With a liquid chromatograph, an autosampler is used so that one liquid sample among a plurality of liquid samples is automatically selected and introduced into a column. FIGS. 1A and 1B show a schematic views of the flow path of an autosampler in a previous liquid chromatograph.
In FIGS. 1A and 1B, high-pressure valve 1 is a flow path switching valve having 6 ports, a through f, and low-pressure valve 2 is a flow path switching valve having 7 ports, g through m. In high-pressure valve 1, port a is connected to a mobile phase flow path to which a mobile phase is supplied; port b is connected to needle 11; port c is connected to port k of low-pressure valve 2; port d is connected to a drain via a solenoid valve 12; port e is connected to an injection port 13; and port f is connected to a flow path that leads to a column. At low-pressure valve 2, port g is connected to a mobile phase; ports h and i are connected to cleaning liquids R1 and R2, respectively; port j is connected to a measuring-pump 14; port k is connected to port c of high-pressure valve 1; port l is connected to a cleaning port 15; and port m is constructed to be connectable to any one of ports g through l while connecting adjacent ports g through l to each other.
FIG. 2 is a block diagram schematically showing a control system for the autosampler. High-pressure valve 1, low-pressure valve 2, measuring-pump 14 and a movement mechanism 6 of needle 11 are connected to a control unit 7. Control unit 7 selects and switches the ports of high-pressure valve 1 and low-pressure valve 2, drives a plunger of measuring-pump 14 and controls the movement of needle 11.
The basic operation of the afore-described autosampler involved in the introduction of samples is described next. To collect a sample, the respective ports of high-pressure valve 1 and low-pressure valve 2 are connected as shown in FIG. 1A to establish a load mode. Needle 11 is then moved to a position above a sample vial, and the tip of needle 11 is inserted into a sample solution. (This is the state shown by the dotted lines in FIG. 1A.) When the plunger of measuring-pump 14 is withdrawn in this state, a predetermined amount of sample solution is drawn in by suction from the vial via a mobile phase (or a cleaning liquid having the same components) that fills the flow path leading from measuring-pump 14 to needle 11 and fills sample loop 16.
After the sample is collected, needle 11 returns to injection port 13 and is connected to injection port 13, switching the connection state of the respective ports of high-pressure valve 1 to that shown in FIG. 1B (injection mode). When this happens, the mobile phase that is supplied from a liquid feeding pump 3 is sent to column 4 via sample loop 16, needle 11 and injection port 13. The sample solution that fills sample loop 16 is supplied to column 4 together with the mobile phase and is separated into components as the sample solution passes through column 4. The components are then sequentially detected by detectors that are not illustrated.
Needle 11 to which a sample solution is adhered due to the afore-described sample collection operation is cleaned in the following way. First, port m and port h of low-pressure valve 2 are connected. The plunger of measuring-pump 14 is withdrawn in this state to draw in cleaning liquid R1 by suction (FIG. 3A). Next, after port m and port l of low-pressure valve 2 are connected, the plunger of measuring-pump 14 is pushed down (FIG. 3B). This causes cleaning liquid R1 to be drawn into and to be held inside cleaning port 15. Next, needle 11 is moved to a position above cleaning port 15 and is immersed into and is cleaned by the cleaning liquid that is present in cleaning port 15. While needle ills so immersed, the cleaning liquid is made to flow into cleaning port 15 from the bottom of cleaning port 15 and to flow out from the top of cleaning port 15 so that the cleaning liquid in cleaning port 15 is kept always clean, thus increasing the cleaning effect on the tip of needle 11. Methods for cleaning the needle proposed in the past include those where a plurality of cleaning methods is used for the cleaning (e.g., Patent Literature 1) and a method where the needle is cleaned by using a high flow rate for the cleaning liquid (e.g., Patent Literature 2).
After needle 11 has been cleaned for a predetermined amount of time by the cleaning liquid, needle 11 is moved to injection port 13. The waste cleaning liquid is discharged to a drain from cleaning port 15.
With the afore-described autosampler, the afore-described cleaning operation of needle 11 is performed every time after the completion of the introduction of a sample. This significantly reduces the cross-contamination of the next sample solution by the previous sample solution but does not completely eliminate the cross-contamination because of the following reasons.
FIG. 4 shows an enlarged view of the connection area between injection port 13 and needle 11. A through-hole 17 is formed through the center of injection port 13. A funnel-shaped sealing-surface 18 is formed at the upper end of through-hole 17. Because the tip of needle 11 is tapered, as needle 11 is lowered and inserted into through-hole 17 of injection port 13, there comes a point where needle 11 has been lowered enough to form a liquid-tight contact and seal between the outer peripheral surface of the tip of needle 11 and sealing-surface 18.
When a sample solution is drawn in by suction from a vial, some of the sample solution adheres to the outer peripheral surface of the tip of needle 11. Because needle 11 is inserted in this condition into injection port 13, some of the sample solution adheres to a part (“contact point 19”) of sealing-surface 18 that comes into contact with needle 11. Even when a mobile phase flows from needle 11 to through-hole 17 of injection port 13, the sample solution that was present at contact point 19 is not washed away by the mobile phase and remains there. This means that when the next sample solution is introduced into injection port 13 by needle 11, it becomes possible for the sample solution that remained on contact point 19 of sealing-surface 18 to be pushed by needle 11 into and mixed with the flow path.
One solution that has been proposed to solve problems such as this is to clean sealing-surface 18 by introducing a cleaning liquid into injection port 13 from needle 11 while keeping the tip of needle 11 slightly elevated above sealing-surface 18 (see Patent Literature 3). When doing this, any cleaning liquid that is introduced into injection port 13 that overflows from injection port 13 contaminates the surrounding areas. To address this problem, with the autosampler according to Patent Literature 3, injection port 13 is surrounded by a partition and the cleaning liquid that overflows into the partitioned area is forcibly expelled by an air pump to prevent contamination by the cleaning liquid (waste cleaning liquid).