Distortion Of Peak Shape Research Articles

Peak shape distortions during capillary electrophoretic separations of multicomponent samples in two co-ion buffers

During the capillary electrophoretic separation of a five-component quatternary ammonium analyte sample in two co-ion background electrolytes prepared from phosphoric acid, lithium hydroxide and tetrabutylammonium hydroxide, grossly distorted analyte peaks were observed. The electropherograms were successfully simulated using an earlier mathematical model of electrophoresis that was extended to handle up to eight nonprotic sample ions and two nonprotic background electrolyte co-ions. Peak shape distortion closely followed the predictions made during the recently described simulations of single analyte-two co-ion background electrolyte systems. Peak shape distortion was shown to depend on the relative mobilities of the particular analyte, the non-comigrating system peak and the governing co-ion. Severe peak shape distortion could occur in every multiple co-ion background electrolyte, such as in the indirect detection background electrolytes and charged interacting agent-containing background electrolytes, when certain analyte peaks and the non-comigrating system peaks overlap.

Peak Shape Distortions in the Capillary Electrophoretic Separations of Strong Electrolytes When the Background Electrolyte Contains Two Strong Electrolyte Co-Ions

A series of 25 mM phosphate buffer background electrolytes were prepared from phosphoric acid and mixtures of lithium hydroxide and tetrabutylammonium hydroxide as pH adjusters and sources of background electrolyte co-ions. These background electrolytes were used for the capillary electrophoretic separation of quaternary ammonium analytes. Abnormally distorted peaks, different from the simple characteristic triangular peaks usually attributed to electromigration dispersion, were observed. In order to understand the origin of the greatly distorted peaks, capillary electrophoretic separations with two co-ion background electrolytes were numerically simulated using a mathematical model of the electrophoretic process. Generalized peak shape rules were derived from the simulations which can be used to predict the shape of the analyte, co-ions, and counterion concentration peaks, as well as the local electric field strength changes. Abnormal peak shape and peak disappearance can occur when the analyte peak and the noncomigrating system peaks overlap.

Capillary zone electrophoresis of two synthetic neuropeptides: examination of detectability and resolution as a function of peptide concentration and buffer concentration

The capillary zone electrophoretic behavior of substance P and methionine enkephalin, when the amount of peptide was increased at a constant injection volume, was examined using a 50-μm I.D. capillary. Peak-shape distortion (asymmetry) increased when the concentration of these two peptides exceeded 1% of the buffer concentration. Increasing the buffer concentration increased the peak height and decreased the peak width, leading to higher detectability and resolution, respectively. Peak area versus analyte concentration remained linear, even under the overloading conditions that distorted the peak shape.

Morphiceptin and beta-casomorphin-5 analogues containing a reduced peptide bond: selective mu-receptor agonists and a novel mu antagonist, H-Tyr-Pro psi (CH2-NH)Phe-Pro-Gly-OH.

In order to prevent enzymatic degradation of beta-casomorphin-5 (1) and morphiceptin, reduced peptide bonds were incorporated at the 2-3 and 3-4 bonds, respectively. The analogues were synthesized by a combination of solid phase methodology and reductive alkylation of resin-bound peptide amines with Boc-amino acid aldehydes (Boc: tert-butyloxycarbonyl) in the presence of NaBH3CN. During reversed phase high pressure liquid chromatography purification, peak shape distortions could be observed. Epimerization was excluded, based on gas chromatography/mass spectroscopy analysis, which indicated acceptable levels of racemization (less than 3%) in the crude product. Instead, the phenomena could be attributed to slow cis/trans isomerizations originating from the Xxx-Pro bonds in the sequence. The presence of different conformational isomers was also established by 1H-nmr spectroscopy in DMSO-d6. All analogues showed high stability in blood plasma, enhanced binding affinity for the mu receptor, and very low binding to the delta receptor. While the Phe 3 psi(CH2-N)Pro4 analogues (3) and (5) displayed agonist activity, the Pro 2 psi(CH2-NH)Phe3 modified analogue (2) showed antagonist activity comparable to D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2.

