Title: Enhanced sensitivity differential refractometer measurement cell
Abstract: A sensitivity-enhanced flow cell to be used in the determination of the differential refractive index increment of a sample fluid relative to a reference fluid is disclosed. The invention permits the use of smaller sample amounts without sacrificing overall sensitivity. Equally important, said improved flow cell produces measurements of increased precision without requirement for increased sample amount. This is achieved by means of two chambers within said cell whose volumes are different. The sample fluid chamber is the smaller of the two with the reference fluid chamber constructed so that the incident illumination beam, upon passage through said sample chamber and displacement by the partition element located therebetween said sample and reference chambers, passes through said reference chamber without grazing any of the confining walls or striking corners of said sensitivity-enhanced flow cell. As the amount of deflection of said transmitted beam depends upon the refractive indices of the fluids relative to the transparent matter of which the containing cell is comprised, as well as the RI difference between said fluids themselves, the reference chamber is designed to transmit said incident light beam without grazing degradation thereof for all practical ranges of expected fluid and cell refractive indices.
Patent Number: 6,975,392 Issued on 12/13/2005 to Larkin
| Inventors:
|
Larkin; Michael I. (Santa Barbara, CA)
|
| Assignee:
|
Wyatt Technology Corporation (Santa Barbara, CA)
|
| Appl. No.:
|
768600 |
| Filed:
|
January 29, 2004 |
| Current U.S. Class: |
356/246; 356/128; 356/73 |
| Intern'l Class: |
G01N 001/10 |
| Field of Search: |
356/246,128-136,337-343,73
250/573-575
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Toatley, Jr.; Gregory J.
Assistant Examiner: Nguyen; Sang H.
Attorney, Agent or Firm: Wyatt; Philip J.
Claims
1. A sensitivity-enhanced differential refractometer flow cell comprised of
a) An aperture (
7) restricting an incident light beam (
1) that
falls onto an entrance face (
8) of
b) A sample chamber (
2), said restricting aperture limiting said incident
beam to illuminate fully sample solution contained in said sample chamber (
2)
of said flow cell without impinging or grazing corners between defining sides of
said sample chamber nor sides upon which said beam should not be incident;
c) A transparent partition (
3) through which said incident light beam
(
1) passes, after having traversed said sample chamber (
2), into
d) A reference chamber (
4) containing a reference fluid and bounded by
said transparent partition (
3) separating said sample chamber (
2)
from said reference chamber (
4) whose dimensions are established prior to
fabrication by
i. first determining the complete range of transmitted beam refractions at said
partition interface (
3), based upon the refractive index of said flow cell
and the complete range of sample and reference refractive indices for which said
enhanced-sensitivity refractometer flow cell will be, used, and then
ii. providing adequate dimensions of said reference chamber (
4) such that
said transmitted beam is restricted to said reference chamber exiting it only at
an exit face (
13) and not grazing other surfaces or corners therein.
2. The sensitivity-enhanced differential refractometer flow cell of claim 1 where
said incident light beam is monochromatic.
3. The sensitivity-enhanced differential refractometer flow cell of claim 1 where
said sample and reference chambers are of similar right triangular cross sections.
4. The sensitivity-enhanced differential refractometer flow cell of claim 1 where
said reference chamber exit surfaces (
12) and (
13) are plane surfaces
which are not parallel.
5. The sensitivity-enhanced differential refractometer flow cell of claim 1 where
said transparent partition (
3) through which said incident light beam (
1)
passes, after having traversed said sample chamber (
2), is not normal to
said incident beam.
6. The sensitivity-enhanced differential refractometer flow cell of claim 1 where
said transparent partition (
3) is at an angle of the order of 45° with
respect to said incident light beam direction.
7. The sensitivity-enhanced differential refractometer flow cell of claim 1 where
said determination of the complete range of transmitted beam refractions is achieved
by calculation based upon said refractive index of said flow cell and the complete
range of sample and reference refractive indices for which said enhanced-sensitivity
refractometer flow cell will be used.
8. The sensitivity-enhanced differential refractometer flow cell of claim 1 where
said determination of the complete range of transmitted beam refractions is achieved
by direct measurement of said transmitted beams for all sample and reference fluids
for which said enhanced-sensitivity refractometer flow cell will be used.
