Note: Descriptions are shown in the official language in which they were submitted.
3~44~
Field of the Invention
This invention relates to circuitry for analog-to-
digital and digital-to-analog conversion and similar operations.
The digital circuit, on the other hand, is usually
faster operating, and thus the time required to complete each
sequence of steps is largely controlled by the response time of
the analog circuit. For example, if the analog circuit includes
a filter for smoothing out ripple and noise, it may require a
few seconds for the output of the analog circuit to reach a new
value. When these few seconds are multiplied by the number of
times the sequence is repeated the lost waiting time can be
considerable. We have dlscovered that the time lost waiting -
for the analog circuit to respond can be shortened by dissipat- ~ -
ing (e.g.,with a switch connected in parallel) the charge-
storage device (e.g., capacitor) in the analog circuit. After
dissipation, the analog circuit and charge-storage device are
allowed to respond normally.
This invention relates to light-beam deflection
instruments, such as refractometers. In such instruments, a
~20 light source typically directs a beam through a test cell to a
detector (e.g., a photocell). A change in some physical
quantity (e.g., refractivity) causes the light beam to move
with respect to the detector. A difficulty is that a change in
the position of the light source (e.g., from movement of the
filament in an incandescent bulb) often cannot be distinguished
from a change in the physical quantity being measured, as both
cause movement of the light beam with respect to the detector.
We have discovered that the position of the light source can be
stabilized with respect to the zone (e.g., test cell) in which
the measurement is made by modulating the light beam position
(e.g., cyclically sweeping it back and forth) through a
preselected amplitude which is independent of light source
-- 1 --
-'` 113fi444
movement or other uncontrollable movements of the light beam
(such as from thermal eddies). Light source movements only
vary the phase of the modulation (e.g., the time at which the
cyclical sweeping starts and finishes), and the electronics
which process the detector output can easily be designed to
ignore such phase changes, such as by determining the average
position at which the beam strikes
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1136444
the detector. The invention provides greatly reduced sensi-
tivity to light source movements, and can be inexpensively
implemented in preferred embodiments. In a second aspect, the
invention features an unfocused light path in a direction per-
pendicular to the direction that the light beam moves during
a measurement. Light from many points of the light source can
thereby be spread evenly across this unfocused direction, thereby
making the measurement insensitive to variations in brightness
of the light source along this direction.
This invention relates to devices which depend upon
accurate measurement of changes in the refractivity of a flowing
fluid, e.g., in liquid chromatography. Because temperature
affects refractivity, the temperature of the flowing liquid
must be carefully controlled. Typically in refractive index
detectors used in liquid chromatography the refractivities of
sample and reference streams are compared at a cell. One tech-
nique for equalizing temperatures in the sample and reference ~;
streams in the cell has been to supply the fluid to the cell
after having passed through~sample and reference inlet tubes
in heat-exchanging relationship with each other and with a large
metal block. Ideally, both streams are at equal temperatures
before entering the~cell. We have discovered that better
control over temperature can be achieved with a heat-exchanging
relationship between the sample inlet ~to the cell~ and sample
outlet ~from the cell) streams. The required hardware is
simpler and inexpensive. Excellent temperature equalization
of sample and reference fluids, and very fast warm up and cool
~.-
down of the device, are made possible.
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113~;444
We have discovered that increased light throughoutcan be achieved in the cell of a refractometer by placing an
integral reflective layer on or within the cell, to reflect ~;
the light beam back through the cell. Parallax between the flow
cell chambers and the reflective surface is reduced. Fewer sur-
faces are exposed to the ambient, thereby reducing losses due
to dust buildup and surface reflection. Manufacturing is simpli-
fied as fewer parts are required.
We have also discovered an improved zeroing technique
which is relatively insensitive to changes in light intensity,
and which, in preferred embodiments, eliminates optical
zeroing. In general, we include in the system output a main
term dependent on the difference between the two optical measure-
ments and an offset term dependent on at least one such measure-
ment. Any change in light intensity will substantially equally
affect the main and ofset terms, so as to maintain seromg
accuracy.
