Direct Measurement of Elemental Mercury using Multi-Dimensional

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Direct Measurement of Elemental Mercury using MultiDimensional Gas Chromatography with Microwave Induced Helium Plasma Atomic Emission Spectroscopy Ronda Gras, Jim Luong, and Robert A. Shellie ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00008 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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ACS Earth and Space Chemistry

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Direct Measurement of Elemental Mercury using Multi-Dimensional Gas

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Chromatography with Microwave Induced Helium Plasma Atomic Emission

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Spectroscopy

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Ronda Gras,1,2,3 Jim Luong,1,2 and Robert A. Shellie*2,3,4

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1

Dow Chemical Canada ULC, Highway 15, Fort Saskatchewan, Alberta T8L 2P4, Canada

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2

Australian Centre for Research on Separation Science (ACROSS), University of Tasmania,

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Private Bag 75, Hobart, Tasmania 7001, Australia

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3

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Tasmania, Private Bag 75, Hobart, Tasmania 7001, Australia

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4

ARC Training Centre for Portable Analytical Separation Technologies (ASTech), University of

Trajan Scientific and Medical, 7 Argent Place Ringwood 3134 Australia

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* Correspondence to: [email protected]

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ACS Paragon Plus Environment

ACS Earth and Space Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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Microwave induced helium plasma atomic emission spectroscopy permits direct measurement of

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picogram levels of elemental mercury in various matrices when combined with multi-

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dimensional gas chromatography. Two columns with different stationary phases provide

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excellent separation for elemental mercury and multi-dimensional analysis improves the

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reliability, performance, and system cleanliness of atomic emission detection. The possibility of

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false positive identification is substantially eliminated and excellent sensitivity for the target

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compound was attained with the use of two selective columns and atomic emission detection at

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254 nm. A flame ionization detector was incorporated as part of system configuration to

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increase analytical platform capability and flexibility. Elemental mercury was measured in gas

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matrices over a range of 0.1−170 µg/m3 having a correlation coefficient of R2 = 0.9995, a

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precision of less than 5% RSD (n = 10), and a measured recovery exceeding 99% in natural gas

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as a model matrix. The total analysis time is less than 10 min. Only a small one mL sample

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volume is needed and the described approach does not rely on any form of sample enrichment.

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The utility of multi-dimensional gas chromatography with microwave induced helium plasma

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atomic emission spectroscopy is demonstrated with challenging industrial applications such as

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the measurement of elemental mercury in natural gas, industrial solvents, and vapour generated

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from ruptured compact fluorescent light bulbs.

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Keywords: Elemental mercury, multi-dimensional gas chromatography, natural gas, solvents,

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CFL bulbs, organometallics, flame ionization detector

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Introduction

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Detectors combining plasma excitation with optical emission spectroscopy have been used to

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detect a wide variety of analytes with great success including the measurement of organometallic

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mercuric compounds in environmental and biological matrices [1-25]. Unlike the ubiquitous

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flame ionization detector (FID) which measures simple gas phase carbon-containing ions created

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in a hydrogen flame, the atomic emission detector (AED) has much broader applicability. AED

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is based on detection of atomic emissions. Molecules eluted from the chromatographic column

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enter a helium plasma where the high temperature is sufficient to break bonds into individual

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atoms. The atoms are excited to a higher electron excited state and specific frequencies of light

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as they transition to lower electron states. The strength of AED lies in the ability to

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simultaneously determine 23 different atomic emissions in analytes that are amenable to gas

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chromatography, like organometallics [26-38]. Therefore a great deal of sample information can

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be achieved with a single analysis. AED is quite sensitive for mercury emission; with a reported

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detection limit in the range of 0.1 pg Hg/s at either 185 nm or 254 nm and selectivity in the range

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of 10,000 [39].

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An AED consists of three major hardware components: a microwave-induced helium plasma

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light source (MIP), a spectrometer, and a photodiode array (PDA) assembly for light detection.

