Note: Descriptions are shown in the official language in which they were submitted.
CA 02389726 2002-06-27
BACKGROUND OF THE INVENTION
FIELD OF INVENTION
This invention relates to a method and apparatus of determining the
concentration
of analytes in sample using an extraction device whereby the concentration of
analytes of
interest can be determined from the diffusion coefficient for said analytes.
More
particularly, this invention relates to a method and apparatus for determining
the
concentration of organic and inorganic compounds in liquid and gaseous
samples.
DESCRIPTION OF THE PRIOR AR'h
It is known to use solid phase microextraction (SPME) and polydimethylsiloxane
1o (PDMS) coated fibers to extract volatile organic compounds (VOC's) in
environmental
samples. PDMS is the most widely used coating for extracting nonpolar volatile
analytes
as well as many polar analytes. However, the sensitivity of mixed phase SPME
coatings,
such as PDMS/DVB and Carboxen/PI~MS was reported to be much higher compared to
PDMS coating for extracting VOC's (see Mani et al, Applications of Solid Phase
15 Microextraction, RSC, (:ornwall, U.K., 1999, Chapter 5). Mixed phase
coatings have
some complementary properties compared to PDMS and are more suitable for
sampling
highly volatile species (see Pawliszyn, Solid-Phase Microextraction: 'Theory
and Practice,
Wiley-VCH, Inc., New York, 1997, Chapter 4). Mixed phase SPME fibers have been
used for sampling and quantifying target VOC's present in indoor air at the
part per
2o billion level and even at the part per trillium level.
Indoor air quality and its potential impact on human health is an increased
concern to the public and government environmental agencies. Many VOC's, such
as
formaldehyde, aromatic compounds, and halogenated hydrocarbons have been found
to
be highly toxic to humans. Large-scale air quality testing by conventional air
sampling
2s methods can often be time-consuming and expensive. Solid phase
microextraction
coupled with gas chromotography has been previously successfully applied to
analyze
various air samples. Chai et al., Analyst, 1993, 118,1541 reported the
determination of
the presence of volatile chlorinated hydrocarbons in air by SPME in 1993.
Martos et al.
developed a new method using a linear temperature-programmed retention index
method
3o to calibrate SPME devices for fluctuations in sampling temperature, and for
air analysis
of total VOC's with (PDMS) fibers (see Martos et al., Analytical Chemistry,
1997, 69,
CA 02389726 2002-06-27
206 and 402). Grote et al used SPMI? for fast quantitative analysis of
acetone, isoprene
and ethanol in human breath with SPME fibers in Analytical Chemistry, 1997,
69, 587.
It is known that the syringe-like SPME device is portable and can be easily
used for field
analysis. When coupled with a field-portable gas chromatograph, both SPME
sampling
and instrumental analysis can be conducted at the test site without the need
for sample
preservation (see Koziel et al, Analytical Chemistry, Acta, 1999, 400(1-3),
153.
In mixed phase coatings, the majority of interaction on porous polymer
particles is
determined by the adsorption process. W ith mixed phase coatings, the
molecules can be
attracted to a solid surface via van der Waals, dipole-dipole, and other weak
to intermolecular forces (see Gorecki et al, Applications of Solid Phase
Microextraction,
RSC, Cornwall, UK, 1999, Chapter 7 j. Hydrophobic interaction and
electrostatic
interaction also occur when extracting analytes from water and ionizable
analytes from
aqueous phase, respectively. Compared to the dilTusion coefficient in liquid
coatings of
PDMS or PA, the diffusion coefficients of VOC's in divinylbenzene and Carboxen
are so
small that, within the frame of SPME analysis, essentially all the molecules
remain on the
surface of the coating. Therefore, the fundamental difference between
adsorption and
absorption is that in adsorption molecules bind directly to the surface of a
solid phase
while, in absorption, they dissolve into the bulk of the liquid phase.
'The Langmuir adsorption isotherm is one of most important adsorption
theories. The
2o Langmuir model assumes there is only a limited number of surface sites that
can be
occupied by analyte molecules, all sites are equivalent, and there is no
interaction
between adsorbate molecules on adjacent sites. The Langmuir adsorption
isotherm was
used to describe the adsorption equilibrium on PDMS/DVB and Carbowax/DVB
coatings. A linear function is found to exist only if the affinity of an
analyte toward the
coating is low or its concentration in the sample is very low. In a real
sample matrix, for
example, air, there are usually more than two components. Since different
components
have different affinities towards the active sites, the presence of mufti-
components must
affect the adsorption of one other. Unlike the non-competitive absorption
process in
liquid coatings, adsorption process onto porous polymer coatings in a mufti-
component
3o system is a competitive process and therefore displacement effect is
expected. Sampling
conditions, the sample matrix composition and concentration can largely affect
the
CA 02389726 2002-06-27
4
amount of analytes extracted by mixed phase fibers. From a practical point of
view, this
makes quantitative analysis using porous polymer SPME coatings more difficult.
The majority of adsorption models are based on the equilibrium theory. In
SPME,
however, the equilibrium time ranges from a few minutes to a couple of hours
depending
on the nature of the analytes and the sampling conditions (see Ai et al.,
Applications of
Solid Phase Microextraction, RSC, Cornwall, UK, 1999, Chapter 2). For porous
solid
coatings, the equilibrium time for the same analyte is usually much longer
than that in
liquid coatings. It may be impractical to wait for partition eduilibrium of
all of analytes
in the matrix if the equilibrium times for some analytes are too long.
to In the direct SPME system, such as sampling in air ur in water, the analyte
movement
proceeds in two steps. The first step consists of the mass transfer of
analytes from the
bulk sample matrix to the surface to the SPME polymer coating followed by
diffusion of
the analytes within the coating. Fick's first law of diffusion (equation 1;)
can describe the
rate of mass diffusion in the sample matrix in the coating as follows:
~' - -DS(aCs ) (eq.l)
c'~ x
Where F is the flux of analyze in the direction x from the sample matrix
bulk to the SPME fiber surface.
