(30) A . W. Morris, Anal. Chim. Acta, 42, 397 (1968). (31) S.Motomizu, Anal. Chim. Acta, 84, 217 (1973). (32) F. G.Lowman and R. Y. Ting, “Radioactive Contamination of the Marine Environment,” IAEA Syrnposiurn$Series 158, Vienna, 1973, p 369. (33) I. M. Kolthoff and J. J. Lingane, Polarography,” Voi. 2, Interscience. New York, N.Y., 1952,p 475.
(34) R . U. Vakhobova and R. D. Yurina, Dokl. Akad. Nauk Tadzhik. SSR, 15, 29 (1972). (Anal. Abstr., 28, 2086 (1974)). (35) H. Gubitsch and J. Schukoff, FreseniuS’ Z.Anal. Chem., 253, 201 (1971).
RECEIVED for review July 1, 1977. Accepted August 22, 1977.
Subnanogram Fluorine Determination by Aluminum Monofluoride Molecular Absorption Spectrometry Kin-ichi Tsunoda, Kitao Fujiwara, and Keiichiro Fuwa Depaflment of Chemistry, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113, Japan
The diatomic molecule of aluminum monofluoride, AIF, gives a sharp absorption band spectrum at 227.5 nm in both flames and a carbon rod furnace. The absorption intensity is high enough so that it can be used for a sensitive analytical method of fluorine determination. The best sedtivity observed Is 0.021 ng of fluorine with the carbon rod furnace, which Is the best so far reported for this element and Is also superior to that obtained with the N20-C2H2 flame, Le., 24 pg F/mL. An excess aluminum solution is applied first to the furnace before the fluorine containing sample is pipetted, so that the AIF molecule is effectively formed inside the furnace. A certain amount of strontium and nickel Is found to enhance the signal with decreasing the background. The method has been successfully applied to biological samples and also to organofluorine compounds.
Atomic absorption spectrometry has been extensively developed as a useful analytical method for various metallic elements. However, its applications to nonmetallic elements have many difficulties, mainly due to the fact that the resonance lines of these atoms are found in the vacuum ultraviolet region of the spectrum. Especially, it is impossible to determine fluorine using its atomic spectrum since its main resonance line is located a t 95 nm. As a method of determination of fluorine by atomic absorption, indirect spectrometry using Zr and Mg atomic absorption was reported by Bond e t al. (1). However, this method is rather insensitive and inconvenient for practical use. On the other hand, molecular emission spectra of some metal monofluorides have long been available for fluorine determination, e.g., CaF, SrF, and BaF in a d.c. arc ( 2 )or in flames ( 3 ) . Recently, Haraguchi and Fuwa ( 4 ) observed aluminum monofluoride, or AlF radical in ai-acetylene flame, which gives a strong absorption spectrum with a linelike shape and a peak a t 227.5 nm in the ultraviolet region. T h e present paper demonstrates that the absorption spectrum of A1F both in flames and in the high temperature cuvette of the carbon rod furnace is useful for the determination of fluorine. The latter is particularly sensitive for both inorganic and organic forms, and it can be applied for the determination of subnanograms of fluorine in the biological samples ( 5 ) . EXPERIMENTAL Apparatus. For the measurement of the absorption spectra, an atomic absorption spectrophotometer, AA-1, Mark-11, from
Nippon Jarrell-Ash Co., Ltd. was used. For the sample dispersing devices, a 10-cm slot burner was used for the air-acetylene and air-hydrogen flames, a 5-cm slot burner for the nitrous oxideacetylene flame, and a carbon rod furnace (Nippon-Jarrell Ash FLA-100) for the high temperature cuvette. As the continuous light source, a deuterium lamp of either the hollow cathode or the thermal cathode type from Hamamatsu TV Co., Japan, was used. Spectral band width of the spectrophotometer was 0.03 to 0.16 nm. and 0.08 nm was used unless otherwise defined. Reagents. All the reagents used were of analytical grade purchased from Wako Pure Chemicals. Fluorine standard solution was prepared by dissolving ammonium or sodium fluoride in distilled water. All the metal ions investigated were in the form of nitrates. Sodium monofluoroacetate, trifluoroacetic acid, and ofluorobenzoic acid were used as organofluorine compounds. These reagents were accurately weighed and dissolved in distilled water. Procedure. The absorption spectrum of A1F was observed by aspirating the mixed solution of ammonium fluoride and aluminum nitrate into the flame at the rate of 3 mL/min. The fundamental conditions