Multiple sector instruments, mass spectrometry

The significance of inorganic mass spectrometry in precise isotope ratio measurements will be further increased for sector field mass spectrometers with multiple ion collector systems and also for mass spectrometers with single ion detection or quadrupole based instruments with and without a collision cell. 

Special mass spectrometry systems are built for isotope ratio measurements. Most isotope ratio mass spectrometers consist of a single-focusing magnetic sector instrument and a multiple Faraday cup detection system. Because the Faraday cup exhibits a stable response, it is an ideal detector for isotope ratio measurements. Simultaneous collection of relevant ion beams from all isotopes provides high-precision isotopic measurements. A three-Faraday cup detection system is shown in Figure 7.9. The multicup assembly is placed at the focal plane of the mass analyzer and can be used for the simultaneous detection of each isotopic form of the analyte species (e.g., m/z 44, 45, and 46 from CO2). Commercial instruments with up to nine collectors are available. 

TIMS, depending on the application [3]. These developments with magnetic sector instruments include multiple Faraday cup detectors (multicollectors) and multiple ion counters. The Faraday cup multicollectors have enabled the determination of isotope ratios with accuracy and precision rivaling TIMS [4], Due to lower sensitivity and differences in instrument design, quadrupole instruments (quadrupole inductively coupled plasma mass spectrometry [Q-ICP-MS]) have not demonstrated the same accuracy and precision as magnetic sector instruments, especially at low concentrations [5], Nevertheless, improvements in the sensitivity of quadrupole instrumentation have also increased their utility for many isotope ratio applications [6]. 

There are two basic instrumental concepts for MS/MS. The first is tandem mass spectrometry in space (or tandem-in-space MS). In order to perform two consecutive mass-analyzing steps, two mass analyzers may be mounted in tandem. Thus, tandem-in-space refers to MS/MS instrumentation where product ion spectra are recorded using spatially separated m/z analyzers. Speedfic m/z separation is performed so that in one section of the instrument ions are selected, then dissociated in an intermediate region, and the products thereof are finally transmitted to a second analyzer for mass analysis (Fig. 9.1). All beam transmitting devices, e.g., multiple sector, ReTOF, TOF/TOF, QqQ, and QqTOF instruments follow this route to tandem MS (Fig. 9.2) [4]. The second approach, tandem mass spectrometry in time (or tandem-in-time MS), enploys a single m/z analyzer (QIT, LIT, FT-ICR) that may be operated by executing the discrete steps of ion selection, activation, and product ion analysis in the very same place but sequentially in time [4]. 

---