Computer-assisted desorption

This step is intended to reduce residual moisture to levels allowing no microbial growth or chemical reactions of the end product. The amount of residual moisture present in a product depends on its desorption isotherms. Such isotherms in turn depend on various factors including the product temperature, pressure chamber, partial vapour pressure in the container and nature of the interaction of the water vapour with the interstitial material formed in the freezing step. The computer should be fed with information on the target sample component. For example, if the component of interest is a protein, then overdrying may alter its configuration and decrease the potency of the end product. Consequently, the computer should control not only the final product temperature but also the partial water vapour pressure and the duration of the desorption step. 

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Figure 6 The SELDI technology. This type of proteomic analytical tool is a class of mass spectroscopy instrument that is useful in high-throughput proteomic fingerprinting of serum. Using a robotic sample dispenser, 1 p,L of serum is applied to the surface of a protein-binding chip. A subset of the proteins in the sample binds to the surface of the chip. The bound proteins are treated with a matrix-assisted laser desorption and ionization matrix and are washed and dried. The chip, which contains multiple patient samples, is inserted into a vacuum chamber where it is irradiated with a laser. The laser desorbs the adherent proteins and causes them to be launched as ions. The TOF of the ion before detection by an electrode is a measure of the mass-to-charge (m/z) value of the ion. The ion spectra can be analyzed by computer-assisted tools that classify a subset of the spectra by characteristic patterns of relative intensity (adapted from www.evmsdoctors.com).

SPE processes may be subjected to computer assisted optimization. As an example, an orthogonal array design was employed for the optimization of an SPE process applied to atrazine, diazinon, ame-tryn, and fenthion in surface water. Seven parameters (type of desorption solvent, type of sorbent, flow rate of the elution solvent, sample pH, sample volume, elution volume, organic modifier addition, and flow rate of the water sample) were studied and optimized. 

MAT 311A instrument operating at 70 eV [electron impact (El) mode] and reported as m/z and relative intensity (%). Field desorption (FD) mass measurements were carried out on a ZAB 2-SE-FDP instrument. Microwave-assisted synthesis was performed using a CEM-Discovery monomode microwave system utilizing an IR temperature sensor and magnetic stirrer in sealed 10 -mL glass vials with aluminum caps and a septum. All reactions were monitored and controlled using a personal computer. 

Finally, from the point of view of the organic chemist, obtaining amino acids from proteins, which is done by either acid or base hydrolysis (after cleaving the sulfur-sulfur bonds) rather than by structural analysis, is of concern. Indeed, at this writing, a significant amount of structural analysis is accomphshed by specialized computer-coupled mass spectrometric (Chapter 2) techniques. The special nature of these techniques derives from the need to put the very large molecules in the gas phase and so desorption from various materials— without decomposition— becomes important and exotic technology, such as matrix-assisted laser desorption ionization (MALDI) MS becomes necessary. 

With the introduction of fast-atom bombardment (FAB) in 1982, and matrix-assisted laser desorption/ ionization (MALDI) and electrospray ionization (ESI), most of the biomedical applications have been directed towards these methods. The 52( f.pD method has been found to have wide applicability, including the study of refractory materials, catalysts, semiconductors and frozen gases. Electronics capable of measuring the timing of events with subnanosecond resolution (the time it takes for a single photon to travel 1 cm) is used by this method as well as event-by-event data acquisition using the computer to make decisions at the molecular level, the basis of correlation mass spectrometry, a unique feature of 252Cf-PD. 

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