Patent Application
20140146940
The invention relates to a sample holder, for use in X-ray measurement apparatus, and X-ray measurement using a sample holder.
In order to measure powder samples in X-ray fluorescence (XRF) apparatus, it is necessary that the sample be mounted in a sample holder which can in turn be placed in the apparatus.
However, conventional sample holders for powder usually have a significant contribution to the observed background intensity under the fluorescence peaks of interest caused by scattering in the sample holder wall material.
A number of different containers have been proposed for use in such
X-ray fluorescence apparatus. In particular, examples are proposed in GB 2 194 636 and U.S. Pat. No. 4,037,109.
However, there remains a need for an improved method of X-ray fluorescence in which the effects of sample holders are minimised.
Further, there is a need for providing a sample holder that gives good results for X-ray fluorescence with a minimal volume of sample.
By way of example, during pharmaceutical research the amount of pharmaceutical prepared can often be very low and powder samples of only 0.1 g may need to be measured. The inventors tried using an existing sample cup for this powder but with such small quantities of sample the usable
Embodiments of the invention provide a sample holder and a method of carrying out X-ray fluorescence using the sample holder, the sample holder having a counter ring and a sample ring, in which a powder sample is held between a pair of films in a through hole in the sample ring.
The inventors have discovered that the sample holder according to the invention can provide good XRF results even with very small quantities of sample. In results presented below XRF results are achieved with a 0.1 g sample that are as good as when using 5 g sample in a conventional holder.
Such results are not achievable with conventional sample holders, such as conventional microcell holders.
The inventors have realised that even when X-rays are directed through the sample, and not directly irradiating the sidewalls of the sample holder, the powder in the sample holder can scatter the X-rays towards the side walls of the sample holder where the X-rays can interact with the sample holder and produce fluorescence or additional scattering. By reducing the thickness of the sample holder significantly, scattering and/or fluorescence from the walls of the holder, is reduced not just absolutely, but also as a fraction of the total measured scattered or fluorescence signal. The inventors have further realised that reducing the thickness of a sample holder reduces the stiffness of the sample holder.
This is particularly a problem when the sample is being held in place by very thin sheets, since the sample holder can become extremely flexible.
Accordingly, the sample holder may be a two-part or three-part sample holder with the inner sample ring fitting into a counter ring.
In this way, a maximum area of powder sample of minimum thickness can be measured. The reduction in sample thickness makes the sample transparent to high-energy X-rays which significantly reduces the background due to scattering. The sample holder shapes low mass loose powders for good measurements.
This in turn allows for improved lower limits of detection when making XRF measurements.
The sample holder thickness reduction allows small amounts of material to be handled and measured in the sample holder while remaining sufficiently stiff for keeping the sample flat and correctly located in the X-ray fluorescence apparatus. The counter ring provides additional stiffness than that provided by the sample ring alone. The sample holder is particularly relevant for determination of trace elements in relative high scattering matrix.
The counter ring may form a press fit with the sample ring. Alternatively, the counter ring may form a snap fit with the sample ring. In this way, the counter ring and sample ring may conveniently be brought together to form an integral unit without tools.
The films may be foils of thin polymer having a thickness from 1 to 50 μm.
Preferably, the materials used have low atomic number, not higher than 26.
The sample ring may be of polymer.
The counter ring may be of aluminium.
In the three-part arrangement, the sample holder further includes an outer ring arranged to engage the counter ring to form a unit of outer ring, counter ring and sample ring, the outer ring having a through hole arranged to align with the through holes of the counter and sample rings to pass X-rays.
One of the opposed films is held between the sample ring and the counter ring and the other of the opposed films is held between the counter ring and the outer ring. In this way, both films may be held between the rings.
The rings may be fitted together to hold the films without the use of adhesive.
In particular, the outer ring may form a press fit or a snap fit with the counter ring.
For a better understanding of the invention, an embodiment will now be described, purely by way of example, with reference to the accompanying drawings, in which:
Referring to
To retain the powder 6, opposed foils 8, 10 are provided, one on each opposed face of the sample disc 2.
The foils are thin—for example 1 μm to 50 μm, especially 3 to 20 μm. In the specific embodiment shown, the foils are 4 μm to 6 μm thick. The sample disc 2 is typically 0.5 mm to 2 mm, preferably 0.75 mm to 1.5 mm thick. The diameter of the central hole may be in the range 5 mm to 50 mm, preferably 10 mm to 20 mm, and the sample space 4 typically holds 10 to 200 mg of powder.
