ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
(13) Issaq, H. J.: Zielinski, W. L., Jr. Anal. Chem. 1974, 46, 1328-1329. (14) Struempler, A. W. Anal. Chem. 1973, 45, 2251-2254. (15) Robertson, D. E. Anal. Chim. Acta 1968, 42, 533-536. (16) ReneF. - - ..- -, .P., Steinnes. - .- ... .- -, -E.. Water ..- .. Res. 1975. 9 . 741-749. Florence, T. M.; Batley, G. E. Taianta-1977, '24. 151-158. Z i f , M . ; Mitchell, J. W. "Contamination Control in Trace Element Anatysis"; John Wiley & Sons: New York, 1976:pp 28-33. Tolg, G. Taalanta 1972, 19, 1489-1521. Truitt, R. E.: Weber, J. H. Res. Rept21,Water Resources Research Center, University of New Hampshire, Durham, N.H., 1979. Truitt, R. E.; Weber, J. H. Water Res., in press.
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(22) Wallace, Jr. G. T.; Fletcher, I . S.;Duce, R. A. J . Environ. Sci. Health 1972, 12, 493-506.
RECEIVED for review May 21, 1979. Accepted July 23, 1979. This research was partially supported by the Office of Water Resources Technology, Grant B004-NH administered by the Water Resources Research Center a t the University of New Hampshire.
Platinum Atomic Lines for Determination of Ultratrace Fluoride by Aluminum Monofluoride Molecular Absorption Spectrometry Kin-ichi Tsunoda, Koichi Chiba, Hiroki Haraguchi, and Keiichiro Fuwa' Department of Chemistry, University
of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Recently, the present authors have developed a method for the ultratrace determination of fluoride ( I ) , chloride and bromide ( 2 ) by aluminum monohalide molecular absorption spectrometry, where a carbon rod furnace was used as a formation source for aluminum monohalides. Aluminum monofluoride is readily formed in the furnace and shows a sharp molecular absorption band near 227.45 nm. Subnanogram fluoride levels in various samples can be determined by the measurement of aluminum monofluoride molecular absorption at 227.45 n m (1). In the previous paper ( I ) , a deuterium lamp was used as a light source, but this approach was inconvenient from the point of view of slit width and background correction considerations. In the case of slit width, t h e sensitive measurement of aluminum monofluoride molecular absorption requires a narrow spectral bandpass, but on the other hand, if the background correction is performed at the same wavelength as the aluminum monofluoride band, a wide spectral bandpass is needed for background correction caused, in particular, by molecular absorption of other species. Otherwise, a wavelength different from 227.45 nm should be used for background correction. These two problems (the use of narrow spectral bandpass and the background correction) may be overcome by using a line-like irradiation source for t h e aluminum monofluoride molecular absorption measurements. In this paper, the potential use of neighboring atomic lines from various elements for ultratrace determination of fluoride is examined for the convenient use of the molecular absorption technique with simultaneous background correction. In consequence, the atomic lines of platinum a t 227,438 and 227.484 n m were found to be most suitable for requirements mentioned above.
EXPERIMENTAL Apparatus. Atomic absorption spectrophotometers, AA-I, Mark-I1 from Nippon Jarrell-Ash Co., Ltd., and AA 170-50 from Hitachi Co., Ltd., were used for the measurement of AlF molecular absorption. The latter instrument offers the possibility of simultaneous background correction with a deuterium lamp. A carbon rod furnace, FLA-100 from Nippon Jarrell-Ash Co., Ltd., was used as the high temperature cuvette. Argon gas (1.0 L/min) was flowed to purge the carbon rod furnace from air. Platinum, cobalt, and nickel hollow cathode lamps, and a deuterium hollow cathode lamp from Hamamatsu TV Co., Japan, were used as the light sources for molecular absorption and background correction, respectively. The spectral band width of the monochromator was usually set at 1.1 nm, when simultaneous background correction was performed using the Hitachi AA 170-50 spectrophotometer. Procedure. The procedure and the experimental conditions for the measurements of AF molecular absorption are summarized in Table I. In practical measurements, the appropriate amounts 0003-2700/79/0351-2059$01.00/0
Table I. Experimental Procedure and Operating Conditions for the AIF Molecular Absorption Measurement with Carbon Rod Furnace procedure conditions 1. 2. 3. 4. 5. 6. 7. 8.
