Instability, baseline

Detector noise in its various forms has already been discussed but, in practice, measures can often be taken to reduce 

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The effect of capillary temperature, which can also have a significant impact on molecule separation, is shown in Figure 4. A higher temperature provides faster migration however, it also causes baseline instability. Our recommendation is to use 20—25°C. 

The off-line measurements of the linear block copolymer "arm" samples were not difficult. However, in most cases, chloroform solutions demonstrated noticeably more LALLS baseline instability than those prepared in THF and toluene intensity readings changed as much as 10% within several minutes regardless of the amount of solution prefiltration. 

Normal CEC has the advantage of high efficiency at the expense of long elution times. Moreover, baseline instability, bubble formation and fluctuation of the current is commonly observed. 

Outlet waste gas stream from the ELSD may contain organic solvent vapors. For safety reasons, it is essential to ensure that the outlet of ELSD is properly directed to a safe vented outlet (e.g., a fume hood). Waste ventilation should occur at atmospheric pressure. A vacuum or restriction may result in pressure changes within the optical detection chamber and cause detector baseline instability. 

Nonaqueous Organic acids Simple method Baseline instability... 

Fluorescence and ED can provide better sensitivity compared to UV detection, but only some phytoestrogens like daidzein, formononetin, and coumestrol have fluorescence response whUe using ED at an operating potential above 1.2 V creates baseline instability." The weaknesses of these detection methods are their low sensitivity and nonspecihcity leading to the possibility of sample matrix interference." ... 

The low efficiency and short migration distances typical of thin-layer chromatography limit useful separations to those with relatively large enantioselectivity factors. Absorption by the chiral selector can cause baseline instability and reduced sample detectability for quantitative measurements by scanning densitometry. The chiral sepa-... 

The heat flow signal is weak, especially for the largest grains, and therefore baseline instabilities are causing the signal to become less accurate. However, it is clear that the smaller the particle size, the larger the heat flow at the beginning of the reaction and the faster the reaction tends to completeness. For the smallest 1.5 /rm particles, the heat flow drops to zero before 500 min of reaction. 

Because it is based on UV absorption, the use of ninhydrin is the least sensitive method for the determination of amino acids. It also has the disadvantage that the reaction coil must be heated to 130°C in order for the postcolmnn chemistry to occur in the minimmn amovmt of time. In addition, the reagent reacts with ammonia, which leads to baseline instability if buffers containing trace ammonia are employed to make up the mobile phase. 

Another problem is that the spectra must have flat baselines in order for these methods to work properly. As seen earlier in the discussions of preprocessing methods for quantitative spectroscopy, there are methods for accomplishing this with little or no user interaction. However, these methods are very sensitive to baseline instabilities, and the correction applied must be very good to have any degree of success with these methods. 

The carrier gas must be of high purity (Ha, at least 99.999 %). Impurities can saturate molecular sieve traps, gas lines and other materials and, if not removed, result in bleeding at higher temperatures which causes baseline instability and shortens the life of the column. 

Phase oxidation was monitored by observing the baseline as air was injected into the column. The full scale baseline deflection, followed by baseline instability, reveals that the phase is severely degraded. 

FIGURE 10.7 Impure carrier gas can cause baseline instability on sensitive detectors. 

Both sample and matrix effects influence quantitative TLC using scanning densitometry. The sorbent matrix influences the shape of calibration curves impurity gradients, resulting from contamination of the layer, or application of derivatising reagents are common sources of baseline instability. Chemical reactions catalysed by the sorbent layer are also a source of sample instability. Fluorescence quenching and fluorescence enhancement effects can influence the reliability of quantification in the fluorescence mode [97]. Densitometry has been reviewed [100]. 

Baseline Compensation Analysis—A baseline compensation analysis, or baseline blank, is performed exactly like an analysis except no injection is made. A blank analysis must be performed at least once per day. The blank analysis is necessary due to the usual occurrence of chromatographic baseline instability and is subtracted from sample analyses to remove any nonsample slice area fijom the chromatographic data. The blank analysis is typically performed prior to sample analyses, but may be useftil if determined between samples or at Ae end of a sample sequence to provide additional data regarding instrument operation or residual sample carry-over from previous sample analyses. Attention must be given to all factors that influence ba%line stability, such as column bleed, septum bleed, detector temperature control, constancy of carrier gas flow, leaks, instrument drift, etc. Periodic baseline blank analyses should be made, following the analysis sequence protocol, to give an indication of baseline stability. 

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