9. A method for designing a differential refractometer flow cell of enhanced
sensitivity for subsequent manufacture comprising the steps of
a) Designing an incident light beam means to impinge upon an entrance face means
of a sample chamber means (
2) within said flow cell,
b) Designing an aperture (
7) restricting said incident light beam such
that it illuminates fully a sample solution contained in said sample chamber (
2)
means without impinging or grazing corners between defining sides of said sample
chamber nor sides upon which said beam should not be incident; said design
c) Including a transparent partition means (
3), separating said sample
chamber (
2) from an adjacent reference chamber that is to contain a reference
fluid, through which said incident light beam (
1) passes, after having illuminated
said sample chamber (
2),
d) Determining the complete range of transmitted beam refractions at said partition
means (
3), based upon the refractive index of said flow cell means and the
complete range of sample and reference solution refractive indices for which said
enhanced-sensitivity refractometer flow cell will be used,
e) Designing said reference chamber (
4) of adequate dimensions, position,
and structure such that said light beam entering thereinto through said partition
means (
3) is totally contained within said reference chamber, exiting it
only at an exit face (
13) and not grazing other surfaces or corners therein.
10. The design method of claim 9 where said incident light beam means chosen
is monochromatic such that said sample and reference fluid refractive indices are
determined at the wavelength of said monochromatic beam.
11. The design method of claim 9 where said transparent partition means (
3)
through which said incident light beam (
1) passes, after having traversed
said sample chamber means (
2), is not normal to said incident beam.
12. The design method of claim 11 where said transparent partition means (
3)
is at an angle of the order of 45° with respect to said incident light beam direction.
13. The design method of claim 9 where said determination of said range of transmitted
beam refraction is made via calculation of said transmitted beams for all sample
and reference fluids for which said enhanced-sensitivity refractometer flow cell
will be used.
14. The design method of claim 9 where said determination of said range of transmitted
beam refraction is made direct measurement of said transmitted beams for all sample
and reference fluids for which said enhanced-sensitivity refractometer flow cell
will be used.
Description
RELATED AND CO-PENDING APPLICATIONS
Expressly incorporated herein are the following related patents and concurrent
applications. These are of importance as the present invention insures that they
may be used and implemented more effectively:
U.S. Pat. No. 4,616,927—"Sample Cell for Light Scattering Measurements,"
(Oct. 14, 1986)
U.S. Pat. No. 5,530,540—"Light scattering measurement cell for very
small volumes," (25 Jun. 1996)
U.S. Pat. No. 6,411,383—"Method for measuring the 2nd virial
coefficient," (25 Jun. 2002).
U.S. Pat. No. 6,651,009—"Method for determining average solution properties
of macromolecules by the injection method," (Nov. 18, 2003)
Ser. No. 10/665,903 filed 18 Oct. 2003, S. Trainoff, "Method for Correcting
the Effects of Interdetector Band Broadening."
Ser. No. 10/723,548 filed 25 Nov. 2003, M. Larkin, "Refractometer Cell for
both Absolute and Differential Refractive Index Measurement of Fluids."
BACKGROUND
The difference in refractive index between a sample and a reference material
is referred to as the differential refractive index, dRI, and is a physical parameter
of considerable importance. The dRI between a sample solution consisting of a solvent
plus a solute and a reference solution comprised of a pure solvent may be used
to determine the solute concentration from the relation
##EQU1##
where the change in concentration, Δc, is directly proportional to the
measured change in solution refractivity, Δn. The constant of proportionality
is the reciprocal of the differential refractive index increment,
##EQU2##
A typical instrument for measuring the dRI is a "walk-off" type differential refractometer.
That instrument contains a cell made of a transparent material with two fluid chambers,
able to accommodate either a liquid or a gas, and having an angled transparent
interface separating the chambers. As pictured in FIG. 1, a beam of light 1
passes into the cell, through sample chamber 2, through the interface 3
separating the two chambers, through reference chamber 4, and finally out
of the cell. For the cell pictured, if the fluids in the two chambers have identical
indices of refraction, then after exiting the cell the transmitted beam of light
5 travels in a path parallel to the incident beam 1. If the two fluids
have different indices of refraction, then the transmitted beam of light 6
travels in a path which is at some angle θ to the incident beam. The angle
θ between the incident light beam and the transmitted light beam is, to first
order, proportional to the difference in refractive index between the two liquids.
