Reference peak

All the chemical shifts are expressed in 5 units ppm of applied field and TMS as reference peak. 

To define the position of an absorption, the NMR chart is calibrated and a reference point is used. In practice, a small amount of tetramethylsilane [TMS (CH )4Si] Is added to the sample so that a reference absorption peak is produced when the spectrum is run. TMS is used as reference for both l H and 13C measurements because it produces in both a single peak that occurs upfield of other absorptions normally found in organic compounds. The ]H and 13C spectra of methyl acetate in Figure 13.3 have the l MS reference peak indicated. 

Where SA R is the specific area of the reference peak, and SA is the specific area of component x. AR is the GC peak area of the reference, Ax is the GC peak area of component x, WR is the weight of the reference, and Wx is the weight of component x. The weight percent of component x can be obtained from the sample chromatogram by using the relative response factors in the following equation ... 

Computer Techniques McLafferty (Ref 63) has pointed out that the usefulness of elemental composition information increases exponentially with increasing mass, since the number of elemental combinations with the same integral mass becomes larger. There are compilations of exact masses and elemental compositions available (Refs 12a, 13 18a). Spectral interpretation will be simplified in important ways if elemental compositions of all but, the smallest peaks are determined. Deriving the elemental compositions of several peaks in a spectrum is extremely laborious and time-consuming. However, with the availability of digital computers such tasks are readily performed. A modern data acquisition and reduction system with a dedicated online computer can determine peak centroids and areas for all peaks, locate reference peaks, interpolate between them to determine the exact masses of the unknown peaks, and find within minutes elemental compositions of all ions in a spectrum (Refs 28b 28c)... 

Figure 4. Improvement in sensitivity with the CAT. PFA and a reference peak at m/e 315. The observed improvement in signal-to-noise ratio results from the longer total scanning time and also the fact that many sweeps are made during this time. The overall improvement in signal-to-noise ratio depends on the detailed power spectrum of the noise (9). Resolution 12,000 here and for all following time averaged spectra...

Perfluoroalkane-225 (PGR, Gainesville, FL) was admitted through a glass inlet system to provide reference peaks. Analytical and reference peaks for the nitrosamines studied are shown in Table I. Sample and reference peaks were scanned alternately at a repetition rate of approximately 1 sec and were monitored on an oscilloscope. When the nitrosamine peak appeared, the oscillographic recorder chart drive was engaged and remained on until the peak disappeared. Nitrosamine quantities were estimated by comparing the sum of sample peak heights measured from the chart (usually 10 to 20 values) with values derived from injection of standard solutions. 

Nitrosamine Composition m/z Relative Intensity Reference Peak... 

There are several examples in the literature of GFC now being utilized for small molecule analysis (17). However, in this case, attempts to obtain monomer concentrations for kinetic modelling were frustrated by irreproducible impurity peak interference with monomer peaks, time varying refractometer responses and insufficient resolution for utilization of a reference peak. This last point meant that injected concentration would have to be extremely reproducible. 

Figures 8 and 9 show the first order kinetic plots for the isomerization and crosslinking reactions, respectively. In the data analysis the area of the isoimide peak was measured between consistent limits chosen to exclude any contribution from the 1775 cm imide band. These data were generated by measuring the area of the appropriate peak in a baseline corrected spectrum and ratioing this area to that of a reference peak (which was invarient during the experiment) in the same spectrum. This concentration indicative number was then ratioed to the concentration ratio observed on the initial scan. Plots of the log of the ratio of the concentration of the functionality at time "t" to the concentration of the functionality at t = 0 were then constructed. In order to insure that the trends in the data were not artifacts of this procedure or of the baseline correction routine, we also plotted the data in terms of peak intensity in absorbance units and observed the same trends but with more scatter in the data.

This function is already aligned with its fiber axis in. vj-direction, and we do not normalize it. Together with the normalization of the reference peak these measures guarantee the conservation of the 3D scattering intensity under the desmearing operation. 

For the decabromodiphenyl oxide (DBDPO) pyrolysis reactions, two different procedures were used to synthesize the series of brominated diphenyl oxides and dibenozofurans employed as the relative retention time standards AlBr3/Br2 in ethylene dibromide and Fe° (metal)/Br2 in tetrachloroethylene. The rate of the initial bromination steps in the former reaction was so rapid that only the higher degree of bromination adducts could be isolated. The rate of the Fe°/Br2 reaction was found to be much slower, especially during the initial stages, and these reactions yielded a broader range of relative retention time reference peaks. 

Peaks are identified from absolute or relative retention times by comparison with data from previously run standards stored in RAM or in libraries on disk. To take account of the variability of retention times from successive runs, retention time windows are used. These are defined as being /R x% for a standard, the unknown being positively identified if its retention time falls within the specified range. The size of the window can be varied by the user to conform with the degree of certainty required. Reference peaks can be selected for the calculation of relative retention times or as internal standards in quantitative analysis (pp. 9, 114). 

Fig. 7.12 Schematic spectra for quantitative J-cor- a reference peak occurring in the same experi-relation experiments. A cross peak is compared to ment.

