102
Anal. Chem. 1986, 58, 102-108
Evaluation of Homogeneity of a Certified Reference Material by Instrumental Neutron Activation Analysis Byron Kratochvil* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
M. John M. Duke and Dennis Ng SLOWPOKE 11Nuclear Reactor Facility, University of Alberta, Edmonton, Alberta, Canada T6G 2N8
The homogenelty of the marlne reference materlal TORT-1, a spray-drled and acelone-extracted hepatopancreatlcmaterlal from the lobster, was tested for 26 elements by Instrumental neutron actlvatlon analysls (INAA). Through a oneway analysis of variance based on six analyses on each of SIXbottles of TORT-1, ll was concluded that the betweenbottle heterogenelty Is no greater than the wlthln-bottle heterogenelty. The analytlcal results for those elements for which values were provlded by NRC agree wlth the NRC values wlthln 95 % confidence Iimlts.
A marine biological reference material has recently been issued by the National Research Council of Canada in response to the need for a homogeneous reference for testing analytical methodologies, especially for trace toxic metals. The development of new analytical techniques that require only small amounts of sample, or that provide improved accuracy and precision, place increasing demands on the uniformity of composition of reference materials. Accordingly, this study was designed to assess the homogeneity of the NRC reference material TORT-1, a spray-dried, acetone-extracted lobster tomalley. Instrumental neutron activation analysis was chosen as the measurement technique because preliminary operations that may introduce contamination or loss, such as dissolution or separation steps, are minimal; only weighing into irradiation containers or the preparation of pellets is required. Also, a number of elements can be determined with good sensitivity in a single irradiation and counting sequence. Activation analysis has been used at the U.S.National Bureau of Standards on a variety of NBS reference materials and has been applied to evaluation of the homogeneity of 12 lanthanide elements in rock standards from the U.S.Geological Survey (1).
EXPERIMENTAL SECTION Preparation of Samples and Standards. Samples were taken from six bottles of the marine reference material TORT-1, obtained from the Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada. The bottles were tumbled end over end for 2ll2 h prior to removal of material. Portions were removed, placed in clean glass weighing bottles, and dried for 24 h in a vacuum oven (National Appliance Co., Model 5851-6) at approximately 0.06 mmHg. These drying conditions were selected after a study of several conditions and times. The dried samples were stored in a desiccator over magnesium perchlorate. For the short irradiations approximately 300-mg portions of material (TORT-1 or NBS reference) were packed directly into nitric acid washed 400-pLpolyethylene micro-centrifuge tubes. The tubes were heat-sealed and then enclosed in a second larger polyethylene vial for irradiation. For the long irradiations pelletized samples were wrapped first in polyethylene film and then in Parafilm. For this purpose pellets were pressed in 12-mm
diameter dies machined from stainless steel in the shop facilities of the Department of Chemistry, University of Alberta, so as to fit into the polyethyleneviala used for irradiation. Hydraulic press pressures of 2000 psi were found satisfactory for TORT-1, but 4000 psi was required for most of the NBS biological standard materials. Six pellets could be accommodated in one mediumsized irradiation vial. Following irradiation each pellet was repacked with new Parafilm into a new vial to reduce background during counting. Standards were prepared from commercial atomic absorption standard solutions (Aldrich, Alfa, and Spex). For the short irradiations single-element standards were prepared by pipetting 5-100 pL of stock solution onto Whatman No. 1filter paper that had been rolled and inserted into micro-centrifuge tubes. For the long irradiations multielement standards were prepared by pipetting aliquots of the standard solutions onto 1.4-cm diameter Whatman No. 2 filter paper in 7-mL polyethylene vials. The solutions were then dried slowly under an infrared lamp. For volatile elements such as arsenic and selenium the drying step was omitted and the filter paper was sealed directly into polyethylene bags. Irradiation and Counting. The irradiation and