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H2S and TRS Measurements – Continuous and Integrated Methods

Prepared by Sanjay Prasad, Yu-Mei Hsu, Michael Martineau, Kendra Thomas

Wood Buffalo Environmental Association

April 25, 2018

1. Background information

Hydrogen sulfide (H2S) is a colourless gas which has the odour of rotten eggs. H2S is considered

an odour nuisance at low levels, and can result in discomforting physiological symptoms of

headache and nausea. Total Reduced Sulfur (TRS) is a collection of H2S and a number of other

sulfur compounds (e.g Carbonyl Sulphide (COS), Carbon Disulphide (CS2) and Methyl

Mercaptan (CH3SH)). The current Alberta Ambient Air Quality Objectives for H2S are:

1-hour average of 14 μg m-3 (or 10 ppb)

24-hour average of 4 μg m-3 (or 3 ppb).

The principle of operation accepted by the Alberta Air Monitoring Directive (AMD) for

continuous ambient air H2S and TRS monitoring methods is Ultraviolet (UV) pulsed

fluorescence: H2S and TRS analyzers are basically Sulfur dioxide (SO2) analyzers with a SO2

scrubber and a converter to convert either H2S or TRS to SO2 for analysis. Some analyzers are

capable of cycling between SO2, H2S, and TRS measurements by switching between sample

pathways with and without SO2 scrubbers and converters. After sulfur compounds are converted,

the SO2 is measured proportional to UV absorption and fluorescence, which is related back to the

H2S or TRS concentration for detection and reporting (USEPA 2017).

2. Current Method for Continuous Measurements

The common method used for a continuous ambient H2S or TRS monitor is based on UV pulsed

fluorescence which is also required by the Alberta AMD (Alberta Environment and Parks 2017).

2.1 Principle of the Method

H2S/TRS is measured using thermal oxidation to convert H2S or TRS molecules to SO2 and the

converted SO2 molecules are then analyzed using continuous fluorescent SO2 analyzers.

H2S

H2S molecules will be oxidized to SO2 in the presence of Oxygen and heat. This is accomplished

by diverting the sample flow through the H2S converter, after a scrubber is used to remove

hydrocarbons. The converter has a stainless-steel body heated to at least 320 °C. Prior to the

converter, the sample flow passes through a scrubber to remove all SO2, and to allow only the

H2S molecules to pass through and enter the H2S converter. The H2S molecules are then

converted to SO2 as Equation 1.

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H2S + O2 + heat → SO2 + H2O Equation 1

The converted SO2 molecules then return to the standard SO2 analyzer for detection and are

reported as H2S (Alberta Environment and Parks 2011a, Alberta Environment and Parks 2011b).

TRS

TRS compounds include a variety of airborne compounds which contain Sulfur. Some of the

common compounds found in Alberta are: Carbonyl Sulphide (COS), Carbon Disulphide (CS2)

and Methyl Mercaptan (CH3SH). Similar to H2S, TRS molecules will be oxidized to SO2 in the

presence of Oxygen and heat. The difference between H2S and TRS is the TRS conversion

requires a higher temperature. This is accomplished by diverting the sample flow through a TRS

converter, after the flow has passed through the hydrocarbon scrubber. The TRS converter is a

Quartz tube heated to a minimum of 800 °C. Prior to the converter, the sample flow must pass

through a scrubber to remove all SO2, and to allow only the TRS molecules to pass through to

enter the TRS converter. The TRS molecules are then converted to SO2 as Equation 2.

TRS + O2 + heat → SO2 + H2O Equation 2

The converted SO2 molecules then return to the standard SO2 analyzer for detection and are reported as

TRS (Alberta Environment and Parks 2011a, Alberta Environment and Parks 2011b).

The primary detection principle utilized in SO2 analyzers is UV fluorescence. Where a valence

electron absorbs UV at a wavelength of 214 nm and enters an excited state (*). When the

electron drops back into its usual state, a photon is emitted at a lower energy wavelength of 390

nm as Equation 3.

