Development of a High-Resolution Laser Absorption Spectroscopy

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Development of a high-resolution laser absorption spectroscopy method with application to the determination of absolute concentration of gaseous elemental mercury in air Abneesh Srivastava, and Joseph T. Hodges Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00757 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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Analytical Chemistry

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Development of a high-resolution laser absorption spectroscopy method with

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application to the determination of absolute concentration of gaseous elemental

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mercury in air

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Abneesh Srivastava* and Joseph T. Hodges

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Chemical Sciences Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899-8393, United States

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*Corresponding author: Abneesh Srivastava, [email protected]

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ABSTRACT

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Isotope dilution cold-vapor inductively coupled plasma mass spectrometry (ID-CV-ICPMS)

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has become the primary standard for measurement of gaseous elemental mercury (GEM) mass

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concentration. However, quantitative mass spectrometry is challenging for several reasons

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including: (1) the need for isotopic spiking with a standard reference material, (2) the

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requirement for bias-free passive sampling protocols, (3) the need for stable mass spectrometry

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interface design, and (4) the time and cost involved for gas sampling, sample processing and

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instrument calibration. Here, we introduce a high-resolution laser absorption spectroscopy

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method that eliminates the need for sample-specific calibration standards or detailed analysis of

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sample treatment losses. This technique involves a tunable, single-frequency laser absorption

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spectrometer that measures isotopically resolved spectra of elemental mercury (Hg) spectra of 6

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1

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modeled from first principles using the Beer-Lambert law and Voigt line profiles combined with

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literature values for line positions, line shape parameters, and the spontaneous emission Einstein

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coefficient to obtain GEM mass concentration values. We present application of this method for

S0 ← 6 3P1 intercombination transition near λ = 253.7 nm. Measured spectra are accurately

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the measurement of the equilibrium concentration of mercury vapor near room temperature.

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Three closed systems are considered: two-phase mixtures of liquid Hg and its vapor, and binary

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two-phase mixtures of Hg-Air and Hg-N2 near atmospheric pressure. Within the experimental

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relative standard uncertainty (0.9-1.5) % congruent values of the equilibrium Hg vapor

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concentration are obtained for the Hg-only, Hg-Air, Hg-N2 systems; in confirmation with

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thermodynamic predictions. We also discuss detection limits and the potential of the present

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technique to serve as an absolute primary standard for measurements of gas-phase mercury

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concentration and isotopic composition.

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Keywords: Mercury, Laser Absorption Spectroscopy, Standard Generator, SI traceability, Dumarey equation, vapor pressure correlation, saturation, detection limit, isotope ratio

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INTRODUCTION

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There are three forms1,2 of atmospheric mercury: gaseous elemental mercury, Hg0 (GEM),

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divalent gaseous mercury (Hg(II)) and particle-bound mercury (Hg-p). The majority (95%) of

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the gaseous mercury is in the form of Hg0, which makes the emission, transport and toxicology

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of GEM an important species in atmospheric science studies.3 GEM emissions arise from a

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combination of anthropogenic and natural geogenic primary sources and secondary re-emission

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sources originating from surface reservoirs3. GEM has an atmospheric lifetime of (0.5 to 1) yr,4

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allowing it to be transportable and prone to bio-accumulation.1,5 Mercury is classified as a

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neurotoxin,6,7 and it is regulated by the United States Environmental Protection Agency ( US

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EPA) and other global government agencies as a potent hazardous pollutant.8,9 The

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environmental and occupational health standards for inhalation exposure to mercury vapor range

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over (0.3 to 100) µg m-3, classified based on time, threshold and trigger for investigation.10


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In order to quantify mercury emissions in the field, continuous emission monitoring systems

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comprising a calibration GEM generator unit and cold-vapor atomic absorption or fluorescence

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spectroscopy analyzers, (CVAAS, CVAFS) unit are commonly employed.11 GEM generators are

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open systems that involve a metered carrier gas (typically air or N2) in equilibrium with a

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reservoir of liquid Hg at a controlled temperature. In principle, this approach, provides a GEM

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mass concentration that depends only on the partial pressure of the liquid mercury and the total

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flow rate of the carrier gas. In practice, temperature and flow rate are used to precisely control

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the amount of GEM in the carrier gas, and linkage to the SI (Système International d’unités) is

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realized through a chain of calibrations to an SI-traceable primary standard for elemental

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mercury mass concentration.

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In the USA, primary standard generators for mercury mass concentration are used to certify

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commercial standard generators to which field-deployed GEM systems are typically certified.

