A Fast Optical Method for the Determination of Liquid Levels in Microplates
Parallel liquid handling systems are widely used in different applications of life sciences. In order to avoid false positive or negative results which lead to higher costs due to the replication of the experiments it is necessary to monitor the process and success of liquid delivery. An easy method for the determination of the liquid levels in microplates has been developed and evaluated. The optical method bases on the measurement of the liquid level using CCD cameras followed by special algorithms for the evaluation and visualization of the measured data. The proposed method was tested in changing environmental lighting for two different liquids. These tests confirm our approach towards optical liquid level determination for smallest volumes in microplates and also show the challenges regarding environmental lighting and different physical properties of fluids.
LLS-Liquid Level Sensing methods in qPCR set-up
One of the main goals of all liquid level sensing techniques in automated pipetting systems is to obtain most precise control over actual pipetting performance, which largely depends on ensuring optimum liquid level (meniscus) in the well and precise immersion of the pipette tip in the liquid. Various techniques for liquid level sensing are reported in literature. All of them have some advantages and limitations. Some of the techniques are used for checking the proper setup of a workstation deck prior to starting a run. Other so-called ‘real time’ level sensing devices are able to track the meniscus in each well at every pipetting step. Real time level sensing systems give the additional benefit of being able to monitor the whole pipetting procedure and provide for a consistently pipetting protocol after each run. Tracking the meniscus in each well at every pipetting step enables the pipette tip to be positioned near the liquid surface. To avoid wetting on the outside of the tip the robot calculates the minimal submersion depth necessary for pipetting the set volume. By these actions both hydrostatic pressure of the liquid and wetting of the pipette tip are minimized. Therefore accuracy in pipetting smaller volumes is enhanced as well as aerosol formation due to droplet transfer at the outside of the tip is minimized. This kind of real time level sensing gives maximum safety control for the precision and safety of all performed pipetting steps. Depending on the field of application only a few of these methods provide adequate performance for setting up polymerase chain reaction (PCR) and quantitative real-time PCR (qPCR) applications.
Design of an optical level using the Talbot effect
An optical arrangement for liquid-level measuring is proposed. The method is based on self- imaging or Talbot effect. Knowing the container dimensions of a liquid, it is possible to obtain the volume changes by the liquid variation in the container. The optical arrangement consists in detecting a diffraction field of a Ronchi grating illuminated by a collimated beam. This field is first reflected by the interface liquid-air, afterwards casting on a second Ronchi grating. This superposition produces a Moire pattern when the Talbot plane of the first grating coincides with the second one. If the travelled path of the wavefront is not a Talbot distance, the Moire pattern is lost. In this way, it is necessary to move the second grating up to see the sharper Moire fringes. The grating change gives directly the level change. Description of the experimental setup and results are shown. Strengths and weaknesses of the method are presented. The future lines of work are given in order to have an automatic and compact instrument.