Abstract No. 21. Excitation Mechanisms in Microwave-induced Plasma Excited with Okamoto-cavity and the Spectrometric Application to Steel Analysis

Transcription

1 Abstract No. 21 Excitation Mechanisms in Microwave-induced Plasma Excited with Okamoto-cavity and the Spectrometric Application to Steel Analysis Institute for Materials Research, Tohoku University, Japan Kazuaki Wagatsuma, Yuuki Arai, and Shigeo Sato A surface-wave-excited non-resonant cavity, published by Okamoto in 1991 (Okamoto cavity) 1), induces a microwave induced plasma (MIP) having excellent features beyond conventional ones, such as a Beenaker cavity and a Surfatron. It makes the electric field of microwave to be concentrated at the outer zone of the plasma torch, resulting in an annular flame-like plasma having a doughnut-like structure so that sample aerosol can be easily introduced through a central tube of the plasma torch, as similar to ICP. The Okamoto cavity can be loaded at higher powers up to 1.5 kw; therefore, wet aerosol from solution samples can be aspirated directly, also like ICP. Furthermore, the Okamoto-cavity MIP has several advantages beyond a conventional ICP. It is the most important feature in them that organic solvents can be directly and easily introduced into the plasma when using nitrogen-oxygen mixtures or air as the plasma gas. 2) The plasma characteristics using nitrogen-oxygen mixed gas are also employed effectively for the analytical applications, where the intensities of particular emission lines are largely elevated when oxygen of up to 20 % is added to the nitrogen plasma. 3) Our previous paper has presented that, on the determination of chromium in steel materials, several emission lines of chromium having excitation energies of ca. 3.0 ev give better detection limits in the mixed gas MIP-OES than the corresponding result in conventional ICP-OES. 3) It is also reported in this paper that several interesting excitation phenomena occur in the Okamoto-cavity MIP, especially when a small amount of oxygen is added to the nitrogen matrix in the composition of the plasma gas. 4) An ion-toatom ratio of iron, which is estimated from the intensity ratio of ion to atomic lines

2 Abstract No. 21 having almost the same excitation energy, is reduced by adding oxygen gas to the nitrogen MIP, eventually contributing to an enhancement in the emission intensities of the atomic lines. The ionization of iron would be caused less actively in the nitrogen-oxygen plasma than in a pure nitrogen plasma, because excited species of nitrogen molecule, which can provide the ionization energy in a collision with iron atom, are consumed through collisions with oxygen molecules to cause their dissociation. Furthermore, Boltzmann plots for iron atomic lines are investigated to discuss the excitation mechanism in more detail. 4) References 1) Y. Okamoto: Anal Sci., 7 (1991) ) T. Maeda and K. Wagatsuma: Microchem. J., 76 (2004) 53. 3) Y. Arai, S. Sato, and K. Wagatsuma, ISIJ Int., 53 (2013) ) K. Wagatsuma: Anal. Sci., to be published.

3 E xcitatio n Mechanisms in Microwaveinduced Plasm a excited with Okam otocavity and th e Spectrom etric A p p lica tio n to Steel Analysis Yuki Arai, Shigeo Sato, and Kazuaki Wagatsuma IMR, Tohoku University, JAPAN wagatuma@imr.tohoku.ac.jp IMR Microwave Induced Plasma with Okamoto Cavity 1) Y. Okamoto: Anal. Sci., 7 (1991) ) H. Yamada, Y. Okamoto: Appl. Spectrosc., 9 (2001) 114. Plasmas sustained with various gases: Nitrogen, Air, Helium, Argon, Nitrogen-Oxygen,... High Robustness against loading of organic solvents 1) Z. Zhang, K. Wagatsuma: J. Anal. At. Spectrom., 17 (2002) ) T. Maeda, K. Wagatsuma: Anal. Bioanal. Chem., 382 (2005) The emission intensity in the nitrogen-oxygen mixed gas MIP was different from the pure nitrogen gas MIP. The reason for this phenomenon, chemical or physical? Excitation mechanism in the Okamoto-cavity MIP using nitrogen-oxygen mixed gas

4 Comparison between ICP and Okamoto-cavity MIP rf ICP Okamoto-cavity MIP Frequency 27.12, MHz 2.45 GHz (2450 MHz) Power kw kw Sample solution aerosol solution aerosol Plasma gas Ar N 2, O 2, Air, He,... Matching sensitive less sensitive "High Robustness" Schematic diagram of Okamoto-cavity MIP Nitrogen-oxygen gas

