Abstract:
An ion mobility spectrometer having an ion source for generating ions; an ion detector for recording ions, and a number of substantially flat diaphragm electrodes arranged substantially perpendicular to a straight system axis that passes through the apertures in said diaphragms, with the diaphragms being arranged in a series of cells with each cell including an entrances and an exit diaphragm and a short region in between. The exit diaphragm of one cell is identical to the entrance diaphragm of the next cell, and the cells of said ion mobility spectrometer are grouped into three parts: an ion-beam forming region, an ion analyzing region, and a decelerating ion gate.

Description:
RELATED APPLICATION 
     This application is the U.S. national stage application of International (PCT) Patent Application Serial No. PCT/US2012/029227, filed Mar. 15, 2012, the entire disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     Aspects of the present invention relate to low-pressure and high-pressure ion mobility spectrometers. 
     2. Related Art 
     Ionized large molecules are analyzed in mass spectrometers and in ion mobility spectrometers. In related art, mass spectrometers molecule ions are analyzed by determining their deflections in electromagnetic fields to determine their molecule weight, which is approximately proportional to the volume of a molecule under investigation. Ion mobility spectrometers molecule ions are analyzed by determining their velocities, v=K*E, when they are dragged through a buffer gas by an electric field “E” and so their mobilities “K” are approximately proportional to their cross sections. 
     Ion mobility spectrometers require that the molecule ions to be investigated are entered as short clouds. What are the measured are then the times these clouds need to pass through the length of an ion mobility spectrometer, as is disclosed in G. A. Eiceman and Z. Karpas in “Ion Mobility Spectrometry” 2. ed. Boca Raton, Fla., 2005. What are very important in such ion mobility spectrometers are the used ion gates that form these ion clouds from a continuous ion beam. Such ion gates are for instance disclosed in: A. M. Thyndal, C. F. Powel Proc. Royal Soc. of London 129 (809), (1930) 162 and N. E. Bradbury, R. A. Nielsen, Phys. Rev. 49 (5), (1936) 388. Both of these ion gates consist of harp-like grids placed perpendicular to the incoming ion beam that allow passage of ions only during short time intervals during which the wires of these grids are all at the same potential. At all other times, no ions can pass since different potentials are applied to neighboring wires, in which case the ions are attracted to one of these wires and are so kept from propagating forward in said ion mobility spectrometer. 
     Related art investigations of molecules have become important in applications for environmental, biological, medical, and pharmacological problems. These related art techniques allow characterization of a molecule not by weight as in a mass spectrometer but by cross section, and thus, by structure since the cross section of a long molecule is certainly bigger when it is stretched out as when it is coiled up. Such characterizations are especially important for the investigation of molecule fragments into which a large molecule breaks up when it absorbs energy, for example, from collisions with buffer gas molecules or atoms. 
     SUMMARY OF THE INVENTION 
     An exemplary, non-limiting embodiment of an ion mobility spectrometer, that includes a “decelerating ion gate,” comprises at least one ion source, the ion mobility spectrometer, and at least one ion detector, wherein the ion mobility spectrometer comprises an arrangement of substantially flat diaphragms arranged substantially perpendicular to a straight system axis that passes through circular, elliptical or polygonal apertures of the diaphragms. In this ion mobility spectrometer properly chosen potentials are applied to the diaphragms establishing electric fields along the system axis that push ions, that were extracted from at least one ion source, to at least one ion detector, with this direction being called the forward direction. Such an electrode arrangement can be understood as a series of cells with each cell comprising an entrance diaphragm and an exit diaphragm and with the exit diaphragm of one cell being identical to the entrance diaphragm of the downstream neighboring cell. 
