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Anal. Chem. 1981, 53, 1727-1728
Ultrapurification of Analytical Reagents Monitored by Laser Intracavity Absorption T. D. Harris" and A. IH. Wllllams Bell Laboratories, 600 Mountain Avenue, Murray Hill, New Jersey 07974
For accurate and precise trace analysis, reagent purity is of primary consideration. This is especially true for spectrophotometric determinations where a pure chromophore and efficient reducing and buffering agents are required. Optimum results are obtained for ultratrace determinations when contamination in reagents is lowest and a minimum concentration of reagents is needed to effect reduction and buffering. For these reasons, simple, efficient, inexpensive methods of producing pure reagents are essential for successful analytical work. A simple chemical system for sub-part-per-billion iron determination has been reported by this laboratory (1). It utilized hydrazine hydrochloride as the reducer, ammonium acetate as the buffer, and Ferrozine as the complexing agent. The reagent purity attained was essential to the successful application of that system. Even though hydrazine hydrochloride has many advantages as a reducing agent for trace analysis, it also has the disadvantage of being a suspected carcinogen with a moderate to high toxic hazard rating (2). To eliminate the use of this hazardous reagent, we sought alternative methods for producing reducers of satisfactory purity. In this paper we report an evaluation of two similar procedures which are easily applied to reagents not amenable to purification by sublimation or distillation methods. Preliminary indications were that one method yielded subpart-per-billion purity of reagents with respect to iron (3). The first method tested was reported by Schilt and Lundgren (4). Their procedure calls for removal of Fe(I1) from aqueous reagents by complexation with a ligand preadsorbed to porous polymer beads (coated column method). The second method was published by Willis and Sangster (5). It calls for addition of 1,lO-phenanthroline to aqueous reagents, and the mixture flowed over the same type porous polymer beads (uncoated column or precomplex method). Both reports claimed quantitative iron removal, but the furnished analysis of effluents was not sufficiently precise to assure that either method would result in a final iron concentration as low as 1ppb. Since the ability to quantitatively determine extremely small concentrations of iroii had been previously demonstrated (I), the two methods were evaluated to assess their potential as ultrapurification techniques.
EXPERIMENTAL SECTION Reagents. Water was demineralized and distilled twice from a quartz still located in a cllass 1000 clean room. Hydrochloric acid and glacial acetic acid were high purity subboiling distilled, purchased from the US. Nakional Bureau of Standards. Ultrex ammonia was purchased from J. T. Baker Chemical Co. and used as received. Hydrazine hydrochloride was purified and prepared as previously reported (2). Solutions of 1,lO-phenanthroline (J. T. Baker Chemical Co.) and Ferrozine Iron Reagent, 3-(2pyridyl)-5,6-bis(4phenylsulfonicacid)-1,2,4-triazine,monosodium salt (Hach Chemical Co., Ames IA)were prepared by dissolution of the solid in 12 M HCl and dilution to the desired concentration with water. Solutions of PDT, 3-(2-pyridyl)-5,6-diphenyl-1,2,4triazine (Hach Chemical Co.), were prepared by dissolution of the solid in 12 M HCl and dilution with methanol (Mallinckrodt, Analytical Reagent Grade). Amberlite XAD-2 copolymer beads were purchased from SGA Scientific and cleaned as described below. Hydroxylamine hydrochloride and ammonium acetate stock solutions for purification were prepared from Analytical Reagent Grade solids (J. T. Baker Chemical Co.). Apparatus. Teflon columns (50 cm X 1cm i.d.1 equipped with Teflon stopcocks [SGA Scientific] were used for all purifications. 0003-2700/81/0353-1727$01.25/0