The water resonance as an alternative pH reference: relevance to in vivo 31P NMR localized spectroscopy studies.

pH measurements require a suitable pH reference within the 31P NMR spectrum with respect to which the chemical shift of Pi, and hence pH, may be calculated. In muscle spectra PCr is prominent and provides a reference frequency. However, recent localized tumor studies have reported the absence of PCr, for example, in breast tumors. The use of the alpha-, beta-, and gamma-ATP peaks as suitable references has been suggested, but the position of the beta- and gamma-ATP peaks is dependent upon the intracellular Mg2+ concentration. The alpha-ATP is not affected by ionic concentrations; however, it contains UDPG and NAD+, the presence of which can lead to peak-shape distortion. This paper considers the use of the H2O resonance from the proton spectrum used for shimming as a suitable pH reference, provided this is also localized to the same region of interest, using a sequence giving rise to eddy current effects comparable to those of the 31P NMR sequence. Localized in vivo measurements in the muscle and brain of volunteers indicate good agreement between the proton and phosphorus chemical shifts, allowing the PCr position to be predicted to within 0.01 ppm in all cases.

Sulfite—disulfite equilibrium on an ion chromatography column

In the course of investigating new eluents for the resolution of sulfite from sulfate using a commercial column, unexpected behavior was observed for the sulfite peak when certain eluents were used, namely, distortion of peak shape and shortened retention time with increased concentration. This behavior is described in terms of a sulfite-disulfite equilibrium occurring on the column. Conditions for a baseline resolution of sulfite and sulfate are also presented

Quantitation of coverages on rough surfaces by XPS: An overview

AbstractSelected methods for the XPS quantitation of surface coverages are summarized with an emphasis on non‐planar samples with overlayers, e.g. powders of high surface area. Equations have been modified for uniform terminology and are presented based on a photoelectron spectrometer with an E−1 transmission function. Approaches which require a minimum of information from independent analytical techniques are given special consideration. Quantitative schemes discussed include calibration curves, surface sensitivity factors, exponential attenuation of bulk signals by overlayers, catalyst models, and the use of subshell twins or peak shape distortions related to photoelectron energy losses.

A solution to the pulse pileup and peak shape distortion problem

The pileup loss from the peak area and peak shape distortion effects are of considerable importance in routine activation analysis where the count rate may vary considerably from one sample to another. In this paper simple linear approximations for the peak shape change and efficiency loss have been used to show that correct results to within 2% are obtainable in the range 1000 cps to 10 000 cps.

“Inverted” peaks and current oscillations in cyclic voltammetry

Inverted peaks and strongly increased peaks have been reported and different explanations of these phenomena have been suggested[1,2]. The present communication discusses the peak shape distortions (“inverted” peaks, current oscillations) in terms of tangential movements of the mercury electrode surface. These distortions are explained similarly to the maxima in classical polarography: inhomogeneous polarization and inhomogeneous adsorption are considered as the two major forces causing the mercury surface movement and as a result, leading to the peak shape distortions. Experimental examples of the phenomena in aprotic and protic media and for organic and inorganic redox couples are given and explained in accordance with the proposed theory.

High Performance Size Exclusion Chromatograph with Computerized Data Reduction for Analysis of Polyethylene and Oligomers

Abstract A liquid chromatograph is described for the size exclusion chromatography (SEC) of polyethylene and oligomers. The rigid column packing material was 10 µm diameter porous silica. An infrared spectrophotometer was used as the detector. The chromatograph was designed and operated to minimize the degree of chromatographic peak shape distortion resulting from factors such as system dead volume, detector cell design, sample injection volume, and sample concentration. To compensate for peak spreading and skewing, the chromatographic data were analyzed with an IBM 370/168 computer using an APL program. The molecular weight range covered by this SEC system was 100-500 000. Precision and accuracy of Jhe calculated weight average molecular weight (Mw) values for polyethylene standards over this range were ±3 and s±4%, respectively.