5 O kam oto-cavity M IP (m icrowave induced plasm a) external conductor microwave plasma gas Okamoto-cavity MIP Plasma internal conductor external conductor microwave surface-wave field internal conductor Surface-wave field is induced at the gap between the internal and external conductors Doughnut-like carrier gas flame + sample plasma Solution sample can be introduced through the central channel. Okamoto-cavity MIP with Nitrogen-Oxygen Mixed Gas N 2 -O 2 -M IP

6 Experimental setup Scanning Spectrometer Microwave generator Power Supply Biconvex lens Waveguide Nitrogen-Oxygen plasma gas Conductors Experimental System Cr solution sample Microwave induced plasma Okamoto cavity Fused Silica Torch Collimator Scanning spectrometer MIP Imaging spectrograph Imaging Spectrograph Two-dimensional spectrometer system Computer for control of spectrometers ICP CCD Detector

7 Em ission Characteristics of th e Nitrogenoxygen Mixed Gas MIP Comparison in the emission intensity of Cr lines between pure N 2 MIP and N 2 -O 2 MIP 1) Z. Zhang, K. Wagatsuma: J. Anal. At. Spectrom., 17 (2002) ) T. Maeda, K. Wagatsuma: Anal. Bioanal. Chem., 382 (2005) Analytical Lines for Cr Determination Wavelength Excitation Energy Rel. Intensity Cr I nm 2.90 ev Cr I nm 3.44 ev Cr I nm 3.46 ev Cr II nm 5.88 ev(+ip 6.72 ev) 1700 Cr II nm 5.86 ev(+ip 6.72 ev) 1100 N nm(band head) ev N nm(band head) ev

8 Cr Atomic Lines Emission Intensity / arb.unit Variation in the emission intensity of Cr atomic lines when oxygen gas is added to a nitrogen MIP CrI CrI Oxygen Gas Flow Rate / L/min Variations in the emission intensity of chromium atomic lines as a function of oxygen as added to nitrogen MIP. Sample: Cr 200 mg/dm 3 ; MV: 800 W; plasma gas: N 2 14 L/min + O 2 ; carrier gas: Ar 0.5 L/min. Cr Ionic Lines Emission Intensity / arb.unit Variation in the emission intensity of Cr ionic lines when oxygen gas is added to a nitrogen MIP CrII CrII The ionization equilibrium moves on towards neutral atomic species. Oxygen Gas Flow Rate / L/min It could improve the analytical performance of the atomic lines. Variations in the emission intensity of chromium ionic lines as a function of oxygen as added to nitrogen MIP. Sample: Cr 200 mg/dm 3 ; MV: 800 W; plasma gas: N 2 14 L/min + O 2 ; carrier gas: Ar 0.5 L/min.

9 Analytical performance Calibration curves for the Cr determination Comparison in the analytical performance between the N 2 -O 2 MIP and a conventional ICP 1) Y. Arai, S. Sato, K. Wagatsuma: Emission spectrometric analysis using an Okamoto-cavity microwave-induced plasma with nitrogen-oxygen mixed gas, ISIJ Int., 53(11) (2013), ) Y. Arai, S. Sato, K. Wagatsuma: Comparative study on the emission spectrometric determination of manganese using nitrogen-oxygen Okamoto-cavity microwave induced plasma and argon radiofrequency inductively-coupled plasma, Microchem. J., 116 (2014) sigma detection limit: Pure N 2 : 0.21 mg/dm 3 N 2 +O 2 : 0.09 mg/dm 3 MIP Line: Cr I nm, Plasma gas: N 2 14L/min+O 2, Carrier gas: Ar 0.5 L/min, Power: 1.02 kw Obs. Height: 10 mm Emission Intensity / arb. unit Calibration curves for Cr I nm at different amounts of oxygen in nitrogen-oxygen mixed gas MIP Chromium Content / mg/dm 3 O L/min O L/min O L/min

