannitol/0.001 M HCl eluent, a peak due to the borate-mannitol complex was observed after 12 min. RESULTS AND DISCUSSION (a) Determination of Boron as t h e Tetrafluoroborate Anion. The results of the analyses for boron as the tetrafluoroborate ion are presented in Table I. The values are in good agreement with data obtained by colorimetric procedures. A standard containing 1ppm BpO3 were easily observed, and the plot of peak heights (conductance) for 1.0, 5.0, 10.0, and 20.0 ppm B2O3standards was linear. At low concentrations (less than 1 ppm), however, the peak for BF4- is somewhat masked by that due to chloride (Figure 1). In the determination of trace concentrations of boron, especially in geological samples, the peak overlap effectively masks the BF,, causing it to be unobserved. The presence of HC1 is required,
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
1649
Table I. Boron Analysis as Tetrafluorobclrate B, ppm obsd obsd by ion by sample chromatography colorimetry howlite 17.3 17.0 urine 40.0 43.0 blood 6.06 6.1
3
2
c 0 E
a
TIME ( m l n u i e c
>
t 2
Figure 2. Chromatogram obtained under “ICE” conditions, for a standard containing 1 ppm boron. Eluent used contained 0.1 M mannltol, 0.001 M hydrochloric acid. Impurlty peaks A and B are assigned to carbonate and phosphate, respectively.
0 0
0 O
1
2 d U TIME (minutes)
Figure ‘I.Chromatogram obtained under standard ion chromatographic conditions, demonstrating the masking of the peak for 0.5 ppm tetrafiuoroborate ion by chloride.
however, to effect conversion of the borate to BF,. Attempts a t conversion without hydrochloric acid were unsuccessful. No formation of BF4- resulted by the atteimpted reaction of samples with HF alone, in spite of a calculated 1Wfold excess for 48 h. The addition of other of HF and heating at 35 concentrated acids gave poor yields of BF4-, and also demonstrated a marked peak: interference. The masking of the tetrafluoroborate peak by chloride was lessened significantly by the addition of silver oxide to the solutions. The chloride peak was greatly reduced in size due to the precipitation of AgC1. Consequently, trace concentrations of BF, were observable. The silver oxide poses two difficulties: the presence of flocculant material and the relatively slow rate of formation of silver chloride due to biphasic conditions. Care must be taken that the silver oxide and resultant silver chloride precipitate are not, injected into the ion chromatograph, otherwise significant blockages in the columns and ancillary tubing may occur. Decantation or filtration of the solution before injection into the ion chromograph alleviates the problem. There is no solution, however, to the relatively slow rate of formation of silver chloride. The biphasiic conditions employed limit the rate of formation. The addition of other soluble silver salts does not help, as the counterion added causes as much peak overlap with BF4- as the removed chloride. The addition of large excesses (up to 1.0 g) of silver oxide and allowing the samples to stand 12 h before analysis are most effective in chloride removal. (b) Determination of Boron by Ion Chromatographic Exclusion. Mannitol (14-16) and other polyhydroxy compounds (17-21) have been commonly employed in the analysis
of borates. The reactivity of borates with polyhydroxy compounds is well-known, and there has been considerable effort in the characterization of complexes formed (22-27). Although much of the data available are somewhat inconsistent, it is apparent that in the presence of excess mannitol, the stoichiometry of the borate-mannitol complex formed is 1:2 (27, 28). The species has a pK, = 5.14, whereas boric acid alone has a pK, = 9.2. Consequently, the borate-mannitol complex can be detected by conductivity, whereas boric acid and other borates cannot. By use of an eluent with a high concentration of mannitol, the borate-mannitol complex will be formed and detected in situ. Borates can thus be directly detected by ion chromatography. The high concentration (0.1 M) of mannitol in the eluent, low solubility of borates at room temperature (approximately 5% wt/vol maximum), and the small sample volume employed (0.100 mL) ensure the mannitol to be present in large excess. At room temperature the greatest concentration of boric acid possible in a 0.100-mL injection volume is 0.08 M, or 1.4 X lo4 mol of B303. The use of ion chromatographic exclusion columns allows the analysis of the mannitol-borate complex, without interference from chloride or other strong anions. A typical chromatogram of borates using the mannitol-HC1 eluent and ICE columns is shown in Figure 2. The peak obtained for the borate under the conditions above is observed after approximately 11 min. By use of standard IC columns, the borate-mannitol complex would be strongly retained, on the columns, and the resultant time required to elute the complex would be too long to make the technique useful. The resin bed of the exclusion separator columns allows the rapid elution of the larger ions, which would normally have extended retention times under standard IC conditions (12, 13). A plot of conductivity vs. concentration of boric oxide is after shown in Figure 3. The plot is linear to 20 ppm B203, which considerable curvature is observed. This curvature somewhat restricts the analytical range of the technique, however, samples containing significantly large B203concentrations may be analyzed either by sufficient dilution of the sample or by the use of several standards combined with a curve fitting analysis of the data. Both techniques yield good data. The resulta of analyses performed using the mannitol eluent ion chromatographic exclusion technique are presented in
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
Table 11. Borate Analysis by Ion Chromatographic Exclusion (a) Kernite Ore Analysis % BaO,,
by
% B,O,, by
sample “ICE” titration 408-6A 26.6 25.58 408-6B (duplicate) 26.6 26.41 408-9A 34.0 38.23 408-9B (duplicate) 34.0 37.26 409-11A 43.5 43.76 409-11B (duplicate) 43.5 43.46 (b) Silt Analysis % B,O, % B,O, sample (ICE) (titration) NVR-5 0.094 0.08 MVR-7 1.22 1.49 (c) Howlite Analysis P P BzO, ~
sample NVR-4 I
0
10
I
I
I
20
30
40
(ICE) 29.36
P P BaO, ~ (colorimetry) 28.2
.-.I---
50
60
CONCENTRATION (ppm)
Flgure 3. Plot of conductivity vs. boric oxide concentration, in the determination of boric oxide by ion chromatographic exclusion.
