GB2105058A - Frustrated multiple total internal reflection absorption spectrophotometer - Google Patents

Abstract

A frustrated multiple total internal reflection absorption spectrophotometer employs a circular rod (20) having conical end faces (38, 40) in a sample chamber (24) through which a fluid sample flows. Unit magnification mirror relay optical systems comprising a pair of facing spherical mirrors (44, 46, 80, 82) having aligned circular apertures in the centre thereof are employed between the source of radiation (22) and one conical end face of the rod (40) and/or the other conical end face of the rod (38) and the detector of radiation (88) for imaging a conical end face of said rod at said source or detector. A rotating disc (66) is located between the source and the detector containing apertures employing one or more radiation filters. The relay systems employ baffles (60, 90, 96) preferably located near their exits extending within the narrowest cone of rays reflected by the mirrors. The system is particularly useful in the infra-red but may also be used at optical and ultraviolet wavelengths. <IMAGE>

Description

SPECIFICATION Frustrated multiple total internal reflection absorption spectrophotometer This application relates to frustrated multiple total internal reflection absorption spectrophotometers.

More particularly it relates to such a spectrophotometer employing a circular rod having conical end faces which is immersed in a moving sample fluid. Identical two mirror unit magnifications systems are utilized advantageously to focus radiation from the source on one conical end face of the rod and for collecting radiation from the other end face and focusing it on a detector. The spectrophotometer provides much greater throughput of energy than heretofore possible and provides a signal to noise ratio one hundred times greater than previously possible. Radiation filters are placed on a rotating wheel between the source and the detector which aid in the determination of sample contents.

In United States Patent Number 3,460,893 of joint inventorPaulA. Wilks, Jr., issued August 12, 1969, entitled APPARATUS FOR ANALYZING A CON TINUOUSLY MOVING STRIP BY MEANS OF ATTENUATED TOTAL REFLECTION, Mr Wilks disclosed circular rods having conical end faces, both extending from the rods and extending into the rods, for utilization in spectrophotometers for analyzing a moving strip or web passing overthe rod.

In United States Patent Number 3,370,502 of joint inventorPaulA. Wilks, Jr., issued February 27, 1968, entitled FRUSTRATED MULTIPLE INTERNAL REFLECTION ROD WITH VARIABLE LENGTH FLUID CONTAINING ENCLOSURE MEANS, Mr Wilks disclosed how such rods might be utilized in analyzing fluid streams surrounding such rods.

In these prior patents Mr Wilks disclosed the use of ordinary thin lenses for illuminating one end ofthe rod and for collecting radiation emanating from the other end of said rod and supplying the same to a spectrophotometer. In U.S. Patent No. 3,370,502 Mr Wilks also disclosed the use of conical mirrors surrounding the ends of said rods for gathering collimated illuminating radiation into one end of the rod and for gathering illumination emanating from the other end of said rod and collimating the same.

We have now discovered an optical system employing opposed mirrors which may be used between the source of illumination and one end of said rod and the other end of said rod and a detector of radiation which is much more efficient than heretofore employed in the art. These identical optical systems employ two opposed mirrors of the same radius spaced such that the conjugate image planes fornied thereby are outside of the mirrors, the mirrors having central apertures therein. Using these mirror relay systems we have found an increase in the signal to noise ratio in an infra-red spectrophotometerof 100.

In the spectrophotometer we preferably employ a rotating disc having multiple apertures wherein the apertures are preferably selected filters.

It is therefore an object of the invention to provide a frustrated multiple total internal reflection absorption spectrophotometer.

Another object of the invention is to provide a spectrophotometer utilizing a rod in which the total internal reflection takes place having concial end faces.

A further object of the invention is to provide such a spectrophotometer of increased sensitivity.

Still another object of the invention is to provide such a spectrophotometer employing improved illumination means.

Yet still another object of the invention is to provide such a spectrophotometer employing improved radiation collecting means.

A further object of the invention is to provide such a spectrophotometer employing simple radiation analyzing means.

Another object of the invention is to provide such a spectrophotometer for anaylsis of moving fluid streams.

Other objects of the invention will in part be obvious and will in part appear hereafter.

The invention accordingly comprises the features of construction and elements and the arrangement of parts which will be exemplified in the construction hereinafter described.

