High mass accuracy analyzers

The m/z values of peptide ions are mathematically derived from the sine wave profile by the performance of a fast Fourier transform operation. Thus, the detection of ions by FTICR is distinct from results from other MS approaches because the peptide ions are detected by their oscillation near the detection plate rather than by collision with a detector. Consequently, masses are resolved only by cyclotron frequency and not in space (sector instruments) or time (TOF analyzers). The magnetic field strength measured in Tesla correlates with the performance properties of FTICR. The instruments are very powerful and provide exquisitely high mass accuracy, mass resolution, and sensitivity—desirable properties in the analysis of complex protein mixtures. FTICR instruments are especially compatible with ESI29 but may also be used with MALDI as an ionization source.30 FTICR requires sophisticated expertise. Nevertheless, this technique is increasingly employed successfully in proteomics studies. 

Ion trap/TOF This combination is advantageous because it enables the MS" capabilities of an ion trap with the high mass accuracy and speed of a TOF analyzer. The ion trap can either be a conventional hyperbolic Penning-type device or a linear trap device. Below we describe the deployment of the former type ion trap/TOF for explosives detection. 

A number of mass analyzers in use today have been coupled to these sources. These include two- and three-dimensional quadrupole field, time-of-flight (TOF), quadrupole-TOF hybrids, magnetic sector, and Fourier transform mass spectrometers. Paramount to the mass spectrometer analyzer used in the analysis is proper sample preparation. With proper preparation of proteins and peptides, their molecular weights can be determined with high mass accuracy. Conversely, a poorly prepared sample will lead to poor or no mass spectrometer results. For peptides and proteins, the mass accuracy is typically better than 0.01%. 

A key factor that has hindered the use of FTICR for imaging applications is the inherently low throughput. For example, the typical imaging speed for FTICR MSI is 4 pixels/min, which is dramatically slower than imaging systems based on TOF mass analyzers or ion traps (30—50 pixels/min). Due to the extended measurement times involved with FTICR, it is recommended to use these other faster imaging methods to preselect areas of interest before the ultra-high-mass accuracy of FTICR is used. 

One of the latest mass analyzer is the linear-trap quadrupole (LTQ) Orbitrap mass spectrometer. In this, the commercial LTQ is coupled with an ion trap, developed by Makarov [73, 74]. Due to the resolving power (between 70000 and 800000) and the high mass accuracy (2-5 ppm), Orbitrap mass analyzers, for example, cab be used for the identification of peptides in protein analysis or for metabolomic studies. In addition, the selectivity of MS/MS experiments can be greatly improved. However, the coupling is not useful with UHPLC for rapid chromatographic pre-separation, as the data acquisition rate is too low for a reproducible integration of the narrow signals produced with UHPLC. 

It is clear that mass spectrometry imaging has great potential as a comprehensive analysis technique for endogenous peptides, particularly with a hardware configuration as evaluated in this chapter, where MALDI produced ions are analyzed in an ion trap - Orbitrap hybrid instrumentation. A single experiment can provide high mass accuracy data in combination with the spatial distribution of peptides in the tissue sample. With MS/MS experiments the molecular identity (accurate mass measurements complemented with MS" sequence data) can be confirmed firom a single or few scans only and from a few laser shots in total. 

The resolution of a mass analyzer is typieally quoted as the unitiess quantity m/Am, where Am is the mass difference between two adjacent peaks that are just resolved and m is the mass of the first peak in the spectrum. TOF-MS can achieve a resolution of up to 50000 (meaning that it can resolve peaks occurring at, e.g., 500.00 and 500.01 atomic mass units (Da), or 50.000 and 50.001 Da], FT-orbitrap up to 500000, and FTICR-MS as much as 500000-1000000. In fact, the latter two mass analyzers offer such high mass accuracy that experiments can even account for the masses of individual electrons (around 0.00054 Da). 

By registering the ion oscillation, the trap can be used as a mass analyzer. Orbitraps have a high mass accuracy (typically 2—5 ppm), a high resolving power (up to 100,000), and a high dynamic range (around 5000) (Makarov et al., 2006a, b). 

TOF mass analyzers have been broadly used for lipid analysis in lipidomics [33, 65, 66]. Although TOF techniques are improved and can measure masses with high mass accuracy/resolution, high sensitivity, and high efficiency, instruments constructed by TOF alone have difficulty in performing MS/MS experiments for lipid analysis. Therefore, hybrid instruments with quadrupoles (i.e., QqTOF) or liner trap (i.e., LIT-TOF) are required to overcome this difficulty. 

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