Mass spectral peaks

The peak at mje = 98 is taken as the arbitrary standard. The height of the other peaks is measmed relative to it. Once this matrix has been established, ordered sets of mass spectral peak heights at mfe = 69, 83, 84, and 98 constitute the experimental b vector for an unknown mixture that contains or may contain the four... 

In a well-tuned (adjusted) instrument, the shape of a mass spectral peak is approximately triangular (Figure 44.7a), but, in an instrument that is poorly tuned the peak will appear misshapen (Figure 44.7b). Usually, the cause of the skewing of the peak arises from incorrectly adjusted... 

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). 

The side-chain chlorine contents of benzyl chloride, benzal chloride, and benzotrichlorides are determined by hydrolysis with methanolic sodium hydroxide followed by titration with silver nitrate. Total chlorine determination, including ring chlorine, is made by standard combustion methods (55). Several procedures for the gas chromatographic analysis of chlorotoluene mixtures have been described (56,57). Proton and nuclear magnetic resonance shifts, characteristic iafrared absorption bands, and principal mass spectral peaks have been summarized including sources of reference spectra (58). Procedures for measuring trace benzyl chloride ia air (59) and ia water (60) have been described. 

Problem 19.26 Tell the prominent IR absorptions and mass spectral peaks you would expect for the following compound ... 

Table I. Mass Spectral Peaks Employed for Identification of Nitrosamines...

MS detection does not necessarily require as highly resolved GC separations as in the case of selective detectors because the likelihood of an overlapping mass spectral peak among pesticides with the same retention time is less than the likelihood of an overlapping peak from the same element. Unfortunately, this advantage cannot always be optimized because SIM and current gas chromatography/tandem mass spectrometry (GC/MS/MS) methods, it is difficult to devise sequential SIM or MS/MS retention time windows to achieve fast GC separations for approximately > 50 analytes in a single method. 

The low molecular weight mJz region of spectra derived from microorganism whole cells is generally comprised of mass spectral peaks derived from glycerides, phospholipids, glycolipids, and small protein fragments. This mass spectral information should be examined as a compliment to protein bio-... 

Variations on the spectral peaks from different species of the same genus were also observed. Three species of Pseudomonas produced the spectra shown in Figure 14.2. These spectra are clearly unique and were used to correctly identify unknown samples. Because of peak ratio reproducibility issues in bacterial protein profiles obtained by MALDI MS,11 a fingerprint approach that had been used for other mass spectrometry approaches has not been used. The profile reproducibility problem was first recognized by Reilly et al.12,13 and later researched by others in the field.14,15 As a later alternative, a direct comparison of the mass-to-charge ratio (m/z) of the unknown mass spectral peaks with a database of known protein masses has been used to identify unknown samples.14... 

Mass Peak Width (A/n50%) The full width of a mass spectral peak at half-maximum peak height [3]. 

Mass spectrometry can be used to measure the molar mass distribution (MMD) of a polymer sample by simply measuring the intensity, Nt, of each mass spectral peak with mass m . This is due to the fact that mass spectrometers are equipped with a detector that gives the same response if an ion with mass 1 kDa or 100 Da (actually any mass) strikes against it. In other words, the detector measures the number fraction and this implies that Nt also represents the number of chains with mass m,. Thus, the number-average molar mass, Mn, is given by ... 

FIGURE 5.31 Histograms of mass spectral peak intensities at m/z 77 for 300 compounds with a phenyl substructure and 300 compounds without a phenyl substructure. The peak intensities are divided into 10 intervals between 0% and 100% base peak intensity. For each interval, the frequency of peaks is given for both groups. 

Mass spectra in Figure 22-4 are computer-generated bar graphs. In contrast, Figure 22-1 shows the actual detector signal. Each mass spectral peak has a width that limits how closely two peaks could be spaced and still be resolved. If peaks are too close, they appear to be a single peak. 

Recently, Kohout and Lampe264 decomposed H2 or D2 by mercury sensitization in the presence of NO. The experiments were done in a cell with a pinhole leak to a Bendix time-of-flight mass spectrometer. They directly observed mass spectral peaks at m/e = 31 or 32, which correspond to HNO or DNO, respectively. 

The first positive identification of the product apparently was made by Bryce and Ingold.65 They thermally decomposed (CH3)2Hg in the presence of NO and bled the reaction mixture through a pinhole into a mass spectrometer, where they observed a mass spectral peak at... 

The most conclusive evidence for reaction (16) was recently given by Kohout and Lampe.264 They studied the mercury-sensitized decomposition of H2 and D2 in the presence of a small amount of NO. The products of the reaction were bled through a pinhole into a mass spectrometer for continual analysis. Results are shown in Figure 8-6 for the D2-NO system. They observed mass spectral peaks for DNO, DzO, and N20, but not for (DNO)2. Initially, DNO is produced, but it subsequently decays to form NsO (and D20). The minimum rate constant reported by them for DNO disproportionation was 3.4 x 106 M 1 sec-1. However, more recently Kohout263 in his doctoral dissertation has reported that DaO and N20 are the products of the bimolecular encounter of two DNO molecules. The rate constant is 4.0 x 105 Af-1 sec-1 independent of pressure over the limited pressure range of 30-70 torr. An increase in surface-to-volume ratio of about 50 had no effect on the results. 

At very high hydrogen-atom concentrations, Kohout and Lampe264 observed a small mass spectral peak, which they assigned to hydroxyl-amine, by bleeding the reaction products through a pinhole into a mass spectrometer. Presumably, the reaction sequence is... 

Resolution The width (in u) of a mass spectral peak at a given m/z value. Also frequently used interchangeably with resolving power below. Along with mass calibration, the mass resolution is the most essential parameter to control in the... 

Fig. 5. Mass spectral peaks of 72Br and 73 r measured as a function of the voltage superimposed on the 40 kV ISOL acceleration voltage each plotted point represents the total area of selected y-ray peaks in measured decay spectra.

D. Single crystals or trace amounts of cimetidine respond to color tests and will give a positive mass spectral peak for the molecular ion. These have been described above. 

The capillary HPLC separation from a selected protein spot provides a base-peak profile shown in Figure 6.2A. The base-peak profile is similar to a total ion current (TIC) profile, but it contains only the most abundant mass spectral peak in each scan. The chromatogram is simplified and the contributions from background ion abundances are eliminated, resulting in an enhanced signal-to-ion ratio for an improved visualization of data. The molecular mass for each component is labeled along with corresponding amino acid residues. This format provides a comprehensive approach for peak selection and peptide identification. 

The mass spectrum of acetylcholine chloride was obtained utilizing a Shimadzu PQ-5000 mass spectrometer, with the parent ion being collided with helium carrier gas. The mass spectrum is shown in Figure 10, and Table 4 shows the mass fragmentation pattern. Clarke lists the principal mass spectral peaks at m/z 58, 43, 57, 149, 71, 42, 41, 55 [3]. 

Mass spectrometry was used in combination with chromatographic methods for the analysis of acetylcholine in biological systems, with the mass spectral peak at m/z 58 being most frequently used for detection. Quantitation of acetylcholine and its related compounds was successfully performed using GC/MS [17-26] and HPLC/MS [27-29]. 

Mass spectral peaks are often seen corresponding to loss of small, stable molecules. Loss of a small molecule is usually indicated by a fragment peak with an even mass number, corresponding to loss of an even mass number. A radical cation may lose water (mass 18), CO (28), C02 (44), and even ethene (28) or other alkenes. The most common example is the loss of water from alcohols, which occurs so readily that the molecular ion is often weak or absent. The peak corresponding to loss of water (the M-18 peak) is usually strong, however. 

---