We have discovered~that increased light throughout
can be achieved;in the cell~of~a refractoDeter by mcorporating
an integral curved~surface with the cell, to act as a lens for
: ~ :
focuslng the light beam. Parallax between the flow cell
chamber and the lensing surface is reduced. Fewer surfaces are
exposed to the ambient; thereby reducing losses rom dust build-
up and surace reflectiQn. Manufacturing is simplified as
fewer parts are required. In a second aspect, our invention
'eatures incorporating an opaqu~ mask ~ithin or on the flow cell.
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113~444
According to a broad aspect of the invention there is
provided in apparatus of the type including a light source
directing a light beam through a measurement zone to a detector,
wherein movement of said beam in a measurement direction with
respect to said detector gives an indication of the measurement
made in said zone, the improvement comprising: means for
modulating, through a preselected amplitude in a modulation
direction, the position of said beam with respect to said
measurement, said amplitude being independent of light beam
movements at said light source or in the path of said beam
between said source and said modulating means, whereby said
measurement can be made substantially independent of said beam
movements in said modulation direction.
Preferred Embodiment
The structure and operation of a preferred embodiment
of the invention will now be described, after first briefly
describing the drawings.
Drawings
Figure 1 is a perspective view of said embodiment.
Figure 2 is a partially cross-sectional view of said
embodiment.
Figure 3 is a cross-sectional view of 3-3 of Figure 2,
showing the photocell end of the optical bench.
Figure 4 is a cross-sectional view of 4-4 o~ Figure 2,
showing the flow cell end of the bench and the outer insulating
cylinder and shields, with internal heat shield/light baffle 77
removed.
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~ 13~i444
Figure 5 is a cross-sectional view at 5-5 of Figure 2,
showing the light source.
Figure 6 is a schematic of the heat exchanger plumbing.
Figures 7a and 7b are cross-sectional views through the
sample and reference heat exchangers, respectively.
Figure 8 is a cross-sectional view at 8-8 of Figure 4,
showing construction of the flow cell.
Figure 9 is an elevation view of the back surface of the
fIow cell at 9-9 of Figure 8.
Figures lOa and lOb are diagrammatic views of the optical
path through said embodiment. ~
Figure 11 is~a block diagram of the electronics that
process the outputs of the photocells.
Figures 12a and 12b are schematics of the electronic
circuits that null the photocell~output and process the nulled
output for display and integration.
Structure
; Turning to~Figure~1, opt1cal bench 10 is~supported inside
20~ an oven on four~insulating posts l2 attached to floor 14 of the
oven. Light source~16 for the~bench is~positioned~below~the -~ ;
bench and outside of~the oven. Fiber-optic cable 18 carries
light from source 16 to the bench. Sample liquid from the out-
let of a chromatographic column tnot shown) positioned inside ;~
tihe oven flows into~the optical bench through inlet~ tube~20
O.009 inch`ID), and out through outlet;tube 22 (0.040 inch OD).
A small diameter sample m let tube is used to minimize band
spreading in the chromatogram.~ Similarly~ reference liquid
flows into and out of the~bench through~lnlet tube 24 (0.020
inch ID) and o~utlet tube 26 (0.040 inch OD). All four tubes
are stainless steel and have 1/16 inch outside diameters. The
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1~3~
outle-t tubes have larger internal diameters than those of the
inlets to lower backpressure and its effect on refractivity.
Outlet tubes 22, 26 are connected together downstream of the
optical bench to equal'ize sample and reference pressures at the
flow cell. Electrical wires 28 fro~ photoceIls'52'(Fig. 2)
lead from the bench'to processing circuits shown in Figs. 11,
12a, and 12b.
Turning to ~igs. 2 through`4, optical bench'l0 -
consists of an irlner cylinder 32, through which a light beam
B is passed, and a concentric outer cylinder 34, which provides
an insulating air gap 36. Two flat shields 3~ (Flg. 1) retard
r~diation of heat to and from the bench, and act as legs (Fiy.
4~ to support the cylinders, via four bolts 40,` on posts 12.
End caps 42, 44 close each end of outer cylinder 34, and end
caps 46, 48 each end of inner cylinder 32. End cap 46 supports
elon~ated outlet 50 ('0.050 inches wide by 0.35 inches high) of
~iber-optic cable 18 and photocell 52. End cap 48 supports
flow cell 54 via cell hridges 56, 58, which are attached to the
cap and each other by screws and epoxy. Sample inlet and outlet
tubes 20, 22 terminate at bridge 58; reference tubes 24, 26
terminate at bridge 56. Recess 60 in end cap 48 behind the ;~
bridges contains about four coils o~ sample inlet tube 20. ` '
Notches 62, 63 in inner cylinder 34 provide entr~ways for the
tubes. The end caps, cylinders, and shields are all made from
alu~inum, to speed warm up o~ the bench while also insulating
the benc'h by virtue of air ~ap 36 between the cylinders.
f urning to Figs. 8 and 9, flow cell 54 has two hollow
chamhers 70, 72, for the sample and reference liquids, respect-
ively. Each chamber has a triangular (ahout 45x45x90 degrees,
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~` 113~ 4
0.062 inches on each short side) cross section (Fig. 8), and ;
is connec-ted to its respective inlet and outlet tubes by
internal passages 73. ~he height (or vertical dimension in
Fig. 9) of the chambers is about 0.50 inches. The flow cell is
manufactured by fusing to~ether, without adhesive, pieces of
borosilicate glass. Teflon~seals 75, compressed against the
cell by the bridges, provide a seal between the sample and
reerence tubes and internal passages 73 of the:cell. The
front surface 74 of the flow cell is ground to provide an
integral lens that has curvature in horizontal but not vertical
planes. The back surface of the cell has a re1ective surface
coating 78 of gold to provide a mirror to reflect light back
through chambers 70, 72 to photocell 52. The focal line of the
le~s is positioned at photocell 52, and the spacing between
mirror 78 and photocell 52 is about 6.0 inches~ As shown in
Fig. 9, the mirror coating 78 is limited to approximately the
area directly behind chamber 72, thereby to limit refle~tion ~ ;.
: principally to light passing through the trian~ular chambers.
Oth.er light is absorbed by black epoxy coating 76 applied over
and around the mirror coating, The mirror coating is slightly
lar~er th~n the chambers to acaommodate variatiQns in the inter-
nal size of chambers 70, 72. The coating StQpS short o~ the to.
of chambers 70, 72 (Fig~ 9) so as not to re1ect li~ht passing
through the top o the chambers, ~here bubbles might form.
To reduce radiant and convective heat.transer to the
flow cell from within the optical bench, a blackened disk 77
with rectangular light-beam ~perture 79 (just larqe enough to
expose the flow cell~ is positioned ahead of the.flow cell~
This di.sk also serves as a light baffle, and is tilted down 10
(Fig. 2).
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Sample and reference liquid are brought into the flow
ce].l throu~h sample and reference counterflow heat exchangers
90, 91 (Figs. 1, 6, and 7), each of which are'formed by bonding
corresponding inlet and outlet tubes together inside a tubular
jac~et. Each bonded pair is then routed along a multi-zone path
beginning outside the: bench and endin~ at the flow.cell. As
shown in Fig. 7, sample heat exchanger 90 is constructed by
placing tubes 20, 22 inside a tubular copper braid 80, heat ~'
shrinking a polyeth.ylene tube 82 over the outside of the copper ~.
braid, and filling the interstices between the braid and the
inlet and outlet tubes with a low-viscosity, moderately-heat- '::
conductive epoxy 84 (Stycast~3051). Reference tubes 24, 26
are bonded without a copper braid by inserting the tubes inside .
a Teflon tube ana filling the tube with the same low-viscosity
epoxy as used for the sample tubes. The braid is omitted be- i~
cause less efficie'nt''heat transfer is needed for !the reference,
as it does not flow during measurement, but only during flushing
~: between measurements.
:~ Turning to Fig. 6, the multi-zone path followed by '
:: 20 the heat exchangers is shown diagra~matically~ The first zone
for both sample and reference heat exchangers beglns outside .'.
the optical bench and extends along the outside len~th Qf the `` ;`~i.' .
bench between outer cylinder 34 and shields 38 (total zone .
length about 8 inches). The sample heat exchan~er 90 is posi- :
tioned on the side of the bench closer to the center of the ' '
,
oven, where temperatures are better controlled. At end cap
42, both heat exchangers turn 180 and enter gap 36 between .
cylinders 32, 34, through a slot (not shown~ in the end cap,
The second zone'for both sample and reference ex'tends along .~'
gap 36 (total length'about 7 inches), The reference heat ex-
changer goes directly from gap 36 into cylinder 32 through notch
oc~/e J~Q r k
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1~3~ 44
62 in the end of cylinder~ Inside cylinder 32, the reference
inlet and outlet tubes are brought directly to flow cell 54
via bridge 56.
