GB2132114A

GB2132114A - Ultrasonic nebuliser for atomic spectroscopy

Info

Publication number: GB2132114A
Application number: GB08236509A
Authority: GB
Inventors: John Frederick Woolley
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-12-22
Filing date: 1982-12-22
Publication date: 1984-07-04
Also published as: GB2132114B

Abstract

A nebuliser for generating a aerosol for introduction into the excitation source of an atomic spectroscopy analyser is constructed so that the sample liquid issues from a capillary 7 in a plug 5 fitted in the base of a delivery tube 1, and a focussed bowl ultrasonic transducer 8 attached to the plug is disposed so that the mouth of the capillary lies at the acoustic focus. A gas admission pipe 2 is arranged tangentially to a tube 3 to produce swirl and to transport the aerosol to the mouth of the tube. An outlet tube 4 removes excess liquid and is connected to a peristaltic pump. <IMAGE>

Description

SPECIFICATION Ultrasonic nebuliser for atomic spectroscopy This invention relates to nebulisers for the generation of aerosols for introduction into thermal excitation sources for determination of elements by atomic spectroscopy.

Hitherto the conventional techniques of flame atomic absorption spectroscopy (FAAS), and atomic emission spectroscopy (AES), in the latter case whether using an inductively coupled argon plasma (lCP) or a DC argon plasma (DCAP) as the excitation source, have generally relied upon the use of pneumatic nebulisation for generating the sample aerosol for introduction into the source.

Pneumatic nebulisation, in which an aerosol is created by blowing gas over the open end of a liquid filled venturi tube, produces a distribution of droplet sizes which is so far from ideal that typically it is only between 5 and 10% of the sample uptake, which may be in the region of 1 to 5 mL min , that is transported into the excitation source in a utilisable form. Some improvement on these figures has been reported to result from using higherthan normal gas pressures for nebulisation, but this introduces problems of ducting the high pressure gas, typically in the region of 100 to 200 psi, safely and compactly to where it is needed without obstructing other parts of the apparatus.

The generation of an aerosol by the use of ultrasound has been used in other applications, its use in the field of atomic spectroscopy has also been examined, and some instrument manufacturers do in fact offer ultrasonic nebulisation as an option with their instruments. In general, however, the ultrasonic nehulisation systems offered tend to be relatively expensive, and lack the simplicity of the pneumatic nebuliser, with the resuit that to date they have not gained wide acceptance for general use. For a description of an ultrasonic nebuliser for atomic spectroscopy reference may for example be made to the article by H. Uchida et al. entitled 'Desolvation System with Ultrasonic Nebuliserfor Plasma Jet Emission Spectroscopy' appearing in Spectroscopy Letters, 11(1), 1-8 (1978).This has a piezo-electric transducer in the base of a coupling bath which contains water into which is placed a vessel containing a quantity of the sample liquid to be atomised.

Ultrasonic energy is coupled from the transducer via the water into the sample liquid in the vessel where it causes an aerosol to be created above the liquid surface. This aerosol is transported to the thermal excitation source by a flow of argon. Meanwhile, the vessel has to be topped up with further sample liquid to maintain the level and thus stabilise the rate of nebulisation.

According to the present invention there is provided an ultrasonic nebuliserforthe generation of an aerosol for introduction into a thermal excitation source, which nebuliser consists of a plug provided with an axial capillary formed by or connected with a sample liquid delivery pipe, and, attached to the outer end of the plug, a focussing bowl piezo-electric ultrasonic transducer configured such that its focus lies at the mouth of the inner end of the capillary.

The invention also provides an ultrasonic nebulis er for the generation of an aerosol for introduction into a thermal excitation source, which nebuliser has an open-ended tube provided with gas inlet and excess sample liquid outlet pipes near one end of the tube which is closed off by a plug provided with an axial capillary formed by or connected with a sample liquid delivery pipe, wherein a focussing bowl piezo-electric ultrasonic transducer is attached to the outer end of the plug such that its focus lies at the mouth of the inner end of the capillary.

The invention further provides a method of generating an aerosol for introduction into a thermal excitation source by causing liquid to emerge from a capillary whose mouth is at the focus of a beam of focussed ultrasonic energy.

There follows a description of a focussed beam ultrasonic nebuliser and nebuliser assembly embodying the invention in a preferred form. The description refers to the accompanying drawing depicting a longitudinal section through the nebuliser assembly.

The nebuliser assembly of the drawing has been designed to fit a Spectraspan DC Plasma instrument, and has an open-ended quartz aerosol introduction tube 1 that is 1cm in diameter and 5.5 cm long. Near one end of the tube is an inlet pipe 2 for the admission of gas, typically argon, to transport the aerosol out of the mouth 3 of the tube and into the excitation source (not shown). This gas inlet pipe is arranged tangentially so as to impart a swirling motion to the flow within the tube. Nearer to the end ofthetube is an outlet pipe4forthe removal of excess liquid from the nebuliser. This is connected to a peristaltic pump (not shown).

