Liquid Cell For X-ray Fluorescence Analysis

Abstract

Solids in a slurry batch or batch mixtures of liquids of different densities the upper surfaces of which are presented in a liquid cell for X-ray analysis are prevented from separating by an impeller causing rotary movement of the slurry or liquid mixture within the cell; the liquid surface being maintained substantially planar by baffles which inhibit the creation of a vortical cavity.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

In X-ray fluorescence analysis, or fluorescent X-ray spectroscopy, primary X-rays from an X-ray source, such as the target of an X-ray tube, are directed against the surface of a specimen which may be either in gaseous, liquid or solid form. Characteristic lines of the elements contained in the specimen emerge in all directions and a beam of this radiation separated by diffraction or dispersion into characteristic wavelengths is directed against a detector. Analysis is accomplished by relating each characteristic wavelength to the chemical composition of the material.

There are two ways of discriminating the characteristic wavelengths for measurement: (1) crystal diffraction, in which only one wavelength is diffracted for a given setting of the crystal, and (2) energy dispersion, in which at a given setting the detector circuitry passes only a selected X-ray energy corresponding to a particular characteristic wavelength. Energy dispersion is sometimes referred to as "non-dispersive optics" because various characteristic X-rays are distinguished without an analyzing crystal to disperse them according to wavelength. Various arrangements for crystal diffraction are described in "Electron Probe Microanalysis" by L. S. Birks, published by Interscience Publishers, a division of John Wiley & Sons in 1963, at Chapter 6, while detectors and energy dispersion are treated in Chapter 7.

Liquids, including slurries some of which may contain as much as 60 percent solid particles, are presented for analysis in apparatus for X-ray fluorescence analysis either by flowing the liquid in a continuous stream past an X-ray translucent surface through which radiation from the primary X-ray source can be directed, or by placing a batch of the liquid in a cell, through an X-ray translucent surface of which such radiation can be directed. The present invention is directed to the improvement of liquid cells of the latter type, which are called static liquid analysis cells and which must be employed when the quantities of materials available for analysis are limited.

2. Description of the Prior Art

Previously known static liquid analysis cells have been of two general types designated as "window up" and "window down," respectively, but each has had a number of disadvantages.

In the "window up" type, the upper surface of a static body of liquid is exposed to X-ray radiation and the radiation produced by the resulting fluorescence emerges from that surface through an X-ray translucent membrane. In such cells, however, the settling of solid particles and/or the separation of mixtures of liquids of different densities introduces inaccuracies into the results of analyses, as do bubbles, either introduced during filling or evolved or enlarged by the heat generated by the X-ray tubes, since such bubbles collect in the area under the window.

In the "window down" type, the lower surface of the liquid, through a similar membrane, is exposed to X-rays; the resulting fluorescence emerging through the same membrane. Bubbles rise away from the membrane in this type of cell and therefore create no problem, but solid particles settle onto the membrane, and mixtures of liquids separate; these occurrences introducing inaccuracies in an inverse sense as compared with those introduced by settling and liquid separation in "window up" cells. Additionally, because very thin membranes must be used, they tend to bulge so that the liquid surface is no longer substantially planar. Also, the membrane may rupture, causing damage to the equipment and requiring lengthy clean-up operations.

SUMMARY OF THE INVENTION

According to the present invention a liquid analysis cell having a window opening is provided with an impeller driven by a variable speed motor, whereby rotary motion is imparted to a slurry or a mixture of liquids in the cell. In order to inhibit distortion of the substantially planar liquid surface by creation of a vortical cavity in the liquid, baffles are disposed within the cell so as to cause the liquid to move in a continuous serpentine path, portions of which are tangent to the surface presented adjacent the window opening for analysis. The cell is designed for operation in "window up" position so that that surface can be inspected visually without inconvenience. It can be operated without any membrane covering its upper opening, without spilling any liquid, although preferably an X-ray transparent membrane is employed.

The impeller may be in the form of a bar magnet within the cell coated with chemically inert material and rotated by another bar magnet outside the cell driven by a small motor; a thin plastic bottom wall of the cell separating the two bar magnets. Where, however, slurries containing magnetic particles are to be analyzed, a non-magnetic impeller driven by a shaft passing through the bottom wall of the cell is provided in order to insure against concentration of the particles of magnetic material adjacent the impeller.

