Load coil

These direct-insertion devices are often incorporated within an autosampling device that not only loads sample consecutively but also places the sample carefully into the flame. Usually, the sample on its electrode is first placed just below the load coil of the plasma torch, where it remains for a short time to allow conditions in the plasma to restabilize. The sample is then moved into the base of the flame. Either this last movement can be made quickly so sample evaporation occurs rapidly, or it can be made slowly to allow differential evaporation of components of a sample over a longer period of time. The positioning of the sample in the flame, its rate of introduction, and the length of time in the flame are all important criteria for obtaining reproducible results. 

Heaviside made a significant contribution to electrical communications when he advocated the introduction of additional inductance in long-distance telephony cables although there was then no practical means to add it. His idea was eventually patented in 1904 by Michael Campbell of AT T after Heaviside and George Pupin of Columbia University had shown it was possible to apply inductance in the form of uniformly spaced loading coils. By 1920 engineers had installed such loading on thousands of miles of cable, particularly in the United States. 

Low-temperature glow-discharges were utilized to cause bond rupture in hexafluoroethane (11). In these experiments, the power to support the discharge was supplied by a radio-frequency discharge that delivered 25 W of power, at a frequency of 8.6 MHz, to the copper coil surrounding the Pyrex reactor (see Fig. 2). The load coil was indue-... 

With the exception of the reactions of trifluoromethyl radicals with sulfur vapor, which is really a separate class of reactions, if the power supplied to the load coil surrounding the reactor (see Fig. 2) was maintained at, or near, the minimum amount needed to support the discharge, in only two cases were compounds found that clearly resulted from reactions other than replacement of halogen by trifluoromethyl. The reaction of tellurium tetrabromide (or the chloride) gave, in addition to the products just reported, very small proportions of such species as BrCF2TeCF2Br and (C2F5)2Te, which were isolated in yields of... 

The temperature of the inductively coupled plasma varies with the distance from the load coil and according to the setting of the ICP rf power and nebulizer gas flow rate. A typical profile of the plasma gas temperature along the torch axis as a function of distance from the load coil is shown in Figure 2.4. With increasing distance from the load coil and with a reduction of ICP rf power the gas plasma temperature decreases. 

Figure 2.4 Plasma gas temperature as a function of distance from load coil.

An inductively coupled argon plasma eliminates many common interferences. The plasma is twice as hot as a conventional flame, and the residence time of analyte in the flame is about twice as long. Therefore, atomization is more complete and signal is enhanced. Formation of analyte oxides and hydroxides is negligible. The plasma is remarkably free of background radiation 15-35 mm above the load coil where sample emission is observed. 

Figure 3 Typical quartz plasma torch positioned within a (induction) load coil. The center (injector) tube can be made of quartz or inert materials (alumina, platinum, or sapphire) to allow corrosive samples (including those containing hydrofluoric acid) to be introduced into the plasma. One-piece quartz torches, torches with demountable injector tubes, or completely demountable tubes are used.

A 1- to 2-kW radio frequency power supply, either free-running or crystal-controlled, drives current through a water- or air-cooled copper tube that acts as the induction coil (often called a load coil). The oscillating current through the load coil produces an oscillating electromagnetic field. 

Several different approaches have been used to minimize formation of a secondary discharge, which results from parasitic capacitance between the plasma and the load coil. A balanced load coil can be used where the two ends of a single load coil are driven by rf signals of opposite phase but nearly equal amplitude [9] (as is done on Perkin Elmer/Sciex instruments, called PlasmaLok). Then the center of the load coil is at 0 V. In some cases the center of the load coil can be directly connected to ground or to the sampling plate of the mass spectrometer. Alternatively, two separate load coils can be interlaced to form a balanced rf drive system (as is done on Varian instruments). 

A grounded, electrical shield can be placed between the load coil and torch to reduce the capacitive coupling between the load coil and the plasma in order to reduce the plasma potential [10]. A thin metal cylinder, split along its length (to... 

Many experimental parameters and components affect sensitivity, including the analyte transport efficiency of the sample introduction system and the mean size and size distribution of the aerosol entering the ICP. The plasma torch design, rf generator, load coil, interface between the atmospheric pressure ICP and mass spectrometer, ion optics, mass spectrometer itself, and detector also affect sensitivity. 

If the plasma power is increased, the location where the plasma begins along the center axis moves upstream (closer to the injector tube of the torch) and the plasma temperature increases. Therefore, if the nebulizer gas flow rate were optimized at a power of 1.0 kW and the power were increased to 1.2 kW, ions would be produced farther from the sampling orifice. There would be more extensive diffusion of the ions before they reached the sampling orifice. This could be overcome by either moving the sampling orifice closer to the load coil or increasing the nebulizer gas flow rate. 

Figure 1 Schematic diagram of a typical commercial inductively coupled plasma mass spectrometry (ICP-MS) instrument (A) liquid sample, (B) peristaltic pump, (C) nebulizer, (D) spray chamber, (E) argon gas inlets, (F) load coil, (G) sampler cone, (H) skimmer cone, (I) ion lenses, (J) quadrupole, (K) electron multiplier detector, (L) computer.

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