Dynamic background subtraction

In this case the survey scan was set as a full scan and the dependent scan as a product ion scan. The problem with data dependent acquisition is to determine the selection criteria. In most cases the system picks up the most abundant ion in the full scan spectrum. An inclusion list with masses of potential metabolites or exclusion list of known interferences significantly improves the procedure. In the example shown in Fig. 1.39, a procedure called dynamic background subtraction (DBS) was applied. This procedure considers chromatographic peak shapes and monitors not the most abundant signal in the spectrum but the largest increase of an ion in a spectrum. The advantage is that once a signal of a peak has... 

Le Blanc, Y.C.Y Bloomfield, N. Dynamic Background Subtraction to Improve Candidate Ion Selection for Information Dependant Acquisition LC-MS/MS Analysis, in Proceedings of the 52nd ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN, May 23-27, 2004. 

On the last released versions of the Analyst software, dynamic background subtraction can be selected with a single click. An independent script is needed for older versions. 

Components of such a system are an amplifier, a m.ulti-channel analyzer, two single-channel analyzers, two scalers, timers, and the dynamic background subtraction module (Kevex model 4840). The dynamic subtraction module produces a signal equivalent to the difference of the count rates from two single-channel analyzers, set to represent an energy window... 

FIGURE 8. Plot of data coitiparing total count result obtained by separately scaling background and iodine counts with total count obtained by dynamic background subtraction. Line of best fit was not significantly different from line of identity. (From ref. 2 by permission.)... 

Table 1. Comparison of calculated imaging characteristics by means of dynamic background subtraction (DBS) and iodine peak only (I).

JCount variation for iodine peak only = I /I x 100 and count variation using dynamic background subtraction = C(I + BKG) /(I-BKG)] x 100. 

Figure 4. Tunneling characteristics of an Al-AlOx-4-pyridine-COOH-Ag junction run at 1.4 K with a 1 mV modulation voltage, (a) Modulation voltage Vu across the junction for a constant modulation current Iu. This signal is proportional to the dynamic resistance of the sample, (b) Second harmonic signal, proportional to d2V/dI2. (c) Numerically obtained normalized second derivative signal G , dG/ dfeVJ, which is more closely related to the molecular vibrational density of states, (d) Normalized G0 dG/d(eVJ with the smooth elastic background subtracted out...

Fig. 3. Data analysis performed by using MicroVigene (VigeneTech, Billerica, MA, USA) software program. Spot finding, regional and local background subtraction, and linear dynamic range finding are performed by the software program. Combined with the calibrator, this method provides for a reproducible and quantifiable value for every sample.

This attests to the proper function of the dynamic background module. An excess in background subtraction would have resulted in an intercept that was significantly below zero. 

Potential modulation techniques are used frequently in electrochemistry. The most well-known potential modulation electrochemical technique is a.c. impedance spectroscopy, in which current modulation caused by a potential modulation is analyzed. The potential modulation technique has also been used for in-situ IR spectroscopy (EMIRS and SNIFTIRS), but its use was aimed only to subtract the solution background and to enhance the S/N ratio of the spectram. If the IR signal caused by a potential modulation is analyzed, some information on electrode dynamics could be obtained as in a.c. impedance spectroscopy. 

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