Method for preparing a sample for laser induced breakdown spectroscopy

Abstract

A method for preparing a sample of organic material for laser induced breakdown spectroscopy (LIBS) may include obtaining granular organic material, forming a portion of the granular organic material into a sample pellet, and searing the organic material. The searing may include searing only an exposed end surface of the sample pellet on which LIBS analysis is to be performed. The method may include pressing the seared sample pellet to consolidate the material comprising the seared end surface.

Claims

1. A method for preparing a sample of organic material for laser induced breakdown spectroscopy (LIBS), the method comprising: obtaining granular organic material, the granular organic material including an organic matrix; loading the granular organic material into an open ended cylindrical die, and applying pressure to the granular organic material that is loaded in the open ended cylindrical die to form the granular organic material into a consolidated mass that is a sample pellet, wherein the sample pellet is retained in the open ended cylindrical die such that the sample pellet presents an exposed end surface that is exposed in the open ended cylindrical die; applying heat to the exposed end surface of the sample pellet while the sample pellet is retained in the open ended cylindrical die to sear at least the exposed end surface of the sample pellet that is exposed through the open ended cylindrical die to induce a thermochemical decomposition of at least a portion of the organic matrix that is included in the sample pellet, to establish a seared surface of the sample of organic material.

2. The method of claim 1, wherein the searing sears only the exposed end surface of the sample pellet.

3. The method according to claim 2, further comprising: re-applying pressure to the sample pellet while the sample pellet is retained in the open ended cylindrical die, subsequently to searing only the exposed end surface of the sample pellet, to consolidate organic material of the seared surface of the sample of organic material.

4. The method according to claim 1, wherein the granular organic material includes plant material.

5. The method according to claim 1, wherein the applying pressure includes applying five tons per square centimeter of pressure to the granular organic material loaded into the open ended cylindrical die for ninety second.

6. A method of performing laser induced breakdown spectroscopy (LIBS), the method comprising: preparing a sample of organic material according to the method of claim 1; directing a laser beam pulse to the seared surface of the sample of organic material to produce a plasma ablation event; and performing a spectrometric analysis of light emitted from plasma generated in the plasma ablation event to identify constituent elements of interest in the sample of organic material by characteristic emission wavelengths of the constituent elements of interest.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other advantages and features will be better understood from a consideration of the following description of one or more exemplary embodiments of the method and the system according to the present invention made with reference to the drawings of the accompanying figures, of which:

(2) FIG. 1 shows a flow chart illustrating an embodiment of the method of the present invention;

(3) FIGS. 2A-2D show Comparative LIBS spectra of seared and un-seared samples of Feed and Soy; and

(4) FIG. 3 shows Comparative LIBS spectra of samples of Feed under different searing durations.

DETAILED DESCRIPTION

(5) An illustrative embodiment of the method according to the present invention will be described with reference to FIG. 1.

(6) A first step 4 of the method 2 generally consists of obtaining granular organic material. In a particular embodiment a sample of plant material (being material from the plant itself or material manufactured using such material, such as animal feed, flour or foodstuff) is processed into unconsolidated granules, for example by shredding, grinding or pulverizing the material. This processing may be achieved manually, for example using a mortar and pestle, or mechanically, for example using a grinder or shredder, and may be done optionally after the material is dried.

(7) A second step 6 generally consists of forming at least a portion of the granular material obtained at the first step 4 into a sample pellet. In a particular embodiment the granular organic material, for example plant material, is loaded into an open ended cylindrical die and pressure is applied to the material in order to form a sample pellet of consolidated granular plant material which is preferably retained in the die to help maintain structural integrity and to enhance ease of handling. The so-formed pellet presents an exposed end surface, at which LIBS analysis will be performed. A conventional hydraulic laboratory pellet press may be employed to form the sample pellet.

(8) A third step 8 generally consists of searing the granular organic material. In an embodiment this step is performed after the step 6 of forming the sample pellet and involves applying heat only to the exposed end surface of the pellet. In an alternative embodiment the step of searing 8′ may be performed on the organic material before the step 6 of forming, for example after the step 4 of obtaining the granular organic material.

(9) A fourth step 10 is performed in embodiments where the step 8 of searing is performed after the step 6 of forming a sample pellet. This fourth step 10 generally consists of pressing (or re-pressing) the sample pellet. At this step 10 pressure is applied to the sample pellet in order to consolidate the material which forms the seared exposed end surface of the sample pellet. The press employed at the step 6 of forming is conveniently employed at this step 10 of pressing (or re-pressing) the sample pellet.

(10) Samples are prepared according to the method (2) described above in respect of FIG. 1 by firstly obtaining granulated plant material (4); then press-forming the granulated plant material into a sample pellet (6) having an exposed end surface for LIBS analysis; then searing the exposed end surface (8); and finally re-pressing the seared exposed end surface (10) before presenting the sample for LIBS analysis. LIBS analysis is performed on the seared exposed end surface of each of the resulting sample pellets. Essentially, this is achieved by directing a laser beam pulse to a seared surface of the sample of organic material to produce a plasma ablation event (12) and performing a spectrometric analysis of light emitted from plasma generated in the plasma ablation event to identify constituent elements of interest in the sample of organic material by their characteristic emission wavelengths (14). Spectra generated by the spectrometer of the LIBS system for seared (solid lines) and un-seared (broken lines) sample pellets are illustrated in FIGS. 2A-2D for sodium (Na) in mixed ration feed (FIG. 2A); for Calcium (Ca) in mixed ration feed (FIG. 2B); for potassium (K) in soy (FIG. 2C) and for phosphorus (P) in soy (FIG. 2D). As can be seen the spectral features associated with the elements in each of the samples are all enhanced in the seared sample pellets.

(11) The effect of searing duration on LIBS spectra from the seared exposed end surfaces of sample pellets of mixed feed ration produced according to the method of FIG. 1 is illustrated in FIG. 3 for calcium (Ca) and includes the step 10 of re-pressing the sample pellet after searing. Five tons per square centimeter is applied to the plant material in the die for ninety seconds at each pressing stage 2, 10. LIBS analysis 12, 14 is performed at five different searing levels: (a) ‘No searing’ (i.e. the method according to FIG. 1 is performed up to and including the step 6 of press-forming particulate mixed feed ration into a sample pellet); (b) ‘Normal searing’ (i.e. searing is stopped when the exposed upper surface of the pellet becomes black according to visual inspection); (c) ‘Heavy searing’ (twice the searing time compared to Normal searing); and (d) ‘Heaviest searing’ (three times the searing time compared to Normal searing).

(12) As can be seen from FIG. 3, the intensity of the LIBS spectral signal due to Ca increases as the duration of the searing increases (i.e. from (a) to (c)). The intensities begin to converge as searing intensity increased and beyond a certain searing level (between the Heavy (c) and Heaviest (d) searing durations) the intensity of the LIBS spectral signal begins to reduce. Thus the optimum searing time for a specific matrix and/or searing temperature can be readily experimentally determined.