10 MIP 3σ DL:0.09 mg/dm 3 ICP 3σ DL:0.39 mg/dm 3 MIP Line: Cr I nm, Plasma gas: N 2 14L/min+O 2 1.5L/min, Carrier gas: Ar 0.5 L/min, Power: 1.02 kw ICP Line: Cr I nm, Plasma gas: Ar 10.5L/min, Carrier gas: Ar 0.55 L/min, Power: 1.0 kw, Obs. Height: 35 mm ICP Line: Cr II nm, Plasma gas: Ar 10.5L/min, Carrier gas: Ar 0.55 L/min, Power: 1.0 kw, Obs. Height: 14 mm Emission Intensity / arb. unit Comparison in the calibration curve for Cr determination between MIP and ICP Chromium Content / mg/dm 3 MIP Cr I nm ICP Cr I nm ICP Cr II nm The analytical performance in the N 2 -O 2 MIP is better than in an ICP. Spatial Distribution of th e Em ission Sptectra Two-dimensional emission images of chromium atomic lines Two-dimensional emission images of band heads of nitrogen molecule (ion) 1) S. Sato, Y. Arai, K. Wagatsuma: Spatially-resolved spectral image of a microwave-induced plasma with Okamoto-cavity for nitridation of steel substrate, Anal. Sci., 30(2), (2014)

11 Two-dimensional Emission Image of Cr I nm Line Two-dimensional images of the intensity of Cr I nm in nitrogen-oxygen mixed gas MIP when the content of oxygen is varied. Sample: Cr 1000 mg/dm 3 ; MV: 800 W; plasma gas: N 2 14 L/min + O 2 ; carrier gas: Ar 0.5 L/min. Spatial distribution of the emission signal of nitrogen excited species Axial Distance /mm N nm N nm Radial Distance / mm Intensity/ arb.unit Simplified energy level diagram of nitrogen molecule 1

12 Two-dimensional Emission Image of N nm Band Head Two-dimensional Two-dimensional images of images the intensity of the of intensity N of N nm in nitrogen-oxygen mixed gas Quenching MIP when the content by oxygen of is molecule: nm in nitrogen-oxygen mixed gas MIP when the content of oxygen is varied. Sample: pure varied. water; Sample: MV: 800 Npure W; water; MV: 800 W; plasma 2* plasma gas: N 2 14 L/min + O 2 ; carrier gas: gas: Ar N L/min. L/min + O 2 ; carrier gas: Ar 0.5 L/min. Drastic decrease in the number density of the nitrogen + O 2 -> excited N 2 + O species + O Penning-type Two-dimensional images ionization: of the intensity of Cr N Nnm in nitrogen-oxygen mixed gas MIP when the content of oxygen is varied. Sample: pure water; MV: 2* -> Cr +* + N 800 W; plasma gas: N 2 + e -, is less caused in 2 14 L/min + O 2 ; carrier gas: the Ar 0.5 oxygen-mixed L/min. gas plasma than in the pure nitrogen plasma. B o ltzm a n n d istrib u tio n of iro n atom ic lin es Comparison in the Boltzmann relationship between pure N 2 and N 2 -O 2 mixed gas MIPs 1) K. Satoh, K. Wagatsuma: Comparative study on contribution of charge-transfer collision to excitations of iron ion between argon radio-frequency inductively-coupled plasma and nitrogen microwave induced plasma, Spectrochim. Acta Part B, 108, (2015) ) K. Wagatsuma: Deviation from Normal Boltzmann Distribution of High-lying Energy Levels of Iron Atom Excited by Okamoto-cavity Microwave induced Plasmas Using Pure Nitrogen and Nitrogen Oxygen Gases, Anal. Sci., 31(6), (2015) in press.

13 40 emission lines of iron atom (Fe I), whose excitation energies range from 3.4 to 6.9 ev, are in the wavelength range of nm. Wavelength Assignment Transition (nm) Upper level (ev) Lower Level (ev) Probability (ga) x10 8 s 1 Fe I d 6 4s4p 5 F d 6 4s 25 D Fe I d 7 4p 5 F d 7 4s 5 F Fe I d 6 4s4p 5 F d 6 4s 25 D Fe I d 6 4s4p 5 F d 6 4s 25 D Fe I d 7 4p 5 F d 7 4s 5 F Fe I d 6 4s4p 5 F d 6 4s 25 D Fe I d 6 4s4p 5 F d 6 4s 25 D Fe I d 7 4p 5 F d 7 4s 5 F Fe I d 6 4s4p 5 F d 6 4s 25 D Fe I d 7 4p 5 F d 7 4s 5 F Fe I d 6 4s4p 5 F d 6 4s 25 D Fe I d 7 4p 5 F d 7 4s 5 F Fe I d 6 4s4p 5 F d 6 4s 25 D Fe I d 6 4s4p 5 F d 6 4s 5 2 D Fe I d 6 4s4p 5 F d 6 4s 25 D Fe I d 7 4p 5 F d 7 4s 5 F Fe I d 7 4p 5 F d 7 4s 5 F Fe I d 7 4p 5 F d 7 4s 5 F F I d F d F Boltzmann plot of iron atomic lines when pure nitrogen is employed as the plasma gas in the Okamoto-cavity MIP. Sample: Fe 1000 mg/dm 3 ; plasma gas: N dm 3 /min; carrier gas: Ar 0.5 dm 3 /min; microwave power: 0.95 kw. in LTE condition in non-lte condition Overpopulation Collisions with energetic electron Thermal Excitation Non-thermal Excitation Penning-type collision with metastables of nitrogen molecule

