Mass spectrometers cycle time

As with ion beam QMFs, quadrupole ion traps have improved ion signal-to-noise ratios in the SIM scan mode versus the full-scan mode. The longer a mass spectrometer spends time detecting a selected ion, the lower the detection hmit will be for that ion. QMF mass spectrometers have an important advantage over the QIT in the SIM scan mode. In QMF instruments, the duty cycle can increase from less than 0.1% for one ion in a scan from miz 100-2000, to nearly 100% in a SIM scan [duty cycle = (ion collection or detection time/total scan time) x 100%]. However, the XQIT mass spectrometers have a duty cycle advantage in the fuU-scan mode of operation over QMF instruments (see Table 9.1). Since in a XQIT the duty cycle is dependent on the sample amount, unUke in the QMF case, low-level ion signals can reach duty cycles levels > 50% in the normal full-scan mode and can increase to levels >90% for the SIM scan mode. 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

Discontinuous chemical-detection systems do not provide a signal that is continuous in time, but rather cycle rapidly through a series of phases such as sample collection, preconcentration, separation, and detection in such a way that the overall system is capable of providing a detection report every minute or so. Examples of such systems include ion mobility spectrometers, mass spectrometers, and chromatography-based systems. Many technologies are possible candidates for each of the different phases.2... 

FIGURE 4.4 Arrow indicates chromatograms from three injections of SC-50267 and an internal standard [D2]SC-50267. The last injection appears near the end of the display. A VG Trio-2 mass spectrometer equipped with a thermospray LC interface was used. Samples were injected every 1.85 min—the cycle time of the autosampler (Waters WISP). (Source Chang, M. and G. Schoenhard, presentation at Pittsburgh Conference and Exposition, 1993. With permission). 

The TOF mass analyzer has a low duty cycle, and the combination with an ion accumulation device such as an ion trap is therefore very advantageous. It offers also MS capabilities with accurate mass measurement. In all acquisition modes, the ions are accelerated into the time of flight for mass analysis. Various other hybrid mass spectrometers with TOF have been described, including quadrupole ion trap [70] and linear ion trap [58]. High energy tandem mass spectrometry can be performed on TOF-TOF mass spectrometers [71, 72]. 

Experiments conducted with mass-selected ions do not bear any direct relevance for applied catalysis, simply because the number densities of the ionic species are rather low (typically about 10 particles per cm ). Nevertheless, the advantages associated with the handling and the detection of ionic species render gas-phase studies as an ideal tool for the investigation of the elementary steps in oxidation reactions. In the same vein, this holds true for the investigation of the separate mechanistic steps, and in appropriate mass spectrometers that are able to store ions for extended timescales this can also be extended to real catalytic cycles [66]. The time-honored prototype of such a catalysis was reported by Kappes and Staley who demonstrated that bare Fe" ions initiate a catalytic conversion of CO into CO2 in the presence of N2O [67]. In the following decades, a number of other catalytic cycles involv-... 

Several scan modes are unique to the triple-quadrupole instrument, and most of these modes are superior in duty cycle versus an ion trap, Fourier transform (FT), or time-of-flight (TOF) mass spectrometers. Different elements of the triple-quadrupole perform different operations for each scan mode. These scan modes, each of which will be described in detail, are single-reaction monitoring (SRM) or multiple-reaction monitoring (MRM), precursor ion scanning (PIS), and constant-neutral-loss scanning (NLS). These scan modes and applications for structural elucidation have been described in detail (Yost and Enke, 1978, 1979). 

The advantage of generic LC/MS run conditions is that it allows the preparation of an LC/MS separation database that can be referenced for compound mixtures from anywhere in the development and manufacturing process cycle. It trades off resolution for consistency, speed, and a decrease in methods development times. It permits creation of a computer-searchable database of information for all of the compounds being investigated in the company. The mass spectrometer provides sensitivity and resolution gain as well as information on retention times and molecular weights. 

Gas chromatographv-mass spectrometry. Gas chromatography-mass spectrometry (GC-MS) was carried out using a Hewlett-Packard 5840 gas chromatograph connected to a VG-70S mass spectrometer operated at 70 eV with a mass range m/z 40-800 and a cycle time of 1.8 s. The gas... 

Flash pyrolysis-gas chromatography-mass spectrometry. Flash pyrolysis-GC-MS analyses were performed as described in detail previously (48), although another mass spectrometer (VG-70S) was used. Electron impact mass spectra were obtained at 70 eV with a cycle time of 1.7 s and a mass range m/z 50-800 at a resolution of 1000. Data acquisition was started 1 min. after pyrolysis. 

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