Abstract:
An interface member plate is provided for use in a mass spectrometer system. It includes an orifice and a desolvation chamber which can be generally cylindrical and elongate. Moreover, the desolvation chamber can be heated to encourage desolvation of any remaining solvant. The orifice largely determines flow into low pressure downstream sections containing sections of the mass spectrometer. The desolvation chamber has a larger cross-section and its characteristic parameters can be set independently of the parameters of the orifice. The interface member can be provided directly downstream from an ion source, or separating an upstream curtain gas chamber from the low pressure mass analyzing sections of the mass spectrometer.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to mass spectrometry. This invention more particularly relates to the interface between an atmospheric ion source and low pressure regions of a mass spectrometer.  
         BACKGROUND OF THE INVENTION  
         [0002]    Samples or analytes for analysis in mass spectrometers are commonly provided dissolved in a suitable solvent. This provides advantages in handling of samples, but when the sample is to be ionized for analysis in a mass spectrometer, there is the problem of removing the solvent. One common ionization technique is electrospray and its derivatives, such as nanospray, which provides a low flow. In all such techniques, a liquid sample, containing the desired analyte in a solvent, is caused to form a spray of charged droplets at the tip of an electrospray capillary. Accordingly, an important step in generating ions is to ensure proper desolvation, i.e. the evaporation and removal of the solvent from the droplets. Proper desolvation improves signal to noise ratio, signal intensity and signal stability. The electrospray source is usually coupled with some means of desolvation in an atmospheric pressure chamber where desolvation can be enhanced by heat transfer to the droplets (radiation, turbulence) or/and counter flow of dry gas.  
           [0003]    As the majority of mass spectrometers operate under reduced gas pressure (typically vacuum conditions of less than 10-4 Torr), there is the problem of transferring ions produced in the atmospheric pressure region into the low pressure chamber of the mass spectrometer, while ensuring adequate separation or rectification of ions from surrounding gas and solvent molecules (neutrals). This process can be roughly separated into three distinct regions, namely: liquid sample charging and its nebulization; transport of ions through an interface between atmospheric and low pressure chambers; and rapid expansion into the low pressure chamber. Correspondingly, the desolvation step can be performed: immediately after nebulization, for example as in Covey et al. U.S. Pat. No. 5,412,208 and Apffel, Jr. et al. U.S. Pat. No. 5,495,108; in the interface region, for example as disclosed in Allen et al. U.S. Pat. No. 5,015,845, Chowdhury et al. U.S. Pat. No. 4,977,320, Chait et al. U.S. Pat. No. 5,245,186, Henion et al. U.S. Pat. No. 4,935,624, and Bajic U.S. Pat. No. 5,756,994; or during expansion in the low pressure region, with the help of an electrostatic or RF field, for example as disclosed in conference abstract entitled “Ion transmission through a multi-capillary inlet and ion funnel interface” by Taeman Kim et al. from Abstracts of the ASMS 2000 Conference. Small translational energy is pumped by the RF field into the total ion energy between every collision preventing ions from cooling. Following collision, there is partial transfer of this energy into the internal energy of ions (clusters). If collisions are more frequent than the RF field cycle, overall effect will be cooling. If the RF field cycles several times between collisions, overall effect will be the step-by-step increase in the internal energy of ions/clusters with high probability for such ions to overcome a dissociation barrier.  
           [0004]    In U.S. Pat. No. 5,756,994, a heated entrance chamber is provided, and is pumped separately. Ions enter this chamber through an entrance orifice, and are then sampled through an exit orifice that is located in the side of the chamber, off any line representing a linear trajectory from the entrance orifice, with the intention of providing for efficient removal of neutral solvent molecules. Pressure in this heated entrance chamber is maintained around 100 Torr. To the extent that this is understood, there is independent pumping arrangement in the entrance chamber, and the shape of the chamber is not conducive to maintaining laminar flow, with the entrance orifice being much smaller than the cross-section of the main portion of the chamber itself. It is expected that significant loss of ion current to the walls of this chamber would occur in addition to obvious inefficiency of sampling from only one point of cylindrical flow (trough the exit orifice).  
