Quantitative determination of isotope ratios from experimental isotopic distributions

Isotope variability due to natural processes provides important information for studying a variety of complex natural phenomena from the origins of a particular sample to the traces of biochemical reaction mechanisms. These measurements require high-precision determination of isotope ratios of a particular element involved. Isotope ratio mass spectrometers (IRMS) are widely employed tools for such a high-precision analysis, which have some limitations. This work aims at overcoming the limitations inherent to IRMS by estimating the elemental isotopic abundance from the experimental isotopic distribution. In particular, a computational method has been derived that allows the calculation of 13C/12C ratios from the whole isotopic distributions, given certain caveats, and these calculations are applied to several cases to demonstrate their utility. The limitations of the method in terms of the required number of ions and S/N ratio are discussed. For high-precision estimates of the isotope ratios, this method requires very precise measurement of the experimental isotopic distribution abundances, free from any artifacts introduced by noise, sample heterogeneity, or other experimental sources.     

超高速低功耗CMOS4:1复接器

研究了在特定工艺条件下进行高速低功耗集成电路设计的相关问题,包括结构设计、电路设计和工艺角的影响。提出用CMOS逻辑电路完成超高速电路设计的思想,利用CSM 0.35μm CMOS工艺设计完成了速率为3.125Gb/s的4:1复接器芯片。该系统采用树型结构,由两个并行的低速2:1复接单元和一个高速2:1复接单元级联而成。核心电路锁存器在低速单元中用带有电平恢复的4_T电路构成,在高速单元中用动态传输门构成;选择器则用CMOS传输门构成的双路开关实现,每一电路都只用4只晶体管实现。芯片面积为0.39mm~2。芯片测试结果表明:在3.3V电源电压下,芯片核心功耗低于40mW,最高工作速率可达4Gb/s。     

Molecular dynamics calculation of the isotope effect for vacancy diffusion

The relative jump frequencies of tracer atoms with masses 1.00 and 1.05 times the host atom mass have been computed by molecular dynamics in a system of 255 Lennard-Jones atoms comparable with solid argon at 80 °K. The chief error of previous theoretical treatments of the isotope effect is found to be their neglect of unsuccessful saddle point crossings (e.g. U turns) which become more prevalent the lighter the diffusing atom. In the system studied, this effect lowers the ΔK factor from its harmonic value of 0.98 to an effective value of 0.89±0.05.     

On the calculation of consistent stress distributions in finite element approximations

The theory of conjugate approximations¹ is used to obtain consistent approximations of stress fields in finite element approximations based on displacement assumptions. These consistent stresses are continuous across interelement boundaries and involve less mean error than those computed by the conventional approach.     

Quantitative analysis of isotope distributions in proteomic mass spectrometry using least-squares Fourier transform convolution

Quantitative proteomic mass spectrometry involves comparison of the amplitudes of peaks resulting from different isotope labeling patterns, including fractional atomic labeling and fractional residue labeling. We have developed a general and flexible analytical treatment of the complex isotope distributions that arise in these experiments, using Fourier transform convolution to calculate labeled isotope distributions and least-squares for quantitative comparison with experimental peaks. The degree of fractional atomic and fractional residue labeling can be determined from experimental peaks at the same time as the integrated intensity of all of the isotopomers in the isotope distribution. The approach is illustrated using data with fractional (15)N-labeling and fractional (13)C-isoleucine labeling. The least-squares Fourier transform convolution approach can be applied to many types of quantitative proteomic data, including data from stable isotope labeling by amino acids in cell culture and pulse labeling experiments.     

Computer calculation of nonstationary processes in multistage cascades for the separation of two-component isotope mixtures

A systemic method of integrating differential equations is used for working out a process for the numerical calculation of nonstationary processes using electronic computers.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 31, No. 3, pp. 506–513, September, 1976.     