     The electrode arrangement of the ion mobility spectrometer that includes at least one decelerating ion gate is divided into three regions:
         an ion-beam forming region, in which the lateral envelope of a continuous ion beam originating from at least one ion source is shaped by static electric forward fields along the system axis that in most cases differ from one cell to the next   a decelerating ion gate according to the present invention that comprises at least two cells, i.e. an initial cell A of length l A , and a final cell B of length l B , in which decelerating ion gate time-varied electric forward fields along the system axis divide the continuous ion beam into short ion clouds   an ion analyzing region in which the ion clouds are moved to at least one ion detector that determines arrival times of the ion clouds and thus the mobilities of the ions contained in these clouds       

     In the ion-beam forming region and in the ion analyzing region the electric forward fields along the system axis are substantially static and have in the n th  cell a magnitude E n ≧E H  with the magnitude of E H  being chosen so that ions of interest of mobility K 0  would move forward with a velocity v H =K 0 E H  of about several meters per second. In the decelerating ion gate, that comprises the cell A and the cell B, the electric forward fields along the system axis are varied over time in three consecutive periods T 1 , T 2 , T 3  whose durations are chosen so in the chosen fields ions of a range of mobilities K 0 ±ΔK can all pass through the decelerating ion gate.
         1. During a first time period of duration T 1  the ions must move from the ion-beam forming region into cell A of the decelerating ion gate where they are decelerated and thus form a short and dense cloud of ions. This is achieved by choosing the potentials of the entrance and exit diaphragms of cell A so that a low field E A,1 ≦E H /10 is established along the system axis in cell A, causing ions of mobilities K 0 ±ΔK to be slowed down to a velocity v A,1 =(K 0 ±ΔK)E A,1  when they enter cell A from the last cell of the ion-beam forming region where they moved forward with a velocity V H =(K 0 ±ΔK)E H  in the high electric forward field ≧E H  along the system axis. In order that not even the fastest ions of mobility K 0 +ΔK have yet reached the exit diaphragm of cell A before the end of T 1 , it is necessary that the duration of T 1  is chosen to be ≦l A /[(K 0 ±ΔK)E A,1 ].   2. During a second time period of duration T 2  the ion cloud is to be pushed out of cell A and into cell B and compressed to a shorter ion cloud. This is achieved by choosing the potentials of the entrance and exit diaphragms of cells A and B so that a high electric forward field E A,2 ≧E H  is established along the system axis in cell A and a low electric forward field E B,2 ≦E H /10 along the system axis in cell B, causing ions of mobilities K 0 ±ΔK to be moved out of cell A with velocities v A,2 =(K 0 ±ΔK)E A,2  and into cell B where they are slowed down to velocities V B,2 =(K 0 ±ΔK)E B,2 . Here T 2  is to be chosen ≧l A /[(K 0 −ΔK)E A,2 ] and ≦l/[(K 0 +ΔK)E A,2 ] so that the ions of lowest mobility K 0 −ΔK have all moved out of cell A and that the ions of highest mobility K 0 +ΔK have all not yet reached the end of cell B at the end of T 2 .   3. During a third time period of duration T 3  the ion cloud is to be pushed out of cell B and into the first cell of the ion analyzing region. This is achieved by choosing the potentials of the entrance and exit diaphragms of cell B so that the electric forward field along the system axis in cell B is E B,3 ≧E H , causing ions of mobility K 0 −ΔK to move out of cell B with a velocity v B,2 =(K 0 ±ΔK)E B,2  and into the first cell of the “ion analyzing region. Here T 3  is to be chosen ≧l B /(K 0 +ΔK)E B,3 ) so that the ions of lowest mobility K 0 −AK have all moved out of cell B at the end of T 3 . These ions then will enter the first cell of the ion analyzing region where the electric forward field along the system axis is ≧E H  and thus comparable to E B,3  so that the velocity of the ions is not changed drastically and the length and shape of the ion cloud stays substantially unchanged.       

     As soon as the ion cloud has been transferred to the first cell of the ion analyzing region a new first time period of duration T 1  can start by establishing again a low electric forward field E A,L ≦E H /10 along the system axis in cell A of at least one decelerating ion gate. Note here also that the transition from one electric field distribution to another at the start of any one of the three time periods is to be short as compared to T 2  and/or T 3 . 
