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Anal. Chem. 1991, 63, 2156-2162
Multielement Prompt y Cold Neutron Activation Analysis of Organic Matter Matthias Rossbach Institute of Applied Physical Chemistry, Research Center (KFA) Jiilich, P.O. Box 1913, 0-5170 Jiilich, Germany
The setup and experlmentai condltlons of a prompt y cold neutron actlvatlon analysis (PGCNAA) instrument at the external neutron gulde laboratory (ELLA) of KFAJullch are described In detali. A novel approach to establlsh a “clean” sample environment wlth mlnlmal lntroductlon of background-produclng material is presented and tested for the analysts of a number of elements (H, B, C, N, 0, F, AI, P, S, CI, K, Ca, V, Mn, Co, Cd and Sm) In dried organic matter applled to blologlcai materials. Mlnlmum detection IlmRs (between ng/g and mg/g) and results are glven for six reference materials. The technlque proved to be sensitive, reliable, and complementary wlth respect to the elements to be determlned by the two methods, INAA and PGCNAA respectively.
INTRODUCTION Chemical characterization of biological materials for the Environmental Specimen Bank (ESB) Project of the Federal Republic of Germany is accomplished by the application of various physicochemical analytical techniques including mass spectrometry, electrochemical methods, different types of atomic absorption, and nuclear analytical techniques like instrumental or radiochemical neutron activation analysis (I). Some of those techniques have oligo- or multielement capabilities. The number of elements to be determined in environmental materials is rather large (30-35 elements of the periodic table), but they are mostly from trace and ultratrace concentration levels only. The bulk of materials consisting of H, C, N, Al, Si, P, S, and Mg has not yet been determined. Great demand exists however for data on some of these elements as they might be somehow related to malnutrition, forest decline, and other environmental problems; e.g. excess A1 in the soil has been suspected of causing root damage in trees and Mg deficit can cause reduced resistance to viral diseases in plants. There are very few data in reference materials given for these elements and so a reliable instrumental technique capable of detecting and quantifying them in biological materials in a single analytical step is highly desirable. For almost 50 years nuclear techniques like instrumental and/or radiochemical neutron activation analysis based upon the thermal or epithermal neutron-induced radioactivity in the sample and measurement of the delayed emission of y radiation by the use of semiconductor detectors have been well-known and applied in numerous ways to biological materials. The detection of spontaneously emitted y rays resulting from neutron absorption (PGNAA)-equally useful for quantification of the chemical composition of a sample-has only rarely been used since their successful demonstration by Henkelmann and Born in 1973 at a thermal neutron beam of the high-flux reactor in Grenoble, France (2). They reported detection limits for various elements that gave rise to a great optimism for the technique. However technical problems and the scarcity of filtered neutron beams did not allow a rapid 0003-2700/9 1/0363-2 156$02.50/0
development of this particular multielement technique until recently. Besides various attempts to determine single elements (B, Cd, N, C1) in vivo for biomedical investigations, multielement application of PGNAA to biological samples are only rarely reported (e.g. see refs 3-5). Another publication describes PGNAA application to food samples (6). H20 in geological materials (7), sulfur in coal (8), and boron in borosilicate glasses (9) are only a few examples of PGNAA application described in the literature. Multielement determination in geological samples is described by a number of authors (e.g. see refs 10-12). Additionally, interesting applications of PGNAA are reported (13, 14). These few examples demonstrate the broad range of potential use of the technique. More and more analysts are becoming aware of the inherent advantages of PGNAA for the chemical characterization of environmental and other materials because it is a multielement technique that is truly nondestructive and allows the same aliquot of a sample to be analyzed by different methods (15). The dramatic improvement in cold source and neutron guide technology that made beam diameters larger and fluxes more intense initiated the installation of neutron guide laboratories at a greater number of nuclear installations worldwide. At KFA-Julich a small neutron guide laboratory that had existed since 1970 was replaced by a new and larger external neutron guide laboratory (ELLA) connected to the Jiilich research reactor FRJ-2. New and more efficient technology was used in the layout and construction of the cold source, the %Ni-coatedneutron guide system, and the building itself. The implementation of the project took from April 1983 until late 1986 and the facilities were erected according to the German Atomic Energy Act (16-18). Altogether 10 instruments use the absolute cold neutron flux of 5.4 X lo8 n/(cm2 s), measured at the entrance of