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The primary standard generators used for these certifications, which are located at the National

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Institute of Standards and Technology (NIST) in Gaithersburg, Maryland span the mass

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concentration range (0.25 to 300) µg m-3 for GEM and are in turn characterized by SI-traceable

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measurements of mercury mass and carrier gas volume. Specifically, the mass of mercury

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emitted by the NIST primary standard generators is measured using an isotope-dilution

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inductively coupled mass spectrometry (ID-ICPMS) method that is referenced to an SI standard

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for mercury amount-of-substance (NIST SRM 3133).

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NIST primary standard generators have relative standard uncertainties between 1 % and 2 % for

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GEM mass concentration.

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Using this scheme, certifications of

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Although mass spectrometric measurements of collected mercury result in acceptable

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accuracy for characterizing the output of the NIST standard generators, the overall approach is



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time consuming, labor intensive, and costly. Therefore, at the expense of introducing additional

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uncertainty because of unaccounted-for system drift, routine certifications of the NIST standard

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generators cannot be implemented. Consequently, we seek an alternative approach that is rapid,

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accurate, and which has the potential for mercury concentration measurement over a wider

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dynamic range than is currently available. The present strategy involves demonstrating the

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feasibility of isotope-resolved, ultraviolet (UV) laser absorption spectroscopy of elemental

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mercury: an SI-traceable, calibration-free method that will be sensitive, rapid and accurate.

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Direct absorption of ground state atoms, as adopted in our approach, provides an advantage over

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excited state-relaxation-based fluorescence detection, which is sensitive to quenching by foreign

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gas. As discussed below, we project that this technique will measure mercury mass

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concentrations greater than approximately 0.1 µg m-3 using single-pass absorption cells

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nominally 1 m in length. Furthermore, combining the high-resolution approach discussed here

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with pathlength-enhanced methods such as cavity ring-down spectroscopy (CRDS) of GEM13-16

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will extend the accessible measurement range to ng cm-3 concentration levels, which are

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representative of ambient-level abundances and well outside the capabilities of current standards

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maintained at NIST.

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In addition to developing a new method for measuring the concentration of GEM, this study

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addresses present discrepancies in the reported temperature dependence of mercury vapor

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pressure discussed below. To this end, we measure mercury vapor concentration in contact with

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liquid mercury in the absence as well as in presence of air or nitrogen. We note that for a variety

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of reasons, use of an open system containing flowing air saturated with mercury could be prone

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to transport losses. Possible loss mechanisms include those caused by sampling, incomplete

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saturation, thermal instability, or chemical reaction, among others. In the following study, we


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have therefore used closed systems in the form of a static sealed quartz cell loaded with either

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pure liquid mercury only or liquid mercury in presence of atmospheric pressure air or nitrogen.

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This arrangement ensures gaseous elemental mercury stays in continuous equilibrium over a pool

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of liquid mercury, and is enclosed in an optically transparent and chemically inert material.

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Inclusion of liquid mercury in equilibrium with nitrogen gas near atmospheric pressure gives us a

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measurement "sample triad". This strategy allows us to screen for the confounding effects of

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Hg-O2 photosensitization17-19 and possible laser fluence population-saturation threshold

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requirements needed for accurate measurement of the Hg-air system vapor concentration. The

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resulting detection parameters help minimize, within our experimental uncertainty, the

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occurrence of these effects by using low ultraviolet laser intensity; and is described in the

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supplementary information. Our experiments use ultra-zero grade commercial source air

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(synthetic high purity oxygen, nitrogen mixture) for the air buffer gas and are not subject to

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photosensitization reactions that could occur in the presence of common flue-gas emission

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components18. Because of the high spectral resolution of the measurements, we can account for

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changes in the spectra caused by collisional effects. This capability enables us to precisely

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measure relative Hg vapor concentrations in the presence or absence of surrounding air.

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Furthermore, we can compare our measured absolute mercury vapor concentration values to

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those predicted by thermodynamic and empirical correlations. This comparison allows us to

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establish which of these formulas provides the most accurate representation of Hg vapor pressure

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for Hg-air standard generators.

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The remainder of this article is organized as follows. First, we describe the experimental

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system and the line-by-line spectroscopic model used for data reduction. We present measured

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and fitted spectra for the Hg-only, Hg-Air, Hg-N2 systems and discuss sources of bias. We



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discuss spectrometer performance metrics including detection limits, measurement uncertainties

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and potential application to absolute mercury isotope ratio measurements. Finally, we report

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room-temperature measurements of mercury concentration at room temperature for the Hg-only,

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Hg-Air, Hg-N2 systems and compare results with literature values and predictions.

