GB2158608A - Sample introduction device - Google Patents

Abstract

A device for the introduction of a sample into a plasma tube in a microwave induced plasma atomic emission spectrometer, consisting of a tapered sample introduction tube (1), a detachable gas inlet tube (6) mounted in one end of the tube (1), a heater e.g. a pair of tungsten electrodes (10, 10') passing through the inlet tube (6) into the tube (1), and a tantalum heating element (14) connected across the electrodes (10, 10'), and a sample delivery port (4) in the side of the tube (1) leading onto the element (14). In use, liquid sample delivered by micropipette or syringe onto the element (14) is vapourised and is swept by gas passing into the tube (1) from the inlet tube (6) into the plasma tube (20) where excitation occurs. <IMAGE>

Description

SPECIFICATION Sample introduction device This invention relates to a sample introduction device for use with an atomic emission spectrometer, in particularfor use with a microwaveinduced plasma (hereinafter referred to as MIP) atomic emission spectrometer. The invention also relates to an atomic emission spectrometer including a sample introduction device.

The MIP in a noble gas (usually argon or helium) has been used for several years as an elementselective detector for both metals and non-metals as discussed by van Dalen eta/(Anal. Chim. Acta 142 p.159 - 171, 1982). Its main advantage over other more widely used atomic emission spectroscopy methods is that it can be used to detect a far greater range of elements, particularly the non-metallic elements. In use, the plasma is induced at either atmospheric or sub-atmospheric pressure in a plasma tube which passes through a microwave cavity within a block of metal. A sample of material to be analysed is swept into the cavity with the noble gas, and the optical signal produced is subjected to spectroscopic analysis.

Because of the relatively low kinetic energy of the induced plasma, individual samples of material to be analysed generally require volatilisation before being introduced into the plasma with the noble gas.

For non-gaseous materials this is normally achieved by dispersing the materials in a solvent, and volatilising a sample droplet of the material-laden solvent in a sample introduction device prior to excitation of the material in the MIP. However, one problem of known sample introduction devices is that during operation they frequently cause the plasma to overload or extinguish or cause the spectrometer to give inaccurate readings due to instabilities produced in the plasma. For these reasons it has not hitherto been generally possible using MIP atomic emission spectroscopy to analyse samples with a speed and accuracy comparable to that achievable by some of the more widely used atomic emission spectroscopy techniques. An example of one such known sample introduction device is described by Nixon eft at (Anal. Chem. 46 p. 210-213, 1974).This device consists of a flat heating element connected across 2 copper terminals and housed within a dome-shaped quartz chamber. Sample liquid is injected through the dome onto the element, and electrical current is passed through the element until the volatile solvent component of the sample is evaporated. The electrical power is then steeped up briefly to vapourise the non-volatile component of the sample, and noble gas flowing through the chamber sweeps the non volatile component into the MIP cavity. During this operation, the plasma in the cavity frequently becomes unstable or is extinguished.

It is an object of the present invention to provide a sample introduction device for use in MIP atomic emission spectroscopy which overcomes or at least mitigates the above disadvantages.

Accordingly, the present invention provides a device for the introduction of a sample into a plasma tube in a microwave induced plasma atomic emission spectrometer, said device comprising a sample introduction tube, having a plasma gas inlet and a tapered, plasma gas outlet, a sample delivery port, which is in communication with the introduction tube and is situated between the inlet and the outlet, and a heating element, which is situated within the introduction tube adjacent the delivery port and which, in use, is operatively linked to an electrical power source, the device being so constructed that, in use, when a sample is passed through the delivery port onto the heating element, it is vapourised by the heating element into a flow of plasma gas passing through the introduction tube and is thereby swept, in vapourised form, into the plasma tube.

The present device is particularly suitable for use with an MIP cavity operated at atmospheric pressure, though it may also be used with an MIP cavity operated at reduced pressure.

