Time constant, detector recorder

The effect of the detector time constant on the apparent efficiency depends only on the time width of the bands. It has been shown by Sch-mauch 41) and by Me William and Bolton 42) that the profile recorded with a detector having a time constant r is wider than the actual profile by a factor (1 -f r/ert), where is the time standard deviation of the profile, provided this factor is less than about 1.2. Moreover, the peak heigh becomes smaller although the peak area remains unchanged. 1 he (list mu ment (retention time) of a peak increases by r and the retention time of the... 

In most cases the time constant of the detector is due to slowness of the electronics this is especially true for optical detectoHi- There would be no technical problem to reduce the time constant tb 20-50 msec, although the noise level is expected to increase somewhan With such fast detectors computer data acquisition becomes necessary, as recorders with a time constant less than O.S sec are rare and expen ve. Since they are difficult to maintain, they are impractical. 

The contribution of slow detector response can be ne ected when the base peak width is at least 40 times larger than r [cf. Eq. 3)]. In practice it is difficult to correct for such distortion because the time constant concept is only an approximation. It is not very reproducible and is sensitive to changes in the characteristics of the various elemehts of the electronics. Furthermore, detectors, amplifiers, and record are not first-order systems mid their response is only iippruxinntted by an exponential function (44). The response time is therefore defined by the time neces-... 

Prior to deconvolution, the convolved spectrum was corrupted with additive noise to form rms signal-to-noise ratios of oo 1, 580 1,115 1, 60 1, and 30 1 for traces (c)-(g), respectively. The noise generated is similar to white noise (or detector noise) after passing through an amplification system with a negligible time constant. A typical spectrum would be recorded with the higher noise frequencies already attenuated, and thus our example may not represent a realistic situation. It should also be pointed out that the simulated noise differs from trace to trace in Fig. 3 only in amplitude and is thus not truly random. 

All manual methods of quantifying peak size make use of the recorder tracing. Some consideration has. already been given to the peak height and width as determined by recorder chart width and chart speed. Both should be maximized for the size measurement technique used. In addition the recorder may have a limiting time constant as far as response to rapid peaks are concerned. This possibility should be considered along with the detector when time constant problems are suspected. 

The time constant of the detector is related to the electronics and should not be confused with the time constant of the recorder (or any data acquisition). 

Figure 1.17 Effect of detector time constant on resolution, system efficiency, and sensitivity (a) 100 msec, (b) 200-msec. Flow cell volume was 2.4 ml, and both chromatograms were recorded at the same sensitivity. (Reprinted from Ref. 41 with permission.)...

The time constant of the detection is the combined effect of the detector ( detector time constant ) and the data handling or recorder system. The time constant of the detector may be partly due to the fundamental kinetics of the detection (e.g. in polarographic detection), but is usually determined by the amplifier and other electronic components. 

It appears from table 7.3b that modern HPLC columns impose very stringent demands on the detection (and recording) system. Typical time constants of current LC detectors are in the range of 0.3 to 0.5 s [710], which is not even sufFicient to allow the use of a 20 cm, 5 pm column (column III in table 7.3b). 

One advantage of a large time constant is decrease in short term noise, which is also called damping. The temptation to improve one s chromatogram by increasing the time constant to decrease the noise must be avoided. Also, consideration must be given to the time constants of all components in the detector network for example, the recorder must have a speed comparable to the detector itself. 

The term appear is used as the solvent profile itself is not actually changed, only the profile as presented on the recorder or printer. The effect of the detector time constant can be calculated and the results from such a calculation are shown in figure 14. The undistorted peak, that would be monitored by a detector with a zero time constant, is about 4 seconds wide. Thus, for a GC packed column operating at 20 ml/min this would represent a peak having a volume of about 1.3 ml. 

The time constant, r, is a measure of how quickly a detector can record apeak. As the detector works together with the data acquisition, the time constant of this combination is an important factor. The time constant can be defined as the minimum time required by a system to reach 63% of its full-scale value (1 — 1/e = 0.63 98% is also often specified). It should not be greater than 0.3 s for detecting very narrow HPLC peaks, i.e. those that are rapidly eluted at a volume of less than 100 pi, and should not... 

Band broadening can occur in other parts of the chromatographic system as well as in the column. Contributions to this extra-column broadening may come from the injector, the detector flow cell, and the connecting tubing. Slow time constants of detectors and recorders may also contribute. These extra-column effects are more severe for early, narrow peaks in the chromatogram than for later, broader peaks. A more detailed discussion of these effects will be given in the section on instrumentation. 

The time constant, r, is a measure of how quickly a detector can record a peak. As the detector works together with the data acquisition, the time constant of... 

The first-order diffraction intensity is recorded on a storage oscilloscope. The YAG pulse is 10 ns long, and the detector time constant is 10 ns. This must be kept in mind when interpreting the scope trace. 

Because of the special properties of the exponential function the light decays with the same time constant r as the population decay. The light decay can be followed by a fast detector connected to fast, time-resolving electronics. If the excited state has a substructure, e.g. because of the Zeeman effect or hyperfine structure, and an abrupt, coherent excitation is made, oscillations (quantum beats) in the light intensity will be recorded. The oscillation frequencies correspond to the energy level separations and can be used for structure determinations. We will first discuss the generation of short optical pulses and measurement techniques for fast optical transients. 

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