Because the separation presented in Fig. 2Dpresents an elevated
selectivity, it is possible to further increase the mobile phase flow
rate, the value of %Binitial and the gradient slope while maintaining
acceptable separation. In this perspective, an 18–30% ACN gradient
in 0.3 min was carried out at a flow rate of 1mL/min to speed up
the original separation asmuchas possible. The final chromatogram
presented in Fig. 3A shows a baseline separation of the eight phenolic
compounds in only 30 s with aminimal resolution of 1.72 for C
and EC (peaks number 1 and 2). For this separation, the generated
backpressure was around 850 bar, and this remained acceptable
even for routine use of the method.
Onthe other hand, the determination ofsomeminorcompounds
contained within a complex matrix such as tea extract, which possesses
hundreds of constituents, requires a high resolving power.
Thus, the conditions used for Fig. 3A cannot be applied to real tea
samples. Additionally, even if the separation in Fig. 2D presents
a high selectivity for catechins, the global resolving power is still
too limited to deal with real samples. The performance in gradient
mode can be estimated with the peak capacity (P), which is the
number of peaks that can be separated in a given time window.
The latter is based on the gradient time (tgrad) and peak width at
the baseline in time units (Wt) according to the following equation: