Anal. Chem. 1982, 5 4 , 761-764
Landis, (7)
., (10) (11) (12) (13)
J. B.; Rebec, M.; Pardue, H. L. Anal. Chern. 1077, 49, 785-788. Holler, F. J.; Enke, C. G.;Crouch, S.R. Anal. Ctiirn. Acta 1080, 117, 99-113. Chattopadhyay, P. K.; Coetzee, J. F. Anal. Chem. 1072, 4 4 , 21 17-21 18. Horton, W. S. J . Phys. Collold. Chem. 1048, 52, 1129-1136. Benson, S. W. "The Foundations of Chemical Kinetics"; McGraw-Hili: New York, 1960; Chapter 4. Miller, M. L.; Gordon, 10. Anal. Chem. 1076, 48, 778-779. Albery, W. J.; Robinson, B. H. Trans. Farad8y SOC.1060, 65, 980-991. Caldin, E. F.; Crooks, J. E.; Queen, A. J . Phys. E . 1073, 6 , 930. Hagelauer, U,; Faust, U.; Ksller, U. Fresenlus' 2.Anal. Chern. 1080, 301, 184.
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(14) Mieiing, G. E.; Pardue, H. L. Anal. Chern. 1878, 50, 1611-1618. (15) Dwass, M. "Probablllty and Statistics"; W. A. Benjamin: New York, 1970; pp 329-330. (16) Liebhafsky, H. A.; Mohammed, A. J . A m . Chem. SOC. 1033, 55, 3977-3986. (17) Holler, F. J.; Engh. S.,manuscript in preparation. (18) Holler, F. J.; Mateyka, W. C. Anal. Chem. 1080, 52, 354-355. (19) Caserta, K. J.; Holler, F. J.; Crouch, S. R.; Enke, C. G. Anal. Chern. 1078, 50, 1534-1541.
RECEIVED for review September 16,1981. Accepted January 12,1982. R.K.C. and S.F.M. are grateful to the Ashland Oil Foundation for summer fellowship support.
Determinatilon of Ultratrace Levels of Fluorine in Water and Urine Samples by a Gas Chromatographic/Atmospheric Pressure Helium Microwave Induced Plasma Emission Spectrometric System Koichi Chlba, Kazuo Yoshida, Kiyoshl Tanabe,' Masanao OzakI,* Hlrokl Haraguchl, * J. D. Winef~rdner,~ and Kellchlro FUWH Department of Chemistry, Faculty of Science, Unlversity of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Fluoride Ion In natural water and urine samples Is extracted with TMCS (trirnethylchiorosilane) and converted to TMFS (trlmethylfluorosllane)lin toluene. The extracted toluene solutlon ls Injected into a gas chromatograph (GC) and detected by atmospheric pressure helium mlcrowave Induced'plasma (MIP) emission spectrometry. The emlssion line of fluorine at 685.6 nm Is used fair detection. The detection limit and linear dynamlc range of the GCMIP measurement system are 7.5 pg/s and mare thari 4 orders of magnltude, respectively. Analytlcal results obtained by the GC-MIP system are condstent w%hthose by a fluoride Ion selective electrode method.
In recent years, the determination of fluorine in various samples has been extensively investigated because of clinical and environmental interest. An ion-selective electrode for fluoride ion (F-ISE) has been widely used as a conventional analytical method in recent papers (1-5) because off its convenience and economy. As is well-known, the F-ISE is selective to only free fluoride ion; however, it is affected by coexisting cations (for example, magnesium in seawater) under ordinary operating conditions, Other analytical methods consist of colorimetry (6, 7) and gas chromatography (8)which are applied in various fields. Recently, the present authors reported on a molecular absorption spectrometric method for fluorine, in which total fluorine was directly determined by measuring aluminum monofluoride (AlF) molecular absorption a t 227.45 nm (4-12). In another way, since the excitation energy of fluorine is very high (14.5 eV), it is difficult to excite fluorine efficiently by flame or ICP atomic emission spectrometry. Only one National Institute for Public Health, Minato-ku, Tokyo 108, Ja an. BAjinomoto Co., Ltd., Buzuki, Kawasaki 210, Japan. 30n leave from the Department of Chemistry, University of Florida, Gainesville, 171, 326 11. 0003-2700/62/0354-0761$01.25/0
paper (13) has reported on emission lines of fluorine with an ICP. However, in the helium MIP, a high energy of metastable helium (19.8 eV) is available and the detection of fluorine is therefore feasible. Several papers have reported on fluorine detection by atomic emission with a reduced pressure helium MIP with gas chromatograph (14,151. The atmospheric pressure helium microwave induced plasma (MIP), which was recently developed by Beenakker (16),has been investigated as an excitation source for spectrochemical analysis of nonmetallic and metallic elements (17-22). The plasma system with a microwave cavity is simple and convenient to operate. Beenakker (17)also showed the potential usefulness of the atmospheric pressure He-MIP as a GC detector with an exponential dilutor, and reported detection limits, linear dynamic ranges, selectivities and relative sensitivities for C, H, C1, Br, I and S. Quimby et al. extensivly investigated the analytical feasibility of a GC-MIP system (18, 19). The present authors previously reported (23) on the simple GC-MIP system, and detection limits, linear dynamic ranges, selectivities, and sensitivities were investigated for nonmetallic elements H, C, F, Br, I, and S. In this paper, the feasibility of this simple GC-MIP system is evaluated as a conventional analytical method of fluoride determination and applied to the determination of fluoride in actual samples, i.e., seawater, tap water, human urine, and pond water. The reliability of this system is investigated by comparing the analytical values obtained by both this system and the fluoride ion selective electrode (F-ISE).
