Method and device for optical analysis of particles at low temperatures

Abstract

Method and device ( 1 b ) for performing the optical analysis of particles ( 2 ) contained in suspension in a fluid ( 3 ) arranged inside a microfluidic device ( 4 ) which maintains it at a temperature significantly lower than the ambient temperature; the formation of humidity on the outer surface ( 8 ) of the cover of the microfluidic device is avoided by applying a thermal flow (F) which determines an increase in the temperature of the outer surface ( 8 ) of the cover to above the condensation temperature (Td), or a reduction in the ambient temperature (and/or humidity) in the vicinity of the cover ( 8 ), so as to bring the condensation temperature (Td) (dew point) to below the temperature of the surface ( 8 ) of the cover determined by the internal operating temperature.

1 . A method for optical analysis of particles ( 2 ) contained in suspension in a fluid, at temperatures lower than dew point temperature, comprising the steps of:
 i. arranging the particles in suspension within at least one microchamber ( 4 ) containing said fluid and delimited between a first and a second surface ( 5 , 6 );   ii. thermally coupling the first surface ( 5 ), by means of a first thermal resistance (RLW), to first cooling means ( 7 ) adapted to subtract heat from the fluid, and thermally coupling the second surface ( 6 ), by means of a second thermal resistance (RHI), to an optical inspection surface ( 8 );   iii. bringing said fluid to a first temperature (T 1 ), lower than dew point temperature, by means of said first cooling means;   iv. while the particles ( 2 ) are being optically analysed, establishing at the optical inspection surface ( 8 ) a thermal flow (F) such that the optical inspection surface is constantly maintained at a second temperature (T 2 ) higher than the dew point temperature (Td) of the ambient humidity contained in the air which laps on the optical inspection surface ( 8 ); said first and second thermal resistances being chosen so that the second thermal resistance (RHI) has a thermal conductivity value, preferably, at least one order of magnitude lower than that of the first thermal resistance (RLW) and, in any case, equal to at least half the thermal conductivity of the first thermal resistance (RLW).   
     
     
         2 . A method according to  claim 1 , characterized in that step (iv) is carried out by heating the optical inspection surface ( 8 ) to above ambient air dew point. 
     
     
         3 . A method according to  claim 2 , characterized in that the optical inspection surface is heated by Joule effect, by arranging on the same, externally to the microchamber ( 4 ), a resistor ( 24   b ) chosen from the group consisting of: a transparent conductive resistive layer ( 25 ), e.g. ITO, applied uniformly on the entire optical inspection surface; a plurality of filiform microresistors ( 26 ) arranged on the optical inspection surface, preferably in a comb shape, uniformly spaced from one another. 
     
     
         4 . A method according to  claim 3 , characterized in that said filiform microresistors ( 26 ) are supplied so that the current density distribution is homogenous by using a current distribution frame ( 27 ) arranged opposite the filiform microresistors, which in turn receives current by means of a plurality of conductor bridges ( 30 ), which connect a plurality of different points ( 28 ) of the distribution frame, arranged on the side opposite the filiform microresistors, to at least one common collector ( 31 ). 
     
     
         5 . A method according to  claim 2 , characterized in that the optical inspection surface ( 8 ) is heated by forcing an air flow over the same. 
     
     
         6 . A method according to  claim 1 , characterized in that step (iv) is carried out by cooling an amount of ambient air immediately surrounding the optical inspection surface ( 8 ) and so lapping the inspection surface at a temperature such that the dew point (Td) of said amount of air is lower than said second temperature (T 2 ) of the optical inspection surface. 
     
     
         7 . A method according to  claim 2 , characterized in that the temperature of the optical inspection surface is feedback controlled by continuously measuring the instant temperature (T 2 ) of the optical inspection surface, preferably by means of a resistor applied to said optical inspection surface ( 35 ), or by means of an infrared sensor arranged facing the optical inspection surface. 
     