Application of aluminum solutiona Drying I Ashing I Cooling of furnace Application of sample solution Drying I1 Ashing I1 Atomization and measurement
0.01 M, 20 pL 25 A, 2 0 s 60 A, 15 s
5 pL 25 A, 20 s 60 A, 30 s 280 A, 7 s
Aluminum nitrate was dissolved in distilled water. For the analysis, iron(II1) nitrate ( 0 . 0 1 M ) and strontium nitrate ( 0 . 0 1 M ) were added t o this solution. (I
of Fe3+and Sr2+were added to the aluminum solution in order to enhance the sensitivity and to improve the precision of analysis according to the previous investigation ( I ) . Chemicals. All reagents used were of analytical grade purchased from Wako Pure Chemicals Co., Japan. Fluoride standard solution was prepared by dissolving sodium fluoride in distilled water. All metal ions were in the form of nitrates.
RESULTS AND DISCUSSION Measurement of A1F Molecular Absorption Utilizing Atomic Lines. As described in the previous paper ( I ) , the A1F molecular band a t 227.45 nm shows line-like absorption, and the most intense molecular absorption of A1F is observed with the slit width as narrow as possible, when a continuum light source is used. On the other hand, the sensitivity of analysis in atomic absorption spectrometry utilizing a hollow cathode lamp (HC1) is not so dependent upon the slit width of the monochromator, because the line widths of atomic lines from a hollow cathode lamp are very narrow (ca. 0.001 nm). Therefore, it was considered that the intensity of A1F molecular absorption would have only a small dependence on the slit width, if a line source replaced the continuum source. In order to investigate the possibility of using line sources for AlF molecular absorption spectrometry, atomic lines of metallic elements close to the A1F molecular absorption band (227.45 nm) were selected from the MIT Wavelength Tables ( 3 ) . Consequently, neighboring lines of cobalt, platinum, and nickel were examined, and the spectral figures for the A1F band and platinum lines are shown in Figure 1, as one example. The experimental results obtained are shown in Figure 2 and Table 11. As can be seen in Table 11, the atomic lines of Co (227.449 and 227.463 nm) and Pt (227.483 and 227.438 nm) gave relatively strong A1F molecular absorption. In the C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
.3-
227.44 nm
1 'AIF bh 227.45nm
.2
0.
226
> i .
I,
227
228
Wavelength (nm)
Flgure 1. Atomic and molecular absorption spectra of aluminum in a carbon rod furnace, and spectral line profiles of platinum near 227.4 nm A
0
Table 11. Absorbance (Relative) of A1F Molecular Absorption for Various Light Sources' slitwidth intenof sity spectrowavelength, of meter absorbance light source nmb linesb pmC (relative) continuum light source
227.45
case of cobalt, however, the best sensitivity is obtained a t a narrow slit width (15 pm), and the absorbance decreases with increase of slit width. This is because cobalt has a strong non-absorption line at 227.56 nm in addition to the absorption lines a t 227.449 and 227.463 nm. The cobalt HCL may, therefore, not be so suitable for A1F molecular absorption spectrometry. On the other hand, although the Pt atomic lines (227.438 and 227.483 nm) gave a slightly lower sensitivity for fluoride than those of cobalt and the continuum light source a t the narrow slit width, the sensitivity did not depend significantly on the slit width. When platinum is contained in the samples, the use of platinum lines for AlF molecular absorption may result in spectral interference. However, the signals of the ALF molecule and atomic Pt are fortunately time-resolved in the furnace under the present conditions, i.e., the A1F signal is more transient than the Pt signal, as can be seen in Figure 3. Therefore, spectral interference due to Pt can be avoided by controlling the experimental conditions for the furnace heating. In consequence, it can be concluded that a Pt HCL is the most suitable light source for A1F molecular absorption spectrometry. Simultaneous Background Correction with a Deuterium Lamp. As mentioned earlier ( I ) , simultaneous background correction is an important