Ultrahigh-speed chromatography and virtual chemical sensors for detecting explosives and chemical warfare agents

Chromatographic separation of organic compounds is a well-known method of producing time-resolved chemical spectra or chromatograms. Whereas conventional chromatography using 10-100-m columns is slow, often requiring minutes to hours, ultrahigh-speed chromatography with short resistively heated metal columns requires only seconds. The performance of an ultrahigh-speed gas chromatograph using a surface acoustic wave (SAW) resonator to measure the mass of eluted chemical compounds is described. Closed-loop temperature programming of a resistively heated 1-m capillary column at rates as high as 20/spl deg/C/s produces near real-time, 10-s chromatograms with chemical spectra peak widths measured in milliseconds. Eluted chemicals are physically adsorbed on an uncoated SAW resonator and frequency deviation versus time produces an eluted mass versus time chromatogram. The derivative of frequency versus time produces a mass/unit time chromatogram of column flux, which is used to measure the retention times of eluted compounds. This paper describes the instrument and process where independent database of chemical spectra are produced by indexing the retention time of specific target chemicals (e.g., explosives and chemical warfare agents) to the retention times of n-alkane standards. It is also shown that assigning time windows centered about specific indices can be used to create arrays of nonoverlapping virtual sensors for specific compounds. Repeated high-speed chromatographic measurements enable virtual sensor readings to be updated in near real time. This work clearly proves that arrays of virtual chemical sensors specific to explosive and chemical warfare agents can detect part per trillion levels of these compounds with high probability of detection and low probability of false alarm.     

Families of distributions arising from distributions of order statistics

Consider starting from a symmetric distributionF on ℜ and generating a family of distributions from it by employing two parameters whose role is to introduce skewness and to vary tail weight. The proposal in this paper is a simple generalisation of the use of the collection of order statistic distributions associated withF for this purpose; an alternative derivation of this family of distributions is as the result of applying the inverse probability integral transformation to the beta distribution. General properties of the proposed family of distributions are explored. It is argued that two particular special cases are especially attractive because they appear to provide the most tractable instances of families with power and exponential tails; these are the skewt distribution and the logF distribution, respectively. Limited experience with fitting the distributions to data in their four-parameter form, with location and scale parameters added, is described, and hopes for their incorporation into complex modelling situations expressed. Extensions to the multivariate case and to ℜ⁺ are discussed, and links are forged between the distributions underlying the skewt and logF distributions and Tadikamalla and Johnson'sL _U family.     

On the determination of grain-size distributions from intercept distributions

A matrix formulation for determining the spatial grain-size distribution of tetrakaidecahedral grains from linear-intercept data is developed. The truncation effect (which stems from the fact that the intersection of single-size grains by a test line gives rise to intercepts of different length) and the sampling effect (which results as a consequence of bigger grains being intersected more frequently than smaller ones) are separately taken into account. The derivation procedure of this formulation is applicable to any other convex shape, provided the linear intercept distribution for single-size grains of the corresponding shape is known. The percentage spatial grain-size distributions obtained by the formulation derived here are similar to those estimated by the Spektor's chord method for spherical grains.     

Perchlorate isotope forensics

Perchlorate has been detected recently in a variety of soils, waters, plants, and food products at levels that may be detrimental to human health. These discoveries have generated considerable interest in perchlorate source identification. In this study, comprehensive stable isotope analyses (37Cl/35Cl and 18O/17O/16O) of perchlorate from known synthetic and natural sources reveal systematic differences in isotopic characteristics that are related to the formation mechanisms. In addition, isotopic analyses of perchlorate extracted from groundwater and surface water demonstrate the feasibility of identifying perchlorate sources in contaminated environments on the basis of this technique. Both natural and synthetic sources of perchlorate have been identified in water samples from some perchlorate occurrences in the United States by the isotopic method.     

Periodic distributions of inclusions

Elastic fields caused by periodically distributed spherical inclusions in an infinite isotropic medium are investigated when eigenstrains in the inclusions are homogeneous polynomials of degree $\text{l}$ of the local coordinates taken at the centers of the inclusions. The stress field can be expressed as a convergent sum of the solutions of individual inclusions when $\text{l}$ is an odd number. The stress field must be modified when $\text{l}$ is an even number. If a distribution of inclusions is on a plane or a line, the stress field can be expressed as a convergent sum of the individual inclusions for all $\text{l}$. The analysis is extended to general ellipsoidal inclusions with arbitrary eigenstrains in anisotropic media.     