     
         8 . A method according to  claim 7 , characterized in that the temperature (T 2 ) at which to maintain the optical inspection surface is calculated as a function of the parameters ambient air temperature and ambient air humidity, which are continuously detected by means of appropriate sensors. 
     
     
         9 . An apparatus ( 1   a,   1   b ) for optical analysis of particles ( 2 ) contained in suspension in a fluid ( 3 ), at temperatures lower than dew point temperature, comprising:
 at least one microchamber ( 4 ) containing said fluid and delimited between a first ( 5 ) and a second ( 6 ) surface;   first cooling means ( 7 ) which are thermally coupled with the first surface by means of a first thermal resistance (RLW) and are adapted to subtract heat from the microchamber by an amount such as to maintain said fluid at a predetermined first temperature (T 1 ), lower than the dew point temperature; and   an optical inspection surface ( 8 ) thermally coupled to the second surface by means of a second thermal resistance (RHI); characterized in that, in combination:   the second thermal resistance (RHI) has a thermal conductivity value, preferably, of at least one order of magnitude, and even more preferably, of two orders of magnitude lower than that of the first thermal resistance (RLW) and, in all cases, equal to at least half the thermal conductivity of the first thermal resistance (RLW); and   the apparatus further comprises means ( 24 ) for establishing at the optical inspection surface ( 8 ), while the apparatus ( 1   a / 1   b ) is operative and manipulation and/or optical analysis of the particles ( 2 ) are being performed with it, a thermal flow (F) such that the optical inspection surface is constantly maintained at a second temperature (T 2 ), higher than the dew point temperature (Td) of the ambient humidity contained in the air which laps the optical inspection surface ( 8 ) in use.   
     
     
         10 . An apparatus according to  claim 9 , characterized in that it comprises means ( 24   b;   24   c ) for heating the optical inspection surface ( 8 ) to above the ambient air dew point. 
     
     
         11 . An apparatus according to  claim 10 , characterized in that said means for heating the optical inspection surface consist of a resistor ( 24   b ) constituted by: a transparent conductive resistive layer ( 25 ), e.g. ITO, applied uniformly on the entire optical inspection surface; or a plurality of filiform microresistors ( 26 ) applied integrally in one piece on the optical inspection surface ( 8 ) and arranged on the same, preferably in a comb shape, uniformly spaced from one another. 
     
     
         12 . An apparatus according to  claim 11 , characterized in that said filiform microresistors ( 26 ) are electrically connected, each on the side of the same one end thereof, to a current distribution frame ( 27 ) constituted by a metal foil which receives electric current by means of a plurality of conductor bridges ( 30 ) which connect a plurality of different points ( 28 ) of the distribution frame, arranged on the side opposite the filiform microresistors, to at least one common collector ( 31 ) arranged at a base element ( 19 ) of the apparatus which supports the microchamber; said filiform microresistors ( 26 ) having a width equal to approximately one tenth of the pitch between the same, in a direction transverse to their longitudinal extension. 
     
     
         13 . An apparatus according to  claim 9 , characterized in that said means ( 24 ) for establishing said thermal flow (F) at the optical inspection surface ( 8 ) comprise forced ventilation means ( 40 ) of the optical inspection surface ( 8 ) and, preferably, heating or cooling means ( 41 ) of the forced ventilation flow. 
     
     
         14 . An apparatus according to  claim 9 , characterized in that it comprises means ( 35 ) for measuring said second temperature (T 2 ) and means ( 36 ; 37 ) for actuating in feedback said means ( 24 ) for establishing said thermal flow (F) at the optical inspection surface ( 8 ); and preferably means for calculating the ambient air dew point (Td). 
     
     
         15 . Apparatus as claimed in  claim 9 , characterised in that it comprises electronic means ( 100 ,  101 ) for performing the manipulation of said particles ( 2 ) in said microchamber ( 4 ).