           [0005]    In U.S. Pat. Nos. 4,977,320, 5,245,186 and 4,935,624, a heated tube made from conductive or non-conductive materials was used for delivering the ions/gas carrier/solvent flow into the low pressure chamber. In such a configuration, the heated tube provides two distinct and separate functions; firstly, due to its significant resistance to gas flow, the tube configuration, namely its length and diameter, adjusts the gas load on the pumping system; secondly, the tube can be heated to effect desolvation and separation of ions from neutrals. With respect to the first function, this resistance can be provided, while keeping the tube length constant, to ensure laminar gas flow in the tube and the widest possible opening for inhaling the ion/gas carrier/solvent flow. Generally, a wider bore for the tube provides increased gas flow and hence more load on the pumping system; correspondingly, reducing the tube length provides less resistance to the gas flow, so as also to increase the gas flow and load on the pumping system. These two geometric parameters, bore and length, are obviously related and can be adjusted to provide the desired flow rate and flow resistance. The second function is provided by mounting a heater around the interface tube. The heat provided to the tube promotes desolvation of the ion flow, and also helps to reduce contamination of the surface of the tube, thereby reducing memory effects. An interface of this sort is able to work only under strictly laminar flow conditions, limiting the variability of the tube length and tube bore. Additionally, the desolvation, which depends on temperature and residence time (inversely proportional to gas velocity through the tube) is related to the pumping requirements. As a rule, it is not possible to optimize all the desired parameters; in particular, it is desirable to minimize total mass flow to reduce pumping requirements, on the other hand to ensure best efficiency for transfer of ions into the mass spectrometer, a large diameter tube with high mass flow rates is desirable. However, it is generally opposite to the desolvation requirements.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with the first aspect of the present invention there is provided an interface number, for use in a mass spectrometer between an ion source operating at a relatively high pressure and a lower pressure chamber, the interface member including:  
           [0007]    An orifice, defining the minimum flow cross-section of the interface member; and a desolvation chamber having an elongate bore, in communication with the orifice, the bore having a larger cross-section than the orifice, whereby, in use, for a given pressure differential across the interface member, the orifice primarily determines the mass flow rate of gas through the interface member.  
           [0008]    In accordance with the second aspect of the present invention, there is provided a mass spectrometer system comprising:  
           [0009]    a ion source in a first chamber for operation at relatively high pressure;  
           [0010]    an interface member;  
           [0011]    at least one-second chamber maintained at a relatively low pressure and separated from the first chamber by the interface member;  
           [0012]    a mass spectrometer within said at least one second chamber; and  
           [0013]    a pump connected to said at least one-second chamber for maintaining the relatively low pressure therein;  
           [0014]    Wherein the interface member includes an orifice and an elongate desolvation chamber that are within communication with one another, with the desolvation chamber having an inlet for receiving ions from the first chamber and the orifice opening into said at least one second chamber, and wherein the desolvation chamber has a larger cross-section than the orifice, whereby, in use, for a given pressure differential between the first and second chambers, the flow rate there between is primarily determined by the cross-section of the orifice.  
           [0015]    Additionally, a third aspect of the present invention provides a method of analyzing an analyte, the method comprising:  
           [0016]    providing the analyte as a liquid sample comprising the analyte dissolved in a solvent;  
           [0017]    forming a spray of droplets of the liquid sample and promoting ionization of the analyte to form analyte ions, in a first chamber at a first pressure.  
           [0018]    passing the analyte ions through a desolvation chamber having a first cross-section;  
           [0019]    passing analyte ions through a orifice having a second cross-section smaller than the first cross-section, into at least one second chamber at a pressure lower than the first pressure; and  
           [0020]    mass analyzing the ions.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings which show preferred embodiments of the present invention and in which:  
         [0022]    [0022]FIG. 1 shows schematically a simple mass spectrometer incorporating an interface in accordance with the present invention;  
         [0023]    [0023]FIG. 2 shows a triple quadrupole mass spectrometer incorporating an interface in accordance with the present invention;  
         [0024]    [0024]FIG. 3 shows a QqTof mass spectrometer incorporating an interface according to the present invention;  
         [0025]    [0025]FIG. 4 shows a cross-section through a first embodiment of an interface in accordance with the present invention;  
         [0026]    [0026]FIG. 5 shows a cross-section through part of the interface of FIG. 4 and skimmers in accordance with the present invention;  
         [0027]    [0027]FIG. 6 shows a cross-section through a second embodiment of an interface in accordance with the present invention;  
         [0028]    [0028]FIG. 7 shows a schematic cross-section of a further embodiment of an interface in accordance with the present invention;  
         [0029]    [0029]FIG. 8 shows schematic cross-sectional view of another interface in accordance with present invention; and  
         [0030]    [0030]FIGS. 9, 10, and  11  show graphs showing variation of of ion signals under different operating conditions. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    Reference is first made to FIG. 1 which shows diagrammatically a prior art nebulizer gas spray analyzer  10  generally as shown in U.S. Pat. No. 4,861,988. The analyzer  10  includes an atmospheric pressure ionization chamber  12 , a gas curtain chamber  14 , and a vacuum chamber  16 . The ionization chamber  12  is separated from the gas curtain chamber  14  by an orifice plate  18  containing an inlet orifice  20 . The gas curtain chamber  14  is separated from the vacuum chamber  16  by an outlet plate  22  containing an orifice  24 .  
