Triple quadrupole instruments

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. 

Relationship Between Velocities of Precursor and Product Ions in Tandem Mass Spectrometry 

The archetypal ion fragmentation reaction is +G +, where conservation of charge requires p = f+g 

Now each of the u values can be positive or negative, but they must be real-valued so their squares are necessarily positive but so are each of mp and mG. So the energy conservation equation can be satisfied only if either mp = 0 = mg (a trivial case of no fragmentation) or if Upj = 0 = Uqj for each of J = X, Y, Z. Converting this nontrivial condition back to the coordinate system fixed in the laboratory gives the final condition imposed by the conservation eqnations  

That is to say, for the special case where the F values are negligible, the two products (Ff+ and formed in the reaction continue with the same velocity components as those of the precursor ion. It can be shown (e.g., Boyd 1994) that when the F valnes are significant, the same result holds to an excellent approximation if now the Vjj values are taken to be the median values of the resulting velocity distributions. Note also that if PP+ is induced to dissociate by collision with a gas molecule, the relevant value of Vpz is the value after the coUision but te/ore the fragmentation event. Fnrther, the restriction to zero external fields strictly applies only to velocity components in directions in which no fields are applied, e.g., the axial Z-direction in an RF-only quadmpole collision cell, but in that case obvionsly not to the radial components (X and Y, see Equations [6.12]). 

Recently, the development of quadrupole instruments has given rise to the introduction of another class of instrument which is more flexible and less costly. These instruments are termed triple quadrupole (QIQIQ), in which the collision cell is located in the second quadrupole, in which no mass separation occurs. These instruments were introduced by Yost, Enke and co-workers [180]. 

This method of parent detection leading to the same neutral fragments with a QIQIQ instrument is interesting, since the other ions remain transparent to the fast analysis and a very good specificity is obtained in comparison to GC/MS. 

Finally, this system was used to study mixtures of polyaromatic compounds. A Townsend Cl discharge source with as reagent gas improves the production of [M-H-fN02] ions, which fragment in the second quadrupole with elevated efficiency to eliminate, among others, NOj radical. Thus, all the molecular ions of the polyaromatic compounds were obtained by studying the NO2 (46 amu) constant neutral spectrum. 

Zakett and Cooks [186] studied mixtures obtained from SRC II refined coal by analysing constant neutral fragment spectra under collision conditions. To increase 

A simplified multiquadrupole type of instrument was introduced by Siegel [185]. This is a double quadrupole analyzer which analyzes primary ion (or precursors) and fragment ions. In place of the second normal quadrupole (for QIQIQ instruments), a ferrite ceramic collision cell is installed which overlaps with the two quadrupoles. The first and second quadrupoles analyzed, respectively, the primary and fragment ions. The collisions occur in the intermediate region. 

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An example of linked scanning on a triple quadrupole instrument. A normal ion spectrum of all the ions in the ion source is obtained with no collision gas in Q2 all ions scanned by Q1 are simultaneously scanned by Q3 to give a total mass spectrum (a). With a collision gas in Q2 and with Q1 set to pass only m+ ions in this example, fragment ions (f, fj ) are produced and detected by Q3 to give the spectrum (b). This CID spectrum indicates that both f, and fj are formed directly from m+. 

There are a variety of possible linked scanning methods, but only those in more frequent use are discussed here. They differ from the linked scanning methods used in triple quadrupole instruments and ion traps in that two of the three fields (V, E, and B) are scanned simultaneously and automatically under computer control. The most common methods are listed in Table 34.1, which also defines the type of scanning with regard to precursor and product ions. 

Linked scanning is particularly easy with a triple quadrupole instrument. 

Triple quadrupole instruments can be used to detect metastable ions or can be used for linked scanning to obtain information about molecular structure. 

MS/MS Instrumentation As was noted previously, a variety of instrument types can perform MS/MS experiments, but because of their popularity, we only discuss MS/MS experiments using triple quadrupole instruments. The principles can be applied to other types of instrumentation. 

The triple quadrupole instrument consists of two mass analyzers separated by an rf-only quadrupole. In the rf-only mode, ions of all masses are... 

At the present time, LC/MS/MS with triple-quadrupole instruments is the analytical method of choice for the determination of residues of sulfonylurea herbicides. We can expect to see improved triple-quadrupole instrumentation become more available and affordable as time passes, so that more analytical laboratories will have this capability. Time-of-flight (TOP) instrumentation may also play an increasingly important role in sulfonylurea analysis. Even though the metabolites are innocuous, stricter regulatory requirements may mandate that they be monitored, and LC/MS/MS is the method of choice for these compounds also. 

Chemical ionization can be used at ambient pressures. Chemical ionization was used in tandem MS, using a triple quadrupole instrument, to characterize the antipsychotic agent 2-amino-N(4-(4-(l,2-benzisothiazol-3-yl)-l-piperazinyl)butyl)benzamide from human plasma.5... 

Alternatively, the translational energy threshold for endothermic proton transfer from MH+ to R can be measured using a flowing afterglow triple quadrupole instrument.127 These data define the proton affinity of M, relative to that of R. Thus, the PA of cyclopropenylidene was found to exceed that of ammonia by 23.3 1.8 kcal/mol (Table 6).128 In order to obtain absolute proton affinities, the enthalpies of formation of both the base and the conjugate acid must be known from other measurements (Eq. 9). Numerous reference compounds with known absolute PA are available.124... 

