Anal. Cheni. 1982, 5 4 , 407-413
407
Proton Activation Analysis for the Measurement of Fluorine in Food Samplles R. E. Shroy,‘ H. W. Kraner, and K. MI. Jones* Brookhaven National Labcwatory, Upton, New York 11973
J. S. Jacobson and l.,, 1. Heller Boyce Thompson Instituter for Plant Research, Ithaca, New York 14850
We have developed a proton actlvatlon method for the determlnatlon of ’‘F In food samples based on the use of the ’‘F(p,p‘y)’’F reaction. Special techniques were used to obtaln reproduclble target condltlons and low background values. Two callbratlon technlques not dependent on chemical analyses for fluorlne gave values comparable to a third method which employed vegetatlon and cellulose contalnlng from about 20 to 500 ppm (pg/g dry welght) of fluorlne. Results are reported for FDA market basket food samples contalnlng less than 10 ppm fluorlnce (dry welght) and are compared with the values obtalned wllh two methods of chemlcal analysis for both Vegetation and food samples. Proton actlvatlon and chemlcal methods gave walues In excellent agreement for the fluorlne content of the hlgh fluorlne vegetation samples; however, substantla! dlsagreement remains for the low-fluorlne food samples.
I. INTRODUCTION Concern over the fluorine (F)content of foods and beverages has increased in recent years. The element fluorine is ubiquitous, amounting to approximately 0.075% of the earth’s crust. It is used therapeutically in potable water, toothpaste, and drugs, and it is emitted to the atmosphere as a byproduct of the manufacture of aluminum, glass, fertilizer, and other items as well as from the combustion of coal and processing of uranium (1). Debilitating effects can occur in both animals and humans after excessive ingestion of F-containing foods. Unfortunately, there is no agreement on the magnitude of safe and unsafe doses of F in the diet or on a satisfactory margin of safety. The dietary intake of F may be increasing so it is particularly important to have accurate and precise estimates of F in foods and beverages (2). The controversial issues surrounding F in the diet cannot be resolved, in part, because current information on the F content of foods and beverages is inadequate ( 3 , 4 ) . Traditional chemical methods of F analysis are known to contain substantial errors (5),but few estimates of the magnitude of these errors have been made for foods and beverages. Two estimates of random error have been reported (6, 7);however, estimates of systematic errors do not seem to be available. Unless alternate methods, not subject to the same type and magnitude of errors, are developed specifically for food samples and verified in extensive interlaboratory comparisons, the quantification of F in the diet will remain controversial. The principal purpose of this study was to develop techniques suitable for the analysis of food samples by proton activation analysis (PAA). A preliminary account of this work has been reported (8). W’e choose proton activation analysis Present address: The Machlett Laboratories, Stamford, CT 06907. 0003-2700/82/0354-0407$01.25/0
as the method of study because it has previously been applied to a variety of substances with high F concentrations, does not depend on the chemical nature of the F compounds, and is insensitive to the nature of the sample matrix. F has been measured by activation analysis in samples of water, teeth, urine, vegetation, and other biological materials (9). Major potential advantages of activation analysis are the capacity to provide estimates of the absolute F content of samples, high sensitivity, and the ease of using high-level comparative standards. I t should be mentioned that elemental analysis by means of nuclear reactions or elastic scattering has been used by nuclear physicists for many years. For example, the technique was used by Williamson et al. in 1951 (10) to correct for the target carbon contamination which can be a serious problem in measurements of nuclear reaction Q values. The use of reaction techniques has gained steadily over the succeeding years with the introduction of more sophisticated detector systems. Indeed, some workers now are inventing specific acronyms for particular types of reactions. Macias et al. (11) have used the term GRALE to denote Gamma Ray Analysis for Light Elements and have measured concentrations of several elements in atmospheric aerosols. However, GRALE is not wholly unique, for Kenny et al. (12) use the term PIGME to describe Proton Induced Gamma Measurements. Proton activation analysis (PAA) complements the acronym for neutron activation (NAA or INAA) (13). The work described in this paper follows an earlier effort (14)which measured fluorine in vegetation samples by proton activation analysis. The sensitivity achieved here represents a substantial improvement, by about 3 orders of magnitude, over experiments such as that reported by Macias et al. (11) which measure trace elements in particles. This improvement results from the use of higher beam currents, longer bombardment times, and thicker targets, as well as higher detection efficiency for the lower energy F y-rays and a cross section for the 19F(p,p’y)1gFreaction which is generally somewhat higher than that for the reactions used by Macias. The problems involved in measuring very low levels of fluorine