AU2021103159A4

AU2021103159A4 - Preparing a pellet for laser induced breakdown spectroscopy

Info

Publication number: AU2021103159A4
Application number: AU2021103159A
Authority: AU
Inventors: Jacob BREINER; Ahmed Fadil; Mathias Moeller KLAUSEN
Original assignee: Foss Analytical AS
Current assignee: Foss Analytical AS
Priority date: 2020-09-22
Filing date: 2021-06-07
Publication date: 2021-07-22
Anticipated expiration: 2029-06-07
Also published as: AR124754A4; WO2022064287A1

Abstract

PREPARING A PELLET FOR LASER INDUCED BREAKDOWN SPECTROSCOPY ABSTRACT A press (2) for preparing a pellet of granulated material for laser induced breakdown spectroscopy comprises a tubular die (8) and a piston (4) arrangement, the tubular die (8) being formed with an inner surface (14) along which the piston (4) can slidably move and which is delimited by an open first end (10) of the tubular die (8;30;48a-d) and an opposing open second end (12) of the tubular die (8). The press (2) further comprises a removable end cap (18) having at least one flat face (24) for sealing the second end (12) of the tubular die (8). 4/6 88 104 8104 100 292 48b 48a 92 9 94 82 94 2106 8604a 86g.

Description

4/6

88

104

8104 100

292 48a 48b 92 9

94

82 94 2106

8604a

86g.

PREPARING A PELLET FOR LASER INDUCED BREAKDOWN SPECTROSCOPY DESCRIPTION

[0001] The present invention relates to a press for and a method of preparing a pellet for laser induced breakdown spectroscopy (LIBS).

[0002] LIBS is a known analysis technique which is employed in the measurement of the concentrations of elemental components of a sample. A high energy density laser pulse is used to generate a plasma at the surface of the sample. The plasma contains a mixture of excited atoms representative of the elemental composition of the sample. As the plasma cools the excited atoms emit optical radiation that is characteristic of the atom emitting it. As the intensity of the characteristic radiation is related to the number of atoms in the plasma and ultimately the number of atoms in the sample then spectrophotometric analysis of the so emitted radiation may be employed to provide information on the concentration of elements present in the sample.

[0003] The amount of energy produced by such a laser source may also generate relatively large amounts of black body (or continuum') radiation, by means of a general heating of the sample. This radiation acts as a background from which the optical radiation emitted by the plasma must be differentiated. Lower energy laser sources may be employed to overcome this problem. However, the use of a lower energy laser source will increase the necessary precision by which the focussing optics must focus the laser beam at the sample in order to ensure a sufficient energy density at its surface for plasma generation. This is especially a problem when investigating granular samples as the surface of the sample presented for interaction with the laser beam tends to present height variations as measured along the optical axis of the focussing optics.

[0004] When analyzing samples of granulated material using LIBS it is known, for example from US 2007/0218556, to present the sample for analysis as a single consolidated mass in the form of a pressed pellet. The pellet is made by placing the granular material, that is, material composed of individual or partially agglomerated granules, pieces or particles, for example, ground, shredded or pulverized plant or soil material; foodstuff or its intermediate products; or pharmaceutical products; into a tubular die of a press via an open end of the die. The granular material in the die is pressed against a sealed end of the die to consolidate it into the pellet utilizing a piston of the press. The piston itself is provided with a substantially flat head that is sized such that it can slidably engage about its outer periphery with an inner surface of the die.

[0005] Once formed, the pellet in the tubular die is moved into the path of a laser beam for the subsequent LIBS analysis of an exposed surface which was contacted by the piston.

[0006] One problem with this is that the position of the exposed surface within the die can vary from sample to sample. This may depend on the amount of granular material in the die. It may also depend on the compressability of the material. Even when using samples of the same material its comprssability may vary, for example due to variations in the size and size distribution of the granuals or due to variations in the composition, such as moisture content, of the samples. Relatively complicated automatically adjustable focussing optics may be employed such as is described in, for example US 2013/0271761, to ensure that the laser beam is focussed at the exposed surface, irrespective of its position within the die.

