definition of response time/time constant

[moon704520]

HI all:

please help to explain what does the response time/time constant in HPLC mean?

[Hfranz]

The Rise Time parameter (also referred to as Response) is a measure of how quickly the detector responds to a change in signal. The rise time is defined as the time it takes the detector's output signal to rise from 10% of its final value to 90% of its final value. The measured value is postulated to (in theory) directly jump from 0% to 100%. The Rise Time (Response Time) is closely related to the Time Constant, as the following approximation shows: Rise (Response) Time = 2.2 x Time Constant.
(This description is taken from Dionex Chromeleon on-line help.)

The faster the data collection rate, the shorter the rise time/time constant should be set. You should find recommended settings in your operator manual.

[HPLCCONSULT]

If your question is really related to what data collection rate should you use (or set your detector for), then :


Quick Version:
(1) Perform an analysis. *Assume you have proper method and peak shape.
(2) Identify the narrowest REAL sample peak in your chromatogram. Measure the peak width (in seconds). Divide this number by 20 or 30 (I am out of time to explain why right now) and use that as the samples per minute. Now you have a ball park number for the data collection rate that will gather enough statistically useful data per peak. Many applications at 1.000 ml/min and UV detection end up with a value of 1 to 10 samples /second.

Example: Peak width = 1 minute (60 seconds). Divide by 20 = 3 seconds; Divide by 30 = 2 seconds. So you could set your collection rate at 2 or 3 seconds in this case. Better to go a little faster than needed due to system delay so 1 second would work well here.

[moon704520]

get it ,I see.thanks and regards

[hexatriene]

I have a follow-up/clarification question On the relationship between response time and data rate. Essentially my question is what is the difference between response time and data rate? For the uv detector, isn't there truly only ONE variable, namely Data Rate in Hz?

It's my understanding that Peakwidth and response time are just ways for the user understand and compute an appropriate value for the data rate, in Hz.

Am I correct?

[tom jupille]

Sampling rate? Sampling rate !?? We don't need no stinkin' sampling rate! Back in the day, we didn't have none of them newfangled "digital integrators" or "data system" whatchamacallits. The time constant (or "response time", or "rise time") was set with RC filters the way the good Lord intended.