An FIA curve, or fiagram, is a plot of the detector’s signal as a function of time. The illustration below shows a typical fiagram for conditions in which both convection and diffusion contribute to the sample’s dispersion. Also shown on the figure are several parameters for characterizing a sample’s fiagram.
Two parameters define the time for a sample to move from the injector to the detector. Travel time, ta, is the time between the sample’s injection and the arrival of its leading edge at the detector. Residence time, T, on the other hand, is the time required to obtain the maximum signal. The difference between the residence time and travel time is t′, which approaches zero when convection is the primary means of dispersion, and increases in value as the contribution from diffusion becomes more important.
The time required for the sample to pass through the detector’s flow cell—and for the signal to return to the baseline—also is described by two parameters. The baseline-to-baseline time, Δt, is the time between the arrival of the sample’s leading edge to the departure of its trailing edge. The elapsed time between the maximum signal and its return to the baseline is the return time, T ′. The final characteristic parameter of a fiagram is the sample’s peak height, h.
Of the six parameters illustrated here, the most important are peak height and return time. Peak height is important because it is directly or indirectly related to the analyte’s concentration. The sensitivity of an FIA method, therefore, is determined by the peak height. The return time is important because it determines the frequency with which we may inject samples. As illustrated below, if we inject a second sample at a time T ′ after injecting the first sample, there is little overlap of the two FIA curves.
By injecting samples at intervals of T ′, we obtain the maximum possible sampling rate.