Applications of polydimethylsiloxane in analytical chemistry: A review

Analytica Chimica Acta 750 (2012) 48–62

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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Review

Applications of polydimethylsiloxane in analytical chemistry: A review夽 Suresh Seethapathy, Tadeusz Górecki ∗ Department of Chemistry, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 Polydimethylsiloxane (PDMS) has numerous applications in analytical chemistry.  Sorptive properties of PDMS are used primarily in sampling.  Partitioning properties are used in separations.  Permeability of PDMS forms the basis for sample introduction and passive sampling.  Advantageous mechanical properties are used in lab-on-a-chip devices.

a r t i c l e

i n f o

Article history: Received 22 February 2012 Received in revised form 26 April 2012 Accepted 3 May 2012 Available online 11 May 2012 Keywords: Polydimethylsiloxane (PDMS) Passive sampling Sorption Permeability Extraction

a b s t r a c t Silicones have innumerable applications in many areas of life. Polydimethylsiloxane (PDMS), which belongs to the class of silicones, has been extensively used in the field of analytical chemistry owing to its favourable physicochemical properties. The use of PDMS in analytical chemistry gained importance with its application as a stationary phase in gas chromatographic separations. Since then it has been used in many sample preparation techniques such as solid phase microextraction (SPME), stir bar sorptive extraction (SBSE), thin-film extraction, permeation passive sampling, etc. Further, it is gaining importance in the manufacturing of lab-on-a-chip devices, which have revolutionized bio-analysis. Applications of devices containing PDMS and used in the field of analytical chemistry are reviewed in this paper. © 2012 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structure and properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Partitioning properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

49 49 50

Abbreviations: CMV, cytomegalovirus; DNA, deoxyribonucleic acid; ESE, equilibrium sorptive enrichment; GC, gas chromatography; GC–IR-C-MS, gas ´ Univerchromatography–isotope ratio-combustion-mass spectrometry; GLC, gas liquid chromatography; GADPH, glyceraldehyde 3-phosphate dehydrogenase; GUT, Gdansk sity of Technology; HSSE, headspace sorptive extraction; LOC, lab-on-a-chip; LTPRI, linear temperature-programmed retention index; MESCO, membrane-enclosed sorptive coating; MESI, membrane extraction with a sorbent interface; MEMS, microelectromechanical system; MIMS, membrane inlet mass spectrometry; MOSFET, metal oxide semiconductor field effect transistor; NIOSH, National Institute of Standards and Health; OCP, organochlorine pesticides; OSHA, Occupational Safety and Health Administration; PAH, polyaromatic hydrocarbons; PCB, polychlorinated biphenyls; PCR, polymerase chain reaction; PDMS, polydimethylsiloxane; PTFE, polytetrafluoroethylene; SBSE, stir bar sorptive extraction; SiSTEx, solvent in silicone tube extraction; SMSE, silicon membrane sorptive extraction; SPME, solid phase microextraction; STE, sorptive tape extraction; sVOC, semi-volatile organic compound; TWA, time-weighted average; US-EPA, United States Environmental Protection Agency; VOC, volatile organic compound; WMS, Waterloo Membrane Sampler. 夽 PDMS versatility makes it a perfect material for analytical chemistry. ∗ Corresponding author. Tel.: +1 519 888 4567x35374; fax: +1 519 746 0435. E-mail address: [email protected] (T. Górecki). 0003-2670/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2012.05.004

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Permeability properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Effect of temperature on the partition coefficient and permeability of PDMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications utilizing the partitioning properties of PDMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. GC columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. SPME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Thin film extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. PDMS rod and PDMS tube microextraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. SBSE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Integrative and equilibrium sorptive enrichment (ESE) using packed bed PDMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. In-tube SPME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8. Membrane-enclosed sorptive coating (MESCO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9. Sorptive tape extraction (STE) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications utilizing the permeability properties of PDMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Permeation passive samplers with PDMS membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Membrane inlet mass spectrometry (MIMS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Membrane extraction with a sorbent interface (MESI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications of PDMS in lab-on-a-chip devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Miscellaneous applications of PDMS in the analytical laboratory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.

3.

4.

5. 6. 7. 8.

Suresh Seethapathy graduated from the University of Waterloo, ON, Canada with a Ph.D. degree in Chemistry in November 2009 under the supervision of Prof. Tadeusz Górecki. Prior to his studies at Waterloo he worked in analytical chemistry departments of various chemical industries including bulk herbal drugs, perfumery raw materials, metal working fluids, polymers and dental pharmaceuticals. Dr. Seethapathy continued his postdoctoral studies in the same group until January 2011. During his tenure at the University of Waterloo, his research focused on passive air sampling, calibration of permeation passive samplers and their applications. The research resulted in successful commercialization of a polydimethylsiloxane based permeation passive sampler used for sampling volatile organic compounds from air and soil gas matrices. Dr. Seethapathy is currently an applications scientist with Thermo Fisher Scientific and is involved in the development of gas chromatography and gas chromatography–mass spectrometry applications. His other areas of interest include fast gas chromatography and multidimensional gas chromatography.

