apparatus described is capable of use with solid, liquid, or gel type samples. Calorimetric techniques have been reported for triplet formation quantum yields and photochemical reaction studies ( 5 ) ,and the applications of OAS to obtain not only spectral data but also information about transient excited states for solid and solution samples should be of growing interest.
1
LITERATURE CITED (1) W. R. Harshbarger and M. B. Robin, ACC. Chem. Res., 6 , 329 (1973).
30
~
C
2
L
6
I
1 -
8
10 x
~
~
Concn Chloride I o n
Figure 2. Effect of chloride ion Concentration on the optoacoustic signal magnitude for a 5 X M quinine bisulfate solution at p H 1
in Figure 1. By this technique, a value for Q for quinine bisulfate (in 0.1 N sulfuric acid) over the concentration range to lo-' M was found to be 0.53 0.02. This value is in good agreement with the accepted literature value of 0.51 (7).
*
CONCLUSION OAS provides a rapid and accurate technique for the determination of absolute quantum efficiencies of solutions; the
(2) A. Rosencwaig, Anal. Chem., 47, 592A (1975). (3) M. J. Adams. A. A. King, and G. F. Kirkbright, Ana/yst(London), 101, 73 (1976). (4) M. J. Adams, B. C. Beadle, A. A. King, and G. F. Kirkbright, Analyst (London), 101, 553 (1976). (5) J. B. Callis, J . Res. Natl. Bur. Stand., Sect. A , 80 (3), 413 (1976). J (6) ~ W . Lahmann and H. J. Ludewig, Chem. Phys. Lett., 45, 177 (1977). (7) W . H. Melhuish, J . Phys. Chem., 6 5 , 229 (1961). (8) M. J. Adams, B. C. Beadle, and G. F. Kirkbright, Ana/yst(London). 102, 569 (1977). (9) E. Lette and W. West, Proc. R . Soc. (London), Ser. A , 121,294 (1928). (10)J. Eisenbrand and M. Raisd, Fresenius' Z. Anal. Chem., 179, 352 (1961). (11) A. Rosencwaig and A. Gersho, J. Appl. Phys., 47, 64 (1976). (12) W. H. Melhuish, J . Phys. Chem., 64, 762 (1960).
for review May 31, Accepted 25r We are grateful to the Science Research Council for the provision of a Fellowship t o one of us (M.J.A.) and also for a studentship to J.G.H. under the CASE award scheme in cooperation with BP Research Ltd.
Determination of Boron Isotope Ratios by Atomic Absorption Spectrometry Peter Hannaford' and R. Martin Lowe CSIRO, Division of Chemical Physics, P.O. Box 160, Clayton, Victoria, Australia 3168
Isotopic analysis of boron by atomic absorption methods is possible if neon-filled (but not argon-, krypton- or xenon-filled) discharge lamps are used as the source of boron resonance lines. For accurate measurement of the isotope ratio, it is advantageous to use enrkhed isotope sources and a sharpiine absorber, such as a water-cooled sputtering cell, and to make the absorption measurements on the 2088.912089.6 A doublet, which has a considerably larger Isotope shift than the main 2496312497.7-A resonance-line doublet. The 'OB abundance of a sample of natural boron, determined with a sputtering absorption cell, was found to be 20.0 f 0.2%. A legs accurate determinatlon of boron Isotope ratios is possible using a nkrous oxide-acetylene flame absorption cell with the enrkhed Isotope sources. A very slmple, approximate method, which uses a commerclai natural-boron source and the nitrous oxideacetylene flame, is also described.
