PASSIVE SAMPLING FUNDAMENTALS

AND APPLICATION FOR MONITORING

VOC’S IN VAPOR INTRUSION

INVESTIGATIONS

2nd October 2014

Atlanta

Nicola Watson

Agenda

• Diffusive sampler theory

• Factors that influence sampler performance

• Sampling complex atmospheres

• Comparison studies

• Real World applications

Diffusive Sampler Theory

• Diffusion is a molecular transport property. It is the process by which matter progresses along a concentration gradient until there is an equalisation of concentration within a single phase.

• Thus, as with any chemical system, a diffusing gas is spontaneously adopting its most probable energy distribution in its quest for an even, maximum dispersion and thus maximum entropy.

• The rate of this migration property is measured by its flux (J), which is the quantity of matter passing through a reference surface area per unit time.

• As Jx is the component of a vector and as matter flows down the concentration gradient away from its source, the coefficient of proportionality, D (the diffusion coefficient) in the matter flux expression must be negative.

Fick’s 1st Law

dx

dCJ x

where:

Jx - rate of diffusion (molcm-2s-1);

dC/dx - concentration gradient (molcm-2).

D - diffusion coefficient (cm2s-1).

dx

dCDJ x

therefore,

Application of Fick's First Law to passive samplers

• A passive sampler is essentially a collection medium, either

– solid sorbent,

– liquid sorbent,

– or chemically impregnated inert support,

which is separated from the atmosphere of interest by a zone of still air.

Axial type samplers

The driving force for

matter flux across the

still-air gap is the

induced concentration

gradient formed

between the sampler

opening and the air-

adsorbent interface

where vapour phase

matter is scavenged.

Application of Fick's First Law to passive samplers

• Ambient concentration of the analyte at the surface of the monitor (Camb) i.e. 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, i.e. 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

For application of Fick's First Law to a diffusive sampler several simplifying

assumptions are necessary:

Calculation of ambient concentrations

When these conditions apply, Fick’s First Law of Diffusion applies, and

analytes will migrate to the surface of the sorbent (uptake rate Uideal) at a rate

that is dependent on:

• the distance between the sorbent surface and the monitor surface (L)

• the cross-sectional area of the sampler (A)

• the diffusion coefficient of the analyte through air (D)

Knowing the,

• the time of exposure (t)

• the mass collected on tube (mt)

Will allow the ambient concentration (Camb) to be calculated

tU

mC

ideal

tamb

L

DAU ideal

Application of Fick's First Law to passive samplers

• The previously derived expression for the quantity of mass sampled by a

passive device holds only for axially configured devices.

• The simplified equation, still holds But Uideal is now,

Radial type samplers

tU

mC

ideal

tamb

)/(

2

ao

idealrrIn

hDU

Factors that influence sampler performance

The uptake rate is directly proportional to adsorbent bed surface area and

inversely proportional to diffusion path length. Commercially available passive

samplers fall into two main categories

• low uptake rate tube-type devices,

• high uptake rate badge-type devices.

Sample geometry

Factors that influence sampler performance

• Temperature

– A 20°C difference in temperature represents a five percent difference in sampled mass.

– However increased temperature does not lead to faster matter transfer and thus a greater quantity of analyte being up taken, but the opposite, which is why sampling devices should be protected from heat sources, such as direct sunlight, in the field.

• Relative humidity

– This is more influential when using polar adsorbents. At high RH, water molecules may preferentially occupy adsorbent sites, leading to premature saturation. Secondly,

– When the compounds of interest are water sensitive, hydrolysis or solution may take place on the adsorbent surface.

– When sampling for prolonged durations from ambient atmospheres, where high relative humidities are commonly encountered, a hydrophobic adsorbent is preferable.

Environmental factors

Factors that influence sampler performance

• Zone A: Represents stagnant, or slowly moving air. The main process by which this

matter is replaced is by diffusion and it has been shown that under these conditions

the replenishment rate is too low.

• Zone C: At the other end of the scale there is non-ideal behaviour through excessive

face velocity. Here due to turbulence in the diffusion air gap, the effective diffusion

path length is decreased and consequently the uptake rate increases.

Environmental factors - Face velocity (air speed)

Recommended minimum face

air velocities are:

Badge type samplers 0.05 to

0.10 ms-1.