of the flames and the concentrations of the solutions are shown in Table I. The measuring procedure with the high temperature cuvette is summarized in Table 11. In practical measurements, the appropriate amount of nickel and strontium was added to the aluminum solution to increase both the sensitivity and precision of analysis. In order to obtain the absorption spectrum of AlF’in the flame, a wavelength scan was performed. For the high temperature cuvette, however, the absorption of AIF was measured at fixed wavelengths at intervals of 0.1-0.2 nm. In practical measurements, the A1F absorption peak at 227.5 nm was used for fluorine determination. In case it was necessary, the background intensity at 228.1 nm was subtracted from that at 227.5 nm. Preparation of Biological Samples. The Standard Reference Material of Orchard Leaves from the National Bureau of Standards (SRM No. 1571) was chosen and prepared as an example for biological samples. A proper amount of powdered and dried (90 “ C ) sample was precisely weighed and ashed with 0.1 g NaZCO3in a porcelain crucible at 550 “C for 10 h. The ash was dissolved in hot water and neutralized with 1 N nitric acid. The solution was diluted to a fixed volume. After the precipitate settled, the fluorine in the supernatant was measured according to the method mentioned above. RESULTS A N D DISCIJSSION A l F Absorption in Flames. The molecular absorption spectra of AlF in the air-acetylene and NzO-acetylene flames are shown in Figure 1. In the air-hydrogen flame, the absorption spectrum of A1F observed was the same as that in the air-acetylene flame. In N20-acetylene flame, the A1 atom is generated, which gives a spectrum near that of All’, i.e., 226.9 nm and 226.3 nm, ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
2035
Table I. Conditions for AlF Measurement in Flames Burner height, cm, from surface
Flow rate, L/min Fuel Oxidant
a
Flame 5 Air-hydrogen 2 Air-acetylene 6 Nitrous oxide-acetylene Best A1F signal was found at an F/A1 ratio of 3.
Aluminum addition to sample 8 0.5 Small amount' 8 0.5 Small amount' 8 0.8 Large excessb 0.8 M A1 solution was found to give the highest signal.
Table 11. Procedure for AIF Measurement with Carbon Rod Furnace Application of Aluminum Solutiona (0.01 M, 1 0 pL) i Dry-I (20 A, 10 s )
"I ,'\ AI I 226 3 I ,
i
stop i
Application of Fluorine Solution or Sample (5P L ) .1
Dry-I1
(20 A, 1 0 s )
4
Ash
(40 A, 30 s )
1
Atomization and Measurement (280 A, 7 s) ' Al(NO,), was dissolved in water. For actual analysis, 0.005 M of both Ni(NO,), and Sr(NO,), were added to this solution. A
01
B
AI I
1269
O0
5 2260
2270
228 0
229 0
Wavelength nm Figure 2. Molecular absorption spectrum of AIF in carbon rod furnace. Dashed line (- - -) is the background spectrum obtained with 5 pL of 0.01 M AI(NO,), solution, and solid line (-) is the spectrum obtained with 4.5 ng F (5 pL of 0.9 pg F/mL NHIF solutlon) added to the dried background solution
Figure 1. Molecular absorption spectra of AIF in flames, (A) in air-C,H, flame and (B) in N20-C2H2 flame. Solutions used are those of AI(NO& and NHIF and their concentrations and combinations are indicated for each spectrum in the figures by the arrows
while the atomic spectrum of A1 could not be observed in the air-acetylene flame, in which the temperature is 300 degrees lower than in the former flame. T h e dependence of the absorption intensity of the A1F spectrum on the flow rate of acetylene and the height of the observed area of the N20-acetylene and air-acetylene flames was investigated. In the N20-acetylene flame, absorption of AlF increased towards the acetylene-rich and at the red feather region (C*H*/N20flow rate ratio = 0.75; burner height 8 mm) where A1 atomic absorption was also maximal. In the airacetylene flame, the A1F band increases in the lower part of the flame independently of the acetylene flow rate. The background absorption observed when a simple. aluminum solution was nebulized into the air-acetylene flame is higher than that in the N20-acetylene flame as shown in Figure 1. The optimum concentrations of aluminum ion to give the highest AlF band were examined. In the N20-acetylene flame, an excess aluminum concentration in sample solution, Le., 0.8 M Al for the range of 0 to 0.2 M F, gave a more intense signal, whereas in the air-acetylene flame, a n equivalent to fluoride or an even less amount of aluminum seemed to give a higher 2036
ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