1. Introduction PDMS belongs to the group of silicones, which are made of silicon, carbon, hydrogen and oxygen, and sometimes other elements as well. Pioneering work on silicones, and PDMS in particular, was done by both Dow Corning and General Electric Company in the middle of the 20th century, and their development continues to this day. A search in the SciFinder Scholar journal database with keyword “polydimethylsiloxane” results in nearly 24,000 hits, which illustrates its extensive use in various areas of chemistry, and science in general. The use of the term “PDMS” in analytical chemistry and polymer-related journals only was also researched. With a few exceptions, the data indicate a steady increase in the number of research articles related to PDMS since 1981. The first notable application of PDMS in analytical chemistry was perhaps its use as stationary phase in gas liquid chromatography (GLC). In the past two to three decades, the applications of PDMS have expanded mainly in the areas of sampling of organic compounds from air, water and soil gas matrices, as well as manufacturing of lab-on-a-chip (LOC) devices. PDMS employed in various analytical techniques might have many different geometric forms, including thin films, tubes and rods; coatings on a fiber, inside a

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50 52 52 52 54 55 55 56 56 56 57 57 57 57 58 59 59 60 60 60 60 60

Tadeusz Górecki is a professor at the Department of Chemistry, University of Waterloo (ON). He obtained his M.Sc. Engineer (1981) and Ph.D. (1986) degrees from the Gdansk University of Technology, Poland, and the Professor of Chemical Sciences degree (2009) from the President of the Republic of Poland. Prof. Górecki’s scientific interests include passive sampling, comprehensive two-dimensional gas chromatography (GC × GC), enhanced extraction techniques, environmental analysis, field analysis, pyrolysis GC–MS and HPLC. He is an author/co-author of 20 books/book chapters, over 130 papers published in peer reviewed journals, over 210 conference papers/abstracts/short communications (including 36 invited lectures) and 5 patents.

capillary column, on a magnetic stir bar etc.; foams or particles packed into sorbent tubes, etc. PDMS can be manufactured into the required shape and form by processes such as extrusion, coating, molding, calendering and soft lithography. Nowadays, it can be easily fabricated using two-part kits available from various vendors. In most cases, one of the parts is a pre-polymer (generally a vinylterminated PDMS), while the other is a cross-linker (dimethyl, methyl hydrogen siloxane) [1]. By combining the two ingredients in specific ratios and curing them at specified conditions, PDMS of different properties can be obtained [1]. The structure and physicochemical properties of PDMS responsible for its wide applicability in analytical chemistry will first be discussed in this article, followed by their applications in various analytical devices. 2. Structure and properties PDMS (CAS number 63148-62-9) consists of a flexible (Si–O) backbone and a repeating (Si(CH3 )2 O) unit [2]. The number of the repeating (Si(CH3 )2 O) units generally defines the molecular weight, and consequently many of the viscoelastic properties of the material [2]. Based on the application needs, alterations to the viscoelastic properties can be accomplished by cross-linking the

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polymer [3] (e.g. vinyl cross-linking), or by adding fillers (e.g. silicon dioxide) to the polymer network [4]. PDMS is a non-toxic, highly hydrophobic, translucent polymer that does not bio-accumulate. It has a very low glass transition temperature of −127 ◦ C [5] and a shear elastic modulus of 250 kPa, which changes at a very small rate of 1.1 kPa per 1 ◦ C change in temperature [6]. The specific gravity of PDMS is generally within the range from 0.91 to 1.00. For PDMS with molecular weight varying from 10,000 to 60,000 g mol−1 , the dielectric constant is very low and ranges from 2.72 to 2.75 (implying low refractive index) [7]. Further, PDMS is optically transparent down to 300 nm, enabling its use in some optical measurements [8]. Even though PDMS is non-reactive to many chemicals, it tends to swell when exposed to hydrophobic solvents. Most hydrophilic solvents such as water, nitromethane, acetonitrile, dimethylsulphoxide and ethylene glycol, used frequently in analytical chemistry, swell the PDMS matrix the least [9]. This makes it possible to use PDMS to extract analytes from water for quantitative analysis. On the other hand, most hydrophobic solvents including pentane, diisoproylamine, triethylamine and xylene swell the PDMS the most [9]. This property is often used for extracting the analytes from the PDMS matrix after the analytes are absorbed in the polymer. Lee et al. determined the swelling of PDMS in a variety of polar, moderately polar and non-polar organic solvents generally used in chemical synthesis and correlated the swelling with the solubility parameter [9]. In analytical chemistry, the partitioning and permeability properties of PDMS are by far the most widely used and will be briefly discussed before their actual use in various applications is described. Mechanical, electrical and optical properties of PDMS have been exploited in some applications and will be presented subsequently. 2.1. Partitioning properties Whenever there is a difference in the chemical potential of a species between two phases in contact with each other, there will always be a net movement of this species in the direction which tends to bring the system to equilibrium. In the case of PDMS, two parameters are often used to describe the sorption properties of the polymer towards a chemical: solubility (S) and partition coefficient (K). These two parameters are thermodynamic properties and define the relative concentration of an analyte at equilibrium between PDMS and the matrix in contact with the polymer. Analyte sorption by PDMS is an absorption phenomenon, where the analyte molecules diffuse into the bulk of the homogeneous polymer [10]. The term “solubility” is generally used to describe the sorption property in air, and is defined as the ratio of the analyte concentration in the membrane (Cm ) to its partial pressure in the air (p), as shown in Eq. (1). This term is often used in the field of pervaporation studies. S=

Cm p

(1)

In analytical chemistry however, the term partition coefficient (or distribution ratio) is more often used. It is defined as the ratio of the concentration of the analyte in the vapor or liquid phase to its concentration in PDMS at equilibrium (Eq. (2)). K=

Cm Cg

Kinetic region

Equilibrium region

Analyte mass

50

Time Fig. 1. Analyte mass accumulation as a function of time.

