Can low-temperature point discharge be used as atomic emission source for sensitive determination of cyclic volatile methylsiloxanes?

Supporting Information (SI)

Can Low-Temperature Point Discharge Be Used as Atomic Emission

Source for Sensitive Determination of Cyclic Volatile

Methylsiloxanes?

Yuan Yang,

_{Yao Wang,}

_XiaolingHou,

_{Yao Lin,}

_{Lu Yang,}

_{Xiandeng Hou,}

Chengbin Zheng

†

_{Key Laboratory of Green Chemistry & Technology of MOE, College of Chemistry,}

Sichuan University, Chengdu, Sichuan 610064, China

‡

_{Chengdu Environmental Monitoring Center, Chengdu, Sichuan 610072, China.}

_{National Research Council Canada, Ottawa, Ontario, Canada K1A 0R6}

*

_{Corresponding author:}

Fax: +86 28 85412907; Phone: +86-28-85415810

E–mails:

(C. B. Zheng)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Table of Contents

1.

Information of Analytes

2.

Typical Operation Conditions of the GC-PD-OES

3.

Typical Operation Parameters of GC-MS

4.

Effect of Carrier Gas on the Spectral Response

5.

Typical Chromatograms of GC-PD-OES for cVMSs Analysis

6.

Optimization of Experimental Parameters for PD-OES

6.1

6.2

1.

Information of Analytes

Table S1.

Molecular formula, structures and boil point of silicon species

analyzed in this study.

Name Abbreviation Formula Structure B.P/°C

Tetramethylsilane TMS C4H12Si 27 Hexamethylcyclotrisiloxane D3 C6H18O3Si3 134 Octamethylcyclotetrasiloxane D4 C8H24O4Si4 175 Decamethylcyclopentasiloxane D5 C10H30O5Si5 211 Dodecamethylcyclohexasiloxane D6 C12H36O6Si6 245 Trimethylsilyloxysilane M4Q C12H36O4Si5 106 S3

2.

Typical Operation Conditions of the GC-PD-OES

Table S2.

Operation Conditions of the GC-PD-OES

parameter

value

GC system Agilent 7820A

initial temperature 40 °C (2 min)

ramp rate 20 °C min-1

final temperature 180 °C (1 min)

carrier gas flow rate Ar, 2 mL min-1

injector temperature 200°C

PD-OES

discharge gas flow rate Ar, 80 mL min-1

discharge voltage 4.0 kV

3. Typical Operation Parameters of GC-MS

The concentrations of target compounds in Shampoos were acquired by selected ion monitoring (SIM) of GC-MS analysis using Agilent 7890B gas chromatograph-5977B mass spectrometric detector equipped with HP-5MS column (30 m× 0.25 mm× 0.25 μm). The temperature of GC injector port (splitless mode) was 200°C.

Helium was chosen as the carrier gas at a flow rate of 1.2 mL min-1_{. The MS was}

operated in electron-impact ionization mode (EI) at 70 eV, ion source temperature of 230°C and quadrupole temperature of 150°C. MS parameters for compounds were summarized in Table S3 [1].

Splitless injection was used (1 μL). The GC column oven temperature was programmed at an initial temperature of 40 °C (held for 2 min), then at a rate of

20 °C min−1 _{to 180°C (held for 1 min). Quantifications for all methyl siloxanes were}

based on the responses of the internal calibration standards. The internal

standard used, tetrakis (trimethylsilyloxy) silane (M4Q), was purchased from

Aladdin (Shanghai, China) with a purity >97%.

Table S3.

MS parameters of target compounds and internal standard.

