Background‐free solution boron NMR spectroscopy

The appearance of background signals arising from the NMR probe and tube is a well‐known problem of boron NMR spectroscopy. Background suppression may be achieved by using DEPTH, which increases the signal‐to‐background (S/B) ratio. Although, the quality of such spectra is often adequate, but in the case of rapid relaxation broadened resonances (T₁ < 1 ms), the residual background signals may still hamper the interpretation of the spectra. It was observed that the background signals are practically invisible in solution ¹⁰B NMR. The unusual isotopic effect on the (S/B) ratio was interpreted as an inherent consequence of the integer versus half‐integer spin of ¹⁰B and ¹¹B, respectively. The practicability of ^10/11B NMR was compared for a selected set of boron compounds covering the typical range of (S/B) ratio. The application of ¹¹B is more favourable than ¹⁰B as long as it is possible to achieve the desired spectral quality by using DEPTH. Otherwise, the ‘background‐free’ appearance of ¹⁰B NMR spectra makes ¹⁰B a reasonable alternative of ¹¹B DEPTH. This was found typical for compounds having relaxation broadened resonances. The variable temperature (VT) NMR study of an adduct formation process was also presented here as an example of the advantage of ¹⁰B over ¹¹B. Copyright © 2012 John Wiley & Sons, Ltd.     

Improvement of the detection power in electrothermal atomic absorption spectrometry by summation of signals : Determination of Traces of Metals in Drinking Water and Urine

In electrothermal atomic absorption spectrometry, the signals corresponding to total absorption (TA), background(BG) adn the required atomic absorption signal (TA - BG) show good temporal reproducibility. For improvement of the detection power, the time-resolved signals can be summed with the aid of a microcomputer. The summed signal height is exponentially related to the number (N) of measurements up to N = 40. The detection power is improved according to the N^{$\text{sol1}\text{2}$} law with a practical limit at summation of ca. 20 signals. Concentrations which are around the detection limit in single measurements (lead and cadmium in drinking water or lead in urine) can readily be determined by summation of signals from, for example, sixteen 20-μl injections of urine. Resulting high summed backgrounds, giving absorbances > 5, can be compensated.     

Estimation of background interference in prompt-gamma neutron activation using MCNP

Prompt-gamma neutron activation (PGNA) is used to measure total-bodynitrogen and hydrogen in humans. Background interference in the gamma spectraarises from both subject and shielding. A Monte Carlo simulation program (MCNP4B2)was used to examine the neutron and gamma signals in the PGNA system ( ²⁴¹AmBe source). N and H peak regions were assessed in the presenceand absence of calibration phantoms. The simulations suggested extracorporealH peak contributions of up to 30%, depending on subject body habitus. MostN background could be attributed to detector pileup events. The MCNP resultsallowed us to improve shielding design and develop background correction algorithmsto improve measurement precision.     

Determination of tannic acid after precipitation with bovine serum albumin by visible light scattering in a flow injection system

Tannic acid can form a precipitate with bovine serum albumin (BSA), and this was explored for its determination using a flow injection system along with detection via light scattering at 600 nm. The results on light scattering are confirmed using a particle sizing method. Scattering signals are negligible compared to the background generated by BSA in solutions with a pH of <4 or >8. Any precipitates on the wall of the flow cell pipeline were effectively removed by using a sodium dodecyl sulfate washing solution. The analytical range is from 70 to 250 µgmL^-1 of tannic acid. The results imply that increasing BSA concentrations will enhance sensitivity and analytical ranges. The system can be used in solutions of pH values between 5 and 7. The results correlated closely (r²?>?0.97) with those obtained by the ferrous tartrate method when analyzing a single tannic acid and two tea tannins.     

