Mass spectrometic detection

When nano LC is combined with mass spectrometer detection, attamole detection can be achieved for low abundance components in biological fluids, drug metabolites, and natural products such as Chinese herb medicines. Nano LC-MS-MS has become an essential tool for complex biological and drug metabolite studies. Nano LC-MS presents two significant differences from conventional analytical HPLC (1) large enhancement factor for sample detection and (2) direct interface to MS without flow splitting. The enhancement in MS ion counts relative to a conventional 4.6 mm ID column is proportional to the ratio of the square of the column diameter ... 

The amount of cresol in the concentrated extract can then be determined by high performance liquid chromatography (HPLC) (DeRosa et al. 1987 Yoshikawa et al. 1986) or gas chromatography (GC) coupled to either a flame ionization detector (FID) or a mass spectrometer detection system (Angerer 1985 Needham et al. 1984). Separation of the cresol isomers by gas chromatography is readily accomplished, and the use of an appropriate internal standard allows the determination of their concentrations. Although exact detection limits were not given for the above GC methods, a concentration of 10 ppm appears to be readily determined. 

A variety of experimental techniques have been used for the determination of uptake coefficients and especially Knudsen cells and flow tubes have found most application [42]. Knudsen cells are low-pressure reactors in which the rate of interaction with the surface (solid or liquid) is measured relative to the escape through an aperture, which can readily be calibrated, thus putting the gas-surface rate measurement on an absolute basis. Usually, a mass spectrometer detection system monitors the disappearance of reactant species, as well as the appearance of gas-phase products. The timescale of Knudsen cell experiments ranges from a few seconds to h lindens of seconds. A description of Knudsen cell applied to low temperature studies is given [66,67]. 

It has been the purpose of this paper to provide an overview of the basic differences and similarities of the various types of Instruments which detect Ionized particles emitted from surfaces by energetic particle bombardment. Since the scope of secondary ion mass spectrometry Is so broad, It is not surprising that no one Instrument has been designed to perform optimally for all types of SIMS analyses. Design aspects of the primary beam, extraction optics, mass spectrometer, detection equipment and vacuum system must be considered to construct an Instrument best suited for a particular purpose. 

In the third common scan mode, both mass spectrometers are scanned together, but with a constant mass offset between the two. Thus, for a mass difference a, when an ion of mass m goes through the first mass spectrometer, detection occurs if this ion has yielded a fragment ion of mass (m — a) when it leaves the collision cell. This is a neutral loss scan , the neutral having the mass a. For example, in chemical ionization the alcohol molecular ion loses a water molecule. Alcohols are thus detected by scanning a neutral loss of 18 mass units. On the other hand, a given mass increase can be detected if a reactive gas is introduced within the collision cell. 

Unfortunately, from the point of view of the physical organic chemist, the mass-spectrometric approach suffers from certain intrinsic limitations. In the first place, the range of pressures accessible to the investigator is severely limited, and most of the available data refer to experiments carried out at pressures well below one torr. In the second place, the mass spectrometer detects only charged species, and the neutral molecules, which represent the final products of the carbonium-ion reactions and are of prime concern to the physical organic chemist, cannot be determined at all. Finally, since the structure of the ionic species, that are analysed exclusively according to their m/e ratio, cannot be directly deduced from mass spectra, it is difficult to discriminate isomeric ions, and to study the isomerization reactions of the carbonium ions, which play such an important role in their solution chemistry. 

P.K. Jensen, L. Pasa-Tolic, K.K. Peden, S. Martinovic, M.S. Lipton, G.A. Anderson, N. Tolic, K.K. Wong and R.D. Smith, Mass spectrometic detection for capillary isoelectric focusing separations of complex protein mixtures. Electrophoresis, 21, 1372-1380 (2000). 

The "soft" ionization MS techniques of and FAB have been employed successfully for quantitative applications with stable-isotope-labeled internal standards and interesting developments in the use of microwave plasma discharges, either alone or in combination with mass spectrometic detection, promise to extend the utility of stable isotope labeling methods in quantitative studies of drug dispositon. New methods have been reported for the measurement of N enrichment in ammonia, based on conversion into hexamethylenetetramine, and for the... 

Spark Source Mass Spectroscopy electrodes spark produces ions mass spectrometer detection limit 0.01-0.05 ppm ... 

Glow Discharge Mass Spectroscopy for a D.C. glow discharge ionized in plasma mass spectrometer detection limit sub-ppb ... 

Mass spectrometers detect highly selectively, depending on their operation settings, but not specifically. 

Another issue in trap-loss detection is the competition between homonuclear and heteronuclear trap loss in a given experiment, making assignments quite difficult. For example, when searching for KRb trap loss just below the %i/2 + 5pi/2 asymptote, stronger trap loss is simultaneously observed from just below the Rb2 5 i/2 + 5pi/2 asymptote. However, for detection of KRb+ ions by REMPI from theX and/or a states of KRb, there is no competition from ions due to Rb2 PA (the Rb+ and RbJ ions are easily distinguishable by their differing TOFs). Thus we believe that REMPI TOF mass spectrometer detection of X- and a-state heteronuclear molecules is normally preferable to trap-loss spectroscopy. 

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