Mass analyser quadrupole

This consists of four rod-like electrodes arranged parallel to each other about a central axis. Potentials with a radio-frequency component are applied to these rods in order to create an electrostatic mass filter. Only masses within a specified range can pass down the axis of the filter and be detected. Ions outside this mass range exhibit unstable oscillations and eventually hit one of the electrodes. By varying the potentials applied to the electrodes mass scanning can be achieved. 

---

An MS-MS instrument only relatively recently made available commercially for LC-MS applications is the Q-ToF system, i.e. the combination of a quadrupole mass analyser for precursor-ion selection and a time-of-fiight analyser for production detection. As described earlier in Section 3.4.1.4, this instrument has the... 

Hybrid mass spectrometer An MS-MS instrument combining magnetic sector and quadrupole mass analysers. 

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. 

Q. What are the main differences between the scanning and peak-hopping mode of operation of a quadrupole mass analyser ... 

Plasma MS is usually based on quadrupole mass analysers. The atmospheric ICP, optimised for ion formation, is placed on its side facing a sample cone (Fig. 4.3). The mass spectrometer operates at reduced pressure and therefore a two- or three-stage differentially pumped interface is needed to transfer the ions from the plasma to the mass analyser. The interface for GC-ICP-MS is generally the same as for ICP emission systems. In one of the earliest GC-MS speciation studies (Chong and Houk, 1987) a packed GC column was used to obtain mass spectra of organic compounds with detection limits in the range 0.001-500 ngs The effects of isotopic fractionation by natural physico-chemical processes were also studied. 

Two techniques that have become preffered for ionisation of proteins/peptides is electrospray ionisation (ESI) and matrix-assisted laser desorption/ionisation (MALDI). Although different combinations of ionisation techniques and mass analyser exist, MALDI usually is coupled with a time-of-flight (TOF) (Figure 7) tube as a mass analyser while ESI is tradionally combined with quadrupole mass analysers. Instruments capable of MS/MS have the ability to select ions of particular m/z ratio from a mixture, to fragment selected ions and to record the precise masses of the resulting fragment ions. If this process is applied to the analysis of peptide ions, in principle the amino acid sequence of the peptide can be deduced. 

Quadrupole mass analysers use two pairs of parallel rods on either side of the ion beam. By modulating the voltage across these rods, species of particular m/z ratios are selected to pass to the detector and other species are lost. 

Atmospheric pressure ionization. A mass spectrometer in which sample ionization is achieved at atmospheric pressure (API) in an external source has been described [127, 128] and applications reported [129]. The source of electrons is a Ni foil and samples, injected in organic solvent, are swept through the reaction chamber (ion source) in a stream of nitrogen carrier gas. Ions and neutral molecules then enter a quadrupole mass analyser via a 25 )u,m diameter aperture. Continuous analysis of the ions, either positive or negative, formed in the reaction chamber, may be achieved by repetitive scanning or by selected ion monitoring. The ion-molecule reactions in the production of sample ions involve the carrier gas and the solvent. Thus the quasi-molecular ion for a compound injected in benzene as solvent may result from the following sequence ... 

A major breakthrough was achieved by Fenn et al. [5-6] using ESI-MS in 1988. They demonstrated the generation of multiple-charge ions from large proteins by ESI-MS, enabling their detection and molecular-mass determination with a relatively inexpensive quadrupole mass analyser. Some of their early mass spectra are shown in Figure 16.2. 

In addition to structural characterization, quantitative analysis of the various species is important in many application areas. Due to the effect of unsaturation on the relative abundance of the [M+H]" and mass discrimination effects in favour of the DAG fragment ion in a quadrupole mass analyser, determination of response factors for the various species is required for an accurate quantitative analysis [24]. Therefore, limiting the number of standards that are required in the analysis of complex TAG mixtures is an important topic in quantitation. From a comparison of four different approaches, Byrdwell et al. [24] concluded that the most accurate method involved the calculation of response factors from contributions from individual fatty acids to the TAG response. This approach was apphed with good results by others as well [17]. 

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]. 

There are, however, several ions which are produced by the torch. Polyatomic ions containing elements from the solvent and/or the analyte and/or Ar are common. Given the resolving power of quadrupole mass analysers... 

---