counting operations were carried out at the SLOWPOKE I1 reactor facility of the University of Alberta. Table I outlines the conditions utilized for the analysis of each element determined in this study. All counting was carried out with an Ortec 86 cm3active volume Win-15 coaxial Ge(Li) detector, coupled to a Nuclear Data (ND) 660 multichannel analyzer with an Ortec 572 amplifier and ND 575 ADC. The detector specifications include a relative efficiency of 18.5%, and a measured fwhm of 2.1 keV and peak-to-Compton ratio of 53:1, for the 1332-keV photopeak of “Co. Signals were assigned to one of 4096 channels, and the spectra collected were stored on floppy disks for later analysis. The specific activities (counts/microgram for each activated nuclide) were determined for each counting period, and the masses of the various elements in both NBS standards and TORT-1 were determined via an adaptation of the comparator (semiabsolute) method for INAA (2), that is, by dividing the photopeak areas of the unknowns by the specific activities of the relevant radionuclides. Deadtime corrections (3) for the decay of short-lived radioisotopes in the presence of active, longer-lived isotopes such as 24Naand 38Clwere applied to each peak; in addition, a correction factor for random summing effects was calculated and applied following the procedure of Wyttenbach (4). The measurement of aluminum by counting the 1779-keV photopeak of %Al produced by 27Al(n,r)28Al suffers an interference from the fast neutron reaction 31P(n,a)28A1.This was corrected from knowledge of the phosphorus content of the material, 0.879%, and an interference factor of 668 based on measurements of potassium hydrogen phosphate (668 pg of P yields 1pg of %All. The potassium salt was analyzed for aluminum by graphite furnace atomic absorption spectroscopy and a value of 7 pg/g was obtained. This was taken into account in the calculation of the interference factor. The phosphorus interference was calculated to be 13.2 pg/g. A second interference is that of 28A1produced from silicon by the reaction 28Si(n,p)28A1.Analysis of TORT-1 for silicon by graphite furnace atomic absorption gave a value of 231 33 pg/g, based on three analyses of -300 mg portions. Using this value, plus an interference factor of 192 calculated from the measured
0003-2700/86/0358-0102$01.50/00 1985 American Chemical Society
*
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
Table I. Summary of Conditions Utilized for Elements Determined by INAA
element
radioisotope measured
half-life
Na
%Na
14.96 h
c1
3sc1
37.29 min
K Ca Mg A1 sc V Cr Mn Fe co
42K 49Ca 27Mg 28A1 46Sc
12.36 h 8.72 rnin 9.46 rnin 9.24 rnin 83.83 d 3.76 rnin 27.70 d 2.58 h 44.50 d 5.27 yr
cu Zn As
Se Br Rb Ag Ag
I La
52v
Wr 56Mn 59Fe MCO
MCU
66Zn 16As %e %Rb ll0%
'loAg
1281 140La
5.10 rnin 243.8 d 26.32 h 119.8 d 17.6 rnin 18.65 d 249.7 d 24.2 s 24.99 rnin 40.28 h
y-ray energy counted'
analysis schemeb
1369 2754 1643 2167 1525 3083 1014 1779 889 1434 320 1811 1099 1173 1332 1039 1115 559 265 617 666 1077 658 658 443 487 1596 145 103 1408 879 108
B&C B C B B A D A D B D D
B D C D B D D A B C
D 141Ce 32.50 d 153Sm 46.7 h C Sm ls2Eu 12.7 yr D Eu D 72.3 d '60Tb Tb 1 6 5 r n ~ ~ 1.26 rnin A DY 'Lederer, C. M.; Hollander, J. M Perlman, I. "Table of the Isotopes", 6th ed.; Wiley: New York, 1967. Values are in kiloelectronvolts. Times of irradiation, delay, and counting for the analysis schemes were as follows: (A) 120 s-10 s-180 s (irradiation at flux of 1 X 10" n cm-2 s-l, counting geometry 6 cm from detector (open); (B)120 9-30 s-600 s (irradiation flux as in A, counting geometry 6 cm from detector in 10-cm Pb cave); (C) 4 h-6 d-l h (irradiation at 1 X 10l2n cm-2 s-l, counting geometry 3 cm from detector in 10-cm Pb cave); (D) 4 h--21 d-10 h (irradiation flux as in C, counting geometry 1 cm from detector in 10-cm Pb cave). Ce
cross section for the reaction, gives a correction of 1.2 pg/g for the silicon reaction. The total interference then was 13.2 + 1.2 = 14.4 pg/g. This value was therefore subtracted from all the aluminum results before they were reported. All computations were carried out off-line in batch mode using standard Nuclear Data software (with modifications made by personnel at the SLOWPOKE Facility) and the University of Alberta Amdahl mainframe computer. The one-way analysis of variance was performed on the University of Alberta Amdahl mainframe computer using the SPSS-X program ONEWAY.
