Helium, as collision gas

Fig. 4 Palladium concentration in different samples from Frankfurt/Main determined after mercury co-precipitation by ID-ICP-Q-MS using helium as collision gas (gas flow 120 mL/min) and measured without helium...

BCR723 Measurement with helium as collision gas Certified value for Pd in road dust reference material 10 4.2 1.4 pg/kg 6.1 1.9 pg/kg... 

In low energy CID, the transfer of momentum plays a more important role than the transfer of energy. Therefore larger molecules, such as nitrogen, are more effective than small atomic species such as helium, as collision gas. 

Alternatively, ions of any one selected m/z value can be chosen by holding the magnetic field steady at the correct strength required to pass only the desired ions any other ions are lost to the walls of the instrument. The selected ions pass through the gas cell and are detected in the singlepoint ion collector. If there is a pressure of a neutral gas such as argon or helium in the gas cell, then ion-molecule collisions occur, with decomposition of some of the selected incident ions. This is the MS/MS mode. However, without the orthogonal TOF section, since there is no further separation by m/z value, the new ions produced in the gas cell would not be separated into individual m/z values before they reached the detector. Before the MS/MS mode can be used, the instrument must be operated in its hybrid state, as discussed below. 

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. 

Helium is used as the collision gas Nitrogen is used as the collision gas... 

The Raman spectra of solids have a more or less prominent collision-induced component. Rare-gas solids held together by van der Waals interactions have well-studied CILS spectra [656, 657]. The face-centered, cubic lattice can be grown as single crystals. Werthamer and associates [661-663] have computed the light scattering properties of rare-gas crystals on the basis of the DID model. Helium as a quantum solid has received special attention [654-658] but other rare-gas solids have also been investigated [640]. Molecular dynamics computations have been reported for rare-gas solids [625, 630, 634]. 

The quadrupole ion trap (QJT) is about the size of a small fist and consists of a ring electrode and two hyperbolic end electrodes (see March and Todd68 for a detailed theory of operation and history of development). Like the linear ion trap (LIT, see below), the QJT operates at relatively high pressure (10 3 torr) with a helium buffer gas that assists the ions to maintain a stable orbital frequency. The buffer gas also serves as the collision gas for collision-induced dissociation (CID) during MS/MS experiments. 

Linear quadrupole instruments are widely spread. Recent developments in the technology now make it possible to work simultaneously with fullscan and selected ion monitoring (SIM) modes in a single run. In GC tandem MS, a first quadmpole acts as a mass selective filter and the second quadrupole is used as the collision cell with addition of a collision gas such as helium or argon. In a third quadrupole the full mode is performed to obtain the full mass spectrum of the product ions. 

The most popular two-dimensional mass spectrometry configuration at present is the QQQ, or triple-sector quadrupole, represented schematically in Fig. 3.9. Three scan modes are possible with this configuration product ion scan, precursor ion scan, and constant neutral loss scan. Product ion scan is the most widely used, and involves using Qj to selectively transmit one precursor ion to Q2 where it is fragmented, normally by collisions with an inert gas such as helium. This type of fragmentation is referred to as collision-induced dissociation, or CID. Q2 is operated in radio frequency mode only, and thus stores ions of a broad m/z range until they are transmitted to Q3 for mass analysis of the product ions. 

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