         [0032]    The vacuum chamber  16 , which is evacuated through outlet  26  by pump  28 , contains a commercially available mass analyzer  28  (for example, a tandem triple quadrupole mass spectrometer as detailed below). Ions from ionization chamber  12  are drawn through orifices  20 ,  24  and are focused by ion lens elements  26  into analyzer  30 . A detector  34  at the end of the analyzer  30  detects ions, which pass through the analyzer, and supplies a signal at terminal  36  indicative of the number of ions per second, which are detected.  
         [0033]    The liquid sample to be analyzed is typically supplied from a liquid chromatograph (LC)  38  through capillary tube  40  into chamber  12 . The flow rate of the liquid through capillary tube  40  is determined by an LC pump  42 . The portion of capillary tube  40  which enters chamber  12  is made of a conductive material and has one pole (depending on the polarity of the ions desired) of a voltage source  44  connected to it. The other pole of source  44 , and plate  18 , are grounded. A source  46  of pressurized gas (e.g. nitrogen) supplies a sheath tube  48  coaxial with and encircling capillary  40  with a high velocity nebulizing gas flow which nebulizes fluid ejected from capillary  40 . The mist of droplets  50  formed is carried toward the orifice  20  by the nebulizing flow. The droplets  50  are charged by the voltage applied to capillary  40 , and as the droplets evaporate, ions are released from them and are drawn toward and through the orifices  20 ,  24 .  
         [0034]    As is conventional, the axis  52  of capillary  40  is aimed slightly off axis, i.e. slightly below the orifice  20 . Thus, large droplets, which do not fully evaporate by the time they reach orifice  20 , simply impact against the plate  18  and run down the plate where they are collected (by means not shown). Ions released from the fine droplets, which have evaporated, are drawn through the orifices  20 ,  24  into the vacuum chamber  16 , where they are focused into the analyzer  30 . As is well known, a curtain gas (typically nitrogen) from curtain gas source  54  diffuses gently out through orifice  20 , i.e., there is a small pressure differential across the orifice plate  18 , to prevent contaminants in chamber  12  from entering the vacuum chamber  16 . Excess gas leaves chamber  12  via outlet  56 .  
         [0035]    It will be understood that while FIG. 1 shows a nebulizing gas flow including the high velocity gas flow through the sheet tube  48 , this is not essential. Various ion sources are known which do not require a nebulizing source. Electrospray emitters that operate without the use of nebulizing gas can be used as sources of ions for this interface. In particular, electrospray sources that ionize liquid flows in the nanoliter to low microliter per minute range operate with a substantial advantage with the interface of the present invention which serves as the means of transferring the ions from the emitter droplet cloud into the vacuum system. The heated chamber serves several purposes with these low flow emitters. First it confines the gas being drawn into the vacuum chamber to the region immediately surrounding the low flow electrospray emitter serving to assist the electrospray charged droplet generation process. Second the heated portion of the chamber assists droplet desolvation and ion declustering. Third the gas confining properties of the chamber guide the dispersing spray cloud toward the ion entrance aperture substantially improving the efficiency of ion transfer into the vacuum system. These combined effects result in the maintenance of a stable generation of ions from this type of emitter over a broad flow range (low nanoliter to low microliter per minute range) and widely varying solvent compositions with substantially different surface tension properties. Electrospray emitters spraying solvents composed of 100% water or 100% organic solvent generate ion currents with equal efficiency and stability when they are operated in conjunction with the interface of the present invention.  
