CA1087866A - Liquid photometer with apertured mask for input radiation - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Disclosed is a photometer of the type utilizing a light source, a sample cell adapted to transmit a continuously-flowing liquid to be analyzed from an inlet port near one end thereof through a flowpath to an outlet port near the other end thereof, and a photodetector for measuring the absorption of light in the sample cell. The photometer comprises a light detector forming means to receive substantially all non-absorbed light, of a pre-selected wavelength, transmitted from the sample cell and an apertured mask for eliminating loss of light by refraction of light onto the walls of the cell. The apertured mask is placed between the light source and the flow cell to prevent light passing through the mask from entering the flow cell from being refracted into contact with an interior wall of said flow cell.

Description

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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 ~!
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 .

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 r,~:

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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 .,:
substantially all of the light emerging from said window, said :~ detection means including a photoelectric element onto which said ::~
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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8~6i 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 .~ :
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 '7~

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 .' ;.
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 - 10 ~

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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 ~ -: - ., ~ , -'7~
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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Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. 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 one longitudinal end of said cell, and a light exit window at the other longitudinal end of said cell; a light source for generating 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 transmitted through said cell without contacting said cell wall for a predetermined maximum liquid-lensing condition, thereby assuring that wall contact does not occur under any expected condition, said maxi-mum condition corresponding to when the liquid in said cell has a distribution of refractive index that causes the greatest expected divergent spreading of said beam by refraction, detect-ion means positioned beyond said exit window for measuring substantially all of the light emerging from said window, said detection means including a photoelectric element onto which said emerging light beam is incident, whereby the amount of light detected by said apparatus is independent 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 emerges from said cell and is detected, thereby making said apparatus independent of variations in the refractive index of said liquid.

2. Apparatus as defined in claim 1 wherein said light source and said detection means are so selected that said apparatus is an ultra-violet absorbance detector.

3. Apparatus as defined in claim 1 wherein said sample cell has a volume of less than 32 microliters and a maximum diameter of less than 2 millimeters.

4. The apparatus of claim 1 further comprising a second cell for holding a reference liquid, said second cell including a wall and entrance and exit windows, said masking means including means to shape said beam so that the portion of the beam entering said second cell is transmitted therethrough with-out contacting the cell walls, and said detection means includ-ing means for measuring substantially all light emerging from said second cell.

5. The apparatus of claim 4 wherein said sample and reference cells have parallel longitudinal axes and said light entrance windows of said cells each include a lens surface for bending said beam emerging from said mask into the general direction of parallel cell axes.

6. The apparatus of claim 5 wherein said entrance windows comprise a single element and said lens surfaces are a common surface on said element.

7. The apparatus of claim 1 wherein said mask is positioned with respect to said light source and said aperture is sized so that light emerges from said aperture at less than a predetermined angle with respect to the longitudinal centerline of said cell and said cell is shaped so that light entering said cell with an inclination of no more than said predetermined angle passes through said cell without contacting said cell wall, for said predetermined liquid-lensing condition.

8. The apparatus of claim 7 wherein said mask is sub-stantially closer to said light source than to said cell.

9. The apparatus of claim 1 wherein said masking means includes means for shaping said beam so that light of a pre-selected wavelength that enters said cell through said window is transmitted through said cell without contacting said cell wall for said predetermined liquid-lensing condition.