GB2116707A - Optical system for a liquid flow absorption cell - Google Patents

Abstract

An optical system is designed to form real images of both its field and aperture stops 21 and 22 at the two window boundaries 13 and 14 of the path through a cell 11. This dual stop imagery permits control of both size and shape of the illuminated space. If this space is made cylindrical, its volume equals the exact fundamental minimum required to transfer the available light flux over a given path length, without reliance on wall reflections. A marginal concentric clearance of the cell bore around the illuminated space serves to avoid interferences from the wall and any adjacent perturbed liquid layer. The embodiment shown is an analytical apparatus for detection of optical absorption in liquid chromatography and cell 11 is tubular and has an entrance 15 and exit 16 for flowing liquid. Hemispherical and hyper - hemispherical lenses may be used. <IMAGE>

Description

SPECIFICATION Optical system for directing a light flux through a liquid flow absorption cell Background of the invention The present invention relates to an apparatus for opto-analytica! measurements and more particularly to a device for detection of optical absorption in liquid chromatography.

In the field of optical absorbance measurement in liquid chromatography, it is generally desirable on one hand to minimize the smallest detectable quantities, for example, by attaining a maximum light flux through a minimum volume of liquid resident in a flow cell and, on the other hand, to minimize the aberrations in absorbance dependent on the flow rate by substantially eliminating the interaction of light with the flow cell wall as well as with any adjacent layer subject to inconsistent optical inhomogeneities such as thermal refractive index gradients. In the past, however, these two objectives were often considered incompatible with each other and have been pursued in separate approaches. For example, the so-called light-piping method has been widely adopted to maximize the light flux through the flow cell.This method basically relies on multiple reflections off the cell wall which is typically a polished metal and a considerable fraction of the light flux from the entrance window can be thereby made to pass through the exit window. The physical throughput, however, is drastically reduced by such multiple reflections although the light flux can be geometrically conserved in light-piping. Whenever liquid corrosion changes the physics of the metal wall, furthermore, the reduction in throughput is by a factor that can be expected to change in magnitude and spectral distribution. Moreover, any optically inhomogeneous liquid layer adjacent to the wall unavoidably distorts the light-piping process due to the gradient in index and thus causes majorflow rate effects. These can only be suppressed at the expense of chromatographic peak-broadening in heat exchangers.

One of the attempts in the past to reduce such undesirable effects resulted in the so-called "flaring" method whereby a physical beam stop is placed at the cell entrance and the cell bore is widened toward the exit in orderto clearthetruncated cone of illuminated liquid. See, for example, U.S. Pat. No.4,011,451 to K.E.

Nelson which shows a photometric apparatus characterized by a conically shaped flow cell. Another method of the same type was by "focusing", or putting an optical stop image anywhere along the light path through the liquid and widening the cell bore to clear the twin truncated cones of illuminated liquid. By either method, however, the cell volume actually becomes larger not by a marginal but a quite significant amount over the fundamental minimum volume of liquid required by the etendue, or geometrical acceptance of the optical system.

It is, therefore, an object of the present invention to provide a highly sensitive apparatus for optical absorbance measurement in flowing liquids and to limit the volume of liquid required to a substantially fundamental minimum.

It is another object of the present invention to provide an optical system for a flow cell whereby the flow cell wall and any adjacent optically disturbing layer can be kept reliably in the dark.

It is still a further object of the present invention to provide an opto-analytical apparatus having the above characteristics and directly interchangeable cells with widely different characteristics.

The optical etendue, or geometrical light acceptance of an optical system, can be measured as follows between subsequent field and aperture images. If the area of the field image is A1, that of the aperture image is A2, their separation is sand the refractive index of the enclosed medium is n, the etendue of the system is given by A1A2n2 s2. Therefore, by designing the optical system in such a way that an image of the field stop be formed on one window and an equal size image of the aperture stop be formed on the other window of the flow cell, the cell's given optical path length is made identical to the separation between subsequent field and aperture images.Thus, by sheer definition, the cylindrical illuminated volume, encompassed between both stop images (and swept by light rays at all angles), equals the exact fundamental minimum required to transmit the available light flux over the given absorbance path. Only a marginal clearance zone between the optically swept cylinder and the cell bore wall is required to keep the gradient layer in the dark.