[0007] According to a first aspect of the present invention there is provided a press for preparing a pellet of granulated material for laser induced breakdown spectroscopy (LIBS) the press comprising a tubular die and a piston arrangement, the tubular die being formed with an inner surface along which the piston can slidably move and which inner surface is delimited by an open first end of the die and an open second end of the die opposite the open first end, wherein a removable end cap is provided, configured with a substantially flat face for sealing the open second end of the die. Once the end cap is removed an external surface of the pellet of granulated material which is retained within the die may be presented for LIBS analysis. This has the advantage that, in use, a pellet of granulated material is prepared in which the external surface has a reproducible flatness, reflecting the flat surface against which it was pressed under the action of the piston during the formation of the pellet of granulated material, and a reproducible location, being determined by the location of the substantially flat face relative to the open second end when it seals the open second end.

[0008] In some embodiments the removable end cap comprises a raised portion engagable about its outer periphery with the inner surface of the die to seal the second end, the raised portion also being configured with the flat face presenting internal of the die when the second end is sealed. Usefully, when the end cap is removed after pressing the granulated material the external surface of the so formed pellet is at a reproducible, preferably known, position within the die. Since the external surface of the pellet lies within the die, the die itself affords a level of physical protection for the pellet surface.

[0009] In some embodiments the inner surface is formed with one or more irregularities, such as one or more grooves into or protrusions from the inner surface. The internal cross section of the die is changed at each of the one or more irregularities. The pellet, when formed, will have an outer surface which conforms to the one or more irregularities so that movement of the pellet along an axis which extends between the first and the second ends of the die is thereby limited.

[0010] In some embodiments the end cap maybe formed with a plurality of the raised portions, each raised portion for sealing an own second end. In this manner a plurality of dies may be presented to the piston through a relative movement of the piston and the end cap for compressing granualated material that is contained in the die. In other embodiments a plurality of pistons may be provided for simultaneous operation to compress granulated material that is contained in the plurality of dies that are each sealed at its second ends by a corresponding raised portion of the plurality of raised portions.

[0011] According to a second aspect of the present invention there is provided a method of performing laser induced breakdown spectroscopy on a pellet of granulated material comprising the steps of: a. forming a pellet of granulated material; and b. loading the formed pellet into a laser induced breakdown spectroscopy analyser to provide an exposed surface for interaction with laser radiation emitted by a laser of the analyser; wherein the step a. comprises: placing the granulated material into a tubular die of a pellet press according to any one of the preceding claims, which tubular die is sealed at its second open end by the removable end cap; and compressing the granulated material in the die by action of the piston at the open first end of the die; and wherein the step b. comprises: removing the end cap to provide the exposed surface of the pellet retained within the die; and placing the die into the analyser with the exposed surface in a light path to intercept laser radiation emitted by the laser.

[0012] In this manner a pellet having an exposed surface for LIBS analysis that is reproducibly at a fixed distance from the second end of the die in which it is retained, as determined by the location of the flat surface of the removable end cap, may be presented. This reduces the focussing requirements for the laser of the LIBS analyser used to ablate a portion of the exposed surface.

[0013] These and further advantages, modifications and embodiments will now be further described with reference to the accompanying figures, of which:

[0014] Fig. 1 Shows a schematic representation of a press according to the present invention;

[0015] Fig.2 Shows a schematic representation of another embodiment of a die usable in the press of Fig. 1;

[0016] Fig.3 Shows a schematic representation of a second embodiment of a press according to the present invention;

[0017] Fig.4 Shows a schematic representation of a die holder of a press according to the present invention;

[0018] Fig. 5 Illustrates the assembled die holder of Fig. 4; and

[0019] Fig. 6 Shows a flow chart illustrating an embodiment of the method of performing LIBS according to the present invention.

