Interface sample cone

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... 

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. 

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. 

Plasma MS is usually based on quadrupole mass analysers. The atmospheric ICP, optimised for ion formation, is placed on its side facing a sample cone (Fig. 4.3). The mass spectrometer operates at reduced pressure and therefore a two- or three-stage differentially pumped interface is needed to transfer the ions from the plasma to the mass analyser. The interface for GC-ICP-MS is generally the same as for ICP emission systems. In one of the earliest GC-MS speciation studies (Chong and Houk, 1987) a packed GC column was used to obtain mass spectra of organic compounds with detection limits in the range 0.001-500 ngs The effects of isotopic fractionation by natural physico-chemical processes were also studied. 

The plasma/mass spectrometer interface allows the import of a stream of ions from the plasma, at atmospheric pressure, into the mass spectrometer, which is under vacuum. The interface has a sample cone and a skimmer cone, usually of nickel and typically with 1.0 mm and 0.8 mm apertures, respectively. The turbomolecular pump is presently the most widely used type in the vacuum system. 

ESI-MS Process Evaluation Experiments. ESI-MS experiments were performed using a Micromass (Manchester, UK) LCT electrospray time-of-flight mass spectrometer (ESI-TOF-MS). A 1.6 x 10 4 M solution of reserpine in a mixture of acetonitrile and water (1 1 v/v) was used as a standard. Acetic acid (0.1%) was added to the solution in order to enhance analyte protonation. For both monolithic and modular interfaces, samples were infused at flow rates ranging between a few tens of nLmin 1 to a few pL min-1. Spectra were acquired at various capillary, cone and extraction voltages and optimized for each of the interfaces. 

Figure 11 - 2 shows schematically the components of a commercial ICPMS system. A critical part of the instrument is the interlace that couples the ICP torch, which operates at atmospheric pressure with the mass spectrometer that requires a pressure of less than lO tnrr. This coupling is accomplished hy a differentially pumped interface coupler that consists of a sampling cone, which is a vsater-coolcd nickel cone with a small... 

---