Elsevier

Journal of Chromatography A

Volume 1421, 20 November 2015, Pages 123-128
Journal of Chromatography A

Back-flushing and heart cut capillary gas chromatography using planar microfluidic Deans’ switching for the separation of benzene and alkylbenzenes in industrial samples

https://doi.org/10.1016/j.chroma.2015.10.027Get rights and content

Highlights

  • Fast (15 min) separation of industrial styrene monomer and Isoparaffin™ solvent.

  • Flexible heart cut/back flush configuration with microfluidic Deans’ switching.

  • PDMS, PEG and ionic sorbent column phases for robust, low cost separations.

  • Trace analysis of benzene 0.8 mg kg−1 and alkylbenzenes <2.8 mg kg−1 with FID.

Abstract

Planar microfluidic devices coupled with modern electronic pressure control have allowed gas chromatography (GC) practitioners to easily manipulate chromatographic systems to achieve heart cut and back-flushing configurations. These planar microfluidic devices have enhanced the connectivity between different components of GC instrumentation and have improved the inertness and minimised system dead volumes compared to classical chromatographic unions and valves. In the present contribution the setup and configuration of two multidimensional GC (MDGC) platforms is described for achieving the separation and quantification of trace level target C6–C8 alkylbenzenes in styrene monomer and Isoparaffin™ solvents, using flame ionisation detection (FID). The performance of these MDGC platforms indicated excellent retention time (0.2% relative standard deviation, RSD) and peak area repeatability (1% RSD) for all analytes of interest. The limit of detection (LOD) was 0.8 mg kg−1 for benzene in styrene monomer, and 2.4–2.8 mg kg−1 for C6–C8 alkylbenzenes such as benzene, toluene, ethylbenzene and xylene in Isoparaffin™ solvent.

Introduction

Back-flushing and heart cutting are important approaches in capillary gas chromatography (GC) achieved by manipulating the net direction of carrier gas flow at the confluence of two or more capillary columns. Contemporary back-flushing and heart cutting approaches build on the work Deans described 50 years ago [1], [2]. These once seemingly complicated approaches are readily accessible today using precise electronic pressure control (EPC) [3] and advanced capillary column connectivity using planar microfluidic devices [4], [5], [6], [7], [8], [9], [10]. Planar microfluidic devices feature low connection void volumes that enhance carrier gas flow and pressure equilibration response times compared to classical setups. The thermal mass of each device is kept low to reduce the potential for thermal hysteresis during temperature programming, and these devices are chemically deactivated to ensure inert surface chemistry, which prevents chemical activity during GC analysis.

Planar microfluidic devices are particularly useful when coupled with modern electronic pressure control, which is able to rapidly achieve and maintain accurate and precise pressure settings. Instrument control software can determine the pressures required to deliver user defined carrier gas flow rates after compartment temperatures, column dimensions, and connectivity between injector modules, microfluidic devices, and detector modules has been entered. Electronic pressure control and event control have enabled fast and simple implementation of column back-flushing, heart cutting or comprehensive two-dimensional GC configurations, minimising the need for laborious iterative system optimisation [9], [11].

Multidimensional GC (MDGC) is increasingly required to address the complexity of samples to permit accurate compound identification and quantification. By utilising the stationary phase selectivity of two columns it is possible to reduce the probability of false-positive or false-negative results arising from peak co-elutions which enhances the confidence in solute identification based on retention time [12], [13]. The present investigation uses planar microfluidic Deans’ switching to achieve MDGC for the separation and quantification of trace levels of benzene in styrene monomer, and C6–C8 alkylbenzenes in Isoparaffin™ solvent. It is important to monitor styrene and Isoparaffin™ solvent for alkylbenzene content to ensure that these compounds are not incorporated into final products, since these compounds have health and hygiene implications that affects the quality of household, industrial, and automotive products [14], [15], [16].