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The microwave helium-induced plasma light source is generated in a water-cooled plasma

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discharge tube contained in a novel re-entrant cavity [40]. Water cooling is critical to prolonging

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the life of the plasma discharge tube from either becoming chemically active or rupture of the

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tube itself. A typical plasma discharge tube can last more than one month of operation under

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proper care. The cavity is waveguide-coupled to a magnetron supplying microwave power. This

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arrangement provides for a stable atomic-emission source that does not require tuning of

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microwaves. An elliptical mirror in the spectrometer focuses light (160-800 nm) from the

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plasma onto a slit. The light is then dispersed according to wavelength by a diffraction

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holographic grating. Each wavelength is focused to a point on a focal curve. The PDA, a

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multichannel optical sensor, can be positioned at any point along the focal curve to measure

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specific wavelengths and thus, the 23 different elements, like chlorine, sulfur, hydrogen, and

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others may be detected. The elements are allocated into different element groupings having

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emission lines within about a 30 nm span of the PDA and using the same reagent gas



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composition allows for the simultaneous detection of up to seven elements in a single analysis.

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This provides a significant advantage over a photomultiplier tube, which can monitor only one

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wavelength at a time. The plot of the detector output at a particular wavelength over time

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produces a chromatogram.

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Although AED is selective, the formation of excess soot or fly-ash as a result of atomizing

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carbon compounds can be quite problematic, leading to false positives from nearby intensive

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emission lines such as C 193 nm. The soot or fly-ash deposit can foul components of the optics

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and may contaminate the plasma discharge tube resulting in severe peak tailing, decreased

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analytical performance, and increased the cost of ownership. To this end, the benefits of

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hyphenating the detection system with a separation approach can be quite beneficial. Critical

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features of MDGC are: 1) improved separation through harnessing the additional

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chromatographic peak capacity from two different stationary phases, 2) the option to selectively

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transfer (i.e. heart cut) only the analytes of interest to the AED, and 3) the capability to backflush

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the matrix to vent if required preserves system fidelity. Recent studies have demonstrated the

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benefits of multi-dimensional gas chromatography (MDGC) to maximize and maintain system

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cleanliness in GC - mass spectrometry instrument platforms [41-44].

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Since elemental mercury is highly amenable to analysis by gas chromatography (GC) [45], it is

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possible to leverage the known benefits of MDGC to overcome the abovementioned limitations

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of AED. Here we introduce an MDGC-MIP-AED approach for the direct measurement of trace

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level elemental mercury in challenging matrices. A flame ionization detector was also

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incorporated as part of the MDGC configuration for matrix profiling and system flexibility. Hg0

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can be well separated from complex matrices with a boiling point equivalent of up to nC24 (284

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⁰C). No pre-concentration step is required, and only a small sample volume of one mL is

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needed.


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Experimental

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All analyses were performed using an Agilent 6890N (Agilent Technologies, Wilmington,

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Delaware, USA), equipped with a split/splitless inlet, one flame ionization detector, and an

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auxiliary pressure control module. An Agilent G-2350 Atomic Emission Detector (AED) was

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used.

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A SilFlow five-port planar microfluidic device, PN# 123726 (Trajan Scientific and Medical,

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Ringwood, Australia) was incorporated into the GC. Pressure switching was conducted with a

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three-port, 24V 5W DC switching valve (Agilent G-2399-60610) and support kit (Agilent G-

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2399-67610). The valve was connected to the five-port device with SilFlow Stainless Steel

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Capillary Tubing (Trajan) 1.1 mm outside diameter, sleeved to 1/16 " at one end for connection

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to the switching valve.

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For the gas samples, manual injections were conducted using a 1 mL Gastight Syringe,

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PN#008131 (Trajan Scientific and Medical). To eliminate the potential of over-pressurization of

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the inlet which could result in the sample contacting the septum or being diverted through the

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septum vent stream, a slow but steady injection technique was used. Liquid samples were

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injected using an Agilent7683B Automated Liquid Sampler equipped with a 10 µL liquid

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syringe; sample size was 1 µL.