         [0036]    Other types of ion sources operating at atmospheric pressure also may serve as sources of ions to be transferred into the vacuum system with the interface of the present invention, demonstrating its versatility. Atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), and sources that produce ions as the result of other forms of energy input into the sample such as laser desorption and the related technique of matrix assisted laser desorption ionization (MALDI) are examples some of the ionization methods that benefit from the new ion transfer device of the present invention.  
         [0037]    Also, FIG. 1 shows, schematically, a mass analyzer indicated at  30 . It will be understood by a person skilled in this art that this mass analyzer  30  can be any suitable mass analyzer or mass spectrometer, and in particular any such instrument that necessarily operates under high vacuum conditions and therefore requires and interface between the atmospheric pressure ion source and the high vacuum conditions of the mass spectrometer. Specific examples of a triple quadurople mass spectrometer and a QqTOF instrument are detailed below in relations to FIGS. 2 and 3, and the invention can also be applied to ion trap mass spectrometers, Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometers, magnetic sector mass spectrometer or 3D quadrupole ion trap mass spectrometer.  
         [0038]    In accordance with the present invention, the orifice plate  18  would be replaced by a combination of an interface plate or member in accordance with the present invention, and, an upstream curtain plate, with the curtain chamber then located upstream of the interface member. The chamber  14  would then form an intermediate pressure chamber as in FIGS. 2 and 3.  
         [0039]    Referring to FIG. 2, there is shown a conventional triple quadrupole mass spectrometer apparatus generally designated by reference  60 . An ion source  62 , for example an electrospray ion source, generates ions directed towards a curtain plate  64 . Behind the curtain plate  64 , there is an orifice plate  66 , defining an orifice, in known manner.  
         [0040]    A curtain chamber  68  is formed between the curtain plate  66  and the orifice plate  66 , and a flow of curtain gas reduces the flow of unwanted neutrals into the analyzing sections of the mass spectrometer.  
         [0041]    Following the orifice plate  66 , there is a skimmer plate  70 . An intermediate pressure chamber  72  is defined between the orifice plate  66  and the skimmer plate  20  and the pressure in this chamber is typically of the order of 2 Torr.  
         [0042]    Ions pass through the skimmer plate  70  into the first chamber of the mass spectrometer, indicated at  74 . A quadrupole rod set Q 0  is provided in this chamber  74 , for collecting and focusing ions. This chamber  74  serves to extract further remains of the solvent from the ion stream, and typically operates under a pressure of 7 mTorr. It provides interface into the analyzing sections of the mass spectrometer.  
         [0043]    A first interquad barrier or lens IQ 1  separates the chamber  74  from the main mass spectrometer chamber  76  and has an aperture for ions. Adjacent the interquad barrier IQ 1 , there is a short “stubbies” rod set, or Brubaker lens  78 .  
         [0044]    A first mass resolving quadrupole rod set Q 1  is provided in the chamber  76  for mass selection of a precursor ion. Following the rod set Q 1 , there is a collision cell of  80  containing a second quadrupole rod set Q 2 , and following the collision cell  80 , there is a third quadrupole rod set Q 3  for effecting a second mass analysis step.  
         [0045]    The final or third quadrupole rod set Q 3  is located in the main quadrupole chamber  76  and subjected to the pressure therein typically 1×10 −5  Torr. As indicated, the second quadrupole rod set Q 2  is contained within an enclosure forming the collision cell  80 , so that it can be maintained at a higher pressure; in known manner, this pressure is analyte dependent and could be 5 mTorr. Interquad barriers or lens IQ 2  and IQ 3  are provided at either end of the collision cell of  80 .  
         [0046]    Ions leaving Q 3  pass through an exit lens  82  to a detector  84 . It will be understood by those skilled in the art that the representation of FIG. 2 is schematic, and various additional elements would be provided to complete the apparatus. For example, a variety of power supplies are required for delivering AC and DC voltages to different elements of the apparatus. In addition, a pumping arrangement or scheme is required to maintain the pressures at the desired levels mentioned.  
