Radio-frequency potential, mass

Mass spectrometer that consists of four parallel rods whose opposing poles are connected. The voltage applied to the rods is a superposition of a static potential and a sinusoidal radio frequency potential. The motion of an ion in the x and dimensions is described by the Mathieu equation whose solutions show that ions in a particular m/z range can be transmitted along the z-axis.1... 

Chemical ionization with an ion trap mass spectrometer is possible at much lower-reagent gas pressures and longer reaction times than typical external sources. A preliminary ionization period is used to establish the reagent gas ion concentration, followed by a relatively long (10s of ms) reaction period at a slightly higher radio frequency potential, during which sample ions are formed and stored. The radio... 

A transmission hexapole, octapole or square quadrupole colhsion ceU to which an osciUating radio frequency potential is apphed that is filled with a collision gas at low pressure and used to generate collision induced dissociation (CID) of ions to form a product ion spectrum. The coUision cell has no mass separating capabihties. Axial fields accelerate product ions to leave the colhsion cell for a fast switch of SRM reactions. 

Quadrupole mass analyzers consist of four rods that produce a radio frequency quadrupole field. Only a single mass-to-charge ratio has a stable trajectory through the mass analyzer, but the electric potentials can be swept rapidly, either continuously or in discrete steps to measure a variety of masses. Quadrupole mass spectrometers are used in a wide variety of applications, but are not widely used in cosmochemistry. 

Quadrupole mass analyzers consist of four parallel metal rods which are either round-shaped or ideally have a hyperbolic section. Ion separation is realized by applying an alternating electrical fleld to the four rods with each pair of adjacent rods having the opposite signs of the potential. The potential applied to two opposite rods is a superposition of a constant potential U and an alternating potential V cos mt (Eq. (5.1)). The other two opposite rods bear the potential . co describes the angular frequency and equals Inv when v is the frequency of the applied radio frequency field t is the time. 

In a quadrupole mass analyzer, only a single mass-to-charge ratio m/z) value is transmitted to the detector for any given combination of radio frequency (RF) and direct current (DC) potentials. Typically, the RF/DC ratio is held constant and scanned to provide a mass spectrum. If, for example, a quadrupole is scanned from m/z 1 to 1000 in 1 second, then any particular m/z is transmitted to the detector for only 1 millisecond, representing a duty cycle of 0.1 %. Thus, a quadrupole mass analyzer has a low transmission duty cycle in the full-scan mode, which results in limited full-scan sensitivity. In contrast, ion-trap and TOF mass analyzers have the theoretical potential to transmit all ions that enter the mass analyzer and yield far better sensitivity across the entire mass spectrum. In reality, the pulse sequences associated with these analyzers devote significant time to functions such as ionization and detection. The actual duty cycles are generally between 10 and 25%, still far better than a scanning quadrupole mass spectrometer. 

A quadrupole mass filter consists of four hyperbolic rods on which a potential U F0cos cot is applied (see Fig. 2.6). The hyperbolic shape lead to the production of a quadrupolar field, where the field intensity is linearly dependent on space. However, these conditions can be validly approximated by the use of cylindrical rods (see Fig. 2.7). The ions are injected in the z direction and experience a field due to the direct current (dc) and radio frequency (rf) potentials applied on... 

Ions prcxluced in the center of the trap (usually by electron impact) are constrained by the magnetic field and applied potentials on the trapping plates to follow orbits within the cell and only slowly diffuse toward the walls. During the time the ions are trapped, they may have maiy collisions with neutral molecules in the trap, and these collisions can lead to thermalization of the ions and may result in chemical reactions. At a neutral pressure of 10 torr, the approximate pseudo first-order rate constant for ion-molecule collisions is 30 s. The ionic products of ionization and subsequent reactions in the cell can be detected at aiy time mass spectrometrically application of a broad range of radio frequencies and detection of resonance at cyclotron frequencies corresponding to m/z values of the ions present. Techniques are available that allow isolation of one or more m/z value in the ion population by ejection of all other ions, and this allows the reactions of a particular ion to be followed directly. Fourier transform methods, intrcxluced by Marshall and... 

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. 

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