Note: Descriptions are shown in the official language in which they were submitted.
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This application is a diyisional of Canadian Patent
Applicat~on Ser~al No. 268,990 filed Decem~er ~1, 1976.
1. Fi-eId-of the Invention - DiffractometriC
refractometers and a novel sample and reference cells
usable in the refractometers.
2 ''De'scr'i' t'i'on'of't'he''P'ri'or'Art - U.S. Patent
P
No. 2,795,~91 teaches an interferometer for indicating
the composition of gases based on changes in refractive
index including use of a comparative known reference gas.
U.S. Patent No. 3,035,482 teaches interferometers
o which are employed for that type of measuring purposes in
which the displacement of the interference fringes is used
for determining the change in the value of a measurement,
for instance, for determining the concentration of a gas.
U.S. Patent No. 3,090,279 describes an inter-
ferometer using a diffraction grating, and either mono-
chromatic or white light can be used.
U.S. Patent No. 3,472,598 describes an apparatus
for determining the refractive index of light'permeable
substances providing for measuring a reference substance
2 o of known refsactive index to compare with the measurement
of the unknown substance by observation of interference
bands.
U.S. Patent No. 3,487,227 describes an inter-
ferometer apparatus especially suitable for measuring gas
purity and having three apertures using a monochromatic
light source. Fraunhofer interference pattern is
produced by this interferometer.
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U.S. Patent No. 3,612,696 describes a
refractometer cell for use in liquid chromatography
comprising a housing forming a chamber through which
radiation can be transmitted, said chamber adapted to receive
a test fluid. A radiation-transparent refracting means is
positioned within the chamber. In one embodiment, this
refractive means is a spherically shaped element. In
another embodiment, the refractive means comprises two
spaced elements having concave spherically shaped surfaces
facing one another. Use with a chromatographic column in
a detector is described.
U.S. Patent No. 3,680,963 describes a
refractometer to measure refractive indices of fluids by
optical fringe counting. Interference fringes are created
by overlapping two coherent beams of light which have
traversed different optical paths. One optical path is
through an unknown sample fluid and the other is through a
known reference fluid. The fringe pattern shifts in
direct relation to differences between the two optical
2 o path lengths.
In none of this prior art are described sample and
reference-cells of the design of the present invention and
none of the sample cells is especially suitable because of
such small sample volume for use in detectors for gas
chromatographs. Thermal conductiv,ity detectors are presently
used with gas chromatographs, and the cell and device of the
present invention has advantages over the thermal con-
ductivity detectors in providing smaller internal volume
sample cells (5-10 ~1 or even 2 ~1 or less), better
sensitivity, can be used with carrier gases which may be
corrosive or with corrosive samples, and is potentially
simpler and lower cost.
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A device useful as a detector for fluid (liquid and
gas) chromatograph effluent comprising a light source, a cell
divided into sample and reference fluids compartments separated
by an opaque diaphragm sufficiently thin to form a Fraunhofer
diffraction pattern, means to introduce and remove sample and
reference fluids to and from said compartments, means to colli-
mate light from said light source at said diaphragm and through
the compartments of said cell, photodetector means capable of
sensing phase shift of said diffraction pattern, and means to
focus! light exiting said compartments on said photodetector.
When used as a detector for chomatograph effluent the sample
compartment should have a volume less than the smallest volume
of eluted fractions from the chromatograph with which the de-
vice is used. An especially significant and novel feature of
the device is the cell used in the refractometer. Obviously,
the device is also useful as a refractive index detector for
other than chromatograph effluent.
In a preferred embodiment of the present invention
there is provided in a device capable of measuring refractive
index comprising light source means, a sample fluid compartment
and a reference fluid compartment, means to form a Fraunhofer
diffraction pattern and photodetector means to receive light
transmitted through said compartments and capable of sensing
phase shift of said diffraction pattern, the improvement making
the device especially useful as a detector for chromatograph
effluent comprising a cell divided into sample and reference
fluid compartments separated by an opaque diaphragm sufficiently
thin to form a Fraunhofer diffraction pattern.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more clearly understood from
the following detailed description of specific embodiments
thereof read in conjunction with the accompanying drawings .
wherein:
FIG 1 is a schematic diagram of a device of the
invention, -
FIG 2 is two graphical presentations of intensity
in the Fraunhofer plane, the first where sample and reference
gases are the same and the other where they are different,
. .