(i) DS is the diffusion coefficient of the analyte in the sample matrix,
(ii) CS is the analyte concentration in the sample bulk.
2o In a static gas system, mass movement results only from molecular
diffusion due to intermolecular collisions. In practice, both molecular
diffusion and bulk
fluid movement must be considered. 'rhe extent of fluid movement (agitation),
reflects
the access of analytes to the surface and is frequently described as a
theoretical parameter
called the boundary layer (Cooper et al, Air Pollution Control: A Design
Approach,
Waveland Press Inc., Prospect Heights, 1994, Chapter 1 ~).
According to the boundary layer theory, a laminar sublayer or the sample
matrix film
is formed when a fluid passes a fixed object. The only way that the analyte
can pass from
the air bulk phase to the surface of the coating is via molecular diffusion
across the
CA 02389726 2002-06-27
boundary layer. In the liquid/solid interface, the thickness of the boundary
layer is
determined by the agitation conditions and the viscosity of the fluid (see
Pawliszyn, Solid
Phase Microextraction: Theory and Practice. Wiley/VCH, Inc., New York, 1997).
In a gas system, air wind velocity is a very important factor in mass transfer
process.
Because the value of wind velocity represents the degree of bulk air movement,
wind
velocity will influence the overall mass transfer rate in the bulk of fluid.
Based on mass
transfer theories, the mass transfer rate of an analyte is proportional to the
mass
diffusivity, and inversely proportional to the Thickness of gas film at the
interface.
Many factors such as temperature, pressure, molecular structure and molecular
weight
can directly affect the molecular diffusion coefficients of VOC's (see Lugg,
G.A.,
Analytical Chemistry, 1968, 40 (7), 1072). Since accurate experimental
measurement of
the diffusion coefficient is difficult, relatively few values for organic
compounds in gas
systems are available from the literature. A number of methods have been
proposed for
estimation of diffusion coefficients of VOC"s in air systems. 'the method by
Fuller,
~ 5 Schettler and Giddings (FSG method) was reported to be most accurate for
non-polar
organic gases at low to moderate temperature (see Lyman et al, Handbook of
Chemical
Property Estimation Method, ACS, McGraw-Hill, Inc., New York, 1982, Chapter
17).
Minimal error is associated with the aliphatics and aromatics. FSG model
describes that
the molecular diffusion coefficient of an analyte is directly proportional to
temperature,
and inversely proportioned to air pressure. The relative humidity of air is
another factor
that can affect VOC extraction on SPME fibers because water molecules
participate in
the adsorption process.
SUMMARY OF THE 1NVEN'rION
A method of determining a concentration of analytes of interest in a sample
using a
solid phase microextraction device having a surface containing an extraction
coating
comprises bringing the sample into contact with the coating while highly
agitating the
sample under controlled conditions to maintain a substantially constant
boundary layer
between the sample and the coating. 'The method further comprises limiting a
time of
contact between the sample and the coating and siring the coating so that all
analytes that
3o pass through the boundary layer are adsorbed by the coating. The method
further
CA 02389726 2002-06-27
comprises terminating the contact, determining the amount of each anaiyte of
interest in
the coating and calculating the concentration of each analyte of interest in
the sample by
using the diffusion coefficient for that analyte.
A method of determining the concentration of analytes of interest in a sample
uses an
extraction device having a membrane. The method comprises bringing the sample
into
contact with the membrane for sufficient time to allow tnicroextraction to
occur. The
method further comprises choosing a membrane with a large surface area and
limiting the
time of contact so that all analytes that contact the membrane are adsorbed by
the
membrane. The method further comprises separating the membrane from the
sample,
1o determining the amount of each analyte of interest in the membrane and
calculating the
concentration of each analyte of interest in the sample using the diffusion
coefficient for
that analyte.
A device for determining the concentration of analytes of interest in a
sample, the
device comprises a solid phase microextraction device having a surface
containing an
15 extraction coating characterized by a large surface area to adsorb all
analytes that contact
said coating and agitation means to highly agitate the sample under controlled
conditions
during microextraction.
In a further embodiment, a device for determining the concentration of
analytes of
interest in a sample uses a membrane having a large surface area to adsorb all
analytes
2o that contact a surface of the membrane in a time allowed for extraction.
The membrane
is sized and shaped to fit into an injection port of an analytical instrument.
BRIEF DESCRIPTION OF 'THE DRAWINGS
Figure 1 is a flow diagram of an air sampling system;
Figure 2 is a further embodiment of an air sampling system for samples having
25 varying humilities;
Figure 3 is a graph showing the effect of wind velocity on adsorption;
Figure 4 is a graph showing the extraction time profiles of toluene under
different
wind velocities;
Figure 5 is a graph of the slopes of toluene extraction time profiles and wind
velocity;
CA 02389726 2002-06-27
7
Figure 6 shows extraction time profiles for difference concentrations of
benzene at
high wind velocities;
Figure 7 is a graph of the amount of benzene adsorbed and concentration after
one
minute of sampling time;
Figure 8 is a graph of the amount of adsorption with temperature;
Figure 9 is a graph showing the relationship between the amount adsorbed at an
exposure time of 5 seconds with increasing temperature;
Figure 10 is a graph showing the amount of adsorption with temperature at a 10
second sampling time with a different fiber than that used for Figure 9;
to Figure 11 is a graph showing the amount of benzene adsorbed with time at
different
humilities;
Figure 12 is a graph showing the amount of toluene adsorbed with time at
different
humilities;
Figure 13 is a graph showing the amount of p-Xylene adsorbed with time at
different
l5 humilities;
Figure 14 shows an extraction device having an electric blower to provide
constant
air agitation;
Figure 1 SA is a schematic side view o P an extraction device having a fiber
with an
extraction coating thereon;
2o Figure 1 SB is a schematic side view of a flaw tube having an extraction
coating on an
inner surface thereof;
Figure 1 SC is a schematic side view of a vessel having an extraction coating
on an
interior surface of the vessel;
Figure 15D is a schematic side view of a vessel containing particulates with
an
2s extraction coating thereon;
Figure 1 SE is a schematic side view of a vessel having a stirrer with an
extraction
coating on the stirrer;
CA 02389726 2002-06-27
Figure 15F is a schematic side view of a vessel having a stirring bar with an
extraction coating on said bar;
Figure 16 is a schematic perspective view of a boundary layer surrounding a
silica rod
having an extraction coating thereon a.nd a graph of concentration profile;
Figure 17 is a perspective view of an expanded membrane attached to a handle;
Figure 18 is a perspective view of the membr~me of F figure 17 rolled around
the
handle;
Figure 19 is a schematic side view of a holder having three fibers; and
Figure 20 is a schematic side view of a device for determining the
concentration of
1 o analytes in a liquid.
