Abstract
In
this work, Laser Induced Breakdown Spectroscopy (LIBS) was used to determine
the constituent of Gum Arabic (Acacia nilotica)
collected from five different locations in Sudan. Gum samples were irradiated
with 80 mJ pulse energy of Nd-YAG laser (1064 nm) and Atomic spectra Database
was used for the spectral analysis of the plasma emitted from these samples. It
was found that the samples contain the elements C, O, H, S, N, P, Na, Mg, Ca,
Fe, Cr, Mn, Co with different amounts. Some elements like (Ti, Br, Ar, Th, Kr,
Sc and Pr) are recorded here for the first time.
1. INTRODUCTION
Laser-induced breakdown spectroscopy (LIBS) is a method of atomic emission spectroscopy (AES) that uses laser-generated plasma as the hot vaporization, atomization, and excitation source [1-4]. Foremost of these is the ability to interrogate samples in situ and remotely without any preparation. In its basic form, a LIBS measurement is carried out by forming laser plasma on or in the sample and then collecting and spectrally analyzing the plasma light. Qualitative and quantitative analyses are carried out by monitoring emission lines positions and intensities [5-7]. LIBS can be used to investigate different materials especially those composed of large molecules such as Gum Arabic. Acacia nilotica Sunt has been found the most valuable timber-producing species [8-10]. An ability to regenerate successfully on flooded sites along the Nile and its tributaries, coupled with timber properties that satisfy most of the utilization standards make the species the most important in the economy of the Sudan. This work aimed to use LIBS for Identification of Gum Arabic (Acacia nilotica).
2. METHODS AND MATERIAL
2.1. Experimental Setup
Figure 1 illustrates the LIBS setup which was used in this work. The LIBS system composed of Q-switched Nd-YAG Laser (Laser wavelength is 1064 nm, pulse duration 10 ns, Pulse Energy 80 mj, Spot size 2-8 mm, and repetition rate 2 Hz), Ocean Optics 4000+ spectrometer, connected with PC.
2.2. The Materials
Four samples of GumArabic (Acacia nilotica) (sunt) obtained from different locations in Sudan. Were used in this work, they are illustrated in Table 1.
2.3. Experimental Procedure
Each sample was put in a quartz cell and irradiated by the Nd-YAG laser where the spark of the sample plasma was collected by a fiber optic to the spectrometer which was interfaces to a computer. The emission spectra were collected in the range from 200-900 nm. In order to test the homogeneity of Gum Arabic samples, several LIBS measurements were performed at its surface. The recorded spectra of the samples were analyzed using Atomic Spectral Database (NIST) data.
Irradiation with pulse energy of 80 mJ resulted in generating exited elemental species and cations in higher oxidation states. This was evident from the emission spectral lines corresponding to species such as Cr3+, Ti2+ and Ar3+.
Although the samples of GA in this study were collected from different locations in Sudan having different soil characteristics, their LIBS emission spectra reflect presence of the same elements in all samples. This may indicate that there is no influence of the soil type on the elemental composition of exudates gums like GA.
3. RESULTS AND DISCUSSION
LIBS emission spectra of Acacia nilotica (sunt)gum samples irradiated with laser of 80 mJ pulse energy are shown in Figures 2-5 while Table 2 shows the results of analysis and elemental composition of the mentioned gum samples, which were obtained with the help of Atomic spectra database and the Handbook of basic Atomic spectroscopy.
The elements constituting the samples observed in the emission spectra were C, O, H, S, N, P, Na, Mg, Ca, Fe, Cr, Mn, Co. This is in agreement with findings of other researches published previously. [11-13]. In addition to these, other elements had been detected for the first time in natural gums, namely
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Figure 1. Schematic diagram of the setup.
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Figure 2. LIBS emission spectrum of sample (1).
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Figure 3. LIBS emission spectrum of sample (2).
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Figure 4. LIBS emission spectrum of sample (3).
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Figure 5. LIBS emission spectrum of sample (4).
(Ti, Br, Ar, Th, Kr, Sc and Pr).
Interestingly the elements Th, Pr, Kr, and Sc have not been observed in any of the previous studies undertaken on Acacia nilotica using the conventional (AAS or ICP) techniques. These elements and their ions are reported here for the first time in the elemental analysis of Acacia nilotica using LIBS, this adds to the many advantages of this technique, compared to other conventional techniques mentioned above. Sample of Acacia nilotica showed the presence of heavy metals like Fe, Cr, Pr and Th which may hinder its application in food and pharmaceutical formulation.
4. CONCLUSIONS
Sample of Acacia nilotica showed the presence of heavy metals like Fe, Cr, Pr and Th which may hinder its application in food and pharmaceutical formulation and the elemental composition of Gum Arabic by LIBS technique enabled observing elements, such as Br, Ti and Ar, to be reported for the first time.