Use of a multi-tube Nafion® membrane dryer for desolvation with thermospray sample introduction to inductively coupled plasma-atomic emission spectrometry

A multi-tube Nafion^® membrane dryer used as a part of a desolvation system in conjunction with thermospray nebulization was optimized and characterized with inductively coupled plasma-atomic emission spectrometry (ICP-AES). Either argon or nitrogen could be used as the sweep gas, and optimum conditions were found to be at low temperature and low sweep gas flow rate. Analyte sensitivity was not significantly affected by placing the membrane between the plasma and the nebulizer, although about 20% of the analyte entering the dryer is lost within the dryer. A dual role of the membrane dryer was demonstrated. As a secondary step within the desolvation system, it enabled a high desolvation efficiency of 99.94% for aerosols from 1% (v/v) nitric acid. Plasma solvent load could be reduced to 0.9 mg min⁻¹ with a tap water cooled condenser combined with the membrane dryer, compared to 21 mg min⁻¹ with the normal chilled condenser desolvation system. Meanwhile, the membrane was also found to act as a pulse dampener, eliminating the plasma pulsation in the central channel caused by thermospray nebulization and thus improving the analytical performance of the system. The average relative standard deviations (RSD) with the optimized membrane/thermospray system were 0.83% and 0.60% for the background and analyte signals, respectively, which were reduced by a factor of 1.9 and 2.7 for the background and analyte signals, respectively, compared to thermospray without the membrane desolvation, and were essentially identical to those obtained with pneumatic nebulization sample introduction. The improvements in detection limits with the membrane/thermospray system were 1.2–3.0 times with an average factor of 1.8 compared to thermospray without the membrane dryer, and 18–68 times with an average factor of 39 compared to the standard pneumatic nebulization sample introduction system without a desolvation unit. The detection limits for Mn, Mg, Cr and Cd with the present thermospray/membrane system were comparable to those reported for pneumatic nebulization ICP mass spectrometry.     

Les possibilités et limitations de l'atomisation électrothermique en spectrom/'etrie d'absorption atomique lors de l'analyse directe des metaux lourds dans l'eau de mer

This work shows the analytical possibilities of an electrothermal atomizer for the direct determination of trace metals in sea-water.The high background signals generated by the matrix perturb in particular volatile elements because of the low decomposition temperature allowed. In the case of cadmium, addition of ascorbic acid to the sample permits the modification of the atomization mechanism and the reduction of the optimum temperature. Under these conditions, absorption peak of the cadmium precedes the background absorption and consequently the analysis is no longer limited by the magnitude of the matrix signal: the determination of cadmium concentrations far below the μg^?1 level is easily possible. Although the direct determination of the other elements should be in principle less disturbed by the background, the analytical performance is poorer than for cadmium. Limits of determination of the order from 0.1 to 1 μg^?1 can be reached for chromium, copper and manganese. Lead and nickel appeared to be the most difficult elements; their direct determination is only possible in polluted coastal or estuarine waters.The injection of the sample as an aerosol into hot graphite tube showed to be well adapted to this kind of investigations. The simultaneous visualization of specific and background signals allows interpretations which until now were impossible with commercially available apparatus.     

Assessment and application of diode laser induced fluorescence spectrometry in an inductively coupled plasma to the determination of lithium

A diode laser based system for the detection of Li in an inductively coupled plasma (ICP) by diode laser induced fluorescence (DLIF-ICP) has been developed and successfully applied to the determination of lithium in several mineral waters and a thermal salt. The experimental setup is based on an unmodulated, continuous wave diode laser, emitting light at around 670 nm and exciting neutral Li atoms on their 2s ²S–2p ²P° transition, which was coupled to a commercial ICP atomic emission spectrometer. The spectrometer's monochromator, photomultiplier detector and built-in data acquisition software were utilized to collect background corrected fluorescence and emission signals. A simple, three-step measurement procedure was devised that corrected for the contribution of lithium thermal emission and scattered laser light in the analytical signals. Despite the facts that lithium was detected on its neutral atom, which accounts for less than 1% of the total concentration of Li in the ICP, and that only about 1–2% of all atoms could be excited by the laser light at any given time, the limit of detection (LOD) was still found to be as good as 8 μg/L. The LODs of the DLIF-ICP technique are therefore expected to be in the low ng/L range for elements that can be detected under more advantageous conditions. The linear dynamic range was found to be around three orders of magnitude.     

