Mass FTMS analyzers

Figure 7b also illustrates the high detection sensitivity of the FTMS instrument. We calculate that the CO peak corresponds to approximately 5000 ions in the analyzer cell. In Figure 7a, the number of ions with m/z 43 was calculated to be approximately 20 million. A point to note is that In FTMS the sensitivity increases with resolution whereas it decreases with other mass spectroscopies. 

Multiple mass analyzers exist that can perform tandem mass spectrometry. Some use a tandem-in-space configuration, such as the triple quadrupole mass analyzers illustrated (Fig.3.9). Others use a tandem-in-time configuration and include instruments such as ion-traps (ITMS) and Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS or FTMS). A triple quadrupole mass spectrometer can only perform the tandem process once for an isolated precursor ion (e.g., MS/MS), but trapping or tandem-in-time instruments can perform repetitive tandem mass spectrometry (MS ), thus adding n 1 degrees of structural characterization and elucidation. When an ion-trap is combined with HPLC and photodiode array detection, the net result is a profiling tool that is a powerful tool for both metabolite profiling and metabolite identification. 

What mass analyzer exhibits the highest achievable mass resolving power (ICR spectrometer, a.k.a. FTMS). 

Orbitrap The newest of the major mass analyzers, the Orbitrap is a hybrid MS consisting of a LIT mass analyzer, or transmission quadmpoles connected to the high-resolution Orbitrap mass analyzer. The Orbitrap utilizes electrical fields between sections of a roughly egg-shaped outer electrode and an inner (spindle) electrode (Chapter 5). Ions orbit between the inner and outer electrodes and their oscillation is recorded on detector plates (Hardman and Makarov, 2003 Hu et al., 2005). As with the FTICR, fast Fourier transform of the raw data is used to convert the data for mass analysis, making the Orbitrap the second major type of FTMS instrument. The resolving power of the Orbitrap is intermediate... 

Figure 7.7 An FTMS mass analyzer cell depicting the motion of trapped ions in the presence of the magnetic field.

Figure 7.8 Excitation (a) and detection (b) of the ion cyclotron motion within an FTMS mass analyzer cell. Reprinted from Marshall, A.G. and Flendrickson, C.L., Fourier transform ion cyclotron resonance detection principles and experimental configurations. International Journal of Mass Spectrometry, 215, 59-75. Copyright (2002), with permission from Elsevier.

Figure 7.10 A commercial FTMS system with a front-end nanoflow LC system. The schematic cutaway view depicts the sequential vacuum stages that are required to transfer ions in the ESI source (formed at atmospheric pressure) to the mass analyzer cell of the FTMS (10 ° mbar).

Time-of-flight (TOF) MS detectors (Fig. 15.7) are commonly used in pro-teomics studies of proteins and protein fragments because this type of detector can handle and analyze very large molecular and fragmentation ions. Fourier transform mass spectrometers (FTMS) are being incorporated into commercial LC/MS systems and offer the advantage of being nondestructive detectors that can trap and repeatedly analyze the same sample in order... 

All experiments were performed using a Nicolet Analytical Instruments FTMS-2000 dual-cell Fourier transform mass spectrometer with optional GC and laser desorption interfaces. The FTMS-2000 dual cell is specially constructed of stainless steel with low magnetic susceptibility. This permits very efficient ion transfer between the source and analyzer cells, if the cells are properly aligned in the magnetic field. 

Figure 2. Peppermint oil analyzed by GC/FTMS (a) total ion chromatogram (b) mass spectrum of component eluting at 15 minutes (c) enlargement of region around m/z 139 from (b) (d) mass chromatogram for m/z 139 +/- 0.5 amu.

For this work, a 5 meter x 50 micron ID fused silica column, coated with a 0.25 micron polydimethylsiloxane film was introduced directly into the source chamber through the transfer line normally used for GC/FTMS. A restrictor was created at the end of the column by using a microflame to draw out the end of a 1 meter portion of deactivated but uncoated column to an inside diameter of approximately one micron. Details of the instrumentation used for SFC have been described elsewhere [19]. With the SFC interface in place, pressures in the source chamber were approximately 5 x 10 5 torr. Despite this high source cell pressure, we were able to obtain relatively high quality mass spectral data with analyzer side detection at 5 x 10"7 torr. 

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