Quadrupole Technology

When the first quadrupole of a triple quadrupole is replaced by a double-focusing mass spectrometer, the instrument is termed a hybrid (i.e. a hybrid of magnetic sector and quadrupole technologies). Figure 3.9 shows the MSi unit as a forward-geometry instrument although there is no reason why this could not be of reversed- or even tri-sector geometry. 

Table 6.30 lists the main characteristics of quadrupole mass spectrometers. QMS is a relatively simple and robust analyser which does not need such a high vacuum as a sector instrument. The maximum admissible pressure at the source of the spectrometer is 10 6mbar in continuous regime and 10-5—10 4 mbar during short time intervals. Quadrupole technology assures reproducible and accurate molecular weight measurements day in and day out. For figures of merit, see Table 6.27. 

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. 

An important configuration of quadrupoles is the triple quadrupole (QqQ), in which there are two analytical quadrupoles (Q) separated by a transmission quadrupole (q). While the predominant use of the QqQ is in quantification, this very versatile format has several scanning modes that enable multiple MS/MS approaches to obtain structural information (Section 3.3.3.1). Extensions of the quadrupole technology are the quadrupole ion trap (QIT) and the more recent linear ion trap (LIT) that has higher ion capacity. The resolutions of these ion traps are similar to those of single quadrupoles. However, an advantage of the traps is the ability to store and manipulate ions prior to their detection, thus enabling MS/MS experiments (Section 2.3.2). 

The simultaneous nature of sampling ions in TOP offers distinct advantages over traditional scanning (sequential) quadrupole technology for ICP-MS applications in which large amounts of data need to be captured in a short span of time. To understand the benefits of this mass separation device, let us first take a look at its fundamental principles. All TOP mass spectrometers are based on the same principle the kinetic energy (KE) of an ion is directly proportional to its mass (m) and velocity (VO. This can be represented by the following equation ... 

Another benefit of the fast acquisition time is that qualitative or semiquantita-tive analysis is relatively seamless compared to scanning quadrupole technology, because every multielement scan contains data for every mass. This also makes spectral identification much easier by comparing the spectral fingerprint of unknown samples against a known reference standard. This is particularly useful for forensic work, where the evidence is often an extremely small sample. 

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. 

Although there are real benefits of using TOF over quadrupole technology for some ICP-MS applications, there are also subtle differences in the capabilities of each type of TOF design. Some of these differences include ... 

FIGURE 10.11 The fundamental principles of triple quadrupole technology used in ICP-MS. 

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