Quadrupole instrument

In a normal quadrupole instrument, the field-free regions are very short. Ions formed in region t will be transmitted by the quadrupole as normal ions. In region 2 there is no differentiation between metastable and normal ions. 

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. 

Most ion sources produce singly charged ions, i.e., z = I and the ranges shown here apply to such ions. Matrix assisted methods may produce ions with r > 1. When = 1, m/z. = m, viz., mass can be measured directly. An ES ion source produces ions with z > 1 and this effectively extends the mass ranges that can be examined. For example, with z = 1 and m = 10,000, the m/z value is 10,000 and this would be beyond tbe capabilities of a quadrupole instrument. 

Electrospray can be used with sector, time-of-flight, and quadrupole instruments. The technique has been used extensively to couple liquid chromatographs to mass spectrometers. 

By using a sampling device, the ions are siphoned from the end of the plasma flame and led into an ion mass analyzer, such as a quadrupole instrument, where the abundances of the ions and their m/z values are recorded. 

A discharge ignited in argon and coupled inductively to an external high-frequency electromagnetic field produces a plasma of ions, neutrals, and electrons with a temperature of about 7000 to 10,000°C. Samples introduced into the plasma under these extremely energetic conditions are fragmented into atoms and ions of their constituent elements. These ions are examined by a mass analyzer, frequently a quadrupole instrument. 

Metastable ions cannot be detected with simple quadrupole instruments. 

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. 

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). 

SiiH2 are completely separated from the peak. Quadrupole instruments are not usually capable of such high mass resolution. 

The choice of mass spectrometer for a particular analysis depends on the namre of the sample and the desired results. For low detection limits, high mass resolution, or stigmatic imaging, a magnetic sector-based instrument should be used. The analysis of dielectric materials (in many cases) or a need for ultrahigh depth resolution requires the use of a quadrupole instrument. 

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... 

A slow deterioration that is not remedied by cleaning the ion source and not related to the multiplier gain is an indication that the analyzer needs to be cleaned. For quadrupole instruments, cleaning the prefilter is often sufficient to restore performance. (A prefilter is not available on all quadrupole instruments.)... 

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). 

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... 

The most important gas phase analytical techniques are mass spectrometry and gas phase chromatography. If the total gas pressure is <10-4-10-8 Torr, a mass spectrometer such an omegatron or a quadrupole instrument may be inserted into the reactant volume. However, in most cases, the pressure is in excess of this, and gas must be delivered to a mass spectrometer via a leak, such as a Metrosil pellet or a capillary constriction, situated as closely as possible to the reaction volume. 

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... 

The layout of an ICP-MS is shown schematically in Figure 8.17 and comprises three essential parts the ICP torch, the interface and the mass spectrometer. The ICP torch differs little from that discussed earlier and the mass spectrometer is very similar to those used for organic mass spectrometry and discussed in Chapter 9. Typically a quadrupole instrument would be used. The construction of the interface is shown in Figure 8.18 and is based on the use of a pair of water-cooled cones which divert a portion of the sample stream into the ion optics of the mass spectrometer whence the mass spectrum is produced by standard mass spectrometer operation. Some modern instruments also incorporate a so-... 

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... 

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