Plasma Spectroscopic Studies Of MnSO₄, MgSO₄ And KBrO₃ By Using The First And Second Harmonics Of Nd:YAG Laser

Laser-induced plasmas are prevalent today worldwide due to its applicability and potentials in a large range of fields and continue to dilate each day because of the preferment of laser science and technology. All applications of laser-induced plasmas require a deeper understanding and characterization of physical and chemical processes occurring in it. As the spectrum of the radiation emitted by the plasmas is a mine of information about the plasma state and therefore plasma spectroscopy remains one of the key techniques for its investigation.Laser Induced Breakdown Spectroscopy (LIBS) is a spectral analysis technique to obtain information on elemental content of the sample material to be analyzed with the aid of atomic or ionic transitions in plasma produced by laser radiation. Its working principle is:a focused pulsed laser beam is incident on a sample under test to generate laser plasma and at this stage plasma begins to cool down after the interaction process of laser pulses and then the excited atoms and ions release energy in the form of light radiation. LIBS spectrum is recorded and resolved with a spectrometer. Elements in the sample can be identified according to the characteristics of spectral line intensity and wavelength distribution. With the help of spectral calibration method content of elements can be further analyzed and quantitative chemical components in the material under investigation may also be calculated by means of stoichiometric ratios. Molecular finger-print can possibly be recognized from the characteristic of the spectra of molecular emissions.LIBS is a unique technique for chemical elementary detection applicable in various different environments. It can be used to analyze solid, liquid and gaseous samples. LIBS is capable of rapid, in-siru, real-time analysis and it can be operated at fixed position or in scan mode, and it can also be classified as one of techniques for real-time and multi-elemental detection. In recent years LIBS technique has been extensively used in many areas including metallurgy, mining industry, water and soil pollution, air pollution, environmental monitoring, space exploration, artwork and dye identification, etc.The main drawback of LIBS is the relatively complex calibration process for different samples to be detected, and moreover, accuracies and sensibility of the detection method itself need to be improved, if compared with other conventional techniques such as the inductively coupled plasma (ICP). In order to solve these problems, several quantitative analytical methods have thereafter adapted and developed in quantitative LIBS analysis, including conventional external standard, and other commonly used methods such as internal standard, calibration-free LIBS (CF-LIBS) and a few chemometrics-based methods (such as PCA,PLS and ANN,etc.). However, the external standard method is usually unable to meet requirement for analytical accuracy due to structural complexity of the analytes. Reliability of quantitative analysis based on the internal standard is often to be verified although the analytical method itself is relatively simple. Powerful chemometrics methods depend on complicated mathematical models too. CF-LIBS method utilizes intensity dependence characteristics of spectral lines upon atomic/ionic energy levels described by the Boltzmann distribution to determine elemental concentrations. No standard curves are needed in quantitative CF-LIBS analysis, but specific conditions such as Local Thermal Equilibrium (LTE) must be met in detection process. In general, all methods mentioned above have both advantages and disadvantages of each and no special effective universal approach has been finally established in LIBS analysis up to date.It is noticed that analysis of plasma radiation spectra is the basis for quantitative LIBS detection. Up till now, the deviation of quantitative LIBS detection results is often rather large if compared with real elemental concentrations. The main reasons include improper measurement parameters selection or inconsistent parameters, influences of self-absorption effect of plasma and matrix effect, etc. From a fundamental point of view, the intrinsic cause of this kind of deviation should be the non-LTE state of LIBS plasma during the process of spectral detection. It is well known that recorded plasma spectrum derives from irradiative atomic and ionic transitions of dominant elements in samples under test and at some extent consequential spectral distribution is under direct influence of plasma state and its evolution behavior. Atoms or ions in plasma obey the Boltzmann distribution only when the plasma is in LTE condition and then spectral line intensities recorded by the spectrometer correlate effectively with elemental concentrations of atoms or ions in plasma. The McWhirter criterion is nowadays the base use to decide whether the plasma is in the LTE condition and it is related to two parameters i.e. plasma temperature and electron number density. Practically the plasma temperature can be determined by Boltzmann plot method and the electron number density can be further obtained according to Stark broadening of spectral lines. From this point of view, quantitative analytical results are