Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to photomcters.
In analysis of very small quantities of liqllids, it has been
recognized that the physical condi~ioning of the fluid must be done very
carefully, Thus, for example, in the field of liquid chromatography wherein
very small, continuously-flowing streams of liquid are measured, care is
taken to minimize mechanical and thermal disturbance of the liquid stream
between the chromatographic column and analytical apparatus in which the
liquid stream from the column is to be continuously analyzecl. The primary
objective is to present~ to a transparent sample cell, the precise sequence
of changing liquid composition that leaves the chromatography column,
The rationale and particulars of such apparatus are described
in the art. For example, see United States Patent 3,674,373 to Waters,
Hutchins and Abrahams which involves a refractome~er particularly well adapted
to receive such a liquid stream. In general, the approach is to minimize
the conduit path through which the liquid to be analyzed must travel and to
provide a maximum thermal-conditioning of the liquid within such a minimized
path, This generally illustrates the art-recognized importance of careful
handling of sample liquid between its point of origin and the sample cell
in which it is to be subjected to analysis, usually analysis which measures
an effect of the sample liquid stream on some radiation directed into a
flow-cell through which the stream passes.
Investigators have also realized that some attention must be
given to the physical condition of the fluid even after it enters the flow-
; cell. Consequently~ flow-cells have been made ever smaller to avoid mixing
and peak-spreading effects and, in some cases, a positive thermal equili-
bration of the cell with the liquid has been sought in order to avoid light-
shimmering effects along the cell walls. Moreover, the cells are usually
positioned with outlets so placed that any entrained gas bubbles tend to
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be carried upwardly out of the cell. It is noted that U,S, Patent 3,666,941
to Watson describes a conical bifurcated cell wherein the larger end of the
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cell faces the light source, thereby forming means to gather a
maximum amount of fluorescence-exciting radia-tion. Applicant's
discovery, to be detailed below, is based upon a major improve-
ment in flow-cell construction which solves a problem quite
different than that described by Watson but which, like Watson's
apparatus, is particularly useful in combination with liquid
chromatography applications.
A recent patent, U.S. Patent 3,792,929, to Alpert, it
has been noted, seems to disclose a conical sample-holding cell.
The patent related to static-sample devices and in no way
involves fluid lenses of any type; although the patent came to
the attention of the instant inventor after an error resulted in
the word "field" appearing as "fluid" in the title of the
~ Alpert patent. ~loreover, the apparent and relative dimensions of
;~ the Alpert cell would not allow its effective use in most con-
tinuous-flow monitoring sys-tems such as are encountered in liquid
chromatographic work and the like.
A principal object of the invention is to provide an
absorptometer which can be utilized with liquids of various
refractive indices without encountering variations in optical
performance of the instrument which will materially interfere
with the quality of the absorption measurements being performed.
It is another object of the invention to provide an
` absorptometer wherein light entering the absorption cells is
carefully processed before entry thereinto in order to avoid any
such light's impinging on the walls of said absorptometer.
~ Thus, in accordance with a broad aspect of the
; invention, there is provided apparatus for measuring the amount
of light absorbed by a portion of a flowing liquid, comprising:
a sample cell for transporting said flowing liquid along the
longitudinal dimension of said cell, said cell including a wall
extending generally longitudinally, a light entrance window at
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one longitudinal end of said cel~, and a light e~it window at the
other longitudinal en~ of said ce~l; a light source for generat-
ing a light beam; masking means including a mask with an aperture
positioned between said light source and said entrance window of
said cell for optically shaping said beam so that the portion of
said beam that enters said cell through said window is trans-
mitted through said cell without contacting said cell wall for
:~ a predetermined maximum liquid-lensing condi-tion, thereby assuring
~ that wall contact does not occur under any expected condition,
said maximum condition corresponding to when the llquid in said
cell has a distribution of refractive index that causes the
~. greatest expected divergent spreading of said beam by refraction,
.. ~ detection means positioned beyond said exit window for measuring
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substantially all of the light emerging from said window, said
:~ detection means including a photoelectric element onto which said
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emerging light beam is incident, whereby the amount of light
detected by said apparatus is inaependent of the amount by which
said beam is bent by refraction within said cell because all of
` said beam entering said cell and not absorbed by said liquid also
20 emerges from said cel.l and is detected, thereby making said
apparatus independent of variations in the refractive index of
` said liquid.
