WO2017181223A1

WO2017181223A1 - Atomic emission spectrometer

Info

Publication number: WO2017181223A1
Application number: PCT/AU2017/050336
Authority: WO
Inventors: Peter John Saunders
Original assignee: Gbc Scientific Equipment Pty. Ltd.
Priority date: 2016-04-18
Filing date: 2017-04-13
Publication date: 2017-10-26

Abstract

An optical instrument and method, the instrument comprising: a light source (12); a first polychromator (86) having a first entrance aperture (90) and configured to detect light admitted by the first entrance aperture (90); a second polychromator (88) having a second 5 entrance aperture (98) and configured to detect light admitted by the second entrance aperture (98); and optical elements (16) arranged to receive a first light beam (64) emitted by the light source (12) in a first direction and to receive a second light beam (70) emitted by the light source (12) in a second direction. The optical elements (16) are configured or controllable to direct the first light beam (64) to the first entrance aperture (90) and the 10 second light beam (70) to the second entrance aperture (98).

Description

Atomic Emission Spectrometer

Related Application

This application is based on and claims the benefit of the filing and priority dates of Australian patent application no. 2016901440 filed 18 April 2016, the content of which as filed is incorporated herein by reference in its entirety.

Field of the Invention

The present invention relates to an ICP (inductively coupled plasma) atomic emission spectrometer or other optical instrument, of particular but by no means exclusive use for performing simultaneous inductively coupled plasma optical spectroscopy.

Background of the Invention

An inductively coupled plasma atomic emission spectrometer is an instrument that is used in analytical chemistry to measure the concentration of elements in a solution. The analytical technique is known as Inductively Coupled Plasma - Atomic Emission Spectroscopy (ICP- AES).

In ICP-AES, the sample containing the elements to be measured is injected (typically sprayed) as a solution into the centre of an argon plasma contained inside a plasma torch, as described in— for example— History of inductively coupled plasma atomic emission spectral analysis: from the beginning up to its coupling with mass spectrometry (Knut Oh Is and Bernhard Bogdain, J. Anal. At. Spectrom, 31 (2016) 22-31). The sample is thereby intensely heated, which results in atomic line emission from the sample. This emission is detected using an optical spectrometer a device in which the light is dispersed or separated in wavelength so the intensity of each of the atomic emission lines can be measured using optical detectors.

Existing instruments have two principal components, a light source and a detection apparatus. The light source includes the parts involved in the creation of the plasma and the generation of the atomic emission; the detection apparatus includes a mechanism for dispersing the light from the light source according to wavelength and a detector for detecting the atomic emission. The light source and the detection apparatus are connected by entry optics comprising an arrangement of optical elements that conduct the atomic emission from the light source to the detection apparatus.

Early instruments employed a system of entry optics that detected the emission coming from the side of the plasma torch (the so-called 'radial view'). Later instruments were developed that detected emission from the top of the torch looking down the centre of the torch (the so- called 'axial view'). Each view has both advantages and disadvantages when it comes to measuring the atomic emission from the plasma. Certain subsequent instruments combined these two views by various techniques. For example, some instruments rotated the torch to successfully obtain the two different views, but this approach has the disadvantage that the analytical work has to stop while the torch is rotated. Other instruments use a system of adjustable mirrors to switch between these views. In these systems also, only one view at a time can be measured and the mirrors have to be moved between measurements. U.S. patent application publication no. 2013/0286390 discloses an optical emission system including dichroic beam combiner system, that includes a detection system with a single entrance aperture for admitting the emission light from the light source. Light beams are emitted from the light source in two directions

(corresponding to the radial view and the axial view respectively) and are received by the dichroic beam combiner, which reflects portions of the radial beam and transmits portions of the axial beam (transmission and reflection having different wavelength ranges) into the single entrance aperture, hence allowing the beams to be analyzed simultaneously but with considerable complexity of manufacture.

U.S. patent no. 5,483,337 discloses an atomic emission spectrometer. A first mirror on the longitudinal axis of the plasma generator receives axial radiation, and a second mirror disposed laterally from the plasma generator reflects radial radiation towards a third mirror. The third mirror passes the radiation to a fourth mirror positioned adjacent to the axial radiation and reflects the radial radiation to the first mirror. The first mirror is rotated to a first orientation to reflect the axial radiation into a detector system, or to a second orientation to reflect the radial radiation into the detector system. Summary of the Invention

According to a first broad aspect of the invention, there is provided an optical instrument, comprising:

a light source;

a first polychromator having a first entrance aperture and configured to detect light admitted by the first entrance aperture;

a second polychromator having a second entrance aperture and configured to detect light admitted by the second entrance aperture; and

optical elements arranged to receive a first light beam emitted by the light source in a first direction and to receive a second light beam emitted by the light source in a second direction;

wherein the optical elements are configured or controllable to direct the first light beam to the first entrance aperture and the second light beam to the second entrance aperture.