DESCRIPTION OF A PREFERRED EMBODIMENT
Compared to the diffusion coefficient and liquid coatings of PDMS or PA, the
diffusion coefficients of VOC's in divinylbenzene and C",arboxen are so small
that, with
the frame of SPME analysis, essentially all of the molecules remain on the
surface of the
15 coating. Sampling conditions, the sample matrix composition and
concentration can
largely effect the amount of analytes extracted by mixed phase fibers. From a
practical
point of view, this makes quantitative analysis using porous polymer SPME
coatings
more difficult.
It has been found that with a very short exposure time (for example, one
minute),
20 when using SPME with PDMS/DVB coating fibers for fast sampling and analysis
of
VOC's in indoor air, there is a linear relationship between adsorption and
concentration.
Within a one minute sampling period, airborne benzene, toluene, ethylbenzene
and
p-xylene (BTEX) extracted on a PDMS/DVB fiber increased linearly with the
sampling
time. The short exposure time before equilibrium produces an advantage due to
the fact
25 that the adsorption rate is controlled by diffusion coefficients of
analytes rather than their
distribution constants. Because the differences between the diffusion
coefficients of
VOC's are much smaller compared to the differences of the distribution
constants are
much smaller than the differences between the distribution constants, all
target VOC's
CA 02389726 2002-06-27
with similar molecular weights produce similar extraction rates when using a
short
sampling time.
The mass transfer parameters through a boundary layer should include both
molecular
diffusion and bulk fluid movement. When using a porous polymer SPME coating
for air
sampling, it can be reasonably assumed that all available analyte molecules
are mobilized
within a very short exposure period. In other words, when the concentration of
analyte
on the coating surface is far from the saturation point, all of the target
molecules are
immediately adsorbed as soon as they contact the surface of the porous solid
extraction
coating. If the matrix composition and sampling conditions are kept constant,
it has been
1o found that the rate of mass diffusion of analyte will be proportional to
its mass diffusion
coefficient in the sample bulk within this short time period. Also, it has
been found that
there exists a quantitative relationship between the amount of analyrtes
adsorbed and
concentration depending on the diffusion coefficient of analytes when using a
very short
exposure time that occurs well before equilibrium.
1 s In the gas-solid interface, the thickness of the gas film is largely
affected by the air
movement or by the wind velocity and the nature of air. In a gas system, air
wind
velocity is a very important factor in the mass transfer process. Since the
value of wind
velocity represents the degree of bulk air movement, wind velocity will
influence overall
mass transfer rate in the bulk of the fluid. Based on mass transfer theories,
the mass
2o transfer rate of an analyte is proportional to the mass diffusivity and
inversely
proportional to the thickness of the gas film at the interface. ~Cherefore,
when considering
air sampling with porous SPME fibers, air wind velocity is a very important
factor related
to the adsorption process, especially for pre-equilibrium extraction.
With the present invention, some of the critical factors, including air/wind
velocity,
25 sampling temperature and air relative humidity have been investigated in
relation to the
adsorption process of VOC's onto porous polymer SPME coatings under non-
equilibrium
conditions.
Extraction Model Development. 'the solid SPME fiber coating can be modeled
as a long cylinder with length L, and outside and inside diameters of b and a,
respectively
30 (Figure I). When the coating is exposed to moving air, an interface (or
boundary layer)
CA 02389726 2002-06-27
with thickness b develops between the bulk of air and the idealized surface of
the fiber.
The analytes are transported from the bulk air to the surface of the coating
via molecular
diffusion across the boundary layer. In most cases, the molecular diffusion of
analytes
across the interface is the rate-limiting step in the whole adsorption
process.
5 The analyte concentration in the bulk air (Cg) can be considered constant
when a
short sampling time is used, and there is a constant supply of an analyte via
convection.
These assumptions are true for most cases of SPME air sampling, where the
volume of
air is much greater then than the volume of the interface, and the extraction
process does
not affect the bulk air concentration. In addition, the SPME solid coating can
be treated
1o as a perfect sink. The adsorption binding is instantaneous and the analyte
concentration
on the coating surface (CO) is far from saturation and can be assumed to be
negligible for
short sampling times and relatively low analyte concentrations in a typical
air. These
concentrations range from parts-per-trillion (by volume) to parts-per-million
(by volume)
for most VOCs of interest and typical industrial hygiene, indoor and ambient
air
concentrations. The analyte concentration profile can be assumed to be linear
from Cg to
C0. In addition, the initial analyte concentration on the coating surface (CO)
can be
assumed to be equal to zero when extraction begins. Diffusion inside the pores
of a solid
coating controls mass transfer from b to a.
The mass of extracted analyte with sampling time can be derived using the
2o analogy of heat transfer in a cylinder with inside and outside diameters of
b and b,
respectively, with a constant axial supply of heat. The steady-state solution
to heat
transfer can be translated into a mass transfer solution by replacing
temperatures with
concentrations, heat with flux of mass and heat transfer coefficient with gas-
phase
molecular diffusion coefficient. As a result, the mass of extracted analyte
can be
estimated from the following equation:
2rcD 1.