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~ le invelltion is based on the discovery that substantial spur-
ious radiation signals are generated by differences in refractive indices
and particularly by a lens-type effect caused by liquids of different re-
-fractive index and especially laminar-flow patterns at the interface of com-
positions diff~ring in refractive index; the effect istroublesome in small
cylindrical photometer sample-cells. These laminar flow patterns will
sometimes be called "dynamic liquid lenses" in this des~ription. In gen-
eral the worst problems have been encountered in 10w-cells in the micro-
liter range, say flow-cells having a diameter of less ~han about 2 milli-
meters. In the usual situation the flow path of an ultra violet absorpto-
meter cell is selected to be one centimeter in length, and a flow cell of
``~ 2 millimeters maximum diame~er will have a volume of less than about 32
microliters. As the diameter increases the lens effect caused by a given
rate of laminar-Elow tends to decrease; but a mere increase in diameter oE
a cylindrical flow path to avoid the lens effect is not practical because
the increased diameter would result in either ~1) a large increase in the
voluma of the tube or (2) a substantial decrease in length of the tube.
~, A large increase in volume is untenable because the ability of thc appara-
tus to detect ve~y small samples would be substantially limited by dilu-
tion facts. The length of the cell cannot be markedly reduced without
proportionately decreasing the magnitude o light absorbed by a given
solution 10wing through a cell. Still other conceivable tube conigura-
tions would give dlsadvantages liquid flow patterns.
Because the problem of these dynamic fluid lenses is primarily
encountered at the point of changing compositions, its solution has been
found to enhance both the qu~ltitative and qualitative analytical capabili-
ties of liquid chromatographic systems and like analytical systems where `
constantly changing compositions are inherent in the method. However, the
apparatus is useful in other lens-inducing situations encountered in the `
process industry; e.g,, where the dynamic fluid lens may be induced by
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tem~erature change or other phenomena tl~at result in formation of a refrac-
tive index gradient wit}lin ~he flow-cell.
On discovering the nattlrO of the problcM associated with such
small flow-cells, al)plicant has devised a simple constructional solution
which substantially eliminates the problem: he has provided a flow-cell
whereby the lens effect is rapidly dissipated by a progressive increase in
the cross-sectional area of the flow-cell along ~he flow path. lhus, the
wall of the flow-cell advantageously forms a diverging surface of rotation
whereby the walls form an angle of divergence of at least about one angular
degree with the axis of the cell, An optical system is advantageously pro-
; vided which avoids any substantial radiation from entering the cell at
sharp angles which would resul-t in the radiation to impinge on the walls of
the cells. An angle of about 1.5 or slightly greater provides sufficient
widening to substantially dissipate the undesirable effect of the dynamic
liquid lens formed at the interface of water and most organic solvents.
The improvement is largely achieved by collecting refracted light, which
would have otherwise been absorbed on the wall of the cell, but it is also
believed the reduction in velocity of the stream during its transit through
the cell--usually a reduction of over 50%--causes a dissipation of the lens
effect itself which reduces the amount of refracted light directed against
the walls of the cell. Angles of divergence between the axis of the flow-
path and the wall of the cell of 1 to 3 are most advantageous; larger
angles only become problems because they usually dictate a larger cell si7e.
