Pulsed extraction continuous ionization

Chapter 7 Pulsed Extraction, Continuous Ionization, and Ion Storage Instruments... 

A 30-lb field-portable quadrupole ion trap TOF (QitTof) mass spectrometer with an atmospheric photoionization source was constructed [17]. The photoionization source has the ability to choose a narrow-band ionization energy that is sufficiently high to ionize and detect most compounds of interest but low enough to avoid ionization of most common air constituents, such as N2, 02, H20, C02, CO, Ar, etc. Fragmentation is minimal because ionization occurs just above the ionization thresholds, with very little excess energy. The ion trap stores ions from a continuous ionization source followed by pulsed extraction into the TOF mass analyzer. 

A major advantage of the TOF mass spectrometer is its fast response time and its applicability to ionization methods that produce ions in pulses. As discussed earlier, because all ions follow the same path, all ions need to leave the ion source at the same time if there is to be no overlap between m/z values at the detector. In turn, if ions are produced continuously as in a typical electron ionization source, then samples of these ions must be utihzed in pulses by switching the ion extraction field on and off very quickly (Figure 26.4). 

On the other hand, there are some ionization techniques that are very useful, particularly at very high mass, but produce ions only in pulses. For these sources, the ion extraction field can be left on continuously. Two prominent examples are Californium radionuclide and laser desorption ionization. In the former, nuclear disintegration occurs within a very short time frame to give a... 

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