     At the end of the first time period T 1  the length of the ion cloud in cell A is Δl A ≈T 1 K 0 E A,L  for ions of mobility K 0  as is stated above. However, the upstream end of this ion cloud may not be defined very well since until the last moment of T 1  ions are moving into cell A. In order to better define the end of the ion cloud it is advantageous to eliminate the last arriving ions by stopping the ion flow into cell A a short time ΔT 1 &lt;&lt;T 1  before the start of T 2  by changing the potential of at least one of the diaphragms in the ion-beam forming region for this short time, ΔT 1 . 
     In some cases it is advantageous to have mainly ions of low mobilities in the final ion cloud and not the usually abundant ions of high mobilities. To achieve this one can divide the period T 3  into two periods T 31  and T 32  and extract during the period T 31  mainly ions of high mobilities out of cell B while ions of low mobilities are left, which after a waiting time ΔT 3  can be extracted during the second period T 32 . Similarly also period T 2  can be divided into two periods T 21  and T 22  with a waiting time ΔT 2  in between. 
     In cases in which slightly longer clouds of ions can be tolerated in the ion analyzing region, the electric forward field along the system axis in cell B of the at least one decelerating ion gate can be chosen to be constant and having approximately the same magnitude as E A,H . In this case the velocity of ions will not be changed substantially when they leave cell A and enter cell B. Thus, the ion cloud will substantially have the same length in cell B it had, when it was still in cell A. 
     Since in any space-charge free and conductor free region div(E) must vanish, one finds that ions that are slowed down along the system axis also experience forces that drive them away from the system axis. During the time period T 1  such forces are rather strong in the neighborhood of the entrance diaphragm of cell A. As a consequence the lateral ion beam extension increases noticeably when the ions move through cell A. Thus, it is advantageous to increase the aperture of the exit diaphragm of cell A as well as the apertures of all diaphragms in the beam analyzing region in order to let this widened ion beam pass. 
     In order to keep this beam widening in limits it is advantageous to reduce the ratio between the cross section of the entering ion beam and the area of the aperture in the entrance diaphragm of cell A. The reason is that in this case the ion beam passes only through the middle of this aperture where the fringing field forces that drive ions away from the system axis are smallest. 
     The best way to reduce the ratio is to reduce the initial lateral width of the ion beam as much as possible before it enters cell A. Such an ion beam of reduced lateral extension can be achieved:
         1. by placing at least one explicit lens into the ion acceleration region between the at least one ion source upstream of the ion-beam forming region and/or   2. by decreasing the electric forward field along the system axis in at least one cell of the ion-beam forming region while increasing the field in at least one of the further downstream cells.       

     Since the ratio between the cross section of the entering ion beam and the area of an aperture is most critical in the aperture of the entrance diaphragm of cell A, one may also increase the aperture in the diaphragm, as long as this increase stays within limits and does not increase the extension of the fringe field in the neighborhood of the diaphragm too much. 
     Though mechanical grids placed over the apertures of the diaphragm of any cell in the ion mobility spectrometer that includes a decelerating ion gate have the disadvantage that they reduce the ion transmission, there are cases in which it is advantageous to use such grids anyhow. The reason is that at least for a short distance upstream and downstream of a gridded diaphragm all equipotential surfaces are substantially parallel to the grid and thus substantially perpendicular to the system axis. Consequently the electrical forces that act on the ions are mainly parallel to the system axis and the shape of an ion cloud is not distorted in a major way when it passes through the grid. 
