WO2002054039A2

WO2002054039A2 - Improved chemiluminescence reaction cell

Info

Publication number: WO2002054039A2
Application number: PCT/US2001/050116
Authority: WO
Inventors: Loren T. Mathews; Radhakrishna M. Neti
Original assignee: California Analytical Instruments, Inc.
Priority date: 2001-01-08
Filing date: 2001-12-21
Publication date: 2002-07-11
Also published as: WO2002054039A3

Abstract

A chemiluminescence analyzer includes a reaction cell, (shown generally as 10), comprising a manifold body comprising a manifold section (16) and a reaction chamber section (18) that includes the reaction chamber (50). The manifold section (16) communicates with a source of reagent ozone through a line (12) and with a source of sample through a line (14). The lines, (12 and 14), enter the manifold section (16) of the reaction cell (10) in parallel or coaxial relationship so the streams of ozone and sample do not impinge one another. The stream of sample and ozone are combined in the manifold section (16) and the sample and ozone flow coaxially to the reaction chamber section (18) of the reaction cell (10). In the reaction chamber (18) the sample and ozone react with the emission of light from the reaction chamber (18). The emitted light from the reaction chamber (18) is focused by the dimensions of the reaction cell opening so that the beam is approximately the same size as the sensing area of the phot detector.

Description

IMPROVED CHEMILUMINESCENCE REACTION CELL

Field of the Invention

This invention relates to apparatus for carrying out chemiluminescence reactions and more particularly to an improved detector cell for chemiluminescence detectors.

Background of the Invention Chemiluminescence reactions between a sought for substance and a suitable reagent, such as, for example, ozone, are particularly useful for the determination of certain pollutants in gases such as nitrogen oxides and the oxides of sulfur. Auto emission test apparatus often includes a chemiluminescence detection apparatus (CD) as part of the emission test for the detection and determination of NOx in auto emissions. The chemiluminescence reaction proceeds in accordance with the following mechanism:

NO + 03 → NO ₂* + O₂ NO ₂* → NO₂ + hv In the case where the sample includes other nitrogen oxide compounds ( NOx) it is necessary to first pass the sample through a converter to reduce the NOx to NO. United States Patent 5 ,633 , 170, Neti, issued May 27, 1997 describes a converter that contains a preconditioned vitreous carbon for the catalytic reduction of NOx to NO.

A key component of the CD apparatus is the detection cell, that is the component in which the chemiluminescence reaction occurs to create the reaction light. The reaction light is detected by a photo detector, such as a photo multiplier tube or a photo diode. The intensity of the reaction light is directly related to the quantity of sought for substance and is conventionally converted by the CD apparatus to concentration units.

Typically a stream of the reagent component in gaseous form, for example ozone, is led into a detector cell. A stream of the sample gas, after suitable treatment to convert the sought for substance into a form that is reactive with the reagent, is also led into the detector cell. The streams are caused to impinge upon each other in a reaction chamber which forms part of the reaction cell to mix the sample gas and the reagent. The chemiluminescence reaction also occurs in the reaction chamber.

The interior of the reaction chamber of the detector cell is conventionally hemispherical in shape. The reaction chamber opens toward the photo detector to focus as much of the emitted light

CAIV/0Q 108 as possible on the photo detector. A suitable optical filter/window is placed between the detector cell and the photo detector.

The chemiluminescence reaction can be subjected to a considerable amount of background noise and also is subjected to a reduction of the emitted light due to interfering reactions, such as the phenomenon known as CO₂ quenching. Such problems are particularly troublesome when determining low concentrations of sought for substance in a gas and it can generally be said that this problem can cause, along with other factors, such as the sensitivity of the photo detector and background noise, a limitation in the sensitivity of the chemiluminescence detector.

Accordingly, it would be highly desirable to provide an improved chemiluminescence detector cell in which the effects of CO₂ quenching and background noise are reduced to provide increased instrument sensitivity and usefulness.

Summary of the Invention

It is an object of the invention to provide a reaction cell for chemiluminescence determination of low concentrations of compounds in a gas. Another object of the invention is to provide an improved cell for chemiluminescence detector apparatus that operates within acceptable noise limits when determining low concentrations of constituents.

