向后迭代区间选择算法及其在近红外光谱模型转移中的应用

近红外光谱（NIR）具有快速、无损、操作方便的特点,故广泛用于食品分析。作为一种间接的分析技术,NIR需要建立光谱与待测浓度之间的统计模型来实现检测。故模型的维护有助于保证NIR的预测准确性。在外界条件发生变化的情况下,诸如样品性状的改变、仪器对理化指标函数关系的变化、湿度和温度等环境因素的改变,会导致相同样品的光谱信号发生偏移,进而使得原有模型的预测精度下降。此时,如果重新建模,虽然可以解决光谱偏移对建模的影响,但是重新建模将耗费大量的人力物力。对此,模型转移可以在避免重新建模的情况下,校正光谱的偏移,进而提高模型预测精度。通常模型转移算法多用全光谱进行模型转移,这种方法计算量较大,且不能找到合适的有化学意义的波段。故提出一种基于模型转移中的变量选择方法：向后迭代区间选择法（IIBS）,通过计算主光谱（用于建模的那组光谱）和从光谱（发生偏移,需要通过模型转移算法将其校正的光谱）中,变量区间的重要性信息（回归系数（β）、残差向量（Res）以及变量重要性投影（VIP））。进而通过计算该区间变量重要性信息的几何平均数,并以此作为该区间的区间重要性指标。接着根据区间的重要性,删除重要性信息较小的变量区间。然后对主光谱和从光谱重复迭代上述过程：计算变量的重要性信息,计算区间的重要性信息,删除重要性信息较小的区间。最后,比较不同的主光谱和从光谱区间组合的验证均方根误差（RMSEV）,选择RMSEV最小的主光谱和从光谱区间作为最优区间。玉米、小麦两套NIR数据测试了该算法。结果显示,与全波段相比,β,Res以及VIP均可以从主光谱和从光谱中选择较少的,有化学意义的区间,提高模型转移的精度。在比较不同变量重要性向量方面,基于β的变量选择算法,模型转移的计算误差较小。     

LIBS结合单变量定标法对土壤中铅元素含量检测

重金属污染一直影响着人们的健康生活,如镉,铅和铜等的污染,故而土壤重金属的快速检测和如何预防,一直受各国学者关注和研究。传统土壤重金属检测方法（如原子吸收光谱法、X荧光光谱法等）样品预处理复杂,分析成本较高,易形成样品的二次污染,不能满足快速分析的要求。激光诱导击穿光谱（LIBS）是一种典型的原子发射光谱,它是基于分析物质中原子和离子受激发而发射的特征谱线信息,进而研究物质成分的分析方法。LIBS技术能够快速检测任何状态（固、液和气态）物质元素的成分和含量,被看作是未来化学检测和快速绿色分析领域的新兴技术。LIBS技术具有对样本简单预处理（或不需要处理）、多元素同步分析、远距离测量、适用性广等优势,被广泛用于生活生产的各个领域,已成为近年来国内外学者广泛关注和研究的热点之一。在农业信息快速感知的大背景下,以激光诱导击穿光谱技术为技术手段,以土壤重金属铅元素为研究对象,运用理论分析和数学建模相结合,建立了多种基于单变量定标曲线的土壤重金属铅检测模型,并进行了模型验证。自制15个已知的铅元素浓度梯度的谱线土壤样本,在获取了土壤LIBS数据之后,对其进行预处理对比,建立了基于谱峰强度、谱峰积分、洛伦兹拟合强度三种定标曲线模型,对土壤中铅元素含量进行定量分析,得出基于三种定标曲线模型对土壤中铅元素含量的预测决定系数R2分别为0.918 0,0.910 1和0.914 3,三种定标曲线分析方法的预测结果都较好,说明了LIBS结合单变量定标曲线法对土壤中铅含量的检测可靠性高。最后选取部分样本数据进行验证,结果较好。研究结果为研发便携式农田土壤污染物检测技术与装备提供技术支撑,也为农田精准管理和科学施肥奠定基础。     

Optical pressure sensors for high-pressure–high-temperature studies in a diamond anvil cell

We review the various optical pressure sensors that are suitable for high-pressure and high-temperature studies in a diamond anvil cell. Two different kinds of sensors are considered: those based on the pressure shift of a fluorescence line (ruby, SrB₄O₇:Sm²⁺) and those based on the pressure shift of a Raman line (c-BN, diamond). The calibration of those sensors are presented in detail, and discussion is made on their useful pressure and temperature ranges.     

A fast automatic calibration system for a sound level meter in an anechoic room

A fast automatic calibration system for a sound level meter in an anechoic environment has been developed. The precise and fast generation of the constant frequency-independent acoustic pressure field and reduction of digital voltmeter readout to sound pressure level were achieved by a newly designed software algorithm, assuming the system linearity. The performance test justified the assumed linearity, and showed that the searching speed was sufficiently fast within the small error limit.     

