Radiofrequency filter

Bailey and Mahadevan have used a radiofrequency filter to measure separately the currents of electrons and of slow negative ions resulting from collisions between O2 and several negative ion species (H", O , OH , 02 ) in the range of reactant ion energies from about 8 to 350 eV. In the case of H 4- O2, the slow product ions are principally O2" from reaction (9), and the cross section shows a maximum of 13 x 10" cm at 8 eV decreasing to 6 x 10 cm at 350 eV. The electron currents observed by Bailey and Mahadevan in reaction of H with O2 were ascribed entirely to collisional detachment... 

Following ionization and fragmentation, the ions produced are separated based on their miz ratio in the mass filter (analyzer). There are several types of analyzers used in GC/MS, including time-of-flight (TOE), magnetic sector, and radiofrequency, which include both quadrupole and ion trap. 

There are two ways to operate a quadrupole filter the voltages V and Frf are maintained constant while v, the radiofrequency, is scanned, or alternatively, v is maintained constant and both voltages are scanned together keeping their ratio at a fixed value. It is thus possible to sequentially obtain the mass spectrum of the compound. 

The maximum resolution is given by equation (16.16) where e represents a unit charge, Vz the acceleration potential, L the length of the quadrupole mass filter, r0 the radius of the filter and l/max the maximum voltage of the radiofrequency. 

The mode of operation of an ion trap can be described in the following way the ions are generated in the central part of the filter by electron ionisation using a short electron pulse. A radiofrequency voltage is then applied to the annular electrode, which confines the ions in the source where they follow complex trajectories in the presence of a low helium pressure of about 0.01 Pa. The mass spectrum is obtained by increasing the radiofrequency amplitude, which destabilises ions of increasing mass. The increase in voltage causes the ions to increase the amplitude of their... 

In considering the molecules in Table 5.2 it should be remembered that the method of detection filters out any molecules with zero dipole moment. There is known to be large quantities of H2 and, no doubt, there are such molecules as C2, N2, 02, II—C=C—H and polyacetylenes to be found in the clouds, but these escape detection by radiofrequency, millimetre wave or microwave spectroscopy. 

Fig. 20. Experimental setup for applications of the SPECIFIC CP experiment in the context of triple-resonance solid-state NMR experiments. After an initial broadband adiabatic CP step from protons to the I nuclei, SPECIFIC transfer to the observed 5 nucleus occurs during the mixing time tm. The resulting signal represents a dipolar and chemical shift-filtered spectrum and can be controlled by variation of the carrier frequencies and the radiofrequency during the SPECIFIC transfer. A conventional HETCOR experiment is obtained by the introduction of an evolution time t. (Adapted from Baldus et al.215 with permission.)...

Figure 4.11 [126]. The experimental techniques are similar to those described in Section III. The alkali resonance doublet emitted by a radiofrequency lamp is separated with a spectral purity of one part in 108 by a grating monochromator in series with an interference filter or filters. The monochromatic beam is then split into two parts One is condensed into a fluorescence cell...

The qua(hupole filter comprises foiu symmdiically ged p allel rods. A field is setup in the space between the rods by tiie use of a radiofrequency field said a d.c. voltage which may be vmied to allow ions of different m/z to reach the detector. 

By allowing multiple-quantum coherence to process during the evolution period of a two-dimensional experiment, Drobny et al. were able to detect its effects indirectly. This idea subsequently blossomed into the new technique of filtration through double-quantum coherence. Multiple-quantum coherence of order n possesses an n-fold sensitivity to radiofrequency phase shifts, which permits separation from the normal single-quantum coherence. This concept inspired the popular new techniques of double-quantum filtered correlation spectroscopy (DQ-COSY) and the carbon-carbon backbone experiment (INADEQUATE), both designed to extract useful connectivity information from undesirable interfering signals. 

Fig. 8 Schematic of a tandem quadrupole MS/MS instrument. A tandem quadrupole MS/MS instrument consists of two quad-rupole MS filters, MSI and MS2, separated by a collision cell. Each quadrupole MS filter consists of four cylindrical or hyperbolic shaped rods. A unique combination of direct current (dc) potential and radiofrequency (rf) potential is applied to each pair of rods (one pair 180° out of phase with the other). A mass spectrum results by varying the voltages at a constant rf/dc ratio. A variety of scan modes (e.g., full scan, product ion, precursor ion, neutral loss) provide unique capabilities for quantitative and qualitative structure analysis. (Courtesy of Micromass, Manchester, UK.)...

The temperature of solid specimens in the plasma was determined by means of an infrared radiation thermometer (Infrascope Model 3-1C00, Huggins Laboratories Inc., Sunnyvale, Calif.) attached to a chart recorder. This device employs a lead sulfide detector and suitable filters to permit remote measurement of 1.2 to 2.5/x radiation emitted by the specimen. Line filters and electrostatic shielding were employed to minimize interaction of the radiofrequency field with the infrared thermometer. To compensate for variations in emissivity and inhomogeneity in the optical field, empirical calibration curves were constructed based on measurements of new and partially oxidized graphite pellets in a con-... 

Figure 16.9 Quadrupolar filter. Depending upon their mass, the ions respond more or less to the demands of the variable field. Their travel time must be superior to the period of the radiofrequency. For this reason they enter the filter with a weak kinetic energy of several tens of electron volts.

Consider now the case of an ion that does not have a velocity component on the x-axis its pathway will remain in the yz-plane. The heavy ions will continue to be inexorably attracted towards the negative bars. Only the light ions that can rapidly react to the influence of the radiofrequency will be transmitted. The yz-plane constitutes a low-pass or highcut filter. 

The calculations related to this technique expose that there are two ways of using this quadrupolar filter one method in which the two voltages, U and V p, are maintained constant while v, the radiofrequency, is scanned, or a second method in which the frequency v is maintained at a given value and the amplitude of the voltages U and are increased regularly in such a way that their ratio remains at a invariable value. 

The maximum resolution is given by expression 16.16 for an ion carrying a unit charge e, accelerated under a PD of volts at the entrance to a filter of length L and of radius by using a radiofrequency whose maximum voltage... 

Originally the radiofrequency spectrum of the output voltage of the optical mixing photomultiplier tube was analyzed with a swept filter... 

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