Quadrupole mass switching

In quadrupole-based SIMS instruments, mass separation is achieved by passing the secondary ions down a path surrounded by four rods excited with various AC and DC voltages. Different sets of AC and DC conditions are used to direct the flight path of the selected secondary ions into the detector. The primary advantage of this kind of spectrometer is the high speed at which they can switch from peak to peak and their ability to perform analysis of dielectric thin films and bulk insulators. The ability of the quadrupole to switch rapidly between mass peaks enables acquisition of depth profiles with more data points per depth, which improves depth resolution. Additionally, most quadrupole-based SIMS instruments are equipped with enhanced vacuum systems, reducing the detrimental contribution of residual atmospheric species to the mass spectrum. 

In both electron post-ionization techniques mass analysis is performed by means of a quadrupole mass analyzer (Sect. 3.1.2.2), and pulse counting by means of a dynode multiplier. In contrast with a magnetic sector field, a quadrupole enables swift switching between mass settings, thus enabling continuous data acquisition for many elements even at high sputter rates within thin layers. 

Q-switches, in YAG lasers, 14 698 Q-switching technique, 14 618, 673-678 Quadrature techniques in sampling, 26 1010-1011 Quadrupole mass analyzers, 24 109 Quadrupole moment, 1 620-621 selected molecules, 1 621t3 Quadrupoles, 15 661-662 Qualimet... 

The precision of any isotope ratio measurement depends to a great extent on the mass spectrometer operating conditions. Because a quadrupole mass spectrometer is a rapid sequential analyser, the frequency at which it switches between the masses to be ratioed and the amount of time spent collecting data at each mass must be optimized. 

Typical residual gas analyzers employ electron impact to generate ions and a quadrupole mass filter to obtain the mass-to-charge ratio of these species. Mass ranges for these analyzers vary 2-80 atomic mass units for small units 1-500 on larger, more expensive gas analyzers. Electron impact ionization of the gas is achieved using a hot filament as the source. Many instruments are equipped with a second filament which can be switched into use in the event of a failure of the first. When a robust detector of moderate sensitivity is needed, a... 

Hunt et al (12) demonstrated the possibility of obtaining positive and negative ion mass spectra at the same time by employing an electrical field (quadrupole) mass analyzer with rapid switching. This may prove to be a valuable technique in many applications, as indicated by Hunt. 

Fig. 1.35. Experimental setup for the investigation of gas-phase catalytic activity of mass-selected metal clusters. The cluster ions are sputtered from solid targets with a CORDIS, mass-selected (Qi), and guided at low energies (Qo and Q2) into the temperature controllable octopole ion trap. By means of appropriate switching of the lenses Li and L2, the reaction products are extracted and subsequently mass-analyzed by another quadrupole mass filter (Q3) [32,186]...

The rapid LC methods employed for array analysis produce typical peak widths at half-height of less than 2 s. It was immediately apparent that the scan rates achievable on a radiofrequency (RF) quadrupole mass spectrometer would not be sufficient to generate high-quality mass spectra that could be used to confirm the presence of a desired chemical entity. Alternatively, commercial orthogonal time-of-flight (TOF) mass spectrometers can acquire full scan profile data at rates approaching 10 Hz so that rapid switching between up to four streams becomes feasible. Therefore this approach was followed in our development of the novel interface based on the Micromass LCT instrument. 

When steady-state conditions were established (after 2-4 hours), a switch from CO to CO in the feed stream was carried out, followed by the opposite switch 5 minutes later (see Figure 4). The transient concentration changes during the switches were monitored on-line by a Balzers QMG 420 quadrupole mass spectrometer. The catalytic activity was determined using a HP 5880 GC equipped with a GS-alumina column (J W Scientific) connected to a FID detector. 

The experimental set-up consists of two separate feed sections converging to a 4-way valve, a multichannel microreactor and a quadrupole mass spectrometer. The variations of the inlet concentration are generated switching the 4-ways valve. 