EXPERIMENTAL SECTION Gas Chromatograph. A Shimadzu GC-6A gas chromatograph with a dual column type (Shimadzu Seisakusho Ltd.) equipped with a thermal conductivity detector (TCD) is employed. The Pyrex glass chromatographic column (1m X 3 mm i.d.) packed with 15% DC-200 on 80/100mesh Uniport B and the other Pyrex glass column (3 m X 3 mm i.d.) packed with 3% OV-17 on 80/100 mesh Uniport HP are used. When the column packed with DC-200 is used, the temperatures of the injection port, the detector oven, and the heated transfer tube are maintained at 160 "C, 160 @ 1982 American Chemical Society
762
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
r
G C - - -1 -- ----
I
Vent
M
I
added. The test tubes are shaked vigorously for 30 min. A part of the TMCS in the organic phase reacted with fluoride ion existing in sample solutions producing TMFS (trimethylfluorosilane); therefore fluoride in the aqueous phase is extracted into the organic phase as TMFS. After the organic phase is separated from the aqueous phase, the organic phase is transferred into a sampling tube and then measured with the present GC-MIP system. The flow rate of carrier helium gas is adjusted to 80 mL/min for both columns, and then the plasma is ignited with a Tesla coil. About 30 min later, the plasma became stable and the temperature of the gas chromatograph reached a constant level. The adjustment of wavelength and observation position in the plasma are performed as follows: a,a,a-trifluorotolueneis injected very slowly into the column packed with OV-17, so that a broad peak of fluorine appeared. During the appearance of the peak, wavelength and observation position in both vertical and horizontal, are adjusted rapidly. The analytical atomic line of fluorine measurement is at 685.6 nm. The measurement of the plasma emission signals of samples is carried out according to the following procedure. One microliter of sample extractant solution is injected into the column. The TMFS fluorine peak appears first and is detected by the MIP detector at its characteristic retention time. Immediately after the TMFS peak appears, the four-way valve is switched to vent the solvent while the solvent peak is monitored by the TCD. This procedure is necessary to avoid extinguishing the plasma and to maintain good plasma stability.
I
Flgure 1. Schematic diagram of the GC-MIP system: (S) helium tank, detector, (H) heated transfer tube, (C) microwave cavity, (0)microwave generator, (L) lens, (M) monochromator, (P) photomultiplier, (V) higher voltage power supply, (A) plcoammeter, (R) chart recorder. (T) thermal conductivity
O C , and 170 OC, respectively, throughout the experiment. The optimum conditions for the DC-200 column are as follows: the column temperature is 40 OC; the flow rate of carrier helium gas is 80 mL/min. On the other hand, when the column packed with OV-17 is used, the optimum conditions are as follows: the column temperature is maintained at 80 "C; the carrier gas flow rate is also 80 mL/min. The temperature of the injection port, the detector oven, and the heated transfer tube are selected at 150 OC, 150 OC, and 160 "C, respectively, when using the OV-17 column, Plasma and Detector System. The microwave cavity with T&lo mode is constructed from copper metal after Beenakker's description (16, 17) with some modification (24, 25). The microwave generator, which provided 20-200 W of microwave power at 2.45 GHz, is run at 75 W of forward power with about 12 W of reflected power. The GC carrier gas is used as the plasma gas and no auxiliary gas is added. The plasma is sustained at a flow rate of 80 mL/min in a quartz discharge tube (6 mm o.d., 1mm i.d.1. High stability of the plasma could be expected from basic plasma parameters such as temperature and electron number density under these operating conditionsfrom our previous studies
(26) An Ebert-type monochromator (0.5 m focal length) purchased +
from Nippon Jarrell-Ash Co., Ltd., is used for spectral measurements. The width and the height of the entrance and exit slita are 10 Fm and 5 mm, respectively. A 1:l image of the plasma (axially viewed) is focused on the entrance slit with a quartz lens (60 mm focal length, 25 mm diameter). An R955 photomultiplier tube (Hamamatau TV Co., L a . ) with low dark current and high gain over a wide wavelength region is used. The fluorine measurement is carried out by observing an atomic line at 685.6 nm. Interface. A schematic diagram of the present GC-ME' system is shown in Figure 1. The effluents from the column are first directed to the TCD and then passed through the high-temperature electromagnetic four-way valve (GC-604-A, Nippon Kuromato Kogyo Co., Ltd.) attached to the TCD oven. This four-way valve is operated by a switch located on the front panel of the gas chromatograph. One of the outlets of the valve is connected with the heated