signal. In other words, a proper aluminum concentration should be pre-fixed for each determination. Comparing the results of A1F absorption obtained in N20-acetylene, air-acetylene, and air-hydrogen flames, the N20-acetylene flame seems to be preferable for the flame determination of fluorine in terms of the sensitivity and of the smaller background level. The sensitivity in the N20acetylene flame, 24 pg F/mL, is 20 times better than that in lower temperature flames (Table V). The effects of various cations and anions in the sample solution have been surveyed. Concomitants of Li, Na, K, Mg, Ca, Sr, and Mn a t equal concentration to fluorine do not cause any effect on A1F absorption. On the other hand, when the sulfate or chloride is used as the aluminum salt, the suppressions to 51% and 67 7'0 in A1F absorption are observed, respectively, comparing to the nitrate. Fifty percent ethanol also suppresses the A1F signal to about 28%. A1F Absorption in the Carbon Rod Furnace. The spectrum ascribed to AlF observed in the cuvette of the carbon rod furnace is shown in Figure 2, where 5 pL of 0.01 M aluminum and 5 p L of 0.9 Wg F/mL fluorine solution were used. The sharp absorption peak of A1F a t 227.5 nm as well as Al atomic absorptions a t 226.9 and 226.3 nm are recorded. The small background peak observed a t 227.5 nm from the aluminum solution alone may be due to fluorine contamination in Al salt and/or the material of the carbon rod. These spectra of AlF and A1 are superimposed on the broad absorption due to A10. Figure 3 shows the dependence of AlF absorption on
OZOi
t ----
1 -
0
01
04
02
08 m m Slit Widlh
Figure 3. Effect of slit width on the intensity of AIF absorption. 1.8 ng F (5 pL of 0.36 pg F/mL NH4F solution) is added to the carbon rod furnace on which 10 pL of 0.01 M Ai(N03), solution were dried. Slit width 0.1 mm corresponds to 0.16 nm of spectral band width
/
: ’,
I’
IO0
300
200
P-OMIZ:+i.
Cb?REIIT
’:
Figure 5. Effect of atomization temperature (or “atomizing current” 5 fiL of (atomizing current 200(A) corresponds to 2400 “C)). (--A--) 0.9 pg F/mL of NH4F -I- 5 pL of 0.05 M AI(N03)3. 5 pL of 0.05 M AI(NO,), only. (-0-) difference of the above two values -a.()
n
-0
J
--
._.-
r-
-.,., *A’
0
0.0 3
0.02
001
CONCENTRATION OF A L U M I N I U M I
20
GShIYG
40
60 C’JGREYT ( A )
,
BO
100
Flgure 4. Effect of preheating temperature (or “ashing current” (ashing current 70(A) corresponds to 1000 O C ) ) on the intensity of AiF absorption. (--A--)5 pL of 9 pg F/mL of NH4F 5 pL of 0.05 M A(N03)3. 5 pL of 0.05 M AI(N03)3 only. (--O-) difference of the above two values
+
[ M
,
004
0.05
Flgure 6. Effect of amount of aluminum on AiF absorption. 5 fiL of aluminum solution of varied concentrations were applied and dried on the carbon rod furnace before the addition of 5 pL of 0.9 pg F/mL of NH,F solution. (--A-) F -I-Ai. AI only. (-0-) difference of the two, or the net AIF absorption (.-.-e)
-e .()
the slit width of the spectrophotometer. The absorption intensity decreases with the slit width, as expected. The effects of the “ashing” (The power supply to the furnace has three steps, “dry”, “ash”, and “atomize”. Therefore, “ashing” has been used as the preheating step even when the sample does not contain any organic materials.) and atomizing temperatures of the carbon rod furnace to the A1F absorption intensity were investigated in Figures 4 and 5. The absorption of A1F is found independent of ashing current up to 70 A or lo00 “C, over which the absorption signal is steeply decreased as is seen in Figure 4. When the atomizing current reaches 200 A, which corresponds to 2400 “C, the net AlF absorption signal becomes maximum and levels off thereafter as is seen in Figure 5 . This temperature also gives the maximum atomic absorption of Al. This fact suggests that the prior production of A1 atoms is indispensable for the formation of A1F radicals. The dependence of AlF absorption on the amount of A1 added is shown in Figure 6. The background-corrected AlF absorption increases with A1 concentration up to 0.01 M, above which the intensity of ALF absorption becomes constant. Therefore, addition of 0.02 M AI solution has been used for the analysis of up to 1ppm of fluorine. Figure 7 shows the effects of various metal cations coexistent in the fluorine solution on A1F absorption. In the figure, the absorption signal a t 228.1 nm was measured as the background. The net absorption (A227.5-A228.1) ascribed to the
ZOO]
-
w.