         [0047]    As indicated, a power supply  86  is provided for supplying RF and DC resolving voltages to the first quadrupole rod set Q 1 . Similarly, a second power supply  88  is provided for supplying drive RF and auxiliary AC voltages to the third quadrupole rod set Q 3 , for scanning ions axially out of the rod set Q 3 . A collision gas is supplied, as indicated at  90 , to the collision cell  80 , for maintaining the desired pressure therein.  
         [0048]    The apparatus of FIG. 2 is based on an Applied Biosystems/MDS SCIEX API 2000 triple quadrupole mass spectrometer. In accordance with the present invention, an interface member or plate with a desolvation chamber would be substituted for the orifice plate  66  of FIG. 2.  
         [0049]    Another example of the application of the interface member of the present invention is shown in FIG. 3, which shows a QqTOF configuration, where in effect the final mass analyzer of a triple quadrupole is replaced with a Time of Flight (TOF) section. For simplicity like components in FIG. 3 are given the same reference as in FIG. 2 and the description of these components is not repeated. The Q-q-time-of-flight (TOF) tandem mass spectrometers shown in FIG. 7 (Q designating a mass analysis section and q a collision cell), is generally indicated at  92 .  
         [0050]    Precursor ions are mass selected in Q 1  and then subject to one of the collision and reaction in Q 2 , within the collision cell  80 . The resultant product ions and any remaining pressure ions are then mass analyzed in a Time of Flight (TOF) section  96 . As indicated at  94 , an RF power supply is provided for the collision cell  80 , and although not shown in FIG. 2, this power supply would be provided in the triple quadrupole configuration as well.  
         [0051]    Reference will now be made to FIGS. 4, 5 and  6 , which show different embodiments with the interface in accordance with the present invention. Each interface can replace the orifice plate variously indicated at  18  and or  66  in FIGS.  1 - 3 . It is first to be noted that, when the interface plate of the present invention is used, it is, in general, optional whether the curtain gas is provided. In general, with nano-electrospray, where there are low desolvation requirements, the gas curtain is not essential. It is more desirable to use a gas curtain, with high flow sources, where there is a greater flow of neutrals, contaminants and the like, which is desired to keep out of the mass spectrometer.  
         [0052]    Referring now to FIG. 4, this shows an interface  100 , in accordance with the present invention. The interface  100  has a generally cylindrical cross-section throughout, i.e. it is formed as a body of revolution about its central axis  102 . It includes a relatively thick outer portion  104  and a thin, plate-like central portion  106 , with only a portion  108  of intermediate thickness there between. The interface plate  100  provides the function of the orifice plate  18 .  
         [0053]    An orifice  110  is provided centrally in the inner portion  106 , on the axis  102 , and as shown, a conical face  112  is provided on the low pressure side of the interface plate  100 .  
         [0054]    On the atmospheric pressure side, a cylindrical body  114  is provided extending axially out from the orifice  110  and defining a cylindrical desolvation chamber  116 . Around the cylindrical desolvation chamber  116 , there is a heater body  118 , which is also generally cylindrical. A heater element is indicated at  120 .  
         [0055]    A sprayer for generating ions is indicated at  122 . As noted above, this can be any suitable sprayer, for example an electrospray, nanospray, etc.  
         [0056]    Here, the desolvation chamber  116  has a larger diameter than the orifice  110 , and is dimensioned to ensure that there is laminar flow.  
         [0057]    In the case where there is no curtain gas, the intermediate pressure region of the mass spectrometer is typically at a pressure of 2 Torr. Due to the high pressure differential from the atmospheric pressure chamber  126  to the low pressure chamber  124 , a supersonic flow jet is created through the orifice  110 . As is known, the mass flow of such a supersonic jet depends solely on the upstream pressure. In the present case, with the desolvation chamber  116  having a relatively large diameter compared to the orifice  110 , the gas velocity through the desolvation chamber  116  is low and there is negligible pressure drop across the length of the desolvation chamber  116 . Consequently, flow through the nozzle  110  is, effectively, determined by the exact pressure of the chamber  126  and the diameter of the orifice  110 .  
         [0058]    This gives the advantage that the properties of the orifice  110  and the desolvation chamber  116  can be separately determined and optimized. The diameter to the orifice  110  is set depending upon the acceptable pumping requirements and desired ion flow into the mass spectrometer.  
