Tip# 112: Determining the
Data Acquisition Rate (Sampling Rate) For Your HPLC Detector
Another common question I am asked is how to set-up the
detector’s sampling rate. This applies in a general sense to UV, VIS and many types
of mass detectors. Most instrument manufacturer’s input default sampling rate values
into their software packages. Often, clients use these values without questioning if they
are correct. The manufacturer can not know what sample(s), method and conditions you have
so you should always calculate the correct value yourself to make sure it is appropriate
for your application.
How Many Points Across The Peak Are Needed? In a typical
chromatographic analysis you must determine the sampling rate of the detector. This value
is required for proper instruments set-up, quantification and integration. In the most
basic sense, the area under a peak requires at least ten points to describe it with some
accuracy. Ten points will provide basic data about the shape of an ideal peak to the
computer. Since peaks are not always perfectly symmetrical, a larger number of points will
provide a more accurate picture of the peak’s shape which will also improve
reproducibility and quantification. We suggest you have between twenty to thirty data
points to allow for a more detailed fit to the peak. Too few points across a peak and you
lose detail and sacrifice reproducibility. Too many points and you start to introduce
noise into system.
With these facts in mind we can next think about the
detector’s data acquisition rate. You must select a data rate (sampling rate) that is
sure to provide the recommended 20 to 30 data points across the peak width (we use the
commonly calculated peak width at half height as the time measurement). Select a detector
sampling rate that will provide you with this degree of detail.
Examples:
(a) If your narrowest peak has a peak width of 1.00 minute
(60 seconds), then divide 60 seconds into 30 parts for a result of 2 seconds per data
point. The preferred sampling rate would be 2 seconds, 0.03 minutes or 0.5 Hz (depending
on the units used by your detector).
(b) If your narrowest peak has a peak width of 0.20 minutes
(12 seconds), then divide 12 seconds into 30 parts for a result of 0.4 seconds per data
point. This equals a sampling rate of 2.5 samples per second or 2.5 Hz.
To Determine the Data Acquisition Rate For Your Detector
You Need To:
(1) Use your existing integration results to identify the
narrowest chromatographic peak in your analysis.
(2) Record the peak width at half height for the narrowest
peak (usually in units of time).
(3) Divide this number by thirty (30) to determine the
preferred sampling rate.
(4) Use this value, or a value close to it, for your
detector’s sampling rate.
> Bill Letter, 05/29/2011.