SO2 + hv1 → SO2* → SO2 + hv2 Equation 3

* = Excited State

hv1 = Exposure Light at 214 nm

hv2 = Emitted Light at 390 nm

The light emitted by the decaying SO2 electrons is filtered and channeled into a photo multiplier

tube and amplified and converted to an electrical signal (Alberta Environment and Parks 2011c).

2.2 Operations/Diagram

The continuous H2S/TRS monitor measures H2S/TRS concentrations in ambient air by thermal

conversion of H2S/TRS to SO2 with a molybdenum catalytic converter and UV fluorescence of

the SO2 gas (Sumner et al. 2005). H2S measurements are made using a low temperature

converter operating typically between 300 to 350 °C. For TRS measurements, the conversion of

sulfur compounds to SO2 occurs at a much higher temperature typically between 800 and 1000

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°C. To maximize converter performance, it is important to operate the analyzers with converters

in the appropriate temperature ranges as specified by manufacturers (Alberta Environment and

Parks 2017).

Figure 1 illustrates a pneumatic diagram of Teledyne Instruments T101 H2S Analyzer. The air

sample is drawn into the monitor through the sample gas inlet and then flows through a sample

filter and a hydrocarbon scrubber (hydrocarbon kicker) to remove particles and hydrocarbons,

respectively, from the air sample. A H2S/SO2 mode valve is used to control the air flow for SO2

measurement or H2S measurement (Table 1).

The air sample then flows into the sample chamber, where pulsating UV Light excites the SO2

molecules. The condensing lens focuses the pulsating UV light into the mirror assembly. The

mirror assembly contains four selective mirrors that reflect only the wavelengths which excite

SO2 molecules. As the excited SO2 molecules decay to lower energy states they emit UV light

that is proportional to the SO2 concentration. The bandpass filter allows only the wavelengths

emitted by the excited SO2 molecules to reach the photomultiplier tube (PMT). The PMT detects

the UV light emission from the decaying SO2 molecules (Thermo Scientific 2015).

Figure 1. Pneumatic Diagram of Teledyne Instruments T101 Analyzer (Adapted from Teledyne

Monitor Labs (2016))

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Table 1. H2S – SO2 Switching Valve Operating States (Adapted from Teledyne Monitor Labs

(2016))

GAS MODE CONDITION OF H2S –SO2 SWITCHING

VALVE

VALVE PORT

CONNECTION

H2S Open to SO2 Scrubber and Molybdenum

Converter COM → NO

SO2 Open to directly to Sample Chamber. Bypasses

SO2 Scrubber and Molybdenum Converter COM → NC

H2S –SO2 Switches between above two states every 10

minutes.

-

2.3 Interferences

It should be noted the Ultraviolet (UV) pulsed fluorescence method for measuring H2S/TRS is

subject to interference from a number of sources, including SO2, hydrocarbons (Luke 1997) and

other species which absorb at about 190 - 230 nm and fluoresces at 340 - 410 nm. Obviously,

since the analyzer measures H2S by converting it to SO2, the most significant interfering gas for

this measurement would be ambient SO2 which is present in the sample gas. A SO2 chemical

scrubber is applied to remove SO2 from the sample gas before the H2S → SO2 conversion takes

place.

The second most common source of interference is from other gases which fluoresce in a similar

fashion to SO2 when exposed to UV Light. The most significant of these gases is a class of

hydrocarbons called aromatic hydrocarbons (Luke 1997) of which xylene and naphthalene are

two prominent examples. A hydrocarbon scrubber (kicker) mechanism removes aromatic

hydrocarbons present in the sample gas before it the reaches the sample chamber.

Because ozone absorbs UV Light over a relatively broad spectrum it could cause a measurement

offset by absorbing some of the UV given off by the decaying SO2* (* = excited state) in the

sample chamber. This can be prevented from occurring by having a very short light path between

the area where the SO2* fluorescence occurs and the PMT detector (Teledyne Monitor Labs

2016).