Axial tube type samplers

0.001 ms-1.

Factors that influence sampler performance

A poor choice of adsorbent can lead to unpredictable sampler behaviour and unusable results that no amount of subsequent calculation or careful

laboratory procedure can correct.

Considerations when selecting a sorbent/compound pair:

• Ensure that the sorbent is strong enough for the compound(s) of interest and also for the duration of the sampling event i.e. the sorbent must behave as a zero sink. This ensures a constant sampling rate and no loss of compounds prior to analysis.

• If under extreme adsorbent loading conditions

– extended exposure to high concentration atmospheres

– misuse of high uptake rate devices

– poor matching of an adsorbent to the target species

a build-up in the vapour phase concentration at the adsorbent surface may be observed. This can lead to deviation from theoretical uptake rates and even sample loss back to the atmosphere

Sorbent selection

Factors that influence sampler performance

• For air monitoring situations, where concentrations of targets are typically high, the most critical factor governing the suitability of an adsorbent for a given compound is its adsorption capacity. An adsorbent’s capacity may be determined from its isotherm.

Sorbent selection – high concentration atmospheres

Isotherm "a" is favourable for

passive sampling.

The linear isotherm and

isotherm "b" are unfavourable

as they predict a rapid build

up in the vapour phase

concentration after only a

minimal uptake of adsorbate.

Factors that influence sampler performance

• When monitoring low concentrations in ambient atmospheres, sampler

blanks and adsorbent inertness to interference from humidity and reactive

atmospheric species also become governing factors for adsorbent choice

Sorbent selection – low concentration atmospheres

Sorbent name Volatility range

Tenax TA C7 – C30

Carbograph 2TD C8 – C20

Carbograph 1TD C5/6 – C14

Carbograph 5TD C3/4 – C6/7

SulfiCarb C3 – C8

Carboxen 1003 C2 – C5

Water retention

Effects of poor sorbent/compound pairings

• This occurs due to the inversion of the concentration gradient within the

sampler air-gap. It occurs when the vapour phase concentration within the

sampler becomes greater than the vapour phase concentration in the

surrounding air. This is particularly acute when sampling poorly retained

compounds from transient atmospheres.

Sample losses i.e. back diffusion

Sampling complex atmospheres

• Three VOC atmospheres –

with the concentration a) static

(blue), b) falling (red) and c)

increasing (green) over the

monitoring period – but with

the same time weighted

average concentration overall

Why is it important to ensure a constant sampling rate?

• Three more VOC atmospheres

with an emission event

occurring

a) early-, b) mid- and

c) late- in the monitoring cycle,

but with the same time weighted

average concentration overall.

T=0 T= t

T=0 T= t

A B C

Sampling complex atmospheres

The result, if sampling rate is not constant e.g. slowly dropping

T=0 T= t

Model does not take in to account back diffusion, which would further skew these results

Sampling considerations

• When designing a passive sampling study, the target compounds and typical

concentrations should be known.

– This will help define

• the type of sampler to use

• the sorbent

• the exposure period

• As a general rule, concentration ranges are,

Compound target concentration

8 h Four week

Axial 2 μg m–3 to 10 mg m–3 0.3 μg m–3 to 300 μg m–3

• Sampling over shorter periods is possible (30 minutes) for higher concentrations, as

is sampling over longer periods (six weeks) for much lower atmospheric

concentrations (sub-ppb), providing the correct sorbent is used.

• However, when atmospheric concentrations are very low (<1 ppb) and when a result

is required within a few hours, in these instances high uptake rate samplers provide a

better solution.

Performance Evaluation

• Assessments covered should include,

– the repeatability and reproducibility of replicate measurements;

– experimentally observed uptake rates at different exposure

doses to identify any deviation from ideal behaviour;

– the influence of environmental factors including temperature,

pressure, humidity and wind velocity on sampler performance;

– the effects of potential interferents on the adsorbent and

adsorbed compounds

• Many publications exist on validation of sorbent based samplers and

international standards are available

Laboratory and field

Performance Evaluation

• ASTM D6246 - 08(2013) Standard Practice for Evaluating the Performance of Diffusive Samplers

• ASTM D4597 – 10, Standard Practice for Sampling Workplace Atmospheres to Collect Gases or Vapors with Solid Sorbent Diffusive Samplers

• ASTM D6306 – 10, Standard Guide for Placement and Use of Diffusion Controlled Passive Monitors for Gaseous Pollutants in Indoor Air

• Protocol for assessing the performance of a diffusive sampler (Method for the Determination of Hazardous Substances No. 27), UK Health & Safety Executive.

