Antec Scientific DECADE II 171, Руководство пользователя

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CHAPTER 5
Noise suppression: ADF™
39
C H A P T E R 5
Noise suppression: ADF
™
Introduction
Besides for its tremendous linear dynamic range and selectivity,
electrochemical detection is well-known for its very low limits of detection. To
further improve these detection limits the Antec Leyden engineers have
developed ADF (Advanced Digital Filter) and the DECADE II has been
equipped with it as a standard. The improvement factor in signal-to-noise
(S/N) ratio depends on the frequency relation of signal and baseline noise.
S/N improvements from a factor 5 up to more than 100 have been obtained.
To understand how a digital filter works, first the importance of frequencies in
chromatographic analysis will be explained. Then we will look at peak width,
filter settings, cut off frequency, amplitude response plots and finally at a few
chromatograms before and after applying ADF.
Frequency
A scientific definition of frequency is “the number of completed alterations per
unit time”. It has two dimensions: count and time. Frequency is usually
expressed in Hz, which is counts per second.
The counts themselves can run in a regular, evenly spaced manner, as with
sine waves whose curve shapes do not change. Alternatively, the counts can
run in an irregular manner within the specified unit of time. If the latter
happens, frequencies would vary if broken down into t smaller units of time.
In the example of Fig. 18 a signal is shown with a frequency of 12 alterations
in 5 minutes. To express its frequency in a more scientific way a full period is
precisely determined and expressed in Hertz (or s
-1
). It is a sine wave with a
frequency of 0.04 Hz (Fig. 19).