View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Journals of Universiti Tun Hussein Onn Malaysia (UTHM) Analytical Methods in Plasma Diagnostic by Optical Emission Spectroscopy: A Tutorial Review H. H. Ley*, A. Yahaya, R. K. Raja Ibrahim Department of Physics, Universiti Teknologi Malaysia, 81310 Skudai Johor. *Corresponding email: [email protected] Abstract This article reviews on the basic data processing technique and plasma diagnosis method by optical emission spectroscopy. The review is focused on the physical concepts of an analysis methods and its application to determine the plasma parameters. Processing procedures such as signal averaging and spectrum smoothing technique such as boxcar are discussed. Next, conditions such as local thermodynamic equilibrium plasma were briefly discussed with the method available to assess its existence. Similarly, the available techniques to quantitatively determine optically thin condition of spectral line are considered including method to calculate the suitable time window for a spectral line to be optically thin. Measurement techniques of plasma temperature and electron density were compared and contrast to gauge the advantages of each technique. Keywords: analytical methods; plasma diagnostic; optical emission spectroscopy 49 1. INTRODUCTION The word ‘plasma’ which means jelly in Greek was coined by Nobel Laureate Irving Langmuir who pioneered the study of this new state of matter. Plasma is simply an ionised gas consisting positive and negatively charged particles of roughly equal charge density. As such, plasma can be formed when any gaseous state is heated to a sufficient temperature such that the particles collide against each other and knocking off electrons in the process producing a mixture of ions and electrons. Plasma can be categorised into high temperature known as fusion plasma, and low temperature plasma. This review paper will focus only for the latter group of plasma. Low temperature plasmas can be further divided into plasma that is in thermal equilibrium and those which are not in thermal equilibrium. The term thermal equilibrium refers to when the temperature of all plasma species such as electrons, ions, neutral particles, is the same [1-7]. However, this condition is only true for stellar and fusion plasma and almost never achieved in laboratory plasma. For that matter, a more lenient condition called the ‘local thermal equilibrium’, or LTE in short, is used to imply that temperature for all plasma species are the same for a localised area in the plasma. Optical emission spectroscopy (OES) is commonly employed in the diagnosis of laboratory plasma such as gas discharge plasma, inductively coupled plasma (ICP), or laser induced plasma (LIP). Numerous analytical techniques have been established to determine the plasma properties such as electron density, plasma temperature, element recognition and quantification of elements present in the plasma. For instance, the Laser Induced Breakdown Spectroscopy (LIBS) technique is widely used to study elemental composition of a sample. Gondal et al.[8] and Trevizan et al.[9] demonstrated such technique to determine the amount of poisonous metal presence in wastewater from paint manufacturing plant and the amount of micronutrient in plant material respectively. The details on LIBS experimental setup, conditions, and analysis are discussed in great length in [1, 2, 10]. Various studies of ICP plasma system such as argon [11, 12], or even combinations of several molecules system [13] were analysed via OES to obtain the plasma properties. Review on mathematical procedures for background correction and analysis in ICP is described by E. H. Van Veen [14]. Similarly, myriads of work have also been done on OES for glow discharge plasma which includes development of mathematical models to describe the plasma [15-17], and a review on its applications [7]. Thus, the purpose of this paper is to describe the basic analytical techniques applied in the research on plasma diagnosis by OES in terms of data processing and determination of plasma properties. Although a few books by Griem [3], Hutchinson [4], and Kunze [5] may have explained extensively the plasma diagnosis method and its underlying physical principles, it is necessary to provide discussion on data pre-treatment 50 method and details on analysis condition. This review would serve to provide and expose initial knowledge to new researchers who are about to embark into this field of study. 2. PRE-PROCESSING OF EXPERIMENTAL DATA Generally, dark background spectrum component is measured without the light source on, and it can be easily eliminated using the software provided by the spectrometer or it can be done manually by measuring the dark spectrum separately and subtracting it from the spectrum obtained. Note that the exposure time for dark background must match that of data collection, for instance, data collected with exposure time of 1 second will need to be applied with dark background recorded with one second exposure time. Figure 1 shows the comparison of spectrum with dark background subtracted and without subtracting the dark background. 