how to measure long duration, time-weighted average

How to Measure Long Duration, Time-Weighted Average Concentrations in Air to Provide

a More Accurate Risk Assessment

By:Harry O’Neill, President

BEACON

Road Map

• Current air sampling practices and challenges with that approach

• EPA guidance documents

• Passive sampler types – benefits and limitations

• Why Long-Duration, Time-Integrated Samples are Important

• Principles of how operate

• Demonstration of long duration linear uptake rates

• Reporting limits and standard analytical method

• Comparison data – Accuracy and Precision

• Questions

The Challenge

Temporal Variability

Indoor air concentrations can vary daily by orders of magnitude

Vapor intrusion has shown to be episodic – anomalous events occur

Source:

Johnson, P. Multi-Year Monitoring of a House Overlying a Dilute Chlorinated Hydrocarbon

Plume: Implications for Vapor Intrusion Pathway Assessment

SERDP & ESTCP Webinar Series, 2014.

WINTER MONTHS SPRING MONTHS

The Challenge

Sources:

Johnson, P. Multi-Year Monitoring of a House Overlying a Dilute Chlorinated

Hydrocarbon Plume: Implications for Vapor Intrusion Pathway Assessment

SERDP & ESTCP Webinar Series, 2014.

Holton et al., ES&T, 2013, 47, 13347-13354

Results: Analysis of Sampling Outcomes

With 24 hr samples:

High potential for false negative result concerning VI occurrence

High potential to incorrectly characterize long-term exposure

High potential to incorrectly characterize maximum short-term

exposure

• About half of all 24-h samples would come back non-detect

• Only about 50% chance that sample results would have a

mean concentration inside a 10X range about the true mean

concentration

The Challenge

Source: Hosangadi et al, 2017

02/02/16 02/11/16

“Vapor concentrations can vary in both the subsurface and

indoor environments due to barometric pumping, soil moisture

dynamics, building ventilation, wind shear, tidal fluctuation, and

other environmental and anthropogenic factors”

Point in Time Measurement – Typically 8 or 24 hours

Method TO-15 -- Summa canister

Method TO-17 -- Sorbent tube and pump

Typical Sampling Strategy

Source: EMS Environmental

Source: H&P Mobile

Point in Time Measurement – Real Time and Continuous

Portable GC/MS

Optional Sampling Strategy – Continuous Monitoring

Source: EMS Environmental

Source: Inficon Source: VaporSafe

Portable GC/PID/FID/ECD

Optional Sampling Strategy - Passive Sorbent Samplers

Radial Sampler, Badge Sampler, & Axial Samplers

US EPA GUIDANCE DOCUMENTS

Passive Samplers for Investigations of Air Quality: Method

Description, Implementation, and Comparison to Alternative

Sampling Methods – July 2015

OSWER Technical Guide For Assessing And Mitigating The

Vapor Intrusion Pathway From Subsurface Vapor

Sources To Indoor Air – June 2015

“All else being equal, a longer collection period for each

individual sample would be expected to yield a more reliable basis

for estimating long-term, time-average exposure than would a

one-day sample collection period.”

US EPA Method 325: Volatile Organic Compounds from

Fugitive and Area Sources – September 2015

New EPA Method 325B – Refinery Fenceline Monitoring

Monitor the emissions of benzene

across the perimeter of refineries

within the United States.

Final Rule Issued Sept. 2015

Requirement to sample minimum

of 12 locations around perimeter

of refinery

Continuously sample using passive

sorbent tube samplers at each of

the sample locations

Two-week sample periods

Time-Integrated, Passive Samplers

Passive samplers allow for the collection of samples

over several days or weeks to measure organic

compounds in indoor and ambient air, which reports an

average concentration over that time period that may

be more representative of health risks

• Passive samplers are easy to use

• No pumps or flow regulators required

• Quality analytical procedures will produce high quality

data while achieving low reporting limits

• Sorbent samplers can target concentrations that span

orders of magnitude

Types of Passive Sorbent Samplers

High Uptake Rate and Low Uptake Rate Samplers

Example:

Uptake Rates for PCE

Range from 54 ml/min (High) to 0.41 ml/min (Low)