) In b~+8 ,~(:"x (t)dt
b ~ 1
()
CA 02389726 2002-06-27
where: n is the mass of extracted analyte over sampling time (t) in ng; Dg is
the gas-
phase molecular diffusion coefficient (em2/s); b is the outside radius of the
fiber coating
(cm); L is the length of the coated rod (cm); b is the thickness of the
boundary layer
surrounding the fiber coating (cm); and Cg is analyte concentration in the
bulk air
(ngJmL). It can be assumed that the analyte concentration is constant for very
short
sampling times and therefore Equation 1 can be further reduced to:
C r
In b+bl x
b /,
(2)
where t is the sampling time (s). The fiber length and th.e outside diameter
of the fiber
coating are constant for each type of the fiber. The nominal length for the 65
p,m
1o PDMS/DVB and the 75 p,m Carboxen'rM/PDMS coatings is L = 1 cm, and the
outside
diameter 2b = 0.0240 cm (~10%) and 2b = 0.026() cm (~l 0%), respectively.
It can be seen from Equation 2 that the amount of extracted mass is
proportional
to the sampling time, Dg for each analyte, bulk air concentration, and
inversely
proportional to 8. This in turn allows for quantitative air analysis. Equation
2 can be
t 5 modified to estimate the analyte concentration in the air in ng/mL for
rapid sampling with
solid SPME coatings:
n 1nC b + 8
a _ b
2~Dg Lt
(3)
The amount of extracted analyte (n) can be estimated from the detector
response.
2o For a special case, where the thickness of the boundary layer is much
smaller than
the outside radius of the fiber (8<<b), the general solution can be reduced to
a flat plate
problem. For such condition, ln(1+8/b) ~ b/b, 2~cbl, _ ~~, and Equation 2
simplifies to:
CA 02389726 2002-06-27
12
D A
n(t) = S Cg t
(4)
where A is the surface area of the sorbent. Equation 4 is analogous to the
mass uptake
model for the TWA sampling with retracted SPME fiber, where the distance
between the
needle opening and the fiber (Z) is replaced by 8.11,12
Under equal conditions, the amount of extracted mass will be greater for an
analyte with a greater gas-phase molecular diffusion coefficient (Dg). This is
consistent
with the fact that the analyte with a greater Dg will cross the interface and
reach the
surface of the fiber coating faster. Values of Dg for each analyte can be
found in the
t o literature or estimated from physicochemical properties. A number of
methods have been
proposed for estimation of diffusion coefficients of VOCs in air systems. The
method by
Fuller, Schettler and Giddings (FSG) was reported to be the most accurate for
non-polar
organic gases at low to moderate temperatures:
0.001x'1.75 x 1._+._._~__
Mair Mo_oc
Dg - r . _-.17
P~~, fair ~3 +~~ vvnc'3
(5)
where Dg is expressed in cm2/s; T is the absolute temperature (K); Mair, Mvoc
are
molecular weights for air and VOC of interest (g/mol); p is the absolute
pressure (atm);
Vair, Vvoc are the molar volumes of air and the VOC of interest (em3/rnol).
According
to the FSG model, Dg is directly proportional to temperature and inversely
proportional
to air pressure. Because the atmospheric pressure changes are relatively low,
the air
temperature is a more important factor than pressure when considering air
sampling.
Regardless, both atmospheric pressure and air temperature are routinely
monitored during
conventional air sampling.
The thickness of the boundary layer (8) is a function of sampling conditions.
The
most important factors affecting 8 are SPME coating radius, air velocity, air
temperature
and Dg for each analyte. The effective thickness of the boundary layer is
determined by
CA 02389726 2002-06-27
13
both rate of convection and diffusion. As the analyte approaches the sorbent
surface, the
overall flux is increasingly more dependent of diffusion than convection. The
analyte
flux in the bulk sample is assumed to be controlled by convection, whereas the
analyte
flux inside the boundary layer region is assumed to be controlled by
diffusion. The
effective thickness of the boundary layer can be described as the location
where this
transition occurs, i.e., where the flux towards cS (controlled by convection)
is equal to the
flux towards the surface of the SPME coating (controlled by diffusion). In the
Nernst
model, the matrix within the boundary layer is stationary. Experimental
research
indicated that convection was also present inside the boundary layer. However,
its
t o effects decreased with the distance to the solid surface. The effective
thickness of the
boundary layer can be estimated using Equation 6, adapted tiom the heat
transfer theory
for an SPME fiber in a cross flow:
8 = 9.52 ~' (6)
Reo'~2Sco~38
where Re is the Reynolds number = 2ublY, a is the linear air velocity (cm/s);
v is the
t5 kinematic viscosity for air (em2/s); Sc is the Schmidt number = vlDg. The
effective
thickness of the boundary layer in Equation 6 is a surrogate (or average)
estimate and
does not take into account changes of the thickness that may occur when the
flow
separates and/or a wake is formed. Equation 6 indicates that the thickness of
the
boundary layer will decrease with an increase of the linear air velocity.
Similarly, when
2o air temperature (Tg) increases, the kinematic viscosity also increases.
Since the kinematic
viscosity term is present in the numerator of Re and in the denominator of,Sc,
the overall
effect on 8 is small.
The gas-phase molecular diffusion coefficient (DR) for each analyte is also an
important parameter controlling & As illustrated in Equation 6, the effective
thickness of
25 the boundary layer will be reduced for analytes with lower D~. This can be
explained
considering that, analytes with low molecular weight will reach the coating
surface faster
then the less volatile analytes under equal experimental conditions and
therefore the point
at which the diffusion is a primary mode of analyte transport to the coating
is located
further away from the surface. The reduction of the boundary layer and the
increase of
CA 02389726 2002-06-27
1 ~1
the mass transfer rate for m analyte can be achieved in at least two ways,
i.e., by
increasing the air velocity and by increasing the air temperature. However,
the
temperature increase will reduce the solid sorbent efficiency. As a result,
the sorbent
coating may not behave as a zero sink for all analytes.
Chemicals and Supplies. The volatile organic compounds under study, i.e.,
benzene,
toluene and p-xylenes were purchased from Sigma-Aldrich (Mississauga, ON). All
VOC
standards had purities >_ 98.0% and used for calibrating GC/FID response
factors.