500 Without dark background With dark background 400 Intensity (a.u)Intensity 200 0 728 736 Wavelength Figure 1: Mercury glow discharge spectra 2.1 Signal Averaging The spectral information obtained from the spectrometer is encoded in the form of electrical signal. Hence, the presence of an undesirable component in the signal called noise is inevitable and it will also impede the accuracy and interpretation of the data. The factor which contributes to noise and methods to overcome it are explained briefly by [18]. For that, the quality of a spectral measurement is measured by signal-to-noise ratio, S/N defined as; S/N = average signal magnitude / rms noise A good spectrum is portrayed by a high signal-to-noise ratio, which can be done by a process called signal averaging. This process is conducted by repeated scanning and taking the summation of individual spectra. Assuming random distribution of noise, time coherent signals improve linearly with number of scans N, mathematically expressed as; Signal magnitude = C1N (1) where C1 is proportionality constant 51 On the other hand, the average magnitude of random noise increases proportional to the square roots of number of scans given by; 1/2 Noise magnitude = C2N (2) where C2 is proportionality constant Dividing (1) by (2) yields the signal-to-noise ratio; signal C1 N 1/ 2 1/ 2 CN (3) noise C2 N The results of equation (3) where C is a constant, shows that the signal-to-noise ratio is proportional to the square roots of the number of scans. The effect of averaging on spectrum is illustrated in Figure 2. 500 Signal averaged spectrum Without signal averaging 400 300 200 Intensity (a.u) 100 0 655 656 657 658 Wavelength (nm) Figure 2: 10 scans are applied in this case hence the average spectrum (top) appeared to be less ‘noisy’. The vertical axis of the spectrum is shifted to avoid spectra from coinciding. 2.2 Signal Smoothing Despite performing signal averaging, a spectrum may still appear to be ‘noisy’ and this can be overcome by signal smoothing process. Note that there are a few methods to perform signal smoothing but in this case we will only elaborate on boxcar averaging which is a simpler method and usually provided in the accompanying software of a spectrometer compared to the others such as polynomial smoothing, Savitzky-Golay smoothing or moving average method. Boxcar averaging is carried out by replacing a centroid average value for a spectral data which are divided into groups of equally spaced and discrete bands [18]. The degree of smoothing is proportional to the number of points averaged. However, over smoothing of spectrum by this method will introduce distortion and loss of spectral resolution. Comparison of different degree of boxcar smoothing applied on the same spectrum is shown in Figure 3. Further explanation of other smoothing method such as moving average method and Savitzky-Golay smoothing are explained in detail in reference [18]. 52 No smoothing Boxcar 1 Boxcar 5 3000 2000 Intensity (a.u) 1000 364 366 368 Wavelength (nm) Figure 3: Comparison of different degree of boxcar smoothing performed via Ocean Optic’s spectrometer software, Spectra Suite. Different degrees of boxcar spectrum are shifted on the y-axis to avoid data overlapping. 2.3 Signal Stacking This method is employed to differentiate data points from noises. From equation (1), the time coherent signal magnitude is directly proportional to the number of scans which is conducted by repeated scans and summing each individual spectrum to a stacked spectrum. This spectrum will unveil any data points which initially have intensities comparable to the noise in one scan. In Figure 4, clearly data point increases linearly with the number of spectra employed in the summation which unveils a data peak close to 715nm which is only distinguishable after 10 sets of spectra applied. This method is used to improve the visibility of peaks that corresponds to data and not to be used for analysis purposes such as to determine the plasma parameters like temperature or electron density. For this matter, signal-averaged spectrum should be utilised to ensure accuracy of the results. 3000 3 spectra 5 spectra 7 spectra 10 spectra 2000 1000 Intensity (a.u)Intensity 0 708 712 Wavelength (nm) Figure 4: Comparison of different numbers of spectra used in signal summation 53 3. LOCAL THERMAL EQUILIBRIUM The radiative behaviour of the atomic species in any plasma can only be predicted if the expected population of its myriad possible states are known. The condition called thermal equilibrium is satisfied if and only if the atoms obeys Boltzmann distribution for every possible states, and the radiation energy density for all transition has the blackbody level of the system temperature [19, 20]. However, this condition is difficult to be achieved by laboratory plasma, and hence LTE is taken into consideration.