Method and Apparatus for Automated Sampling With an Atomic Absorption Spectrometer

This invention relates to method and apparatus for introducing a sample to be analyzed through an introduction aperture into a graphite tube of a heated graphite atomizer of a flameless atomic absorption spectrometer.

In flameless atomic absorption, the atomic cloud, whose absorption is measured, is generated in a graphite tube of a heated graphite atomizer. The sample to be analyzed is in general introduced through a wall aperture into the graphite tube. Only very small quantities, of the order of up to about 20 microliters, for example, are normally introduced by means of a micro-pipette into the graphite tube. However, this procedure has certain disadvantages when carrying out a series of atomic absorption measurements, as an unproportionately large amount of time and the continous presence of an operator are required. It is known from gas chromatography, as referred to in German AS 1 157 817, to use so-called micro-dipper bars instead of micro-pipettes, i.e. capillary containers into which liquid volumes of up to 20 microliters are aspirated by means of capillary action.

An object of the present invention resides in the provision of a procedure and a device of the above-mentioned type which, with the use of sample containers open at both ends, carries out an automated measuring series.

According to the invention the aforesaid problem is solved by the provision of a sample container, open at both ends, which is inserted by means of an introduction device into the introduction aperture of the graphite tube. After a sample has been introduced into the graphite tube, the sample container is returned to the sample holding device and, upon return of the sample container, the next following sample container is transported to the introduction device. In this procedure, the sample containers are filled, independently of the analysis step and placed in the sample holding device. A specific advantage of this procedure consists in the fact that, during the time when the measuring series is being carried out, the sample containers can be inserted one after the other into the introduction aperture of the graphite tube, whereby the sample is introduced in a particularly simple manner.

According to one aspect of the invention, the sample container, positioned above the introduction aperture of the graphite tube, is forced into the introduction aperture by means of the introduction device, against the tension of a spring, and kept in this position until the sample has been transferred into the graphite tube. The application of the spring tension makes sure that the sample container always follows the individual movements of the introduction device.

According to another aspect of the invention, the sample is transferred into the graphite tube by means of a gas flow directed through the introduction device, when it is in its pressed-down position. Part of the inert gas stream, passing through the heated graphite atomizer, can be used for this purpose by directing it through the sample container when the introduction device is in the down-position.

It will be appreciated that the sample containers offer the advantage of retaining the sample liquid in them by capillary action, but then additional measures have to be taken for transferring the sample from the sample container into the graphite tube. Part of the inert gas, indispensable for the operation of the heated graphite atomizer, is advantageously used for this purpose.

In one form of the invention there is provided a new and improved device for introducing a sample to be analyzed through an introduction aperture into a graphite tube of a graphite tube cell of an atomic absorption spectrometer comprising a sample holding device for stepwise movement into a sampling position above the heated graphite atomizer. Sample containers are provided in the form of micro-pipette tips, open at both ends, which when in their sampling positions, are positioned in alignment with the introduction aperture of the graphite tube. The introduction device comprises a sliding member movable vertically in a guide, which places the sample container into the introduction aperture. In the down-position of the sliding member the sample container is connected to a pressure source to press the sample into the graphite tube. A control device is provided which controls the operation of the introduction device and the sample holding device, thereby controlling the analysis of one sample after the other.

According to one aspect of the invention, a common support is provided for the sample holding device and the introduction device, and the heated graphite atomizer is removably attached to this support. Thus, the individual elements are located and maintained in their mutual positions with respect to each other.

In one form of the invention the sample holding device is in the form of a turntable having mounting means for the micro-pipette tips disposed adjacent the circumference thereof. This mounting means includes a spring-loaded holding member adapted for vertical reciprocating movement with respect to the turntable. The holding member has an arm portion extending outwardly of the periphery of the turntable, and an aperture is provided therein for receiving the micro-pipette tips.

Further, according to the invention, the sliding member is mounted above the turntable and is provided with a lower punch-like element that engages the sample container to urge the micro-pipette into the introduction aperture of the graphite tube when in its down-position. Spring means are provided to urge the sliding member towards its upper position. The sliding member is provided with a longitudinally extending gas duct having an exit opening at the bottom thereof. The upper end of the gas duct is connectible to an inert gas supply through a control valve so that, when the sliding member is in its down-position, gas passes downwardly through the gas duct to force the sample out of the micro-pipette tip into the graphite tube. This gas flow also serves to seal the top of the micro-pipette. In one form of the invention, the inert gas is supplied from the inert gas stream passing through the heated graphite atomizer.

According to the invention, means are provided for effecting the vertical movement of the sliding member. In one form thereof, this means comprises an electric motor, which is controlled by a device, provided for the purpose. In another form of the invention, the means for effecting vertical movement comprises a pneumatic drive system. In this system the sliding member is in the form of a piston and rod assembly and the guide is in the form of a pneumatic cylinder. Control means are provided for admiting inert gas at the top of the cylinder to urge the sliding member downwardly. According to one aspect of the invention this inert gas is supplied from the inert gas stream passing through the heated graphite atomizer.

FIG. 1 is a side elevation, partially in section, of a device for introducing a sample to be analyzed through an introduction aperture into a graphite tube of a graphite tube cell of an atomic absorption spectrometer according to the invention, when the device is in its rest position;

FIG. 2 is a view similar to FIG. 1, but showing the device in its operative position;

FIG. 3. is a plan view of the device of FIG. 1; and

FIG. 4 is a fragmentary view, partially in diagramatic form, showing alternative means for moving the sliding member.

