Skin
4
Sample
PPm
1 2 3 4 5
0.03 0.05
NDa ND‘ 0.001
Table 111. Samples of Potatoes Analyzed for Fentin Bulk l/r-Inch layer Potato, Potato, PPm Potato, Z PPm
z
8 12 8 8 8
0.01 0.001 0.01 0.01 0.002
70 47 60 51 64
0.001 0.001
NDa ND‘ NDQ
z
22 41
32 41 28
Total ppm 0.008 0.007 0.005 0.005 0.001
ND None detected.
by cathodic stripping voltammetry (28) does not constitute an interference in the present method. The object was to establish both the total residue level and also the distribution of the triphenyltin residue within the potato. Each sample was analyzed for fentin in the skin, the first Il4-in. layer of flesh, and in the bulk of the potato. The residual liquor resulting from extraction, clean-up, and distillation of the acetonitrile extract was transferred to the cell for stripping analysis ; buffer and Triton X-100 were added and the final solution was diluted to 15 ml so that it contained 50% vjv ethanol. Standard additions of fentin to the potato samples prior to the extraction process indicated that the procedure was within 10% of quantitative recovery for 0.4 ppm of fentin. The sensitivity of the method compares very favorably with con-
ventional spectrophotometric procedures (5) which are approximately one hundred times less sensitive. Table I11 shows clearly that the bulk of the fentin residue is located in the first l/4-in. layer. Although the greatest concentration of fentin is found in the skin itself, peeling of the potato only results in the removal of some 10% of the total residue. It should be pointed out that the levels involved are below the recommended tolerance levels (29).
(28) B. Fleet and M. J. D. Brand, Analyst, in press.
(29) J. B. Stoner, Brit. J. Ind. Med., 23, 222 (1966).
ACKNOWLEDGMENT
We thank L. Pratt for carrying out the NMR analysis. RECEIVED for review December 3, 1969. Accepted April 15, 1970. The Agricultural Research Council provided a research assistantship for one of us (M.D.B.).
Nondestructive Determination of Fluorine by Photon Activation Using a Betatron Lado Kosta and Jaroslav Sluneckol Department of Chemistry, Unicersity of Ljubljana and Institute “Joief Stefan,” Ljubljana, Yugoslavia The reliability and the versatility of the photon activation method was demonstrated by determining fluorine in concentrations between 0.01 and 5% in a variety of materials, such as organic and inorganic fluorine containing pharmaceuticals, fluorapatite and vanadium concentrate, using amounts of sample from 0.1 to 1.0 gram. Interferences from the elements, such as chlorine, bromine, arsenic, tin, silver, chromium, or zinc can be eliminated or reduced to minimum by adjusting irradiation time, waiting period, and the energy of the primary electron beam. Results by activation are in good agreement with those obtained by the distillation-titration method, and give a precision better than 2%.
METHODS FOR THE DETERMINATION of fluorine have recently been reviewed ( I ) . Among them, nuclear activation offers the advantage of a nondestructive approach. There is a selection of possible nuclear reactions, depending upon the available source: thermal neutrons ( 2 ) , fast neutrons ( 3 , 4), protons 1 On leave from the Institute for Nuclear Research CSAV, Rei, Praha. Czechoslovakia.
(1) J. K. Foreman, Analyst, 94, 425 (1969). (2) H. P. Yule, ANAL.CHEW, 38, 818 (1966); 37, 129 (1965). (3) E. A. M. England, J. B. Hornsby, W. T. Jones, and D. R. Terrey, Anal. Chim.Acta, 40, 365 (1968). (4) W. Leonhardt, Kernenergie, 6,45 (1963).
(5,6), as well as photons (7, 8). All produce radionuclides from fluorine. Among new interesting alternatives is activation by fast neutrons from a 241Am-242Cm-Besource, as proposed by Wing and Walgren (9), producing 16Nas the basis for fluorine determinations by the I 9 F (n, a)leNreaction. In this work, photon activation has been applied to the determination of fluorine in a number of samples varying widely in compositon, in order to evaluate the selectivity and precision of the method, and to determine the character and the extent of important interferences. Basic work on the technique has been done by Engelmann (10). Later, V . Guinn (11)et al. have studied the feasibility of nondestructive determination of a number of elements, in(5) J. M. Bavers and F. C. Flack, Analyst, 94, 1 (1969). (6) T. B. Pierce, R. F. Peck, and D. R. A. Cuff, ibid., 92,143 (1967). (7) R. 3. Horsley, R. N. H. Haslam, and H. E. Johns, Phys. Reo., 87, 756 (1952). (8) G. H. Andersen, Trans. Amer. Nucl. SOC.,10, 63 (1967). (9) J. Wing and H. A. Walgren, J. Radioanal. Chem., 3, 47 (1969). (10) C. Engelmann, Analyse par activation aux photons. Rapport CEA-R 3307, Centre $Etudes Nuclearies de Saclay, 1967. (11) G. H. Andersen, F. M. Graber, V. P. Guinn, H. R. Lukens, and D. M. Settle, Nuclear Activation Techniques in the Life Sciences, (International Atomic Energy Agency, Vienna, 1967), p 99. ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
831
Table I. Nuclear Characteristics and Contribution to Apparent Fluorine Content of Important Interferences. AF/&
Q
Reaction
TUZ,min.
