Faraday collectors

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.

All MC-ICPMS instruments are equipped with a multiple Faraday collector array oriented perpendicular to the optic axis, enabling the simultaneous static or multi-static measurement of up to twelve ion beams. Most instruments use Faraday cups mounted on motorized detector carriers that can be adjusted independently to alter the mass dispersion and obtain coincident ion beams, as is the approach adopted for MC-TIMS measurement. However, some instruments instead employ a fixed collector array and zoom optics to achieve the required mass dispersion and peak coincidences (e.g., Belshaw et al. 1998). 

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 ... 

Figure 8. Schematic outline of a second-generation MC-ICPMS instrument (Nu Instalments Nu Plasma), equipped with a multiple-Faraday collector block for the simultaneous measurement of up to 12 ion beams, and three electron multipliers (one operating at high-abundance sensitivity) for simultaneous low-intensity isotope measurement. This instmment uses zoom optics to obtain the required mass dispersion and peak coincidences in place of motorized detector carriers. [Used with permission of Nu Instruments Ltd.]...

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.]...

Faraday collector, simultaneously with U, U and U during the first sequence. This shortens the analysis routine, consuming less sample. Ion beam intensities are typically larger in MC-ICPMS than in TIMS due to the ease with which signal size can be increased by introducing a more concentrated solution. While this yields more precise data, non-linearity of the low-level detector response and uncertainties in its dead-time correction become more important for larger beam intensities, and must be carefully monitored (Cheng et al. 2000 Richter et al. 2001). 

Thorium. Multiple-collector measurement protocols by TIMS for thorium isotopic analysis typically involve the simultaneous measurement of Th and °Th (for silicate rocks), or Th and °Th, then Th and Th (for low- Th samples), using an axial ion counter and off-axis Faraday collector (Table 1). Various methods are used to correct for the relative gain between the low-level and Faraday detectors and 2a-uncertainties of l-5%o are typically obtained (Palacz et al. 1992 Cohen et al. 1992 McDermott et al. 1993 Rubin 2001). Charge-collection TIMS protocols enable Th, °Th and Th to be monitored simultaneously on a multiple-Faraday array and can achieve measurement uncertainties at the sub-permil level (Esat et al. 1995 Stirling et al. 1995). 

Schematic representation of the experimental setup is shown in Fig 1.1. The electrochemical system is coupled on-line to a Quadrupole Mass Spectrometer (Balzers QMS 311 or QMG 112). Volatile substances diffusing through the PTFE membrane enter into a first chamber where a pressure between 10 1 and 10 2 mbar is maintained by means of a turbomolecular pump. In this chamber most of the gases entering in the MS (mainly solvent molecules) are eliminated, a minor part enters in a second chamber where the analyzer is placed. A second turbo molecular pump evacuates this chamber promptly and the pressure can be controlled by changing the aperture between both chambers. Depending on the type of detector used (see below) pressures in the range 10 4-10 5 mbar, (for Faraday Collector, FC), or 10 7-10 9 mbar (for Secondary Electrton Multiplier, SEM) may be established.

Each ion beam passed through a resolving slit and gives up its charge to the appropriate Faraday collector. This generates an ion current proportional... 

Ordinarily electrical amplification is used to compensate for differences in isotope abundances in the gas being measured. Thus, for carbon dioxide all three Faraday collectors are used with relative signal amplification at m/z = 44, 45, and 46 of 1 91 500 (since the normal abundance ratios 12C/13C 91, and 160/180 500). The amplified signals from all three detectors are thus comparable in intensity. Because of this feature, however, IRMS should only be used on gases with isotope composition close to natural abundance. Enriched material should not be used without careful recalibration since there is no guarantee of a linear response of electric signal to ion current for widely different isotope ratios. 

Modem isotope ratio mass spectrometers have at least three Faraday collectors, which are positioned along the focal plane of the mass spectrometer. Because the spacing between adjacent peaks changes with mass and because the scale is not linear, each set of isotopes often requires its own set of Faraday cups. 

