Patent Publication Number: US-6989529-B2

Title: Plasma torch

Description:
TECHNICAL FIELD 
   This invention relates to a plasma torch for spectrochemical analysis, for example for producing an inductively coupled plasma or a microwave induced plasma. 
   BACKGROUND OF THE INVENTION 
   A common requirement in spectrochemical analysis with inductively coupled or microwave induced plasma torches is to analyse liquid samples having relatively high concentrations of dissolved solids. In extreme cases the concentrations of such samples might approach the saturation point of one of the dissolved components. Liquid samples are usually introduced a plasma torch as aerosols produced by a nebulizer. Attempts to analyse highly concentrated samples can result in salts coming out of solution as the nebulised aerosol is generated and as it is transported through the torch. Salt particles are often deposited in the nebulizer or in the torch, ultimately causing blockages that require the analysis to be stopped and the apparatus to be disassembled and cleaned. This wastes valuable time. Nebulisers that are resistant to obstruction by salts are known in the art, but obstruction of the plasma torch has continued to cause difficulty. 
   An object of the present invention is to provide a torch for producing a plasma for use in spectrochemical analysis that is resistant to obstruction by salts deposited from samples containing high levels of dissolved solids. 
   SUMMARY OF THE INVENTION 
   The invention provides a torch for producing a plasma for use in spectrochemical analysis including 
   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 at least a substantial portion of its length such that its cross-sectional area gradually and smoothly reduces towards its outlet along said at least a substantial portion of its length. 
   As the aerosol travels from the inlet of the tube of the torch (typically 4 to 8 mm ID) to the outlet of the tube (typically 0.8–3 mm ID) the velocity of the aerosol increases. This change in velocity (and associated changes in pressure) ought to occur as smoothly as possible, such that turbulence and pressure changes are minimized. The invention meets the stated object by providing a gradual reduction in cross-sectional area between the inlet and the outlet and by providing a smooth transition between the said tapered portion and any parallel portions at each end. The taper may be constant (that is, linear), exponential or of any other shape that can provide an appropriately smooth pathway of decreasing cross-sectional area, blending smoothly into any parallel portions. Such a shape is produced naturally when glass or quartz tube is heated and stretched to produce the taper. If the tube were to be manufactured from ceramic material (either by machining and sintering or moulding), or by shrinking quartz onto a mandrel, care would need to be taken in machining, or in the design and construction of the mould or mandrel, to avoid sharp or abrupt transitions between the tapered portion and any adjacent parallel portions. The length of the tapered portion may be at least five times the internal diameter of the inlet of the tube and is advantageously from five to ten times the internal diameter of the inlet of the tube. Such a design eliminates relatively sudden transitions in the cross sectional area of the tube of the plasma torch, as have been provided in prior art torches, at which salts are preferentially prone to deposit. 
   Any turbulence and changes in pressure that may be associated with the transition between a parallel portion of tube at the outlet end and the said tapered portion should occur at a location sufficiently remote from the plasma to be largely unaffected by the heat radiated from the plasma or conducted down the tube from the plasma. This can be achieved in the invention by ensuring that the final change in internal diameter occurs at a distance of 40 to 50 mm from the outlet end of the tube. The known relatively sudden transition in cross-sectional area of a tube in a prior art torch is typically located near the outlet end of the tube and is thus nearby the heat of the plasma. The invention reduces the influence of this factor given the tapered portion provides a gradual and smooth transition between the inlet and outlet cross sections and also that it may be located as remotely as possible from the heat of the plasma, for example, the tapered portion may commence at the inlet. 
   For a better understanding of the invention and to show how it may be carried into effect, preferred embodiments thereof will now be described, by way of non-limiting example, with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically shows a typical prior art inductively coupled plasma torch. 
       FIG. 2  illustrates a known modification for a tube of the torch of  FIG. 1  for conveying a flow of a gas carrying sample aerosol. 
       FIG. 3  illustrates an embodiment of a tube for conveying a flow of a gas carrying sample aerosol for a torch according to the invention. 
       FIG. 4  illustrates a second embodiment of a tube for a torch according to the invention. 
       FIG. 5  schematically shows a torch according to an embodiment of the invention. 
       FIG. 6  is a graph illustrating spectrochemical analysis results from a plasma sustained by a plasma torch according to an embodiment of the invention. 
       FIG. 7  is a graph illustrating performance of a plasma torch according to an embodiment of the invention in comparison to a prior art plasma torch. 
   