Equivalent to sampling 77 L to 0.59 L per day

or 539 L to 4.1 L per week

Long Duration Sampling Strategy

Passive Samplers allow for the collection of samples over

longer time periods – days or weeks

Passive Samplers – Principles of Operation

For application of Fick's First Law to a

diffusive sampler several simplifying

assumptions are necessary:

Ambient concentration of the analyte at the

surface of the monitor (Camb); that is, does not

take matter from its surrounding environment

faster than it can be replaced

Zero concentration of the analyte at the surface

of the sorbent; that is, the adsorbent is a zero

sink and therefore there is no saturation of the

adsorbent (Cads = 0)

A linear concentration gradient between the

two. Steady state conditions always exist

Axial type samplers

Source: Markes International

Principles of Passive Sampling

The adsorbent must be compatible with the target VOCs

The VOCs are retained on the sorbent for the full duration of

the sample period -- no back diffusion

Analytical method must completely desorb the VOCs from the

adsorbent -- no irreversible sorption

Calculate concentration:

C=M/U x t

C = Concentration ug/m3

M = Mass (nanograms x 1000)

U = Uptake rate (ml/min)

t = time (min)

Passive Samplers – Uptake Rates Theoretical

Calculate uptake rates:

U = A/Z x D

uptake rate (U) = ml/min

Area (A) = cm2

Distance (Z) = cm

Diffusivity Coefficient in Air (D) = cm2/sec

(e.g., cm2/cm x cm2/sec x 60 = cm3/min or ml/min)

Passive Samplers – Uptake Rates Theoretical

Uptake Rate

U = A/Z x D

ChemicalDiffusivity in Air (D)

(cm2/s)

Area

(cm2)

Distance

(cm)

Uptake Rate

(ml/min)

Vinyl Chloride 1.06E-01 0.18857 1.5 0.80

1,1-Dichloroethylene 9.00E-02 0.18857 1.5 0.68

Cis-1,2- Dichloroethylene 8.86E-02 0.18857 1.5 0.67

Trichloroethylene 7.90E-02 0.18857 1.5 0.60

1,1-Dichloroethane 7.42E-02 0.18857 1.5 0.56

Tetrachloroethylene 7.20E-02 0.18857 1.5 0.54

Trans-1,2- Dichloroethylene 7.03E-02 0.18857 1.5 0.53

Uptake Rates Theoretical vs. Measured

Chemical

Theoretical

Uptake

Rate

(ml/min)

Measured

Uptake

Rate

(ml/min)

Relative

Percent

Difference

(RPD)

Vinyl Chloride 0.80 0.78 2.48%

1,1-Dichloroethylene 0.68 0.57 17.43%

Cis-1,2- Dichloroethylene 0.67 0.58 14.15%

Trichloroethylene 0.60 0.50 17.50%

1,1-Dichloroethane 0.56 0.57 1.83%

Tetrachloroethylene 0.54 0.48 12.33%

Trans-1,2- Dichloroethylene 0.53 0.58 8.96%

Need for new uptake rate studies

Beacon Funded Passive Sampling Rate Study

Beacon worked with MCBA and the UK Health and Safety Laboratory (HSL) - recognized specialists in the field

Determine uptake rate stability over 7-day, 14-day and 26-day periods

Test two proprietary adsorbents in standard axial samplers

Include some Carbopack-X samplers to ensure that this work replicates previously published data (BTX added to mix along with chlorinated compounds)


Sampling study set up


Comparison to Published References


Chlorinated Compound Uptake Rate Linearity with Time

VC 1,1-DCE

T-1,2-DCE 1,1-DCA


Chlorinated Compound Uptake Rate Linearity with Time

C-1,2-DCE 1,2-DCA

TCE (A) TCE (B)

