Analysis of Hydrocarbon Materials by Use of Nonpolar Matrices in Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry (MALDI-MS).
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In this study, polyaromatic hydrocarbon compounds including anthracene, pyrene, acenaphthene and perylene were investigated as nonpolar matrices in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for the analysis of selected nonpolar analytes. First, the influence of matrix and the analyte ionization potentials on a charge-transfer ionization mechanism were determined. Results of these studies demonstrated that formation of radical molecular cations in MALDI-MS depends on the ionization energy difference between the matrix and the analyte. Charge-transfer ionization occurs only when the ionization potential of the matrix is higher than that of the analyte. Next, nonpolar matrices were investigated for the analysis of low molecular weight nonpolar polymers including polybutadiene, polyisoprene, and polystyrene. Conventionally, these types of polymers are analyzed by MALDI-MS using acidic matrices, such as all-trans-retinoic acid, with an additional metal salt as a cationization reagent. Nonpolar matrices were shown to be more effective than acidic matrices, as nonpolar matrices provide better matrix isolation and enhanced spectra reproducibility. Potential difficulties associated with background silver salt clusters present during the analysis of nonpolar polymers by MALDI-MS are reported. Silver cluster ions are observed with m/z values ranging from 1500 to about 7000 when acidic, polar matrices are used with silver salts. It was found that these background signals could be greatly reduced by the use of the nonpolar matrices. Alternatively, using copper salts with acidic matrices instead of silver salts substantially improves the mass spectral quality due to reduced background signals. Finally, the nonpolar matrices were applied to other polymeric compounds including amine functionalized polystyrene, platinum (II) linked modified polystyrenes, and poly[(o-cresyl glycidyl ether)- co-formaldehyde] (PGF). These studies demonstrated that nonpolar matrices can be used for the MALDI-MS analysis of polymers of varying polarity. As with the prior studies, nonpolar matrices were found to be generally more effective for the analysis of these analytes than acidic matrices. Taken together, these studies demonstrate that nonpolar matrices provide high quality MALDI mass spectra of nonpolar and moderately nonpolar polymers. These improvements arise due to improved matrix-analyte miscibility as well as reduction in background salt clusters when silver salts are used as cationization reagents.Keywords:
Matrix (chemical analysis)
The effect of uneven fissioning of mass and charge from electrospray droplets on the amount of analyte charged during the electrospray process was explored. A surface selectivity factor (S) was developed to describe the affinity of an analyte for the droplet surface, and both theoretical and experimental response curves were compared for analytes with various S values. The theoretical response curves were generated by calculating the overlap between the charge and analyte spawned from parent droplets to determine the amount of analyte charged. This overlap was then graphed as a function of analyte concentration. Differences in the amount of analyte charged during droplet fission were predicted for analytes of varying surface affinities. The issue of analyte partitioning between the surface and interior phases of the ESI droplet was also included in the discussion. This was accomplished by applying the equilibrium partitioning model to a set of offspring droplets to determine the amount of analyte on their surfaces and then calculating the overlap between fissioning analyte and excess charge. Experimental response curves resembled theoretical ones, and S values predicted from theory were in excellent agreement with those predicted on the basis of the structural characteristics of the analytes.
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Bioanalysis
Internal standard
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In this Article, we describe a microfluidic enzyme-linked immunosorbent assay (ELISA) method whose sensitivity can be substantially enhanced through preconcentration of the target analyte around a semipermeable membrane. The reported preconcentration has been accomplished in our current work via electrokinetic means allowing a significant increase in the amount of captured analyte relative to nonspecific binding in the trapping/detection zone. Upon introduction of an enzyme substrate into this region, the rate of generation of the ELISA reaction product (resorufin) was observed to increase by over a factor of 200 for the sample and 2 for the corresponding blank compared to similar assays without analyte trapping. Interestingly, in spite of nonuniformities in the amount of captured analyte along the surface of our analysis channel, the measured fluorescence signal in the preconcentration zone increased linearly with time over an enzyme reaction period of 30 min and at a rate that was proportional to the analyte concentration in the bulk sample. In our current study, the reported technique has been shown to reduce the smallest detectable concentration of the tumor marker CA 19-9 and Blue Tongue Viral antibody by over 2 orders of magnitude compared to immunoassays without analyte preconcentration. When compared to microwell based ELISAs, the reported microfluidic approach not only yielded a similar improvement in the smallest detectable analyte concentration but also reduced the sample consumption in the assay by a factor of 20 (5 μL versus 100 μL).