The use of a tube with a tapered outlet for sample introduction by the present device ensures that the plasma gas passing into the plasma tube does so in a uniform manner and with the minimum of turbulence. This reduces to a minimum plasma instability within the MIP cavity, and is a major advantage of the present device since in the past plasma instability has often caused the plasma to extinguish and the results from MIP atomic emission spectroscopy to be irreproducible.

The present inventors have found that the flow of the plasma gas into the plasma tube from the present device is particularly uniform if the sample introduction tube is tapered inwards towards the plasma gas outlet at an angle of taper to the longitudinal axis of the tube of less than 15", preferably less than 10 .

The heating element and the delivery port are preferably situated close enough to the plasma gas outlet that, when the device is in operation, the internal surfaces of the introduction tube receive sufficient radiant heat from the element that none of the vapourised sample plates out onto the internal surfaces.

The ratio of the internal diameter of the body of the sample introduction tube to that of its plasma gas outlet is preferably less than 15:1, most preferably less than 7:1. As the angle of taper is preferably fairly gentle (generally less than 15),a ratio of more than 15:1 will usually result in an unacceptably long taper on the tube. A long taper cannot be adequately heated by the element when the device is in use, so that some of the vapourised sample may plate out onto the cool internal surface of the taper and thereby give inaccurate results.

In one particularly preferred embodiment of the present device the sample delivery port is also a tube which extends outwards from the introduction tube.

The tubular port will generally be closed by a removable seal. Hitherto it has not been possible to remove the seal from the delivery port, even when a sample introduction device has been operated at atmospheric pressure. This has been because the removal of the seal under these circumstances usually caused the ingress of air sufficient to exting uish the plasma. Thus, in these known devices, sample delivery could only be effected by injection through the seal using a syringe. By contrast, in the present particularly preferred device, the entrance to the delivery port is sufficiently removed from the introduction tube that removal of the seal from the port for short periods (whilst the device is connected to a MIP cavity operating at atmospheric pressure) will not extinguish the plasma.This permits the use of a relatively wide bodied means of delivering the sample through the port and into the introduction tube (eg a micro pipette). Wide bodied delivery means, such as a micro-pipette, are more readily adapted than syringes to automatic sample delivery.

If automated sample delivery is employed, such automated delivery will also include means for automatically removing the seal before and replacing the seal after each delivery. Furthermore, unlike syringes, pipettes and the like will not introduce impurities to the sample through direct contact with the seal. The tubular port is preferably no wider than that required to permit the passage of the delivery means with ease.

The sample introduction tube is preferably transparent, at least in part, so that sample delivery onto the heating element can be observed and so that any plating out onto the internal surfaces of the introduction tube can easily be detected. Since the introduction tube must also be capable of withstanding the high temperature generated by the heating element, the tube is conveniently constructed of a high temperature glass eg a borosilicate glass.

The heating element may consist of a strip or cup of refractory material of high electrical conductivity suitably shaped to receive a volume of sample liquid. Preferably, however, the element is in the form of a filament (eg a wire) of a refractory metal such as tantalum, platignum, rhenium, tungsten or molybdenum, which filament will retain a drop of sample liquid on its surface by surface tension effects. In either case the element is conveniently connected between two electrodes which support it.

The advantage of a filament is that it will retain a droplet regardless of the position in which the device is mounted. It will normally be capable of retaining a sample droplet of 5-20,a1 volume, which is a sufficient volume to permit the detection of very low concentrations of elements present in the droplet by atomic emission spectroscopy, but which is a small enough volume to permit rapid evaporation of the solvent without unduly upsetting or even extinguishing the plasma.

The plasma gas inlet may comprise an opening in the sidewalls of the introduction tube, but, in order to reduce gas turbulence within the tube, the plasma gas inlet preferably comprises an opening in one end of the tube at the opposite end to the plasma gas outlet. Similarly, the electrodes supporting the heating element may extend transversely through the sidewalls of the introduction tube, but preferably they extend into the tube and along its length to more efficiently utilise the space within the tube.

