Quadrupole devices

The most widely regarded approach to accomplish the determination of as many pesticides as possible in as few steps as possible is to use MS detection. MS is considered a universally selective detection method because MS detects all compounds independently of elemental composition and further separates the signal into mass spectral scans to provide a high degree of selectivity. Unlike GC with selective detectors, or even atomic emission detection (AED), GC/MS may provide acceptable confirmation of the identity of analytes without the need for further information. This reduces the need to re-inject a sample into a separate GC system (usually GC/MS) for pesticide confirmation. Through the use of selected ion monitoring (SIM), efficient ion-trap or quadrupole devices, and/or tandem mass spectrometry (MS/MS), modern GC/MS instruments provide LODs similar to or lower than those of selective detectors, depending on the analytes, methods, and detectors. 

In a quadrupole device, not as accurate and precise as double-focusing instruments but fast, a quadrupolar electrical field comprising radio-frequency (RF) and direct-current components is used to separate ions. Quadrupole instruments as mass analyzers are used together with ESI as the ion source the configuration employing a three-dimensional quadrupolar RF electric field (Wolfgang Paul, University of Bonn, 1989 Nobel prize for physics) is referred to as an ion trap analyzer (see below). 

Positive Ion Chemical Ionization Photo-Induced Dissociation Post-Source Decay Pyrolysis Mass Spectrometry Quadrupole device used in RF-only mode Quadrupole... 

The various means of obtaining mass spectroscopy with laser ionization will be discussed. The production of ions at a point in space (and during a short time interval) by pulsed laser ionization is particularly well suited to time-of-flight TOF, as compared to quadrupole, mass analysis. The increased resolution of reflection TOF spectrometers is comparable with that attainable with quadrupole devices and yet maintains the advantage that the entire mass spectrum is obtained with each laser pulse [5]. 

Tandem Mass Spectrometry with Multiple-Quadrupole Devices... 

In the operation of 2D and 3D quadrupole instruments, the modification of ion trajectories by ion/ neutral collisions must be considered. In rf quadrupole fields, an ion/neutral collision reduces both ion kinetic energy and ion excursions such that the ions are cooled and focused to the center of each field. Collisional cooling is an important aspect of the behavior of ions in a quadrupole field for example, in a 2D quadrupole device, collisional cooling is employed to limit the excursions of ions so as to form a tightly focused ion beam of diminished kinetic energy constrained close to the central axis. A focused beam of ions may be transmitted through a relatively small orifice from one section of an instrument to the next such that pumping requirements are reduced, and a focused ion beam can be accelerated with reduced ion loss. When the axial motion of a focused ion beam is arrested within a rod array such that the confined ions can be excited resonantly, a linear ion quadrupole trap is obtained. 

Figure 6.9 Sketches of the two commonly used types of RF quadrupole devices (a) the linear m/z filter (two-dimensional quadrupole field), and (b) the Paul ion trap (a three-dimensional quadrupole field). Reproduced from Dawson, Mass Spectrom. Revs. 5, 1 (1986), with permission of John Wiley Sons, Ltd. (c) Diagram of the cross-section of a linear quadrupole m/z analyzer at z = 0, showing the potentials applied to the two pairs of electrically connected rods the rod spacing is 2tq at the closest approach, as shown. Reproduced from Farmer, in Mass Spectrometry (CA McDowell, Ed), McGraw-Hill (1963), with permission of John Wiley Sons, Ltd.

It is important to distinguish the following somewhat similar symbols a non-italicized lower-case q denotes a linear quadrupole device operated using only RF fields, an upper-case Q denotes a similar device operated with both RF and DC fields, an italicised q denotes the trajectory stability parameter described above, and qj, represents the charge on an ion (in coulombs and containing the sign indicating the polarity). 

Linear quadrupole devices are also used in RF-only mode (U = 0), i.e. only the g -axis in Figure 6.10(b) is considered so that a range of m/z values will correspond to stable trajectories. Such devices (denoted as nonitali-cized q) are used as ion lenses (guides) to transport ions from one part of an apparatus to another (e.g., from ion source to analyzer) and as collision cells to effect collision induced dissociation an extensive review (Douglas 2005) covers the literature on aU these topics up to 2003. The emphasis is placed on linear quadrupole devices, essentially the same as those discussed above in the context of quadrupole mass filters. 

