Background artifacts

Since the background artifact signal is no longer present with the new optical arrangement, negative VCD intensities may actually be present as negative transmission intensities. In order for VCD spectra to be measured properly, a new phase correction procedure needs to be invoked. Such a procedure has been published previously by McCoy and de Haseth [62], and we have implemented their method. 

Fig. 1 SSNMR spectra of ibuprofen, 75.6 MHz. (A) Bloch decay experiment (single pulse), no decoupling, static, 240-sec pulse delay, 100 scans, 400-min experiment time. (B) Same as (A) but with decoupling ( 60 kHz). (C) Same as (B) but with 5-kHz MAS. (D) Cross-polarization experiment, with H decoupling ( 60 kHz), 5-kHz MAS, 1.5-msec contact time, 3-sec pulse delay, 100 scans, 5-min experiment time. (E) Same as (D) with the TOSS pulse sequence applied to suppress spinning sidebands. Note Asterisk ( ) denotes spinning sidebands sharp ( ) denotes spectrometer background artifact.

As background artifacts can be a problem in nonradioactive detection systems, any damage of the membrane surface should be avoided... 

In practice, it is found that reducing the constant background artifact also reduces the severity of the absorption-dependent artifact. It has also been found... 

Since no background correction can be made, dot maps of minor and trace constituents are subject to possible artifacts caused by the dependence of the bremsstrahlung on composition, particularly with EDS X-ray measurement. 

Case III - Analyte Detection (A - random). One of the most reliable modes of simple (single) analyte detection obtains when y, B and A are each measured (observed) for every sample processed. Such is often the case, for example, in radiocarbon dating where the age of each unknown artifact is estimated from sequential measurements of the sample, the background, and the radiocarbon dating standard. Thus,... 

In addition, color film has an inherent artifact in areas of overexposure, in which the color of the film becomes yellow, no matter what the color of the exposing light. This produces apparent areas of enhanced concentration, which may or may not reflect the true intensity of label, especially in areas in which a low-level background image is added to the specific signal. This can lead to misinterpretations about selective areas of concentration of label. 

The filter material of choice is a thin teflon membrane since it minimizes artifact formation and maximizes analytical sensitivity by X-ray fluorescence analysis. Although X-ray fluorescence (XRF) may not be the only analytical technique used, it is generally accepted as being the most cost effective analysis for source apportionment. ( 2) Its background and therefore, analytical sensitivity, is dependent on the filters surface density. The analytical sensitivity of XRF for aerosols deposited on a stretched teflon membrane with a density of about 0.3 to 0.4 mg/cm, for example, is about three times greater than an aerosol deposited on a cellulose based filter with a surface density of about 4 mg/cm. This difference can be translated into either more information for the same analytical costs or the same information for a lower analysis cost. 

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