That angular deflection of the light beam may be measured by a variety of well
established techniques, and so the dRI may be measured and reported.
Although the incident beam, as shown in FIG. 1, strikes the sample chamber
interface normal to the entrance surface, in general, the incident beam will be
oriented at an angle to it. In this manner, for example, it becomes possible to
have the finally transmitted beam reflected by a mirror back into the flow cell
chambers so that exits through the same surface. By such mirror means the sensitivity
of the cell will be doubled. The emerging beam will not be parallel to or co-linear
with the incident beam and may be detected more easily.
Conventionally, the angle of the transparent interface between sample
and reference chambers is of the order of 45° with respect to the direction
of the incident beam, though the greater this angle is the greater will be the
angular deflection of the transmitted beam due to the difference between refractive
indices of the sample and reference fluids. For the geometry shown, increasing
this angle results in a requirement for a sample fluid chamber of increased volume
while decreasing it decreases the angular deflection due to the refractive index
difference between the sample and reference fluids.
Applications which utilize measurements of the dRI between two liquids
typically benefit if the amount of sample required for a measurement is reduced.
For many applications, sample preparation requires a great expenditure of time
and resources, and reducing the quantity of sample required for measurements has
a direct financial benefit. In addition to a reduction of costs and effort associated
with sample preparation, the quality of measurements are in many cases enhanced
if the quantity of sample required for a measurement is reduced. Liquid chromatographic
systems are one example where the quality of the measurements are in some cases
enhanced if the volume of sample required for measurement is reduced. In a liquid
chromatographic system a material potentially consisting of many species is dissolved
into a solvent and then injected into a fluid stream. The fluid stream is made
to traverse some medium or device which preferentially delays species in the medium
or device based upon some physical parameter, such as size, chemical affinity,
thermal properties, electrical properties, etc., and so separates the species from
one another. The different species thus exit the medium or device at different
times. In keeping with traditional nomenclature, this medium or device will here
be referred to as a column, although the physical form and function of the device
may be quite different from a column. The fluid passing through the column typically
exits into a small diameter tube, and so at any one moment in time different species
reside at different locations along the length of the tube. If a measurement device,
such as a differential refractometer, is situated such that the fluid flows from
that tube through the measurement device, then the species which make up the material
may be individually measured. The measurement of constituent species of a material
is an essential purpose of chromatographic systems. Since a finite volume of liquid
is always required for measurement, the species within some volume of the tube
necessarily contribute to the signal at any moment in time. The measurement device
is therefore always measuring an average over the species which reside along the
length of the tube which corresponds to the measurement volume. This averaging
over species negates in part the separation accomplished by the column, and results
in a reduction in the quality of data. Reducing the volume of sample required for
measurement minimizes the averaging over species, resulting in higher quality data.
In addition to the negative effects on data quality due to the measurement averaging
over a finite volume of sample, some volume of sample is mixed together as it traverses
the measurement system. Many chromatographic systems consist of several measurement
devices placed serially along the fluid stream, each measuring different physical
parameters concerning the sample. If a measurement device mixes some volume of
fluid together, then all subsequent measurements on that fluid are negatively impacted
by the resulting averaging over multiple species in the measurement volume. Typically,
the larger the volume required for measurement, the larger the volume of sample
which is mixed together, and the greater the negative impact on data quality for
instruments placed later in the fluid stream.
In addition to their application in the field of liquid chromatography, differential
refractometers of various types are used in many different fields. By accurately
determining refractive index differences between a reference standard and a sample,
such determinations may be used to determine sucrose concentration, fluid densities,
the concentrations of a myriad of industrial fluids such as sulfuric acid, sodium
chloride, ethanol, etc. A variety of instruments have been designed around the
concept of measuring and using such refractive index differences as a means to
measure various derivative quantities.
There are clearly advantages in reducing the volume of sample required for
a dRI measurement. However, for a walk-off type differential refractometer, a tradeoff
exists between reduction of the sample volume and sensitivity of the dRI measurements.
There are at least three reasons for a reduction in dRI sensitivity with a reduction
in sample volume. The first reason for a reduction in sensitivity is a reduction
in averaging over the sample. For even perfectly stable systems, fundamental laws
of thermodynamics predict local fluctuations through time of the temperature, density,
and solute concentrations across the sample and reference liquids. This was explained
at length by Albert Einstein in his 1910 seminal paper on "The theory of opalescence
of homogeneous fluids and liquid mixtures near the critical state," published in
Annelen der Physik, volume 33, pages 1275-1298. Real world systems are never
perfectly stable, and those fluctuations are in general enhanced in real systems.