Brief description of the drawing The sole accompanying drawing is a schematic illustation, enlarged in the transverse direction, of the optical path in an opto-analytical apparatus embodying the present invention.

Getailed description of the invention Referring now to the sole drawing which schematically illustrates the optical paths in an apparatus according to one of preferred embodiments of the present invention, flow cell 11 is a tubular chamber of length about 4 mm with cylindrical inner wall 12 of cross sectional diameter about 1.2 mm, front window 13 and back window 14. Wall 12 is provided with inlet 15 connected to a source of a liquid to be examined such as a liquid chromatography column (not shown) and outlet 16 so that the liquid will flow generally uniformly in the direction parallel to the symmetry axis 20 of flow cell 11. Front and back windows 13 and 14 are perpendicular to axis 20 and cover the entire cross-sectional area of flow cell 11.

The optical system for conducting a light flux through the flow cell 11 includes a source of light (not shown), a limiting system field stop 21, limiting system aperture stop 22 and positive lenses 31,32 and 33.

Field stop is essentially a screen with an opening placed at a position where an image of the light source is formed. The opening can be circular or, for example, have a shape bounded by two arcs of a circle and two parallel sides to form the exit slit of a monochromator. As illustrated by three representative rays 41,42 and 43 emanating from a common field point 44, field stop 21 plays the role of a secondary light source. Positive lenses 31,32 and 33, arranged in that order on the axis 20 are so designed that the rays entering lens 31 from a same point on the field stop opening 21 such as rays 41,42 and 43 will emerge as a collimated beam after passing through it and form a real image on front window 13 after passing through lenses 32 and 33.

Aperture stop 22 is a circular opening in a screen placed on and perpendicular to the axis 20 so as to limit the cross section of the light beam. Lenses 31, 32 and 33 are further so designed that an image of aperture stop 22 will be formed on back window 14 in the presence of liquid inside the flow cell 11. The dimensions of stops 21 and 22 are so adjusted that their images formed respectively on front and back windows 13 and 14 are circles, or confined within circles the diameters of which are nearly equal to but slightly smaller than that of the internal bore of cell 11.This small difference between the bore of cell 11 and the size of the images represents the zone near wall 12 inside which aberrations of the absorbance signal are generated when this apparatus is used without thermal equilibrium between wall and liquid as, for example, in liquid chromatography. Therefore, it is desired to keep a layer adjacent to the wall reliably in the dark. If the illustrated optical system is adjusted as discussed above, it is possible to illuminate the exact fundamental minimum volume of liquid with only a marginal clearance zone around it required to keep the wall 12 and the adjacent gradient layer reliably in the dark. Evey ray which passes through stops 21 and 22 front is bound to pass through front window 13 and also through back window 14 without being internally reflected on cell wall 12.

The rays which pass through cell 11 are measured by detector 45. Positive lenses 34 and 35 are placed so that an image of front window 33 will be formed (or, rays 41,42 and 43, for example, will be focused) at the position of detector 45.

According to a preferred embodiment of the present invention, lenses 33 and 34 are hemispherical (planoconvex) lenses with their plane surfaces forming the boundaries of the liquid column in cell 11, or, in effect, the front and back windows 13 and 14, and so dimensioned that the immersed focus of each lens will be at the opposing boundary. This configuration will lead to field imagery which is telecentric to one side and collimated to the other, while it is the other way around with the aperture imagery.

Alternatively, lenses 33 and 34 may be hyperhemispherical lenses with their plane surfaces forming the boundaries of the liquid column in cell 11 and so dimensioned that the immersed aplanatic point of each is at its plane boundary while its immersed focus is at the opposing boundary. It can be shown that flow cell systems with hemispherical and hyper-hemispherical lenses can be so dimensioned that they become equivalent in relation to the external optical system and hence directly interchangeable within it.With the parameter n defined as the design refractive index of the system, all dimensional, optical and chromatographic quantities of such interchangeable cells scale in powers of n as follows: Quantity Hemisphere Hyper-Hemisphere Lens radius n 1 Cell path length n2 1 Cell diameter n 1 Cell cross-section n2 1 Cell volume n4 1 Signal-to-Noise Ratio (at equal concentration) n2 1 Detectable concentration (at equal signal-to-noise ratio) 1 n2 Thus, the use of hemi spherical and hyper-hemispherical lenses permits direct interchangeability of flow cells with widely different characteristics. Moreover, both these types of lenses are economical to produce, self-centering in assembly and compatible with mountings that yield high liquid pressure integrity.

