Concentration detectors peak shapes

Peak Shapes. In the case of the Wesslau MWD, the shapes of the peaks from the three detectors are always the same. For the Flory-Schulz distribution, the peak shapes are slightly different and the differences increase with increasing polydispersity. As the polydispersity increases, the LS and viscosity signals become narrower relative to the concentration detector signal and they also become less skewed. Figure 3 shows the peak variance of the viscosity and LS signals relative to the concentration detector peak variance as a function of polydispersity. The concentration detector peak variance increases from 0.25 mL when the polydispersity is 1.1 to 3.65 mL when the polydispersity is 3.3. The LS peak variance increases more slowly. The viscometer variance is in between the two but closer to the LS peak behavior. Figure 4 shows the relative skew of the peaks compared with the refractometer, where the skew is defined as... 

Following this procedure, recovery of greater than 80% was obtained from DBA-spiked urines at levels of from 0.05 ppm to 100 ppm. The electron capture detector was linear for chloro-benzilate from 0.1 yg/ml to 2 yg/ml and for DBP from 0.005 yg/ml to 0.5 yg/ml. The lowest concentrations which produced reasonably good peak shapes (relative to noise) were 0.2 yg/ml for CB and 0.02 yg/ml for DBP. Background at the retention time of DBP averaged 0.014 ppm for control rat urines. Although this is a low value, it is highly recommended that further application of this method should include an adsorption column clean-up step such as the alumina column used by Bartsch and coworkers (3). 

The simplest analytical information that can be obtained with the aid of FFF is the homogeneity of the sample or evidence for the presence of a compound of interest in the fractionated sample by the appearance of a peak in the expected interval of retention volume. In some cases, comparison of the retention volume and the peak shape of the investigated component with the peak shape of a reference sample can provide sufficient qualitative analytical information on sample purity and homogeneity. The peak areas in the fractogram can be used to evaluate quantitatively concentrations of the detected components provided that the relationship between detector response and concentration or quantity of the detected component is known. This relationship is usually determined by a calibration procedure. However some sample is lost in the void peak so that it is not possible to relate the detected concentration to that of the original sample consequently, concentration determinations can more advantageously serve to compare the relative concentrations of the fractionated components. 

Typical signal tracings for the Flory-Schulz MWD are shown in Figure 2. The tracings have similar shapes but different peak positions. From equation 5 it can be shown that the elution fraction at the maximum in the concentration detector signal has a DP of... 

The computer models described provide a functional simulation of SEC-viscometry-LS analysis of linear polymers. The results for the Flory-Schulz MWD are in qualitative agreement with previous results for the Wesslau MWD. Both models emphasize the importance of determining the correct volume offset between the concentration detector and molecular weight-sensitive detectors. For the Flory-Schulz model, the peak shape, as well as the peak elution volume, can provide information about molecular weight polydispersity. Future work will extend the model to incorporate peak skew and polymer branching. 

Figure 24.1 Peak shapes as obtained with different column diameters and separation performances (as a function of particle diameter at a given column length). Columns 1 and 3 are packed with a coarse stationary phase, columns 2 and 4 with a fine one. The packing quality, defined as reduced plate height h, is the same in all four cases. Separation performance is independent of column inner diameter therefore peaks 1 and 3 as well as 2 and 4, respectively, are ofthe same width. The peak height in the eluate can be calculated from equation (15) it is higher when the column is thinner and the separation performance is better. Peak areas cannot be compared although the same amount is injected in any case if a concentration-sensitive detector is used optimum flow rate depends on particle size and hence also the residence time in the detector.

Passage of the sample through the detector causes a variation in the monitored radiation, and the related transient signal is recorded as a peak which is ideally proportional to the analyte concentration in the sample. Figs 2.5 and 5.5 show typical recorder tracings from a segmented flow analyser, where a tendency towards a plateau is observed as a consequence of the low sample axial dispersion involved. Important parameters related to the recorded peak shape are the lag phase, peak width at... 

The actual amount of stationary phase per unit column length on a capillary column is much less than that supported on the particles of the typical packed column. Therefore the capacity for analytes of the capillary is much less than that of the packed column. When the capacity is exceeded, the analytes will spread out over the front of the column after injection, and the improved resolution of the capillary will be lost. This results in the phenomenon of leading or fronting peak shape described in Section 11.4. In capillary GC, one must decrease the concentrations of the injected analytes, often by dilution with solvent to levels which are at concentrations of only the part per thousand or ppm level. For this to be practical one must have detectors orders of magnitude more sensitive than the TCD described in Section 12.1. 