Yet another object of the invention is to provide a reaction cell in which CO₂ quenching is substantially reduced. The obj ects and advantages of the invention are achieved by a reaction cell which comprises a manifold section into which the reagent gas, for example ozone, and the sample gas are introduced and a reaction chamber section in which is located the reaction chamber. The sample stream is introduced into the manifold section so not to impinge on the reagent gas stream as contrasted to the counter current flow of the sample and reagent gas streams that is conventional in the prior art. The opening of the reaction chamber that faces the photo detector is the point of the greatest dimension of the reaction chamber and is dimensioned to be no larger than the dimension of the sensing area of the photo detector so that substantially all of the reaction light emitted is detected by the photo detector thus producing a stronger signal for a given concentration and increasing the sensitivity of the instrument . The photo detector may consist of a photo multiplier tube or a photo diode. Preferably, the photo detector is a photo diode.

In a preferred embodiment of the invention chemical noise and C0₂ quenching is reduced

CAIV/00 108 even further by the incorporation of a diffuser in the line leading ozone into the reaction cell. The diffuser may consist of a porous Teflon® tube or a stainless steel tube that is perforated along its length. The ozone diffuser facilitates the reaction of ozone and NO to increase the sensitivity of the instrument and in addition reduce chemical noise. The use of a diffuser also lowers the so-called C0₂ quench effect.

Other objects and advantages of the invention will become apparent from the following description of the preferred embodiments and the accompanying drawings.

Brief Description of the Drawings

FIG. 1 is a schematic block diagram of a chemiluminescence detector including the improved reaction cell of the invention;

FIG. 2 is a side view in section of the reaction cell of the invention;

FIG. 3 is an elevation of the reaction chamber section of the reaction cell of the invention as viewed from the rear face;

FIG. 4 is an elevation of the reaction chamber section of the reaction cell of the invention as viewed from the front face; and

FIG. 5 is an elevation of the manifold section of the reaction cell of the invention illustrating the manifold path;

Description of the Preferred Embodiments

The invention will be described herein in connection with the chemiluminescence determination of nitrogen oxides. It will be understood, however, that the present invention can be applied equally as well to the determination by chemiluminescence analysis of sulfur oxides, H₂S ammonia and the like. Referring to FIG. 1 and FIG. 2, a chemiluminescence analyzer includes a reaction cell, shown generally as 10, comprising a manifold section 16 and a reaction chamber section 18 that includes the reaction chamber 50. The manifold section 16 communicates with a source of reagent ozone through a line 12 and with a source of sample through a line 14. The lines, 12 and 14, enter the manifold section 16 of the reaction cell 10 in parallel or coaxial relationship so the streams of ozone and sample do not impinge one another. In the embodiment illustrated the streams from the two lines are flowing essentially parallel to each other as they enter the manifold section 16. The stream of sample and ozone are combined in the manifold section 16 and flow coaxially through a passage 48 to the reaction chamber 50. In the reaction cell the NO reacts with the ozone to produce NO₂ with emission of light. A beam 21 of the emitted light from the chemiluminescence reaction exits the reaction cell 10 through an optical filter 20. A photo diode 22 is disposed in the path of the beam 21 of emitted light and a signal processor 24 receives the signal emitted by the photo diode. The signal processor 24 includes suitable amplifier and readout and recording means. Power for the signal processor 24 is supplied by a suitable power source 25.

The sample stream line 14 communicates with a converter 26 for converting nitrogen oxides in the sample stream to nitric oxide. The converter 26 communicates with a source of unconverted sample, such as for example auto emissions. Various converters for converting the nitrogen oxides in the sample to nitrous oxide are well-known in the art and do not per se form a part of this invention. By way of example, however, a suitable converter which contains preconditioned vitreous carbon is described in United States patent 5,633, 170 issued May 27, 1997 to R.M. Neti. A power supply 28 provides power to the photo diode 22 and the signal processor 26. Referring to FIG. 2, the forward face 30 of the manifold section 16 and the rear face 32 of the reaction chamber section 18 are contiguous when the sections are assembled. The manifold section 16 and the reaction chamber section 18 are secured by suitable machine bolts (not shown) which extend through aligned bolt passages (not shown) in the two sections. A gasket (not shown) is preferably provided between the manifold section 16 and the reaction chamber section 18 to insure a gas tight seal between the sections. A channel 34 (FIG. 5) is formed on the forward face 30 of the manifold section 16. The channel 34 is closed at its sides and bottom and is open to the forward face 30 of the manifold section 16 . When assembled, the rear face 32 of the reaction chamber section 18 closes the channel 34 to form the manifold 36. As most clearly shown in FIG. 5 the channel 34 defining the manifold 36 follows a generally U-shaped path with a first leg 38 in communication with the inlet port 39 of the ozone line 12 and a second leg 40 in communication with an exhaust port 42 for removal of excess reactants and reaction products.