RESULTS AND DISCUSSION A summary of the analytical results obtained on 300-mg TORT-1 samples from six randomly selected bottles is shown in Table TI for the major elements and in Table I11 for the trace elements. The NBS standard reference materials SRM 1566 (oyster tissue), SRM 1573 (tomato leaves), and SRM 1577a (bovine liver) were run as controls; results for these analyses are given in Tables IV and V, along with the NBS certified values. The precision of the counting statistics is somewhat less than can be obtained using a reactor of higher flux or where no limitations are placed on irradiation or counting times. Two approaches were used in an effort to reduce the standard deviation in the results. Firstly, selected pellets from the long irradiation were counted a second time for 10 h. The precision improved, but the extent of improvement was not sufficient
9
103
to warrant the additional detector time that was required. Secondly, the test portions used for the short irradiations were reanalyzed by the 120 s-10 9-180 s irradiation-decay-count scheme (Scheme A in Table I) several months later to provide another six replicate measurements on each of the six test portions. Values for the second set of measurements are shown in parentheses in Tables I1 and 111. Again the precision of the counting statistics is improved, but the extent of improvement does not warrant in our judgment the additional time and instrument commitments necessary to extend this approach to remaining schemes. We conclude that although the precision attained in this study is not as great as might be attained by the method when other reactor-counter systems are employed, we consider it sufficient to assess the homogeneity of the test material for the elements studied at the test-portion sizes investigated. Evaluation of the heterogeneity of the subsampling operation, along with the results of a one-way analysis of variance for the data of Tables I1 and 111,are summarized in Table VI. The term expressing the relative magnitude of the betweenbottle to within-bottle homogeneity is the F ratio (Table VI, second column from right). Values of F near unity indicate that between-bottle homogeneity is not significantly greater than within-bottle homogeneity. For the 6 X 6 matrix used in this study the analysis of variance falls into the single classification components of variance, model I1 (5). From Table A-7 on p 470 of ref 5 the value for F at the 95% confidence level for 5 between-group and 30 within-group degrees of freedom is 2.53. All save one of the F ratios for those elements determined by long irradiation and counting fall below this value. The exception, europium, is the result of a high value for bottle 155 relative to the other bottles (for europium in bottle 155 % = 0.077 ppm, compared to an average of 0.065 f 0.006 for the remaining five bottles). The other five lanthanide elements that were measured did not show this anomaly. Among the remaining elements the largest F ratio is for iron, with a value of 1.92. The values for the elements determined by short irradiations show larger F ratios in several cases, especially for sodium and the halides. For example, values for the sodium F ratios were 1.1and 5.1 for the long and short irradiations, respectively. Because of the greater uncertainty in the results for sodium in the short runs, due largely to pulse pile-up corrections, we consider the results of the long irradiations to be more reliable. Our overall conclusion is that the between- and within-bottle homogeneity is not statistically different for any of the elements studied.
Subsampling Uncertainty Relative to Measurement Uncertainty. An important factor in assessing sampling hQmogeneity is differentiation of measurement uncertainty from subsampling uncertainty. If the measurement uncertainty is large, the precision with which the subsampling variability can be assessed will be low. Instrumental neutron activation analysis has the advantage that preliminary operations such as sample dissolution, with the attendant possibilities for sample contamination or loss, are not required. When a reactor of constant flux such as the SLOWPOKE is used, the major source of random uncertainty in INAA is counting statistics. (Systematic error, or bias, caused by interferences such as that of phosphorus in the determination of aluminum described earlier is important from the standpoint of accuracy but is not usually a problem in the evaluation of variability in composition.) Instrumental neutron activation analysis also has the capability of simultaneous multielement determination of a range of elements and is nondestructive. Fot short-lived isotopes in particular it is possible to repeat determinations on the same sample, either to check a suspect value or to obtain an improved estimate of the uncertainty in the measurement step.