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The carrier gas was helium (He). A split/splitless inlet in split mode with a 4 mm ultra-inert liner

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at a split ratio of 5:1. The inlet pressure was 31 psig at 250 oC. In the backflush mode, the inlet

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was held at 31 psig (4.0 min) to 0.2 psig @ - 99 psig/min and maintained at 0.2 psig for 4 min.

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The auxiliary pressure for Deans switching was maintained at 20.7 psig throughout the entire

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analysis. The flame ionization detector temperature was set to 250 oC, with hydrogen flow at 35

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mL/min, air flow at 350 mL/min and nitrogen flow at 25 mL/min. The temperature was

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programmed from 60 oC (2 min) to 280 oC @ 20 oC/min (2 min). A Deans Switch Calculator

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PN# G2855-80010 (Agilent) was used to calculate the target column dimensions and pressures.

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A 20 m × 0.25 mm id × 1 µm VF-1ms UI™ GC column was used in the first-dimension and

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operated with a flow rate of 2.5 mL/min He. A 30 m × 0.32 mm id × 1 µm VF-200ms™ GC



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column was used in the second-dimension and operated with a flow rate of 5.0 mL/min He. A

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1.5 m × 0.15 mm id uncoated deactivated fused silica column was used for flow balancing.

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Data were collected with Agilent ChemStation software A.10.02. Figure 1 shows a simplified

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diagram of the analytical system. The AED transfer line temperature and cavity temperature

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were both operated at 250 oC. Helium makeup gas flow rate for the AED was set at 180

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mL/min. The optics are purged with filtered dry nitrogen at 0.5 L/min. AED reagent gas

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pressures were set by the gas flow controller module used. Hydrogen and oxygen plasma

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pressures were set at 30 psig each. Carbon was measured at 248 nm while Hg was measured at

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254 nm with this reagent gas mixture. Other elements that are measured under the same settings

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are silicon at 252 nm, lead at 261 nm, germanium at 265 nm, manganese at 259 nm, and tin at

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271 nm.

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Carrier and utility gases such as helium, nitrogen, hydrogen, and air used for system performance

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studies were acquired from Air Liquide (Edmonton, Canada). A test mixture containing 10 mg/g

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nC8, 500 µg/g of aliphatic hydrocarbons from nC10 to nC16, 100 µg/g of nC18 to nC24, and 500

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µg/g of benzene, toluene, ethyl benzene, and xylenes in n-hexane was prepared in-house.

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Chemicals and solvents used for syringe cleaning and dilution such as hydrocarbons, aromatics,

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and hexane were acquired from Sigma-Aldrich (Oakville, Canada). Compact fluorescent light

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(CFL) bulbs were acquired from a local thrift store located in Fort Saskatchewan, Canada.

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Hg0, 99.9999% grade, PN#261017 (Sigma-Aldrich) was placed in a sealed vial filled with

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mercury-free air, which was equilibrated in a water-filled Dewar flask. The injection of known

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amounts of air saturated with mercury vapor into the analytical system was used for calibration.

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The vapor densities of Hg0 in air at 1 atm and various temperatures (T) were calculated by using

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the ideal gas law and mercury vapor pressure fitted to a linear least squares equation in log vp vs

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1/T [r]. One milliliter of air at 20 °C and 1 atm at equilibrium over Hg0 is equal to 13.19 ng of

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Hg0. Secondary standards were prepared from the primary standard by serial dilution.

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Mercury-free air was generated using a Parker Balston Zero Air generator model HPZA-7000 to

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remove any mercury or chromatographic interferences present.