Conveniently, the electrodes are sideably mounted in the introduction tube so that by sliding the electrodes back and forth the heating element may be positioned accurately in line with the delivery port to facilitate sample delivery. Furthermore, the electrodes and the element are preferably detachably mounted within the introduction tube so that when the electrodes are detached from the tube, the element may be inspected and replaced as necessary. In one especially preferred embodiment of the present invention, the plasma gas inlet comprises a tubular member detachably connected to and slideably located within the said one end of the introduction tube, the electrodes being mounted within the member along its length so asto extend through the member and into the said tube.Conveniently, the tubular member is perforated at one end by two or more perforations so as to admit the flow of plasma gas into the introduction tube and direct the flow outwards against the sidewalls of the introduction tube rather than along its length. Directing the flow outwards is found to promote a less turbulent gas flow within the introduction tube.

The sample introduction device in accordance with the present invention may either form a separate part of an MIP atomic emission spectrometer consisting of two or more separate spectrometer units, or form an integral part of a complete MIP atomic emission spectrometer constructed as a single unit. Alternatively, the sample introduction device may form an integral part of a separate unit forming part of the spectrometer consisting of, for example, a combined sample introduction device and microwave cavity, or a combined sample introduction device, microwave cavity, and monochromator.

The present invention will now be described by way of example only with reference to the accompanying drawings, in which Figure 7 is a disassembled plan view of a preferred sample introduction device according to this invention, and Figure 2 is a sectional view along the line AA of the sample introduction device of Figure 1,when assembled, attached to a plasma tube and Figure 3 is a sectional view along the line BB of the sample introduction device of Figure 1, when assembled.

The sample introduction device illustrated in the Figures consist of an introduction tube 1 of borosilicate glass having a tapered end 2 and an externally threaded end 3 at the other. The side walls of the tube 1 open into a short tubular delivery port 4 which extends transversely from the tube 1. The open end of the delivery port 4 is closed by a rubber sealing cap 5 which is a tight fit over the end of the port.

An inner tube 6 of borosilicate glass closed at both ends is located in co-axial alignment with the introduction tube 1 with its forward end 17 located within the tube 1 through its threaded end 3. The inner tube 6 is demountably attached to the threaded end 3 of the introduction tube 1 by means of a threaded plastic cap 7 adapted to engage with the threaded end 3, a rubber ring 8 and a polytetrafluoroethylene (PTFE) washer 9. The ring 8 and washer 9 fit about the inner tube 6 and are urged against both the inner tube 6 and the threaded end 3 of the introduction tube 1 when the cap 7 is screwed into the threaded end 3. A pair of electrical terminals comprising two parallel tungsten rods 10, 10' extend longitudinally through the inner tube 6 and into the introduction tube 1. Within the inertube 6, the tungsten rods 10,10' are housed in glass tubes 11, 11'.The tubes 11, 11' are attached at either end to the closed end of the inner tube 6. The rods 10,10' are sealed within the tubes 11, 11' by means of a slug of lead sealing glass 18 at the forward end 17 of the inner tube 6. The tungsten rods 10, 10' are thereby isolated from one another and from the internal atmosphere of the sample introduction device.

The side walls of the inner tube 6 are perforated by an inlet port 12 positioned outside the introduction tube 1, and by two outlet ports 13,13' positioned within the introduction tube 1, when the sample introduction device is assembled as illustrated in Figure 2.

A loop of tantalum wire 14 is attached across the end of the tungsten rods 10, 10' within the outer tube 1 such that the bottom 15 of the loop 14 is directly in line with the delivery port 4 when the sample introduction device is assembled as illustrated in Figure 2. To ensure that the loop 14 is held firmly and in good electrical contact with the tungsten rods 10, 10' whilst enabling the loop to be easily and quickly replaced when necessary, the loop is attached between the forwards ends 19, 19' of the rods, 10, 10' by two silica sleeves 16, 16'. The sleeves 16, 16' are a close fit over the ends 19, 19' of the rods 10, 10', and urge the loop 14 against the rods when the loop is pushed into the sleeves to overlap with the ends 19, 19'. The other ends of the rods 10, 10' are connected to an electrical power supply (not shown).The use of the cap 7, ring 8, and washer 9 allow the inner tube some longitudinal movementso that the loop 14 may be positioned accurately in line with the port 4 while still providing an adequate seal between the inner tube 6 and the introduction tube 1.

