Sampling discharge ionization source

McLuckey S. A., Glish K. G., Asano K. G. and Grant B. C. (1988) Atmospheric sampling glow discharge ionization source for the determination of trace organic compounds in ambient air, Anal Chem 60 2220—2227. 

The ion source is an essential component of all mass spectrometers where the ionization of a gaseous, liquid or solid sample takes place. In inorganic mass spectrometry, several ion sources, based on different evaporation and ionization processes, such as spark ion source, glow discharge ion source, laser ion source (non-resonant and resonant), secondary ion source, sputtered neutral ion source and inductively coupled plasma ion source, have been employed for a multitude of quite different application fields (see Chapter 9). 

Different from the El source, the APCI source contains a heated vaporizer which facilitates rapid desolvation/vaporization of the droplets by a very brief heating period up to 500 °C. Reagent ions obtained from the solvent vapour by a corona discharge ionize the vaporized sample molecules through an ion—molecule reaction. Chemical ionization of sample molecules is very efficient due to the numerous ion—molecule collisions (Figure 16.23). This technique leads to multicharged ions of type (M-F nH)"+ by proton transfer (in the positive mode). Unfortunately this technique is difficult to miniaturize as the required flow rate is higher than that of the electrospray and as such is convenient only for volatile and thermally stable molecules of mass less than 1000 Da. 

The following ionization sources are used mainly in inorganic (atomic) MS, where the elemental composition of the sample is desired. The glow discharge (GD) and spark sources are used for solid samples, while the inductively coupled plasma (ICP) is used for solutions. All three sources are also used as atomic emission spectroscopy sources they are described in more detail with diagrams in Chapter 7. 

Schematic of a typical ion mobility spectrometer is shown in Fig. 1. An ion mobility spectrometer consists of an ionization source, an ion mobility drift tube, a detector, and supporting electronics. The samples are usually ionized by radioactive Nickel-63, electrospray ionization source, corona discharge, or photoionization source. The ions travel through the drift tube while colliding with the medium molecules, usually air or nitrogen, at atmospheric pressure. The resulting ion velocity is proportional to the applied electric field and mobility of the ion.

Currently, there are four commercial companies offering handheld and portable gas phase time-of-flight ion mobility spectrometers, with drift tubes shorter than 5 cm Smiths Detection, Bruker-Daltonics, GE-Interlogix, and G.A.S. Gesellschaft fiir analytische Sensorsysteme mbH. Table 1 presents their respective instruments and their major applications. These instruments generally use an applied electric field of around 250 Vcm , and they operate at either ambient or elevated temperature and at atmospheric pressure. The samples are ionized by radioactive nickel-63 source mostly but also by corona discharge and photoionization source. 

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