Analyzers Quadrupole Technology

Mass separation devices all have their own strengths and weaknesses, which will be discussed in greater detail over the next four chapters. Let us first begin with the most common type of mass separation device used in ICP-MS—the quadrupole mass filter. 

FIGURE 7.1 The mass separation device is positioned between the ion optics and the detector. 

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ToF analyzers as well as hybrid instruments that combine two or more mass-resolving components, such as quadrupole-ToF (Q-ToF), ion-mobility ToF, and ion-trap-ToF, as well as the high-resolving Fourier transform (FT) analyzer Orbitrap and ion cyclotron resonance (ICR). For targeted analysis, a multiple-reaction monitoring instrument based on triple-quadrupole technologies (QQQ) has provided unrivaled sensitivity for MSI of pharmaceuticals, yet its targeted nature renders it unsuitable for discovery-based research. 

Before we go on to discuss these in greater detail and how these parameters affect the data, it is important to remind ourselves how a scanning device like a quadrupole mass analyzer works. Although we wiU focus on quadrupole technology, the fundamental principles of measurement protocol wiU be very similar for all types of mass spectrometers that use a sequential approach for multielement peak quantitation. 

The first commercial instrument to use this approach was called dynamic reaction cell (DRC) technology Similar in appearance to the hexapole and octapole collision/reaction cells, the DRC is a pressurized multipole positioned prior to the analyzer quadrupole. However, this is where the similarity ends. In DRC technology, a quadrupole is used instead of a hexapole or octapole. A highly reactive gas such as ammonia, oxygen, or methane is bled into the cell, which is a catalyst for ion-molecule chemistry to take place. By a number of different reaction mechanisms, the gaseous molecules react with the interfering ions to convert than into either an innocuous species different from the analyte mass or a harmless neutral species. The analyte mass then emerges from the DRC free of its interference and is steered into the analyza quadrupole for conventional mass separation. 

It should be noted that up until now, only single multipole-based cells have realized commercial success, but a recent development has placed an additional quadrupole prior to the collision/reaction cell multipole and the analyzer quadrupole. This first quadrupole acts as a simple mass filter to allow only the analyte masses to enter the cell, while rejecting all other masses. With all nonanalyte, plasma, and sample matrix ions excluded from the cell, sensitivity and interference removal efficiency is significantly improved compared to traditional collision/reaction cell technology coupled with a single quadrupole mass analyzer. 

Fig. 4. 32. Schematic (a) and photograph (b) of a linear quadrupole mass analyzer. By courtesy of JEOL, Tokyo (a) and Waters Corp., MS Technologies, Manchester, UK. (b).

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