Abstract:
A spectroscopy system for spectro-chemical analysis of a sample includes a plasma torch ( 50 ) for generating a microwave induced plasma ( 90 ) as a spectroscopic source. The plasma forming gas is nitrogen which can contain an oxygen impurity. Thus the system includes a nitrogen generator ( 70 ) which is preferably supplied with compressed atmospheric air from a compressor ( 75 ) for oxygen to be removed from the air by adsorption. The invention allows the use of an on-site nitrogen gas generator and thus gives cost savings because the need to obtain supplies of bottled high purity gas is eliminated.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a spectroscopy system having a gas supply for sustaining a plasma as a part of the system. The system is for spectro chemical elemental analysis of a sample. 
       BACKGROUND 
       [0002]    The following discussion of the background to the invention is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was, in Australia, published, known or part of the common general knowledge as at the priority date established by the present application. 
         [0003]    All spectroscopic based elemental analysers (except those that use X-ray techniques) require a gas supply, for example acetylene and nitrous oxide for flame atomic absorption spectroscopy (FAAS), or argon for inductively coupled plasma (ICP) emission or mass spectrometry. For a microwave induced plasma spectroscopic source, a preferred plasma forming gas is nitrogen, as disclosed in the applicant&#39;s International application No. PCT/AU01/00805 (WO 02/04930 A1) at pages 9-10 (which has been granted as U.S. Pat. No. 6,683,272 B2). 
         [0004]    Providing a gas supply for spectroscopy systems is typically very expensive, for example the annual gas supply costs can amount to as much as the initial purchase price of a spectroscopy instrument and possibly be even higher if the gas has to be supplied to a remote location. A significant factor in this cost is the high purity customarily required of the supplied gas. For example, commercially supplied nitrogen is typically specified as containing less than 0.1% by volume of oxygen and argon together. 
       DISCLOSURE OF THE INVENTION 
       [0005]    The present invention provides a spectroscopy system including a torch for generating a microwave induced plasma as a spectroscopic source, a generator for generating a supply of nitrogen gas, the generator being connected to the torch for supplying the nitrogen gas for sustaining the plasma, wherein the generator generates the nitrogen gas from atmospheric air. 
         [0006]    A microwave induced plasma source for spectroscopy can operate satisfactorily, and in certain conditions give improved performance for the spectroscopy system, on substantially nitrogen which contains some oxygen as the plasma sustaining gas. This means that the gas supply can be provided by a nitrogen generator located at the site of the spectroscopy instrument for which the gas input is atmospheric air (that is, the air at the location of the generator). For example, it is possible to create a nitrogen enriched gas supply from atmospheric air that is compressed at the location of the spectroscopy instrument by use of a gas-selective filtration membrane, or by pressure-swing adsorption of oxygen by use of a suitable sorbent such as a carbon molecular sieve. Such a generator can supply nitrogen containing typically between 0.1% and 5% by volume residual oxygen and only small or trace amounts of the rarer atmospheric gases such as argon, CO 2  etc. A microwave induced plasma spectroscopy system according to the invention can operate satisfactory on such a nitrogen supply. 
         [0007]    Thus the invention allows for the use of an on-site nitrogen gas generator and this gives significant cost savings because the need to obtain supplies of bottled high purity gas is eliminated as is the cost of transportation of the bottled gas supplies to the location of the spectroscopy instrument. In remote locations, at sites where access is difficult, or in countries with a low level of infrastructure, the cost savings could be so large as to make the difference between being able to operate the spectroscopy instrument and not being able to operate it. 
         [0008]    Preferably the generated nitrogen for the invention contains between about 0.1% to about 3.0% by volume of oxygen. More preferably the nitrogen contains between about 0.1% to about 2.0% by volume of oxygen. Even more preferably the nitrogen contains between about 0.5% to 1.5% by volume of oxygen. 
         [0009]    It is thought that an improved sensitivity of a spectroscopy system according to the invention increases from an oxygen content of the nitrogen of about 0.1% by volume and is maximised when the nitrogen contains between about 1% to 2% by volume of oxygen and then decreases for concentrations greater than about 2% by volume of oxygen. Further experiments are being conducted to determine these ranges for the oxygen content. 
         [0010]    Preferably the generator of the invention is one which operates by adsorption of oxygen from an air supply. 
         [0011]    For a better understanding of the invention and to show how the same may be performed, a preferred embodiment thereof will now be disclosed by way of non-limiting example only, with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  schematically illustrates a spectroscopy system according to an embodiment of the invention. 
           [0013]      FIG. 2  schematically illustrates a nitrogen generator for use in the system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Tests were undertaken to determine the sensitivity of a microwave induced plasma spectrochemical system to nitrogen purity. These tests measured the signal level in A/D counts received in 1 second for a 1 mg/L solution of the element of interest. Representative results are shown in the tables below. The spectroscopy instrument was optimised differently for the two sets of data, but the set-up was unchanged within each set of results. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Counts with pure 
                 Counts with oxygen- 
               
               
                   
