a new laser ablation capillary absorption spectrometer ( la - cas ) and process are described that provide highly sensitive ( down to 100 femtomoles ) and accurate ( 0 . 2 ‰ or better ) stable isotope measurements of molecular isotopologues present in samples including samples containing biological and other organic materials . the term “ isotopologue ” as used herein refers to molecular species that differ only in their isotopic composition , i . e ., the number of isotopic substitutions . each member of an isotopologue family has at least one atom with a different number of neutrons than the parent molecule . water , as an example , has isotopologues that include : light water ( hoh or h 2 o ); “ semi - heavy water ” with the deuterium isotope present in equal proportion to protium ( hdo or 1 h 2 ho ); “ heavy water ” with two deuterium isotopes of hydrogen per molecule ( d 2 o or 2 h 2 o ); and “ super - heavy water ” or tritiated water ( t 2 o or 3 h 2 o ), where hydrogen atoms are replaced with tritium isotopes . oxygen - related isotopologues of water include heavy - oxygen water ( h 2 18 o ) and the slightly lighter isotopologue containing the 17 o isotope , ( h 2 17 o ). in the doubly labeled water isotopologue , both oxygen ( o ) and hydrogen ( h ) are replaced by isotopes ( d 2 18 o ). further aspects of the present invention are detailed in a publication by kelly et al . in [ review of scientific instruments 83 , 023101 ( 2012 )], which reference is incorporated herein in its entirety . the following description includes a best mode of at least one embodiment of the present invention . it will be clear from this description that the invention is not limited to these illustrated embodiments but that the invention also includes a variety of modifications and embodiments thereto . while the invention is susceptible of various modifications and alternative constructions , it should be understood , that there is no intention to limit the invention to the specific form disclosed , but , on the contrary , the invention is to cover all modifications , alternative constructions , and equivalents falling within the spirit and scope of the invention as defined in the claims . therefore the present description should be seen as illustrative and not limiting . fig1 is a schematic showing a laser ablation ( la ) capillary absorption spectrometer ( la - cas ) 100 of the present invention . la - cas spectrometer 100 may include a modulation device 2 coupled to an excitation laser 4 . modulation device 2 may be configured to deliver a current - controlled waveform to excitation laser 4 . excitation laser 4 may be configured to supply a non - pulsed excitation beam 6 to an absorption ( or transmission ) device 30 . the current - controlled waveform 6 may include : a dc offset current that establishes the initial frequency of beam 6 emitted from laser 4 , a repetitive linear current ramp ( i . e ., repeated cycles of up and down ramping ) that sets the range over which the frequency of laser 4 is scanned that may be added to or superimposed atop a dither - modulated current that serves to modulate the output frequency of beam 6 emitted from laser 4 . the output frequency of beam 6 may be modulated around a center value determined by the sum of the dc offset current and the linear ramped current as a function of time . “ dither - modulated ” as used herein means a variation in a selected frequency that is on the order of the transition line width relative to the total frequency range being scanned . the term “ transition ” as used herein means a discrete absorption frequency characteristic of a selected molecular gas ( or target isotopologue ) defined by the difference in energy levels being probed by the excitation laser . the terms “ dither ” or “ dithering ” as used herein refer to a sinusoidal variation in current added in the modulation device to a dc current offset and a linear ramp current resulting in a variation in laser frequency that is smaller ( 10 times to 20 times lower ) in amplitude compared to a total range being scanned [ e . g ., ½ wavenumber ( cm − 1 )] and on a time scale that is faster ( at least a factor of 5 greater ) than the frequency of the repetitive current ramp . for example , if the linear current ramp is repetitively ramped ( cycled ) at 50 khz , the dither frequency may be 250 khz or greater . continuous wave lasers 4 suitable for use may include , but are not limited to , e . g ., quantum cascade ( qc ) lasers , distributed feedback ( dfb ) lasers , inductively coupled ( ic ) lasers , external cavity ( ec ) qc lasers , diode lasers , including combinations of these various lasers . laser 4 may be contained within a dewar vessel 8 that maintains a stable temperature for laser 4 . in some embodiments , laser 4 may be cooled , e . g ., with liquid nitrogen , or with a thermo - electric cooler ( not shown ). temperatures may be selected that assist the shift in laser beam frequencies to a desired region of interrogation . in some embodiments , excitation beam 6 delivered by excitation laser 4 may be modulated with a sine wave at a modulation index that is less than or equal to the doppler width of the molecular absorption line widths . the term “ modulation index ” as used herein is defined as the frequency range over which a sine wave varies the