[0020] An embodiment of a press 2 according to the present invention is illustrated in Fig. 1. The press 2 comprises at least one piston (in this embodiment one piston 4) which is moveable along an own movement axis X, typically by means of an actuator 6, such as a known hydraulic actuator. The press 2 also comprises at least one removable tubular die (in this embodiment one die 8) which is locatable along the movement axis X with a first open end 10 facing the piston 4. The die 8 has a second open end 12, opposite the first open end 10, which will also lie along the movement axis X. An inner surface 14 of the die 8 connects the first open end 10 and the second open end 12. The piston 4 is provided with an outer surface 16 which is dimensioned to slide along the inner surface 14 as the piston 4 moves along the movement axis X.

[0021] An end cap 18 is also provided as a part of the press 2 and in the present embodiment is configured with an upper surface 20 at which is provided at least one raised portion (in this embodiment one raised portion 22). The raised portion 22 is dimensioned to engage about an outer periphery with the inner surface 14 of the die 8 and thereby seal the second open end 12 of the die 8. The raised portion 22 is provided with a substantially flat face 24 which presents internal of the die 8 when the raised portion 22 is engaged with the open second end 12.

[0022] In some embodiments, as is illustrated in Fig. 1, one or more irregularities, here in the form of one or more protrusions (three protrusions 26a, 26b, 26c illustrated), are formed in the inner surface 14 of the die 8 in order to proved localized changes in the internal cross sectional area of the die 8 and thereby help retain a pellet of the compressed granulated material in the die 8 when the end cap 18 is removed to unseal the second end 12.

[0023] In some embodiments, such as that illustrated in Fig.1, a machine readable tag 28, for example an RFID chip, a linear bar code or a matrix barcode (such as a QR code), may be incorporated with the die 8 to provide information relating to a granulated sample contained in the die 8. This information may include one or more of: information regarding the origin of the material, information related to previous handling or storage of the material, and information for controlling subsequent processing of the material.

[0024] Another embodiment of a die 30 that is to form part of a press according to the present invention is illustrated in Fig. 2. The die 30 comprises a tubular body 32 having a first open end 34; second open end 36 opposite the first 34; and an inner surface 38 connecting the first 34 and the second 36 open ends. An irregularity, here in the form of one or more (here one) grooves 40, is formed in the inner surface 38 to provide a localized change in the internal cross sectional area of the die 30. An outer periphery of the raised portion 22 of the removable end cap 18 fits snugly against the inner surface 38 to reversibly seal the second open end 36 of the tubular body 32 of die 30 so that its flat face 24 presents internal of the tubular body 32, past the second open end 36.

[0025] A second embodiment of a press 42 according to the present invention is illustrated in Fig. 3. The press 42 comprises a plurality of pistons (here three pistons) 44a,b,c each moveable along an own movement axis, Xi, X 2 , X 3 respectively, by means of an actuator 46. In the present embodiment, by way of example only, the actuator 46 is operable to move each piston 44a,b,c along its associated movement axis Xi, 2 ,3 simultaneously. In some embodiments, the actuator 46 may operate to move each piston 44a,b,c individually along its associated movement axis Xi, 2 ,3

. In other embodiments the actuator 26 may comprise a pluralilty of simultaneously or individually controllable actuators, operable to achieve the effects described above in respect of the single actuator 46. The press 42 also comprises a plurality of, here three, tubular dies 48a,b,c and may for example be those illustrated in Fig.1 or Fig. 2, as well as an end cap 50.

[0026] The end cap 50 comprises an upper surface 52 on which are provided a plurality of, here three, raised portions 54a,b,c, each having an associated flat surface 60,a,b,c. Each raised portion, 54a say, is dimensioned for sliding engagement about an outer periphery with an inner surface, 56a say, of an assicated tubular die, 48a say, to releasably seal a second open end, 58a say, of that die, 48a say, and present a flat surface, 60a say, internal of that die, 48a say.