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RESULTS AND DISCUSSION

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The developments, benefits, and impact of planar microfluidics based multi-dimensional gas

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chromatography have been reported elsewhere in the literature [41-44]. One critical requirement

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for successful operation is that the Deans Switch pressures are well balanced to ensure the device

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functions as a switch and not a splitter. This is particularly important in the present instrument

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platform because the AED plasma cavity is at an elevated pressure at approximately 16.2 to 16.7

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psia, whereas the FID is 14.7 psia. Proper choice of dimensions of the restrictor capillary

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between the Deans Switch and FID ensure appropriate pneumatic conditions for stable operation.

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Figure 2 shows chromatograms of a liquid test mixture of aromatics and hydrocarbons with the

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effluent of the column being sent solely to the FID. All the compounds were measured only on

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the FID channel (Figure 2c). Using the same test mixture, the capability to quarantine the solvent

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in the first dimension is clearly demonstrated in Figure 3. The column effluent is diverted to the

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AED channel at 3 min after the solvent is eluted. No peaks reach the FID after 3 min while on

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the AED, the presence of aromatics and hydrocarbons were detected on the C 248 nm carbon

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channel.

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The strength of AED lies in its ability to simultaneously determine the atomic emissions of

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different elements in those analytes eluted from the column. Natural gas can contain many trace

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metals, the majority of these entrained elements exist as nano-particles which can be removed by

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mechanical filtration [46]. Elemental mercury; however, remains in the gas phase and can be

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directly measured concurrently with the hydrocarbons in natural gas. As an illustration, Figure 4

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shows two chromatograms simultaneously collected at 248 nm for C and 254 nm for Hg for a

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sample of 6.3 µg/m3 Hg0 in natural gas is directed to the AED. As shown by the chromatograms,

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the classical profile of commercially available natural gas (C1 to C7s) was detected on the C 248

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nm chromatogram. In contrast, on the Hg 254 nm chromatogram, only Hg0 was detected

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demonstrating the high degree of selectivity for Hg0 as well as the enhanced analytical capability

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AED has to offer. The absence of response in Figure 3b for aromatics and hydrocarbon

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compounds on the mercury selective 254 nm channel also demonstrates AED selectivity. The

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incorporation of MDGC is an advantage for AED system cleanliness and stability, as only the

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effluent fractions of interest are transferred to the AED while the rest of the analysis is left on the

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monitoring channel (FID). This strategy is successful for the determination of Hg0 as it



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minimizes the amount of soot/fly-ash accumulated on the plasma discharge tube and the optics of

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the detector. Deposition of carbon in the discharge tube can lead to asymmetric peaks. Figure

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S1 shows a photograph of a discharge tube that is in operation both with and without carbon

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deposits. In most cases, a tube replacement is required; however, on occasion, an on-line

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oxidation process can clean the plasma tube through the successful combustion of carbon with

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oxygen as reagent gas. Figure S2 demonstrates the impact of carbon deposits of

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chromatographic peak symmetry before and after carbon removal.

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Until recently, analyzing elemental mercury by GC was met with trepidation as analysts

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anticipated difficulties in method development and lengthy process optimization. This is partly

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due to the reactivity of Hg0 along the sample flow path, with untreated metal surfaces, and

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especially with inter-column interfaces such as unions and connectors. To ensure successful

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performance, components involved in the sample flow path including the Deans Switch device

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used for MDGC have been chemically deactivated to maximize inertness. Two inert column

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phases with dissimilar selectivity were chosen; dimethyl polysiloxane and trifluoropropyl methyl

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polysiloxane. With MDGC, undesired compounds can be quarantined in the first dimension.

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Figure 5 shows chromatograms of C 248 nm and Hg 254 nm of the vapor phase above an

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aliphatic solvent comprising of C8 to C11 branched and straight chain hydrocarbons to which 10

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µg/m3 of Hg0 has been spiked. On the C 248 nm chromatogram, only carbon-containing

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compounds that were transferred within the first 2.5 min were detected while the rest of the

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matrix of the sample was quarantined to the first-dimension column and detected by FID. Hg0

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was accurately detected using the two-dimension separation, demonstrating the high degree of

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flexibility and enhanced analytical capability of the total analytical system.

