Multi-collector devices

In contrast to the reverse geometry commonly used for high-resolution single-coUector (SC) sector field ICP-MS instruments, double-focusing MC-ICP-MS devices have the ESA before the magnetic sector, in the forward Nier-lohnson geometry. This is essential for MC instruments. 

Compared to TIMS, MC-ICP-MS has the potential to produce data of equal quality for a greater range of elements (including those with first ionisation potentials 7 eV) as a result of the high ionisation temperature of its source (approximately 10 000 K vs approximately 2000 K for TIMS). Walder and Freedman were the first to describe the use of such an instrument and MC-ICP-MS has since established itself as a valuable addition to TIMS and ICP-quadrupole(Q)-MS techniques. 

Relatively new developments in MC technology include the virtual amplifier and zoom optics. Instead of each Faraday cup having a dedicated amplifier, the virtual or rotating amplifier 

Inductively Coupled Plasma Mass Spectrometry Handbook 

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This short chapter finishes with multi-collector devices for true simultaneous multi-element analyses. 

The Isoprobe MC-ICP-MS (GV instruments, Wythenshawe, UK), a multi-collector device, is equipped with a pressurized hexapole collision cell to reduce the energy spread of the incoming ion before it enters the magnetic sector. Since it is a prerequisite that all ions must be accelerated by the same potential, all ions exit the hexapole at < 1 V potential and are then accelerated. The optional wide aperture retarding potential (WARP) filter is applied to enhance abundance sensitivity in this device. The WARP filter was originally developed for thermal ionization mass spectrometers in an attempt to improve the abundance sensitivity by two orders of magnitude. The principle of the WARP filter is that it excludes those ions that are not at the full accelerating potential. Any ions that have collided with residual gas molecules in the analyser are not transmitted by the WARP filter. 

Whereas multi-collector devices are operated at a constant magnetic field and collect the isotopes of interest simultaneously in different detectors, single-collector instruments have to scan over... 

These essentially consist of a pair of closely spaced, vertical rectangular plates bounded on the sides by the electrodes. The sample and carrier buffer are fed from the top of the slit and travel down in laminar flow to a battery of fraction collectors at the bottom. Unlike the Philpot-Harwell device, which is essentially adiabatic, the thin-film separator can be cooled at the plates. The commercially available device, the Elphor , has a throughput of around 0.1 g/h of protein when operated for multi-component separation. It has been used to separate not only proteins, but cells and other particulate materials. Like the Philpot-Harwell apparatus, it uses a relatively large quantity of carrier buffer and the products are substantially diluted during separation. 

The theoretical treatment14,18) of such a device, assuming no self-absorption nor scattering, connects the efficiency r]Q of the multi-stacked collector with the radiation collected at each stage Ck = Sk(l - L). Here, Sk is the radiation entering the k th collector and L the losses due to the critical cone. Hence, t/q is the sum of the m differing Ck values. For PMMA with the refractive index 1.49 the collection efficiencies for m plates are... 

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