[0027] The end cap 50 is removably locatable in the press 42 relative to the pistons 44a,b,c so that a first open end 62a,b,c of each of the dies 48a,b,c is located along the associated movement axis Xi,2 ,3 . Each piston, 44a say, can be moved through the first open end, 62a say, of its associated die, 48a say and along the inner surface, 56a say when the actuator 46 is operated. As each piston, 44a say, is moved within the associated die, 48a say, towards the associated sealed second open end, 58a say, granulated material within the associated die, 48a say, is compressed against the flat surface, 60a say, of the associated raised portion, 54a say, to form a pellet having an outer surface which conforms to the inner surface, 56a say, of that die, 48a say and to the associated flat surface 60a say of the end cap 50. The inner surface, 56a say, of each die, 48a say, is provided with, in the present embodiment, a single irregularity in the form of a groove, 64a say. This irregularity 64a say provides a localized change in the internal cross section of the associated die, 48a say. The outer surface of the pellet will also conform to this irregularity, groove 64a say, so that movement of the pellet between the open ends, 58a, 62a say, is inhibited when the die, 48a say, and the end cap 50 are separated.

[0028] In another embodiment, there may be provided a single piston, 44a say. The end cap 50 and the single piston 44a may then be moveable relative to one another in a plane perpendicular to the movement axis, Xi say, of the piston 44a so that each raised portion 54a,b,c of the end cap can be brought in turn to locate it associated die 48a,b,c with its first open end 62a,b,c along the movement axis, Xi. Granulated material within each of the dies 48a,b,c may then be sequentially formed into a pellet.

[0029] A die holder 80 is illustrated in exploded view in Fig 4. This die holder 80 is useable in and forms a part of a press according to the present invention, for example the press 42 which is described above with respect to Fig. 3. The die holder 80 advantageously facilitates the simultaneous handling of a plurality of dies (three say) 48a,b,c. The die holder 80 comprises a die grip 82; a top 84 and a bottom 86. In some embodiments the bottom 86 is omitted and in other embodiments the bottom 86 is formed integral with the die grip 82. Optionally, and as illustrated in the present embodiment, a plurality of press shafts (three say) 88 may be provided as components of the die holder 80 and each serve as an interface between sample in a corresponding one of the plurality of dies 48a,b,c and a corresponding piston 44a,b,c (not shown) of actuator 46 (not shown) in the press 42 (not shown).

[0030] An end cap 50' which, as with the end cap 50, is common to all of the pluralitiy of dies 48a,b,c is in the present example shaped to conform with the shape of the of the die holder 80 and usefully may be provided with pins 51 for engaging with corresponding holes 90, 92 in the bottom 86 and the die grip 82 respectively and facilitate the correct relative orientation of these elements 50',82,86 so that the raised portions 54'a,b,c can pass through through-holes 106 in the bottom 86. In some embodiments, the end cap 50, 50' may be formed, at least in part, of a magnetic material so that magnets 102 employed to releasably connect the bottom 86 to the die grip 82 can also be employed to releasably hold the end cap 50, 50' against the bottom 86 (or against the die grip 82 in embodiments where the bottom 86 is omitted).

[0031] The die grip 82 comprises a plurality of retainers 94 for releasably retaining a corresponding one of the plurality of dies 48a,b,c orientated with its second open end 58a,b,c exposed to a corresponding one of the raised portions 54'a,b,c of the end cap 50'. In some embodiments one or more shoulders 96 are provided which extend from an upper surface 98 of the die grip 82. These shoulders 96 releasably engage with recesses 100 in the top 84 to help collocate the two elements 86,84 during assembly of the die holder 80 (as illustrated in Fig. 5). Releasable connectors are provided between the bottom 86 and the die grip 82 and the die grip 82 and the top 84. In the present embodiment magnets 102 between the bottom 86 and the die grip 82 and the die grip 82 and the top 84 act as these releasable connectors and are arranged with their magnetic poles orientated to provide an attractive magnetic force between the magnets 102. In other embodiments press-fit or bayonet-fit or similar known connectors may be employed instead of the magnets 102.