T h e isotopic composition of elements can in some cases be determined by spectroscopic techniques, such as atomic emission or atomic absorption. The atomic absorption technique, which was suggested some time ago as a possible method of isotopic analysis ( I ) ,is applicable to those elements for which the isotope shifts of the resonance lines are comparable to or greater than the widths of the emission- and absorption-line profiles defined by the experimental condi1852
*
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
tions. For conventional atomic absorption measurements, in which uncooled sources and uncooled absorption cells are used, the elements having suitably large isotope shifts are: (i) the very light elements, for which the large relative mass difference of the isotopes can result in large isotopic mass shifts, and (ii) the heavy elements, for which differences in the nuclear charge distribution of the isotopes can result in large isotopic field shifts. Thus, to date, the only successful reported determinations of isotope ratios by atomic absorption (or fluorescence) measurements have been for lithium (2-6), mercury (7), lead (8, 9) and uranium ( I O , 11). After lithium, the next light element having a natural isotope is boron, for which the natural abundance is usually 20% loB to 80% llB. The determination of boron isotope ratios is of interest for a number of reasons: (i) the natural abundance of the isotopes is known to be variable (12), (ii) the large relative mass difference of the isotopes can lead to separation during the manufacture of boron compounds, and (iii) 'OB-enriched materials, which are commonly used as neutron absorbers in reactor technology, can become depleted in O ' B during exposure to high fluxes of neutrons. For the main boron resonance line, 2497.7 A, the isotope shift (of the IIB component relative to log)is C0.0085 A (13),which is comparable to the typical widths of emission lines from hollow-cathode lamps ( 1 4 ) . This suggests that boron should be a suitable element for isotopic analysis by atomic absorption methods. In an attempt to determine boron isotope ratios
T
700 . .K ( b ) 35C K ( c ) IOOK (2)
I ~~
11
"r
67
208889
91
Figure 1. Isotopic splitting, fine structure splitting, and computed profiles of the boron doublets. The horizontal bars represent the total widths of the hyperfine structures due to nuclear-spin splitting
by atomic absorption, Goleb (15) used water-cooled hollow-cathode sources, containing the pure isotopes O ' B and "B, with a water-cooled sputtering absorption tube containing boron of natural abundance. However, rather surprisingly, no difference could be detected in the measured absorption when the two sources were interchanged. Recently it has been shown (16)that, for boron hollow-cathode lamps filled with argon, krypton, or xenon, an unusual excitation mechanism occurs which results in the emission of intense boron lines with extremely large Doppler widths, of the order of 0.15 8, (FWHM). The occurrence of this unexpected line-broadening accounts for the lack of sensitivity in Goleb's experiments, in which lamps filled with krypton were used. The abnormal line-broadening, which occurs for emission lines only, is negligible in neon- and helium-filled lamps (16). This paper reports the successful determination of boron isotope ratios by atomic absorption, using neon-filled sources and both sputtering and flame absorption cells. Measurements have been made on the main 2496.8/2497.7-8, (2s22p 2P1,2,3/2 - 2s23s 2S1,2) resonance-line doublet and also on a second resonance-line doublet a t 2088.9/2089.6 A (2s22p 2P1,2,3,2 - 2s2p2 2D),which has a considerably larger isotope shift (-0.025 8, (17)). Although the absorption for the 2088.9/2089.6-8, doublet is only about half t h a t for 2496.8/2497.7 8, (f249a = 0.087, f2089 = 0.045 (18)),the markedly better resolution of the isotopic components of the 2088.91 2089.6-A doublet makes it more suitable for isotopic analysis. The large isotope shift of the 2088.9/2089.6-A doublet is due to a relatively large contribution of the specific mass shift of the excited 2D state (17). T h e specific mass shift for the excited state is zero and the isotope shift of the 2496.81 2497.7-A doublet is determined mainly by the small positive contribution of the normal mass shift of the ?Sstate.
