Spark ionization (also known as spark source ionization (deprecated)) is a method used to produce gas phase ions from a solid sample. The prepared solid sample is vaporized and partially ionized by an intermittent discharge or spark. This technique is primarily used in the field of mass spectrometry. When incorporated with a mass spectrometer the complete instrument is referred to as a spark ionization mass spectrometer or as a spark source mass spectrometer (SSMS).
Spark ionizaton was developed by Dempster in 1936 for analysis of trace elements. It never gained popularity due to low sample throughput rate and high equipment costs. Development of other trace element detection techniques with improved resolution and accuracy also led to spark ionization becoming obsolete.
For spark ionization, there exist two ion sources: the low-voltage direct-current (DC) arc source and the high-voltage radio-frequency (rf) spark source. The rf spark source is the ion source discussed here since it is the one that gained traction commercially. Typically, samples are physically incorporated into two conductive electrodes (usually either carbon or silver) between which a high-voltage electric spark is produced, ionizing the material at the tips of the pin-shaped electrodes. If conductive materials are being analyzed, the sample may serve as an electrode. Non-conductive samples are first mixed with graphite, homogenized, and then formed into electrodes. When the pulsed current is applied to the electrodes under high vacuum, a spark discharge plasma occurs in the spark gap in which ions are generated via electron impact. Within the discharge plasma, the sample evaporates, atomizes, and ionizes. The total ion current may be optimized by adjusting the distance between the electrodes.
The spark source creates ions with a wide energy spread (2-3 kV), which necessitates a double focusing mass analyzer. Mass analyzers are typically Mattauch-Herzog geometry, which achieve velocity and directional focusing onto a plane with either photosensitive plates for ion detection or linear channeltron detector arrays. Advantages of SSMS include high sensitivity with detection limits in the ppb range and simple sample preparation. SSMS yields more extensive fragmentation than electron ionization spectra; however, poor resolution and accuracy make it impossible to deduce an unambiguous structure from spark ionization spectra.
- IUPAC gold book definition
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