Spectral Baseline

Baseline noise resulting from either chemical or electronic noise is always present in MS analysis particularly by using a type of QqQ mass spectrometer [14] however, it is continuing to improve with instrument development. The contribution of the noise to ion intensity may be negligible for quantification of the abundant lipid species since the degree of the baseline ion current (or ion counts) is only a few percent or less of 

The presence of baseline noise is obvious when mass spectra are examined in the case of shotgun lipidomics. Therefore, this can be easily corrected as demonstrated [1]. However, such noise may not be directly visible in the case of LC-MS analysis of lipids, particularly when a mobile-phase gradient is employed. It is difficult to access the magnitude of this noise level. Many researchers use the extfacted ion intensity at neighboring elution times to evaluate the baseline however, it does not entirely represent the level of noise at the real elution time. 

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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. 

In spectroscopy applications, a first derivative effectively removes baseline offset variations in the spectral profiles. As a result, first derivatives can be very effective in many spectroscopy applications, where spectral baseline offset shifts between samples are rather common. 

Perhaps, the simplest method for minimizing sample-induced spectral baseline fluctuations is by calculating derivative (first, second, etc.) spectra. In derivative spectra, broad baseline fluctuations have much smaller slopes and curvatures than vibrational absorption bands in the original spectra, and as a consequence, the resulting derivative spectra have diminished broader features. This effect is seen in the Fig. 4.6,... 

Phase 2 - data preprocessing. There are many ways to process spectral data prior to multivariate image reconstruction and there is no ideal method that can be generally applied to all types of tissue. It is usual practice to correct the baseline to account for nonspecific matrix absorptions and scattering induced by the physical or bulk properties of the dehydrated tissue. One possible procedure is to fit a polynomial function to a preselected set of minima points and zero the baseline to these minima points. However, this type of fit can introduce artifacts because baseline variation can be so extreme that one set of baseline points may not account for all types of baseline variation. A more acceptable way to correct spectral baselines is to use the derivatives of the spectra. This can only be achieved if the S/N of the individual spectra is high and if an appropriate smoothing factor is introduced to reduce noise in the derivatized spectra. Derivatives serve two purposes they minimize broad... 

Most organometallic EDA complexes of arenes with titanium tetrachloride [116] in solution also follow the general reaction scheme in Eq. 15 in that no net chemical reaction is observed upon charge-transfer irradiation due to rapid back electron transfer (A et 10 ° s ). For example, the transient absorption spectrum of bro-moanthracene (BrAnt) cation radical generated by 532-nm laser excitation of the [BrAnt, TiCU] complex in cyclohexane (see Figure 7) decays completely to the spectral baseline within about 1 ns (see inset) due to back electron transfer [116], (Eq. 18) ... 

The latter process results in an overall second-order annihilation of the radicals as observed in the complete decays of the transient absorption to the spectral baseline on the microsecond time scale (see Figures 12 and 13). Since dimerization of the 17-electron radicals is orders of magnitude slower than the highly exothermic back electron transfer, no net photochemical transformations are observed even after prolonged charge-transfer irradiation. 

Unless otherwise known, proceed with a level of safety precaution at least as rigorous as for the parent species. Samples should be provided in the smallest vessel possible, preferably with a conical bottom, to reduce loss. Once the sample has been transferred to the NMR tube or syringe, inspect the tube for any residual solids. Solids decrease both the sensitivity and the resolution achievable and may produce a broad hump in the spectral baseline. Finally, before placing the sample tube into the magnet or injecting into an LC-NMR system, be sure to eliminate any air bubbles, since bubbles will make shimming more challenging and will cause adverse effects in the LC-NMR system. 

The character of a spectrum, its pattern of peaks and valleys due to the chemistry of a sample, varies little with changes in tablet hardness. However, as described, a significant shift in the spectral baseline of a tablet occurs with in-... 

Both evidence of the browning caused by the high-intensity absorption of conjugated chromophoric groups and upward shifts in the ultraviolet-visible spectral baselines indicate that higher hydroquinone concentrations produce more polymers. Free radical coupling is probably involved in the polymerization since the existence of free radicals is correlated to hydroquinone concentration. The ultraviolet-visible absorption profiles of these polymers formed in the systems that are not sterilized (Kung and McBride, 1988) appear to be similar to those of phenol-derived polymers in the systems free... 

