Antec Scientific DECADE II 171, User Manual, Page: 46 / 87

46 DECADE II user manual, ed. 11
Prerequisite for a ‘good’ noise filter for data acquisition in liquid
chromatography is that it improves the S/N ratio without significant distortion
of the signal of interest. This is particularly difficult if the frequency of the
signal is close to the frequency of the noise.
The DECADE II has a number of filter settings to optimise for best possible
signal-to-noise ratio. The width of the peaks of interest is important because
wider peaks allow stronger filter settings simply because of the lower
frequency of such peaks. Advised filter setting to start further optimisation is
given as:
Filter setting = 1 / [2 * (peak width)]
So at a 10 s peak width a 0.05 Hz filter setting is advised. If peaks are 50 s a
0.01 Hz filter is advised to start with. Note that if a chromatogram has
interesting peaks of 10 s as well as 50 s, it may not be possible to work with
one filter setting. In that case it is advisable to switch to a stronger filter
setting for the second half of the chromatogram using a timed event. To
optimise for the best S/N ratio, use the lowest acceptable cut-off frequency.
After optimisation, do not change the cut-off frequency setting during analysis
of a calibration sequence. Use the same settings for analysis of samples and
calibration standards.
Fig. 28. Analysis of 20 pmole/L 5 hydroxytryptophan, 5-HIAA and 5HT using
ADF for improving detection limits.
The S/N improvement depends on the composition of the frequency
spectrum. Improvement up to a factor 100 may be obtained compared to an