Those fluctuations cause the path of the light beam traversing the fluids to change
through time, and so cause the angle θ at which the light beam 6 exits
the cell to fluctuate with time. The fluctuations through time of the beam angle
are seen as noise in the dRI measurement. Increasing the volume sampled by the
beam causes the beam to better average over these local fluctuations, reducing
their overall effect.
A second reason that a reduction in sample volume results in a reduction in sensitivity
of the dRI measurement is a reduction of optical power through the system. For
the cell design picture in FIG. 1, as the sample volume is reduced, the area of
sample through which light may be sent is reduced. To obtain the same optical power
through the system, the light intensity must be increased. Typically, a system
used to measure the angular deflection of the light beam has its sensitivity increase
in some proportion to the optical power supplied to it. Therefore, to obtain with
a smaller volume sample the same sensitivity in the determination of the beam angular
deflection as with a larger volume sample, the light intensity must be increased.
Since these systems are typically already using the most intense light sources
practicable, a reduction in sample volume necessarily results in a reduction of
optical power through the system and a corresponding reduction in the sensitivity
with which the angular deflection of the light beam may be determined. A reduction
in the sensitivity with which the angular deflection of the light beam may be determined
corresponds directly to a reduction in sensitivity of the dRI measurement.
A third way that reducing sample volume reduces sensitivity of the dRI measurement
is once again due to a reduction in the area through which the light beam may be
sent. As the area through which the light beam is sent is reduced, diffraction
effects limit the sharpness with which the beam may subsequently be focused. The
smaller the area through which the beam passes, the more diffuse the focal point
becomes. Typically, a system used to measure the angular deflection of the light
beam has its sensitivity increase as the sharpness of the focused beam increases.
And so yet again reducing the area through which the light beam passes results
in a decrease in sensitivity in determining the beam angular deflection, corresponding
to a reduction in sensitivity of the dRI measurement.
It is an important objective of my invention to increase the sensitivity of a
dRI measurement while at the same time minimizing the amount of sample required.
Another objective of my invention is to reduce diffraction effects by increasing
the dimension of the clear aperture through which the beam must pass without increasing
the sample volume. A further objective is to provide for a broad range of instrument
response without the beam moving too closely to any side of the cell.
BRIEF DESCRIPTION OF THE INVENTION
In order to achieve the objectives of this invention, a new type of cell for a
dRI instrument is disclosed. Rather than integrate a conventional structure comprised
of two juxtapositioned chambers of approximately the same size, the novel cell
of the invention employs chambers of two different sizes. The first chamber containing
the sample solution, and into which the incident beam first enters the cell, is
smaller that the successive chamber containing the reference fluid. Before entering
the sample cell, the incident beam passes through a mask limiting its cross section
to fill the sample chamber while not passing too closely to the chamber edges.
In the preferred embodiment of the invention, the two chambers have cross sections
comprised of similar triangles with the reference chamber made large enough to
accommodate all beam displacements at the interface/partition separating the two
chambers. The reference chamber is made large enough so that the displacement of
the beam passing through the interface between the chambers will not strike or
graze the edges of said reference chamber for a large range of liquid refractive
indices. These include fluids whose refractive indices are less than the refractive
index of the glass or transparent material of which the cell is fabricated as well
as fluids whose refractive indices are great than said cell materials. In this
manner, the invention allows a reduction of the sample volume without compromising
the sensitivity of the dRI measurement, or conversely an increase in sensitivity
without increasing the sample volume.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a conventional dRI cell design illustrating the displacement of
the transmitted beam.
FIG. 2 shows a conventional cell with the beam filling most of the sample chamber.
FIG. 3 shows the preferred embodiment cell of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The objective of all dRI detectors is to measure the refractive index difference
between the reference and sample fluids. For a walk-off type dRI detector, this
is accomplished by measuring the angular deflection of the light beam emerging
from the cell after traversing it. The translation of the emerging beam relative
to the incident beam contributes to limit the sensitivity of the conventional cell
structure by reducing the region within the sample chamber which may be illuminated
and still have that light reach and traverse the reference chamber. This translation
has a major dependence upon the refractive index difference between the sample
fluid refractive index and that of the transparent material of which the cell is
fabricated, and a minor dependence on the refractive index difference between the
sample and reference fluids. For most practical applications, especially those
related to the field of liquid chromatography, the refractive index between the
two fluids is small compared to that between the fluids and the cell material.
