A New Method for the Analysis of ppb Levels of Mercury in Air and

Water

Dr. Jack N. Driscoll & Jennifer Maclachlan

PID Analyzers, LLC

Cape Cod, MA

Pittcon Symposium: Environmental Portable Instrumentation

Paper # 1100-6

AM Session 10:25

March 10, 2015

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Introduction

• Hg has long been detected by non specific techniques such as UV absorbance (253.7 nm). Fluorescence is also a popular non specific technique for Hg

• The reaction of mercury (Hg) with gold (Au) is used to concentrate the Hg while removing any potential interferences

• The Hg is desorbed from the amalgam by rapidly heating to 500o C

• This amalgamation reaction turns non specific Hg methods into specific ones.

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Introduction (cont.)• Photoionization is a technique that has been used since

the 1970’s primarily for ppb detection of VOC’s.• We report here, for the first time, that Hg0 with its

ionization potential (IP) of 10.4 eV can be detected at low ppb levels with the 10.6 eV lamp in our PID.

• Instead of Au/Hg amalgam, we have chosen NIOSH method 6009 for collection(on a Hopcalite® tube) & concentration of a Hg air sample followed by dissolution in Nitric/HCl, reduction with NaBH4 and specific analysis by GC/PID.

• We will describe our work on the detection of ppb levels of Hg in air & water in this presentation.

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Photoionization ProcessR + hn => R+ + e-

Where:

R is any molecule with an Ionization Potential 10.6 eV or less- Hg IP = 10.4 eV

hn is a high energy photon (10.6 eV)

The R+ ions are collected in an ionization chamber adjacent to the UV lamp; the current is proportional to concentration over a range of 10exp7. So it has a very wide dynamic range.

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Photoionization Detector

PID Schematic & Photo Compounds Detected

• Hg, & As, Pb, Fe, Cd… Hydrides

• Aromatics, Alkenes, Alkanes >C4+

• Hydrogen sulfide, ammonia

• Any compound with an ionization potential <10.7 eV

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List of Ionization Potentials

Name Formula Ionization Potential (eV)

Methane CH4 12.48

Oxygen O2 12.08

Water H2O 12.59

Arsine AsH3 10.5

Phosphine PH3 10.1

Mercury Hg 10.4

Benzene C6H6 9.24

Toluene C7H8 8.82

O-Xylene C8H10 8.56

Ethylene C2H4 10.15

Hydrogen telluride H2Te 9.14

Germane GeH4 10.53

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Applications of the GC/PID-HC Analysis• First commercial PID was introduced by HNU Systems (Driscoll) at Pittcon

1976 in Cleveland. The detector was described as being 50x more sensitive than the FID for aromatic HCs

• 1978- EPA method 602 for aromatics in drinking water recommended the HNU Model PI52 since it was the only PID commercially available

• Drinking water- EPA 602b, 502.3, 506,

• Solid waste- EPA SW846. 8021, 8260, 1996B, OSW 5021

• Soil –OSW 8021B, EMMI 1997, OSW 8020B,

• Air- TO3, TO14

• OSHA- Tetra ethyl lead, Ethylene oxide

• PID/FID ratios for HC identification was a basis of more than 40 State & EPA methods for gasoline hydrocarbon samples instead of dual column confirmation

• SINCE 1976 the primary application for the PID has been HC Analysis

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Air and Water Hg Methods

• Hg- Air

– Is collected and concentrated (on a Hopcalite® tube) which make the sample specific for Hg via reduction with NaBH4 then GC-PID for detection

• Hg- Water

– Use acid (HCl) then NaBH4 to reduce mercury to free mercury then complete analysis via GC/PID

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Air Sample Collection System

Description

• NIOSH method 6009 uses a Hopcalite® tube to collect Hg0. The Hg is removed from the Hopcalite® by treatment with nitric acid (first) then HCl. This liquid is transferred to the sparging vessel for analysis by GC/PID.

Collection

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Mercury in Air

• Ambient air sampling usually involves looking for low levels (0.5 to 5ng/m3) of mercury (typical sampling times can range from 5 to 30 min.

• For workplace air sampling, the levels are typically higher (0-50 mg/m3) and prone to sudden fluctuations. Shorter sampling times are used with an increased sampling frequency.

• The OSHA Permissible Exposure Limit (PEL) for Hg is 0.5 mg/10m3

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Description of the Specific Hg System• The air/Hg sample is purged through the GC 10 port

valve where the Hg is injected with a short inject time (10 sec.) and goes through the column to the PID. Other sample components (hydrocarbons, water, oxygen etc.) are back flushed, to vent, remove them and prevent them from going onto the analytical column.

• This method is specific for Hg because all potential interferences (HCs) are back flushed before going to the gc column. Then, only the Hg is transferred to the GC column and then to the PID.

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Hg in N2 detection by PID

100 ppb Hg direct injection Description

A VICI® mercury permeation tube (at 35oC) was purged with a constant flow of nitrogen and injected into the PID. The results are shown in the figure on the left. The response to Hg is quite strong and reproducible. The detection limit for Hg is < 10 ppb for direct injection of 1 cc of Hg in N2. Note here that there is NO COLUMN, injection goes from the 10 port into the oven at 40 degrees C.