Ionization mechanism in gel permeation chromatography mass spectrometry (GPC-MS) with atmospheric pressure chemical ionization (APCI) interface

The ionization mechanism in gel permeation chromatography (GPC) coupled with mass spectrometry (MS) using an atmospheric pressure chemical ionization (APCI) interface is explained when chloroform is used as GPC solvent. The even electron anions [CHCl₃+Cl]^? generated by thermal electrons in negative APCI mode are responsible for further ionization of the solute compounds as [M?+?Cl]^? adducts. In positive APCI mode, the contaminants in chloroform generate a relatively high background noise. These cations can be suppressed by adding 10% of tetrahydrofuran (THF). Supporting the described mechanism are key experiments done in the absence of solvent, in GPC solvents and using a mixture of anionic polystyrene (PS) standards from dimer up to 12,000?g/mol, covering the molecular masses of great interest in studies related to Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH) and Food and Drug Administration (FDA). Advantages of suppressing the oligomer signals for polymer additive analyses were presented using Irgafos^® 168.     

19F VT NMR: Novel Tm3+ and Ce3+ Complexes Provide New Insight into Temperature Measurement Using Molecular Sensors

In the past few decades, magnetic resonance spectroscopy (MRS) and MR imaging (MRI) have developed into a powerful non-invasive tool for medical diagnostic and therapy. Especially ¹⁹F MR shows promising potential because of the properties of the fluorine atom and the negligible background signals in the MR spectra. The detection of temperature in a living organism is quite difficult, and usually external thermometers or fibers are used. Temperature determination via MRS needs temperature-sensitive contrast agents. This article reports first results of solvent and structural influences on the temperature sensitivity of ¹⁹F NMR signals of chosen molecules. By using this chemical shift sensitivity, a local temperature can be determined with a high precision. Based on this preliminary study, we synthesized five metal complexes and compared the results of all variable temperature measurements. It is shown that the highest ¹⁹F MR signal temperature dependence is detectable for a fluorine nucleus in a Tm³⁺-complex.     

A retention‐time‐shift‐tolerant background subtraction and noise reduction algorithm (BgS‐NoRA) for extraction of drug metabolites in liquid chromatography/mass spectrometry data from biological matrices

A retention‐time‐shift‐tolerant background subtraction and noise reduction algorithm (BgS‐NoRA) is implemented using the statistical programming language R to remove non‐drug‐related ion signals from accurate mass liquid chromatography/mass spectrometry (LC/MS) data. The background‐subtraction part of the algorithm is similar to a previously published procedure (Zhang H and Yang Y. J. Mass Spectrom. 2008, 43: 1181–1190). The noise reduction algorithm (NoRA) is an add‐on feature to help further clean up the residual matrix ion noises after background subtraction. It functions by removing ion signals that are not consistent across many adjacent scans. The effectiveness of BgS‐NoRA was examined in biological matrices by spiking blank plasma extract, bile and urine with diclofenac and ibuprofen that have been pre‐metabolized by microsomal incubation. Efficient removal of background ions permitted the detection of drug‐related ions in in vivo samples (plasma, bile, urine and feces) obtained from rats orally dosed with ¹⁴C‐loratadine with minimal interference. Results from these experiments demonstrate that BgS‐NoRA is more effective in removing analyte‐unrelated ions than background subtraction alone. NoRA is shown to be particularly effective in the early retention region for urine samples and middle retention region for bile samples, where the matrix ion signals still dominate the total ion chromatograms (TICs) after background subtraction. In most cases, the TICs after BgS‐NoRA are in excellent qualitative correlation to the radiochromatograms. BgS‐NoRA will be a very useful tool in metabolite detection and identification work, especially in first‐in‐human (FIH) studies and multiple dose toxicology studies where non‐radio‐labeled drugs are administered. Data from these types of studies are critical to meet the latest FDA guidance on Metabolite in Safety Testing (MIST). Copyright © 2009 John Wiley & Sons, Ltd.     

