3) is q (pawhich is 1.7 X lo5 ph/sr/s for 1pg/cm2 of Cd as given in Table 11. Step 10. As with the characteristic line, we multiply by 0.35 for the sample area (Step 51, by .n to get the scattered intensity per steradian (Step 6), and by 8 X lo4 for the spectrometer efficiency (Step 7). F
X
lo5X
0.35 X 8 X 10-6/4n = 3.8 X
counts/s
The line/back ratio becomes
640/(3.8X
lo-’)
=
1.7 X
lo4
One must remember that this favorable line/background ratio is for 1 pg/cm2 of Cd without any supporting substrate. If we include the scattering from, for example, 0.5 mg/cm2 of carbonaceous substrate (like Mylar), Table I1 shows the scattering from it to be 8.7 X lo6 ph/sr/s which is 51 times larger than the scattering from the Cd itself giving a total scattering of 2 counta/s. The practical line/background ratio becomes
640/2= 320 Step 11. To estimate the limit of detection, CL, we must use the total line and background counts, Np and NB,collected in a selected counting interval, t. For t = 100 s and the counting rates above, we get Np = 6.4 X lo4 counts and NB = 2 X lo2 counts. The definition for limit of detection ( I ) is that the line must exceed the background by 3 standard deviations of the background or 3& = 42 counts. Now we can form the ratio to determine CL.
6.40x 104
42 counts = -counts
1Pg/cm2 C L = 42/6.4X
CL lo4 = 6.8X
,ug/cm2 0.7 ng/cm2
Again to compare with measured results, our experimental estimate of CL was also 0.7 pg/cm2 for Cd which happens to agree exactly with the prediction.
DISCUSSION The prediction of absolute sensitivity and limit of detection for x-ray fluorescence analysis of pollution samples can be done
with simple mathematics and parameters available in the literature. The expression for the measured total counts from a characteristic x-ray line is the product of the parameters in Table I.
Similarly the total background counts which interfere with the characteristic line come from the parameters in Table I and can be expressed as
The calculated sensitivity and limit of detection for the example illustrated were in amazingly good agreement with the experimental values, but even disagreement by a factor of three to five times would have been completely satisfactory as a prediction method. Although it was not demonstrated, the x-ray predictions can be made for energy dispersion as well as for crystal spectrometers. Perhaps advocates of other analytical techniques will be challenged to devise similar predictive capabilities for their methods so that various methods can be compared on paper and the most effective method selected scientifically rather than by intuition. A word of warning should be noted about trying to predict absolute x-ray response for bulk specimens! In bulk specimens, absorption of both the incident and emerging radiation would alter the intensity so radically as to make the predictions impractical.
LITERATURE CITED (1) IUPAC Report on Data Interpretation, Pure Appl. Chem., 45. 99 (1976); also Anal. Chem., 48, 273R (1976). (2) D.B. Brown, J. V. Gilfrich, and M. C. Peckerar, J. Appl. Phys., 46, 4537 (1975). (3) W. H. McMaster et al., Lawrence Radlatbn Laboratoty, UCRL Rept. 50174 (revlsion l),May 1969. (4) R. W. Fink et al., Rev. Mod. Phys., 38. 513 (1966). (5) J. V. Gilfrlch, D. B. Brown, and P. G. Burkhalter, Appl. Spectrosc., 29,
322 (1975).
RECEIVED for review April 25, 1977. Accepted June 21, 1977.
Determination of Fluorine by Neutron Activation Analysis H. Gene Knight,* A. Keith Furr, and T. F. Parkinson Neutron Activation Analysis Laboratory, Virginia Poiytechnic Institute and State University, Blacksburg, Virginia 2406 1
The analysls of fluorlne by Instrumental neutron actlvatlon analysls Is hampered by the short half-llfe (11.41 8 ) of fluorlne-20 and by frequent Interference from chlorlne. A procedure Is descrlbed whlch mltlgates these problems. Using thls procedure, Teflon standards were analyzed and an estlmated mlnlmum detectable welght of 14 pg of fluorlne was determined.