THEORY Isotopic a n d Fine S t r u c t u r e Components. Figure 1 shows the isotopic and fine structure components (vertical bars) of the 2496.8/2497.7-A and 2088.9/2089.6-A doublets for the case of naturally-occurring boron. For both doublets, t h e hyperfine structure due to nuclear-spin splitting is very small compared with the isotope shifts and fine structure
splitting, and only the total widths of the hyperfine splittings are indicated in Figure 1. The fine structure splitting in the case of the 2D excited state of the 2088.9/2089.6-A doublet is only 0.014 A (In,which is comparable to the isotope shifts of the lines. The values of the isotope shifts are taken from Burke (13)and Edl6n et al. ( I n ,and the hyperfine splitting data for the 'P states from Harvey et al. (19) and from Lew and Title (20). The hyperfine splitting for the 'Sl,2 state was estimated from the known splittings of the first 2S1/2states of other Group 3 elements, and the hyperfine splitting for the 'D states was assumed to be zero. The relative intensities of the various fine structure and hyperfine structure components are taken from the theoretical values tabulated by White and Eliason (21). The profiles shown in Figure 1 were computed by summing Gaussian distributions corresponding to each of the individual components of the lines (using Equation 2), assuming vapor temperatures of (a) 700 K, (b) 350 K, and (c) 100 K. These values are considered to be representative of the temperatures in the uncooled, high-intensity hollow-cathode sources (700 K ) , in the water-cooled hollow-cathode sources and sputtering-type absorption cell (350 K), and in a liquid-nitrogen cooled source or absorption cell (100 K). The profiles in Figure 1 indicate that, for the 2496.81 2497.7-A doublet, the loB and llB components are only partially resolved a t 100 K, whereas for the 2088.9/2089.6-A doublet, the isotopic components are almost completely resolved a t temperatures up to a t least 700 K. T h e o r e t i c a l Calibration Curves. The absorbance of a boron absorption cell, having an effective path length 1, can for either of the boron doublets be expressed as
I(v)is the frequency distribution of the emission line, which for hollow-cathode sources operated under conditions of negligible self-absorption, is assumed to be 10
10
l ( v ) = isz:
cjloe
~ p [ - ( w ~ ' ~ )-t-~(1 * ]-
i s ) c C;' j=1
j= 1
exP[-(w,"
(2)
)S'l
where the summations are over all (ten) fine structure and hyperfine structure components in each doublet for each isotope; the superscripts are used to denote the isotopes O 'B and 'lB, and the subscript S refers to the source. In Equation 2, is is the isotopic abundance of O ' B in the source, C," is the (normalized) peak intensity of the j t h component, and ( w , ' ~ ) ~ is given by (O,l0)s =
[2(v-
vJ10)s/(nv,'o)),]flE2
(3)
where ullo is the center-frequency of the j t h component and ( I v D ' O ) ~ is the Doppler width (FWHM) of the O ' B components. In Equation 1, k ( v ) is the absorption coefficient, which, for t h e general case of combined Doppler and collisional broadening (e.g. in the nitrous oxide-acetylene flame), is assumed to be 10
k ( v ) = h , l o ~c , ' o v [ ( o j ' o ) A ,
a101
+
J=l 10
h (J =p1 q " V[(W,"),,
(4)
all]
V[(U,")~ alO] , is the Voigt function V[(w,'O)A, a ' ' ] =
-.
a10
n
e-Y2dy
m
I -m
(a1')*
+
-
[(W,")A-y]'
ANALYTICAL CHEMISTRY, VOL 49, NO. 12, OCTOBER 1977
(5) 1853
where
and
koio = [ ~ / ( A v , ~ ~ )~~( ]n e Z / r n , c ) ~ N ' o(8) f which is the peak-absorption coefficient for an unsplit, Doppler-broadened O ' B absorption line. In Equations 6-8, NIO is the number density of ground-state loBatoms, f is the total oscillator strength of the boron doublet, AvL is the collisional width (FWHM), and 3 u s is the collisional shift. In most of the calculations for the nitrous oxide-acetylene flame, Avs and AuL are assumed t o be related (22) by
I 0
500 DOPPLER
A v S = -0.36 A v ,
(9)
which is applicable for a simple van der Waals interaction (23). A computer program (BORIS) was developed to compute the theoretical absorbance A (Equation 1) as a function of the fraction of '% in the absorption cell, N'o/(N1O + N"),for given values of the various parameters. The Whiting approximation (24) t o the Voigt function was used. T h e theoretical absorbances were used t o obtain theoretical calibration curves of the fractional absorbance ratio A(lo)/A(lO'+ A"") vs. N1O/(Nl0 N"), where A(1oiand A("' represent the absorbances for '%-enriched and l'B-enriched sources, respectively. For moderately low absorption levels, the quantity A('''/ (A(1o' + A''1)) is essentially independent of N f l , where N = N'O + N". As the computed calibration curves are almost linear (for moderately low absorption levels), it is convenient to describe the sensitivity of a particular curve to changes in isotopic composition by the gradient