FIG. 1 shows a very thin light beam for purposes of clarity. The cell when filled
with as much light as possible, for the reasons discussed in the background section,
is shown in FIG. 2. Note that the beam undergoes a translation d as it passes through
the partition
3 of thickness t between the sample and reference chambers.
That translation need not contribute to the dRI measurement, since using simple
optics it is possible to separate angular deflection of the beam from its translation.
However, that translation does limit the volume within the sample chamber which
may usefully be illuminated. Light near the edges of the sample chamber
2
could miss the reference chamber
4, and thus not contribute to the measurement,
reducing, thereby, its sensitivity. If light is allowed to enter the sample chamber
2 that misses or grazes the reference chamber, it will reflect and scatter
from various surfaces, corners, and discontinuities of the cell typically contributing,
thereby, a spurious and undesirable signal to whatever device is measuring the
angular deflection of the light beam. To prevent that situation, an aperture
7
is typically placed before the flow cell, as shown in FIG. 2, ensuring that light
which would miss or graze the reference chamber
4 is not permitted to enter
the sample chamber
2.
The direction of the translation depicted in FIG. 1 and in FIG. 2 is appropriate
when the index of refraction of the fluid is less than the index of refraction
of the material comprising the cell. That is the common case, but in some instances
the fluid to be measured has a higher index of refraction than that of the cell
material. For example, a fluid such as toluene has an index of refraction of 1.5
that is greater than a typical cell material made of fused silica, with an index
of refraction of 1.46. Since dRI detectors are typically designed to operate with
a variety of fluids, the aperture is made small enough and positioned such that,
over the desired range of fluid refractive indices, no light will graze the reference
chamber walls. For a typical low volume type flow cell, the light beam may be restricted
to enter only the central 65% of the sample chamber
2; the rest of the chamber
being essentially unused, but nevertheless filled with sample fluid. It is the
thickness of the partition
3 that results in a translation of the light
beam, and the negative consequences described above associated with that translation.
As the thickness of the partition is brought to a negligible thickness, the translation
goes to zero, as do the consequences associated with the grazed surfaces of the
reference chamber
4. However, it is practically difficult to make that dimension
small. Furthermore, as the partition dimension is reduced, pressure differences
between the sample and reference chambers cause the partition between them to flex,
adding noise and distortion to the measurement. Pressure differences inevitably
exist between the sample and reference chambers, especially in the case where fluid
is flowing through one or both chambers.
The reference chamber
4 of the invention, whose preferred embodiment is
shown in FIG. 3, is made sufficiently larger than the sample chamber to allow the
light beam to traverse said reference chamber without impinging on any of its corners
located at the intersections of said chamber's faces, nor grazing any side through
which said beam is not intended to pass after said beam has entered the sample
chamber
2 at any position along face
9, traversed the sample chamber,
and passed onto and through the partition
3 into said reference chamber.
This allows the full volume of the sample chamber to be illuminated by the incident
beam. As the fraction of the sample chamber volume illuminated is increased, the
sensitivity of the measurement is increased without increasing the size of the
sample chamber. Conversely, using the invention described, the sample volume may
be made smaller than in a conventional flow cell with no loss in measurement sensitivity
since, in the preferred embodiment, a greater fraction of the sample chamber volume
is illuminated. For conventional chromatographic measurements, the reference chamber
4 is filled with a reference fluid at the beginning of a measurement and
then sealed during the course of the measurement. Thus an increase in volume of
the reference chamber
4 does not affect the volume of the sample required
for a measurement.
