Improvement of Analytical Sensitivity by Ar-N2 Inductively Coupled Plasma in Axially Viewing Optical Emission Spectrometry

Abstract

An inductively coupled plasma (ICP) is widely used for trace element analysis as an appropriate excitation source and an ionization one for optical emission spectrometry (ICP-OES) 1 and mass spectrometry (ICP-MS), 2,3 respectively. Although innovative efforts have been conducted to achieve further improvements in analytical sensitivity for both ICP-OES and ICP-MS, no reports have been found for axially viewing ICP-OES. In the 1980's, many examinations were performed on ICP-OES to improve the analytical sensitivities of elements. Mixed-gas ICPs such as Ar-N2 ICP, Ar-O2 ICP and Ar-H2 ICP, molecular-gas ICPs such as N2 ICP and O2 ICP, and He ICP were also examined to improve the analytical sensitivity of elements for OES analysis instead of Ar ICP-OES. 1,4 These ICPs sometimes showed better analytical performances compared with Ar ICP-OES; however, they have not been applied to commercial ICP-OES instruments because of some difficulties and complexities concerning the plasma operating conditions and optimization with respect to OES analysis. 1,4-6 It should also be remarked that these reported ICPs were examined only in radially viewing ICP-OES. 1,[4][5][6] In the present study, we found an effective sensitivity enhancement of elements in axially viewing ICP-OES by adding N2 gas into the Ar plasma gas. This is the first report concerning an examination of a mixed-gas ICP in axially viewing ICP-OES. In previous studies examined in radially viewing ICP-OES, 4-6 a shrunk plasma was obtained when N2 gas was added to Ar ICP. A shrunk plasma is considered to enhance the plasma-sample interaction; therefore, a sensitivity enhancement of elements can be obtained. In the present study, N2 gas was only added at flow rates of up to 1.0 L min -1 into 15 L min -1 of Ar plasma gas in a commercially available ICP-OES instrument; a sensitivity enhancement was then obtained. Except for a modification of the gas line for the addition of N2 gas into Ar plasma gas, no modifications were made to the ICP-OES instrument. The torch geometry was not changed, and optimization of the spectrometer was not performed for Ar-N2 ICP, but for Ar ICP. It is concluded that the developed Ar-N2 ICP technique used in the present study is simple, useful and effective as an advanced analytical technique in axially viewing ICP-OES. Experimental InstrumentationThe ICP-OES instrument used was an Optima 4300 DV (PerkinElmer, USA). Though the instrument can be used for both axially viewing ICP-OES and radially viewing one, the former was used. Table 1 gives the operating conditions for the ICP-OES instrument. Except for the plasma gas flow rate of N2, all parameters were fixed during all experiments in the present study. The ICP torch used was a Fassel-type long one, which is usually supplied with the ICP-OES instrument. The ICP-OES instrument was designed for Ar ICP-OES, that is, the spectrometer was optimized with respect to the Ar ICP. In the present study, except for a gas line of N2 added to the Ar plasma gas, no modifications were performed on the ICP-O...

Spectroscopic Characteristic and Analytical Capability of Ar-N2 Inductively Coupled Plasma in Axially Viewing Optical Emission Spectrometry

“…Since the central channel of Ar-N2 ICP became slightly narrower with an increase of the N2 gas flow rate, larger enhancements of the elements were observed for higher N2 gas flow rates, especially for up to 0.5 L min -1 observed in our previous study. 32 Although strong correlation between the excitation temperatures and the N2 gas flow rates could not be seen in the present study, improvements of the analytical sensitivity of the elements on the Ar-N2 ICP reported in our previous study 32 could be elucidated qualitatively.…”

“…This trend was similar to that of the Mg II/Mg I intensity ratio, which was related to the electron number density of the analytical zone in the Ar-N2 ICP, observed in our previous study. 32 The increase in the excitation temperature was considered to be attributed to the plasma shrunken by the thermal pinch effect 4,5,[29][30][31] which resulted in the enhancement of the interaction between the plasma and the sample. Since, the excitation temperature increased, the Ar-N2 ICPs could be expected to possess a higher analytical capability than that of the standard ICP for axially viewing ICPOES.…”

“…The wavelengths of the elements can be found in Table 2. Since enhancements of the emission intensities were observed for the addition of the N2 gas flow rates between 0.1 and 0.5 L min -1 and the enhanced emission intensities became almost constant for more than 0.5 L min -1 in our previous study, 32 the flow rates of 0.1 and 0.5 L min -1 were chosen for detailed evaluations. If the emission intensities between the Ar-N2 ICP and the standard one are the same, the plotted relative intensities reveal unity.…”

“…32 The torch geometry was not changed, and optimization of the spectrometer was not performed for Ar-N2 ICP, but for Ar ICP. Since only the gas line to add N2 gas into Ar plasma gas was modified in the commercial ICPOES instrument and instrumental optimization of the alignment between Ar-N2 ICP and the spectrometer was not needed, the proposed Ar-N2 ICP in axially viewing OES The spectroscopic characteristics and analytical capability of argon-nitrogen (Ar-N2) inductively coupled plasma (ICP) in axially viewing optical emission spectrometry (OES) were examined and figures of merit were determined in the present study.…”

Effect of N2 on the emission profile and excitation temperature in axially viewed plasma-ICP OES

“…[7][8][9][10][11][12][13][14][15][16] In addition, Ar-N 2 -ICP proved usefulness in reducing oxides and Ar polyatomic ions, which are interferents in ICP-MS. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] Researches dealing with the use Ar-N 2 -ICP in axially viewed plasma in ICP OES have been quite limited. Ohata et al 36 demonstrated that plasma robustness (i.e., Mg(II)/Mg(I) ratio) were improved by using 1.3% of N 2 in the outer gas, 2 | J. Name., 2012, 00, [1][2][3] This journal is © The Royal Society of Chemistry 2012…”