High temperature differential refractometry apparatus

A differential refractometry apparatus that maintains optimal optical alignment of components while accurately providing differential refractometry measurements at elevated temperatures. The differential refractometry apparatus has a first thermal zone, a thermal isolation zone and a second thermal zone. The first thermal zone is configured to be located in an oven and exposed to higher temperatures. The thermal isolation zone is located adjacent to the first thermal zone and acts as a barrier to the conduction of heat from the first thermal zone into the second thermal zone. The second thermal zone is at a relatively lower temperature than the first thermal zone and its temperature is regulated using a thermal electric cooler located at its base. A flow cell, a mirror which reflects the incoming light beam, and an imaging lens are located in the first thermal zone. An LED and a photodiode detector are located in the second thermal zone and are encased in thermally stable blocks with low coefficients of thermal expansion. The LED sends a light beam up through the thermal isolation zone and into the first thermal zone. The light beam passes through the sample and gets reflected by the mirror. The light beam passes through the sample a second time. The light beam then passes through an imaging lens before traveling back to the photodiode in the second thermal zone. In this manner the optimal optical geometry is preserved while allowing analyses to be conducted with the sample at elevated temperatures.

FIELD OF THE INVENTION 
The present invention relates to differential refractometry 
instrumentation, and more particularly to a method and apparatus 
implementing systems conducting differential refractometry of samples at 
high temperatures. 
BACKGROUND OF THE INVENTION 
The ability of a medium to refract light is its refractive index ("RI"). RI 
is the ratio of the velocity of light in a vacuum to the velocity of light 
in a medium. It is a physical property of the medium and is represented by 
a dimensionless integer "n". Differential refractometry is the art of 
measuring small differences in RI between a reference solution and a 
sample solution. The difference in RI is referred to as ".DELTA.n". 
.DELTA.n is measured in RI units ("RIU"). 
Differential refractometers in the prior art generally consist of a light 
emitting diode (LED), a flow cell containing a sample side and a reference 
side, and a dual element photodiode detector. As illustrated in FIG. 1, 
known differential refractometers utilizing a dual pass optics bench 
contain a mirror which reflects the light causing the light beam to pass 
through the flow cell twice before reaching the photodiode. 
After the light beam passes through the flow cell the second and final 
time, it passes through an imaging lens and then falls upon the dual 
element photodiode. When the flow cell contains just solvent, the position 
of the light beam is centered on the elements of the photodiode as shown 
in FIG. 2a. This position of the beam creates a baseline signal. When 
sample is inserted into the flow-cell, light beam is refracted further, 
causing a deflected image on the photodiode as illustrated in FIG. 2b. The 
deflected image creates a signal that differs from the base-line signal. 
The changing signal from the photodiode results in a change in the output 
voltage of the refractometer. An integrator or chart recorder then 
registers the changes in output voltage as peaks on a chromatogram. 
One factor that always creates issues in refractometry is temperature. The 
sample has to be maintained in a very thermally stable environment. Even 
slight changes or variations in temperature affect the density of the 
sample thereby changing its refractive index. Temperature also poses 
problems in elevated temperature polymer characterization and other high 
temperature analyses because the LED and the photodiode detector are 
electronic devices which generally cannot withstand very high 
temperatures. 
It is generally known in the industry that taking one or both of the LED or 
the photodiode detector out of the high thermal environment is a solution 
to problems caused by high temperatures. However, taking both devices out 
of the high temperature thermal environment creates alignment, reliability 
and cost issues, and the refractometer is no longer a self contained unit. 
Taking one or both of the LED or photodiode detector out of the high 
thermal environment is usually accomplished by means of fiber optic cables 
that complicate the optical alignment. The fiber optic cables can degrade 
over time due to temperature and mechanical stress, causing reductions in 
light transmission. In addition to the cost of fiber optic cables, there 
are other costs associated with mounting parts at the fiber optic cable 
ends. 
A known refractometer implementation aimed at allowing analyses at high 
temperatures is the Waters 150C Refractometer available from Waters 
Corporation, Milford, Mass. In this implementation, the light source, a 
tungsten lamp, is positioned outside of the high thermal environment while 
the photodiode detector is positioned within the high thermal environment 
to maintain the optical geometry of the system. As a result the Waters 
150C Refractometer can only be used for analyses at 150.degree. C. or 
below or the integrity of the electronic photodiode detector will be 
compromised. Also, the signal to noise performance is generally negatively 
affected at temperatures above 100.degree. C. due to increased photodiode 
electrical noise at such elevated temperatures. 
SUMMARY OF THE INVENTION 
The present invention provides a differential refractometry apparatus that 
maintains optimal optical alignment of components while accurately 
providing differential refractometry measurements at elevated 
temperatures. 
According to the invention, the differential refractometry apparatus 
comprises a first thermal zone, a thermal isolation zone and a second 
thermal zone. The first thermal zone is configured to be located in an 
oven and exposed to higher temperatures. The thermal isolation zone is 
located adjacent to the first thermal zone and acts as a barrier to the 
conduction of heat from the first thermal zone into the second thermal 
zone. The second thermal zone is at a relatively lower temperature than 
the first thermal zone and its temperature is regulated using a thermal 
electric cooler located at its base. In an illustrative configuration the 
refractometer is disposed vertically so that the first thermal zone is 
located above the thermal isolation zone which is above the second thermal 
zone. 
