Sampling cones

The end or front of the plasma flame impinges onto a metal plate (the cone or sampler or sampling cone), which has a small hole in its center (Figure 14.2). The region on the other side of the cone from the flame is under vacuum, so the ions and neutrals passing from the atmospheric-pressure hot flame into a vacuum space are accelerated to supersonic speeds and cooled as rapid expansion occurs. A supersonic jet of gas passes toward a second metal plate (the skimmer) containing a hole smaller than the one in the sampler, where ions pass into the mass analyzer. The sampler and skimmer form an interface between the plasma flame and the mass analyzer. A light... 

Schematic diagram of an orthogonal Q/TOF instrument. In this example, an ion beam is produced by electrospray ionization. The solution can be an effluent from a liquid chromatography column or simply a solution of an analyte. The sampling cone and the skimmer help to separate analyte ions from solvent, The RF hexapoles cannot separate ions according to m/z values and are instead used to help confine the ions into a narrow beam. The quadrupole can be made to operate in two modes. In one (wide band-pass mode), all of the ion beam passes through. In the other (narrow band-pass mode), only ions selected according to m/z value are allowed through. In narrow band-pass mode, the gas pressure in the middle hexapole is increased so that ions selected in the quadrupole are caused to fragment following collisions with gas molecules. In both modes, the TOF analyzer is used to produce the final mass spectrum.

The part that marries the plasma to the mass spectrometer in ICPMS is the interfacial region. This is where the 6000° C argon plasma couples to the mass spectrometer. The interface must transport ions from the atmospheric pressure of the plasma to the 10 bar pressures within the mass spectrometer. This is accomplished using an expansion chamber with an intermediate pressure. The expansion chamber consists of two cones, a sample cone upon which the plasma flame impinges and a skimmer cone. The region between these is continuously pumped. 

The skimmer has a smaller aperture than the sample cone, which creates a pressure of 10 atmospheres in the intermediate region. The ions are conducted through the cones and focused into the quadrupole with a set of ion lenses. Much of the instrument s inherent sensitivity is due to good designs of these ion optics. 

By for the most simple acid to work with in ICPMS is nitric acid. This has minimal spectral interferences and in concentradons under 5% does not cause excessive wear to the sample cones. Other acids cause some spectral interferences that often must be minimized by dilution or removal. When HF is used, a resistant sampling system must be installed that does not contain quartz. 

The ions formed are then directed though a sampling cone at 90° to the direction of vapour flow - to minimize the chances of blocking of the entrance to the mass spectrometer - into the source of the mass spectrometer, while the vast majority of the vapour generated by the mobile phase is removed by a pump directly opposite the capillary. 

Slightly downstream of the end of the capillary are usually to be found a filament and/or a discharge electrode, which provided secondary methods of ionization, while opposite or slightly downstream of the sampling cone is a repeller or retarding electrode. Their nse will be described in more detail later. 

Although the detection limit of an ICP-MS is about 1 ppt, the device is rather inefficient in the transport of the ions from the plasma to the analyser (interface efficiency of about 1 %). The influence of the ICP-MS sampling cone is still to be worked out. Introduction of organic solvents into an ICP-MS decreases the sensitivity, due to excessive solvent loading of the plasma. 

Figure 11.2 A schematic of the electrospray process, showing the release of charged droplets from the Taylor cone and the Z-spray arrangement with respect to the sample inlet, sample cone, and the subsequent path of the ions into the analyzer.

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