When PDMS is exposed to the sample matrix, analyte concentration in the polymer changes with time as shown in Fig. 1 [12]. The relationship is generally represented by a first-order one-compartment model described by Eq. (3), where CPDMS is the analyte concentration in PDMS, Cmedium is the analyte concentration in the sample matrix, t is the time, and k1 and k2 are the uptake (into the membrane) and elimination (into the matrix) constants [12]. CPDMS = Cmedium

k1 [1 − e−k2 t ] k2

(3)

For each sampling system based on the partitioning properties of PDMS, the extraction time profile (or kinetics) at a specific temperature is generally dictated by the volume of the PDMS phase, its surface-to-volume ratio, sample agitation conditions, partition coefficient of the analyte of interest, boundary layer conditions, as well as the analyte’s diffusion coefficient within the PDMS network [13]. In quantitative applications, the amount of analyte trapped in PDMS after exposure to a sample is determined, and the concentration in the sample is computed based on which part of the curve shown in Fig. 1 is used. In the linear region, the analyte mass collected is directly proportional to its concentration in the sample and the time for which the PDMS is exposed to the sample. The intermediate region is seldom used, but is currently getting more popular owing to the introduction of kinetic methods of calibration [14]. In the equilibrium region, the analyte mass is related to its concentration through the partition coefficient K as shown in Eq. (2). Mathematically, it can be shown that the extraction yield R at equilibrium in a PDMS phase is given by [15]; R=

1 (ˇ/K) + 1

(4)

where ˇ is the phase ratio (ratio of sample volume to PDMS volume) and K is the PDMS-matrix partition coefficient. The general approach is to have high ˇ in order to achieve R greater than the quantitation limit of the method. Consequently, for a specific ˇ, the quantitation limits generally decrease with increasing K. 2.2. Permeability properties

(2)

The two terms are easily convertible using ideal gas law [11]. The partitioning property forms the basis of such techniques as GLC, solid phase microextraction (SPME), thin film extraction, stir bar sorptive extraction (SBSE), PDMS rod extraction, PDMS particle bed extraction, membrane-enclosed sorptive coating (MESCO), etc.

Whenever there is a difference between the concentrations of a soluble analyte on either side of a polymer membrane, there will be a net flow of the chemical from one side of the membrane to the other. This process is termed permeation. For rubbery polymers like PDMS, this process was first described by Graham using his solution-diffusion model for organic compounds as early as in

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Membrane

Concentration

Sample

“Zero sink” sorbent

Analyte concentration at the surface of the membrane

Analyte concentration at the surface of the membrane

Analyte concentration in air

Distance Fig. 2. Ideal steady state concentration profile when vapor phase concentration is practically zero at the membrane-sorbent interface.

the 1800s [16]. According to this model, the transfer of gas or vapor across a polymer takes place in three steps: dissolution of the vapor molecule in the polymer, diffusion of the molecule across the polymer under a concentration gradient, and release of the vapor from the polymer at the opposite side of the membrane [5]. It can be shown that the permeability coefficient P of a molecule is the product of its diffusion coefficient D in the polymer and its partition coefficient K. P = DK

(5)

The transport of molecules within the membrane itself is described mathematically using Fick‘s first law of diffusion, which states that the flux J of a chemical species across a membrane of unit area per unit time is proportional to the concentration gradient between the two surfaces of the membrane. The constant D is the diffusion coefficient of the chemical in PDMS. J = −D

∂C ∂x

(6)

During the permeation process, the partitioning at the PDMSsample interface occurs instantaneously, and the diffusion process within the membrane determines the kinetics of the analyte transfer (rate determining) [17]. Generally, vapor molecules permeate faster through rubbery polymers such as PDMS than through glassy polymers such as Teflon® [5]. In the PDMS structure, the (–Si–O–Si–) backbone is highly flexible compared to many other rubbery polymers, and interactions between the individual PDMS segments are relatively weak. Consequently, PDMS has one of the lowest glass-transition temperatures for polymers of −127 ◦ C, which allows long-range segmental motions even at very low temperatures resulting in one of the lowest diffusivity selectivity for permeation [18]. Because of the low diffusivity selectivity, the differences in permeability of vapor molecules through PDMS are mostly governed by their partition coefficients rather than the diffusivities in the polymer [5]. In practice, concentrations of analytes in the sample matrix can often change over the duration of the time the PDMS is exposed to the sample. When such changes occur, there is an intermittent period when steady state concentration profile does not exist within the membrane. A measure of the time the sampling system takes to respond to this change in concentrations and reach the steady state again is quantitatively expressed using various terms such as residence time [19], relaxation time [20], lag time [21], and response time [20]. The term “residence time” as applied to passive sampling was introduced by Tompkins and is defined as the average

residence time of an analyte in the diffusion region at steady-state conditions [19]: tr =

