I would like to adjust the length of the HPLC output tubing and therefore I can collect exactly at the same time as the targeted compound(s) associated peak(s) appear on the screen. Thanks
No need to use any "colored" material. You may contaminate the flow path of the system in doing so. The answer to the question is found by knowing the flow rate used and the volume of liquid contained in the outlet tubing. If you know what the retention time is for your fraction or peak on your HPLC system, then you know when it appears at the detector (a slight signal buffering delay-time may need to be added to this time). *You can use any detectable standard for this same purpose of determining the delay time. To find out how long it will take for the peak to move from the detector's flow cell (which is when you "see" it) to your collection tube outlet, measure the volume of the outlet tubing. The volume contained plus the flow rate (which provides you with: volume/time) are all you need to determine the delay time for collection purposes.
- To measure the volume in the tubing, you can either estimate it using the known ID and length of tubing from a Table like this one; "HPLC Capillary Tubing Connection Volumes" [https://hplctips.blogspot.com/2012/10/capillary-tubing-connection-volumes.html] OR, fill the tubing with pure water, then weigh the water inside it on an analytical balance to determine its weight (which gives you the volume).
To add to William Letter 's good reply- Assuming UV detection, there will always be a delay time from when the peak is displayed to when it leaves the tubing from the capillary after the detector, unless there is a significant buffering delay to the display as he described, so if you are manually collecting fractions, there will be a delay between the time the peak appears and when it leaves the tubing. If you are using a fraction collector, the delay can be programmed into the fraction collector.
Some detectors, such as mass spectrometers or light scattering detectors, have a longer, but consistent, response delay because of the time needed for sample evaporation/transit. Depending on the flow rate, you may need extra tubing after the detector so that the peak doesn't leave the tubing by the time the signal is even detected. Companies that manufacture preparative LC systems spend a lot of time getting these delay tubes correct, and find that the delay sometimes depends on solvent viscosity.
Kindly check..
https://www.researchgate.net/deref/https%3A%2F%2Fhplctips.blogspot.com%2F2012%2F10%2Fcapillary-tubing-connection-volumes.html
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