Mass spectrometers linear scan mode

TOF analyzers are especially compatible with MALDI ion sources and hence are frequently coupled in aMALDI-TOF configuration. Nevertheless, many commercial mass spectrometers combine ESI with TOF with great success. For proteomics applications, the quadrupole TOF (QqTOF) hybrid instruments with their superior mass accuracy, mass range, and mass resolution are of much greater utility than simple TOF instruments.21,22 Moreover, TOF instruments feature high sensitivity because they can generate full scan data without the necessity for scanning that causes ion loss and decreased sensitivity. Linear mode TOF instruments cannot perform tandem mass spectrometry. This problem is addressed by hybrid instruments that incorporate analyzers with mass selective capability (e.g., QqTOF) in front of a TOF instrument. 

The operation of magnetic sector (Chap. 4.3), linear quadrupole (Chap. 4.4), or quadrupole ion trap (Chap. 4.5) mass spectrometers in the repetitive scanning mode is useful for the identification of the components of a mixture. If quantitation is a major issue (below), selected ion monitoring (SIM) is preferably employed the term multiple ion detection (MID) and some others are also in use. [33] In the SIM mode, the mass analyzer is operated in a way that it alternately acquires only the ionic masses of interest, i.e. it jumps from one m/z value to the next. [34-39] The information obtained from a SIM trace is equivalent to that from a RIC, but no mass spectra are recorded. Thus, the scan time spent on a diagnostically useless m/z range is almost reduced to zero, whereas the detector time for the ions of interest is increased by a factor of 10-100. [40] An analogous improvement in sensitivity (Chap. 5.2.3) is also observed. 

A new generation of linear ion trap mass spectrometers has been developed and exhibits increased performance compared to traditional three-dimensional (3D) ion traps (Hopfgartner et al., 2003 Douglas et al., 2005). A further evolution of the triple-quadrupole family and ion trap class of instruments is the production of the hybrid triple-quadrupole/linear ion trap (QQQ/LIT) platform. Hybrid instruments of this nature allow for operation in space and not just in time when performing MS/MS analysis. This feature allows for increased performance compared to classical ion traps. A powerful combination possible on a hybrid LIT/QQQ instrument is the ability to use highly sensitive and selective precursor ion, constant neutral loss, and multi-MRM as a survey scan for dependent LIT MS/MS. Compared to a simple MS experiment, these comprehensive triple-quadrupole and LIT modes can be more complex to setup. 

Among these, both LC-MS and LC-MS/MS approaches have been described using different mass analyzers operating in the positive ion mode scan such as triple stage quadrupole (TSQ) mass spectrometers [145, 194, 226, 227, 235, 239, 240, 242-247] and hybrid quadrupole-linear ion trap (LTQ) [173, 193, 218, 238, 241] mass spectrometers working in SRM mode as well as time-of-flight (TOF) [195] and hybrid quadrupole-TOF (Q-TOF) [236,237] mass spectrometers working in the MS mode. 

The equipment used for this work consisted of a Hewlett-Packard 5710A GC, with on-coluitm injection, directly coupled to a VG7070E medium resolution double-focusing mass spectrometer. The GC coluitm employed was a 60 m fused silica capillary colurrm coated with a cross-linked methyl silicone stationary phase, DB-1. The initial temperature of the colurrm was 30°C, and after 4 minutes, the temperature was linearly programmed at 8°C/min to 270°C, and held at this final temperature for 15 minutes. The mass spectrometer was operated in the electron impact (El) mode, and the mass range of 20-700u was scanned once a second. The mass spectrometer was linked to an Incos data system which stored the acqnired mass spectra, and allowed these to be compared to the EPA/NIH Mass Spectral Data Base to assist with the identification of compounds detected. 

The triple quadrupole (QqQ) mass spectrometer is comprised of two linear mass hlters, Q1 and Q3, and a radiofrequency (RE) only quadrupole, q2, as shown in Eigure 4.5(a). The QqQ analyzer is capable of MS and MS-MS scans and is commonly coupled to an ESI source for proteomic analysis. Each of the three quadrupoles consists of four linear rods that have a high voltage RE potential (I to 3 MHz) and a DC potential applied to them. A triple quadrupole instrument performs three types of MS-MS scan modes precursor, product, and neutral loss. The RF/DC... 

Sierra Analytics and Applied Biosystems investigated the metabolic profile of indinavir, an HIV protease inhibitor with a very short half-life (2 hours), readily metabolised via CYPs to produce phase I and subsequent II products. Aliquots (10 pL) of indinavir incubated with rat liver S9 fractions were separated by online liquid chromatography and analysed with an Applied Biosystems hybrid triple quadrupole/linear ion trap mass spectrometer in information dependent acquisition (IDA) mode. This consisted of an enhanced MS survey scan followed by enhanced product ion scans for the two most intense parent ions as dependent MS/MS experiments. 

The simplest MS technique is the linear programmed heating of the sample in the vacuum chamber of the mass spectrometer. The evolved gases are analyzed continuously as the mass spectrometer scans the mass range of interest. Either a plot of ion current versus time or temperature for each ionic mass may be made or a single ionic mass may be monitored as a function of time or temperature of the pyrolysis. Both modes are commonly employed. 

The wide linear dynamic range of the mass spectrometer, even running in full scan mode, permits target compounds to be accurately quantified over a concentration range of at least three decades. Full-scan operation enables unknown peaks to be automatically detected and identified. Even after a period of several weeks of unattended operation, highly sensitive and reproducible results are achieved. 

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