2 Lm 2Dr

(7)

where tr is the residence time of the analyte with diffusion coefficient Dr in the PDMS phase, and Lm is the thickness of the phase. The lower this value is, the shorter is the time required for the sampler to respond to a change in analyte concentration in the sample matrix. Response time on the other hand is considered to be the time taken for the permeation rate to increase from 10% to 90% of the steady state value, and is normally used in the field of membrane inlet mass spectrometry (MIMS) [22]. An often overlooked factor when comparing results obtained with PDMS manufactured by different manufacturers or researchers is the effect of fillers used during the fabrication process. PDMS is very often fabricated with fillers such as SiO2 (up to 30%) to increase its mechanical strength [23]. While the filler was not found to have any effect on the partition coefficients, it increases the tortuosity of the diffusion path and decreases the analytes diffusion coefficient [23]. Consequently, the permeability of the polymer towards an analyte also decreases in the presence of a filler. In practice, the permeability property is used in one of two major ways. In the first method, the analyte concentration on the receiving surface is always kept at zero, generally using a strong sorbent. In such cases, the steady state concentration profile across the membrane surface can be represented by Fig. 2. In the second method, the permeating analytes are swept away from the membrane surface, typically using a sweep gas or by maintaining vacuum conditions at the receiving side. If the stripping phase effectively removes the permeated analytes, the concentration profile will be similar to that shown in Fig. 2, but could often be complicated depending on the partition coefficient of the analytes, the effectiveness of the stripping process and the boundary layer conditions [24]. Any attempt to use the permeability properties should always consider the analyte transport mechanism in the phases in contact with PDMS on its both sides, and necessary corrections in the mathematical models should be made. Whenever the fluid flow velocity at the sampling device’s location is not sufficient to supply analytes to the sampling surface faster than their rate of transfer into the sampler, there will always be a region adjacent to the membrane where the analyte concentration is lower than that in the bulk of the sample [13]. In the field of passive sampling, this introduces a negative bias in the analyte concentration determined using the sampling device, which is said to be due to “starvation effect” [10]. For PDMS-based permeation sampling systems, the magnitude of

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Table 1 GC column manufacturers and specific codes used for PDMS stationary phases. Manufacturer

Phase code

Agila Technologies Agilent Technologies Altech GS-Tek Machery Nagel Ohio Valley Perkin Elmer Phenomenex Quadrex Restek SGE Supelco/Sigma–Aldrich Thermo Fisher Scientific United States Pharmacopeia Varian/Chrompack

DA-1 DB-1, DB-1MS, HP-1, HP-1MS, Ultra-1, HP-101, DB-1ht, SE-30, DB-Petro, DB-PS1, CP-Simdist AT-1, SE-30, AT-1MS, EC-1 GsBP-1, GsBP-1MS Optima 1, Optima 1MS OV-1, OV-101 PE-1, Velocity-1, Elite-1 ZB-1, ZB-1MS, ZB-1HT 007-1 Rtx-1, MXT-1, MXT-1HT, Rxi-1MS BP-1, BPX-1 SPB-1, SPB-1MS, Petrocol DH, SPB-1sulfur, Equity-1, SP-210, MDN-1 TR-1, TR-1MS G1, G2, G38 VF-1MS, CP-Sil 5CB, CP Sil 5CB-MS

this starvation effect depends on the permeability of PDMS towards the analyte, the geometry of the sampling system, the fluid flow velocity and pattern across the surface of the PDMS, and the diffusion coefficient of the analyte in the matrix. Under the absence of the starvation effect, the uptake rate is polymer-controlled. As the starvation effect increases, the uptake rate is progressively boundary layer-controlled. Using appropriate models, it is often possible to estimate the boundary layer thickness for specific analytes, at which the net analyte transfer rate into the sampling device is controlled equally by the polymer and the boundary layer [13]. Permeability of PDMS is a function of both the partition and the diffusion coefficient of the chemical, both of which are temperature dependent. It is therefore important to consider the thermodynamic parameters involved in these processes to make necessary changes in the models used in calculations, as well as take advantage of the effects while using the PDMS for various purposes. 2.2.1. Effect of temperature on the partition coefficient and permeability of PDMS The temperature dependence of the partition coefficient of an analyte between PDMS and air is related through the heat of solution as shown in Eq. (8), where K0 is the pre-exponential factor, Hs is the enthalpy difference between analyte sorbed in PDMS and in its pure phase (heat of solution), R is the gas constant and K is the partition coefficient at temperature T [18]. The heat of solution for the partitioning of the majority of volatile organic compounds into PDMS is negative, indicating that the partition coefficient decreases with increasing temperature.