Styrene monomer is an important industrial chemical that is for the production of synthetic rubber. Styrene is synthesised industrially from benzene, and residual benzene can be found in crude styrene, intermediate process products, or as an undesirable impurity in purified styrene. Analysis of benzene in styrene is normally performed using long, polar stationary phases, such polyethylene glycol (PEG), as specified by ASTM Method D5135-14 [17]. However these methods are not ideal for process monitoring and trace level quantification due to the high probability of false positive measurements arising from incomplete separation, and poor long-term method stability. The upper temperature limit (250 °C) for PEG phases limits the ability to elute high molecular weight compounds that are present in styrene and petroleum derived samples.

Isoparaffin™ solvents are light petroleum products that are derived from petroleum feedstock. They are industrially useful due to their high chemical stabilities, well-defined boiling points, low surface tensions, low freezing points and low electrical conductivities. They are used extensively in industrial applications during fuel refinery, process chemistry and are further used as cleaning agents, functional fluids, and fuels. The analysis of benzene and C6–C8 alkylbenzenes in Isoparaffin™ solvent is complicated by numerous peak co-elutions that are typical of one-dimensional separations. For this reason GC coupled with mass spectrometry (GC–MS) operated in the selective ion monitoring mode (SIM) is often utilised for the analysis of benzene and the alkylbenzenes [18]. GC–MS with SIM alleviates the need for complete temporal separation prior to detection, and reduces the possibility of false positive results. Unfortunately GC–MS systems have a high cost of ownership, which precludes them from being implemented universally; therefore other methods should be explored to complement GC–MS for routine analysis. An alternative MDGC method for the analysis of aromatic compounds in finished gasoline using FID is the ASTM D5580 method which uses a polar 1,2,3-tris (cyanoethoxy)propane (TCEP) column to selectivity trap aromatic compounds, which are then eluted to a polydimethylsiloxane (PDMS) column for additional separation. While this method is effective for the separation of a range of aromatic compounds, the temperature stability of the TCEP phase is limited (145 °C) and prone to contamination by high molecular weight compounds making this method not ideal for routine analysis.

MDGC with a combination of non-polar PDMS columns and highly polar PEG or ionic sorbent PLOT columns can be effective at separating benzene and C6–C8 alkylbenzenes in styrene monomer and petroleum derived samples which lessens the need for expensive MS detection [19], [20], [21], [22]. This approach enabled the quantitation of trace levels of C6–C8 alkylbenzenes compounds in styrene monomer and Isoparaffin™ solvent while using low cost flame ionisation detection. The approach is fast and robust, with potential for deployment in field quality control laboratories where time, space and resources are limited.

Section snippets

Measurement of benzene in styrene monomer

An Agilent 7890A gas chromatograph (Agilent Technologies, Wilmington, DE, USA) equipped with a Spilt/Splitless injector, Agilent 7683B series Automated Liquid Sampler, two FID modules, and an auxiliary Pressure Control Module (PCM) was used for benzene analysis in styrene monomer.

A highly inert, non-polar VF-1ms column coated with 100% dimethylpolysiloxane phase, 30 m × 250 μm ID × 1 μm df (Agilent, #CP8913), was connected between the injector and was connected to the central port of a planar

Deans switch installation and GC configuration

Installing a planar microfluidic Deans’ switch into a GC instrument is fast and straightforward. The Deans’ switch is mounted inside the oven cavity and connected to a three-way solenoid valve and PCM mounted outside the GC oven. The first-dimension column is connected between a GC inlet and the central port of the Deans’ switch, as shown in Fig. 1. Columns are installed using metallic ferrules and nuts to ensure leak free and inert connections. Metallic ferrules ensure that ferrule

Conclusions

Planar microfluidic Deans’ switching coupled with modern electronic pressure control and instrument control software was utilised for low cost, rapid, robust, and repeatable MDGC analysis of trace amounts of benzene in styrene monomer and C6–C8 alkylbenzenes in Isoparaffin™ solvent. MDGC separations utilising non-polar first-dimension columns coupled with polar second-dimension columns were demonstrated to be very effective for the separation of benzene in styrene monomer and benzene and C6–C8

Acknowledgements

Matthew Jacobs is the Recipient of an Australian Postgraduate award 2012 (APA). Robert Shellie is the recipient of an Australian Research Council Australian Research Fellowship (project number DP110104923).

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