the hall for scattering experiments. Full experimental conditions for Prompt Gamma Cold Neutron Activation Analysis (PGCNAA) in these facilities will be described in detail in this report. Basic Considerations for PGCNAA. Neutrons interacting with matter are scattered and/or absorbed. The magnitude of the c r w section (in barns, 16%cm2)of a nucleus determines the fraction of neutrons for either reaction. If absorption takes place, the surplus of energy resulting from the neutron velocity and additional binding force transferred to the compound nucleus is emitted from the resulting excited level in the form of y rays until the isotope has reached a stable or metastable level within s (prompt). The absorption cross sections of most elements are inversely proportional to the velocity of the neutrons (u l/u). Therefore slow (=cold) neutrons with a wavelength of about 5 8,enlarge cross sections by a factor of 2.8 compared to thermal (1.8 A) neutrons. The critical angle for the total reflection of neutrons on 58Ni-coated beam tubes is proportional to the wavelength. This means that cold neutrons are transmitted with higher efficiency than thermal neutrons, so they are more effectively guided away from the reactor core into a low y ray background environment. A t the ELLA laboratory of KFA-Jiilich this was accomplished by the installation of a H2 cold source near the
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
core of the FRJ-2 reactor. Three independent neutron beam tubes guiding neutrons into the neutron guide laboratory (ELLA) are connected to the reactor biological shield (16,19). Prompt y rays are emitted from =lo0 keV to =lo MeV by virtually all elements of the periodic table with different neutron absorption efficiency. y rays with energies of more than 1.02 MeV produce more than one peak in the detector. Due to pair production single and double escape peaks separated by 0.51 MeV make these spectra much more complicated than delayed y spectra (up to ~ 2 . 0MeV) usually are. As the resolution of y detectors declines with increasing energy (typically 2 keV a t 1.33 MeV and 4.5 keV a t 7.5 MeV), the evaluation of peak areas from prompt y spectra of natural materials with 4W500 identified peaks calls for sophistication in the evaluation software greater than is currently available for nuclear spectroscopy. Spectral contrast is proportional to the peak to background ratio. Background is produced predominantly by Compton scattering of y rays in the detector itself. The counting error of a given peak is a function of the inverse square root of the time:
f = peak error in percent c = constant
Therefore increasing the irradiation time by a factor of 4 will increase the percent error in spectral contrast by a factor of only 2. The use of a Compton suppressor, either an NaI or a BGO crystal, surrounding the central Ge detector to prevent an appreciable amount of Compton-scattered events from being collected by the multichannel analyzer will increase the peak to background ratio by factors of 6-10 (20). A total of 16 K channels are available today and should be used for the sake of better peak resolution. The detection limit of the method is a function of the neutron beam intensity, the sample size, and the detector array efficiency. Geometric considerations, e.g. an increase of the solid angle between the detector and the sample position, compete with count rate limitations and dead time corrections of the whole system. Therefore high count rate systems and fast analog to digital converters (ADC's) are necessary. Depending on the diameter of the neutron beam the sample should cover almost the entire beam area but the thickness of the sample can cause severe flux depression in the sample by scattering out of neutrons. A 6 mm thick bovine serum sample acts almost as a beam stop for cold neutrons, visualized by indium autoradiography (see Figure 1). Hydrogen-rich materials should therefore be prepared as thin samples, preferably of i l - m m thickness. Figure 1 seems to document the experimental evidence for a low and tolerable influence of the sample slab on the attenuation of the neutron beam. Optical thickness is low, and the influence of scattering appears to be low compared to the absorption probability. A theoretical paper by Copley and Stone (21)treats the matter more on a statistical basis. They calculated macroscopic correction factors C for the reaction rate R for absorption of neutrons in slab samples (eq 2.38 of ref 21). m
c=ccj j=O
C is a sum of individual c, depending upon the macroscopic cross section ratio x3 = Cs/CA for scattering and absorption, respectively, and the optical thickness 7 = x A t * (t* = "effective thickness" in the incident beam direction). A geometry factor CY is defined as the ratio between lateral dimension 1 and thickness t* in the light of which multiple
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Figure 1. Indium autoradiography (5 min irradiation, 2 min decay, 5 min exposure time) of biological samples. Right: 13-mm pellets of Bovine Liver SRM 1577a of different thickness, 1, 2, 4, and 6 mm. Left: 2794.6 mg of Mixed Diet RM 8431a pressed into a flat slab (0.8 X 47 X 64 mm).
