Quadrupole Filter

Rapid scanning mass spectrometers providing unit resolution are routinely used as chroaatographic detectors. Ion separation is accomplished using either a magnetic sector, quadrupole filter or ion trap device. Ions can also be separated by time-of-flight or ion cyclotron resonance mass analyzers but these devices are not widely used with chromatograidiic inlets and will not be discussed here [20]. 

Different mass analysers can be combined with the electrospray ionization source to effect analysis. These include magnetic sector analysers, quadrupole filter (Q), quadrupole ion trap (QIT), time of flight (TOF), and more recently the Fourrier transform ion cyclotron resonance (FTICR) mass analysers. Tandem mass spectrometry can also be effected by combining one or more mass analysers in tandem, as in a triple quadrupole or a QTOF. The first analyzer is usually used as a mass filter to select parent ions that can be fragmented and analyzed by subsequent analysers. 

The compatibility is excellent with continuous ion sources such as ESI, dynamic SIMS, CF-FAB, ICP, El, Cl, etc. Sector instalments are not well-suited for pulsed ionization methods, although there are examples where MALDI sources have been utilized [225-229]. Sector instruments are usually larger and more expensive than other mass analyzers, such as TOFs, quadrupole filters, and traps. 

However, nowadays some other different mass spectrometers are used for ICP-MS time-of-flight (TOP) systems for multielemental analysis of transient signals, ion trap analysers for ion storage, multicollector instruments for precise isotope ratio measurements and double-focusing sector field mass spectrometers for high mass resolution, but still the majority of instruments are equipped with quadrupole filters, which are simpler and cheaper. 

Thus, the potential at each point in the quadrupole filter as a function of time will be the sum of expressions (16.13) and (16.14) ... 

Figure 16.10—Quadrupole filler. Ions, depending on their mass, will react differently to the AC voltage. A model of a quadrupole filter with a pre-filter is shown (Fisons).

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. 

Figure 16.11 —Partial spectrum of a hydrocarbon obtained with a gas analyser operating under the principle of a quadrupole filter. In this case, the instrument has been used with the constant Am over the mass range.

The collimated ion beam, having left the source, is separated into groups of ions of different mass to charge ratio m/z) usually by employing a magnetic sector or a quadrupole filter. The ability of the mass spectrometer to separate these ions is termed the resolution, and mass spectrometers may be of low, medium, or high resolution. 

Quadrupole filter mass spectrometer, 255 Quality assessment, external, 118, 123 Quality assurance, 118 Quality control, analysis of results, 123 choice of method, 126 data collection, 120 in clinical analysis, 118-127 in immunoassays, 156 internal, 118 serum pools, 119 Quantification, general notice, xvi in gas chromatography, 190 in high pressure liquid chromatography, 208 in mass spectrometry, 262 in thin-layer chromatography, 167 Quartamon, 379 Quarzan, 473... 

The resolution achievable by a quadrupole filter depends on the selection of the U/ Vo ratio and can thus be adjusted throughout the mass range. For this reason the resolution varies throughout a scan of the mass range, with AM in the resolution equation remaining constant. 

The resolution of this spectrometer is not high, but the spectrometer are of relatively low cost and fast they collect a full spectrum of the investigated substance in about 0.3 s. In the spectrometer (also called quadrupole filter), there appears an electromagnetic field which causes the oscillation of ions. At given voltages, the oscillation amphtude of ions under a certain mass is larger than the distance between one of the pairs of electrodes such ions are thrown out of the filter. Through the filter to the detector, there arrive only the ions of a determined mass. 

The resolution of a quadrupole filter (for details see section 16.8.3) is related to numerous parameters. The instrument can be used either in a mode where Am is constant — in which case the resolution increases with the mass (Figure 16.10) — or in a mode in which the resolution is constant, close to 1 u. 

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