Phosphor Assembly For Ultraviolet Light Absorption Detector

Abstract

An inert phosphor package or enclosure assembly, especially for use with an ultraviolet light absorption detector in liquid chromatography, and a process for manufacture. A wave length converting phosphor for location in a lamp housing is protected from the surrounding environment and abrasion by an encapsulating container comprised of a rigid base with a phosphor receptacle or pocket, a protective cover or window over the pocket transparent to ultraviolet radiation, and a heat-fused seal between the window and base. The package is fabricated by filling the pocket with a phosphor powder, covering the powder with the window, applying a fusible sealing strip, heating the assembly to a first temperature to dry the phosphor without fusing the sealing strip, and heating to a higher temperature without intermediate cooling to seal the window to the base.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a phosphor package or assembly, particularly for use with an ultraviolet radiation absorption detector in liquid chromatography, and to a method for making the assembly.

2. Prior Art

Ultraviolet radiation absorption detectors are used with flow cells in liquid chromatography to monitor effluent that contains components to be detected or analyzed. A typical wave length of ultraviolet radiation used in detectors by life scientists is 254 nm (nanometers), which is emitted by a low pressure mercury discharge lamp. However, certain biologically interesting compounds are more absorbing and certain useful solvents less absorbing at wave lengths of 280 nm than they are at 254 nm, and a phosphor can be used to convert the 254 nm radiation to 280 nm radiation where the latter wave length is desired. The phosphor is located within a lamp housing of the detector, along with the mercury lamp. A flow cell assembly, including a light sensing detector, is secured to the lamp housing. Radiation from the lamp causes the phosphor to emit ultraviolet radiation at the different or "converted" wave length, which is then directed through the effluent in the flow cell for the detection and analysis of certain compounds.

Phosphors are susceptible to serious degradation when irradiated with ultraviolet light in a moist atmosphere, which is common in the ultraviolet lamp housing of a detector. In addition, an exposed phosphor surface is subjected to abrasion, contamination, as by solvent leakage from the flow cell, and film build up on the phosphor surface, which degrades the phosphor and reduces efficiency.

SUMMARY OF THE INVENTION

The present invention provides a phosphor package or assembly that seals the phosphor against moisture and other elements of the surrounding environment, facilitates the use of a phosphor in particulate form, provides a viewed or emitting phosphor surface that can be cleaned in the event of cell leakage of film build up, and that physically protects the phosphor from abrasion. Further, the assembly can be conveniently fabricated, facilitates the provision and maintenance of a viewed surface that is uniform and smooth, and facilitates the drying of the particulate phosphor and sealing of the assembly in an efficient manner during fabrication.

Briefly, the phosphor assembly, which because of its form is referred to as a "button," is in the nature of a package that seals and protects the phosphor and maintains the phosphor in a desired location, arrangement and configuration without the need for binders, adhesives, or the like in the phosphor. The assembly is comprised of a backing and a protective cover, between which the phosphor is sealed. The protective cover is transparent to ultraviolet radiation and protects the phosphor against moisture or other surrounding atmosphere, and against abrasion. It also provides a surface to be viewed in use, which is smooth and cleanable. Preferably, the protective cover is of fused silica, i.e., quartz.

In its preferred form, the backing is a receptacle for containing a quantity of phosphor in particulate, i.e., powdered form. Preferably the backing is metal, suitably aluminum with an inner ledge and outer flange surrounding the receptable to form a step for receiving, locating and providing a seat and seal for the protective cover. Preferably, the receptacle or cavity is of uniform shallow depth to provide a phosphor layer of uniform thickness and emissivity with a minimum of phosphor material.

The seal between the backing and protective cover must be resistent to degradation from ultraviolet radiation and, for convenience in fabrication of the assembly, is advantageously of a material that is heat fusible above a temperature suitable for drying the phosphor. Fluorinated ethylene propylene (Teflon) is especially suitable.

Manufacture or fabrication is best accomplished by placing the powder within the receptacle or cavity of the backing, using enough powder to fully fill the cavity. The protective covering is applied to flatten the phosphor powder and is then removed for cleaning. The cover is replaced and the sealant applied between the cover and flange of the backing. The assembly is then heated to a first temperature suitable for drying the phosphor but below the fusion temperature of the sealant, for a sufficient time to dry the phosphor. Subsequently, the assembly is heated to a temperature at which the sealant will fuse. Advantageously, the heating to the higher temperature immediately follows the drying without intermediate cooling, to avoid moisture pick-up and to shorten the fabrication time.

It is an object of the present invention to provide a phosphor assembly that is in part transparent to ultraviolet radiation and in which the phosphor material is supported, protected, and sealed from the surrounding atmosphere.

A more specific object is to provide a phosphor assembly that provides a uniform, smooth, viewable, surface of phosphor powder, that seals the powder from the surrounding atmosphere and protects the surface of the powder against abrasion, that can be cleaned after use, and that is economical convenient to fabricate.

A further object is to provide a method of fabrication of the phosphor assembly that facilitates drying the phosphor material and sealing the assembly against moisture.