Thus, the optical instrument— which may be, for example, an optical emission

spectrometer— allows the simultaneous measurement of two different views of the light source.

I n an embodiment, the optical elements are controllable to direct the first light beam to the second entrance aperture and the second light beam to the first entrance aperture. In one example, the spectrometer comprises a movable mirror controllable to move between a first position and a second position , wherein when the movable mirror is in the first position the optical elements direct the first light beam to the first entrance aperture and the second light beam to the second entrance aperture, and when the movable mirror is in the second position the optical elements direct the first light beam to the second entrance aperture and the second light beam to the first entrance aperture. I n another embodiment, the light source comprises a plasma torch adapted to generate a plasma, the first light beam comprises light emitted in an axial direction by the plasma and the second light beam comprises light emitted in a radial or lateral direction by the plasma.

Thus, the spectrometer allows a simultaneous axial view in the ultraviolet part of the spectrum and a radial view in the visible part of the spectrum, or vice versa.

It will be appreciated by those skilled in the art that the axial direction relative to a plasma generated by a plasma torch is aligned with the direction in which the jet of plasma (such as a jet of ionized argon gas) emerges from the plasma torch. This is typically upwardly, so an axial view is typically downwards. Likewise, it will be appreciated that the radial or lateral direction relative to a plasma generated by a plasma torch is the direction generally perpendicular to the axial direction; this is typically horizontally, so a radial or lateral view is typically a horizontal view (though it may be noted that there are instruments in which the torch is horizontal) .

I n one embodiment, the first polychromator is adapted for detection of a first wavelength range of light and the second polychromator is adapted for detection of a second wavelength range of light that is different from the first wavelength range of light. In an example, the first wavelength range of light is principally or entirely ultraviolet light, and the second wavelength range of light is principally or entirely visible light.

I n an embodiment, the optical elements include a rotatably mounted mirror in a path of the first or second light beam, the rotatably mounted mirror being controllable to control the reception of the respective first or second light beam from different portions of the light source.

I n another embodiment, the light source comprises a plasma torch adapted to generate a plasma, the first light beam comprises light emitted in an axial direction by the plasma and the second light beam comprises light emitted in a radial or lateral direction by the plasma, and wherein the optical elements include a rotatably mounted mirror in a path of the second light beam, the rotatably mounted mirror being controllable to control the reception of the second light beam from different radial portions of the plasma.

The optical instrument may be an optical emission spectrometer or an inductively coupled plasma-atomic emission spectrometer.

According to a second broad aspect of the invention , there is provided a light detection apparatus, comprising :

a first polychromator having a first entrance aperture, a first diffraction grating, a first inner sector, and a first detector disposed at least in part on the first inner sector, the first detector being configured to detect light admitted by the first entrance aperture and diffracted by the first diffraction grating; and

a second polychromator having a second entrance aperture, a second diffraction grating , a second inner sector, and a second detector disposed at least in part on the second inner sector, the second detector being configured to detect light admitted by the second entrance aperture and diffracted by the second diffraction grating ;

wherein the first inner sector is integral with the second inner sector.

I n an embodiment, the first polychromator further comprises a first outer sector and the second polychromator further comprises a second outer sector, wherein the first detector is disposed in part on the first inner sector and in part on the first outer sector, and the second detector is disposed in part on the second inner sector and in part on the second outer sector.

I n another embodiment, the first detector comprises a plurality of CCD sensors, and the second detector comprises a plurality of CCD sensors. I n a further embodiment, the first polychromator is adapted for detection of a first wavelength range of light and the second polychromator is adapted for detection of a second wavelength range of light that is different from the first wavelength range of light.

The first wavelength range of light may be principally or entirely ultraviolet light, and the second wavelength range of light principally or entirely visible light.

According to a third broad aspect of the invention, there is provided a method of performing optical analysis, the method comprising:

receiving with optical elements a first light beam emitted by a light source in a first direction and a second light beam emitted by the light source in a second direction ; and directing with the optical elements the first light beam to a first entrance aperture of a first polychromator configured to detect light admitted by the first entrance aperture and the second light beam to a second entrance aperture of a second polychromator configured to detect light admitted by the second entrance aperture.

