The present invention generally relates to a mechanism for automatically supplying samples, and, in particular, relates to such a mechanism adapted for supplying a liquid sample to the measuring loop of a liquid chromatograph.
A liquid chromatograph comprises a separating column, through which a solvent is pumped. A liquid sample to be tested is inserted into this solvent flow and interacts with a separating substance present in the separating column. The sample substances are transported by the solvent flowing through the separating column with different transporting speeds depending on the strength of the interaction between the separating substance and the sample substance. Consequently, the components of the sample emerge successively from the exit of the separating column and can be captured individually, for example, by means of a fractional collector. The quantity of sample is supplied into the solvent fluid by means of a "measuring loop." The measuring loop is often a length of tube connected between a solvent pump and the entrance of the separating column, whereby the solvent flow through the tube carries the sample contained therein into the separating column.
In a prior art device for automatically supplying sample to the measured loop of a liquid chromatograph (Auto--Sampler Model 420 of The Perkin-Elmer Corporation, Norwalk, Connecticut) individual samples are contained in separate bottles, each of which is closed with a diaphragm, or septum. A needle, provided with two bores, pricks through the diaphragm and immerses with the lower bore into the sample. The upper bore of the needle remains above the liquid surface of the sample. Nitrogen is supplied under pressure into the bottle through the upper bore and the sample liquid is thus forced through the lower bore of the needle to a connecting conduit and the measuring loop. The measuring loop is then connected into the solvent flow of the chromatograph. The needle is removed from the bottle and is ready for the next sample.
The sample vessels described are required to be sealed by a diaphragm, which requirement involves considerable expenses. In addition, the transfer of the sample requires the availability of pressurized nitrogen. Another particular disadvantage in the prior art arrangement is that a considerable portion of the sample liquid is used for washing the needle, the connecting conduit and the measuring loop. Further, the dosing volume, i.e., the volume of the liquid sample actually supplied to the liquid chromatograph, is determined by the volume of the measuring loop and can be varied only by changing the measuring loop.
Devices for supplying samples to the graphite tube of a graphite tube atomizer in the flameless atomic absorption spectroscopy (see, for example, U.S. Pat. No. 4,111,051, issued on Sept. 5, 1978) or to a burner of a flameless atomic absorption spectrometer (see, for example, U.S. patent application Ser. No. 56,751, filed on July 12, 1979) are known, which devices are able to take in an accurately defined quantity of sample liquid from an open vessel and supply that quantity of liquid to the analytical instrument. To avoid cross-contamination between samples in these prior art instruments, the dosing tube thereof is washed inside and outside via a washing liquid prior to each sample feeding. For this purpose, the rear end of the dosing tube is connected via a washing liquid pump to a washing liquid container. After each sample feeding, the dosing tube is immersed into a washing vessel and washing liquid is pumped out of the washing liquid container through the dosing tube into the washing vessel. The sample liquid is sucked out of a sample vessel by means of a sample pump connected to the rear end of the dosing tube and supplied to the analytical instrument after a motion of the dosing tube.