In liquid chromatographic applications, best results will be
a~hieved if the apparatus to be used with the flowcell is selected to achieve `
~ the most ideal flow pattern possible, i.e., the flow pattern most nearly
-I achieving plug flow. This is true of all flow in a liquid chromatographic
system: flow from sample injection to the column and flow between the column
and the analytical component of the system. Such apparatus is available:
an injector advantageously used is that available under the trade description ~ ;
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~lodel U6K Injector by ~aters ~ssociates, Inc A pumping system, advanta-
geously used to ced liquid into a high pressure colum~, is that availablc
from the same source under the trade designation Model 6000 Solvent Delivery
System. Ilowever, as will be obvious to ~hose skilled in the art, otller such
apparatus will be generally use~ul in many applications in which ~he instan~
invention is advantageously used~
It will also be obvious to those skilled in the art tha~ a number
of modifications can be made in the shape of the wall structure of the flow-
cell. For example, further enlargement of the cell conduit over that de-
fined minimal conical shape will yield an operable cell that will avoidthe effect of the dynamic liquid lens but will also be larger in size and
therefore less favorable for many applications. Such enlar~ement is non-
functional with respect to the present invention. However other such shapes
including such as catenoidal horns, hyperbolic horns, parabolic and hyper-
bolic surEaces as well as similar surfaces of revolution are all intended
to be covered by the term "generally truncated cone" as used in this applica-
tion. Such shapes may on some occasions be favorable in view of effects ~ -
caused by special flow properties of the fluid components which form the
dynamic lens, temperature profiles across the cell, friction ef~ects along
the surface of the wall or the like. "Generally conical", thereore, is
meant to include any flow-cell wherein the inle~ port is smaller than the
outlet port and the cross section of the cell ls progressively larger as
measured closer to the outlet port.
;~ It is realized that the most important structural aspect of the
invention relates to the relationship of the conical cell to the direction
o~ the lightpath: the larger end of the cone must be toward the detector.
It is possible, however, to reverse the direction of flow o~ the liquid to
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be analyzed through the cell. Best practice is to avoid this siutation or,
if for some reason it is desirable, to arrange the attitude of the cell so
`` 30 that any minute gas bubbles can be displaced upwardly toward the outlet port
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of the cell.
In chrolllatograp}lic related analytlcal operations and other such
operations which monitor microliter qu~ntities o~ a flowing sample, the
length-to-average diameter ratio of the flo~ cell is advantageously at least
5 to 1. It is primarily the monitoring of such small samples, rather than
inherent optical considerations J which make angles of divergence greater
than 3 undesirable for many applications
One additional advantage of the apparatus disclosed herein is
that ~act that, for some applications, it allows the light source to be
brought (physicallyJ or by optical means) closer to the sample cell without
undue losses of light by refraction and light scattering occuring primarily
at the interfaces of gas-lens and liquid-lens interfaces.
Although, the above invention has been described largely in terms
` of flow cells, it should be recognized that it also has advantage in non-flow
cell situations wherein liquids of substantial difference in refractive
index are used with the same optical system.
In this application and accompanying drawings there is shown
and described a preferred embodiment of the invention and suggested various
alternatives and modifications thereof, but it is to be understood that
these are not intsnded to be e~laustive and that other changes and modii-
cations can be made within the scope of ~he invention, These suggestions
are selected and included for purposes of illustration in order that others
skilled in the art will more fully understand the invention and tlle prin-
ciples thereof and will be able to modify it in a variety of forms, each
as may be best suited in the condition of a particular case.
Figure 1 is a schematic diagram of an analytical apparatus.
Figure 2 is a section of a flow-cell.
Figure 3 is a graph illus~rating the output signal of an ultra-
violet absorption-measuring apparatus using a conventional cylindrical flow-
cell.
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Figure 4 is a grapl~ illustrating a chart similar to that sllown
in Figure 3 but obtainecl utilizing a flow-cell constructed according to
the apparatus ~isclosed herein.
Figure 5 is another schematic diagram showing a particularly
advantageous mode of the invention.
