Linear Quadrupole Instruments

Linear quadrupole instruments are widely spread. Recent developments in the technology now make it possible to work simultaneously with fullscan and selected ion monitoring (SIM) modes in a single run. In GC tandem MS, a first quadmpole acts as a mass selective filter and the second quadrupole is used as the collision cell with addition of a collision gas such as helium or argon. In a third quadrupole the full mode is performed to obtain the full mass spectrum of the product ions. 

TOF instruments come in a variety of sizes and complexity, typically correlated with cost, ranging from the bench-top linear instruments to the several-hundred-kilogram TOF-TOFs. Generally TOF instruments are somewhat larger than quadrupole instruments but smaller than sector and FTICR instruments. 

With the advent of linear quadrupole analyzers the full width at half maximum (FWHM) definition of resolution became widespread especially among instruments manufacturers. It is also commonly used for time-of-flight and quadrupole ion trap mass analyzers. With Gaussian peak shapes, the ratio of / fwhm to Rio% is 1.8. The practical consequences of resolution for a pair of peaks at different m/z are illustrated below (Fig. 3.17). 

Geometries composing of an oaTOF as MS2 bear the advantage that advanced TOFs offer accurate mass measurements close the the accuracy of magnetic sector instruments. Currently, QqTOF systems can be regarded as the commercially most successful hybrid. While the linear quadrupole serves as MSI in MS/MS experiments, it is operated in RF-only mode when tandem MS is not intended, because... 

The main advantages that ICP-MS has over other techniques are its low detection limits, in the 1-10 pg ml i range for quadrupole instruments (Fig. 5.9), large linear dynamic range and rapid multi-element capability. However, ICP-MS also suffers from a number of interferences. 

Essentially, the ion storage trap is a spherical configuration of the linear quadrupole mass filter. The operations, however, differ in that the linear filter passes the sorted ions directly through to the detector, whereas the ion trap retains the unsorted ions temporarily within the trap. They are then released to the detector sequentially by scanning the electric field. These instruments are compact (benchtop), relatively inexpensive, convenient to use, and very sensitive. They also provide an inexpensive method to carry out GC/MS/MS experiments (Section 2.2.7) (GC is gas chromatography). 

Collision induced reactions are also applied in ICP-MS with an ion trap for high mass resolution measurements. Two instrument configurations of the plasma source ion trap (PSIT) are shown in Figure 5.8 a and b, respectively. PSIT I utilizes a linear quadrupole for ion transport and mass selection (Figure 5.8 a) while the PSIT II applies direct injection of ions into the trap (without... 

Once a targeted list is assembled, the appropriate LC-MS instrument can be set up to acquire both MS and MS/MS data (or MS" data for traps) in an automated fashion. The MS/MS acquisitions would only be triggered by detection of a targeted precursor ion (from the list) at a minimum specified intensity. Linear ion trap quadrupole instruments are increasingly popular for this type of work (Hopfgartner and Zell, 2005) and are discussed in Chapter 3 of this book. 

A quadmpole, used as part of a mass spectrometer, can be found in either a single- or triple-element linear configuration. Single-quadrupole instruments are of limited use for metabolite identification because of relatively poor sensitivity, only moderate resolving power, and the inability to perform MS/MS experiments for structural... 

Linear quadrupole mass analyzers consist of four parallel metal rods aligned along the instrument s z-axis (Fig. 6). 

A 10% aliquot of the digestion mixture was analyzed on a Vydac C18 250 im ID fu silica cap aiy column connected on-line to ESI-MS (4). A linear gradient of 2% B to %% B in 45 minutes using solvent A (0.1% TFA) and solvent B (90% acetonitrile, 0.07% TFA) was used with a flow rate of 2 jul/min. Sample elution was monitored by UV detection at 200 nm. Mass spectra were recorded in the positive ion mode using a TSQ-700 triple quadrupole instrument (Fiimigan-MAT, San Jose, CA) with an electrospray ion source opo ating at atmospheric pressure. Scans were continuously acquired every three seconds between la/z 500 and 2(XX) in the centroid mode. 

An impressive diversity of mass analyzers are utilized in modem analytical instrumentation. An overview of the common mass spectrometer analyzers follows, with particular emphasis on linear quadrupole mass analyzers, quadrupole ion traps, and time-of-flight mass analyzers, as they arguably constitute the quantitative MS workhorses of the pharmaceutical industry. The description of alternate analyzer systems should provide a framework in which the utility of these three particular systems provides the most cost-effective analytical mass spectrometer systems for pharmaceutical analysis. 

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