Multifunctional Core–Shell Silica Nanoparticles for Highly Sensitive 19F Magnetic Resonance Imaging

¹⁹F magnetic resonance imaging (¹⁹F MRI) is useful for monitoring particular signals from biological samples, cells, and target tissues, because background signals are missing in animal bodies. Therefore, highly sensitive ¹⁹F MRI contrast agents are in great demand for their practical applications. However, we have faced the following challenges: 1) increasing the number of fluorine atoms decreases the solubility of the molecular probes, and 2) the restriction of the molecular mobility attenuates the ¹⁹F MRI signals. Herein, we developed novel multifunctional core–shell nanoparticles to solve these issues. They are composed of a core micelle filled with liquid perfluorocarbon and a robust silica shell. These core–shell nanoparticles have superior properties such as high sensitivity, modifiability of the surface, biocompatibility, and sufficient in vivo stability. By the adequate surface modifications, gene expression in living cells and tumor tissue in living mice were successfully detected by ¹⁹F MRI.     

Development of continuous inflow tritium measurement in water technology using electrolysis and a plastic scintillator

Our goal was to develop a mobile tritium monitor for continuous inflow system for water sample. The system is based on electrolysis and a plastic scintillator detection system. The minimum detectable activity (MDA) of the prototype system is 431 kBq L^?1, while the MDA of a commercially available product is 740 kBq L^?1. We expected to achieve a 5.73-times lower MDA by optimizing detection geometry using a multi-hydrogen-gas-channel. The system can be applied either as a mobile leakage surveying method or as a fixed-type monitor for detecting tritium in drinking water by adapting conventional background reduction technologies.     

Underground sources of radioactive noble gas

It is well known that radon is present in relatively high concentrations below the surface of the Earth due to natural decay of uranium and thorium. However, less information is available on the background levels of other isotopes such as ¹³³Xe and ^131mXe produced via spontaneous fission of either manmade or naturally occurring elements. The background concentrations of radioxenon in the subsurface are important to understand because these isotopes potentially can be used to confirm violations of the comprehensive nuclear-test-ban treaty during an on-site inspection. Recently, Pacific Northwest National Laboratory measured radioxenon concentrations from the subsurface at the Nevada Nuclear Security Site (NNSS—formerly known as the Nevada Test Site) to determine whether xenon isotope background levels could be detected from spontaneous fission of naturally occurring uranium or legacy ²⁴⁰Pu as a result of historic nuclear testing. In this paper, we discuss the results of those measurements and review the sources of xenon background that must be taken into account during OSI noble gas measurements.     

The Formation of Analytical and Background Signals in Radioisotope X-ray Fluorescence Analysis Using a 241Am Radioactive Source and a Si(Li) Detector

Reasons for the appearance of a background signal in the relatively soft x-ray range (2–15 keV) upon fluorescence excitation with the ²⁴¹Am radioisotope are discussed. Mathematical models are proposed for the interaction of the radiation of the radioactive source with the sample and the Si(Li) detector, capable of describing the appearance of the analytical and background signals. It was shown that processes occurring in the Si(Li) detector should be taken into account to explain and calculate the background signal in this range. The results of calculations agree well with the experimental data obtained using an ²⁴¹Am radiation source for fluorescence excitation.     

Evaluation of a radio frequency plasma for oxygen-selective detection in capillary gas chromatography

A radio frequency plasma detector for capillary GC has been modified for oxygen-selective detection. Purification of the plasma gas and purging of both ends of the discharge region with helium were crucial to minimizing oxygen background emission from impurities in the plasma. With a pure helium plasma, eluting hydrocarbons released oxygen from the discharge region resulting in interfering signals on the oxygen channel. These interfering signals were efficiently reduced by using a methane-doped (0. 15%) low power RF plasma (15 W) sustained in a high make-up flow (150 mL/min). With this plasma, a 10³:1 oxygen-to-carbon selectivity and a 100 pg oxygen/s detection limit were obtained. The detector was linear over three orders of magnitude. The detection system has been used to screen for oxygenated compounds in two environmental samples.     

