Investigating Population Heterogeneity With Factor Mixture Models.

Sources of population heterogeneity may or may not be observed. If the sources of heterogeneity are observed (e.g., gender), the sample can be split into groups and the data analyzed with methods for multiple groups. If the sources of population heterogeneity are unobserved, the data can be analyzed with latent class models. Factor mixture models are a combination of latent class and common factor models and can be used to explore unobserved population heterogeneity. Observed sources of heterogeneity can be included as covariates. The different ways to incorporate covariates correspond to different conceptual interpretations. These are discussed in detail. Characteristics of factor mixture modeling are described in comparison to other methods designed for data stemming from heterogeneous populations. A step-by-step analysis of a subset of data from the Longitudinal Survey of American Youth illustrates how factor mixture models can be applied in an exploratory fashion to data collected at a single time point. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Feasibility of formation of hot ions in electrospray

Internal energy changes during the electrospray process have been studied. Our results suggest that, contrary to conventional wisdom, ions that are quite hot are formed in electrospray, even without acceleration in the cone-skimmer region. The main role of the curtain gas seems to be not to break up existing clusters but, rather, to cool down the nascent ions and to prevent cluster formation by shielding them from solvent vapors.     

Atmospheric pressure gas-phase H/D exchange of serine octamers

The recently discovered homochiral serine octamer has been a focus of interest because of its possible implications for the origin of homochirality in living systems. Electrospray ionization (ESI) and sonic spray ionization (SSI) tandem mass spectrometry have been used to generate this unusually stable magic number cluster. Several structures have been suggested for the serine octamer, based on tandem mass spectrometry, ion mobility measurements, and quantum mechanical calculations. We now report experimental hydrogen/deuterium (H/D) exchange data, which demonstrate the existence of two different structures for the serine octamer. These forms undergo exchange at significantly different rates. One form may correspond to solution-phase assembled clusters and the other to octamers formed during the ionization process. Experiments done at higher resolution confirm that the experimental observations made here apply to the serine octamer without interference from metaclusters, namely, higher order clusters (Ser(16) + 2H)(+2), etc., the (12)C isotopes of which have mass-to-charge ratios identical to the protonated octamers. H/D exchange of racemic serine shows predominantly the extensively exchanged ion population, as well as providing evidence that racemic serine generates abundant metaclusters. The evidence presented here shows that one type of serine octamer is responsible for the strong chiral effects associated with the formation of these magic number clusters. These slowly exchanging more fragile clusters are the octamers that might have played a role in homochirogenesis.