The above and other objects, features and advantages of the invention will become more apparent from the detailed description that follows, when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic sectional view, with parts in elevation, of an ultraviolet radiation detector assembly for use in liquid chromatography;

FIG. 2 is a top plan view of a phosphor assembly or button embodying the present invention;

FIG. 3 is a sectional view of the assembly or button of FIG. 2, taken along the line 3--3; and

FIG. 4 is a diagrammatic exploded view of the assembly or button of FIGS. 2 and 3 on an assembly fixture, diagramatically illustrating a preferred manner of fabrication.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference to FIG. 1 of the drawings, an ultraviolet radiation detector assembly 10 for use in liquid chromatography is shown diagramatically. The detector assembly 10 includes a lamp housing 12, a low pressure mercury lamp 14 having two lobes in the configuration shown, a phosphor assembly or button 16, a flow cell assembly 18 carried by the lamp housing, and an ultraviolet radiation detector 20 carried by the flow cell assembly.

The lamp housing 12 has an opening 22 in one wall 24 on which the flow cell assembly 18 is mounted. The phosphor assembly or button 16 is secured inside the lamp housing on a wall 26, generally opposite the opening 22, so that the surface of the button is within the view of the flow cell assembly 18, including the ultraviolet detector 20. The lamp 14 is offset from the opening 22 so that the detector is shielded from direct rays. Ultraviolet radiation of a wave length of 254 nm is produced by the mercury lamp 14, strikes the phosphor assembly 16, and causes the phosphor to emit radiation having a wave length of 280 nm, some of which is directed to the flow cell assembly. The ultraviolet radiation from the phosphor is collimated by a quartz lens 29, passes through two parallel fluid cells, one containing a liquid sample and the other a reference liquid, and passes through a second quartz lens or window 30 to the ultraviolet detector 20.

The construction of the phosphor assembly 16 is best shown in FIGS. 2 and 3 of the drawings. As shown, the phosphor assembly includes a base 36, phosphor 38 in particulate form, a cover 40, and a seal 42 between the cover and base.

The base 36 serves as a receptacle for the phosphor powder, facilitates securing the assembly 16 within the lamp housing, as by a screw 43. The base is of rigid material, preferably metal, and advantageously aluminum, which is light and readily fabricated. In the preferred embodiment shown, the base is generally cylindrical in shape, having a front or top surface 44 and a base or back surface 46 at an angle to the top surface, 15.degree. in the embodiment shown, to orient the top or front surface 44 in a proper relationship to both the mercury lamp and the flow cell of the apparatus with which the phosphor assembly is used. A threaded hole 48 in the base surface 46 receives the screw 43 and is located directly under the center of the interface between the cover 40 and the phosphor 38. By way of example, the base shown is, in a preferred embodiment approximately 1 inch in diameter and 0.3 inch in height, at the shortest portion of the periphery. The top or front surface 44 has a recess 50 surrounded by a narrow ledge 52 and has a narrow rim 54 extending above the ledge for a short distance, for example one-sixteenth of an inch. The recess 50 is shallow, for example about 0.05 inch and is filled to the top with powdered phosphor 38. The recess 50 is of uniform depth so that the phosphor is of uniform thickness. The recess holds approximately one gram of General Electric Type X--401 lanthanum fluoride phosphor powder.

The cover 40 is a disc of a thickness essentially equal to the height of the rim 54 and is shaped and sized to rest on the ledge 52 with a small peripheral clearance, for example, 0.025 inch, between the disc and the rim. This surrounding clearance permits relative sliding between the disc and base during assembly, and provides space for the seal 42. In the preferred embodiment shown, the cover is of quartz, i.e., fused silica, which is substantially transparent to ultraviolet radiation. Such a cover disc can be procured commercially and a suitable product is manufactured by General Electric, identified as Type 125 polished quartz disc.

The seal 42 is a fused strip of material that wets the surfaces of the quartz cover and the rim 54 of the base, that is chemically resistant, and which further resists deterioration in an ultraviolet environment. The fused material of the preferred embodiment is fluorinated ethylene propylene, marketed under the trademark Teflon. The seal extends completely around the cover disc to a height approximately one-half that of the rim 54, to provide a continuous hermetic seal between the quartz disc and the metal base. The seal is achieved with a 5 millimeter strip (0.005 inch) of fluorinated ethylene propylene film 42' (shown in FIG. 4) approximately 1/8 inch in height and 3 1/4 inches long, so that it completely surrounds the perimeter of the cover.

