Instrumentation mass analyzers

Positive ions are obtained from a sample by placing it in contact with the filament, which can be done by directing a gas or vapor over the hot filament but usually the sample is placed directly onto a cold filament, which is then inserted into the instrument and heated. The positive ions are accelerated from the filament by a negative electrode and then passed into a mass analyzer, where their m/z values are measured (Figure 7.1). The use of a suppressor grid in the ion source assembly reduces background ion effects to a very low level. Many types of mass analyzer could be used, but since very high resolutions are normally not needed and the masses involved are quite low, the mass analyzer can be a simple quadrupole. 

The Z-spray inlet causes ions and neutrals to follow different paths after they have been formed from the electrically charged spray produced from a narrow inlet tube. The ions can be drawn into a mass analyzer after most of the solvent has evaporated away. The inlet derives its name from the Z-shaped trajectory taken by the ions, which ensures that there is little buildup of products on the narrow skimmer entrance into the mass spectrometer analyzer region. Consequently, in contrast to a conventional electrospray source, the skimmer does not need to be cleaned frequently and the sensitivity and performance of the instrument remain constant for long periods of time. 

Commercial mass analyzers are based almost entirely on quadrupoles, magnetic sectors (with or without an added electric sector for high-resolution work), and time-of-flight (TOE) configurations or a combination of these. There are also ion traps and ion cyclotron resonance instruments. These are discussed as single use and combined (hybrid) use. 

The choice of a mass spectrometer to fulfill any particular task must take into account the nature of the substances to be examined, the degree of separation required for mixtures, the types of ion source and inlet systems, and the types of mass analyzer. Once these individual requirements have been defined, it is much easier to discriminate among the numerous commercial instruments that are available. Once suitable mass spectrometers have been identified, it is then often a case of balancing capital and running costs, reUability, ea.se of routine use, after-sales service, and manufacturer reputation. 

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. 

Once inside the hot plasma, which is at a temperature of about 8000 K and contains large numbers of energetic electrons and ions, the sample molecules are broken down into their constituent elements, which appear as ions. The ions are transported into a mass analyzer such as a quadrupole or a time-of-flight instrument for measurement of m/z values and ion abundances. 

The instrumentation for SSIMS can be divided into two parts (a) the primary ion source in which the primary ions are generated, transported, and focused towards the sample and (b) the mass analyzer in which sputtered secondary ions are extracted, mass separated, and detected. 

The basic instrumental set-up for dynamic SIMS is the same as for SSIMS (Sect. 3.1.2). Depending on the intensity, beam diameter, and ion species needed, dif ferent ion sources are used. Several mass analyzers with different characteristics enable a broad field of applications. 

Apart from the quadrupole and TOP analyzers described in Sect. 3.2.2, the most important types of mass analyzer used in common dynamic SIMS instruments employ a magnetic-sector field. 

Instruments are available that can perform MS/MS type experiments using a single analyzer. These instruments trap and manipulate ions in a trapping cell, which also serves as the mass analyzer. The ion trap and fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers are examples. 

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... 

In the case of carbamate insecticides, both ESI and APCI can be used. However, in this study, the sensitivity of APCI was 3-5-fold less than that of ESI. In this case, the Z-spray configuration was used with APCI, which gives a lower efficiency of ions reaching the mass analyzer than is achieved with other instrumental configurations. 

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