Chemical mass shift

Individual collision events are another possible source of chemical mass shift during a mass-selective instability scan. Based on results from simulations using ITSIM, Plass et al. [96] reported that elastic collisions causing a change in the ion s oscillation amplitude can reduce the normal ejection delay, particularly when there are scattering events causing a sudden increase in axial displacement. 

In modem ion trap mass spectrometry, weak higher-order multipole and/or dipole electric fields are always superimposed upon the quadrupole electric field. The weak superimposed higher-order multipole and/or dipole electric fields are found to enhance the mass spectral performance with respect to mass resolution, charge capacity, scan speed, and to chemical mass shift. In fact, all commercial ion trap mass spectrometers are non-linear ion traps [7-9]. In the Finnigan ion trap instrument, the positions of the two end-cap electrodes are stretched out from the theoretical positions to add the higher-order multipoles. In the Bruker ion trap, the profiles of the end-cap electrodes are crafted to create the superimposed higher-order multipoles. In the Varian 4000 ion trap, a dipole and higher-order multipoles are superimposed upon the quadrupole field by a switchable electric circuit [10],... 

The proteins in the mortars can be modified by gradual oxidation or other chemical processes. In mass spectra the peaks that can be interpreted as oxygen incorporation (the mass shift of +16 Da) or ammonia release (—15 Da) can be sometimes indicated. This observation is not surprising as several amino acids (Met, Trp, Tyr, etc.) can be oxidised under these conditions similarly, Gin and Asn can gradually release their ammonia by long-term hydrolysis in a wet inorganic matrix. 

The peptide mixture on the MALDI target can be exposed to a chemical derivatization to confirm the identity of a peptide by the mass shift associated with the sequence-specific derivatization. A large number of possible derivatization reactions can be combined with the MALDI-TOF analysis. Their usefulness depends critically on the kinetics of the derivatization reaction, whether the reaction is complete with small amounts of peptides and whether only one product is generated. A visible MALDI signal can be generated from low atomole of peptide present under the laser beam (Vorm et al., 1994), but these amounts are often not sufficient... 

The quantification of metabolites in dried blood spots primarily ensures that the quality of the isotopes standards is excellent in terms of chemical and isotopic purity. When using MS/MS, it is essential that the fragments produced by the collision cell and the product ions detected ensure that both labeled and unlabeled metabolites are identical. Most importantly, the choice of the isotope label and the structural positions must be such that they are stable and do not exchange with other isotopes during sample preparation. Finally, it is imperative that the mass shift is sufficiently high (at least 3 Da) for small molecules less than 1000 Da and that the label occurs at a mass free from other compound interference. Figure 4 illustrates the concepts of quantification using stable isotope with Phe measurement in a dried blood spot as an example. 

FIGURE 7.44 MALDI mass spectra from on-CD analysis of the phosphopeptide-con-taining sample, (a) Peptide mass spectrum after concentration/desalting. A database search showed that the sample contained bovine protein disulfide isomerase. (b) Phosphopeptide enrichment by IMAC. Two phosphopeptides at m/z 964 and 2027 were recognized (c) Phosphopeptide enrichment followed by enzymatic on-column dephosphorylation using alkaline phosphatase. Two phosphopeptides at m/z 884 and 1947 were recognized from the mass shifts of 80 Da from (B), which were resulted from dephosphorylation [794]. Reprinted with permission from the American Chemical Society. 

The protein footprinting approach utilizes chemical modifications to generate a mass shift either on amino acid side chains or the protein amide backbone, followed by MS analysis. This approach can capture minor changes in protein solvent accessibilities that are induced by protein-ligand (ligand small molecule, protein, DNA, etc.) interactions or protein conformational changes. 

Chemical ionization MS, with various deuterated reagent gases, has been used to probe the number and nature of active hydrogens in a molecule. For example, based on the mass shifts of the protonated ions that are observed when switching from ammonia (NH3) to deuterated ammonia (ND3), the number of active hydrogens in morphine and several other drugs have been demonstrated. 

Most analytical methods use a bulk property to distinguish, separate, or identify an analyte. Properties such as chemical reactivity, chromatographic retention time, and optical absorbance or emission reveal information about an aggregate of analyte molecules, giving a collective response value. A remarkable thing about a mass spectrum, and one of the unique attributes of mass spectrometry, is that the analyte molecules or atoms are separated by mass, and the detector records the mass of each individual analyte molecule. The isotopic composition of the analyte is revealed as are mass shifts due to modifications of very large molecules, even when only a fraction of them have been modified. 

Many variations on ion trap scan functions are known and utilized for all manner of ion manipulation and detection coUisionally induced dissociation (CID), selective ejection or trapping, high mass resolution, ion reaction, chemically selective excitation of motion via the mass shift , etc. The interested reader is referred to the significant ion trap literature already cited. 

The adsorption of molecules into a sorbent layer (e.g. a polymer or a molecular film) produces a change of mass and the measurement of these mass shifts can allow the evaluation of the amount of adsorbed molecules. The measure of small mass changes is made possible by piezoelectric resonators. A piezoelectric resonator is a piezoelectric crystal properly cut along a well specified crystalline axis. Due to the piezoelectric effect, the mechanical resonance of the crystal is coupled with an electric resonance. Since crystal resonance is extremely efficient the electric resonance is characterised by a very large quality factor. This property is largely exploited in electronics to build stable oscillators as clock references. The same effect is exploited for chemical sensing adopting particularly shaped crystals such... 

The methods listed thus far can be used for the reliable prediction of NMR chemical shifts for small organic compounds in the gas phase, which are often reasonably close to the liquid-phase results. Heavy elements, such as transition metals and lanthanides, present a much more dilficult problem. Mass defect and spin-coupling terms have been found to be significant for the description of the NMR shielding tensors for these elements. Since NMR is a nuclear effect, core potentials should not be used. 

The section on Spectroscopy has been expanded to include ultraviolet-visible spectroscopy, fluorescence, Raman spectroscopy, and mass spectroscopy. Retained sections have been thoroughly revised in particular, the tables on electronic emission and atomic absorption spectroscopy, nuclear magnetic resonance, and infrared spectroscopy. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon ICP, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-29, and phosphorus-31. 

An example of enhanced ion production. The chemical equilibrium exists in a solution of an amine (RNH2). With little or no acid present, the equilibrium lies well to the left, and there are few preformed protonated amine molecules (ions, RNH3+) the FAB mass spectrum (a) is typical. With more or stronger acid, the equilibrium shifts to the right, producing more protonated amine molecules. Thus, addition of acid to a solution of an amine subjected to FAB usually causes a large increase in the number of protonated amine species recorded (spectrum b). 

A number of analytical methods have been developed for the determination of chlorotoluene mixtures by gas chromatography. These are used for determinations in environments such as air near industry (62) and soil (63). Liquid crystal stationary columns are more effective in separating m- and chlorotoluene than conventional columns (64). Prepacked columns are commercially available. ZeoHtes have been examined extensively as a means to separate chlorotoluene mixtures (see Molecularsieves). For example, a Y-type 2eohte containing sodium and copper has been used to separate y -chlorotoluene from its isomers by selective absorption (65). The presence of ben2ylic impurities in chlorotoluenes is determined by standard methods for hydroly2able chlorine. Proton (66) and carbon-13 chemical shifts, characteristic in absorption bands, and principal mass spectral peaks are available along with sources of reference spectra (67). 

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