Automatic gain control, AGC

If resolution is not important, scanning of QITs can be very fast, a property that can be employed to make a pre-scan. Then, the result of the pre-scan is used to adjust the number of ions inside the QIT close to the optimum for the subsequent analytical scan. Such an automatic gain control (AGC) [138,162] gives increased sensitivity at low sample flow and avoids overload of the QIT at high sample flow. 

For MS work, the electron impact (El) mode with automatic gain control (AGC) was used. The electron multiplier voltage for MS/MS was 1450 V, AGC target was 10,000 counts, and filament emission current was 60 pA with the axial modulation amplitude at 4.0 V. The ion trap was held at 200°C and the transfer line at 250°C. The manifold temperature was set at 60°C and the mass spectral scan time across 50-450 m/z was 1.0 s (using 3 microscans). Nonresonant, collision-induced dissociation (CID) was used for MS/MS. The associated parameters for this method were optimized for each individual compound (Table 7.3). The method was divided into ten acquisition time segments so that different ion preparation files could be used to optimize the conditions for the TMS derivatives of the chemically distinct internal standard, phenolic acids, and DIMBOA. Standard samples of both p-coumaric and ferulic acids consisted of trans and cis isomers so that four segments were required to characterize these two acids. The first time segment was a 9 min solvent delay used to protect the electron multiplier from the solvent peak. 

Stafford, G.C. Jr. Taylor, D.M. Bradshaw, S.C. Syka, J.E.P. Uhrich, M. Enhanced sensitivity and dynamic range on an ion trap mass spectrometer with automatic gain control (AGC). Pmc. 3 ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, June 5-10,1988. 

Applications of WRs include automatic gain control (AGC) circuits, electronic attenuators, electronically variable filters, and oscillator ampHtude control circuits. 

Automatic gain control (AGC) An electronic circuit that compares the level of an incoming signal with a previously defined standard and automatically amphfies or attenuates the signal so it arrives at its destination at the correct level. 

Automatic gain control (AGC) was turned off in order to maintain the same number of laser shots at each point. To avoid deleterious space-charge effects, the number of laser shots and the power of the laser were first determined by interrogating one spot of the tissue sample. Typically, 10 laser shots with about 20-30% laser power (arbitrary units) were used for all full-scan analyses and 15-20 laser shots were used for MS/MS and MS experiments. 

The detected cantilever deflection signal is routed back to the piezo actuator through a phase shifter and an automatic gain control (AGC) circuit to form a self-oscillation loop. In this selfoscillation circuit, the cantilever works as a mechanical resonator and determines the oscillation frequency of this circuit. 

The automatic gain controller (AGC) presented in Chapter 4 regulates the quantity of formed ions. It absolutely does not control the quantity of neutral molecules eluted by the chromatograph and therefore does not allow solution of the autoprotonation problem. Consequently, with an internal ionization ion trap, one must be cautious in accessing a database to find the mass spectrum of a product to be identified from the foot of the chromatographic peak rather than its top, especially if the mass spectra are different according to the position of the peak at which they are observed. 

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