requirement for the accurate and precise determination of fluoride via A1F molecular absorption spectrometry. In atomic absorption spectrometry, background correction utilizing a deuterium lamp is the conventional procedure. The method is based on the fact that atomic absorption with a line source is usually much more sensitive than with a continuum light source, while molecular absorption does not depend on the nature of the light source. Such a possibility of the background correction in A1F molecular absorption spectrometry was investigated, utilizing a deuterium lamp as a light source. The relationship between the slit width of the monochromator and the intensity
1.00
227.449 9 20 1.30 H.C. lamp 227.463 8 platinum 227.438 30 15 0.99 H.C. lamp platinum 227.484 25 15 0.58 H.C. lamp platinum 227.43ad 30 260 0.81 H.C,lamp 227.484 25 nickel 227.466 12 25 0.61 H.C. lamp ' 5 p L of 0.08 yg/mL fluoride standard solution was tested, and absorbance of the A1F molecular absorption obtained with the continuum light source (slit width 20 p m ) was normalized to 1.00. These data were cited from tha MIT Wavelength Tables ( 4 ) . The Nippon Jarrell-Ash AA-I atomic absorption spectrophotometer Atomic lines were not resolved. was used. cobalt
.32 .64 .96 Spectral Band Width (nm)
Flgure 2. Dependence of AIF molecular absorption intensity on the spectral band width of the monochromator for various light sources: (A) platinum lamp, (B) deuterium lamp, (C) cobalt lamp. (A - B) represents the difference of the absorbances obtained with a platinum lamp (A) and a deuterium lamp (B)
20
B -.
-~
- ...
-
-
-
-
--
A
~
--
Flgure 3. Formation of AIF molecule and atomic Pt in a carbon rod furnace. (The upper signals are the corrected ones, and the lower signals are the background ones.) (A) AIF only. (B) AIF f platinum
of A1F molecular absorption with various light sources is shown in Figure 2. The absorbance of AlF molecular absorption with a deuterium lamp decreases to about a quarter of t h a t with a Pt HCL a t a wide slit width, Furthermore, the intensity ratio of AlF molecular absorption with a Pt HCL and a deuterium lamp is almost constant over 300 wm. Therefore, the measurement of the difference of the absorbances with Pt and deuterium lamps enables simultaneous background correction to be performed. In the present system, the spectral band width is fixed a t 1.1 nm. In order to examine the feasibility of simultaneous background correction with the present system, the background absorption due to NaC,l a t the A1F molecular band was investigated by using the Pt HCL only, and by using both the Pt HCL and deuteium lamp, respectively. The results are shown in Figure 4. Figure 4 shows t h a t the present system
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
data of A1F molecular absorption spectrometry with and without the simultaneous background correction system are summarized in Table 111. The sensitivity of the method with the present system (Pt HCL used) is about half of that reported with a continuum light source in the previous paper ( I ) . The dynamic range with the present system is three or four times larger compared to the previous method, while the relative standard deviation was almost the same. These experimental results suggest that the method proposed here is very useful for the practical analysis of fluoride. The sensitivity of fluoride analysis by the present technique was examined, using the various commercially-available furnaces from different companies. I t varies with the types of furnaces, apparently more than the measurement of atomic absorption. The size and the stream of innert gas within the furnace must affect significantly the production of diatomic molecules. Therefore, it should be noticed that the sensitivity shown in Table I1 is the data obtained with the furnace and experimental conditions employed in the present work. T h e present method was applied to the determination of fluoride in a freeze dried urine sample (SRM 2671 of NBS). The observed value of 7.10 f 0.1 Mg/mL (at 100 times dilution) was obtained for the certified value of 7.14 f 0.48 Mg/mL.