Table 11. The values obtained are compared with data from colorimetric and titrimetric analyses. Again excellent agreement between the techniques is noted; however, significantly better correspondence of results is observed between repeated analyses performed by ion chromatography than by either colorimetric or titrimetric procedures. The reproducibility is undoubtedly due to the “closed loop” nature of the ion chromatograph. With the fixed volume of the sample injection loop, and the sealed circuit comprised of the eluent, pump, columns, and detector, ion chromatography is independent of all external sources of error that may have a significant effect on the results. Furthermore, the use of chromatographic techniques limits matrix effects and intermolecular interferences as sources of error. With proper dilution, the ICE technique has been effectively employed in the analysis of samples containing Bz03in the range of parts per billion to percent. Furthermore, a wide range of sample types have been analyzed, with no apparent loss in accuracy due to the different sample matrices. No current techniques presently demonstrate the range of applications, with an equivalent sensitivity and accuracy. The only significant difficulty which has yet to be overcome is the unavoidable deposition of halides in the ion exclusion suppressor column, which causes a gradual increase in pressure and loss in signal sensitivity as the suppressor bed becomes expended. While the concentration of hydrochloric acid in the eluent is kept low to minimize its effect on the suppressor, its presence is nonetheless necessary in the formation of the boratemannitol complex. Eventually, the suppressor is totally contaminated with halides and must be replaced. There is presently no procedure available to regenerate expended ICE suppressor columns. Typically, the suppressor column will last for 1 to 2 weeks of extensive use before replacement is required. The cost of the columns is low and replacement of the columns is facile. In summary, the technique of ion chromatography and, in particular, ion chromatographic exclusion offers a new method
of analysis for borates in a wide range of sample matrices. This procedure can be applied to the determination of borates over a wide concentration range, with no matrix or intermolecular interferences. With its enhanced sensitivity and reproducibility, the ion chromatographic exclusion technique offers the analyst a simple and efficient method for the determination of borates.
LITERATURE CITED (1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 4 7 ,
1801. (2) Wetzel, R. A.; Anderson, C. L.; Schielcher, H.; Crook, G. D. Anal. Chem. 1979, 57, 1532. (3) “Ion Chromatographlc Analysis of Environmental Pollutants”; Ann Arbor Science Publishers: Ann Arbor, MI, 1979; Vol. Iand 11. (4) Lipski, A. J.; Vario, C. J. Can. Res. 1980 (Feb), 45-48. (5) Murray, A. D.; Varlo, C. J.; Lett, R. E. Twenty-Second Rocky Mountain Conference on Analytlcai Chemistry, Denver, CO, Aug 1980; Paper No. 21. (6) Nemodruk, A. A.; Karalova, 2. K. “Analytlcal Chemlstry of Boron”; Academy of Science U.S.S.R.: Moscow, 1964. (7) Silverman, L.; Trego, K. Anal. Chem. 1958, 25. 1264. (8) Spicer, G.; Strickland, J. Anal. Chlm. Acta 1958, 78, 231. (9) Hayes, M.; Metcalfe, J. Analyst (London) 1982, 8 7 , 956. (10) Uppstrom. L. Anal. Chlm. Acta 1988, 43, 475. (11) Rich, W.; et al., I n “Ion Chromatographlc Analysis of Environmental Pollutants”; Mullk, J. D., Sawlckl, E., Eds.; Ann Arbor Science Publishers: Ann Arbor, MI, 1979; Vol. 11, Chapter 2. (12) Rich, W.; et 81. I n “Liquid Chromatography in Clinical Anaiysls”; Kabra, P. M., Martin, L. J., Eds.; Humana Press Inc.: Clifton, NJ, 1981. (13) HIII, C. J.; Lash, R. P. Anal. Chem. 1980, 52, 24. (14) Chapin, W. H. J. J. Am. Chem. SOC. 1908, 30, 1691. (15) Wilson, W. J. Anal. Chlm. Acta 1958, 79, 516. (18) Gilmour. G. Analyst (London) 1921, 4 6 , 3. (17) Haider, S. Z. Analyst (London) 1954, 7 9 , 454. (18) Sciarra, J. J.; Zapotocky. J. J. J. Am. Pharm. Assoc., Sci. Ed. 1955, 44, 370. (19) Weatherby, S.; Chesny, N. N. Ind. Eng. Chem. 1928, 820. (20) Hahn, F. L. Anal. Chem. 1981, 33,316. (21) Mellon, M. S.; Morris, V. N. Proc. Jnd. Acad. Sci. 1934, 33, 85. (22) Boeskln, J. Adv. Carbohydr. Chem. 1949, 12, 81. (23) Roy, G. L.; Laferriere, A. L.; Edwards, J. 0. J. Inorg. Nucl. Chem. 1957, 4, 106. (24) Angyai. S. J.; McHugh, D. J. J. Chem. SOC. 1957, 1423. (25) Larsson, R.; Nunziata, G. Acta Chem. Scand. 1970, 2 4 , 2145. (26) Mazurek, M.; Perlin, A. S. Can. J . Chem. 1983, 4 1 , 2403. (27) Davis, H. B.; Mott, C. J. B. J. Chem. Soc., Faraday Trans. 1980, 7 6 , 1991. (28) Antlkalnem, P. Ann. Acad. Scl. Fenn., Ser. A2 1954. 5 6 , 3 .
RECEIVED for review January 11, 1982. Accepted April 19, 1982.
