Headspace-gas chromatography: The influence of sample volume on analytical results

Summary The role of the volume of the sample and the sample vial in equilibrium headspace-gas chromatography is discussed. A new term, thesample phase fraction (_S) is introduced. It is shown that if the value of _S is kept constant, the vial's volume has no influence on the sensitivity of the headspace analysis (which is proportional to the concentration of the analyte in the headspace). In a given headspace sampling system, concentration of the compound of interest in the headspace (c _G ^* ) at equilibrium is related to the value of _S: a higher _S will increase c _G ^* . However, the influence is important only in the case of low distribution coefficients: in the case of higher distribution coefficients this influence is negligible. This conclusion is also true for small changes in the sample volume in duplicate analyses: exact reproducibility of the sample volume is important only in the case of low distribution coefficient values.     

Some exact expressions for the mean and higher moments of functions of sample moments

Examples of exact expressions for the moments (mainly of the mean) of functions of sample moments are given. These provide checks on alternative developments such as asymptotic series for n, and simulation processes. Exact expressions are given for the mean of the square of the sample coefficient of variation, particularly in uniform sampling; Frullani integrals studied by G. H. Hardy arise. It should be kept in mind that exact results for (joint) moment generating functions (mgfs) are of interest as they produce a means of obtaining exact results for (cross) moments—including moments with negative indices. Thus an exact expression for the joint mgf of the 1st two noncentral moments can be used to obtain the mean of the (c.v.)² (but not for the mean of the c..). A general expression is given for the moment generating function of the sample variance. The limitations of Fisher's symbolic formula for the characteristic function of sample moments (or more general statistics) are noted.This research was sponsored by the Applied Mathematical Sciences Research program, Office of Energy Research, U. S. Department of Energy under contract DE-AC0584OR21400 with the Martin Marietta Energy Systems. Inc.     

Determination of As, Sb, Se, Te and Bi in milk by slurry sampling hydride generation atomic fluorescence spectrometry

A simple and fast analytical procedure has been developed for the determination of As, Sb, Se, Te and Bi in milk samples by hydride generation atomic fluorescence spectrometry (HG-AFS). Samples were treated with aqua regia for 10 min in an ultrasound water bath and pre-reduced with KBr for total Se and Te determination or with KI and ascorbic acid for total As and Sb, the determination of Bi being possible in all with or without pre-reduction. Slurries of samples, in the presence of antifoam A, were treated with NaBH₄ in HCl medium to obtain the corresponding hydrides, and AFS measurements were processed in front of external calibrations prepared and measured in the same way as samples. Results obtained by the developed procedure compare well with those found after microwave-assisted complete digestion of samples. The proposed method is simple and fast, and only 1 ml of milk is needed. The values obtained for detection limit are 2.5, 1.6, 3, 6 and 7 ng l⁻¹ for As, Sb, Se, Te and Bi respectively in the diluted samples, with average relative standard deviation values of 3.8, 3.1, 1.9, 6.4 and 1.2% for three independent analysis of a series of commercially available samples of different origin. Data found in Spanish market samples varied from 3.2±0.3 to 11.3±0.2 ng g⁻¹ As, from 3.1±0.2 to 11.6±0.4 ng g⁻¹ Sb, from 10.7±0.5 to 25.5±0.4 ng g⁻¹ Se, from 0.9±0.2 to 9.4±0.6 ng g⁻¹ Te and from 11.5±0.1 to 27.7±0.4 ng g⁻¹ Bi.     

Selective and sensitive detection of organic contaminants in water samples by on-line trace enrichment–gas chromatography–mass spectrometry

Liquid chromatographic (LC) type trace enrichment is coupled online with capillary gas chromatography (GC) with mass spectrometric (MS) detection for the analysis of aqueous samples. A volume of 1–10 ml of an aqueous sample is preconcentrated on a trace-enrichment column packed with a polymeric stationary phase. After cleanup with HPLC-grade water the precolumn is dried with nitrogen and subsequently desorbed with ethyl acetate. A fraction of 60 μl is introduced on-line into a diphenyltetramethyldisilazane-deactivated retention gap under partially concurrent solvent evaporation conditions and using an early solvent vapor exit. The analytes are separated and detected by means of GC–MS. The potential of the LC–GC–MS system for monitoring organic pollutants in river and drinking water is studied. Target analysis is carried out with atrazine and simazine as model compounds; the detection limits achieved under full-scan and multiple ion detection conditions are 30 pg and 5 pg, respectively. Identification of unknown compounds (non-target analysis), is demonstrated using a river water sample spiked with 168 pollutants varying in polarity and volatility.     

复合地基中桩与桩间土应力分配实验

在实验室条件下，用碎石或灰土圆柱芯增强体与黏性土组成的复合地基单位桩模型，通过复合试样的三轴实验模拟现场碎石桩和灰土桩的工作状态，探讨桩与土的相互作用问题。实验结果表明：复合体的竖向强度不是两种材料按一定几何比例简单叠加。复合试样在荷载及应变不大时，桩土应力比增加很快，随后荷载逐渐向桩周土集中，桩土应力比减少。这种规律与现场测试的桩土应力比是一致的。实验揭示了桩土应力比变化规律及其机理。     

布朗单样本轨道的重分形分析

主要研究布朗单W＝{W（s1,s2)s1，s2≥0}样本轨道的重分形分析问题．在布朗单一致连续模的基础上讨论“α—快点集”的重分形分析性质，得到了两类不同增量形式的“α—快点集”的Hausdorff维数．     

省级工业多目标抽样最佳样本数目分析法初探

通过对我国工业统计抽样调查实际工作中的有关方法制度的分析研究，阐明了关于我国省级工业抽样调查中的样本框的构造和分解方法；对于各分解后的样本进行特征值分析和相关分析，提出了多目标(指标)抽样调查的样本数目确定的综合排序分析方法。     

NO2分子Faraday效应磁强计

采用Faraday光谱方法,系统研究了交流(Zeeman调制)、直流(光强调制)磁场下,NO2气体分子的光谱信号随外加磁场的变化关系.实验结果与理论分析表明,NO2分子的光谱信号与外加磁场呈线性关系,由此提出了一种NO2分子Faraday效应磁强计,预期可以测量5×10-3～100 mT,DC-1 MHz范围内的磁场,并具有良好的线性度.     

敏感性问题中的抽样调查方法与均方误差

讨论了敏感性问题中的几种抽样调查方法，给出了它们相应估计量的均方误差的计算公式。     

重型落地铣镗床焊接立柱有限元分析

对Ｔ６９１６重型落地铣镗床焊接立柱进行了有限元动静态分析，力学模型网格中有３５２２个八节点等参元，６９３０个节点；计算了两种工况，前五阶固有频率和相应的振型．文中以图形和表格方式显示部分计算结果，这些结果是设计焊接立柱的依据．