     In the exemplary embodiment of an ion mobility spectrometer that includes a decelerating ion gate such grids are assumed to be placed over the apertures of at least one of three diaphragms:
         1. over the aperture in the exit diaphragm of cell A of the decelerating ion gate which is also the entrance diaphragm of cell B. This grid substantially eliminates the otherwise during the period T 2  existing fringe field caused by the difference in the high electric field E A,H  along the system axis in cell A and the low electric field E B,L . along the system axis in cell B.   2. over the aperture in the exit diaphragm of cell B which is also the entrance diaphragm of the first cell of the ion analyzing region. This grid substantially eliminates the otherwise during the period T 3  existing fringe field that is caused by the difference in the electric field E B,H  along the system axis in cell B and the electric field ≧E H  along the system axis in the first cell of the ion analyzing region.   3. over the aperture in the entrance diaphragm of the first cell of the ion-beam forming region. This grid substantially eliminates the otherwise existing fringe field in the neighborhood of that diaphragm that separates the ion-beam forming region from the ion acceleration region, i.e. the region in which the ions are extracted from the at least one ion source and pushed into the ion-beam forming region. Such a grid also protects the ion-beam forming region from possible high-voltage discharges to the at least one ion source.       

     Since fringing fields may be detrimental between neighboring cells throughout the mobility spectrometer that includes a decelerating ion gate it is in many cases also advantageous to modify the fringing fields by placing an extra tubular electrode between the entrance and exit diaphragms of a cell under investigation. Herein the potential of this tubular electrode is advantageously chosen to be in the range between the potentials of the entrance and exit diaphragms of a respective cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects and features will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic view of the mechanical design of a first exemplary, non-limiting embodiment of an ion mobility spectrometer that includes at least one decelerating ion gate built from a series of diaphragms placed at different electric potentials. 
         FIG. 2  is identical to  FIG. 1  except that in this embodiment grids are placed over the apertures of the diaphragms  5 ,  8 , and  9 , that two additional static voltage generators  33 ,  34  and two additional pulse generators  35 , 36  are installed and that a conductive tube is placed around the ion mobility spectrometer with this tube being divided into several sections. 
         FIGS. 3 a -3 c    are a schematic view of the potentials of the diaphragms of the ion-mobility spectrometer that includes at least one decelerating ion gate during said different periods T 1 , T 2 , T 3  for an exemplary, non-limiting way to change the potential of a single diaphragm in order to achieve the necessary field strengths throughout the ion mobility spectrometer wherein the potentials of all diaphragms are static except one. 
         FIGS. 4 a -4 c    are a schematic view of the potentials of the diaphragms of the ion-mobility spectrometer that includes at least one decelerating ion gate during said different periods T 1 , T 2 , and T 3  for an exemplary, non-limiting way to change the potentials of two diaphragms in order to achieve the necessary field strengths throughout the ion mobility spectrometer as shown in  FIG. 3 , wherein, however, the range of the overall potentials is reduced, which requires that during the time T 3  a voltage V 00  is added to the potentials of at least three diaphragms. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Exemplary embodiments will be described in greater detail with reference to the accompanying drawings. In the following description, the same drawing reference numerals are used for the same elements in all drawings. The matters defined in the description such as a detailed construction and arrangement of elements are only those provided to assist in a comprehensive understanding of the invention. Thus, it is apparent that the present invention can be carried out without being limited to those defined matters. Also, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. 
       FIG. 1  is a schematic view of the mechanical arrangement of an exemplary, non-limiting embodiment of a mobility spectrometer that comprises a decelerating ion gate. In total ions are moved from an ion source  1  through the ion mobility spectrometer to an ion detector  3  from where collected ion charges are conducted to an amplifier  4 . The electric fields throughout the mobility spectrometer are formed by potentials applied to diaphragms  5 ,  6 ,  7 ,  8 ,  9 ,  10 ,  11 ,  12 ,  13 ,  14 ,  15 ,  16  shown in  FIG. 1 . These potentials are provided from a static voltage supply  17  and a resistive voltage divider  18  as well as by two static voltage generators  19  and  20  and by two pulsed voltage generators  21  and  22 . 
