Baseline flatness

Wavelength accuracy and reproducibility, stray light, resolution, photometric accuracy and reproducibility, noise, baseline flatness, stability, and linearity... 

Baseline flatness refers to the average deviation from 0.0 AU for a blank versus blank spectrum in the wavelength range in which the spectrum is taken. Note. This may be dependent on scan speed. 

In Table 3 some essential parameters are listed, which should be referred to in a manual, since they influence the performance of a spectrometer in addition to such common parameters as range, accuracy, and reproducibility of wavelength, stray light bandpass (spectral bandwidth or slit width of the spectrometer) photometric accuracy, reproducibility, and linearity baseline flatness absorbance zero stability noise level scan speeds response times and data intervals. Furthermore possible modes of the axis are of interest absorbance, transmittance, derivative, Kubelka-Munk function [9], and the possible scaling of the axis. Most of these parameters are given in manuals, determine the limitations of the instrument, and affect each other. 

Every spectrophotometer will exhibit a change in the flatness of its baseline measurement. The degree of flatness in transmittance, reflectance, or absorbance is equal to the difference between the minimum and maximum values for a baseline reference scan. Baseline flatness is more precisely defined as the maximum difference in photometric values of a regression line drawn through all the data points of a baseline reference scan. A different term, baseline drift, refers to the change in photometric value of a spectrometer s baselines at a specific wavelength with respect to time... 

Baseline correction is a procedure used to make the spectral baseline flat (parallel to the abscissa axis), when a measured spectral baseline is curved and/or has a slope for reasons which are not always clearly known. This correction helps clarify spectral features, and makes it easier to compare band intensities between different spectra. This correction should be applied to absorbance spectra (not to transmittance spectra). 

Expiration The expiratory limb is a smooth curve. At high lung volumes, the compliance is again low and the curve flat. The steep part of the curve is around FRC as pressure returns to baseline. 

A pre-requisite for the successful extraction of key NMR parameters from an experimental spectrum is the way it is processed after acquisition. The success criteria are low noise levels, good resolution and flat baseline. Clearly, there are also experimental expedients that can further these aims, but these are not the subject of this review per se. In choosing window functions prior to FT, the criteria of low noise levels and good resolution run counter to one another and the optimum is just that. Zero filling the free induction decay (FID) to the sum of the number acquired in both the u and v spectra (in quadrature detection) allow the most information to be extracted. 

Different baseline correction methods vary with respect to the both the properties of the baseline component d and the means of determining the constant k. One of the simpler options, baseline ojfset correction, nses a flat-line baseline component (d = vector of Is), where k can be simply assigned to a single intensity of the spectrum x at a specific variable, or the mean of several intensities in the spectrum. More elaborate baseline correction schemes allow for more complex baseline components, such as linear, quadratic or user-defined functions. These schemes can also utilize different methods for determining k, such as least-squares regression. 

Altius current bromine geochemical study utilizes core samples from holes PF-2, Flat Bay-101-1 and Captain Cook-1 (Fig. 3). This study was undertaken to establish baseline bromine profiles for the St. Georges Bay region. 

The fact that the absorbance in the flat, postdecay region, is subtracted from both the numerator and denominator implies that in order to fit to correct value of feexp aU that is required is to shift the baseline to the infinite level, rather than the preexcitation level. 

Non ideal baselines, deviating from horizontal flat lines, are introduced either by... 

The Frankfort LPA instrument (51-53) departs from both of these instruments in two principal ways it achieves the necessary path length within a 6-m folded-path cell, and it rapidly scans a narrow-band frequency-doubled dye laser across the spectral region of interest (the Qi(2) line group) in a process sometimes called differential optical absorption spectrometry (DOAS). The scanning rate is sufficient to ensure that the observed air volume is chemically and physically stationary during each scan (the baseline standard deviation is less than 2 x 10-4 for a 0.2-ms scan). The laser output is actively feedback-stabilized to provide a flat spectral baseline, and a detection limit better than 10"5 in optical density has been claimed. A summary of published LPA configurations is given in Table II. 

---