Mass analyzers operation modes

Mass Analyzer Operation Modes and Tandem Mass Spectrometry... 

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. 

Most mass analyzers for ICP/MS instruments are operated in a scanning mode. These mass analyzers operate as bandpass filters, passing a single mass-to-charge ratio at a time for detection by the ion collection system. The mass analyzers are scanned either by peak-hopping or by continuous scan. 

Most mass analyzers operate under high-vacuum conditions to ensure that charged particles do not deviate from their trajectories due to collision with residual gas. Mass spectrometers can be grouped into different types of operation mode continuous mode (magnetic sector, quadrupole), pulsed mode (time of flight), and ion trapping mode (quadrupole traps, Fourier transform ion cyclotron, orbitrap). 

The most common modes of operation for ms/ms systems include daughter scan, parent ion scan, neutral loss scan, and selected reaction monitoring. The mode chosen depends on the information required. Stmctural identification is generally obtained using daughter or parent ion scan. The mass analyzers commonly used in tandem systems include quadmpole, magnetic-sector, electric-sector, time-of-flight, and ion cyclotron resonance. Some instmments add a third analyzer such as the triple quadmpole ms (27). 

Figure 3.9 Conceptual view of tandem mass spectrometry with a tandem-inspace triple quadrupole mass analyzer." The first mass analyzer (Ql) selects the precursor ion of interest by allowing only it to pass, while discriminating against all others. The precursor ion is then fragmented, usually by energetic collisions, in the second quadrupole (q2) that is operated in transmissive mode allowing all fragment ions to be collimated and passed into the third quadrupole (Q3). Q3 performs mass analysis on the product ions that compose the tandem mass spectra and are rationalized to a structure.

Perhaps the simplest mass analyzer of all, the TOF mass spectrometer [46] has experienced a reemergence in the past several years. Like the 3D quadrupole ion trap, the TOF analyzer has come to commercial prominence several decades after its initial introduction. The limitations of electronic components in the 1960s constrained the capabilities of the instrument, limiting its mass range and resolving power. The TOF analyzer operates in a pulsed mode, requiring either a pulsed ion... 

Thermospray (TSP) [29-31] unites three modes of operation. In pure TSP, a solution of the analyte and a volatile buffer, usually 0.1 M ammonium acetate, is evaporated from a heated capillary at a flow rate of 1-2 ml min into a heated chamber, hence the term thermospray. As the solvent evaporates, the analyte is forming adducts with ions from the buffer salt. While most of the neutrals are removed by a vacuum pump, the ions are extracted orthogonally from their main axis of motion by use of an electrostatic potential. The ions are transferred into a quadrupole mass analyzer through a pinhole of about 25 pm in diameter (Fig. 11.2). The quadrupole was employed according to its tolerance to poor vac-... 

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. 

Phenolic compounds in Sicilian wines were directly detected by La Torre et al. [373] using an HPLC with a DAD coupled on-line with a MS system equipped with ESI source operating in the negative-ion mode and a quadruple mass analyzer. The structure was elucidated by recording MS spectra at different voltages, in addition to the molecular mass information. The method allowed both the identification and determination of 24 phenolic compounds in 22 different commercial Sicilian red wines by direct injection without any prior purification of the sample. Figure 19.10 reproduced an HPLC trace obtained in this work. 

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