A theoretical comparison of electron energy analysers degraded so as to obtain high sensitivity

The signals and signal-to-noise ratios are calculated for Lorentzian and Gaussian inputs to unmodulated electron spectrometers with wide slits and to over-modulated spectrometers. Data which characterise the distortion of the output as compared to the input are given in graphical form. It is concluded that unmodulated spectrometers give a small but useful increase in signal-to-noise ratio for peaks on low backgrounds whilst introducing the least distortion of peak shape.

A simple method for generation of background-free gamma-ray spectra

A simple and versatile method of generating background-free γ-ray spectra is presented. This method is equivalent to the generation of a continuous background baseline over the entire energy range of spectra corresponding to the original ones obtained with a Ge(Li) detector. These background curves can not be generally expressed in a single and simple analytic form nor in the form of a power series. The background-free spectra thus obtained make it feasible to assign many tiny peaks at the stage of visual inspection of the spectra, which is difficult to do with the original ones. The automatic peak-finding and peak-area calculation procedures are both applicable to these background-free spectra. Examples of the application are illustrated. The effect of the peak-shape distortion is also discussed.

Non-linear resonances in quadrupole mass spectrometers due to imperfect fields: II. The quadrupole mass filter and the monopole mass spectrometer

Peak-shape distortions and peak-splitting are commonly observed phenomena with all members of the quadrupole family of mass spectrometers. Errors in electrode shape, or spacing, or harmonics in the r.f. field may produce non-linear resonances. The resonances result in peak splitting by causing some ion trajectories to become unstable that otherwise, in a perfect field, would be stable and limited in amplitude. In Part I a theory of non-linear resonances in quadrupole fields was applied to the three-dimensional rotationally symmetric quadrupole ion trap. In the present paper, the theory of non-linear resonances is applied to the quadrupole mass filter and to the monopole mass spectrometer. Numerical computation of ion trajectories was used to determine the extent of peak-splitting for various amounts of third and fourth order spacial distortion and to determine the positions of resonance lines in the stability diagram. The calculations were used in the case of the mass filter to relate the performance to possible spacing errors in assembling the electrode structure. The monopole operating point can be chosen to avoid the resonance lines. The focusing quality is, however, affected by the presence of distortions. For the monopole, peak shapes were determined from the computed ion trajectories. A small amount of one type of third-order distortion is shown to be beneficial.

Non-linear resonances in quadrupole mass spectrometers due to imperfect fields I. The quadrupole ion trap

Peak-shape distortions and peak-splitting are commonly observed phenomena with all members of the quadrupole family of mass spectrometers. Errors in electrode shape, or spacing, or harmonies in the r.f. field may produce non-linear resonances. The resonances result in peak splitting by causing some ion trajectories to become unstable that otherwise, in a perfect field, would be stable and limited in amplitude. A theory of non-linear resonances in quadrupole fields was applied to the three-dimensional rotationally symmetric quadrupole ion trap to predict resonance locations in the Mathieu stability diagram. Numerical computation of trajectories was used to derive peak shapes for various amounts of distortion and to determine the extent of peak-splitting. The calculations were used to relate the performance to the spacing errors in assembling the electrode structure. The occurrence of resonances, in agreement with this theory, was experimentally demonstrated. A method is presented for obtaining good peak shapes in spite of the presence of field distortions, but with some loss in sensitivity. This method was investigated both experimentally and theoretically.

Distortion of peak shape in fast scanning of mass spectra

When a mass spectrometer is operated under fast scanning conditions the apparent resolving power and sensitivity as exhibited on a recorded spectrum are, in general, less than the static values, due to the limited bandwidth of the recording system. The present paper offers a theoretical treatment of the problem, the variation of these characteristics with scan time and time constant of the recording system being derived for various values of static resolution. Results have been computed from the theory, assuming a triangular peak shape under slow scan conditions, and a comparison is made with experimental observations both on a single and on double focusing instrument. Good agreement is obtained.