In recent years the fast and accurate determination of fluorine concentration has become increasingly important to
researchers in many diverse fields. More work is now being done requiring fluorine determination over a wide range of concentrations. This range varies from the low concentrations of fluorine as it is found as an environmental pollutant to the high concentrations found when a fluorine compound is used as a solvent in coal liquefaction. The increased interest in determination of fluorine content emphasizes the importance of a single analytical technique which is suitable for use on a wide variety of materials and over a wide range of concentrations. The conventional analysis methods include titration, x-ray spectrometric analysis, atomic absorption spectrometry, and ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1507
neutron activation analysis (NAA), Other methods are available for fluorine determination in liquid media only; thus these methods are limited to specific types of samples. The advantages and problems associated with each major analytical method for fluorine determination are summarized below. The chemical analysis for fluorine utilizes a thorium nitrate titration method. The titration end point is so poorly defined as to require computer refinement for determination. This ill-defined end point causes the analysis results to be questionable. X-ray spectrometric analysis (or x-ray fluorescence) has a major handicap in that the absorption of the primary x-rays in the beryllium x-ray tube window becomes significant a t the long wavelengths required to excite elements of atomic number less than 22 (I). With elements of low atomic number, the excited x-rays will be attenuated in the beryllium window and will not impinge on the Si(Li) detector; thus the fluorine content cannot be determined. Fluorine has an atomic number of 9 which precludes it from determination by the x-ray spectrometric method. Any element may be determined by atomic absorption spectrometry (AA) if the resonance line is in the portion of the light spectrum which the AA instrument can utilize. Most instruments currently in use operate with wavelengths of greater than 1900 A (2) which is in the upper portion of the ultraviolet and in the visible part of the spectrum. The fluorine resonance line has a wavelength of 955 A and is thus below the operational limit of most atomic absorption instruments. Fluorine has been a difficult element to determine by NAA because of interferences from other substances within a particular material, the very short half-life (11.41 s (3)) and the relatively high radiation levels associated with irradiated high density materials. NAA utilizes the activation of stable fluorine-19 and the subsequent decay of fluorine-20 ( 4 ) to stable neon-20:
on' + yF'y+ (9F20)* + loNe20+ 13-(5.41 MeV) + ~(1.6331MeV) + Q
(1)
The procedure discussed in this paper has been quite effective in overcoming the problems normally encountered and has proved to be a viable procedure for fluorine determination when used in conjunction with the soft-ware analysis program (5, 6) used a t the VPI & SU Laboratory.
EXPERIMENTAL The equipment used for this analysis consists of a 100 kW Argonaut type nuclear reactor used for sample irradiation in a thermal neutron flux of 1.2 X 10l2neutrons/cm2-s,a y radiation counting system featuring a high resolution Ge(Li) detector, a Nuclear Data Model 4420 Function Control Unit with a 100 MHz Analog-to-Digital Converter (ADC), an internal 32 000 word computer central processing unit, a computer standard tape unit, an optional video-data terminal or teletype and the VPI & SU main computer system IBM 370-158. Two Ortec Model 775 scalers were added to the standard counting system to totalize counts for the dead-time correction. See Figure 1. The observed weights of the standards are based on an absolute determination of fluorine content using the known composition of the standard materials, weights, half-lives, activation times, delay times, and neutron flux. The VPI & SU research reactor has an extremely stable neutron flux in the activation region over a long period of time. The initial determination of the absolute flux levels was made by absolute activation measurements ofoC@ ' and periodic rechecks affirm the stability of the flux. An optimum activation time must be determined which will vary with the material composition. This time must be long enough to allow the fluorine-19 in the sample to become sufficiently activated while maintaining overall sample radiation levels low enough to allow data acquisition. Because of the short time interval (less than one half-life) between the end of activation 1508
