Identification of peaks

Figure Bl.25.9(a) shows the positive SIMS spectrum of a silica-supported zirconium oxide catalyst precursor, freshly prepared by a condensation reaction between zirconium ethoxide and the hydroxyl groups of the support [17]. Note the simultaneous occurrence of single ions (Ff, Si, Zr and molecular ions (SiO, SiOFf, ZrO, ZrOFf, ZrtK. Also, the isotope pattern of zirconium is clearly visible. Isotopes are important in the identification of peaks, because all peak intensity ratios must agree with the natural abundance. In addition to the peaks expected from zirconia on silica mounted on an indium foil, the spectrum in figure Bl. 25.9(a)...

In addition, the appHcation of the mass spectrometer (ms) as a detector for gas—Hquid chromatography has made the positive identification of peaks possible. High performance Hquid chromatography (hplc), which involves various detectors, can be used to measure hydrophilic and hydrophobic organic compounds in water. 

Before the advent of modern hyphenated techniques (GC/HS, GC/FTIR), numerous qualitative physical and chemical tests were devised for the identification of peaks in a gas chromatograa [705]. For the most part these tests were simple to perform, inexpensive, required minimum instrument modification and, in a few instances, provided a simple and easy solution to an otherwise complex problem. They still have some value today as spectroscopic techniques do not solve.all problems. 

Identification of Peaks from Crystallographic Data. Crystallography is not an issue of X-ray scattering. However, even in materials science crystallographic data are frequently consulted11. Based on such data the crystallizing species (component of a blend, block of a block copolymer, one of the crystal modifications possible) can... 

Identification of Peaks for Hydrogen Adsorption on the Disordered Low Index Planes. Besides the major objective for studying electrocatalysis on single crystal stepped surfaces mentioned above, these studies offer a wealth of information on the behaviour of polycrystalline surfaces, of preferentially oriented surfaces and, as we suggested recently (12), of disordered low-index surface. 

Another potential source of peaks in the NIR is called Fermi resonance. This is where an overtone or combination band interacts strongly with a fundamental band. The math is covered in any good theoretical spectroscopy text, but, in short, the two different-sized, closely located peaks tend to normalize in size and move away from one another. This leads to difficulties in first principle identification of peaks within complex spectra. 

The system used by these workers consisted of a Microtek 220 gas chromatograph and a Perkin-Elmer 403 atomic absorption spectrophotometer. These instruments were connected by means of stainless steel tubing (2mm o.d.) connected from the column outlet of the gas chromatograph to the silica furnace of the a.a.s. (Fig. 13.2). A four-way valve was installed between the carrier gas inlet and the column injection port so that a sample trap could be mounted, and the sample could be swept into the gas chromatographic column by the carrier gas. The recorder (lOmV) was equipped with an electronic integrator to measure the peak areas, and was simultaneously actuated with the sample introduction so that the retention time of each component could be used for identification of peaks. 

Fig. 16.21 Reversed phase HPBC-ED chromatogram of alkylphenols from SPE extract of Miller oil. See Table 16.7 for identification of peaks. Reprinted from Bennett B, Barter SR (1997) Partition behaviour of alkylphenols in crude oil brine systems under subsurface conditions. Geochim Cosmochim Acta 61 4393-4402. Copyright 1997 with permission of Elsevier...

Chromatography of one or two samples spiked with AdoMet (210 nmol/1) and AdoHcy (160 nmol/1) is performed within each batch to aid in identification of peaks. Typical elution times are 28-34 min for etheno-AdoMet and 8-10 min for etheno-AdoHcy. The separations obtained for AdoMet and AdoHcy obtained with a normal plasma sample are shown in Fig. 2.2.8. 

The conventional approach is to extract from the TIC profile the mass spectra of all peaks above a predetermined intensity and to perform either manual or computer-assisted [39] identification of each mass spectrum. Analysis of mass spectra should be carried out only by properly trained technologists, under the supervision of a qualified laboratory director. Libraries of mass spectra should be available for identification of peaks that are not readily recognized. This library should be user-created, indexed by retention time and molecular weight, and have the capacity to be expanded and edited. 

Figure 4. Weight-percent data for each peak for three sections of two representative cores, G26 (for the lower portion of Twelve Mile Creek) and G56 (for Lake Hartwell). Depth in core and weight-percent summaries for peaks 1-22 and 25-64 are given. Figure 2 gives sample locations and identification of peaks not quantified and sample locations.

Table G1.5.8 Identification of Peaks for Cold-Pressed Versus Distilled Lime Oil1...

An example of the use of PDA UV libraries to help in the identification of peaks is provided by the identification and quantification of tricyclic antidepressants.7 Although the structures of the compounds are very similar, making separation fairly difficult, the UV spectra of most tricyclic antidepressants are fairly distinct, as shown in Figure 7.4. On the occasions where the UV spectra are similar, the retention times are sufficiently different to prevent misidentification. 

GC peaks were identified using the Finnigan Ion Trap Detector (ITD) and its programs for libraiy comparison. To aid in the positive identification of peaks, a library of ITD mass spectra was generated using standards of compounds equivalent to those found in this work. Capillary GC (Perkin-Elmer Sigma-300) for the ITD was carried out similar to the conditions above, i.e., splitter ration, 20 1, oven temperature, 50 °C (5 minute hold) followed by a 5 C/min ramp to 180 C the 10 minute hold was followed by a 5 C/min ramp to 200 °C (20 minute hold to remove impurities). 

Figure 4.2 shows the SIMS spectrum of a promoted iron-antimony oxide catalyst used in selective oxidation reactions. Note the simultaneous occurrence of single ions (Si+, Fe+, Cu+, etc.) and molecular ions (SiO+, SiOH+, FeO+, SbO+, SbOSi+). Also clearly visible are the isotope patterns of copper (two isotopes at 63 and 65 amu), molybdenum (seven isotopes between 92 and 100 amu), and antimony (121 and 123 amu). Isotopic ratios play an important role in the identification of peaks, because all peak intensities must agree with natural abundances. Figure 4.2 also illustrates the differences in SIMS yields of the different elements although iron and antimony are present in comparable quantities in the catalyst, the iron intensity in the spectrum is about 25 times as high as that of antimony ... 

Figure 6.7. Dual channel presentation of GC analysis of air contaminants in parking lot. For identification of peaks, see Table 1. Reprinted with permission from W. Nutmagul, D. R. Cronn, and H. H. Hill, Jr., Anal. Chem. 1983,55, 2160. Copyright 1983, American Chemical Society.

X-ray spectrometry has benefited, as have other physico-chemical analysis methods, from the many recent advances in electronics and micro-computing. We have seen that, for qualitative analysis, software is used for the automatic identification of peaks. For quantitative analysis, the majority of equipment manufacturers provide highly extensive correction software packages. Finally, the automatic handling of samples in the spectrometer is well established (sample handlers with a capacity of 72 to 100 samples). 

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