Focusing quadrupole

Fig. 1.7 Single stage pumping atmospheric pressure ionization interface with curtain gas. The size of the orifice is ca. 100 pm, qO acts as a focusing quadrupole and the nitrogen curtain gas prevents neutrai molecules being introduced into the mass spectrometer. T = Temperature of the cryoshells (in Keivin) p = pressure.

Fig. 1.18 Schematic of a triple quadrupole instrument. Stage qO focusing quadrupole Ql, Q3 mass analyzing quadrupoles q2 collision cell. In the present configuration the collision energy (CE) is determined by the potential difference between qO and q2.

Common combinations of electric (E) and magnetic (B) sectors, quadrupoles (Q) and collision cells (C). The lowercase q indicates a focusing quadrupole. 

Through the use of sequential electric (electrostatic) and magnetic fields (sectors) and various correcting lenses, the ion beam leaving the ion source can be adjusted so that it arrives at the collector in focus and with a rectangular cross-section aligned with the collector slits. For the use of crossed electromagnetic fields. Chapter 25 ( Quadrupole Ion Optics ) should be consulted. 

All mass spectrometers analyze ions for their mass-to-charge ratios (m/z values) by separating the individual m/z values and then recording the numbers (abundance) of ions at each m/z value to give a mass spectrum. Quadrupoles allow ions of different m/z values to pass sequentially e.g., ions at m/z 100, 101, 102 will pass one after the other through the quadrupole assembly so that first m/z 100 is passed, then 101, then 102 (or vice versa), and so on. Therefore, the ion collector (or detector) at the end of the quadrupole assembly needs only to cover one point or focus for a whole spectrum to be scanned over a period of time (Figure 28.1a). This type of point detector records ion arrivals in a time domain, not a spatial one. 

Peak matching can be done on quadrupole and magnetic-sector mass spectrometers, but only the latter, particularly as double-focusing instruments, have sufficiently high resolution for the technique to be useful at high mass. 

Upon emerging from the quadrupole, the ions are accelerated through about 40 V and focused into the time-of-flight (TOF) analyzer. A pusher electrode is sited alongside this focused ion beam. Application of a pulse of high electric potential (about 1 kV) to the pusher electrode over a period of about 3 ps causes a short section of the ion beam to be detached and accelerated into the TOF analyzer. A positive potential is used to accelerate positively charged ions and vice versa. 

Electric and magnetic fields can be used sequentially to focus the beam of ions. The use of crossed electromagnetic fields is described in the discussion of quadrupoles (Chapter 25). 

A simple mass spectrometer of low resolution (many quadrupoles, magnetic sectors, time-of-flight) cannot easily be used for accurate mass measurement and, usually, a double-focusing magnetic/electric-sector or Fourier-transform ion cyclotron resonance instrument is needed. 

Mass spectrometer configuration. Multianalyzer instruments should be named for the analyzers in the sequence in which they are traversed by the ion beam, where B is a magnetic analyzer, E is an electrostatic analyzer, Q is a quadrupole analyzer, TOP is a time-of-flight analyzer, and ICR is an ion cyclotron resonance analyzer. For example BE mass spectrometer (reversed-geometry double-focusing instrument), BQ mass spectrometer (hybrid sector and quadrupole instrument), EBQ (double-focusing instrument followed by a quadrupole). 

The skimmer has a smaller aperture than the sample cone, which creates a pressure of 10 atmospheres in the intermediate region. The ions are conducted through the cones and focused into the quadrupole with a set of ion lenses. Much of the instrument s inherent sensitivity is due to good designs of these ion optics. 

Resolution Quadrupole instruments are not capable of achieving the high resolution that is common with double-focusing magnetic-sector instruments. In GC/MS analyses, a compromise is struck between sensitivity (ion transmission) and mass resolution. In the quadrupole instrument, the resolution is set to the lowest possible value commensurate with resolving peaks differing by 1 Dalton (unit resolution). 

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. 

Selection of a suitable ionisation method is important in the success of mixture analysis by MS/MS, as clearly shown by Chen and Her [23]. Ideally, only molecular ions should be produced for each of the compounds in the mixture. For this reason, the softest ionisation technique is often the best choice in the analysis of mixtures with MS/MS. In addition to softness , selectivity is an important factor in the selection of the ionisation technique. In polymer/additive analysis it is better to choose an ionisation technique which responds preferentially to the analytes over the matrix, because the polymer extract often consists of additives as well as a low-MW polymer matrix (oligomers). Few other reports deal with direct tandem MS analysis of extracts of polymer samples [229,231,232], DCI-MS/MS (B/E linked scan with CID) was used for direct analysis of polymer extracts and solids [69]. In comparison with FAB-MS, much less fragmentation was observed with DCI using NH3 as a reagent gas. The softness and lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Most likely due to higher collision energy, product ion mass spectra acquired with a double-focusing mass spectrometer provided more structural information than the spectra obtained with a triple quadrupole mass spectrometer. 

In recent years several new instruments have been developed based on different mass-spectrometer principles. Two different categories of ICP-MS instruments are currently commercially available low-resolution instruments (using either QMS, ITMS or ToF-MS) and focusing high-resolution instruments (DFS, FTMS). Selected specifications for these two categories are shown in Table 8.63. Both the quadrupole-based and the double-focusing instruments allow a sequential multielement measurement, whereas ICP-ToFMS allows... 

Focusing Elements. Two ion optical components are usually placed between the accelerator and the sample chamber. A magnetic field separates any He-, He, or He+ from the He++ beam. A quadrupole magnetic lens then shapes the beam and focusses it into the sample chamber. 

The ions are pushed out of the electron beam, toward the central aperture of the source electrodes by a positively charged repeller plate. They accelerate through the source by a potential gradient applied to the source electrodes, and they are then focused toward the mass analyzer. Typically a quadrupole has been used to separate the ions, although more recently time of flight has been investigated (Letarte et al.,2004). 

Various analyzers have been used to analyze phenolic compounds. The choice of the MS analyzer is influenced by the main objective of the study. The triple quadrupole (QqQ) has been used to quantify, applying multiple reaction monitoring experiments, whereas the ion trap has been used for both identification and structure elucidation of phenolic compounds. Moreover, time-of-flight (TOF) and Fourier-transform ion cyclotron resonance (FT-ICR) are mainly recommended for studies focused on obtaining accurate mass measurements with errors below 5 ppm and sub-ppm errors, respectively (Werner and others 2008). Nowadays, hybrid equipment also exists, including different ionization sources with different analyzers, for instance electrospray or atmospheric pressure chemical ionization with triple quadrupole and time-of-flight (Waridel and others 2001). 

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