in organic compounds are far removed and more difficult than those encountered in the analysis of the much more stable inorganic materials, such as air-borne particles, which are not as vulnerable to radiation damage or beam-heating effects. PAA can, of course, be used for many elements other than fluorine. For example, Na is present in large amounts in food and can be easily determined by observation of the 440-keV y-ray produced by excitation of the first excited state in the 2SNa(p,p’y)23Nareaction. Other elements such as C or S can be measured if desired. Measurement of 12Cby observation of the 4,4-MeV y-ray from the 12C(p,p’y)12Creaction would require a higher bombarding energy than was used in the present work. The experimental details and parameters are generally quite variable and can be optimized for each particular element and matrix to be studied. 0 1982 American Chemical Society
408
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
Here, our attention is focused on the detection of fluorine in food samples since, in this case, the measurement by chemical means has been particularly troublesome and uncertain. Indeed, it is not possible to emphasize too often the urgent need for collaborative round-robin measurements to establish sound standard materials and methodology. At the present time, to our knowledge, reliable and accurate chemical methods for analysis of fluorine in food samples have not been demonstrated by a comparison of interlaboratory results. The lgF(p,p’y)lgFreaction is particularly attractive because the energies of the reaction y-rays specifically identify the emitting nucleus. This is in sharp contrast to the situation that would be encountered, if, for example, a linac or betatron were used to produce the lgF(y,n)18Freaction. In this case the resulting 18Fpositron activity would have to be identified from the multicomponent time decay curves arising from other positron emitting activities such as llC and 13N. Accurate differentiation of 18F is thus impossible when working with fluorine concentrations in the range of 1 part per million (pprn). The l9F(p,ay)l6Oreaction also produces a specific y-ray signature for lgF and has been studied but is not emphasized here. The y-rays are of much higher energy and are not so readily detected as those of the reaction to be described. 11. SELECTION AND PREPARATION OF SAMPLES Five sets of samples were prepared in order to assist the development and testing of techniques for the measurement of F in food samples using PAA. The first set contained samples of plant foliage with between 35 and 485 ppm F obtained from a collaborative study of three chemical methods of F analysis (15). A second set of samples was prepared by adding known amounts of F, equivalent to between 18 and 60 ppm, to weighed portions of four of these vegetation samples. The third set of samples contained inorganic salts of F (ammonium,potassium, sodium, and tin) added to cellulose powder (Cellex), to provide final F concentrations of between 20 and 25 ppm. A fourth set of samples, with varying F content and chemical composition, was prepared by adding either 0,0.5,1.0,2.0, or 3.0 g of powdered potatoes containing less than 1ppm F to 1.0 g of cellulose powder containing 75 ppm F as NaF. These samples were thoroughly mixed, and the final F concentrations were 75,50,37.5,25, and 18.8 ppm F, respectively. The fifth set of samples contained composite food materials obtained from the Food and Drug Administration’s (FDA) FY 1975 markebbasket collection no. 19 which was collected under the Total Diet Study/Selected Minerals in Foods Program (16-18)- Samples, received in the frozen conditions, were freeze-dried for 24 h in sorption-pumped vacuum and pulverized to a powder with a mortar and pestle. All F concentrations reported are given on a dry weight basis. 111. CHEMICAL ANALYSIS OF SAMPLES Data on the F content of vegetation samples with and without added F were taken from a collaborative study in which each of 25 different laboratories used one of three chemical methods to measure F (15). The three methods were (1) the Willard-Winter method, (2) the potentiometric method, and (3) the semiautomated method, all of which have been recommended by the Association of Official Analytical Chemists, the American Society for Testing and Materials, and the Intersociety Committee on Methods of Air Sampling and Analysis (1941). Cellulose powder and potato samples enriched with F were analyzed by the potentiometric method. Food samples could not be analyzed by any of the methods described above because these methods are imprecise with samples containing less than 10 ppm; consequently, an alternative method was developed. Food samples which ranged from 0.2 to 1.2 g in mass were charred under infrared lamps for 31/2h and then
ashed in covered platinum crucibles in a nickel-linked furnace at 550-600 OC using 50 mg of special prepared low-F calcium phosphate and NaOH as F fixatives (22). Ash was transferred from crucibles with deionized water and perchloric acid in a nitrogen atmosphere to avoid carbonate formation. F was microdiffused from perchloric acid for 13 h at room temperature (23,241into a NaOH solution and measured with a selective-ion electrode and the known addition technique (25) with a citrate-acetate buffer at pH 5.5.