Another type of commercially available triple-quadrupole known as the TSQ Quantum was recently reported100 to achieve significantly better resolution than a traditional triple quadrupole instrument without any significant loss of transmission. Based on the improved inherent resolution, assay development of an analyte on a classic TSQ that requires extensive sample preparation... 

Fig. 11.16. Representation of three tandem mass spectrometry (MS/MS) scan modes illustrated for a triple quadrupole instrument configuration. The top panel shows the attributes of the popular and prevalent product ion CID experiment. The first mass filter is held at a constant m/z value transmitting only ions of a single mlz value into the collision region. Conversion of a portion of translational energy into internal energy in the collision event results in excitation of the mass-selected ions, followed by unimolecular dissociation. The spectrum of product ions is recorded by scanning the second mass filter (commonly referred to as Q3 ). The center panel illustrates the precursor ion CID experiment. Ions of all mlz values are transmitted sequentially into the collision region as the first analyzer (Ql) is scanned. Only dissociation processes that generate product ions of a specific mlz ratio are transmitted by Q3 to the detector. The lower panel shows the constant neutral loss CID experiment. Both mass analyzers are scanned simultaneously, at the same rate, and at a constant mlz offset. The mlz offset is selected on the basis of known neutral elimination products (e.g., H20, NH3, CH3COOH, etc.) that may be particularly diagnostic of one or more compound classes that may be present in a sample mixture. The utility of the two compound class-specific scans (precursor ion and neutral loss) is illustrated in Fig. 11.17.

MS—MS was applied to characterise alkyl quats. The blend of cetyl-dimethyl-ammonium bromide was examined by FIA—APCI—MS— MS(+). Besides the nitrogen-containing fragment at m/z 46, mainly alkyl chain fragments from the C46 chain could be observed at m/z 43, 57, 71, and 85. The APCI(-I-) product ion spectrum is presented in Fig. 2.12.12. The inset contains the pattern of the CID fragmentation behaviour observed on a triple quadrupole instrument [39]. 

Scan Modes for Tandem MS with Triple Quadrupole Instruments... 

In triple quadrupole instruments Qi and Q3 are operated independently as MSI and MS2, respectively, making MS/MS a straightforward matter. The experimental setups for product ion, precursor ion, and neutral loss scanning are summarized in Table 4.3. 

Classically, high-resolution work is the domain of double-focusing magnetic sector instruments. More recently, TOP and to a certain degree triple quadrupole instruments are also capable of resolutions up to about 20,000. However, the rapid development of FT-ICR instruments has established those as the systems of choice if ultrahigh-resolution (>100,000) and highest mass accuracy (1 ppm) are required (Chap. 4.6). 

Fig. 1.6 (A) Electron impact spectrum obtained on a single quadrupole mass spectrometer of a compound with Mr = 355. (B) Product ion spectrum after atmospheric pressure ionization obtained on a triple quadrupole instrument. Chemical ionization and atmospheric pressure ionization give in both cases protonated precursor ions, which is ideal for tandem mass spectrometry.

A triple quadrupole instrument (QqQ) is a combination of two mass quadrupole mass filters (tandem mass spectrometry) separated by a collision cell which is also a quadrupole operating in RE-only mode (Pig. 1.18). A common nomencla-... 

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.

LC-MS/MS has dramatically changed the way bionalysis is conducted. Accurate and precise quantitation in the pg ml scale is nowadays possible however one has to be aware of certain issues which are specific to mass spectrometric detection such as matrix effects and metabolite crosstalk. With the current growing interest in the analysis of endogenous biomarkers in biological matrices, quantitative bioanalysis with MS has certainly the potential to contribute further in this field with the development of multicomponent assays. Modern triple quadrupole instruments have the feature to use very short dwell times (5-10 ms), allowing the simultaneous determination of more than 100 analytes within the timescale of an HPLC peak. Due to the selectivity of the MS detection the various analytes... 

Figure 16.23—Triple quadrupote MS MS instrument. In the triple qnadrupole arrangement, the middle quadrupole is used as a collision chamber. It is operated in the radiofrequency voltage mode only, where it will transmit all masses. A gas pressure introduced in the second quadrupole is responsible for collision activation. Triple quadrupole instruments can conduct all three types of MS — MS analysis described above.

Figure 3.2. Schematic of ion path for hybrid QQQ/LIT (QTRAP) instrument. The ion path has three quadrupoles (Q1, Q2, and Q3) like a standard triple-quadrupole instrument, but Q3 also has ion trap capabilities.

Several scan modes are unique to the triple-quadrupole instrument, and most of these modes are superior in duty cycle versus an ion trap, Fourier transform (FT), or time-of-flight (TOF) mass spectrometers. Different elements of the triple-quadrupole perform different operations for each scan mode. These scan modes, each of which will be described in detail, are single-reaction monitoring (SRM) or multiple-reaction monitoring (MRM), precursor ion scanning (PIS), and constant-neutral-loss scanning (NLS). These scan modes and applications for structural elucidation have been described in detail (Yost and Enke, 1978, 1979). 

As mentioned in the previous section, triple-quadrupole instruments are very good at finding low levels and structurally related compounds in the presence of biological matrices as well as being the gold standard technique for quantitation. Ion trap mass spectrometers, on the other hand, have the capabilities to obtain high-sensitivity full-scan MS and MS/MS spectra therefore, they are widely used for qualitative analysis, such as structural elucidation and unknown identification. For complete metabolite identification, it is important to have both the sensitivity and selectivity of triple-quadrupole instruments and the full-scan data quality of ion traps. 

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