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
Table 11. Results of Homogeneity Study of -300-mg Samples by INAA for Major Elements in TORT-1” element concn in % bottle no.
Nab
c1c
Ca
Mg
5
3.51 f 0.06 (3.52 f 0.05) 3.51 f 0.07 (3.50 f 0.05)
5.71 f 0.14 (5.80 f 0.06) 5.68 f 0.12 (5.84 f 0.04)
0.85 f 0.12
0.81 f 0.16 (0.77 f 0.06)
0.22 f 0.04 (0.27 f 0.03)
155
3.52 f 0.07 (3.48 f 0.04) 3.53 f 0.08 (3.44 f 0.04)
5.59 f 0.11 (5.80 f 0.05) 5.59 f 0.15 (5.78 f 0.05)
0.78 f 0.17
0.84 f 0.09 (0.82 f 0.05)
0.22 f 0.08 (0.27 f 0.02)
811
3.53 f 0.04 (3.44 f 0.05) 3.54 f 0.04 (3.42 f 0.04)
5.44 f 0.03 (5.72 f 0.04) 5.49 f 0.08 (5.73 f 0.03)
0.79 f 0.36
0.78 f 0.08 (0.88 f 0.08)
0.24 f 0.07 (0.26 f 0.03)
881
3.55 f 0.07 (3.44 f 0.05) 3.56 f 0.07 (3.43 f 0.05)
5.63 f 0.09 (5.73 f 0.06) 5.62 f 0.07 (5.76 f 0.03)
0.78 f 0.18
0.84 f 0.10 (0.83 f 0.07)
0.23 f 0.02 (0.24 f 0.02)
887
3.51 f 0.04 (3.47 f 0.05) 3.53 f 0.05 (3.44 f 0.03)
5.56 f 0.05 (5.73 f 0.04) 5.61 f 0.09 (5.77 f 0.03)
0.87 f 0.15
0.84 f 0.12 (0.82 f 0.06)
0.21 f 0.03 (0.26 f 0.03)
908
3.57 f 0.04 (3.50 f 0.04) 3.59 f 0.03 (3.46 f 0.04)
5.71 f 0.14 (5.86 f 0.05) 5.71 f 0.07 (5.81 f 0.03)
0.88 f 0.27
0.85 f 0.15 (0.93 f 0.08)
0.24 f 0.06 (0.26 f 0.02)
av
3.54 f 0.06 (3.46 f 0.03)
5.61 f 0.13 (5.78 f 0.05)
0.82 f 0.21
0.83 f 0.12 (0.84 f 0.06)
0.23 f 0.05 (0.26 f 0.01)
NRC value
3.67 f 0.20
5.58 f 0.10
1.041 f 0.040
0.895 f 0.058
0.255 f 0.025
K
“Values in table are averages of six counts on each of six individual sample portions. Uncertainties given are f1 standard deviation. Values in parentheses are for a replicate set of six counts on the same test portions by Scheme A in Table I. NRC uncertainties represent 95% tolerance limits for an individual subsample. *First value is for peak at 1368 keV, second for peak at 2754 keV. CFirstvalue is for peak at 1643 keV, second for peak at 2167 keV. Table VI lists the standard deviation in overall results for a number of elements, so, along with the standard deviation in the measurement (counting) step,.,s From these values the standard deviation in the subsampling step, s, can be obtained by
the between-bottle variance still is not statistically significantly greater than the within-bottle variance. It should be kept in mind, though, that for these elements the overall homogeneity is provided by the magnitude of Ingamells’sampling constant, described in the next section.