The scope of the invention is indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description taken in connection with the accompanying drawings in which: FIGURE 1 is a diagrammatic view of a spectrophotometer according to the invention.

FIGURE 2 is a diagrammatic view of the filter chopper of the spectrophotometer of FIGURE 1; FIGURE 3 is a cross sectional view taken along the line 3-3 of FIGURE 2; FIGURE 4 is a diagrammatic view of an alternative embodiment of the invention shown in FIGURE 1; and FIGURE 5 is a diagrammatic view of an alternative chopper filter for the spectrophotometer illustrated in FIGURE 1.

The same reference characters refer to the same elements throughout the several views of the drawings.

Now referring to FIGURE 1, a spectrophotometer according to the invention comprises a circularly cylindrical rod 20 formed of material which transmits the illumination supplied by source 22. Rod 20 is mounted within a sample chamber 24, which may be of glass, by means of end caps 26, 28 and seals 30 and 32. Afluid sample may be introduced into sample chamber 24 via inlet 34 and exits via outlet 36.

Rod 30 is provided with conical ends 38 and 40.

In the infra-red spectophotometer shown in FIG URE 1 source 22 may be a heated wire or other infra-red source. Rod 20 may be of infra-red trans mitting material such as sapphire, diamond, zinc selenide, zine sulfide, germanium, silicon, KRS-5 (a thalium bromide iodide eutectic crystal), these materials being chosen to have a higher index of refraction than the sample, so that infra-red radiation passing within the rod 20 as shown will be multiply reflected by total internal reflection.

Total internal reflection being a surface phenomena involves the penetration of the infra-red radiation to a small extent into the sample 42 surrounding the rod 20. A certain mount of the infra-red radiation is absorbed and this absorption may be measured in a known manner.

The problem of the prior art has been to ;betain maximum illumination ofthe rod 20, maximum amount of radiation passing in multiple total internal reflection through the rod 20 and thereafter collection of the radiation which has passed through the rod and to some extent been absorbed by the sample 42. To this end, between the source and the conical end face 40, we provide a pair of spherical mirrors 44 and 46. Each mirror is provided with a circu lar central aperture generally indicated at 48 and 50.

Source 22 and mirrors 44 and 46 may be conveniently mounted within a housing generally indicated at 52 provided with exit window 54 of infra-red transparent material such as barium fluoride.

The angle of the apex of the cone 40 is preferably the same angle as the cone formed by the central or average rays generally indicated by dotted lines 56 and 58. This angle is chosen such that the rays within the rod 20 are all totally internally reflected at an angle greater than the critical angle defined by the ratio of the index of refraction of the rod 20 and the sample 42. The choice of identical angies between the rays 56 and 58 and the apex of the cone 40 provides for maximum transmission into the rod 20.

Reflection at angles as close as possible to the critical angle provides for maximum pene- tration into the sample 42.

In order to prevent direct radiation from the source 22 from entering the rod 20 where it might not be totally internally reflected and thus would contribute to noise in the system, a baffle 60 is provided which preferably just fills the cone between the innermost rays 62 and 64 reflected from mirror 46. This is also preferably located as close to the end face 40 as practical, such as on the exit window 54 as shown in order to also block stray light.

For spectral analysis chopper disc 66 is provided, see also FIGURES 2 and 3. It is provided with a plurality of apertures generally indicated at 68,70, 72 and 74, for example. More apertures could be provided or as few as two, The apertures are provided with at least two transmission media indicated in FIGURE 2 as A, and A2-these preferably being filters at a reference wavelength A1 less affected by the material sought for analysis and at A2 a wavelength greatly affected by the material sought for analysis.

Disc 66 is rotated by means of a motor 76 (Figure 1) and gears generally indicated at 78.

Alternatively, for determination of more than one constituent of the sample 42 additionai filters may be employed. For example, in order to determine the concentration of three components four filters are employed, that is, Ai, A2, Xa, A, as shown in FIGURE 5.

Light is collected from conical end face 38 from rod 20 by means of an optical system which is identical to the illuminating optical system. It comprises circular mirrors 80 and 82 providing the same 60O cone angle, 84 matching the 60O cone angle of conical end face 38 of rod 20. The mirrors 80 and 82 are mounted within a housing generally indicated at 86, as is an infra-red detector, generally indicated at 88.

