Abstract:
An apparatus for generating a defined environment for particle-shaped samples, comprises a support element with a rest end for a particle-shaped sample and an apparatus for generating a humid gas flow at a mouth end thereof. The mouth end is directed to the rest end. A gas provider provides gas having a first dew-point temperature. A cooler cools the gas to a cooler temperature under condensation of moisture to adjust a second dew-point temperature of the gas. A guide guides the gas with the second dew-point temperature to the mouth end, preventing condensation of moisture from the gas. A controller adjusts the relative humidity of the gas at the mouth end by adjusting the cooler temperature and adjusting the temperature of the gas at the mouth end.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of International Application No. PCT/EP03/07606, filed Jul. 14, 2003, which designated the United States and was not published in English and is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to apparatus and methods for generating a defined environment for particle-shaped samples, and in particular for particle-shaped samples having to be kept in an environment of defined humidity, such as protein crystals. 
     2. Description of the Related Art 
     The protein crystallography is a method of structural analysis of proteins in which these are exposed to X-ray or synchrotron radiation in order to explore the molecule structure from diffraction images. By the attachment of irregularly shaped proteins in a protein crystal, channels develop which are filled with crystallization solution. Protein crystals are very sensitive due to the high solvency content as well as by the weak contacts in comparison with crystals of small molecules and are only stable in a special environment. 
     In order to guarantee such a stable environment, previously protein crystals, together with some mother liquor, i.e. the solution in which the crystal has grown, are mounted in a glass capillary, which is then closed at both ends. Thus, in the capillary, an atmosphere arises, in which the crystal may be kept. This procedure, however, is disadvantageous, because it is a closed system, so that manipulations at the crystal are no longer possible. Furthermore, it is known to subject protein crystals to quick-freezing in a so-called loop representing a loop fixture and to measure same at low temperatures. Apart from temperature annealing, the crystal can also no longer be manipulated here. 
     Recently systems have become known, in which protein crystals are kept stable in a humid airflow, wherein by the adjustment of the humidity of the airflow the relative humidity at the crystal may be checked in a simultaneous analysis of the crystal state at an X-ray camera. 
     Such systems are known for example in Reiner Kiefersauer et al., “Free-mounting system for protein crystals: transformation and improvement of diffraction power by accurately control humidity changes”, J. Appl. Cryst. (200), 33, pp. 1223-1230, and EP-A-0987543. These known systems include a measuring head including both a fixture for a protein crystal to be examined and a gas channel for feeding a humid air flow to the protein crystal. In these known systems, the humidity of the airflow is adjusted using a humidity regulation system, in order to adjust the mixing ratio of a dry airflow and a wet airflow depending on the humidity detected by means of a humidity sensor, in order to thus regulate the humidity of the airflow. 
     A similar method of adjustment of the humidity is also known in R. Kiefersauer et al., “Protein-Crystal Density by Volume measurement and Amino-Acid Analysis”, J. Appl. Cryst. (1996), 29, pp. 311-317. In T. Sjögren et al., “Protein crystallography in a vapour stream: data collection, reaction initiation and intermediate trapping in naked hydrated protein crystals”, J. Appl. Cryst. (2002), 35, pp. 113-116, there is also described a system for protein crystallography in a humid airflow. In this known system a bubbler is used to impart the gas with a desired humidity. In this, the gas is let to rise through a liquid, wherein the humidity of the gas may be manipulated by changing the temperature of the liquid or by changing the composition thereof. The gas with the humidity so adjusted is fed via a buffer vessel to a nozzle, at the outlet end of which a crystal is arranged at a fixture, so that a laminar gas flow strikes the crystal. 
     The known systems for adjusting the humidity of a gas flow are disadvantageous in that exact adjustment of the humidity, in particular in an interesting range slightly below 100% relative humidity, is difficult to realize therewith. In the methods mentioned first, using a humidity sensor, the employment of the sensor for the measurement of the relative humidity directly at the measurement location for the regulation of a humidifying means is not possible due to the spatial closeness at the crystal. Moreover, commercially available relative humidity sensors do not have sufficient accuracy and long-term stability in the required humidity range. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide apparatus and methods for generating a defined environment for particle-shaped samples, which enable highly accurate and long-term-stable humidity conditioning at the location of a particle-shaped sample. 
