Gradient delay volume

However, those who have worked with smaller diameter columns have often experienced lower performance and other difficulties. This is primarily due to extra-column effects the bandspreading in the injector, detector and tubing, or the gradient delay volume of the instrument. Troubleshooting guidelines for sorting out the causes of these difficulties are available in Reference f. With proper care, 2-mm columns can be run on a standard modern HPLC instrument with few difficulties. Smaller i.d. columns require special instrumentation. 

Also known as gradient delay volume, system dwell volume is the liquid holdup volume of the HPLC system from the point of solvent mixing to the head of the column. This includes the additive volumes of the injector, the sample loop, all fluidic connection tubing, and any internal pump volumes of a low-pressure mixing system. The typical dwell volume of a modem HPLC is 0.5-2 mL, but can be as high as 5-7 mL in older systems. 

Fig. 7. Delayed sample injection to eliminate the gradient delay volume.

Gradient 100% MeCN against 100% MeCN with 0.1% benzyl alcohol, secondary flow rate 20 piL min, UV 254 nm, no column -rather a thin capillary, approx, pressure in each case 50 bar, gradient delay volume approx. 1000 p.L... 

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