Improved stochastic resonance algorithm for enhancement of signal-to-noise ratio of high-performance liquid chromatographic signal

Based on the theory of stochastic resonance, an improved stochastic resonance algorithm with a new criterion for optimizing system parameters to enhance signal-to-noise ratio (SNR) of HPLC/UV chromatographic signal for trace analysis was presented in this study. Compared with the conventional criterion in stochastic resonance, the proposed one can ensure satisfactory SNR as well as good peak shape of chromatographic peak in output signal. Application of the criterion to experimental weak signals of HPLC/UV was investigated and the results showed an excellent quantitative relationship between different concentrations and responses.     

Improved accuracy of on-line heavy water measurement using infrared spectroscopy by investigation of signal-to-noise ratio and temperature influences

Experimental protocols for the on-line measurement of heavy water concentration in nuclear power plants have been established, and important parameters, such as the temperature and signal-to-noise ratio, which govern the accuracy of measurement, have been studied. The temperature of a sample should be controlled tightly because the temperature variation introduces non-linear baseline variations and leads to an increase of the partial least squares calibration error. Furthermore, the variation in the signal-to-noise ratio of spectra sensitively influences the calibration. For reliable prediction, it is critical to maintain the signal-to-noise ratio at a certain level. When the sample spectra were collected at a higher temperature, it was possible to acquire spectra with an improved signal-to-noise ratio and better calibration. In addition, a single beam spectrum of water shifts to a lower frequency, and the maximum transmission intensity at around 2500 cm(-1)(the heavy water band location) increases at an elevated temperature. Overall, an on-line infrared spectroscopic scheme is presented for measuring heavy water. The scheme can be applied to an actual process without practical difficulties. If the spectra could be collected at elevated temperature over 2 min with the use of a high throughput light source, the prediction error could reach to 1.0 ppm.     

Monte Carlo optimization of signal-to-noise ratio for electrothermal atomizers

This work uses Monte Carlo simulation models for Cu and Ag to study the change in the signal-to-noise ratio (S/N) with variations on the electrothermal atomizer length and diameter. A 5 × 5 grid corresponding to different lengths (2.0–4.0 cm) and diameters (0.30–0.70 cm) is filled with S/N data from the simulations. It is assumed that the S/N is shot-noise limited. The dosing hole diameter is held at 1.5 mm in all cases. A double-desorption type mechanism for Ag atomization was tested against experimental profiles and good agreement was achieved. Spherical microdroplets release Ag(g) with a fractional order (2/3) and an activation energy of 238 kJ mol⁻¹. Ag(g) readsorbs as dispersed atoms on the wall upon collision. The secondary desorption is first order and has a lower activation barrier (117 kJ mol⁻¹). The S/N predicted by the Monte Carlo simulation does not strongly depend on the heating rate or the nature of the analyte being determined. The optimum with respect to geometry is broad. The optimum furnace diameter is near 0.50 cm, while the optimum furnace length is at the 4.0-cm limit used in these studies. Most commercial atomizers have diameters close to the optimum but shorter lengths, probably reflecting the pragmatic consideration that the longer tubes require larger power supplies and prolonged heating times which may substantially reduce the useful life of the furnace. Further increases in length may also accentuate atomizer nonisothermality which deters analytical improvement.     

Minimum required signal-to-noise ratio for optimal precision in HPLC and CE

In a survey, results of more than 100 assay determinations of various analytes (active pharmaceutical ingredients, methylparabene, chlorthalidone, and proteins) at different concentration levels were collected with signal-to-noise ratio (S/N) levels between 2 and higher than 10?000. It must be concluded that without having a S/N of at least 50, repeatabilities (combination of injection, separation process, and integration) of 2% cannot be achieved, in contrast to the general assumption that a S/N of 10 is sufficient for analytical HPLC. These data now confirm an earlier assumption with a much broader fundamental of measurements. The empirical functions %RSD = 58/(S/N) + 0.30 (for HPLC data) and %RSD = 73/(S/N) + 1.07 (for CE data) could be derived. It was shown that a S/N of greater than 100 is necessary for optimal precision. Before optimizing HPLC or CE methods, for example, for sample pretreatment, S/N > 100 should be the prerequisite, else optimal precision will not be achieved. Only after the detection-related scatter is sufficiently reduced, other sources of variation can be successfully tackled.     