     In accordance with a first aspect, the present invention provides an apparatus for generating a defined environment for particle-shaped samples, having a support element having a rest end for a particle-shaped sample; an apparatus for generating a humid gas flow at a mouth end thereof, wherein the mouth end is directed to the rest end, wherein the apparatus for generating the humid gas flow has a gas provider for providing gas having a first temperature and a first relative humidity, so that the gas has a first dew-point temperature; a cooler for cooling the gas to a cooler temperature under condensation of moisture to adjust a second dew-point temperature of the gas corresponding to the cooler temperature; a guide for guiding the gas with the second dew-point temperature to the mouth end, preventing condensation of moisture from the gas; and a controller for adjusting the relative humidity of the gas at the mouth end by adjusting the cooler temperature and adjusting the temperature of the gas at the mouth end. 
     In accordance with a second aspect, the present invention provides a method for generating a defined environment for particle-shaped samples, with the steps of supporting a particle-shaped sample at a rest end of a support element; generating a humid gas flow at a mouth end directed to the rest end, with the sub-steps of providing gas having a first temperature and a first relative humidity, so that the gas has a first dew-point temperature; cooling the gas to a cooler temperature under condensation of moisture in order to adjust a second dew-point temperature of the gas corresponding to the cooler temperature; guiding the gas with the second dew-point temperature to the mouth end, preventing moisture to condense from the gas; and adjusting the cooler temperature and the temperature of the gas at the mouth end for adjusting the relative humidity of the gas at the mouth end. 
     According to the invention, in order to achieve high required accuracy of the humidity values of a test gas, typically air, at the location of a particle-shaped sample, it is done without regulation of the humidity by means of a closed loop of humidifier and humidity sensor. Instead, the adjustment of the humidity according to the invention takes place via the precondition of the dew-point temperature without active humidity measurement. According to the invention, for the humidity adjustment of the dew point, a two-stage conditioning is employed, which takes place using a humidifying unit and a condensation unit in the form of a recooler. In the humidifying unit, the test gas is provided with a too high moisture by adjusting both dew-point and the temperature of the test gas to a value above the desired value. Thereafter, the gas is cooled again in the recooler, wherein excess water in the gas condenses again. Thus, the return-cooler temperature to which the gas is cooled sets the desired gas dew-point. The dew-point temperature corresponds to the gas temperature at which just 100% relative humidity is present in the gas, i.e. the gas is completely saturated, even minimal additional amounts of water can no longer be taken up by the gas. This state is achieved by the controlled condensation. 
     The so-conditioned test gas, typically air, is guided to the measurement head and takes on the desired gas temperature in the heat exchanger there. In order to prevent condensation of moisture therefrom while guiding the conditioned test gas to the measurement head, usually heated conduits holding the test gas above the cooler temperature are used. The temperature of the gas at the mouth end is controlled so that the humidity content of the measuring gas, expressed in relative humidity, results clearly by calculation from the quantities “dew-point temperature” and “gas temperature at the mouth end”. Changes of the humidity content may now be adjusted by adjusting either the dew-point temperature, i.e. the cooler temperature, or the gas temperature at the mouth end correspondingly. 
     The connection between relative humidity f rel , dew-point temperature T dp  and gas temperature T g  is given by the so-called Magnus formula. This reads: 
               F   rel     =     exp   ⁢     {       a   w     ⁢       b   w     ⁡     [         T   dp     -     T   g           (       b   w     +     T   dp       )     ⁢     (       b   w     +     T   g       )         ]         }     ×   100   ⁢           ⁢   %           
With the constants a w =17.50 and b w =241.2 K.
 