While the decay of SO2* to SO2 happens quickly, it is not instantaneous. Since it is not

instantaneous, it is possible for the extra energy possessed by the excited electron of the SO2*

molecule to be given off as kinetic energy during a collision with another molecule. This

process, in effect, heats the other molecule slightly and allows the excited electron to move into a

lower energy orbit without emitting a photon. The most significant interferents in this regard are

nitric oxide (NO), carbon dioxide (CO2), water vapor (H2O) and molecular oxygen (O2). In

ambient applications the quenching effect of these gases is negligible (Teledyne Monitor Labs

2016).

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To monitor H2S/TRS concentrations in ambient air, the H2S/TRS is oxidized to SO2 by a

Molybdenum converter. However, the converter unit does not oxidize all H2S/TRS to SO2, rather

it uses a conversion efficiency determined by the manufacturer. The conversion efficiency varies

from instrument to instrument, and is typically above 80% (Bluhme et al. 2016).

Sumner et al. (2005) have assessed the effect of potential interferant gases (Table 2) on the

response of the SO2 analyzer ( Teledyne Model 101E) with seven (7) gases in zero air and a 100-

ppb H2S standard. No interference effect was observed in the Model 101E response to SO2, a

blend of C1 to C6 alkanes, and ammonia. The Model 101E showed an interference effect for

carbonyl sulfide in zero air of 20% and in 100-ppb H2S of 6%. Carbon disulfide resulted in an

interference effect of 6% in zero air and 9% in the 100-ppb H2S matrix. The interference effect

of methyl mercaptan on the Model 101E was 33% in both zero air and 100-ppb H2S. Dimethyl

sulfide resulted in a 12% interference effect in both matrices.

Table 2. Interference Effect Evaluation (adapted from Sumner et al. (2005))

Interferant Approximate

concentration (ppb)

Interference Effect (%)

Zero Air Matrix 100-ppb H2S Matrix

Sulfur dioxide 100 0 0

Carbonyl sulfide 100 20 6

Carbon disulfide 100 6 9

Methyl mercaptan 100 33 33

Dimethyl sulfide 100 12 12

Hydrocarbon blend 500 (total) 0 0

Ammonia 500 0 0

3. Designations – US, Australia, New Zealand, Europe

H2S or TRS are not criteria pollutants in the United States of America, therefore the United

States Environmental Protection Agency (USEPA) does not have a Federal Reference Method

(FRM) for H2S or TRS (USEPA 2017).

4. AMD and Manufacturer Performance Specifications

In Alberta, as required by the Alberta AMD, Chapter 4: Monitoring Requirements and

Equipment Technical Specifications, the operating principle for measurement of H2S and TRS is

UV pulsed fluorescence. The minimum performance specifications required by the AMD are

listed in Table 3. The manufacturer performance specifications of two analyzers including

Teledyne T101 and Thermo Scientific Model 450i are listed in Table 4 and Table 5, respectively.

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Table 3. Alberta AMD performance specifications for H2S and TRS continuous analyzers

(adapted from Alberta Environment and Parks (2017))

Criteria Performance Specification

Required Operating Range* (Full Scale): 0.1 ppm, 0.5 ppm or 1.0 ppm

Zero Noise: 0.5 ppb RMS

Lower Detection Limit: 1.0 ppb

Zero Drift (24-hr): 1.0 ppb

Span Drift (24-hr) 1% of full scale

Linearity: 1% of full scale

Precision: 1.0 ppb or 1% of reading

Rise time: Maximum of 120 seconds

Fall time: Maximum of 120 seconds * The typical operating range is 0.1 ppm with 0.5 ppm and 1.0 ppm used in unique circumstances such as

emergency monitoring. RMS is the root mean square of differences.

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Table 4. Teledyne Model T101 Basic Unit Specifications (adapted from Teledyne Monitor Labs

(2016))

Parameter Description

Ranges

H2S: Min 0-50 ppb Full scale; Max 0-10 ppm Full scale

SO2: Up to 0-20 ppm Full scale

(selectable, independent ranges and auto ranging supported)

Measurement units ppb, ppm, µg/m3, mg/m3 (selectable)

Zero Noise

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Table 5. Thermo Scientific Model 450i Specifications (adapted from Thermo Scientific (2018))