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
CHANGER
FUNCTION CONTROL UNIT
VIDEO-DATA TERMINAL
CHANGER
MAGNETIC TAPE UNIT
Figure 1. ND4420 counting system
and start of count, care must be exercised to avoid saturating the ADC or the Ge(Li) detector. To increase the activity at which a sample may be analyzed, a restricted region must be set up on the ADC to eliminate the effects of all other y peaks not contained within the region of interest. The upper and lower level discriminators on the ADC must be adjusted to encompass the region immediately surrounding the fluorine-20 y peak. This restricted region includes the fluorine peak at 1633.1 keV, a chlorine peak at 1642.4 keV and part of the Compton edge for aluminum. Since the ADC is affected only by the interactions occurring within the restricted region, the sample activity as sensed by the ADC will be quite low; thus the saturation of the ADC will no longer be a limiting factor. The hard /3 particles (up to 5.41 MeV) emitted will cause an energy shift and broadening of the fluorine peak. It will also contribute to the overall radiation levels of the sample. To minimize the effects of this /3 particle and the associated bremsstrahlung, a sheet of lead 0.74 mm thick was installed between the sample and the Ge(Li) detector. Lead was selected because of sample and detector geometry considerations. Aluminum, with a lower atomic number, would have been the ideal shielding material but would have necessitated too great a change in the sample-to-detector geometry. This thickness of lead will not seriously attenuate the y from fluorine-20 decay. The lead absorber decreases the energy shift, the peak broadening, and the radiation levels of the sample. Thus, a sample with a higher total activity may be analyzed with little peak distortion and without saturating the Ge(Li) detector, provided that the sample activity as sensed by the Ge(Li) detector is maintained at less than approximately 40% deadtime at which point the Ge(Li) detector will become too highly saturated. There is a deadtime correction factor incorporated into the analysis program (7) which will correct for error due to the percent deadtime. A pile-up correction (7,8)must be used to compensate for the loss of data due to the detector transmitting fluorine-20 photoelectric event pulses to the ADC which are not recognized as coming from fluorine-20. The time the detector requires to transmit a pulse from a single interaction to the ADC is called the resolving time ( 7 ) . During this period of time, two or more pulses may arrive at the detector and, since the detector cannot separate the multiple pulses, they will be summed together and transmitted as a single pulse of a higher energy. Their energies thus combined will be interpreted by the ADC as a y photon of a higher energy than that for fluorine, resulting in an observed fluorine concentration which is lower than the true concentration. The resolving time has been determined to be 5.89 ps by the split-source method (9). The total number of interactions occurring within the Ge(Li) detector will determine the magnitude of the effect that pile-up will have upon a particular sample. The i n s u e d scalers will show the indicated count rate in the detector from the input signal to the ADC whether the ADC utilizes the signal or not; thus it is a measure of the total interactions occurring within the Ge(Li) detector. Pulse pile-up affects the entire spectrum. However, the loss of counts from the photopeak area, and hence the amount of
Table I. Results of Analysis of (C,F,), Standards
Sample 1 2 3 4 5 6 7 8
9
10 11 12
Calculated weight, pf 76.0 152 380 760 1520 2 960 5 930 1 2 400 2 3 800 47 500 95 000 1 9 0 000
Measured weight, p g b 65.9 156 381 815 1770 3 210 6230 1 2 300 21 800 39 300 71 900 135 000
i.
t t t
* t i
* * f
i f
11.2 4.8 28.5 45.9 106 257 154 447 1090 1260 6 210 5 960
1314k;k11:
6574
a Calculated weight (micrograms) = (Molecular weight of fluorine x weight x 106)/(Molecularweight of C,F,) = micrograms ( 3 ) . [ 4 ( 1 8 . 9 9 8 4 ) x 1 gram X l o 6 ] / [2(12.01115) + 4 ( 1 8 . 9 9 8 4 ) ]= 759 815 pg == 760 000 pg. * Measured weight = mean t standard deviation
fluorine present, can be computed by the method of Roscoe and Furr (7) using Equation 2.