1500
IO00 TEMPERATURE
( O K
2000 )
Figure 2. Effect of Doppler temperature on the gradient of the computed calibration curves for the two boron doublets. Equal Doppler temperatures are assumed for the source and absorber
+
(10)
m = y i -yo
where y1 and y o are the values of A'lo'/(A'lo' + A'"') a t N'O/(N'O N") = 1.0 and 0.0, respectively. Thus m = 1.0 indicates maximum isotopic sensitivity (perfect resolution of the isotopic components), whereas m = 0 indicates zero sensitivity. In Figure 2, the gradients m of computed calibration curves for the two boron doublets are plotted as a function of Doppler temperature for the idealized case of equal Doppler broadening in the source and absorber, zero collisional broadening (a = 0), pure loB and llB sources, and low absorption levels. T h e calculations clearly illustrate the superiority of the 2088.9/2089.6-A doublet for isotopic analysis: the isotopic sensitivity should be very good for this doublet over a wide range of temperatures, but rather poor for the 2496.8/2497.7-A doublet a t temperatures much above room temperature. When only a single source is used, the calibration curve is plotted as the absorbance A vs. NIO/ (N'O + N1') for a constant total boron concentration. In this case it is convenient t o describe the isotopic sensitivity in terms of a reduced gradient
+
m ' = (Ai -Ao)/(Ai + Ao)
(11)
where AI and A. are the values of the absorbance a t Nio/(N1o N") = 1.0 and 0.0, respectively. The A vs. N"/(N" + N") calibration curve has more curvature than A'1o'/(A'"' + A'"') vs. N"'/(N'O W1),and m'shows some sensitivity to large changes in Nfl.
+
+
EXPERIMENTAL Light Sources. The sources used for most of the atomic absorption measurements were two uncooled, high-intensity 'B hollow-cathode lamps ( 2 5 ) ,which had cathodes enriched in O and "B, respectively. The cathodes were prepared by pressing 1854
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
L
-> -
I
r+
RECORDER
Figure 3. Schematic diagram of the double-beam system used for the absorption measurements and sintering powders of isotope-enriched crystalline boron. The boron isotopes, which were obtained from Oak Ridge National Laboratory, contained 94.89% O ' B and 97.15% "B, respectively. The lamps were filled with neon to 10 Torr. The hollow-cathode discharges were operated from current-regulated, square-wave modulated power supplies at 25 mA average current, and the secondary (booster) discharges from current-regulated dc supplies at approximately 300 mA. With the lamps operating under these conditions, the measured absorptions were found to be independent of the hollow-cathode discharge current, and the emission intensities of the two lines of each boron doublet were always very close t o the theoretical ratio of 2:l. These results indicate that the boron resonance lines emitted by the high-intensity hollow-cathode lamps were essentially free from self-absorption effects. The natural-boron source was a standard Varian-Techtron boron hollow-cathode lamp (neon-filled), operated at 10 mA average current. Optics. Figure 3 is a schematic diagram of the apparatus used for the double-beam absorption measurements. The O ' B and "B sources were operated from separate power supplies at different modulation frequencies. Their light output was combined t o a single beam which was focused through the absorption cell and onto the entrance slit of the monochromator (Varian-Techtron 0.5-m focal length, spectral bandpass 3 A). The output signal from the photomultiplier was fed to two synchronous amplifiers, each of which was tuned to one of the source-modulation frequencies. This detection system allowed simultaneous recordings of the absorptions of the resonance lines from the two isotope sources and hence the measured ratio of absorbances was independent of any variations in the rate of vaporization. The beam-combining device was a thin, polished quartz plate which was positioned so that one beam could be reflected from its surface to coincide exactly with the second beam which was transmitted by the plate (see Figure 3). A high angle of incidence (75') was used to increase
