How to Detect Dielectric Barrier Discharge Ion Source by Spectroscopy

author: Susan

2022-01-06

How to Detect Dielectric Barrier Discharge Ion Source by Spectroscopy

Introduction
Plasma is composed of ions, electrons and unionized neutral particles. The overall state of matter is neutral. The dielectric barrier discharge ion source uses helium as the excitation gas to produce ionized He⁺, excited He^*, and metastable helium. Atomic He^m, high-energy electrons, and unionized helium atoms. These particles all have a certain energy. For example, the ionized state He⁺ has an ionization energy of 24.6 eV, and the metastable state He^m has an ionization energy of 19.8 eV (2³S₁) or 20.55 eV (2S¹S₀). The typical density of high-energy electrons is 10¹⁰-10¹²cm^-3, They can produce a large number of free radicals, electrons, ions and excited atoms in the process of collision with the surrounding gas molecules. Therefore, in addition to the metastable helium atoms, He^m and the compound molecules in the air (such as water molecules) undergo Penning ionization to produce In addition to hydrate ions and electrons, other particle components will also participate in the characteristic ion process of the sample. Therefore, it is very important to understand the substance composition in the plasma to study the mechanism of DBDI ionization samples.
Challenge
Because the atomic structure of different substances, the energy of the excited state and the upper and lower energy levels of the spectral transition are different, the type of particles and the energy state can be determined according to the plasma emission spectroscopy, and the plasma composition can be qualitatively determined. Analysis, this experiment uses the spectroscopy system to study the emission spectra of helium and argon plasmas, combined with the mass spectra of the actual samples, and elaborates the composition of their plasma and the reaction mechanism between particles, and combines the plasma The components in the sample feature ion mechanism are discussed.
Experiment System

1. Reagents and materials: Methanol, Acetonitrile, distilled water, microelectrode glass capillaries (glass capillaries, B10024F, od=1mm,id=0.59mm, L=100mm), helium and argon.

2. Instruments and devices

DBDI-100 ion source, DART-SVP, the mass spectrometer is Thermo Finnigan LTQ ion trap mass spectrometer, the optical fiber spectrometer is Optosky ATP2000P (customized, measuring range 200nm-1100nm), and the low-temperature plasma experimental power supply CTP-2000K.
When measuring plasma spectra, ATP2000P and DBDI-100 ion source are placed at 90° vertically, as shown in Figure 1(a) below. The distance between the fiber spectrometer and the ion source nozzle is 1cm horizontally, and the vertical distance from the plasma is 1cm, and the helium discharge is measured. For plasma spectroscopy, the helium flow rate is set to 4L/min, the heating temperature is 100°C, and the ATP2000P integration time is set to 150ms. When measuring the plasma spectrum generated by argon discharge, CTP-2000K is used to power the ion source, as shown in Figure 1(b), the output voltage of CTP-2000K is set to 11KV, the flow rate of argon is set to 4L/min, and the heating temperature is 100 ℃, ATP2000P integration time is set to 2000ms. When measuring actual samples, the helium flow rate of the DBDI-100 ion source is also set to 4L/min, and the heating temperature is 100°C. The ion source is placed horizontally and sprays the plasma beam toward the inlet of the mass spectrometer. The outlet of the ion source is 2cm away from the inlet of the mass spectrometer. , Use capillary siphon injection to make the distance between the sample end of the capillary and the injection port of the mass spectrometer 0.5cm, as shown in Figure 1(c) below.

Fig.1 (a) Schematic of measuring plasma spectrum (b) plasma produced by DBDI-Ar (c) measuring actual samples

Test Results

1. Emission spectrum of He-DBDI plasma in air
The spectrum of the plasma generated by DBDI-100 using helium as the discharge carrier gas is shown in Figure 2. From the figure, it can be observed that a series of spectral lines (337nm, 357nm, 380nm, 391nm) are released after nitrogen is excited and returned to the ground state , 427nm, 470nm) and multiple spectral lines of He atoms (587nm, 667nm, 706nm and 728nm). Among them, the intensity of the 706nm line of He atom
and the 391nm line of the first negative band line of N₂⁺ is the highest.

