Mass profile spectra

This last m/z value is easy to measure accurately, and, if its relationship to the true mass is known (n = 10), then the true mass can be measured very accurately. The multicharged ions have typical m/z values of <3000 Da, which means that conventional quadrupole or magnetic-sector analyzers can be used for mass measurement. Actually, the spectrum consists of a series of multicharged protonated molecular ions [M + nWY for each component present in the sample. Each ion in the series differs by plus and minus one charge from adjacent ions ([M + uH] + n -an integer series for example, 1, 2, 3,. .., etc.). Mathematical transformation of the spectrum produces a true molecular mass profile of the sample (Figure 40.5). 

Fig. 1.3. Three representations of the molecular ion signal in the field desorption mass spectrum (Chap. 8) of tetrapentacontane, C54H110 (a) profile spectrum, (b) bar graph representation, and (c) tabular listing.

Figure 6. Peptide mass profiling of a silver stained protein spot from a narrow range, pH 4-7, 2DE separation of human heart (ventricle) proteins. A MALDI-TOF mass spectrum of tryptic peptides is shown, analyzed using a Micromass Tofspec 2E spectrometer (Manchester, UK) operated in the positive ion reflectron mode at 20 kV accelerating voltage with time-lag focusing enabled. The protein spot of interest was identified as human vimentin, (courtesy of J.A.Westbrook and R. Wait) (unpublished data). 2DE gels were silver stained using a modified Amersham Biosciences kit.

FIGURE 15.39 Extracted ion profile taken from the TIC shown in Figure 15.34 at m/z 314 representing the compound chlorpyriphos. The full mass scan spectrum of chropyriphos is shown in the bottom part of the figure. 

A laser pulse strikes the surface of a specimen (a), removing material from the first layer, A. The mass spectrometer records the formation of A+ ions (b). As the laser pulses ablate more material, eventually layer B is reached, at which stage A ions begin to decrease in abundance and ions appear instead. The process is repeated when the B/C boundary is reached so that B+ ions disappear from the spectrum and C+ ions appear instead. This method is useful for depth profiling through a specimen, very little of which is needed. In (c), less power is used and the laser beam is directed at different spots across a specimen. Where there is no surface contamination, only B ions appear, but, where there is surface impurity, ions A from the impurity also appear in the spectrum (d). 

Several features of ISS quantitative analysis should be noted. First of all, the relative sensitivities for the elements increase monotonically with mass. Essentially none of the other surface spectroscopies exhibit this simplicity. Because of this simple relationship, it is possible to mathematically manipulate the entire ISS spectrum such that the signal intensity is a direct quantitative representation of the surface. This is illustrated in Figure 5, which shows a depth profile of clean electrical connector pins. Atomic concentration can be read roughly as atomic percent direcdy from the approximate scale at the left. 

In quadrupole-based SIMS instruments, mass separation is achieved by passing the secondary ions down a path surrounded by four rods excited with various AC and DC voltages. Different sets of AC and DC conditions are used to direct the flight path of the selected secondary ions into the detector. The primary advantage of this kind of spectrometer is the high speed at which they can switch from peak to peak and their ability to perform analysis of dielectric thin films and bulk insulators. The ability of the quadrupole to switch rapidly between mass peaks enables acquisition of depth profiles with more data points per depth, which improves depth resolution. Additionally, most quadrupole-based SIMS instruments are equipped with enhanced vacuum systems, reducing the detrimental contribution of residual atmospheric species to the mass spectrum. 

Figure 5.19 MALDI-ToF mass spectrum, providing a molecular-weight profile of the tryptic peptides derived from spot 22 (see Figure 5.18) of the silver-stained two-dimensional gel of the proteins extracted from the yeast S. cerevisiae. From Poutanen, M., Salusjarvi, L., Ruohonen, L., Penttila, M. and KaUddnen, N., Rapid Commun. Mass Spectrom., 15, 1685-1692, copyright 2001. John Wiley Sons Limited. Reproduced with permission.

The kinetics study [38] utilized a Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer to measure the pathway branching ratios. The ability to eject selected masses and the extremely high mass resolution of this technique ensured that the observed CD3CH2 was in fact a primary product of the reaction. Temporal profiles from this reaction are shown in Fig. 1. Noticeably absent from the mass spectrum are the cations C2D2H3 and... 

To measure the strength of the forces exerted on particles, various analytical techniques have been developed [6, 7]. Unfortunately, since most of these techniques are based on hydrodynamics, assumption of the potential profiles is required and the viscosities of the fiuid and the particle sizes must be precisely determined in separate experiments, for example, using the viscous flow technique [8,9] and power spectrum analysis of position fluctuation [10]. Furthermore, these methods provide information on ensemble averages for a mass of many particles. The sizes, shapes, and physical and chemical properties of individual particles may be different from each other, which will result in a variety of force strengths. Thus, single-particle... 

Static SIMS is appropriate for obtaining information on the lateral distribution of surface chemical species. A broad, defocussed ion beam is often used in order to minimise surface damage. In dynamic SIMS sample erosion takes place quite rapidly, and depth profiles are obtained by monitoring peak intensities in the mass spectrum of sputtered ions as bombardment proceeds. 

As already stated, the mass spectrum is a two-dimensional graph that reports the m/z ratio of ions (abscissa) and their relative intensity (ordinate). The most abundant ions are assigned as 100%. A mass spectrum can be displayed as peak profiles or as bar graphs corresponding to the peak centroids, i.e. the weighted centre of mass of the peak, or as a table (Figure 2.16). The most abundant ions in a mass spectrum constitute the base peak whose intensity is assumed equal to 100%. 

Figure 2.16 Different ways to represent a mass spectrum peak profile (a), bar graph (b) and table (c)...

The profile is characterized by substantial amounts of dicarboxylic acids, mainly formed in the oxidation of polyunsaturated Cl8 acids (linoleic acid, 08 2 and linolenic acid, 08 3), which are no longer detected after ageing. The presence of short chain dicarboxylic acid, and in particular of oxalic acid, is also characteristic of aged lipid layers. Moreover, in the 30 35 min interval, several peaks are detected and attributed to oxidized species of unsaturated 08 acids. Figure 7.7 reports the mass spectrum of the peak at 31.84, attributed to 13-OH-9-octadecenoic acid or 14-OH-9-octadecenoic acid. Its... 

We have used accurate mass measurements obtained by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS) to differentiate and profile saponins from M. truncatula roots. An example is provided (Fig.3.11) showing the MALDI-TOFMS spectra of a solid-phase extract of M truncatula root tissue. In this spectrum, we can identify multiple saponins. 

Fig. 21.11. Mass spectra of the unknown off-flavor compound after spectral subtraction from the co-eluting peak and the matching spectrum from the NIST library. (Redrawn/redrawn from J. Chromatogr., 351, R.A. Sanders, and T.R. Morsch, Ion profiling approach to detailed mixture comparison. Application to a polypropylene off-odor problem, 525-531, Copyright (1986) with permission from Elsevier.)...

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