Compounds Abbreviation Quantization

Ion (m/z) Characterized Ions (m/z) Retention Time (min) Hexamethylcyclotrisiloxane D3 207 96, 191,207 3.98 Octamethylcyclotetrasiloxane D4 281 133,265,281 5.71 Decamethylcyclopentasiloxane D5 267 73,267,355 7.01 Dodecamethylcyclohexasiloxane D6 429 73,341,429 8.28 Trimethylsilyloxysilane M4Q 369 147,281,369 7.26 S5

4. Effect of Carrier Gas on the Spectral Response

It is well-known that the type of discharge gas strongly affects the analytical application of PD. No information about the determination of Si by PD-OES is available in earlier literatures. In this work, three different gases, including Ar, He, and N2, were tested as the discharge gas to investigate the effect

of carrier gas on the spectral response. All the tested gases could maintain the PD plasma easily and provided several typical emission bands of OH (283, 309 nm), N2 (358, 380 nm), CN (387 nm), and

NH (337 nm), in agreement with the reported results obtained from PD plasma or other different type plasmas [2-4]. It can be seen that no emission lines around 251.61 nm were observed when Ar and He used as discharge gas, while N2 generates much stronger molecular emission bands around the atomic

emission of silicon at 251.61 nm which resulting in serious spectral interferences on subsequent measurements. Therefore, both Ar and He plasma can be used as background spectrum.

Figure S1. Silicon atomic emission lines and background emission spectra generated with PD-OES

using different discharge gases (N2, He, Ar).Experimental conditions: TMS concentration, 15 mg

L−1_{;discharge voltage, 2.9 kV; discharge gas flow rate, 200 mL min}−1_{; and CCD integration time, 200}

5. Typical Chromatograms of GC-PD-OES for cVMSs Analysis

Figure S2.Typical chromatograms of GC-PD-OES for a mixed standard solution containing 10 mg L−1

D3, 10 mg L−1 D4, 10 mg L−1 D5, 10 mg L−1 M4Q and 10 mg L−1 D6 , their individual standard (for

qualitative analysis, 10 mg L−1_{) and a blank solution.Ar flow rate = 60 mL min}-1_{, discharge voltage =}

2.9 kV, discharge gap = 2 mm.

6. Optimization of Experimental Parameters for PD-OES

6.1 Effect of Ar Flow Rate. Argon acted as not only the discharge gas for plasma but

also the carrier gas to transfer the analytes to the PD plasma. Figure S3a shows

the effects of flow rate of argon discharge gas on responses of D3, D4, D5, and D6.

Lower Ar flow rate resulted in inefficient generation of PD plasma and low efficiency of analyte transport to the PD detector; higher flow rate resulted in significant dilution of analyte in the carrier gas. Therefore, a transport gas flow

rate of 80 mL min−1 _{was selected for all subsequent experiments.}

6.2 Effect of Discharge Voltage.The effect of discharge voltage on response is shown in Figure S3b; responses of the tested cVMSs increased significantly as the discharge voltage increased from 3.3 kV to 4.0 kV and then level off at higher voltage. Lower discharge voltage cannot generate and maintain a stable plasma, whereas the stabilities of responses were deteriorated slightly when a higher voltage was used. Finally, a discharge voltage of 4.0 kV was selected as the optimal input voltage, in consideration of both efficient excitation of the cVMSs and stability of plasma, as well as minimization of power consumption.

6.3 Effect of Discharge Gap. According to previous works [2,5] the discharge gap, which is defined as the distance between the two tungsten electrodes, seriously influenced the analyte responses. Therefore, the effect of the discharge gap was also investigated, as shown in Figure S3c. The maximum responses for four cVMSs can be simultaneously achieved when the gap is 2 mm. Although the microplasma can be maintained steadily in the range of 1−4 mm, the microplasma zone is too small to completely cover the outlet of the GC capillary column as the discharge gap is <2 mm, thus resulting in lower responses. On the other hand, the excitation capability of PD is too weak to excite the analytes when the gap is >4 mm.

Figure S3. Effect of instrumental parameters on the PD microplasma (four cyclic volatile methyl

siloxanes (cVMSs) standard solution, 10 mg L−1_{, all with injection volume, 1μL and CCD integration}

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