Mass analyser detectors

Sample inlet Ionisation sources Mass analysers Detectors... 

For measuring the inert species, some of which are present in the majority of gases, the thermal-conductivity detector (TCD) is often the detector of choice for gas analyses. Since the TCD is a concentration detector and its sensitivity is lower than that of mass-flow detectors such as the flame-ionization detector (FID), relatively high concentrations of compounds in the carrier gas are needed. This means that packed columns, with their high loadability, are still quite popular for such analyses. 

The operation of this type of device is fundamentally different to those described previously in which ions of one m/z ratio at a time enter the mass analyser. By varying the conditions in the mass analyser, e.g. magnetic field, quadrupole field, etc., ions of different m/z values are brought to the detector and a corresponding mass spectrum obtained. 

Magnetic sector A low-resolution mass analyser in which the variation of a magnetic field is used to bring ions of different m/z ratios to a detector. 

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. 

Sample inlet Source (ion production) Mass analyser Ion detector... 

Figure 4.2 is a block diagram that illustrates the principle of the SIMS technique. The apparatus includes a primary ion source, a vacuum chamber where the objects under study are placed, a mass analyser and a secondary ion detector. 

This is used in rubber analysis. HPLC combined with a mass selective detector enables unknown samples to be analysed. 

GC analyses of the pupal secretion of E. borealis have indicated the presence of vitamin E acetate and other tocopherol derivatives [49,50]. However, in tests with ants, these compounds proved to be essentially inactive, whereas the secretion itself was potently deterrent. To find and identify the active components in the pupal Epilachna borealis secretion, NMR spectroscopic studies on the fresh secretion were carried out. One and two-dimensional NMR experiments revealed that the tocopheryl acetates account for only a relatively small percentage of the beetles5 total secretion (20%), whereas the major components represented a group of previously undetected compounds. By analysis of the COSY, HSQC and HMBC spectra of the mixture, these components were shown to be esters and amides derived from three (co-l)-(2-hydroxyethylamino)alka-noic acids 44-46. HPLC analyses coupled to a mass spectrometric detector revealed that the secretion contain a highly diverse mixture of macrocyclic polyamines, the polyazamacrolides (PAMLs) 47-52 (Fig. 8). 

P33 Analyses wereper/ormed on a gas chromatograph equipped with an electron capture detector (ECD) and a gas chromatograph coupled to a mass-selective detector working in mass spectrometry-mass spectrometry (MS-MS) mode, to achieve better limits of detection and selectivity. The proposed method yields high sensitivity, good linearity, precision, and accuracy. (From Dellinger et ah, 2001)... 

Atomic emission detectors (AEDs) and mass selective detectors (MSDs) are also being used to enhance selectivity and sensitivity for air analyses (Yamashita et al. 1992). 

Figure 1.2 shows the basic instrumentation for atomic mass spectrometry. The component where the ions are produced and sampled from is the ion source. Unlike optical spectroscopy, the ion sampling interface is in intimate contact with the ion source because the ions must be extracted into the vacuum conditions of the mass spectrometer. The ions are separated with respect to mass by the mass analyser, usually a quadrupole, and literally counted by means of an electron multiplier detector. The ion signal for each... 

Since the launch of the first commercial quadrupole ICP-MS instrument in 1983, the technology has evolved from large, floor-standing, manually operated systems, with limited functionality and relatively poor detection limit capabilities, to compact, sensitive and highly automated routine analytical instruments. In principle, all ICP-MS systems consist of similar components a sample introduction system, the ICP ion source, an interface system, the mass analyser, the detector and a vacuum system [8,11]. 

Finally, successful operation of the mass analyser requires a collision-free path for ions. To achieve this, the lens system, mass analyser and detector are operated in a high-vacuum environment (below 10 Torr). 

Fig. 17.7. The MALDI schematic design of a MALDI ToF instrument (Micromass ToF Spec 2E, MMUK) showing ion generation via laser desorption (UV/IR), followed by ion acceleration through the field-free ToF-mass analyser, where the ions are reversed as they enter the reflector to end at the detectors.

Talmi and Bostick [221] extracted methylarsenic compounds from seawater with cold toluene, then analysed the extract by gas chromatography using a mass spectrometric detector. Down to 0.25mg L 1 of organoarsenic compunds were detectable. 

Early mass spectrometers were simply equipped with a SQ mass analyser merely suitable for full scan MS mode and selected ion monitoring. Even though SQ spectrometers are highly superior to UV-detectors with respect to selectivity, they still bear the risk of deterioration by matrix compounds of similar m/z values. Especially, peptides and small proteins may cause a series of diverse m/z values due to their multiple charge states after ES ionization potentially interfering with the analytes or IS [103, 105],... 

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