 H  s

K = K0 exp −

RT

(8)

The permeability of PDMS is dependent on both K and D (Eq. (5)). Consequently, temperature dependence of permeability is decided by the temperature dependences of both K and D. While the heat of solution defines the temperature dependence of K (Eq. (8)), the energy of activation of diffusion defines the temperature dependence of the diffusion coefficient of the analytes within the PDMS network. This relationship is shown in Eq. (9), where D0 is the pre-exponential factor and Ed is the energy of activation of diffusion. Ed is positive for most compounds, and consequently the diffusion coefficients of chemicals in PDMS increase with increase in temperature [25]. The temperature dependence of permeability of PDMS can be represented by Eq. (10), where P0 is the standard permeability and Ep is the energy of activation of permeation.

 E  d

D = D0 exp −

RT

 E  p

P = P0 exp −

RT

(9) (10)

Combining the above three equations, it is apparent that Ep is related to Ed and Hs as shown in Eq. (11). Therefore the direction and quantitative variation of permeability through PDMS with a change in temperature is decided based on which of these two parameters defining the activation energy of permeation is dominating. Many researchers have determined the Ep values for many chemical species and found them to be negative [25,26]. This shows that for PDMS, Hs is the dominating factor. In other words, the permeability of most gas-phase organic compounds through PDMS decreases with increasing temperature. Ep = Ed + Hs

(11)

3. Applications utilizing the partitioning properties of PDMS Many devices make use of the partitioning properties of PDMS for various applications. Irrespective of how the PDMS is used, the challenges in practical applications are not due to the material itself, but to difficulties with incorporating the environmental effects, boundary layer effects, uncertainties in quantitation, calibration data and temperature effects into the models used for quantitative purposes. Nevertheless, simple mathematical models have been developed for all the devices discussed in this section for successful use in quantitative analysis. 3.1. GC columns PDMS was one of the earliest stationary phase materials tested for GLC separations, and to date it is the most widely used stationary phase for GC columns [27]. GC columns coated with PDMS are available from many manufacturers, each with their own product code. Examples are listed in Table 1. Many standard methods in chromatographic analysis published by regulatory agencies such as US-EPA, NIOSH, OSHA, etc., require the use of PDMS as the stationary phase for the analysis of specific groups of compounds. The stationary phases used in GLC have to fulfill many functional requirements; the popularity of PDMS in this application is due to the fact that it satisfies most of them [28]. The stationary phase needs to be chemically inert, thermally stable (low bleed) and handle many temperature cycles during its usage time. Further, it should be possible to uniformly distribute the stationary phase inside the column, reproducibly from one batch to another, during the manufacturing process. Apart from these fundamental requirements, the stationary phase should exhibit high selectivity towards volatile and semi-volatile organic compounds of interest that need to be analyzed by GC. The selectivity of PDMS stationary phases arises from the differences in the partition coefficients of the analytes, hence the use of

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53

Fig. 3. Decomposition mechanism of PDMS stationary phase (based on reference [39]).

PDMS as the stationary phase falls under the category of partition gas chromatography techniques [29]. The retention properties of a compound in such chromatographic separations are the function of the analyte’s partition coefficient between the carrier gas and the stationary phase (PDMS in this case). Under isothermal conditions, the partition coefficient of a solute at a given temperature is related to the retention time of the solute in the following manner [30], tr − tm Vs tr = =K tm tm Vm t

(12)

where r is the adjusted retention time, tm is the retention time of an un-retained compound, tr is the retention time of the solute, K is the partition coefficient of the solute, Vs is the volume of the stationary phase, and Vm is the volume of the mobile phase. The differences in the partition coefficients of the analytes can be explained based on the fundamental molecular interactions between the analyte and PDMS [31]. Since PDMS is hydrophobic, the partition coefficients generally increase with increasing hydrophobic interactions. Further, the partition coefficients of homologous series of compounds are simple functions of their vapor pressure. Consequently, such analytes elute from the column in the order determined by their vapor pressure, from high to low. The partition coefficient of an analyte can be easily manipulated by changing the temperature of the chromatographic column during sample elution, which is used in temperatureprogrammed gas chromatography to overcome the general elution problem. Retention times are functions of column dimensions, temperature and linear flow velocity of the carrier gas. To be able to compare chromatographic results obtained with different columns and under different conditions, Kovats introduced a system of retention indices, which are dimensionless numbers obtained by comparing the retention times of an analyte with those of a standard set of compounds, such as n-alkanes, under the same isothermal conditions. Kovats retention indices are determined using Eq. (13), where tr is the retention time of the analyte, tn is the retention time of the n-alkane eluting directly before the analyte, tn+1 is the retention time of the n-alkane eluting directly after

the analyte, and n is the number of carbon atoms in the n-alkane eluting directly before the analyte. I = 100

 log(t ) − log(t )  r n log(tn+1 ) − log(tn )

+ 100n

(13)

While Kovats retention indices are based on isothermal gas chromatographic separation, Van den Dool and Kratz [32] introduced the concept of linear temperature-programmed retention indices (LTPRI), which involves calculation of the retention indices while performing the separation under the conditions of linear temperature programming. LTPRI is defined as: LTPRI = 100

 t −t  r n tn+1 − tn

+ 100n

(14)