scattering of neutrons within the samples is treated. By Monte Carlo calculation they found out that "C increases with increasing 7 when CY is greater than 1 and becomes independent of CY when CY is much greater than unity: this is the case of an infinite plane slab sample". In our case CY = 70 and so the correction factor C for the reaction rate R should only be dependent upon the optical thickness 7 . In the limiting case of a strong absorber where x3 becomes 0, the only contribution to C is the self-shielding factor C,, which for a slab sample is given by (eq 5.1 of ref 21)
C,(7) = [l - e x p ( - ~ ) ] / ~
(3)
and accounts for the fall in neutron flux with increasing depth of penetration into the sample. As the sample becomes more and more protonated, the multiple scattering effect becomes increasingly predominant. For biological material rich in hydrogen (5-10 wt To), the fate of slow neutrons will be governed by the magnitude of the cross sections for scattering and absorption on hydrogen, respectively. The cross section for incoherent scattering is independent of the wavelength (C, = 80 X cm-2 for hydrogen) whereas the absorption cross section increases with increasing wavelength (CA(H)= 1 barn for 4 A neutrons). A rough estimate of the beam attenuation by scattering can be carried out by applying Lambert-Beer's law:
I x = Ioe-px
(4)
with j~ = nC3. For a pure water sample with 11.1 w t %, hydrogen, n is equal to 2 X X 6.023 X loB, hence it follows that the l / e thickness is 0.18 cm. For the human diet sample (RM 8431a) shown in Figure 1with equal specific density (2.52 cm3 and 2.795 g) but only 6.67% hydrogen (see ref 6), the thickness of the sample by which the neutron beam will be attenuated to a factor of l / e by elastic scattering is approximately 0.31 cm. For our 0.08 cm thick sample this first-order approximation reveals a correction factor for the intensity of the beam of 0.773. The whole problem of hydrogen scattering in biological samples is greatly reduced by applying the standard comparison method and taking great care that standards and samples are of same (or very similar) matrix types. For their preparation see the section Preparation of Standards and Samples below. The most difficult manipulation to enhance sensitivity for PGCNAA is related to the neutron flux intensity. Most ex-
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901
- -
I”
bo0 125
Figure 3. Irradiation chamber for biological materials in PGCNAA analysis.
“-4
+ Hv
973Amp1
Graphite
: 1-Teflon
HBA
Somple
T 5200 PC
Fl~m 2. Schematic setup of the PGCNAA device at EUA, KFAJiilii.