The sample heat exchanger 90 continues into a third
zone beyond the end cap, where it is bent intb coil 92, con-
sisting of four turns (total coil length about 24 inches)
positioned in the space hehind end cap 48. The last coil is
adjacent to the back of the end cap. From the coils the sample
heat exchanger enters cylinder 32 through notch 63. Inside the
cylillder, sample outlet tube 22 is connected directly to the
flow cell~ Sample inlet tube 20 is wound in another coil 94
(total length about 12 inches) before entering the flow cell. ~ ~ -
Coil 94 is positioned in recess 90, and potted with a heat-
conductive epoxy to provide good conductivity with the end cap
and cell bridges.
Turning to Figs. 2 and 5, light source 16 includes an
incandescent bulb 100 (~hillips 6336, H3 base, 6 V, 55W, oper-
ated at 4.8 V) with verticall~-extending filament I01, a
concave light-focusing mirror 102 (gold-coated glass), and a
rotating prism 104. The prism is rotated at about 50 to 60
rpm along an axis parallel to the ilament axis b~ a shaded-
pole AC motor 106~ The motor also dri~es a an 10~ which sup- -
plie~s cooling air to the bulb. Prism 104 is about 0.37 inches
high, is made of ~lass, and has a rectangular cross section.
Two opposite surfaces 110 of the prism are clear and about 0.3
inches wide. The other two surfaces 112 are opaqued with a ~ -
white opaque silicone rubber, and are about 0.25 inches wide. ~-~
Fiber-optic inlet 114 is round tabout 0.150 inches in diameter~ -
and is positioned opposite th~ prism from the bulb. Mirror
1~3~4~L4
102 is positioned so as to focus an image of filament lOl on ~-
the face of inlet 114. Bulb 100 has a peak output in the near
infra-red spectrum at a wavelength of about 1000 nanometers.
Fiber optic cable 18 is broken internally into sub-
bundles and the sub-~undles are intentionally disordered at
one end to randomize the liyht path between inlet 114 and
outlet 50.
PhotoceIl 52 has two adjacent triangular dual photo-
voltaic cells 180, 1~2 (gold-bonded silicon) arranged so
th~t their lon~ di~ensions e~tend horizontaliy, which is the
direction o movement of the light beam. Each triangle is
about 0~150 inches long and 0.05 inches high. The spacing be-
tween the triangles is about 0.008 to 0.010 inches. The shunt ;
impedance at operating temperature (about 150C) is maximized,
as is the sensitivity to long wavelengths.
The oven in which the optical bench resides is heated
by proportionally-controlled eleatrical resistance elements. ` --
~ithin the oven, temperatures can vary as much as 5 to 7C
from point to point, but by much less (e.g., 0.3C) at the same
point over time. The time period during which the resistance
ele~ents are on is varied in proportion to the dif~erenae be-
tween the actual oven te~perature and the desired temperature
and in proportion to the integral o~ this difference. To make
oven temperature less sensitive to variations in AC line voltage,
the time period is also made inversely proportional to the square -
of the line voltage, as the heat generated by the elements is
proportional to the square of the line voltage. The elements
are SCP~ controlled, and are turned on and off only at zero
crossin~s of current.
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" 113~4~4
~ igs. 11, 12a, and 12b show the electrical circuits
that process the outputs of photocell 52. Fig. 11 shows the
overall circuitry in block diagram form. PaneI inputs 118
(e.g., recorder gain) are fed to a central pxocessor 120. The
central processor (CPU) initiates the automatic eLectrical
zeroing (nulling) o~- the photocell outputs~ and sends signals
via buffers 122 and gain latches 124 to circuitry shown in Fig.
12b to set the gain for display of the chromatogram on a recorder.
An analog data ac~uisition voltage (D.A.V.) is converted to
10 digital and sent by the processor via the input buffers to a ~ ~-
panel display 126.
Fig. 12a shows the circuitry for electrical zeroing.