     The adjustment of the test gas temperature at the mouth end here preferably takes place by the adjusting of the temperature of a measuring head, through which the test gas is guided and which comprises the mouth end. The temperature of the measuring head may be adjusted by means of arbitrary known means, for example using heating windings, using Peltier elements, or using a liquid heat exchanger. Apart from heating the test gas in the measuring head, also cooling is possible by the employment of cooling aggregates, so that a further temperature range of the test gas may be adjusted. By cooling it is also possible to adjust very high humidities depending on the outside temperature. 
     According to the invention, preferably the temperature at the sample head and thus the gas temperature is controlled or regulated to a constant value, whereas the dew-point temperature, i.e. the return-cooler temperature, is varied corresponding to the desired humidity. Alternatively, the dew-point temperature may be held constant and the temperature of the sample head may be varied corresponding to the desired humidity. 
     In order to increase the accuracy of the adjustment of the relative humidity at the mouth end and thus at the particle-shaped sample, which is preferably arranged immediately at the mouth end, preferred embodiments of the present invention include means to compensate for the flow-through-dependent pressure loss in the gas conduit between the cooler and the mouth end in the adjustment of the relative humidity, i.e. the adjustment of the gas dew-point or the adjustment of the gas temperature at the mouth end. 
     In further embodiments, apart from the conditioning of temperature and humidity of the test gas, the inventive apparatus further enables mixing-in one or more additional foreign gases and/or a liquid converted to a gaseous state with a vaporizer. The apparatus comprises corresponding mass flow controllers in order to dose all fluid flows, i.e. measuring gas, foreign gases and/or liquid, so that from the ratio formation of the mass flows lowering or raising the dew-point adjusted in the recooler or the gas temperature at the mouth end may be calculated. 
     Instead of corresponding mass flow controllers, also other means for generating a defined flow or volume stream may be used, for example means containing pumps driven by a step motor in order to thereby cause a defined volume stream. 
     In the inventive apparatus and methods, the accuracy of the humidity adjustment does not depend on humidity sensors, but only on long-term-stable temperature sensors used for the adjustment of the gas temperature at the mouth end as well as for the adjustment of the cooler temperature, wherein these temperature sensors do not have to come into contact with the test gas directly. Thus, according to the invention, the disadvantages of relative humidity sensors do not occur, which consist in that in such sensors aging of the sensor material shifts the characteristic curve, that such sensors have poor accuracy in the range from 90% relative humidity to 100% relative humidity, that the aging mentioned is even more extreme in this range, and that humidity sensors with adequate accuracy are extremely expensive. Moreover, cheaper sensors mostly have only a limited measuring range. Dew-point probes also do not enable dew-point constancy in the range of one hundredth degree, whereas this may be guaranteed with the use of temperature probes as taking place according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects and features of the present invention will become clear from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of a system in which an inventive apparatus may be used; 
         FIG. 2  is a schematic illustration of an embodiment of an inventive apparatus; 
         FIG. 3  is a schematic illustration of an alternative embodiment of an inventive apparatus; and 
         FIG. 4  is a schematic illustration of a further alternative embodiment of an inventive apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a system in which the present invention may find application, comprising a control computer  10 , control electronics  12 , a humidifying unit  14 , and a sample head  16 . 