ternal irradiation facilities offer neutron intensities, ether cold or thermal, of about 108-109n/(cm2 s). Focusing of cold neutrons with the aid of super-mirror focusing devices is possible (221, and compressing a 50-cm2beam area to about 1 cm2 and using a 70% transmittance of such a device could allow a 5 X 10s n/(cm2 s) beam to approach in the near future highly attractive 1O’O n/(cm2 s). Natural background radiation problems are normally not considered to be serious, as the count rates under irradiation conditions are orders of magnitude higher than from natural radiation. Passive shielding of the detector by lead and boron-carbide may, however, be advisable in a crowded experimental area, as an external neutron guide laboratory normally is, where changing neutron- and y-ray fields might be produced by neighboring experimenters. The only serious background consideration is due to the fact that neutrons from the irradiating beam hit materials other than the sample and this is always the case, as scattering cannot be predicted and prevented. Therefore y rays producing background are emitted from all sample-supporting and -surrounding materials and most of the efforts to decrease background radiation should be devoted to creating a “clean” sample environment, by using either materials with low absorption cross section (Teflon, graphite) or high cross section but no y ray emission (6Li) (see ref 23). EXPERIMENTAL SECTION The irradiation position is at the end of neutron guide NL1 about 50 m away from the biological shield of the reactor. The cross section of the beam is 50 X 100 mm. Neutrons are homogeneously distributed across the beam area as indium autoradiography demonstrates. The Gamma-X HPGe-detector (EG & G Ortec) with pop-top cryostat configuration (72 H LNlholding time) has an active area of 49 mm2and a 52-mm length, a 0.5-mm beryllium window, and a resolution of 1.9 keV at 1.33 MeV. Peak Compton ratio (@To) is 50:1, and the relative efficiency for 1.33-MeVy rays is 20.0%. The detector crystal is connected to a chargesensitive transistor reset preamplifier. The output pulses are fed through a 973 Ortec high count rate amplifier at 2.5-ps shaping time and stored in an ADCAM 919 multichannel buffer (16K channels, 10-ps fixed
foll
lOtm
Figure 4. Graphite frame for sample support. conversion time ADC). The whole system is controlled from a portable Toshiba T 5200 personal computer with 40 hfbyte hard disk (see Figure 2). The software for the evaluation of the spectra is from EG & G Ortec, Omnigam and runs under MS-DOS 4.0. The Doppler-broadened peak of boron at 478 KeV was always read out by hand. Contributions from Na (472.3 keV) were subtracted when necessary by applying a correction derived from pure sodium and its peak ratios (398214’72.3keV). For the identification of major prompt y lines, refs 24 and 25 were used. Detector Shielding. As the irradiation position can be used only part time for PGCNAA, the whole system must be flexible and easy to mount and dismount. The detector shielding consists of a lead cave (63 X 59 X 47 cm) with a tungsten slit collimator of 50-mm thickness at the front. The whole box is surrounded by 10-mm boron-carbide and is adjustable in height by a slow motion motor drive. The support is a massive table of stainless steel moveable on four wheels. A table connected to the shielding construction at its front carries the irradiation box for the samples. Irradiation Chamber. The configuration of the special irradiation device designed to position the samples in the beam in front of the detector is shown in Figure 3. As PGCNAA at the present stage is evaluated on the basis of standard comparison, a reproducible geometry of samples and standards is mandatory. As was mentioned in the section Basic Considerations for PGCNAA, one has to take great care to minimize background from neutron absorption in materials other than the sample. Teflon foil 0.014.1 mm thick (CRP Inc., Ronkonkoma, NY) is a very stable and clean material that can be used for the entrance window and in the form of heat-sealed bags to contain and support the samples. These bags are then clipped between two graphite frames that fit into each other (seeFigure 4). The frame can be manually dismounted sidewise, in the same way as with an old fashioned slide projector, after each irradiation to change the samples. The box is made of aluminum (2-mm thickness) and with the exception of the entrance window is internally lined with 2.5-mm
