An evaluation of the detection limits possible for competitive capillary electrophoretic immunoassays.

Using a high-affinity antibody for estradiol, thermodynamic and experimental limitations on detection limits for competitive capillary electrophoretic immunoassays were examined. Theoretical modeling of the dose-response curves for such assays allowed for optimization of experimental conditions. Through the examination of experimental and theoretical results generalizations could be made as to the ability of capillary electrophoretic immunoassays to achieve low detection limits. An experimental detection limit of 310 pM, corresponding to 2100 molecules, was achieved. A minimum theoretical detection limit for the antibody of interest was approximated to be 125-525 pM depending on the standard deviation. An overview of the optimization process is given as well as commentary on theoretical predictions.     

Electron energy loss spectrometry mapping

Among electron beam microanalytical techniques, electron energy loss spectrometry (EELS) offers unique advantages in terms of information content, sensitivity, limits of detection. This paper describes new methods and tools for acquiring families of spectra over many pixels on the specimen, i.e. spectrumimages, and for processing them. Applications in different fields of research, both in materials science and in life sciences, demonstrate the potential impact of the technique for characterizing nano-sized structures.     

Demonstration of an optimized flow cell SFC-FTIR interface

Summary An on-line application of capillary SFC-FTIR for the analysis of five steroids has been demonstrated. Baseline resolution is achieved for all the components of the mixture and IR spectra are obtained which match well with spectra of the neat materials. Also, the injected minimum detectable quantity (IMDQ) of caffeine (approximately 2 ng) under similar chromatographic conditions is demonstrated for this interface. Very low IMDQ's achieved using this flow cell interface will facilitate the use of this technique for the analysis of biological fluids and other materials where very low detection limits are required.     

Identify fine microstructure of multifarious iron oxides via O K-edge EELS spectra

Relying on the electron energy loss spectrum (EELS) of metallic elements to obtain microstructure analysis is an investigation method of the reaction mechanisms of transition metal oxides (TMOs) in catalysis, energy storage and conversion. However, the low signal from K shell owing to insufficient electron beam energy, and the complicated electronic structure in L shell of the metal element restrict the analysis of the coordination environment of the TMOs. Herein, density functional theory (DFT) calculation, Fourier transform (FT) and wavelet transform (WT) were employed to probe the relationship between the four individual peaks in O K-edge spectra of iron oxides and the microstructure information (chemical bonds and atomic coordination). The findings show that the peak amplitude ration is in a linear correlation with the valence state of Fe element, and that the coordination number obtained by radial distribution function (RDF) is favorably linearly correlative with that from the standard coordination structure model. As a result, the quantitative analysis on the change of valence state and atomic coordination in microstructure can be realized by EELS O K-edge spectra. This study establishes EELS O K-edge spectrum as a promising pathway to quantitatively analyze the valence state and atomic coordination information of TMOs, and offers an effective method to conduct microstructure analysis via the EELS spectra of the non-metal element.     

Fundamental limits of optical microrheology

We estimate the fundamental limits of different microrheological techniques based on optical detection. It is suggested that particle tracking systems using nondifferential detection have a minimum detectable displacement given by 0.2(lambda0/NA)(1/square root of (SNR)), where lambda0 is the wavelength, NA is the numerical aperture of the focusing objective, and SNR is the signal-to-noise ratio of the system. This limit has important consequences in microrheology, since the noise contributes with an apparent diffusion constant of Dl approximately 0.02(lambda0/NA)2(B/SNR), where B is the bandwidth of the detection unit. As the SNR of ordinary microscopes is limited, one should be extra careful when probing soft materials with low diffusion constants. On the other hand, in differential systems based on laser detection, the SNR is considerably increased due to reduced laser noise, and the minimum detectable displacement is given by 0.4(lambda0/NA)(1/SNR). One may therefore expect to measure the diffusion constant with higher accuracy if the SNR is large. Finally, we find that total internal reflection microscopy (TIRM) has a minimum detectable displacement given by 0.1lambda0/SNR.     

