the raman based flow cytometry of the present invention counts individually identified particles within a sample by combining non - destructive , molecule specific raman spectroscopy with a flow cytometer &# 39 ; s ability to sequentially observe individual molecules . one difficulty in implementing raman based flow cytometry is that raman scattering has a very low cross - section , i . e ., only a very small percentage of the photons in a light beam focused on a sample are raman ( in - elastically ) scattered . the effective raman cross - section of a sample may , however , be increased by the technique of surface enhanced raman scattering , in which each molecule is associated with a noble metal colloid or colloidal aggregate . this association can be done by , for instance , sonification of cells in a noble metal colloidal solution . the cytometric technique of hydrodynamic focusing may then be used to arrange the prepared molecules in a single file , flowing as a core stream within a fluid sheath , ready for individual interrogation by a light source . hydrodynamic focusing requires precision pumping and control of the sample and sheath fluids within appropriately designed channels , as described in detail below . an intense light source , preferably a laser source , may be focused onto the molecules flowing in the cytometer core - stream , and the in - elastically scattered light collected , detected and analyzed by raman spectroscopy . for the identification of the molecules , the detection system needs appropriate wavelength selecting elements that provide sufficient detail of the molecule &# 39 ; s characteristic raman spectrum . an exemplary embodiment of the present invention , having the elements for performing raman detection based cytometer on individual molecules will now be discussed in detail by reference to the accompanying drawings in which , as far as possible , like numbers represent like elements . fig1 is a cross - sectional view of an exemplary embodiment of a raman detection based flow cytometer , comprising an instrument housing 10 , a removable cartridge 12 , a pressurization module 14 , control electronics 16 , an illumination source 18 , source steering optics 20 , source focusing optics 22 , raman scattered light collecting optics 24 , a light dispersing optical element 26 and a light detecting element 28 . fig2 is a schematic view of an exemplary , portable embodiment of a raman detection based flow cytometer , comprising a lower section 32 having batteries 34 , illumination source 18 , illumination steering and collimating optics module 38 , control electronics 16 ; an upper section 30 , comprising a scattered light detection and dispersion module 40 , a light detecting element 28 , a manual pressurizing unit 44 , first pressure chambers 72 a , 72 b and 72 c , second pressure chambers 74 a , 74 b and 74 c , first valves 76 a , 76 b and 76 c and second set of valves 78 a , 78 b and 78 c ; and a removable cartridge 12 , comprising a sample supply lumen 48 , a fluid control unit 50 , a flow stream 52 , a raman chamber 61 and a waste reservoir 54 . the removable cartridge 12 may be made primarily of materials such as molded plastic that allow the cartridge to be mass produced and disposable . fig3 is a schematic view of an exemplary embodiment of a removable cartridge 12 of the present invention , in which the fluid control unit 50 comprises an ultrasonic source 55 , a sample conditioning reservoir 56 , a sample storage reservoir 58 , a sheath fluid reservoir 60 , flow sensors 62 a , 62 b and 62 c , a hydrodynamic focusing module 64 , a flow channel 52 in which a sheath fluid 51 surrounds a core stream 51 . the sample may be prepared off - line or in the sample conditioning reservoir 56 . examples of sample preparation techniques that enhance the raman cross - section of molecules include loading cells with colloidal gold by fluid - phase uptake or sonification , as described in detail in , for instance , in the article entitled “ surface - enhanced raman spectroscopy in single living cells using gold nanoparticles ”, by kneipp et al , applied spectroscopy , volume 56 , number 2 , 2002 , pp 150 - 154 , the contents of which are hereby incorporated by reference . the molecules may also have enhanced raman cross - sections by attaching them to micro - beads filled with noble - metal ( copper , gold , silver , platinum , palladium and iridium ) colloids , as described in , for instance , in the article entitled “ flow analysis - based surface - enhanced raman spectroscopy employing exchangable microbeads as sers - active surfaces ” by lendl et al . in applied spectroscopy , volume 54 , number 7 , 2000 , pp 1012 - 1018 , the contents of which are hereby incorporated by reference . noble metal colloids can also be made to aggregate into clusters suitable for individual molecules to attach to , as described in detail in , for instance , the article , “ single molecule detection using surface enhanced - raman scattering ( sers )” by kneip et al . in physical review letters , volume 78 , number 9 , 1997 , pp 1667 - 1670 , the contents of which are hereby incorporated by reference . molecules may also be attached to microstructures , such as liposomes , that are filled or coated with noble metal colloids . molecules themselves may also be coated with noble metal colloids . other possible methods for associating the cellular particles with the nobel metal colloidal particles include , but are not limited to , the manual injection of the metal colloids into individual cells and treating the metal colloidal particles as projectiles and firing them into the cellular particles . in a preferred embodiment , intended for raman based cytometry of individual cells , the sample conditioning reservoir 56 is adjacent to an ultrasonic source 55 , which may be controlled by control electronics 16 . the sample conditioning reservoir 56 comprises a cell supporting , buffer solution containing aggregated clusters of 60 nm gold nano - particles . once sample cells have been fed into the buffer solution , a short burst of ultrasound ruptures the cell membrane , permitting gold colloid uptake by the cell . on cessation of the ultrasound , the cell membrane typically self - anneals within seconds . the prepared sample fluid is then fed from the sample conditioning reservoir 56 into the sample storage reservoir 58 . the sample fluid is then fed into the hydrodynamic focusing module 64 , which may be a fluidic circuit engineered to perform hydrodynamic focusing , as described in detail in , for instance , u . s . pat . no . 6 , 597 , 438 , titled “ portable flow cytometery ” issued to cabuz et al . on jul . 