Helical distributions of stokeslets

Helical distributions of stokeslets can valuably model microbial locomotion through a fluid, and also the flow field generated, wherever a flagellum actively executes helical undulations (as in many single-celled algae and protozoa) or where (as in many bacteria) the action of rotary motors causes a passive structure of helical shape (which may be a flagellum or else the cell body itself) to rotate. Here, previous biomechanical studies of such modes of locomotion are extended to include analyses of three-dimensional flow fields. In some cases, a rotlet field (curl of a stokeslet) needs to be incorporated in the models. For example, spirochete swimming is modelled by combined helical distributions of stokeslets and rotlets; the computed flow field being confined to within distances of less than twice the radius of the cell body's helical shape from its axis, while including a powerful jet-like interior flow through the coils of the swimming spirochete.     

Evaluating chlorine isotope effects from isotope ratios and mass spectra of polychlorinated molecules

Compound-specific chlorine isotope analysis receives much interest to assess the fate of chlorinated hydrocarbons in contaminated environments. This paper provides a theoretical basis to calculate isotope ratios and quantify isotope fractionation from ion-current ratios of molecular- and fragment-ion multiplets. Because both (35)Cl and (37)Cl are of high abundance, polychlorinated hydrocarbons consist of molecules containing different numbers of (37)Cl denoted as isotopologues. We show that, during reactions, the changes in isotopologue ratios are proportional to changes in the isotope ratio assuming a nonselective isotope distribution in the initial compound. This proportionality extents even to fragments formed in the ion source of a mass spectrometer such as C 2Cl 2 (double dechlorinated fragment of perchloroethylene, PCE). Fractionation factors and kinetic isotope effects (KIE) may, therefore, be evaluated from isotope, isotopologue or even fragment ratios according to conventional simple equations. The proportionality is exact with symmetric molecules such as dichloroethylene (DCE) and PCE, whereas it is approximately true with molecules containing nonreactive positions such as trichloroethylene (TCE). If in the latter case isotope ratios are derived from dechlorinated fragments, e.g., C 2HCl 2, it is important that fragmentation in the ion source affect all molecular positions alike, as otherwise isotopic changes in reactive positions may be underrepresented.     

Improving predictive distributions

Summary Consider a sequence of decision problemsS ₁,S ₂, ... and suppose that in problemS _i the statistician must specify his predictive distributionF _i for some random variableX _i and then make a decision based on that distribution. For example,X _i might be the return on some particular investment and the statistician must decide whether or not to make that investment. The random variablesX ₁,X ₂ ... are assumed to be independent and completely unrelated. It is also assumed that each predictive distributionF _i assigned by the statistician is a subjective distribution based on his information and beliefs aboutX _i. In this context, the standard Bayesian approach provides no basis for evaluating whether the statistician’s subjective predictive distribution forX _i is good or bad, and does not even recognize this question as being meaningful. In this paper we describe models in which the statistician can study his process for specifying predictive distributions identify bad habits, and improve his predictions and decisions by gradually breaking these habits.     

Stationary scientometric distributions

Stationary distributions, i.e. distributions involving no time dependence, are considered. It is shown that all these distributions in scientometrics can be approximated by the Zipf distribution at high values of variables. The sample moments appear to depend significantly on the sample size. Accordingly, the approximation of these observational data by probability distributions converging to a stable distribution different from the normal one proves to be the only correct approximation. The conclusion is formulated that the use of non-Gaussian statistics is necessary in the science of science and other social sciences.     

Stationary scientometric distributions

Stationary distributions, i.e. distributions involving no time dependence, are analysed. The rank and frequency forms of statistical distributions are considered. On the basis of this consideration the approximations of stationary scientometric distributions are reviewed.     

Stationary scientometric distributions

The non-Gaussian character of scientific activity is discussed. This character makes correct only non-Gaussian approximations of stationary distributions of scientific activity. Deviation of different non-Gaussian approximations from the Zipf distribution can be explained in some cases by distortion introduced by the observer. The hypothesis that latent stationary distributions of scientific (and generally human) activity for separate person are always described by the Zipf distribution is formulated using the considerations connected with the variational entropy and the Zigler principles.     

Mie-scattering calculation

The new Mie-scattering calculation is a robust and efficient algorithm used to compute light scattering from spheres. It calculates the ratio between Riccati-Bessel functions instead of the complicated logarithmic derivative. The Kapteyn inequality is used to estimate the number of significant digits of the calculated Riccati-Bessel functions and their ratio. This new algorithm is stable and accurate for both large and small particles. The implemented C++ code yields the same accurate results for both small and large particles compared with Wiscombe's MIEV0 code in double precision. Suggestions are provided for the porting of the MIEV0 code.