June 11, 2015, 715004752 Rev. A
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If the value for the narrowest peak of interest is less than 15, you must specify a
higher sampling rate in the instrument method. If the value is greater than 30,
specify a lower sampling rate in the instrument method.
Set the sampling rate to the lowest value required to achieve 15 or more points
across the narrowest peak. Excessively high sampling rates can slow the system with
more data than you need for your analysis.
1.2.1.4
Filtering noise
The detector provides a Hamming filter to minimize noise. The Hamming filter is a
digital finite impulse response filter, which creates peak height degradation and
enhances the filtering of high frequency noise.
The behavior of the filter depends on the filter time constant you select. You can
program a filter time to be Fast, Slow, Normal, or Other. If you select Fast, Slow, or
Normal, you do not need to enter a value. The filter constant is determined by the
sampling rate. If you select Other, you can enter a value. However, the value you
enter is rounded up or down to a value based on the sampling rate.
The filter time constant adjusts the time window over which the data is filtered, thus
controlling the degree of baseline smoothing and the impact on peak height
degradation. Optimizing this parameter in the method ensures the highest
signal-to-noise ratios are achieved for a particular application.
Decreasing the time-constant settings produces these effects:
•
Narrow peaks with minimal peak distortion and time delay.
•
Very small peaks become harder to discriminate from baseline noise.
•
Less baseline noise is removed.
Increasing the time-constant settings produces these effects:
•
Greatly decrease baseline noise.
•
Shorten and broaden peaks.
The software includes fast or normal filtering constants at each sampling rate that are
appropriate for high speed or high sensitivity applications respectively.
The following figure shows the relationship between increased filter time constant and
absorbance.
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