Gases collision

This is entirely analogous to the problem with simple chemical ionization, and the solution to it is similar. To give the quasi-molecular ions the extra energy needed for them to fragment, they can be passed through a collision gas and the resulting spectra analyzed for metastable ions or by MS/MS methods (see Chapters 20 through 23). 

By introducing a collision gas into Q2, collision-induced dissociation (CID) can be used to cause more ions to fragment (Figure 33.4). For example, with a pressure of argon in Q2, normal ions (mj ) collide with gas molecules and dissociate to give mj ions. CID increases the yield of fragments compared with natural formation of metastable ions without induced decomposition. 

An example of linked scanning on a triple quadrupole instrument. A normal ion spectrum of all the ions in the ion source is obtained with no collision gas in Q2 all ions scanned by Q1 are simultaneously scanned by Q3 to give a total mass spectrum (a). With a collision gas in Q2 and with Q1 set to pass only m+ ions in this example, fragment ions (f, fj ) are produced and detected by Q3 to give the spectrum (b). This CID spectrum indicates that both f, and fj are formed directly from m+. 

In (a), an ion and a gas atom approach each other with a total kinetic energy of KE, + KEj. After collision (b), the atom and ion follow new trajectories. If the sum of KE, + KEj is equal to KE3 + KE4, the collision is elastic. In an inelastic collision (b), the sums of kinetic energies are not equal, and the difference appears as an excess of internal energy in the ion and gas molecule. If the collision gas is atomic, there can be no rotational and no vibrational energy in the atom, but there is a possibility of electronic excitation. Since most collision gases are helium or argon, almost all of the excess of internal energy appears in the ion. 

Rather than looking at just the low-abundance metastable ion processes occurring in the second quadrupole, extra fragmentation can be induced by having a neutral collision gas present in this quadmpole. 

Tandem mass spectrometry (MS/MS) is a method for obtaining sequence and structural information by measurement of the mass-to-charge ratios of ionized molecules before and after dissociation reactions within a mass spectrometer which consists essentially of two mass spectrometers in tandem. In the first step, precursor ions are selected for further fragmentation by energy impact and interaction with a collision gas. The generated product ions can be analyzed by a second scan step. MS/MS measurements of peptides can be performed using electrospray or matrix-assisted laser desorption/ionization in combination with triple quadruple, ion trap, quadrupole-TOF (time-of-flight), TOF-TOF or ion cyclotron resonance MS. Tandem... 

Flow rate Injection volume Retention time Ionization mode Polarity Nebulizer gas Auxiliary gas Nebulizer temperature Collision gas Acquisition... 

Mode Interface Spray voltage Sheath gas Auxiliary gas Capillary temperature Manifold temperature Collision gas Total scan time QIMS resolution voltage added Q3MS resolution voltage added Parent masses... 

Figure 9. Apparatus for the determination of CID thresholds. Electrospray ions are produced as in Figures 4 and 7, however pressure reduction capillary CAP leading to 10 torr low pressure chamber LPS is coaxial with orifices leading to triple quadrupole. The IT chamber at 10 torr is used for ion thermalization. Collision chamber at Q2 is usually used with collision gas Ar or Xe. The last quadrupole Q3 is not shown on Figure.

The multiple reaction monitoring (MRM) conditions for each analyte were optimized by infusing 0.1 jxglmL of analyte in mobile phase. The Ionspray needle was maintained at 4.0 kV and the turbo gas temperature was 650°C. Nebulizing gas, auxiliary gas, curtain gas, and collision gas flows were set at 35, 35,40, and 4, respectively. In the MRM mode, collision energies of 17,16, and 15 eV... 

A Waters Micromass triple quadrupole mass spectrometer was used with an electrospray ionization interface in positive ionization mode desolvation gas (400), cone gas (70), collision gas (2.74 x 10 3 mbar), capillary (3.0 kV), cone (14 (kV), source temperature (105°C), and desolvation temperature (300°C). The detection and quantitation of amlodipine and nimodipine were performed... 

Where Ecom is the center-of-mass energy, Et is the translational energy of the ion, mn is the mass of the neutral collision gas, and m is the mass of the ion. Dissociations often occur with Ecom < 10 eV. 

Fluoro compounds, particularly fluorocarbon molecules, are generally regarded as being less reactive than other halo molecules. The compound SF6 has been regarded as inert and often used as a collision gas in gas-phase experiments, but this molecule reacts with some bare metal ions. The group 3 (IIIB), 4 (IVB), and 5 (VB) transition metal ions (92) and Pr+ (120), remove fluorine atoms from SF6. In the most extreme case, Ta+ forms a series of T iFJ+ and SF ions. 

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