Dispensers feed measured amounts of sample material into trays, vessels, microplates, or centrifuges, without human contact with the material. These devices may be operated manually, automatically timed, or computer controlled for more complicated projects. Dispensers are used instead of simply pouring out materials because they are more accurate, help to cut down on waste, and lessen the risk of sample contamination.
The most common types of dispensers are bottle top and burette dispensers. Bottle top dispensers portion out small volumes of sample from a bottle safely and reliably without contamination or waste. Pouring introduces the possibility of contamination each time the bottle is opened and invariably leads to waste. Burette dispensers are used for the volumetric transfer of reagents for titration. They consist of a calibrated barrel, a delivery tip, and a valve (stopcock). There are other far less common types of dispensers, but these tend to be designed for proprietary needs.
When determining which of the many dispenser types is best for a given set of applications, or expected uses, there are a number of criteria that should be specified. It is important to be aware of the level of accuracy that will be needed. Most dispensers function with a degree of accuracy between .5 to 1.0% of accepted values. If the dispenser will be used in very sensitive tests, it may be better to tend towards those devices with a higher degree of accuracy. Similarly, the volumetric increments to which the dispenser is geared will help to gauge its reliability for certain applications. While milliliters are the most common increment, they are certainly not the only scale. Additionally, it is important to be aware of the sample capacity of the device. If the dispenser is going to be used often to produce or maintain reactions, especially in situations where it will not be directly supervised, or the experiment may take place over a period of days, a large capacity dispenser is suggested.