The current outputs (AC signals) of photocells 180, 182 are
brought via shielded cable to current-to-voltage con~erters
130. The AC voltages A, B produced by the converter are summed
and amplified by a gain of 2.2 at amplifier 132, to form the
expression -2.2(A+B), which is called SUM. Amplifier 134
subtracts voltage A from voltage~B, and adds to the difference -
the sum of three volta~es: SUP~, FINE ZERO, and COARSE ZERO.
~20 The~latter two voltages are produced by multiplying SUM hy a
negative scale factor. Thus the output of amplifier 134 `
(ZEROED OUTPUT) can be expressed as
[~ - Al - 2.2[.33 - .67KC - .0033R ]~A ~ ~]
where KC is the coarse æero scale factor and Rp is the fine zero
scale~factor. Scale factors Kc, KF are set between about zero
and about one by the digital circuitry of block 136, whenever
~ a signal is sent across the ~UTOZERO COMMAND lead. Normally
; zeroing would be done before a chromatogram was generated, but
can be done at any time.
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13~ 4
The above expression for ~he Z~ROED OUTPUT can be
presented in simplified form as
~ B - A] -K[A ~ B]
where K is the overall scale factor. The expression is inde-
pendent of variations in the overall brightnes's of the light
beam striking photocell 52 because the zer'oing term (K~A + B])
is not a constant, but, like the difference''term ~B - A) is
proportional to bea~ brig~tness. For example,' if the brightness
were to xise b~ 10~, both'the difference term and *he zeroing
term would similarly ~ise by 10~, and thus the'whole expression ~ '
would still remain equal to zero. When beam deflection '
does occur, as the result of refractivity changes, the zeroing
term remains roughly const~nt because of the complemontary shape
of the two cells 180, 182~ which'at any horizontal lo^ation '~'~
h~ve xoughly the same cor.lbined vertical height'.
,. .
Two successive-approximation registers 138, 1~0 ' '
.
drive a pair of digital-to-analog converters 142, 144 to form
the ~INE ZERO and COARSE ZEP~O signals. Each o aonverters
142, 144 multiplies the SUM slgnal by a scale'fact~r set by the
20 digital output of registers 138, 140. Registers 138, 140
follow a conventional successive approximatlon algorithm to
select the digital outputs or scale factors. About onae'a
'~ second, the registers~receiVe a clock pulse from ahip 148, which
produaes a slow c}ock from the much faster proces`sor clock
:: ~
sl~nal, At each clock pulse, the output of a register is
adjus~ed in response to the output of comparàtor ~1~6 which
indicates whether the applied FINE/COARSE ZERO signal lS too
l~rge or too small, The input to comparator 146 is the DC
OUTPUT, produced at filter amplifier 150 (Fig. 12~). A
FILTER RESET connection between the zeroing circuitry and filter
1~
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1~3~4
amplifier 150 is used during the zeroing process to discharge
capacitors in the filter and reset the DC OUTPUT to zero.
This allows for a more rapid autozero sequence. Register 138 -
works first to set the coarse scale factor Kc, and then register
140 to set the fine scale factor KF. The AUTOZERO CO~D~AND is
used by the CPU to star~ the autozero sequence. The AUTOZEROI~G
- signal is used to alert t'ne central processor thàt the refract-
o~eter is autozerGing.
Turning to Fig. 12b, there is shown circuitry for
processing the ZEROED OUTPUT. Amplifier 152 raises or lowers
the si~nal level in response to command signals 151 from the
central processor 120 via the data latch 124. Demodulator 154
(with the help of phase computing block 153) converts the AC
signal to DC~ and filter amplifier 150 smooths the DC signal.
Switching block 156 operates during zeroing to turn off the
RECORDER and INTEGRATOR signals. It also is used to change the -
polarity of the DC signal in response to a POL RI~Y signal from
the central processor lZ0 via data latch 12A. Downstream-of ~ `
,
~lock ~56 the DC signal is processed by amplifier 158, and sup-
plied to an integrator output lead. The DC signal is also
processed by attenuator 160, under control o the cen~ral
processor via signals 162. The attenuator produces a recoxder
oUtput 164, whic~ is supplied to a recorder output terminal and
to ~mplifier 166, and a data acquisition voltage (D.~.V.~,
which is supplied to the central processor for panel display.