• CEN EN 838: Workplace atmospheres – Requirements and test methods for diffusive samplers for the determination of gases and vapours.

• EN ISO 16017: Air quality – Sampling and analysis of volatile organic compounds in ambient air, indoor air and workplace air by sorbent tube/thermal desorption/ capillary gas chromatography. Part 2: Diffusive sampling.

• MDHS 80: Volatile organic compounds in air. Laboratory method using diffusive solid sorbent tubes, thermal desorption and gas chromatography (August 1995).

• E.R. Kennedy et al., Protocol for the evaluation of passive monitors, US National Institute for Occupational Safety and Health (NIOSH), published in: ‘Diffusive sampling: An alternative approach to workplace air monitoring’ (CEC Publication No. 10555EN), 1986.

Publications/Methods

Comparison Data

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 diffusion sorbent tubes and

summa canister data for the two primary compounds of concern

(PCE and TCE)

Passive diffusion samplers allow you to sample over several days

or weeks to measure organic compounds in indoor and ambient

air providing a sample that may be more representative of average

concentrations

Passive Indoor Air Sampling – NAS JAX

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 µg/m3

Passive Indoor Air Sampling – TN Sites

Real World applications of

passive sampling

Using Sorbent Tubes for Diffusive Sampling of

Outdoor Air

• 100 sampling sites - black

dots

• Yellow dots = pollution

hot spots

Mapping urban pollution concentrations with low-cost diffusive

(passive) sampling1

1 TDTS 10 Use of diffusive sampling with TD-GC for ambient air monitoring

Passive sampling for air monitoring in industry

Industrial perimeter

monitoring

Workplace air monitoring

Monitoring guidelines (sorbents, uptake

rates, sampling volumes, etc.) available from

standard methods

(Occupational hygiene)

In situ monitoring of

underground contamination

Sampling in confined atmospheres e.g.

Sub slab soil gas sampling

• Undesirable effect – starvation depends on two factors

– At low air velocities, a static layer of air external to the sampler will cause ‘starvation’ or an extension of the diffusive path and hence a flattening of the concentration gradient.

– If the air to which the sampler is exposed is stagnant, then the sampler may remove VOC vapors from the air faster than they are replenished, hence disturbing the equilibrium between the soil/water/vapour phases, also the sampler itself will impose a reduction in the concentrations it is intended to measure.

Ambient

concentration

Camb

Concentration

at adsorbent

surface

Cads

Diffusion

Path-length L

Air Velocity

Ambient conc. of

vapours in the

environment

Zero conc. of

vapours at the

sorbent surface

Air g

ap

Uptake rates can be reduced by:

• Making the diffusive path length longer

• Reducing the surface area of absorbent exposed

• Or a combination of both

Combating the Starvation Effect

Indoor Air and Personal Exposure

• TD-GC/MS is used for

several applications relating to

poor IAQ and ‘sick building

syndrome’.

• In this case residents were

complaining of poor air quality in

their home.

– Diffusive sampling with ‘axial’ sorbent

tubes was used to monitor indoor &

outdoor air quality at the house and to

monitor the personal exposure of

residents.

1st floor

2nd floor

3rd floor

Garage

Living

Quarters

Summary

• Many types of sorbent diffusive samplers (i.e. different geometries) have

been designed since the introduction and validation of sorbent based

samplers years ago.

• How do we decide which samplers should be included in standards, what

needs to be shown before we consider them? What properties MUST they

have

– Constant sampling rate

– Zero sink

• Passive sampling is a useful tool for VI monitoring and studies have

shown good correlation between these and other sampling devices.

• Applications could include, not only indoor air sampling, but personal and

soil gas (Sub slab).

Take home messages

Questions?

We would like to

acknowledge the work of

Beacon Environmental

Services

Contact us/Learn More

1-866-483-5684

enquiries@markes.com

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