This review offers an introduction to the principles and generic applications of FT-ICR mass spectrometry, directed to readers with no prior experience with the technique. We are able to explain the fundamental FT-ICR phenomena from a simplified theoretical treatment of ion behavior in idealized magnetic and electric fields. The effects of trapping voltage, trap size and shape, and other nonidealities are manifested mainly as perturbations that preserve the idealized ion behavior modified by appropriate numerical correction factors. Topics include: effect of ion mass, charge, magnetic field, and trapping voltage on ion cyclotron frequency; excitation and detection of ICR signals; mass calibration; mass resolving power and mass accuracy; upper mass limit(s); dynamic range; detection limit, strategies for mass and energy selection for MSn; ion axialization, cooling, and remeasurement; and means for guiding externally formed ions into the ion trap. The relation of FT-ICR MS to other types of Fourier transform spectroscopy and to the Paul (quadrupole) ion trap is described. The article concludes with selected applications, an appendix listing accurate fundamental constants needed for ultrahigh-precision analysis, and an annotated list of selected reviews and primary source publications that describe in further detail various FT-ICR MS techniques and applications. © 1998 John Wiley & Sons, Inc., Mass Spec Rev 17, 1–35, 1998

Fourier transform ion cyclotron resonance mass spectrometry: A primer

Bacteriorhodopsin and the purple membrane of halobacteria

Gas-phase chemistry of collisionally activated ions

4.04 – Pyrazoles and their Benzo Derivatives

The purple membrane of Halobacterium halobium contains the protein bacteriorhodopsin which resembles the visual pigment, rhodopsin, in many aspects. The isomeric configurations of its chromophore, retinal, were studied by a combination of methylene chloride extraction and analysis by high-pressure liquid chromatography. The light-adapted form bR570LA yields solely all-trans-retinal, while the dark-adapted form of bacteriorhodopsin, bR560DA, yields a mixture of 13-cis and all-trans with a ratio of similar to 1;1. The photointermediate M412 in a membrane modified by ether at high NaCl concentration also yields an approximately 1:1 mixture of 13-cis-and all-trans-retinals, while a similar M405 species produced by illumination in 2 M guanidine hydrochloride at high pH yields mainly 13-cis-retinal. These results indicate that the photochemical cycle of bR570LA may involve an isomerization of the retinal chromophore from the all-trans to the 13-cis form.

Identification of retinal isomers isolated from bacteriorhodopsin.

ions generated from a number of different presursors have been studied by high kinetie energy ion—molecule reations. It has been shown that at least four distinct stable species oeeur, of which acetonitrile and methyl isoeyanide retain their original structure. With imidazole or pyrazole as precursors, a mixture of open thain radical cations, not identical to the above species and probably interconvertible via the 1H-azirine radocal cation, is formed. From butrynitrile, pyrrole, crotonitrile, allyl interconvertible via the and cyanocyopropane a fourth species, probably the vinylidenimine ion, is formed.

The formation and structure of radical cations

The mass spectra of ten pyrazole compounds have been determined. Fragmentation schemes have been derived by means of the metastable defocusing method. The predominant process is cleavage of the nitrogen-nitrogen bond resulting in expulsion of HCN. The process second in prominence is the loss of a nitrogen molecule after initial removal of a hydrogen radical or a substituent, giving the species [C3H2R]+, probably a cyclopropenyl ion. In general, the fragmentation pattern is strongly influenced by the substituent.

The mass spectra of some pyrazole compounds

IN the photolysis of the visual pigment rhodopsin the intermediate first observed is bathorhodopsin1,2 (formerly called prelumirhodopsin). It is generally held that isomerisation of 11-cis to all-trans retinal occurs during this initial step. On theoretical grounds, however, an hexaene–amine structure (Fig. 1) has been proposed as an alternative for the chromophore of bathorodopsin (ref. 3 and unpublished results of K. Van der Meer, J. J. C. Mulder and J.L.). This chromophore has an exomethylene double bond between the atoms C5 and C18 and the nitrogen of its ene–amine moiety derives from the ɛ-amino group of a lysine residue of opsin. It is a retrotautomer of the chromophoric group in native rhodopsin which is a protonated Schiff base of 11-cis retinal. We now present two lines of evidence which support the structure depicted in Fig. 1: first, the interpretation of a recently published laser resonance Raman spectrum of bathorhodopsin4, and second, experiments which establish hydrogen (deuterium) exchange during photolysis of rhodopsin.

Structure of the chromophoric group in bathorhodopsin

For compounds C6H5X (XCl, Br, I) under chemical ionization conditions, methylamine causes ipso substitution of X by [NH2CH3]+ and by [NH2]+˙. C6H5F is less reactive; it gives some [C6H5NH2]+˙. Nitrobenzene gives an adduct ion [M+CH3NH3]+, a reduction product ion [C6H5NO2]+˙, and an ion at m/z93, probably a substitution product [C6H5NH2]+˙, but no [C6H5NH2CH3]+. It is also shown that the ion m/z94, formed from nitrobenzene with ammonia as reagent gas, is a substitution product rather than a reduction product ion. Carbonyl compounds C6H5. CO. X give adduct ions and some substitution, mainly [C6H5NH2]+˙.

Electrophilic aromatic substitution under chemical ionization conditions. Methylamine and ammonia as reagent gases

The relative losses of unlabelled vs. labelled HCN from the [M]+˙ and [M – 1]+ ions of a number of specifically labelled 1-methylimidazoles (I) and 1-methylpyrazoles (II) have been determined. Hydrogen randomisation in the molecular ions prior to fragmentation is insignificant. Expulsion of HCN follows two distinct pathways: elimination involving positions 2 and 3 (predominant in I) and elimination involving the methyl group and the nitrogen atom at position 1 (predominant in II). The molecular ions eject H˙ from the methyl groups to a high degree of specificity. In both cases some contribution by position 5 is observed. The resultant [M – 1]+ ions exhibit extensive, but incomplete hydrogen randomisation. Loss of HCN from these ions is consistent with intermediacy of ring-expanded ions, but notably in II a proportion of the HCN is generated from the group. A mechanism for this observation is presented.

J. van Thuijl

Papers

The formation and structure of radical cations

The mass spectra of some pyrazole compounds

Structure of the chromophoric group in bathorhodopsin

Electrophilic aromatic substitution under chemical ionization conditions. Methylamine and ammonia as reagent gases

Fragmentation behaviour and ring expansion of 1‐methylimidazole and 1‐methylpyrazole upon electron‐impact