A baffle 90 is preferably mounted within the cone formed by the innermost rays 92 reflected from mirror 82 and may be mounted by means of a spider 94.

The baffle 90 is close to the detector 88 in order to block stray light as well as direct rays from the rod 20. However, alternatively it may be mounted as shown at 96 to merely block direct rays and in some cases it may be dispensed with.

Those skilled in the art will understand that the motor 76 provides an output synchronization signal on line 98 which is combined with the detector signal on line 100 (which varies with time due to the action of chopper-filter 66) to determine the concentration of constituents of the sample 42 by appropriate electronic means.

Now referring to FIGURE 4, alternatively a circular rod 120 may be provided with depending conical end faces 122 and 124, with a cone angle which is substantially equal to the angle of the optical axis rays 56 and 58, FIGURE 1. It is mounted within the chamber 24 by means of end caps 26 and 28 and elastomeric seals 30 and 32 shaped as shown. The end caps 26 and 28 slightiy constrict the ends of the sample chamber 24 and constrict the elastomeric end seals 30 and 32 about the rod 120. End seals 30, 32 may be of such material as tetrafluorethylene or the like which is impervious to the sample 42.

Those skilled in the art will understand that the illumination system conveniently retained within the housing 52 and the collection system conveniently retained within the housing 86 are unit relay magnification systems; that is, they provide an image of the end face 40 of the rod 20 at the source 22 and vice versa of the same size and similarly an image of the end face 38 of the rod 20 at the detector 86 of the same size. The mirrors may be, for example, of a radius of six inches and are spaced close enough together to provide the conjugate image planes thereof outside of the housings 52 and 86. In this manner the housings 52 and 86 may be self contained and completely sealed and provided with windows 48 and 148 of infra-red transmitting material.

Those skilled in the art will understand that similar systems may be provided in the visibie spectrum using sapphire, plastic or diamond rods and in the ultraviolet using sapphire rods with appropriate sources and detectors of the chosen radiation.

It will thus be seen that the objects set forth above among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above systems without departing from the scope of the invention it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also 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 as a matter of language might be said to fall therebetween.

Having described our invention what we claim as new and desire to secure by Letters Patent is:

Claims (13)

1. A spectrophotometer of the character described, comprising a radiation transmitting circular rod having conical end faces, a source and a detector of radiation; the improvement comprising: an optical system disposed at one end of said rod comprising a pair of facing spherical mirrors having aligned circular apertures in the centers thereof for imaging a conical end face of said rod at said source or said detector.

2. A spectrophotometer as defined in Claim 1 wherein said optical system images said conical end face without magnification.

3. A spectrophotometer as defined in Claims 1 or 2 and a baffle located in said optical system to prevent direct rays which have not been reflected by said mirrors from imaging.

4. A spectrophotometer as defined in Claim 3 wherein said baffle extends throughout the smallest cone of rays of radiation between said end face of said rod and said mirror located farthest from said rod.

5. A spectrophotometer as defined in Claim 3 wherein said mirrors are spaced such that said end face and the image thereof lie outside of said mirrors.

6. A spectrophotometer as defined in Claim 5 wherein said mirrors have the same radius.

7. A spectrophotometer as defined in Claims 1 or 2 wherein said mirrors have the same radius.

8. A spectrophotometer as defined in Claims 7 wherein said mirrors are spaced such that said end face and the image thereof lie outside of said mirrors.

9. Aspectrophotometer as defined in Claims 1 or 2 and an identical optical system at the other end of said rod.

10. A spectrophotometer as defined in Claim 9 wherein the average angle rays reflected by said mirrors is substantially equal to the angle of the apex of said conical end face.

11. A spectrophotometer as defined in Claims 1 or 2 wherein the average angle rays reflected by said mirrors is substantially equal to the angle of the apex of said conical end face.

12. A spectrophotometer as defined in Claims 1 or 2 and a chopper located between said source and said detector having apertures therein for transmitting radiation from said source of at least two differing spectra.

13. The spectrophotometer as defined in Claim 12 wherein there are n spectra and at least 2n apertures alternatively transmitting said spectra.