Ion Counting Techniques

The velocity mapping-ion counting technique increases the resolution of imaging data. This is clearly seen when it is compared with the data from the conventional Wiley-McLaren imaging apparatus2 or even the... 

In this paper, the photofragmentation of transition metal cluster complexes is discussed. The experimental information presented concerning the gas phase photodissociation of transition metal cluster complexes comes from laser photolysis followed by detection of fragments by ionization (5.). Ion counting techniques are used for detection because they are extremely sensitive and therefore suitable for the study of molecules with very low vapor pressures (6.26.27). In addition, ionization techniques allow the use of mass spectrometry for unambiguous identification of signal carriers. 

Ions exiting the drift tube are mass analyzed in mass spectrometer MS2, an important feature if reactions are occurring in the drift cell. Ions are generally detected after MS2 by ion counting techniques. The mass spectrometers MSI and MS2 are typically quadrupole mass filters, and either one or the other can be run in RF-only mode for better signal but without mass selection, if desired. 

To obtain spectra of very dilute molecular beams with low laser intensities requires extensive signal averaging. Using ion counting techniques the molecular beam intensity is measured with the laser on and off over many thousands of cycles of a mechanical chopper to obtain the absolute fraction of molecular loss. Signal to noise is limited only by the counting statistics. 

The most accurate and precise quantitative measurements do not employ the current measurement methods described thus far, but instead use ion counting techniques. The use of ion counting techniques in mass spectrometric analyses of very small samples was first intoduced (Barton 1960 Ihle 1971) for determination of relative isotope abundances in radioactive metals, but the first published report of application to trace level quantitation of organic analytes by GC/MS appears to have been somewhat later (Picart 1980) modem practice uses the same fundamental principles but with greatly improved detector and electronics technology. 

Electroanaly deal (EA) methods can also be considered as ion counting techniques that are used to selectively measure the potential generated between two electrodes or the currents produced by certain types of ions that are present in a liquid sample. The selectivity is achieved by deploying a specially tailored coating on an electrode that allows only a specific ion type to register a current. In other embodiments, elec-troanalytical methods can be used to preconcentrate a specific ion and then it can be detected and quantified by stripping the charge from the same electrode or by... 

The ICR voltage signal strength at the detector plates is inversely proportional to ion mass if the monitoring circuit is predominantly resistive, and is independent of ion mass if the circuit is predominantly capacitive [200]. Image current detection at room temperature is typically less sensitive than ion counting techniques in... 

In the type of ion source used the trend seems to be turning over from El to Cl (4) or, better, to use both. FI or FD or API (46) are to my knowledge still untried in the field. An ion counting technique has been used by Abramson (34), other workers have used ion current measurement. 

Hunter, K. L. and Gray, J. W. (1993) Ion Detection in Mass Spectrometry using the Ion Counting Technique. Technical Publication, FTP Scientific Inc., Auburn, MA, USA. 

A scintillation ion detector, described in detail elsewhere (41), detected virtually every ion which entered the detector chamber. Pulse counting techniques were used. 

An alternative to the bridge technique was recently reported for thorium analysis in silicate rocks for which both Th and Th are measured on a single lon-counting detector (Rubin 2001). With careful chemistry and mass spectrometry, °Th/ Th ratios of igneous rocks can be measured with this technique with a precision that is similar to the bridge method. The disadvantage of this technique is that °Th ion-count rates are extremely low (around 10 cps) with normal silicate thorium ratios and are therefore subject to perturbations from background variation and low-level isobaric interferences in normal samples. 

Multiple-collection techniques. Uranium. Table 1 shows a typical protocol used by multi-collector instruments (equipped with one ion counting channel) both in MC-TIMS, MC-ICPMS and LA-MC-ICPMS (e.g., Cohen et al. 1992 Stirling et al. 1995 Luo et al. 1997 Stirling et al. 2000 Pietruszka et al. 2002). A first sequence monitors the atomic ratios between and by aligning Faraday collectors for masses (10 ... 

Spectroscopic techniques require calibration with standards of known analyte concentration. Atomic spectrometry is sufficiently specific for a simple solution of a salt of the analyte in dilute acid to be used, although it is a wise precaution to buffer the standards with any salt which occurs in large concentration in the sample solution, e.g. 500 pg ml-i or above. Calibration curves can be obtained by plotting absorbance (for AAS), emission signal (for AES), fluorescence signal (for AFS) or ion count rate (for MS) as the dependent variable against concentration as the independent variable. Often the calibration curve will bend towards the concentration axis at higher concentrations, as shown in Fig. 

In 1977, Jochum et aZ.12,14 developed the multiple ion counting (MC) technique using an old spark source mass spectrometer with 20 separate channeltrons 1.8 mm wide for simultaneous electrical ion detection. The sensitivity was increased by a factor of 20 compared to SSMS with ion detection using a photoplate and the precision of the analytical results was improved. 

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