Performance evaluation of spectral deconvolution analysis tool (SDAT) software used for nuclear explosion radionuclide measurements

The Spectral Deconvolution Analysis Tool (SDAT) software was developed to improve counting statistics and detection limits for nuclear explosion radionuclide measurements. SDAT utilizes spectral deconvolution spectroscopy techniques and can analyze both β-γ coincidence spectra for radioxenon isotopes and high-resolution HPGe spectra from aerosol monitors. Spectral deconvolution spectroscopy is an analysis method that utilizes the entire signal deposited in a gamma-ray detector rather than the small portion of the signal that is present in one gamma-ray peak. This method shows promise to improve detection limits over classical gamma-ray spectroscopy analytical techniques; however, this hypothesis has not been tested. To address this issue, we performed three tests to compare the detection ability and variance of SDAT results to those of commercial-off-the-shelf (COTS) software which utilizes a standard peak search algorithm.     

Systematic calculation of detection limits in INAA of single element matrices with a SLOWPOKE reactor

A computer code using a simple model for the simulation of γ-ray spectra has been written to evaluate detection limits in INAA of single-element matrices. The data base includes cross-sections for the (n, γ), (n, p) and (n, α) reactions that contribute to the activity of samples irradiated in the inner irradiation sites of a SLOWPOKE reactor. The values of the detection limits of 63 elements in 60 matrices are presented in a tabular form. Up to 4 radioisotopes per element were considered for a total of 114 radioisotopes. The program also lists the set of experimental conditions, selected from 7 sets which cover the range of practically used values of irradiation, decay and counting time, which yields for each radioisotope the lowest limit of detection. It also indicates, via a reduction factor, the maximum allowable sample size.     

An ion exchanger-epoxy resin pelletization method for sample preparation in X-ray fluorescence analysis. Microanalysis of metal ions in industrial waste water

Flame atomic emission and atomic absorption detection limits are compared on an absolute basis for the easily determined element, copper. Remarkably similar detection limits are obtained with a variety of atomization and sample introduction system combinations. Heated graphite atomizers offer significantly lower minimum detectable quantities.     

Trends,applications and results in X-ray fluorescence analysis

X-ray fluorescence (XRF) has reached a mature state and represents a powerful analytical tool for the qualitative and quantitative determination of almost all the chemical elements in a sample. Standard XRF methods as well as special techniques to improve the detection limits will be presented, emphasizing the versatility of the method. With modern instrumentation the detectable number of elements ranges from Be to U. The minimum detectable quantities under optimizied excitation and detection conditions for medium Z elements are a few hundred femtograms. Other features like rapid analysis because of the multielement capability and in some cases the nondestruction of the sample are advantageous for a wide field of applications.     

HRTEM-EELS cross-sectional structural analyses of glassy carbon substrate irradiated by platinum ions using a coaxial arc plasma gun

A glassy carbon (GC) substrate was irradiated by Pt ions using a coaxial arc plasma gun in vacuum. The structure of the substrate was studied in the atomic scale using high-resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) with a thin foil specimen prepared by cross-sectioning using focused ion beam milling. HRTEM combined with the EELS measurements of the cross-sectioned sample indicated a disordering of the GC substrate surface and the detection of phenol-type species, carbon atoms in aliphatic chain molecules, and carboxyl groups, which are considered to have been formed by chemical reaction between the carbon atoms of the GC substrate surface and hydrogen or oxygen atoms. Penetrated Pt atoms were observed inside the GC substrate, and the surface position of the GC substrate was located from the depth profiles of the EELS spectra. STEM-EELS analyses of a pristine GC substrate without Pt ion irradiation were also conducted for reference.     

Analysis of light elements in superposed layers by Monte Carlo simulation of EELS spectra

A Monte Carlo code, previously set up to simulate electron energy loss spectra of carbon films on silicon at 100 kV, has been extended to the analysis, at 300 kV, of a Si/SiO₂/Si structure; the final goal is the determination of the oxygen concentration in SiO_x precipitates embedded in a Si matrix. The upgrading of the programme has required the introduction of relativistic kinematics and relativistic corrections to elastic and inelastic cross sections.The Si/SiO₂/Si samples have been prepared by CVD deposition of a 16 nm thick silicon film onto a silicon wafer covered with a 11 nm thick thermal oxide. The thickness of both films has been checked by transmission electron microscopy on cross sections. The EELS experiments have been performed on planar sections, in regions of different thickness; the EELS spectra have been acquired with a parallel 666 Gatan spectrometer, fitted to a Philips CM 30 TEM/STEM, operating at 300 kV. The stoichiometry of the SiO_x can be obtained by the ratioing of the areas under the OK and SiK edges, taking advantage of the possibility given by the Monte Carlo simulation to separate the background electrons from the one suffering the characteristic energy loss. The agreement between experiments and calculations in the case examined is satisfactory, so that the application of this procedure to SiO_x precipitates is promising.     