The exact correlation between LTPRI and the partition coefficient is complicated, and involves fluid dynamics inside the capillary column [33]. However, various researchers working on determining empirical relationships and/or mathematical approximations have found that LTPRI for a homologous series of compounds is related to the partition coefficient of the analyte at a particular temperature as [34] LTPRI = N ln K + B

(15)

where N and B are constants. The Kovats retention indices and LTPRI values have been determined for numerous compounds with PDMS stationary phase and are extensively reported in the literature [35]. The retention index database published by the National Institute of Standards and Technology (NIST) is perhaps one of the largest and compiles data from numerous literature sources. In addition, many researchers reported ways to estimate the retention indices based on structure–property correlations [36–38]. It is therefore possible to use these retention data to estimate the partition coefficient values required for the calibration of many sampling devices that will be presented in the rest of the article. One of the common problems in GLC is the decomposition of the stationary phase, often referred to as column bleeding, which tends to increase the baseline levels affecting the detectability of compounds eluting from the column. While PDMS is generally accepted as being thermally stable and inert, minor thermal decomposition

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Fig. 4. Solid phase microextraction device used for TWA sampling (based on Ref. [10]).

is still unavoidable and the detectors of today are sensitive enough to detect these decomposition products. While using mass spectrometry for detection, various decomposition products as shown in Fig. 3 can be observed, and their mass fragments can be used to monitor the decomposition. Discernible peaks are generally found at m/z 207, 281, and 355 [39]. During thermal decomposition, the PDMS first forms a cyclic trimethylsiloxane trimer, which gives rise to a peak at m/z 207. The high molecular weight decomposition products are one of the reasons why the ion source in a mass spectrometer needs to be cleaned regularly for proper functioning. Many strategies are used to reduce column bleeding, including polymer cross-linking, chemically bonding PDMS to the column wall, and carefully avoiding introduction of oxygen into the column (which tends to increase the decomposition) [40]. Currently, the use of various methods to reduce the bleeding as described above makes it relatively easy to increase the column temperature up to 350 ◦ C and even beyond in some cases. A successful separation of closely eluting compounds and quantification using GLC also depends on the column efficiency (measured through the number of theoretical plates N). Among the variables described by the van Deemter equation that affect the efficiency, the diffusion coefficient of the analyte in the stationary phase is critical [40]. It has been shown that the mass transfer coefficient in the stationary phase is inversely proportional to the square of the diffusion coefficient of the analyte [40]. As was discussed in detail in Section 2.2, the diffusion coefficients of analytes are relatively high in PDMS, which tends to decrease the plate height (or increase the efficiency). A specific application of the PDMS stationary phase in chromatography is the determination of the distribution of the boiling points of the components of petroleum fractions (so-called simulated distillation) [34]. This application is possible owing to the direct correlation of the retention times determined using PDMS stationary phase with the boiling points of the generally non-polar components of these fractions. Recently, PDMS stationary phases in metal capillary columns were introduced with ladder technology, which involves systematic cross-bond formation between the individual PDMS chains in a stationary phase [41]. Combining this with effective surface deactivation of the column, simulated distillations of petroleum fractions containing up to 116 carbon atoms in the molecules have been performed reproducibly [41]. This also indicated that properly manipulated PDMS stationary phases could be stable up to 435 ◦ C during a temperature cycle in a chromatographic run. Partitioning of analytes into PDMS is an absorptive process; hence complications such as competitive sorption hardly occur. However, as the concentration of the analytes increases beyond its solubility limit, the sorption isotherm deviates from linearity [42]. This phenomenon is observed very easily in partition

chromatography, where high sample loadings result in peak fronting [42]. 3.2. SPME PDMS was one of the earliest extraction phases tested for use in SPME. The SPME device consists of an extracting phase such as PDMS coated on a short fused silica fiber. The coated section of the fiber is typically 1 cm long [43] and is housed inside a stainless steel needle. The PDMS-coated fibers are available commercially in three different thicknesses: 7, 30 and 100 ␮m, as both bonded and non-bonded phases. The fiber arrangement allows for the extraction phase to be exposed to the sample matrix by moving the silica fiber in and out of the needle, as well as for introducing the fiber into a GC injector for thermal desorption and quantification of the extracted analytes. When the extraction phase is exposed to the sample matrix, the analytes partition between the sample matrix and the PDMS, and the analyte uptake follows the profile illustrated in Fig. 2. As illustrated in Fig. 4, the same fiber can be used for time-weighted average (TWA) concentration measurements by retracting the fiber into the metal housing to have a well-defined diffusion distance for the analyte before it can be sorbed. Chen and Pawliszyn first demonstrated the application of such a system for the analysis of VOCs in air [44]. Under given fluid flow conditions around the fiber and for a particular geometry of the sampling device (length and thickness of the coating), an analyte takes a specific amount of time to equilibrate between the sample and the PDMS phase. Proper agitation of the sample and higher diffusion coefficient of an analyte result in reaching the equilibrium concentration in the PDMS extraction phase faster. The amount of analyte extracted by an absorption-type SPME coating at equilibrium when the extraction phase is in direct contact with the sample matrix (two-phase system) is given by Eq. (16): n=

