Peaks peak stripping

Fig. 4.3 Example of sections taken from a CBCA(CO)NH experiment of the transcription factor CDC5. Strips are shown for the same set of amino acids as in Fig. 4.2. The two peaks per strip indicate the 13C-frequencies of the a- and the...

IV.la) Natural Isotopic abundances An element can be definitively identified if its correct isotopic abundance is found, either directly or indirectly after peak stripping. For instance, the peak height intensities in the 92-100 mass range in Fig. 17 closely match the abundance ratios of the... 

The influence of the accumulation potential on the peak stripping current was studied in the range of 0.0 to —0.6 V. The results are shown in Fig.4. The highest peak current was obtained with accumulation at —0.40 V. 

Fig.4. Influence of accumulation potential on the peak stripping current. Timolol concentration is 5x10 M and the other conditions as in Fig.3.

The major shortcoming of the method is the ct that most of the residual error is assigned to the last peak stripped in a region of peak overlaps, lypically, this last peak is also the smallest and can least tolerate the additional intensity error. In addition, the method is slow where many peaks must be stripped. Some relief can be gained by using calculated spectra rather than measured reference spectra. This is particularly helpful where a pure element standard is unavailable. Here again it is important to check for the absorption-edge problem described in Sec. 6.10.1. 

Resolve multiple peaks, either by peak stripping or by deconvolution. 

If one component of a doublet must be measured it is worth considering whether a simple peak strip could be used. The procedure is demonstrated in Figure 9.11. The peak to be measured is labelled A and the interfering peak Bj. Nuclide B is known to have one or more... 

In subsequent sample analyses, the area of peak B2 can be multiplied by this empirical correction factor and subtracted from the total peak area of the doublet A + B. While this all seems very straightforward, it is not always as useful as expected. An oft-quoted example of the use of peak stripping is the resolution of the 186keV peak in the spectra of naturally occurring radionuclides. This is a composite of the 185.72 keV peak from and the 186.21 keV from 26Ra. 235 j 143.76keV... 

Although the procedure is simple and direct it will not necessarily be available within a commercial spectrum-analysis program. Programs which do are GammaVision and the comparative analysis program CompAct. In the absence of facilities within the spectrum analysis program, a simple spreadsheet could be used to perform an off-line peak strip on the output from the analysis program. 

We should perhaps make clear that peak strip is not the same as the spectrum strip option often provided on old MCA systems. That would subtract one spectrum, or a proportion of it, from another on a channel-by-channel basis. Spectrum stripping permanently alters the spectrum data and is not recommended. One particular problem with it is that the statistical scatter of the stripped spectrum is not representative of the actual data. 

More complicated is the situation where several nuclides have mutually interfering peaks. Genie 2000 and Sampo 90 use what the manual refers to as a Common Algorithm Nuclide Identification . That identifies unresolved mutual interferences by a process of least squares minimization of a set of simultaneous equations, one for each nuclide, involving all of the peaks measured. The process is a more general treatment of the peak stripping explained in Section 9.11. 

Of particular concern in the gamma spectrometry of NORM is the mutual interference between (185.72keV) and Ra (186.21 keV). These peaks are so close together that deconvolution in real environmental spectra is unlikely to give results that one can have confidence in. In principle, it would be possible to perform a peak stripping operation using other peaks in the spectrum to estimate its contribution to the 186 keV peak. Unfortunately, the emission probability of the next most intense peak at 143.76 keV is only 1/5 of that of the... 

Figure 5.11 A mass spectrum of the negative secondary ion emission from a Silicon wafer. The solid line represents the collected signal and the dashed lines represent the peak stripped spectra, i.e. the spectra not suffering the hydride interferences. This mass spectrum was collected on a Quadrupole-based SIMS instrument at unit mass resolution (from Figure 5.1).

Care must, however, be taken to ensure that the mass scale (or more precisely the m/q scale) of the instrument is well calibrated, and that there are no isobaric interferences under the conditions used (isobaric interferences are covered in Section 5.3.1.3). If there are isobaric interferences, these should be removed using one of the procedures outlined within subsections of Section 5.3.1.3. These include the use of HMR, KE filtering, or peak stripping. 

Peak stripping A retrospective (mathematical) approach to remove isobaric interferences... 

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

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