the intensity of the reflected beam. This type of beam-combiner was found to be more efficient at 2089 8, than semi-aluminized mirrors. For measurements with the sputtering absorption cell, a small correction for background attenuation of the incident beam was required. A deuterium lamp was mounted on the opposite side ' B lamp and the background attenuation of the optical axis to the O determined sequentially by rotating the quartz plate to focus the continuum radiation through the absorption cell. The background attenuation is the result of scatter from small particles which are formed by the aggregation of sputtered metal vapor near the cathode surface. Sputtering Absorption Cell. In order t o minimize spectral overlap of the isotopic components of the absorption lines, most of the ratio measurements were made with a sputtering absorption cell which operated with water-cooled specimens at a pressure of typically 5 Torr of argon. At this pressure, collisional broadening effects are negligible and the absorption line profiles are determined mainly by Doppler broadening, corresponding to the average temperature of the sputtered vapor (approximately 350 K). The sputtering cell was of the hype described in detail by Gough (26) for operation with a continuous flow of argon gas. It is designed to allow the easy interchange of flat specimens which seal onto the sputtering chamber with an O-ring. The inlet for the argon flow is immediately adjacent to the cathode surface and produces a fast flow of gas in the surface region. The fast flow has the advantage that: (i) it rapidly sweeps away any gaseous impurities released during sputtering. and (ii) it greatly reduces lateral and back diffusion of the sputtered vapor with the result that the boron atom density within the cell is greatly enhanced. The cell was operated with an argon flow rate of approximately 0.3 L/min. The sputtering current was 50 mA dc. The sputtering voltage varied slightly from specimen to specimen but was typically 600 V. Calibration samples containing known ratios of the two boron isotopes were prepared in the form of flat pellets. The pure enriched isotopes were ground to -325 mesh (43 pm) and the required amounts of each isotope were dried, weighed, and thoroughly mixed. The isotope mixture was then diluted in copper powder (-400 mesh, 37 wm) so that the final samples contained a copper to boron atom ratio of 21. The copper-boron mixture was pressed (400 MPa) into flat pellets, which were set into brass blocks to facilitate sealing to the sputtering chamber. These pressed pellets contained appreciable amounts of trapped gases, the release of which during sputtering increased the time taken to reach a steady sputtering rate. Typical equilibration times for these pressed samples were 10-15 min, compared with 1-8 min reported (26) for solid metallic samples. Flame Absorption Cell. Meawrements of boron isotope ratios were also made using a fuel-rich. nitrous oxide-acetylene flame as the absorption cell. The flame was produced with a Varian-Techtron burner and spray chamber. Standard samples containing known values of the isotope ratio in solution were prepared by weighing the required amounts of the pure boron isotopes for each mixture and dissolving in warm concentrated nitric acid. The dissolution process was very slow and in some cases took up to 5 days. The solutions were diluted with de-ionized water to give total boron concentrations of 2 g/L. RESULTS AND DISCUSSION At the monochromator bandpass used for the absorption measurements (3 A), the fine structure components of the respective boron doublets were not resolved, and for convenience the doublets are hereafter referred to as single lines, Le., 2498 A and 2089 A. S p u t t e r i n g Absorption Cell. The measured absorptions in the sputtering cell were fairly low and scale expansion ( x 5 for 2089 A, X 2 for 2498 A) was used. T h e results are presented by plotting the measured fractional absorbance ratio, A"o'/(A'lo' + A'"'), vs. the known fraction of loB atoms in the sample, N"/(N" + N " ) . In the ideal case, with completely resolved isotopic components, 100% pure isotope sources and low absorption levels, this plot would be very close t o a straight line with a gradient of m = 1 (Equation 10).