14 Semi-log plots of the enhancement factor as a function of excitation energy when oxygen is added to the nitrogen plasma Pure Nitrogen Nitrogen + Oxygen Flow rate of O dm 3 /min (open circle) or 1.2 dm 3 /min (filled triangle). Sample: Fe 1000 mg/dm 3 ; plasma gas: N dm 3 /min; carrier gas: Ar 0.5 dm 3 /min; microwave power: 1.02 kw. Addition of oxygen towards LTE condition Normal Boltzmann relationship Schematic energy level diagram of nitrogen molecule, where four electronic states are only illustrated, together with a dissociation level of oxygen molecule and the ionization energy of iron atom. Fe + Fe * Fe + N 2 * Fe + + N 2 + e - Fe + + e - Fe * stepwise de-excitation N 2 * + O 2 N 2 + O + O Decrease of excited nitrogen molecule

15 Summary IMR The emission intensities of Cr atomic lines are enhanced when oxygen gas is added to the nitrogen MIP, while those of Cr ionic lines are reduced. The band spectra of nitrogen molecule species are strongly quenched by adding oxygen gas. The emission of the Cr atomic lines is observed not only from the central channel but from the surrounding zone of the plasma when oxygen gas is added to the nitrogen MIP. The addition of oxygen changes the characteristics of the nitrogen MIP, so that the excited levels of Cr atom having low excitation energy can be favorably populated. In the nitrogen-oxygen MIP, the number density of N 2 excited states decreases due to collisions with O 2,, which predominantly controls the excitation mechanism of analyte species. IMR Thank you very much fo r your kind attention.

16 Instrumentation and the Operating Conditions Operating parameter Description Microwave generator MKN-153-LR-OSC(Nippon Koushuha) Mictowave frequency 2.45 GHz Power supplier KN-153-3T-LR-PS(Nippon Koushuha) with a maximum power supply of 2 kw Working power 1.0 kw Microwave cavity Okamoto cavity(hitachi) Plasma torch (Hitachi) Plasma gas flow rate 14 L/min(N L/min(O 2 ) + 2 ) Observetion height 20 mm above the plasma torch Nebulizer system Pneumatic concentric nebulizer(model , Hitachi) equipped with a double-pass spray chamber Carrier gas flow rate 0.5 L/min(Ar) Spectrometer P5200-ICP emission analysis system (Hitachi) Monochromator Focal length: 0.75 m; grating: 3600 grooves mm -1 ; blaze wavelength: 300 nm; slit width (both entrance and exit): 30 μm Photomultiplier tube R955 (Hamamatsu Photonics Corp.) Experimental apparatus in MIP optical emission spectrometry Cr aqueous solution Scanning monochromator Photomultiplier

17 Two-dimensional Emission Image of Cr I nm Line Two-dimensional images of the intensity of Cr I nm in nitrogen-oxygen mixed gas MIP when the content of oxygen is varied. Sample: Cr 1000 mg/dm 3 ; MV: 800 W; plasma gas: N 2 14 L/min + O 2 ; carrier gas: Ar 0.5 L/min. Two-dimensional Emission Image of N nm Band Head Two-dimensional images of the intensity of N nm in nitrogen-oxygen mixed gas MIP when the content of oxygen is varied. Sample: pure water; MV: 800 W; plasma gas: N 2 14 L/min; carrier gas: Ar 0.5 L/min.

18 MIP: Plasma gas: N 2 14L/min+O 2 1.5L/min, Carrier gas: Ar 0.5 L/min, Power: 1.02 kw Obs. Height: 10 mm Emission Intensity / arb. unit Calibration curves for several Cr I lines in a nitrogenoxygen mixed gas MIP Cr I nm Cr I nm Cr I nm Cr I nm The Cr I nm line Chromium is the best Content analytical / mg/dm line 3 in the N 2 -O 2 MIP.