excitation laser 4 . laser beam 6 transmitted from laser 4 may be focused in concert with various spatial filtering devices including , e . g ., focusing lenses 10 and 12 , slits 14 , pin holes 16 , distributed feedback ( dfb ) gratings , including combinations of these various devices . spatial filtering devices may be further used to suppress reflections of light back to excitation laser 4 . eliminating reflected light may eliminate feedback noise in the absorption signal . for example , spatial filters including , e . g ., slits 14 and pin holes 16 may be positioned at a tilt angle that selects a subset of rays from excitation beam 6 at a selected pointing angle ( defined by an angle phi ) and selected beam spread defined by a divergence angle ( theta ) into the absorption ( transmission ) device 30 . “ pointing angle ( phi )” as used herein is an angle representing the angular variation from the central axis of laser beam 6 , described further in reference to fig2 . “ divergence angle ( theta )” as used herein refers to the angle at which the laser beam diverges or converges from the central axis that defines the spread of the of laser beam 6 . “ central axis ” as used herein means the optical axis of the laser 4 . in some embodiments , lenses 10 and 14 may be collimating ( focusing ) lenses that deliver excitation beam 6 from laser 4 such that the excitation beam 6 is delivered off - axis ( i . e ., at an angle that is offset from the central axis of excitation beam 6 ) into absorption ( transmission ) device 30 . in some embodiments , lens 10 may be an external aspheric ( e . g ., f / 1 ) lens constructed of a transmission material such as zinc selenide ( znse ) that includes a broadband anti - reflective ( ar ) coating configured to transmit wavelengths from about 0 . 6 μm to 16 μm suitable for medium wavelength infra - red ( i . e ., mwir ) operation . in some embodiments , excitation beam 6 may be extracted through an off - axis pin hole 16 to absorption device 30 that suppresses feedback noise back to excitation laser 4 . in some embodiments , lens 14 may be a plano - convex , ar - coated silicon ( si ) lens ( e . g ., 50 - mm focal length , with & gt ; 98 % bandpass in the mwir ) positioned , e . g ., in front of pin hole 16 ( e . g ., ˜ 1 . 5 - mm diameter ), but lens types are not intended to be limited thereto . laser 4 may be coupled to an optical isolation device 18 positioned between excitation laser 4 and absorption device 30 ( discussed further in reference to fig2 a - 2 c herein ). “ optical isolation device ” as used herein means a device that delivers turbulence 26 , e . g ., through a medium 24 or turbulence to elements present within the medium 24 as a function of time . the term “ turbulence ” as used herein refers to random spatial and temporal variations introduced in the medium through which the laser beam traverses or that is delivered across the propagation path of the laser beam that alters the index of refraction of the medium . the spatial and temporal variations prevent light from reaching the excitation laser when reflected from the absorption device reaching back to the excitation laser . turbulence may be induced in concert with various gradients in the medium that introduce variations across the propagation path of the light beam from the laser en route to the absorption ( transmission ) device . the term “ gradient ” used herein means a randomly varying distribution of elements ( e . g ., particulates ) present within the medium that introduce variations in the propagation path of the light beam . gradients include , but are not limited to , e . g ., velocity gradients , temperature gradients , heat gradients ( i . e ., gradients defined by a random distribution of heat waves ), density gradients , sonic gradients , aerosol gradients , particulate gradients , and / or other gradients . medium 24 may be a fluid medium , a gaseous medium , a liquid medium , an aerosol - containing medium , a particulate - containing medium , or combinations of these various media . medium 24 may be an open medium an enclosed medium . for example , in some embodiments , medium 24 may be contained within an enclosure 23 or vessel 23 . elements in medium 24 may include , e . g ., liquids , gases , particulates , aerosols , heat waves , sonic waves , and like elements that can define a gradient . “ optical isolation ” as used herein refers to the process whereby the excitation laser may be isolated from reflected light that returns back to the excitation laser stemming from an optically coupled absorption ( or transmission ) device . reflected light introduces feedback noise into the absorption ( transmission ) spectrum . optical isolation in concert with the present invention may be achieved by inducing random spatial ( i . e ., 3 - d positions in space ) and temporal ( time ) variations in the index of refraction of a medium positioned between the excitation laser and the absorption ( transmission ) device . these random variations may be introduced by a turbulence generator into the medium across the propagation path through which the laser beam traverses en route to the absorption device , or a particulate ( e . g ., aerosol ) generator that disperses particles of varying size and velocity in the medium through which the light beam from the excitation