Liquid analysis cells constructed in accordance with the present invention, therefore, present a planar liquid surface for analysis and at the same time maintain a uniform temperature in the material under analysis and prevent the separation of liquids of different densities and the settling of solid particles in liquids and slurries presented for such X-ray fluorescence analysis. Bubbles entrained in such liquids or slurries are moved away from the surface presented for analysis, as is material heated by the X-ray tube, and any pressure build-up which might bulge a covering membrane is avoided by the provision of a suitable vent. When the cell is oriented in the preferred "window up" position, a very thin membrane or none at all may be employed to cover the surface presented for analysis, which can be visually inspected without inconvenience.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a typical arrangement for wavelength diffractive fluorescent X-ray spectroscopy employing curved crystal X-ray optics;

FIGS. 2 and 3 are diagrammatic views of typical alternative arrangements for energy dispersive fluorescent X-ray spectroscopy;

FIG. 4 is an elevational view in vertical section of a liquid analysis cell embodying the present invention;

FIG. 5 is a plan view in section taken on the line 5--5 of FIG. 4;

FIG. 6 is a plan view in section taken on the line 6--6 of FIG. 4;

FIG. 7 is a detail view of the impeller employed in the cell illustrated in FIGS. 4 to 6;

FIG. 8 is an exploded perspective view in detail of the baffles employed in both embodiments of the liquid cell of the present invention;

FIG. 9 is an elevational view in vertical section of an alternative embodiment of the present invention; and

FIG. 10 is a plan view in section taken on the line 10--10 of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the typical arrangement for wavelength dispersive fluorescent X-ray spectroscopy illustrated diagrammatically in FIG. 1, primary X-rays from the target of a sealed off X-ray tube 11 emerge through the window 12 of the tube and strike the surface 14 of the material to be analyzed. X-rays characteristic of the elements contained in the specimen emerge in all directions and are focused by X-ray optics including slits 16 and 17 so that a focused beam of radiation strikes an angularly adjustable curved crystal 18. At each of a range of positions of the crystal on the focal circle 19 a single wavelength will be diffracted and pass on to a detector 20 which must always be positioned the proper angle on the same focal circle 19 to be in a proper position to intersect the diffracted radiation. Analysis is accomplished by relating each characteristic line to the composition of the material.

In a typical arrangement for energy dispersive fluorescent X-ray spectroscopy illustrated diagrammatically in FIG. 2, radiation from a source 22, such as a radioactive material, contained in a shielding housing 24 strike the surface 26 of the material to be analyzed. X-rays characteristic of the elements contained in the specimen surface 26 are collimated by the opening 28 in the housing 24 through which opening they pass to a detector 30 such as a gas proportional or scintillation counter which gives an electrical pulse the amplitude of which is proportional to the X-ray quantum energy.

The energy of each characteristic X-ray quantum is invariable; each CuK quantum, for instance, having an energy of 8,000 v. The number of quanta is a measure of the relative number of atoms of an element in the specimen, and the different elements in it are distinguished by the different pulse amplitudes associated with their characteristic X-ray quantum energies.

In another typical energy dispersive arrangement illustrated diagrammatically in FIG. 3, radiation from a similar source 32 shielded by a lip 34 of a housing 36 from the surface 38 of the material to be analyzed, strikes a target 40 secured within the housing 36. X-rays emitted by the target 40 under such bombardment, which are selectively controlled as to wavelength by selection of the target material, strike the surface 38 of the material to be analyzed, and X-rays characteristic of the elements contained in the specimen surface 38 are collimated by an opening 36a in the housing 36 through which opening they pass to a detector 30 identical with the detector 30 of the arrangement illustrated in FIG. 2.

The embodiment of the liquid cell of the present invention illustrated in FIGS. 4 to 8, which is useful in all of the types of arrangements described above, comprises a cell body 25 of non-magnetic and chemically inert plastic material, such as polypropylene, a vertically oriented sample receptacle 27 having a window opening 29 at its upper end is formed in the upper end of the body 25; the lower end of the sample receptacle 27 being closed by a wall 31 separating the sample receptacle 27 from a cavity 33 formed in the lower portion of the cell body 25.

An X-ray translucent membrane 35 which may be of polyimide film such as that marketed under the trademark "Kapton" which may be of a thickness of up to 25 .mu.m. (0.000025 m.), is secured to a cover member 37 adapted to be frictionally secured in position over the upper end of the cell body 25; a liquid tight seal between the cell body 25 and the cover member 37 being effected by an O-ring 39.