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FIG 3 is a graphical presentation of diffraction
angle vs. sensitivity for different beam diameters,
FIG 4 is a graphical presentation of baffle width
vs. cell length,
FIG 5 is a drawing of the details of a cell of
the invention,
FIG 6 is a block diagram of the electronics for
a vibrating slit type device of the invention,
FIG 7 is an optical schematic both top and plan
o views using a tungsten lamp in a device of the invention,
and
FIG 8 is an electronics schematic for a split
segment photodiode device of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The primary goal of this invention is to provide
a simple, low cost, small volume detector which will be
more reliable than the widely used thermal conductivity cell.
The diffractometric refractometer (DR) cell of the present
invention can readily be fabricated of materials which can
operate continuously at temperatures above 400C. Since
there need be no oxidizable constituents in the cell,
accidental injection of air into the column will not damage
the detector. When utilizing low cost laser sources currently
available, mean operating time between failures in excess of
10,000 hours can readily be achieved, and other system com-
ponents can be designed to have even lower failure rates.
The expense and inconvenience of repair and recalibration
should, therefore, be negligible with such a DR detector
system.
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A schematic diagram of a DR detector system is
shown in FIG 1. The basic components are the light source,
the cell, the detection slit, the photodetector, and the
photocurrent processing electronics. Each of these elements
will be discussed in more detail below.
THE DIFFRACTIO~ CELL
When an opaque plane with a long narrow slit is
placed in the path of a light beam, a FraunhofeT diffraction
pattern is formed on a distant screen, consisting of
o uniformly spaced light and dark bands whose spacing is
determined by the slit width and the wavelength of the
light. If an opaque strip, equal in width to the slit, is
placed over the slit and the opaque plane is then removed,
the diffraction pattern on the distant screen remains the
same except for the region where the direct beam strikes
the screen. The equi~ale~ce of the diffracted intensity
for positive and negative diffraction objects having
identical geometry is a general law, and we use it as the
starting point for the diffractometric reractometer design.
If the isotropic media on either side of the opaque strip
are identical, the usual symmetrical diffraction pattern is
produced in the Fraunhofer plane, as shown in FIG 2a. If
the refractive index of one medium is slightly higher than
the other by ~n, a relative phase shift of ~ = 2~nL/~
radians is introduced between the upper and lower sections
of the light beam causing the entire pattern to shift in
the direction of the higher refTactive index (RI) medium, as
shown in FIG 2b. Here L is the path length in the
reg;on having differential index and ~ is the wavelength of
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the light. By performing the ~raunhofer integration in the
presence of small phase shifts, and including the region of
the "direct beam", a plot of fringe shift per RI increment
~the "sensitivity") as a function of position in the
Fraunhofer plane can be made. Such plots for different
beam diameters are shown in FIG 3. By judicious selection
of beam diameter, the sensitivity at the center of the
pattern can be made almost as high as in the wings. Operation
in the "direct beam" results in greater optical signal into
the detector and, in general, better signal to noise ratios,
particularly with non-laser light sources.
The length of the cell is an important parameter.
The Fresnel diffraction in the near field of the leading
edge of the diaphragm results in reflection from the face
of the diaphragm, which interferes with the diffracted wave
at small angles. To avoid this spurious interference, it
may be desirable to place a thin opaque strip on the entrance
window, which is sufficiently wide to reduce the diffracted
light intensity which is incident upon the diaphragm surface
2 o to a negligible value. The geometrical arrangement is
shown in the inset of FIG 4, and the relation between the
width of the strip and the length of the diaphragm is
shown for the case where the diffracted intensity at the
far edge of the diaphragm is about 1% of the direct beam
intensity. It is clear from this relationship that as the
cell length increases, the beam diameter must also increase,
so that the active cell volume increases. Under these
conditions, the sample cell volume is approximately
Vs ~ 4~L2, where ~ is the mean wavelength of the beam and
L is the length of the cell. The optimum length of the cell
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is that length which has a sample volume somewhat smaller
than the smallest volume of the eluted fractions from the
particular chromatographic system being used. The
sensitivity of the detector to RI changes increases as ~r~,
so the longest cell, consistent with the maximum sample
volume for the system, should be used.
For example, cell compartments design to couple
efficiently to a Meret FIP 307 gallium arsenide laser
might have the following dimensions: 1 cm long (light path)
by 0.041 cm wide (both compartments) and 0.318 cm high
having a 0.004 cm wide diaphragm separating the sample and
reference gas compartments.