Measuring acetylene concentrations using a frequency chirped continuous wave diode laser operating in the near infrared

Two frequency chirped continuous wave diode lasers operating in the near infrared (IR) at wavelengths of lambda approximately 1.535 microm and lambda approximately 1.520 microm have been used to measure acetylene concentrations using the P(17) and R(9) rotational lines of the (nu1 + nu3) vibrational combination band. The diode lasers were frequency chirped by applying an electrical current pulse to the laser driver at a repetition rate of greater than 1 kHz. As the laser is operated at high repetition rates, more than 1000 spectra per second can, in principle, be acquired and summed, allowing fast accumulation of data, rapid averaging and consequent improvement of the signal to noise ratio and detection limit. Experiments were performed using a single-pass cell with a path length of 16.4 cm, and also an astigmatic multi-pass absorption cell aligned to give a path length of 56 m. Detection limits corresponding to minimum detectable absorption coefficients, alpha(min), of 5.6 x 10(-5) and 7.8 x 10(-8) cm(-1), respectively, were obtained over a 4 s detection bandwidth. These detection limits would correspond to mixing ratios of 21 parts per million by volume (ppmv) and 59 parts per billion by volume (ppbv) of acetylene at 1 atm in air, with the deleterious effects of pressure broadening accounted for. The single-pass cell was used to perform breakthrough volume (BTV) experiments for the low volume adsorbent traps used to pre-concentrate organic compounds in air, taking advantage of the capability of the system to measure concentrations in real time.     

Capability of detection and three-way data

Due to the possibility of making analytical determinations in the presence of non-modelled interferents and to identify the analyte of interest, calibrations based on scores of PARAFAC decomposition of three-way data are becoming increasingly important in routine analysis.Furthermore, the IUPAC and EU (European Decision 2002/657/EC) have accepted the definition given by the ISO 11843 for the capability of detection as the minimum net quantity detectable with a pre-set probability of false positive and false negative. What is more, recently our research group has generalised this definition of capability of detection, CCβ, to multivariate calibrations. In practice, CCβ is a good measure of the quality of the calibration because in its definition it brings together analytical sensitivity with precision in analytical determinations.This paper studies the effect of the pre-treatment of the sample, the signal/noise ratio and the second-order advantage on CCβ when using second-order signals modelled by PARAFAC. All of them are experimental factors which influence the quality of the calibration. Analytical pre-treatment is habitual in the analysis of real samples. Specifically, we analyse the effect of the extraction phase and the clean-up of milk samples on the determination of chlortetracycline by HPLC-DAD. It is shown that it is more efficient to do the joint PARAFAC decomposition of the pure standards with the milk samples.Secondly, the effect of asymmetry on CCβ, according to the path of the noise of the signals, is studied. Specifically, in the determination of naphthalene by excitation-emission spectroscopy, EEM, it is the emission spectrum which limits the capability of detection. It is shown that by eliminating the spectra with the poorest signal/noise ratio in this path, the capability of detection can be substantially improved.Thirdly, the impact on CCβ when the second-order advantage is used, that is when PARAFAC calibration is used over samples with an unknown interference not modelled in the calibration step. This is important to apply a PARAFAC calibration to routine analysis in the IUPAC and European Decision framework. Specifically, in the determination of enrofloxacine in poultry feeding water through excitation-emission fluorescence CCβ is evaluated when the PARAFAC is built only with calibration samples or with the calibration samples plus the test samples with uncalibrated and unknown interferent.     