Day
P+, IT B+, ECP+, P-, EC
11.6
31.99 6.5 17. 6(E0Br) 4.38h (gomBr) 38.4
9.4 10.2 8.5 12.9
23.96 35.0 39.5 41.9
Of
Wl(y,n)34mC1 7gBr(y,n)T8Br 8 lBr(y,n)*OfsomBr
12.5 10.7 10.2
64Zn(y,n)53Zn lO'Ag(y ,n)lOBAg 1lZSn(y ,n)111Sn ( 1z4Sn(y,n)123mSn) 6°Cr(y,n)4gCr
IT
B+, EC B+, EC
6Be, EC
E , MeV 0.145, 2.127, (0.511) (0.511), 0.614 (0.511), 0.618, 0.037 (0.511), 0.669, 0.962 (0.511) (0.511) 0.160 (0.511), 0.091, 0.063,0.153
20 min
2 hr
5.2 1.5
23.5 23
0.66
2
0.3 11.7
49
2.6 53 140
All radionuclides listed (except lasmSn)are positron emitters, measured by the annihilation peak at 0.511 MeV. Gamma rays emitted by some of these radionuclides sometimes offer the possibility of correcting for the contribution of the respective interference. The intensities, however, are in general weak, and therefore of limited practical value.
ucts by activation in the bremsstrahlung beam of a 32 MeV Betatron, also reported high sensitivity for fluorine (13). Energies for the primary beam beyond 18.7 MeV, however, are not suitable for analyzing samples containing carbon, because of the formation of the 20.34 minute I C , which is also a pure positron emitter. Carbon interference is entirely eliminated by reducing the energy of the photon beam close to the threshold value of the I2C (y, n)"C reaction (18.7 MeV). There is also a parallel decrease in the available flux, but for samples with fluorine content in the range of one milligram or more there is enough induced activity to allow precise measurements. In Table I are listed some other interferences, in terms of the ratio of the activity induced in the respective element and the one obtained under similar conditions by the same amount of fluorine, for two waiting periods: 20 minutes and 2 hours, respectively. For all elements listed, except bromine, this ratio improves with time. Of the three radionuclides produced in photon activation of bromine, 78Brand 80Br have much shorter half lives than 18F. The presence of 4.38-hour SomBr, however, results in a relative increase of the Br/F activity ratio after the decay of the shortlived bromine radionuclides. From the curves in Figure 1, representing the decay of the activity induced in bromine, it follows that this interference is at minimum approximately two hours after the end of irradiation.
h 2 = 6.5min 1
EXPERIMENTAL
I
I
I
1
2
3
I
1
5
lime (hours) Figure 1. Decay of activity induced in bromine cluding fluorine, using bremsstrahlung from a linear accelerator. Recently, photon activation was applied to the determination of fluorine in sea water (12). D. Brune, who determined iodine in pharmaceutical prod(12) P. K. Wilkniss and 530 (1968).
832
V. J. Linnebom, Limnol. Oceanogr., 13,
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
Irradiations in the present investigation were carried out in the bremsstrahlung beam of the IJS betatron, purchased from Brown Bovery and additionally stabilized in this Institute (14). The experimental arrangements are represented by Figure 2. The characteristic bremsstrahlung spectra were measured at the full energy of the electron beam (32 MeV) and at 18.7 MeV, and are shown in Figure 3. The calibration curve for fluorine (Figure 4a,b) was prepared by irradiating pellets prepared from lithium fluoride, and sodium fluoride, respectively, spaced by Teflon @u Pont) monitors of the same diameter. In the same diagram is also represented the percentage decrease in photon flux along the polyethylene capsule used as the sample holder. The distribution of the photon flux density around the peak in cross section perpen(13) D. Brune, J. Mattsson, and K. Liden, Anal. Chim. Acra, 44, 9 (1969). (14) D. Jamnik, Nucl. Znstrum., 1, 324 (1957).