   DETAILED DESCRIPTION 
   A typical inductively coupled plasma torch  9  is shown in  FIG. 1 . It consists of three concentric tubes,  10 ,  11  and  13 , usually made out of fused quartz.  FIG. 1  shows a torch  9  in which the three tubes are permanently fused together, but it is known in the art to provide a mechanical arrangement whereby the three tubes  10 ,  11  and  13  are held in their required positions and wherein one or more of the tubes  10 ,  11  and  13  can be removed and replaced. Such an arrangement is called a demountable torch. 
   Tube  13  is the outermost of the three tubes. Tube  11  is the intermediate tube and may be provided with a portion  12  of larger diameter, which may in some designs extend over the entire length of tube  11 . The purpose of portion  12  is to provide a narrow annular gap between tubes  11  and  13  for the passage of a plasma forming gas (typically argon) that is supplied though a gas inlet  15 . The narrow gap imparts a desirably high velocity to the gas. Radiofrequency induction coil  16  is supplied with radiofrequency current from a power supply (not shown). Plasma  17  is initiated by momentarily applying a high-voltage spark (by means known in the art and not shown) to the gas entering through gas inlet  15 . Plasma  17  is sustained by inductive coupling of the radiofrequency electromagnetic field generated by coil  16  with plasma  17 , as is known in the art. A small flow of gas is supplied to tube  11 - 12  through gas inlet  14 . This serves to keep plasma  17  at an appropriate distance from the nearby ends  19  of tubes  11 – 12  and  10 , so that the ends  19  of tubes  11 - 12  and  10  do not overheat. 
   For a microwave induced plasma instead of an inductively coupled plasma, the coil  16  would not be present and the torch  9  would be suitably associated with means for applying a microwave electromagnetic field to the torch  9 , for example the torch  9  may be appropriately located through a resonant cavity to which the microwave energy is supplied. 
   A flow of gas carrying sample aerosol (not shown) for analysis is introduced by known means (not shown) into the end of tube  10  remote from the plasma (i.e. the tube&#39;s inlet  33 ). The aerosol-laden gas emerges from the other end (i.e. the outlet  35 ) of tube  10  adjacent to plasma  17  with sufficient velocity to pass through plasma  17 . The passage through plasma  17  of gas and aerosol emerging from tube  10  forms a central channel  18  in plasma  17 . Aerosol droplets passing from the outlet  35  of tube  10  into central channel  18  are progressively dried, melted, and vaporised by the heat of plasma  17 . The vaporised sample is subsequently converted to atoms and ions by the heat of plasma  17 , and these atoms and ions are excited to emit radiation by the heat of plasma  17 . Radiation emitted by excited atoms and ions can be used for spectrochemical analysis by optical emission spectrometry, as is known in the art. Furthermore, ions in central channel  18  can be used for analysis by mass spectrometry, as is also known in the art. 
   In order that aerosol emerging from outlet  35  of tube  10  may effectively penetrate plasma  17  and form central channel  18 , it is known to provide a narrow parallel-walled path through at least a portion of tube  10  adjacent to its outlet  35 , so that the flow therethrough is substantially laminar. In  FIG. 1  such a narrow parallel-walled path is shown extending the entire length of tube  10 . It is also known, however, that such a long, narrow passage or capillary is readily obstructed by salts deposited from the aerosol when aerosols generated from samples containing high levels of dissolved solids are introduced into tube  10 . Accordingly, it is also known to provide a tube as shown in  FIG. 2 , which has a wide parallel-sided portion  21 , extending for a substantial length of the tube from the end through which the aerosol enters, and a narrow, parallel-sided short portion  22 , extending from the end through which the aerosol outlet ends. Portions  21  and  22  are joined by a short tapered portion  23 . A tube  10  according to  FIG. 2  is more resistant to blockage by deposited salts than is a tube  10  as shown in  FIG. 1 , in which the narrow parallel-side portion extends the entire length of tube  10 . None the less, a torch equipped with a tube  10  according to  FIG. 2  is still subject to obstruction by deposited salts. The deposition of salts is particularly evident in the short tapered portion  23 . 