laser is delivered that randomizes the pointing angle and the spread of the light beam as a function of time while the light beam is en route to the absorption ( or transmission ) device . the turbulence or particulates prevent feedback noise from appearing in the resulting absorption spectrum ( see fig7 ). optical isolation device 18 may be configured to scatter light delivered from laser 4 through medium 24 en route to capillary waveguide 30 detailed hereafter such that light incident upon , and reflected from , the capillary waveguide 30 experiences scattering conditions and shifts in the pointing angle and beam spread that are different than conditions present for the incident light . the change in pointing and beam spread may serve to isolate laser 4 from reflected light , which serves to suppress optical feedback and resulting feedback noise in the absorption spectrum . in particular , turbulence may yield a random spatial and temporal variation in the index of refraction of medium 24 or a selected gradient in delivery medium 24 that may suppress feedback noise induced by reflection of the excitation beam 6 incident upon , and from within , the absorption device 30 back to excitation laser 4 . turbulence induced in the medium may provide a signal - to - noise ratio ( snr ) enhancement of at least about a factor 2 or better . optical isolation device 18 may include a turbulence generator 20 or randomization device that generates turbulence 26 in a medium ( open or enclosed ) 24 across the propagation path through which the laser beam 6 traverses en route to absorption device 30 . turbulence generators 20 may include , but are not limited to , e . g ., heat sources , gas sources , forced gas sources , flowing gas sources , aerosol devices , nebulizing devices , flame sources , sonic sources , liquid sources , forced liquid sources , flowing liquid sources , including combinations of these various devices . in some embodiments , the turbulence generator may be a heat gun configured to deliver a random spread or distribution of heat waves of varying temperature across the propagation path of light beam . in some embodiments , turbulence generator may be coupled to an enclosed liquid through which the light beam is directed that provides the variation in the index of refraction of the light beam when delivered from laser device to absorption ( or transmission ) device when directed through the liquid . in some embodiments , the randomization source may be an aerosol generator configured to deliver a random spread of aerosols or particulates dispersed in the medium with a random distribution of sizes and velocities as a function of time across the propagation path of the light beam that is delivered through the volume of aerosols to the absorption ( or transmission ) device that induces random spatial and temporal variations in the scatter of the beam as it passes through the aerosols or particulates in the medium while en route to the absorption ( transmission ) device . in some embodiments , the turbulence generator may be a vapor - nebulizing device . in some embodiments , the randomization source may be a sonic device configured to deliver a random spread of sonic waves at a plurality of temperatures above ambient across the propagation path of the light beam from the laser that provides the random spatial and temporal variation in the medium when the light beam is delivered through the spread of sonic waves to the absorption device and / or the transmission device . the la - cas spectrometer 100 may also include an absorption ( or a transmission ) device 30 such as a hollow waveguide . in some embodiments , absorption device 30 may be a capillary waveguide configured to receive a sample gas for analysis containing analytes that absorb ( or transmit ) in the infra - red wavelength region when an excitation beam from an excitation laser is introduced into the sample gas therein . in some embodiments , the waveguide may have an internal diameter ( i . d .) that is 1 - mm and a length that is 0 . 8 meters . however , dimensions are not intended to be limited . in some embodiments , the waveguide may have an internal diameter that is constant . in some embodiments , the waveguide may be of a tapered design , with an internal diameter that varies as a function of length ( i . e ., one end has a smaller diameter than the other end ) to suppress noise from reflections within the capillary . in some embodiments , gas volumes in the absorption ( transmission ) device may below about 0 . 6 cm 3 . in other embodiments , gas volumes may be greater than about 0 . 6 cm 3 . no limitations are intended . in some embodiments , waveguide 30 may include coatings optimized for selected laser wavelengths . in some embodiments , waveguide 30 may include louisville optics , e . g ., as detailed by harper et al . in laser - based spectroscopic detection techniques , u . s . pat . no . 7 , 102 , 751 , which reference is incorporated herein in its entirety . capillary waveguide 30 can allow analyses of samples at total gas pressures below about 25 torr ( 0 . 03 atm ), more preferably below about 2 torr ( 0 . 003 atm ), and yet more preferably between about 1 torr ( 0 . 001 atm ) and 2 torr ( 0 . 