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Analytical figures of merit for the MDGC-MIP-AES approach are demonstrated. Figure 6

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shows an overlay of three standards of 1.4, 8.3, and 61 µg/m3 of Hg0 in air. Much better peak

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symmetry was obtained for Hg0 when compared to previously reported results [45]. The

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improved peak shape was a direct result of improvements in system inertness and the elimination

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of void volume in the system. A repeatability of < 5% (n = 10) was obtained at two different

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concentrations, namely, 8.3 µg/m3 and 83 µg/m3 of Hg0 in natural gas. With a signal to noise

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ratio of 3:1 using peak area measurements, an instrument detection limit of 50 ng/m3 was

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achieved. Sample detection limit is consistent with the detection limit of the instrument using a


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standard. The gas sample must be collected in an inert vessel. Preferably the sample should be

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analyzed at a temperature about 20 ºC to ensure accuracy. The linear range of quantification was

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generated with seven standards over the range of 100 ng/m3 to 170 µg/m3, with a correlation

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coefficient of R2 = 0.9995. The average recovery of Hg0 was measured over five spiked

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samples, ranging in concentration from 5 µg/m3 to 60 µg/m3, and found to have a measured

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recovery of 99% (n = 3). In Figure S3, the chromatogram generated from a sample of mercury-

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free air was analyzed. No response to Hg⁰ was detected, highlighting the system integrity. The

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utility of the described approach was demonstrated by using the technique to determine Hg0 in

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compact fluorescent light bulbs. Compact fluorescent light bulbs are used in many parts of the

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world as an effort to reduce energy consumption and accidental breakage or improper disposal of

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can lead to the release of Hg0 into the environment. Figure 7 shows an overlay chromatogram of

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C 248 nm and Hg 254 nm of the vapour of a generic compact fluorescent light bulb immediately

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after it has been released in an enclosed 1000 mL polyethylene vessel. No hydrocarbons were

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detected in C 248 nm while Hg0 was present at a concentration of 6.4 µg/m3 in the vessel. Total

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mercury content detected was 6.4 ng per bulb.

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The full potential of MIP-AES can be realized by hyphenating MDGC with MIP-AES. This

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approach has the possibility of “sparking” the development of new analytical methodologies.

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The inherent element specific and compound independent calibration characteristics of the AED

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can assist in simplifying the calibration process. As mercury response is independent of the

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molecular composition, other volatile mercury containing compounds like methyl mercury,

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phenyl mercury, or unknowns can be quantified with an independent single compound

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calibration. Thus, elemental mercury can be used to quantify other mercury-containing

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compounds that are either difficult to obtain or to avoid of use of highly toxic molecules [29-35].

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Among the many techniques for determination of mercury [1-25] cold vapor atomic absorption

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spectroscopy (CVAAS) and cold vapor atomic fluorescence spectroscopy (CVAFS) [21-25] are

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most commonly employed. CVAAS and CVAFS are approved methods by health and

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environmental agencies such as ASTM, OSHA, USEPA, and others. These techniques are

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popular, because of their sensitivity and simplicity in addressing liquid and solid matrices where

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mercury can be found. More often than not, a large sample volume ranging from 10 mL to 1 L

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and an enrichment step is required to reach appropriate method detection limits, which range



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between 0.2 ng/m3 and 1 ng/m3. With large sample volume and enrichment these techniques

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have limits of detection for total mercury on the order 250 times lower than the approach

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described herein. However, they may not be suitable for mercury speciation, and cannot generate

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additional information about the sample content.

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CONCLUSIONS:

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A novel approach for direct measurement of Hg0 by incorporating MDGC with MIP-AES has

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been successfully developed and implemented. This approach is suitable for a variety of

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matrices including ambient air, inert atmospheres, and fuels up to nC24. Detection of Hg0 in

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various matrices was conducted directly without sample pre-concentration over a range of

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0.1−170 µg/m3 with a one mL sample. The incorporation of MDGC enhances the

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chromatographic resolution of Hg0 against possible interferences, enabling the analytical system

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to selectively transfer the analyte of interest to the atomic emission detector, and either

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quarantine the matrix to the first dimension or backflush to vent. These capabilities substantially

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increase the robustness of the detector against coking from soot and fly-ash generated by carbon-

248

containing compounds.

