Unexpected peaks

Once your problem takes on a 13 C dimension, you are of course, obligated to examine the 13 C data with the same level of dispassionate scrutiny that you apply to the proton data. Chemical shifts cannot be fudged and unexpected peaks cannot be glossed over. You have to be able to account for everything you see to have confidence in your product. 

Discuss troubleshooting procedures for spectrophotometers for 1) failure of electrical components, 2) unexpectedly high absorbance readings, and 3) an unexpected peak in an absorption spectrum. 

Unexpected peaks can arise from components from a previous injection that moved slowly through the column, contamination from either the reagents used to prepare the sample or the standards, or a contaminated septum, carrier, or column. Solutions to these problems include a rapid bakeout via temperature programming after the analyte peaks have eluted, use of pure reagents, and replacement or cleaning of septa, carrier, or column. 

What can a GC analyst do to solve the problem of unexpected peaks on the chromatogram ... 

Detection of impurities by separation, which almost always involves the finding of unexpected peaks in some kind of chromatogram, and which may lead to identification. 

In starting a residue analysis in foods, the choice of proper vials for sample preparation is very important. Available vials are made of either glass or polymeric materials such as polyethylene, polypropylene, or polytetrafluoroethylene. The choice of the proper material depends strongly on the physicochemical properties of the analyte. For a number of compounds that have the tendency to irreversible adsorption onto glass surfaces, the polymer-based vials are obviously the best choice. However, the surface of the polymer-based vials may contain phthalates or plasticizers that can dissolve in certain solvents and may interfere with the identification of analytes. When using dichloromethane, for example, phthalates may be the reason for the appearance of a series of unexpected peaks in the mass spectra of the samples. Plasticizers, on the other hand, fluoresce and may interfere with the detection of fluorescence analytes. Thus, for handling of troublesome analytes, use of vials made of polytetrafluoroethylene is recommended. This material does not contain any plasticizers or organic acids, can withstand temperatures up to 500 K, and lacks active sites that could adsorb polar compounds on its surface. 

The FTIR spectrum of the nano hydroxylapatite product is shown in Fig. 16.6. On the spectrum, 3570 cm-1 and 632 cm-1 are the absorption peaks reflecting the expandingcontracting and winding vibrations of OH", respectively, suggesting that the oxhydryls do exist in the synthesized molecules 566 cm 1, 604 cm"1, 959 cm 1030 cm 1 and 1094 cm"1 are the typical absorption peaks of phosphoric acid radical. Somewhat unexpected peaks are those at 1420 cm 1 and 1470 cm 1, which are the typical... 

Ionophoresis of the buffer salts. When considering evaporation (Section 1.1.1.1) mention was made of sodium determination in eluates of the strip after the run. Measurements of potential on the strip have confirmed the presence of salt zones (unpublished and P21). When current is applied, the sodium peak due to evaporation moves partially toward the cathode and diminishes at the same time. A second new and unexpected sodium peak appears near the anode (Fig. 16). Determinations of both sodium and barbiturate proved the presence of both ions together on the same spot (P8). Experiments with sodium barbiturate as anodic electrolyte and potassium barbiturate as cathodic electrolyte proved that it is an accumulation of the cation leaving the anodic buffer vessel which causes this new and unexpected peak (Fig. 17). The reason appears to lie in the unequal mobilities of the two ions which tend to occupy a zone from which the faster ion (sodium) needs the same time to reach the cathode as the slower ion (barbiturate) needs to reach the anode (G26, P7). It is, of course, necessary that one anion be discharged at the same moment as one cation. 

This effect and similar ones are sometimes responsible for unwanted, unexpected peaks in chromatograms, and then they are called ghost peaks or pseudo peaks. A recent review of this entire subject55 calls them system peaks, and it is a good source of further details and references. 

Truly reproducible unexpected peaks are system peaksas shown in Figures 13.3... 

The stability of arsenobetaine and DMA in the tuna fish was tested over a period of 9 months at -20°C, +20°C and +40°C. In addition, the stability of the material and of the raw extract was also verified by a qualitative control of the chromatograms no unexpected peaks containing arsenic were detected. The results showed that no instability could be demonstrated for both arsenobetaine and DMA at +20°C and +40°C [12]. 

Errors in DEPT editing may arise from a number of sources. The most likely is incorrect setting of the proton pulses, especially the 0 pulse used for editing, which may often be traced to poor tuning of the proton channel. Even small errors in 0 can lead to the appearance of small unexpected peaks in the DEPT-90 if 0 is too small it approaches DEPT-45 whilst too big and it approaches DEPT-135. Usually, because of their low intensity, these spurious signals are easily recognised and should cause no problems. Even with correct pulse calibrations, errors can arise when the setting for the delay period is very far from that demanded by Uch [35] and in particular CH3 resonances... 

The interpretation of mass spectra of compound libraries obtained from direct injection measurements is problematic for two main reasons (i) The yield of ionization of the various components of a library can vary significantly depending upon the structure of the side chain and poorly ionized members are therefore easily overlooked and (ii) even with soft ionization techniques, fragmentation is possible and the additional unexpected peaks impede the interpretation of the mass spectrum. 

Unexpected peaks in the chromatogram can have multiple causes air bubbles, impurities, late elution solutes or memory effects. How do you recognize a memory effect and how can you eliminate it ... 

Truly reproducible unexpected peaks are system peaks,as shown in Figs. 13.3 and 21.2. They always occur when the mobile phase is a mixture of several... 

In addition to the buckling measurements, toll irradiations were used to determine the lattice parameters tt, t,t, p, and L . Particular attention was paid to the resonance flux distributions as obtained from In, An, Co, W, and U-238 foil irradiations. Calculations of the effectiveness of the-Internal surface of the fuel in absorbing resonance neutrons agreed, within 5%, with activation experiments using U-238 foils. However, in some cases the observed distributions of the resonance flux through the fuel assemblies showed an unexpected peaking inside the assemblies. 

The temperature dependence of the phonons has also been measured in SmBe by Alekseev (1993) and by Alekseev et al. (1993b). They reported an unusual increase of the longitudinal acoustic branches, but a softening of the longitudinal optic branches with decreasing temperature. In addition, they found an unexpected peak at 20meV at high temperatures, and this was interpreted as a resonant gap mode. This mode apparently disappears at low temperature. 

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