I
,
None
,
t i Na K Cu Mg Ca Sr Mn Fe Ni Co
Flgure 7. Effects of cations on AIF absorption in the carbon rod furnace. Sample solution contains 0.36 fig of F/mL and 0.01 M of each cation. T h e upper levels are the intensities at AIF peak of 227.5 nm and t h e lower levels are the background intensities at 228.1 nm. Figures are relative absorptions compared with no cation added taken as 100 control solution of added “none” is set IDthe relative intensity of 100. All the metal ions tested except Li’ enhance the AlF absorption. Notably, Co2+and Sr2+have a strong enhancing effect, whereas Co2+,Ni2+,and Fe3+ decrease the background absorption. Therefore, for the practical determination of ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
2037
Table IV. Reproducibility of Fluorine Determination
Table 111. Elimination of Cations Interferences by Addition of Ni and S P Relative Cation absorbance None added 100 Li 92
by A1F Absorptiona
Re1 std
Re1 std
Fluorine, Absorbance dev, Absorbance dev, ng (SBW: 0 . 0 3 n m ) % (SBW: 0 . 0 8 n m ) 5%
+
Na+ K' MgZ+
Ca2+ Sr2 Mn2+ Fe3+ +
co2
+
92 90 96 98 96 105 92 105
Blankb 0.214 0.536 1.072
0.061 i 0.002 0.117 F 0.003 0.185 r- 0.004 0.279 t 0.012
3
0.053
3 2 4
0.081 0.121 0.177
*
0.001
2
0.002
3
t t
0.003 0.003
3 2
i
0.01 M aluminum solution, containing Ni and Sr
0.01 M Al(NO,), solution containing 0.005 M of both Ni(NO,), and Sr(NO,), is used; 5 p L of sample solution containing 0.36 pgF/mL and 0.01 M of each cation is examined.
(0.005 M each) was used; 6 measurements at each of the The blank is due to fluorine amounts were performed. the residual fluorine in the aluminum salt and/or in the carbon rod furnace besides the background absorption, due to AI0 molecules. The other readings include the
blank.
C O N C E N T R A T I O N OF A C I D Figure 8. Interference of acids on AIF absorption. (-0-) (-A-) H3P04,(-A-) HCI, and (-0-) HNO,
H,SO,,
fluorine, the addition of some other metal ions is expected to be useful in the viewpoint of both increasing the sensitivity and decreasing the background. Although the addition of cobaltous ion, seems to be the best from the intensity chart shown in Figure 7 , the atomic line of cobalt at 227.5 nm interferes with the A1F absorption. Therefore, the addition of Sr2+and Ni2+to the aluminum solution has been selected and tested for the purpose of increasing the sensitivity by the former ion and decreasing the background by the latter ion. As a result, a substantial increase in sensitivity was actually found (Table V). Furthermore, additims of Ni2+ a c d Sr2+ to the sample act as a spectral buffer for eliminating the influence of concomitant ions, as is shown in Table 111. The interferences of HC1, H2S04,"OB, and H3P04in AlF absorption were surveyed as shown in Figure 8. Although strong suppression was found when a simple A1 solution was used, the addition of Ni2+and Sr2+was also found effective for decreasing these effects. More than N of HC1, H3P04, and H2S04, however, did suppress the A1F absorption even if the Ni2+and Sr2+were present. HNOBcauses the smallest decrease. Table 1V shows the reproducibility of measurements for the various concentrations of fluorine standard solution, where 2-4% relative standard deviation was obtained in terms of t h e 0.03 and 0.08 nm spectral band width. These values of relative standard deviation are comparable to those of atomic absorption spectrometry in a graphite furnace. The 0.03 nm of spectral band width gives higher sensitivity than 0.08 nm. A 3% precision was found for 0.214 ng F, and an example of the actual chart of repeated fluorine measurements is shown in Figure 9. 2038
ANALYTICAL CHEMISTRY, VOL. 49, NO. 13,NQVEMBER 1977
, ~
Flgwe 9. Chart profile of
repeated AIF measurement. 0.01 M aluminum soiution, containing Ni and Sr (0.005 M each) was used. 5 WL of 0.107 Kg/mL fluorine standard solution was applied
10'
1 aConcentration o f Fluorine
I
10
P Q / rnl
Figure 10. Calibrations of fluorine by AIF absorption in t h e carbon rod furnace. 0.01 M aluminum solution, containing Ni and Sr (0.005 M each) was applied on the furnace and dried, before 5 p l of a series of fluoride
