Baseline Perturbations

If the perturbations are in the form of spikes of an irregular nature, the problem is likely to be detector contamination. Such spikes are especially observed when dust particles have settled into the FID flame orifice. Of course, the problem may also be due to interference from electrical pulses from some other source nearby. Regular spikes can be due to condensation in the flow lines causing the carrier, or hydrogen (FID), to pulse, or they can be due to a bubble flow meter attached to the outlet of the TCD, as well as the electrical pulses referred to above. Baseline perturbations can also be caused by pulses in the carrier flow due to a faulty flow valve or pressure regulator. 

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Unfortunately, not much experimental work has been carried out on the combination of Supercritical fluid extraction and liquid chromatography systems (43, 44). One of the reasons for this arises from the difficulties in achieving compatibility between the extraction solvent and the FC mobile phase. Baseline perturbations have been... 

Short term noise consists of baseline perturbations that have a frequency that is significantly higher than the eluted peak. Short term detector noise is not often a serious problem as it can be easily... 

Drift results from baseline 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 mobile flow rate, or column bleed in GC in LC drift can be due to pressure changes, flow rate changes or variations in solvent composition. As a... 

Short-term noise consists of baseline perturbations that have a frequency that is significantly higher than that of the eluted peak. Short-term detector noise is usually not a serious problem in practice, as it can be easily removed by appropriate electronic noise filters that do not significantly affect the profiles of the peaks. The source of this noise is usually electronic, originating from either the detector sensor system or the amplifier electronics. 

Values of AH(ad) were typically near 55 10 kcal/ mole O2 and 25+5 kcal/mole CO. Values for H2 appeared to be around 20 kcal/mole H2 after a correction was made for a baseline perturbation caused by the difference in thermal conductivities between H2 and Ar. The AH(ad) values on Pd/Ti02 reduced at 773K were not markedly different from those on the "typical" Pd catalysts, and no significant support effect on heats of adsorption was found for Pd. For all three gases, heats of adsorption increased as Pd crystallite size decreased from 6 nm to below 2 nm. 

Fig. 5. The reversibility of the unfolding of Sac7d demonstrated by DSC with termination of the first DSC scan at 100°, rapid cooling (2 deg/min), and rescanning to 120°. The two scans coincide to within the error expected for repetitive scans (see Fig. 2). The pretransition slopes have been adjusted to zero since rapid cooling leads to baseline perturbation.

In extraction techniques (LLC, SPE, etc.), normal-phase liquid chromatography (NPLC), and thin-layer chromatography, aliphatic hydrocarbons (e.g., -hexane, -heptane) are usually used. The elution strength of these solvents is often modified by addition of more polar solvents. The fundamental problem with the eluents in NPLC is dissolved water and trace amounts of olefins. These contaminations can induce a change in the wavelength cut-off values (UV detection, spectrophotometry), baseline perturbation, and poor reproducibility of retention data. Halogenated solvents such as dichloromethane can react with some organic solvents (e.g., acetonitrile) to form crystalline products. 

The pumps required for flow calorimetry have to be carefully considered because they can be sources of error. Accentuated or irregular pulsing can cause severe baseline perturbation and the pump rollers can damage the cells. The choice of flow rate is crucial for several reasons. If it is too fast, heat will be carried away from the measuring vessel in the distal transmission tubing. If it is too slow, then there may well be problems of cell sedimentation and oxygen... 

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