Dual-wavelength absorption detector adapted for continuous-flow detection

• US 5,408,326 A
• Filed: 04/28/1993
• Issued: 04/18/1995
• Est. Priority Date: 04/28/1993
• Status: Expired due to Term

• Current Assignee: Shell Oil Company (Shell Plc)
• Original Assignee: Shell Oil Company (Shell Plc)
• Inventors: Wang, Priestley J.
• Primary Examiner(s): McGraw, Vincent P.
• Assistant Examiner(s): WOLFE, RUSSELL
• Application Number:
  US08/053,784
• Time in Patent Office: 720 Days
• Field of Search: 356/410, 356/411, 356/402, 356/425, 356/436, 356/440, 250/573, 250/576
• US Class Current: 356/410
• CPC Class Codes: G01N 2030/027 Liquid chromatography
• CPC Class Codes: G01N 21/05 Flow-through cuvettes G01N21/09 takes precedence; handling fluid samples G01N1/10
• CPC Class Codes: G01N 21/255 Details, e.g. use of specially adapted sources, lighting or optical systems
• CPC Class Codes: G01N 30/74 Optical detectors measurement of intensity, velocity, spectral content, polarisation, or phase of infrared, visible or ultraviolet light G01J

Abstract

A dual-wavelength absorption detector is provided. The detector includes an electronics section, a three branched optical guide and a detector-cell. Each branch of the optical guide has a plurality of fibers, and at one end the branches are separated and connected to different portions of the electronics section. Two of the three branches are terminated next to LEDs having different wavelengths, a measuring wavelength and a reference wavelength. The third branch is terminated next to a photodetector. At the opposite end of the optical guide the three branches are combined to form a common end which is terminated within the detector-cell. The detector-cell includes a continuous-flow sample cell connected to a sample line. Within the detector-cell, the common end'"'"'s fiber ends are held at a fixed location on one side of the sample cell with a concave mirror being located on the opposite side. Sequential pulses of light from the LEDs are guided through their respective branches to the common end where they cross the sample cell to be absorbed by sample material flowing through the cell. Portions of each pulse are reflected by the concave mirror back across the sample cell to the common end and strike the photodetector branch fiber ends. At the photodetector, signals proportional to the illumination intensity of the pulses striking the fiber ends are produced. The signals are then compared to provide an output signal which represents the absorption of the sampled material.

Claims

1. A method for determining absorption of a species using dual-wavelength, continuous-flow chemical analysis whereby the method reduces bubble induced noise and provides for utilizing optical energy diffusion to enhance the capability of the method to determine the absorption of the species, comprising the steps of:

(a) aligning a longitudinal sample cell having an opening at each end to an approximately vertical orientation such that there is one opening on the bottom and one opening on the top;

(b) attaching a sample line to the bottom opening of the a sample cell;

(c) directing the species to flow through the sample line into the sample cell as a continuous-flow having a direction of flow such that the species flows from the bottom to the top of the sample cell, said vertical orientation of the sample cell enabling air bubbles within the species to quickly rise through the sample cell, whereby entrapment of air bubbles within the sample cell is prevented thereby reducing air bubble noise;

(d) alternately pulsing at least two light sources having different wavelengths to produce a first pulsed light having a reference wavelength and a second pulsed light having a measuring wavelength;

(e) guiding the first and second pulsed lights to a fixed location adjacent an approximately first vertical side of the sample cell;

(f) transmitting the first and second pulsed lights across the sample cell such that the pulsed lights diffuse as they cross the sample cell, whereby the second pulsed light is absorbed by the species to a greater extent than the first pulsed light along optical paths traveled by the diffused pulsed lights across the species'"'"' flow path;

(g) reflecting portions of the first and second pulsed lights back to the fixed location with a concave mirror, the mirror being located adjacently to a second approximately vertical side of the sample cell, the approximately first vertical side and the second approximately vertical side being located on opposite sides of the sample cell, whereby in crossing the sample cell, the reflected second pulsed light is absorbed to a greater extent than the reflected first pulsed light along the optical paths traveled by the reflected lights to back to the fixed location;

(h) collecting portions of the reflected first and second pulsed lights at the fixed location, whereby reflecting diffused pulsed lights back to the fixed location increases the optical path lengths of the pulsed lights without increasing the sample cell'"'"'s dead zone and also concentrates the diffused pulses so that substantial amounts of illumination which has been exposed to the species are available for collection at the fixed location;

(i) guiding portions of the reflected first and second pulsed lights away from the fixed location;

(j) converting the guided portions of the reflected first and second pulsed lights to a first and second electrical signal, the strength of the first signal being proportional to the illumination intensity of the portions of the reflected first pulsed signal guided away from the fixed location and said second signal being proportional to the illumination intensity of the portions of the reflected second pulsed signal guided away from the fixed location; and

(k) comparing the logarithmic ratio of the signals to determine the absorption of the species.