The lower end of the tube is closed off with a plug 5 that reaches above the level of the exhaust pipe 4 but does not seal it off. O-rings 6 provided a seal between the plug 5 and the tube 1 which has an axial bore threaded to a platinum iridium capillary 7, whose upper end is flush with the top of the plug.

The lower end of the capillary is connected to a second peristaltic pump (not shown), and the capillaryforms a delivery pipe for the sample liquid whose flow rate is controlled by the pump. The lower end of the plug is curved so as to fit the inside surface of a focussing bowl piezo-electric resonator transducer 8. The length of the plug is chosen so that the focal point of acoustic energy provided by the transducer lies at the mouth of the upper end of the capillary 7. Typically the transducer may be secured to the plug with epoxy resin, and the assembly of transducer and plug is designed to resonate at a frequency typically lying in the range 1 to 4 MHz. To avoid the risk of the plug itself forming a source of contamination it is convenient to make it of a machinable glass ceramic such as that sold by Corning under the designation 'Ceram'.

Optionally a baffle plate 9 may be located above the capillary 7 to obstruct the central region of the cross-section of the delivery tube 1 and hence prevent entrainment of any large droplets arising from a central ultrasonic "fountain" of sample liquid generated by the transducer. Entrainment of such large droplets in the flow from the mouth of the tube is also inhibited by the swirling flow of argon introduced via pipe 2 which will tend to retain such larger droplets in the centre of the tube by a mass/gas velocity relationship.

With a greaterthan 50% transport efficiencythe delivery rate for the sample liquid into the capillary could be as low as 0.2 to 0.3 ml mix~, which can be accurately controlled by the peristaltic pump. Cleaning of the apparatus between samples may be simply achieved by increasing the flow rate to overflow conditions to the extent that the whole of the upper surface of the plug is flooded. The excess liquid is then drained off by way of exhaust pipe 4.

This gives an operating procedure similar to that used with a pneumatic nebuliser assembly. This waste pipe also serves to drain excess liquid falling back onto the plug in normal operation of the apparatus.

It will be evident that the shape of the top of the tube may need to be modified for different types of excitation source. Thus for instance, in the case of an inductively coupled plasma torch the top end of the tube will normally be required to neck down to fit around the sample introduction tube of the torch which typically will have a diameter of a few millimetres. This will still involve vertical delivery, but an atomic absorption burner head will normally require horizontal delivery, in which case the top of the aerosol introduction tube of the nebuliser assembly may incorporate a right-angle bend.

Claims

1. An ultrasonic nebuliser for the generation of an aerosol for introduction into a thermal excitation source, which nebuliser consists of a plug provided with an axial capillary formed by or connected with a sample liquid delivery pipe, and, attached to the outer end of the plug, a focussing bowl piezo-electric ultrasonic transducer configured such that its focus lies at the mouth of the inner end of the capillary.

2. A nebuliser as claimed in claim 1, wherein the plug is made of a machinable glass.

3. A nebuliser as claimed in claim 1 or 2, wherein the plug capillary is formed by a length of tubing made of platinum iridium.

4. Afocussed beam ultrasonic nebuliser substantially as hereinbefore described with reference to the accompanying drawing.

5. An ultrasonic nebuliser assembly for the generation of an aerosol for introduction into a thermal excitation source, which nebuliser has an open-ended tube provided with gas inlet and excess sample liquid outlet pipes near one end of the tube which is closed off by a plug provided with an axial capillary formed by or connected with a sample liquid delivery pipe, wherein a focussing bowl piezo-electric ultrasonic transducer is attached to the outer end of the plug such that its focus lies at the mouth of the inner end of the capillary.

6. A nebuliser assembly as claimed in claim 5, wherein the gas inlet pipe is tangentially oriented to provide a swirling gas flow within the tube.

7. A nebuliser assembly as claimed in claim 5 or 6, wherein the tube is provided with an internal baffle plate obstruction in the central region of the tube cross-section.

8. A nebuliser assembly as claimed in claim 5, 6 or 7, wherein the plug is made of a machinable glass.

9. A nebuliser assembly as claimed in claim 5, 6, 7 or 8, wherein the plug capillary is formed by a length of tubing made of platinum iridium.

10. Afocussed beam ultrasonic nebuliserassembly substantially as hereinbefore described with reference to the accompanying drawing.

11. A method of generating an aerosol for introduction into a thermal excitation source by causing liquid to emerge from a capillary whose mouth is at the focus of a beam of focussed ultrasonic energy.

12. A method as claimed in claim 11, wherein the aerosol is generated near one end of a tube, and is transported to the other end and beyond by a gas flow disposed to provide a swirling motion to the aerosol.

13. A method of generating an aerosol as claimed in claim 11 or 12 using a nebuliser as claimed in any claim of claims 1 to 4 or a nebuliser assembly as claimed in any claim of claims 5 to 10.