Elsevier

Talanta

Volume 144, 1 November 2015, Pages 427-431
Talanta

Modification of hydrophobic sorbents by cobalt chloride in order to concentrate low molecular polar organic substances from the air for subsequent gas chromatographic determination

https://doi.org/10.1016/j.talanta.2015.04.069Get rights and content

Highlights

  • We propose cobalt chloride as a modifying agent of hydrophobic sorbents (activated carbon, carbon nanotubes, porapak).

  • Modified sorbents exhibit 5–10 times higher retention capacity for methanol and ethanol than the original ones.

  • Modification of some sorbents has little influence on the retention of non-polar and weakly polar compounds.

Abstract

The article presents a new method of modification of hydrophobic sorbents. To improve sorption pre-concentration of polar organic compounds in the air analysis, these sorbents are coated with cobalt chloride. This modification increases retention volume of lower alcohols by 5–10 fold as compared to that of unmodified sorbents and solves the problem of gas-chromatographic determination at 1–2 ppb (micrograms/m3) by using the most common flame ionization detector. It should be noted that the modification of hydrophobic sorbents by cobalt chloride has little influence on their porosimetry parameters (specific surface area, proportions of meso- and micropores) and modified sorbents are capable of retaining hydrophobic nonpolar and weakly polar analytes as well as original unmodified sorbents. Thus, a fairly simple procedure leads to a large positive effect.

Introduction

Determination of organic compounds in the air at the limit level and at background concentrations usually includes a step of pre-concentration. Low molecular weight organic compounds, which are discussed in the article, are usually present in the air in their vaporized state. The main methods of concentrating vapors of volatile organic compounds (VOCs) are cryogenic concentration [1], [2], [3], solvent extraction, performed in the impinger [4] and denuders [5], [6]; active [7], [8] and passive [8], [9] sorption of substances by sorbents contained in cartridges or tubes, as well as solid-phase microextraction (SPME) [10], [11].

SPME and passive sampling are more suitable for prolonged atmospheric air control, rather than for operative monitoring the concentration of VOC, because of their inherent slow mass transfer. Solvent extraction suggests the use of liquid analysis methods and it is not suitable for determination of readily volatile substances such as methanol. A serious problem of cryogenic VOC pre-concentration is the interfering effect of moisture that condenses in the trap. Active sampling on solid sorbents (dynamic sorption) is the most common and effective method of low molecular weight VOCs pre-concentration in the air analysis [7], [8], [12], [13]. This is done by passing the analyzed air sample through a tube filled with sorbent. The absorbed VOCs are then desorbed by heating into a flow of carrier gas and transported to a gas chromatograph [13], [14].

The greatest difficulties arise in the sorption pre-concentration of low molecular weight polar VOCs, such as methanol. All the known sorbents can poorly retain similar substances at room temperature. In this context previously suggested in gas chromatography sorbents based on salts of transition and alkaline earth metals appear to be the most promising [15]. In this context previously in gas chromatography suggested sorbents based on salts of transition and alkaline earth metals appear to be the most promising. However, these sorbents cannot provide retention of non-polar substances. According to the literature [16], these salts have a high solubility not only in water, but also in polar organic solvents. These solvents allow us to apply salts to non-water wet hydrophobic sorbents which are widely used for sorption pre-concentration of organic vapors in the air analysis [17].

The aim of this study was to evaluate the possibility of modifying the hydrophobic polymer sorbents by sorption-active, non-porous salts (SANS) and to assess their sorption properties with regard to retention of polar and non-polar volatile organic compounds.

Section snippets

Reagents and preparation of solutions and model gas mixture

All the chemicals used in this work were of analytical reagent grade (Vekton Co. Ltd, St. Petersburg, Russia). Solutions of organic substances were prepared by a volume–volume methodology. Aliquots of analytes were collected using a measuring pipette, administered into a volumetric flask, wherein the level was brought to a flask label by addition of the required liquid (water, petrolatum oil). If necessary solutions thus prepared were diluted by pure liquid.

Sorbents

The study involved modification of

The laws of VOCs retention from the gas phase by sorbents modified with cobalt chloride

Previously [20], we have found that many completely dehydrated salts of transition and alkaline earth metals have the property of strong retention of polar VOC vapors. The most probable mechanism of this retention is the ion–dipole interaction between the VOCs molecules and nodes of the salts crystal lattice. A detailed study showed that Mg(ClO4)2 provides the strongest retention of methanol vapor and acetone. However, this salt completely loses water of crystallization at temperatures above 250

Conclusion

We propose a simple method for modification of hydrophobic sorbents to increase their sorption capacity of the low molecular weight polar organic compounds from the gas phase. It has been established that the application of 10–20% of cobalt chloride(II) of sorbent mass results in the increase of sorption parameters of alcohols 5–10 times. Upon that, the application of salt has little effect on the retention volumes of nonpolar and weakly polar components. The negative effect of water vapor can

Acknowledgments

The authors thank Russian Foundation for Basic Research, Russian Federation (Project 15-03-05082a) for the support of this work. Studies were carried out using the equipment centers SPSU: Centre for Innovative Technologies of Composite Nanomaterials and Chemistry Education Centre.

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