Scanning quadrupoles

Mondello, L., Casilli, A., Tranchida, P.Q., Dugo, G., Dugo, P. (2005) Comprehensive two-dimensional gas chromatography in combination with rapid scanning quadrupole mass spectrometry in perfixme analysis. J. Chromatogr. A 1067 235-243. 

In a quadrupole mass analyzer, only a single mass-to-charge ratio m/z) value is transmitted to the detector for any given combination of radio frequency (RF) and direct current (DC) potentials. Typically, the RF/DC ratio is held constant and scanned to provide a mass spectrum. If, for example, a quadrupole is scanned from m/z 1 to 1000 in 1 second, then any particular m/z is transmitted to the detector for only 1 millisecond, representing a duty cycle of 0.1 %. Thus, a quadrupole mass analyzer has a low transmission duty cycle in the full-scan mode, which results in limited full-scan sensitivity. In contrast, ion-trap and TOF mass analyzers have the theoretical potential to transmit all ions that enter the mass analyzer and yield far better sensitivity across the entire mass spectrum. In reality, the pulse sequences associated with these analyzers devote significant time to functions such as ionization and detection. The actual duty cycles are generally between 10 and 25%, still far better than a scanning quadrupole mass spectrometer. 

Two almost simultaneous papers reported on the feasibility of rapid-scanning quadrupole mass spectrometers (qMS) with an electron-capture negative ion (ECNI) option as detector for GCxGC analysis of PCBs [26] and PCDD/Fs [25]. In both studies, the instrument was operated in the single ion monitoring (SIM) mode. The limitation in the number of scanned ions resulted in the desired 33-50 Hz acquisition rate. However, the selected mass range should still ensure that... 

The linear ion trap analyser is a variation of the ion trap analyser introduced to combat some of the problems that had been encountered with the ion trap in relation to the space-charge effects. The Unear ion trap utilises two trapping electrode rings on the end of a quadrupole. This allows the instrument to be used in a normal scanning quadrupole mode it also allows for the end electrodes to be switched on to retain only specific ions in the trap for collision with the damping gas. 

PeikinElmer uses a patented scanning quadrupole-based collision/reaction cell, called a universal cell, which can operate with collision/KED or reactive gas. The scanning quadrupole in the cell removes side reaction product and new interferences, so that only the analyte passes to the analyzing quadrupole. 

Fig. 2.4 Triple quadrupole MS, QQQ, where Q1 and Q3 indicate scanning quadrupoles and Q2 indicates the collision cell consisting of either an RF-only quadrupole, hexapole, or octapole. In some hybrid instruments Q3 is replaced by either a TOF or a linear quadrupole ion trap. (See text for more details and references for further reading)...

Full scan ion trap analysis is well known to be more sensitive (often by an order of magnitude) than full scan quadrupole analysis. Fig. 1 illustrates capillary column GC/MS mass chromatograms obtained during the analysis of 100 pg standard mixture containing DMMP (2), DEMP (3), soman (4) and CR (8). 

Mondello L, Sciarrone D, Casilli A, Tranchida PQ, Dugo P, Dugo G (2007) Fast gas chromatography-full scan quadrupole mass spectrometry for the determination of allergens in fragrances. J Sep Sci 30 1905-1911... 

The conditions under which a mass spectrometer operates places certain restraints on on-line chromatographic methods. While ionization can be assumed to be instantaneous, mass analysis is not. As an example, fast-scanning quadrupole and magnetic sector instruments provide scan speeds... 

Another benefit of the fast acquisition time is that qualitative or semiquantita-tive analysis is relatively seamless compared to scanning quadrupole technology, because every multielement scan contains data for every mass. This also makes spectral identification much easier by comparing the spectral fingerprint of unknown samples against a known reference standard. This is particularly useful for forensic work, where the evidence is often an extremely small sample. 

So, based on counting statistics, it is logical to assume that the more ions are counted, the better the precision will be. To put this in perspective, it means that at least 1 million ions need to be counted to achieve an RSD of 0.1%. In practice, of course, these kinds of precision values are very difficult to achieve with a scanning quadrupole system because of the additional sources of noise. If this information is combined with our knowledge of how the quadrupole is scanned, we begin to understand what is required to get the best precision. This is confirmed by the spectral... 

But it should be emphasized that while a reactive gas efficiently removes interferences, it is also capable of creating new interferences if not properly controlled. For that reason, an optimized reaction cell (coupled with a single mass analyzer) requires the use of a scanning quadrupole to prevent these new interferences from forming through the creation of a unit-resolution bandpass filter, in order to allow only a single mass to pass through. 

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