Optimizing the signal-to-noise ratio of scoring functions for protein--ligand docking

Empirical scoring functions provide estimates of the free energy of protein-ligand binding in situations when atomic-scale simulations are intractable, for example, in virtual high-throughput screening. Currently, such scoring functions are often inaccurate, and further improvements are complicated by the lack of reliable training data, the complex interplay between scoring functions and docking algorithms, and an inconsistent statistical treatment of positive and negative training data. In comparison to various other performance measures of scoring functions, "analysis of variance" provides a well-behaved objective function for optimization, which focuses on the signal-to-noise ratio of ligand-decoy discrimination. In combination with a large database of ligands and decoys, an in situ optimization of scoring function parameters was able to generate improved, target-specific scoring functions for three different proteins of pharmaceutical interest: cyclin-dependent kinase 2, the estrogen receptor, and cyclooxygenase-2. Statistical analysis of the improvements observed in "receiver-operating characteristic" curves showed that the optimized scoring functions achieved a significantly (between p < 0.0001 and p < 0.05) higher enrichment of true ligands. A scaffold dependence of the resulting binding modes was observed, which is discussed in conjunction with the rigid receptor hypothesis commonly made in protein-ligand docking. In summary, the approach described here represents a well-adapted statistical method for setting up scoring functions.     

方波激发非接触式电导检测器信噪比的影响因素研究

研制了用于毛细管电泳的方波激发非接触式电导检测器(CCD)并详细考查了对检测器信噪比影响较大的几个重要因素,包括激发频率和运算放大器、基线分压电路和A/D转换器、数字低通滤波;以及常见缓冲体系2-(N-吗啡啉)-乙磺酸/组氨酸(MES/His)体系中电解质浓度,添加剂十六烷基三甲基溴化铵(CTAB)对金属离子的检出信号的影响。所得的结果对于该类检测器的设计和优化具有参考价值。     

Enhancing signal-to-noise ratio and resolution in low-field NMR relaxation measurements using post-acquisition digital filters

The traditional way to enhance signal-to-noise ratio (SNR) of nuclear magnetic resonance (NMR) signals is to increase the number of scans. However, this procedure increases the measuring time that can be prohibitive for some applications. Therefore, we have tested the use of several post-acquisition digital filters to enhance SNR up to one order of magnitude in time domain NMR (TD-NMR) relaxation measurements. The procedures were studied using continuous wave free precession (CWFP-T₁) signals, acquired with very low flip angles that contain six times more noise than the Carr–Purcell–Meiboom–Gill (CPMG) signal of the same sample and experimental time. Linear (LI) and logarithmic (LO) data compression, low-pass infinity impulse response (LP), Savitzky–Golay (SG), and wavelet transform (WA) post-acquisition filters enhanced the SNR of the CWFP-T₁ signals by at least six times. The best filters were LO, SG, and WA that have high enhancement in SNR without significant distortions in the ILT relaxation distribution data. Therefore, it was demonstrated that these post-acquisition digital filters could be a useful way to denoise CWFP-T₁, as well as CPMG noisy signals, and consequently reducing the experimental time. It was also demonstrated that filtered CWFP-T₁ method has the potential to be a rapid and nondestructive method to measure fat content in beef and certainly in other meat samples.     

Improve the signal-to-noise ratio of a quartz crystal microbalance in an impedance analysis method

A new data treatment method for the improvement of the signal-to-noise ratio of a quartz crystal microbalance (QCM) was proposed, where an averaged resonant frequency was calculated according to its conductance peak in an impedance analysis method. The relationship between the averaged resonant frequency and the medians of the conductance peak at different sampling heights was derived. It was shown that the signal-to-noise ratio of the averaged resonant frequency was about eight times of that of the resonant frequency calculated directly from its equivalent circuit parameters. The averaged resonant frequency of the QCM was applied to monitor the self-assembled process of a 6-O-(2′-(α-thiohydroxyacetamide)-ethyl)-diethoxylsilyl-β-cyclodextrin (OTED-β-CD), on gold surface as well as the adsorption of nitrophenol isomers onto the OTED-β-CD self-assembled monolayer film.     

The use of simulation for the optimisation of the signal-to-noise ratio in cyclic activation analysis

It is always useful to predict the result of an experiment without having to carry it out. In cyclic activation analysis where the signal-to-noise ratio for the detection of short-lived nuclides in a given matrix is particularly sensitive to the value of the timing parameters, determination of the best conditions for the detection of a nuclide of interest, without resort to preliminary experiments, is most helpful. Simulation and optimisation of a gamma-ray spectrum is derived for a sample irradiated in cyclic mode. The simulation includes consideration of photopeak, Compton continuum, single and double escape peaks and bremsstrahlung. The simulation output forms the input to the signal-to-noise ratio optimisation routine. Consequently the sensitivities, and detection limits for the isotopes of interest can be deduced from the optimised spectrum. Interference from other gamma-lines is identified, if applicable. In addition, a graphical output of the simulated spectrum and a listing of the optimised activation parameters, detection limits and sensitivites are produced. The programme has been extended to include simulation of conventionally neutron activated samples. Examples for standard reference materials are given as illustrations, together with measured spectra.     