The foil is of polymer, for example nylon or Mylar. Likewise, the sample disc 2 is of polymer, and may conveniently be of the same material. Low atomic number materials are preferred, to avoid fluorescence.
The foil does not cover the whole of the opposed surfaces of the sample disc 2.
In view of the materials choice and thickness of the sample disc 2, the sample disk 2 and the films are flexible which can give rise to handling difficulties, especially in manufacturing environments.
Accordingly, a counter ring 20 is provided that fits together with the sample ring 2 to provide additional support. The counter ring 20 is in the form of a ring 22 with a central through hole 24 arranged to correspond with the central hole 4 of the sample disc 2 to allow X-rays to pass.
The counter ring has a raised outer ring 26 in which the radially inner face is arranged to engage the outer circumference of the sample disc 2 to engage the sample disc.
In the embodiment shown, the counter ring 20 engages with the sample disc 2 by a press fit. However, in alternative embodiments detent means may be provided to allow a snap fit.
The complete sample holder with counter ring 20 engaged with sample disc 2 is sufficiently robust and in particular rigid to be able to be used in industrial environments. The counter ring provides some measure of protection to foil 10 to allow the sample holder to be placed on a surface with the counter ring against the surface without damage.
In some cases, such sample holders may be used in existing equipment replacing existing sample holders with the same form factor but providing improved performance.
The counter ring may be made of plastics, for example the same plastics as the sample ring.
Alternatively, the counter ring may be made, in all or in part, of metal. Preferably, a metal of atomic number no higher than 26 is used. The counter ring may in particular be made of aluminium.
The sample ring need not be in the form of a circular disc, but instead may be for example square or rectangular. Likewise, the through holes may be square, circular or rounded as appropriate for individual X-ray fluorescence apparatus.
Results using 0.1 g of pharmaceutical powder in a prototype sample holder were measured and compared with those using a 5 g self standing pellet of the same material and with 5 g of powder in a conventional “P1” cup.
To compare samples, the root mean square deviation (RMS) in parts per million were obtained using XRF for three elements, Ru, Pd and Pt, for the three samples.
These results demonstrate that similar levels of deviation may be obtained using very small quantities of sample.
An alternative arrangement is shown in
The three rings are arranged to fit together with the outer ring 50 having an outer retaining ring 60 on its lower side adapted to mate with an outer step 62 on the upper edge of the counter ring 52. The counter ring 52 has an inner step 64 arranged to mate with an outer flange 66 on the sample ring 54.
In use a first foil 68 is stretched across the lower surface of sample ring 54 and then the sample ring 54 is mounted into counter ring 52 which retains the foil stretched along the bottom surface of inner ring 54. A powder sample 74 is then mounted above this foil 68 in central cavity 70.
A second foil 72 is then mounted on the upper surface of the counter ring 52 and sample ring 54 extending to the outer edge of the counter ring. The counter ring 52 is then mounted to the underside of outer ring 60 stretching the second foil. Thus, the powder sample 74 is held between the first and second foils 68, 72. Note that in
In this way the powder sample can be held in place without the need for glue. In particular, in this embodiment the rings are held together by a press fit. Alternatively, a snap fit may be used.
Alternatively, for greater security, glue or other adhesive can be used to fix the rings together.
In this arrangement all the rings can be made of the same or of a combination of different materials such as polymer and/or aluminium.
The sample holder of embodiments of the invention can minimise the exposure of holder material to the excitation X-ray beam. This reduces the amount of holder material that can scatter and so the spectral background component can be reduced.
The sample holder of embodiments of the invention does not need to be absolutely leakproof unless the sample is toxic or biologically active.
The sample holder of embodiments of the invention does not require screws and so is easy, simple and safe to use. The holder can avoid contamination of the sample by humidity.
Thus, while there have been shown, described, and pointed out fundamental novel features of the invention as applied to several embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the illustrated embodiments, and in their operation, may be made by those skilled in the art without departing from the invention. Substitutions of elements from one embodiment to another are also fully intended and contemplated. The invention is defined solely with regard to the claims appended hereto, and equivalents of the recitations therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 12165715.9 | Apr 2012 | EP | regional |
