Time constant, detector

Dispersion Resulting from the Overall Detector Time Constant... 

Figure 4. Derivative 1H NMR spectra of the measured clinoptilolite samples at room temperature. The modulation amplitude was 0.22 G, phase detector time constant 0.1 s, speed of the field 6.9 mGs -1. The lines are averaged out of eight accumulated repetitions.

Figure 42 shows the temperatures measured by two-color pyrometry for step changes in temperature compared with the true temperature and calculated temperatures based on the response characteristics of the detectors (time constant = 0.311 s). The response speed of the detectors in this case was too slow to follow the actual temperature decrease, but the temperature rise is reasonably well detected. Spjut and Bolsaitis reported that two-color temperatures are unreliable when the optical properties of the microparticle change during the experiment, but they showed that single-wavelength temperatures can yield consistent results and, with some caveats, are adequate for particle temperature measurement. 

Figure 5. The effect of detector time constant on the GPC separation of a liquid epoxy resin. Column Perkin-Elmer/ PL gel 2.00 Angstrom. Eluent THE at 2.0 ml/min.

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... 

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 detector time constant is the response time of the detector to the signal passing through it. A slower time constant will result in less apparent noise, but it will compromise signal and also resolution for closely eluting peaks. For closely eluting peaks, therefore, it is important to use a faster time constant. If there is plenty of resolution, a slower time constant will provide a smoother baseline. The effect of the time constant is illustrated in Figure 8.6. 

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. 

Figure 7.5. Effect of detector time constant on peak characteristics t, < t2 < t3 < t4.

Similarly the detector time constant must be adjusted because peaks can be only a second or two wide for short, narrow-bore columns run at high flow rates a large time constant would make narrow peaks appear artificially broad. If the column length is not drastically reduced, such sub-2-pm packings demand ultrahigh pressure pump systems to overcome the high back pressure (>400 bar). 

The detector time constant (or digital filter for modern instruments) is used to remove high-frequency noise. If the detector time constant is too slow, the observed peaks will be broadened. The 1/3 rule may be applied here as well As a rule of thumb, the maximum detector time constant (seconds) tolerable is about 1/3 the standard deviation of the peak in seconds. Peaks 1-2 seconds in width require a time constant no larger than 0.1 second. Figure 17-19 illustrates these points. Although noise-free chromatograms are desirable, resolution and sensitivity can be adversely affected by excessively large time constants due to peak distortion. 

Figure 17-19. Effect of UV detector time constant on peak shape and noise for fast LC separation of phenones (0.05pg/mL). Time constants, from top to bottom, are 0.5sec, 0.2sec, 0.1 sec, and Osec (no time constant). Peak widths are approximately 1 sec sampling rate is 20 Hz colnmn is 2.1 mm x 50 mm.

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. 

Detector Time Constant - (D ) - The overall time constant of the sensor and electronics and is usually given in milliseconds. It is mainly of interest in high speed chromatography. 

Since the resistive chain together with the capacitances C behaves also as a diffuse RC-line, it takes some time for the charges to reach the ends of the detector, The resolving time — this is the time over which the charges have to be collected — has to be not much less than To = RdCd> tlis detector time constant, where ... 

The BaX reaction products are detected by observing the laser-induced fluorescence of the C Tl-X L+ band systems that lie near 500 nm. Fig. 3-6 present typical scans showing the variation of fluorescence intensity with excitation wavelength. The scan rates are between 0.1 and 0.5 nm min , the detector time constant being 1 s. The variation of laser intensity with wavelength is small for all these scans ( < 10 %). Within experimental error identical results are obtained using the second-... 

The detector time constant. The detector mnst respond sufficiently fast to the changes in concentration or mass flow in the effluent, otherwise the peaks are distorted. 

The detector time constant and detector cell volume are both involved. The slit width along the length of S the capillary is proportional to the latter. A value of ... 

The contribution of the detector time constant r is modeled by the following relation ... 

Slow detector time constant Adjust time constant to match peak width. 

The small peak volumes and resolution demands from high-throughput analyses require faster data collection rates for accurate results. Adequate detector time constants should also be used. 

A pressure-optimized column has L = 2.2 cm, dp = 6.9 )Xm and A/ = 2.3 bar Shorter columns are preferred for simple problems with a separation factor of ca. 1.2. Both time and solvent are saved by using columns 3-5 cm in length (also available commercially), but the injection and extra-column volumes and the detector time constant must be kept small in order not to deteriorate the separation performance. 

The peak width in seconds (equation 9) is influenced by the mobile phase flow rate, whilst the peak width in microlitres (the peak volume, equation 9a) is not, if we assume that the decrease in plate number by a flow rate other than the optimum one is negligible. Yet the peak width in seconds is an important parameter for the calculation of the detector time constant (equation 12) ... 

Detector time constant (or response time, rise time) < 1 s... 

Detector time constant (or response time, rise time) < 1 s Column LiChrospher 100 RP 8, 5 pm, steel, 12.5 cm x 4 mm i.d. Temperature ambient... 

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