The Quadrupole Mass Analyzers

In a quadrupole mass analyzer, ions enter an oscillating electric field that is created by four identical rods placed parallel to each other. Both opposite pairs of rods are 

When the ions have an unstable trajectory, they collide with one of the quadru-poles and will not be detected. When they have a stable trajectory, they do not collide with the quadrupole and will be detected. Only specific mjz values are able to pass the quadrupole when certain D C and RF values are applied. In this way, by varying DC and RF values in a controlled way, whole mass spectra can be obtained. Quadrupole MS has unit resolution (R 1000), moderate accuracy ( lOOppm) at a high scan speed, and, when operated in the most sensitive way, picogram sensitivity. 

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The quadrupole mass analyzer utilized by Q-ICPMS is comprised of quadrupole rods with combined DC and RF potentials that can be set to allow analyte ions with a specific... 

The quadrupole mass analyzer [1] consists of four parallel metal rods arranged as in Figure 1. 

Fig. 1.8 Di fferential pumping design with heated capilla. This configuration requires a dual stage pumping system before the ions are introduced into the quadrupole mass analyzer which needs to be operated at high vacuum. The role of the lenses is to focus ions. In some systems the lenses are replaced by hexapoles or octapoles.

Fig. 1.17 The quadrupole mass analyzer is formed by four circular or hyperbolic rods placed in parallel. O Quadrupolar potential.

The quadrupole mass analyzer is a popular economical choice when known compounds are being analyzed and the filter can be set to a limited number of ions. 

A static calibration is used to accurately park the quadrupole mass analyzer on a specific mass of interest. If only a static calibration is performed, the instrument is calibrated for acquisitions where the quadrupoles are held at a single mass as in SIM or SRM. 

In the quadrupole mass analyzer, focusing electrodes direct and accelerate the ionized fragments into a mass filter consisting of four cylindrical electrodes in a vacuum. Tire cylindrical electrodes establish a combination radio-frequency and direct-current electrical field that permits only those ions with a specific, selected mass-to-charge ratio to pass all the way through the filter. The rest of the ions impact die electrodes and do not travel to the exit. Varying the electrical field allows ions with other masses to pass through the filter. 

In the geometric separation method, ions having different m/e ratios are separated according to their geometric position at the collecting spot. The magnetic mass analyzer, the quadrupole mass analyzer, and the ion trap are based on these principles of operation. The magnetic mass spectrometer will not be discussed in this chapter as it has not been used for the detection of explosives. 

The quadrupole mass analyzer [1] consists of four parallel metal rods arranged as in Fig. 1. Two opposite rods are electrically connected and have an applied potential of U+V cos (ot, and the other two rods, also electrically connected, have a potential of -(U+V cos cot), where U is a DC voltage and V cos cot is an RF voltage, co being the angular frequency (co = 2irf). 

As in the quadrupole mass analyzer, the motion of an ion in the trap having an m/e ratio can be described by the Mathieu differential equations ... 

The quadrupole mass analyzer is much smaller and cheaper than a magnetic sector instrument. A quadrupole setup (seen schematically in Figure 1.10) consists of four cylindrical (or of hyperbolic cross-section) rods (100-200 mm long) mounted parallel to each other, at the corners of a square. A complete mathematical analysis of the quadrupole mass analyzer is complex but we can discuss how it works in a simplified form. A constant DC voltage modified by a radio frequency voltage is applied to the rods. Ions are introduced to the tunnel formed by the four rods of the quadrupole in the center of the square at one end to the rods, and travel down the axis. 

Figure 32F-2 A tandem mass spectrometry system. The ions produced in the source are filtered in the first quadrupole so that only a selected ion passes through to the collision cell. A collision gas in this cell causes fragmentation of the selected ion. The fragment masses are sorted by the quadrupole mass analyzer and detected. Usually, the collision cell is also a quadmpole operated in such a way that the fragment ions are directed into the mass analyzer.

Table 3 Single-Stage and Tandem MS Experiments of the Quadrupole Mass Analyzer Summarizing the Qualitative and Quantitative Applications of Each Configuration... 

One limitation of the quadrupole mass analyzer is that when configured for optimal quantitative performance, e.g., SIM or MRM, virtually no qualitative information can be obtained from an assay (aside from some limited applications, such as in-source fragmentation of a metabolic conjugate). Quantitative analysis... 

Other Mass Analyzers. Other analyzers, such as quadrupole ion trap (QIP) and Fourier transform mass spectrometer (FTMS), are of some interest for proteomics. The quadrupole ion trap mass analyzer was devised by Wolfgang Paul it works on the principle of trapping ions with a particular RF in the quadrupole mass analyzer. This device provides a way to eject ions of certain radio frequency and retain the others, only the latter are allowed to reach the detector by scanning ions of a particular radio frequency. In this method, the selected ions can be subjected to fragmentation by collision-induced dissociation (CID), which is useful for the analysis of peptides. 

The most common type pf mass spectrometer used in atomic mass spectroscopy is the quadrupole mass analyzer shown in Figure 11-6, This instrument is more compact, less expensive, and more rugged than most other types of mass spectrometers, it also has ihe advantage of high scan rales so lhat an entire mass spectrum can be obtained in less than I fX) ms, ... 

The quadrupole mass analyzer does not use a magnetic held to separate ions. The quadrupole separates ions in an electric held (the quadrupole held) that is varied with time. This held is created using an oscillating radio frequency (RF) voltage and a constant direct current (DC) voltage applied to a set of four precisely machined parallel metal rods (Fig. 9.22). This results in an AC potential superimposed on the DC potential. The ion beam is directed axially between the four rods. 

The quadrupole mass analyzer consists of four hyperbolic rods at right angles to each other (see Fig. 10.6). A DC voltage is applied to all rods (adjacent rods have opposite signs) and the signs of the voltage are rapidly... 

The radio-frequency (RF)-ICP source was described in Section 7.3.1, and the quadrupole mass analyzer was described in Chapter 9. The interfacing between the ICP sonrce and the quadrupole mass analyzer will be looked at with a little more detail to explain how ICP-MS provides the information it does. 

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