s, = (so2 - Sm2)1/2
Estimation of Ingamells’ Sampling Constant for Selected Elements in TORT-1.A useful method of estimating
From the values for sa, column 4 in Table VI, it can be seen that the fraction of the overall uncertainty contributed by the counting step, column 6 in Table VI, varies greatly. The subsampling contribution varies accordingly,ranging from 0.02 to 0.93 of the total uncertainty for those elements determined following long irradiation. For iron, which has a relatively high F ratio of 1.9, the value for the counting uncertainty is of the same order as the overall value. This indicates that subsampling is not the major contributor to the larger F ratio. In contrast, for sodium subsampling is the major contributor to the uncertainty in the results. The F ratio of 1.1for sodium indicates, however, that between-bottle variance is of the same order as within-bottle variance. It appears, therefore, that the material, though well mixed, possesses a heterogeneity for sodium yielding a relative standard deviation of the order of 1%.The reason for this is not clear, but may be the result of segregation of sodium, perhaps as sodium chloride, into some of the particles during the spray drying operation. F ratios for bromine and iodine are also high. This suggests the possibility of the other halogens being carried along with chloride during NaCl formation. The fraction of the overall uncertainty derived from test portion variability is greater than 0.5 for several other elements. These include Sc, Co, Zn, Ce, Mg, and Mn. But although test portion variability is greater for these elements,
the amount of subsample that should be taken for chemical analysis so that the sampling uncertainty does not exceed a specified level is that proposed by Ingamells (7,8). He showed that for unsegregated material the product of sample weight W times the square of the relative standard deviation R (in percent) is a constant. This constant K,, termed Ingamells’ sampling constant, corresponds to the weight of sample required to limit the sampling uncertainty to 1%at the 68% confidence level. The magnitude of K, may be obtained by calculating s, from a series of measurements of samples of weight W. Once K, is calculated for a given element in a sample, the minimum weight W required for any preselected maximum relative standard deviation of R percent can be found. It is advisable to estimate K , from measurements on subsamples appreciably different in size to establish whether segregation is present. If K, is reasonably constant over the range of sample sizes taken, then confidence in its use is increased. Results of long irradiations of single 50-mg samples from each bottle of TORT-1 showed, as expected, that the uncertainties in counting statistics owing to the smaller sample size were larger by about a factor of 21/2for cobalt, zinc, and silver. The overall increase in standard deviation is primarily the result of increased measurement uncertainty. Therefore for these elements increwed heterogeneity owing to smaller
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
Table IV. Results of Analyses of NBS Standard Reference Materials for Major Elements (All Concentrations in Percent)
c1
Na SRM 1566 (oyster tissue) found NBS value SRM 1573 (tomato leaves) found NBS value lit. a P SRM 1577A (bovine liver) found NBS value
0.503 f 0.004 0.51 f 0.03
K
0.98 f 0.02
0.80 f 0.15
Mg
0.13 f 0.02 0.15 f 0.02
0.969 f 0.005
(l.O)b
0.0618 f 0.0018
0.12 f 0.01 0.128 f 0.009
4.18 i 0.40 4.46 f 0.03 4.36 f 0.18
0.0534 f 0.0064 0.245 f 0.003 0.243 f 0.013
Ca
0.28 f 0.01 0.28 f 0.01
1.15 f 0.17 0.996 f 0.007
0.0571 f 0.0057 0.0600 f 0.0015
‘Values from “Compilation of Elemental Concer ation Data for NBS Biological ant Environmental Standard Reference Materials”, Report LA-8438-MS,Los Alamos Scientific Laboratory, LOBAlamos, NM, January 1981. ’ Noncertified value, provided by NBS. Table V. Results of Analyses of NBS Standard Reference Materials for Trace Elements” (All Concentrations in p g / g ) .