     The sample head  16  may be a conventional sample head, as it is for example described in the above-mentioned EP-A-0987543. This sample head includes an outer part  18  and an inner part  20 , which is preferably moveable with reference to the outer part  18 . Attached to the inner part  20  is a support element  22  for supporting a particle-shaped sample, in particular a protein crystal. The present invention, however, is not limited to the use for particle-shaped samples in the form of protein crystals, but may be advantageously used for any particle-shaped samples having a high liquid content, i.e. having to be held in a humid environment. Among these are biological objects, such as biological cells or cell components, or also synthetic non-crystalline objects with high solvent content. 
     In  FIG. 1  the support element  22  is illustrated as a loop fixture having a loop as rest end in which a protein crystal may be inserted. Such loop fixtures are known from the protein crystallography in particular for quick-freezing of samples. Alternatively, the fixture may include a hollow capillary (vacuum tweezers) operated with negative pressure or also a compact, elongated, tip-shaped component, at the end of which a rest for the particle-shaped material sample is given. In the inventive apparatus, any fixture devices are applicable, in which the particle-shaped sample may adhere at the rest end of a support element under the effect of absorption forces, electric forces, a glue material or the like. 
     As shown in  FIG. 1 , the sample head  16  further includes a gas channel  24 , via which a humid airflow may be fed to the rest end of the support element  22  and thus the particle-shaped sample. Attached to the gas channel  24  is a gas conduit  28 , via which the sample head  16  is connected to the humidifying unit  14 . The humid gas flow  26  is guided under pressure from the humidifying unit  14  via the gas conduit  28  and the gas channel  24  to a mouth end  30  of the gas channel  24 . 
     The rest end of the support element  22  is preferably immediately at the mouth end  30 , wherein, however, a certain spacing, for example in the order of 1 to 10 mm, preferably 2 to 3 mm, is usually provided to enable simultaneous analysis of the crystal state using an X-ray camera. Preferably, the gas channel  24  and the mouth end  30  thereof are formed to guarantee a substantially laminar gas flow  26  in the area of the rest end of the support element  22 . To this end, it may be advantageous to provide the gas channel directly leading to the mouth end  30  without change of direction with a sufficient length. The flow rate of the humid airflow  26  is adjusted to support the generation of a laminar stream in the area of the rest end of the support element  22 , wherein a good flow rate may be in the range of 0.6 to 2.0 l/min. 
     The sample head  16  further comprises tempering means  32  to adjust the temperature T p  thereof. The gas channel  24  through the sample head  16  is embodied such that the humid airflow takes on the temperature T p  of the sample head  16  when flowing through the sample head. To this end, the gas channel  24  may have a corresponding course or may for example also be embodied as several sub-channels. For adjusting the temperature of the sample head  16  also a temperature sensor (not shown) is provided thereat in a known manner. 
     The tempering means  32  may be an arbitrary known means for adjusting the temperature. In preferred embodiments of the present invention, the tempering means  32  is a liquid heat exchanger for sample head tempering enabling both heating and cooling the sample head  16 . Thus, also gas temperatures below or close to the room temperature may be adjusted quickly. Alternatively, conventional Peltier elements or heating elements might be used for the adjustment of the sample head temperature and thus the humid airflow flowing through the sample head. At this point it is to be noted that in  FIG. 1  only the gas conduit  28  is illustrated as the connection of the sample head to the rest of the system for clarity reasons, whereas further connections, for example electric connection lines, conduits for feeding a tempering fluid, conduits for feeding a vacuum for a supporting capillary and the like, are not shown. 
     The present invention is not limited to a sample head comprising a support element and a gas feed in combination. Rather, a support element and separately therefrom an apparatus directing a humid gas flow to the support element may be provided. Such an apparatus may for example have an elongated nozzle of sufficient length to support the generation of a laminar gas flow. 