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Hg in Air conc. sample 1000 ppb by GC/PID- not N2 purged

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Note: The strong negative oxygen peak due to quenching from this and it takes a few seconds for the mercury peak to recover. This was a 30M by .32mm 5 micron methyl silicone column

Hg in Air conc. Sample- 1000 ppb by GC/PID- N2 purged

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Since this sample was purged with nitrogen, we don’t see the decrease in the baseline that’s caused by oxygen.There is a little tail on this which eventually goes away-we suspect it’s an intermediate- but we still see reasonable reproducibility

Conversion from mg/m3 to ppm

Hg values are frequently quoted in units of mg/m3 to convert to ppm see below:

MW Hg= 200.59 g/mole

ppm = mg/m3 x 24.45/ MW

ppm = mg/m3 x 0.12

ppm = app. 1/10 mg/m3

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Hg in Water Method

Reaction: Conversion of mercuric salt to free Hg using a reducing agent-we like sodium

borohydride

HgX2 + H+ + reducing agent => Hgo

IP Hg= 10.4 eV

Energy of lamp = 10.6 eV

Forms Hg+

Hg+ ions are proportional to concentrationover a range > 107

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What Metals have been Analyzed by GC/PID?• Vien 1988 Arsenic, Selenium, Tin and Antimony (as hydrides) in

natural waters• Yamamoto 1992 Arsenic and Antimony in natural waters• Cutter 1991, 2006 Arsenic and Antimony in natural waters• Haque 2007 Arsenic in natural waters• Mergemeyer- no GC-just single element-Analysis has been by hydride generation (with strong acidic reducing agents)- generally with the metalloids using a liquid N2 trap/concentrator- These methods were not very popular and produced fewer than 7 papers in the last 25 years. This very sensitive method has been under utilized!Driscoll & Maclachlan described a more sensitive GC/PID method at ACS/NOLA (2013) for As in Juices at sub ppb levels.No liquid N2 was needed to concentrate the sample! We also

described a HG/GC/PID methods for Pb, Cd & Fe at ACS in SF (2014).

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Jack and Jennifer at the 2013 R&D 100 Awards, Orlando, FLA

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Mercury in water• Mercury in water is generated by a hydride type

method, that is an acidic solution with a strong reducing agent, but instead the mercuric salt is reduced to elemental mercury instead of a hydride.

• The mercury vapor is injected into the GC-PID with a short injection time while any organics, water and other compounds are back flushed to vent.

• The capillary column separates the elemental Hg vapor that is detected by the PID.

• EPA drinking water standard for Hg0 is 2 ppb

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Cold Vapor Mercury Analysis

One of the most popular methods for Hg analysis involves the conversion of Hg salts to Hg vapor using an strong acidic reducing agent such as combining tin and HCl or NaBH4/HCl as shown below:

3H2O+ 2NaBH4 + HCl => 8H. + 2H3BO3

8H. + Hg+2 => Hgo + 3 H2

Atomic Absorption or Atomic Fluorescence are common detectors for this analysis. Now GC/PID is also available for this analysis!

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Mercury Generator for Air/WaterDescription

• 25 ml of an aqueous sample containing ppb level of mercuric salt is pipetted into the sparging vessel.

• Start the nitrogen purge at 15 cc/min and continue for 3-4 min.

• Add 1 ml of concentrated HCl and screw the top on tight. Start the Teflon stirring bar, then add 1 cc of 4% NaBH4 with a syringe via the septum. For the air samples, no acid step is needed.

• Inject a number of samples via the 10 port valve. At low levels, it may take 10-13min to reach the maximum value

Hydride Generator

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Hg Analysis SystemDescription Analysis System

• The acidic liquid from air sampling is transferred to the sparger. N2 is used for purging for 2-3 min. to remove residual O2, then NaBH4 is added and samples are injected with the 10 port valve.

• For water samples 1 ml of concentrated HCl is added. N2is used for purging for 2-3 min. to remove residual O2, then NaBH4 is added and samples are injected with the 10 port valve.

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Hg Generation & Detection by GC/PID

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Hg Analysis for Water Samples

Hydride generation

The Hg concentration as a function of time is shown in the adjacent figure. There is no column in this system but if we use a column, we can improve the sensitivity and specificity. The Hg concentration takes about 12 min. to a maximum value.

Variation of Hg concentration (f) time

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Hg Analyzer for Air and Water

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Connect with us on Linked-In

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Comparison of HGAAS & HG/PIDHG/AAS

• Cleans up sample

• Concentrates the sample

• Uses a Hg lamp/uv absorption

• Detection limits in low or sub ppb

• Sample concentrated using Au amalgam

HG/PID• Cleans up the sample

• Reduces the Hg salt to elemental Hg that the PID can detect

• Major improvements in PID sensitivity and optimization, reduces the detection limit to < 10 ppb

• Air sample is concentrated using Hopcalite® and both air and water samples are concentrated using hydride generation

• PID is 10-50 times more sensitive than AAS

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Conclusions• Mercury (IP= 10.4 eV) is detected with low ppb sensitivity by the PID (first

time reported) with a 10.6 eV lamp

• Mercury vapor in the air can be collected and concentrated using a Hopcalite™ tube (NIOSH method 6009). Following acid desorption, and Hg reduction, the analysis is by GC/PID with a capillary column. Sub ppb Hg detection levels are achievable

• Mercury in water can be detected by PID or with improved specificity by GC/PID after reduction with NaBH4. Sub ppb levels of Hg can be detected.

• This new method of analysis for Hg has significant potential for measuring ppt levels of Hg in air and water.

• Want to learn more? We’ve got a booth in the Pittcon expo, #1326.

• Check us out on the web at hnu.com

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