Further improvement in the reduction of chemical noise and the decrease of CO₂ quench is accomplished by the incorporation of an ozone diffuser 43 in the reagent gas line 12, normally at or just before it enters the manifold 36. The diffuser 43 may consist of a tube, formed either of Teflon or of stainless steel, with openings drilled along the length of the tube or, preferably the diffuser is section of a porous Teflon tube such as the porous Teflon tubes made by W.L. Gore Associates. The diffuser may be consist of any material inert to ozone and NOx. A second groove 46 formed in the rear face 32 of the reaction chamber section 18 is closed by the forward face 30 of the manifold section 16 to define a path of communication between the base of the U-shaped manifold channel 34 and a passage 48 leading to the reaction chamber 50 in the reaction chamber section 18. The point at which the second groove 46 communicates with the 5 manifold channel 34 defines an enlarged mixing chamber 52 that receives the stream of sample gas. The sample gas flows through the ozone in the mixing chamber 52 and both the sample stream and the ozone flow coaxially through the passage 48 to the reaction chamber 50.

The reaction chamber 50 is considerably smaller than the reaction chamber of a conventional chemiluminescence detector. Being semi-hemispheric in shape, the opening of the reaction chamber

10 50 is the greatest dimension of the reaction chamber. The diameter of the opening is selected so that the emitted beam is focused to approximately the same size as the sensing area of the photo-detector and little or no emitted light is undetected by the photo-detector. In the embodiment illustrated the opening of the reaction chamber is approximately the same as the diameter of the photo diode 22. By way of example, the opening of the reaction chamber of a conventional chemiluminescence

15 detector normally ranges between 25 mm to 50 mm. The opening of the reaction chamber 50 of the reaction cell 10 of the invention is about 10mm to conform to the dimension of the photo diode 22. In this manner a maximum amount of emitted light is directed to the sensing portion of the photo diode 22. This is to be contrasted with conventional chemiluminescence detectors where the photo detector is smaller than the opening of the reaction cell and a portion of emitted light bypasses the

20 photo- detector and is thus undetected.

The sample stream is a led from the sample line 14 into the mixing chamber 52 through a capillary tube 54, preferably at a flow rate of between 30 cc/min and 400 cc/min. Although the material from which the capillary tube 54 is formed is not critical, it has been found that certain materials perform better than others as capillaries for the sample stream. For example, best results

25 thus far have been achieved using capillaries made from polyether ether ketone (PEEK). Stainless steel capillary tubes can also be used with acceptable results but they are not preferred as it has been found that background noise is greater using the stainless steel capillaries than when PEEK capillaries are utilized. Consequently maximum sensitivity is achieved using the PEEK capillary tubes as compared to stainless capillary tubes.

30 As pointed out above, the sample stream and ozone stream flow coaxially from the mixing chamber 52 through the passage 48 to the reaction chamber 50. Conventional chemiluminescence detectors attempt to create a point source for the chemiluminescence reaction by causing a stream

CAIV/00 108 of ozone and a stream of the sample gas to be directed counter currently or to impinge one another in the reaction chamber. Although it is not fully understood, it has been found that avoiding any substantial impingement between the sample stream and ozone substantially reduces loss of sensitivity due to a phenomenon known as CO2 quenching. CO2 quenching is believed to be due to degradation of the ozone which may occur as a result of heating and degradation of the ozone molecule by molecular friction produced when the streams impinge one another. In the reaction cell 10 of the present invention the ozone is permitted to diffuse into the manifold 36 where it contacts a sample stream in the mixing chamber 52 and a portion of the ozone is carried along with the sample stream to the reaction chamber. It has been found that, contrary to expectations, the chemiluminescence reaction cell of the present invention produces a signal that is almost two and one half times stronger than predicted based on the signal strength produced by larger prior art reaction chambers carrying out the same determination. Moreover, the background chemical noise generated by the reaction cell of this invention is on the order of 0.01 ppm or less of the chemical noise generated by the prior art reaction cells.