Kfs C0 Vs Vf Vs + Kfs Vf

(16)

where Kfs is the partition coefficient of the analyte between air and the extraction phase, Vf is the volume of the extraction phase, Vs is the sample volume and C0 is the concentration of the analyte in air [43]. When the sample volume is very large compared to the volume of the extraction phase multiplied by the analyte partition coefficient, the amount of the analyte (n) collected by the fiber at equilibrium is given by [43]: n = Kfs C0 Vf

(17)

From the above equation it is clear that under the conditions of near-infinite sample volume, the equilibrium concentration of an

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analyte in an absorption-type phase (Cf in the case of SPME) is given by: Cf =

n = Kfs C0 Vf

(18)

Apart from being useful in SPME, this equation forms the basis of many equilibrium sampling techniques used in sediment and soil analysis (among others). Calibration of SPME can be done in many ways as reported in the literature [45,46]. Of all the methods, kinetic calibration needs a special mention, as it is well developed and applied for SPME, but not so widely used for other sampling systems based on the partitioning property of PDMS. The possibility of such calibration arises as analyte sorption into and desorption out of PDMS is isotropic [47]. In this method, the PDMS coating is first doped with a known amount of an isotopic analogue of the analyte of interest and the extraction process is started. After a certain time (generally less than the time required to reach equilibrium), the analyte mass and the isotope analogue mass on the fiber are determined by chromatography. The knowledge of these masses along with the knowledge of the amount of isotope analogue originally doped onto the fiber allows the mass of the analyte that would have been in the fiber at equilibrium to be calculated. Consequently, analyte concentration in the sample can be determined. The isotope analogue of the analyte is referred to as performance reference compounds (PRCs). This method is valuable when the partition coefficient of the analyte is relatively high and the sample agitation conditions are below optimal, both of which tend to increase the time required to reach equilibrium. Variation of the iso-kinetic principle can also be applied for determining sorption coefficients and quantify analytes in a sample matrix. In this method, the sorption phase is first loaded with analyte and allowed to desorb into the sample matrix. This is particularly useful for highly hydrophobic compounds which take a long time to reach equilibrium between the sample and the PDMS phase. For example, Ter Laak et al. used this method to determine sorption coefficients of PAHs and quantified them in contaminated soil [48]. Kwon et al. have published the theoretical principles behind such a quantifying method and used it to quickly determine the sorption coefficients of PAHs in water [49]. It is often not possible to calibrate SPME for each analyte when a large number of analytes are involved in the analysis of samples such as gasoline and diesel fuel. For such applications, Martos et al. introduced a simple method to calibrate SPME with PDMS extraction phase based on the LTPRI values of the analytes determined using a capillary column coated with a PDMS stationary phase [34]. This method is possible because the calibration constants for SPME are a function of the partition coefficients of the analytes between the PDMS extraction phase and the sample, and so are the retention indices measured on capillary columns coated with PDMS. Since the retention index values can be determined with good accuracy, they can be used to estimate the calibration parameters of SPME. Using this method, Martos et al. showed that total parameters such as total petroleum hydrocarbons (TPH) can be determined by SPME. SPME with a PDMS extraction phase (apart from other extraction phases) has been widely used as a sample introduction technique for compound-specific isotope ratio determination using gas chromatography–isotope ratio-combustion-mass spectrometry (GC–IR-C-MS) [50]. In the field of GC–IR-C-MS, it is critical to avoid any fractionation while sampling so as to determine the compound-specific isotope ratios of the analyte in the sample with sufficient accuracy. Since sampling with SPME involves phase transfer (from liquid or air to the PDMS phase), and phase transfer phenomena involve isotopic fractionation, care has to be taken while interpreting data based on this technique. Hunkeler and Aravena determined such isotopic fractionation effects for