                   
                 nitrogen (from 
                 depleted air (principally 
               
               
                   
                   
                 evaporation of 
                 nitrogen with ~1.5% 
               
               
                   
                 Element 
                 liquid nitrogen) 
                 oxygen) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Aluminium 
                 105492 
                 103315 
               
               
                   
                 Arsenic 
                 7665 
                 12570 
               
               
                   
                 Cadmium 
                 68427 
                 88959 
               
               
                   
                 Cobalt 
                 23636 
                 38009 
               
               
                   
                 Chromium 
                 74641 
                 75860 
               
               
                   
                 Copper 
                 221123 
                 342675 
               
               
                   
                 Manganese 
                 58920 
                 75524 
               
               
                   
                 Molybdenum 
                 74310 
                 79514 
               
               
                   
                 Nickel 
                 47786 
                 70486 
               
               
                   
                 Lead 
                 11160 
                 13039 
               
               
                   
                 Strontium 
                 1352067 
                 1447932 
               
               
                   
                 Zinc 
                 36386 
                 51618 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Counts with pure 
                 Counts with oxygen- 
               
               
                   
                   
                 nitrogen (from 
                 depleted air (principally 
               
               
                   
                   
                 evaporation of 
                 nitrogen with ~2.5% 
               
               
                   
                 Element 
                 liquid nitrogen) 
                 oxygen) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Aluminium 
                 97125 
                 125676 
               
               
                   
                 Arsenic 
                 28513 
                 21716 
               
               
                   
                 Cadmium 
                 174468 
                 195209 
               
               
                   
                 Cobalt 
                 15028 
                 35807 
               
               
                   
                 Chromium 
                 65815 
                 66955 
               
               
                   
                 Copper 
                 139458 
                 353904 
               
               
                   
                 Manganese 
                 116046 
                 99425 
               
               
                   
                 Molybdenum 
                 36292 
                 70762 
               
               
                   
                 Nickel 
                 26250 
                 63983 
               
               
                   
                 Lead 
                 12688 
                 11772 
               
               
                   
                 Strontium 
                 1249902 
                 945604 
               
               
                   
                 Zinc 
                 94081 
                 93208 
               
               
                   
                   
               
             
          
         
       