003 atm ). absorption device 30 may also include transmission windows 32 and 38 positioned at respective ends of absorption device 30 that deliver selected ir wavelengths that are absorbed by analytes in absorption device 30 that provide for analysis of these analytes . in various embodiments , the waveguide may be a capillary waveguide that includes ir transmission windows composed of , e . g ., zns that transmit wavelengths from about 0 . 6 μm to 16 μm . in some embodiments , the transmission windows may be composed of sapphire that transmits wavelengths in the range from about 0 . 15 μm to 5 μm . in other embodiments , transmission windows may be made of other transmission materials known in the art may be that transmit ir wavelengths within selected and suitable ranges for absorption by analytes in the waveguide . all transmission materials as will be selected by those of ordinary skill in the art in view of the disclosure are within the scope of the invention . no limitations are intended . absorption device 30 may be coupled to low - dead - volume or vacuum tees 34 and 40 and vacuum valves 36 and 42 that allow sample gases and analytes to be introduced into waveguide 30 and exchanged from waveguide 30 for absorption measurements therein . in some embodiments , a gas inlet 48 may be employed to provide gas transfer rates into absorption device 30 at low inlet pressures , e . g ., between about 0 . 5 torr ( 7 × 10 − 4 atm ) and about 20 torr ( 0 . 03 atm ). inlet 48 may be of a small dead volume design to provide gas - tight operation into waveguide 30 . gas inlet 48 may couple to any of a variety of instrument systems including , but not limited to , e . g ., gas chromatographs ( gc ), laser - ablation and catalytic reactor devices detailed further herein , and other analytical devices . no limitations are intended . in some embodiments , waveguide 30 may be operated in a static mode . in some embodiments , waveguide 30 may be operated in a continuous ( dynamic ) mode . for example , in some embodiments , outlet valve 36 may couple to a vacuum system ( not shown ) allowing exchange of static samples or a controlled continuous flow of a sample in a dynamic or a continuous sampling mode at selected and selected and / or suitable vacuum pumping speeds and gas transfer rates . position of valves in the present invention is not intended to be limited to exemplary embodiments described herein . in some embodiments , waveguide 30 may be coupled to a laser ablation device and a catalytic reactor as detailed hereafter , which permits conversion of solid samples into molecular gases suitable for absorption measurements within waveguide 30 . spectrometer ( la - cas ) 100 may include a laser ablation device 44 coupled with a catalytic ( or combustion ) reactor 46 that provides ablation of solid samples and conversion of the ablated particulates forming molecular gases . the molecular gases may be introduced to absorption device 30 for analysis in spectrometer ( la - cas ) 100 . the la - cas spectrometer ( la - cas ) 100 may also include a detector 52 that is coupled to absorption device 30 and to a detection device 54 configured to detect absorption features for one or more analytes in a sample gas when introduced into absorption device 30 . the detector transmits absorption ( transmission ) signals . detectors suitable for use may include infra - red sensitive detectors such as indium - antimonide ( insb ) detectors configured to produce an output current that is proportional to the intensity of the laser light transmitted from the absorption device that is now incident on the detector resulting from detection of absorption ( transmission ) signals generated in the waveguide during analysis . fig2 a shows a turbulence generator 20 coupled to a medium 24 containing a gas dispersed in an open volume . in the figure , turbulence generator 20 induces turbulence in the gas 24 by delivering random thermal and velocity gradients across the propagation path of excitation beam 6 delivered through the gas in the open medium 24 . in some applications , turbulence may be induced in the gas by delivering heated and turbulent air from the generator . in the figure , excitation beam 6 may be delivered through medium 24 to absorption device 30 . when delivered through the turbulent medium , laser beam 6 may be spread from a parallel delivery path by a distance defined by a divergence angle ( theta ) 60 . any reflected light 56 from absorption device 30 must pass through medium 24 again . the random spatial and temporal variations in the index of refraction of the medium 24 due to the induced turbulence ensures that laser beam divergence angle ( theta ) 60 and laser beam 6 pointing angle ( phi ) 58 are different for reflected light 56 , which prevents reflected light 56 from being incident on laser 4 and prevents feedback - induced noise in the absorption spectrum . fig2 b shows a turbulence generator 20 ( e . g ., a sonic generator or a heat generator ) coupled to an enclosed volume 24 containing , e . g ., an enclosed fluid or an enclosed gas . again , turbulence generator 20 induces turbulence in the fluid or gas contained within the enclosed volume 24 that delivers , e . g ., random sonic and / or thermal agitation in the enclosed medium 24 . in the figure , excitation beam 6 may be delivered through medium 24 to absorption device 30 . when delivered through the turbulent medium 