Parameter Description

Preset Ranges

0-0.05, 0.1, 0.2, 0.5, 1, 2, 5, and 10 ppm,

0-0.2, 0.5, 1, 2, 5, 10, 20, and 25 mg/m3

Extended Ranges

0-0.5, 1, 2, 5, 10, 20, 50, and 100 ppm,

0-2, 5, 10, 20, 50, 100, 200, and 250 mg/m3

Custom Ranges 0-0.05 to 100 ppm, 0-0.2 to 250 mg/m3

Zero Noise

Manual SO2 or

Combine Sulfur

Automatic mode

SO2 or H2S

1.0 ppb 3.0 ppb (10 second averaging time)

0.5 ppb 1.5 ppb (60 second averaging time)

0.25 ppb 0.75 ppb (300 second averaging time)

Lower Detectable

Limit

Manual SO2 or

Combine Sulfur

Automatic mode

SO2 or H2S

2.0 ppb 6.0 ppb (10 second averaging time)

1.0 ppb 2.0 ppb (60 second averaging time)

0.5 ppb 1.5 ppb (300 second averaging time)

Zero Drift (24 hour) Less than 1 ppb

Span drift (24 hour) ± 1% Full Scale

Response Time 80 seconds (10 second average time)

110 seconds (60 second average time)

320 seconds (300 seconds average time)

Precision 1% of reading or 1 ppb (whichever is greater)

Linearity ±1% full scale 80% H2S to SO2 (Note: Various other Sulfur compounds can be converted

at varying %)

Operating

Temperature 20⁰C to 30⁰C

Power Requirements 100 VAC, 115 VAC, 220-240 VAC ±10% @ 300W

Size and Weight 16.75"(W) x 8.62"(H) x 23"(D), 48 lbs. (21.8 kg)

Outputs

Selectable Voltage, RS232/RS485, TCP/IP, 10 Status Relays, and Power

Fail Indication (standard). 0-20 or 4-20 mA Isolated Current Output

(optional)

Inputs 16 Digital Inputs (standard), 8 0-10 Vdc Analog Inputs (optional)

Available Options Teflon particulate filter, Rack mounts, Rear extenders

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5. Other Methods for H2S and RSC Measurements

Both passive and active measurements have been developed for H2S and Reduced Sulfur

Compounds (RSCs) measurements. A general outline of the analytical protocols commonly

employed in RSC analysis is illustrated in Figure 2.

Figure 2. General protocols of Reduced Sulfur Compounds (RSCs) analysis in air matrices. The

abbreviations used in figure are as follows: LC-AFS = liquid chromatography-atomic

fluorescence spectrometer, IC = ion-chromatography, SPME = solid phase microextraction, FPD

= flame photometric detector, PFPD = pulsed flame photometric detector, MS = mass

spectrometer, AED = atomic emission detector, SCD = sulfur chemiluminescence detector, and

TD = thermal desorber. (adapted from Pandey and Kim (2009).)

5.1 H2S Passive Measurement

Many H2S passive samplers are made commercially available (Shooter et al. 1995, Tang et al.

2002, Venturi et al. 2016) for the remote locations with no power accesses. The H2S passive

collection media are coated with chemicals (e.g., Silver nitrate, zinc acetate) to collect H2S for

sample analysis (Tang et al. 2002, Pavilonis et al. 2013).

Sampling stage Preconcentration stageDetection stage

(Detector type)

LC-AFS

IC

Conductomery

Potentiometry

Coulometry

Voltametry

FPD

PFPD

MS

AED

SCD

Bag or Canister

Non-GC methods

GC method

Alkaline aqueous Phase

Diffusion scrubbers

SPME

Tube

TD

Cryogenic trappingSolid sorbentsSorption on metal

surfaces

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5.2 Gas Chromatograph (GC) Methods

For RSC analysis in the air, the gas chromatography (GC) technique has been and is the most

common methodology because of its excellent separation capability and quantitative recovery

(Helmig 1999, Willig et al. 2004). The GC-based analysis commonly involves a sample

collection stage, injection, and separation on a chromatographic column. The final detection can

then be carried out through various Sulfur-selective (or universal) detectors.