The device of Figures 1, 2 and 3 may be designed to be used with any MIP cavity, the inner diameter of the tapered end of the tube 1 being chosen such that it is substantially the same as the internal diameter of the plasma tube (typically made of silica or alumina) of the cavity. In use the device is supported by clamps (not shown) and is abutted against in axial alignment with the open end of a plasma tube 20 of an MIP cavity (not shown) at the tapered end 2 of the introduction tube 1. The tapered end 2 and plasma tube 20 are preferably detachably connected to one another by means of a short silicone rubber tubular sleeve 21 which fits over the plasma tube 20 and the tapered end 2. Conveniently, the sample introduction device is supported horizontally with the delivery port 4 pointing upwards, so that samples may be delivered downwards into the device.

An inert plasma gas, such as helium or argon, is then introduced continuously under slight positive pressure (eg from a gas cylinder) into the sample introduction device through the inlet port 12. The gas flows through the inner tube 6, into the outer tube 1 through the outlet ports 13, 13', and into the plasma tube 20 of the cavity through the tapered end 2 of the introduction tube 1. Once the device is fully flushed with inert gas, the flow rate of the inert gas is adjusted accordingly and the gas in the cavity is initiated by conventional means to establish a microwave-induced plasma therein.

To introduce a sample of material into the cavity for analysis, the material is first dissolved in a solvent and a small drop of the resulting solution of know volume (typically 5 to 25 ul) is delivered onto the bottom 15 of the loop 14 through the delivery port 4. The drop of the solution may be delivered either by injecting through the rubber sealing cap 5 directly onto the loop 14 using a syringe (not shown), or alternatively the drop may be delivered by removing the rubber sealing cap 5, placing the drop onto the loop 14 using a micro-pipette employing a glass capillary sample tube (not shown), and then replacing the cap. Sample delivery may be effected by a hand-held or automatic means of delivery.A small amount of air is entrained when the sample is delivered by micro-pipette, but delivery can easily be effected in a matter of seconds without extinguishing the plasma, and the plasma rapidly retains equilibrium once the sealing cap 5 is replaced. Direct current (D.C.) power is then applied across the tungsten rods 10,10 by the electrical power supply so that the solvent evaporates slowly and steadily on the loop 14 at a sufficiently slow rate not to upset or extinguish the plasma in the plasma tube 20. The power is then increased rapidly to atomise the dried sample. The atomised sample is then swept through the tapered end 2 and into the MIP cavity by the inert gas, thereby allowing atomic emission spectroscopic analysis of the sample to be effected.

The sample introduction device as illustrated in Figures 1 2 and 3 above was used in conjunction with MIP atomic emission spectrometer equipment to measure the concentration of indium dissolved in water. This equipment included a Beenakker TMo1o microwave cavity having a 2.5 mm inside diameter, 5 mm outside diameter plasma tube of silica, an Electro Medical Supplies Microton* Mark Ill microwave generator with reflected power meter, a Hilger Monospek* 1000 monochromator including readout equipment, and a Capilettor* micro-pipette. The outer tube 1 of the sample introduction device comprised a Quickfit* borosilicate glass SO 18 screw thread.The outer tube 1 had a 15.5 mm outside diameter, 11.5 mm inside diameter, and a 35 mm long tapered portion which tapered the tube down at an angle of about 8 to the longitudinal axis of the tube to the size of the plasma tube (2.5 mm inside diameter, 5 mm outside diameter). The delivery port 4 was 25 mm long and had an inside diameter of about 2mm. The inner tube 6 had an outer diameter of 8 mm and was attached to the outer tube 1 at its threaded end by a Quickfit* QC 18/11 plastic cap 7, a QR 18/7 rubber ring 8, and a QW 18/7 PTFE washer 9.

