Baseline compensation

Selected entries from Methods in Enzymology [vol, page(s)] Absorption Spectrophotometer Application, 24, 15-25 baseline compensation, 24, 8-10 computerized, 24, 19-25 light scattering, 24, 13-15 monochromator, 24, 4 photometer, 24, 5-8 recorder, 24, 8 sample compartment, 24, 5 single-beam, 24, 3-4 spectral characteristics, 24, 10-12 split-beam, 24, 3 stray light, 24, 12-13. 

The energy dispersive x-ray fluorescence spectrometer, which had been developed recently as a qualitative analysis instrument, showed promise of meeting the goals of the new laboratory (1). Its unique features, which earned it the name, The Curators Dream Instrument, are The measurements require neither sampling nor alteration of the object in any way. Systems for obtaining quantitative analysis data are now operational (I). Concentrations of up to thirty elements above chlorine (Z = 17) can now be printed out simultaneously. Techniques have been developed that minimize errors caused by sample size, shape, position, overlapping spectral peaks, matrix effects, and baseline compensation. Interpretative procedures have been established that recognize the shallow depth of penetration of the excitation radiation (2). 

Some minor elements displayed may be fictitious because of improper baseline compensation. Information on the questionable elements as well... 

Janata E. (1986) A baseline compensation circuit for measmement of transient signals. Rev Sei Instrum 57 273-275. 

Janata E. (1992) Instrumentation of kinetic spectroscopy-8. The use of baseline compensation in the subnanosecond time domain. Radiat Phys Chem 39 319-320. 

Baseline Compensation Analysis—A baseline compensation analysis, or baseline blank, is performed exactly like an analysis except no injection is made. A blank analysis must be performed at least once per day. The blank analysis is necessary due to the usual occurrence of chromatographic baseline instability and is subtracted from sample analyses to remove any nonsample slice area fijom the chromatographic data. The blank analysis is typically performed prior to sample analyses, but may be useftil if determined between samples or at Ae end of a sample sequence to provide additional data regarding instrument operation or residual sample carry-over from previous sample analyses. Attention must be given to all factors that influence ba%line stability, such as column bleed, septum bleed, detector temperature control, constancy of carrier gas flow, leaks, instrument drift, etc. Periodic baseline blank analyses should be made, following the analysis sequence protocol, to give an indication of baseline stability. 

Baseline Compensation Analysis—To compensate for baseline drift and signal offset, subtract an area slice profile of a blank run from the sample run to obtain corrected area slices. This profile is obtained as follows ... 

Note 7—Some commerdally available gas chromatographs have the capability to make baseline corrections (from a stored blimk analysis) directly on the detector signal. Further correction of area slices may not be required with such systems. However, if an electronic ofl is added to the signal after baseline compensation, additional area slice correction may be required in the form of offset subtraction. Consult the specific instrumentation instructions to determine if an offset is applied to the signal. 

The main difference when compared rebreathing with the normal trace is that the baseline is not zero. Consequently the Petco2 may rise. If the patient is spontaneously breathing, the respiratory rate may increase as they attempt to compensate for the higher Petco2. 

Figure 6.29 Schematic representation of the adjustment which is made to compensate for the sloping baseline in stripping voltammetry, itself due to charging of the electric double-layer at the WE.

Suppose that another series of samples is to be predicted using tlie DCLS model. Figure 5.24 displays t ie spectral residuals from the predictions. A problem is indicated because the residuals do not resemble those from the validation data (see Figure 5.18). Figure 5-25 display the spectra of the unknown samples, which reveal a random linear baseline witli variable offset. It is not obvious from the residuals that tliis is the problem, because CIS attempts to fit the baseline feature with the pure spectra. Wlien unexpected features appear in the spectra of the prediction samples, CLS compensates by overestimating the concentration of one or more of the pure components. Hie result is that the residuals have features tliat come from the pure speara as well as some remnant of the original unexpected features. This makes interpretation of the residual spectra difficult. 

With ID WIN-NMR the compensation of DC offsets is the only baseline correction option for ID FIDs. It is automatically performed prior to any processing when confirmed in the dialog box that appears on screen. The baseline of the FID is corrected with the zero-line set to the mean value of the last part of the FID. 

The refractive index detector, considered to be almost universal, is often used in series with a UV detector in the isocratic mode to provide a supplementary chromatogram. This detector, which is not highly sensitive, has to be temperature controlled, as does the column (0.1 °C). The baseline of the chromatogram has to be set to an intermediate position because it can lead to either positive or negative signals (Fig. 3.18). The detector can only be used in the isocratic mode because in gradient elution the composition of the mobile phase changes with time, as does the refractive index. Compensation, which is easily obtained in the case of a mobile phase of constant composition, is difficult to carry out when the composition at the end of the column differs from that at the inlet. 

This simple method is used in industry for repetitive analyses. For such analyses, the chromatograph must be equipped with an autosampler, including a sample tray and an automatic injector. The single reference solution, periodically injected for control purposes, can be used to compensate for baseline drifts. It is not necessary to add an internal standard to each of the samples, as discussed below. 

A basic assumption in DSC kinetics is that heat flow relative to the instrumental baseline is proportional to the reaction rate. In the case of temperature scanning experiments the heat capacity of the sample contributes to the heat flow (endothermic), and this is compensated by the use of an appropriate baseline under the exo- or endothermic peak produced by the reaction. It is also assumed that the temperature gradient through the sample and the sample-reference temperature difference are small. Careful control of the sample size and shape, and the operating conditions are necessary in order to justify these assumptions. 

What constitutes a significant difference between two spectra When the differences are small, the answer depends on sample preparation and sample stability as well as accuracy of concentration determination, identification of and compensation for drift in the spectrometer, correct baseline correction, absence of bubbles in the sample, reproducible cleanliness of the cuvette, and the level of general handling procedures. Ultimately, an assessment of significance depends on the experience, competence, and confidence of the operator. 

An Aerograph 204 dual column gas chromatograph with compensating flame ionization detectors to minimize baseline drift because of column bleed during temperature programming. 

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