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EXPERIMENT

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We used a commercial, continuous-wave (CW) light source, consisting of a tunable, narrow

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linewidth (sub-MHz) laser system to probe the 6 1S0 ← 6 3P1 absorption transitions of elemental

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mercury near the UV wavelength λ = 253.73 nm. The light source is a tapered-amplifier, fourth

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harmonic generator (TA-FHG), which comprises an external cavity diode seed laser emitting at

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λ = 1.015 µm, followed by a semiconductor amplifier and two cascaded second-harmonic

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generation stages for non-linear frequency quadrupling to the UV region. The resulting single-

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frequency output can be continuously scanned over more than 40 GHz and has a short-term

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linewidth < 500 kHz. The system produces UV laser output power of more than 20 mW with a

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specified residual intensity noise less than 0.3 % of the mean value.

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At each spectrum step, the frequency-doubled beam near λ = 507.5 nm is measured with a

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wavelength meter (1 MHz frequency resolution) and the transmission of the UV laser beam

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through the sample cell is recorded. The laser and wavelength meter are used in a closed-loop

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configuration to actively stabilize the probe laser frequency to the desired setpoint values.

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Spectral scans were collected over 40(80) GHz at UV frequency steps ranging from 200 MHz to

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1 GHz (400 MHz to 2 GHz) depending on the sample system and desired resolution. The laser

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frequencies at each setpoint had a standard deviation of approximately 2 MHz over the data

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collection time. The narrow linewidth and precise frequency tunability enable the acquisition of

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highly resolved absorption spectra (MHz-level precision) that were analyzed as discussed below. 6 ACS Paragon Plus Environment

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The experimental configuration is schematically shown in Figure 1. The mercury vapor

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absorption cells were custom fabricated to have a quartz cylindrical body with parallel but tilted

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(2°) faces, thermally sealed with fused silica wedged (2°) windows. This geometry minimized

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back reflection and etalon effects. The cell path length (nominal value of 10 mm) was measured

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at several orientations and had an overall relative standard uncertainty of 0.1 % about its mean

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path length (NIST, Dimensional Metrology Group). The cells contained a small volume of liquid

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mercury localized in a short stem extending below the main cell body. The sensitivity of path

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length to cell orientation and multiple reflections is provided in supporting information, S3.

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Three separate sealed cells containing mercury in the absence of added gas (p < 10-6 Pa),

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added air (ultra-zero grade) and N2 (99.9999% purity) both at 80 kPa nominal pressure were used

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for the present studies. These cells contain liquid mercury at natural isotopic abundance in

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equilibrium with its vapor in the surrounding gas and are subsequently referred to as Hg-only,

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Hg-Air, and Hg-N2, respectively.

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calibrated against an ITS-90 standard platinum resistance thermometer (NIST, Thermodynamic

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Metrology Group) was used to monitor the cell temperature. All measurements were conducted

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at room temperature near 22 °C.

A four-wire industrial platinum resistance thermometer

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The laser beam was intensity modulated with an acousto-optic modulator (AOM) at a

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modulation frequency of 20 kHz and used in first-order. The intensity-modulated laser beam

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was expanded and then split into two paths (sample and reference legs), where each path was

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terminated by a GaP photodiode with a bandwidth of 300 kHz under a load resistance of 1 kΩ

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and noise-equivalent-power of 13 fW/Hz1/2. With this arrangement, the sample path was used to

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irradiate the sample cell and the reference path was used for signal normalization to compensate

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for variations in the laser intensity. The sample-path and reference-path photodiode signals were 7 ACS Paragon Plus Environment


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measured using separate phase-sensitive (lock-in) amplifiers, both of which were referenced to

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the same AOM modulation signal. A multifunction data acquisition device was used to digitize

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the voltage outputs. Customized software was written to record the signal and control the

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stepwise frequency tuning of the laser across mercury absorption profile.

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characterization studies, the total power and spatial profile of the beam incident on the cell were

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measured using a calibrated power meter and camera beam profiler, respectively.

For saturation

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SPECTROSCOPIC MODEL

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6 1S0 ← 6 3P1 transition mercury isotope absorption spectra near λ = 253.7 nm

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It is well established that the relatively high mercury absorption cross section of the 6 1S0 ←

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6 3P1 intercombination transition provides an extremely sensitive measurement of mercury

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concentration.

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GEM concentration by probing this transition. Although, the spectral resolution was limited by

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the linewidth of their pulsed laser, ∆ν = 7.5 GHz, they were able to demonstrate ultrasensitive

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detection of GEM in ambient air at a molar fraction of 7 pmol mol-1. Subsequently, Fain et al.13

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developed a prototype CRDS spectrometer using a pulsed laser with a linewidth of 0.9 GHz

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which enabled them to resolve the Doppler-broadened spectra of mercury and to achieve a

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detection limit of 0.04 pmol mol-1 for an averaging time of 1 s. This work was extended by

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Pierce et al.15 to incorporate a frequency-locked probe laser with the CRDS method for

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automated field-deployable measurement of GEM.