I n an embodiment, the method comprises controlling the optical elements to direct the first light beam to the second entrance aperture and the second light beam to the first entrance aperture. I n an example, the method comprises controlling a movable mirror to move between a first position and a second position, wherein when the movable mirror is in the first position the optical elements direct the first light beam to the first entrance aperture and the second light beam to the second entrance aperture, and when the movable mirror is in the second position the optical elements direct the first light beam to the second entrance aperture and the second light beam to the first entrance aperture.

I n another embodiment, the light source comprises a plasma and the method includes generating a plasma with the plasma torch , the first light beam comprising light emitted in an axial direction by the plasma and the second light beam comprising light emitted in a radial or lateral direction by the plasma. In an example, the optical elements include a rotatably mounted mirror in a path of the second light beam, the method including rotating the rotatably mounted mirror to control the reception of the second light beam from different radial portions of the plasma. I n a further embodiment, the first polychromator is adapted for detection of a first wavelength range of light and the second polychromator is adapted for detection of a second wavelength range of light that is different from the first wavelength range of light. For example, the first wavelength range of light may be principally or entirely ultraviolet light, and the second wavelength range of light principally or entirely visible light.

In a certain embodiment, the optical elements include a rotatably mounted mirror in a path of the first or second light beam, the method including rotating the rotatably mounted mirror to control the reception of the respective first or second light beam from different portions of the light source.

The method may comprise optical emission spectrometry or inductively coupled plasma- atomic emission spectrometry.

It should be noted that any of the various individual features of each of the above aspects of the invention, and any of the various individual features of the embodiments described herein including in the claims, can be combined as suitable and desired. Brief Description of the Drawing

In order that the invention may be more clearly ascertained, embodiments will now be described, by way of example, with reference to the accompanying drawing, in which:

Figure 1 is a schematic view of an ICP atomic emission spectrometer according to an embodiment of the present invention;

Figure 2 is a more detailed schematic view of the spectrometer of figure 1 according to an embodiment of the present invention;

Figure 3 is a schematic view of the spectrometer of figure 1 according to an embodiment of the present invention, with its swing away mirror either moved out of the optical path or omitted; and

Figure 4 is a schematic view of the ultraviolet polychromator of the spectrometer of figure 1 according to an embodiment of the present invention.

Detailed Description of the Invention

Figure 1 is a schematic view of an ICP atomic emission spectrometer 10 according to an embodiment of the present invention. ICP atomic emission spectrometer 10 is adapted to perform simultaneous ICP atomic emission spectrometry, as is explained in detail below. Spectrometer 10 includes a light source 12, a detection apparatus 14 and entry optics 16 for coupling light from light source 12 to detection apparatus 14. Light source 12 includes a regulated argon supply 18, mass flow controllers 20, a plasma gas line 22, an auxiliary gas line 24, a plasma torch 26 and an induction coil 27 located around plasma torch 26. Plasma gas line 22 and auxiliary gas line 24 conduct argon from argon supply 18 to plasma torch 26. Light source 12 also includes a sample receptacle 28, a peristaltic pump 30, a nebulizer 32, a sample gas line 34 and a spray chamber 36 coupled to the base of plasma torch 26. The sample is conducted from sample receptacle 28 to peristaltic pump 30 by a first sample line 38, and from peristaltic pump 30 to nebulizer 32 by a second sample line 40. The sample— once nebulized by nebulizer 32— is mixed with argon in spray chamber 36, which injects the mixture into plasma torch 26.

Light source 12 further includes an RF generator and power supply 42, signal processing and control electronics 44 and a host computer 46. RF generator and power supply 42 provide the required RF signal to induction coil 47 around plasma torch 26. Signal processing and control electronics 44 are electrically coupled to and control mass flow controllers 20 and peristaltic pump 30, under the control of host computer 46.

