False peaks

Artifacts False peaks, noise, or other unwanted signals in the NMR spectrum. 

In the ESI MS studies some care must be taken due to the possible formation of false peaks (73). The time-of-flight (TOF) ESI MS equipment of a new generation with an orthogonal interface design allows one to avoid somewhat experimental artifacts of this kind. 

Guard column is placed anterior to the separating column. This serves as a protective factor that prolongs the life and usefulness of the separation column. They are dependable columns designed to filter or remove particles that clog the separation column and compounds and ions that could ultimately cause "baseline drift", decreased resolution, decreased sensitivity or create false peaks... 

HPLC, high performance (or pressure) liquid chromatography, is particularly suited for small water-soluble molecules and proteins. Most used for analysis of DNA fragments is the reverse phase HPLC. Detection with electrochemical detectors are preferred. There are many different brands of ECD detectors and electrodes/cells. In our laboratory we have found that the ESA Coulochem is working excellent for our purposes and provides excellent sensitivity. Similar experience with other detectors can be found. We have found that for HPLC-ECD analysis of urine, separation is critical due to electrochemically active peaks eluting close to that of 8-oxodG. Ways to detect a false peak is given in details elsewhere [3]. 

The results of an automatic peak search can be further improved by adding real and/or removing false peaks manually. An example of such a peak search is shown in Figure 4.8. 

Figure 4.8. Automatic peak search conducted using a second derivative method (top) and manually corrected reduced pattern (bottom). The upward arrow placed on the digitized pattern shows a false peak (which was eliminated manually) and the downward arrows show the missed peaks (which were added manually).

The major difference between the two Fourier maps shown in Figure 6.12, Figure 6.13, Table 6.6, and Table 6.7 is that peak heights of the correctly placed atoms are much stronger than the heights of false peaks. Furthermore, the coordinates of false peaks vary but the coordinates of true maxima remain the same. As is easy to verify by the calculation of distances, none of the peaks listed below peak No. 3 in Table 6.7 has a reasonable distance to the La and Ni atoms already located in the unit cell. 

False peaks (e.g. peak No. 4 in Table 6.6, which is easily recognizable in Figure 6.12) appear on Fourier maps due to a variety of reasons i) the largest contribution comes from the truncation of the Fourier summation (Eq. 2.133) because only a limited amount of diffraction data is available (see Table 6.4) ii) the structure amplitudes are not exact, especially when powder diffraction data were used in combination with Le Bail s extraction, and iii) phase angles calculated using atomic parameters, which are not fully refined, are still imprecise because we used randomly assigned displacement parameters and assumed completely random distribution of Ni and Sn in two possible sites. 

False peaks may sometimes be stronger than the real peaks on the Fourier map, especially when a structural model is incomplete and/or structure factors accuracy is relatively low, which is the case here. One atom, which is still missing from the model, is Rh it is the second strongest scattering atom and, therefore, phase angles are relatively imprecise thus causing the appearance of a strong false peak. 

Other Fourier peaks either were too close to the already present atoms or made no chemical sense at all all were treated as false peaks both caused by the limited resolution of the pattern and by the sizeable truncation of the Fourier summation 69°). 

Earlier we demonstrated that scattered radiation may lead to instrumental deviations from Beer s law (p. 733). Another undesirable effect of this type of radiation is that it occasionally causes false peaks to appear when a spectrophotometer is... 

Figure 2 6-6 Spectra of cerium(IVj obtained with a spectrophotometer having glass optics (A) and quartz optics (B). The false peak in A occurs when stray radiation is transmitted at long wavelengths.

Figure 4.18 Powder pattern of Pb myristate, Pb(Ci4H2702)2, taken with CuKa radiation and variable divergence slit. The closing of the divergence slit produces a false peak at 29 0.5° (Schreiner, 1986. )...

The neutron diffraction studies on myoglobin by Benno Schoenborn, reported elsewhere in this symposium, sound an important note of warning. Whereas in 1971 he had reported 106 water molecules per myoglobin molecule by neutron diffraction (33) and Takano in refined x-ray diffraction studies had found about 80 (34), Schoenborn in very careful work has now reduced his estimate to 42. It is known that false peaks of density can often be produced in calculations from x-ray or neutron diffraction. How many alleged water molecules in other structures may there be, that will not withstand closer scrutiny in future This is a disturbing question that can only be answered by further research. 

Figure 6.5. Example of a fit to the data in Figure 6.4 at 250 K. The line appears to be asymmetric because of a strong spectrometer-related false peak at about 2 cm-1. Reprinted with permission from Li et al Copyright 2000 American Chemical Society.

Thus, there is a negative deviation from Beer s law. Errors due to stray light are more commonly found near the wavelength limits of the instrument components. Many reports of spectra in the UV region below 220 nm should be carefully checked, since false peaks have been reported. Visible radiation usually presents the most serious stray-light problem for ultraviolet-visible spectrophotometers, because both the spectral radiance of most visible sources and the spectral response of most detectors to visible radiation are high. 

A more challenging example was demonstrated by Stanek and Kozmihski [85], who applied their algorithm to 3D N- and C-labeled NOESY spectra of ubiquitin without suppression of diagonal peaks. The efficiency of artifact suppression was investigated by comparison of the reconstruction with the conventionally acquired reference spectrum. Less than 2% of peaks were missing, and about 1.5% false peaks were reported. The correlation coefficient between peak volumes of... 

Peak detection is an important step in the identification process. Sometimes only a few experimental peptide masses in the fingerprint match the theoretical masses, and therefore the failure to detect a relevant peak can hinder the correct identification of a protein. However, if too many false peaks are considered, this may lead to erroneous database matches causing false identifications, as well as increasing search duration. Furthermore, it is important to precisely determine the peptide masses. 

Figure 5.18 Performance of triple-rank correlation method with moment filtering for antamanide. For the two overlapping HR resonances of Pro3 (red (gray in print version)) and Phe5 (black), (C and D) show 2D HSQC and TOCSY (tm 90 ms) strip plots (A and B) show 3D FT strip plots at constant frequency (E and F) show 3R planes before filtering and (G and H) show those after filtering. Peak cross-sections in (C) and (D) demonstrate differential line positions and line widths that permit identification of false peaks in (E) and (F) and their successful suppression in (G) and (H) based on moment filtering. Reprinted with Permission from Bingo et al. [15]. Copyright 2010 American Chemical Society.

A comparison of LC methods for determination of cis-trans isomers of P-carotene was made on Vydac C18 201 TP and calcium hydroxide columns (186). The purity and relative distribution of P-carotene and its isomers in several commercially available products were evaluated by HPLC on several columns using a mobile phase of methanol/water (97 3) (187). Because carotenoids and chlorophylls are very sensitive to the nature of injection solvent, such as acetone, sample-solvent interaction may give rise to distorted and even false peaks (188). Because metal column frits may damage carotenoids. Teflon column frits should be used (189). Also artifacts may be produced on the column by reactions among the carotenoids, injection solvents, and mobile phase. Losses that occur during extraction and saponification can be reduced by use of suitable antioxidants (189). 

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