The tungsten rods 10, 10 were 1.6 mm in diameter and attached between them was a 3 cm long loop 14 of 0.5 mm diameter, 99.9% pure tantalum wire.

In order to analyse the indium-containing solution, helium gas at atmospheric pressure was first fed through the sample introduction device into the MIP cavity at a flow rate of 650-1850 ml min -', and the plasma was initiated. A 10ffi1 drop of the indium containing solution was then delivered by the handheld Capilettor* micro-pipette onto the loop 14. The drop was dried by passing 4 amps at 0.7 volts potential difference across the tungsten rods 10,10' for 30 seconds, and was then atomised by momentarily raising the current through the loop 14to 21 amps at 5.8 volts potential difference. Further samples were analysed at a sample analysis rate of 50 per hour.The spectrometer was found to be capable of detecting Indium at concentrations down to 1.0 ng my.~'. Analysis results were found to be accurate to within about 5%, which is typical for analysis procedures employing hand-held micropipetting.

The loop was found to be capable of withstanding between 100 and 500 sample analyses before failing.

The life of the loop was found to depend on the temperature to which it was heated to effect sample atomisation, and on the sample material itself.

The above arrangement of MIP atomic emission spectrometer equipment was found to be equally suitable for analysing solutions containing two or more metals or non-metals of differing boiling points eg copper and lead. In the case of copper and lead, the atomisation was carried out in two steps, the first step consisting of raising the temperature of the loop momentarily to atomisethe lower boilingpoint lead, and the second step consisting of momentarily raising the loop temperature still furthear to atomisethe copper. In this way two measurable signals were produced.

Claims (12)

1. A device for the introduction of a sample into a plasma tube in a microwave induced plasma atomic emission spectrometer, said device comprising a sample introduction tube, having a plasma gas inlet and a tapered, plasma gas outlet, a sample delivery port, which is in communication with the introduction tube and is situated between the inlet and the outlet, and a heating element, which is situated within the introduction tube adjacent the delivery port and which, in use, is operatively linked to an electrical power source, the device being so constructed that, in use, when a sample is passed through the delivery port onto the heating element, it is vaporized by the heating element into a flow of plasma gas passing through the introduction tube and is thereby swept, in vapourised form, into the plasma tube.

2. A device according to claim 1, wherein the heating element and the delivery port are situated close enough to the plasma gas outlet that, when the device is in operation, the internal surfaces of the introduction tube receive sufficient radiant heat from the element that none of the vapourised sample plates out onto the internal surfaces.

3. A device according to claim 1 or claim 2 wherein the sample introduction tube is tapered inwards towards the plasma gas outlet at an angle of taper to the longitudinal axis of the tube of less than 15".

4. A device according to claim 3 wherein the angle of taper is less than 10 .

5. A device according to any one of the preceding claims wherein the sample delivery port comprises a tubular port which extends outwards from the introduction tube.

6. A device according to claim 5 wherein the tubular port is closed by a removable seal.

7. A device according to any one of the preceding claims, wherein the plasma gas inlet comprises an opening in one end of the introduction tube, the plasma gas outlet being disposed at the other end of the sample introduction tube.

8. A device according to any one of the preceding claims wherein the heating element is connected between two electrodes extending into and along the length of the sample introduction tube.

9. A device according to any one of the preceding claims wherein the heating element is connected between two electrodes which are slideably located within the sample introduction tube.

10. A device according to claims 7,8 and 9 wherein the plasma gas inlet comprises a tubular member detachably connected to and slidably located within the said one end of the sample introduction tube, the electrodes being mounted within the member along its length so as to extend through the member and into the said tube.

11. A device for the introduction of a sample into a plasma tube in a microwave induced plasma atomic emission spectrometer, substantially as hereinbefore described with reference to the drawings.

12. A microwave induced plasma atomic emission spectrometer including the device according to any one of the preceding claims.