Current status and need for standards in ion mobility spectrometry

Ion mobility spectrometry (IMS) is a well established technique for the detection of many compounds of interest based on the reduced mobility (K₀) values of their ions. While having the advantage of small size, weight, and power, IMS has been subject to low specificity and is subject to interferences that can cause false alarms in detectors used for security applications. The rate of false positive alarms is directly related to the detection window width required to maintain a high rate of true positive detections. These window widths are in turn a result of the historically available accuracy of reference measurements and the range of responses by multiple detectors. The windows cannot be arbitrarily reduced without risking an increase in the rate of false negative responses. Ongoing work has focused on high accuracy calibration as a means of decreasing the false alarm rates by reducing the variability between detectors which would allow for narrower detection windows. Central to the calibration procedure is the selection of an appropriate calibrant (or reference standard) that can be easily characterized and known with a high degree of certainty across a range of instrumental conditions. This review evaluates a number of previously proposed and potential calibrants against seven recommended criteria of suitability. We examine the sources of false positive alarms in IMS-based detectors and propose a calibration procedure based on high accuracy reference measurements. Initial results of applying this procedure in a post-processing manner are promising towards reducing detector variability and detection window width.     

Phase behavior of cyclic siloxane-based liquid crystalline compounds

A series of 24 cyclic siloxane-based liquid crystalline compounds was synthesized using conventional hydrosilation chemistry. Variables examined included ring size, spacer group length, and type and composition of pendant mesogeric groups. Both pentamethylhydro- and tetramethylhydrosiloxane rings were reacted with mesogens based on cholesterol, biphenyl, or equimolar mixtures of both. Four different length spacer groups containing terminal vinyl groups were used to attach the mesogens to the cyclic siloxane core. The thermotropic liquid crystalline phase behavior was studied using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). A lengthening of spacer groups resulted in lower crystallization temperatures for the biphenyl-based compounds and lower glass transition temperatures for the cholesterol- and mixed mesogen-based compounds. The tetramethyl ring derivatives exhibited higher glass transition or crystallization temperatures than their pentamethyl counterparts. Biphenyl-based compounds exhibited low temperature crystalline phases while the cholesterol-based compounds exhibited low temperature glassy phases. © 1994 John Wiley & Sons, Inc.     

Cure state detection for pre-cured carbon-fibre epoxy prepreg (CFC) using Temperature-Modulated Differential Scanning Calorimetry (TMDSC)

Carbon-fibre prepregs have found widespread use in lightweight applications. They are based on a carbon-fibre fabric impregnated with reactive epoxy resin. Prepreg materials are generally pre-cured so that they have a higher molecular weight than typical resins in order to reduce resin flow, which facilitates storage and later processing properties.The measurements were carried out using commercially available materials and follow the published DMA investigations of the same material [1]. TMDSC was used to find the correlation between curing conditions, the degree of cure and glass transition temperature. TMDSC has the advantage over standard DSC that it enables better determination of the glass transition temperature, which is often accompanied by an exothermic curing reaction, and thus overshadowed. The influence of the amplitude of temperature modulation was tested in preliminary experiments. For non-cured material a glass transition temperature of approximately 0 °C was determined; whereas for the totally cured material it was approximately 230 °C. The changes in degree of cure, temperature of actual glass transition and post-reaction are given as a function of curing time at 180 °C. The correlation between actual glass transition temperature and degree of cure is derived.     

Investigation of the profiling depth in matrix-assisted laser desorption/ionization imaging mass spectrometry

Matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry is generally considered to be a surface analysis technique. In this report, the profiling depth of imaging mass spectrometry was examined. MALDI matrix solution was found to be able to gain access to the tissue interior and extract analyte molecules to the tissue surface. As a consequence, prazosin, a small molecule pharmaceutical compound, located as deep as 40 microm away from the surface was readily detected after matrix application. Likewise, cytochrome c, a 12 kDa protein, was also detectable from the tissue interior. Moreover, for prazosin, not only the extent of matrix effect, but also the extraction efficiency of the matrix solvent appeared to be dependent on the type of tissue. These results indicated that experimental conditions that decrease the matrix solvent evaporation during matrix application may increase analyte extraction efficiency and hence sensitivity of the analysis. Furthermore, thin sections should be used to avoid differential extraction efficiency of matrix solvent in different tissues for whole-body analysis.