There have been many studies of the most effective way to address a common feature in NIR and Raman spectra of reacting systems, viz. the change in spectral baseline. Figure 3.48(a) shows a typical output from a NIR diffuse-reflectance spectral measurement of 12 kinds of ethylene-vinyl acetate (EVA) copolymers differing in vinyl acetate content (Shimoyama et al, 1998). 

Finally, even when all these parameters are taken into account, spectral baselines are never flat. The situation has improved considerably as instruments have gone fully digital and taken advantage of efficient digital filtering, but setting the baseline remains a significant source of error. 

Using the spectral baseline correction mode, determine the peak spectral absorption differences (AA) between 420-490 nm (P420) and 450-490 mn (P450) of CO-reduced-reduced (Fe CO-Fe" ) samples -... 

Still anotiier attempt to cope with the problem of mass discrimination in MALDl has been reported. In this, an off-line correction of the detector response is made, eliminating spurious components in the signal and generating a new spectral baseline from which the molar mass of the polymer can be calcu-lated. The method utilizes the MALDl spectrum in continuous extraction to get tire full ion yield from tire detector. The results obtained up to the present... 

Baseline distortions can be caused by a variety of instrumental and processing conditions. Ideal spectral baselines have an average mathematical value of zero over all segments of data. Distorted baselines, on the other hand, have varying local averages. Truncation of discrete data can cause serious distortions in the Fourier... 

The IR spectra were recorded with a ReactIRe4000 equipped with a DiComp ATR probe (ASI Applied Systems, Mettler Toledo). The following steps of the experiments have been performed (1) THF was cooled to -75°C (2) the spectral baseline was reset to zero and the spectra recording was started (3) the substrate was added (4) the base was added dropwise and (5) the reaction was quenched by deuterium oxide. 

Measurement of the reflectivity AR/R from experimental spectra of thin dielectric layers on metals is complicated by the fact that if the absorption depth AR is taken as the band intensity then the experimental values of reflectivity, (AR// )exp will differ from the calculated values of AR/R. The reasons are the following. First, the band wings are distorted by the presence of the layer, effectively causing a shift of the spectral baseline relative to the reflectance of the bare... 

In the case of samples that produce scatter in transmission or diffuse reflectance spectra, a number of factors corrupt the linearity of the Beer-Lambert absorbance concentration relationship. Sample scattering of radiation results in an alteration of the proportion of absorbed and reflected radiation so that pathlength becomes another unknown in the Beer-Lambert relationship. Particle size, particle shape, crystalline form, bulk, density, and the nature of the pore space (filled with air, water, or oil) are all variables that dictate the effective pathlength of the radiation. Sample surfaces also reflect specular energy that has not interacted with molecular structures. This form of energy has an overall effect on spectra, contributing primarily to the curvilinearity of the spectral baseline. 

A dominant feature of NIR diffuse reflectance spectra is the increase in absorbance values from 1100 to 2500 nm. This trend is curvilinear for densely packed samples. A second-degree polynomial function has proved to be an adequate model to linearize the spectral baseline. 

The quahty of microbial mass spectra was assessed visually immediately after data acquisition with regard to the following criteria first and foremost, the SNR and the presence of a sufficient number of mass peaks were evaluated. Further quality criteria were a fiat spectral baseline and the absence of interfering, or confounding, mass peaks from plasticizers and other synthetic polymer additives. Outliers, that is, spectra failing to meet one or more of the quahty requirements were not accepted for subsequent analyses. The remaining spectra were de-noised in the following by... 

Curved surfaces, debossing, and scoring are factors that affect the spectrum of a tablet as positioning is varied, but the spectral effects of such factors can be reduced by the methods just discussed. Natural variations in tablet mass and hardness will affect a tablet s spectrum, primarily through spectral baseline shifting. Some work by Baxter [126] involved a unique method of normalizing tablet weight variations. 

A Practical Approach for Determination of Mass Spectral Baselines... 

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