LSIMS spectrum

If the liquid that is being bombarded contains ions, then some of these will be ejected from the liquid and can be measured by the mass spectrometer. This is an important but not the only means by which ions appear in a FAB or LSIMS spectrum. Momentum transfer of preformed ions in solution can be used to enhance ion yield, as by addition of acid to an amine to give an ammonium species (Figure 4.3). 

Desorption/ionisation techniques such as LSIMS are quite practical, as they give abundant molecular ion signals and fragmentation for structural information. In the conditions of Jackson et al. [96], all the molecular ion and/or protonated molecule ion species were observed in the LSIMS spectrum when only 1 pmol of each additive was placed on the probe tip. However, as mentioned above, in LSIMS/MS experiments the choice of the matrix (e.g. NBA, m-nitrobenzylalcohol) is very important. Matrix effects can lead to suppression of the generation of molecular ions for some additives. LSIMS is not ideal for the quantitative detection of polymer additives, as matrix effects are very important [96]. 

Figure 1. Mass spectra of a tryptic peptide from the heavy chain of monoclonal andb y CEA11.H5. A. ESI spectrum showing the doubly charged (655.6) and single charged (1310.6) ions. B. LSIMS spectrum for HPLCfiacdtm 50 (m/z 1310.1). C. MALDI spectrum for die same firacdon (m/z 1310,2258,2906, and 4118). An external standard was used far calibration (bovine insulin).

The inset shows the LSIMS spectrum after C-18 fractionation where the signal is barely distinguished from the noise. After further purification over a phenyl column the signal noise is ca. 15 1. Our study identified a number of sites of hemoglobin modification with styrene oxide, including the nucleophilic cysteine [5(93), and these results will be published in detail elsewhere. 

Table 7.83 lists the main characteristics of TLC-FAB-MS/LSIMS. A key difference between EI/CI and FAB/LSIMS/LD is the fact that sampling in FAB and LSIMS is from a specified location that corresponds to the impact footprint of the primary particle beam. The natural compatibility of FAB, LSIMS and LD with the direct mass-spectrometric analysis of TLC plates is readily apparent. Most mass-spectrometric measurements are destructive in nature, but FAB and LSIMS are surface-sensitive techniques in which the material actually consumed in the analysis is sputtered only from the top few microns of the sample spot. The underlying bulk is not affected, and can be used for further probing. The major limitation of TLC-FAB depends on the capability of the compounds to produce a good spectrum. 

The fragmentation patterns and characteristic fragment ions for the carotenoids observed in FAB-MS and LSIMS tandem mass spectra are also observed in the tandem mass spectra obtained following ESI (see Basic Protocol 4), APCI (see Basic Protocol 5), and other methods. A detailed account of structure determination of carotenoids using FAB ionization with CID and MS/MS is presented in van Bree-men el al. (1995). Finally, another advantage of MS/MS is that matrix ions formed during FAB-MS or LSIMS, and any other contaminating ions, are eliminated, which simplifies interpretation of the mass spectrum. 

MALDI-TOF-MS facilitates the analysis of carotenoids and other natural products with detection limits that are lower than most other techniques. For example, subpicomole quantities can be detected (Wingerath et al., 1999). The enhanced sensitivity is the result of the efficiency of the pulsed ionization and detection system in which a complete mass spectrum is recorded with each laser flash. Like FAB and LSIMS, molecular ions are the most abundant sample ions, although some protonated molecules and [M-H]+ ions may be formed as well. Abundant molecular ions of carotenoid esters have been observed using MALDI-TOF-MS (Kaufmann et al., 1996 Wingerath et al., 1996),... 

The increased mass accuracy available when the instrument was operated in the reflectron mode was important for the analysis carried out. For example, fractions 4b and 7 or fractions 5 and 8 from C. ermineus appeared to be the same species when measured with the instrument operated in the linear mode. Only in the reflectron mode were we able to reliably distinguish the masses of each species. The high sensitivity of MALDI-TOF is particularly important for the analysis of native peptides such as conotoxins where often the venom of many milkings must be collected to obtain sufficient material for sequence analysis. The increased sensitivity of MALDI over LSIMS is illustrated in the analysis of fraction 5 from C. striatus venom (see Table I). Despite the two orders of magnitude difference in the amount of material consumed in the LSI experiment we did not discern any intact species in fraction 5, whereas the MALDI measurement yielded useful information. However, the comparisons in Tables I and II reveal that some components may be detected by LSIMS but not observed in the MALDI mass spectrum (measured with any of the matrices or sample preparation methods). The contrary is most likely more prevalent, i.e. that a large number of the species detected by MALDI with one or more of the matrices are difficult species to ionize with LSIMS. 

FIGURE 36. Collisional activation spectrum (nitrogen collision gas) of the m/z 93 ions produced by collisional deiodination of (a) protonated 4-iodoaniline and (b) protonated 3-iodoaniline, produced under LSIMS conditions... 

NR spectrum together with a significant peak at m/z 17 for reionized ammonia. The protonation-deiodination sequence to the 4-iodoaniline and 3-iodoaniline has been applied using the LSIMS ion source. The high-energy CA spectra (Figure 36) of the protonated para-iodoaniline (4-I-ANI-H+) and meta-iodoaniline (3-I-ANI-H+) forms obtained in these conditions are indeed completely different from the spectra shown in Figure 35, and feature very intense peaks at m/z 76, 74 and 50. These peaks, associated with the dehydroanilinium structure b-d in the Cl experiments, are expected for such a distonic ion structure. 

Hoffmaniolide (180) has been isolated from Prorocentrum hoffmannianum The gross structure of hoffmaniolide (180) was obtained by analyzing the liquid secondary ion mass spectrometry (LSIMS)-MS/MS spectrum as well as 2D NMR such as H- H COSY, TOCSY, and HMBC, and the relative stereochemistry of a tetrahydropyran ring was obtained from J coupling data. Isolation of hoffmanniolide (180) from P. hoffmannianum suggested a biosynthetic capability of this genus to produce either linear or macrocyclic polyethers. 

As electrospray is a very soft ionisation technique, the spectra are dominated by molecular ion- or quasimolecular ion-related species. Ionic, nonionic and zwitterionic surfactants are all amenable to the technique. Unlike FAB or LSIMS, electrospray does not suffer from discrimination effects [6] and it is therefore applicable to the analysis of mixtures. Sensitivity is high 10 pg of cationic surfactants can be detected with a good signal-to-noise ratio. The spectrum contains peaks from the intact cation. It has been reported [7] that the limit of detection for positive-ion electrospray is of the order of several femtograms (1 fg = 10 g). 

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