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Jongma et al.14 first demonstrated the application of CRDS to measure

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The emergence of narrow linewidth, tunable CW diode lasers combined with non-linear

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frequency conversion schemes for the generation of UV radiation has enabled measurements of

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the 6 1S0 ← 6 3P1 intercombination transition with improved spectral resolution well below the 1

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GHz level. Specifically, Alnis et al.21 were the first to resolve the Doppler-broadened mercury 8 ACS Paragon Plus Environment

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isotope lines at 253.7 nm. They used sum frequency generation (SFG) of visible diode laser

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beams to generate 1 nW of UV power at 254 nm with a tuning range of 35 GHz. Carruthers et

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al.23 significantly improved the SFG conversion efficiency achieving output powers of 50 nW

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and demonstrated a tuning range of 70 GHz which allowed them to probe both the Doppler- and

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air- broadened spectra of mercury with a resolution of approximately 10 MHz. Anderson et al.22

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subsequently developed a ruggedized diode-laser-based sensor package for mercury detection at

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253.7 nm. Their system achieved a rapid spectral scan rate (up to 4 kHz) as well as a tuning

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range of 100 GHz to fully capture the air-broadened absorption spectra of mercury. Based on an

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inspection of published spectra Anderson et al. demonstrated the highest spectral resolution and

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signal-to-noise ratio (SNR) when compared to Alnis et al. and Carruthers et al. More recently

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Lou et al.24 used a frequency-doubled multimode diode laser as a simple and rugged laser source.

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However, for quantitative measurements, the multimode character of the probe laser required a

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sample-reference cell configuration to extract the concentration of GEM. The potential for long-

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term (14 h) frequency stabilization and signal averaging was recently exploited by Almog et al.20

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using Doppler-free saturation spectroscopy of this transition.

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As discussed below, the absorption spectrum of pure mercury vapor at room temperature and

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natural isotopic abundance for the 6 1S0 ← 6 3P1 intercombination transition exhibits six peaks

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originating from a total of 10 transition lines spanning approximately 22 GHz (supporting

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information, S1). When the mercury vapor mixes with a bath gas such as air or N2 the spectrum

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(as will be shown) the spectrum is dramatically altered due to pressure-broadening-induced

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blending of individual component transitions.



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Beer-Lambert Law and Signal Analysis

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We begin by assuming that the composite absorption spectrum can be modeled from first

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principles using the Beer-Lambert law, which relates the absorption coefficient, α (ν ) , to the

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product of the absorber number density, n, times its local cross section σ (ν ) , where ν is the

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optical frequency.

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The dual-path arrangement of Fig. 1 yields a pair of electronic signals sr and ss, which

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correspond to the reference and signal channels, respectively. Assuming that the reference

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photoreceiver responds linearly to the laser irradiation then sr = Piεrℜr , in which Pi is the beam

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power at the location where the incident beam splits into the two paths, ε r is the fraction of this

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power that reaches the reference detector, and ℜr is the transimpedance gain of the reference

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detector and phase-sensitive amplifier system. Similarly, for the sample channel and in the

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absence of absorption then ss = Piεsℜs . For an absorbing sample of path length l , the Beer-

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Lambert law gives the transmittance as Ts = exp(−α(ν )l) . Dividing the sample signal by the

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reference signal, yields the normalized transmission signal as,

z(ν ) =

ss Piε sℜs exp(−α (ν )l) = sr Piε r ℜr

β (1 + κ∆ν1)exp(−α (ν )l) ,

(1)

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in which β and κ are measured system constants and ∆ ν is the frequency detuning relative to

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the first point of the scan. Here, β and κ are obtained by the first-order Taylor series expansion

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of the ratio (εsℜs / εrℜr ) = f (∆ν) such that β = f (∆ν = 0) and κ = (df / dν)∆ν=0 / f (0) . The latter quantity

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accounts for any small difference in the frequency dependence of the collection and

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measurement efficiencies between the two paths that leads to imperfect normalization. Physical


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mechanisms contributing to this complication include slight variations in beam steering,

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reflection efficiencies, and etalons etc. as the laser frequency is tuned. This term can also capture

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slowly varying wavelength-dependent losses like Rayleigh scattering and the wings of distant

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absorption lines by species such as O2. We also note that spectra acquired with a similar-sized

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but empty cell also were conducted to rule out the contribution of any absorption features from

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the cell and air medium.

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For optically thin conditions that correspond to the limit αl

Development of a High-Resolution Laser Absorption Spectroscopy

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