22 , 2003 , the contents of which are hereby incorporated by reference . the hydrodynamic focusing causes molecules of the sample to fall into single file a long a core stream 53 surrounded by a sheath fluid 51 within the flow channel 52 . the velocity of the sheath fluid 51 is preferably about 9 times that of the core stream 53 . additionally , the velocity of the sheath fluid 51 and the core stream 53 remain sufficiently low to maintain laminar flow in the flow channel 52 . in a preferred embodiment , the required velocity of the sheath fluid and the sample fluid are provided by a combination of a manual pressurizing unit 44 , coupled via pressure chambers 46 b and 46 c to fluid reservoirs 58 and 60 and fluid flow sensors 62 b and 62 c , all acting in a closed feed - back loop under control electronics 16 . the manual pressurizing unit 44 may , for instance , be manually powered plungers or bulbs with a check valve . in either case , the manually generated pressure is preferably provided to first pressure chambers 72 a , 72 b and 72 c . first valves 76 a , 76 b and 76 b are then provided for controllably releasing the pressure in the first chambers 72 a , 72 b and 72 c to the second pressure chambers 74 a , 74 b and 74 c . second valves 78 a , 78 b and 78 c may be provided in the second pressure chambers 74 a , 74 b and 74 c for controllably venting the pressure in the second pressure chambers 74 a , 74 b and 74 c . the control electronics 16 , which typically comprises a programmable micro - processor , opens the first valves 76 a , 76 b or 76 c when the fluid flow in the corresponding downstream fluid stream drops below a first predetermined value and opens the second , vent value 78 a , 78 b or 78 c when the fluid flow in the downstream fluid increases above a second predetermined value . each value is preferably an array of electrostatically actuated micro - valves that are individually addressable and controllable . the fluid flow sensors 62 a , 62 b and 62 c are preferably thermal anemometer type flow sensors . the hydrodynamically focused core stream 53 , containing sample particles arranged in single file , surrounded by a sheath fluid 51 , flows through flow channel 52 into raman chamber 61 . fig4 is a cross - sectional view of the cytometer flow channel of an exemplary embodiment of a raman detection based cytometer , comprising the flow stream 52 , an upper and a lower glass substrate 56 and 54 , an illuminating micro - lens 58 , a detecting micro - lens 60 , a source pass filter and source absorbing filters 64 and 66 , a focused core stream 53 , containing sample particles . fig5 is a schematic view of the illumination and detection optics of an exemplary embodiment of a raman detection based cytometer , comprising an illumination source 18 , source focusing optics 22 , a focused core stream 53 containing sample particles 68 , raman - scattered light - collecting optics 24 , a light dispersing optical element 26 and a light detecting element 28 . in a preferred embodiment , the raman chamber 61 comprises glass substrates 56 and 54 , flow channel 52 , micro - lenses 58 and 60 , source - pass filter 62 , and source - absorbing filter 64 and 66 . light emitted by light source 18 is directed as a collimated beam onto micro - lens 58 by beam steering and collimating optics 38 . micro - lens 58 focuses the light though source - pass filter 62 onto particles in the core stream 53 . light that is in - elastically ( raman ) scattered from particle in the core stream 42 passes through source - absorbing filter 56 and is directed towards detector 28 by micro - lens 60 and steering and discrimination optics 40 . light source 18 may be a laser or array of lasers having a wavelength , a power and a polarization suitable for raman spectroscopy . prior art light sources include argon - ion pumped ti : sapphire lasers operating at 830 nm , providing 200 mw of power focused down to about 2 × 10 5 w / cm 2 at the sample . other suitable light sources include nd : yag laser operating at 1064 nm and helium neon lasers operating at 632 . 8 nm . a vertical cavity surface emitting laser ( vcsel ) is particularly suited to portable implementation . vcsel are available that operate at wavelengths in the range from 850 nm to 670 nm . a 10 × 10 micron emitting surface vcsel typically has a power of about 1 mw , producing a power density of about 1 × 10 3 w / cm 2 when focused to a gaussian spot of about 10 microns . any suitable solid state laser may be used as light source 18 . the light source 18 may also be a single light source , such as a solid state laser or a diode laser , movably attached to one or more stepper motors so that either the sources lateral position or distance from the core - stream 53 , or both , may be adjusted to achieve optimal scattering . adjustment of such a movable source in focus and / or lateral displacement with respect to the core stream 53 may include a feed - back loop responsive to light reflected by a difference in refractive index between the core stream 53 and the sheath fluid 51 . the adjustment feed - back loop may also be accomplished by , for instance , using light reflected or scattered from tracking or calibration micro - beads that are interspersed with the molecules of interest in the core - stream steering and discrimination optics 40 and detector 28 may be , for instance , a sentinal raman spectrometer having a charge couple device detector ( ccd ), as supplied by bruker optics , inc . of bilerica , mass . detector 28 may also be any other low noise , high quantum efficiency multichannel detector or ccd array . steering and discrimination optic module may also comprise any suitable combination of filters , acoustoptic tunable filters ( aotf ), gratings , diffraction optics and holographic optics . in a preferred embodiment , the steering and discrimination optic module is a suitable holographic diffraction grating that combines wavelength selection with focusing the collected raman scattered light onto the detector 28 . source - pass filter 62 is a coating that band passes only the source wavelength , thereby eliminating collecting raman scattered light that occurs before the flow channel . any suitable well - know multi - layer or holographic band pass filter may be used . source - absorbing filters 64 and 66 are coatings that reject the source wavelength , thereby eliminating raman scattering after the flow channel . any suitable well - known multi - layer or holographic band pass filter may be used . once the core stream 53 and surrounding sheath fluid 51 have passed through raman chamber 61 , they drain into waste reservoir 54 . although the invention has been described in language specific to structural features and / or methodological acts , it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described . rather , the specific features and acts are disclosed as exemplary forms of implementing the claimed invention .