It is important in systems of the character illustrated in FIGS. 1, 2 and 3 that the surface of the sample to be analyzed be maintained substantially planar during analysis in order to make possible proper focusing or collimation of the X-ray optics. Therefore, in order to prevent bulging of the membrane 35 due to the expansion of liquid materials in the sample receptacle 27 or the formation of bubbles in such materials, either of which would affect the planarity of the liquid surface, a vent 41 connecting an annular pocket 42 in the sample receptacle 27 with an overflow cavity 43 in communication with the atmosphere is provided in the wall of the cell body 25. The annular pocket 42 is defined by a depending lip 44 of the cover member 37 and lies above the normal liquid level which is defined by the free edge of that lip, so that any bubbles in the liquid will tend to move into the pocket and thence out through vent 41.

Disposed within the sample receptacle 27 is an impeller 45 illustrated in detail in FIG. 7. The impeller 45 comprises a bar magnet having north and south poles at its opposite ends, as indicated in FIG. 7, and a coating of an inert material such as polytetrafluoroethylene, marketed under the trademark "Teflon." The impeller 45 is also provided with a diametrical belt 47 of the material with which it is coated, the belt being disposed at the center of the impeller and thus serving to space the main body of the impeller 45 from the wall 31 so as to facilitate rotation of the impeller by the means hereinafter described.

Driving means for the impeller 45 are provided in the form of a motor unit 47 which comprises an electric motor which may be connected to a source of power by leads not shown and the speed of which is variable according to the voltage supplied, together with speed reduction gearing through which the motor drives an output shaft 49. The motor unit 47 is removably retained in a conforming recess 51 in an adapter 53 by cementing with a cement which may be dissolved for the purpose of removing the unit 47 from the adapter 53. The adapter 53 is, in turn, retained in position in the cavity 33 of the cell body 25 by a shouldered set screw 55 screwed into a threaded bore 57 of the cell body 25 and provided with a portion of reduced diameter adapted to engage in a bore 59 in the adapter 53.

The cell body 25 is mounted on a base 61 secured to the cell body 25 by a plurality of nylon screws 63 spaced equidistantly around the ring shaped base 61; the screws 63 passing through an insulating ring 65 and a spacer 67 into the threaded bores 69 in the cell body 25.

Secured to the upper end of the motor unit output shaft 49 is a magnet holder 71. At its opposite ends the magnet holder 71 carries bar magnets 73 and 75, respectively, the magnet 73 being oppositely oriented with respect to the magnet 75 as to polarity. For example, with the magnet of the impeller 45 having the polarity orientation indicated in FIG. 4, the magnet 73 should have its south pole uppermost and its north pole lowermost, while the magnet 75 should have its north pole uppermost and its south pole lowermost. A third permanent magnet 77 U-shaped in cross-section as shown in FIG. 4 but of cylindrical configuration in plan as shown in FIG. 6, is secured to the magnet holder 71 centrally thereof and is magnetically oriented with its north pole toward the magnet 73 and its south pole toward the magnet 75. The magnets 73, 75 and 77 are secured to the magnet holder 71 by an adhesive material such as an epoxy resin.

This arrangement is such that upon rotation of the shaft 49 by the motor unit 47, the magnets carried by the rotating magnet holder 71 attract the respectively opposite poles of the magnet forming a part of the impeller 45 and thus cause rotation of the impeller 45 upon its central belt 47 as a pivot, thus imparting rotary motion to any liquid contained in the sample receptacle 27.

In applications involving the employment of fluorescent X-ray spectroscopy for the analysis of materials such as slurries containing iron oxide or other magnetic particles, which would be attracted to a magnetic impeller, the use of the modified embodiment of the present invention illustrated in FIGS. 9 and 10 eliminates any possibility of concentration of such magnetic particles at the bottom of the sample receptacle 27.

All of the elements illustrated in the embodiments shown in FIGS. 9 and 10 to which the same numbers have been applied are identical with the elements to which those numbers have been applied in FIGS. 4 to 8, inclusive, so that a detailed description of them need not be repeated.

In the embodiment of the liquid cell of the present invention illustrated in FIGS. 9 and 10, the lower end of the sample receptacle 27 is only partially closed by a wall 100 having a central opening 102 closed by a threaded sleeve 104 the threads of which are engaged by a nut 106; the margin of the opening 102 being clamped between an enlarged upper portion of the sleeve 104 and the nut 106.