FIG 5 shows the construction of a cell of the
invention. A pair of stainless steel blocks each con-
taining a channel through the blocks together with the
steel diaphragm when fitted together form the sample and
reference compartments of the cell, two holes are provided
through each steel block communicating with the channel
therein to serve as passages for introducing or removing
either sample gas or reference gas, and two bolt holes
are provided through each block parallel to the channel
for use in assembling the cell. A pair of split glass
plates at each end of the blocks fit in recesses in the
blocks, sandwich the diaphragm between a pair at each
end, and serve as closures for the ends of the compartments.
0-rings fit in each recess around the split glass plates
to seal the ends of the cell. Retainer plates are
provided each with a hole in the center to provide a path
for the passage of light through the cell compartments
and with bolt holes for assembly of the cell. FIG 5 shows
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all parts slightly sepaTated and unassembled for better
understanding of the cell construction. It should be noted
that unlike the diaphragms of FIGs 1 and 4 no opaque strip
or baffle is provided on the forward edge of the diaphragm
to reduce interference since this interference may not be
sufficient to bother with as a practical matter.
THE LIGHT SOURCE
A low power (~1.0 mw) HeNe laser would be a
suitable source for a practical instrument. For a more
o compact system a gallium arsenide semiconductor laser or
an incandescent lamp operated with a suitable aperture slit
are desirable.
Currently available GaAs semiconductor lasers are
about l/lOth the cost of the HeNe gas laser, but must be
operated in short pulse mode. This is not a severe
restriction since the pulse repetition rate can be quite
high, and can be synchronized with the slit vibration drive,
if necessary. The emitting area of the diode is a narrow
slit, which should be aligned parallel to the diaphragm plane
to insure adequate spatial coherence. The slightly longer
wavelength of 0.90 ~m (compared to 0.63 ~m of the HeNe
laser) will not significantly influence performance, aside
- from requiring slightly larger cell volume for the same
length. A minor disadvantage in initial alignment is the
non-visibility of the radiation, requiring adjustment by
observing sîlicon detector response, perhaps using an
image converter for the initial coarse alignment.
The tungsten (or other non-laser) source has
much lower brightness. To provide adequate spatial
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coherence approaching that of the HeNe laser, but utilizing
the entire spectral band from 0.6 ~m to 1.0 ~m, a tungsten
source with a circular aperture can provide only 5 x 10 8 watts
compared to 10 3 watts for the laser. If a slit source
is used, this could be increased to about 10 6 watts, but
cell volume would have to be increased also to pass the
wider beam. An input optical power of 10- 6 watts will be
adequate to approach the fluctuation noise limited per-
formance of the laser system.
Q Since the tungsten lamp has a much lower brightness
than the other sources, some care must be exercised to
couple it efficiently to the cell. The highly incoherent
nature of the light emission from non-laser light sources
resulted in the development of the theory of partial
coherence, so that the diffraction and interference
characteristics of optical instruments working with such
light sources could be understood and predicted. With
such sources, the spatial coherence of light falling on a
given area can only be increased at the expense of reduced
2Q angular size of the light source. Roughly speaking,
coherence is maintained only over an area which falls well
within the central diffraction maximum of an incoherently
illuminated aperture corresponding to the source size.
Therefore, the tungsten lamp is imaged onto a slit, the
light from which is then collimated and passed through,the
cell, as shown in FIG 7.
The lower light level available with the tungsten
lamp source would make it difficult to detect the audio
frequency modulation of the vibrating slit. Since photon
noise and detection system instabilities do not appear to
,. .
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be limiting factors even in this case, simple D.C.
differential sensing of the photocurrents in two detector
segments can be utilized. The detector segment gap is about
0.1 mm, so the diffraction pattern at the detector must
be large enough that the central maximum for the symmetrical
case is at least 2 or 3 times the gap width. This requires
that the Fraunhofer plane be at least 100 mm from the cell.
A 105 mm F.L. achromatic lens is used to focus the
diffraction pattern on the detector plane. An auxilliary
slit is placed in front of the detector, having about the
width of the primary maximum, to eliminate the effect of
subsidiary maxima on the detector output, since the
detector segments are much wider than the central peak.