The present invention has been described above in terms of a few embodiments but these are intended to be illustrative rather than limiting. For example, the word "light" used throughout herein must be interpreted broadly to include electromagnetic radiation of all wave lengths such as ultraviolet radiation. The word "window" is to be interpreted in both the optical sense, i.e. either a field or aperture image and in a material sense, i.e. a transparent boundary to a liquid. It is a significant characteristic of this invention that the optical "windows" coincide with the material "windows", bounding the liquid column along which absorbances are measured. The stops are preferably circular or nearly circular in shape, but matching optical systems economically may lead to stops of different shapes and sizes, and cell 11 need not necessarily be circular in section or cylindrical in shape. The detector means can be of any design and its positioning need not exactly conform with the drawing herein. It is to be understood that the present invention is limited only by the following claims.

Claims (13)

1. An analytical apparatus for the measurement of light absorbance through a column of flowing liquid, said apparatus comprising a tubular absorbance flow cell, a light source, a light detector and an imaging optical system, said flow cell comprising a first transparent window and a second transparent window, both said windows facing said liquid, and forming respectively a first boundary surface and a second boundary surface which confine said column of liquid and are perpendicular to the axis of said column, said optical system being for conducting a light flux from said light source through said cell to said detector, said optical system comprising imaging means, a limiting system field stop and a limiting system aperture stop, said imaging means and both said stops being dimensioned and disposed to form both a real image of said field stop at said first boundary surface and a real image of said aperture stop at said second boundary surface.

2. The apparatus of claim 1 wherein the internal wall of said tubular absorbance flow cell clears all possible light rays through said column of liquid between both said real images by a prescribed safety margin.

3. The apparatus of claim 2 wherein said prescribed safety margin is determined by the thickness of the optically perturbed liquid layer adjacent to said internal wall and from aberrations and tolerances in said imaging optical system.

4. The apparatus of claim 2 wherein said imaging means comprises a positive hemispherical planoconvex lens, a portion of the plane surface of said lens coinciding with one of said boundary surfaces, said lens being so dimensioned that the immersed focal point thereof is at the other of said boundary surfaces.

5. The apparatus of claim 2 wherein said imaging means comprises a positive hyper-hemispherical planoconvex lens, a portion of the plane surface of said lens coinciding with one of said boundary surfaces, the immersed aplanatic point of said lens being at the plane surface thereof, and the immersed focal point of said lens being at the other of said boundary surfaces.

6. An opto-analytical apparatus comprising a tubular cell which has a front window and a back window, and an optical system for passing a beam of light through said cell from said front window to said back window substantially without allowing said beam to pass through a zone inside and within a prescribed short distance from the wall of said cell, said optical system comprising a light source, a field stop between said light source and said front window, an aperture stop between said field stop and said entrance window, and a focusing means for forming an image of said field stop on one of said windows and an image of said aperture stop on the other of said windows.

7. The apparatus of claim 6 wherein said images of said stops are of a predetermined diameter.

8. The apparatus of claim 6 wherein said focusing means comprises a plurality of positive lenses.

9. The apparatus of claim 8 wherein at least one of said positive lenses is a hemispherical lens.

10. The apparatus of claim 9 wherein the plane surface of said hemispherical lens is at said front window.

11. The apparatus of claim 8 wherein at least one of said positive lenses is a hyper-hemispherical planoconvex lens.

12. The apparatus of claim 11 wherein the plane surface of said planoconvex lens is at said front window.

13. The apparatus of claim 6 further comprising a detector means for measuring the energy of light passing through said cell.