0
02 ISOTOPIC
0 4 COMPOSITION
OF ABSORBER
08
06
10
N '0 N"+
Nl1
Figure 4. Calibration curves for the sputtering absorption cell using uncooled high-intensity hollow-cathode sources. The broken lines are computed curves corresponding to a source temperature of 700 K and an absorber temperature of 350 K
Figure 4 shows the calibration curves obtained a t 2089 and 2498 A with the sputtering absorption cell, together with theoretical curves (broken lines) which were computed using the relationships given above (Theory). For the computed curves, it was assumed that the source temperature was 700 K, the absorber temperature was 350 K, and that there was no collisional broadening. T h e experimental curve for 2089 A shows high sensitivity to changes in the isotopic Composition, its gradient, m = 0.88, being fairly close to the ideal value (1.0). The good agreement between the computed and experimental curves indicates that the actual resolution of the isotopic components must be similar to the resolution predicted by the theoretical model (Figure 1). T h e difference between the gradients of the experimental curve and that of the ideal case (rn = 1) is due mainly to the isotopic impurity in the source lamps, Le., 5.11% llB in the 'OBsource and 2.85% O ' B in the IlB source. If 100% pure isotope sources were available, then for the same source and absorber conditions, the calibration curve would have a gradient of about 0.96. For the 2498-A line, the isotope shifts are considerably smaller than for the 2089-b, line and the increased overlap of the isotopic components results in a marked reduction in the slope of the calibration curve (Figure 4). T h e sensitivity of isotope-ratio measurements ( m = 0.33) for this line is thus considerably lower than for the 2089-A line. However, in terms of the precision of an isotope-ratio measurement, the larger measured absorptions a t 2498 A would partially compensate for this loss in isotopic sensitivity. It was shown in Figures 1 and 2 that decreasing the source temperature can cause a significant reduction in the spectral overlap of the isotopic components of the 2496.8/2497.7-A doublet. Some measurements of the isotope ratio were attempted using water-cooled hollow-cathode sources, rather than the high-intensity sources, with the sputtering absorption cell. For a discharge current of 10 mA in the sources, the slope of the 2498-8, calibration curve ( m = 0.38) was slightly greater than in the case of the uncooled high-intensity sources (rn = 0.33). When the discharge current in the water-cooled lamps was increased to 40 mA, the lines were broadened by selfabsorption effects, and the resulting increase in the spectral overlap reduced the sensitivity of the calibration curve to the extent that its slope was similar to that for the uncooled high-intensity sources a t 2498 A. At the lower current (10 mA), the intensity of emission of the 2089-A line was too low to allow useful measurement of isotope ratios, and consequently for the majority of isotope-ratio determinations the uncooled high-intensity sources were preferred. T h e higher operating ANALYTICAL CHEMISTRY, VOL 49, NO 12, OCTOBER 1977
1855
1
lor
P
k V W
a LT
5: 4
0 ISOTOPIC COMPOSITION OF ABSORBER
Calibration curves for the nitrous oxide-acetylene flame absorption cell using uncooled high-intensity sources
temperature of the high-intensity sources results in only a slight decrease in the resolution of the isotopic components of the 2088.9/2089.6-A doublet (see Figure 1). Absorption measurements in the sputtering cell were also used to determine the isotopic compositions of two samples of naturally-occurring boron, one being pure amorphous boron and the other pure crystalline boron. Three accurate calibration samples with isotopic compositions [N"/ (N'O + N")] of 0.15,0.20, and 0.25 were used to obtain a partial calibration curve a t 2089 A for the region of interest, and the naturalboron compositions then determined from a least-squares fit to these calibration points. For the amorphous natural boron, the isotopic composition was found to be N1'/(N'' + N") = 0.200 i 0.002, and for the crystalline natural boron, 0.2015 h 0.002. The reported range of isotopic compositions of natural boron is 0.188-0.203 (12). Flame Absorption Cell. For measurements with the nitrous oxide-acetylene flame, the absorptions were higher than with the sputtering cell and no scale expansion was required. In the flame, where there are high temperatures (= 2880 K (27)) and significant collisional broadening effects, the absorption lines are much broader than