Long capillary waveguide raman cell

The chemical properties of an aqueous liquid, the liquid comprising a solvent having an index of refraction which is the same as or closely approaches that of water, are determined by Raman spectroscopy wherein a sample of the liquid is delivered into an optical waveguide. The waveguide is in the form of a capillary having a reflective surface defined by a material having a refractive index of less than 1.33. Excitation light is transmitted axially into the liquid at an end of the waveguide. The excitation light is transmitted the length of the waveguide, by reflection from the reflective surface, causing the fluid to emit Raman spectra. The Raman spectra is transmitted along the waveguide, collected and delivered to a spectrometer.

FIELD OF THE INVENTION 
The present invention relates to the detection of analytes present in an 
aqueous solution and particularly to the performance of light spectrum 
measurements to determine the chemical properties of a liquid analyte. 
More specifically, this invention is directed to analytical cells 
configured for Raman spectroscopy which employ a liquid core waveguide as 
the cell in which fluid analyte is placed while being illuminated by a 
monochromatic light whereby line spectrum induced in the analyte may be 
measured. Accordingly, the general objects of the present invention are to 
provide novel and improved methods and apparatus of such character. 
BACKGROUND OF THE INVENTION 
The irradiation of certain sample materials with an intense monochromatic 
light causes pertubations of the molecular energy levels of the sample. As 
a result, secondary light waves with wavelengths which are different than 
that of the monochromatic light are produced and radiated from the sample. 
This phenomenon is known as Raman scattering. 
Raman spectroscopy is a well established technique that enjoys wide 
application in industry and many fields of research. However, Raman 
scattering is a very weak effect. Consequently, conventional Raman 
spectroscopy requires a highly focused and intense laser beam and a very 
efficient detection system. Typically, Raman light is collected from a 
point source, within a small solid angle. 
In 1972, Stone and Walrafen demonstrated that the use of a liquid core 
waveguide can enhance the Raman signal detected from a sample by a factor 
of 10.sup.2 to 10.sup.3. This improvement resulted from using a sample 
solution as the core and the cell wall as the waveguide cladding. 
Accordingly, both the excitation light and the Raman light are guided 
along the waveguide/cell and thus through the liquid sample. Increasing 
the length of the tubing increases the number of interactions between the 
excitation light and the sample solution, thereby magnifying the Raman 
signal. Stone and Walrafen used quartz tubing as their cell and were 
unable to measure Raman spectra in fluids having refractive indices equal 
to or lower than the refractive index of quartz (1.46). Restated, the 
described technique relied upon reflection from the interface between the 
sample/core and the cell wall and the requisite reflection, in turn, 
requires that there be an interface between the core and a material having 
a lower index of refraction, 
In 1987, Schwab and McCreery disclosed a long capillary Raman cell 
consisting of uncoated glass tubing exposed to air. Total reflection 
theoretically occurred at the glass/air interface on the exterior surface 
of the tubing. With a refractive index of 1.0, the air total reflection 
surface virtually removed the constraint on the refractive index of the 
liquid sample. However, this design is subject to several limitations. The 
excitation light intensity is almost evenly distributed within the total 
diameter of the cell. Consequently, the cell wall must be very thin to 
retain effective excitation intensity within the sample. Therefore, cells 
in accordance with this design are exceedingly fragile and are likely to 
break when subjected to a slight amount of flexing. Furthermore, the 
outside of the cell must be kept extremely clean in order to maintain 
efficient waveguide action. 
Due to the above-discussed limitations of the previously proposed 
apparatus, the liquid core waveguide method for intensifying Raman signals 
has not been exploited. The failure to exploit this technology can be 
primarily attributed to the unavailability of an appropriate material with 
which to construct a commercially viable aqueous waveguide cell. 
SUMMARY OF THE INVENTION 
The present invention overcomes the above-briefly discussed and other 
deficiencies and disadvantages of the prior art by providing a Raman cell 
having a long path length. This invention thus also encompasses a method 
of achieving the enhancement of Raman light signals. 
In accordance with a preferred embodiment of the present invention, a 
suitably shaped vessel, i.e., a waveguide, is fabricated from a glass or 
quartz which has been coated on a surface with a material having a 
refractive index lower than that of water (1.33). Alternatively, the 
vessel may be fabricated from a material having a refractive index lower 
than 1.33. 