As illustrated in FIGS. 1 to 3, the device, according to the invention, includes a support member 10, a heated graphite atomizer 12 detachably mounted thereon, and an introduction device indicated generally at 14 mounted on the support member 10 above the atomizer 12. A sample holding device in the form of a turntable 16 is mounted on a shaft 18 for stepwise rotation by conventional means, not shown. The shaft 18 is carried by the support member 10, as at 20, whereby, the support member 10 determines the relative positional relationships of the turntable, the heated graphite atomizer and the introduction device, with respect to each other. The turntable 16 carries a sample holding member 22, which is arranged for vertical movement with respect thereto. A spring 24 serves to urge the holding member towards its upper position. The holding member has an arm portion 26 extending outwardly of the periphery of the turntable and an aperture 28 is provided therein for receiving a sample container. The sample containers are in the form of micro-pipette tips 30, that are open at both ends, the sample 32 being contained therein by means of capillary action. Accordingly, a large number of sample containers 30 can be inserted in the holding members 22 of the turntable and kept therein without the sample liquid being lost.

The heated graphite atomizer 12 contains a graphite tube 34 of a graphite tube cell having an introduction aperture 36, which is so-positioned that, when a sample holder holding a micro-pipette tip 30 is in its sampling position, it is in alignment with the introduction aperture 36. A stream of inert gas is passed through the heated graphite atomizer from a source 38 via a pipe line 40, in the conventional manner.

The introduction device 14 includes a sliding member which, in the form of the invention illustrated in FIGS. 1-3, includes a piston and rod assembly 42 mounted in a pneumatic cylinder 44 for vertical reciprocating action. A punch-like element 46 is provided on the lower end of the piston rod for engaging the sample container of the micro-pipette tip 30 to urge it downwardly into the introduction aperture 36. The cylinder 44 has an upper connection 48 connected to a gas line 50. Inert gas may be supplied from the inert gas source 38, through valve 52, line 54 and valve 56. Alternatively, or in addition, inert gas may be supplied from an external source 58. The downstroke of the piston and rod assembly is effected by applying gas under pressure to the top of the piston, as at 60, and the return stroke is effected by relieving the gas pressure on the top of the piston, back through the connection 48, line 50, valve 56 and outlets 62, while spring means 64 acts upwardly on the bottom of the piston.

In addition, the cylinder 44 is provided with a second connection 66, which is connected to the inert gas line 50 via line 68 containing a valve 70. The piston and rod assembly 42 contains a longitudinally extending gas duct 72 having an exit opening 74 at the bottom thereof and an inlet opening 76 at the upper end. As best seen in FIG. 2, the upper inlet opening 76 is in alignmemt with the connection 66 when the piston and rod assembly is in its lower position and, at this time, valve 70 is opened to allow a flow of inert gas under pressure to pass down through the gas duct 72 and into the open upper end of the micro-pipette tip 30 to force the sample 32 out of the micro-pipette tip into the graphite tube 34 of the atomizer 12. It will be appreciated that this gas flow also serves as a seal between the top of the micro-pipette tip 30 and the bottom of the punch-like element 46. A control unit 78 serves to interrelate and control the functions of the valve 56, valve 70 and turntable 16, as will be described more fully hereinafter.

FIG. 4 shows an alternative introduction device. In this embodiment, a motor 80 vertically reciprocates a sliding member 82 in guide 84. The sliding member 82 is provided with a punch-like element 46' and a longitudinally extending gas duct 72' which function in the same manner as their corresponding parts described hereinbefore in connection with the embodiment of FIGS. 1 to 3. A control unit 78' serves to interrelate and control the functions of the motor 80, valve 70' and the turntable 16.

In operation, a plurality of sample containers 30 containing samples are placed in the sample holding members 22, and the control unit 78 actuates the motive means for turntable 16 so that it rotates one step or until one of the sample containers is under the introduction device and over the introduction aperture 36. Then, the control unit 78 actuates valve 56 to close the outlet 62 and open the flow therethrough so that the inert gas under pressure from the supply source flows through line 50 and into the top of the cylinder 44 to thereby urge the piston and rod assembly downwardly from its position as seen in FIG. 1 to its position as seen in FIG. 2. This movement causes the punch-like element 46 to push the micro-pipette tip 30, carried in the holding member 22, downwardly against the force of the spring 24 into the introduction aperture 36 of the graphite tube 34. At the bottom of the stroke, as seen in FIG. 2, the inlet opening 76 is aligned with the connection 66 and the control unit actuates the valve 70 so that inert gas under pressure flows down through the gas duct 72 and into the open top of the micro-pipette tip 30 to thereby force the sample 32 out of the micro-pipette tip and into the graphite tube cell 34 of the atomizer 12. After discharging the sample, the control unit 78 closes the valve 70 and closes the valve 56 with respect to the supply source and opens valve 56 with respect to the outlet 62. This action relieves the pressure on the top of the piston 42 so that the spring means 64 moves the piston and rod assembly back to its upper position. At the same time, the spring 24 returns the holding member 22 to its upper position, as seen in FIG. 1. Thereafter, the control unit 78 actuates the motive means for the turntable 16 so that it rotates one step to position the next adjacent sample container under the introduction device and over the introduction aperture. The aforesaid procedure is then repeated for the next sample analysis.

Thus, an improved introduction device has been shown. Although specific embodiments have been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from the spirit of the invention, which is intended to be limited solely by the appended claims.