SRM 1566 (oyster tissue) found NBS value SRM 1573 (tomato leaves) found NBS value lit. av SRM 1577A (bovine liver) found NBS value
A1
sc
V
Cr
Mn
Fe
co
cu
252 i 6
0.071 f 0.003
2.5 f 0.2 2.3 i 0.1
1.45 f 0.31 0.69 i 0.27
17 f 1 17.5 1.2
178 f 32 195 f 34
0.317 f 0.014 (0.4)*
69 f 14 63.0 f 3.5
674 f 97 690 25 500 f 120
*
0.507 f 0.020 (O.6lb 0.490 f 0.110
181 f 28
0.244 f 0.012 0.21 f 0.05
0.175 f 0.004 (0.13)b 0.170 i 0.040
1.0 f 0.3
Zn
As
SRM 1566 (oyster tissue) found 850 f 14 NBS value 852 f 14 SRM 1573 (tomato leaves) found 68.5 f 1.7 NBS value 62 f 6 lit. av 61 f 3 SRM 1577.4 (bovine liver) found 127 f 3 NBS value 123 f 8
SRM 1566 (oyster tissue) found NBS value lit av. SRM 1573 (tomato leaves) found NBS value lit. av SRM 1577A (bovine liver) found NBS value lit. av
5.15 f 0.29 4.5 f 0.5 3.5
13.0 f 0.6 13.4 f 1.9
Se
2.3 f 0.3 2.1 f 0.5
9.8 f 0.4 9.9 h 0.8
Rb
194 f 20
Mo
Ag
4.2 f 0.6 4.45 f 0.09
0.79 f 0.18 0.047 f 0.006 0.71 f 0.07
12.2 f 0.7 12.5 f 0.1
0.33 f 0.11
0.040
0.63 f 0.09 (0.9)* 0.5
3.4 f 1.2 3.5 f 0.5
Sm
Eu
0.41 f 0.18
0.063 f 0.015
0.015 f 0.008
1.28 f 0.18
La
0.046 f 0.020
Ce
(1.6)6
Sb
1.14 f 0.13 0.89 f 0.09
16.8 i 0.9 16.5 f 0.1 16.0 f 0.8
164 f 10 158 f 7
0.027 f 0.007 (0.04)b 0.020
Tb
cs
Hf
0.064 f 0.021
0.25 f 0.02
0.050
“Conditions and uncertainties as in footnote a of Table 11. bNoncertified value, provided by NBS. sample size is not observed. Since the aim of this study was to assess heterogeneity in general, and not to determine the optimum sample size for a given technique, standard deviations for the sampling step were calculated separately, and values for K , were calculated on the basis of s, only. If a
measurement method having an uncertainty of the order of the sampling uncertainty is used, an overall standard deviation incorporating the s, of the measurement method should be included in the estimation of K,. Values of K, based on s, only are given in the last column of Table VI. In general, for those
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
107
Table VI. Standard Deviation in hg/g due t o Sampling for Elements on -300-mg Samples of TORT-1 by INAA measmt std dev, element
overall std dev, so
Sm
sampling std dev, s,
fraction of uncertainty from sample in g
F ratio
K,, based on s,
Long Irradiations Na (1368 keV) Na (2754 keV)
K sc Cr Fe Co (1173 keV) Co (1332 keV) Zn As Se Rb Ag (657.7 keV) Ag (884.5 keV) La (487 keV) La (1597 keV) Ce
Sm Eu Tb
536 610 2104 0.0022 0.329 16.7 0.0361 0.0711 4.51 0.833 0.212 0.413 0.261 0.434 0.359 0.263 0.668 0.031 0.008 0.010
141 191 2431 0.0014 0.242 21.6 0.0213 0.0239 2.45 0.795 0.392 0.409 0.229 0.252 0.402 0.261 0.194 0.023 0.007 0.008
0.40 0.54
0.0018 0.223
0.67 0.46
0.0292 0.0670 3.79 0.241
0.65 0.89 0.71 0.09
0.0575 0.125 0.353
0.02 0.23 0.66
0.036 0.639 0.020 0.003 0.006
0.02 0.92 0.42 0.14 0.36
1.07 1.12 0.22 0.35 1.08 1.92 0.53 0.61 0.24 0.85 0.04 1.35 0.98 0.34 0.78 0.78 1.57 0.31 3.16 0.62
0.75 0.71 0.77 0.58 0.53 0.30 0.82 0.01 0.98 0.87
4.72 5.55 0.24 5.92 3.45 0.30 1.38 0.40 0.41 3.05
1.3 1.4 108 1.0 0.71 16 4.1 0.16 76 260
0.58 0.20 0.28
0.30 0.30 1.72
43 154 172
0.93 0.90
518 579
15.7 9.6 13.3, 12.2n 57 1.60, 1.43" 0.34 0.91 0.25, 0.62" 2.11, 2.43" 0.14 74 9.7 8.1 44
Short Irradiations (10 min count) Na (1368 keV) Na (2754 keV) Mg C1 (1643 keV) C1 (2167 keV) Ca Mn cu Br I
831 889 520 1341 1185 1152 3.28 43.4 60.1 9.18
417 482 250 868 815 961 1.39 43.3 9.6 3.36
719 747 456 1022 861 636 2.97 3.00 59.4 8.54
Short Irradiations (3 min count) A1 V DY a
8.86 0.56 0.085
5.78 0.60 0.072
6.72 0.25 0.045
Values of K. obtained for 50-mg samples.
elements where the counting uncertainties are small, of the order of 1%relative, K, values range from 0.25 to about 2 g. Larger K, values are usually observed for those elements determined by short irradiations, where the measurement uncertainties are greater (especially for the 3-min counting), and for the lanthanides, which are present a t the part-perbillion level.