2. The method in accordance with claim 1, wherein:

in step (a) the sample cell has two parallel flat sides, a first flat side and a second flat side, and the sample cell is aligned such that the two flat sides are approximately transverse to the optical paths of the pulsed lights;

in step (e) the first vertical side is the first flat side of the sample cell; and

in step (g) the second vertical side is the second flat side.

3. The method in accordance with claim 2 wherein in step (d) the light sources are light emitting diodes.

4. The method in accordance with claim 2 wherein in step (d) the light sources are laser diodes.

5. The method in accordance with claim 2 wherein in step (d) the light sources are a two-color light emitting diode, the colors being alternately pulsed.

6. The method in accordance with claim 3, wherein:

in step (d) the light emitting diodes are mounted together;

step (e) also includes guiding the pulsed lights to the fixed location on an optical guide having a plurality of branches, a first branch guiding the first and second pulsed lights between the fixed location and the light emitting diodes; and

step (i) also includes guiding the portions of the reflected first and second pulsed lights away from the fixed location with a second branch of the optical guide.

7. The method in accordance with claim 3, wherein:

in step (d) the light emitting diodes are mounted separately;

step (e) also includes guiding the pulsed lights to the fixed location on a three branch optical guide, the three branches are combined at one end of the optical guide to form a common end, the first and second pulsed lights being guided on a first and second branch of the guide the fixed location, the first and second branches extending between the fixed location and its associated separately mounted light emitting diode; and

step (i) also includes guiding the portions of the first and second reflected pulsed lights away from the fixed location on a third branch of the three branch optical guide.

8. The method in accordance with claim 7 wherein each branch has a plurality of optical fibers, the fibers from all the branches being combined in forming the common end such that branch fiber ends are randomly distributed across the common end.

9. The method in accordance with claim 7, wherein each branch has a plurality of optical fibers, the fibers being combined in forming the common end such that branch fiber ends from separate branches are evenly distributed across the common end.

10. The method in accordance with claim 7, wherein each branch has a plurality of optical fibers, each branch'"'"'s fibers being a group of fibers wherein the group of fibers from one of the branches is located at the center of the common end with the groups of fibers from the remaining branches forming concentric circles about the center group of fibers.

11. The method in accordance with claim 2, wherein:

in step (g) the concave mirror is a spherical mirror and the fixed location is located at the center of curvature of the spherical mirror.

12. A dual-wavelength absorption detector adapted for continuous-flow detection, the absorption detector being functional to connect to a sample line having the capability of providing a continuous stream of material for sampling, the absorption detector comprising:

a detector-cell, the detector-cell includes a detector-cell body and a longitudinal sample cell;

the detector-cell body has four openings connecting to a central cavity within the body, the four openings including a top opening and a bottom opening located opposite each other and a third opening and a fourth opening located opposite each other and approximately transverse to the top and bottom openings;

a means for connecting the top opening to the sample line;

a means for connecting the bottom opening to the sample line wherein flow from the sample line is from bottom to top through the detector-cell;

the longitudinal sample cell has two flat parallel sides with the sample cell being located within the central cavity such that its longitudinal length is aligned approximately vertical, the sample cell including a bottom inlet and a top outlet which are functional to allow sample material from the sample line to flow through the sample cell, wherein bubbles are swept out of the sample cell by an upward flow of the sample material thereby preventing entrapment of bubbles within the sample cell and reducing the ability of the sample cell to cause bubble induced noise;

wherein the means for connecting the top opening to the sample line includes a means for removing the sample cell through the top opening for inspection, cleaning and replacement when the absorption detector'"'"'s performance may have been reduced as a result contact with the sample material;

a means for alternately producing a first pulsed light having a reference wavelength and a second pulsed light having a measuring wavelength;

an optical guide, the optical guide having a plurality of branches, each branch of the optical guide having at least one optical fiber, fibers of the branches are combined at one end of the optical guide to form a common end which is connected to the detector-cell body'"'"'s third opening such that fiber ends of the common end'"'"'s fibers are located at a fixed location, the fixed location being adjacent to one of the parallel sides of the sample cell, the optical guide being functional for the guiding of light into and away from the detector-cell whereby the first and second pulsed lights are guided to at least one of the fiber ends where the pulsed lights diffuse upon leaving the guide to irradiate the sample material;

a concave mirror, the concave mirror is located within the fourth opening and on the opposite side of the sample cell from the fiber ends, the concave mirror being functional to reflect some of the pulsed lights back to the fiber ends whereby some of the reflected pulsed lights are collected by at least one of the fiber ends where they are guided away from the detector-cell;

a means for converting the guided first and second pulsed lights into a first electrical signal and a second electrical signal, the first electrical signal and the second electrical signal being respectively proportional to the intensity of the reflected first and second pulsed lights guided away from the detector-cell; and

a means for comparing the first and second electrical signals such that the comparison represents a determination of the absorption of the sample material.

13. The absorption detector of claim 12 wherein the means for alternately producing the first and second pulsed lights includes at least two light emitting diodes which are alternately pulsed.

14. The absorption detector of claim 12 wherein the means for alternately producing the first and second pulsed lights include at least two laser diodes which are alternately pulsed.