ANALYTICAL CHEMISTRY, VOL. 63,NO. 19, OCTOBER 1, 1991
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6LiF ceramic; the rear is covered by 7.5-mm 6LiF and acta as a beam stop. Carbon and fluorine have very low absorption cross sections (3.37 and 9.5 mb, respectively) whereas 6Li absorbs neutrons with high probability (0th= 940 b) and has the unique attribute of not producing any y rays: 6Li(n,a)T
(5)
This reaction produces tritons (T) of energy 2.73 MeV with isotropic angular distribution. Nuclear interaction between these tritons and 6Li or 'Li leads to the production of high-energy neutrons via several reaction channels. According to ref 26 a thick target of 6LiF would produce a total of =lo4 fast neutrons per triton with energies up to 16 MeV. In the case of our beam with 9 X lo9 n/s being absorbed in 7.5-mm %iF, 9 X lo6 fast neutrons are created by triton/6Li and triton/fluorine interactionsand considering 1/Pattenuation about 2500 fast neutrons/s will hit the sample 30 cm away from the rear. 6LiF was produced by the reaction of hydrofluoric acid with commercially available 6Li2C03. After thorough drying, the material was slightly moistened with D20 and pressed into rectangular pellets which were then sintered at about 750 "C. These ceramic slabs (32 X 45 X 2.5 mm) were glued to the aluminum surface by waterglas@LiF paste. This material is superior to all other glues because it is almost hydrogen-free and H2 is a strong neutron absorber and scatterer. It is very useful to avoid all materials that would introduce any great amount of hydrogen into the vicinity of the sample. The measured absorption coefficient for cold neutrons of the 6LiF cermaic is ~1 = 1.18, and using eqs 3 and 5, one can calculate the half-thickness to dl12= 0.587 mm; 4 1 2 = 1n 2/r (6) that means with 2.5" thickness 95.4% of the scattered neutrons is absorbed. Residual H2 background introduced by this material is 0.33 counts/s. The box can be flushed with He in order to minimize background from N2 or water vapor. Preparation of Standards and Samples. Biological materials are pressed into slabs 47 X 67 mm wide and 51-mm thickness. The dried, homogenized materials were filled in appropriate amounts into Teflon foil bags, mounted in a stainless steel frame, and covered by thin A1 foils of the size of the slabs. These sandwiches were put between the heating plates (100-mm diameter) of an ordinary pill press, heated to 50-80 OC and manually pressed with up to 15 tons for about 2 min. The thickness of the resulting sample slab is given by the thickness of the frame and the covering foils; surplus material is squeezed out at the edges. After cooling, the sample is cut out of the Teflon foil, the covering foil is removed, the sample is weighed and moved into a new, clean Teflon bag which is heat-sealed airtight. This kind of sample preparation turned out to be careful and nondestructive, and the relatively large samples (0.8-3.0 g) are easy to handle and stable. Moisture content and volatile5 particularly in oily samples (e.g. fish) are in some cases squeezed out, and therefore care must be taken to select appropriate conditions not to alter the sample matrix by the preparation process. Standards were prepared by mixing known amounts of high-purity (suprapure or ultrapure grade) compounds or elements with chromatographic grade cellulose powder and then pressing them into slabs of the same size and thickness as the samples. Due to uneven thickness of samples and/or standards, geometric effects in the counting process can occur with such large samples and therefore the spectra of some of the samples and standards were taken while the samples were rotated from time to time in all four directions. The standard deviation for the count rates of single peaks from these measurements is typically between 3 and 5% and does not deviate significantly from the normal counting statistics. Therefore this effect does not need special consideration.
RESULTS AND DISCUSSION Preliminary experiments were devoted to the characterization of the neutron beam itself. By Au-foil activation the neutron flux rate a t the end position of neutron guide NL1-the experimental area-was determined to be 1.5 X 10s n/(cm2 s) a t a reactor power of 23 MW with the cold source
Figure 5. Time-of-flight spectrum of the cold neutrons from NL-1 at the ELLA laboratory, KFAJulich.