National Institute of Standards and Technology (NIST) traceable certified
permeation
tubes of benzene, toluene and p-xylene were purchased from Kin-Tech (La
Marque, TX),
1o and used for the generation of a standard gas mixture. Ultrahigh purity
hydrogen,
nitrogen, air were purchased from Praxair (Waterloo, ON). SPME fibers with 65
p,m
PDMS/DVB, 75pm Carboxen/PDMS and SPME holders were purchased from Supelco
(Oakville, ON).
Standard Gas. A standard gas-generating device with a flow-through sampling
chamber, was constructed to provide a wide range of target VOC concentrations
at
constant temperature. Ultrahigh purity air (zero gas) was supplied from a
Whatman air
generator (Haverhill, MA) and maintained at 50 psi head pressure. Permeation
tubes of
benzene, toluene and p-xylene were held inside a glass permeation tube adapter
(Kin-
Tech, La Marque, TX) and swept with a constant flow of dilution air. The
adapter was
2o placed inside a cylindrical aluminum coven, which was heated by two heating
elements
( 100 W), and its temperature was controlled by K-type thermocouple (Omega
~~M,
Stamford, CT) and an electronic heat control device (Science Shops, the
University of
Waterloo, ON). The air flow rate was controlled by two Sidetrack ~~M mass flow
controllers (Sierra Instruments, Monterey, CA) placed on both the primary and
the
dilution loops in the system. Wide ranges of concentration for target VOC's
were
obtained by adjusting both the air flow rate and the permeation tube
incubating
temperature.
Design for Air Wind Velocity Study. As shown in Figure l, an air sampling
system
2 consists of a main cylindrical glass chamber 4 and four additional
cylindrical glass
3o chambers 6, 8, 10, 12. All of the glass chambers 4, 6, 8, 10, 12 have
different diameters
CA 02389726 2002-06-27
with the chamber 4 being the largest. The chamber 6 has the smallest diameter
with the
chambers 8, 10, 12 each increasing in diameter in chronological order. The
main
chamber 4 contains an SPME device 14, which is described in more detail
subsequently.
A 1.0 L glass sampling bulb 16 (Supelco, Oakville, ON;1, was constructed and
installed
5 downstream from the standard gas generator. The gas flow rate varied from
1,000
standard cubic centimeters per minute (scan) to 4,000 scan for generating a
wide range
of air wind velocities. This new sampling system can provide both a dynamic
airflow
with different wind velocities and a static gas mixture. Experiments for
estimation of the
range of air velocities in typical indoor environments were conducted in a
mechanically
10 ventilated building using an OMEGA ~ M HHFS 1 Temperature and Air Velocity
Meter
(OMEGA, Stamford, CT). The indoor air velocities were found to vary with the
distance
between the measured location and the air vent, which is shown in the Table 1.
The
average indoor air velocity varied from 0 to 10 cm/s, and the average wind
velocity at
ventilating zones (near vent) varied from 15 to 40 cm/s. The data listed in
Table 1 were
15 consistent with the values reported by Wasiolek et al. ( 1999) Indoor Air-
International
Journal Indoor Air Quality and Climate, 9 (2) 125, who found that the average
indoor
wind velocities (at 19 locations in a workroom) varying from 1.4 to 9.7 cm/s,
and the
average wind velocity at the breathing-zone height varying from 9.9 to 35.5
cm/s by
using an accurate three-dimensional sonic anemometer. T'he glass chambers 4,
6, 8, 10,
12 allowed for sampling under dynamic flow conditions, and the chamber 16
allowed for
static sampling when stopcocks 18 and 20 were closed. A stopcock 22 opens and
closes
an exhaust line 24. The average wind velc>cities were calculated by dividing
the airflow
rate by the cross-section area of gas sampling chambers. When the air flow
rate was set
at 1,000 scan, the air velocities ranged from 0.2 to 20.8 cm/s. When the air
flow rate
was increased to 4,000 scan, the air velocities ranged from 0.8 to 83.2 cm/s.
Since the
Reynolds numbers in all chambers were less than 1,200, the air flow in the
sampling
chambers was in a laminar flow ecrndition. A 65 qm PDMS/DVB fiber was used to
sample the VOC gas mixture in each sampling port under different average air
velocities.
A short exposure time of 20 seconds was used to examine the effect of wind
velocity on
3o the VOC adsorption process onto PDMS/DVB fiber. The extraction time
profiles of
airborne BTEX were also constructed under various wind velocities.
CA 02389726 2002-06-27
16
Design for Temperature Study. The gas flow rate was maintained at 1,000 sccm,
and the permeation tubes were incubated at 60 °C. The main sampling
chamber 4 in
Figure 1 was used to provide a steady-state mass flow of VOC's at different
temperatures. The air temperature in the vicinity of the SPME fibers was
maintained
within ~ 0.3°C at the range of room temperature. A 65 pm PDMS/DVB fiber
and a 75
pm Carboxen/PDMS fiber were used to sample the VOC gas mixture in the chamber.
The temperature of the air stream in the chamber varied from 22 to
40°C:. The SPME
fiber exposure times were 5 seconds and 1 U seconds, respectively.
Design for Air Humidity Study. As shown in Figure 2, to create a dynamic
airflow
1o under different humidities, an in-line impinger trap 26 (Supelco, Oakville,
ON), and a
humidity meter 28 (Radio Shack, Waterloo, ON) were installed in the air
sampling
system. The components of Figure 2 that are identical to the components of
Figure 1 are
described using the same reference numerals as those used in F figure 1
without further
description. Relative humidities of 47% and 75% were obtained by maintaining
the
water level in the impinger trap at 1.0 cm and 8.0 crn height, respectively. A
65 pm
PDMS/DVB fiber was used to sample VOC's in the gas mixture under different
humidities.