The manner in which the phosphor assembly is fabricated is best shown in FIG. 4 of the drawings. The base 36 is placed on a jig or fixture 60 that has a top surface 62 that slopes at the same angle as the base surface 46 of the base 36, so that the top surface 44 of the base 36 can be held horizontal. A pin 63 extending from the surface 62 of the fixture 60 is received in the threaded aperture 48, with a slight clearance fit, to retain the base on the fixture. Approximately 1 gram of phosphor powder 38 is heaped into the recess or pocket 50, as illustrated. The fused silica window 40 is pushed down on the phosphor powder while being rotated slowly and at the same time slid back and forth within the tolerance provided in the window step to uniformly distribute the phosphor powder within the recess. After the powder is uniformly distributed and there are no gaps or vacancies at the interface between the powder and the fused silica surface, the cover or window is removed, excess powder removed from the ledge 52, and the window is cleaned and then replaced in the ledge, within the surrounding rim 54. The strip of fluorinated ethylene propylene film 42' is inserted edgewise in the gap between the perimeter of the cover and the rim of the base. The assembly is placed in an oven on the fixture 60 and is heated to dry the phosphor powder. This initial drying is accomplished at a temperature below that at which the fluorinated ethylene propylene film will fuse. By way of example, in the preferred embodiment, the assembly is heated to a temperature between 225.degree. centigrade and 250.degree. centigrade for 3 to 4 hours to dry the phosphor powder. At the end of the drying operation, the temperature is elevated and held at the elevated level for sufficient time to fuse the strip 42'. In the present instance, the assembly is heated to a temperature of 350.degree. centigrade and held at this temperature for 1/2 hour, to fuse the strip 42' to form the seal 42. Most advantageously, the assembly is not cooled between the drying and fusing operations, to avoid heat loss and to eliminate the change of moisture condensation prior to sealing.

The resulting assembly is durable and completely seals the phosphor from any surrounding environment. The durability of the assembly has been tested by submerging an assembly in an aqueous dye solution for 72 hours. At the end of this period the assembly was boiled in the dye solution for ten minutes and cooled to room temperature. The assembly exhibited no penetration of the dye into the phosphor material. Decay of the phosphor material was checked by comparing the light emitted from an assembly exposed to ultraviolet radiation in a lamp housing for approximately 2 weeks, with the light emitted from a newly fabricated assembly. Relative photocell light intensities for the two phosphor assemblies were measured in the same lamp housing with the same photocell sensor or detector, within 20 minutes of each other. The results of this experiment show a meter reading from the photocell detector for the phosphor button irradiated for 2 weeks of 0.359, and a meter reading of the freshly prepared assembly of 0.360. Accordingly, the irradiated assembly showed no decrease in emission intensity, within experimental error.

While a preferred embodiment of this invention has been described with particularity, it will be appreciated that various modifications or alterations may be made without departing from the spirit and scope of the invention set forth in the apended claims.

Claims

1. In a method of making a sealed phosphor assembly, the steps comprising:

placing loose particulate phosphor in a cavity of a base in an amount sufficient to fill the cavity,

covering the cavity with a material substantially transparent to electromagnetic radiation of at least certain transparent to electromagnetic radiation of at least certain wave lengths,

heating the assembled base, phosphor and covering material to dry the phosphor, and

2. A method as set forth in claim 1, wherein the covering material is a quartz window, and the phosphor is dried and the covering material is sealed to the base by placing a heat-fusible sealant about the cavity between the base and window and heating the base, phosphor, window and sealant first to a drying temperature below the fusion temperature of the sealant and then subsequently to a temperature at which the sealant fuses.

3. A method as set forth in claim 2, wherein the temperature is raised to fuse the sealant without lowering the temperature below the drying

4. A sealed wave length-converting phosphor assembly for use in an ultraviolet radiation detector, comprised of a base for attachment of the assembly to a support, a phosphor receptacle formed in said base, a phosphor in particulate form in said receptacle, a cover over said receptacle and phosphor, retaining the phosphor in the receptacle, protecting the phosphor from moisture and abrasion, and substantially transparent to ultraviolet radiation, said base includes a ledge about said receptacle forming a seat for the cover, and a rim about the ledge for locating the cover, the area circumscribed by said rim being greater than the area of said cover, and means surrounding said receptacle forming

5. A sealed wave length-converting phosphor assembly for use in an ultraviolet radiation detector, comprising a metal base for attachment of said phosphor assembly to a support, a phosphor receptacle formed in said base, a phosphor in particulate form in said receptacle, a fused silica cover over said receptacle and phosphor, said cover serving to retain the phosphor in the receptacle and to protect the phosphor from moisture and abrasion, said cover being substantially transparent to ultraviolet radiation, and fluorinated ethylene propylene sealing means disposed along a continuous path surrounding said receptacle to form an hermetic seal

6. The assembly of claim 5 wherein said base comprises a ledge about said receptacle forming a seat for the cover, and a rim about the ledge for locating the cover, the area circumscribed by said rim being greater than

7. In an ultraviolet radiation detector for use in chromatography, an ultraviolet wave length-converting phosphor powder enclosed within and filling a dry closed hermetically-sealed chamber, said chamber being formed by a base member having a cavity and by a cover member over said cavity, said base member comprising a ledge about the perimeter of said cavity, said ledge forming a seat for said cover member, said base member and said cover member defining the chamber filled by said phosphor powder, said cover member being transparent to ultraviolet radiation.