     The ion mobility spectrometer shown in  FIG. 1  can be understood as being divided into three parts:
     1. An ion beam-forming region  23 , that consists of three cells formed between diaphragms  5 ,  6  and  6 ,  7  and  7 ,  8  that all are shown to have circular, elliptical or polygonal apertures of substantially equal areas ≈σ 0 . Through this ion beam-forming region  23  a continuous ion beam is pushed by electric forward fields E 5,6 ≈E 6,7 ≈E 7,8 ≧E H  along the system axis  2  formed by the potentials of the diaphragms  5 ,  6 ,  7 , and  8 . Here the magnitude of E H  is to be chosen so that ions of interest of mobility K 0  move forward with a velocity V H ≈K 0 E H  of several meters per second. Often used is here an arrangement in which E 6,7 &gt;E 5,6  and/or E 7,8 &gt;E 6,7  since this causes a reduction of the lateral width of the passing ion beam.   2. An ion analyzing region  24 , that consists of the six cells formed between the diaphragms  10 ,  11 ,  12 ,  13 ,  14 ,  15 , and  16  that all have circular, elliptical or polygonal apertures of substantially equal areas which, however, are noticeably larger than those in the diaphragms  5 ,  6 ,  7 , and  8  of the ion-beam forming region  23  and thus allow a widened ion beam to pass. Through this ion analyzing region  24  clouds of ions are moved by electric forward fields E 10,11 ≈E 11,12 ≈E 12,13 ≈E 13,14 ≈E 14,15 ≈E 15,16 ≧E H  formed by the potentials of said diaphragms  10 ,  11 ,  12 ,  13 ,  14 ,  15 , and  16 .   3. A decelerating ion gate  25  in which the continuous ion beam injected from the ion-beam forming region  23  is split into short ion clouds of high ion density. This ion gate  25  comprises a cell A  26  of length l A  formed between the diaphragms  8 , 9  and a cell B  27  of length l B  formed between the diaphragms  9 , 10 . The apertures in the entrance and exit diaphragms  9  and  10  of cell B here are shown in  FIG. 1  to be substantially equal to the large apertures in the diaphragms  10 ,  11 ,  12 ,  13 ,  14 ,  15 , and  16  of the ion analyzing region, while the aperture in the entrance diaphragm  8  of cell A  26  is shown in  FIG. 1  to be equal or only slightly larger than the apertures in the diaphragms  5 ,  6 , and  7  of the ion-beam forming region  23 . The potentials of all diaphragms are shown to be fixed by corresponding taps of the potential divider  18  while the potentials of the diaphragms  8  and  9  are shown to be determined by the sum of static voltage generators  19  and  20  and pulse generators  21  and  22  the output of which may vary during three time periods T 1 , T 2 , and T 3 . The durations of these time periods are chosen so that with proper potentials applied to the diaphragms  8 ,  9 , and  10  during the time periods T 1 , T 2 , and T 3  ions of mobilities K 0 ±ΔK are passed through said decelerating ion gate  25  and compressed to short ion clouds.
       3.1 During a first period T 1 , which in most cases lasts for many milliseconds, these potentials are to be chosen so that they cause the electric forward field along the system axis in cell A  26  of length l A  to be E A,1 ≦E H /10, an electric field that it is much smaller than said static electric forward field E 7,8 ≧E H  along the system axis in the last cell  28  between the diaphragms  7  and  8  of the ion-beam forming region  23 . Consequently all ions move out of this cell  28  with high velocities and are slowed down when they enter cell A  26  thus, forming high density ion clouds in cell A, wherein such ion clouds are shorter for ions of low mobilities than for ions of high mobilities. The duration of T 1  here should be chosen to be ≦l A /[(K 0 +ΔK)E A,1 ] so that at the end of T 1  even the ions of the highest mobilities (K 0 +ΔK) form an ion cloud of length ≦l A  and thus are contained in cell A  26 . At the end of the period T 1 , however, some of the ions of mobilities ≧(K 0 +ΔK) have already passed through the full length l A  of cell A  26  and thus are lost.   3.2 During a second period T 2  that in most cases lasts for ≈1 ms, these voltages are chosen so that they cause the electric field along the system axis in cell A  26  of length l A  to be E A,2 ≧E H  and in cell B  27  of length l B  to be E B,2 ≦E H /10. Consequently all ions move out of cell A with high velocities and are slowed down when they enter cell B thus forming even denser ion clouds of lengths Δl B ≈Δl A (E B,2 /E A,1 ) in cell B if they had lengths ΔL A ≦l A  in cell A.
