108
Anal. Chem. 1986, 58, 108-109
(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, it 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
ANALYTICAL CHEMISTRY, VOL. 58, NO. 1, JANUARY 1986
109
Table 11. Determination of Nitrogen in Standard Steel Samples (Mg gel) certified value
first assay
second assay
32 65 293
40 f 3 59 f 5 350 f 30
48 f 4 68 f 5 331 f 28
Table 111. Determination of Nitrogen in Silicon Samples by Photon and Neutron Activation (qg g-l) 1
8
10
l
20
l
30
L
I
50
40
60
l
70
Flgure 1. Variation of the count rate, R , of llquid scintlllation as a function of added aqueous volume to total volume (% Ca).
alternate method for silicon is to oxidize it in a molten NaOHNaN03 bath. After the residue is cooled, it is dissolved in HNOs under argon flow, and carbon dioxide is evolved. Aliquota of the aluminum nitride standard were dissolved under argon flow in a bath containing 20 mL of concentrated H2S04, 20 mL of concentrated H$04, and 1g of KI04at 300 "C. Evolved carbon dioxide was absorbed as before. No carrier was added, as reagents contain a sufficient amount of carbon. Liquid Scintillation Counting, The KOH solutions were mixed with 10 mL of liquid scintillation solution (Instagel from Beckmann) and put in polyethylene vials. The measurements were achieved with a Kontron MR 300 apparatus. Nitrogen concentrations were calculated by comparing carbon radioactivities in samples and standards. RESULTS AND DISCUSSION Chemical Yield. The chemical yield is very close to 100% as checked in our previous studies with llC. This yield is achieved by using two KOH absorbers. Liquid Scintillation Efficiency. The efficiency is the same for samples and standards. We performed studies in order to raise the efficiency to its optimum value taking into account the miscibility of the liquid phases. We have reported in Figure 1the variation of the count rate as a function of the added aqueous volume to total volume ratio. Between 10 and 25% water, we observe two phases. Beyond 25% there is a gel. In practice we work at 33% water; the efficiency decrease is not significant. The time stability of the efficiency was also controlled. Sensitivity. The measurements on nitrogen standards gave an average of 10 pu1ses.s-l-pg-l of nitrogen, which could be compared to the calculated value of 9.4 Bq. For the above irradiation conditions and for a counting time of 1h with a background of 2.3 counts.s-l, the detection limit calculated with 95% confidence limits is 0.012 pg (10). This limit could be decreased with the now available neutron fluxes of 1015 cm-2 .s -1 (ILL reactor at Grenoble). Long checks by y counting showed that the separation is selective and that the measurements are not disturbed by radioisotopes other than 14C. Causes of Error. An error could result from contamination by '*C coming from the environment or from previous separations. The apparatus must be carefully cleaned and periodically checked for blanks. We experimentally checked that the above cited interferences (Table I) from carbon and oxygen are negligible. Analytical Results. In order to control the accuracy, we applied the method to standard steel samples from LECO (Table 11). Statistical error intervals are given for 95% confidence limits. The results are very close to the expected ones, with however a low but significant discrepancy for the 32 and 293 ppm samples. As this discrepancy is not systematic for all samples, it is difficult to attribute it to the analytical '
sample
~
C O % ~
photon activation
metallurgical 1 metallurgical 2 zone-refined
4.3 f 0.4 16.4 40.4
* 1.3
neutron activation 3.2 f 0.4 2.8 f 0.5 14 f 1.2 90.08
method. In fact the accuracy on the standard samples is unknown. For silicon we did not have standards samples, but we could compare to results obtained by photon activation (Table 111). This comparison should be very interesting because in this method the determination is performed through another radioelement. The results are quite consistent and are good arguments for assuming that both methods are accurate. The little discrepancy for metallurgical 1sample may be explained by sample heterogeneity as observed in previous experiments (2). The precision depends mainly on the counting statistics. For the zone-refined sample, the detection limit was considerably lower as compared to photon activation. The value of 0.08 pg-g-l was achieved with a 0.2-g sample and for 4 puls e s d p g - l for the nitrogen standard. In this experiment the highest neutron flux was not available. The detection limit could also be lower if higher sample weight and longer counting time were available. CONCLUSION As with other nuclear activation methods, these determinations are not disturbed by nitrogen contamination, and a very low detection limit can be achieved. The use of a long-lived isotope allows careful surface etching and chemical separation. The availability of neutron fluxes and the possibility of simultaneous irradiation are also favorable factors for the spreading of this method. The disposal of previously developed carbon separations should allow the application of this method to many different materials. ACKNOWLEDGMENT We are grateful to F. Le Goffic and his co-workers for their active cooperation and access to liquid scintillation. LITERATURE CITED (1) Fedoroff, M.; Loos-Neskovic, C.; Samosyuk, V. N.;Chapyzhnikov, B. A. J. Radioanal. Chem. 1982, 72, 715-723. (2) Fedoroff, M.; Loos-Neskovlc, C.; Rouchaud, J. C.; Samosyuk, V. N.; Chapyzhnlkov, B. A. J. Radioanal. Nucl. Chem. 1985, 88, 45-49. (3) Engelmann, C. J . Radioanal. Chem. 1971, 7 , 89-101. (4) Engelmann, C. J. Radioanal. Chem. 1971, 7 , 281-298. (5) Alley, W. E.; Lessler, R. M. Nuci. Data Tables 1973, 7 1 , 621-826. (6) Shukla, P. N.; Kothary, B. K.; Goel, P. S. Anal. Chim. Acta 1978. 96, 259-826. (7) Lavrukhina, A. K.; Alekseev, V. A.; Ivllev, A. I. J. Radioanal. Nucl. Chem. 1985, 88, 145-152. (8) Fedoroff, M.; Loas-Neskovic, C.; Revel, G. J . Radioanal. Chem. 1977, 38. 107-113. (9) Fedoroff, M.; Loos-Neskovic, C.; Revel, G. J. Radloanal. Chem. 1980, 55, 219-232. (10) Currie, L. A. Anal. Chem. 1968, 4 0 , 568-592.
RECEIVED for review July 5, 1985. Accepted September 9, 1985.