SUMMARY AND CONCLUSIONS It has been shown by a one-way analysis of variance that the between-bottle heterogeneity of TORT-1 is no greater than the within-bottle heterogeneity for 26 elements. Concentrations of the elementa determined by INAA in this work agree within experimental uncertainty with the values provided by NRC, bearing in mind that the sensitivity of INAA varies greatly among elements. Thus, the uncertainty in the INAA values for potassium is much larger than for an element such as arsenic. Values for elements not certified by NRC should be taken as estimates only. A breakdown of the overall standard deviation in the analyses into measurement and sampling components shows that the fraction of the total caused by sampling varies from element to element. Values for Ingamells' sampling constant, the weight of subsample required to hold the sampling uncertainty to 1%relative a t the 68% confidence level, tended to be lower for those elements in which the relative standard deviation in the measurement step was small. This indicates
that for the number of measurements made in this study, the measurement uncertainties were sufficiently great that calculation of the sampling error was affected. Put another way, the sampling uncertainties tended to be of the same order as the measurement uncertainties, indicating that the homogeneity of TORT-1, especially for the trace elements, is adequate for evaluation of many analytical methodologies in use today.
ACKNOWLEDGMENT The assistance of R. Thapa in sample preparation and of E. Stubley with data treatment is gratefully acknowledged.
Registry No. Na, 7440-23-5; C1-, 16887-00-6; K, 7440-09-7; Ca, 7440-70-2; Mg, 7439-95-4; Al, 7429-90-5; Sc, 7440-20-2; V, 7440-62-2; Cr, 7440-47-3; Mn, 7439-96-5; Fe, 7439-89-6; Co, 7440-48-4; Cu, 7440-50-8; Zn, 7440-66-6; As, 7440-38-2; Se, 7782-49-2; Br-, 24959-67-9; Rb, 7440-17-7; Ag, 7440-22-4; I-, 20461-54-5; La, 7439-91-0; Ce, 7440-45-1; Sm, 7440-19-9; Eu, 7440-53-1; Tb, 7440-27-9;
Dy,7429-91-6.
LITERATURE CITED (1)
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Kosiewlcz. S. T.: Schornbera.P. J.: Haskln. L. A. J . Radioanal. Chem. 1974, 20, 619-626.
(2) Zikovsky, L.;Gallnler, J.-L.J . Radioanal. Chem. 1981, 67, 193-203. (3) Takeuchl, 1.;Uehara, S.; Hayashl, T. J . Radioanal. Chem. 1980, 56, 25-35. (4) Wyttenbach, A. J . Radioanal. Chem. 1971, 8 , 335-343. (5) Dlxon, W. J.; Massey, F. J. "Introductionto Statistical Analysis",3rd ed.; McGraw-Hili: New York, 1969. (6) Ingarnells, C. 0.; Switzer, P. Talanta 1973, 20, 547-568.
108
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(7) Ingamells, C. 0.Talanta 1074, 27, 141-155. (8) Ingamells, C. 0. Talanta 1076,23,263-264.
RECEIVED for review October 9,1984. Resubmitted July 22, 1985. Accepted July 22,1985. This research was supported
by the National Research Council of Canada under Contract OSC83-00225 for the Marine Analytical Chemistry Standards Program, the University of Alberta, and the Natural Sciences and Engineering Research Council of Can8da through an infrastructure grant to the SLOWPOKE I1 reactor facility.
Determination of Nitrogen in Metals and Semiconductors by Thermal Neutron Activation Jean-Claude Rouchaud and Michel Fedoroff* Centre d'Etudes de Chimie Metallurgique, Centre National de la Recherche Scientifique, 15 rue Georges Urbain, F 94400 Vitry, France
A method of nitrogen determination through the 14N(n,p)14C reaction is presented. Carbon is chemlcally separated from molten sal baths by methods previously developed for carbon determlnatlon. Radioactivlty measurements are performed by liquld sclntlllatlon. A limit of detection of less than 1 pgg-' of nltrogen can be achieved. The method was applled to steel and silicon samples.