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The biosensor described below capitalizes on the specificity and reversibility of antibody-analyte interactions, and is designed to operate continuously. Detection of the analyte of interest relies on the specific displacement of labelled-analyte from the antibody binding site. When an aqueous sample containing the analyte is introduced into the buffer flow, the analyte competes for the analyte binding sites with the labelledanalyte already bound by the antibody. An amount of the labelled analyte proportional to the amount of the analyte present is displaced, enters the buffer flow, and is subsequently detected downstream. In the model system, radioisotopes and fluorophores were used as analyte labels in the development of a sensor capable of detecting small molecular weight analytes. Dinitrophenol @NP) was used as the analyte to construct a model system and could be detected at the nanomolar level. The assay does not require the introduction of exogenous reagents throughout the analysis of a sample. In addition, incubation of the sample in the detection column was not required since significant displacement of the labelled DNP-analyte occurred in continuous flow. Due to the wide range of specificities that antibodies can express, this system has the potential of being designed to detect many different analytes.
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Chemiresistors are polymer-based sensors that transduce the sorption of a volatile organic compound into a resistance change. Like other polymer-based gas sensors that function through sorption, chemiresistors can be selective for analytes on the basis of the affinity of the analyte for the polymer. However, a single sensor cannot, in and of itself, discriminate between analytes, since a small concentration of an analyte that has a high affinity for the polymer might give the same response as a high concentration of another analyte with a low affinity. In this paper we use a field-structured chemiresistor to demonstrate that its response kinetics can be used to discriminate between analytes, even between those that have identical chemical affinities for the polymer phase of the sensor. The response kinetics is shown to be independent of the analyte concentration, and thus the magnitude of the sensor response, but is found to vary inversely with the analyte's saturation vapor pressure. Saturation vapor pressures often vary greatly from analyte to analyte, so analysis of the response kinetics offers a powerful method for obtaining analyte discrimination from a single sensor.
Chemiresistor
Saturation (graph theory)
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In capillary electrophoresis, the relative orders of mobilities of analyte, additive, and the complex formed determine the analyte peak shape in a way similar to the way the binding isotherms determine the peak shapes in chromatography. The three mobilities allow six possible orders; each produces a characteristic peak shape in CE. Equations describing the analyte migration in a CE system with the presence of mobility-changing additives can be implemented into computer programs to predict the migration times of the analyte peak maximums, and the predicted migration times agree well with the experimental results.
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One of the strongest analytical qualities of energy-dispersive x-ray fluorescence (EDXRF) is the wide range of analyte elements that can be detected and analyzed. Historically, the technique has covered all the elements from sodium (Z=11) and above. A useful measure of specific spectrometer performance is analyte sensitivity. X-ray spectrometric sensitivity is usually expressed in terms of minimum detectable amount of analyte or rate of change of analyte line intensity with change in amount of analyte. Many factors affect analyte sensitivity in EDXRF. These include excitation conditions, specimen conditions, system geometry, atmosphere, detector and readout conditions, and of course the specific analyte line. Typically, EDXRF sensitivity is very good, and low ppm concentrations of analytes are routinely analyzed–until one encounters the light elements.
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Electromigration
Dissociation constant
Stoichiometry
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The analytical performance of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is strongly influenced by the method of analyte and matrix preparation. We report a nonintrusive method based on laser confocal microscopic imaging technology to examine the MALDI samples prepared by various protocols. In this method, the analyte is tagged with a fluorescent group. The matrix and analyte are prepared under the same conditions as those used in conventional MALDI experiments. It is demonstrated that confocal microscopy can provide clear, three-dimensional images of sample crystals as well as the analyte distribution within the crystals. It is shown that the analyte is incorporated into the matrix crystals for all the sample preparation protocols examined. Moreover, the confocal microscopic images reveal that, with the use of a dried-droplet method for sample/matrix preparation, the analyte is not uniformly distributed within the matrix crystals. In some crystals, no analyte is incorporated. In addition, it is found that large crystals formed using a slow growth process display a more uniform analyte distribution. Relatively more uniform analyte distribution is observed for samples prepared with the formation of microcrystals. The possible correlation between the ion signal variations observed in MALDI and the uniformity of the analyte distribution obtained by the confocal microscopic imaging method is discussed. Finally, a double-imaging method involving the use of two analytes with different labeling groups is demonstrated. It is found that different analytes are not coherently distributed in the matrix crystals.
Matrix (chemical analysis)
Sample Preparation
MALDI imaging
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