24 , laser beam 6 may be spread from a parallel delivery path by a distance defined by a divergence angle ( theta ) 60 . any reflected light 56 from absorption device 30 must pass through medium 24 again . random spatial and temporal variations in the index of refraction of medium 24 containing the gas due to the induced turbulence ensures that laser beam 6 includes a divergence angle ( theta ) 60 and a pointing angle ( phi ) 58 that are different for the reflected light 56 , which prevents reflected light 56 from being incident on laser 4 and prevents feedback - induced noise in the absorption spectrum . fig2 c shows a generator 22 of an aerosol or particulate delivery design used as a component of an optical isolation device 18 . in the figure , generator 22 is configured to deliver a random spread of aerosols or particulates with a random distribution of sizes and velocities across the propagation path of the light beam 6 when the light beam 6 is delivered to the absorption ( or transmission ) device 30 . for example , when the light beam 6 is directed through the spread of aerosols or particulates in the medium 24 , the pointing angle beam 6 and the spread of the light beam as a function of time are randomized . in particular , when the laser beam 6 is delivered through the aerosols or particulates in the medium , any reflected light 56 from absorption device 30 must pass through the medium 24 again . the random distribution of aerosols of various sizes and velocities in medium 24 ensures that the divergence angle ( theta ) 60 and the pointing angle ( phi ) 58 of laser beam 6 are different for the reflected light 56 , which prevents reflected light 56 from returning back to the laser 4 , thereby preventing feedback - induced noise in the absorption spectrum . fig3 shows exemplary devices that may be coupled to cas 100 to convert solid samples into molecular gases suitable for analysis by the present invention . as shown in the figure , cas 100 may further include a laser ablation device 44 that couples to a catalytic reactor 46 . laser ablation device 44 may ablate the sample ( including , e . g ., biological materials ) and form sample particulates , e . g ., as detailed by moran et al . ( rapid commun . mass spectrom . 2011 , 25 , 1282 - 1290 ), which reference is incorporated herein in its entirety . ablated sample particulates from ablation device 44 may be delivered through a catalyst in catalytic reactor 46 , which converts the particulates into a molecular gas containing isotopologues representative of the ablated sample . molecular gases formed in reactor 46 may be subsequently transferred via a transfer device 50 ( described further herein in reference to fig4 and fig5 ) into the wave guide 30 for determination therein . sample particulates may be combusted forming gases that may be analyzed at pressures below 25 torr ( 0 . 03 atm ). molecular gases stemming from the converted and combusted sample may be transferred through a transfer device detailed hereafter to the capillary absorption device 30 ( described previously in reference to fig1 ) for analysis . fig4 a shows one embodiment of a transfer device 150 of a cryogenic design configured to transfer ( deliver ) analytes in a molecular gas from a catalytic reactor 46 at a first ( e . g ., high ) pressure and deliver the analytes in the molecular gas into absorption device 30 at a second ( lower ) pressure without loss of analytes . transfer device 150 may include a cryogenic trap 63 with a trapping volume 68 that is configured : 1 ) to receive the molecular gas delivered from the catalytic reactor 46 , e . g ., through a first valve 64 at a first ( high ) pressure . trapping volume 68 of cryogenic trap 63 may be coupled , e . g ., to a condensing ( freezing )/ evaporation device 70 that condenses and traps the analyte - containing molecular gas . for example , in the cryo - trapping phase , valve ( v 1 ) 64 may be opened to receive a flow of gas from catalytic reactor 46 into trapping volume 68 of cryogenic trap 63 at a temperature below the freezing point of the molecular gas such the sample gas is condensed , frozen , and trapped as a solid therein while allowing a carrier gas such as helium ( he ) with a higher freezing temperature to flow through the trap 63 . a purge and / or carrier gas 62 such as helium ( he ) may be introduced into cryogenic trap 63 , e . g ., through a second valve ( v 2 ) 66 to assist with transfer of sample gases from trapping volume 68 to capillary absorption spectrometer 30 . valve ( v 1 ) 64 may be closed and trapping volume 68 containing the cryogenically trapped sample may be evacuated ( e . g ., through a vacuum system valve of the la - cas described in reference to fig1 ) to remove excess carrier gas that yields a lower pressure , e . g ., between about 1 torr ( 0 . 001 atm ) and about 25 torr ( 0 . 03 atm ) suitable for introduction of the molecular gas into capillary absorption device 30 for analysis therein . in this manner , analytes in cryogenic trap 63 may be isolated to give gas pressures in trap 63 that match the second ( lower ) pressure of absorption device 30 . following evacuation , valve 36 to vacuum system may be closed and freezing / evaporation device 70 may heat the cryotrapped gas in trapping volume 68 above the evaporation temperature of the molecular gas , allowing it to fill absorption spectrometer 30 at the second ( lower ) pressure . pressures suitable for use in the transfer device are not intended to be limited . however , pressures may include a first ( high ) pressure of from about 760 torr ( 1 atm ) down to 100 torr ( 0 . 