     Before it is subsequently gone into the inventively used humidifying unit  14  in detail, it is to be pointed out briefly that all programming thereof for the performance of humidity experiments and the like may take place via the control computer  10  and control electronics  12 . In the construction illustrated, control electronics  12  serve to feed the commands of the control computer  10  to the humidifying unit  14  as well as to the sample head  16 . Since neither the control computer  10  nor the control electronics  12  are the subject matter of the present invention, they do not need to be explained further. 
     In the following, the embodiment schematically illustrated in  FIG. 2  of an inventive apparatus for the generation of a defined environment for particle-shaped samples is explained in greater detail. It includes a fluid module  34  connected to the sample head  16  via a plurality of fluid conduits  36 ,  38 ,  40 , and  42 . Via the fluid conduit  36 , the measuring gas is fed to the sample head and provided at the mouth end  30  thereof. The gas fed to the location of the particle-shaped sample, i.e. substantially the mouth end, to adjust the desired relative humidity is designated as measuring gas. As explained above, the measuring gas takes on the temperature T p  of the sample head  16 . 
     The humidifying unit for generating the humid gas flow guided through the fluid conduit  36  includes a humidifier  44  and a recooler  46 . The recooler  46  has a temperature control means  48  for adjusting the cooler temperature of the recooler  46 . The input of the humidifier  44  is connected to a compressed air conduit  50 , whereas the output thereof is connected to the input of the recooler  46 . The output of the recooler  46  is connected to the fluid conduit  36 . The recooler  46  further has an output connected to a condensate conduit  52 . 
     The fluid conduit  38  is connected to a vacuum pump  54  and also to a support capillary (not shown) provided in sample head  16 , in order to hold a particle-shaped sample at the rest end of the support capillary. 
     The two fluid conduits  40  and  42  represent conduits for feeding and withdrawing a tempering fluid for adjusting the temperature of the sample head  16 . To this end, these fluid conduits are connected in a known manner to a tempering control means  56  and a pump  58  for providing a tempering fluid flow through the tempering fluid conduits  40  and  42 . 
     In the embodiment shown in  FIG. 2 , a gas flow of defined humidity at the mouth end  30  is generated as follows. 
     Depending on a humidity desired at the sample location and a default sample head temperature T p , the dew-point temperature T dp  required for the desired relative humidity F rel  is determined, using the above-mentioned Magnus formula. This determination may take place in the control computer  10 . 
     The cooler temperature is adjusted to the determined dew-point temperature T dp , in order to thereby adjust the desired humidity at the location of the sample. For generating the gas flow with the dew-point temperature T dp , at first compressed air imparted with a too high humidity in the humidifier  44 , i.e. a dew-point at a temperature above the desired value, is fed to the humidifier  44  via the conduit  50 . This too humid gas is fed to the recooler  46  and cooled to the cooler temperature T K . Thereby excess water in the gas is condensed, so that the cooler temperature T K  sets the desired gas dew-point and thus the dew-point temperature T dp  of the measuring gas. The condensate generated in this is withdrawn via the conduit  52 . 
     The measuring gas with the desired dew-point temperature T dp  is guided to the mouth  30  under pressure in the above-described manner via the fluid conduit  36 . In this, it has to be ensured that between the recooler and the measuring head  16  no condensation takes place, so that no reduction of the dew-point temperature can take place. To this end, the gas conduit  36  is preferably formed by a heated gas conduit, for example a flexible heated Teflon conduit. If it is ensured that the cooler temperature T K  always lies below the ambient temperature in which the system is operated, the provision of a heating for the fluid conduit  36  is not required, because then it is ensured without providing a heating that the temperature of the measuring gas after leaving the recooler does not sink below the cooler temperature and thus condensation does not take place. 
