Quadrupole, separating system

Even without solving the differential equation, it is possible to arrive at a purely phenomenological explanation which leads to an understanding of the most important characteristics of the quadrupole separation system. 

The concept of peak capacity is rather universal in instrumental analytical chemistry. For example, one can resolve components in time as in column chromatography or space, similar to the planar separation systems however, the concept transcends chromatography. Mass spectrometry, for example, a powerful detection method, which is often the detector of choice for complex samples after separation by chromatography, is a separation system itself. Mass spectrometry can separate samples in time when the mass filter is scanned, for example, when the mass-to-charge ratio is scanned in a quadrupole detector. The sample can also be separated in time with a time-of-flight (TOF) mass detector so that the arrival time is related to the mass-to-charge ratio. 

Dynamic mass separation systems use the fact that ions with different masses (accelerated with the same voltage) possess several velocities and consequently their flight times are different. There are about 50 dynamic separation systems known2 using several types of ion movements (linear straight ahead, linear periodic or circular periodic as a function of the electric or magnetic sector field applied). The simplest dynamic mass separation system is a linear time-of-flight (ToF) mass analyzer, and a widely applied mass separation system is the quadrupole analyzer. 

Beside the most commonly used ion separation systems, such as the quadrupole mass analyser, the ion-trap mass analyser and the double-focusing and tri-sector mass analyser, a new system has encountered increasing interest. This system, the time-of-flight (TOP) mass analyser offers fast scanning opportunities coupled with the possibility of detecting ions with high m/z ratios. This provides a useful tool for rapid quality control of sensorial profiles as well as the detection and identification of toxins and proteins or protein-bound substances [42]. 

For ions produced in the gas phase, typical of most ion sources coupled to separation systems, a combination of quadrupole injection and an orthogonal accelerator produces a focused ion beam with a small kinetic energy distribution from virtually any ion source. Early interfaces between the ion source and mass analyzer merely provided... 

The development of the concept of linking analytical instrumentation is motivated by the need for chemists to unequivocally detect, characterize, and quantify compounds in unknown mixtures. Mass spectrometry has long been recognized as a valuable analytical tool because of its superior reproducibility, repeatabdity, specificity, and limits of detection. The coupling of the universal detection power of mass spectrometry (MS) with the relatively inexpensive and versatile separation capabilities of gas chromatography (GC), together with the technological advances that have been achieved over the past 30 years, has led to widespread use of GC/MS instrumentation, some 80% of which is represented by GG/MS quadrupole-based systems. 

FIGURE 2.9 Separation on Sm and Nd in irradiated mixed oxide sample after injection of 1 mL containing 20 pg fuel mL- measured with the HPLC-Quadrupole ICPMS system. (From Gunther-Leopold, I. et al.. Characterization of spent nuclear fuel by an online combination of chromatographic and mass spectrometric techniques, in Proceedings of the Seventh International Conference on Nuclear Criticality Safety, ICNC, Tokai, Japan, October 2003, JAERI-Conf 2003-019, 2003. With permission.)... 

The range of systems that have been studied by force field methods is extremely varied. Some force fields liave been developed to study just one atomic or molecular sp>ecies under a wider range of conditions. For example, the chlorine model of Rodger, Stone and TUdesley [Rodger et al 1988] can be used to study the solid, liquid and gaseous phases. This is an anisotropic site model, in which the interaction between a pair of sites on two molecules dep>ends not only upon the separation between the sites (as in an isotropic model such as the Lennard-Jones model) but also upon the orientation of the site-site vector with resp>ect to the bond vectors of the two molecules. The model includes an electrostatic component which contciins dipwle-dipole, dipole-quadrupole and quadrupole-quadrupole terms, and the van der Waals contribution is modelled using a Buckingham-like function. 

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. 

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