Matrix Orbitrap

One recent advance in MS hardware that has been found to be useful for metabolite identification studies is the Orbitrap. This MS has a mass resolution of 30,000 to 100,000 (two models). For many applications, 30,000 mass resolution capability is sufficient. While only a few current literature references cite the Orbitrap MS for metabolite identification, it is safe to predict that the Orbitrap will be the subject of many references in the future. Two references related to its use for metabolite identification are Peterman et al.190 and Lim et al.182 Lim s group related an an impressive example of the use of high mass resolution to differentiate a metabolite from a co-eluting isobaric matrix component, as shown in Figure 7.14. 

Technological advances of ion-trap mass spectrometers are the ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and the recently released technique, the Orbitrap Fourier transform mass spectrometry (Hu et al., 2005), which enable the determination of molecular formulae with a high mass resolution and mass accuracy in mixtures. Today these ion-trap mass spectrometers are most frequently coupled with atmospheric pressure ionization (API) techniques such as electrospray ionization (ESI) (e.g., Fievre et al., 1997 Qian et al., 2001 Kujawinski et al., 2002 Llewelyn et al., 2002 Stenson et al., 2002,2003 Fard et al., 2003) or matrix-assisted laser desorption/ionization (MALDI) (e.g., Solouki et al.,... 

Landgraf R, Conaway M, Garrett T, Stacpoole P, Yost R (2009) Imaging of lipids in spinal cord using intermediate pressure matrix-assisted laser desorption-liner ion trap/orbitrap MS. Anal Chem 81 8488-8496. doi 10.1021/ac901387u... 

Mass spectrometers that are routinely used in drug discovery include the single quadrupole [48], triple quadrupole [49], quadrupole ion trap [50], time-of-flight [51], quadrupole time-of-flight [52], and Orbitrap [53] mass spectrometers. Mass spectrometers are primarily used to perform two functions (1) discriminate against matrix interference and (2) maximize the signal for the ion(s) of interest [29],... 

A variety of mass analyzers have been used in MSI experiments such as TOP, quadrupole ion trap (QIT), linear ion trap (LIT), QqQ, Fourier transform ion cyclotron resonance (FTICR), and Orbitrap. Also, various tandem configurations of these mass analyzers such as QqTOF, QqLIT, and TOF/TOF have been used. Due to the many possible interferenees from endogenous compounds or from the matrix, the use of tandem mass speetrometry (MS/MS or MS") or the ability to perform high-resolution and aeeurate mass measurements for the analysis of drugs by MALDI is essential. An overview of some of the established instrumentation and their respeetive eapabilities is presented below. 

Fig. 25.5. (a) Example of direct tissue MS and MS/MS data (American cockroach neurohemal glands (cc and ca-complex) sprayed with the DHB matrix). MS profiling of ca spectrum averaged over the ca area of the tissue sample (see Note 5). Molecular mass measurement errors in ppm are calculated (Protein Calculator) and indicated for obsen/ed peptides (see Table 25.1 for peptide identifications), (b) Orbitrap (FTMS) MS/MS spectrum of m/z 996.6467, yielding high accuracy fragment ions confirming peptide sequence [LVPFRPRLamide Pea-PK-lll]. 

Fig. 25.6. Cockroach ca prepared with dried-droplet matrix deposition (a) before matrix addition, (b) after matrix addition, (c, d) MALDI LTQ Orbitrap images green color represents selected lipid ion at m/z 610.444 (c), and Pea-PK-I peptide MH+ ion at m/z 1,010.587 (d). Dark blue color shows intense DHB matrix crystals from the overlaid photograph (optical image). In contrast to the 610.444 ion, the PK-I peptide is clearly diffused out of the tissue into the matrix droplet. The MS image is indicated using the Rainbow color scheme, where relative abundance is coded as red>yellow>green>blue = zero intensity (see Notes 6 and 7).

Solvent-free separation using IMS-MS when combined with solvent-free sample preparation and appropriate ionization methods provides TSA that are independent of analyte solubility and is applicable to complex mixtures. A new ionization method, LSI, which operates at AP with laser ablation of samples prepared in a MALDI matrix, has been effectively interfaced with a SYNAPT G2 IMS-MS instrument. Early results show separation of protein mixtures and that protein ion structures from LSI and ESI are similar. Further, a new matrix allows LSI multiply charged ion formation to be extended to solvent-free matrix preparations. Thus TSA by LSI-IMS-MS is a new approach to tissue imaging at high spatial resolution on high-end mass spectrometers such as the SYNAPT G2 and Orbitrap instruments. LSI has... 

To couple a low-pressure MALDI source to a QIT, LQIT, or orbitrap mass analyzer, the primary concern other than having an efficient MALDI source is to be able to trap the gas-phase ions efficiently inside the analyzer. In the low-pressure MALDI experiment, ions can be formed inside the ion trap or they can be generated outside the device and injected into the analyzer. Ions are lifted by the matrix plume at a velocity of 500 m/s with several electron volts of energy and may undergo additional reactions in the plume. Once in the gas phase, the ions are accelerated as a result of either (a) an RF field in the case of internal ionization or (b) an external lens system for the external ion formation/injection scheme. Ions can be trapped efficiently in a QIT or LQIT if they have low kinetic energies (typically <20 eV), but the trap must be set to the appropriate RF trapping potentials and filled with helium buffer gas. In two similar configurations from 1993, the QIT was... 

Detection limits of analytes measured in ESI and MALDI using the mass analyzers covered in this chapter are quite noteworthy. Table 9.3 lists detection limits for various molecules analyzed by LC, CE, and EIA coupled to ESI QIT, LQIT, and the orbitrap or with the use of MALDI. The lowest limits of detection are measured when an isolation step is used to accumulate and/or isolate only the ion(s) of interest. For example, in Table 9.3, the detection limits for reserpine in SIM and SRM scan modes are better than in the full-scan mode. Although ion trap full MS scans have a higher duty cycle over QMF ion beam instruments, detection limits can be reduced in the full-scan MS mode due to matrix effects. In this latter case, at low analyte amounts, the QIT is filled up primarily with unwanted matrix ions, leaving little space to store the ions of interest. The advantage of the high-charge-capacity... 

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