Skimmer cone

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. 

Figure 5. Front end of triple a quadrupole mass spectrometer with an ion source and reaction chamber (see Figure 4) mounted in front of an evacuated space containing skimmer cone CB, AC only quadrupole lens Qo, and first quadrupole Qi. The second and third quadrupoles, Q2 and Q3, are not shown. CR denotes cryopumping surfaces. IQ is an interquad lens.

The skimmer cone is another metal cone, the tip of which has an orifice approximately 0.7 mm in diameter, that protrudes into the zone of silence, and is axially in-line with the sampling orifice as shown in Fig. 5.3. The ions from the zone of silence pass through the orifice in the skimmer cone, into a second intermediate vacuum chamber held at < 10 atm, as an ion beam The ion beam can then be focused by means of a series of ion lenses, which deflect the ions along a narrow path and focus them on to the entrance to the mass analyser. 

Several effects, like instrumental mass bias, isobaric interferences, instrumental background, contamination of the solution introduction system and the sampler and skimmer cone and lens... 

Instrument and maintenance costs are high. Prices range from about 180,000 (U.S. dollars, 1998) for quadrupole instruments to almost 1,000,000 (U.S. dollars, 1998) for a fully capable multicollector sector-based instalment and laser ablation sampling. About 10 to 20 L/min of Ar is used by the ICP. Sampling and skimmer cones cost 800 to 3000 (U.S. dollars, 1998), depending on material. Detector lifetime may be less than 1 year. Vacuum pumps have limited lifetimes. Of course, the rapid multielement analysis capabilities, low detection limits, and isotope measurements often provide information that makes ICP-MS successful financially as well as scientifically. 

Micromass has developed a potentially powerful new technique that eliminates many of these molecular interferences and also removes ions with an energy that differs from that of the analyte, such as components of the Ar support gas. This has a dramatic effect on the performance of the instrument. The technique deploys a hexapole ion lens (Szabo, 1986) located behind the skimmer cone and surrounded by a gas cell (Fig. 8.5). The hexapole uses a hexagonal array of rods between which a 400-V rf field is applied, confining the ions of interest to stable trajectories be-... 

Micromass has applied this new hexapole technology to a fast scanning magnetic sector multiple-collector instrument (Fig. 8.6). The source is at ground potential so most of the lens system and analyzer float at —6 kV. The ions are extracted into the hexapole through a sample cone with a 1.1-mm orifice, a 0.8-mm orifice skimmer cone, and finally a 2-mm orifice transfer lens held at —400 V. The hexapole is inclined to prevent line-of-sight transmission and damage to the detectors from the source. A lens system transfers the ions into the mass analyzer. 

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