[0032] The top 84 is provided with a plurality (here 3) of through-holes 104 which, when the die holder 80 is correctly assembled (see Fig. 5), overlie corresponding retainers 94 of the die grip 82 and permits external access to the first open ends 62a,b,c of dies 48a,b,c retained by the die grip 82.

[0033] In the present embodiment, each of the plurality of press shafts 88 is dimensioned to pass through a through-hole 104 and contact sample within a die 48a,b,c retained in the die grip 82.

[0034] With reference to Fig. 6, a method 66 of performing LIBS is illustrated as comprising a first step 68 of forming a pellet of granulated material followed by a second step 70 of loading the so formed pellet into a LIBS analyser of known construction to provide an exposed surface in a light path of laser radiation emitted by the laser of the LIBS analyser in order to cause ablation at that exposed surface.

[0035] The first step 68 includes a step 68a of placing a granulated material into a tubular die 8,30,48a-c of a press 2, 42 according to the present invention, which tubular die 8,30,48a-c is sealed at its second open end 12,36,58a-c by an associated removable end cap 18,50, 50'. In some embodiments the granulated material may be charred or ashed before placing in the die 8,30,48a-c. In some embodiments charring may be performed later, before ablation, with the granulated material in the die 8,30,48a-c and in some embodiments only at the surface of the granular material that was compressed against the flat faces 24,60a-c of the removable end cap 18,50, 50' .

[0036] The first step 68 also includes a step 68b of compressing the granulated material which is in the tubular die 8,30,48a-c by action of the piston 4,44a-c via the first open end 10,34,62a-c of the tubular die 8,30,48a-c, opposite the second open end 12,36,58a-c. The piston 4,44a-c exerts a force on the granulated material in the tubular die 8,30,48a-c, compacting it to form a pellet, a surface of which conforms to the flat face 24,60a-c of the removable end cap 18,50,50' which seals the second open end 12,36,58a-c of the tubular die 8,30,48a-c. In some embodiments the inner surface 14,38,56a-c of the tubular die 8,30,48a-c is formed with one or more irregularities, such as protrusions 26a-c or grooves 40,64a-c, which provide localised changes in the internal cross section of the tubular die 8,30,48a-c. During the formation of the pellet in the tubular die 8,30,48a-c the granular material is compressed around these irregularities 26a-c,40,64a-c which helps retain the pellet fixed within the tubular die 8,30,48a-c.

[0037] The second step 70 includes a step 70a of removing the end cap 18,50,50' to expose the surface of the pellet which was previously pressed against the flat face 24,60a-c of the removable end cap 18,50,50'. This exposed surface advantageously is both flat and is located at a fixed, usefully known, distance inside the tubular die 8,30,48a-c relative to the open second end 12,36,58a-c as a result of the configuration of the removable end cap 18,50,50'. The second step 70 also includes a step 70b of placing the tubular die 8,30,48a-c in which the so formed pellet is retained into the LIBS analyser such that the exposed surface lies in a light path to intercept laser radiation emitted by the laser of the LIBS analyser so that laser ablation of this exposed surface can occur.

[0038] In some embodiments the second step 70 may include a further step 70c of heating the pellet in the tubular die 8,30,48a-c to cause a charring of at least the exposed surface to induce a thermochemical decomposition of the sample matrix, at least at the exposed surface. The thermochemical decomposition may provide at least two useful effects; 1) the emission lines for minerals are stronger since the elements are more easily ionized; 2) the accuracy for a quantitative elemental abundance analysis is improved since the seared matrices, when comparing different samples, have more in common with respect to chemical composition than have the un-seared matrices.