           The duration of T 2  here should be chosen to be ≧l A /[(K 0 −ΔK)E A,2 ] and ≦l B /[(K 0 +ΔK)E B,2 ] so that at the end of T 2  even the ions of lowest mobilities (K 0 −ΔK) are transferred out of cell A  26  and into cell B  27 , while ions of highest mobilities (K 0 +ΔK) have not yet reached the end of cell B  27 . Some of the ions of mobilities ≧(K 0 +ΔK), however, have already passed through the full length l B  of cell B and thus are lost at the end of T 2 , while some of the ions of mobilities ≦(K 0 −ΔK) have not yet left cell A and thus are lost as well.   
           3.3 During a third period T 3 , that also lasts for ≈1 ms in cell B these voltages must be chosen so that they cause the electric field along the system axis in cell B  27  of length l B  to be E B,3 ≧E H  while the electric field along the system axis in the first cell  29  between the diaphragms  10  and  11  of the ion analyzing region  24 , is about equal to said static electric forward field E 10,11 ≧E H  along the system axis. Consequently all ions move with about equal velocities from cell  27  into cell  29  and thus form there ion clouds of length Δl 10,11 ≈Δl B (E 10,11 /E B,3 ) if they had lengths Δl B  in cell B. However, since E B,3  and E 10,11  are not drastically different, the lengths, shapes and densities of the ion clouds stay more or less unchanged relative to what they were in cell B  27 .
           The duration of T 3  here is chosen to be ≧l B /[(K 0 −ΔK)E B,3 ) so that at the end of T 3  even the ions of lowest mobilities (K 0 +ΔK) are transferred out of cell B  27  and into cell  29  of the ion analyzing region  24 , while some of the ions of mobilities ≦(K 0 −ΔK) have not yet left cell B and thus are lost. As soon as the ion cloud has been transferred to the first cell  29  of the ion analyzing region a new time period T 1  can start by establishing again a low electric field E A,1 ≦E H /10 along the system axis in cell A  26 .   
           
       

     Since in any space-charge free and conductor free region div(E) must vanish, ions that are slowed down along said system axis also experience forces that drive them away from this axis. During the relatively long time period T 1  such forces are rather strong for a short distance downstream of diaphragm  8 , the entrance diaphragms of cell A  26 . As a consequence the lateral ion beam extension increases noticeably when it enters cell A  26  in which case it is advantageous to increase the aperture of the diaphragm  9  of cell A  26  as well as the apertures of the diaphragms  10 ,  11 ,  12 ,  13 ,  14 ,  15 , and  16  in the beam analyzing region in order to let this widened ion beam pass. 
     In order to keep said beam widening in limits, it is advantageous to reduce the ratio between the cross section of the ion beam and the area of the aperture of the diaphragm through which the ion beam passes, since in this case the ion beam passes only through the middle of this aperture where the fringing field forces that drive ions away from said system axis are smallest. Especially important is for the ion beam to pass through the apertures of said diaphragms  8  and  9  the entrance and exit diaphragms of cell A. 
     The best way to reduce this ratio is to reduce the initial lateral width of the ion beam as much as possible before it reaches these diaphragms. Such ion beams of reduced lateral extensions can be achieved:
     1. by placing at least one explicit lens (not shown in  FIGS. 1, 2 ) into the ion acceleration region between the ion source  1  upstream of the ion-beam forming region  23  and/or   2. by decreasing the electric forward field along said system axis in at least one cell of said ion-beam forming region  23  while increasing said electric forward field in at least one of the further downstream cells.   3. by increasing the aperture in the entrance diaphragm  8  of cell A slightly as long as this increase stays within limits and does not increase the extension of the fringe field in the neighborhood of diaphragm  8  too much.   

