A torch for producing an inductively coupled or microwave induced plasma for use in spectrochemical analysis. The torch includes a central tube ( 25 ) for conveying a flow of a gas carrying sample aerosol to a plasma ( 17 ) produced in the torch. The tube ( 25 ) has an inlet ( 31 ) and an outlet ( 39 ) of smaller size than the inlet and is shaped to deliver a substantially laminar flow of the gas at the outlet ( 39 ). The tube ( 25 ) is tapered along at least a substantial portion of its length such that its cross-section area gradually and smoothly reduces towards its outlet ( 39 ) along at least a substantial portion of its length. It has been found that such a tapered tube is resistant to obstruction by salts deposited from samples containing high levels of dissolved solids.
1. A torch for producing a plasma for use in spectrochemical analysis comprising:
a tube for conveying a flow of a gas carrying sample aerosol to a plasma produced in the torch by an electromagnetic field, the tube having an inlet and an outlet of smaller size than the inlet, and being shaped to deliver a substantially laminar flow of the gas at the outlet, wherein the tube is tapered along its length for a distance that is at least five times the internal diameter of the inlet of the tube such that its cross-sectional area gradually and smoothly reduces towards its outlet along at least such portion of its length.
2. A torch as claimed in claim 1 , wherein the tube is tapered along its length for a distance that is from five to ten times the internal diameter of the inlet of the tube.
3. A torch as claimed in claim 1 , wherein the tube comprises a parallel walled portion extending to the outlet, wherein the tapered portion of the tube smoothly blends into the parallel walled portion.
4. A torch as claimed in claim 1 , wherein the tapered portion of the tube commences at the inlet.
5. A torch as claimed in claim 1 , wherein the tube is tapered along substantially is whole length.
6. A torch as claimed in claim 1 , wherein the tube comprises:
an inlet portion that is smoothly curved through an angle of about 90° and smoothly bends into the tapered portion.
7. A torch as claimed in claim 1 , wherein the taper of the tapered portion of the tube is constant.
8. A torch as claimed in claim 1 , wherein the tube is concentrically located within a second tube and the second tube is concentrically located within a third tube, the third tube including an inlet for supplying a flow of a plasma forming gas to the annular outlet formed between the second and third tubes, the second tube also including inlet for supplying a flow of gas to the annular outlet formed between the first defined tube and the second tube for keeping a plasma away from the outlet ends of the first defined tube the second tube.