Reporting Limits

Target Analyte 3 Days 7 Days 10 Days 14 Days 21 Days

1,1-DCE 0.41 0.17 0.12 0.09 0.06

1,1-DCA 0.41 0.17 0.12 0.09 0.06

cis-1,2-DCE 0.40 0.17 0.12 0.09 0.06

1,2-DCA 0.41 0.17 0.12 0.09 0.06

1,1,1-TCA 0.45 0.19 0.14 0.10 0.06

Carbon Tetrachloride 0.45 0.19 0.14 0.10 0.06

TCE 0.46 0.20 0.14 0.10 0.07

PCE 0.48 0.21 0.14 0.10 0.07

Benzene 1.73 0.74 0.52 0.37 0.25

Toluene 2.23 0.95 0.67 0.48 0.32

Ethylbenzene 2.52 1.08 0.75 0.54 0.36

m,p -Xylene 2.52 1.08 0.75 0.54 0.36

o -Xylene 2.52 1.08 0.75 0.54 0.36

Detection Limit (ug/m3)

Reporting Limits

Target Analyte 3 Days 7 Days 10 Days 14 Days 21 Days

TCE 0.46 0.20 0.14 0.10 0.07

Detection Limit (ug/m3)

Average reporting limit also indicates that:

TCE conc never exceeded 1.4 ug/m3 in any 24 hour period

TCE conc never exceeded 4.2 ug/m3 in any 8-hour period

Note the 24-hr and 8-hr periods do not need to be continuous

Analysis by EPA Method TO-17

Analysis by thermal desorption-gas chromatography/mass spectrometry (TD-GC/MS) following EPA Method TO-17

• Analytical results based on 5-point initial calibration

• Internal standards and surrogates included with each analysis

• Daily continuing calibration checks

• Laboratory control samples

• System daily tunes and

• Method blanks

• Limit of Detection and Quantitation (LOD and LOQ) Studies

• Meets requirements of Level III/Level IV data quality objectives

• Able to recollect sample split to perform dilutions and re-analyses

TD-GC/MS

Laboratory Accreditation

It is important to have samples analyzed by an

accredited laboratory that has a demonstrated ability

to produce quality data within defined accuracy and

precision limits…

and ask for copies of their proficiency test (PT) results

Summa Canisters compared to Sorbent Tubes

Comparison Data

Passive Samplers (TO-17) – 14 Day Sample

Summa Canister (TO-15) – 1 Day Sample

Sample Location

Compound

TO-17

(14 day)TO-15 RPD

TO-17

(14 day)TO-15 RPD

TO-17

(14 day)TO-15 RPD

TO-17

(14 day)TO-15 RPD

Tetrachloroethene 3.4 3.4 0% 3.6 3.6 0% 0.45 J 0.43 J 5% 0.82 J 0.73 J 12%

Trichloroethene 1.3 1.2 8% 1.8 1.3 32% 0.11 U 0.46 U NA 16 13 21%

Samples collected in June 2012

Units in micrograms/cubic meter (ug/m3)

AI103 AI103P AI106 AI109

Strong correlation between passive sorbent tubes and summa canister

data for the two primary compounds of concern (PCE and TCE)

Project Management:

CH2M – Eric Davis and Kimberly Stokes

Passive Samplers -- Ambient Air (Method 325B)

Max RPD = 8.55 %

Max RPD = 9.05 %

Duplicate Sample Results

Duplicate samples

suspended side by side show

excellent correlation with a

14-day sampling period.

Sample Location

Methylene Chloride 0.60 0.59 1.7%

Trichloroethene 0.85 0.87 2.3%

Toluene 34.0 33.8 0.4%

Tetrachloroethene 306.7 305.6 0.4%

Ethylbenzene 4.11 4.01 2.5%

p & m-Xylene 10.98 10.59 3.6%

o-Xylene 2.66 2.64 0.8%

Sample Location

1,1,1-Trichloroethane 1.45 1.39 4.2%

Trichloroethene 2.44 2.58 5.6%

Tetrachloroethene 29.24 31.27 6.7%

Ethylbenzene 1.15 1.07 7.2%

Indoor #1

Beacon Passive

Diffusion Sampler

(14 Days)

Beacon Passive

Diffusion Sampler

(14 Days)

RPD

Compound

Compound

Indoor #2

Beacon Passive

Diffusion Sampler

(14 Days)

Beacon Passive

Diffusion Sampler

(14 Days)