IV. PROTON ACTIVATION ANALYTICAL METHODS The proton activation measurements were made by observation of 110- and 197-keV y-rays from the lgF(p,p’y)l9F reaction in the following experimental arrangement. The proton beam from the Brookhaven National Laboratory 3.5MV Research Van de Graaff accelerator was collimated with 2-mm tantalum apertures spaced 26.4 cm apart followed after 8.9 cm by a 3-mm tantalum scraping aperture. Two tantalum antiscattering slits with 3-mm apertures were spaced at 8.7-cm intervals between the collimating slits. Lead cylinders with a 7-mm hole were used as spacers between the collimators to attenuate y-rays produced upstream by the beam. The target assembly was a glass tube of 3.2-cm diameter which was separated from the accelerator vacuum system by a 0.625-pm nickel foil to allow the use of a target cooling gas. The distance from the last collimator to the foil was 3.0 cm and from the foil to the target was 10.0 cm. An in-line liquid nitrogen trap was used to ensure a good vacuum at the foil to reduce buildup of possible fluorine-bearing contaminants. Gas pressures of 0.3-atm helium were typically used for cooling. The beam was either stopped in the target or, for background measurements, could pass on to a well-shielded Faraday cup 2 m from the target. The target could also be cooled by circulation of liquid nitrogen through a small copper tube soldered to the assembly. y-Rays were observed at 90’ to the incident beam with a Ge(Li) y-ray detector shielded from background by 5 cm of lead. Cross sections for production of the 110-and 197-keVy-rays produced in the 19F(p,p’y)lgFreaction have been measured by Ranken, Bonner, and McCrary (26). During the course of this experiment some discrepancies were noted in their cross section results, and a remeasurement was undertaken. Accurate values of this cross section are required for the absolute fluorine determination that the nuclear method promises, which is of course independent from comparative standards. A 250 pg/cm2 target of CaF2on a 20 pg/cm2 carbon foil was used. y-Rays were detected with a 10% efficient, relative to a 7.6 X 7.6 cm NaI(Tl), coaxial Ge(Li) y-ray detedor calibrated with a NBS SRM 4215-B mixed radionuclide standard (27) and a calibrated WOsource at the beam position. Overall uncertainties in the absolute cross section scale resulting from uncertainties in the target thickness, current integration and detector solid angle and efficiency are estimated at =k15%. Our results for the 197-keV y-ray are in essential agreement with those given by Ranken et al. (26) although they are about 20% lower for energies from 3.0 to 3.5 MeV. Our results for the 110-keVy-ray are in disagreement with the previous result and are approximately a factor of 2 lower. Values obtained for the cross sections are given in Table I. The relative efficiency for production of fluorine y-rays by proton bombardment of organic samples can be readily calculated by using the measured energy dependence of the cross s,ections. The thick target yield for these reactions is given by the expression
where N y is the number of y-rays produced, NBthe number
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
Table I. Cross Sections for the 110- and 197-keV 7-Rays Produced in the 19F (p, p' y ) 19F Reaction 0,
E,, MeV
110 keV
1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.85 2.9
12 4 49 18
10 25 113 41 47 48 58 71
mb
a,
-197
lev
MeV
110 keV
7 15 72 98 19 35 109 87 56 107 102 66
2.95 3.0 3.05 3.1 3.15 3.2 3.25 3.3 3.35 3.4 3.45 3.5
111 74 56 48 58 46 54 96 94 103 117 89
Eg,
mb
197 keV 171 160 132 105 88 86 66 87 122 131 172 170
200 2 00
l3 -----z 1
,,I
/ y ,, "4'
,