It will be understood that the improved chemiluminescence reaction cell of the invention is not limited to the determination of NO_x through the reaction with ozone but is used with advantage in the determination of any substance that is capable of reacting with a reagent to quantitatively produce light as a reaction byproduct. As will be understood by those skilled in the art, various arrangements which lie within the spirit and scope of the invention other than those described in detail in the specification will occur to those persons skilled in the art. It is therefor to be understood that the invention is to be limited only by the claims appended hereto.

Having described our invention, we claim:

Claims

1. An improved reaction cell for chemiluminescence determinations comprising: a. a manifold section defining a manifold having a sample inlet in communication with a line from a source of sample and an ozone inlet in communication with a line from a source of ozone for non-impinging introduction of a stream of sample gas and a stream of ozone into said manifold; b. a reaction chamber section defining a rear face contiguous with said manifold section and a forward face, c. a reaction chamber formed in said reaction chamber section, said reaction chamber opening on the forward face of said reaction chamber section, said reaction chamber being in communication with said manifold for receiving a mixed stream of sample and ozone therefrom.

2. The reaction cell for chemiluminescence determinations of claim 1 adapted to be matched with a photo detector for the detection of the emission of light therefrom, said opening of said reaction chamber being no greater than a sensing area of said photo detector with which said reaction cell is matched thereby to direct essentially all light emitted from said reaction chamber to the sensing area of the photo detector.

3. The reaction cell for chemiluminescence determinations of claim 2 wherein said reaction cell is semi-hemispheric in shape and said opening defines the greatest dimension thereof.

4. The reaction cell for chemiluminescence determinations of claim 1 wherein said manifold defines a first leg communicating with said ozone inlet for receiving a stream of ozone and a second leg in communication with an exhaust port for removal of excess reactants and reaction products, said manifold section further including a passage communicating between said manifold and said reaction chamber, said point of communication between said passage and said manifold defining a mixing area into which said stream of sample is received.

5. The reaction cell for chemiluminescence determinations of claim 4 wherein said stream of sample and said stream of ozone are combined in said mixing area and a portion of said ozone flows coaxially with said sample stream to said reaction chamber.

CAlV/00 108

6. The reaction cell for chemiluminescence determinations of claim 1 wherein a channel is formed on said forward face of said manifold section, said channel being closed at its sides and bottom and opening to said forward face, said rear face of said reaction chamber section closing said channel to form said manifold.

7. The reaction cell for chemiluminescence determinations of claim 4 wherein a capillary tube is utilized for communication between said sample inlet and said mixing chamber.

8. A chemiluminescence analyzer comprising: a. a photo detector having a light sensing area for receiving light emissions and creating a signal in response thereto; b. a reaction cell including a manifold section defining a manifold having a sample inlet in communication with a line from a source of sample and an ozone inlet in communication with a line from a source of ozone for non-impinging introduction of a stream of sample gas and a stream of ozone into said manifold, a reaction chamber section defining a rear face contiguous with said manifold section and a forward face, a reaction chamber formed in said reaction chamber section, said reaction chamber opening on the forward face of said reaction chamber section, said opening of said reaction chamber being no greater than the sensing area of said photo detector thereby to direct essentially all light emitted from said reaction chamber to the sensing area of said photo detector, said reaction chamber being in communication with said manifold for receiving a coaxially flowing stream of sample and ozone therefrom. c. a signal processor for processing and displaying said signal as a function of the concentration of sought for substance reacting with the emission of light in said reaction chamber; d. an ozone line and a sample line for the non-impinging flow of sample gas and ozone to said manifold inlets; and e. a power source.

9. The chemiluminescence analyzer of claim 7 further including gas diffuser means in said

CAlV/00 108 ozone line at said manifold inlet.