Discharge ionization process

Only a low net ionization in the discharge is produced. The ionization efficiency is estimated to be 1 % or less. However, glow discharge produces an atomic vapour representative of the cathode constituents and the discharge ionization processes are also relatively non-selective. Also, because most elements are sputtered and ionized with almost the same efficiency in the source, quantitative analysis without a standard is possible. 

Figure 1 Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

Typical ion sources employ a noble gas (usually Ar). The ionization process works either by electron impact or within a plasma created by a discharge the ions are then extracted from the region in which they are created. The ions are then accelerated and focused with two or more electrostatic lenses. These ion guns are normally operated to produce ions of 0.5-10 keV energy at currents between 1 and 10 pA (or, for a duoplasmatron, up to 20 pA). The chosen spot size varies between 100 pm and 5 mm in diameter. 

APCI and APPI are probably the ionization methods of choice for unconjugated, underivatized steroids. They are related techniques in APCI a corona discharge initiates the ionization process, while this is initiated by ultraviolet light in APPI. There are probably no absolute rules regarding sensitivity, it being very much dependent on the instrument design. A particular source on a particular instrument may be... 

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). 

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