of incident protons, NF(.E)the number of fluorine atoms per square centimeter in a target thickness corresponding to a proton energy loss dE at energy E. g(E) the production cross section, and EB the incildent beam energy. To evaluate the integral, we transform to an integration over range by using the relationship with NF(x)dx the number of fluorine atoms in a thickness dx at depth x . Organic materials, such as foods, are mainly composed of carbon, hydrogen, and oxygen with approximate abundances of 40, 7, and 53% by weight, respectively. (We use the composition of sucrose as typical of the composition of plant materials). Variations of the relative composition of light elements by as much as 20% and small admixtures of heavier materials do not change the stopping powers by more than 10%. Therefore, it is possible to calculate the absolute production efficiency for fluorine y-rays produced by proton bombardment. The valules obtained for the 110- and 197-keV and, for comparison, the 6.13-MeV y-rays from the J9F(p,ay)la0 reaction, using values for stopping powers obtained from the compilation of Northcliffe and Schilling (28) are shown in Figure 1. Tho total uncertainty is dominated by the contribution from the cross section measurement and should also be about 15'%. The discussion given here deals only with food samples that have been lyophilized but inot modified in any other way. The ability to make determinations with such samples is extremely important since the possibility of losing or adding fluorine to the sample material is minimized relative to chemical methods where the samples must be dissolved. However, determination of fluorine is not limited1 to such samples but can be easily applied to ashed samples. Variations in the ash/mineral content would not be troublesome since the stopping power is not a strong function of composition. As a result, the fluorine content of foods with high NaCl content can be equally well determined using either type of target. Higher level beam currents can also be used with an ashed target without damage to the target. Targets were preparedl in basically the same way as that described by Jones et al. (14). Target materials in the form of a finely divided powder prepared as described above in section I1 were pressed into a disk about 2 mm thick and 2.5 cm in diameter using premures up to 110 bar. In some of the FDA market basket samplles there appeared to be residual oils which made high pressures inadvisable because of the possible loss of target material. A careful study was made of the effects produced by continued proton bombardment of the target. A sample of camellia with a fluorine content of 469 ppm (parts per million by
409
2.4 2.8 Ep (MeV)
3.2
Calculated production of 110- and 197-keV y-rays from the lgF(p,p'y)'gFreaction and, for comparison, of the 6.13-MeV y-rays from the IgF(p,ay)"Oreaction are given as a function of proton energy. The following parameters were assumed: a thick organic target containing 1 ppm lgF, an integrated beam current of 1 pC, and productlon into a solid angle of 4ir sr. Flgure 1.
. 5 'P
; 80008
7000-
He ATMOSPHERE t LN,
0 INTEGRATED BEAM CURRENT ( pC)
Flgure 2. Relationship between production of 197-keV y-rays and integrated beam current for three different target cooling techniques. A 35-nA beam of 3.5-MeV protons was used to bombard a thick target of pulverized camellia leaves. The conditlons of target cooling are indicated on the figure.
weight) was used so that runs with good statistical accuracy could be made in a time short compared to that observed for significant changes in the target. Measurements made with helium gas cooling or liquid nitrogen cooling alone showed substantial variations with the total beam fluence. However, a combination of the helium cooling with the liquid nitrogen cooling gave constant concentrations in this target and in a variety of the food samples as demonstrated in Figure 2. We believe that the use of helium ensures good heat transfer from target to metal holder and that the use of liquid nitrogen temperatures results in retention of volatile gases that are produced by radiation damage effects at their point of production. For some of our work with samples containing more than 20 ppm fluorine it was possible to dispense with the liquid nitrogen cooling because of the short bombardment times required. Stable values of yield were obtained for beam currents up to 200 nA. For this current the y detector was counting-rate limited, and no effort was made to use higher currents. In practice the number of counts in the 110- and
I
l
l
1
I
I
1
1
110 heV
10800-
10600 10400-
-
= 4 8 k 5
1
1
I
370
I .
