Time-delayed exponential decay procedure

A convenient and accurate method of force-field programming in SdFFF uses a time-delayed exponential decay (TDE-SFFF) procedure (9, 10, 15). In this method, mobile-phase flow is initiated after sample injection, and the initial force field is held constant for a time equal to r, the time constant of the subsequent exponential decay. After this delay, the force field is decayed exponentially with a time constant r. In the TDE-SFFF mode, a simple log-linear relationship is obtained with both particle mass (and size) as a function of retention time. This simple relationship permits a convenient calculation of the quantitative information desired for the sample. 

So, when it has been decided that the decay is exponential, the best procedure is to cut off the data at some maximum delay, beyond which the rms deviation for the estimated increases. A way to do this is to calculate the rms deviation as a function of the cutoff time for a trial run and decide when to stop taking data (like at 10% of full scale, for example). The only exception to this occurs when Mq has to be determined directly. 

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