Detector Response Time - PerkinElmer 200 UV/VIS Series User Manual

Principles of Operation Application Notes

Detector Response Time

Noise is always present in absorbance detectors. Even when no sample is present, there are
random fluctuations in the detector signal. Noise becomes a problem when it widens the
baseline sufficiently to disguise small signals; these small signals subsequently cannot be
distinguished from the baseline noise. By choosing an appropriate rise time value, however,
HPLC performance may be optimized by keeping noise at a minimum level.
Noise consists of an infinite number of individual frequencies; as more frequencies are elim-
inated, the noise level drops. Signals also consist of frequencies, but the number of different
frequencies is very limited. In order to obtain the best signal-to-noise ratio (S/N), an elec-
tronic filter is added to eliminate noise frequencies while allowing all signal frequencies to
pass unattenuated. However, some frequencies are common to both, and therefore, a filter
should be chosen that reduces noise to as low a level as possible without distorting or reduc-
ing the signal level. In an actual electronic circuit, resistors and capacitors together act as
the filter. A circuit that consists of only one resistor coupled with a capacitor is a one pole fil-
ter. A two pole filter consists of a pair of resistors and a pair of capacitors along with an
active device such as an operational amplifier. Two pole filters provide a larger S/N ratio
than single pole filters, and therefore PerkinElmer currently uses two pole filters in their
detectors (see Figure 8-3).
Figure 8-3. Effect of a one pole versus a two pole filter on S/N.
8 - 8