Average theoretical void volume of some HPLC column is given in the articles.
But I didn't get much information on UPLC system. ...Can any provide detail information ?
I have used Uracil to find void Rt ...It comes around 0.2 Rt .... with 0.23 ml/min.
If you have measured the Tzero time using an unretained compound on your column, then you have your answer, but your observations are not consistent with the information presented. BTW: Names such as HPLC vs UHPLC do not matter here (esp names such as "UPLC" as that is a trademark of Waters Corp, not a type of chromatography. It is all called "HPLC"). For gradient programs, the dwell volume of the system (not column volume) becomes very important as this directly contributes to the time it takes for the programmed mobile phase changes to reach and move through the column. *I think you may be confusing these facts with the column's volume.
Now, to estimate the column volume in your above example for a L1 C18 column we find that a 2.1 mm ID x 100 mm long column with 1.7u particles has an approx void volume of 0.240 ml (please refer to the free link below for the calculation and table of estimated column volumes). At a flow rate of 0.230 ml/min your Tzero time should be ~ 1.04 minutes, not 0.2 minutes. Only 46 ul of mobile phase have eluted off your column by 0.2 minutes so the sample has not had enough time to migrate through the column or elute from it. *BTW: You may want to take an empty, very small graduated cylinder and place it under the column's outlet line (or detectors) and leave it there for 20 or more minutes and use it to confirm the actual flow rate.
I have linked a few free chromatography articles related to these fundamentals below and hope you find them helpful.
BTW: If you are trying to determine the dwell volume of your HPLC system, then before measuring it, please remove the column from the system so its volume does not contribute to the total.
http://hplctips.blogspot.com/2011/05/determination-of-hplc-column-dead-time.html
http://hplctips.blogspot.com/2011/07/hplc-to-uhplc-conversion-notes.html
http://hplctips.blogspot.com/2015/08/terminiology-which-is-it-uplc-uhplc-or.html
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