380
I
,
,
,
q
,
390
400
410
420
430
440
*
10600 10400
1
197 keV
I0200 10000
* .
9600
*\
94001 9200
9000’ O 6;
640
640 7d0 71b 710 CHANNEL NUMBER
i . --. ’
760
740
Figure 3. Typical pulse height spectra observed for the 110- and reaction at 3.5 MeV. A thick 197-keV y-rays from the lBF(p,p‘y)lBF target of root vegetables with an appropriate fluorine content of 2.6 ppm was used. The solid lines show the method used to determine the background level. 197-keV y-ray peaks could be determined with standard deviation of 3-6% with runs of about l min for vegetation samples at the 100-ppm level and 30-60 min for food samples at the 1-lo-ppm level. The detector used in the experiments was a coaxial detector with a 4.5-cm length and 1.2-cm and 4.3-cm inner and outer radii, respectively, with a nominal 10% efficiency for 6oCo y-rays. The full-widths at half-maximum for the photopeak from the 122-keV y-ray from 57C0was 1.3 keV. A typical spectrum obtained for a root vegetable sample containing about 2.6 ppm fluorine is shown in Figure 3. The simple method used for obtaining peak areas is indicated on the figure. Fluorine is a rather ubiquitous element, and careful measurements of background contributions are necessary. One particularly large source resulted from gas leaking into the beam pipes from the accelerator pressure tank. The insulating gas contained 10% SFGand thus contaminated all surfaces. The use of the nickel window to the target chamber removed the possibility of target contamination from the source. Background produced by the beam striking slits or other parts of the apparatus was determined by passing the beam through an empty target position and stopping it in the remote beam dump. Fluorine y-rays were detected in an amount equivalent to 0.16 f 0.05 and 0.34 f 0.05 ppm in an organic target for the 110- and 197-keV ys, respectively. The measured target yields were corrected for the this background. Two backgrounds are present which arise from the target proper. First, the target contains oxygen and therefore l80 in particular. Proton bombardment of lSO produces neutrons from the 180(p,n)1sFreaction which has a threshold of 2.59 MeV. These neutrons can then interact with the fluorine which was present in the solid materials of the Ge(Li) detector cryostat to produce the low-energy fluorine y-rays by the l9F(n,n’y)lgFreaction. The magnitude of this contribution was determined by using a 3 mm thick lead absorber which transmitted 92% of the neutrons but absorbed 96% of the y-rays. The correction made for this effect was 0.23 f 0.11 and 0.63 f 0.13 ppm for the 110- and 197-keV ys, respectively, at a bombarding energy of 3.5 MeV. The second target contribution arises directly from the l8O(p,y)l9Freaction. Accurate and extensive absolute cross sections for this reaction do not appear to be available. We measured the magnitude of the correction by correlating the intensity of the l8O(p,p’y)l80 1.982-MeVy-ray taken as a measure of the intensities of the 110-keV and 197-keV 19Fy-rays originating in the
180(p,y)19Freaction with the fluorine concentrations in a variety of the food samples. Concurrent observation of the intensity of the 6.13-MeV line made possible allowance for the contributionfrom the actual fluorine present. A correction amounting to approximately 0.1 ppm was found at a proton energy of 3.5 MeV. Such a correction is small enough to be ignored in the present work. There are two other background problems which made difficult the accurate determination of peak areas. A partially resolved unidentified weak line was sometimes observed at an energy about 1 keV higher than the 19F197-keV line. A careful separation of the two lines was necessary during the data analysis to ensure an accurate evaluation of the true yield of the 197-keV y. An even more important problem is that both lines sit on a rather intense background caused by the Compton plateaus of higher energy y-rays. One major contribution was 511-keV annihilation radiation from the positron decay of 13N produced in the 12C(p,y)13Nreaction. This identification was verified by a measurement of the half-life of the decay of the Compton background