Block 168 supplies a mark si~nal for the recorder in response ~ -
to the AUTOZERO COMMAMD, to indicate on the chromatogram the
point at which the sample injection occurs. The CPU issues
the AUTOZERO CO~AND at the time of sample injection.
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Operation
In operation, the oven surrounding the optical bench
and chro~atographic colu~n is turned on, and ahout an hour and
one half warm up period is allowed for temperature equalization
within the bench. After warm up, solvent is pumped through the 'i-
sample and reference :circuits within the be'nch. ''i~en solvents
are changed, sufficient time is allowed for flushing both
circuits. Flow is then stopped in the`'reference'circuit (hut
reference chambex 72 remains filled with'referen.c~.liquid). A
sample is then injected into the sample column. T~..e electrical
output of the refractometer is zeroed by initiatin~ the
automatic zeroing sequence described above.' Sample passes
through the'chromatographic column and into the optical
bench. Generally speaking, variations in refractivit-r of the -
sample cause movement of the light beam with'respect t~ photo~
cell 52, and thereby';chanye the electrical output, which is :~
plotted against time on a chart recorder, producing a chrcmato-
gra~.
Temperatures within chambers 70, 72 of the flow cell
are maintained within about O,OOOl~C of each'other during
operation to minimize error. A temperature difference betwee~.i .
the two flow cell.chambers results in a refractivity differ-
enae. Temperature equalization is achieved by pxovidin~ good
thermal insulation around the flow cell, in the'form of air
gap 36 between the inner and outer cylinders, shields 38, :
and blackened disk 77; surrounding the flow cell with a thermal
.~ mass, in the form of bridges 56, 58 and end cap 48; and
directin~ incoming sample 10w through a very efficient ''
counterflow heat exchanger to bring the temperature of the
sample..to the flow cell temperature. Incoming sample upstream
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of the heat exchanger is typically ~s much as 1C (and possibly
2 to 3C) different in temperature than the flow cell because
of spatial differences in oven temperature and because of heat
generated by viscous heating inside the inlet tube. This diff- -
erence in temperature is gradually reduced along the length ~`
of the heat exchanger by ther~al conduction between the inlet
and outlet tubes. At t~e end of the heat exchanger, whatever
very small temperature difference remains is minimized by heat
transfer between end cap 48 and coil 94 just prior to entry into
t~e 10~ cell~
The sample heat exchanger is divided into three zones `
to improve its efficiency, with each successive zone being mare
thermally stable and closer to the temperature o the flow
cell. The construction of the heat exchanger provides good
thermal conduction between tubes but very low conduction along
the flo~ direction bf the tubes. There is signifiaant heat
transfer between the tubes and the surrounding air; thus ther-
mal interaction between the heat exchanger and the re~ion sur-
rounding it must be considered. The`first zone, between outer
c~linder 34 and shield 38,~provides a gradual approach in
temperature before the heat exahanger enters the optical bench. -~
The length of this æone is greater than 10~ of the length of
the ~ample inlet tube within the bench. Withou~ the first -
zone, i.e., if the inlet and outlet tubes were joined just
outside the entry to end cap 42, there would be a steeper
approach in temperature along the heat exchanger, and much of
this approach in temperature would occur along portions of
the heat exchanger inside air gap 36, thereby undesirably
,
fi4~
transfexring heat to or from the bench. ~ith the preferred
arrange~ent of a first zone outside the bench, the heat ex~
changer temperature 'is closer to that of the bench when
entering the air gap.
The heat exch~nger enters the gap at the photocell end
of the bench, thereb~ assuring that whatever heat transfer to -'
or fro~ the bench does occur is at a location we'll separated
from the flow cell.
This same concept of routing the heat exchanger through
increasingly more thermally stable'regions is also applied to
the second and thi~d zones, In the second zone, th~' sampIe ~'
heat exchanger is directed along air gap 36 from the photocell
end to the flow cell end, where temperature stability is highe~st.~-
In the third zone, the sample heat exchanger~is coiled behind ~ ;
the flow cell end cap, with each successive coil being closer ; ~
, . .. .
to the end cap and flo~J cell. -
As a final step,~ the sample inlet~tube a~lone is coiled ~ '
in recess 60 of end~cap 48~to minimize whatever small tempera-
ture dif~erence~remalns~betwe~en the incomi;ng samp1e and the
20~ flow cell. ~ -
Because the reference solvent does not~10w during
a measurement, the reference heat exchanger i~ 1QSS sophisti- -
: :
catsd. It lacks the third~coiled zone, and ha~ no copper,heat-conductive ~raid to surround inlet and outlet tubes.