     Though mechanical grids placed over the apertures of the diaphragm of any cell in the ion mobility spectrometer that includes a decelerating ion gate have the disadvantage that they reduce the ion transmission, there are cases in which it is advantageous to use such grids. The reason is that at least for a short distance upstream and downstream of a gridded diaphragm all equipotential surfaces are substantially parallel to said grid and thus substantially perpendicular to the system axis. Consequently the electrical forces that act on the ions are substantially parallel to the system axis and the length and shape of an ion cloud is not distorted substantially when it passes through said grid. 
       FIG. 2  is very similar to  FIG. 1  and also shows an exemplary embodiment of an ion mobility spectrometer that includes a decelerating ion gate. The difference is that in  FIG. 2  mechanical grids  30 ,  31 , and  32  are assumed to be placed over the apertures of at least one of three diaphragms:
     1. A grid  30  placed over the aperture in diaphragm  9  substantially eliminates the otherwise during said period T 2  existing fringe field caused by the difference in the high electric field E A,2 ≧E H  along the system axis in cell A  26  and the low electric field E B,2 ≦E H /10 along the system axis in cell B  27 .   2. A grid  31  placed over the aperture in diaphragm  10  substantially eliminates the otherwise during said period T 3  existing fringe field caused by the difference in the high electric field E B,3 ≧E H  along the system axis in cell B  27  and the about equally large electric field ≧E H  along the system axis in cell  29 , the first cell of the ion analyzing region  24 .   3. A grid  32  placed over the aperture in diaphragm  5  substantially eliminates the otherwise existing fringe field caused by the difference in the ion-beam forming region  23  and in the ion acceleration region in which the ions are extracted from the ion source  1  and pushed into the ion-beam forming region  23 . Such a grid also widely protects the ion-beam forming region  23  from possible high-voltage discharges to the ion source  1 .   

     In order to allow more flexibility in steering the electric fields in said decelerating ion gate, it may be advantageous to provide additional DC and pulsed power supplies  33 , 34  and  35 , 36  that can vary the potential of diaphragm  10 . Analogously and also advantageously one could also supply such steering voltages (not shown) to other diaphragms upstream or downstream of the decelerating ion gate. 
     Since fringe fields may be detrimental between neighboring cells throughout a mobility spectrometer that includes a decelerating ion gate it is in many cases also advantageous to modify said fringing fields by placing extra tubular electrodes between the entrance and exit diaphragms of any cell under investigation. Such a tubular electrode  37  is shown between the diaphragms  6  and  7 . Herein the potentials of such tubular electrodes are advantageously chosen to be within the range between the potentials of the corresponding entrance and exit diaphragms. 
     In order to protect the ion mobility spectrometer, that includes an ion decelerating ion gate, from the influence of outside electric fields it is advantageous to place shielding tubes  38 ,  39 ,  40  around sections of said ion mobility spectrometer. Applying different potentials to said shielding tubes  38 ,  39 , and  40  allows to influence the potential distribution in the cell around which the shielding tubes are placed. 
     Though the decelerating ion gate provides narrow ion clouds of high intensity, it may be useful to further reduce their length by placing a Bradbury-Nielson Gate (not shown) within or downstream of the decelerating ion gate. Such a Bradbury-Nielson Gate could for instance replace the grid  30  placed over the aperture of the exit diaphragm of the cell B  27 , the diaphragm  10 . 
     In  FIGS. 3, 4  examples are shown how the potentials of the different diaphragms could be chosen to form the above described electric fields during said time periods T 1 , T 2 , and T 3 . Naming the potential of a diaphragm N as U N  one may choose the potentials of diaphragms N in the ion-beam forming region as well as in the ion analyzing region as static potentials, wherein the potentials U 1  is substantially more ion repelling than the potentials U i+1  with i=5, 6, 7 and i=11, 12, 13, 14, 15. The potentials U 8 , U 9 , U 10 , however, are varied to achieve the required electric fields during said time periods T 1 , T 2 , T 3  in cell A and in cell B by activating said static voltage generators  19 ,  20 ,  33 ,  34  as well as said pulsed voltage generators  21 ,  22 ,  35 ,  36 . 