closely related to plasma conditions and, therefore, it is necessary to study characteristics of laser-induced plasma in order to determine elemental concentrations of analytics both accurately and effectively.Manganese sulfates (MnSO4), magnesium sulfate (MgSO4) and potassium bromate (KBrO3) are three important chemical compounds which are widely used in industry, agriculture, pharmacy and food processing. It is well known that manganese sulfate is a minor element necessary for crop used to synthesize fatty acid. It is used as fertilizer to increase drop production, as animal feeds to fatten. Manganese sulfate is also the raw material and analytical reagent used to produce other manganese salts and can be used in industrial production processes of electrolytic manganese, dye, paper and ceramics. Magnesium sulfate is used to produce leather, dynamite, paper, porcelain, fertilizer, and oral laxative, etc. It is one of the main components of chlorophyll and it is especially used as a kind of fertilizer in agriculture to fertilize potted plants or magnesium deficient crops. Potassium bromate is mainly used as analytical reagents, oxidants and food additives. Although these chemical compounds such as manganese sulfate, magnesium sulfate and potassium bromated are increasing their important values in technical applications as in agriculture, medicine and food industries, but their consumptions should be controlled in practice, due to the fact that these chemicals are toxic. Therefore, real-time and in-situ methods of quantitative detection of these compounds need be introduced during the manufacturing process of the agriculture, medicine and food products. To provide a perfect quality is usually very expensive but with the use of LIBS method the cost seems to be reduced. As mentioned previously, LIBS is a real-time and in-situ elemental analytical technique. Concentrations and even distributions of these chemical compounds (MnSO4, MgSO4, and KBrO3) in the end products can be determined by on-line or off-line checking of the exact concentrations of the major elements (Mn, Mg and K) and other elements respectively. Therefore, quantitative LIBS technique seems obligatory for quality assurance and quality control during the manufacturing process of the agriculture, medicine and food products and in this sense it definitely plays active roles in reducing unwanted harm caused by these chemicals to human bodies and living environment.In this study the technique of LIBS is familiarized for the first time to detect the three chemical compounds of manganese sulfate (MnSO4), magnesium sulfate (MgSO4) and potassium bromate (KBrO3) which has important uses in the fields such as industry, agriculture, pharmaceutical and food. Theses samples MnSO4, MgSO4 and KBrO3 are investigated by means of time-resolved and spectral analytical methods based on LIBS analysis technique with the fundamental harmonic (1064 nm) and second harmonic (532 nm) light of a Q-switched Nd:YAG pulsed laser sequentially. During experimental investigation, laser light is incident on the sample surface placed in normal atmospheric pressure and the resultant plasma spectrum recorded by a spectrometer. Experimental results have shown that plasma spectra are dominated by spectral lines from atomic and ionic transitions of major elements (such as Mn, Mg and K). In order to study special distributions of plasma temperature (Te) and electron density (Ne), major elements of Mn, Mg and K in the samples were investigated by means of the Boltzmann plot and results from Stark broadening of spectral lines. Further the behavior of these two characteristic parameters was studied under different laser irradiances. Similar results and demeanors were obtained in all three types of chemical compounds samples. Experimental results show that, under the prerequisites that laser beam focusing and orientation of spectral detector are kept constant, the electron density in case of laser excitation with a wavelength of 532nm is higher than that of the 1064 nm laser and electron temperature obtained after excitation of a 1064 nm laser is higher than that of a 532 nm laser. It has also checked the validity of the assumption of local thermal equilibrium conditions based on results.By means of analytical method based on atomic physics plasma spectra have been analyzed spectroscopically and some atomic and ionic spectral lines were assigned. This spectral information is important to qualitative analysis of samples. On this basis, some plasma parameters such as electron temperature and electron density in plasma obtained from experiment and their corresponding behaviors related to laser irradiances on the sample surface are also helpful to understand characteristics of laser induced plasma. As far as accurate quantitative analysis of major elements in the samples is concerned, analytical method and scope of investigation described in this study are indeed the prerequisites for quantitative elemental analysis by means of the calibration-free LIBS technique. Therefore, these research results are beneficial to develop an in-situ, real-time, and fast quantitative analytical technique used in analyzing the chemical compound samples, and moreover, helpful to quality control of these chemical compounds in the processes of manufacture and application in agriculture, pharmacy and food.