Reagent volumes were measured by using Finnpipette automatic pipeters with disposable pipet tips (Markson Science, Inc.). Column effluents were collected in 100-mLpolypropylene beakers and samples prepared in 25-mL polypropylene volumetric flasks. Procedures. Columns were packed by introducing a methanol slurry of the copolymer to a height of 30-35 cm. Trace metallic impurities on the copolymer or columns were removed by rinsing after loading with 100 mL of 6 M HC1, 200 mL of H20, 50 mL 48% HF, and finally 200 mL of H20. Polypropylene volumetric followed flasks were cleaned by soaking 24 h in 80 "C 8 M "03 by 24 h in 80 "C 6 M HC1 and 2 h in 80 "C 24% HF. Pipette tips were cleaned by filling and draining four times with 48% HF and six times with water. Iron removal using the precomplex procedure was investigated by flowing through the columns at 1mL/min, 100-mL aliquots of phenanthroline spiked 10% hydroxylamine hydrochloride or 10% ammonium acetate solutions. The solutions had been prepared with a phenanthroline concentration of 5 X lo4 M and a pH of 6. For minimum contamination, the first 30 mL of each 100-mLbatch was discarded. For purification by coated columns, 200 mL of 0.024 M PDT in methanol was passed through the columns at a flow rate of 1mL/min, followed by 200 mL of water at the same rate. Again 100-mL batches of pH 6 10% hydroxylamine hydrochloride or 10% ammonium acetate were flowed through the columns at 1mL/min. No phenanthroline was used, and as with uncoated columns, the first 30 mL of each batch were discarded. Analytical procedures for samples from both methods were identical and similar to those previously published (1). The specific sequence of reagents was 0.2 mL of Ferrozine, 0.4 mL of hydrazine hydrochloride, 0.4 mL of acetic acid, 0.5 mL of 12 M HCl, 10 mL of analyte, and 0.5-1.0 mL of 25% ammonia to give a solution of pH 4.5. The solutions were then diluted to volume and mixed. When desired, standard additions of iron were made following the analyte. Absorbance measurements were taken 30 min to 2 h after dilution. Hydroxylamine hydrochloride concentrations in the column effluents were determined titrimetrically (6). Absorbance measurements were made either on a Hitachi Model 100-80 UV-Vis spectrophotometer in 10-cm cells or by laser intracavity absorption spectrophotometry as previously described (7).
RESULTS AND DISCUSSION Two procedures described previously ( 4 , 5 )as preconcentration methods for iron determinations in the part-per-million range have been reexamined because they also represent significant potential for producing reagents with very small iron contamination. Although Lundgren and Schilt tested their coated column method as a purification technique ( 4 ) , the utility at very low levels was not established quantitatively. In this study a comparison has been made between two methods which have different factors limiting iron removal. In the case of iron phenanthroline, adsorption of the previously formed complex exploits an extremely large formation constant. This occurs with the sacrifice of some adsorptivity because both the ligand and the complex must be water soluble. Alternatively, precoating the column with the ligand from nonaqueous solvents permits use of ligands and complexes having extremely low aqueous solubility, which should result in higher adsorptivity of the complex to the polymer beads. The approach also has the advantages of simplicity and increased capacity. However, there will be an unavoidable reduction in the formation constant of the iron complex due to steric hindrance. For complete iron removal, complexes must be both efficiently formed and adsorbed. These factors have been considered previously in part, but have not been 0 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 11, SEPTEMBER 1981
Table I. Determination of Fe (ppb), Coated and Uncoated Columns, by Laser Intracaviry Absorption uncoated coated Hydroxylamine Hydrochloride 0.9 t O . l a blank 10-mL sample 1.0 i 0.1 10-mL sample t 0.5 ppb Fe 1.5 2 0.1 Ammonium Acetate blank 0.7 c 0.2 10-mL sample 0.6 i 0.2 10-mLsample + 0.5 ppb Fe 1.1i 0.1
0.6 t 0.1 1.1 f 0.2 1.6 i 0.1 0.3 i 0.1
0.7
f
0.1
1.2
t
0.2
An average of four measurements quoted as
x
20.