15. The absorption detector of claim 12 wherein the means for alternately producing the first and second pulsed lights includes a two-color light emitting diode whereby the colors are alternately pulsed.

16. The absorption detector of claim 13 wherein the means for alternately producing the pulsed lights also includes pulsing the light emitting diodes such that the duty cycle of the diodes is less than ten per cent and drive currents of the diodes are increased above a value designated as the normal one hundred per cent duty cycle rating.

17. The absorption detector of claim 13 wherein:

the light emitting diodes are mounted together; and

the optical guide has at least one branch which guides pulsed lights to the detector-cell and at least one branch which guides reflected pulsed lights away from the detector-cell, whereby the pulsed lights produced by the diodes mounted together are guided by the same branch to the detector-cell.

18. The absorption detector of claim 13 wherein:

the light emitting diodes are mounted separately; and

the optical guide includes a separate branch for each separately mounted diode to guide pulsed light produced by its respective diode to the detector-cell and also includes at least one separate branch to guide the reflected pulsed lights away from the detector-cell.

19. The absorption detector of claim 12 wherein the concave mirror is a spherical mirror having as its center of curvature the fixed location.

20. The absorption detector of claim 12 wherein the sample cell has a tapered top, the tapered top having a round exterior top surface such that an o-ring may provide a pressure seal between the tapered top and the means for connecting the top opening to the sample line.

21. The absorption detector of claim 12 wherein the central cavity and the sample cell have dimensions which work in conjunction with each other to ensure a proper alignment of the sample cell with respect to the fixed location, the mirror and the means for connecting the top opening to the sample line.

22. The absorption detector of claim 12 wherein each branch of the optic guide has a plurality of optical fibers with the fibers being combined to form the common end.

23. The absorption detector of claim 22 wherein the fibers forming the common end have their fiber ends arranged in a random order.

24. The absorption detector of claim 22 wherein the fibers forming the common end have their fiber ends arranged such that fiber ends from each branch are evenly distributed across the common end.

25. The absorption detector of claim 22 wherein the fibers for each branch form a group of fibers at the common end and their fiber ends are arranged such that one group of fibers is located at the center of the common end and the remaining groups of fibers form concentric circles about the fiber ends of the group of fibers located at the center of the common end.

26. A dual-wavelength absorption detector adapted for continuous-flow detection, the absorption detector being functional to connect to a sample line having the capability of providing a continuous stream of material to be sampled, the material including at least one species for which absorption is to be determined, the detector having a configuration which substantially reduces bubble induced noise and at least doubles the sensitivity of the detector without increasing the detector'"'"'s dead zone, the absorption detector comprising:

a means for alternately producing a first pulsed light having a reference wavelength and a second pulsed light having a measuring wavelength;

an approximately vertically aligned longitudinal sample cell having two parallel flat sides, a first flat side and a second flat side, the flat sides being opposite each other, the sample cell also having an opening at the bottom and an opening at the top, the bottom opening being functional to connect to the sample line such that the sample line provides the species as a continuous-flow of material into the sample cell, whereby the approximately vertically aligned sample cell allows bubbles to quickly pass through the cell to the top opening of the cell thereby reducing air bubble noise by preventing bubbles from being entrapped within the sample cell;

a three branched optical guide, each branch of the optical guide having a plurality of optical fibers, at one end of the optical guide, the fibers of each branch are combined to form a common end, the common end having fiber ends that are located at a fixed location adjacent to the sample cell, the sample cell being aligned with respect to the common end such that optical energy emerging from the fiber ends would be approximately transverse to the flat sides;

a first branch of the three branched optical guide, the first branch being functional for guiding the first pulsed light to the fixed location where the first pulsed light diffuses upon emerging from the first branch'"'"'s fiber ends to irradiate the species;

a second branch of the three branched optical guide, the second branch being functional for guiding the second pulsed light to the fixed location where the second pulsed light diffuses upon emerging from second branches fiber ends to irradiate the species, whereby the second pulsed light is subjected to greater absorption along optical paths across the sample cell than the first pulsed light;

a concave mirror, the mirror being located adjacently to the second flat side of the sample cell, the concave mirror being functional to reflect some of the first and second pulsed lights back across the sample cell such that first and second reflected pulsed lights are concentrated at the fixed location with some of the first and second reflected pulsed lights striking the common end, whereby the second reflected pulsed light is subjected to a greater absorption along the optical paths back across the sample cell than the first reflected pulsed light;

a third branch of the three branched optical guide, the third branch being functional to guide the first and second reflected light pulses striking the third branch'"'"'s fiber ends away from the fixed location, whereby reflecting the first and second light pulses from the concave mirror back across the sample cell to the common end at least doubled the optical path length thereby at least doubling the sensitivity of the detector;

a means for converting the guided first and second reflected pulsed lights into a first electrical signal and a second electrical signal, the first electrical signal and the second electrical signal being respectively proportional to the intensity of the reflected first and second pulsed lights guided by the third branch away from the fiber ends; and

a means for comparing the first and second electrical signals such that the comparison represents a determination of the absorption of the species.