Determination of isotopic ratios of uranium samples using passive gamma spectroscopy with multiple detectors

Uranium samples of various enrichments have been passively counted on the University of Texas detector gamma–gamma coincidence system. By observing gamma rays emitted from ²³⁵U and its daughters compared to gamma rays emitted by ²³⁸U daughters and comparing the data to standards of known enrichments, a technique has been developed to take a uranium sample of unknown enrichment and passively count it to determine its uranium isotopic concentration. Because the gamma rays from ²³⁵U are generally in the low-energy regime, there is a strong susceptibility to background interferences, especially from the Compton background produced from higher energy gamma rays. Other interferences, such as those from the decay series of uranium also exist for ²³⁵U gamma rays. In this light, we have collected data using list-mode to produce two-dimensional gamma–gamma coincidence spectra, which allows us to gate the low-energy gamma rays from ²³⁵U with gamma rays that are in coincidence. In doing this, much of the low energy interferences are reduced, and one can analyze the ²³⁵U gamma rays with high precision. Because of the high density of uranium, self-shielding has significant effects especially in the low-energy regime. To correct for this attenuation the detector system has been modeled by MCNP and self-shielding factors have been calculated across the energy spectrum. A big advantage to this method is the capability of performing this analysis with small (<1 g) samples in a non-destructive and relatively inexpensive manner. If necessary, this analysis can be performed within 24 h if an urgent nuclear forensics scenario arises.     

The analysis and attribution of the time-dependent neutron background resultant from sample irradiation in a SLOWPOKE-2 reactor

The Royal Military College of Canada (RMCC) has commissioned a Delayed Neutron Counting (DNC) system for the analysis of special nuclear materials. A significant, time-dependent neutron background with an initial maximum count rate, more than 50 times that of the time-independent background, was characterised during the validation of this system. This time-dependent background was found to be dependent on the presence of the polyethylene (PE) vials used to transport the fissile samples, yet was not an activation product of vial impurities. The magnitude of the time-dependent background was found to be irradiation site specific and independent of the mass of PE. The capability of RMCC’s DNC system to analyze the neutron count rates in time intervals <1 s facilitated a more detailed data analysis than that obtained in previous DNC systems recording cumulative neutron counts. An analysis of the time-dependent background behaviour suggested that an equivalent of 120 ng of ²³⁵U contamination was present on each irradiated vial. However, Inductively Coupled Plasma—Mass Spectroscopy measurements of material leached from the outer vial surfaces after their irradiations found only trace amounts of uranium, 0.118 ± 0.048 ng of ²³⁵U derived from natural uranium. These quantities are insufficient to account for the time-independent background, and in fact could not be discriminated from the noise associated with time-independent background. It is suggested that delayed neutron emitters are deposited in the vial surface following fission recoil, leaving the main body of uranium within the irradiation site. This hypothesis is supported by the physical cleaning of the site with materials soaked in distilled water and HNO₃, which lowered the background from a nominal ²³⁵U mass equivalent of 120 to 50 ng per vial.     

A Portable Liquid Scintillation Counter for General LSC and High Sensitivity Alpha-Counting Applications