         [0059]    Contrary, for example, to U.S. Pat. Nos. 4,977,320, 5,245,186 and 4,935,624, increasing the diameter of the desolvation chamber  116  will not increase the mass flow; instead, as the mass flow is determined by the orifice  110 , increasing the diameter of the desolvation chamber  116  will reduce the flow velocity through chamber  116 . It is generally favorable for the desolvation process to increase the residence time of ions in the desolvation chamber. It can be noted that as the gas is heated in the desolvation chamber  116 , this will have the effect of reducing the gas density, for a given pressure, which theoretically will reduce the mass flow rate of gas into the low pressure chamber  124 . The effect of this is expected to be small.  
         [0060]    Thus, the interface functions are separated, as compared to earlier proposals, and this enabled the orifice function, determining the mass flow rate, and the desolvation function to be configured separately.  
         [0061]    It can be shown that the power P required to heat gas (disregarding the droplet population) in the cylindrical desolvation chamber  116  from a temperature T to a temperature To is independent from the geometry (length and bore) of the desolvation chamber  116 , and it is determined by:  
           P=T   c   A   o   c   p ( T   o   −T )  
         [0062]    Where T c  is the continuum mass flux per unit area in the orifice  110 , A o  is the orifice (nozzle, sample) area, c p  is specific heat at constant pressure. For a nitrogen curtain gas flow, the orifice diameter was 0.25 mm and T o =500K we have P=1.9W.  
         [0063]    The geometry is particularly important for keeping laminar flow. Below a Reynolds number of 2300 one is fairly certain that the flow is laminar. For a desolvation chamber  116  of 1 cm length and 1 mm bore the nitrogen flow speed inside the desolvation chamber is equal to 10.2 m/s, residence time 0.98 ms, and the Reynolds number is 676, where the Reynolds number is determined from the tube (desolvation chamber) diameter.  
         [0064]    Similarly, the desolvation chamber  116  can be designed for a larger interface orifice  128 , which is appropriate for a multi stage interface as depicted in FIG. 5. In this embodiment, chambers  130 ,  132  between the orifice  128  and a skimmer  134 , and between the skimmer  134  and reducer  136  are pumped separately allowing significant increase in orifice size. For a 0.6 mm orifice, a curtain gas flow of nitrogen, and To=500K, we have P=10.6W. The desolvation chamber is here indicated at  138  and is 2 cm long and has a 3 mm bore on diameter, giving a flow speed inside the desolvation chamber  138  equal to 6.3 m/s, a residence time of 1.6 ms, and a Reynolds number of 1258. Therefore, the geometry of the desolvation chamber is separated from the gas load on the pumping system of mass spectrometer and can be selected based on different considerations. For example, compatibility of an electrospray needle, Taylor cone, or size of nebulized droplet flow with the chamber bore. It can be done with the purpose to “inhale” the entire droplet population from the ion source into the desolvation chamber. To prevent contamination and turbulent losses the Reynolds number should be below  2300 . Turbulent flow promotes diffusion of ions to wall of chamber and hence deposition of ions on the chamber walls. Also, turbulent flow is inherently unstable and affects gas dynamics. As an additional benefit, keeping the desolvation chamber bore much bigger than a needle, used for generating electrospray etc., relaxes requirements for precise positioning of interface parts.  
         [0065]    A further embodiment of the invention is disclosed in FIG. 6. This embodiment includes different materials for the desolvation chamber. Otherwise, for simplicity and brevity, like components in FIG. 6 are given the same reference as in earlier Figures.  
         [0066]    In FIG. 6, the desolvation chamber is indicated at  150 , and is made from resistive ceramic which is held in place be insulating ceramic insert  152 . In this case an axial electric field between entrance of the desolvation chamber and interface orifice can be created, which should assist in ion drift through the chamber bore. This is achieved by providing a potential between the ends of the ceramic insert  152 .  
         [0067]    Reference is made to FIG. 7, which shows a further embodiment to the present invention, generally designated by the reference  160 . This shows just a curtain plate at  162  and a nozzle or orifice plate  164 , defining a curtain gas chamber  166 . It will be understood that these elements would be introduced to a mass analyzer of FIG. 1, 2, or  3  in known manner, or into any other suitable mass analyzer. The curtain plate  162  includes a relatively large opening or orifice  163 , while the orifice plate  164  includes an orifice  165 .  