Techniques to investigate the characteristic of CR-39 detectors using Am---Be neutron source

Two techniques have been developed to investigate the characteristic of the sensitivity of CR-39 (polyallyldiglycol carbonate) track detector using Am---Be neutron source. The firstis to measure volume track density created by fast neutrons in CR-39 detectors. The second is to measure areal asymptotic track density on the surface of CR-39 sheets. Two types of CR-39—SY-2 CR-39 made in China and another made in USA were calibrated using the techniques. It shows that the two types of CR-39 detectors have nearly the same sensitivity to detect fast neutrons. The techniques developed in this work can be applied to calibration of other track detectors.     

Metrology in Iceland: a status report

 The metrology system in Iceland has been reviewed and developed in the light of the increased needs of industry and other users of the system, in some cases to comply with the requirements of the agreement between the European Union and the European Economic Area which the Icelandic government signed in 1992. After a short historical review, the current legislation is explained, the metrological infrastructure is described, and the plans for the coming years are discussed. Received: 27 November 1995 Accepted: 18 December 1995     

Temperature profiles in a controlled-stress parallel plate rheometer

Actual melt temperatures are rarely measured directly when conducting rheological characterisations of materials in non-ambient conditions despite the potentially large influence temperature may have on the rheological data. For rheometers that use only a temperature-regulated lower plate, it is likely that the set point and true melt temperatures differ, an effect that becomes significant when characterising melts or suspensions close to phase-change events like crystallisation. This work investigates the magnitude of these effects for a controlled-stress rheometer featuring a temperature-controlled lower plate. The lower plate was fitted with a serrated cover disc that was found to exacerbate temperature deviations from the desired set point. Steady-state radial and vertical temperature profiles within the sample were measured and compared with the predictions of a finite element analysis model. The deviations between set point and measured temperatures were successfully predicted by the simulation for two typical gap heights for a thermoplastic, ceramic paste. The non-ideal heat transfer characteristics were also investigated numerically for a representative polymer system that demonstrated the increased deviations from ideal values for lower thermal conductivity materials.

Isotope Studies of Hydrogen and Oxygen in Ground Ice - Experiences with the Equilibration Technique

Abstract

Equilibration technique suitable for a large amount ofsamples is described for hydrogen and oxygen isotope analyses of ground ice, especially ice wedges, including the sampling strategy and the analytical procedure as well as the calibration of the Finnigan MAT Delta-S mass spectrometer in June, 1999. Since for future analyses of ice wedges, a higher sampling resolution with limited sample volume is required, the limit of the equliibration technique for small water samples size of between 0.05 and 5 ml was checked. For water samples smaller than 1ml, corresponding to a molar ratio [H₂O]/[H₂] of smaller than 0.994, a balance correction has to be applied. The experimental errors due to partial evaporation during evacuation, the balance calcultion of the isotope equilibration process, the linearity as well as memory effects of the mass spectrometer for smaples with large differences in δ¹⁸O and δD are tackled in this paper. In the polar regions of Northern Siberia without Late Pleistocene and Holocene glaciation, ground ice is used as an archive for paleoclimate studies. First results of stable isotope measurements on ice wedges clearly show a shift towards heavier isotopes and thus warmer winter temperatures as well as a change in the source of the precipitation between Late Pleistocene and Holocene. These results indicate the high potential of ground ice for paleoclimate studies.     

Improved calibration transfer between near-Infrared (NIR) spectrometers using canonical correlation analysis

The application of mobile near-infrared (NIR) spectrometers in field measurements is growing. Calibration transfer techniques offer simple solutions for enabling models constructed on benchtop instruments for use on mobile spectrometers. Since different types of spectrometers with different components, scanning ranges and resolutions cause great differences in the spectral response, calibration transfer is difficult to apply. In this paper, we focus on calibration transfer among benchtop, portable and handheld spectrometers by a method of calibration transfer based on canonical correlation analysis (CTCCA). Its capability was illustrated by the example of a group of NIR spectra dataset for predicting reducing sugars, total sugar, and nicotine contents in tobacco leaves. The experimental results showed that the transferability of CTCCA was superior to other conventional calibration transfer methods, including piecewise direct standardization, spectral space transformation, calibration transfer based on independent component analysis, and calibration transfer based on the weight matrix. Moreover, the best transfer results were obtained in the three cases by canonical correlation analysis method executing transfer while the spectra were not interpolated, which shows that this approach has the advantage of easy implementation for calibration transfer. Therefore, CTCCA without interpolation calculation offers a new and simple solution for transferring the spectra acquired by mobile spectrometers to the optimized spectral models built on benchtop devices to improve the accuracy of the results. Additionally, the results show that the benchtop spectrometer is more suitable as the master instrument for calibration transfer with more accurate prediction than using a portable device as the master.     

Flexible and accurate implementation of a binocular structured light system

This paper presents a practical implementation of a binocular system for three-dimensional (3D) surface measurement. Manufacturing procedures are easy to implement because this system does not require that the optical axes of two cameras’ lenses be coplanar and the structured light sheets be perpendicular to that plane. The method of structured light calibration is proposed to obtain the parameters of the structured light sheets. This calibration method can tolerate the surface condition of the calibration board. To eliminate the effect caused by the distortion of projector lens, each structured light sheet is fitted to a conicoid. Meanwhile, data acquired from two cameras are transformed into a unique coordinate system to avoid 3D data matching. Experiments are conducted to validate the flexibility, accuracy, and desirability of our system.