31
9
'
TIME 140Ie1l
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'
Figure 4. Elimination of influence of NaCl by the present system. (The upper signals are the corrected ones, and the lower signals are the background ones.) Background absorption due to NaCl up to 0.05 M could be corrected
Table 111. Data on AlF Molecular Absorption this workn previous workb 1%absorption, ng 0.042 0.021 rel. std. dev., % 2 2 (at 0.500 ng F) (at 0.536 ng F ) dynamic range, ng u p to ca. 2 ng up to ca. 0.6 ng a Platinum HCL and D, lamps were used for the A1F absorption and the background correction, respectively. A continuum light source was used at 227.45 nm with 0.03 nm of spectral band width. These values were cited from the previous paper ( I ) .
LITERATURE CITED K. Tsunoda. K. Fujiwara, and K. Fuwa, Anal. Chern., 49, 2035 (1977). K. Tsunoda, K. Fujiwara, and K. Fuwa, Anal. Chem., 50, 861 (1978). "Wavelength Tables", G. R . Harrison, Ed., The MIT Press, Cambridge, Mass.. 1969.
(using both the Pt HCL and D2 lamp) can correct the background absorption due to NaCl up to 0.05 M. The background correction is incomplete beyond 0.05 M NaC1, because the molecular absorption of NaCl is larger than the capability of this background correction system. Analytical
RECEIVED for review September 28, 1978. Accepted July 24, 1979. This research has been supported by a Grant-in-Aid for Special Project Research under grant No. 310503 and for Environmental Science under grant No. 303022 from the Ministry of Education, Science, and Culture, Japan.
Chemical Reactivation of Silica Columns R. A. Bredeweg,
L. D. Rothman," and C.
D. Pfeiffer
The Analytical Laboratories, Dow Chemical Company, Midland, Michigan 48640
A problem frequently encountered in liquid-solid chromatography is variation in column selectivity caused by changes in the water content of silica column packings. As discussed by Snyder ( I ) , silica is prepared for use in adsorption chromatography by thermal dehydration to a desired activity level. This is accomplished by heating the silica in an open container or in a packed column with a flowing carrier gas a t 15C-250 "C. Silica may also be activated over desiccants such as phosphorus pentoxide and sulfuric acid. or by washing with polar solvents followed by nonpolar solvents. Of all these approaches, only that of solvent treatments may be routinely used to reactivate microparticulate high performance liquid chromatography (HPLC) columns without significantly reducing column efficiency. The quantities of solvent necessary to reactivate a column (on the order of 100-200 mL for a typical analytical HPLC column) plus reequilibration of the column can make reactivation very time-consuming. This report describes a rapid procedure for reactivating silica columns by chemical conversion of water to more easily-eluted species. The procedure is based on the chemical reaction of 2,2-dimethoxypropane (DMP) with water in the 0003-2700/79/035 1-2061$01.OO/O
presence of an acid catalyst ( 2 , 3). 0 OCH, I1 I H,O + CH,CCH, CH,CCH, + 2 CH,OH I OCH, Water is destroyed in situ by pumping a solution containing D M P and a n acid, such as acetic acid, through the HPLC column. This procedure is readily accomplished without dismantling the HPLC system and can return a column to "like new" performance in 25-45 min.
-
EXPERIMENTAL Apparatus. A conventional HPLC system was used in conjunction with a 4 mm i.d. X 300 mm gPorasi1 column (Waters, Associates, Milford, Mass.). Reagents. 2,2-Dimethoxypropanewas obtained from Aldrich Chemical Company, Milwaukee, Wis., 53233. Bio-Si1 A, 100/200 mesh silicic acid, was obtained from Bio-Rad Laboratories, Richmond, Calif. Removal of Water from Bio-Si1 A. Water in Bio-Si1 A was determined by Karl Fischer titration. A 4.6 mm i.d. X 250 mm stainless steel column was packed with 2.1 g of Bio-Si1 A. A total
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1979 American Chemical Society