   Having observed that salt deposited preferentially in the short tapered portion  23 , the inventor decided to make the tapered portion as remote as possible from the heat of the plasma (to reduce the temperature of that portion) and also to make the transition of flow into the narrow parallel-walled portion as gradual as possible. This led to the design of a tube  25  as shown in  FIG. 3 , which has a tapered portion  27  of greatly increased length compared to section  23  of tube  10  of  FIG. 2 . The tube  25  is substantially constantly tapered along at least a substantial portion of its length such that its cross-sectional area gradually and smoothly reduces between is inlet  37  and its outlet  39  along said at least a substantial portion of its length Tube  25  includes a narrow parallel sided portion  29  similar to portion  22  of tube  10  of  FIG. 2 . It is probable, but not yet experimentally verified, that the tapered portion  27  could extend over the entire length of tube  25 , the taper at the outlet end  39  approximating the narrow parallel-sided portion  29 , such that the flow of sample aerosol laden gas within tube  25  at outlet  39  is substantially laminar. 
     FIG. 4  shows an embodiment of a tube  25  of the invention wherein aerosol is introduced into tube  25  through a smoothly curved tube  31  that is continuous with tapered portion  27 . Curved tube  31  is advantageous for the particular spectrometer in which the invention was tested. In another spectrometer, for example one in which the torch is mounted vertically, curved tube  31  might not be required. 
     FIG. 5  shows an inductively coupled plasma torch  40  equipped with a central tube  25  according to the invention. Components of this torch that are the same as in torch  9  of  FIG. 1  have been accorded the same reference numeral. Note that a torch  40  according to the invention in its broadest form involves provision of the three tubes  25 ,  11  and  13  without the radio frequency induction coil  16 . 
   To assess the performance of the tube  25  of the invention, tests were carried out in which a solution containing 250 grams of sodium chloride per liter was nebulised continuously into either an operating prior art torch having a tube  10  according to  FIG. 2  or into an operating torch having a tube  25  according to this invention (as in  FIG. 4 ). The sample introduction system and the operating conditions of the inductively coupled plasma were the same in each of the tests and were typical of those that would be used in normal operation of an inductively coupled plasma atomic emission spectrometer. The prior art torch was blocked within 30 minutes, but the torch  40  according to the invention was still operational after 24 hours. Observation of the blocking process revealed that salt built up continuously in the prior art torch, leading to blockage. In the torch according to the invention, salt was deposited in the curved portion  31  and in the wide part of the tapering portion  27  of tube  25  just after curved portion  31 . This deposited salt was in the form of a fine granular material, some of which was from time to time blown right through tube  25  and into central channel  18 . A torch according to the invention is thus to some extent self-clearing. It is emphasised that continuous introduction of sample is a very severe test. In real analytical practice, the system would be rinsed between samples by aspirating a blank solution and this would prolong the useful analytical time considerably. 
   Example dimensions for a tube  25  are: length approximately 90 mm, inlet diameter 5 mm, outlet diameter 2.3 mm, length of tapered portion approximately 40 mm extending to approximately 45 mm from the outlet. 
   Further test results are illustrated in  FIG. 6  which shows a plot of reported test element concentration versus time for a solution containing 1 milligram of each of three test elements (barium, zinc and magnesium) and 250 grams of sodium chloride per liter of solution. The solution was introduced continuously into an inductively coupled plasma sustained in a torch having a tube  25  (as in  FIG. 4 ) according to the invention while the concentration of each test element was monitored on the basis of the measured intensity of the emission lines indicated in the Figure (that is, Ba 455.403 nm, Zn 206.200 nm, Mg 280.270 nm, Mg 285.213 nm). These results show that the torch was operating satisfactorily after 24 hours&#39; continuous operation. 
     FIG. 7  shows a plot of reported manganese concentration versus time for a solution containing one milligram of manganese and 250 grams of sodium chloride per liter of solution. The solution was introduced continuously into an inductively coupled plasma sustained (a) in a prior art torch and then (b) in a torch having a tube  25  (as in  FIG. 4 ) according to the invention while the concentration of manganese was monitored on the basis of the measured intensity of the 257.610 nm emission line. The same sample introduction system was used with each torch. The prior art torch blocked after two hours, but the torch according to the invention was still operating satisfactorily after 8 hours&#39; continuous operation. 
   Table 1 shows the detection limit for a range of elements in dilute nitric acid solution (a) in a prior art torch and (b) in a torch according to the invention. This solution is easily handled by the prior art torch, yielding state-of-the-art detection limits. Similar detection limits were obtained with the torch according to the invention. 
   