Gas Chromatography. A Varian 3400 GC (Varian Associates, Sunnyvale, CA),
equipped with a FID and a carbon dioxide-cooled septum programmable injector,
was
2o used to analyze air samples extracted by SPME fibers and liquid samples of
standard
compounds. An SPB-5 capillary column (30 m x 0.25 mm i.d., 1.0 ~m film
thickness)
was installed in the GC, and UHP helium was used as the carrier gas with a
flow rate of
2.0 mL/min at 26 psi head pressure. 'The oven temperature program was 50
°C for 1 min,
15 °C/min to 240 °C and held for 2 min. For SPME fiber
desorption, the injector
temperature was isothermally set at 300 °C for Carboxen/PDMS fibers,
and at 250 °C for
PDMS/DVB fibers. For liquid injection, the injector was programmed from 45
°C to 225
°C at a ramp of 300 °C /min. The quantification of target VOC's
in standard gases was
based on the response factors obtained from the F1D signals by liquid
injection of VOC
standards in the test range.
CA 02389726 2002-06-27
17
Effects of Air Velocity. Figure 3 shows the effect of wind velocity on the
adsorption of benzene, toluene, p-xylene and ethylbenzene on a 75 micrometers
Carboxen/PDMS coating for Ss sampling of airborne BETEX. Each data point
represents
a normalized mass, i.e., the ratio of adsorbed mass and the analyte
concentration in air,
and is shown with ~ one standard deviation for three samples. Figure 3 clearly
indicates
that two distinct regimes of mass transfer are present: regime (1) where the
extracted
amount depends on the air velocity and regime (2) where the air velocity has a
less
significant effect on the amount of extracted mass ("semi- plateau" region).
The two zone phenomena can be explained by considering an interface between
l0 air and the porous solid sorbent. 'The first region in Figure 3 describes
diffusion of
analytes through the static, well-developed boundary layer surrounding the
SPME
coating. In this region, the increase in air velocity causes a reduction in
the boundary
layer thickness and more of each analyte can be extracted per unit of time.
This finding
is consistent with the theory summarized by Equation 2. In the second region,
above
15 some critical velocity, the thickness of the boundary layer is further
reduced, but it is
small enough that the mass transfer is controlled by the diffusion inside the
pores of the
SPME coating. Therefore the increase in air velocity has only a small effect
on the
amount of extracted analyte.
The critical velocity for which the effects of the boundary layer thickness
are
2o negligible is approximately 10 cm/s for the analytes in this study.
Although this range is
lower than the average air velocities in ambient air, the critical velocity is
close to the
range of measured air velocities in typical indoor air. Reported average
indoor air
velocities at the breathing-zone height varied from 9.9 to 3S.S cm/s, with the
average of
19 locations in a workroom varying from 1.4 to 9.7 cm/s. Particular care must
be taken to
25 ensure the reproducibility of extraction conditions with porous SPME fibers
in field
sampling. This is because a small change in air velocity in the vicinity of
solid SPME
fiber can have a significant effect on the amount of adsorbed analyte,
particularly in the
first mass transfer region (Figure 3).
Considering the fact that the amount of extracted mass for solid SPME fibers
can
3o be enhanced when sampling is conducted at greater air velocities, i.e., in
the "semi-
CA 02389726 2002-06-27
18
plateau" region (Figure 3), an external fan or an attachment to an air
sampling pump can
be used to provide greater rate of mass transfer. Such a device could be used
by air
sampling professionals wishing to equalize the extraction conditions and
provide
reproducible effective thickness of the boundary layer for each sample. The
use of a
higher air velocity for sampling with solid SPME coatings leads to enhanced
sensitivity.
Preliminary results indicate that the use of solid PDMS/DVB 65 ~m fiber
coating, 30 s
sampling and average air velocity of 1 m/s allows for detection of BTEX at 10
ppt (by
volume) range.
The greatest amount of mass was adsorbed for benzene, followed by toluene, p-
xylene and ethylbenzene. This finding is consistent with theory presented in
Equation 2,
i.e., the mass of adsorbed analyte using rapid sampling is proportional to the
Dg for each
analyte, when all other sampling conditions are equal. The 75 p,m
CarboxenTM/PDMS
coating was acting as a zero sink for short sampling times. The ratio of
normalized
masses in Figure 3 for benzene and toluene was close to the ratio of their Dg
s estimated
by the FSG method. Normalized masses for ethylbenzene andp-xylene were smaller
than expected. This discrepancy is likely associated with experimental errors.
Figure 4 shows the extraction time profiles of toluene using a 65pm PDMSIDVB
fiber under different wind velocities ranging from 0.8 to 83.2 cm/s. These
curves
illustrate the variation of toluene uptake within the whole range of indoor
air wind speed.
The toluene mass loading on PDMS/DVB fiber linearly increases with the
sampling time
within a short period of time ( 1 min). Furthermore, within this short
sampling time, the
toluene uptake rises with the increase of wind velocity. However, the
equilibrium mass
loading of toluene generally decreases with the increase of wind speed. 'This
is caused by
the fact that other, more strongly bound compounds extract faster as well
resulting in a
faster occurring displacement effect.
A further examination of the wind speed effect indicates that there are
different
influences on toluene adsorption on PI)MS/DVB fiber when sampling at different
wind
velocities. Generally, the slope of the toluene extraction time profiles
increases with the
increase of wind velocity. if we plot the slopes of toluene extraction time
profiles against
3o the average wind velocities applied, we can obtain Figure 5. This figure
demonstrates
CA 02389726 2002-06-27
l9
that the slope increases approximately linearly as the wind speed increases
from 0.8 to
8.7 cm/s. This means that the toluene mass loading on PDMS/DVB fiber was
significantly affected by the variation within the average indoor wind
velocity range (0-
em/s), and an approximately linear increase of mass loading can be expected
within
this wind range. Only a slight increase of toluene extraction was found as the
wind speed
increased from 8.7 to 83.2 cm/s. This range of wind velocity is usually found
at indoor
air ventilating zones. This indicates that the mass loading of toluene is only
slightly
affected by the variation of wind speed within indoor air ventilating zones (>
10 em/s).