Laterally propagating light 50 and axially propagating light 52 emitted by a plasma 48 generated by plasma torch 26 (that is, to the left and upwards, respectively in the view of figure 1 ) is collected by entry optics 16 and separately directed by entry optics 16 to detection apparatus 14. Laterally propagating light 50 and axially propagating light 52 are admitted to detection apparatus 14 through separate entrance slits or apertures (not shown) of detection apparatus 14, as is described in detail below. Detection apparatus 14 includes two dispersing elements for respectively dispersing the laterally propagating light 50 and the axially propagating light 52 according to wavelength, and detector elements (not shown) for detecting the light, also as described in detail below. Detection apparatus 14 outputs signals in response to that detection , which are transmitted to host computer 46— by a suitable wired or wireless transmission mechanism (not shown)— for storage and analysis. Figure 2 is a more detailed schematic view of spectrometer 10, and shows various functional elements of the spectrometer. Referring to figure 2, spectrometer 1 0 includes plasma torch 26, a flushing tube 62 arranged to receive a radial light beam 64 from a plasma generated by plasma torch 26, providing a radial or lateral view, and a first MgF₂ window 66 for allowing radial light beam 64 to exit the plasma chamber. Spectrometer 10 also includes a water cooled cone 68 positioned above (in this view) the plasma generated by plasma torch

26, through the centre of which is transmitted an axial light beam 70 from the plasma, providing an axial view, and a second MgF₂ window 72 for allowing axial light beam 70 to exit the plasma chamber. The entry optics 16 of spectrometer 10 include an adjustable radial periscope mirror 74 for receiving radial light beam 64 upon its exiting first MgF₂ window 66 and controllably directing radial light beam 64 optically downstream, an optional swing away mirror 76 for receiving radial light beam 64 from radial periscope mirror 74, a first fixed mirror 78 for receiving radial light beam 64 reflected from swing away mirror 76, and a second fixed mirror 80 for receiving radial light beam 64 reflected from first fixed mirror 78 and directing radial light beam 64 towards detection apparatus 14. Radial periscope mirror 74 is adjustable by rotation about an axis that is horizontal in the view of figure 2. This allows the portion of the plasma from which light is collected for measurement in the radial view to be adjusted, within the limits imposed by the height of the entrance aperture of flushing tube 62. The entry optics of spectrometer 1 0 also include an axial periscope mirror 82 for receiving axial light beam 70 upon its exiting second MgF₂ window 72 and controllably directing axial light beam 70 towards optional swing away mirror 76, and a third fixed mirror 84 for receiving axial light beam 70 reflected from swing away mirror 76 and directing axial light beam 70 towards the detection apparatus.

Detection apparatus 14 of spectrometer 10 includes an ultraviolet polychromator 86 for measurement of (generally) ultraviolet light, and a visible polychromator 88 for measurement of (generally) visible light. In broad terms, ultraviolet polychromator 86 is depicted on the near side of spectrometer 1 0 in the view of figure 2, while visible polychromator 88 is depicted on the far side of spectrometer 10 in the view in figure 2.

Ultraviolet polychromator 86 includes an aperture in the form of an entrance aperture 90, a diffraction grating 92 and a detector comprising a plurality of CCD line cameras 94a, 94b (comprising a total in this embodiment of 21 CCD sensors) arranged in a circular arc. Light admitted by entrance aperture 90 is diffracted off diffraction grating 92 and then focused onto CCD line cameras 94a, 94b by respective parabolic or cylindrical mirrors (not shown). These mirrors reflect the light by approximately 90 degrees, thereby focussing incident light in one direction so as to reduce the height of the line image to the respective CCD.

Alternate mirrors point in opposite directions.

When CCD sensors are arranged adjacent to one another, there are gaps between the light- sensitive portions of the CCD sensors. Accordingly, CCD line cameras 94a, 94b are arranged on two sectors: an inner sector 96a (shown in partial cutaway view) on which are mounted CCD line camera 94a, and an outer sector 96b on which is mounted CCD line camera 94b. Respective parabolic mirrors point towards the respective CCD sensors mounted on inner and outer sectors 96a, 96b. The parabolic mirrors are the same length as the light-sensitive portions of the respective CCD sensors and are arranged as closely as possible to one another, so the effect of the dead areas between the light-sensitive portions of adjacent CCD sensors is minimized or eliminated.

Similarly, visible polychromator 88 includes an entrance aperture 98 (behind a third MgF₂ window 100) , a diffraction grating 1 02 and a detector comprising a plurality of CCD line cameras 1 04a, 104b (comprising a total in this embodiment of 18 CCD sensors) arranged in a circular arc.

First, second and third MgF₂ windows 66, 72, 100 are provided because oxygen in the air absorbs ultra violet light below 1 80 nm in wavelength. To enable ultraviolet polychromator 86 to measure below this wavelength , oxygen has to be excluded from the light path. This is done either by evacuating the air or by purging with pure nitrogen or argon (which do not absorb ultraviolet light). The windows hence seal off purged/evacuated areas and allow the light to enter. The purge gas exits the optics via the end of flushing tube 62 so that oxygen is eliminated at the gap across to the plasma. In the axial view, the argon plasma impinges on cone 68 and oxygen is excluded from the axial light path. Visible polychromator 88 does not require purging so third MgF₂ window 1 00 seals off the purged area.