A flow cell, a mirror which reflects the incoming light beam, and an 
imaging lens are located in the first thermal zone. An LED and a 
photodiode detector are located in the second thermal zone and are encased 
in thermally stable blocks with low coefficients of thermal expansion. The 
LED sends a light beam up through the collimating lens and then through 
the thermal isolation zone and into the first thermal zone. The light beam 
passes through the flow cell and gets reflected by the mirror. The light 
beam passes through the flow cell a second time. The light beam then 
passes through the imaging lens before traveling back to the photodiode in 
the second thermal zone. In this manner the optimal optical geometry is 
preserved while allowing an analyses to be conducted with the sample at 
elevated temperatures. 
Features of the invention include RI measurements at elevated temperatures, 
preservation of the optimal geometric alignment of the optical devices, 
and an accurately controlled dual temperature zone minimizing RI noise and 
drift thereby improving the refractometer detector's precision. The 
present invention is designed to minimize temperature fluctuations within 
each of the two thermal zones as well as to ensure that the temperature of 
the zones relative to each other remains constant.

DETAILED DESCRIPTION 
The present invention, referring to FIG. 3, comprises a differential 
refractometry apparatus containing a first thermal zone 10, a thermal 
isolation zone 12 and a second thermal zone 14. The three zones are 
disposed substantially vertically in an illustrative configuration so that 
the first thermal zone 10 is located above the thermal isolation zone 12 
which is above the second thermal zone 14. 
As illustrated in FIGS. 3-5, the first thermal zone is located in an oven 
16 with an elevated internal temperature. The first thermal zone of the 
refractometer according to the invention comprises a shield 18, seen best 
in FIG. 4, which covers the mirror 24, the flow cell body 60 and a 
stainless steel tube heat exchanger 20 through which sample is transmitted 
to a flow cell 22, as known in the art. The stainless steel tube heat 
exchanger is wrapped around the outside diameter of a metallic cylinder 
portion of the flow cell body a plurality of times. This allows the sample 
to attain the temperature of the flow cell body before it enters the flow 
cell. 
A mirror 24 for reflecting light, is configured in a cylindrical housing 
which is inserted into a counter bore of the flow cell body. The mirror is 
anchored by a plurality of screws. The mirror 24 is disposed proximate to 
the flow cell 22. The flow cell contains first and second compartments 
(not shown). The first compartment contains a reference solution, while 
various sample solutions are cycled through the second compartment. In 
this illustrative embodiment, the flow cell is made of fused quartz. 
An imaging lens 26 (best seen in FIG. 5) is located adjacent to the flow 
cell 22 and focuses rays of light reflecting off of the mirror 24 into the 
dual element photodiode detector 44. 
A cylindrical insulation body 28 abuts against an end surface of the flow 
cell body 60. The cylindrical insulation body 28 in this illustrative 
embodiment is made of a machinable, fully-dense ceramic material, such as 
AREMCOLOX 502 series ceramics. Such a material has low thermal 
conductivity, and in the illustrative embodiment, a low coefficient of 
thermal expansion. The cylindrical insulation body therefore minimizes the 
transfer of heat by conduction from the first thermal zone, which is at an 
elevated temperature, to the second thermal zone. 
The cylindrical insulation body 28 is effectively a portion of the thermal 
isolation zone 12. An insulation shield 30, forming another portion of the 
thermal isolation zone 12, engages an end of the cylindrical insulation 
body 28 distal to the flow cell 22. The insulation shield 30 is made of 
Teflon, which is a material with low thermal conductivity. A thermal 
shield 32 engages a portion of the insulation shield 30. The thermal 
shield in turn engages with an insulator plate 34. The insulator plate 34 
includes orifices for passage of light to and from the mirror 24. The 
thermal shield 32 and the insulator plate 34 are made of polyphenylene 
sulphide (PPS) which has a low thermal conductivity. 
A chassis component 36, configured with a groove to minimize thermal 
conduction, is encapsulated by the thermal shield 32, insulation shield 
30, insulator plate 34 and insulation body 28. 
The second thermal zone 14 contains a first thermally stable block 40 and a 
second thermally stable block 42. The first thermally stable block 40 
contains a dual element photodiode detector 44, a void 48 for passage of 
light therethrough, and embedded in the block next to the dual element 
photodiode detector, a temperature sensor such as a thermistor 59. The 
thermistor allows for monitoring and control of the block temperature. The 
second thermally stable block 42 contains an LED 46. The blocks, in this 
illustrative embodiment, are made of INVAR, a 36% nickel-iron alloy having 
a very low coefficient of thermal expansion. The INVAR blocks do not 
undergo dimensional changes with changes in temperature and, therefore, 
make an ideal material which substantially ensures the stability of the 
electrical devices. The first thermally stable block 40 and the second 
thermally stable block 42 are electrically connected, and both are 
electrically insulated from the remaining assembly by a mica sheet 50. A 
shield 54 is configured to house the thermally stable blocks 40, 42, to 
prevent temperature fluctuations due to moving air. 