90 1764 I484
I484
1764 1484
1764 1404
1764
ENERGY (KEV)
Figure 2. (A) Spectrum plot
for MgF2 standard. (6)Spectrum plot for C2F4standard. (C) Spectrum plot for treated coal. (D) Spectrum plot for contaminated vegetation
(2)
The overall effect of pile-up will be greater as the sample activity is increased. Each sample analysis was replicated five times to minimize statistical variations and activation timing errors due to limitations of existing timing devices. The mean value and standard deviation of the mean were then calculated by the sum of squares method (IO). A more complete discussion of errors due to the curve-fitting algorithm and to counting statistics is given in Ref. (7). ,-
RESULTS AND DISCUSSION
Table 11. Double Pulse Pile-up Effects Sample No. Calculated weight, pg Measured weight, pg Activation time, s Measured weight, p g Activation time, s
I O ~
IO~
In Figure 2 are shown typical y spectra from samples containing fluorine. The fluorine content in the contaminated vegetation (Figure 2D) was found to be 106 f 43 wg/g. This result demonstrates the capability of the analytical procedure for analyzing fluorine in the presence of chlorine. The results of calibrating the system with Teflon standards, (C2F4)n,are summarized in Table I and Figure 3. As can be seen in Figure 3, a plot of experimental results for (C2F4)n,the fluorine weight from NAA vs. the calculated weight correlates well until the higher mass samples begin to show an increasing systematic deviation. This deviation was tentatively attributed either to self-shielding effects or to double pulse pile-up. A self-shielding correction factor was calculated by the methods of Kruger (11)and Zweifel(12) and was found to be negligible. Samples exhibiting this deviation were then reactivated under the same conditions at approximately one-half the original activation times. The rationale for this procedure was that if this deviation were indeed caused by double pulse pile-up, then, by decreasing the activation time, the total activity and ultimately the effects of double pulse pile-up would be minimized. With less error contributed by the double pulse pile-up effect, the correlation between measured and calculated weights should be much
io3
105
IO'
Comparison of measured and calculated weights in C2F,. The error bars are smaller than the symbols
Figure 3.
improved. As can be seen in Table 11,the double pulse pile-up effect was quite significant a t very high fluorine concentrations. With the correction to decrease the effects of double pulse pile-up, the disparity between measured and calculated weights of fluorine was indeed decreased. Double pulse pile-up error may be minimized by decreasing the total activity of the sample while still maintaining sufficient activation for determination of the fluorine-19. This procedure, however, will not eliminate the error entirely. Hence a discrepancy will still exist between measured and calculated weights. A mathematical correction for double pulse pile-up, described by Cohen (13), may be used for the very high fluorine concentrations where this has been found to be a problem. We have defined the minimum detectable weight for fluorine to be three times the background level. This corresponded to 14 pg for the Teflon, (CzFJ,,, standards analyzed.
10
11
12
47 500 39 300 1 2 6 0 30 41 900 3 180 15
9 5 000 71 9 0 0 * 6 210 15 85 400 * 2 030 9
1 9 0 000 135 000 i 5 960 9 154000 f 3850 5
+_
+_
I O ~
CALCULATED WEIGHT (pg)
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1509
LITERATURE CITED (1) E. P. Bertin, “Principles and Practice of X-ray Spectrometric Analysis”, Plenum Press, New York-London, 1970. (2) W. Slavin, “Atomic Absorption Spectroscopy”,Interscience Publishers, New York, N.Y., 1968. (3) 0 . Erdtman and W. Soyka, Nucl. Insfrum.Methods, 121, 197-201,(1974). (4) C. M. Lederer, “Table of Isotopes”,6th ed., Wiley and Sons, New York, N.Y.. 1967. (5) B. A: Roscoe, “Acquisition and Analysls of Neutron Activation Data”, Master of Science Thesis In Nuclear Science and Engineering, VPI & SU, 1976. (6) B. A. Roscoe and A. K. Furr, Nucl. fnstrum. Methods, 137, 173-178 (1976). (7) B. A. Roscoe and A. K. Furr, Nucl. Instrum. Methods, 140, 401-404 (1977).
DeAisenberg, I. M. Cohen, R. 0. Korob, and M. D. Rudelli, Nuclear Activation Techniques in the Life Sciences, IAEA-SM-157/72. (9)G. H. Simmons, “A Training Manual for Nuclear Medicine Technologists”, Public Health Service, BRH/DMRE 70-3. (10) A. M. Selby, Standard MathematicalTables, 21st ed.,The Chemical Rubber Co., Cleveland, Ohlo, 1973. (11) P. Kruger, “Principles of Activation Analysis”, Wiley-Intersclence,New (8) Y.
York, N.Y., 1971. (12) P. F. Zweifel, Nucleonics, 18 (ll),174-175 (1960). (13) E. J. Cohen, Nucl. Instrum. Methods, 121, 25 (1974).
RECEIVED for review February 3,1977. Accepted June 6,1977.
