Base-line shifts

Fraction 8 (Figure 5 5) is mostly alkylated phenols and indanols with a trace amount of smaller alkanes. The base line shift is due to the co-elution of several large phenolic species in many isomeric forms. Fraction 9 (Figure 5-6) does not contain any alkanes. The ratio of the o-cresols to m, p-cresols increases from fraction 8 to 9. Both m-cresol and p-cresol are structurally longer than o-cresol. Some long aromatic species such as biphenyls also appear in this fraction. Compared to fraction 8, the phenols in fraction 9 are of shorter size... 

The circadian process C is implemented by a simple cosine function with period t of 1 day (1 day = 1440 min), a phase shift (tphase) and an arbitrary amplitude (amp) and base line shift (base). 

Typically, in gradient elution liquid chromatography, electrochemical detection has been difficult due to base-line shifts that result as a consequence of the altered mobile phase composition. However, a unique property of micelles allows for much improved compatibility of gradients (i.e. gradient in terms of micellar concentration or variation of small amount of additive such as pentanol) with electrochemical detectors. This has been demonstrated by the separation and electrochemical detection of phenols using micellar gradient LC (488). A surfactant (apparently non-micellar) gradient elution with electrochemical detection has also been successfully applied for the assay of some thyroid hormones by LC (491). 

Another common method used for measuring T Is thermal analysis. The glass transition is associated wfth changes in specific heat, not with latent heat. Thus the transition occurs as a base-line shift rather than as distinct endotherms in DSC or differential thermal analysis (DTA). As shown In Figure 3, wet determinations are more difficult than dry determinations, since the wet fibers must be sealed In DSC capsules to prevent moisture vapor from escaping during the determination. Further, the baseline shift is usually very small, and an accurate measure is difficult to accomplish. 

Since the magnitude of the absorption varies from one solvent to another and from one wavelength to another, base-line shifts are usually observed during gradient elution. These base-line shifts are often linear with concentration of the second solvent. The size of such shifts depends on the extent of the compositional change and on the absorbance characteristics of the mobile phase, the optical path-length and the detector sensitivity. 

Typical DTA curve of the ZAS base glass is shown in Figure 1. The DTA curve is characterized by some endothermic peak. The endothermic peak is very obvious at 746°C, 824.6°C, 963°C and 1024°C, which indicates there exists transformation of crystal configuration. The endothermic peak is not very obvious at 963°C and 1024 and the endothermic base line shift at 650°C 740°C gives the beginning of glass nucleation temperature zone. 

Fig. I show the DTA curves for LZS parent glass. The endothermic base line shift at 500°C indicates the glass transition temperature and the exothermic peak at about 680°C is a crystallization temperature for this system. Normally speaking, nucleation temperature was about 50°C above the transition temperature. Because the anneal temperature was 500°C, which was very close to the nucleation temperature, so the one-step heat-treatment was adopted.

Starting from the lowest temperature, the first discontinuity observed is the glass transition, which appears as a shift of the base line corresponding to the heat capacity difference of the sample before and after the transition. There are various methods to mark the exact location in the curve where the glass-transition temperature Tg is recorded. This will be discussed below. The magnitude of base line shift (ACp) during the glass transition is related to the concentration of amorphous chains in the sample and is affected by other factors such as molecular mobility and free volume. 

The two chromatograms in Figs. 23-3 and 23-4 reveal that there is a base-line shift after the CFC-12 peak. The base-line returns to the original level after the CH2CI2 peak. This shift is due to a pressure change at the turn of valve 4, and shows that the fore-... 

With direct injection below 5 mg/kg sulfur the base line shift error due to the needle septum blank may become significant. Such error can be avoided by inserting the syringe needle into the hot inlet and allowing the needle septum blank to be titrated before injecting the sample. 

Two simple elements accordingly became basic to the interpretation of a calorimetric signal namely, the thermal effect (endo- or exothermic peak), and a base line shift due to the heat capacity drop across the peak. The single contribution (peak and base line shift) can be described according to either a thermodynamic or kinetic approach (see Appendix Al), with the former being more adequate for traces obtained at low heating rates, and the latter at high rates. 

Figure 6. DSC trace from a vegetal the base-line shift at 7 g is followed by an exotherm related to a partial ice formation at 7 g and, an endotherm, at higher temperature, related to ice melting (modified from ref [50]).

The only part of the DSC trace that is not affected by the presence of these compounds is the base line shift at the starch Tg, although it can be concealed by signals related to sugar solubilization, fat fusion, gel-sol transition of hydrocolloids, etc., all occurring in the same temperature range [79],... 

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