Fig.2 The emission spectra of He-DBDI plasma in air

The 706nm spectrum line of He atom is emitted when He (3³S₁) transitions to He (2³P₁) and He (2³P₂);
The first negative band line of N2+ (391 nm, 427 nm and 470 nm) is mainly emitted by the transition of

; The N2 second positive band line (337nm, 357nm and 380nm) is mainly emitted by the transition of

.
Generally, the wavelength range of visible waves is between 400 and 760 nm, and different wavelengths cause different color perceptions in the human eye. The human eye colors corresponding to different wavelengths are shown in the table below.

It can be seen from the table that the 391nm, 427nm, 470nm, 337nm, 357nm, and 380nm spectra generated by the nitrogen gas returned to the ground state after being excited belong to the blue-violet range, which is consistent with the observation that the plasma turns into a blue-violet phenomenon.

2. He-DBDI verifies the proton affinity of different samples
The mass spectra of water, water-methanol (v/v=1:1) mixed solution, methanol-acetonitrile (v/v=1:1) mixed solution and acetonitrile collected by mass spectrometry are shown in Figure 3.

Fig.3 (a)Mass spectrum of background signal by He-DBDI-MS, (b)mass spectrum of aqueous solution by He-DBDI-MS, (c) mass spectrum of water and methanol (v/v = 1:1) mixed solution by He-DBDI-MS,
(d)mass spectrum of acetonitrile solution by He-DBDI-MS, and mass spectrum of methanol and (e) acetonitrile (v/v = 1:1) mixed solution by He-DBDI-MS

3. Emission spectrum of Ar-DBDI plasma in air

The spectrum of plasma produced by argon ionization is shown in Figure 4. From the figure, we can observe the second positive band line of N₂ (337nm, 357nm and 380nm) and a series of spectral lines of Ar atoms concentrated in the range of 690nm-800nm. Within (696nm, 697nm, 706nm and 762nm, etc.), the intensity of the 761nm spectral line of Ar atoms and the 357nm spectral line of the second positive band of N₂ is the highest, but the intensity of the spectrum is much weaker than that of plasma produced by helium ionization And there is no N₂⁺ first negative band line (391nm, 427nm and 470nm).

Fig.4 The emission spectra of argon plasma in the air

Among all the spectral lines of argon, the spectral lines with wavelengths of 696.5nm and 763.5nm are produced by the high-energy metastable Ar^m(³P₂) transition of argon, and the 772.4nm spectral line with wavelength is the same as the high-energy metastable state of argon atoms. Produced by the Ar (³P₀) transition, the intensity of these lines in the spectrum is relatively high.
The N2+ first negative band line (391nm, 427nm and 470nm) is not observed in the emission spectrum of Ar. This is because the N2+ first negative band line (391nm, 427nm and 470nm) is mainly emitted by the transition of

; The second positive band line of N2 (337nm, 357nm and 380nm) is observed in the emission spectrum of Ar, which is mainly emitted by the transition of

4. Comparison of mass spectra of Ar-DBDI and DART ionized samples

Fig.5 (a) Mass spectra of methanol by Ar-DBDI-MS, (b)Mass spectra of methanol by Ar-DART-MS,
(c)Mass spectra of acetonitrile by Ar-DBDI-MS, (d)Mass spectra of acetonitrile by Ar-DART-MS

Conclusion

Helium plasma includes not only ionized He⁺, excited He^*, and metastable helium atoms He^m, but also include , , and . In addition, the ability of proton transfer between substances with different proton affinity was verified. The greater the proton affinity, the stronger the ability to take away protons.
The argon plasma contains not only ionized Ar⁺, excited Ar^*, and metastable argon atoms Ar^m, but also and . Since the energy of the metastable Ar^m is less than the ionization energy of water, Ar^m cannot ionize water, but the sample can be ionized by adding an energy-assisted solvent with an ionization energy lower than Ar^m;
When DBDI ionizes a sample, the proton affinity of the analyzed sample molecule must be greater than that of the solvent molecule. Because there are a large number of water molecules in the environment, DBDI cannot analyze water with a lower proton affinity when ionizing the sample. The molecular system of proton affinity.
It can be seen from the above process that the ionization mechanism of DBDI is similar to the transient microenvironment mechanism of DART, but the difference is that the plasma of DART contains He^m and (H₂O)_n H⁺, while the plasma of DBDI is in addition to He^m and (H₂O)_n H⁺, there are He⁺ and the excited state He^*, in addition to a large number of high-energy electrons.

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