An Ion Mobility Spectrometer Mass Spectrometer

An ion mobility spectrometer consists of an ion-molecule reaction chamber, incorporating an ionization region, coupled to a drift region via a shutter grid. A schematic diagram is shown in Figure 1. The drift region contains a screen grid and an ion collector. A typical cell consists of metal guard rings, separated by insulators, connected to a resistance network with a high voltage attached to one end of the resistor chain, to produce a uniform electric field along the cell, usually of the order of a few hundred Vcm-1. Clean carrier gas is ionized by irradiation, usually with beta particles from a 63Ni radioactive source, to form positive and negative reactant ions and consequently RIPs. The ion-molecule chemistry can be altered by the introduction of a dopant chemical at

Maquina Inyeccion Plastico
Figure 1 Schematic of an ion mobility spectrometer.

a controlled rate. Samples introduced into the ion mobility spectrometer may react to form product ions, the equilibrium concentrations of which are governed by proton affinity or electron affinity. If introduced into an electric field they will migrate according to their polarity and that of the applied field as, between collisions, individual ions have a component of acceleration in the direction of the applied field. Ions pass from the reaction region to the drift region via a shutter grid, which is pulsed open to allow a finite number of ions to enter the drift region. Operation of the shutter grid starts the timing sequence, which measures drift time. A counter-flow of clean drift gas enters the drift region near the collector electrode, which is shielded by a screen grid in order to prevent induced charge, which would lead to a distorted current peak. By monitoring the collector electrode from the instant the voltage pulse is applied to the grid, a mobility spectrum (see Figure 2) is generated. Mobility spectra can be generated con-

Drift time (ms)

Drift time (ms)

Figure 2 An ion mobility spectrum.

tinuously by repetitive pulsing of the grid. Typically, 25 ms is sufficient time to allow all ions to drift from the grid to the collector electrode. The signal-to-noise ratio is relatively noisy because only small ion currents are involved. The signal-to-noise ratio may be improved by averaging the signal over several scans.

In a mass spectrometer, molecules are ionized by any one of a number of techniques. These ions are then analysed using either magnetic or electric fields or a combination of both and are separated according to their mass-to-charge ratio before being detected. In mass spectrometers using magnetic field separation, a repeller plate directs ions to a set of accelerator plates, used to produce a beam of rapidly moving ions, which are directed into a uniform beam by focussing slits. Neutral molecules are drawn off by vacuum pumps. In a quadrupole mass spectrometer, an oscillating electrostatic field is set up between four rods, two diagonally opposite rods having direct current voltage applied and the other two rods having radio frequency applied. Ions acquire an oscillation in the electrostatic field set up according to the ratio of the direct current to the radio frequency amplitude. Ions of the correct m/z value undergo a stable oscillation of constant amplitude and pass through the analyser to reach the detector. Other ions undergo unstable oscillation and the amplitude of the oscillation increases until the ions strike one of the rods.

Solar Panel Basics

Solar Panel Basics

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