Baseline subtraction

Linear extrapolated sample baseline used to calculate the enthalpy of transition 

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Figure 3 Membrane osmometry. Top concentration series with multiple injection of polystyrene (PS, 5,250 g/mol) in toluene. Concentrations 1.07,1.91, 2.94, 4.11, and 5.03 g/L. Multiple solvent injections establish the baseline (subtracted). Inset data evaluation for concentration series of PS (5,250 g/mol) and PS (47,400 g/mol) in toluene. Bottom design of osmometer equipped with flow cell. Reproduced with permission from Lehmann et al. [16]. Copyright 1996 American Chemical Society.

Fig. 21 Microcalorimetric endotherms for aqueous solutions of PVCL samples. PVCL-1500 (a), PVCL-1300 (b), PVCL-330 (c), PVCL-30 (d), PVCL-21 (e). An endotherm prior to baseline subtraction is shown for PVCL-330 in the inset. (Reprinted with permission from Ref. [146] copyright 2004 American Chemical Society)...

Beyond simple data storage and instrument control, modern data systems provide extensive data analysis capabilities, including fitted baselines, peak start and stop tic marks, named components, retention times, timed events and baseline subtraction. Further, they provide advanced capabilities, such as multiple calibration techniques, user-customizable information and reports and collation of multiple reports. If a Laboratory Information Management System (LIMS) is available, the chromatographic data system should be able to directly transfer data files and reports to the LIMS without user intervention. The chapter by McDowall provides a terse but thorough description of the... 

Selected entries from Methods in Enzymology [vol, page(s)] Aspartate transcarbamylase [assembly effects, 259, 624-625 buffer sensitivity, 259, 625 ligation effects, 259, 625 mutation effects, 259, 626] baseline estimation [effect on parameters, 240, 542-543, 548-549 importance of, 240, 540 polynomial interpolation, 240, 540-541,549, 567 proportional method for, 240, 541-542, 547-548, 567] baseline subtraction and partial molar heat capacity, 259, 151 changes in solvent accessible surface areas, 240, 519-520, 528 characterization of membrane phase transition, 250,... 

A further problem that is sometimes encountered is that the spectrum obtained after baseline subtraction does not return to zero above 320 nm (in near-UV CD) or 250 nm (in far-UV CD). If, above these wavelengths, the spectrum runs parallel to the zero base, a provisional spectrum may be derived by subtracting the difference in ellipticity, as a constant, from the whole spectrum. For quantitative assessment of a sample, however, the spectrum should be redetermined, paying attention to the correct buffer blank, cell reproducibility, and instrumental drift (see Strategic Planning). 

Figure B3.6.1 Rayleigh and Raman bands in fluorescent spectra, as seen in scans for solvent baseline and hen egg white lysozyme (EWL) solutions (solid lines). Circles represent the spectrum of EWL with baseline subtracted. Parameters EWL A2ao = 0.05 Xex = 280 nm excitation and emission bandwidths, 2.5 nm scan rate, 100 nm/min five scans accumulated. Spectra were measured using a Perkin Elmer LS50B fluorescence spectrometer.

Apart from the Raman band, a buffer solution should give an almost flat baseline of low intensity (see Fig. B3.6.I). At the usual working concentrations of protein (A280 >0.05), buffer fluorescence constitutes only a minor contribution to the fluorescence spectrum of the protein and is accounted for by the appropriate baseline subtraction. 

Fig. 10.6. Baseline subtracted OSW voltammograms of the difference between the voltammograms obtained before and after metal ion accumulation in test sample number 1 for 10 min at 25°C. Test sample 1 contained 0.100 pM Cu2+, 3.00 pM Cd2+ and 0.600 pM Pb2+ in 50 mM ammonium acetate (pH 7.0). The electrodes were modified with TA-Gly-Gly-His, TA-glutathione, TA-angioten-sin I and TA. OSW voltammograms were measured in 50 mM ammonium acetate (pH 7.0) and 50 mM NaCl at 25°C at a pulse amplitude of 0.025 V, a step of 0.004 V and frequency of 25 Hz.

Figure 7. High resolution carbon, Cls, spectra from Tokai glassy carbon samples following smoothing and baseline subtraction for various treatments. The treatments include a, O (Linde, 99.6%) RFP b, O (liquid carbonic, USP grade) RFP ...

Although the ionic strength of the eluant may remain the same throughout the run, the background conductivity can decrease due to the changing dielectric constant. These baseline changes can be compensated either by chemical means or by computer baseline subtraction. Often, mobile phase ion chromatography is used to elute ions which are very... 

The MALS measurements illustrated by Fig. 2 also may be used directly to calculate the differential mass weight fraction distribution, x(M) = dW(M)ld(logioM) by using the measured logjoM as a function of the elution volume V. Thus, if the concentration detector s baseline subtracted response is h(V), then dWIdV = h(V)IJ h(V) dV, the integral representing the sum over all contributing concentrations to the peak. It is then easily shown [6] that... 

The sample baseline of an experimental curve, as illustrated in Figure 10.8, is the line that connects the curve before and after a peak, as if no peak had developed. Accuracy of enthalpy change measurement relies on correctly constructing the sample baseline from the curve. Construction of the sample baseline from the measured curve may not be straightforward. Figure 10.9 shows examples of how to construct a baseline using interpolation methods for various curve shapes. The procedure of baseline subtraction is often done automatically as it can be incorporated in the software provided with the instmment. 

Standard baseline subtraction and smoothing methods are applied to each individual MALDI spectrum after measurement or on-the-fly during acquisition in order to reduce the influence of chemical noise (Figure 2D) (35). While this is performed independently on each mass spectrum, other processing steps such as normalization, recalibration, and peak picking have to account for the large number of individual spectra in MSI data. 

FIGURE 3.9 (a) Raman spectrum (A. = 514.5nm) of the carbyne deposit shown in Figure 3.7. Top spectrum as-measured, bottom spectrum baseline subtracted and deconvoluted. (b) UV-Raman spectrum (X = 325nm) recorded at different surface points of the carbyne deposit shown in Figure 3.7. The inset shows the down-shifting of the diamond peak position from 1332 to 1328 cm with decreasing crystallite size. 

The computer-controlled system acquires raw spectral data within the range of 200-400 nm at specified time intervals, calculates the results with the use of validated software, and stores the data in a secured computer database. Except for the dissolution apparatus itself, the system contains no moving parts. The spectral data can be corrected for turbidity-related scatter with the use of either a simple baseline subtraction or a second-derivative-based algorithm, and test results and/or profiles can be viewed in real time. 

Figure 6.4 Hyperspectral FT-IR data processing performed simultaneously on four adjacent tissue sections from a cervical biopsy sample. The numbers 1 to 4 identify the individual sections in the figure, (a) A univariate chemical image obtained from the integrated area under the 1275-1190cm region after baseline subtraction (b) A four-cluster map derived from analysis over the 1272-950cm spectral window. The cluster...

S. H. Lee, H. Sun, P. Chen, S. Doddapaneni, J. Hunt, and H. Mahnowski, Sensitivity/reh-abihty of the time-matched baseline subtraction method in assessment of QTc interval prolongation. Clin Pharmacol Ther 75(2) P56 (2004). 

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