In given concentrations of sample vapor neutrals, spectra for product ions can be altered by control of temperature, and this was seen at cryogenic tanperature, at which ion clusters not usually observed in mobility spectra were formed and appeared in mobility spectra. For example, proton-bound trimers of alcohols were observed when temperatures were decreased to -20°C and dissociated at temperatures from -20°C to +10°C. " Increases in temperatures will lead to dissociation of complex ions, such as proton-bound trimers and proton-bound dimers. As temperature is increased, the intensity of peaks for protonated monomer increase, and the peak abundance of proton-bound dimers decreases. This has been developed and explored for dimethyl methyl phosphonate (DMMP), " amines, and ketones. For example, proton-bound dimers of alkyl amines underwent dissociation above -30°C on a 2- to 20-ms time scale, which is within the range of drift times for these ions. Consequently, the dissociation pathway can be observed as a distortion in the peak shape and baseline of a mobility spectrum since an ion entering the drift region as a proton-bound dimer dissociates to a protonated monomer before arriving at the detector. These studies permitted the determination of kinetics of dissociation for thermalized ions and illustrated that the appearance of an ion in a mobility spectrum is governed by ion lifetimes in comparison to ion residence times in drift tubes, and ion lifetimes are controlled by temperature. 

Operation of all detectors is optimized when their internal volumes are small, since band broadening is thereby minimized. However, concentration detectors have a cell volume in which detection occurs and the magnitude of that volume has special importance. Suppose the cell volume of a concentration detector is so large that the entire sample could be contained in one cell volume. The shape of the resulting peak would be badly broadened and distorted. 

Figure 3.2 The elution curve of a single component, plotted as the analyte concentration at the column exit (proportional to the detector response Rj,) as a function of V, the total volume flow of mobile phase that has passed through the column since injection of the analytical sample onto the column. (V is readily converted to time via the volume flow rate U of the mobile phase.) The objective of theories of chromatography is to predict some or all of the features of this elution curve in terms of fundamental physico-chemical properties of the analyte and of the stationary and mobile phases. Note that the Plate Theory addresses the position of the elution peak but does not attempt to account for the peak shape (width etc.). The inflection points occur at 0.6069 of the peak height, where the slope of the curve stops increasing and starts decreasing (to zero at the peak maximum) on the rising portion of the peak, and vice versa for the falling side the distance between these points is double the Gaussian parameter O. Modified from Scott, www.chromatography-online.org, with permission.

Before the connection to the laser system, the MIC system was optimized using a pure Pb standard solution (SRM-981) at 0.1 ng/mL. Th and U were added to this solution for peak center optimization, specifically the five counters (IC2, IC3, IC4, IC5, IC6) and the two Faraday cups (H3 and H4) see Table 31.1. The Th and U concentrations were adjusted to achieve 100-mV signals on the Faraday detectors. The test solution was run to determine the most efficient instrumental settings. The peak shape and the peak superposition of aU masses were optimized by adjusting the optic zoom lenses. In the low mass range, the MIC system consists of two movable blocks IC2, IC3, L4 and IC4, IC5, IC6, L3, the ion counters being fixed relative to each other in each block (Table 31.1). The dark noise was measured... 

Gerard et al. [664] studied the retention of 16 hydroxy and epoxy fatty acids. Cyanopropyl, diol, and silica colunms were tested in conjunction with various solvent gradients. Results using cyclohexane, iso-octane, or hexane modified with an alcohol/acid combination (chosen from IPA, butanol or isoamyl alcohols and sulfuric, formic or acetic acids) were presented. The optimal separation was obtained with a silica column (ELSD) using a 40 min 99/1 - 0/100 (99.3/0.5/0.2 hexane/ IPA/acetic acid)/(79.8/20/0.2 hexane/IPA/acetic acid) gradient. Excellent resolution between hydroxyoctadecanoic acid positional isomers (e.g., 2-hydroxy, 4-hydroxy, lO-hydroxy, and 12-hydroxy) was obtained and peak shapes were very good. Linear concentration vs. detector response plots were generated from 5 to 200 pg injected and 1 pg detection limits were reported. 

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