Collision / reaction cells cell gases

The figures of merit of quadrupole-based ICP-MS, such as the precision of isotope ratio measurements and the detection limits, can be improved significantly, especially for elements which are difficult to determine due to the appearance of isobaric interferences (e.g., by the trace, ultratrace and/or isotope ratio measurements of Ca, Fe, S, As, I or Se).16-22 The occurrence of interference problem can be minimized by the insertion of a collision/reaction cell in ICP-MS as the result of defined collision induced reactions using selected collision/reaction gases or gas mixtures (such as H2, He, NH3, 02, CH4 and others). For each analytical problem, which is different, e.g., for U or... 

More recently, the advent of the collision/reaction cell technology has revolutionised commercial quadrupole ICP-MS systems. A gas, such as hydrogen, helium or ammonia, is introduced into the reaction cell (placed inside the mass spectrometer and preceding the analyser quadrupole), where it reacts and dissociates or neutralises the polyatomic species or precursors. Through collision and reaction with appropriate gases in a cell, interferences... 

A multipole cell at pressures around 1 to 15 mtorr, placed between the sampler-skimmer interface and the mass spectrometer, can serve two functions reduce the kinetic energy of the ions to nearly thermal energies (<0.5 eV) and carry out reactions with analyte or background ions. Of particular interest for ICP-MS are reactions that would dramatically reduce spectral overlaps due to elemental or polyatomic ions. Two potentially undesirable processes must be considered for successful use of a collision-reaction cell. Scattering losses can be severe if the mass of the collision or reaction gas is high compared to that of the analyte ion... 

M. Iglesias, N. Gilon, E. Poussel, J. M. Mermet, Evaluation of an ICP-collision/ reaction cell-MS system for the sensitive determination of spectrally interfered and non-interfered elements using the same gas conditions, J. Anal. Atom. Spectrom., 17 (2002), 1240D1247. 

Ion-molecular reactions are used to resolve isobaric interferences, as discussed, in ICP-MS with a collision/reaction cell or by utilizing ion traps. The mass spectra of Sr, Y and Zr (Fig. 6.10a) without O2 admitted into the collision cell and (Fig. 6.10b) with 10 Pa Oj are different. By introducing oxygen, selective formation of YO and ZrO, but not SrO, is observed. This behaviour of different oxide formation is relevant for an interference free determination of Sr. Ultrahigh mass resolving power ICP mass spectrometry (at m/Am 260 000) selectively removes unwanted ions prior to transfer to the FTICR analyzer cell by gas-phase chemical reactions, e.g., for separation of Ca from " Ar+ obtained with a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer equipped with a 3 tesla superconducting magnet. ... 

Introduction of a collision/reaction cell (CRC) into the ICP/MS is a recent technique for interference reduction (Koppenaal et al., 2004). Interfering ions are eliminated by their gas phase reaction in a gas-filled cell inserted between the... 

PeikinElmer uses a patented scanning quadrupole-based collision/reaction cell, called a universal cell, which can operate with collision/KED or reactive gas. The scanning quadrupole in the cell removes side reaction product and new interferences, so that only the analyte passes to the analyzing quadrupole. 

In on-mass mode, both Ql and Q2 are set to the same target mass. In this case, Ql controls which ions enter the collision/reaction cell. All masses except the analyte mass and any direct mass interferences are rejected by Ql only the analyte mass and any direct on-mass interferences pass into the collision/reaction cell. The ORS separates the analyte from interferences by neutralizing the interference or shifting it to a new mass. Q2 then rejects any cell-formed interferences except for the target analyte ion. The on-mass mode removes the variability seen in reaction cell systems caused by different samples (ions) changing the reaction processes and product ions. The on-mass mode is used when the analyte is not reactive, and the interferences are reactive with a particular cell gas. 

Alternatively, the use of a modified spray chamber in line between the tubing and the plasma connection to increase the signal duration and improve the signal stability has been reported in another archaeometric application [108]. The use of this approach permits the simultaneous aspiration of a nebulized aerosol or of a gas (e.g., N2), which represents another possibility for damping the signal before the plasma. This procedure has been demonstrated to be preferable over the use of a gas inside a collision/reaction cell, as it is able to buffer the effect of the matrix on the mass discrimination [101]. 

MS. Oxygen was used as the reaction gas in the collision/reaction cell to produce PO" by reacting with phosphorus in the gas phase, thereby effectively eliminating the interferences for phosphorus that are normally seen at m/z 31. 

A multipole collision/reaction cell consists of a multipole (consisting of 2n -F 2 parallel metallic rods) in an enclosed cell that can be pressurized with a gas [76-81], It is located between the interface and the mass analyzer (Figure 2.20). 

Collisional damping in a collision/reaction cell provides a significant improvement in the isotope ratio precision [98, 99]. This effect is created by pressurizing the cell with a nonreactive collision gas, typically Ne. As a result, ions extracted from the ICP at slightly different moments in time are admixed in the cell, thereby damping the short-term variations in the ion beam to some extent. The effect of the use of Ne as a non-reactive collision gas on the isotope ratio precision observed in practice is illustrated in Figure 2.24. 

Figure 2.24 Internal isotope ratio precision (RSD) for ° Ag/ ° Ag as obtained using a quadrupole-based instrument equipped with a collision/reaction cell. Filled squares, vented cell open squares, with Ne as an inert collision gas, introduced into the cell at a flow rate of 2 ml min The dotted line represents the precision as predicted by Poisson counting statistics. Reproduced with permission of the Royal Society of Chemistry from [99].

With inert collision gas in collision/reaction cell Sector field ICP-MS -0.05... 

The other thing to be wary of with semiquantitative analysis is the spectral complexity of unknown samples. If you have a spectrally rich sample and are not making any compensations for spectral overlaps close to the analyte peaks, it could possibly give you a false-positive for that element. Therefore, you have to be very cautious when reporting semiquantitative results on completely unknown samples. They should be characterized first, especially with respect to the types of spectral interferences generated by the plasma gas, the matrix, and the solvents/acids/chemicals used for sample preparation. Collision/reaction cells/interfaces can help in the reduction of some of these interferences, but extreme care should be taken, as these devices are known to have no effect on some polyatomic interferences, and in some cases can increase the spectral complexity by generating other interfering complexes. 

More recently there has been an explosion of interest in using collision/reaction cell/ interface technology for the analysis of biomedical samples, because of the benefits it brings to the determination of many of the toxologically and nutritionally significant elements such as As, Se, Cr, Fe, and Cu. Traditionally, these elements have been very difficult to analyze by ICP-MS because of the spectral interferences derived from a combination of the matrix, solvent/acid, and plasma gas ions. This approach is allowing significant improvements in detection capability for both the total and speciated forms of these elements in biomedical-related samples, such as blood serum and tissue samples. 

It should also be pointed out that some collision/reaction cells require high-purity gases with extremely low impurity levels, because of the potential of the contaminants in the gas to create additional by-product ions (refer to Chapter 10). 

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