         [0068]    In accordance with the present invention, a desolvation chamber  168  is provided, mounted on the inner surface of the curtain gas plate  162 . The desolvation chamber  168  has a bore  190  having a diameter less than that of the opening orifice  163  while being significantly larger than the orifice  165 . As shown, a small spacing or gap  172  is left between the desolvation chamber  168  and the orifice plate  164 . This enables curtain gas to flow from the chamber  166  as indicated by the arrows  174  through the bore  170  into an upstream atmospheric pressure chamber  178  containing the ion source (not shown). Curtain gas also flows as indicated by arrows  176  through the orifice  165  into lower pressure downstream chambers of a mass spectrometer.  
         [0069]    The ion beam is indicated at  180 . Consequently, curtain gas flows countercurrent to the ion beam  180  through the desolvation chamber  168 , and in the same direction as the ion beam through the orifice  165 .  
         [0070]    Reference will now be made to FIG. 8, which shows a further embodiment, comparable to that of FIG. 7. For simplicity and brevity, like components are given the same reference numeral in FIG. 8 and the description of these components is not repeated.  
         [0071]    Here, the interface arrangement as a whole is indicated at  190 , and the desolvation chamber is provided as first and second desolvation chambers  192  and  194 . The desolvation chamber  192  is mounted on the curtain plate  162 , comparable to the arrangement in FIG. 7. The second desolvation chamber  194  is mounted to the orifice plate  164 . The desolvation chambers  192 ,  194  have respective bores  193 ,  195  that are axially aligned and, as before, are aligned with the orifices  163 , 165 . A gap  196  is left between the first and second desolvation chambers  192 , 194 .  
         [0072]    As in FIG. 7, this enables curtain gas to flow both countercurrent and co-current with the ion beam again indicated at  180 . Arrows  174  again indicates the flow of curtain gas countercurrent to the ion beam  180 , through the bore  193  of the desolvation chamber  192  into the upstream ion source chamber  178 . Additionally, the curtain gas flows in the same direction as the ion beam  180  as indicated by arrows  176 , through the second desolvation chamber  194  and the orifice  185  into downstream low pressure chambers.  
         [0073]    It is to be appreciated that various configurations of the desolvation chambers are possible. Thus, while the desolvation chambers  168  and  192  in FIGS. 7 and 8 are shown as mounted in the curtain gas chamber, it is conceivable that they could be mounted on the other side of the curtain gas plate  162 , i.e., in the atmospheric pressure chamber  178 . Alternatively, such a desolvation chamber could be mounted extending through the curtain gas plate, so that part of it is in the chamber  178  and another part of it in chamber  166 .  
         [0074]    Referring to FIGS. 9, 10, and  11 , they show test results obtained utilizing the interface chamber of FIG. 4. These graphs show the measurement utilizing a standard solution generated by using an electrospray type source. The ion intensity was measured in the mass range 35 to 800, and as shown, is indicated in counts per second, as measured over an interval of approximately 5 minutes.  
         [0075]    [0075]FIG. 9 shows the result obtained when a curtain gas of nitrogen was applied and heat was also applied to the desolvation chamber. This shows a steady, uniform signal at around 2.0×10 8  counts per second. FIG. 10 shows the results obtained when heat was provided but no curtain gas was supplied. While the initial signal is slightly stronger, this falls off significantly, and it can be noted that the signal level shows much greater variation in noise. Finally, FIG. 11 shows the results obtained when no heat and no curtain gas were supplied. As can be seen the signal fluctuates significantly, and the average intensity was reduced. These figures clearly show that providing both a heated desolvation chamber and a curtain gas provides a steady, constant signal level.  
         [0076]    While the desolvation chamber has been described as being cylindrical, it is to be appreciated that further configurations are possible. For example, the desolvation chamber need not necessarily have a cylindrical cross-section. It is possible that both the desolvation chamber and the orifice could be, to at least some extent, slit-shaped or elongate in cross-section. Additionally, it is possible that the cross-section of the desolvation chamber could taper downwards from the inlet to the orifice. This should enable more available ions to be inhaled into the desolvation chamber. In such a case, it is expected that care would be need to be taken to ensure that the gas flow is not accelerated excessively as the cross-section of the desolvation chamber narrows or reduces since such acceleration may promote unwanted turbulence.