Carbon, nitrogen, and oxygen are the light elements that are most often present as impurities in solids. Even a t low concentrations they alter the properties of metals and semiconductors. Among these elements, the role of nitrogen is the least known, due to a lack of analytical results a t low concentrations. The main nonnuclear techniques for nitrogen determination are the Kjeldahl method and the vacuum or oxidizing fusion method. For concentrations of parts per million or less these methods are not reliable. Errors come from contaminations from the environment. Nuclear activation methods can remove these contaminations by postirradiation etching of the sample. Among activation techniques, we have already used irradiation in gamma photons through the 14N(y,n)13Nreaction (1,2). This method requires an electron accelerator, which may not be readily available. Chemical separations must be performed with a 10-min half-life isotope. Nitrogen may also be determined by charge particle activation (3, 4 ) . This method requires a charged particle accelerator, which may not be readily available. Due to charged particle absorption in matter, definite sample geometries and special standardization methods are needed. In this method only one sample can be irradiated at a time. Neutrons from nuclear reactors are the most available means of irradiation in many countries. Since the neutron flux is practically homogeneous in a large volume, several samples can be irradiated at a time. Nitrogen can be activated through the 14N(n,p)14Creaction. The thermal neutron cross section is rather high 1.8 b (5) for a 99.64% abundancy. However this method has been very scarcely used up to now. T o our knowledge, i t was only applied to rocks (6, 7). The long half-life of 14C (5736 years) necessitates a long irradiation time. For a 70-h irradiation in a thermal neutron flux of 1.26 X 1014cm%-l, we calculated a radioactivity of 9.4 Bqpg-l of nitrogen. Oxygen and carbon may interfere in the determination through the 13C(n,y)14Cand 170(n,a)14C reactions. Cross sections and isotopic abundancies are very
Table I. Intensity of Interferences of Carbon and Oxygen on Nitrogen Determination cross
abundancy,
apparent pg of N for 1 pg of
element
section, b
%
interfering element
N C 0
0.001
1.11
6.7
0.27
0.039
5.0 x 10-5
isotopic
1.8
99.64 X
lo4
low for these elements as shown in Table I. The cross sections decrease for higher neutron energies. For these reasons the interferences are negligible except for samples containing oxygen or carbon as major elements. 14C is a pure p emitter, whose maximum energy is 0.15 MeV. A selective chemical separation of carbon must be performed in order to compensate for the lack of selectivity of p counting. In our previous studies on carbon determination by photon irradiation we developed a series of chemical separations by extraction from molten salts (8, 9). These methods can be applied for the present purpose. Another problem that arises from the low /Y energy is that errors can arise from the self-absorption of the radiation during measurement. One way of avoiding this effect is to use a gas-filled counter. But the technique of filling the counter is rather complicated. We chose liquid scintillation, which also has a high counting efficiency. The only problem is to put the extracted carbon in a mixture appropriate for scintillation counting. The purpose of this study was to set up an analytical procedure and to check it for accuracy and sensitivity. We chose silicon and steel as examples of analyzed matrixes.
EXPERIMENTAL SECTION Irradiation. Samples ranging from 25 to 500 mg and aluminum nitride standards of about 300 mg were wrapped in aluminum foil. They were irradiated for 70 h in the Osiris reactor of the Nuclear Center of Saclay in a thermal neutron flux from 1x to 2.5 x 1014cm-2-s-1. After the samples were irradiated, they were kept 1 month for cooling. Etching. In order to remove superficial contamination, samples were chemically etched after irradiation three times by 15 mL M HF solution for silicon samples and three of a 4 M "03-9 times by 15 mL of aqua regia for steel samples. Weighting was performed after etching. Chemical Separations. Iron and silicon samples were treated by methods whose details have already been published (8,9).In these methods iron or silicon is oxidized in a molten Pb3O4-B203 bath under argon flow. Carbon dioxide is absorbed in KOH. An
0003-2700/86/0358-0 108$01.50/0 0 1985 American Chemical Society