Figure 1 illustrates an anlytical system 10 comprising a source
12 of a liquid to be analyzed, a liquid dlromatography column 14, and an
ultra-violet absorbtometer 16 comprising a light source 18J an interfer
ence filter 20, a lens system 22, front windows 23, main housing wall of a
sample cell 24, a rear window 26 and photoelectric detector 28. Signals
from photo detector 28 and a reference detector 28a are processed according
to known techniques to provide a suitable electronic signal which may be
used as a control means or as is more frequent, to provide a visible record-
ing on a recorder means 30.
An important feature in Figure 1 is the sample cell 24 which
incorporates the cQnical flowpath 32. HoweverJ this innovation directly
enhances the performance of the entire system by providing means to take
the liquid output from chromatographic column 14 and process it in the
ultra-violet absorptiQn apparatus so that the resulting light reacting de-
tector 28 is substantially free of detrimental loss of light due to the
influence of dynamic liquid lenses.
In the apparatus o ~igure 1, the light source is rated at 2.
watts and has principal ~ave length of 253.7 nanometers. The volume of
the sample cell, best seen in Figure 2, is about 12.5 microliters: it is
about 0.04 inches in diameter at the inlet end, about 0.06 inches in diameter
at the outlet end and about 0.394 inches in length. A reference flow-cell
34 is positioned within cell assembly 36, as is common in the photQmetric
analysis of liquids. This cell may be emptyj full of a stagnant liquid or
have a flowing reference fluid therein.
Figure 3 illustrates graphically the type of detection problem
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which can be encoun~ered in radiatlon-absorption analysis because of inter-
ference in ~lltra-violet transmittance by dynamic liquid lens as they move
through a thin cylindrical sample cell.
In each of ~igures 3 and 4, there is an initial peak 60 caused
by a calibration fluid - a standard dichromate solution flowing through the
cells at a rate of one millilitar per minute. The next rise 61 in each
; curveJ is merely an adjustment of the 7ero level of the recorder, At this
point, eacll curve has a relatively flat reference level indicative of the
low ultra-violet absorption of water.
This reference level is flat for the continuous feed in Figure
3 but interrup$ed by abrupt drops in light transmission when injections of
; aqueous methanol solution are introduced into the column, These apparent
increases absorptivity in absorption are caused by the refraction Erom dy-
namic fluid lens formed by the methanol-water interface and the interfaces
of various mixtures thereof. Once refracted~ a substantial portion of
light is absorbed on the parallel walls of the conventional flow-cell.
The valleys 64 of Figure 3 illustrat~ the effect caused by a
transition from water flow of ,3 ml/minute to a flow of 0,3 ml per minute
of a 10% aqueous solu~ion of methanol. This solution is added through a
sample loop over a period of about 3,0 minutes, Then, as water returns
flushing the loop, there is an upward displacement 65 of the curve caused
by the dynamic liquid lens now being formed o:F the water flush ~lowing be-
hind the m~thanol solution~ After the flushing with water is completed
fluid-lens induced displacement subsides until another injection of water-
methanol solution is starte~l.
Equivalent injections made in the same system, except for the use
of a flow-cell as shown in Figure 2 result in no reduction in transmission,
. when methanol is added. Nor is there any substantial increase in transmis-
; sion when the water flush occurs. Such points are identified as 64a and 65a
in Figure 4.
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,~1 a~lvantageous means for assuring tilat the ligllt entering the
cells does not refrac~ against the ~alls is disclosed in Pi~ure 5 and compri-
ses an aporture scrving to mask tho ligllt source at a point betwoen the
source and the flow cell structure itself. ~Iis pre-masking procedure as-
sures that no light entering the cell from a large source can be refracte~
at such an angle as to impinge on the tapered walls of the cell. ~nother
advantage of the apparatus sllown in Figure 5 is to combine the lens and
front window of the cell. This procedure allows one to minimize the dis-
tance between the light source (aperture) and the Elow cell thereby pro-
viding a more efficient use of ligllt gonerated in the absorptometer appara-
~; tus.