Light admitted by entrance aperture 98 is diffracted off diffraction grating 102 is focused onto CCD line cameras by respective parabolic mirrors (not shown) in a manner similar to that employed by ultraviolet polychromator 86. Figure 2 depicts a first or inner CCD line camera 1 04 mounted on an inner sector (not shown, but essentially the rear of inner sector 96a of ultraviolet polychromator 86); the second or outer CCD line camera of visible polychromator 88 is not shown (being obscured in this view by inner sector 96a of ultraviolet polychromator 86), but is mounted on outer sector 1 06 of visible polychromator 88. The parabolic mirrors of visible polychromator 88 are the same length as the light-sensitive portions of the respective CCD sensors and are arranged as closely as possible to one another, to minimize or eliminate the effect of the dead areas between the light-sensitive portions of adjacent CCD sensors. The CCD line cameras of ultraviolet polychromator 86 are selected to be most suitable for the detection of ultraviolet light, while the CCD line cameras of visible polychromator 88 are selected to be most suitable for the detection of visible light.

Thus, in order to measure light in the ultraviolet range in an axial view and light in the visible range in a radial view, swing away mirror 76 is located— in this embodiment— at 45° to the paths of radial light beam 64 and axial light beam 70, as shown in the view of figure 2. In this configuration , second beam 70 (i.e. the axial view) is reflected off the top side (in this view) of swing away mirror 76 and directed towards ultraviolet polychromator 86; radial light beam 64 (i.e. the radial view) is reflected from the bottom side (in this view) of swing away mirror 76 and directed towards visible polychromator 88. Ultraviolet polychromator 86 and visible polychromator 88 can thus operate simultaneously, allowing for the faster analysis of the sample. This arrangement also has the advantage that the higher sensitivity axial view is directed to ultraviolet polychromator 86, the ultraviolet being the wavelength range in which most emission measurements are made.

I n order to measure light in the ultraviolet range in a radial view, swing away mirror 76 is moved out of the path of radial and axial light beams 64, 70 (or, in another embodiment, omitted from spectrometer 10) . This configuration of spectrometer 1 0 is shown at 10' in figure 3, in which swing away mirror 76 has also been omitted for clarity. Radial light beam 64 passes via flushing tube 62, first MgF₂ window 66, radial periscope mirror 74 and third fixed mirror 84 onto entrance aperture 90 of ultraviolet polychromator 86. Likewise, light in the visible range can then be viewed in an axial view, if desired: with swing away mirror 76 out of the path of radial and axial light beams 64, 70, axial light beam 70 passes via the central hole of water cooled cone 68, second MgF₂ window 72, axial periscope mirror 82, first fixed mirror 78, second fixed mirror 80 and third MgF₂ window 100 onto entrance aperture 98 of visible polychromator 88. As mentioned above, however, swing away mirror 76 is optional; it may be omitted, in which case the spectrometer will exclusively measure light in the ultraviolet range in a radial view and light in the visible range in an axial view. Such an spectrometer would also be suitably illustrated by spectrometer 10' of figure 3. I n another embodiment, swing away mirror 76 may be replaced by a fixed mirror, in which case the spectrometer will operate as shown in figure 2, and will exclusively measure light in the ultraviolet range in an axial view and light in the visible range in a radial view.

Figure 4 is a further schematic view of ultraviolet polychromator 86, illustrating the operation of entrance aperture 90, diffraction grating 92 and exemplary CCD sensors 1 10a, 1 1 0b,

1 10c. Figure 4 depicts the admittance to ultraviolet polychromator 86 of axial light beam 70, but it will be appreciated that, if swing away mirror 76 is swung out of the paths of radial light beam 64 and axial light beam 70 or omitted, radial light beam 64 will be admitted to ultraviolet polychromator 86.

Axial light beam 70, after being admitted to polychromator 86 by entrance aperture 90, impinges diffraction grating 92 and is dispersed by diffraction grating 92 according to wavelength. Figure 4 schematically illustrates three exemplary, diffracted components 1 12, 1 14, 1 16 of axial light beam 70, that with longest wavelength 1 12 being detected by CCD sensor 1 10a, that with an intermediate wavelength 1 14 being detected by CCD sensor 1 1 0b, and that with shortest wavelength 1 16 being detected by CCD sensor 1 1 0c. Visible polychromator 88 operates in like manner.