13 atm ). second pressure may be a pressure below about 25 torr ( 0 . 03 atm ). in the instant configuration , cyrotrapping volume 68 of cryogenic trap 63 is separate and distinct from the capillary absorption spectrometer 30 volume . fig4 b shows another embodiment of a cryogenic transfer device 250 of the present invention . in this embodiment , freezing / evaporation device 70 is positioned in thermal contact with a selected section or portion of capillary waveguide 30 such that the trapping volume 68 of cryogenic trap 63 is positioned within , and at the leading end of , capillary waveguide 30 to provide cryotrapping of the molecular gas in that section or portion of capillary waveguide 30 . in the present embodiment , the capillary waveguide 30 functions both as the transfer device at the first high pressure and the capillary waveguide 30 at the second lower pressure . the present embodiment eliminates potential for introducing dead volumes during the evaporation phase immediately prior to analysis which can reduce molar sensitivity or diluting the cryotrapped sample that can result from addition of an extra volume external to capillary waveguide 30 . all other components and operation are as detailed above for fig4 a . fig5 a shows yet another embodiment of a transfer device 350 of a conduction - limited design configured to deliver analytes in a molecular gas from a catalytic reactor 46 at a first ( e . g ., high ) pressure and deliver the molecular gas containing the analytes into absorption device 30 at a second ( lower ) pressure without a loss of analytes . pressures suitable for use are not intended to be limited , as detailed hereinabove . transfer device may include a pin hole ( orifice ) 72 positioned to impose a conductance barrier for transfer of molecular gases between two different pressures , e . g ., a first high pressure and a second lower pressure . dimensions of pin hole 72 are not limited , and may be selected such that molecular gases are transferred through pin hole 72 at a selected rate and at a selected pressure into absorption device 30 . for example , transfer of the gas through the pin hole 72 reduces pressure of the gas from the first high pressure to the 2 nd lower pressure such that it is compatible with the inlet pressure into absorption device 30 at the lower pressure . fig5 b shows yet another embodiment of a conduction - limited transfer device 450 that operates under a graded pressure reduction regime , providing continuous transfer of analytes from the catalytic reactor 46 to the absorption device 30 . in the figure , transfer device 450 has the form of a capillary tube 74 positioned to impose a conductance barrier for transfer of molecular gases between two different pressures , e . g ., a first high pressure and a second lower pressure . length and internal diameter dimensions of capillary tube 74 are not limited , and may be selected such that analytes in the molecular gas received from catalytic reactor 46 may be transferred into capillary tube 74 at a selected rate and at a first high pressure , and then undergo a selected and graded pressure reduction , and then to exit capillary tube 74 at a second ( lower ) pressure compatible with introduction into ir absorption device 30 . analytes may be continuously transferred at selected rates from catalytic reactor 46 to ir absorption device 30 for analysis . fig6 shows a schematic of a ( signal ) modulation device circuit 2 of la - cas 100 . modulation device 2 may be configured to modulate the wavelength of excitation beam 6 delivered by excitation laser 4 , which modulations may be delivered as outputs to excitation laser 4 . modulation device 2 may include a linear current ramp source 122 [ e . g ., a digital signal oscillator ( dso )] that provides a repetitive linear ramp current to a dc power ( offset ) ( summing ) device 124 . the dc offset device 124 provides a dc offset current . the linear current from the linear current source 122 determines the wavelength range over which the laser frequency is scanned . the value of the dc offset current determines the starting value of the laser frequency in the frequency scan . the maximum value of the current from the linear current ramp source 122 determines the ending value of the laser frequency scan . the slope of the current from the linear ramp device 122 may determine the rate at which the laser frequency is scanned . the dc offset device 124 then sums the linear ramp current obtained from the linear current source 122 together with the dc offset current from the dc offset device 124 , e . g ., as detailed by taubman et al . in u . s . pat . no . 