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81.
82.
Prolate spheroidal wave functions, because of their many remarkable properties leading to new applications, have recently experienced an upsurge of interest. They may be defined as eigenfunctions of either a differential operator or an integral operator (as observed by Slepian in the 1960s). There are various ways of calculating their values based on both approaches. The standard one uses an approximation based on Legendre polynomials, which, however, is valid only on a finite interval. An alternative, valid in a neighborhood of infinity, uses a Bessel function approximation. In this letter we present a new method based on an eigenvalue problem for a matrix operator equivalent to that of the integral operator. Its solution gives the values of these functions on the entire real line and is computationally more efficient. 相似文献
83.
Sample enrichment technology and instrumental sensitivity are no longer limiting factors in arson debris analysis. When pyrolyzed, petroleum based composite materials may produce artifacts. Urban air is another major source of interference since some of the components typically found in air, in particular alkylated aromatic hydrocarbons, are customarily used as indicators for the presence of gasoline. It is suggested to replace qualitative analysis with a quantitative approach, taking the sample matrix into consideration. 相似文献
84.
85.
本文论述了“中国家庭经济与生育研究”全国抽样调查中用聚类方法对全国25省(市、区)和农村发达与落后地区进行分层,并采用多阶多次分层抽样;给出了在此方案下各层次目标量的估计方法和估计公式的变换形式. 相似文献
86.
G. Schomburg H. Behlau U. Husig B. Hoening W. Roeder 《Journal of separation science》1989,12(3):142-148
Separations of high efficiency and/or speed can be achieved in capillary GC by capillary columns of lower internal diameter (< 50 μm). Sampling techniques for the analytical application of narrow bore fused silica columns have been evaluated with regard to quantitation. On-column injection cannot be applied. Therefore liquid samples have to be vaporized in external devices before they enter the chromatographic system. Sample introduction by syringe with subsequent splitting must and can be applied but requires special syringes with perfect piston sealing because of the high inlet pressures needed even with hydrogen as carrier gas. For general analytical applications, valve systems should be developed to eliminate both the syringe and the septum from instrumental GC set-up's. In SFC using either narrow bore capillary or packed microbore columns, time-controlled valve sampling with partial displacement of the sample from the loop seems to be an adequate technique because of the very high inlet pressures involved. Splitting in combination with valve operation can also be applied in capillary SFC at least to samples of good solubility in the mobile phase. A disadvantage of splitting in SFC is that another restriction for the adjustment of the split flow is necessary. 相似文献
87.
Compliance with legal requirements: definition of limits, sampling and measurement uncertainty 总被引:1,自引:0,他引:1
Hermann Glaeser 《Accreditation and quality assurance》2002,7(10):388-391
In many cases compositional requirements for foodstuffs (e.g. limits for the fat, protein, dry matter, or water content) are
established by legislation. Adequate compliance testing is possible only if limits are clearly defined, taking measurement
and sampling uncertainty into consideration. Furthermore, decisions on compliance must be based on samples which reflect the
composition of the quantity to be evaluated. The resulting sample sizes are normally regarded by food inspection authorities
as being much larger than acceptable. Consequently, an alternative strategy should be developed. Autocontrol data (i.e. inspection
results obtained by the factory) in principle provide an adequate data basis for decisions on compliance. However, they must
be reliable and the food inspection authority must have access to these data on request. Using these data and on condition
that they show an approximate normal distribution, an inspection strategy based on arithmetic mean and standard deviation
can be developed. Reliable and transparent decisions on compliance can thus be made. In many cases an adequate verification
of food authenticity requires a comparison of raw material and product composition. Maximum acceptable differences, taking
the relevant sources of variation into consideration, have to be defined and should be used instead of limits.
Received: 17 April 2002 Accepted: 23 June 2002 相似文献
88.
《Journal of separation science》2003,26(18):1650-1656
In Stir Bar Sorptive Extraction (SBSE) organic analytes are enriched from aqueous samples by sorption onto a thick film of polydimethylsiloxane (PDMS) coated on a glass‐enveloped magnet. Sampled analytes are recovered by thermodesorption (TDS) and analysed on‐line by cGC‐MS. This study evaluates the SBSE recovery as a function of KO/W, the phase ratio (β), PDMS volume (20, 40, and 110 μL), sample volumes (4, 10, 100, and 1000 mL), and sampling time (0.67, 1, 2, 4, 5, 6, and 24 h) for three pesticides with different KO/W [parathion methyl (KO/W: 7.24×102), β‐endosulfan (KO/W: 6.8×103), and buprofezin (KO/W: 2.0×104)]. Experimental recoveries with SBSE in reasonable sampling times were found to be mainly conditioned by β, which must be as low as possible either by operating on small sample volumes or by using stir bars coated with high PDMS volume when samples of relatively large volume are analysed. 相似文献
89.
A practical approach to assessment of sampling uncertainty 总被引:1,自引:0,他引:1
Paolo de Zorzi Maria Belli Sabrina Barbizzi Sandro Menegon Andrea Deluisa 《Accreditation and quality assurance》2002,7(5):182-188
The paper reports the approach followed in the SOILSAMP project, funded by the National Environmental Protection Agency (ANPA)of
Italy. SOILSAMP is aimed at assessing uncertainties associated with soil sampling in agricultural, semi-natural, urban, and
industrial environments. The uncertainty assessment is based on a bottom-up approach, according to the Guide to the Expression
of Uncertainty in Measurement published by the International Organization for Standardization (ISO). A designated agricultural
area, which has been characterized in terms of elemental spatial distribution, will be used in future as a reference site
for soil sampling intercomparison exercises.
Received: 19 November 2001 Accepted: 6 January 2002 相似文献
90.
Sample processing is a very important component of uncertainty in analytical results. In order to have reliable results, the laboratory sample should be properly processed to obtain statistically homogenous matrix—before the representative test portions are withdrawn for analysis. The use of 14C-labeled compound is preferable because the analyte can be quantified without cleanup. The method is based on surface treatment of cucumber with 14C-chlorpyrifos, determination of 14C-chlorpyrifos activity in the replicate test portions of different size, and determination of the uncertainty of sample processing. 相似文献