       FIG. 3  illustrates one example how to choose potentials for the diaphragms  8  and  9  as U 8 =U 7 −V 1  and U 10 =U 8 −V 2 =U 11 +V 3  with V 1 , V 2  and V 3  being ion repelling voltages determined by the resistive voltage divider  18  while V 9  is varied so, that V 9 =V 8  during the time periods T 1  and T 3  and V 9 =V 10  during the time period T 2 .
         In  FIG. 3 a    the potential distribution is shown during the time period T 1 , wherein the continuous ion flux in the ion-beam forming region between the diaphragms  5  and  8  is indicated as a dashed arrow and the compressed ion cloud at the end of the time period T 1  in cell A  26  between the diaphragms  8  and  9  as a short arrow,   In  FIG. 3 b    the potential distribution is shown during the time period T 2 , wherein the transfer of the ion cloud from cell A  26  between the diaphragms  8  and  9  into cell B  27  between the diaphragms  9  and  10  is indicated as a curved arrow and two solid arrows indicate that the ion cloud in cell B  27  is shorter than it was in cell A  26 .   In  FIG. 3 c    the potential distribution is shown during the time period T 3 , wherein the transfer of the ion cloud from cell B  27  between the diaphragms  8  and  9  into the first cell of the ion analyzing region  29  between the diaphragms  10  and  11  is indicated as a curved arrow and two solid arrows indicate that the ion clouds in cell B  27  and the first cell of the ion analyzing region  29  are approximately equal in length. By a dashed arrow also the path is indicated along which the ion clouds of different mobilities move through the ion analyzing region between the diaphragms  10  and  16 .       

     The same field distribution could be achieved by choosing U 10 =U 11 +V 3  as a fixed potential and by establishing
         U 8 =U 9 =U 10  during the time period T 1 ,   U 8 =U 10 +ΔV 1  and U 9 =U 10  during the time period T 2 , and   U 8 =U 10  and U 9 =U 10 +ΔV 1  during the time period T 3 .       

     In both mentioned examples the ions are still streaming into cell A during the time period T 2 , when the accumulated ion cloud moves from cell A into cell B. 
     Consequently the ion cloud that is extracted from cell A has a small tail which, however, in most cases is negligible. However, this tail is eliminated when during the last milliseconds or so of the time period T 1  the influx of ions into cell A is prohibited by raising the potential of one of the last diaphragms in the ion-beam forming region  23  as has been proposed already above. 
     Besides the listed examples of how to properly choose the potentials of the diaphragms  8 ,  9 ,  10  in the decelerating ion gate there are several alternate ways that all would achieve similar electric fields along the system axis in cell A and in cell B during said times T 1 , T 2 , and T 3  and thus similar ion clouds. 
     The voltage difference between the diaphragms  5  and  16  is large and thus there is always the danger of high voltage discharges. Thus it usually is usually rewarding to reduce this potential difference. One way is to permanently subtract, as is illustrated in  FIG. 4 , from the potentials of the diaphragms  5 ,  6 ,  7 ,  8 ,  9 ,  10 , and  11  (see  FIG. 3 ) a voltage V 00  and to add this voltage V 00  again to the diaphragms  9 ,  10 ,  11  during the time period T 3 . The resultant potential distribution is shown in  FIGS. 4 a , 4 b , and 4 c    for the three time periods T 1 , T 2 , and T 3 . This procedure requires, however, that a grid is placed over the aperture in the diaphragm  9 . In case there is no such grid it is necessary to add this voltage V 00  additionally to diaphragm  8  during the time period T 3 . 
     The foregoing exemplary embodiments are merely exemplary and are not to be construed as limiting. The present teaching can be readily applied to other types of apparatuses. Also, the description of the exemplary embodiments is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art.