in the sputtering cell. Consequently there is more overlap of the isotopic components of the absorption lines and the calibration curves are less sensitive to changes in the isotopic composition. Figure 5 shows the calibration curves obtained with the uncooled high-intensity sources and a nitrous oxide-acetylene flame. For the 2498-A line there is almost complete overlap of the isotopic components and the calibration curve shows little sensitivity to changes in composition ( m = 0.10). However, for the 2089-A line, the calibration curve shows a reasonable degree of sensitivity ( m = 0.61), indicating that boron isotope ratios can readily be determined using a flame absorption cell with this resonance line. It is interesting to note that, in the case of boron 2089-A absorption in the flame, the slope of the isotope-ratio calibration curve can provide some information about the profiles of the absorption lines in the flame. As there is partial overlap of the 2089-A isotopic components in the flame, the extent of this overlap and hence the sensitivity of the isotope-ratio measurements depends on the actual profiles of the absorption lines, particularly in the wings of the lines. Figure 6 shows a series of computed calibration curves of the 2089-A absorbance in the nitrous oxide-acetylene flame for three values of the a-parameter (Equation 7), assuming source temperatures of 700 K, absorber temperatures of 2880 K, and collisional shifts given by Equation 9. T h e a values shown in Figure 6 are actually for the 'OB isotope (a" as defined by Equation 7 ) . T h e points are the same experimental values as are shown in Figure 5 . From these results it can be 1856
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
0.4
0.6
ISOTOPIC COMPOSITION OF ABSORBER
N lo N l 0 + N"
Figure 5.
0.2
0.8
1.0
ti'0 N'O t
N"
Figure 6. Effect of the a-parameter on computed calibration curves
for 2089-A absorption in the nitrous oxide-acetylene flame (source temperature 700 K, absorber temperature 2880 K). The points are the experimental values shown in Figure 5
concluded that, for boron 2089-A absorption lines in the nitrous oxide-acetylene flame, alo is approximately 0.15. This relatively low value is not unexpected considering the low mass of boron and the short wavelength of the line. The collisional width corresponding to an a"-value of 0.15 is 0.10 cm-' (0.005
A). Simple, Approximate Method of Determining Boron Isotope Ratios. For approximate determinations of the isotope ratio, it is possible t o dispense with the dual-beam system and the enriched isotope sources, and to use a single, natural-boron hollow-cathode source with the nitrous oxide-acetylene flame. Although a pure isotope source would give slightly better isotopic sensitivity, natural-boron sources are readily available commercially and, for reasons which are given below, are particularly suitable for this type of measurement. When only a single source is used, the calibration curve is constructed by plotting the measured absorbance A, rather than the ratio A('o'/(A''o' A'"'), as a function of isotopic composition. However, the absorbance A depends on the total boron concentration of the samples as well as their isotopic compositions, and thus in order to determine isotopic compositions using a single source, it is also necessary to have a measure of the total boron content of each sample. Information about the total boron content can be obtained by making atomic absorption measurements on the 2498-A line. The gradient of an isotope-ratio calibration curve depends on the isotopic composition is of the source and, by a fortunate coincidence, this dependence in the case of the 2498-A calibration curve (Figure 7, solid circles) is such that the gradient becomes zero just near is = 0.2, the isotopic composition of natural boron. Consequently, when using a natural-boron source, the 2498-81 calibration curve (for a constant total boron concentration) is found to be flat (Figure 8), and absorption measurements a t this wavelength are independent of the isotopic composition of the samples. The solid lines in Figure 7 show the reduced gradient m' (Equation 11) of theoretical 2498-8, calibration curves computed as a function of is, for several values of Aus, the collisional shift of the absorption line, assuming an a"-value of 0.2 (Curves 1-3), a hollow-cathode source temperature of 400 K (14), an absorber temperature of 2880 K and N f l = 1.5 X 10" cm212.The broken line, for which the m' = 0 intercept occurs at the "symmetrical" value is = 0.5, corresponds to the hypothetical case of equal Doppler widths for the O ' B and "B isotopic components and a zero collisional shift. T h e above calculations indicate that the observed occurrence of the m ' = 0 intercept near is = 0.2, rather than 0.5, results mainly from the presence of a red collisional shift, Aus = 4 . 0 5 cm-' (+0.003