The vessel or cell described above acts as a waveguide when filled with an 
aqueous solution or any other liquid which has a refractive index which is 
greater than that of the coating. The excitation light radiation is 
confined within the sample liquid and the cell wall by means of the total 
reflection surface defined by the coating. Typically, the cell will be in 
the form of a capillary tube. Within a finite cone, Raman emission in 
forward and backward directions relative to the excitation propagation is 
integrated over the length of the capillary. 
A Raman cell in accordance with the present invention intensifies the Raman 
light signal for aqueous samples. Additionally, a Raman cell in accordance 
with the present invention is more flexible, sturdier and easier to use 
than conventional Raman cells.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS 
In the practice of the present invention, a liquid core waveguide is 
employed as the sample cell of a Raman spectrometer. In order to propagate 
light therethrough with negligible losses, it is necessary that the core 
region of a waveguide/cell in accordance with the invention effectively be 
surrounded by a material having a lower refractive index to the incident 
light than the liquid material comprising the core. This arrangement 
results in most of the light which seeks to escape through the wall of the 
light conductor being reflected from the interface at the core side of the 
low refractive index material and, therefore, confined within the core 
region provided, of course, that the incident light is transmitted into 
the core material within an appropriate acceptance angle relative to the 
axis of the core. 
As discussed above, the use of an aqueous liquid as the core material has 
heretofore been impractical because of limitations in the materials which 
defined the surface from which the reflection occurred and/or the 
materials which comprised the body of the waveguide. In the practice of 
the present invention, a waveguide 10 is constituted by a suitably shaped 
vessel 12, for example a capillary, for containing a liquid core 14, i.e., 
an aqueous sample. Capillary 12 may be fabricated from glass, quartz, 
transparent polymers such as polymethyl methacrylate (PMMA), 
polyvinylidene fluoride (PVDF) and ethylene tetrafluorothylene (ETFE), or 
similar materials. 
A material which possess a refractive index which is less than 1.33 may be 
used to clad a surface of the capillary 12. Materials usable for cladding 
the interior surface 16 of the capillary 12 must be non-reactive and 
insoluble in water. Suitable amorphous polymers with sufficiently low 
refractive indices can be created if their structural elements include 
some or all of the fluorocarbon groups --CF.sub.3, --CF.sub.2 O, 
--CF(CF.sub.3).sub.2 and --CH(CF.sub.3).sub.2, A commercially available 
fluorocarbon material having a refractive index which is suitable for use 
in the practice of the present invention is sold by the Dupont Company 
under the trademark "Teflon AF". This commercially available fluorocarbon 
material has a refractive index in the range of approximately 1.29 to 1.31 
and is insoluble in water. 
In accordance with the preferred embodiment, the total reflection surface 
is located at the exterior surface of capillary 12. Thus, the outer 
surface of capillary 12 is clad with, for example, an inorganic material 
having a very low index of refraction. These materials may be soluble in 
the core liquid. Appropriate exterior cladding materials, in addition to 
the preferred fluorocarbon materials described above, include beryllium 
fluoride glasses such as BeF.sub.2, Be-RbF, BeF.sub.2 --KF, BeF.sub.2 
--LiF, BeF.sub.2 --NaF, and fluorophosphate glasses. An arrangement 
wherein reflection of excitation light occurs at the exterior surface 18 
of capillary 12 also allows the interior surface 16 of the capillary to be 
modified to reduce the adhesion or retention of molecules in the liquid 
due to surface potential. For example, a silanization procedure may be 
used to change the charge on the interior surface 16 from negative to 
neutral or positive. 
Referring to FIG. 1, the waveguide/cell 10 of a Raman spectrometer 
comprises a length of tubing defining a capillary 12 which may be wound 
around a cylindrical form 20 for convenience. The capillary 12 is 
optically coupled to a spectrometer 22 in the manner to be described 
below, and to a laser excitation light source 24. This coupling is 
accomplished by means of a coupler which is indicated generally at 26. The 
excitation light source 24 typically produces light having wavelengths in 
the range of 400 nm to 900 nm. The coupler 26 includes an optical fiber 
bundle 28 as shown in FIG. 5. Fluid samples are introduced into the cell, 
i.e., into capillary 12, via an orifice 30 in the coupler 26 and the gap 
between the end of capillary 12 and the end of fiber optic bundle 28. The 
width of this gap is minimized to that which will allow air to escape from 
the capillary 12 as the fluid sample is introduced. Fluid may exit the 
cell via a discharge "port" 32. 