The degree by which the flow cell reference chamber
4 must be made larger
than the sample chamber depends upon the physical details of the flow cell and
the range of solvent refractive indices to be measured. For any embodiment, the
reference chamber
4 must be made large enough and positioned correctly for
the light incident anywhere in the sample chamber
2 to successfully reach
and traverse the reference chamber. In FIG. 3 we show a preferred embodiment of
the invention. In this embodiment, the reference chamber
4 has a triangular
cross section which is a similar triangle to the sample cell cross section. The
planes
8,
9,
12, and
13 are parallel to one another,
and the planes
10 and
11 are parallel to one another. The sample
and reference chambers are separated from one another by a partition of thickness
t, and the light beam depicted is translated by a distance d due to passage through
the partition. The fluid in the sample chamber has refractive index n
1,
that in the reference chamber has refractive index n
2, and the partition
between the sample and reference chambers has refractive index n
g. The
refractive index difference between the sample and reference fluids n
1
and n
2 is typically of the order of 1×10
-3 or less,
while that between n
1 or n
2 and n
g is of the order
of 0.1. For the purposes of simplifying the derivation below, we will assume n
1≈n
2≡n
l.
With this assumption, the translation d may be seen to be:
##EQU3##
Note that in the case when the liquid has a higher refractive index than the
transparent material of the cell, i.e. m>1, d is negative and the beam displacement
is downward.
For the conventional embodiment of the invention shown in FIG. 3 to accept a
range of fluid refractive indices m
max>1.0>m
min,
the length of side
12 must be greater than the incident beam width by at
least a
1+a
2, where a
1=d(m
min) and a
2=-d(m
max)
are the dimensions indicated in FIG. 3. Here m
max is the maximum value
of the refractive index ratios anticipated and m
min is the smallest.
From a practical point of view, the largest m will be of the order 1.5/1.46=1.03,
corresponding to toluene in a fused quartz cell. The smallest m would be of the
order of 1.33/1.62=0.82, corresponding to water in a cell made of F2 glass. Taking
α of the order of 45°, we have the requirement for a downward increase
of side
12 by a
2=0.023 t and an upward increase by a
1=0.29t.
If t is a typical value of 1.0 mm, the reference chamber wall
12 need be
about 0.29+0.023=0.313 mm longer than the sample chamber. A conventional embodiment
of a refractometer cell may have a sample chamber dimension of 1.4 mm, and so the
reference chamber of the corresponding preferred embodiment of the invention shown
in FIG. 3 is made at least 22% larger than a 1.4 mm sample chamber, and allows
for a 22% increase in the illuminated area of the sample chamber.
The above conditions are required for the beam of light, impacting the partition
between the sample and reference chambers anywhere on the sample side, to enter
the reference chamber successfully, i.e. without grazing the cell edges or being
truncated thereby. The derivation above was specific to the geometry of the conventional
embodiment of FIG. 3, and given the assumption that n
1≈n
2≡n
l.
However it will be clear to those skilled in the art of differential refractive
index measurements that relaxing the assumption of n
1≈n
2
adds somewhat to the complexity of the equations above, and adds the requirement
that the reference chamber be expanded further to account for translation of the
beam as it traverses the reference chamber due to angular deflection of the beam,
resulting from the differential refractive index Δn=n
1-n
2.
Calculation is not the only method by which the appropriate dimensions
and placement of the reference cell may be determined. It is possible to determine
the dimensions and placement of the reference cell by experimental methods, such
as measurement of all pertinent angular deflections and translations over the range
of interest of all pertinent indices of refraction and wavelengths. It is also
possible to determine the dimensions and placement of the reference cell by computer
simulation, using well established optical simulation techniques.
It will be clear to those skilled in the art that any technique or device which
incorporates an incident beam of light that is not perpendicular to the partition
between two fluid chambers will result in a translation of that light beam in some
proportion to the refractive index difference between the fluids and the material
comprising the partition. It should be evident also that for virtually every range
of fluid refractive indices, the fraction of the fluid in the sample chamber illuminated
by the incident beam may be maximized by enlarging the reference chamber relative
to said sample chamber. My invention permits, thereby, an increase of instrument
sensitivity without increasing the size of the sample required. Conversely, the
sample size required to provide an accurate measure of its refractive index difference
relative to that of the reference fluid may be decreased significantly by increasing
the fraction of said sample illuminated by said incident beam. The greater the
fraction of the sample illuminated within the sample chamber, the greater will
be the sensitivity of the dRI detector to refractive index difference between sample
and reference.
