Quadrupole MS and Mass Selection

The first research instruments were based on the ungainly magnetic sector mass spectrometers that used very large permanent magnets to establish the electromagnetic field and had very slow response times. The accelerated ions of different masses were detected at different impact points on the detector plate and mass ratios were measured (Fig. 15.2). 

Another commonly used type of mass spectrometer is the tandem mass unit, also referred to as an MS/MS (Fig. 15.4) or a triple quad mass spectrometer. Originally, this was made up of two or three mass spectrometers used in series. One MS is used to separated ions, the middle unit is used as a collision chamber in which selected ions are allowed to impact heavy gas molecules and fragment, and the last MS is used to separate and measure the fragment ions. In one 

The quadrupole MS detector was the first, and is still the most common, detector used for LC/MS, but a number of other mass spectrometers have been adapted to this application. Both three-dimensional spherical (ITD) and linear (LIT) ion trap detectors offer tremendous potential for general, inexpensive LC/MS systems. They both offer the ability to be used as either a mass spectral detector or as a MS/MS detector. The 3D ITD (Fig. 15.5) allows ions to be trapped in the ion trap where they can be fragmented by heavy gas collision and the fragments released by scanning the dc/RF frequency of the trap. 

The linear ion trap (Fig. 15.6) is essentially a quadrupole detector with an electrically controlled ion lens at either end. It can trap a much larger volume of ions in its trap, allowing much higher sensitivity in fragment ion detection for trace analysis as well as MSn-type of experiments in which fragmentation ions can be trapped and further fragmented to aid in structure studies. 

Time-of-flight (TOF) MS detectors (Fig. 15.7) are commonly used in pro-teomics studies of proteins and protein fragments because this type of detector can handle and analyze very large molecular and fragmentation ions. Fourier transform mass spectrometers (FTMS) are being incorporated into commercial LC/MS systems and offer the advantage of being nondestructive detectors that can trap and repeatedly analyze the same sample in order 

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