Liquid dispensers, testing

Flammability of Liquids, No. 340, Underwriters Laboratories Northbrook, 111., 1972 Flame Arresters, No. 525, 1973 Flammable Liquid Dispensing, No. 1238,1975 Group Classification of Flammable Liquids and Gases, Test Report MH8593, 67C2889,1967. 

The gravimetric method is suitable for measuring the volume of liquid dispensed by pipettes, and it is commonly adopted to assess the performance of such instruments. Trained personnel are required to carry out such tests. The balances used for this purpose may have a precision of four to seven decimal places, depending on the volume to be measured and the desired accuracy. It should be weU established that the experimental setup is free from environmental disturbances... 

A related topic is dispensing tests. These are especially important for the in-tank toilet bowl cleaners. Usually the brute force method is used whereby the product is placed in an actual toilet tank, and the number of flushes to exhaust the product is measured. Alternatively, the product can be placed in a container of water the time required for the solid tablet or puck to dissolve completely is measured. (This is not relevant for liquid automatic cleaners.) The difference between the two methods is that the first uses repeated aliquots of fresh water to dissolve the item (as in real use), whereas the second uses the same volume of water for the whole experiment. 

Blocking of plastic film can be determined by a parallel plate method. Several devices and sample assemblies are described in the standard, ranging from very simple equipment based on a modified technical balance in which one arm is attached to a parallel assembly of tested samples, and the other arm has a liquid dispensed from a buret to apply a load sufficient to separate two surfaces. Also, an electromechanical device is described. The ISO standard has a similar title but a different principle of measurement. 

In the initial stages of searching for active compounds, the number of compounds to be screened is often greater than 100,000 and can exceed 500,000. Even 1 million compounds or more may be screened in a single assay.14 For this reason, biochemical screens must be inexpensive and quick. Ideally, the screen is automated and performed by a robot. Solutions of all necessary reagents and compounds to be tested are loaded into the instrument. The robot, called a liquid handler, dispenses reagents and solutions into microtiter plates. 

Test the pH of each of the aqueous solutions. Do the test by first dipping a clean glass rod into the solutions and then transferring a drop of liquid to pH paper. Use a broad pH indicator paper (e.g., pH range 1-12) and read the value of the pH by comparing the color to the chart on the dispenser. Record the results on the Report Sheet (2). 

The accuracy of liquid-handling operations in HTS is analyzed by three key parameters derived from test experiments in which equal volume aliquots are pipetted or dispensed over the plate. The trueness over the plate is a measure of the deviation of the average of the actual volume dispensed over a plate from the intended volume. It is measured gravimetrically and is defined as... 

This important testing procedure is best performed using either unit of use volume (e.g., 5 mL of oral liquid or a spray actuation of an oral inhalation product) or sampling from a well-mixed dispensing container from the top, middle, and bottom of the suspension. 

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