Investigation of chemometric calibration performance based on different chemical matrix and signal-to-noise ratio.

The chemometric calibration performance was systematically investigated by two parameters (changing the chemical matrix as well as the signal-to-noise ratio) of the NIR (near-infrared) spectrum. Three different analytes (hexane, cyclohexane, toluene) were selected and heptane was used as a solvent. The degree of spectral difference significantly affected the calibration performance. The largest structural difference between the analyte and the solvent provided the best calibration result for a given signal-to-noise ratio. Additionally, the signal-to-noise ratio of the spectra also directly influenced the calibration performance. Overall, the spectral difference and signal-to-noise ratio were the major factors for governing the chemometric calibration performance, especially in the low-concentration range.     

Wavelet transform as a new approach to the enhancement of signal-to-noise ratio in anodic stripping voltammetry

De-noising signals is a frequent aim achieved by signal processing in analytical chemistry. The purpose is to enable the detection of trace concentrations of analytes. The limit of detection is defined as the lowest amount of analyte that still causes signals greater than the background noise. Appropriate de-noising decreases only the noise and maintains the measurement signal, so that signal-to-noise ratios are enhanced. One adequate mean of signal processing for this purpose is wavelet transform, which still is not a common tool in analytical chemistry. In this paper, the ability of de-noising by wavelet transform is shown for measurements in anodic stripping voltammetry using a hanging mercury drop electrode. The calculation of limits of detection and signal-to-noise ratios on the basis of peak-to-peak noise is exercised to quantify the performance of de-noising. Furthermore, signal shape with regard of easing the application of base lines is discussed. Different wavelet functions are used, and the results are compared also to Fourier transform. Coiflet2 was found out to reduce noise by the factor of 330 and is proposed as the adequate wavelet function for voltammetric and similar signals.     

基于SAO-VMD-FFT的激光诱导荧光光谱信号信噪比提升方法

针对基于激光诱导荧光光谱法(LIFs)的便携式场地地下水检测技术装备在原位检测地下水中典型重金属时,光传感信号含噪声严重,影响后续定量分析准确度的问题,提出了一种雪消融优化器(SAO)寻优算法优化的变分模态分解(VMD)结合快速傅里叶变换(FFT)的LIFs光传感信号信噪比提升算法。利用SAO算法对VMD分解关键参数分解模态个数K和惩罚因子α进行优化,实现含噪信号的自适应最佳VMD分解,得到一系列本征模态分量。进一步结合FFT算法寻找频率突变点从而筛选出相关模态。累计相关模态得到信噪比提升后的信号。通过仿真和实测信号分析,结果表明与EMD、WTD和S-G等常用算法相比,本文算法能更好地去除LIFs信号中的噪声分量,保留有用信息,验证了该方法的可行性和有效性,可为后续利用LIFs传感信号原位定量分析地下水中重金属元素含量提供支持。     

Molecular contrast optical coherence tomography: a review

This article reviews the current state of research on the use of molecular contrast agents in optical coherence tomography (OCT) imaging techniques. After a brief discussion of the basic principle of OCT and the importance of incorporating molecular contrast agent usage into this imaging modality, we shall present an overview of the different molecular contrast OCT (MCOCT) methods that have been developed thus far. We will then discuss several important practical issues that define the possible range of contrast agent choice, the design criteria for engineered molecular contrast agent and the implementability of a given MCOCT method for clinical or biological applications. We will conclude by outlining a few areas of pursuit that deserve a greater degree of research and development.     

Significant improvement of signal-to-noise ratio in capillary electrophoresis through optimization of aperture width for UV absorption detection.

In capillary electrophoresis (CE), light flux passes through a capillary cell and is in most cases detected photometrically. Due to the thinness of the cell, a part of the light passes through the wall and misses hitting the sample. In most CE apparatuses, incident light is focused by converging lenses in order to condense light beams passing through the capillary. Considering the aberration of lenses and lens effects of capillary, we assumed that light beams inside were approximately parallel. Although the path lengths of light beams vary depending on their tracks, we could estimate the virtual light path length, L, by measuring absorbance when concentration and molar absorptivity of the sample solution were known. A light-restricting device consisting of narrow slits makes effectively L longer and signal intensity higher. On the other hand, noise increases as light width narrows. The signal-to-noise ratio showed a maximum at 68 microm of light width for a capillary with diameter of 75 microm. The optimized L was evaluated by the simulation. The experimental data verified it even in indirect UV detection. Our approach could help to design the optics of CE apparatuses.     