     By inventively cooling the measuring gas starting from a higher temperature and humidity under condensation to a desired dew-point temperature T dp , the dew-point temperature, i.e. the temperature at which the relative humidity at a given pressure is 100%, is exactly adjustable. After the recooler, each further condensation of moisture from the measuring gas is prevented. Thus, the relative humidity at the mouth end  30  and thus at the crystal only depends on the dew-point temperature of the measuring gas and the temperature of the measuring gas at the mouth end  30  corresponding to the above-referenced Magnus formula. In the given embodiment, the sample head  16  is regulated to a given temperature T P  using the tempering fluid, so that the relative humidity of the measuring gas flow may be changed via an adjustment of the dew-point temperature of the measuring gas. This dew-point temperature of the measuring gas corresponds to the cooler temperature T K , so that by adjusting the cooler temperature the relative humidity of the measuring gas at the mouth end  30  can be adjusted. 
     The described system enables the adjustment of the relative humidity at the mouth end of the sample head in an exact manner for the case that in the recooler and at the mouth end identical pressures prevail. Since this is very difficult to realize in practice, in preferred embodiments a correction means is provided to take pressure differences between the mouth end and the recooler into account. 
     For such a pressure correction, the vapor pressure curve of the pure substance system water/water vapor or ice/water vapor is used. This curve p(T) indicates the corresponding water vapor pressure p arising above a water or ice surface for each temperature T. For this curve, which is measured with high precision and which is about exponential in course, there are calculation equations. 
     Furthermore, it is started from the fact that in the mixture of humid air the water vapor fraction may thermodynamically be considered almost independent of surrounding gases (ideal gas mixture), so that the vapor pressure curve p(T) also applied for the water vapor partial pressure e(T) in the mixture. At a certain temperature T of the humid gas, not more than the water vapor partial pressure e(T) indicated by the vapor pressure curve can be present in the mixture. 
     In the recooling principle, air conditioned to a high humidity is cooled to a temperature T K  and excess humidity condensed out. The water vapor partial pressure e(T K ) arises in the gas, wherein the cooler temperature T K  corresponds to the dew-point temperature T dp  due to the recooling principle used. 
     In humid air, Dalton&#39;s law set up for ideal gases also applies, according to which the overall pressure of a mixture results from the sum of the partial pressures of the components, i.e. in humid air p=p air +e. If the pressure of the gas mixture changes, all partial pressures change proportionately. This fact is taken into account in a correction for the compensation of the flow-through-dependent pressure loss in the measuring gas conduit  36 . 
     For performing the inventive pressure correction, the pressure difference between the pressure in the recooler and the pressure at the mouth end has to be determined. As pressure at the mouth end or at the location of the sample a typical ambient pressure p P  of 980 mbar may be assumed in a simplifying manner. Alternatively, to this end, an absolute pressure sensor may be provided at the sample head to detect the exact ambient pressure. Furthermore, the pressure P K  present in the recooler  46  is detected by means of a pressure sensor, preferably a differential pressure sensor. The current pressure in the recooler p K  varies depending on adjusted gas flow-through and temperature. 
     For the performance of the pressure correction, the accompanying dew-point temperature T dp  is now determined from the desired relative humidity and the default sample head temperature via the above Magnum formula. From this dew-point temperature, the accompanying water vapor partial pressure e P  is calculated via the vapor pressure curve. This can be directly calculated by the control computer or be determined by access to a look-up table. 
     From this determined partial pressure e P  necessary at the sample head, the water vapor partial pressure e K  to be adjusted in the recooler is determined corresponding to the ratio of the pressures in the recooler and at the location of the sample (ambient absolute pressure) as follows: 
     
