Nothing Lasts Forever: Environmental Discourses on the Collapse of Past Societies

The study of the collapse of past societies raises many questions for the theory and practice of archaeology. Interest in collapse extends as well into the natural sciences and environmental and sustainability policy. Despite a range of approaches to collapse, the predominant paradigm is environmental collapse, which I argue obscures recognition of the dynamic role of social processes that lie at the heart of human communities. These environmental discourses, together with confusion over terminology and the concepts of collapse, have created widespread aporia about collapse and resulted in the creation of mixed messages about complex historical and social processes

Electrospray, technique and applications

Electrospray makes use of ions present in electrically charged droplets in an aerosol. The generation of an aerosol by electrospray has already been published by Zeleny in 1917. The feasibility of electrospray as an ionization technique was demonstrated by Fenn and coworkers, and by a group of Russian scientists in Leningrad (St. Petersburg) in 1984. Interest from the mass spectrometric community and the instrument manufacturers rose dramatically in 1988. when Fenn and coworkers showed series of multiply charged ions produced by electrospray of protein solutions.The essential steps of electrospray: dispersion of a sample solution into a charged aerosol and the liberation of ions from droplets take place at atmospheric pressure. The sampling of ions from the atmospheric pressure region into the vacuum of the mass analyzer is a technical problem that has been solved in different ways by researchers and manufacturers. The principle of ion formation by electrospray seems simple but the mechanism is not known in detail. The majority of applications is in drug metabolism, peptide and protein research. The use of electrospray will spread to other fields of research that may benefit from the direct observation of ions in solution by mass spectrometry

Mechanistic aspects of electrospray ionization

Electrospray ionization (ESI) mass spectrometry can be divided into three steps: Nebulization of a sample solution into electrically charged droplets, liberation of ions from droplets, and transportation of ions from the atmospheric pressure ionization source region into the vacuum and mass analyzer of the mass spectrometer. A sample solution is fed through a capillary tube and a high electric field at the tip of the tube pulls positive charge towards the liquid front. When electrostatic repulsion becomes stronger than the surface tension, a small electrically charged droplet leaves the surface and travels through the surrounding gas to the counter-electrode. Under the majority of experimental liquid chromatography-mass spectrometry and capillary electrophoresis-mass spectrometry conditions, positive charge on droplets is generated by the removal of negative charge via electrochemical discharge of negative ions against the metal wall of the spray capillary. When the ESI source is set up for the detection of negative ions, all power supplies are at reversed polarity. Removal of positive ions inside the tip of the spray capillary provides droplets depleted of positive charge. The supply of negative charge to the solution may also take place; electrons released from the spray capillary can be captured by sample molecules having a high electron affinity. Droplet size decreases and charge density at the droplet surface increases after droplet disintegration and solvent evaporation. When the electric field at the surface of a droplet has become sufficiently high, ions are emitted from the droplet surface into the surrounding gas and are sampled by the mass analyzer. Sample ion intensity is dependent on ion structure and is affected by solvent composition and presence of additives. ESI behaves as a concentration sensitive detector for chromatography. When the sample concentration is increased above 10 mu M, the sample ion signal saturates, which can be explained by the assumption that the surface of ion-emitting droplets is full at 10 mu M. Sample ion abundance over a wide m/z range is further affected by inherently mass-dependent efficiencies of ion transportation, ion separation and ion detection. (C) 1998 Elsevier Science B.V

ATMOSPHERIC-PRESSURE-IONIZATION MASS-SPECTROMETRY .1. INSTRUMENTATION AND IONIZATION TECHNIQUES

Mass spectrometer ion sources are normally located inside a high-vacuum envelope. Such low-pressure ion sources can make use of a range of different ionization methods and are in routine use in analytical mass spectrometers. An ion source operating at atmospheric pressure is better suited, and may be essential, for a growing number of applications. Mass spectrometric analysis of samples pyrolyzed under controlled conditions, the combination of liquid chromatography and capillary electrophoresis with mass spectrometry, and the determination of the molecular mass of proteins by electrospray ionization all benefit from, or require, an atmospheric pressure ionization source