Low-field 1H NMR spectroscopy: Factors impacting signal-to-noise ratio and experimental time in the context of mixed microstructure polyisoprenes

Low-cost, high-accuracy characterization of polymeric materials is critical for satisfying societal demand for high-quality materials with ultra-specific requirements. Low-field nuclear magnetic resonance (NMR) spectroscopy presents an opportunity to replace costlier or destructive methods while utilizing nondeuterated solvents. Many factors play key roles in the ability of low-field NMR spectroscopy to accurately analyze polymer systems. Sample characteristics such as polymer concentration, composition, and molecular weight all directly affect the capability of low-field spectrometers to accurately determine polymer microstructure compositions. In addition to inherent sample properties affecting instrumental accuracy, many choices concerning instrumental parameters (including number of scans, relaxation delay, spectral width, and points per scan) must be made that impact the quality of the resulting NMR spectra. In this work, we benchmark the capability of a 60-MHz low-field NMR spectrometer for analyzing polymer materials using mixed microstructure polyisoprenes as a model polymer system of interest. The aforementioned critical sample and instrumental variables are varied, and we report on the ability to quantitatively characterize polyisoprene microstructure to within 1–2% of a higher field NMR spectrometer (400 MHz). We anticipate our findings to be generally applicable to other low-field spectrometers of similar field strength and other polymer systems.     

Improved isotope ratio measurement performance in liquid chromatography/isotope ratio mass spectrometry by removing excess oxygen

A low dead volume oxygen scrubbing system was introduced in a commercially available liquid chromatography/isotope ratio mass spectrometry (LC/IRMS) interface to enhance the analytical capability of the system. In the LC/IRMS interface carbon from organic samples is converted into CO(2) inside the mobile phase by wet chemical oxidation using peroxodisulfate (Na(2)S(2)O(8)). After passing the hot reaction zone, surplus oxygen (O(2)) remains dissolved in the liquid phase. Both CO(2) and O(2) diffuse through a transfer membrane into the helium carrier and are transferred to the mass spectrometer. The presence of O(2) in the ion source may have detrimental effects on measurement accuracy and precision as well as on filament lifetime. As a remedy, a new on-line O(2)-removing device has been incorporated into the system.The new O(2) scrubber consists of two parallel hot copper reduction reactors (0.8 mm i.d., active length 120 mm) and a switch-over valve between them. One reactor is regenerated using He/H(2) while the other is actively scavenging O(2) from the gas stream. The capacity of each reduction reactor, expressed as usage time, is between 40 and 50 min. This is sufficient for a single LC run for sugars and organic acids. A further increase of the reduction capacity is accompanied by a peak broadening of about 100%. After switching to a freshly reduced reactor the oxygen background and the delta(13)C values of the reference gas need up to 500 s to stabilize. For repeated injections the delta(13)C values of sucrose remain constant (+/-0.1 per thousand) for about 3000 s. The long-term stability for measurements of sucrose was 0.11 per thousand without the reduction oven and improved slightly to 0.08 per thousand with the reduction oven. The filament lifetime improved by more than 600%, thereby improving the long-term system stability and analytical efficiency. In addition the costs per analysis were reduced considerably.     

A novel electric ion resonance cell design with high signal-to-noise ratio and low distortion for Fourier transform mass spectrometry

Performing wideband ion image current detection mass spectrometry experiments with an electric ion trap—e. g., the Paul trap—is a difficult task, as there is a strong crosstalk current induced by the high voltages of the radio frequency (rf) storage field. In a classic Paul trap the metallic hyperbolic electrodes (a ring electrode and two end cap electrodes) are shaped following the isopotential lines of the quadrupole potential distribution. In our new design the ring electrode is replaced by a cylindrical series of ring electrodes with a parabolic potential distribution, whereas the end cap electrodes are used without modification. Thus the quadrupole field within the trap remains unchanged but the capacitances between the electrodes and therefore the crosstalk currents are significantly reduced. The remaining crosstalk is balanced out by an electronic compensation technique. As a consequence the weak signals of the ion-induced charge can be detected with a wideband low-noise amplifier to perform Fourier transform mass spectrometry experiments with improved signal-to-noise ratio.