Purification of dinosterol for hydrogen isotopic analysis using high-performance liquid chromatography-mass spectrometry

A semi-preparative normal-phase high-performance liquid chromatography-mass spectrometry (HPLC-MS) method is presented for the purification of various alcohol fractions from total lipid extracts derived from sediments, for the purpose of hydrogen isotopic measurement by gas chromatography-isotope ratio mass spectrometry (GC-IRMS). 4-methylsterols, including the dinoflagellate-specific marker dinosterol (4,23,24-trimethylcholestan-22-en-3beta-ol), were successfully separated from notoriously co-eluting plant-derived pentacyclic triterpenoid alcohols and alkyl alcohols. We find that substantial hydrogen isotope fractionation occurs during chromatographic separation, demonstrating the importance of recovering the entire peak when subsequent hydrogen isotope analyses are to be performed. This is the first report of such hydrogen isotopic fractionation for a natural unlabelled compound.     

Stability of probability effects in utility elicitation

The present study investigates the impact of probability levels on the response mode bias in utility elicitation and its stability across a wide range of conditions. Experiments are performed with subjects from two different cultures, Austria and Pakistan, and a variety of methods to measure risk attitudes of subjects on a cardinal scale are used. Results indicate robust influences of probability levels on both the Certainty Equivalent method and the Probability Equivalent method of utility elicitation. Both methods are affected by probability changes in the opposite direction, creating the characteristic “tailwhip” pattern observed in previous research. Our study shows that this effect remains stable across different cultural backgrounds, measurement methods, and problem parameters, and is thus not an artefact but a reproducible phenomenon.     

Shrinkage estimation in the frequency domain of multivariate time series

In this paper on developing shrinkage for spectral analysis of multivariate time series of high dimensionality, we propose a new nonparametric estimator of the spectral matrix with two appealing properties. First, compared to the traditional smoothed periodogram our shrinkage estimator has a smaller L₂ risk. Second, the proposed shrinkage estimator is numerically more stable due to a smaller condition number. We use the concept of “Kolmogorov” asymptotics where simultaneously the sample size and the dimensionality tend to infinity, to show that the smoothed periodogram is not consistent and to derive the asymptotic properties of our regularized estimator. This estimator is shown to have asymptotically minimal risk among all linear combinations of the identity and the averaged periodogram matrix. Compared to existing work on shrinkage in the time domain, our results show that in the frequency domain it is necessary to take the size of the smoothing span as “effective sample size” into account. Furthermore, we perform extensive Monte Carlo studies showing the overwhelming gain in terms of lower L₂ risk of our shrinkage estimator, even in situations of oversmoothing the periodogram by using a large smoothing span.     

Simulating cumulative distributions under transient conditions in well-mixed,continuous and batch polymerization reactors

Instantaneous property methods are applied to the simulation of cumulative distributions of polymer chain architecture in well-mixed, continuous and batch reactors. We show how cumulative distributions can be simulated under transient conditions using a general convolution integral and suitable output from a dynamic reactor simulation package such as POLYRED^TM. We also show how the state equations that describe a polymerizing system can be augmented for direct simulation of these distributions. Examples of the impact of catalyst kinetics, reactor operating policies, and process upsets on the molecular weight distribution of bimodal high molecular weight polyethylene produced in well-mixed, stirred-bed gas-phase reactors are simulated for a binary transition-metal catalyst system.