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Planar microfluidic based multi-dimensional gas chromatography when coupled with MIP-AES

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propels this detection technology to new heights and can be employed in different and

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challenging applications.

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ACKNOWLEDGMENTS

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The authors would like to acknowledge Professor Dr. Paul Haddad, of the Australian Centre for

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Research on Separation Science (ACROSS) for his support and guidance. Special thanks to Mr.

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Mark Misunis of the Canadian Food Inspection Agency for his technical support and fruitful

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discussions on the topic of MIP-AES. Dr. Wayde Konze, Dr. Tonya Stockman, and Dr. Yujuan

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Hua of the Dow Chemical Company are acknowledged for their support. Dr. Matthias Pursch

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and Mr. Myron Hawryluk, also of the Dow Chemical Company are acknowledged for their

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assistance in preparing the manuscript. This work was supported by the ARC Industrial

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Transformation Training Centres program (IC14010002).


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SUPPPORTING INFORMATION

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Three figures



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List of Figures

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Figure 1: A simplified diagram of the analytical system.

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Figure 2: A test mixture of aromatics and hydrocarbons with the effluent of the column being

266

sent solely to the FID.

267

Figure 3: A test mixture of aromatics and hydrocarbons with the capability to quarantine the

268

solvent in the first dimension.

269

Figure 4: Two chromatograms collected at two wavelengths of 248 nm for C and 254 nm for

270

Hg⁰ of 6.3 µg/m3 Hgº in natural gas.

271

Figure 5: Chromatograms of C 248 nm, Hg 254 nm, and FID of a 10 µg/m3 of Hg0 spiked into

272

an aliphatic solvent.

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Figure 6: Overlay of 3 standards of 1.4, 8.3, and 61 µg/m3 of Hg0 in air.

274

Figure 7: Chromatograms of C 248 nm, Hg 254 nm and FID of the vapour of a generic CFL

275

bulb immediately after it has been released.

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Figure S1 A photograph of a discharge tube in operation both with and without carbon deposits.

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Figure S2 The impact of carbon deposits in a plasma tube on the peak symmetry.

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Figure S3 Chromatograms of mercury-free air.


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Figure 1: A simplified diagram of the analytical system 188x92mm (96 x 96 DPI)



Figure 2: A test mixture of aromatics and hydrocarbons with the effluent of the column being sent solely to the FID, 1% nC8, 500 ppm BTEX, 500 ppm nC10 to nC16, 100 ppm nC18 to nC24 248x158mm (150 x 150 DPI)


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Figure 3: A test mixture of aromatics and hydrocarbons with the capability to quarantine the solvent in the first dimension. 264x159mm (150 x 150 DPI)



Figure 4: Two chromatograms collected at two wavelengths of 248 nm for C and 254 nm for Hg of 6.3 µg/m3 Hg0 in natural gas. 249x99mm (96 x 96 DPI)


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Figure 5: Stacked chromatograms of C 248 nm, Hg 254 nm and FID of a 10 µg/m3 of Hg0 spiked into an aliphatic solvent range from c8 to c11 214x120mm (128 x 128 DPI)



Figure 6: Overlay of 3 standards of 1.4, 8.3, 61 µg/m3 of Hg0 in air

204x108mm (96 x 96 DPI)


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Figure 7: Stacked chromatograms of C 248 nm, Hg 254 nm, and FID of the vapour of a generic CFL bulb immediately after it has been released – 6.4 µg/m3 242x142mm (112 x 112 DPI)



TOC 253x93mm (150 x 150 DPI)


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Direct Measurement of Elemental Mercury using Multi-Dimensional

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