Determination of 13 Elements with Atomic Numbers between 12 and 47 by 14-MeV Helium-3 Activation Analysis C. S. Sastri, H. Petri,” and G. Erdtmann Zentralabteilung fur Chemische Analysen, Kernforschungsanlage Julich GmbH, 5170 Julich, West Germany
Nuclear reactions for the trace determination of the elements Mg, AI, TI, V, Cr, Mn, Fe, Ni, Zn, Zr, Nb, Mo, and Ag by activation analysis with 14-MeV 3He ions were Investigated. For these reactions, thick target yields were measured and interference-free detection limits were calculated. For an irradiation of 1 h or 1 half-life, whichever is shorter depending on the product nuclide, at 2 PA, the detection limits are in the range 1-50 ppb for Ai, Ti, V, Mn, Ni, Zn, and Nb; 50-100 ppb for Mo; and 100-500 ppb for Mg, Cr, Fe, Zr, and Ag.
metals and other elements with 2 > 42 can be investigated for their low 2 impurities. In previous papers (18,19), the determination of impurities in the matrices Nb, Ta, and W, by activation analysis with 14-MeV 3He-particles was described. At this energy, the commonly observed nuclear reactions are (3He, a),(3He,2p), (3He, p), (3He,p2n), (3He,n) and (3He,2n). In the present work, 13 elements between 2 = 12 and 2 = 47 were irradiated and from the y-ray spectra of the irradiated targets, the optimum detection reactions having high specific activities and low nuclear interferences were found and the detection limits based on these reactions were calculated.
Charged particle activation analysis has gained great importance in the detection of light elements at the sub-ppm level. The particles that are commonly used are protons, deuterons, tritons, helium-3 and helium-4 ions (1-5). Protons have been used (6-8) also to find heavy element impurities in high purity metals, minerals, etc. Sometimes other techniques like proton activation followed by x-ray counting and PIXE (particle induced x-ray emission) have been used (9,10) for trace element study. Markowitz and Mahony (11) were the first to suggest 3He ions for activation analysis of light elements. Following this, to a limited extent, this technique has been used for investigating heavy element impurities (12, 13). Ricci and Hahn (14) have calculated sensitivities for elements from Be to Ca for 18-MeV 3He ions. Kormali and Schweikert (15) have measured thick target yields for elements from Zr to Cs with 40-MeV 3He ions. In our laboratory the light elements C and 0 in metals are being investigated by the 14-MeV helium-3 activation technique (16,17). Concurrently systematic studies have been made to see if these irradiation conditions are useful also for the determination of heavy elements by nondestructive analysis. This energy corresponds to the Coulomb barrier of 14.2 MeV between 3He nucleus and a target nucleus with 2 = 42, assuming both the nuclei to be spheres with radii of 1.4 X A1/3 cm. This means that elements lighter than molybdenum will undergo nuclear reactions and therefore can be determined. With heavier elements, reactions take place to a very limited extent because of the tunnelling effect. Thus,
EXPERIMENTAL
1510
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
High purity metal foils of natural isotopic composition (suppliers: Goodfellow Metals and Ventron) of the size 20 X 20 mm (approx.) were irradiated in the form of thick targets. The range of the 14-MeV 3He ions was from 30 to 60 mg/cm2 in the targets used; the target thickness varied from 100 to 150 mg/cm2, in all cases being thicker than the range of the beam in the material. Most of the irradiations were made in the internal beam of the isochronous cyclotron JULIC at Kernforschungsanlage Jtilich. The irradiations were made with 14-MeV 3He ions at currents ranging from 50 to 500 nA and for times ranging from 10 to 30 min. It has been found that these irradiation conditions can be reproduced without difficulty. As a check, the thick target yields for some of the standards were measured periodically. Short irradiation times of a minute or less were avoided to minimize the errors due to occasional fluctuations in currents, lasting a few seconds, that are likely to happen with the machine. If for some reason a large fluctuation in beam current had occured during an irradiation, such a measurement was discarded. A current integrator was not available for the present measurements. Because of the high magnetic field in the cyclotron, Fe and Co targets could not be irradiated in the internal beam of JULIC. They were irradiated in the external beam of the Compact Cyclotron CV 28 of Kernforschungsanlage Julich. Irradiations were made for 5 to 10 min at 400 nA to 1 WAcurrent. At the isochronous cyclotron JULIC, the internal beam (-2 mm diameter) strikes the rectangular target onto a side. In the compact cyclotron CV 28, the external beam (- 10 mm diameter) strikes the rectangular target at the center. In both cases, the