Figure 5 illustrates a plan view flow cell assembly 70 comprising
conical reference cell 72 and sample cell 74. Sample cell 74 is usually
equipped with flow inlet and outlet ports as described in Figure 1. The
ports are not shown in Figure 5 to leave cells appearing as unencumbered
as possible. The front wall and back wall of the cell assembly are formed
of lens 76 and window 78. The light entering the cells originates at ultra-
violet lamp 80. A mask 82 comprises a means to intercept light from source
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80 that would be undesirable were it to reach the cells 72 and 74. Light
passing through aperture 84 in mask 42 is so masked that the extreme light
rays enter either light cell so that they cannot be refracted at an angle
which would allow them to impinge upon the tapered walls of the cells by
any commonly used liquid.
It has been found desirable and convenient to use lens 76 as a
window. This procedure allows the aperture 84, and consequently the lamp
80 to be placed closer to the sample cell,
In a typical arrangement as shown in ~igure 5, the axes of the
cells are spaced apart by 0.160 inch, the lens has an edge thickness of 0.04
inch; the radius of curvature of the lens is 0.2559 inch; the mask and aper- -
ture are spaced 0.58 inch from that edge of the lens nearest entrance to
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cells 72 c~nd 74; the aperture is 0.04~ înch. The length of each ~ell is
0.394 inches, the diameter of the front aperture of the cells is O.O~tO inches,
and this tapers to 0.060-inch rear aperture.
The lower edge of aperture 84 of masking means 82 effectively
masks any potential light ray beyond limit~ng ray 90 from approaching lens
76 at such c~n angle that it will be diverted by lens 76 into hitting the
upper ~looking at Figure 5) wall of cell 72. Similarly ~he upper edge of
aperture 84 effective masks any light ray beyond limiting ray 93 from ap-
proa~ling lens 76 at such an angle that it will be diverted into hitting
the lower wall of cell 74.
Substantially all of the light entering the cells is either ab-
sorbed in the liquid or transmi~ted through the cell and thus made available
for measurement by the light ~letector 28~ It will be understood ~hat a
light filter 20 i5 sometimes used to filter out waves lengths of light which
are not to be measured. In this sense, the filter is merely that part of
the detector apparatus which selects what pre-selected quality of light is
-1 to be allowed to reach the photo-sensitive elements thereof.
It will be noted that optimum practice of the invention will
include use of a flow cell, the crossection of which is enlarged from the
end which the light enters towards the end which light leaves. Such a ta-
pered configuration reduces unnecessary Elow cell volume, and this is be-
lieved to be an impor~ant factor in many applications, e.g. wherein very
J small samples are being studied and wherein ancillary apparatus is selected
to avoid gross peak spreading before the liquid enters the absorbtometer.
~lowever, where one is willing to tolerate the disadvantages of a flow cell
which is somewhat larger than required, the advantages of using an apertured
mask to control a cone of light entering the cell and consequently avoid
incidence of light on any portion of the walls of reference cell and sample
cell would still be considerable whether or not the cell were tapered.
, 30 It is stressed that is is intended to cover the apparatus of ~ -
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the invention, wlletller or not it exists in non-assembled parts, wherein
somo intrinsic or extrinsic system is so rclated to such parts that the
system facilitates the collcction of the parts for assembly at a particular
place or places. Sucll a system could include co-ordinated shipping instruc-
tions, a coordinated par~s-packaging system, assembly instructions or any
; other system which facilitates assembly of appa:ratus into a fuctioning
system as defined in claims explicitly relating to assembled systems,
It is to be understood that the following claims are intended
to cover all of the generic and specific features of the invention herein ?
described and all statements of the scope of the invention which might be
said to fall therebetween.
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