Modifications within the scope of the invention may be readily effected by those skilled in the art. It is to be understood , therefore, that this invention is not limited to the particular embodiments described by way of example hereinabove.

I n the claims that follow and in the preceding description of the invention, except where the context requires otherwise owing to express language or necessary implication , the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, that is, to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Further, any reference herein to prior art is not intended to imply that such prior art forms or formed a part of the common general knowledge in any country.

Claims

CLAIMS:

1 . An optical instrument, comprising:

a light source;

2. An optical instrument as claimed in claim 1 , wherein the optical elements are controllable to direct the first light beam to the second entrance aperture and the second light beam to the first entrance aperture.

3. An optical instrument as claimed in claim 2, comprising a movable mirror controllable to move between a first position and a second position, wherein when the movable mirror is in the first position the optical elements direct the first light beam to the first entrance aperture and the second light beam to the second entrance aperture, and when the movable mirror is in the second position the optical elements direct the first light beam to the second entrance aperture and the second light beam to the first entrance aperture.

4. An optical instrument as claimed in claim 1 , wherein the light source comprises a plasma torch adapted to generate a plasma, the first light beam comprises light emitted in an axial direction by the plasma and the second light beam comprises light emitted in a radial or lateral direction by the plasma.

5. An optical instrument as claimed in claim 4, wherein the optical elements include a rotatably mounted mirror in a path of the second light beam, the rotatably mounted mirror being controllable to control the reception of the second light beam from different radial portions of the plasma.

6. An optical instrument as claimed in any one of claims 1 to 5, wherein:

i) the first polychromator is adapted for detection of a first wavelength range of light and the second polychromator is adapted for detection of a second wavelength range of light that is different from the first wavelength range of light; or

ii) the first polychromator is adapted for detection of a first wavelength range of light comprising principally or entirely ultraviolet light, and the second polychromator is adapted for detection of a second wavelength range of light comprising principally or entirely visible light.

7. An optical instrument as claimed in claim 1 , wherein the optical elements include a rotatably mounted mirror in a path of the first or second light beam, the rotatably mounted mirror being controllable to control the reception of the respective first or second light beam from different portions of the light source.

8. An optical instrument as claimed in any one of claims 1 to 7, wherein the optical instrument is an optical emission spectrometer or an inductively coupled plasma-atomic emission spectrometer.

9. A light detection apparatus, comprising:

wherein the first inner sector is integral with the second inner sector.

1 0. A light detection apparatus as claimed in claim 9, wherein the first polychromator further comprises a first outer sector and the second polychromator further comprises a second outer sector, wherein the first detector is disposed in part on the first inner sector and in part on the first outer sector, and the second detector is disposed in part on the second inner sector and in part on the second outer sector.

1 1 . A light detection apparatus as claimed in either claim 9 or 1 0, wherein the first detector comprises a plurality of CCD sensors, and the second detector comprises a plurality of CCD sensors.

12. An optical instrument as claimed in any one of claims 9 to 1 1 , wherein :

1 3. A method of performing optical analysis, the method comprising :

14. A method as claimed in claim 13, comprising controlling the optical elements to direct the first light beam to the second entrance aperture and the second light beam to the first entrance aperture.

1 5. A method as claimed in claim 14, comprising controlling a movable mirror to move between a first position and a second position, wherein when the movable mirror is in the first position the optical elements direct the first light beam to the first entrance aperture and the second light beam to the second entrance aperture, and when the movable mirror is in the second position the optical elements direct the first light beam to the second entrance aperture and the second light beam to the first entrance aperture.

16. A method as claimed in claim 13, wherein the light source comprises a plasma torch and the method includes generating a plasma with the plasma torch, the first light beam comprising light emitted in an axial direction by the plasma and the second light beam comprising light emitted in a radial or lateral direction by the plasma.

1 7. A method as claimed in claim 16, wherein the optical elements include a rotatably mounted mirror in a path of the second light beam, the method including rotating the rotatably mounted mirror to control the reception of the second light beam from different radial portions of the plasma.

1 8. A method as claimed in any one of claims 13 to 17, wherein:

1 9. A method as claimed in any one of claims 13 to 18, wherein the optical elements include a rotatably mounted mirror in a path of the first or second light beam, the method including rotating the rotatably mounted mirror to control the reception of the respective first or second light beam from different portions of the light source.

20. A method as claimed in any one of claims 13 to 19, comprising optical emission spectrometry or inductively coupled plasma-atomic emission spectrometry.