Spark ionization (also known as spark source ionization) 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 introduced by Dempster in 1936 for analysis of metal isotopes. Metals were a class of material that could not be previously ionized by thermal ionization (the method formerly used for ionizing solid sample). Spark ion sources were not commercially produced until after 1954 when Hannay demonstrated its capability for analysis of trace impurities (sub-part per million detection sensitivity) in semiconducting materials. The prototype spark source instrument was the MS7 mass spectrometer produced by Metropolitan-Vickers Electrical Company, Ltd. in 1959. Commercial production of spark source instruments continued throughout the 50s, 60s, and 70s, but they were phased out when other trace element detection techniques with improved resolution and accuracy were invented (circa 1960s). Successors of the spark ion source for trace element analysis are the laser ion source, glow discharge ion source, and inductively coupled plasma ion source. Today, very few laboratories use spark ionization worldwide.
How it works
The spark ion source consists of a vacuum chamber containing the electrodes, which is called the spark housing. The tips of the electrodes are composed of or containing the sample and are electrically connected to the power supply. Extraction electrodes create an electric field that accelerate the generated ions through the exit slit.
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. It was the rf spark source that gained traction commercially due to its ability to analyze both conducting and non-conducting samples.
Low-voltage direct current arc source
In this mode of ionization, a high voltage is applied to the two conducting electrodes to initiate the spark, followed by application of a low-voltage direct current to maintain an arc between the spark gap. The duration of the arc is usually only a few hundred microseconds to prevent overheating of the electrodes, and it repeated 50-100 times per second. This method can only be used to ionize conducting samples, e.g. metals.
High-voltage radio-frequency spark source
Typically, samples are physically incorporated into two conductive electrodes between which an intermittent (1 MHz) high-voltage (50-100 kV using a Tesla transformer) electric spark is produced, ionizing the material at the tips of the pin-shaped electrodes. When the pulsed current is applied to the electrodes under ultra-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. This mode of ionization can be used to ionize conducting, semi-conducting, and non-conducting samples.
If conducting or semi-conducting materials are being analyzed, the sample may serve as the electrodes. Non-conductive samples are first mixed with a conducting powder (usually high purity graphite), homogenized, and then formed into electrodes.
Spark source mass spectrometry (SSMS)
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
- H. E. Beske, A. Hurrle and K. P. Jochum (1981). "Part I. Principles of spark source mass spectrometry (SSMS)". Fresenius' Journal of Analytical Chemistry 309 (4): 258–261. doi:10.1007/BF00488596.
- Kraj, Agnieszka (2009). Mass Spectrometry: Instrumentation, Interpretation, and Applications. http://onlinelibrary.wiley.com/book/10.1002/9780470395813: John Wiley & Sons. pp. 19–20. ISBN 9780470395806.
- Adams, F.; Vertes, A. (1990). "Inorganic mass spectrometry of solid samples" (PDF). Fresenius' Journaly of Analytical Chemistry. Retrieved 23 February 2015.