Scanning mass analyzers

Several dil fercti I types of spectra can be obtained from the MS/MS experiment. Tirsl, the producl-ion spec-iriiiu can be obtained by scanning mass analyzer 2. while mass analyzer 1 is held constant, acting as a mass selector to seleel one precursor ion. [ igure 20-22 shows product-ion spectra for dibutylphthalaie and sulfamethazine. Both compounds produce molecular ions with miz values of 270, However. Ihe produet-ion spectra of the two compounds are very different,... 

By scanning both analyzers simultaneously with an offset in mass between them, a neutral loss spccirum can be obtained. This gives the identity of those precursor ions that undergo the same loss such as the loss of a HjO or CO neutral. Finally, by scanning mass analyzer 1 and obtaining the product-ion spectrum for each selected precursor ion, a complete three-dimensioruil MS/MS spectrum can be obtained. 

Since the width of GC peaks is on the order of a few seconds, a fast-scanning mass analyzer is necessary. The combination of gas chromatography and mass spectrometry (usually given the acronym GCMS) is perhaps the most versatile and sensitive tool in mixture analysis and is often used in petroleum, environmental, and biochemical research. 

Besides the ability to count ions, the EM detector responds to changes in the ion beam intensity instantaneously (on a nanosecond timescale), whereas the high feedback resistance associated with FC detectors results in a long time constant for the measurement (typically 1 to 10 seconds). Fast scanning mass analyzers such as QMF, ITMS, and TOF analyzers almost always use EM type detectors because the FC response is too slow. 

Figure 4.2. Pictorial representation of four scan modes of tand mass spectrometry. AD refers to ion activation and dissociation, and the filled and open circles stand for fixed and scanning mass analyzers, respectively.

The combination of electrospray ionization with magnetic sector mass spectrometers proved to be less technically challenging than the interfacing of MALDI. Electrospray ionization produces ions continuously, and thus it combines easily to scanning mass analyzers such as the magnetic sector. Thus, no modifications of the mass analyzer or detector are required for electrospray ionization. However, the ions produced by electrospray must be accelerated to kilo-electron-volt energies, and this requires some innovation with the design of the ion source. One solution is to float the entire electrospray source and... 

The most common modes of operation for ms/ms systems include daughter scan, parent ion scan, neutral loss scan, and selected reaction monitoring. The mode chosen depends on the information required. Stmctural identification is generally obtained using daughter or parent ion scan. The mass analyzers commonly used in tandem systems include quadmpole, magnetic-sector, electric-sector, time-of-flight, and ion cyclotron resonance. Some instmments add a third analyzer such as the triple quadmpole ms (27). 

Rapid scanning mass spectrometers providing unit resolution are routinely used as chroaatographic detectors. Ion separation is accomplished using either a magnetic sector, quadrupole filter or ion trap device. Ions can also be separated by time-of-flight or ion cyclotron resonance mass analyzers but these devices are not widely used with chromatograidiic inlets and will not be discussed here [20]. 

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