Measuring the loop volume

The ACQUITY UPLC System uses a "Volume Detection Device" (VDD), see figure below, to detect the movement of an air bubble generated by the needle as it travels through the needle and sample injection valve. The air gap is transported until the bubble detector senses the bubble. The VDD has a light source and a window portion, when the air gap crosses the window the light is defracted and the sensor is tripped. This happens twice: once with the sample loop in the load position and once with the loop in the inject position. The measured volumes are then compared and the sample loop apparent volume and the needle apparent volumes are determined. These are called apparent volumes since there are many fluid dynamics in play within this measurement. However, the measurement does simulate the actual transport process and the measured volumes correspond to the amount that the sample needs to be transported in terms of motor steps etc. One factor that should be considered is that the VDD device measures volume based on the air gap. Since bubbles are somewhat sensitive to surface tension, there will be some additional variance associated with the method and the mobile phase used. If the calibration is done in a solvent with high organic modifier or with a compressible solvent like Hexane, the size of the air gap may need to be increased to get consistent results. Assuming that the measured volumes are consistent, the calibration volumes become a "map" for the Sample Manager as it positions the sample in the loop for partial loop injections (PLNO and Pressure Assisted). The volumes for the loop and the needle are inserted into the positioning algorithm that the Sample Manager uses to ensure that the sample is placed in the loop correctly. The loop volume should be characterized once prior to setting up a given method with fresh mobile phase. This can be done automatically by the system when it is primed and prepared for the analysis initially, using the "Sys Prep" function in the Console Software. The user selects this in the appropriate Characterization tab of the SysPrep setup menu.

.

Figure