+
0
"
I"
0
0 2
04
06
O S
ISOTOPIC COMPOSITION
I O
OF SOURCE,
is
Figure 7. Effect of isotopic composition of the source (fraction of 'OB) and collisional shift of the absorption line ( I u s ) on the reduced gradient ( m ' ) of computed 2498-A calibration curves (source temperature 700 K, absorber temperature 2880 K, Nf/= 1.5 X 10'' cm-'). The broken line was computed for i u s = 0 and equal Doppler widths for t h e O 'B and "B isotopic components. The points are experimental values obtained using a nitrous oxide-acetylene flame absorption cell 2490;
a
isotopic composition of the samples. In practice it is not necessary for the boron concentrations to be exactly the same, as the effect of small variations in concentration on the 2089-A absorbance can be estimated from the corresponding variations in the 2498-A absorbance. Figure 8 shows experimental calibration curves of absorbance vs. isotopic composition for both the 2089-A and 2498-A lines obtained using a natural-boron hollow-cathode source and a nitrous oxide-acetylene flame. Each sample was made up to the same concentration of total boron. The reduced gradient m'for the 2089-curve is 4 . 4 2 (compared with a value of -0.59 obtained using a "B-enriched source). To illustrate the level of accuracy of the simple single-source method, the isotopic composition of natural boron was determined for a (N'O + N") = 0.20 number of samples and found to be .V0/ f 0.03. The single-source method, although not as accurate as the method which uses enriched boron sources and a sputtering absorption cell, is very convenient and can be used with a conventional atomic absorption spectrophotometer and a commercially-available, boron hollow-cathode lamp. It may prove useful, for example, in monitoring the O ' B content of 'OB-enriched materials used as neutron absorbers in nuclear reactors. In this case the variations in '9 content can be large and a high degree of accuracy is not always required. Finally, it is interesting to note that, by using the main 24962312497.7-A resonance-line doublet for absorption measurements in a nitrous oxide-acetylene flame, it is possible to make accurate chemical analyses of boron which are independent of the isotopic composition of the samples.
ACKNOWLEDGMENT
03t
The authors thank E. S. Pilkington, CSIRO Division of Mineral Chemistry, for accurate chemical analysis of the enriched boron isotopes, J. J. McNeill, CSIRO Division of Chemical Physics, for suggesting the beam-combining device, and A. Walsh for his continuous interest in this project. I 0
I
0.2
0.4
0.6
ISOTOPIC COMPOSITION OF ABSORBER
1
1
0.5
1.0
N 10
~
+ N" Figure 8. Calibration curves for t h e case of the natural-boron hoilow-cathode source and nitrous oxide-acetylene flame N''
A), of the absorption line in the nitrous oxide-acetylene flame, and to a lesser extent from the unequal Doppler widths of the O ' B and "B isotopic components. (The effect of the unequal Doppler widths is in the opposite direction to that of I u s . ) T h e fairly strong dependence of the reduced gradient m'on IUS is due to the relatively small isotope shift of the 2498-A line (4.138 cm-' (13)). The calculations (Curves 3 and 4) also indicate that the a-parameter has only a slight effect on the position of the m ' = 0 intercept, and affects mainly the rate of change of m' with is. The experimental points are in reasonable agreement with the a l o = 0.2, SUS= -0.05 cm-l curve (Curve 3). The collisional width of the 2498-A line corresponding to this a'O-value is 0.12 cm-', which is close to the collisional width, 0.10 cm-', found for the 2089-A line under the same flame conditions (Figure 6). From the above considerations, it is clear that when a natural-boron source is used with a nitrous oxide-acetylene flame absorption cell, measurement of the absorption of the 2498-A line can be used to monitor the total boron concentration of the samples, thereby allowing samples of differing concentrations to be adjusted to have the same boron content. T h e 2089-A absorptions can then be used to determine the
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RECEIVED for review May 19, 1977. Accepted July 19, 1977.
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, 'OCTOBER 1977
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