7 , 957 , 441 , which reference is incorporated in its entirety herein . modulation device 2 may further include a dither - modulated current source 128 [ e . g ., a dso ] that delivers a dither - modulated current ( e . g ., ac delivered at 1f ) as an output in which the frequency of the modulated current is ½ the in - quadrature condition of a residual amplitude modulation distortion frequency ( response ) of excitation laser 4 . the term “ in - quadrature ” as used herein means the frequency oscillations ( modulations ) in the excitation beam differ or are separated in phase by 90 ° ( π / 2 radians or λ / 4 ) from the amplitude oscillations ( or modulations ). in some embodiments , dither - modulated current source 128 may be operated at frequency ( f ) that includes a crossover frequency near 95 khz , e . g ., as detailed by kelly et al . [ review of scientific instruments , 83 , 023101 ( 2012 )]. no limitations are intended . the output ( ac ) from the dither - modulated current source [ dso f ] 128 may be combined with the repetitive linear ramped current delivered from the linear current ramp source [ dso ] 122 along with the dc offset current from the dc offset source 124 and provided as an output to a 2 nd order or higher bias - tee ( diplexer ) 126 positioned in modulation device 2 . bias - tee ( diplexer ) 126 may deliver a combined , summed , and thus total modulated current signal to excitation laser 4 as a function of time . bias - tee 126 may also suppress harmonics from the repetitive linear current ramp source 122 and sub - harmonics from the dither - modulated current source 128 such that the amplitude of the harmonics and sub - harmonics is at least 60 db lower than the total modulated current waveform selected and provided to excitation laser 4 . the term “ harmonic ” means an integer multiple of a fundamental oscillation frequency delivered by a signal oscillator that may cause undesired noise in a detected signal . the term “ sub - harmonic ” means a fraction of the fundamental oscillation frequency delivered by a signal oscillator that may cause undesired noise in a detected signal . fig6 further shows a phase - sensitive detection device circuit 54 that couples with modulation device circuit 2 . phase - sensitive detection circuit 54 suppresses noise in the absorption signal arising from the ram distortion frequency received from excitation laser 4 and from harmonic distortion frequencies received from dither - modulated current source 128 . phase - sensitive detection circuit 54 may include a first circuit segment ( leg ) 102 having a diplexer / band - pass filter 112 of a 2 nd or greater order that delivers a detection signal 106 from absorption device 30 as an input to a double - balanced mixing device 114 . first circuit segment 102 may also include a bias - tee 108 of a 2 nd or greater order configured to receive the detection ( absorption ) signal 106 from the detector ( fig1 ). detection signal 106 may be amplified , e . g ., with a pre - amp device 110 prior to being introduced to diplexer / bandpass filter 112 that filters out amplification noise from detection signal 106 before it is introduced to double mixing device 114 . phase - sensitive detection circuit 54 may also include a second circuit segment ( leg ) 104 that includes a band - pass filter ( bpf ) 118 configured to deliver a reference ( drive ) signal free of harmonic and intermodulation distortion as a separate input to double - balanced mixing device 114 . in some embodiments , the reference drive signal delivered to the bandpass filter 118 may be at a frequency ( i . e ., 1f ) provided by the dither - modulated current source 128 that is ½ the frequency for the in - quadrature condition of the residual amplitude modulation distortion frequency ( response ) of excitation laser 4 . the term “ 1f ” refers to the drive frequency of the dither - modulated current source 128 . signal recovery can be achieved at ( 1f ) when the amplitude of the laser device is constant over the wavelength range being scanned . thus , no limitations are intended . in some embodiments , a 2 nd dso device 120 may be inserted between device [ dso f ] 128 and bandpass filter 118 to deliver a reference or drive ( 2f ) signal [ e . g ., for ( 2f ) signal recovery ] to bandpass filter 128 at a frequency ( i . e ., 2f ) that is 2 times the frequency provided by the dither - modulated current source [ dso f ] 128 . the 2f drive frequency provided by dso 120 device may be synchronized such that it is in - phase with the output ( 1f ) frequency delivered from dither - modulated current source 128 . the term “ 2f ” ( or second harmonic detection or waveform modulated detection ) refers to a drive frequency or a detection frequency that is twice the magnitude of the reference frequency delivered from the dither - modulated current source 128 . a ( 2f ) frequency may be used when the amplitude of the intensity of the laser beam is not constant over the wavelength range being scanned in order to eliminate slope in the baseline , delivering a flat baseline in the detection signal . double - balanced mixing device 114 may then deliver an output signal made up of various combinations of signals ( i . e ., mixed signals ) received from the respective inputs . for example , double - balanced mixing device ( dbm ) 114 may couple to a 2 nd or greater order diplexer ( bias - t ) 122 configured to : 1 ) receive the mixed output signal from the dbm 114 as an input and 2 ) to isolate a selected absorption response signal ( see discussion fig7 ) for one or more selected analytes present in the mixed signal . the 2 nd - order or greater diplexer 122 may be configured to receive an output signal from the double balanced