In the field of liquid chromatography, the dRI detectors discussed in the section
on background are generally also used to determine sample concentration once the
differential refractive index, dn/dc, has been measured. For such measurements
used in conjunction with multiangle light scattering measurements, in addition
to the need to measure the instantaneous concentration of samples eluting following
fractionation by column or other means, the quantity dn/dc itself is an essential
element of subsequent determinations of molar mass and size. Since dn/dc, and more
generally the refractive index of all materials, changes as a function of the wavelength
of light, measurement of both dn dc and concentration by such detectors is generally
required at the same wavelength as the light scattering measurement itself. Thus
the light beams frequently employed in such dRI detectors are monochromatic and
designed to operate at the same wavelength as used to make the associated light
scattering measurements.
There is, of course, no truly monochromatic light source, which is defined
as a light source having a single perfectly defined wavelength. In the context
of this patent disclosure a "monochromatic" light source corresponds to a light
source having a range of wavelengths which is narrow enough that over such range
of wavelengths, dn/dc of the solute in solution as well as the index of refraction
of sample and reference fluids and the material comprising the cell changes by
an amount which is acceptable for a particular application. For example, a "monochromatic"
light source in this context may have a range of wavelengths over which the value
of dn/dc changes by less than 1%, and all pertinent indices of refraction change
by less than 0.01%. A laser may produce a range of wavelengths which is less than
1 nm while a non-lasing light emitting diode may have a range of wavelengths of
30 nm, but both may meet the above criteria for "monochromatic." Although the preferred
embodiment of this invention would employ such monochromatic light beams, there
are other applications of dRI detectors for which the dRI detector light sources
are not monochromatic. Indeed, some commonly used dRI detectors employ so-called
white light sources producing a light beam made up of contributions from a broad
range of wavelengths or even just a few.
As has been described in considerable detail, the conventional implementation
of a differential refractometer cell as shown, for example, in FIG. 1 includes
two identical right triangular chambers. Extending this conventional structure
to enhance the sensitivity of the device results in a structure similar to that
shown in FIG. 3 wherein the two chambers are not identical but remain, nevertheless,
of similar triangular cross sections. My invention, however, does not require that
the triangles be similar nor does it require that the exit plane
13 be parallel
to the entrance plane
8. Accordingly, there is no restriction that the chambers
be of right triangular cross section. Indeed in my earlier co-pending application,
"Refractometer Cell for both Absolute and Differential Refractive Index Measurement
of Fluids," Ser. No. 10/723,548, the exit plane defined by the two surfaces
12
and
13 includes an inner surface
12 that is not parallel to surface
13. The reference sample chamber cross section is not even a right triangle.
Such a pair of dissimilar chambers provides means to measure the refractive index
of fluids directly. Key to the success of my invention is the requirement that
the exiting beam of light, after having passed into and through said reference
chamber, passes out of said reference chamber through an exit face such as
13.
The acceptable traversal of said reference chamber by the incident beam requires
that said chamber must be of structure sufficient larger than the sample chamber
to prevent said traversing beam from grazing boundaries thereof. The beam translation,
d, as presented above, may be determined for any form of exit surface, including
a wedged plane form as used for my aforereferenced co-pending refractometer application.
The fabrication and implementation of my inventive refractometer cell requires
consideration of the following key elements:
- 1. A sample chamber whose smaller volume, and its correspondingly smaller
sensitivity, is compensated by illuminating as much of the contained sample as
possible without grazing the containing walls thereof: The sample chamber preferentially
should have an entrance surface on which the incident light beam falls after having
been restricted by aperture or other means and should have also an exit surface
through which said light beam may pass after traversing said sample. Said exit
surface should be at an angle to said traversing beam to insure the refraction
of said beam as it passes into a reference chamber and to provide a boundary separating
said sample chamber from said reference chamber. The greater said angle with respect
to the direction of propagation of said light beam, the greater will be the angular
deflection of the exit beam for a given refractive index difference between the
sample and reference fluids.
- 2. A reference chamber, generally of size greater than said sample chamber,
whose dimensions are determined by first establishing the maximum range of fluid
refractive indices that will be employed in any measurement anticipated by said
refractometer and then determining the corresponding range of beam displacements
corresponding thereto; all constituent beam elements corresponding to said displacements
exiting said chamber and cell via an exit surface without grazing other containing
boundaries therein.
There are many embodiments of my invention that will be obvious to those skilled
in the art of differential refractive index measurements that are but simple variations
of my basic invention herein disclosed. Accordingly,
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