Mass analyzers desirable features

Ions exiting the drift tube are mass analyzed in mass spectrometer MS2, an important feature if reactions are occurring in the drift cell. Ions are generally detected after MS2 by ion counting techniques. The mass spectrometers MSI and MS2 are typically quadrupole mass filters, and either one or the other can be run in RF-only mode for better signal but without mass selection, if desired. 

The performance of a mass analyzer is evaluated on the basis of the following desirable features ... 

It is appropriate at this time to discuss some of the limitations associated with LC-NMR. It is more accurate to say the limitations of the NMR spectrometer in an LC-NMR instrument. As compared to MS, NMR is an extremely insensitive technique in terms of mass sensitivity. This is the key feature that limits NMR in its ability to analyze very small quantities of material. The key limiting factor in obtaining NMR data is the amount of material that one is able to elute into an active volume of an NMR flow-probe. The quantity of material transferred from the LC to the NMR flow-cell is dependant on several features. The first being the amount of material one is able to load on an LC column and retain the resolution needed to achieve the desired separation. The second is the volume of the peak of interest. The peak volume of your analyte must be reasonably matched to the volume of the flow-cell. An example would be a separation flowing at lml/min with the peak of interest that elutes for 30 s. This corresponds to a peak volume of 500 pi, which clearly exceeds the volume of the typical flow-cell. This is the crux of the problem in LC-NMR. There is a balance that must be struck between the amount of compound needed to detect a signal in an... 

Since a large number of polycarbonates possessing the desired structural features can be prepared from a variety of diols, it is useful to develop simple methods to predict their behavior as resist materials. To this effect, we have devised spectroscopic methods to follow the degradation of the polycarbonates under a variety of thermolysis or acidolysis conditions. For example, the thermolysis of the solid polymers can be followed conveniently by gas-chromato-graphy-mass spectrometry. The thermolysis is a very clean reaction which proceeds as shown in Scheme III without side-product formation. Figure 4 shows the gas chromatographic trace obtained when polymer II is subjected to thermolysis near 250 the products analyzed by the mass spectrometer have the expected structures as shown in Figure 4. 

Molecularly imprinted polymers are highly cross-linked thermosets, and therefore porosity has been a necessary feature of their morphology to allow permeability and transport of template molecules to the bulk polymer phase. A high internal surface area ensures that the vast majority of the polymer mass is within several molecular layers of the surface and allows access of the template molecules to the majority of the polymer mass. A broad distribution of pore sizes is desirable for the use of these materials in chromatographic applications. Mesoporosity of amorphous porous materials is most commonly evaluated using a porosimeter by analyzing the N2 adsorption/desorption isotherms. Parameters that can be obtained from the measurements include surface area, average pore size, and pore size distribution. 

Electrospray analysis can be performed in positive and negative ionization modes. The polarity of the ions to be analyzed is selected by the capillary voltage bias. A novel feature of the ESI mass spectrum is the formation of intact molecular ions of the analyte. Fragmentation, if desired, can be induced in the ion-transport region of the ESI source by increasing the sampling cone voltage. This process is known as in-source collision-induced dissociation (CID) or nozzle-skimmer (NS) dissociation. 

The need to couple IMS and MS can either be driven by the desire to identify IMS features by mass analysis or by the wish to shape-analyze mass-selected ions. Although the two motivations approach the problem from opposite directions, they lead to instrument designs with basically identical capabilities as far as shape-mass analysis is concerned. The first perspective, upgrading an ion mobility spectrometer by adding an MS detector, logically leads to an IMS-MS configuration. The second proach may lead to a modified tandem mass spectrometer with the collision cell replaced by a drift cell. 

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