Limited heat exchange i's provided on the reference side to
,
maintain rough temperature equaliæation during flushing of the
reference circuit,~ thereby shortening the period needed to
stabilize temperatures ~fter~flushing.
The optical elements of the refractometer are shown ;
diagrammatically in Figs. IOa and 10~. For clarity the optical -'
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1~ 3Ç;~44
path is shown unfolded, with mirror 78 treated as a window. ~ -
Fi~. lOa shows a horizontal section through'the optical path;
Fig. lOb shows a vertical section.
Turning to Fig. lOA, a single light ray B is shown to
illustrate beam move~ent~s. Lens surface 74 on flow cell 54
focuses the' light emerging from fiber-optic cable'outlet 50
onto photocell 52~ The focused image on the photocell is shown
diagramm~lically in the views on the left side o~ the Figure.
To illustrate the effect caused by rotation of prism 104, four
views (~ through D) of the prism in different angular positions
are shown along with the corresponding positions of the light
beam on the photocell.
Light passing through chambers 70, 72 is bent in pro- -
portion to the difference in the'refractive index of t'ne liquids ~'
in the two chambers. Referring to Fig. 8, the chambers are
conventionally constructed so that~surface 190 in chamber 70 ` ~- '`;~
is paxallel to surface 188 in chamber 72 and, similarly, so that
surfac.es 184 and I86 are~parallel. These four~surfaces are
the ~four at which~light is bent by refraction. ~If~the liquid
of tlle same refractive index is in both chambers, light will
be bent by the same amount at eaah of the correspondin~ parallel
surfaces, and will emerge fro~ the flow cell alon~ a path B which ~' -'
is éssentially unaffeated by changes in refractivity common to
both chambers. If liquid in the two chambers differs in re~
fractive index, light will be bent differentially at these '
parallel surfaces, and will emerge along a path'skewed from
the equal-refractivity path. Such a condition is illustrated
in Fig. lOa by light ray B'. The amount by which'the light beam
is s~ewed or bent at the flow cell is measured by detecting the ~ ~
:'.
.
1~3~
position of the image of the beam at photocell 52. The dif~er- -
ence between the electrical outputs of the two triangular cells
180, 182 can very finely resolve the` horizontal position of the
light beam. Imperfections in alignment of the phbtocell with
the flow cell and other tolerances in the syst'em typically
cause these eIectrical oUtputs of the two cel'ls to be unequal
eVen when sample and reference liquids have'tXe''s'ame refractive
index. This initial electrical difference is nulled by the
automatic zeroing procedure described above.
0 Ideally, the light beam location on the ~ho~ocell 52 -
should only be a function of the difference in refractive index ~ ~ '
between sample and reference (and not a function of the location
of bulb filament 101~. To achieve this, the light intensity
distribution across the fiber optic outlet 50 must be spatially
stable over the time period of chromatographic interes' (1 ~-
second to several'ho'urs). This requires that the light in-
tensity distribution into the fiber optics be stable. A, viewed -
from the fiber optics inlet 114, the apparent position of bulb
filament 101 varies due to filament distortion and thermal
eddies in the ~ir path between the filament and the inlet.
