Background peak

Figure 41.4 shows a typical XRD (X-Ray Diffraction) pattern of TUD-1, along with a TEM image (12). Similar to other mesoporous materials, TUD-1 has a broad peak at low 20. However, it has a broad background peak, commonly called an amorphous halo, and lacks any secondary peaks that are evident for example in the hexagonal MCM-41 and cubic MCM-48 structures. The TEM shows that the pores have no apparent periodicity. In this example the pore diameter is about 5 nm. 

Fluorescence. Upon excitation at 370 nm, the emission spectrum from the digests of glucose-exposed dentin slices showed a broad peak (maximum 420 nm), which was strongly increased compared with the background peak in buffer-exposed dentin. This background peak was no contamination of buffer salts, since it was also present in demineralized water. The broad peak at 420 nm also overlapped a broad shoulder peak at 480 nm that occurred in the buffer-exposed dentin (fig. 2). The 370/440 nm fluorescence of glucose-exposed dentin slices was significantly increased compared with controls (table 2). 

Calculate the Inter sUy Ratio between the Foreground and Background Peaks ot the Same Nominal Mass... 

A typical application of GC to the determination of a drug in plasma is in the determination of the anti-epileptic drug valproic acid after solid phase extraction (see Ch. 15) by GC with flame ionisation detection. In this procedure, caprylic acid, which is isomeric with valproic acid, was used as an internal standard. The limit of detection for the drug was 1 pg/ml of plasma. The trace shown in Figure 11.25 indicates the more extensive interference from background peaks extracted from the biological matrix which occurs in bioanalysis compared to the quality control of bulk materials. 

The use of various sorbents and desorption systems permits (often necessitates) gas chromatographic detectors other than the most commonly used flame detector or electron capture detector. The use of a selective detector can greatly simplify a difficult analytical problem by reducing interferences and background peaks. Many applications are discussed in the literature (21,22,23,24). 

MS sensitivity depends on both the type of instrumentation and the nature of the analytes, but, typically, a minimum sample size of 5-10 ng is in most cases sufficient. Limited sensitivity in a certain application is often not due to the inherent sensitivity of the MS but rather the level of background impurities that are in the isolate. It is not always appreciated that very slowly eluting LC solutes from previous separations can create substantial MS background peaks that obscure analyte identification. Thus, it is important to use a blank sample to ensure that the background of the trapped fraction is adequately free of possible interferences to the desired identification. 

Photopeaks observed in the background include the more prominent x-rays and gamma-rays of the natural uranium and thorium decay series and the natural 40K. As reported by Cooper et al. (5), background peak heights are relatively unaffected by suppression, but the reduction in the background continuum is significant. For our 8-cm.3 Ge(Li) detector, the background at 150 k.e.v. was reduced from 0.1 to 0.02 counts/min./ k.e.v., and that at 1.5 M.e.v. was reduced from 0.0013 to 0.0005 counts/ min./k.e.v. 

One limitation on the use of isotope peak intensities to determine the molecular formula is that the molecular ion must be relatively intense, otherwise the isotope peaks will be too weak to be measured with the necessary accuracy. Difficulty may also arise from spurious contributions to the isotope peak intensities from the protonated molecular ion, from weak background peaks or from impurities in the sample. In any event the method is only reliable for molecules having molecular weights up to about 250-300. 

Figure 3.17. Pictorial representation of how DBS works to minimize background peaks and enhance actual signals in EMS-IDA experiments.

Figure 3. High resolution mass chromatograms for m/z 139 (a) mass chromatogram for C10H19+ (b) mass chromatogram for C9H150+ (c) mass chromatogram for C11H17+ (background peak).

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