Mass analyzers duty cycle

Typical quadrupoles transmit ions with mass-to-charge ratios ranging from 1-300 or greater with a mass-dependent resolution of twice the mass. More sophisticated instmments can achieve m/z of 4000 with comparable resolution. The suitability of a quadrupole mass analyzer depends in large part on the duty cycle... 

Quadrupole mass spectrometers has been used mainly in CE—MS because they can be obtained at relatively low cost, possess small dimensions, and are easy to operate. As previously mentioned, the scanning process is relatively slow and allows operation of only a small fraction of the available ions, which is not suitable with very narrow CE peaks. The use of selected ion monitoring (SIM) mode greatly improves sensitivity and duty cycle, but it is not always appropriate for the detection of complex mixtures. This kind of analyzer is currently used as a low-resolution instrument and for quantitative determination. 

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]. 

One critical feature of mass spectrometry when combined with chromatographic or electrophoretic separation techniques remains the duty cycle of the mass analyzer. A conventional LC chromatographic peak lasts about 10 s, which is sufficient to perform various MS and MS/MS experiments on various types of instruments. In the case of fast LC, the peak width can be in the range 1-2 s which is too fast for most mass analyzers except for TOF mass spectrometers. 

The quadrupoles in an MS instrument serve as selective mass filters to isolate ions with m/z-values specific for the analytes of interest. The triple quadrupole MS/MS instrument is typically operated by a pneumatically assisted electrospray source with an additional heated auxiliary gas flow for higher flow rates. There is a trade-off between resolution (favored by lower flow) and sensitivity (favored by higher flow) of the quadrupole analyzers. The biological molecules can be proto-nated or deprotonated at multiple sites to produce ions of n charged states [M nH]n . The MRM-MS/MS scan mode has a high duty cycle for the detection... 

In a quadrupole mass analyzer, only a single mass-to-charge ratio m/z) value is transmitted to the detector for any given combination of radio frequency (RF) and direct current (DC) potentials. Typically, the RF/DC ratio is held constant and scanned to provide a mass spectrum. If, for example, a quadrupole is scanned from m/z 1 to 1000 in 1 second, then any particular m/z is transmitted to the detector for only 1 millisecond, representing a duty cycle of 0.1 %. Thus, a quadrupole mass analyzer has a low transmission duty cycle in the full-scan mode, which results in limited full-scan sensitivity. In contrast, ion-trap and TOF mass analyzers have the theoretical potential to transmit all ions that enter the mass analyzer and yield far better sensitivity across the entire mass spectrum. In reality, the pulse sequences associated with these analyzers devote significant time to functions such as ionization and detection. The actual duty cycles are generally between 10 and 25%, still far better than a scanning quadrupole mass spectrometer. 

Along with advances in various ionization sources, significant improvements have been made in the area of mass analyzers. Mass analyzers can be differentiated based on several attributes such as scan speed, duty cycle, mass resolution, mass range, and cost [126], The most common analyzers used for metabonomics analyses include the quadrupole and TOF-based analyzers [125-127], Some other analyzers that have been reported for use in MS-based metabonomics analyses are the ion traps, Orbitraps, and Fourier transform mass spectrometers [128,129],... 

Analyzer Mass range (amu) Mass resolution (M/AM) Duty cycle Electromagnetic fields1... 

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