Halogen determination in Arctic aerosols by neutron activation analysis with Compton suppression methods

The study of halogens particularly bromine and chlorine in Arctic aerosolshas received a great deal of attention in the past decade in ozone depletionduring polar sunrise studies. Iodine has also been studied as part of geochemicalcycling. We have shown that all three of the above elements can be determinedsimultaneously with very low detection limits using epithermal NAA in conjunctionwith Compton suppression methods. Besides lowering the background considerably,Compton suppression can eliminate or minimize the overlapping peak of the620 keV photopeak arising form the 1642 keV double escape peak of ³⁸Cl interfering with the 616.9 keV photopeak of ⁷⁹Br(n,) ⁸⁰ Br reaction. Iodine is ideally determined by epithermal NAAbecause of its very good resonance integral cross-section. Although chlorineis usually determined using thermal neutrons via the ³⁷Cl(n,) ³⁸Cl reactions, epithermal NAA is still feasible for the Arcticaerosol, since it has a major sea-salt component.     

Compton suppression instrumental neutron activation analysis performance in determining trace- and minor-element contents in foodstuff

In 2003–2004, several food items were purchased from large commercial outlets in Coimbra, Portugal. Such items included meats (chicken, pork, beef), eggs, rice, beans and vegetables (tomato, carrot, potato, cabbage, broccoli, lettuce). Elemental analysis was carried out through INAA at the Technological and Nuclear Institute (ITN, Portugal), the Nuclear Energy Centre for Agriculture (CENA, Brazil), and the Nuclear Engineering Teaching Lab of the University of Texas at Austin (NETL, USA). At the latter two, INAA was also associated to Compton suppression. It can be concluded that by applying Compton suppression (1) the detection limits for arsenic, copper and potassium improved; (2) the counting-statistics error for molybdenum diminished; and (3) the long-lived zinc had its 1115-keV photopeak better defined. In general, the improvement sought by introducing Compton suppression in foodstuff analysis was not significant. Lettuce, cabbage and chicken (liver, stomach, heart) are the richest diets in terms of human nutrients.     

An innovative Compton suppression spectrometer and its capability of evaluation in multi-elemental neutron activation analysis

A new Compton suppression system was established. A low energy HPGe detector (LO-AX) is used as the analyzing detector. It is completely shielded by a 50%n-type HPGe and two NaI(TI) detectors. Experiments show that a substantial Compton suppression effect is also achieved in the low energy region down to 15 keV. The capability of applying this system in INAA was evaluated for 13 elements (As, Cd, Co, Cr, Hg, Mo, Ni, Sb, Se, Sr, Th, U and Zn) in biological and environmental samples. The benefits and problems of using this Compton suppression spectrometer in INAA are examined for each element.     

Determination of zinc in geological samples using Compton suppression with thermal and epithermal instrumental neutron activation analysis

The determination of Zn in geological samples using instrumental neutron activation analysis is usually done using the ⁶⁴Zn(n,γ)⁶⁵Zn reaction and its 244 day half-life. However this analysis has proven to be potentially difficult. This is due to its relatively low neutron absorption cross section and gamma ray intensity, and the relatively high neutron absorption cross section and gamma intensity of ⁴⁶Sc, which has an energy peak that is only 5 keV greater than ⁶⁵Zn. The use of a high resolution detector makes it possible to differentiate between the ⁶⁵Zn and ⁴⁶Sc photopeaks peaks. However, the dominating ⁴⁶Sc gamma ray can even make peak fitting routines unsuccessful in the proper determination of ⁶⁵Zn. The use of a Compton suppression system suppresses the ⁴⁶Sc peak, which has two coincident gamma-rays, and this greatly improves the ratio of the height of the ⁴⁶Sc 1120.5 keV photopeak to the ⁶⁵Zn 1115.4 keV photopeak. Irradiating the sample with epithermal neutrons also improves the measurement since ⁶⁵Zn has a higher cross section for epithermal neutrons rather than thermal neutrons, whereas ⁴⁶Sc has a higher thermal cross section. Another technique to determine zinc is the use of ⁶⁸Zn(n,γ)^69mZn reaction with its 13 h half-life using epithermal neutrons and Compton suppression INAA. However, the 438 keV gamma ray of ^69mZn has no interference with any adjoining photopeak. A critical comparison of these two methods is given.     

Analytical capabilities of anticoincidence INAA for biological materials

Sensitivities and limits of detection (LODs) for 39 elements were determined for cellulose filters, foods, and biological reference materials by anticoincidence instrumental neutron activation analysis. Compton background reduction improved many LODs by about a factor of 2, but increased LODs for elements whose radioisotopes decay with cascading γ-rays. As and Hg analyses were aided by reduction of Br and Se photopeak intensities, respectively. LODs of 0.03–0.8 μg/kg were achieved for 16 elements in cellulose filters. Typical biological material and food LODs were higher by factors of about 10–600.     