Direct chromatographic analysis or direct injection (DI) of RSCs into a GC injector is highly

recommended when the concentration of samples is in the detectable range of a given GC-setup.

The use of the DI method can reduce possible loss (or gain) due to contact with different surface

types, and reduces analysis time by eliminating time-consuming procedures such as

supplementary sample treatments by which contamination or loss of analytes can occur (Kim

2006, Pandey and Kim 2008, Pandey and Kim 2009). However, the application of the DI

approach is often limited, as most detectors of the GC method are not sensitive enough to cover

ambient samples which are typically below a few ppb in concentration. Therefore, research has

sought to improve and develop GC methods based on Sulfur-specific detectors, including

electron capture detector (ECD), flame photometric detector (FPD), pulsed flame photometric

detector (PFPD), sulfur chemiluminescence detector (SCD), atomic emission detection (AED),

Hall electrical conductivity detector (HECD), and photo ionization detector (PID) with combined

application of a mass spectrometer (MS) and GC system (Nielsen and Jonsson 2002, Dincer et

al. 2006, Oostdijk et al. 2007, Ras et al. 2008). USEPA CFR Promulgated Test Method 15

(USEPA 2017) has employed an online GC-FPD method for monitoring hydrogen sulfide,

carbonyl sulfide, and carbon disulfide emissions from stationary sources

To allow the analysis of increasingly smaller quantities of target compounds, the range of

instrumental detectability needs to be improved. As a means to extend the detectability of a given

GC system, one can increase the total amount of analytes injected by adopting some

preconcentration (or sample enrichment) stages: (1) sorption on certain metal surfaces (Braman

et al. 1978, Barnard et al. 1982, Ferek et al. 1986), (2) sorption on solid adsorbents (Maier and

Fieber 1988, Sunesson et al. 1995, Inomata et al. 1999), and (3) cryogenic trapping (Kim 2005,

Kim 2005, Ras et al. 2008, Pandey and Kim 2009). However, the analysis of low-level sulfur

species through such modifications can be subject to positive blanks (e.g., memory effect) or

sorptive loss in the chromatographic system (Sulyok et al. 2001, Sulyok et al. 2002).

Sampling and Preconcentration Strategies. RSCs from air can be collected in vessels such as

glass bulbs, canister bags, polymer bags, and Tedlar film bags. Considering the highly reactive

nature of Sulfur compounds, the sampling vessels should be inert enough to reduce adsorptive

loss (Sulyok et al. 2001). Moreover, careful attention should be given to tubing and connecting

materials used for the sampling of Sulfur compounds. The reason for this is certain materials

can act as significant sources of bias in the determination of RSC concentrations (Sulyok et al.

2001, Sulyok et al. 2002, Kim et al. 2006, Winkel and Tangerman 2008).

ASTM Method D5504-01(ASTM International 2001) use

canister samples for H2S collection

and require samples to be analyzed by GC-PFPD or GC-SCD within 24 hours. ASTM Method

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D5504-01 has been demonstrated that H2S in passivated canisters does not degrade over 24

hours; therefore, detailed holding time tests are not necessary. However, the acceptability of

this holding time should be verified by analyzing an ambient air sample several times over the

period (at least 24 hours) following sample collection. The H2S concentration at 24 hours

following sample collection should be within 15% of the initial measured value (ASTM

International 2001, Battelle 2005).

A major difficulty in the sampling of RSCs is interference caused by atmospheric oxidants such

as SO2, ozone, and NOx. To help overcome these difficulties, many substances were developed

as scrubbers, including PTFE, Tygon, glass fiber filters, chromosorb, anakrom, and glass beads

with a coating of chemicals (ex., Na2CO3, MnO2, KOH, and NaOH) (Braman et al. 1978,

Andreae et al. 1985, Saltzman and Cooper 1988, Bates et al. 1990, Ayers et al. 1991, Kittler et

al. 1992, Watanabe et al. 1995).

Another considerable difficulty in determining RSCs is relative humidity. Water significantly

lowers the capacity of adsorbents and can clog cryogenic traps (Haberhauer-Troyer et al. 1999).

