Fast PDMS Quantitation Using ICP-OES

Fast PDMS Quantitation Using ICP-OES

By Barbara Pavan and Katherine Robertson

 

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Abstract
The reliable quantification of polydimethylsiloxanes (PDMS) in aqueous media is made challenging by their ubiquitous presence in the environment and their ability to “stick” to surfaces. Herein we present a method developed for quantifying Antifoam C (30 wt % PDMS) in aqueous solutions, by inductively coupled plasma-optical emission spectroscopy (ICP-OES), using a Thermo iCAP 6300 Duo. Our method, which determines the PDMS content from the total silicon, is based on a simple extraction in organic solvents and produced good recoveries for both high and low silicon contents. With this method we proved that PDMS can be measured reliably and with minimal sample preparation (i.e. avoiding acid digestion) via total silicon content.

Introduction
Organosilicon compounds are widely used in a variety of applications ranging from food additives and pharmaceuticals, health and beauty products, to adhesives and sealants, making them ubiquitous in the environment. Their reliable quantification poses an analytical challenge, especially at low concentrations, due to the generally high background of inorganic silicates and the high probability of external contamination from silicone rubber septa, O-rings, lubricants, sealants, tubing, etc. Different analytical techniques have been used to quantify, and in some case identify, organosilicons. Most noticeably, Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) have been successfully employed to determine traces of PDMS in extracts from pharmaceutical preparations or plastics [1,2]. High performance liquid chromatography and gas chromatography were also employed for the analysis of organosilicon compounds from water or extract samples [3,4]. All these methodologies have in common cumbersome sample preparation procedures or complex experimental set-up, which are hard to translate in industrial settings where validation and/or monitoring might be required. Herein we present a method tailored to the rapid determination of PDMS in aqueous matrix, without the need to predigest the sample, thus simplifying the sample preparation. Our approach not only reduced the time associated with the analyses, but it also reduced their costs because there was no need of a microwave digestion apparatus.

Conclusions
• A successful method for measuring Antifoam C in aqueous solutions was developed without the need of sample digestion, but simply by extraction in organic solvent. This allowed the rapid determination of Antifoam C both in low (0-18 µg/g) and high (0-1600 µg/g) concentrations from the measured silicon.

• Three extractions and an additional rinse of the original container produced good recoveries.

• The use of an internal standard is necessary at high silicon concentrations to compensate for the viscosity difference of calibration standards and the samples.

Experimental Sample Preparation

Aqueous solutions at 1000 µg/g and 8 µg/g of Antifoam C (Sigma Aldrich Cat # A8011-250ML) were prepared and approximately 10 mL were transferred into clean 50 mL polypropylene vials. An aliquot of 10 mL of xylenes (Fisher Cat # X5-4 ACS grade) was added, the vial was placed on a low shear mixer for 30 minutes, allowed to stand to phase separate, and finally the xylene layer was removed and analyzed on the ICP-OES. The just described extraction was repeated two more times (for a total of three extractions) in the 50 mL polypropylene vial, each time using a fresh aliquot of 10 mL of xylene. A blank was also prepared
by following this procedure, and using 10 mL of de-ionized (DI) water. Because PDMS easily adheres to surfaces, 10 mL of xylene was added to the original vial containing the sample, after the aqueous solution was discarded, and the vial was placed on a low shear mixer for 30 minutes, and the xylene was then analyzed by ICP-OES.

Instrument Conditions
The ICP-OES analysis was performed using a Thermo Scientific iCAP 6300 Duo spectrometer under the following conditions:

Fig 1 Fast PDMS

The ICP-OES was equipped with a sample introduction system designed for the analysis of organic solvents. A baffled spray chamber, v-groove nebulizer, and 1 mm injector were used to minimize plasma loading. The plasma view was set to radial to reduce interference from the solvent, and Viton® pump tubing was used to insure optimal compatibility with the organic solvent. The instrument was also equipped with an internal standard kit, which allowed the continuous addition of the internal standard solution to both calibration standards and samples alike. A 20 ppm solution of Yttrium in xylenes was used as internal standard.

Calibration
The calibration standards were prepared by using a purchased silicon standard, and diluted by weight with xylene. Two fivepoint calibration curves were constructed, one for low content silicon samples (from 0 to 2 μg/g) and one for high content silicon samples (from 0 to 200 μg/g). Calibration curves were found to be linear with an R2 value greater than of 0.99.

Fig 2 Fast PDMS

Results and Discussion
The calculation used to determine Antifoam C from silicon concentrations is based on the chemical structure of PDMS, and the fact that the MSDS lists PDMS in Antifoam C at 30%. The chemical structure of PDMS is (C2H6SiO)n making silicon approximately 37.8% of the total weight of PDMS.

Fig 3 Fast PDMS

Extraction Efficiency

• The efficiency of extraction was evaluated on 5 Antifoam C solutions prepared at approximately 1000 μg/g.

• Three extractions removed > 90% of silicon from the original solution.

• A fourth extraction would not significantly affect the final measured silicon, and thus the calculated Antifoam C.

Fig 4 Fast PDMS

Low Silicon

• Good recoveries were obtained for the low concentration samples, indicating that the extraction was successful.

• When the samples were prepared directly in xylenes (QC) the recoveries improved, thus indicating that the method conditions were properly set-up.

Fig 5 Fast PDMS

High Silicon

• The rinse of the original container extracted significant amounts of Si.

• The use of the internal standard allowed significant improvements to the recoveries for the high concentration samples.

• The difference in viscosity between the standards and the samples is more relevant at high concentrations of Antifoam C.

Fig 6 Fast PDMS

 

References

[1] Faux, P. Analyst, 1989, 114, 445−449.

[2] Faux, P. Analyst, 1990, 115, 179−183.

[3] Dorn, S. B., Frame, E. M. S. Analyst, 1994, 119, 1687−1694.

[4] Mahone, L. G. et al. Environmental Toxicology and Chemistry, 1983, 2, 307-313.

Acknowledgments

The authors wish to acknowledge N. Martin and M. Murphy for the initial method development work and the low silicon analyses.