Epithermal instrumental neutron activation analysis with Compton suppression spectrometry for the determination of iodine in food samples

Epithermal instrumental neutron activation analysis (EINAA) together with Compton suppression system were optimized and used to analyze several food samples for the determination of low levels of iodine. The method involved the irradiation of samples in the outer epi-cadmium site of the Dalhousie University Slowpoke-2 reactor facility. The samples were then counted directly without any chemical treatment on an anticoincidence counting system. This system comprised a 25 cm³ hyperpure Ge detector, a guard detector consisting of a 10"×10" NaI(Tl) annulus with five photomultiplier tubes (PMTs) and a 3"×3" NaI(Tl) plug with one PMT. Iodine was quantitatively analyzed using the 443 keV photopeak of ¹²⁸I. The precision and accuracy of the method were evaluated using real samples and biological reference materials, respectively. The precision of the method was calculated as percent relative standard deviation and in all cases was within ±5%. The agreement between our iodine values and those of the certified values was generally within ±10%, suggesting an excellent accuracy of the method. The detection limits of the various samples calculated, with the lowest value of 20 ppb. The values of iodine determined ranged between 24 to 3080 ppb. The methods and results are presented.     

Evaluation of atmospheric airborne particles in Lisbon, Portugal using neutron activation analysis

Summary We have employed Compton supression neutron activation analysis in conjunction with thermal and epithermal neutrons to detemine trace elements in airborne particulate matter in Lisbon, Portugal. As a result of the proximity of ocean marine, elements such as sodium and chlorine can signficantly decrease the analytical sensitivities of many elements of interest due ot the high backgrounds arising from ³⁸Cl and ²⁴Na. Compton suppression has resulted in the increase of the analytical sensitivities using thermal neutrons of Al, Ba, Ce, Cr, Cu, Ni, Rb, Se, Th, Ti, V, and Zn. The use of Compton suppression and epithermal neutrons significantly reduced the detection limits for As, I, K, Si, and W, while the utilization of solely epithermal neutrons improved the analyses of In.     

Accuracy, precision and sensitivity in cyclic INAA with short-lived radionuclides

Repeated irradiation enhances the precision and sensitivity of INAA based on short-live radionuclides, but entail systematic losses due to dead time. The relative standard deviation in the net peak area decreases with the square root of the number of cycles while the systematic bias increases with it. The limits of decision, detection and determination decrease in a somewhat more complicated way with the number of cycles. The derived formulation is applied to the determination of selenium in hair by the 161.9 keV photopeak of^77mSe,T _1/2=17.8 s.     

Comparison between the diurnal trends of ozone and radon gas concentrations measured at ground in the semi-rural site of Central Italy

The effectiveness of a Compton suppression system (CSS) for instrumental neutron activation analysis of plant materials was evaluated. Suppression factors were measured with ¹³⁷Cs sources. Five certified reference materials were analyzed and the detection limits calculated from both suppressed and unsuppressed spectra were compared. The CSS demonstrated to be useful for lowering the detection limits of ten out of sixteen elements tested, showing a maximum improvement factor of 3.9. The system performance was strongly influenced by the sample composition and also by the measurement conditions, indicating the importance of testing each individual sample type and analytical protocol.     

The instrumental determination of cadmium in biological samples at nanogram levels with the aid of a Compton suppression system and epithermal neutron activation analysis

Instrumental epithermal neutron activation analysis in conjunction with Compton suppression methods has been used to determine cadmium concentrations in seven biological reference materials. The¹¹⁴Cd(n, )¹¹⁵CD(t_1/2=53.3h^115mIn(t_1/2=4.5h) reaction using the 336.3 keV photopeak was successfully employed to achieve an overall precision between 4%–15% and detection limits between 10–20 ng/g. The accuracy of the results as compared to the certified or compilation values was in excellent agreement.     

The use of neutron activation analysis in gold prospecting in small-scale mining in Ghana

A set of rock and soil samples from Dome Beposo in the Amansie-West district of Ashanti Region of Ghana, suspected to contain gold, have been analyzed using instrumental neutron activation analysis (INAA) coupled with conventional counting techniques. The identification and quantification of the elements, gold, arsenic, mercury and antimony were done using 411.8 keV photopeak of ¹⁹⁸Au, 559.1 keV photopeak of ⁷⁶As, 77.3 keV photopeak of ¹⁹⁷Hg and 564.2 keV photopeak of ¹²²Sb. The precision and accuracy of the method were evaluated using standard reference materials. The precision and bias was found to be less than 6%. The first set of samples consists of ten rocks (GS), four of which retain moderate to quite high concentrations of gold, 0.27±0.01 mg/kg, 1.58±0.09 mg/kg, 7.51±0.44 mg/kg and 8.06±0.35 mg/kg, respectively. The second set comprises two soil samples taken from the upper and bottom layers of a gold exploration pit. Gold concentrations in upper (UL) and bottom (BL) layers are 0.06±0.002 mg/kg and 0.47±0.02 mg/kg, respectively. Arsenic was found in the soils as well as the rocks, and the levels ranged from 9.3±0.5 to 274±15.6 mg/kg. Mercury and antimony were found in the rocks only. Mercury levels in the rocks ranged between 0.11±0.004 and 9.67±0.42 mg/kg whilst antimony levels ranged from 0.21±0.01 to 6.88±0.38 mg/kg. This revised version was published online in July 2006 with corrections to the Cover Date.     

INAA with anticoincidence counting significantly reduces interferences from the 554.3-keV photopeak of <Superscript>82</Superscript>Br to allow reliable measurements of nanogram levels of arsenic in solid biological materials via the 559.1-keV photopeak of <Superscript>76</Superscript>As

It is rather difficult to measure low levels of arsenic by instrumental neutron activation analysis (INAA) in biological materials containing high levels of bromine and antimony in particular, as well as sodium and potassium. The 559.1-keV photopeak of ⁷⁶As is interfered with by the 554.3- and 564.1-keV photopeaks of ⁸²Br and ¹²²Sb, respectively. The use of INAA in conjunction with anticoincidence spectrometry (INAA-AC) was found to reduce the background under the 559.1-keV photopeak by factors of 4–16 for the biological reference materials analyzed and to decrease the detection limit to 0.35 µg kg^?1 making the measurement of nanogram amounts arsenic in them possible.     

Chemical characterization of graphite by instrumental neutron activation analysis

Nuclear and commercial grade graphite samples were analysed by instrumental neutron activation analysis (INAA) using high flux reactor neutrons. Eleven elements (Na, K, As, Sc, Fe, Cr, Co, Zn, La, Ce, and Sm) were determined in eight samples of graphite (two nuclear grade and six commercial grade) by irradiating at a neutron flux of 3?×?10¹³ cm^?2 s^?1 in CIRUS reactor and assaying the activity by high-resolution gamma ray spectrometry using 40% relative efficiency HPGe detector coupled to an MCA. Concentrations of elements were determined by relative method of INAA. Results of both types of graphites as well as detection limits achieved by INAA method are discussed in the paper.     

Evaluation of Compton scattering and self-attenuation coefficient after <Emphasis Type="Italic">γ</Emphasis>-ray analysis of naturally occurring radioactive elements in environmental samples

Comparison of Compton scattering and Compton scattering cross section with self-attenuation coefficient were explained based on the kinematic equation and Klein-Nishina formula. Naturally occurring elements, ²³⁸U (²²⁶Ra), ⁴⁰K, ²³²Th (²²⁸Ra) and ¹³⁷Cs were determined in sediments and water from Ismailia canal in Egypt which were found in the range of permissible level. Self-attenuation coefficients, K, the ratio between photopeak detection efficiency using solid and liquid standards were determined. They fit well comparing to Compton scattering, Compton scattering cross section while inversely fit to energy-absorption Compton scattering cross section based on the Klein-Nishina formula.     

Further investigating the determination of phosphorus in plants by INAA using<Emphasis Type="Italic"> bremsstrahlung</Emphasis> measurement

Phosphorus is an essential element for plants and animals, playing a fundamental role in the production of biochemical energy. Despite its relevance, phosphorus is not commonly determined by instrumental neutron activation analysis (INAA), because ³²P does not emit gamma-rays in its decay. There are alternative methods for the determination of phosphorus by INAA, such as the use of beta counting or the measurement of bremsstrahlung originated from the high energy beta particle from ³²P. Here the determination of phosphorus in plant materials by measuring the bremsstrahlung production was further investigated, to optimize an analytical protocol for minimizing interferences and overcoming the poor specificity. Eight certified reference materials of plant matrices with phosphorus ranging between 171 and 5,180 mg kg⁻¹ were irradiated at a thermal neutron flux of 9.5 × 10¹² cm⁻² s⁻¹ and measured with a HPGe detector at decay times varying from 7 to 60 days. Phosphorus solutions added to a certified reference material at three levels were used for calibration. Counts accumulated in the baseline at four different regions of the gamma-ray spectra were tested for the determination of phosphorus, with better results for the 100 keV region. The Compton scattering contribution in the selected range was discounted using an experimental peak-to-Compton factor and the net areas of all peaks in the spectra with energies higher than 218 keV, i.e. Compton edge above 100 keV. Amongst the interferences investigated, the production of ³²P from sulfur, and the contribution of Compton scattering should be considered for producing good results.     