Filament movements along the length of the filament (vertical
in Fig~ 2~ are relatively nonaxitical~ Si~ilaxly, changes
in the filament distance from the fiber optics inlet are nQt
observable and thus are noncritical. Along the thlrd axis
of moVe~ent (vextical in Fig. 5) the apparent filament location
' as viewed by the fiber optics inlet may be spatially stabilized
for the beam location at the photocell to be independent of
filament location~ ~o ~chieve stabilization, a Spatially
Homogenizing Optical Modulator (SHOM) in the form of rectangulax
``` ` - 1136444
prism 104 is employed in the light path between the filament
and the fiber optics inlet. The prism provides an optical path
offset which is a function of its rotation position. When the
prism rotates, the filament optically appears to sweep across
the face of the fiber optics inlet 114. In position A, the prism
is so oriented that the light from filament 101 is bent out-
side the acceptance angle of the fibers in cable 18, and
negligible light is transmitted to the bench. In position B,
the prism has rotated sufficiently for light to be transmitted
through at least some of the fibers in the cable. In position
C, the prism has swept the filament image across the face of the
fiber optics inlet. In position D, the prism has moved the imaxe
to a position beyond the acceptance angle of the fibers, and
again negligible light is transmitted. As the prism rotates
further, the beam first reappears beyond the acceptance angle
of the fibers, as in position A, and then another sweep begins.
The sweeping action,~including the perlod of negligible ~
light transmission,~ occurs two times during each revolution of ~ ~ ;
the prism, or about lOO times per second. ~ :
If filament lOl moves or appears to move, this has `~
the effect of changing the ti e at~whlch the beam starta and
finishes its sweep across the fiber optics inlet. That is, only
the phase of the beam movement is altered by movement of the
filament. The electronics described above compute the average
or middle position swept by the-image. The electronics are
m sensitive to such phase~or time shifts, and thus the ~ :`
undesirable effects of filament shift are minimized.
The apparent light source position is further stabil-
ized by using the randomized fiber optics bundle 18. In
- 19 -
,~
' ' ' . ' . ' ~
s , : . ~
113~i444
a perfectly randomized fiber optics bundle, adjacent fibers at
one end of the bundle are randomly distributed at the other end.
Therefore, increasing the light on one side of the bundle in-
put while decreasing it on the other side results in no change
in the light distribution across the fiber optics output end.
In actual practice, the randomization in a bundle is not
perfect, and some change does occur at the output end. But
using the randomized fiber optics does further decrease the -
effect of filament motion on beam movement at photocell 52.
As can be seen in Figure 10b, the optics do not focus -
the beam onto the photoceli in the vertical direction, as done
in the horizontal direction. Instead, light emerging from
outlet 50 of cable 18 remains unfocused in vertical planes,
thereby producing for each point of light at the outlet a ver-
tical line of light at the photocell. The vertical height of
this line is limited by the vertical height of mirror 78, which
acts as a mask. Light rays from ind-ividual points, e.g.,
points X and Y, on the cable outlet 50 fan out, but only rays
inside of limlt rays Xl, X2 ~Yl~, Y2 for point Y) reach the
~20 photocell. (Other rays are not reflected through the photocell.
The vertical heights of mirror 78, photocell 52, and cable out-
let 50 and the sFacing between the ~low cell and photocell
ends of the bench are all selected so that the limit rays or ;
all points on the cable outlet strike fully above and ully
below triangular cells 180, 182 of the photocell. Limit rays
for point X and point Y, at the top and bottom extremities of
the cable outlet, are shown in ~igure 10b. Thus each point on
the cable outlet produces a line o uniform intensity at the
photocell. And these lines all overlap over the photocell,
thereby assuring a uniform vertical intensity across the photo-
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:
~3~ 4
cell no matter what vertical variation in intensity may exist
at the cable outlet (e,g., due to variation in filament inten-
sity in the vertical dir'ection), The end result is the light
intensity profile shown at the left side of Fig. 10b. Across
the vertical height of the photocells the intensity is uniform;
outside the photocells the intensity falls off to zero. Vert~
ical uniformity of light intensity at the photocells is needed
to linearly determine the'horizontal light beam location on
the triangular-shaped cells 180, I82. (A vertica'l variation in
intensity would be indistinguishable from a horizontal movement
of the light beam.)
Other Embodiments
~ ther embodiments are within the following claims. ' '~
For examplel reflective coatings other than qold could be used
(e.~,, aluminum, silver, or a multilayer coating); an anti~
reflection coating could be substituted for black epoxy coatinq
~;~ 76, with a liqht trap positioned behind and external to the
cell to absorb light passin~ through the coating; quartz glass
nd the like could replace the borosilicate glass used for the
flow cell; and the glass pieces of the flow cell could be joine~
together by diffusion bondin~ or with adhesive.
' Claims
What 19 claimed is:
' ~ ,f~;,;' ,~J/ "
' .'' '
