Resolution quadrupole analysers

Resolution Quadrupole instruments are not capable of achieving the high resolution that is common with double-focusing magnetic-sector instruments. In GC/MS analyses, a compromise is struck between sensitivity (ion transmission) and mass resolution. In the quadrupole instrument, the resolution is set to the lowest possible value commensurate with resolving peaks differing by 1 Dalton (unit resolution). 

Quadrupole mass filters, Qs, which are still widely used for flavonoid analysis, isolate ions of a selected miz ratio. They are mainly able to perform low-resolution mass analyses,... 

Quadrupole mass spectrometers (mass filters) allow ions at each m/z value to pass through the analyser sequentially. Therefore, the ion collector at the end of the quadrupole unit needs only to cover one point or focus in space and can be placed immediately behind the analyser. A complete mass spectrum is recorded over a period of time (temporarily), which is set by the voltages on the quadrupole analyser. The resolution of m/z values is dependent solely on the analyser and not on the detector. 

However, an enhanced resolution quadrupole (Schoen 2001 Yang 2002) can in principle provide increased selectivity in quantitative analyses using SIM or MRM. An example in which this advantage is clearly evident is shown in Figure 6.12, where the SRM chromatogram obtained using a RP yielding a peak width (FWHM) of... 

Enhanced Mass Resolution with Quadrupole Analysers... 

A recent development for the improved mass resolution of quadrupole analysers has been published and commercialized for a SIM technique in MS/MS named H-SRM (< 0.4 Da) or U-SRM (< 0.2 Da), selected reaction monitoring by using enhanced mass resolution. Hyperbolic quadrupole rods of a true hyperbolic pole face, high-precision four-section design with a rod length of 25 cm length... 

Figure 2.178 Low resolution (left, TSQ 7000) and enhanced mass resolution (right, TSQ Quantum HyperQuad ") quadrupole analysers. (Thermo Fisher Scientific.)...

Recently, an extremely powerful hybrid Q-TOF system has been introduced, based on a quadrupole analyser as the front-end and a TOF analyser as the back-end (Q-q ipTOF geometry). As a result of the orthogonal geometry of the quadrupole and TOF, the TOF analyser enables high-resolution operation and thus accurate mass determination (up to 5 ppm), which can be extremely useful in structure elucidation by MS-MS, e.g. in peptide sequencing and impurity profiling. 

The resolution obtainable with a UTI-100C quadrupole mass analyser is m/Am 2m (Jjt)). These three peaks are also separated to baseline resolution In Figure 7a however, they appear as one peak due to the wide mass range which is displayed. 

Specificity is unsurpassed. Traditionally, MS was performed on very large and expensive high-resolution sector instruments operated by experienced specialists. The introduction of low-resolution (1 amu), low-cost, bench-top mass spectrometers in the early 1980s provided analysts with a robust analytical tool with a more universal range of application. Two types of bench-top mass spectrometers have predominated the quadrupole or mass-selective detector (MSD) and the ion-trap detector (ITD). These instruments do not have to be operated by specialists and can be utilized routinely by residue analysts after limited training. The MSD is normally operated in the SIM mode to increase detection sensitivity, whereas the ITD is more suited to operate in the full-scan mode, as little or no increase in sensitivity is gained by using SIM. Both MSDs and ITDs are widely used in many laboratories for pesticide residue analyses, and the preferred choice of instrument can only be made after assessment of the performance for a particular application. 

In general, the mass resolution required for most analyses is such that the singly charged isotope patterns of the detected ions are readily discernible. For applications involving molecular weights up to about 1500 Da, this can be provided by magnetic sector, quadrupole, and time-of-flight mass spectrometers. 

ToFs can also be used in combination with other mass analysers. Both hybrid sector and quadrupole systems are available. oaToF-MS has been interfaced to a quadrupole mass filter and hexapole gas collision cell, such as to allow recording of mass spectra and product ion spectra with good mass resolution (ca. 10000), high sensitivity, high mass range (ca. 10 000 Da) and high mass accuracy (<5ppm) [177,178]. QqToFMS may be fitted with API sources with flow-rates from nL... 

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