Secondary electron multipliers

Ion intensities up to a count rate of 2 x 10 are measured using a secondary electron multiplier (SEM). When it becomes saturated above that value, it is necessary to switch to a Faraday cup. Its ion-current amplification must be adjusted to fit to the electron multiplier response. 

Photocells and photomultipliers (secondary electron multipliers, SEM) are mainly employed in photometry. These are detectors with an external photo-effect . 

Photomultipliers Secondary electron multipliers, usually known as photomultipliers, are evacuated photocells incorporating an amplifier. The electrons emitted from the cathode are multiplied by 8 to 14 secondary electrodes dynodes). A diagramatic representation for 9 dynodes is shown in Figure 18 [5]. Each electron impact results in the production of 2 to 4 and maximally 7 secondary electrons at each dynode. This results in an amplification of the photocurrent by a factor of 10 to 10. It is, however, still necessary to amplify the output of the photomultipher. 

Depending on their positioning the dynodes are referred to as being head-on or side-on . Commercial scanners mostly employ side-on secondary electron multipliers where, as the name implies, the radiation impinges from the side — as in Figure 19. Their reaction time is shorter than for head-on photomultipliers because the field strength between the dynodes is greater. 

Richter S, Goldberg SA, Mason PB, Traina AJ, Schwieters JB (2001) Linearity tests for secondary electron multipliers used in isotope ratio mass spectrometry. Inti J Mass Spectrom 206 105-127 Rihs S, Condomines M, Sigmarsson O (2000) U, Ra, and Ba incorporation dining precipitation of hydrothermal carbonates imphcations for Ra-Ba dating of impure travertines. Geochim Cosmochim Acta 64 661-671... 

Once they have left the separation system the ions will meet the ion trap or detector which, in the simplest instance, will be in the form of a Faraday cage (Faraday cup). In any case the ions which impinge on the detector will be neutralized by electrons from the ion trap. Shown, after electrical amplification, as the measurement signal itself is the corresponding ion emission stream . To achieve greater sensitivity, a secondary electron multiplier pickup (SEMP) can be employed in place of the Faraday cup. 

Figure 4.2 Basic principles of the secondary electron multiplier (SEM).

We should not forget that an appropriate detector, a Faraday cup or a secondary electron multiplier equipped with a conversion dynode, is needed for ion detection. Most commercial instruments are equipped with a secondary electron multiplier, which can be operated in a low amplification mode, the analogue mode, and with a high gain, the counting mode, where each ion is counted. With this dual mode, a linear dynamic range of up to nine orders of magnitude can be achieved, so that major and minor components of the sample can be measured in one run. 

Figure 1. Experimental set-up for performing transient two-photon ionization spectroscopy on metal clusters. The particles were produced in a seeded beam expansion, their flux detected with a Langmuir-Taylor detector (LTD). The pump and probe laser pulses excited and ionized the beam particles. The photoions were size selectively recorded in a quadrupole mass spectrometer (QMS) and detected with a secondary electron multiplier (SEM). The signals were then recorded as a function of delay between pump and probe pulse.

To intensify the ion current, an amplification via electron multiplication is advantageous. For this purpose a MCP is introduced into the PIMMS. An MCP rectangular cut in shape from a standard MCP [25,26] works as a secondary electron multiplier (SEM) as shown in Fig. 11. The MCP is hybrid-integrated into the spectrometer by clamping with flexible silicon springs 20 pm in thickness. They serve both as mechanical fixture and electrical contact. The MCP is operated between 400 and 1,300 V and amplifies the current by more than 1,000, which allows for measurements into the <100 ppm region. 

A MCP is used in the PIMMS as a secondary electron multiplier (see Sect. 3.7). The electron current measured after MCP compared to the initial ion current is amplified by a factor of 10-1,000. The secondary electron emission coefficient is an averaged number of secondary electrons emitted after each impact. This number depends on the initial energies of the electrons or ions and so on the voltage applied to the MCP. The amplification factor of a MCP configuration is expressed as ... 

The principles, sampling systems, control of the measuring device and application of MS for bioprocesses have been summarized by Heinzle [157,158] and Heinzle and Reuss [162]. Samples are introduced into a vacuum (< 10 5 bar) via a capillary (heated, stainless steel or fused silica, 0.3 x 1000 mm or longer) or a direct membrane inlet, for example, silicon or Teflon [72,412]. Electron impact ionization with high energy (approx. 70 eV) causes (undesired) extensive fragmentation but is commonly applied. Mass separation can be obtained either by quadrupole or magnetic instruments and the detection should be performed by (fast and sensitive) secondary electron multipliers rather than (slower and less sensitive) Faraday cups (Fig. 21). 

Regelous et al have reported ou the use of the isotope dilutiou techuique (using a Pa spike with a half-life of 26.97 days) for the quantitative measurement of 20 fg of protactinium in silicate rocks after chemical separation of the actinide from the rock matrix by MC-ICP-MS (Neptune, Thermo Fisher Scientific, Bremen - equipped with uiue Faraday detectors, oue secondary electron multiplier and a retarding potential quadrupole for high abundance sensitivity measurements). 

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