samples
polyethylene capsule
monitors
a
I
I
l
beam 18.7MeV [ m a r l
25 mm
2 2.3 a? 2.2 = 21 -
-5
E?2.0
O\r
-
= 1.9. -
a L 2
= 1.8 E
.-c .--.1.7 L 1.6
-
'1.5
Figure 2. Accelerating tube with target assembly
'0
i
L
170
i
3
i
I
i
I
i
I
175 180 185 190 Distance from the target ( m m l
30 i
195
&
35
Figure 4. Details of a sample capsule, showing the distribution of samples and monitors as used for calibration (a), and the decrease in the photon flux density (b)along the sample capsule in the direction of the beam Legend to the calibration curve: 0 Teflon 0 NaF 8 LIF I
1.0
0.9 9
0.8
0
.O
5
zc* 0.7
0
A
u W
A
- 0.6 x
A
3 0 c L
0
A
zf 0.5
I
0.4
I
I 0
A 1
I
I
5
10
15
0.3 I
I
I
I
20 25 30 35 E (&VI Figure 3. Bremsstrahlung spectrum of betatron measured at 32 MeV 0 and 18.7 MeV A energy of the primary electron beam, respectively
dicular to the axis of the propagation of the beam is shown in Figure 5 ; maximum flux at the center is taken as unity. At a sample diameter of 10 mm, the photon flux density at the edge is still over 90% of the maximum at the center. Irradiation times in fluorine determinations were typically 1 hr, and waiting time 1 hr. The activity was normally measured in a single-channel pulse-height analyzer, using a 3 inch X 3 inch NaI(T1) detector, but first measurements on every
I
I
I
10
20
I I
I
\ I
I
I
10
0
I
20 mm
1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' 1 ' I ' I ' ~
6
1
2
0
2
1
6 degrees
Figure 5. Decrease in photon flux density across beam diameter at position of sample capsule. Maximum flux in peak center is taken as unity new type of sample were also checked on a 512-channel analyzer.
RESULTS AND DISCUSSION The reliability of nondestructive flourine determination by photon activation was demonstrated by irradiating a set of samples, mainly fluorine-containing preparations as used in dental practice for protection against caries. All semiANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
*
833
Table 11. Determination of Fluorine in Commercial Products using Photon Activation Amount Number irradiated, of mg F Sample“ g detmn found Std dev Calcipot 1 .oo 4 11.0 +0.2 Fluor calcium (a) 0.11 8 8.5 10.11 Fluor calcium (b) 0.35 3 0.16 zk0.04 Natrium fluoraturn* 0.57 3 0.25 Hormonal pillsc 0.10 2 0.2 iz0.02 Dental prophylactic paste 0.44 2 1.1 10.1 a Samples 1 to 4 are pelleted prophylactic preparations from different manufactures containing calcium and/or sodium fluoride. I, Average of ten pellets activated simultaneously. Active component, 9cr-fluoro-ll/3-hydroxy-17cr-methyltestosterone.
5 x103 -\
L\ \
- 1x10~ E
CT 0
..->
h
c
c.
2 5x102 logarithmic decay curves changed after 20 minutes into straight lines with a slope corresponding closely to the half life of 109.7 minute ISF. The fluorine content was obtained from the calibration curve, and varied in a wide range, depending on the compound used as the source of fluoride ions. In the case of the lesssoluble calcium fluoride, as much as 11 mg of fluorine per pellet was found. Other components in the pellets included magnesium and/or calcium phosphates, and phosphite, starch, and sugar. The steep sections in the decay curve of Figure 6 represent the decay of short-lived components with a half life close to 2.5 min, which are I5O or 30P. The dotted line, 3(f), represents the decay of sample 3 after irradiation under similar conditions, but using 32-MeV energy of the electron beam. Due to the high excess of IICactivity, it is not possible to obtain a precise value for fluorine, The relative precision of the results, as can be seen in the last column of Table 11, is better than 2 %, except for samples with very low fluorine content. For pharmaceutical preparations, a precise average value is still meaningful and can be obtained by irradiating a larger number of pills, as was done in the case of Ultandren, a synthetic fluorine-containing steroid showing hormonal activity, or in the case of a preparation for dental protection containing as little as 160 pg fluorine per pill. Two other samples included mineral fluorapatite and a vanadium concentrate obtained as a byproduct in the production of aluminum from bauxite. Besides vanadium, the concentrate contained phosphate, arsenate, sulfate, and fluoride in appreciable concentrations. These samples were also analyzed chemically by the Willard-Winter titration method, using a modified distillation procedure for the separation of fluoride (15). In the case of fluorapatite, sulfuric acid was first used, but the results were low because of the precipitation of calcium sulfate, which prevented complete recovery of (15) G. Pietzka and P. Ehrlich, Angew. Clzern., 65, 131 (1953).
1 x102 Time (hours) Figure 6. Decay curves of a set of samples irradiated in photon beam of betatron operating at 18.7 MeV electron energy; the 0.511-MeV annihilation peak was measured Legend to the curves : l(s) calibration 2 Calcipot 3 Ultandren A Individual pills 0 Ten pills irradiatedand measured as a group 3w Ultandren, irradiated at full energy of primary beam, 32 MeV fluoride. By distilling from phosphoric acid, which dissolves fluorapatite, the average results were closer to those obtained by activation, but the precision was considerably better by the latter method. The agreement between the chemical and the activation values for fluoride in the vanadium concentrate is very good. The results have been corrected for the contribution by 17.7 day 74As. At a concentration of arsenic of 2.07& as in this sample, this correction amounts to 5 % of the total activity. There is also a detectable activity (