when the beam was switched off. Another strong y-ray, which was the most prominent feature of many spectra, was the 440-keV y-ray produced in the 23Na(p,p’y)2sNa reaction. The rate from this line was so intense for some samples that it was the major factor in determining the allowable beam currents. Evaluation of the peak area in the presence of this background is straightforward but may in worst cases determine the sensitivity and accuracy of a given F measurement. It is possible to cope with the range of Na and F contents encountered in the actual FDA food samples measured in this work by adjusting the beam currents and time used in each determination. For these samples, the Compton plateau from all higher energy y-rays, including the 23Na(p,p’y)line, was such that detection of l9F at concentrations less than about 0.1 ppm would be difficult. Conversely, for fluorine concentrations in the range observed of a few parts per million, it would be possible to make measurements on samples with 10-100 times the Na concentrations actually encountered. Thehonversion of the observed quantity, y-ray counts per unit integrated incident proton charge (in microcoulombs, pC) to the F content (in parts per million), was performed in three ways. The results given in Figure 1were combined with the value of the solid angle the detector subtends at the target and with its efficiency for detection of the 110- and 197-keV y-rays to calculate the absolute sensitivity for F detection. A second method of calibration, also independent of chemical techniques, utilized a thick F-containing target of known composition. Teflon is a convenient material, and the result was converted to an equivalent figure for an organic material by correcting for differences in stopping powers. Measurements were also made on paired samples of vegetation, one of which contained an intrinsic amount of F and the other of which was treated with NaF, and on the set of vegetation samples measured by chemical methods. A detailed discussion of the calibration technique is given by Shroy et al. (8). The average of the absolute values quoted in Table I of Shroy et al. was used here which are 11.7 f .7 counts/(& ppm) and 11.5 f .6 counts/(pC ppm) for the 110- and 197-keV y-rays, respectively. Our results do not depend on chemically determined F standards.
V. RESULTS Two of the virtues expected for the PAA method are insensitivity to the chemical form of F in the sample and to matrix effects that involve absorption of the 19Fy-rays in the target material. To demonstrate the first property, we measured the yield of I9F y-rays from samples of cellulose containing SnF2,NaF, NH4F,and KF (Figure 4). The yields of y-rays for these samples are equivalent within the estimated
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
411
IlOkeV
I
3.0r
&
30c
I
I
I
i
‘O0I
/ I
I
I
0.25
0.5
0.75
I
I.o
”F CONCENTRATION ( a r b . units)
,,o U SnF2
- - I 1 Na F
N Hq F
KF
Figure 5. Number of 197-keV y-rays from the ’gF(p,p’y)’gF reaction produced by 3.0-MeV proton bombardment of a thick target containing a mixture of cellulose with a nominal 75 ppm I9F concentration treated with Varying amounts of potato containing about 2 ppm F. The solid line is a least-squares fit to the points.
Figure 4. Yleld of 110- and 197-keV y-rays from the iBF(p,p’y)lsF reaction at a proton energy of 3.0 MeV normalized to a thick target lSF concentration of 1 ppm and an Integrated beam current of 1 pC. The targets were of cellulose treated with SnF,, NaF, NH,F, and KF to give samples containing ribout 20 ppm F. The nominal chemical concentrations were used to normalize the data. The width of the error bars for the NH,F samples is an estimate of the standard deviation for a single measurement. The shaded regions are estimates of the standard deviation for the average of three measurements for each sample. errors of counting and addition of F. The error bar shown on the NH4F portion and the shaded areas in Figure 4 are estimates of the counting statistical error to which must be added a systematic error estimate for the mixture to obtain the overall error estimate for the measurement. Measurements were made a