Sampling from Electrical Discharges

Note Normally, commercial MALDI instruments tolerate such a sample supply however, great care is recommended when performing this sort of experiments in order not to loose the sample inside the ion source or to damage the instrument s electronics from electric discharges. 

The understanding of the reactions in the plasma [64] is largely impeded due to the complex nature of plasmas. The most difficult task is to determine the relative importance of ions and neutral species in the reactions of plasma. To explain the process many different models have been postulated from the analysis of chemical products observed in the plasma. Models for plasma reactions have invoked ion-molecule reactions, radical-radical reactions, and radical-molecule reaction. However, the extent of their participation is still unclear. The mechanism of polymer production in gas discharge has also been the subject of some controversy [65]. Many reports of mass spectrometric sampling of electric discharges have appeared. Few attempts [57,66] have been made to obtain correlations between the ions and neutral species. 

Safety notice Unconventional sample supply techniques on commercial MALDI instruments require great care. Possible pitfalls include i) blocking of the vacuum lock due to unacceptable thickness of the target, ii) loss of the sample inside the ion source or inside the vacuum lock. Hi) damage to the instrument from electric discharges. 

To increase the number of ions, a plasma or corona discharge is produced in the mist issuing from the capillary. The electrical discharge induces more ionization in the neutrals accompanying the few thermospray ions. This enhancement increases the ionization of sample molecules and makes the technique much more sensitive to distinguish it from simple thermospray, it is called plasmaspray. 

Spark (source) ionization. Occurs when a solid sample is vaporized and partially ionized by an intermittent electric discharge. Further ionization occurs in the discharge when gaseous atoms and small molecular moieties interact with energetic electrons in the intermittent discharge. It is recommended that the word source be dropped from this term. 

The essential advantage of shock tubes over electric discharge devices is the capability of producing a homogeneous gas sample (HGS) with enthalpy and pressure which can be dependably calculated from the measured shock velocity and the conservation laws. 

Before 1800, electricity meant static electricity, generated by friction. It could be stored in jar-like condensers, and a number of these condensers could be discharged simultaneously, like an artillery battery, producing a very hefty shock—up to half a million volts. The sparks from such discharges could ignite gas mixtures and decompose relatively small samples of some substances. Then in 1800, Volta published a description of a new piece of apparatus, the electric pile. It was called a pile because it consisted literally of a pile of alternating disks of metals and blotting paper moistened with a salt solution. It was... 

Four techniques based on mass spectrometry are widely used for multi-elemental trace analysis of inorganic compounds in a wide range of sample types. These techniques are thermal ionization (TI), spark source (SS), glow discharge (GD) and inductively coupled plasma (ICP) mass spectrometry. In these techniques, atomization and ionization of the analysed sample are accomplished by volatilization from a heated surface, attack by electrical discharge, rare-gas ion sputtering and vaporization in a hot flame produced by inductive coupling. 

In spark sources, electrical discharges are used to desorb and ionize the analytes from solid samples [98]. As shown in Figure 1.42, this source consists of a vacuum chamber in which two electrodes are mounted. A pulsed 1 MHz radio-frequency (RF) voltage of several kilovolts is applied in short pulses across a small gap between these two electrodes and produces electrical discharges. If the sample is a metal it can serve as one of the two electrodes, otherwise it can be mixed with graphite and placed in a cup-shaped electrode. 

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