Manual baseline correction

One potential problem with this method is that it is extremely susceptible to baseline offset and slope effects in the spectrum. When calculating the path-length normalization constants, the spectra must be baseline corrected before creating the training set or a local baseline must be calculated on-the-fly under the thickness correction band for each spectrum. The latter approach is usually recommended because the former method requires separate manual baseline correction of each unknown spectrum before prediction against the calibration model. 

In this section, we present the development of an automated protocol for prostate tissue histology [164] from infrared spectroscopic imaging data as an example of the techniques described (Fig. 8.11). The data is three dimensional with x-y—axes representing the image plane and the 2-axis representing the spectral dimension. After data acquisition, two important pre-processing steps, namely baseline correction and de-noising, are performed. Since the entire data set is derived from human tissue samples, the spectra have similar characteristics and, therefore, a manually chosen set of pre-defined wave number could be used as the reference points for baseline correction. It is... 

A scaled saline or water reference spectrum was manually subtracted from each sample spectrum such that the region of the spectrum between 1700 cm and 2000 cm"1 was flat. Each subtracted sample spectrum was baseline corrected between 900 cm "1 and 2000 cm "1, and the peak height of characteristic protein and polysaccharide bands was determined using the baseline correct function and the cursor, respectively, on the Nicolet 660 data station. 

FIGURE 21 Dissolution profiles for buffered aspirin tablets obtained with the fiber-optic dissolution system (260-350 nm) and manual sampling with HPLC analysis. The dissolution was performed with USP apparatus 2 at 75 revolutions per minute. A second-derivative baseline correction was performed on the fiber-optic raw spectral data to correct for scattering due to the turbid solution. 

The next example of an OTC map was treated first using a direct classic least square (DCLS) method, and then with more sophisticated multivariate analysis methods. The tablet was mapped over 800 X 800 gm with 10 gm steps. The data were baseline-corrected and normalized before being subjected to an unsupervised multivariate analysis. The first set of results was produced using univariate analysis (Figure 11.8a), when a manual exploration revealed three distinguishable and... 

For further details on the baseline correction function of ID WIN-NMR the reader is referred to the relevant pages of the ID WIN-NMR manual which is part of the teaching software. 

When the polynomial is fitted to signal points, which are manually selected, that can be attributed only to baseline (background), then baseline (background) removal is achieved and the drawbacks of detrending are overcome. A variant for baseline correction is to do adopt a weighted least squares automatic procediue (asymmetric least squares [63]). This is an automatic approach to determine which points most likely belong to baseline only, by... 

The H-NMR spectra of FCC feeds were recorded on a Bruker DRX 400 MHz NMR spectrometer. The concentration of the samples of 5 wt% in CDCI3 was recommended by Molina, Navarro Uribe, and Murgich [2] to avoid concentration dependence of the chemical shift. A 30° pulse sequence was applied, with 4.089 s acquisition time, 2 s pulse delay [2], 8012.8 Hz spectral width, and 64 scans. Hexamethyldisiloxane (HMDSO) was used as a reference. NMR processing was realized using MestReNova software. The phase and baseline of the resulting spectra were manually adjusted and corrected. The spectra were integrated six times and average values were taken for the purpose of calculations. The spectra were divided... 

Attention With spectra measured on spectrometers equipped with digital filters (DMX, DRX spectrometers), the automatically performed phase correction (DMX Phase Corr.) will be applied twice when the newly created FID is Fourier transformed again. This will introduce the baseline roll characteristic for the data of these type of spectrometers. A first order phase correction must then be performed manually by setting the PHCl value close to -22000 for the data available in the NMR data base. 

The laboratory should verify and document the proper functioning of the software immediately after any new data acquisition or management systems have been installed. The baseline verification consists of manual calculations to confirm the correctness of all computer calculations. Ongoing verification takes place during laboratory data review process whenever a reviewer replicates one of the results generated by the computer or a manual calculation from a bench sheet. All information used in the calculations (raw data, calibration data, laboratory QC checks, and blank results) is kept on file for the reconstruction of the final result at a later date, should it become necessary. Bench sheets that document sample preparation are also kept on file for the same purpose. 

Baseline and phase correction are also important factors in determining the accuracy of the NMR result - it is usually the case that these have to be done manually to obtain best results. It is also quite likely that different operators will perform these functions differently and this may be a cause of reproducibility problems (see Section 4.4.5 below). A recent paper [199], though not aimed at quantitation directly, has a useful discussion of the difficulties of obtaining precise integrals, particularly with respect to the phasing of signals. 

If the baseline shows any drift, peak heights are often more correctly determined manually than by an automatic system (important for trace analysis). The peak area is very seldom calculated manually, and if it is, a peak area equivalent is used, which is the product of peak height and peak width at baseline. In older publications, the peak area was often calculated by cutting out the peak and weighing it [88). This method also gave very good results. 

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