Air Sampling under Wind Velocities Equal to or above a Critical Air Velocity.
to Figures 4 and 5 suggest that air sampling with a PDMS/DVB fiber should be
conducted
above some critical air velocity, above which the mass transfer and the
analyte uptake are
not affected by air velocity variation. Figure 6 shows extraction time
profiles for benzene
using a 65 p.m PDMS/DVB fiber to sample a standard VOC gas mixture at wind
velocities equal to or above 10.2 cm/s. Three VOC concentrations were obtained
by
setting different incubation temperatures (35, 40 and 60"C",) for VOC
permeation tubes
and a constant air flow at 2,000 scan. Another VOC concentration was generated
by
maintaining the incubation temperature at 60°C and increasing the air
flow rate to 4,000
sccm. These curves illustrate that the benzene uptake increased with the
sampling time
before reaching its equilibrium level. The higher the concentration, the less
time was
2o required for the PDMS/DVB fiber to reach the equilibrium. However, only
within a very
short sampling time ( 1 min), was benzene mass loading approximately linear
with
sampling time. Figure 7 shows that benzene uptake or response increased
linearly with
the concentration when 1 min sampling time was used under an air velocity
equal to or
above 10.2 cm/s. For other target VOC's, i.e., toluene andp-xylene, similar
results were
also observed (data not shown).
Temperature Effect on VOC Adsorption on Porous SPME Fibers. Figure 8
shows that amounts of toluene andp-xylene adsorbed on the PDMSIDVB fiber for a
Ss
exposure increase linearly as the temperature increases from 22 to
26°C, while benzene
uptake remains almost constant in this temperature range. As the temperature
increases
3o continuously, the amounts of toluene and p-xylene adsorbed increase
slightly, while the
CA 02389726 2002-06-27
amount of benzene adsorbed decreases. This indicates the displacement of
benzene
molecules byp-xylene or toluene molecules, which have higher affinities to the
PDMS/DVB coating than benzene. T'he results indicate that increasing
temperature
within a certain range will enhance the adsorption of VOC's on PDMS/DVB
coating,
5 especially for analytes with higher affinity to the coating. Similarly to
Figure 8, Figure 9
shows that the mass of toluene and p-xylene adsorbed onto the PDMS/DVB fiber
for a
lOs exposure increase with the increase of temperature from 22 to 25°C.
Benzene
adsorbed remains almost constant from 22 to 25°C, but decreases as the
temperature
increases further ftom 25 to 40°C. In fact, this situation should be
expected because the
active surface sites become saturated as the adsorption on the coating
proceeds, and some
benzene molecules are possibly displaced by toluene or p-xylene molecules.
In Figure 10, amounts of all three analytes adsorbed on the CarboxeWPDMS fiber
increase linearly as the temperature increases from 22 to 25°C when
using l Os sampling
time. As the temperature increases continuously, the mass of analytes adsorbed
only
~5 increases slightly. Generally, a similar adsorption behavior between
PDMS/DVB and
Carboxen/PDMS was observed. However, the Carboxet~/PDMS fiber has a higher
adsorption capacity than PDMS/DVB fiber for extraction of benzene, the
smallest
molecule among the analytes. Unlike the DVB particle consisting of mainly
mesopores
and a smaller fraction of macropores and micropores, the Carboxen polymer
particle has
2o an even distribution of micro, meso, and macro pares. Therefore, Carboxen
particles are
better for sampling smaller molecules (CZ-C~2) compared to PDMS/DVB fiber.
Unfortunately, the weakness of CarboxenLPDMS is the difficulty for analyte
desorption.
Peak tailing is often observed even with the GC injector temperature of 300
°C.
As one of the most important experimental parameters in SPME sampling, the
extraction temperature has been discussed in several previous papers related
to SPME air
sampling. When a pure-phase liquid SPME fiber is used, an increase in
extraction
temperature usually causes an increase in extraction rate, but simultaneously
a decrease
in the distribution constant. Since the extraction by the SPME coating is an
exothermic
process, a decrease in mass loading at equilibrium is usually expected as the
extraction
3o temperature increases. In contrast to liquid fiber, however, an opposite
trend of
temperature effect was found in this study when mixed-phase porous SPME fibers
were
CA 02389726 2002-06-27
21
used. Within a very short sampling time (far .from equilibrium), VOC analyzes
on a
porous SPME fibre can linearly increase as the extraction temperature
increases in a
narrow range. Since VOC', adsorption on a porous SPME fiber is controlled by
the
diffusion process or diffusion coefficient rather than the extraction
equilibrium or
distribution constant, an increase in diffusion coefficients with an elevated
temperature
should increase VOC uptake on the solid SPME coating.
Effect of Humidity on VOC Adsorption onto PDMS/DVB Fiber. Figures 11-13
are the extraction time profiles of benzene, toluene and ~-xylene using a 65
p.m
PDMS/DVB fiber to extract a standard VOC gas mixture at different humidities.
These
to figures indicate that a humidity level of 7S% resulted in a significant
decrease in the
VOC uptake on the PDMS/DVB coating at equilibrium, especially for smaller
molecules,
e.g., benzene. Due to the high affinity to the PDMS/DVB porous polymer
coating, water
molecules compete with other VOC molecules and occupy a portion of active
surface
sites on the coating surface. Therefore, fewer active suri:ace sites are
available to VOC
15 molecules, especially to smaller molecules with lower affinity to the
coating. However,
within a very short sampling time, i.e., 1 minute, no significant difference
was observed
among the conditions with different humidities. This indicates that the active
surface
sites are not saturated within a very short extraction time, and still
available to VOC
molecules. Thus, a short sampling time (far before equilibrium) minimizes the
effect of
2o humidity on adsorption of VOC's on the PDMS/DVB coating. Table 4 shows the
effect
of relative humidity for decreasing VOC uptake onto the YDMS/DVB coating. The
humidity effect can be neglected if using PDMS/DVB fiber for a very short time
air
sampling in a low humidity (< 50%). However, the result suggests that a mass
loading
decrease of VOC on PDMS/DVB can be expected if the relative humidity is above
50%.
25 Air wind velocity and temperature are important parameters related to the
diffusion
process on porous polymer SPME fibers, particularly at non-equilibrium
conditions.