t l9F concentrations of nominally 20 ppm to minimize the problem of cmsuring the uniformity of the samples. Since the point of these measurement is to demonstrate that the results do not depend on the chemical form of the fluorine compound, it is only necessary to make the measurements a t the same concentrations. It is not necessary to employ the actual concentrations found in food samples to demonstrate the independence of chemical binding effects. Matrix effects were investigated by measuring the yield of y-rays from a sample of Cellex as a function of target composition when treated with varying amounta of low-F potato (Figure 5). No dependence on target composition was found within the estimated precision of the treatment procedure. This conclusion held for beam currents up to 100 nA, the maximum used in the experiment. Reproducibility was demonstrated with a FDA marketbasket sample of root vegetables cooled by liquid nitrogen and helium gas. The samples were prepared by the method described earlier in the paper, by pressing disks from a lyophilized powder. Two measurements were made on each of four samples on each of four different days (Figure 6 ) . Agreement was excellent and indicated that the variation was explainable by the counting errors. Sample inhomogeneity effects were also shown to be smaller than the statistical counting uncertainties by these results. The analytical techniques described for proton activation analysis and chemical analysis were applied to a set of vegetation samples with relatively high F content,. The results are given in Table 11. The F’AA results use only the absolute calibrations given in (8) and previously described and are entirely independent of chemical techniques. The agreement between the PAA values and the chemical values obtained
30
1
I
~
1
1
TARGET I DAY I
197keV
1. TARGET 2 DAY 2
i. TARGET 3 DAY 3
TARGET 4 DAY 4
Flgure 8. Yield of y-rays from the lsF(p,p’y)lgFreaction produced by 9.5-MeV proton bombardment of four different thick targets of market blasket samples of root vegetables. Resuits have not been corrected for the first two backgrounds discussed in the test, since they are small and are constant from run to run. Error bars represent estimated counting error of individual measurements and shaded regions show the estimated statlsticai uncertainty of the unweighted average of all riasuits. The results show the reproducibility of the method for successive runs on a given target and from target to target.
in the present work as well as the average values obtained from a collaborative study involving 31 laboratories (15)is excellent. This gives confidence in the use of the PAA method at lower F concentrations since calibration and sensitivity are essentially independent of fluorine concentration. It is important to test the accuracy and reproducibility of the method on samples that are as close as possible to the alctual food samples which are subject of the investigation. For this reason as mentioned above, we chose to use an actual food sample obtained from the FDA market basket no. 19 to show that reproducible results were obtained at low fluorine levels found in actual food samples. In considering the performance of recovery experiments it was decided that the preparation of homogeneous and uniform small solid (not liquid, as for miany chemical techniques) samples in the region of 1 ppm
412
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
Table 11. Comparison of Results of Fluorine Analyses of Vegetation Samples by Proton Activation Analysis and Chemical Methodsa fluorine content, ppm
sample
by PAAb
pine alfalfa clover alfalfa corn spruce camellia
32 66 95 101 95 149 447
by chemical methodsC by (micro- chemical diffusion) methodscld 35 69 95 118 123 159 496
35 66 88
97 106 156 469
a Values are reported on a dry weight basis. Average of values obtained for 110- and 197-keV 7-rays. Absolute sensitivity calibration has been used. Measurement was made at 3.5 MeV. Relative values are uncertain by less than 5%, while the absolute scale is uncertain by < i 15%. Precisions of the chemical values are estimated at i 10%. These results are taken from ref 15.
