Instrumentation quadrupole

The dwelltime of ions within the ion source is defined by the extraction voltages applied to accelerate and focus them into an ion beam and by the dimensions of that ion source. In standard El ion sources the freshly formed ions dwell about 1 ps before they are forced to leave the ionization volume by action of the accelerating potential. [41] As the ions then travel at speeds of some 10 m s they pass the mass analyzer in the order of 10-50 ps (Fig. 2.9). [9] Even though this illustration has been adapted for a double focusing magnetic sector mass spectrometer, an ion of m/z 100, and an acceleration voltage of 8 kV, the effective time scales for other types of instruments (quadrupole, time-of-flight) are very similar under their typical conditions of operation (Table 2.4). 

Fig. 1.28 Schematic of a quadrupole-time of flight instrument. Quadrupole qO is used for collisional cooling and ion focusing.

Figure 5.2 Overview of ICP-MS instrumentation quadrupole-based ICP-MS with and without colli-sion/reaction cell, sector field ICP-MS with single ion collector (ICP-SFMS) and multiple ion collector ICP-MS (MC-ICP-MS) and time-of-flight ICP-MS.

The ions from the ion beam are separated by their mass to charge ratio (m/z) (where commonlyz= 1), providing the series mi/zi, m2/z2, m3/z3,...mn/Zn that will give the mass spectrum. The ion separation can be done using special ion optics that differentiate the mass spectrometers as follows magnetic sector instruments, quadrupole, time-of-flight, ion trap, Wein filter, ion cyclotron resonance, etc. 

Figure 6.19 Peak identification by LC-MS, LC-MS-MS, LC-UV and LC-NMR [reproduced with permission from J.L. Wolfender, S. Rodriguez and K. Hostett-mann, J. Chromatogr. A, 794, 299 (1998)]. Chromatogram (vertical) sample, extract from Gentiana ottonis column, 15cm x 3.9mm i.d. and precolumn stationary phase, Nova-Pak Cqg, 4p,m mobile phase, 1 ml min water/acetoni-trile with 0.05%trifluoroacetic acid, gradient from 5 to 65% acetonitrile in 50 min detector, UV 254 nm. MS interface, thermospray instrument, quadrupole. UV diode array. NMR D2O instead of H2O stop-flow 500 MHz. Peak 33 is the glucosylflavone swertisin.

As a result of these advances in conunercial ion trap instruments, quadrupole ion traps have been ineluded in several USEPA methods. Examples are USEPA methods 524.2 [54], 525.2 [53], 527 [55], 528 [56], 529 [57], 521 [58], and several SW-846 wastewater and solid waste methods, such as 8260 and 8270 [59]. EPA method 521 requires specifieally the utilization of MS/MS due to interferences that were encountered in sample extracts these interferences could not be overcome by using a full mass scan and SIM mass spectrometry. 

In scanning instruments (quadrupoles, traps, magnetic) electric or magnetic fields must be varied continuously to obtain spectra... 

Radiofrequency (RE) electric quadrupole mass (really miz) filters represent a considerable majority of analyzers in current use, particularly in trace quantitative analysis for this reason the operating principles of these devices will be discussed in some detail to emphasize their advantages and limitations. (The RF range corresponds to a few MHz.) These analyzers are used as stand-alone (non-tandem) MS detectors, as the components of the workhorse QqQ tandem instruments and as the first analyzer in the QqTOF hybrid tandem instrument. Quadrupole mass filters (Q) are essentially the same device as the RF-only collision cells and ion guides (q) discussed later in this section and are intimately related to the RF ion traps described in Section 6.4.5. In this regard, it can be mentioned that Q and q devices are not called quadrupoles because they are constructed of four electrodes (rods), but because a quadrupolar electric field (see Equation [6.11]) is formed in the space between the rods indeed the three-dimensional (Paul) trap (Section 6.4.5) creates a quadrupolar field using just three electrodes ... 

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]. 

Fig. 4 Precursor ion scanning in a triple quadrupole instrument. Quadrupoles 1 and 3 behave as mass filters, scanning as shown in the lower part of the diagram. The instrument is analysing kaempferol-glucose, and the third quadrupole is set constantly to the mass of kaempferol. The first quadrupole scans. The spectrum resulting from this method shows any precursor ion that fragments to die mass of kaempferol...

Fig. 5 Neutral loss scanning in a triple quadrupole instrument. Quadrupoles 1 and 3 now scan in parallel...

In the case of the third requirement, instrument manufacturers have developed highly specialized Magnetic Sector mass filter-based instruments and time-of-flight mass filter-based instruments. Quadrupole mass filter-based SIMS instruments are ineffective in this area because of their inability to provide the high mass resolution required. Fourier Transform Ion Cyclotron Resonance (FT-ICR) as well as Orbitrap mass filter-based instruments, on the other hand, show significant promise. Mass filters are discussed in Section 4.3.2.I. 

Instrumentation Quadrupole mass spectrometers consist of an ion source, ion optics to accelerate and focus the ions through an aperture into the quadrupole filter, the quadrupole filter itself with control voltage supplies, an exit aperture, an ion detector and electronics, and a high vacuum system. 

If the response versus mass of a typical modern ICP-MS is considered under normal operating conditions, then for most if not all commercial instruments (quadrupole, time of flight and magnetic sector), sensitivity increases with mass. Therefore, for example, 1 ng/mL of is found to yield a larger signal than 1 ng/mL of Li. However, when one considers this in terms of the actual number of atoms in the solution, then in the case of the 1 ng/mL of Li and U example, there are 238/7 times more Li atoms in that solution than there are atoms, since 7 g of Li contains... 

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