     
         [0015]    As the results in the above two tables show, a small amount of residual oxygen (up to 2-3%) in the gas supplied to the plasma is beneficial and actually improves the sensitivity. This improvement is also reflected in the detection limits obtainable. These improvements may be modest, but they are certainly worthwhile. 
         [0016]    In the spectroscopy system schematically illustrated by  FIG. 1 , a representative portion of a liquid analytical sample  5  is pumped through a probe  6  to sample transfer tube  7  by a pump  10  and passes into an aerosol generating device  15 . Many suitable aerosol generating devices are known in the art. In the example shown the aerosol generating device  15  is a pneumatic nebulizer supplied with nitrogen at an appropriate pressure (50-500 kPa gauge, typically 120-250 kPa gauge) controlled by a pressure regulator  20 . Aerosol generating device  15  converts the liquid taken as described from analytical sample  5  into an aerosol (not shown) in a spray chamber  25  within which larger aerosol droplets settle out and are drained away through a drain tube  30  by a second pump  35  to a waste outlet  40 . The aerosol consisting of fine droplets suspended in nitrogen passes through an aerosol transfer tube  45  to an injector tube  46  of a plasma torch  50 . 
         [0017]    The above described arrangement illustrates merely a preferred way in which an analytical sample can be converted into a form suitable for introduction into a plasma torch for spectrochemical analysis. Many other arrangements are known in the art and are widely used in conjunction with other types of spectrochemical plasmas, such as the inductively coupled plasma. Any such sample introduction arrangements may be substituted for the arrangement just described. 
         [0018]    Plasma torch  50  is supplied with two gas flows through inlets  55  and  56  from a manifold  60 . The flow required through inlet  55  is less than that required through inlet  56 . A restrictor  57  is placed between inlet  55  and manifold  60  to achieve the required flow. A pressure regulator  65  provides constant gas pressure in manifold  60 . Details of torches suitable for microwave induced plasmas that may be used for torch  50  are described in the applicant&#39;s International applications Nos. PCT/AU01/00805 (WO 02/04930 A1—at pp 11-12) and PCT/AU03/00615 (WO 03/098980 A1). 
         [0019]    According to an embodiment of the invention nitrogen is supplied to manifold  60  and to pressure regulators  20  and  65  from a nitrogen generator  70 , which is supplied with compressed atmospheric air from an air compressor  75 . 
         [0020]    Plasma torch  50  is located in a microwave cavity  80 , which is provided with microwave power by a microwave power supply  85 . A plasma  90  is generated in torch  50  by the action of microwaves in the microwave cavity  80 . Details of cavity  80  and its use to generate a nitrogen plasma for spectrochemical analysis are described in the above mentioned U.S. Pat. No. 6,683,272 B2 and in the applicant&#39;s International application No. PCT/AU02/01142 (WO 03/069964 A1). 
         [0021]    The plasma  90  is viewed through an optical interface  95  by an optical spectrometer  100  for spectrochemical analysis. Optical interface  95  is protected from plasma  90  by an air curtain  105  generated by passing air through a nozzle arrangement  110 . Air is provided to the nozzle arrangement  110  from an air compressor  75  via an air line  115 . A pressure regulator  120  is provided in line  115  to provide an appropriate flow of air through the nozzle arrangement  110 . 
         [0022]    The optical interface  95  and optical spectrometer  100  can be replaced by any one of several types of mass spectrometer as known in the art, and in such circumstances an air curtain  105  is not required. Details of interfacing a plasma to a mass spectrometer for spectrochemical analysis are known in the art. 
         [0023]    An electronic control and data processing system  125  is provided to control the operation of the system and to collect and process the data generated by spectrometer  100 . 
         [0024]    An embodiment of a nitrogen generator  70  is schematically illustrated by  FIG. 2 . In this generator  70 , atmospheric air from an air compressor (not shown) passes through an air filter  205  into a first manifold  210  provided with flow restrictors  215  and  220  and solenoid valves  225 ,  230 ,  235  and  240 . 
         [0025]    Flow restrictors  215  and  220  can be implemented as a single flow restrictor (not shown) between filter  205  and manifold  210 . For simplicity of exposition of the operation of the apparatus it will be assumed that the flow is controlled by solenoid valves  225 ,  230 ,  235  and  240  as shown in  FIG. 2 , but it is to be understood that the set of individual valves  225 ,  230 ,  235  and  240  can be replaced by any appropriate set of valves providing the same functionality as known to those skilled in the art. 
         [0026]    Initially solenoid valve  225  is open, solenoid valve  230  is closed, solenoid valve  235  is closed and solenoid valve  240  is open. The switching of solenoid valves  225 ,  230 ,  235  and  240  is carried out by an electronic control device  300 . Air from the first manifold  210  flows through valve  225  into a first pressure vessel  245 , having a volume of for example 11 litres, which is packed with an appropriate adsorbent medium  250 , such as a carbon molecular sieve. A suitable carbon molecular sieve is CMS-190 manufactured by the China Yancheng Baode Chemical Co Ltd, Baota Town, Yancheng, Jiangsu, China. As air flows at high pressure (˜530 kPa) over the adsorbent medium  250  in pressure vessel  245  oxygen is selectively adsorbed by adsorbent medium  250  and the air is progressively depleted of oxygen. The air depleted of oxygen passes from pressure vessel  245  into a second manifold  255 . A small fraction of the air in the second manifold  255  passes through a flow restrictor  260  into a second pressure vessel  246  having a volume of, for example, 11 litres, that is also packed with adsorbent material  250 . The second pressure vessel  246  is vented to atmosphere through a waste outlet  295  via the open solenoid valve  240 , so the pressure in the second pressure vessel  246  is much lower than that in the first pressure vessel  245 . On its way through the second pressure vessel  246  air from the flow restrictor  260  sweeps adsorbed oxygen from the adsorbent medium  250  and passes through solenoid valve  240  to a muffler  290  and exits through the waste outlet  295 . The major portion of oxygen-depleted air from the second manifold  255  passes through a first one-way valve  265  into a nitrogen reservoir  275 . 
         [0027]    After a pre-determined period of time (typically one minute) the states of solenoid valves  225 ,  230 ,  235  and  240  are switched by electronic control device  300  so that solenoid valve  225  is closed, solenoid valve  230  is open, solenoid valve  235  is open and solenoid valve  240  is closed. The air from the first manifold  210  now flows at high pressure (˜530 kPa) through the valve  235  into the second pressure vessel  246 . As air flows at high pressure over the adsorbent medium  250  in the second pressure vessel  246  oxygen is selectively adsorbed by adsorbent medium  250  and the air is progressively depleted of oxygen. The air depleted of oxygen passes from the second pressure vessel  246  into the second manifold  255 . A small fraction of the air in the second manifold  255  passes through the flow restrictor  260  into the first pressure vessel  245 . The pressure in the first pressure vessel  245  is now much lower than that in second pressure vessel  246  because valve  230  is open to the waste outlet  295 . On its way through first pressure vessel  245  air from flow restrictor  260  sweeps adsorbed oxygen from the adsorbent medium  250  in the first pressure vessel  245  and passes through solenoid valve  230  to muffler  290  and exits through the waste outlet  295 . The major portion of oxygen-depleted air from the second manifold  255  passes through a second one-way valve  270  into the nitrogen reservoir  275 . 
         [0028]    After a pre-determined period of time (typically one minute) the states of solenoid valves  225 ,  230 ,  235 , and  240  are switched again by electronic control device  300  and the cycle repeats. After several cycles the oxygen-depleted air in the nitrogen reservoir  275  contains less than about 5% by vol oxygen and consists predominantly of nitrogen. This gas can be withdrawn through an outlet  280  via appropriate gas pressure regulating and flow control means (not shown). 
         [0029]    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 which fall within the scope of the following claims.