The difference between the test peak and the reference peak is known as the chemical shift of the test. This is a scale of frequency normalised to give the reference peak a value of zero. 

Fig. 3.21. Reproduction of a PFK calibration table of a magnetic sector instrument covering the m/z 1 05 range. In order to expand the PFK reference peak hst to the low m/z range, H, He and peaks from residual air are included, but for intensity reasons H, He and CO2 have not been assigned in this particular case.

Note Mass accuracy is highly dependent on many parameters such as resolving power, scan rate, scanning method, signal-to-noise ratio of the peaks, peak shapes, overlap of isotopic peaks at same nominal mass, mass difference between adjacent reference peaks etc. An error of 5 mmu for routine applications is a conservative estimate and thus the experimental accurate mass should lie within this error range independent of the ionization method and the instrument used. [37] There is no reason that the correct (expected) composition has to be the composition with the smallest error. 

Cl37Cl2, respectively, and thus these can be identified in the next step. The numbers of all other elements must remain the same, i.e., here C16H14N must be part of any formula. In this example, R = 8000 is the minimum to separate the PFK reference peak at m/z 417 from that of the analyte. If separation could not have been achieved, the mass assignment would have been wrong because the m/z 417 peak would then be centered at a weighted mass average of its two contributors. Alternatively, such a peak may be omitted from both reference list and composition list. 

A with respect to the corresponding reference peaks in Fig. 2.19B, C. Note that all spectra are normalized to the same intensity scale and were obtained using 10 pL of the mixture where the protease concentration was 20 pM in each sample. Reprinted from reference [13] with permission from John Wiley Sons. 

Retention time calibration. In spite of all effort to obtain reproducible retention time values these varied for the same component between different chromatographic runs, mainly because of different sample amounts. To solve this problem all retention time values were calibrated. Values for a limited number of peaks, that could easily be found in all the chromatograms in a set, were manually entered into the computer. For each of these reference peaks the mean value M j) over all the runs was calculated. New retention tim.es, Rtcal(i), for the peaks in the data set were then calculated by the straight-line expression ... 

The coefficients A(i) and B(i) are based on the mean values M(j) of the nearest reference peaks on both sides of the peak to be calibrated. 

Columns packed with silica gel Purification 60 (230-400 mesh ASTM) hexane/ethyl acetate gradient Perkin-Elmer 1730FT1R Bruker WP 250 MHz using the solvent peak as a reference peak... 

Bruker WP 4(K) MHz using the solvent as a reference peak Polymer Laboratories DMTA... 

Analytical. The feeds and recovered oils were analyzed for 1-alkene using an F M Model 720 gas-liquid chromatograph equipped with an SE-30 gum-rubber column with helium carrier gas and 99% purity Humphrey Chemical Co. 1-alkenes for reference peaks. 

Twenty-two peaks, 5 of which were designated as retention reference peaks, were selected arbitrarily without identification based on reproducibility of integration over the entire data set. 

FTIR-spectroscopy was performed in a Nicolet 5 SXC instrument using KBr pellets. The spectra were baseline corrected and the 1125 cm-1 band was used as reference peak to compare bleached and unbleached spectra. The 1505 cm-1 peak was used as the reference peak to obtain the subtraction spectra. 

Polynomial order = 4. Used for calibrations that include the lower end of the mass scale, with closely spaced reference peaks. This is suitable for calibrations with polyethylene and poly propylene glycols (see Section 13.3.3) that extend below 300 amu. 

Derivatized polyethers such as polyether sulfate have been investigated for both positive- and negative-ion calibration [11]. Although poly ether sulfates are not commercially available, they are easily synthesized. Lauryl sulfate ethoxy-lates were also used as calibrants for negative-ion ESI. Polyether amines and quaternary ammonium salts were used as positive-ion calibration solutions [11]. These commercially available compounds do not exhibit significant sodium or potassium adducts, and they are more easily flushed out of the mass spectrometer ion source than are nonderivatized polyethers. In addition, doubly charged poly ether diamines can produce reference peaks at low m/z values. 

Water Cluster. Numerous groups have used water clusters successfully as calibration solutions [10,11,20-22]. Water clusters do not produce any source contamination in ESI-MS and provide closely spaced reference peaks with a calibration range up to m lz 1000. In positive-ion mode, protonated water clusters with up to 70 water molecules are observed. In negative ESI singly deprotonated water clusters are observed [OH (HjO), with n > 20], as well as solvated electrons [(H20)m with m > 11]. 

Acetate Salts. Sodium acetate and sodium trifluoracetate clusters were used and produce useful reference peaks for both positive and negative ESI [10,11,23] ... 

Failed calibration. There are number of reasons for a calibration to fail. If an automated calibration method is used, it is possible that the reference peaks are not recognized when the reference file and calibration file are compared. This can be due to the following reasons ... 

Peak picking is usually performed to measure chemical shifts (in ppm relative to a reference peak in the spectrum) or to measure line separations (in Hz) in multiplets in order to calculate or at least to estimate coupling constants. 

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