mixing device 114 as an input and to deliver a distortion free response signal as an output at a selected frequency therefrom . in various embodiments , the 2 nd - order or greater diplexer 122 may deliver the absorption response signal isolated from 1 - times ( 1f ) or 2 - times ( 2f ) the dither - modulated frequency delivered from the dither - modulation device 128 that is also 90 degrees out - of - phase with the ram distortion frequency provided to excitation laser 4 . in some embodiments , the 2 nd - order or greater diplexer may deliver a response signal with a frequency selected in the range from about 100 hz to about 30 khz with an isolation of about 30 db per decade at the upper cutoff frequency . the isolated response signal may be observed in concert with a cpu 130 or other transient signal recording device . fig7 compares ( 2f ) absorption signals for representative isotopologues of carbon dioxide containing 13 c and 12 c , respectively , recovered with and without use of the optical isolation device of the present invention . in the figure , signals were obtained at twice the frequency of the dither - modulated current source . results show the reduction in feedback noise produced by the incident laser beam on the capillary wave guide stemming from reflection from the absorption device back to the excitation laser . results collected with the optical isolation device ( fig1 ) of the present invention shows that feedback noise is reduced by a factor of at least about 50 compared with results collected absent the optical isolation device . however , reduction factors are not intended to be limited . the reduction factor may be calculated as the ratio of the total integrated intensity of the absorption features to the standard deviation ( sd ) of variations ( noise ) observed in the baseline . in table 1 , isotopic precision (‰) is stated in standard delta ( δ ) notation used in isotopic measurements , in which stable isotopic compositions of low - mass ( light ) elements such as oxygen , hydrogen , carbon , nitrogen , and sulfur are normally reported as “ delta ” ( δ ) values in parts per thousand ( denoted as ‰) enrichments or depletions relative to a standard of known composition . the symbol (‰) is spelled out in several different ways : per mil , per mil , per mill , or per mille . the term “ per mill ” is the iso term , but is not yet widely used . delta ( δ ) values may be calculated , as given by equation [ 1 ] below : here , “ r ” is the ratio of the heavy to light isotope in the sample or standard . for the elements sulfur , carbon , nitrogen , and oxygen , the average terrestrial abundance ratio of the heavy to the light isotope ranges from 1 : 22 ( sulfur ) to 1 : 500 ( oxygen ); the ratio 2 h : 1 h is 1 : 6410 . a positive ( δ ) value means that the sample contains more of the heavy isotope than the standard . a negative ( δ ) value means that the sample contains less of the heavy isotope than the standard . as an example , a δ 15 n value of + 30 ‰ means that there are 30 parts - per - thousand ( or 3 %) more 15 n isotopes in the sample compared with the standard . as shown in table 1 , la - cas 100 of the present invention provides a sensitivity exceeding other infrared absorption detection methods . la - cas 100 with 1 - mm id hollow waveguides ( hwgs ) of nominal 1 - m path length can resolve δ 13 c variances to less than 1 ‰ with total sample quantities down to 100 picomoles ( pmols ) using commercial standards . the la - cas has also demonstrated molar sensitivities down to 2 picomoles at 1 ‰, with projected improvements down to 100 femtomoles . the la - cas of the present invention enables spatially resolved isotopic imaging in a variety of sample types for fundamental biological and chemical research and forensic applications , including , e . g ., thin films , polymers , forensic samples including , e . g ., hair ( both human and animal ), tree growth rings , and microbial systems . and , la - cas can be performed on samples under ambient atmospheric conditions that require no preparation beyond mounting the sample . stable isotope analyses permit tracking of physical , chemical , and biological reactions in various sample materials in physical , biological , and chemical processes , interactions , and mechanisms at various spatial scales ranging from the atmosphere to individual microorganism cells . the la - cas may also be configured to study isotopic ratios of co 2 and other gases produced by catalytic combustion of evolved particulates created by uv laser ablation of nonvolatile organics , and can be used to study co 2 and other molecular gases directly from various niche bio - compartments including waste gases generated from soil bacteria and other environments . the la - cs also provides highly sensitivity isotope ratio measurements on extremely small samples with volumes less than or equal to about ˜ 0 . 5 ml . the present invention also eliminates optical feedback in continuous ( cw ) quantum cascade ( qc ) lasers . while exemplary embodiments of the present invention have been shown and described , it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its true scope and broader aspects . the appended claims are therefore intended to cover all such changes and modifications as fall within the spirit and scope of the invention .