       
         
           
             
               e 
               K 
             
             = 
             
               
                 e 
                 P 
               
               · 
               
                 
                   
                     P 
                     K 
                   
                   
                     P 
                     P 
                   
                 
                 . 
               
             
           
         
       
     
     Via the vapor pressure curve, this water vapor partial pressure e K  may again be converted to a dew-point temperature to be adjusted at the recooler. The recooler temperature T K  is adjusted to this dew-point temperature to obtain the desired humidity at the location of the sample. 
     Thus, highly accurate adjustment of the humidity at the mouth end  30  may take place even at a pressure drop occurring across the measuring gas conduit  36 . 
     Preferred embodiments of the present invention enable the addition of foreign gases and/or liquids into the measuring gas flow. A modified fluid module  64  provided with means required for this is shown in  FIG. 3 . To this end, foreign gas conduits  70  and  72  are connected to the measuring gas conduit  36  via respective mass flow controllers MFC. Via these conduits  70  and  72 , a foreign gas  1  and a foreign gas  2  may be introduced into the measuring gas. 
     The system shown in  FIG. 3  also enables introducing a vaporized liquid into the measuring gas. A liquid  80  may be fed via a liquid conduit  68 , in which a mass flow controller MFC is provided, to a direct vaporizer  82  connected to the measuring gas conduit  36  via a gas conduit  84 . The direct vaporizer causes residue-free vaporization of the liquid fed to it, so that the mass flow of the liquid fed to it corresponds to the gas leaving it. 
     Since in the system shown in  FIG. 3  all fluid flows, both the measuring gas and the foreign gases  1  and  2  and the liquid are dosed via mass flow controllers, from the ratio formation of the mass flows, a lowering or raising of the dew-point adjusted in the recooler may be calculated, so that these may be taken into account in the adjustment of the humidity of the measuring gas at the mouth end  30 . The liquid  80  fed may be a water-free liquid, for example isopropanol. If the liquid is not water-free, this has also to be taken into account in the adjustment of the humidity. 
     The present invention thus enables the arbitrary dosing-in of foreign gases or liquids via an internal vaporizer to the measuring gas, wherein the dosing of all fluids takes place via the mass flow controller MFC, so that the respective dosing-in may be taken into account in the adjustment of the humidity by correspondingly lowering or raising the dew-point adjusted in the recooler. 
     The inventively used humidifier for imparting the gas with increased humidity and increased temperature before it is subjected to recooling may be a conventional bubbler. An inventively preferred design of such a humidifier, however, is shown in  FIG. 4 . The humidifier is realized as a circulating humidifier comprising a humidifier unit  90  having an input connected to the compressed air conduit  50  and an output connected to an input of a separator  92 . The output of the separator  92  is connected to the recooler  46 . The separator  92  is also connected to a water supply  96  via a conduit  94 . The water supply further comprises an output connected to a further input of the humidifier unit  90  via a pump  98 . In the humidifier shown in  FIG. 4 , water from the water supply  96  is fed to the humidifier unit  90  via the pump  98 , wherein the measuring gas, i.e. the compressed air, is humidified with the water in the humidifier  90 . Liquid water is separated in the separator  92  and guided back to the water supply  96  via the conduit  94 . A circulating humidifier as it is shown in  FIG. 4  is advantageous as compared to an unwieldy bubbler since it may be embodied in a more compact manner. 
     The inventive apparatus and the inventive method enable highly exact adjustment of the humidity across a large range and in particular highly exact adjustment of the humidity in the interesting range for protein crystallography slightly below 100% relative humidity, for example between 80% and 100% relative humidity. According to the invention, particle-shaped samples may be examined at arbitrary temperatures, wherein only the corresponding gas temperature has to be adjusted correspondingly via the sample head temperature T P . Depending on the temperature of the sample head, the dew-point temperature may be adjusted by correspondingly regulating the temperature of the recooler, to obtain a desired humidity. Advantageously, Peltier elements enabling increased stability of the temperature regulation may be used for this. Furthermore, by the use of recoolers with increased cooling power, an extended dew-point adjustment range is possible, wherein it is preferred to use a long gas path in the recooler, to obtain improved flow-through independence of the generated humidity values. Extended life of the recooler may also be achieved when it is embodied as a stainless steel recooler. The possibility of a highly flexible humidity adjustment and also the possibility of the examination of protein crystals at increased temperatures is provided by an exemplary adjustment range of the gas temperature of 5° C. to 60° C. and an exemplary adjustment range of the gas dew-point from 1° C. to 60° C. 
     The inventive apparatus is particularly suited for the application in the field of protein crystallography. It is known that by crystal shrinking the crystal order in protein crystals may be improved, wherein this process may be controlled directly via the water fraction in the crystal. As explained above, this water fraction may be controlled exactly by the present invention. Preferably, the control computer  10  as well as the control electronics  12  are formed to perform predetermined humidity experiments. The inventive apparatus may preferably comprise means enabling to adjust various parameters, such as starting value humidity, end value humidity and humidity gradient, selectively. Furthermore, the present invention may comprise means enabling to track the change of the crystalline order in X-ray during such humidity experiments. 
     An exemplary humidity experiment, which may be performed by the inventive apparatus for the generation of a defined environment for particle-shaped samples for example consists in at first mounting a protein crystal in its native state and then passing through a humidity ramp for the characterization of the crystal system. As a starting humidity the relative humidity of the native state may be chosen, whereas as an end humidity a humidity value is used which corresponds to the starting humidity minus 20%. The change in humidity may for example take place in steps of 0.25% each, so that with a humidity difference of 20% eighty humidity levels result. As dwell-time on a respective level, a time of 30 seconds may be implemented, so that the overall duration of such a humidity experiment would be 50 minutes. The reaction of a crystal to the humidity change may continuously be recorded with X-ray pictures. 
     While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.