Finally in this section, it is noted that single quadrupole mass filters (as opposed to triple quadrupole instruments) are generally used in trace quantitative analyses only for GC-EI(CI)-MS methods for thermally stable analytes (e.g. pesticides in various matrices) in SIM mode. (It is interesting that GC/MS instruments still appear to account for a large fraction of mass spectrometer units sold worldwide.) The same linear quadrupole devices can also be operated as 2D ion trap mass spectrometers, discussed in Sections 6.4.5 and 6.4.6. 

An important quantity in all electric quadrupole devices (linear and three-dimensional) is the low mass cutoff, the minimum value of m z that has stable trajectories in the device under the stated operating conditions, i.e., the ion that has q just less than 0.908 (see discussion of Eignres 6.10 and 6.18). Then for this trap operated at Vg = 757 V, m/z) = 60.27/0.908 = 66.4, i.e., in practice the low mass cutoff for trapping of ions would be m z 67. The converse question, (i.e. what change would have to be made to the operating conditions to permit a specified low mass cutoff) amounts to determination of the required value of Vg since the RF frequency is considerably more difficult to adjust once fixed. Then the required value of Vg (in volts) for this particular hypothetical trap (i.e., rg, Zg and w all fixed) is given by Equation [6.36] as [(0.908/0.0796).( v z)iiiio] = H-41.(ffV z)imo-... 

Qn/r- notation conventionally used to denote a linear quadrupole device that can be used either as a conventional m/z filter or as a 2D trap mass spectrometer. 

Because tandem MS is in high demand, QqQ instruments have become popular. The three quadrupole devices aligned in series provide several scanning modes for various... 

For certain applications, compared to a MS combination, FT-IR instrumentation has the advantages that it is inherently more simple - no vacuum involved - and it is research-grade quality, whereas a MS is likely to be a basic quadrupole device. While the MS could be more sensitive and would possibly provide more information as an evolved gas analyser, it would also have a much greater need for maintenance than an FT-IR instrument. 

MS spectra with fragmentation of molecules require collision-activated dissociation (CAD) and triple quadrupole analyzers. In these instruments, the analysis is performed as follows the first quadrupole selects the interesting ion (parent ion), the second produces the fragments from the isolated ion, and the third quadrupole analyzes the fragmentation products (daughter ion spectmm). These steps (ion isolation, fragmentation, and analysis) can be repeated by addition of n quadrupole devices (multisector mass spectrometer) to allow multiple MS/MS experiments (MS") to be performed. 

The most common RF ion trap is a Paul trap [42], a 3-D quadrupole device in which ions are confined in a small volume of typically a few tens of millimeters [2] between a hyqterbolically shaped inner surface of a ring electrode and two end-cap electrodes, also of hyperbolic shape (Fig. 1). Elach end-cap electrode has a central hole for loading and ejection of irais. As these traps are compact, commercially available, and allow mass-selection of stored ions, they have become an increasingly popular technically simple solution for cryogenic ion spectroscopy. Paul traps have several drawbacks for cold-ion spectroscopy, however inefficient ion injection an intrinsically limited ability to cool ions low storage volume and inconvenient optical access to the ions by laser beams. 

The basic theory of operation of quadrupole devices was enunciated almost 100 years before the quadrupole ion trap was invented by Paul and Steinwedel ( ]. The pioneering work of the inventors was recognized by the award of a 1989 Nobel Prize in Physics to Wolfgang Paul [2]. The quadrapole ion trap functions both as an ion store for gaseous ions and as a mass spectrometer. The mass-selective axial instability mode of ion trap operation developed by Stafford et al. [3] made possible the commercialization of the quadrupole ion trap. It should be recalled that the trapping parameter is given by... 

There are detailed accounts by Dawson and Whetten [4] and by Dawson [5] of the early development of quadrupole devices a full account of ion trap theory by March et al. [6] reviews by Todd [7], Cooks et al. [8], Glish and McLuckey [9], and March [10] and three volumes entitled Practical Aspects of Ion Trap Mass Spectrometry [11] that contain accounts from 30 laboratories en gaged in ion trap research. 

---