The bench-top quadrupole mass spectrometers have been around longer than the ion trap MS, which was introduced in the late 1980s. In addition to low cost, quadrupole is the most popular in the perfume in-dustries/laboratory than ion trap mass spectrometers [59] such as higher sensitivity in the full-scan mode switching between El and chemical ionisation (Cl) without conversion and ability to perform MS/MS experiments. 

Improvement with respect to these SRM methods was rendered possible by the availability of data-dependent acquisition or information-dependent acquisition (IDA), by which a tandem mass spectrometer can automatically switch from a survey mode to a dependent (or confirmation), full-spectrum MS/MS mode. In addition, the introduction of linear ion-trap-triple quadrupole (LIT-QqQ) hybrid instruments further extended the possibilities of LC-MS/MS in STA or GUS. In this instrument, the second mass analyzer can be used as either a conventional quadrupole mass analyzer or a linear ion trap, which by accumulation of ions provides enhanced full-spectrum sensitivity compared to a conventional quadrupole. The group of Weinmann used targeted SRM with up to 700 transitions as the survey detection mode, and the enhanced product ion (EPI) spectrum mode as the dependent mode (11). Whereas this procedure seems to be a more specific approach to STA as it allows searching rich spectra against those entered in libraries, the use of SRM as the survey mode cannot answer the more general clinical question as to whether an individual has been intoxicated at all, rather than intoxicated with a compound from a predefined list (12). Also, the use of only the positive-ion mode narrows the detection window. 

The data in a magnetic sector instrument is collected sequentially as the magnetic field of the magnetic analyzer is varied to focus ions of a particular mass/ charge ratio on the detector. If multiple detectors are installed, more than one mass/ charge ratio could be detected simultaneously. Mass switching is slower than in a quadrupole instrument. 

The concentration of feed components and reaction products is monitored directly at the reactor outlet using a quadrupole mass spectrometer (QMS) with a time resolution below 100 xs. Since QMS requires at least lOjXS to switch between different atomic mass units (AMUs), only one AMU per pulse can be monitored. In order to detect all feed components and possible reaction products during pulse experiments, QMS switches from the measured AMU to the next one after the data collection interval is completed. Usually 10 pulses for each AMU are recorded and averaged to improve the signal-to-noise ratio. Recently, Cleaves and co-workers equipped their TAP reactor with a time-of-flight (TOF) mass... 

The fundamental principles of a quadrupole mass analyser were described in 1953 by Paul and Steinwegen [112]. The device itself consists of four rods in a parallel arrangement, either of circular or hyperbolic cross-sections, and utilises the principle of stable ion trajectories within an oscillating electric field to separate tn/z ratios. For example a positive ion entering a quadrupole will be attracted to a negative rod, however if the polarity of the rod switches to positive before the ion has chance to discharge on it, then the ion will change its path direction. 

Figure 21.9. Eight-channel parallel LC-MS system consisting of gradient HPLC pumps, a multiple probe autosampler, a single quadrupole mass spectrometer equipped with an electrospray ion source, eight microbore columns (10mm x 1 mm i.d. 3 pm, HQ-C18), and a switching valve. The volume of the sample loops is 20 pL and full loop injections were used for all experiments. Total mobile phase flow rate is 2.0 mL/min (0.25 mL/min for each column). The eight-channel LC-MS system allows up to four plates of compounds (or 4 x 1152 samples) to be run in a single day.

The m/z value transmitted by a quadrupole mass analyzer can be scanned or switched very rapidly. The most common ways to acquire data with one of these devices are illustrated in fig. 19. The spectrum can simply be scanned continuously once, as shown in fig. 19a. In the peak-hopping mode (fig. 19b), the mass analyzer transmits a certain m/z value for a chosen time and then quickly switches to another value. This peak hopping is illustrated for three separate m/z positions spanning each of two peaks. This peak-hopping process is generally done repetitively with each mass peak monitored many times. The background can also be determined by hopping to an m/z position devoid of ions. 

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