Moreover, it can cause baseline perturbations and retention time-shifts in chromatography to

deteriorate detection (Haberhauer-Troyer et al. 1999). To overcome the humidity problem, a

number of approaches have been developed by applying dryers (Juhani and Hannu 1988,

Hofmann et al. 1992, Haberhauer-Troyer et al. 1999).

Sorption on Metal Surfaces. The extent of RSC sorption can be affected greatly by the type of

metals (mainly gold, palladium, and platinum) used for such a reaction (Braman et al. 1978,

Barnard et al. 1982, Ferek et al. 1986, Kagel and Farwell 1986). These metallic materials are

also found in modified forms such as glass or quartz tubes filled with gold wool, gold plated

sand, or metal foils (Braman et al. 1978, Ferek et al. 1986, Davison and Allen 1994, Swan and

Ivey 1994, Davison et al. 1996, Curran et al. 1998).

Sorption on Solid Sorbents. Solid sorbent surfaces are regarded as the most general tools for

the preconcentration of volatile species, e.g., activated charcoal, silica gel, aluminum oxide,

graphitized carbon black, molecular sieves, and porous sorbents (Davison et al. 1996, Wylie

and Mora 1996, Lewis et al. 1997, De Bruyn et al. 1998). Porous sorbents, such as Tenax have

been the most popular choice for Sulfur species (Pio et al. 1996, De Bruyn et al. 1998, Ras et al.

2008), but the breakthrough volume of Tenax is not altered by changes in humidity because of

its hydrophobic property (Steinhanses and Schoene 1990). The trapping efficiency of Tenax

tubes is often limited for organosulfur compounds with low boiling points (Tangerman 1986,

Shooter et al. 1992). The capacity of a sorbent can increase significantly by adding a

cryofocusing system (Kroupa et al. 2004). Many studies have been carried out to determine the

affinity and/or breakthrough volume of different sorbents (Przyjazny 1985, Devai and Delaune

1996, Devai and DeLaune 1997) for the adsorption capacity. The application of various solid

sorbents (i.e., silica gel, molecular sieve, and Carbosieve SIII) has been validated by

measurements of RSC concentrations or fluxes under various environmental settings (Delaune

et al. 2002).

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Cryogenic Trapping. Trapping efficiency of adsorbent traps increases exponentially with a

decrease in temperature (Pollmann et al. 2006). The cryogenic trapping technique can increase

the capacity of sorptive materials effectively. The cryogenic traps (i.e., sampling loops) are

generally composed of PTFE, borosilicate glass, and quartz tubes (Wardencki 1998). For

sampling of RSCs, the cryogenic traps are normally immersed in a strong cryogen such as

liquid nitrogen or in liquid argon (Simo et al. 1993). Cryogenic traps can also be used with a

number of sorbents as packing materials to increase sorption efficiency. Commonly used

packing materials include glass-fiber wool, glass beads, Tenax, Porapack Q, activated carbon,

and carbopack. Based on the technique of cryogenic trapping, and GC-PHD, an automated

Sulfur instrument was developed to analyze RSCs continuously (Von Hobe et al. 2008).

Solid-Phase Microextraction (SPME). The solid-phase microextraction (SPME) method, a

potential solvent-free sample preparation technique, allows a single-step treatment for

sampling, isolation, and enrichment (Xiong et al. 2003, Ouyang and Pawliszyn 2006, Ouyang

and Pawliszyn 2006, Demeestere et al. 2007). For air matrices, the SPME fiber can be used to

extract analytes either by direct exposure to raw samples or by the use of the headspace method

on pretreated samples (Arthur and Pawliszyn 1990, Li et al. 2001, Ouyang and Pawliszyn

2006).

In addition to GC methods, there are alternative methods which include liquid chromatograph

(LC), ion chromatograph (IC), and sensor-based methods. The reliability of these alternative

methods for practical application of real environmental samples has not been sufficiently

researched. Methodologies for online monitoring at ambient concentration levels would help

overcome the time-consuming and tedious processes of sampling, enrichment, and offline

analytical protocols in the laboratory (Pandey and Kim 2009).

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6. References

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