Surface‐Enhanced Raman Scattering Based on Controllable‐Layer Graphene Shells Directly Synthesized on Cu Nanoparticles for Molecular Detection

Graphene shells with a controllable number of layers were directly synthesized on Cu nanoparticles (CuNPs) by chemical vapor deposition (CVD) to fabricate a graphene‐encapsulated CuNPs (G/CuNPs) hybrid system for surface‐enhanced Raman scattering (SERS). The enhanced Raman spectra of adenosine and rhodamine 6G (R6G) showed that the G/CuNPs hybrid system can strongly suppress background fluorescence and increase signal‐to‐noise ratio. In four different types of SERS systems, the G/CuNPs hybrid system exhibits more efficient SERS than a transferred graphene/CuNPs hybrid system and pure CuNPs and graphene substrates. The minimum detectable concentrations of adenosine and R6G by the G/CuNPs hybrid system can be as low as 10^?8 and 10^?10 M , respectively. The excellent linear relationship between Raman intensity and analyte concentration can be used for molecular detection. The graphene shell can also effectively prevent surface oxidation of Cu nanoparticles after exposure to ambient air and thus endow the hybrid system with a long lifetime. This work provides a basis for the fabrication of novel SERS substrates.     

Analysis of polyurethanes using core excitation spectroscopy. Part I: Model polyurethane foam polymers

The C 1s, N 1s, and O 1s excitation spectra of model methylenediphenyldiisocyanate (MDI) polyurethanes with well known structures have been recorded using electron energy loss spectroscopy (EELS) in an electron microscope. These spectra are compared to the core excitation spectra of selected small molecule analogue species (recorded by gas phase EELS) in order to identify transitions characteristic of various structural components found in polyurethanes. A more detailed report on the small molecule analogue spectra is presented in the following article. Spectral features characteristic of the different structural components in polyurethanes are identified in the spectra of the model polymers. These can be used as the basis for chemical studies of micron or submicron sized segregated phases in flexible polyurethane polymers. © 1995 John Wiley & Sons, Inc.     

Quantum mechanical limits to the control of atom-diatom chemical reactions through the polarisation of the reactants

This article considers the extent to which one can control the reactivity of atom-diatom systems through reactant polarisation. Three different limits for reactivity manipulation are defined: "absolute" limits that do not depend on the reaction dynamics but can only be obtained for particular combinations of quantum numbers, "unconstrained" limits that depend on dynamics but not on constraints imposed by any particular experimental setup, and "constrained" limits that depend on dynamics and also on the constraints imposed by a particular experimental setup. Methods for calculation of these limits are presented and applied to the benchmark F + H2 reaction. The variations of the maximum and minimum reactivity one can obtain are analysed in terms of reaction mechanisms and steric constraints. Tables listing the minimum and maximum values of angular momentum polarisation moments of rank up to 4, and integer and half-integer quantum numbers up to 5, are also presented.     

Absolute and/or relative detection limits in laser-based analysis: the end justifies the means

The absolute limit of detection usually expresses the minimum amount of analyte detectable, while the relative limit of detection refers to the minimum concentration of analyte detectable. These concepts and their differences are obviously familiar to all analytical spectroscopists. Nevertheless, the two definitions are used liberally in the literature. For example, it is not uncommon to refer to exceptional sub-femtograms detection limits for a technique used to analyse ultratrace levels of an element in water and to a modest part per million detection limit of another technique used to characterise the microdistribution of an element in a sample mass of about one microgram. In this paper, an attempt is made to point out that the terms "ultratrace analysis" and "microanalysis" must refer to two conceptually different approaches and that there are cases in which one definition is more appropriate than the other. It is argued that, while there is no objection in reporting both detection limits when a single technique is evaluated, one has to be careful in choosing the most appropriate definition when different analytical techniques are compared.     

Microsolvation of F- in water

Microsolvation of F- in water is studied by ionization and double ionization spectra of (H2O)1-3F- calculated by ab initio methods. It is shown that the presence of the fluorine electrons introduces many-body properties in the spectra which cannot be reproduced by the presence of a negative point charge. The increase of the solvation shell increases the complexity in particular of the double ionization spectra. Ionization and double ionization energies slowly increase with continued solvation, and many-body effects in the inner valence spectra become more prominent.