RPD

Results in ug/m3

Passive Samplers -- Indoor Air

400+ Home Monitoring Program

Passive sorbent tube samplers are being used to collect samples over 14

to 26 days as part of an annual monitoring program of residences

overlying a groundwater plume contaminated with chlorinated solvents

Sample during the winter seasons

Previously, summa canisters were collected over 24 hour periods


Residents continue to have a very positive response to sorbent samplers

being used instead of summa canisters

Low-profile and easy to use

Plus for sampling team…, logistics are a lot easier to transport samplers


Good correlation with prior data collected at homes over past decade

when summa canister samples were collected

Data for blind duplicate passive samples

Passive Sampler Duplicate Passive Sampler

ug/m3 ug/m3

2016-2017 0.12 0.12

2016-2017 0.23 0.23

2015-2016 0.25 0.24

2015-2016 0.26 0.26

2016-2017 0.26 0.26

2015-2016 0.31 0.37

2015-2016 0.63 0.61

2015-2016 1.36 1.32

2015-2016 2.09 2.24

2015-2016 2.86 2.52

2015-2016 8.28 7.8

2015-2016 10.12 9.97

Season



Full Range of Concentrations



Low End Range of Concentrations

ASU Study House: Time-integrated passive samples collected over

20+ days vs. daily average samples

• Daily 24 hour samples collected on sorbent tubes with pump

(EPA Method TO-17)

• Time-integrated samples collected over multiple days using

Beacon Passive Samplers (Analysis by EPA Method TO-17)

Time Integrated Sampling Study – Indoor Air

Study Set-Up

On going study to compare results of the passive samplers vs. average daily

concentrations measured using Method TO-17 with pumped samples

Time-Integrated Beacon Passive Samplers compared to

Daily 24-hour average measurements

Passive Samplers collected in triplicate; exposed for duration of sampling periods

Pumped samples collected at a flow rate of 10 ml/min with a total volume of 14.4 L

Beacon Passive Samplers – Analysis of Accuracy

Correlation of Beacon Passive Sampler to

Average of 24-hour Active Samples

Beacon Passive Samplers – Analysis of Precision

Samples Collected in Triplicate

Analysis of Trichloroethene (TCE) Results

Sampling Sampling Average Stnd Dev COV

Event Days ug/m3

ppbv ug/m3

ppbv ug/m3

ppbv ppbv1 26 7.56 1.41 6.58 1.22 6.69 1.24 1.29 0.10 0.08

2 23 20.04 3.73 17.92 3.33 17.29 3.22 3.43 0.27 0.08

3 20 16.69 3.11 15.25 2.84 17.00 3.16 3.04 0.17 0.06

4 30 10.48 1.95 9.28 1.73 10.15 1.89 1.86 0.11 0.06

5 52 4.22 0.78 3.96 0.74 3.39 0.63 0.72 0.08 0.11

6 20 5.87 1.09 6.86 1.28 5.45 1.01 1.13 0.14 0.12

7 7 12.86 2.39 11.81 2.20 9.53 1.77 2.12 0.32 0.15

8 7 4.27 0.80 3.76 0.70 4.19 0.78 0.76 0.05 0.07

9 6 5.34 0.99 5.52 1.03 4.61 0.86 0.96 0.09 0.09

10 30 U U U U U U

11 43 2.24 0.42 1.33 0.25 1.78 0.33 0.33 0.09 0.26

12 35 2.23 0.41 2.36 0.44 2.28 0.42 0.42 0.02 0.04

13 36 1.71 0.32 1.66 0.31 1.81 0.34 0.32 0.02 0.05

AVERAGE 0.10

B-X-01 B-X-02 B-X-03

Data Source: Arizona State Univ. Study House

Drs. Paul Johnson, Paul Dahlen, Yuanming Guo

Summary

• Passive adsorbent samplers are easy to use and less

obtrusive than other sampling techniques

• Quality analytical procedures will produce high quality

data while achieving low reporting limits

• Passive samplers can target concentrations that span

orders of magnitude

• Passive samplers allow for the collection of samples over

several days or weeks to measure organic compounds in

indoor and ambient air, which reports an average

concentration that may be more representative of the

health risks to building occupants

Beacon Environmental Services, Inc.

Harry O’Neill

Forest Hill, MD USA

1-410-838-8780

[email protected]

www.beacon-usa.com

Thank you!

Any Questions?

Beacon… We can be your guide

how to measure long duration, time-weighted average

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