When the optical system is properly adjusted, the
diffracted intensity drops off rapidly enough for subsidiary
maxima that ît may not prove necessary to include this slit
in a simple leak detection module. The alignment of the
segment gap of the detector so that it is parallel to the
gas cell diaphragm is critical; an error of 0.05 mm over
20 the 2.5 mm sensitive area height (or ~1.2) can severely
degrade the differential response.
DISPLACE~D3;1T DETECTION
Differential refractive index changes in the
cells produce a displacement of the diffraction pattern in
the Fraunhofer plane, as discussed above. The position of
the central maximum for the symmetrical cell configuration
is the optimum position for the displacement sensor because
the largest excursions of intensity with refractive index
occur there, and the temporal coherence requirement is
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~069722 - 0369
lowest. Two sensing schemes are proposed, and appear to
be capable of approximately the same sensitivity. The first
method uses a split silicon photodetector with a differential
preamplifier. A shift in the diffraction pattern produces
an unbalance in the detector which is proportional to the
sine of the optical phase difference. A narrow ~and
synchronous detector should be used to avoid D.C. drift
effects in the amplifiers and to average fluctuation noise.
The second detection scheme uses an oscillating
slit in the Fraunhofer plane and a single element fixed
photodetector behind it. The light source is either constant
or pulsed at a much higher frequency than that of the
oscillating slit. A block diagram of such a system is
shown in FIG 6. The photodiode current is A.C. amplified
and the component synchronous with the slit frequency is
detected. This component is proportional to the sine of
; the optical phase shift when the slit is at the center of
the undeviated pattern. For small displacements, the
synchronously detected signal is, therefore, linearly pro-
portional to the refractive index change. Components
inside dashed lines of FIG 6 are needed only if large
refractive index differences need to be measured
(i.e. Qn>10 5 for 1 cm length cell). If larger excusions of
.
refractive index are anticipated, a full quadrature phase
detection system can be implemented by also detecting the
second harmonic of the slit frequency synchronously. The
second harmonic is proportional to the cosine of the optical
phase shift, and could be used with a quadrature phase angle
integrator to read out linear refractive index difference
over a wide dynamic range.
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For either detection system, the synchronous
detector frequency should be in the medium audio frequency
band to minimize low'frequency flutter noise introduced by
fluid turbulence, both in the cells and in the air paths of
the beam outside'the'cell. While careful design can minimize
these turbulence'effects, they will tend to be the limiting
noise source - particularly with laser light sources. The
integration time of the detectors should be of the o-rder
of a second or longer for the same reason.
The two segment photodiode detector requires a
completely different electronic system. The circuit is
shown in FIG 8. It consists of dual amplifiers feeding a
unity gain difference amplifier and a summing output. The
output of the difference amplifier is proportional to sin ~,
~ where ~ is the optical phase angle defined above, while the
sum terminal permits monitoring of detector and slit
alîgnment to insure that the detector is properly centered
and aligned.
SU~ARY
1. The detector'senses optical phase shift
due to presence of a high refractivity molecular component
in a low refractivity carrier fluid on one side of a thin -~
diaphragm, which serves as a diffraction mask.
2. The sample cell can be made entirely of
refractory and chemically inert materials such as stainless
steel and fused silica, so that it can be operated at high
temperatures and with oxidizing or reducing sample con-
stituents without damage to the cell.
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.
3. The light source is required to have high
spatial coherence over the cell aperture, but need not have
high temporal coherence ~i.e. monochromaticity not required),
thus a suitably apertured conventional source can be used
as well as a laser.
4. The optical detector system must be sensitive
to slight displacements of the diffraction pattern intensity
distributi:on.
5. Mechanical and electronic means for minimizing
o the effect of low frequency fluctuations in the diffraction
pattern due to fluid turbulence and mechanical vibrations
should be provided.
6. Reasonable sensitivity can be achieved with
active cell volumes as low as 2 ~1, a reduction of several
orders of magnitude below those of thermal conductivity
detectors.
7. Small cell volume should make practical
true differential sensing of peaks at column limited
resolution.
; 20 Although the invention has been described in
terms of specified embodiments which are set forth in
considerable detail, it should be understood that this
is by way of illustration only and that the invention is
not necessarily limited thereto, since alternative
embodiments and operating techniques will become apparent
to those skilled in the art in view of the disclosure.
Accordingly, modifications are contemplated which can
be made without departing from the spirit of the described
;nvention.
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