Ion counting

If the primary ion beam is used to continuously remove material from the surface of a specimen in a given area, the analytical zone is advanced into the sample as a function of the sputtering time. By monitoring the secondary ion count rates of selected... 

A mass scan is acquired in cases when a survey of all impurities present in a volume of material is needed. Rather than measuring the secondary ion count rates of preselected elements as a fimction of sputtering time the count rates of all secondary ions are measured as a fimction of mass. Because a mass scan is continuously acquired over a mass range, no depth profiling or lateral information is available while operating in this mode. Figure 4 shows a mass scan acquired from a zirconia... 

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. 

Figure 6. Schematic outline of the first commercially available multiple collector ICPMS, the Plasma 54, after Halhday et al. (1995). This instrument uses Nier-Johnson double-focusing and is equipped with eight independently adjustable Faraday collectors. The axial collector can be wound down to provide access to a Daly detector equipped with ion counting capabilities and a second-stage energy filter for high abundance sensitivity measurements. The sample may be introduced to the plasma source by either solution aspiration or laser ablation.

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 ... 

A) and (10 A), while the low intensity ion beam (10 to 10 A) is measured simultaneously in the low-level detector. A second sequence shifts all masses to monitor U in the ion counting channel relative to U. A comparison of the two sequential measurements using different collector configurations provides an... 

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.]...

Isotope dilution measurements using a and/or spike tracer can be performed separately, or combined with the isotopic composition run. Spike tracers can be measured on an ion counting channel, or alternatively, it may be preferable to concentrate the tracer (provided the spike corrections are small) and measure it on a... 

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... 

The first dedicated Curie-point PyMS system was built by Meuzelaar and Kistemaker11 at the FOM Institute (Institute for Atomic and Molecular Physics) in Amsterdam. This was followed shortly afterward by the construction of the first fully automated instrument, the Autopyms, which used highspeed ion counting and computerized data processing.18,32 The Autopyms led to the manufacture of two commercial machines the Extranuclear 5000 (Extranuclear Laboratories, Pittsburgh, PA) and the Pyromass 8-80 (VG Gas Analysis Ltd., Middlewich, Cheshire, UK). However, neither of these machines proved popular, probably because machine cost was in excess of 100000. 

It is evident from the data in Figure 3, that maximum gas-phase ion intensities for a given analyte ion are obtained with 10-4 to 10-3 mol/L analyte in the solution. The 3 x 106 ion counts/s observed at such concentrations are intensities after mass analysis at low resolution where the gas was admitted through a 100 pm orifice into a vacuum chamber which contains a triple quadrupole mass spectrometer.51... 

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. 

Figure 2. Schematic drawing of the AEl IM-20 ion microprobe. The magnetic analyzer and ion counting system are linked to an on-line computer for automated...

Cosmogenic Radionuclides in the 105-107 Year Region Results and Prospects for High-Energy Ion Counting...J. R. Arnold. 

Column diameter is an important parameter to consider in life science applications in which sample amounts are very limited and the components of interest may not be abundant. Researchers have reviewed micro HPLC instrumentation and its advantages.910 Nano LC-MS offers 1000- to 34,000-time reductions in the dilution of a sample molecular zone eluted from nano LC columns of 25 to 150 [Mi IDs in comparison to a 4.6 mm ID column. This represents a large enhancement of ion counts in comparison to counts obtained for the same amount of sample injected into a conventional 4.6 mm column. Solvent consumption for an analysis run or sample amount required for injection in a nano LC application may be reduced 1000 to 34,000 times compared to amounts required by an analytical column operated at a 1 mL/min flow rate. 

When nano LC is combined with mass spectrometer detection, attamole detection can be achieved for low abundance components in biological fluids, drug metabolites, and natural products such as Chinese herb medicines. Nano LC-MS-MS has become an essential tool for complex biological and drug metabolite studies. Nano LC-MS presents two significant differences from conventional analytical HPLC (1) large enhancement factor for sample detection and (2) direct interface to MS without flow splitting. The enhancement in MS ion counts relative to a conventional 4.6 mm ID column is proportional to the ratio of the square of the column diameter ... 

For a 75 /.mi ID nano LC column as an example, the MS detection enhancement factor (ion count) in comparison to a 4.6 mm column is much higher than (4.6/0.075)2 = 3761 because of the reduction in sample molecular zone dilution and because a nano LC solvent flow rate at 0.02 to 2 /iL/min can be 100% directly sprayed into the MS ion source. No post-column flow splitting is required for nano-LC-MS as that required when 1 mL/min is used in a 4.6 mm ID column. This large enhancement of MS detection and the ability to directly interface with MS presents nano LC-MS as the best tool for life science research. 

---