Artifact peak

Structure calculation algorithms in general assume that the experimental list of restraints is completely free of errors. This is usually true only in the final stages of a structure calculation, when all errors (e.g., in the assignment of chemical shifts or NOEs) have been identified, often in a laborious iterative process. Many effects can produce inconsistent or incorrect restraints, e.g., artifact peaks, imprecise peak positions, and insufficient error bounds to correct for spin diffusion. 

Figure 1.30 (a) Normal NMR spectrum resulting from the correct selection of spectral width, (b) When the spectral width is too small, the peaks lying outside the spectral width can fold over. Thus a and b represent artifact peaks caused by the fold-over of the a and b signals. 

Phase cycling is widely employed in multipulse NMR experiments. It is also required in quadrature detection. Phase cycling is used to prevent the introduction of constant voltage generated by the electronics into the signal of the sample, to suppress artifact peaks, to correct pulse imperfections, and to select particular responses in 2D or multiple-quantum spectra. 

Dummy scans are the preparatory scans with the complete time course of the experiment (pulses, evolution, delays, acquisition time). A certain number of these dummy scans are generally acquired before each FID in order to attain a stable steady state. Though time-consuming, they are extremely useful for suppressing artifact peaks. 

Quadrature images Any imbalances between the two channels of a quadrature detection system cause ghost peaks, which appear as symmetrically located artifact peaks on opposite sides of the spectrometer frequency. They can be eliminated by an appropriate phase-cycling procedure, e.g., CYCLOPS. 

The use of urea must be approached with caution, because urea solutions often contain ammonium cyanate, the concentration of which increases with temperature and pH. This contaminant can react with the amino group of lysines and the amino terminus of the polypeptide chain, thus leading to artifact peaks. This effect is minimized by the presence of ampholytes, whose primary amines are cyanate scavengers, and by deionizing the urea solution with a mixed-bed resin prior to adding the ampholytes and detergent. 

With the standards of Dataset B, however, the 1.5 ng value had a range of 1.3 to 1.9 ng. This example shows that with more standards a higher precision was obtained (assuming the quality of the standards was equivalent). If the analyst used Dataset D standards, those containing an artifact peak in the standards would have to report that at 1.6 ng his range would be a much larger 0.9 to 2.7 ng. Clearly a client would prefer the former over the latter. 

Dataset D. In this data set the compound sought had superimposed near it an artifact peak such that at at lower levels the response values were significantly inflated. Compare with Dataset E. 

Dataset E. This data set is the same as Dataset D except that artifact peak data was removed. Compare with Dataset D. 

FIGURE 9 HPLC chromatogram of an extract of a controlled-release tablet with two APIs showing the presence of an artifact peak stemming from sonication. 

Figure2. Unimolecular dissociation spectrum (MIKES) of H30) 1(CH3)20]3 illustrating major loss of (CH3>20 m/z 111) and H2O + (043)20 (m/z 93). The peak near m/z 150 is an artifact peak due to ion reflections from the ESA walls.

Blank solution to show no interference with any HPLC system artifact peak. 

Recently, a GC-MS method for the separation and quantitative identification of extracts from Cephalotaxus species (97) has been described. Most of the alkaloids were resolved, particularly the biologically active esters. The seven Homoerythrina alkaloids were only resolved into two groups of five and two components, respectively, under the conditions described. Acetylcephalotaxine (106), 11-hydroxycephalotaxine (114), and desmethyl-cephalotaxinone (113) were not resolved by retention time, but could be identified within the mixture by their MS fragmentation patterns. Cepha-lotaxinone (112) gave two GC peaks after silylation, presumably due to a contribution from the enol component. The artifact peak overlaps partly with the peak for drupacine (115) and hence introduces a slight error for this component and makes it difficult to quantify cephalotaxinone. 

The subdistribution method is extremely sensitive to operator input and consistently yielded the poorest results of the three methods compared. In general CONTIN yields good results but tends to shift MWDs to higher molecular weights and sometimes produces artifact peaks or shoulders for broad unimodal or multimodal distributions. Of the three methods, the proposed GEX fitting technique seems to provide results that are most consitent with the input distributions and is the most operator independent. CONTIN and GEX fitting are not significantly affected by noisy data. 

Elkington and Wilson [48] examined narrow size distributions of particles and resolved an additional artifact peak, on the coarse side of the main or normal distribution, which was generated by particles moving non-axially through the aperture. They used a Coulter Zg, a Coulter Channelyzer C100 with edit on and edit off and a Coulter TF. 

Fig. 4 reveals the capillary LC separation and collection of peptide fragments generated from CNBr degradation of transferrin. Five peaks were collected and subjected to sequence analysis. It was similar to carbonic anhydrase in that some of fragments contained transferrin sequences and others appeared to be artifacts. Peaks 2,3 and 4 yielded sequences from transferrin. All of them were generated from the cleavage at Met by CNBr. 

Fig. 2 Gas chromatography-flame ionization detection chromatograms containing early artifact peaks from different solvent extraction methods Soxhlet, ASE (pie), SEE, and subcritical water extraction of a soil sample collected from a manufacturing gas plant site. The numbers refer to PAHs identified in the legend of Fig. 1. (From Ref. [12].)...

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