Collector resolution

A point light source is imaged onto the specimen by the objective and the transmitted light collected by the collector lens and detected by a broad-area detector in the case of reflection microscopy, the objective lens also serves simultaneously as a collector (see figure Bl.18.10. The resolution is solely detennined by the objective lens, because the collector has no imaging fimction and only collects the transmitted light. The... 

Since the microchannel plate collector records the arrival times of all ions, the resolution depends on the resolution of the TOP instrument and on the response time of the microchannel plate. A microchannel plate with a pore size of 10 pm or less has a very fast response time of less than 2 nsec. The TOP instrument with microchannel plate detector is capable of unit mass resolution beyond m/z 3000. 

Quadmpole mass spectrometers (mass filters) allow ions at each m/z value to pass through the analyzer sequentially. For example, ions at m/z 100, 101, and 102 are allowed to pass one after the other through the quadmpole assembly so that first m/z 100 is transmitted, then m/z 101, then m/z 102, and so on. Therefore, the ion collector at the end of the quadmpole unit needs to cover only one point or focus in space and can be placed immediately behind the analyzer (Figure 30.1). A complete mass spectram is recorded over a period of time (temporally), which is set by the voltages on the quadmpole analyzer. In this mode of operation, the ions are said to be scanned sequentially. The resolution of m/z values is dependent solely on the analyzer and not on the detector. The single-point collector is discussed in detail in Chapter 28. 

The array system is discussed in Chapter 29. With array detection, resolution of m/z values depends both on the analyzer and the collector. Historically, the method for recording ions dispersed in space was to use a photographic plate, which was placed in the focal plane such that all ions struck the photographic plate simultaneously but at different positions along the plate, depending on m/z value. This method of detection is now rarely used because of the inconvenience of having to develop a photographic plate. 

Unlike the array collector, with a microchannel plate all ions of only one m/z value are detected simultaneously, and instrument resolution does not depend on the number of elements in the micro-channel array or on the separation of one element from another. For a microchannel plate, resolution of m/z values in an ion beam depends on their being separated in time by the analyzer so that their times of arrival at the plate differ. 

However, for quantities substantially less than this level, 7- to 10-mm i.d. analytical columns can often be used in a semipreparative mode. By repeatedly injecting 300 to 500 ju,l of up to 1% polymer, reasonable quantities of polymer can be isolated. An autosampler and automated fraction collector can be setup to perform such injections around the clock. Although the larger injections and higher concentrations will lead to a loss of resolution, in some situations the result is quite acceptable, with a considerable savings in time being realized over other means of trying to make the same fractionation. 

There are three main reasons for this choice. Firstly, it becomes more and more difficult to obtain recordable, molecular-ion signals from un-derivatized carbohydrates as their M, increases significantly above 3000. Secondly, the mass spectrometers that have been used in all high-mass-carbofiydrate studies published at the time of writing this article are not capable of very sensitive analysis above —3800 mass units (see later). Thirdly, at masses >4000, it is usually not practicable to work at the resolution necessary for adjacent peaks to appear as separate signals in the spectrum. To do so would require that the source and collector slits be narrowed to such a degree that there would be an unacceptable loss in sensitivity. Thus, spectra acquired at mass >4000 are usually composed of unresolved clusters. 

Figure 9. Schematic diagram showing a second-generation MC-ICPMS instrament (ThermoFinnigan Neptune). This instrument utilizes double-focusing and is equipped with a motorized multiple-Faraday collector block with two channels that can be operated in high-resolution mode. Optional multiple-ion counting channels are also available for the simultaneous measurement of low-intensity ion beams. [Used with permission of Thermo Finnigan.]...

WWe collector slii Simple gu molecules MullfcnllecU (Faraday Cups) Low Resolution... 

So how does the IRMS get its stability Collector slits are several times the width of the ion beams. This gives a flat-topped peak shape (Fig 6) which makes the ion current intensive to drift. The main source of drift is temperature variation which both affects the electronic components used for mass selection and caused expansion and contraction of mechanical parts. Simultaneous measurement of ion beams using a double or triple collector is more precise than sequential measurement by mass scanning with a single detector. Finally, frequent comparison of sample gas under identical conditions also contributes to stability. Ion beam stability is more important than resolution for isotopic measurements. 

Collectors are placed at the focal points. At sufficiently high vacuum genuinely large separation factors are obtained in electromagnetic separators (high resolution in the language of the mass spectroscopist), but this important advantage is offset by sev-... 

Special features high linear dynamic range simultaneous isotope analysis zoom lens, fixed collectors pseudo high resolution simultaneous isotope analysis simultaneous isotope analysis... 

Re-assessment of silicon isotope reference materials (IRMM-018 and NBS 28 isotope standard) has been performed using both high resolution multiple ion collector ICP-MS and gas mass spec-... 

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