Mass spectrometers, isotopic analyses magnetic sector

Thermal ionization mass spectrometers (TIMS) with magnetic sector are thus the basic instruments for Pu and U isotopic analysis in safeguards inspection samples. The performance of commercial instruments has improved tremendously, particularly in the last 30 years, in terms of vacuum capability, design of the ionization source and detector assembly, stability of electrical supplies, sensitivity and linearity of ion current amplifiers, and includes full automatization of the measurement and data reduction. 

The potential of the most commonly used mass spectrometers (quadrupoles and magnetic sector-field single collectors) for elemental analysis has been discussed in detail in Parts 2.1 and 2.2.1 of this chapter. However, some intrinsic limitations still remain with these sequentially scanned systems, particularly when transient or time-dependent signals (such as those produced by laser ablation (LA), electrothermal vaporisation (ETV), flow injection (FI) and chromatography) are used to analyse a large number of isotopes. These scan-based systems can measure only a single m/z at a unit of time. Hence, truly simultaneous determination of multiple isotopes, particularly when fast transient signals are analysed, is not possible without the introduction of spectral skew . ... 

At the heart of the TIMS ion source are one or more hot filaments that serve to vaporize and ionize atoms or molecules of interest. Once generated, the ions are accelerated, focused, and directed into the mass analyzer for measurement. The classic TIMS instrument consists of an ion source, a single magnetic sector mass separator, and an ion detector. Such an instrument is capable of measuring isotope ratios as small as 1 x 10 6, sufficient for the isotopic analysis of most elements. For radionuclide analysis, smaller isotope ratios are often encountered. Specialized mass spectrometers include multiple magnetic and electric sectors and sector instruments with retarding quadrupole lenses (Smith, 2000) to measure down to the 10-9 range. 

Applications Sector instruments are applied for niche applications such as high-resolution measurements and fundamental ion chemistry studies. Magnetic sector mass spectrometers remain the instrument of choice in areas of target compound trace analysis, accurate mass measurement and isotope ratio measurement. 

The most widely used method for ionization is electron impact (El). In an El source the sample is placed in the path of an electron beam. Although many newer kinds of ion sources have been developed, El is the method commonly used in classical isotope-ratio mass spectrometers (IRMS), i.e. mass spectrometers designed for precise isotopic analysis. In this type of spectrometer the ions, once formed, are electrostatically accelerated, and then ejected through a slit into a magnetic field held perpendicular to the ion trajectory. In the magnetic sector part of the instrument the particles are deflected in an arc described by ... 

Magnetic sector ion microprobes are becoming increasingly important in isotopic analysis of extratenestrial materials as spot size decreases and the precision and accuracy of the measurements improve. The first of the commercially available ion microprobes used in cosmochemistry were the Cameca ims 3f-7f series machines, which initially became available in the mid-1980s. These multipurpose instruments are able to measure isotopic compositions of most elements of interest in cosmochemistry and can also be used to measure trace element abundances. Their main drawback is that the relatively small mass spectrometer can only be operated at mass-resolving powers below about 9000, and at this mass resolving power, the transmission of the mass spectrometer is very low. 

A quite different type of mass spectrometer - the first 180° magnetic sector field mass spectrometer (see Figure 1.7), with directional focusing of ions for isotope analysis, was constructed by Dempster, independently of other instrumental developments in mass spectrometry, in 1918. 

Another combination of magnetic and electric sector fields, together with a tandem accelerator, is realized in different types of accelerator mass spectrometers (AMS)17 applied for carbon-14 dating and extreme ultratrace analysis of long-lived radionuclides at natural isotope abundances (see Chapter 5). 

Modem mass spectrometers are usually one of four types magnetic sector, quadrupole, time-of-flight, or ion trap. A brief introduction to each is provided. Classical applications are discussed to illustrate how these devices have shaped the landscape of elemental and isotopic chemical analysis. 

Most mass spectrometers measure one m/z value at a time. A single channel ion detector is used for these instruments, either an electron multiplier or a Faraday cup. TOF, ion trap, and FTICR mass spectrometers have the ability to extract ions with many m/z values simultaneously, so simultaneous detection of these ions is desirable. One approach to multiple ion detection has been to use multiple detectors. Multiple detectors are also used for high-resolution magnetic sector MS instruments designed for very precise isotope ratio determination and for quantitative analysis using isotope dilution. Instruments with... 

Schultheis, G., Prohaska, T, Stingeder, G., Dietrich, K., Jembrih-Simbtirger, D., Schreiner, M. (2004) Characterisation of ancient and art nouveau glass samples by Pb isotopic analysis using laser ablation coupled to a magnetic sector fleld inductively coupled plasma mass spectrometer (LA-ICP-SF-MS). Journal of Analytical Atomic Spectrometry, 19,838-843. 

Historically, mass spectroscopy was developed to separate atoms and to determine the masses of isotopes accurately [1]. The predecessor of magnetic sector field mass spectrometers was built in 1918 by Dempster [2]. Later, some of the first instruments for the analysis of molecules [3] were used by analytical chemists in refineries to determine hydrocarbons (the first article in the first issue of the journal Analytical Chemistry dealt with mass spectrometry of a mixture of hydrocarbons [4]). Combinations of infrared and mass spectroscopy were reported shortly afterwards [5]. 

The corabination of an inductively coupled plasma ion source and a magnetic sector-based mass spectrometer equipped with a multi-collector (MC) array [multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS)] offers precise and reliable isotope ratio data for many solid elements. In fact, MC-ICP-MS provides data, the trueness (accuracy) and precision of which is similar to, or, in some cases, even superior to, that achieved by thermal ionization mass spectrometry (TIMS), considered the benchmark technique for isotope ratio measurements of most solid elements [1], The basic strength of ICP-MS lies in the ion source, which achieves extremely high ionization efficiency for almost all elements [2, 3]. Consequently, MC-ICP-MS is likely to become the method of choice for many geochemists, because it is a versatile, user-friendly, and efficient method for the isotopic analysis of trace elements [4-8], The ICP ion source also accepts dry sample aerosols generated by laser ablation [9-16], The combination of laser ablation (LA) with ICP-MS is now widely accepted as a sensitive analytical tool for the elemental and isotopic analysis of solid samples. 

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