Advances in TIMS-techniques and the introduction of multiple collector-ICP-MS (MC-ICP-MS) techniques have enabled the research on natural variations of a wide range of transition and heavy metal systems for the first time, which so far could not have been measured with the necessary precision. The advent of MC-ICP-MS has improved the precision on isotope measurements to about 40 ppm on elements such as Zn, Cu, Fe, Cr, Mo, and Tl. The technique combines the strength of the ICP technique (high ionization efficiency for nearly all elements) with the high precision of thermal ion source mass spectrometry equipped with an array of Faraday collectors. The uptake of elements from solution and ionization in a plasma allows correction for instrument-dependent mass fractionations by addition of external spikes or the comparison of standards with samples under identical operating conditions. All MC-ICP-MS instruments need Ar as the plasma support gas, in a similar manner to that commonly used in conventional ICP-MS. Mass interferences are thus an inherent feature of this technique, which may be circumvented by using desolvating nebulisers. 

Boron isotope ratios have been studied in ageothermal system from New Zealand (Ngawha) by MC-ICP-MS (Axiom with eight movable Faraday collectors, from Thermo Electron).188 The 8nB values range between — 3.1 %o and —3.9%e, which does not indicate any marine input into the system. A direct determination of boron isotopes (8nB) on natural and synthetic glass samples at < %o precision at the ng level has been proposed using LA-ICP-MS with multiple electron multipliers.129... 

Figure 14 Detectors (a) Discrete dynode electron multiplier, (b) Dual-mode discrete dynode electron multiplier detector, (c) Channeltron electron multiplier, (d) Faraday collector. (f) Daly detector.

The Faraday collector F is supported on chromel arms to which molybdenum points are attached for pivots. A small nickel rod, at the lower end of a 1-mm molybdenum wire attached to the lower chromel arm, serves as a magnetic control for rotation of the collector about an axis which lies in the face of the crystal and intersects the incident beam at an angle of 90°. 

The modern TIMS instrument consists of an ion-source chamber, flight-tube, sector magnet, and ion-collector chamber. Depending upon the manufacturer and age of the instrument as many as 20 samples can be mounted together in the ion-source chamber and analysed sequentially. The ion-collector chamber may include an array of up to 9 multiple Faraday collectors as well as secondary electron amplification devices allowing ion currents from 1 x 10 A to less than 1 X 10 A to be measured. Since these instruments are designed to measure isotope abundance ratios with high accuracy and precision, the mass spectrum is well-resolved and the spectral peaks are broad with flat tops. 

Samples weighing 2-5 mg are then dissolved in 5-molar nitric acid. The strontium fraction is purified using ion-specific resin and eluted with nitric acid followed by water. This solution is loaded onto a titanium filament for placement in the instrument (Fig. 4.20). Isotopic compositions are obtained on the strontium fraction thermal ionization mass spectrometer (TIMS). This is a single focusing, magnetic sector instrument equipped with multiple Faraday collectors. Strontium is placed on a thin filament and measured. Sr/ Sr ratios are corrected for mass fractionation using an exponential mass fractionation law. Sr/ Sr ratios are reported relative to a value of 0.710250 for the NIST 987 standard (e.g., if the Sr/ Sr ratios for the standards analyzed with the samples average 0.710260, a value of 0.000010 is subtracted from the ratio for each sample). 

The Sr/ Sr ratios of calcite and dolomite in 12 samples were determined after washing the samples with distilled water to remove the pore salts that result from drying. The calcite samples were then reacted with dilute acetic acid and the dolomite samples with 0.1 HCl, and analysed using an automated Finnigan 261 mass spectrometer equipped with nine Faraday collectors. All analyses were performed in the static multicollector mode using rhenium filaments. Correction for isotope fractionation during the analysis was made by normalization to Sr/ Sr = 0.1194. The mean standard error of mass spectrometer performance was 0.00003 forNBS-987. 

Fig. 2. Essential elements of electrical detection-type LEED apparatus. A diffraction pattern is discovered one beam at a time with a Faraday collector. All beams at a given angle of diffraction are measured by rotating the crystal about the axis of its support, keeping the collector fixed. Other angles of diffraction are monitored by motion of the collector along an arc.

Fig. 3. Combined post-acceleration and electrical detection. The diffraction pattern is displayed on the screen. Individual beams can be separately measured by Faraday collector. Rotation of the collector about the dotted axis, and rotation of the supporting base of the collector about the gun axis, permit electrical measurement over the entire back-scattering solid angle except for a small area near the gun. (Photo courtesy of Perkin-Elmer Corp.)...

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