Wind speed or bulk air movement significantly affects the VOC mass transfer
process
from the bulk air to the fiber in a certain range. This indicates that the
thickness of the
gas-phase boundary layer between the fiber and air is diminished as the wind
speed
3o increases, and the mass transfer rate was accelerated between 0 and S cm/s.
This wind
CA 02389726 2002-06-27
22
speed range is typical to average air velocities in indoor air. Therefore, air
sampling with
porous polymer coated SPME fibers should be conducted above some critical air
velocity, above which the mass transfer and the analyte uptake are not
affected, and the
extraction can be reproduced.
When using a liquid SPME fiber, a decrease in mass loading at equilibrium is
usually
expected as the extraction temperature increases because the extraction is an
exothermic
process. An opposite trend of temperature effect was found in this study when
mixed-
phase porous SPME fibers were used with a short sampling time. Within a very
short
sampling time far from equilibrium, analytes extracted on a porous polymer
coated
SPME fiber increased linearly as the extraction temperature increased in a
narrow range
from 22 to 25°C. In this case, the adsorption process is controlled by
diffusion
coefficients instead of distribution constants of analytes. The effects of
wind and
temperature on adsorption by porous polymer coated SPME fibers under non-
equilibrium
conditions have not been addressed by previous researchers. The analytical
data indicate
~ 5 that there is a direct relationship between the rate of mass transfer and
the analyte
diffusion coefficients. Therefore appropriate diffusion coefficients obtained
either from
the literature, calculated or experimentally determined can be used to
calibrate the
relationship between amount of analyte extracted versus concentration for
given
extraction time. During the experiment constant agitation condition are
necessary, or if
different agitation conditions are used than the appropriate adjustment
coefficients needs
to be calculated. Figure 14 shows the example of a simple device 34 based on
an electric
blower 36 having a fan (not shown) that sucks air into an inlet 38 on a
cylindrical head 40
and exhausts the air through an outlet 42. The flower 36 is able to provide
constant air
agitation. An SPME device 14 has a holder 44 mounted in an SPME insert 46. An
O
ring 48 is located between the holder 44 and insert 46. A sleeve 49 extends
through the
head 40 to position a fiber 50 within the head 40. The blower 36 has a handle
52. The
reported data can be extended to liquid sample analysis. In this case,
diffusion of
analytes in a liquid matrix can be used to calibrate the response. Analogous
devices to
one showed on Figure 14 for air analysis can be designed to provide constant
agitation of
3o the sample matrix. For solid samples, indirect headspace or liquid
extraction can be used.
CA 02389726 2002-06-27
23
For air analysis, high humidity was found to decrease VOC uptake on the
PDMS/DVB coating. However, the humidity effect can be minimized by using a
very
short exposure time, in which case the active surface sites are not saturated.
The humidity
effect can be neglected when using PDMS/UVB fiber for a very short time air-
sampling
in a low humidity, but a mass loading decrease of VOC's on PDMS/DVB can be
expected if air samples are taken from a high humidity environment when the
calibration
might be necessary. It is expected that both temperature and humidity will be
monitored
during the field measurement and appropriate correctic>r~ coefficients will be
calculated, if
necessary to adjust the response.
1o The proposed diffusion based extraction and calibration approach is
expected to be
the fastest possible sampling/sample preparation approach in the field and in
the
laboratory. Various different arrangements of the extraction phase can be used
to
practically implement this technology (see Figures 15A to 1 SE).
In Figure I SA, a vessel 60 containing a sample 62 has an extraction phase
coating 63
15 on a tubular member 64. In Figure 1 SB a tube 66 has an extraction phase
coating 63 on
an inner surface thereof. Sample 62 contacts the extraction phase coating as
it flows
through the tube 66.
In Figure 15C, the vessel 60 has an extraction phase coating 63 lining an
inner surface
thereof. The sample 62 contacts the coating 63 when it is contained in the
vessel.
20 In Figure 15D, the vessel 60 contains the sample 62. Particles 72 are
located within
the sample and each particle is surrounded by extraction phase coating 63. In
Figure
15E, the vessel 60 has a sample 62 with a stirrer 74 extending into the
sample. The stirrer
has paddles 76 with extraction phase coating 63 on the paddles. In Figure 15f,
there is
shown a vessel 60 containing a sample 62 with a stirring bar 78 located within
the
25 sample. The stirring bar 78 contains extraction phase coating 63.
In Figure 16, there is shown a schematic perspective view of a silica rod 60
surrounded by a solid extraction phase coating 82 that is porous and contains
pores 86. A
cylindrically shaped boundary layer 84 surrounds the cylindrically shaped
coating 82 and
analytes, designated by Dg pass through the boundary layer and are adsorbed by
the
3o coating 86. A graph at the bottom of Figure 16 shows that the boundary
layer has a
CA 02389726 2002-06-27
24
thickness delta, the rod 80 has a radius a and a radius b equals the distance
from a center
of the rod 80 to the exterior surface of the coating 86. 'The concentration at
Co is the
concentration at the interface between the boundary layer and the coating and
the
concentration at Cg is the concentration of the gas.
In Figure 17, a membrane 90 is supported on a handle 92 and the membrane is in
an
unfolded position. In Figure 18, the membrane 90 is rolled around the handle
92. Figure
l7 shows the collection mode where the handle and membrane are brought into
contact
with a sample (not shown). After rapid extraction has occurred, the membrane
is moved
out of contact with the sample and the membrane 90 is rolled around the handle
92 to
1o make it more compact. 'The membrane and handle can then be inserted into a
cylindrical
sheath (not shown), which is airtight. The membrane can then be transferred to
an
analytical instrument, which can be located in the field where the sample has
been taken
or to an instrument located away from the test site. The sheath prevents the
membrane
from becoming contaminated during transport.
In Figure 19, there is shown a further embodiment of the invention where a
brush 100
has bristles 102 extending therefrom and each bristle has an extraction phase
coating 104
thereon. In Figure 20, a device 107 has a SPME syringe 108 supporting a fibre
109 with
a coating 110. A motor l 12 powers a stirrer 114 in a bracket 116.
