HPLC HINTS & TIPS for Chromatographers
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 detectors sampling rate. This applies in a general sense to UV,
VISand many types of mass detectors. Most instrument manufacturers 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 peaks 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 detectors 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.
(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 detectors sampling rate.
> Bill Letter, 05/29/2011.
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