Preferably, light for excitation is supplied via optical fiber 34 located 
at the center of bundle 28. The axis of excitation fiber 34 is aligned 
with the axis 36 of the capillary 12 (see FIG. 3) to efficiently couple 
the excitation light into the cell. For proper operation, the excitation 
light is transmitted exclusively into the liquid sample 14. Such action is 
required because, in the embodiment of FIGS. 1--3, the refractive index of 
the capillary 12 is smaller than that of the exterior cladding 38 and the 
liquid sample 14. If the excitation radiation directly enters the 
capillary wall, it may be trapped inside the capillary wall by means of 
total reflection at the two opposite surfaces 16, 18. The remaining 
optical fibers 40 of bundle 28 collect the Raman emissions and deliver the 
thus collected light to spectrometer 22. 
As shown in FIG. 2, the waveguide 10 is comprised of a long capillary 12 
clad with a polymer material 38 having a refractive index lower than that 
of the sample liquid. The capillary wall and the sample liquid 14 together 
constitute the core. The cladding 38 should have a thickness of at least 
four (4) times the wavelength of the light to be propagated by the 
waveguide, i.e., a cladding thickness of 2 .mu.m to 3.6 .mu.m is 
appropriate, and may be applied by dipping, spraying or other means known 
in the art. 
The cladding 38 is preferably applied immediately after the capillary wall 
is drawn. The cladding 38 protects the capillary from degradation due to 
light, moisture, oxidation and environmental contaminants. Such 
degradation typically causes the capillary to become brittle. Therefore, a 
Raman cell manufactured in accordance with the present invention is more 
flexible than conventional Raman cells. For example a Raman cell 
manufactured in accordance with the present invention may be wound into a 
three inch coil. 
Since capillary 12 supports the disclosed circular cross-sectional shape of 
the cell, the physical strength requirement for the cladding material is 
reduced. A protective outer coating or jacket 42 of stainless steel or 
other suitable material may be employed to protect the cladding material 
from scratching and mechanical abrasion. 
The following relation determines the conical zones within which the 
excitation and Raman emission are transmitted through the cell: 
##EQU1## 
where .theta. is the angle of incidence with the interior surface of the 
capillary and n.sub.14 and n.sub.38 denote refractive indices of the 
liquid sample and the cladding, respectively. Light energy is evenly 
distributed within this cone over the cross section of the capillary 12 
and liquid sample 14 by means of successive transmission and reflection. 
Therefore, the capillary wall should be thin relative to the capillary 
diameter so that most of the excitation energy is distributed within the 
liquid sample 14. This distribution does not affect the collection of 
Raman emission if the fiber bundle 28 is large enough to accept the light 
output from both the core liquid 14 and the capillary wall. 
An alternative embodiment is shown in FIG. 4. This embodiment has the 
advantage of being compact and convenient. The FIG. 4 arrangement is best 
suited for a Raman cell with a relatively small inner diameter which in 
turn allows a small coil diameter. The capillary body 12' of the cell 10', 
which preferably has the same construction as depicted in FIG. 2, is 
coiled inside a rigid protective cover 52. The radius of the coil should 
be large enough to insure that bending light loss is minimal. Generally, 
the bending radius is a function of the numerical aperture of the 
excitation light delivery fiber and the diameter of the fiber. In a 
preferred embodiment, the bending radius is approximately equal to 100 
times the cladding diameter. A first end portion 58 of the capillary body 
12' extends outside of the cover 52. The first end portion 58 is shielded 
by a rigid and chemically inert sleeve 60. The second end portion 56 of 
the capillary body 12' is coupled to the fiber optic bundle 28 by means of 
a coupler/pipetter 62. 
The liquid sample may be drawn into the capillary 12' by immersing the open 
end of the capillary in the sample solution, closing the valve 68 and then 
releasing the end 66 of piston 64. A spring 70 biases the end 66 of the 
piston 64 away from the capillary 12' to create a suction which draws the 
sample into the capillary. A stop 72 prevents the piston 64 from being 
pushed out of the coupler/pipetter 62. Valve 68 provides a means of 
quickly changing the pressure inside coupler 62 and capillary 12'. 
As an alternative to the use of a cladding on capillary 12, as described 
above, the capillary may itself be constructed of Teflon AF. 
While preferred embodiments have been shown and described, various 
modifications and substitutions may be made thereto without departing from 
the spirit and scope of the invention. Accordingly, it is to be understood 
that the present invention has been described by way of illustration and 
not limitation.