A shaft 110 is rotatably received in a central opening in a sleeve 104 extending beyond the ends thereof and retained by a fluid-tight seal. Secured to the upper end of the shaft 110 is an impeller 112 disposed within the sample receptacle 27, and secured to the lower end of the shaft 110 is a disc 114 having peripheral slots 116 for the reception of driving pins 118 fixed in a disc 120 secured to an output shaft 122 of the drive unit 47.

In order to maintain the surface presented for analysis substantially planar, for the reasons previously explained, means are provided in both embodiments of the invention for preventing the formation of a vortical cavity in such liquid as an incident to the rotary motion imparted to it by either the impeller 45 or the impeller 112. The surface of the liquid presented for analysis is maintained closely adjacent the window opening 29 of the sample receptacle 27 at all times. This means comprises a pair of interlocked baffle plates 81 and 83 shown in detail in FIG. 8, which are disposed within the sample receptacle 27 as shown in FIGS. 4 and 5. The lower portion of each of the plates 81, 83 is cut away as at 85 in order to provide ample clearance for rotational movement of the impeller 45 or the impeller 112, and each is formed with an upper edge having an arcuate portion, as at 87, for the purpose of leaving a space 89 between the upper edge of the plates 81, 83 and the window opening 29 for the circulation of liquid in a horizontal plane at the window opening, whether or not the membrane 35 carried by the cover member 37 is in place.

This arrangement is such that the rotary motion imparted to the liquid within the sample receptacle 27 by the impeller 45 is caused to move in a continuous serpentine path extending vertically along the sides of the baffle plates 81, 83 and tangent to the surface presented for analysis before returning to the area being agitated by the impeller 45. This arrangement insures that solid particles contained in a liquid placed in the sample receptacle 27 will be maintained in suspension in the liquid by the motion imparted to the liquid by the impeller 45; that the temperature of the liquid and any entrained solid particles presented for analysis is maintained substantially uniform by circulation of the liquid produced by the impeller 45; that bubbles, whether initially introduced or formed due to heating of the liquid, are continuously removed by circulation of the liquid under the annular pocket 42 and thence into the vent 41 and to atmosphere; and that at the same time the surface of the liquid presented for analysis is maintained substantially planar at the window opening 29 without any vortical cavity.

Claims

I claim:

1. A liquid cell for X-ray fluorescence analysis comprising a receptacle having a window opening adjacent the top thereof, means including an impeller in said receptacle adjacent the bottom thereof for imparting rotary motion to liquid in said receptacle, baffle means within said receptacle disposed sufficiently below the upper end thereof to permit said baffle means to be covered by liquid in said receptacle whereby continuous mixing of liquids and other materials in said receptacle may be achieved while maintaining a substantially planar liquid surface below said window opening, a removable cover for said window opening having an X-ray translucent portion and a depending lip defining a pocket in said receptacle above the level of said portion, and a vent to atmosphere communicating with said pocket.

2. A "window up" type liquid cell for X-ray fluorescence analysis comprising a walled receptacle having a top and bottom, a window opening adjacent the top of said receptacle, means for imparting rotary motion to liquid contents of said receptacle, and means for inhibiting creation of a vortical cavity in the liquid contents of said receptacle during operation of said rotary motion imparting means comprising baffle plate means in said receptacle extending from the center of said receptacle toward its walls and from adjacent said window opening toward the bottom of said receptacle.

3. A liquid cell according to claim 2 in which said baffle plate means comprises a pair of plates interlocking at the center of said receptacle and extending into engagement with the walls thereof.

4. A "window up" type liquid cell for X-ray fluorescence analysis comprising a walled receptacle having a top and bottom, a window opening adjacent the top of said receptacle, means for imparting rotary motion to liquid contents of said receptacle comprising an impeller in said receptacle and means including a motor drive unit outside said receptacle for rotating said impeller; and means for inhibiting creation of a vortical cavity in the liquid contents of said receptacle during rotation of said impeller comprising baffle plate means in said receptacle extending from the center of said receptacle toward its walls and from adjacent said window opening toward the bottom of said receptacle.

5. A liquid cell according to claim 4 in which said impeller comprises a free bar magnet and said motor drive unit includes means for producing a rotating magnetic field encompassing said bar magnet, whereby said bar magnet may be rotated within said receptacle.

6. A liquid cell according to claim 4 in which said motor drive unit includes a drive shaft extending through the bottom of said receptacle and connected to said impeller.