Simultaneous Determination of Caffeine and Phenolic Compounds in Tea and Coffee - Caffeine: Chemistry, Analysis, Function and Effects - page 147

d n 0 t 2 n g | 0

Figure 8.7 System back pressure using different column temperatures. This figure

illustrates the effect of temperature on system pressure and the potential
of increasing flow-rate by the use of higher temperatures. Reproduced
from Manchon et al 2011 with permission from Elsevier.

simultaneously determined in a single run, which can be expected to increase as
stationary phase technology advances.
Column temperature can have a great impact not only on the separation
achieved by the column but also in analysis time. Higher temperatures reduce
mobile phase viscosity, decreasing system backpressure, which can be used to
increase the flow-rate and reduce analysis time. Figure 8.7 illustrates the
influence of temperature on the system backpressure. Increasing the
temperature from 25 to 50 uC may reduce pressure by more than 2-fold,
implying that a proportional increase in the flow-rate is achievable. Although
the use of higher temperatures can be advantageous in the case of polyphenols
and alkaloids, columns are normally kept at ambient or slightly above ambient
temperatures.
Detection of target analytes is often carried out by UV and UV-diode array
detection (DAD) in the range of 270-320 nm. Mass spectrometry is also used
but to a smaller extent. Multiple UV detection wavelengths are also used to
improve sensitivity and to selectively detect different compound classes
present. Fluorescence detection is also sometimes explored and can be
especially useful for the determination of catechins. This type of detection is
very sensitive and can provide low detection and quantitation limits.
Moreover, selective detection also minimizes interference from some peaks
and improves separation. There are several examples where selective detection
was used for the separation of different compound classes and a general
overview of the results indicate that this is the most adequate approach for the
simultaneous determination of alkaloids and polyphenols. It is clear that using
a combination of different detectors (fluorescence, MS, etc.), and UV