Applicability of a new portable, single tube liquid scintillation (LS) spectrometer was demonstrated for general LS applications, such as wipe tests and low level alpha counting, especially radon (²²²Rn) measurements in water. Wipe tests were performed with wad sticks. They were counted conventionally in a small volume in Eppendorf tubes with less than 200µl of LS cocktail, thus minimizing costs and waste. Small volume is specially recommended because low background can be achieved without heavy lead shielding, thus maintaining portability. Typical NRC recommended LLD's were reached for ³H and other typical LS isotopes. For ²²²Rn in water a biphasic extraction system was adopted where radon is extracted from water into a water-immiscible LS cocktail. The method is sensitive because radon can be extracted from a large water volume. It was observed that common non-evaporating "safe" cocktails with di-isopropyl naphtalene solvent are convenient to use and quite suitable for extraction. Also the isotope ²²⁶Ra can be reliably measured via production of its daughter ²²²Rn. The instrument includes pulse shape electronics to perform alpha/beta separation. This is based on the fact that in LS cocktails alphas generate pulses with longer duration than betas. The alpha/beta separation can be visualized with a two dimensional graph where the x-axis represents pulse amplitude (MCA channels) and the y-axis its length. The graphical operations are all done in standard Excel/Windows environment. Due to their longer pulses, alphas have greater y-coordinates than betas with the same x-coordinate (amplitude). With this graph, one can select a region occupied only by alphas and exclude betas. The above mentioned "safe" cocktails posses good alpha/beta separation properties. Because natural background (cosmic-rays and environmental gammas) produces beta-like pulses, they can be stripped away giving low background for alpha counting, typically a few counts per hour for the extraction samples. The LLD for ²²²Rn was 0.1 Bq/I.     

A quantitative method for determination of Co(II) based on the inner filter effect of reagents on the Raman scattering signals of water

It is known that Raman scattering signals are one of main interference sources leading up to determination errors in spectrofluorometry, and thus the signals can be easily detected with a common spectrofluorometer. In this contribution, we propose a quantitative method based on the inner filter effect (IFE) of reagents on the Raman scattering signals of solvent by taking the complexation of divalent cobalt ion with 4-[(5-chloro-2-pyridyl)azo]-1,3-diaminobenzene (5-Cl-PADAB) as a model system. By adjusting the excitation wavelength of the spectrofluorometer, we could easily detect the Raman scattering signals of water at 424 nm where the maximum absorption of 5-Cl-PADAB reagent is located. In a solution of 5-Cl-PADAB, the Raman scattering signals of water are decreased owing to the IFE of 5-Cl-PADAB. If Co(II), which could form the binary complex of Co(II)/5-Cl-PADAB and consumes the 5-Cl-PADAB reagent, is present in such a case for a given amount of 5-Cl-PADAB solution, recovered Raman scattering signals could be observed and measured with a spectrofluorometer. It was found that the intensity of the enhanced Raman scattering signals is proportional to the Co(II) concentration over the range from 2.0 × 10⁻⁷ mol L⁻¹ to 1.0 × 10⁻⁵ mol L⁻¹, and the detection limit could reach 1.2 × 10⁻⁷ mol L⁻¹. With that, Co(II) in samples could be detected with R.S.D. values lower than 2.6% and recoveries over the range of 97.2-104.7%.     

Background-Free Detection of Spin-Exchange Dynamics at Ultra-Low Magnetic Field

Ultra-low field nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI) inherently suffer from a low signal-to-noise ratio due to the small thermal polarization of nuclear spins. Transfer of polarization from a pre-polarized spin system to a thermally polarized spin system via the Spin Polarization Induced Nuclear Overhauser Effect (SPINOE) could potentially be used to overcome this limitation. SPINOE is particularly advantageous at ultra-low magnetic field, where the transferred polarization can be several orders of magnitude higher than thermal polarization. Here we demonstrate direct detection of polarization transfer from highly polarized ¹²⁹Xe gas spins to ¹H spins in solution via SPINOE. At ultra-low field, where thermal nuclear spin polarization is close to background noise levels and where different nuclei can be simultaneously detected in a single spectrum, the dynamics of the polarization transfer can be observed in real time. We show that by simply bubbling hyperpolarized ¹²⁹Xe into solution, we can enhance ¹H polarization levels by a factor of up to 151-fold. While our protocol leads to lower enhancements than those previously reported under extreme Xe gas pressures, the methodology is easily repeatable and allows for on-demand enhanced spectroscopy. SPINOE at ultra-low magnetic field could also be employed to study ¹²⁹Xe interactions in solutions.