Claims

CLAIMS:

1. A press (2;42) for preparing a pellet of granulated material for laser induced breakdown spectroscopy, the press (2;42) comprising at least one tubular die (8;30;48a-c) and complementary piston (4;44a-c) arrangement, each tubular die (8;30;48a-c) being formed with an inner surface (14;38;56a-c) along which the complementary piston (4;44a-c) can slidably move and which inner surface (14;38;56a-c) is delimited by an open first end (10;34;62a-c) and an open second end (12;36;58a-c) opposite the first end (10;34;62a-c); the press (2;42) further comprising a removable end cap (18;50;50') having at least one flat face (24;60a-c) for sealing the second end (12;36;58a-c) of the tubular die (8;30;48a-c); wherein the inner surface (14;38;56a-c) is formed with one or more irregularities (26a,b,c;40;64a-c).

2. The press (2;42) as claimed in claim 1 wherein the removable end cap (18;50;50') comprises a raised portion (22;54a-c;54'a-c)engagable about its outer periphery with the inner surface (14;38;56a-c) of the tubular die (8;30;48a-c) to seal the second end (12;36;58a-c), the raised portion (22;54a-c) being configured with the at least one flat face (24;60a-c) presenting internal of the tubular die (8;30;48a-c) when the second end (12;36;58a-c) is sealed.

3. The press (2;42) as claimed in claim 1 wherein the one or more irregularities comprises one or more grooves (40;64a-c).

4. The press (2;42) as claimed in claim 3 wherein each of the one or more grooves (40;64a-c) extends in an own plane parallel with a cross sectional plane through the tubular die (8;30;48a c).

5. The press (2) as claimed in claim 1 wherein the one or more irregularities comprises one or more protrusions (26a,b,c).

6. The press (42) as claimed in claim 1 wherein the end cap (50; 50') is formed with a plurality of the raised portions (54a-c; 54'a-c), each for slidable engagement about its periphery with an own tubular die (48a-c).

7. The press (42) as claimed in claim 6 wherein a die holder (80) is provided for releasably holding the plurality of tubular dies (48a-c) and is configured for releasable engagement with the end cap (50'); the die holder (80) comprising a die grip (82) formed with a plurality of retainers (94) adapted to releasably holding a corresponding one of the plurality of dies (48a-c) orientated with its second open end (58a-c) exposed to a corresponding one of the raised portions (54'a-c) of the end cap (50').

8. The press (42) as claimed in claim 7 wherein the die holder (80) further comprises a top (84) configured with a plurality of through-holes (104) which when the top (84) is engaged with the die grip (82) each overlies a corresponding one of the plurality of retainers (94).

9. A method (66) of performing laser induced breakdown spectroscopy on a pellet of granulated material comprising the steps of: a. forming (68) a pellet of granulated material; and b. loading (70) the formed pellet into a laser induced breakdown spectroscopy analyser to provide an exposed surface for interaction with laser radiation emitted by a laser of the analyser; wherein the step a. (68) comprises: placing (68a) the granulated material into a tubular die (8;30;48a-c) of a pellet press (2;42) according to any one of the preceding claims, which tubular die (8;30;48a-c) is sealed at its second open end (12;36;58a-c) by the removable end cap (18;50;50'); and compressing (68b) the granulated material in the tubular die (8;30;48a-c) by actuation of the piston (4;44a-c) to slidably move along the internal surface (14;38;56a-c) via the open first end (10;34;62a-c) of the tubular die (8;30;48a-c); and wherein the step b. (70) comprises: removing (70a) the end cap (18;50;50') to provide the exposed surface of the pellet retained within the tubular die (8;30;48a-c); and placing (70b) the tubular die (8;30;48a-c) into a laser induced breakdown spectroscopy analyser with the exposed surface in a light path to intercept laser radiation emitted by the laser of the laser induced breakdown spectroscopy analyser.

FOSS Analytical A/S

Patent Attorneys for the Applicant/Nominated Person

SPRUSON&FERGUSON