standard, NaF, is determined. Calibrations were attempted at three band widths. The blanks were subtracted, and the net absorbances were plotted. (-0-) 0.03 nm. (-m-) 0.08 nm. (-A-) 0.16 nrn Figure 10 shows the calibration curves, obtained when the spectral band width was adjusted to 0.03,0.08, and 0.16 nm. By means of extending the slit width, the sensitivity of A1F absorption is decreased, the concentration ranges are extended, and as a result the higher concentration of fluorine becomes measurable. However, each calibration curve is bent at the
Table V. Sensitivities of Fluorine Determination by A1F Absorption 1%Absorp-
tion
Method Flame Carbon rod furnace
Air-H, Air-C, H, N,O-C,H, None Added
400 Mg/mL 400 pg/mL 24 pg/mL 0.085 ng
+ N i and Sr
0.021 ng
Table VI. Determination of Fluorine in Orchard Leaves (SRM 1571 of NBS) and Organofluorine Compoundsa Fluorine
Found, vg/g ( d w ) Orchard Leaves
3.8 3.3
Found, Compound
ng
ng
Sodium fluoroacetate
0.68
o-Fluorobenzoic acid Trifluoroacetic acid
0.63
0.56
t t +_
Calculated,
0.03 0.03
0.62
0.03
0.52
0.59
a Compounds were weighed, dissolved in distilled water, and an aliquot was analyzed. Aqueous solution of sodium fluoride was used as the standard.
higher concentration region of fluorine and the linearities of the calibration curves are maintained up to approximately 0.08 and 0.12 ppm for the spectral band width of 0.03 and 0.08 nm, respectively. T h e slower formation rate of the diatomic molecule in the furnace seems to be one of the causes of decreasing of linearity in the calibration curve a t the higher fluorine concentration. Table V summarizes the sensitivities of fluorine determination adopted in this paper, expressed either as kg/mL or ng fluorine, which gives 1%absorption. The most sensitive value of 0.021 ng of fluorine from the carbon rod furnace with the addition of Ni2+ and Sr2+is the best of those so far re-
ported in spectrochemical methods for this element. As examples of the actual application of the present method, the contents of fluorine in biological standard material of Orchard Leaves which is the Standard Reference Material, 1571 of National Bureau of Standards, and in organofluorine compounds of the commercial grade, i.e., sodium monofluoroacetate, trifluoroacetic acid, and o-fluorobenzoic acid, have been measured. The results are shown in Table VI. The analytical conditions were the same a3 those mentioned in the previous sections. The values of fluorine obtained for Orchard Leaves were in good agreement with that recommended by the National Bureau of Standards, and those found for organofluorine compounds are all within 10% deviation from those calculated. These results show that the present method can be applicable to the determination of both inorganic and organic fluorine. Recently, organofluorine compounds such as Freon, etc. have been recognized as pollutants, and their toxicities to bioorganisms are serious as an environmental problem. Conversely, these compounds are difficult to decompose, and suitable and simple analytical methods for fluorine in these materials are not available. The ion-selective electrode is sensitive only to ionic fluorine and not to the organic compounds. The present method of “fluorine molecular absorption spectrometry” may be most useful for total fluorine determination. The characteristics of the present method, such as rapidity, sensitivity, and accuracy, seem to be sufficient as the determinant and prominent method for the quantification of subnanogram of fluorine. ACKNOWLEDGMENT The authors are deeply indebted to H. Haraguchi of the National Institute for Environmental Studies and K. Notsu of Tsukuba University for their valuable discussions. LITERATURE CTTED A . M. Bond and T. A. O’Donnell, Anal. Chem., 40, 560 (1968). K. Fuwa, J . Chem. SOC.Jpn., Pure Chem. Sect., 7614 (1955). E. Gutsche and R. Hemrrann, Fresenius’ 2.Anal. Chem., 269, 260 (1974). H. Haraguchi and K. Fuwa, Spectrochim. Acta, Part B , 30, 535 (1975). (5) K. Tsunoda et al., 25th Annual Meeting 01 Japan SOC. for Anal. Chem., Oct. IO, 1976.
(1) (2) (3) (4)
RECEIVED for review March 14,1977. Accepted August 1,1977. This work is supported by the Government Grant-in-Aid No. 120504, No. 112004, and US-Japan Clooperative Project No. 6R023.
ANALYTICAL CHEMISTRY, VOL. 49, NO. 13, NOVEMBER 1977
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