Drifts of the base line

For CD and MCD observations at temperatures above ambient, many types of cells equipped with jackets through which thermostatting fluid can be circulated from an external constant-temperature bath are useful. Otherwise, a thermostatting arrangement involving a sample holder through which fluid from a constant-temperature bath can be circulated is used. In the latter case, it can take up to 30 min for the system to equilibrate thermally. Thermal fluctuation often causes a drift of the base-line. 

The silver chloride electrode gave poor response to iodide and bromide, and so did the silver bromide electrode to iodide. Although the silver iodide electrode responded to all three halides, the peaks are not sufficiently resolved and they are asymmetric. Further, there was a drift of the base line after detection of a halide ion which was not a component of the electrode and this drift caused disturbance in the following peak. This difficulty is eliminated by using hydrous zirconium oxide instead of the anion exchange resin for the chromatography since it reverses the elution order for halide ions. The silver bromide electrode is then the most suitable as the detector for both bromide. 

It is easiest to reduce base-line drift and keep the displacement of the base line at a minimum when metal blocks are used and probably nearly as easy with ceramic blocks in... 

The important errors were produced by the drifts of base line and by the improper peak limits and base line settings. For example, the presence of a low-molar-mass tail in a polystyrene sample inflicted an error of 800% in its value. In some cases, the low molecular tail was overlapped with the system peaks, while in other laboratories the software cut-off the low-molar-mass tail of chromatogram. It was likely considered just the base line perturbation by the computing system. 

Both Zeeman and frequency modulation are used for line observation but each has its drawbacks frequency modulation is often accompanied by a steady drift of the recorder base line, while Zeeman modulation becomes inefficient when the electric field gradient is highly asymmetric, especially for the v lines. 

The base-line reading - this should be set only after a suitable quantity of mobile phase has passed through the column (prior to injection of the sample) and stability is established. The chart recorder is usually set a little above the edge of the chart paper grid, to allow for base-line drift. 

Make sure you know the direction of the horizontal axis of the chromatogram (usually, either volume or time) - it may run from right to left or vice versa - and make a note of the detector sensitivity on the vertical axis. Ideally, the base line should be flat between peaks, but it may drift up or down owing to a number of factors including ... 

Typical Response Peaks. Figure 2 shows some typical response peaks for the HPLC separation of the saturates and aromatics in a typical vacuum gas oil. The curve in the upper portion of the figure is the response for the sample using the RI detector. After the saturates peak appeared, the backflush valve was switched and the attenuation changed as indicated in the figure. The peaks were very sharp and symmetrical and appeared in less than 10-min lapsed time from the point of injection. Base line drift was minimal and the areas of the response peaks were obtained with a ball and disc integrator on the strip chart recorder. 

If baseline drift has occurred then the peak height is measured from the mid point on the line joining start and end points of the peak and peak width at half height by drawing the horizontal vector to the line drawn at Aq.s parallel to the base line (Figure 8.4). Measurement of peak data assumes symmetrical peaks. Data from asymmetric peaks with an asymmetry ratio of greater than 1.2 will be less accurate. 

The base line drifts are avoided by the use of the flow system shown in Fig. 1, with the capillaries R, R, and R having large flow resistances. The modified reactor is shown in Fig. 2b the reactor tube is jacketed so... 

The usual way of minimising drift effect is to consider as useful response the difference between the base line, obtained by presenting the sensor array to pure reference air, and the signal obtained after stabilisation in the polluted atmosphere. However, such solution requires operating by cycling between reference air and tainted air, which is not convenient for on-site applications. That requires carrying in the field heavy gas cylinders. Alternatively, generating the reference air by a simple... 

Original NIR spectra of total 99 filter samples are shown in figure 2, and the spectra of samples after pre-processing with MSC are shown in figure 3. It is obviously seen that the base line drift of the spectra is reduced in the figure 3 compered to figure 2 by the... 

Drift consists of base line perturbations that have a frequency that is significantly larger than that of the eluted peak. Drift is almost always due to either changes in ambient temperature, changes in solvent composition, or changes in flow rate. All these factors are easily constrained by careful control of the operating parameters of the chromatograph. 

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