The second thermal zone is configured so that its base contains a 
thermoelectric cooler (TEC) 52. The TEC is a solid state heat pump, such 
as a MELCOR TEC, that utilizes the Peltier effect for heat exchange. A 
heat sink 56 is adjacent to the TEC which is in thermally conductive 
communication with the thermally stable blocks. Accordingly, heat is 
conveyed away from the electronic devices in the thermally stable blocks 
40, 42, and toward the heat sink 56. 
That is, during operation, DC current flows through the TEC causing heat to 
be transferred from one side of the TEC to the other. As a result, a hot 
side and a cold side are created in the TEC. The TEC is configured so that 
the cold side is adjacent to the thermally stable blocks 40, 42, while the 
hot side is exposed to the heatsink 56 which dissipates the heat. A fan 58 
is placed adjacent to the heatsink to dissipate the heat. 
There are many advantages to using a TEC to regulate and control the 
temperature of the second thermal zone. The TEC is small, lightweight, and 
completely silent. It does not produce any vibrations that might adversely 
affect the optics. With no moving mechanical parts, the TEC is extremely 
reliable and thus provides a cost effective way of regulating the 
temperature of the second thermal zone. Finally, the TEC provides 
extremely precise temperature control which, as mentioned earlier, is 
crucial to obtaining accurate RI readings. 
The differential refractometer according to the invention is used for high 
temperature analysis of selected samples. The refractometer is configured 
with the first thermal zone 10 and a significant portion of the thermal 
isolation zone 12 disposed in an elevated temperature environment, e.g. 
oven. The second thermal zone 14 is typically subject to an ambient 
temperature environment (see FIG. 3). 
Light is emitted from the LED 46 and passes through a collimating lens 27 
and a mask (not shown) in the first thermally stable block 40. The light 
traverses the insulation body 28 and passes through the sample and 
reference chambers of the flow cell 22, which are maintained at an 
elevated temperature in the oven. The light having passed through the 
sample/reference, is reflected by the mirror 24. After passing through the 
sample/reference a second time, the light is focused by the imaging lens 
26 adjacent to the flow cell body 60. The reflected light (which is 
refracted to some extent as a function of the sample under analysis), 
impinges upon the photodiode detector 44 in the first thermally stable 
block 40. According to the invention the electronic devices (i.e., LED 46 
and detector 44) are maintained at a lower and relatively safe temperature 
with respect to the first thermal zone 10 temperature. The electronic 
devices are maintained in a thermally stable condition via the containment 
in the thermally stable blocks, in thermally conductive communication with 
the TEC 52. Accordingly, extremely stable and reproducible high 
temperature analysis is performed. It should be appreciated that 
significant temperature stability is achieved by orienting the 
refractometer according to the invention in a vertical manner, which 
avoids introduction of optical aberrations due to convective air 
movements. The refractometer is slightly tilted from the vertical position 
to help purge air bubbles from the sample and reference sides of the flow 
cell. 
Although the refractometer described herein includes an insulation body 28 
formed of a low thermal conductivity ceramic, it should be appreciated 
that other low conductivity materials could be used such as machinable or 
non-machinable ceramics, PPS, PEEK, RYTON, glass, high density PYROPEL or 
the like. 
While the thermal isolation zone 12 described herein specifically includes 
an insulation shield, thermal shield, a chassis portion with a groove and 
an insulator plate, it should be appreciated that other combinations of 
elements could be configured to provide thermal isolation. For example, 
the chassis portion (with or without a groove) could be omitted, and/or 
the thermal shield could be omitted or alternatively configured. 
Similarly, such components could be made from materials with low thermal 
conductivity (i.e., other than PPS), such as PEEK, high density PYROPEL, 
Teflon, ceramic or the like. 
Although INVAR, a material with a low coefficient of thermal expansion, is 
specified for forming thermally stable blocks 40, 42, flow cell body 60 
and chassis 36, it should be appreciated that other materials could be 
used, such as stainless steel or the like. 
While a TEC is used to exchange heat in the second thermal zone, it should 
be appreciated that other heat exchange devices could be implemented, such 
as liquid recirculation through a heat exchanger, variable air flow 
cooling or the like. 
Although a vertical orientation is described for the refractometer 
implemented herein, it should be appreciated that alternative orientations 
can be implemented such as a horizontal orientation and/or an inverted 
orientation wherein the orientation of the thermal zones is inverted. 
Similarly, it should be appreciated that a refractometer could be 
implemented without a mirror. That is, in an alternative implementation 
the light source and the photodiode detector could be disposed in extreme 
ends of the refractometer in two different and separate "second" thermal 
zones (i.e. low temperature zone). Each of these "second" thermal zones 
would be separated from the first thermal zone by a respective thermal 
isolation zone. 
Although the invention has been shown and described with respect to 
exemplary embodiments thereof, various other changes, omissions and 
additions in the form and detail thereof may be made therein without 
departing from the spirit and scope of the invention.