addressed with the production of ultrapure reagents as the objective. It is not discernible from published data which system will yield greater reagent purity or if either will reduce iron concentrations to below 1ppb. In addition when purification to extremely low levels is sought, simple equilibrium and kinetic calculations are often in severe disagreement with measured concentrations. Initial application of both procedures gave erratic results. Iron concentrations in the effluent, measured with the conventional UV-Vis spectrophotometer, varied randomly in the 1-3 ppb range. Filtration of the feedstocks through 0.6-pm pore size membrane filters yielded consistent effluent iron concentration of =2 ppb. Particulate contamination was originally suspected as the source of the erratic results. If particulates were responsible for the residual iron, the particle size must be very small. Acidification and reneutralization of the feedstocks resulted in consistently low effluent iron concentrations. The procedure using both coated and uncoated columns yielded iron concentrations below the 0.5 ppb detection limit of the conventional spectrophotometer. This corresponds to an iron concentration in the effluent of less than 1.2 ppb. All subsequent measurements were done by laser intracavity absorption spectrophotometry. The purity of the effluent for the precomplex method was first tested by collection of 25-mL fractions. The iron concentrations found in three successive fractions were 0.4,0.3, and 10.2 ppb. Contamination in the initial portion of effluent is clearly evident. The problem was solved by discarding the first 30 mL of effluent. The results for both coated and uncoated columns with hydroxylamine and ammonium acetate solution as monitored by laser intracavity absorption are shown in Table I. As before (1) the detection limit was 0.1 ppb iron in the solution measured. This corresponds to =0.2 ppb in the column effluent, For columns of 35 cm length the procedure with
phenanthroline gave iron concentrations below the detection limits. When processed through 30-cm columns by the precoated method, sample iron concentrations were consistently ~ 0 . 5 ppb. This corresponds to 1.2 ppb in the column effluent, just below the conventional spectrophotometer detection limit. While this is exceedingly pure by most standards, it is clearly inferior to the phenanthroline precomplex method. Significantly increased column length may yield results similar to the precomplex method, but at a sacrifice in speed and convenience. The variation of blanks shown in Table I is at first disturbing. Each of the four sets of data was run on a different day. The blanks on any one day were constant within experimental uncertainty. The reliability and accuracy of the method are demonstrated by the accurate detection of added iron. Standard additions from 0.5 to 2.0 ppb showed equal consistency. The source of the day to day blank variation has not been resolved but is under investigation. The 0.5 ppb iron concentration detection limit for the conventional spectrophotometer is a result of spectral artifacts due to the high salt concentration in the sample. The actual displayed noise of the spectrometer corresponds to Bin equivalent absorbance of 2 X or 0.05 ppb iron in a 10-cm cell. Experience in our laboratory has been that an absorbance change from blank to sample smaller than 2 x absorbance units cannot be reliably detected, and thus the 0.5 ppb detection limit. CONCLUSION When properly executed the precomplex procedure is a highly effective purification technique. It should be readily applicable for all noncomplexing neutral salts. The method is applicable to all media in which iron phenanthroline spectrophotometry has been done. Under the proper conditions the general method can be made selective for removal of any cation for which a ligand exists that has significance aromatic character. The sensitivity of the laser intracavity technique is essential for establishing optimum conditions. LITERATURE CITED (1) Harris, T. D.; Mitchell, J. W. Anal. Chem. 1980, 52, 1706-1708. (2) Mitchell, J. W.; Harris, T. D.; Blltzer, L. D. Anal. Chem. 1980, 51, 774-776. (3) J. M. Harris, University of Utah, personal communicatlon. (4) Lundgren, J. L.; Schilt, A. A. Anal. Chem. 1977, 49, 974-980. (5) Wlllls, R. B.; Sangster, D. Anal. Chem. 1976, 48, 59-62. (6) Kolthoff, I. M. J. Am. Chem. SOC.1924, 46, 2009-2016. (7) Shlrk, J. S.;Harris, T. D.; Mitchell, J. W. Anal. Chem. 1980, 52, 1701-1705.
RECEIVED for review March 10,1981. Accepted June 4,1981.
CORRECTION Automated Instrument for Absorption-Corrected Molecular Fluorescence Measurements by t h e Cell Shift Method
D. R. Christmann, S. R. Crouch, and Andrew Timnick (Anal. Chem. 1981,53, 276-280). On page 277 eq 4 is missing brackets and a superscript as shown in the correct version below:
