Concentration detectors signal tracings

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... 

In operation, the viscometer of Figure 2 will generate two separate signal detector traces for recording. The differential log-anplifier will generate a viscosity (In hi) trace vdiile the concentration detector will generate a concentration (c) trace. 

Chromatographic analysis is based on a chromatogram. This chromatogram is a two-dimensional diagram (Fig. 1.3) traced on chart paper or a screen that reveals, as a function of time, a parameter that depends on the instantaneous concentration of the solute as it exits the column. Time (or alternatively elution volume) appears on the abscissa and the detector signal appears as the ordinate. 

Figure 2. Signal tracings from the three detectors showing excess LS intensity, specific viscosity, and concentration signals, for a sample with a Flory-Schulz MWD, polydispersity of 2, and a Mark-Houwink exponent of 0.725.

Element concentrations in the eluent are also recorded if a suitable detector is used. This can be processed in a similar fashion to the UV signal to yield an eluent-based size distribution. If the element detector signal is divided by the mass detector signal, we obtain a quantity which is proportional to the concentration in the sample particles. When this quantity is plotted against particle diameter, we obtain the element concentration distribution for the sample (Fig. Ic). It is often useful to plot the element atomic ratio distributions for elements of interest. This graph is particularly useful for deducing size-based speciation data for trace elements. 

One of the simplest ways to determine both LOD and LOQ is based on the ratio of the peak height to the noise in the chromatogram baseline, mostly called the signal-to-noise ratio (S/N). It is common practice to define the LOD at S/N > 3. For the LOQ, the respective rule is S/N > 10. The LOD (and LOQ) of a method is thus low if the method shows a steep slope of the calibration curve combined with a low baseline noise. The slope of the calibration curve expresses the change of detector signal as a function of change in analyte concentration (or mass). This slope is defined as the so-called sensitivity of a method, which is often confused with the LOD or trace detection performance. Both are related, but they are not identical and measured in different units. Since LOD and LOQ are strictly proportional, we only discuss LOD in this section. 

As in any kind of column chromatography, the raw data of SEC (Figure 1(a)) is the trace of the signal from the concentration detector over the retention volume, S(V), which is proportional to the concentration, C(V) ... 

Temperature-programmed desorption (TPD) can be described as the measurement of the rate of desorption of preadsorbed molecules as a function of temperature. It involves heating of sample while contained in a sample holder—as the temperature rises, certain absorbed species will have enough energy to desorb and will be detected simultaneously by means of specific detector (for example, mass spectrometer). In TPD experiments, temperature increases linearly and the concentration of desorbed gas is recorded as a function of temperature. Therefore, TPD profile is traced as desorption rate versus time (or temperature). If detector signal is properly calibrated, concentration of desorbed gas can be plotted as a function of temperature. 

Figure 11 illustrates an SEC separation of a sample of 3-conponent polystyrene mixture with the dual concentration and viscosity detectors of Figure 10. The top trace shows the concentration elution profile of the SEC separation as detected by a uv-photometer. The bottom trace records the same SEC separation, except with the viscometer signal from the log-amplifier output.

For a detector to be of use in quantitative analysis, the signal output should be linear with concentration for a concentration-sensitive detector and with mass for a mass-sensitive detector. Some detectors have an additional time constant purposely introduced to remove the high-frequency noise. This should always taken into consideration, since a slow detector response can significantly broaden and attenuate chromatographic peaks relative to those actually sensed. Moreover, a versatile detector should have a wide linear dynamic range so that major and trace components can be determined in a single analysis, over a wide concenua-tion range. 

The system presents the result to the operator either visually and/or audibly in another few seconds. A trace explosives detection portal system will provide a signal (e.g., red light/green light) output depending on the concentration of explosives reaching the detector. If needed, the particular explosive detected can be relayed to the screener. 

The third atmospheric technique analyses the residual liquid and/or condensate hydrocarbon traces on aerosols carried into the atmosphere by thermals (Barringer, 1981). The aerosols are created by gas bubbles which exsolve into the atmosphere from the sea in areas where microseeps create gas bubbles which reach the sea surface. The aerosols are concentrated from large volumes of air and collected by an airborne cyclone sampler carried aboard an aircraft which is flown at 30 m (100 feet) above the sea surface. Hydrocarbons adsorbed on the aerosols are measured by a flame ionisation detector which yields a total hydrocarbon signal. This system is claimed to produce... 

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... 

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