Table I. Mean Values of Different Prompt (10 Measurements, 300 s Each (counts/s)) 341.9
y
energy, keV 1380.9 1585.4
Lines from Ti
4881
mean f S 512.8 f 28.2 495.1 f 19.2 57.6 f 2.4 7.63 k 0.623 % 5.5 3.9 4.2 8.2 providing neutrons of a mean wavelength of 5.11 A. The Cd ratio of the beam was determined to be 104:1(27). The energy distribution of the neutrons was determined by time-of-flight measurements. A Cd-covered chopper was put downstream of the end window of the neutron guide and rotated at 50 Hz. The slit opened the beam for some 0.3 ms/cycle and an He counter recorded the neutrons 5.5 m apart according to their velocity. The resulting spectrum can be seen in Figure 5. Several dips in the distribution pattern are attributable to the extraction of certain wavelengths by different experiments (time of flight = TOF, diffuse scattering spectrometer = DNS, triple axis spectrometer = HADAS) and the Bragg-scattering of the bismuth filter of the plane (222) (22). More information about the laboratory ELLA,the experiments installed, and the beam characteristics are given in refs 18 and 19. Pure elements in the form of foils or metals with the hghest purity grade available were irradiated, and y energies and intensities were recorded to detect possible interferences and to generate a library for the evaluation of spectra. T i (SRM 354) (237.5 mg) was used as a flux monitor to detect alterations in neutron beam intensity due to changing reactor power and/or influences by other experiments upstream of our beam. During 1week of continuous measurements, the count rate of different prompt y lines from T i (10 measurements of 300 s each) scattered by only 3.9-8.2% (see Table I). Background count rates while a blank Teflon bag was irradiated remained stable to the same extent. Resulta of several elements with sufficient counting statistics are given in Table
11. Detection limits for a number of elements determinable by PGCNAA in selected reference materials under various irradiation conditions were evaluated by using the certified concentrations and relating the actual count rates to 3 d B G (BG = background) under the peak of interest. From Table I11 it can be seen that B, Cd, Sm (V), F, and Co can be determined under real experimental conditions at the ppb level. Other elements can be easily quantified in biological materials in their natural concentration levels. It is an advantage of the method that sensitivities for low-2 elements correspond closely to their natural abundance5 in biological materials; e.g. C in natural matrices in the percent level has
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Table 11. Results of Several Background Measurements under Various Conditions (counts/s) element (energy, keV) B (478) no. 235 no bag
annih rada (511)
C (1261)
F (1633)
N (1884)
A1 (1779)
H (2223)
5.38
62.9
1.94
0.63
1.61
4.81
1.91
17.9
64.07
2.86
8.17
2.69
4.82
2.39
18.3
63.99
2.89
7.73
2.32
5.19
2.64
12.97
51.28
2.90
7.69
2.9
4.92
2.23
9.44
50.34
3.04
7.76
2.75
5.65
2.18
11.08
51.44
2.87
7.71
2.84
5.11
2.17
9.31
51.04
2.74
8.08
2.75
4.95
2.14
10.38
50.99
3.05
7.75
3.07
5.53
2.07
9.58
49.12
2.71
7.81
2.92
5.13
1.88
9.96 f 7.6
51.0 f 2.2
2.88 f 4.2
7.84 f 2.3
2.78
5.12 f 5.8
2.21 f 10
33.3 pg
64.1 pg
5.86%
0.48 mg
1390pg
3.08%
2.27 mg
600 s
no. 237 old bag 600 s
no. 236 new bag 600 s
no. 248 new bag 900 s
no. 251 new bag 900 s
no. 254 new bag 900 s
no. 257 new bag 900 s
no. 268 new bag 900 s
no. 262 new bag 60.500 s
MW f S (excluding no. 235), % mass
1.96 pg
5.47 mg
for comparison: mass values given by NIST
0.24 pg
4.7 mg
7.96
1982 (34)
Annihilation radiation. Table 111. Detection Limits in Selected Reference Materials (3v'BG) under Various Irradiation Conditions (All Values in d k g )
element (energy, keV) Co (230) Mn (314) Sm (335) B (478) Cd (558) V (645)
K (770) C1 (787.4) S (841) S (2379) S (3221) C (1261) F (1633) A1 (1778) Ca (1942) P (2153) 0 (2185) H (2223) N (5269)
Citrus Leaves SRM 1572 1000.8 mg
Pine Needles SRM 1575 3607 mg
Vehicle Exhaust NIES No. 8 334.7 mg
Milk Powder SRM 1549 1479.5 mg
Bovine Liver SRM 1577a 2347.2 mg
Mixed Diet RM 8431a 2794.6 mg
50000 s
52000 s
15000 s
12000 s
15000 s
43000 s
0.0046 0.7 0.036 0.028 0.04 0.038
0.007 7.38 0.146 0.039