     
       
         
             
             
           
             
                 
               TABLE 1 
             
           
          
             
                 
                 
             
             
                 
               Detection Limit, micrograms/liter 
             
          
         
         
             
             
             
             
          
             
                 
                 
                 
               (b) torch 
             
             
                 
                 
                 
               according to the 
             
             
               Element 
               Wavelength (nm) 
               (a) prior art torch 
               invention 
             
             
                 
             
          
         
         
             
             
             
             
          
             
               Al 
               167.019 
               0.30 
               0.40 
             
             
               As 
               188.980 
               3.4 
               3.6 
             
             
               Ba 
               455.403 
               0.12 
               0.13 
             
             
               Be 
               234.861 
               0.05 
               0.05 
             
             
               Ca 
               396.847 
               0.03 
               0.03 
             
             
               Cd 
               214.439 
               0.17 
               0.14 
             
             
               Cu 
               327.396 
               0.67 
               0.64 
             
             
               Fe 
               238.204 
               0.24 
               0.29 
             
             
               Mg 
               279.553 
               0.016 
               0.013 
             
             
               Mn 
               257.610 
               0.05 
               0.057 
             
             
               Mo 
               202.032 
               0.58 
               0.56 
             
             
               Ni 
               231.604 
               0.88 
               1.0 
             
             
               Pb 
               220.353 
               2.0 
               2.3 
             
             
               Se 
               196.026 
               4.9 
               6.4 
             
             
               Zn 
               213.857 
               0.14 
               0.15 
             
             
                 
             
          
         
       
     
   
   Table 2 shows the detection limit for a range of elements in a solution of 250 grams of sodium chloride per liter in dilute nitric acid, measured with a torch according to the invention. This solution rapidly blocks the prior-art torch ( FIG. 7 ), but a torch according to the invention yields stable signals for prolonged periods ( FIG. 6 ) and also provides satisfactorily low detection limits. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
                 
                 
               Detection Limit, 
             
             
                 
               Element 
               Wavelength (nm) 
               micrograms/liter 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
                 
               Al 
               167.019 
               1.5 
             
             
                 
               As 
               188.980 
               7.5 
             
             
                 
               Ba 
               455.403 
               0.40 
             
             
                 
               Be 
               234.861 
               0.30 
             
             
                 
               Ca 
               396.847 
               0.20 
             
             
                 
               Cd 
               214.439 
               0.90 
             
             
                 
               Co 
               238.892 
               2.0 
             
             
                 
               Cr 
               267.716 
               1.0 
             
             
                 
               Cu 
               327.396 
               3.0 
             
             
                 
               Fe 
               259.94 
               6.0 
             
             
                 
               Mg 
               279.553 
               0.20 
             
             
                 
               Mn 
               257.610 
               0.20 
             
             
                 
               Pb 
               220.353 
               11 
             
             
                 
               Ti 
               334.941 
               0.90 
             
             
                 
               V 
               292.401 
               1.5 
             
             
                 
               Zn 
               213.857 
               1.0 
             
             
                 
                 
             
          
         
       
     
   
   The discussion herein of the background to the invention and what is known is included to explain the context of the invention. This is not to be taken as an admission that any of the matters referred to were part of the common general knowledge in Australia as at the priority date of this application. 
   The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions that fall within the scope of the following claims.