Selenium and mercury determination in biological samples using gamma–gamma coincidence and Compton suppression

Biological materials containing trace amounts of mercury and selenium were examined using neutron activation analysis. They were analyzed using Compton suppression and γ–γ coincidence counting. The 279 keV photopeak of activated mercury (²⁰³Hg) was analyzed in order to observe the mercury content in these samples. Selenium, an element found in many biological samples, interferes with the analysis of ²⁰³Hg when activated (⁷⁵Se). Because the selenium interference comes from a cascading emission, Compton suppression was utilized to reduce this interference. In order to fully characterize the selenium content in the samples, γ–γ coincidence was used which reduced the background and eliminated bremsstrahlung interference produced from neutron activated phosphorous through the ³¹P(n, γ)³²P reaction which is a pure beta emitter. As a result, we determined the mercury and selenium concentrations in three standard reference materials, which contain varying ratios of mercury to selenium concentrations. This study also showed that these types of concentrations can be determined from small (<500 mg) sample masses. Further work needs to be done on wet samples that require dehydration, as mercury can be lost through this process.     

William D. Ehmann, radioanalytical chemist and radiochemistry educator

Copper (Cu) is an essential element and is incorporated in many biomolecules that are involved in protecting the brain from oxidative damage. Many brain regions strongly affected by neurodegene rative diseases are small. A sensitive nondestructive procedure to determine Cu is desirable to preserve samples for additional studies. Copper is not easily determined by instrumental neutron activation analysis (INAA) due to high activity levels produced by major abundance elements such as sodium (Na) and chlorine (Cl), which produce a high Compton background. An INAA method involving a short epithermal neutron irradiation and counting with a Compton suppression system was developed to determine Cu in brain, via 5.1-min⁶⁶Cu. These short irradiation results are compared to those based on coincidence spectrometry of annihilation photons from positron emitting 12.7-h⁶⁴Cu after a long irradiation.     

Sensitivity improvements in the determination of mercury in biological tissues by neutron activation analysis

The possible association of dental amalgam surface exposure, brain mercury (Hg) levels, and pathological markers of Alzheimer's disease (AD) in the brain is the subject of an on-going study in our laboratory. Two radiochemical neutron activation analysis methods and the use of instrumental neutron activation analysis (INAA) with Compton suppression spectrometry have been evaluated for improving our INAA Hg detection limit (2.8±0.6 ng/g, wet-weight basis) in human tissue. Large numbers of samples dictated the use of a purely instrumental method or rapid, simple radiochemical separations. Human brain tissues and NIST biological standards were analyzed using a precipitation of Hg₂Cl₂, a solvent extraction utilizing sodium diethyldithiocarbomate, conventional INAA, and INAA with Compton suppression. The radiochemical precipitation of Hg₂Cl₂ proved to be the most useful method for use in our study because it provided a simultaneous, quantitative determination of silver (Ag) and a Hg detection limit in brain tissue of 1.6±0.1 ng/g (wet-weight basis).     

Determination of arsenic and gallium in standard materials by instrumental epithermal neutron activation analysis

Epithennal neutron activation analysis has been applied to the determination of arsenic and gallium in standard materials at trace concentrations. The reduction of ²⁴Na activity compared to thermal neutron activation is advantageous. Arsenic detection limits (1σ) are 0.04 and 0.015 μg for inorganic and organic materials, respectively. The corresponding gallium detection limits, for the best cases, are 0.13 and 0.29 μg. Gallium determinations with the 834-keV photopeak of ⁷²Ga suffered interferences attributed to the threshold reaction ⁵⁴Fe(n,p) ⁵⁴Mn; the less intense ⁷²Ga peaks at 629 and 2200 keV provided quantitative results for all samples tested. Gallium detection limits with the less intense, but more reliable 629-keV peak were 0.9 and 0.1 μg for inorganic and organic materials, respectively. Arsenic determinations are best performed with the more intense ⁷⁶As 559-keV line, as the 657-keV line has an unknown interference.     

Compton suppression spectrometry for analysis of short-lived neutron activation products in foods

Compton suppression spectrometry was used to analyze foods for elements with short-lived neutron activation products (half-lives of about 2 minutes to 1.5 days). Analysis conditions were optimized to provide quality assurance analyses for iodine in FDA’s Total Diet Study. Iodine mass fractions (0.075 to 2.03 mg/kg) were measured in 19 of 42 foods analyzed, with limits of detection (LODs) ranging from 0.03 to 1.4 mg/kg, mostly depending on NaCl content. LODs were lowered by up to a factor of 2 for 16 elements. Suppression factors ranged from about 2 to 8 over the energy range 400 to 3200 keV.