concentrations was not a t all straightforward and would not be a reliable check on the analytical methodology. As an alternate approach, NBS orchard leaves, Standard Reference Material 1571, were used. The use of PAA yielded a vaue of 3.9 f 0.6 ppm in agreement with the uncertified NBS value of 4 ppm, and the chemical determinations, 3.80 f 0.32 ppm, reported by Dabeka et al. (5) and 4.3 ppm by Singer et al. (29) for a sample taken from the material used for the PAA work and 5.0 ppm for an independent sample. The good agreement between these values is an excellent confirmation of the accuracy of the PAA technique with an actual biological sample at a fluorine level similar to those found in most food samples. Good agreement between three different laboratories using different methods is therefore demonstrated for the case of a standard sample. Following the test work on vegetation samples and the recovery experiment, measurements were made on 10 food samples by PAA. Most of them were also measured by the two independent chemical methods described. Further independent chemical measurements were made by Fortin (30). The results are given in Table 111 and Figure 7, which also shows results from the previously discussed vegetation samples. The agreement between the three measurements is poor with the greatest disagreement occurring between the chemical results of the present work and those of Fortin et al. (30). The PAA results fall midway between the chemical measurements. It was also found that there was a systematic difference of about 27% between the values obtained from the 110-keVand' 197-keV y-rays. This could arise from the uncertainty in the determination of the absolute sensitivity for 19Fdetection for each y-ray or from the previously discussed doublet structure of the 197-keV line. The results given in Table 111 are the average of the two determinations weighted inversely by the square of the standard deviation of each measurement. The differences between the results obtained with the two y-rays are small compared to the differences between the average PAA value and the results obtained by chemical methods. VI. DISCUSSION AND CONCLUSIONS Measurements of the fluorine content of FDA FY 75 Market Basket No. 19 food samples have been measured by PAA. These determinations required the development of careful sample cooling methods before reproducible results were obtained for the very low fluorine levels encountered in these materials. The values reported are in the range of 1-8 ppm dry weight which correspond to much smaller values if re-
- / -,0.0
u 0.01
0.1
1.0
IO PAA (pprn)
100
IO00
l0000
Flgure 7. Comparison of results obtained for the F content of vegetation and food samples by chemical analysis as a function of the unweighted mean values obtained by PAA. The chemical values for vegetation are the results shown In Table I, (W) and have F concentrations greater than 10 ppm. The chemical results for food samples are from the present experiment (0)and the experiment of Fortin (30) (0). The PAA results are taken from Tables I I and I11. Curves a and c are linear least-squares fits to the food sample data and are given by the following equations: CHEM = 0.32PAA 0.1 and CHEM = 1.2PAA 4- 0.4, respectively, and CHEM and PAA denote values obtained by the chemical and proton activation methods. Curve b shows the ideal relationship, CHEM = PAA.
+
Table 111. Comparison of Fluorine Analyses of FDA Market-Basket No. 19, FY 1975, Food Samples by Proton Activation Analysis and Chemical Methodsa fluorine content, ppm by chemical methods present Fortin sample byPAAb exptC (30)d dairy products 1.84 i 0.42 0.20 j: 0.07 meat, fish, poultry 2.09 i 0.25 1.29 i 0.07 1.56 1.35 i 0.17 0.18 i 0.03 3.53 grain and cereal 2.27 f 0.52 0.79 i 0.06 2.97 potato 5.11 i 0.19 2.07 i 0.05 leafy vegetables 4.35 f 0.20 2.49 k 0.68 7.17 legumes 2.66 i 0.23 1.15 f 0.05 root vegetables 7.28 f 0.50 1.62 f 0.08 garden fruits 1.13 i 0.14 0.56 i 0.05 2.13 fruits sugars and adjuncts 3.42 i 0.23 0.91 i 0.07 3.93 a Values are reported on a dry weight basis. Mean values f standard deviation of individual measurements. Deviation from absolute values estimated to be within 20%. Mean values i standard error of regression coefficients. Deviation from absolute values unknown. Estimates of precision and accuracy associated with mean values not available. ported in terms of wet weight as is often done in the literature. Measurements of such food samples with fluorine concentrations well below 1ppm dry weight are possible, as can easily be inferred from the spectrum shown in Figure 3. The PAA technique has several very positive attributes which constitute a substantial contribution to the techniques for low-level fluorine analysis. The sample preparation is very simple, and the risks of contamination from the reagents and rather complicated treatments such as are used in chemical methods are minimized. Since PAA is a technique that identifies fluorine by the use of a nuclear reaction, it is, by its very nature, not dependent in any way a t all on the chemical form is which it is bound in the sample. This feature of PAA has been demonstrated explicitly in the data shown in Figure 4. It follows inevitably that PAA measures the total fluorine content of the sample and can be used to check that
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
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chemical determinations are not perturbed by the chemical form. On the negative side, PAA requires the use of proton beams which are not generally available for analytical use and analysis times of several hours for samples with F concentrations at the parts per million level. Matrix effects are straightforwardto cansider. As discussed, a rough knowledge of the stopping power is needed for determination of the fluorine content. The stopping power of organic matrices does not change greatly (
