**3.4 Time of flight mass analyzer**

*Mass Spectrometry - Future Perceptions and Applications*

Full scan mode (Q1 MS) RF

Full scan mode (Q3 MS) RF

Product ion scan (PI) RF

Precursor ion scan (PC) RF

Multiple reaction monitoring (MRM) RF

*Modes of triple quadrupole operation and analytical requirements.*

There are more than a few important features which impact the time necessary to attain a mass spectrum (duty cycle): (i) injection time (0.5–500.0 ms), (ii) scan speed (5000–20,000 m/z units/s), (iii) separation of the parent ion and

**Mode Q0 Q1 Q2 Q3 Analysis type**

RF/ DC

RF only

RF/ DC

RF only

RF/ DC

RF only

RF only

RF only

RF only

RF only RF only

RF/ DC

RF only

RF/ DC

RF/ DC

Qualitative

Qualitative

Qualitative

Qualitative

Quantitative

only

only

only

only

only

is not performed in space but in time. Fragmentation happens with the assistance of helium as collision gas. Also, duty cycles for fragmentation (MS/MS) are much shorter in linear ion trap when compared to triple quadrupole mass analyzer. One major challenge in linear ion trap is to trap precursor ion and fragment in the same space. Often, due to this disadvantage, fragmentation spectra generated in linear ion trap differs from that of triple quadrupole CID. Also, number of MRM transitions that can be monitored in linear ion trap are quite less [4–8] when compared to

for qualitative analysis in drug metabolism, metabolomics and proteomics studies. Similar sensitivities to QqQ mass analyzer can be achieved for quantitative analysis

While linear ion traps mainly function on radial ejection, next generation mass analyzers called quadrupole linear ion trap use axial ejection. This led to discovery of hybrid triple quadrupole mass analyzers, where Q3 performs the function of both quadrupole and linear ion trap [48, 49]. Unlike linear ion trap that fragments precursor in time, these hybrid analyzers perform fragmentation in space.

QqQ mass analyzers (~100 MRM transitions can be monitored).

on linear ion trap, but at the price of precision and accuracy.

. Contrarily to the triple quadrupole, MS/MS

mode, ion traps are particularly attractive

fragmentation in tandem MS or MSn

*Schematic diagram of linear ion trap mass analyzer.*

Due to the high sensitivity in MS<sup>n</sup>

**46**

**Figure 7.**

**Table 1.**

Discovered in 1940's, time of flight mass analyzers achieved popularity after 1990's. Time of flight operates on principle of "time that ions need to cross in a field free tube of about 1 m length" [50, 51]. The motion of an ion is characterized by its kinetic energy Ec = 0.5 m x v2 (m = mass, v = speed). Therefore, the time ions fly through the tube is directly proportional to their m/z value. The velocity of the ions formed is generally low and they are accelerated by strong electric fields (2000– 2030,000 V) in the direction of the detector. Low mass ions reach the detector more rapidly than high mass ions. Due to the short flight time (50–100 msec) and the good transmission, a spectrum can be generated within 100 ms over an almost unlimited mass range. Mass resolution of time of flight mass analyzer depends on the length of flight tube and reduced kinetic energy spread of the ions. Length of flight tube is directly proportional to mass resolution. Kinetic energy spread can be reduced by increasing time delay between ion formation and acceleration, also known as delayed pulse extraction. Also, positioning of electrostatic mirror in the drift region of ions increases the mass resolution (**Figure 8**).

Briefly, the ions with high energy penetrate deeper into the ion mirror region than those with the same m/z at a lower energy. Because of the different trajectories, all ions of the same m/z reach the detector at the same time. With the reflectron the flight path is increased without changing the physical size of the instrument. Commercial TOF instruments are available to operate in either linear mode or reflectron mode. Even though ESI can be coupled with TOF, but the combination of MALDI and TOF is most popular as both operate on the principle of pulsed technique. Coupling of ESI with TOF needs orthogonal acceleration to drive continuous beam of ions [52].

Time of flight instruments are designed to use for qualitative analysis with MALDI or atmospheric pressure ionization. MALDI hyphenated with time of flight analyzer enables the identification of large molecules such as proteins, peptides, lipids and polymers. MS/MS information can also be obtained by CID in drift tube with the assistance of nitrogen or argon as collision gas. However, as quadrupole

**Figure 8.** *Schematics representation of a quadrupole time of flight mass spectrometer.*

technology is so successful for both qualitative and quantitative analysis, TOF analyzers are used as a hybrid platform with quadrupole analyzers. In these hybrid systems, TOF analyzer replaces Q3 of a triple quadrupole system. These hybrid systems are termed as QTOF mass spectrometers. QTOF systems offer high mass resolution (~40,000) and sensitivity. Accurate mass measurements are especially useful in metabolite identification studies and peptide analysis [53]. Various other hydrid TOF platforms have been reported including, linear ion trap, quadrupole ion trap and TOF-TOF mass spectrometers [54–56].