transfer tube to the plasma discharge tube. The heated transfer tube is constructed from an inner nickel tube, a copper protecting tube, a thermocouple, shielding glass tape, nichrome wire, and outer glass tape insulation. The other outlet of the valve is used to vent the excess solvent in order to maintain good plasma stability. Chemicals. All chemicals used are of analytical reagent grade. The stock solution of fluoride is prepared by dissolving sodium fluoride in distilled water. All the analytical standard solutions are obtained by diluting the stock solution with distilled water to a series of proper concentrations. The extractant solution is prepared by dissolving 0.5 g of trimethylchlorosilane (TMCS) in 1L of toluene. All cations used for investigating their interferences are present as nitrates, except for NaC1, KCl, B(OH)B, and NazSiOa. Procedures. The extraction is performed as follows: 4 mL of sample solutions is placed in 15-mL test tubes with ground glass stoppers; 1 mL of 9.5 N HC1 and 1 mL of extractant are then
RESULTS AND DISCUSSION Optimization. Optimum long-term stability resulted by setting the microwave power at 75 W throughout all experimenta. However, the fluorine wavelength and the observation position, in both vertical and horizontal, are adjusted before measurements every day by the means mentioned above. When the column packed with DC-200 is used, the optimum carrier helium gas flow rate and the column temperature are 80 mL/min and 40 "C, respectively. The effects of the carrier gas flow rate and the column temperature on the fluorine signal are presented in Figure 2. As can be seen from Figure 2a, the fluorine signal decreased by more than 2-fold as the carrier gas flow rate is reduced from 80 to 50 mL/min. Previously (26),it was found that the best plasma stability obtained for a helium flow rate was even higher, or 125 mL/min. The carrier gas flow rate of 125 mL/min is, however, much too high for the adequate GC separation of TMFS. On the other hand, the reduced column temperature is helpful for fluorine separation even a t higher gas flow rate, without decreasing i b intensity too much (Figure 2b); the compromised best condition was found at 80 mL/min for carrier gas flow rate with 40 "C of column temperature. The most intense UV-VIS line of fluorine at 685.6 nm is selected as the analytical line for measurement; the wavelength tables for nonmetallic elements (25) facilitated this selection. A typical chromatogram obtained by using the selected conditions is shown in Figure 3. As can be seen in Figure 3, only the TMFS signal corresponding to 1pg/mL fluorine is observed in the chromatogram with the MIP detector (retention time of 24 s). On the other hand, in the chromatogram obtained with the TCD, no TMFS peak is detected because of its low sensitivity, with many other peaks corresponding to the major constituents, including TMCS, benzene (i.e., impurity in toluene), and toluene (i.e., solvent). It takes only 6 min to complete the emission measurement of one sample because only fluorine is detected in this system. When the column packed with OV-17 is used, the flow rate of carrier gas and the column temperature are found in a similar manner to be optimum at 80 mL/min and 80 OC, respectively. Detection Limit and Linear Dynamic Range. The detection limit is defined as the signal level corresponding to twice the standard deviation of background emission a t the
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
Table I. Relative Intensity of Fluorine Atom in Various Compounds with MIP Excitation compound
intens
compound
re1 intens
C,H,CF, C,H,F CF,CH,OH CF,CF,CH,OH
0.90 1.80 1.78
l.OOa
CF,(CF,),CH,OH CH,SO,F (CH,),SiF
1.24 1.12 0.60
Wl
a
a
1.0-
0.5.
Nomalized to 1.00.
Table 11. Analytical Figures of MeritQfor the GC-MIP System with a DC-200 C'.o 1umn sample, sine, DL, EE, RSD, LDP, % decades plLl pg/mL 7 preconcentrated 1 0.02 95 3.0 3.3 in extraction, 4 times preconcentrated 4 0.004 75 6.5 4.1 in extraction, 10 times a DL = detection limit; EE = extraction efficiency; RSD = relative standard deviation; LDR = linear dynamic range.
Table 111. Investigation of Interferences Observed with the GC-MIP Method for Fluorine (1pg/mL) interferconcn of encea element interferent factor none KCl NaCl Mg*
Ca'+ cu2+ Fe3+ SiO,,La3+
Al 3+ B(OH),
so,,-
763
1.0%
3.0% 1500 pg/mL 500 pg/mL 100 pg/mL 1.00 pg/mL 100 pg/mL 100 pg/mL 100 pg/mL 10 pLg/mL I pg/mL 10 pg/mL 0.04 M
1.00 1.01 0.98 1.01 0.99 1.02 1.00 0.99 0.96 0.53 0.93 0.99 0.99 0.99
60
50
70
80
Gas Flow Rate (mL/min)
bl c n
3 1 z
3
0.5
=
I
60
40
80
Column Temperature ("C) Figure 2. (a) The effect of carrier gas flow rate on fluorine Intensity (arbltrary units). (b) The effect of column temperature on fluorine Intensity (arbltrary unlts).
a A factor of 1.00 means no interference. A factor >1.00 means an enhanclement; a factor