55

PDMS-coated SPME and found very small isotopic effects relative to experimental error for chlorinated methanes and chlorinated ethanes and ethenes [50]. Bruner et al. found that heavier isotopes were generally retained less (except at low temperatures) when using a GC column with a PDMS stationary phase by determining the retention behaviors of CH4 –CD4 , C2 H6 –C2 D6 , C2 H4 –C2 D4 and C7 H8 –C7 D8 [51]. Bermejo et al. observed similar results when chlorinated products of (1 H10 )1,4-dimethylbenzene and (2 H10 ) 1,4dimethylbenzene were analysed using PDMS stationary phase. The observation was generally attributed to the higher vapor pressure of the heavier isotope analogues (isotope vapor pressure effect) [52]. Even though PDMS is biocompatible, its use in in vivo sampling of biological fluids is not preferred mainly because of the nonspecific adsorption of proteins to the hydrophobic surface of the polymer [53]. Therefore, surface or chemical modification of PDMS is often necessary for its use in sampling biological fluids. The use of SPME (not restricted to PDMS phase alone) has been extensively studied, reviewed and reported. Consequently, rather than referring to individual journal research articles, the readers are directed to the many review articles specifically published on the calibration, development and applications of SPME [54–76]. 3.3. Thin film extraction In this technique, a thin film of PDMS is used for the extraction and pre-concentration of analytes from air, water or soil matrices. Compared to an SPME or SBSE device, thin films have larger surface and hence mass uptake is faster [67,77]. Further, since the total volume of the thin film PDMS is generally higher than that in SPME (and possibly also SBSE), larger analyte amount can be extracted leading to higher sensitivity of the sampling method without compromising the time required for the extraction [78]. Analytes from the PDMS film can either be solvent-extracted and analyzed by conventional liquid injection GC analysis, or they can be thermally desorbed directly into the GC inlet for higher sensitivity. Qin et al. used such thin films for the extraction and analysis of five polycyclic aromatic hydrocarbons (PAHs) from water. They also showed that thin PDMS films could be used to mimic the bioaccumulation of PAHs by black worms owing to the hydrophobicity of the polymer [78]. Similar studies were done by Bragg et al. for the quantitation of PAHs in the Meuse River in the Netherlands and Hamilton Harbour in Ontario, Canada [79]. Jahnke et al. studied the applicability of thin PDMS films for the in-tissue extraction of PCBs from fish [80] and found that equilibration of PCBs into PDMS was rapid for lipid-rich fish, but slow for low lipid-containing fish. Kinetic calibration is possible with thin film extractions just like with SPME, as was demonstrated by Bragg et al. [79]. Thin films can also be coated on the inside of containers (e.g. vials, beakers, jars, etc.), and the sample (soil, sediment, water) equilibrated with the PDMS layer. For example, Maenpaa et al. coated PDMS on the inside surface of 20 and 120 mL vials and jars, respectively, for the sampling and analysis (by solvent desorption) of PCB-contaminated soil and sediments [81]. This method is also sometimes referred to as the immobilised liquid extraction (ILE). 3.4. PDMS rod and PDMS tube microextraction An excellent review of the applications of PDMS rods and PDMS tubes for sample preparation was recently published by van Pinxteren et al. [82]. The ultimate goal of these devices is to increase the sensitivity of extraction by increasing the PDMS volume. The review indicated that studies to date used PDMS phase volumes ranging from 8 to 635 ␮L in the form of tubes or rods for the extraction of analytes from various matrices such as air, water, milk and tea. The analytes of interests extracted from these matrices included

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many common pollutants, such as PAHs, PCBs, chlorobenzenes, polybrominated diphenyl ethers, halogenated anisoles, pesticides, and other VOCs and sVOCs. PDMS tubes can be used both as partition sampling devices, as well as permeation sampling devices. In the latter case, the outside of the tubes serve as the sampling surface, and the analytes permeating to the inside of the tube are stripped using a solvent or gas for further analysis [83,84]. Ouyang et al. compared SPME operated in the TWA mode by retracting the fiber into the needle housing, PDMS rod and thin film for the extraction of PAHs in Hamilton Harbour (Ontario, Canada). The PDMS rod used in the study was 1 cm long with a diameter of 1 mm (volume of about 7.85 ␮L), while the 127 ␮m thick PDMS film corresponded to a volume of 63.5 ␮L. As expected, they found that the passive sampling rate increased in the order thin film > PDMS rod > SPME (in TWA mode). The comparatively large surface areato-volume ratio of thin films was responsible for this observation [85]. Zhao et al. described the theory of PDMS rod extraction from aqueous solutions for the kinetic calibration method and applied it to the quantification of PAHs [86].

3.5. SBSE Introduced in 1999 for analyte pre-concentration from aqueous samples, SBSE is now commercially available from Gerstel GmbH (Germany). The SBSE device consists of a magnetic stir bar, a coating of extraction phase on the outside, and a thin glass layer between the two (to avoid metal-catalyzed decomposition of PDMS) [87]. For the extraction, the bar is allowed to stir the sample solution to speed up the partitioning of the analytes between the matrix and the coating. Once the extraction process is finished, the SBSE device is usually thermally desorbed, and the analytes are introduced into a GC column for quantitation. Unlike SPME fiber, which can be desorbed directly in the injector of a GC, the SBSE device requires a separate module for desorption and re-focusing, as it takes a considerably longer time for analyte release from the much larger volume of PDMS (which leads to severe peak broadening if no re-focusing is applied). SPME fiber contains about 0.5 ␮L of PDMS as opposed to 25–125 ␮L for the SBSE device. The desorption module commercially available from Gerstel GmbH (Germany) is designed to allow cryogenic re-focusing of the analytes before their release into the GC column for separation. The main application of SBSE to-date has been direct extraction of analytes from aqueous samples, but it can also be used for sampling analytes from sample headspace (headspace sorptive extraction, HSSE) [87] or from the air [88]. In SBSE, an analyte’s PDMS–water partition coefficient required for quantitative purposes can be determined through calibration or can be easily estimated based on the analyte’s octanol–water partition coefficient (Kow ) [89]. Because the Kow values are widely available in the literature, development of applications for various analytes in aqueous solutions is relatively easy. While many researchers experimented with different coatings on the stir bars, only PDMS and PDMS/ethylene glycol phases have been available commercially at the time of writing of this paper [90]. Yun Nie and Kleine-Benne studied the use of a mixture of ethylene glycol (EG) and PDMS, as well as polyacrylate (PA) alone as extraction phases for SBSE [91]. Their experiments indicated that EG-PDMS phase had better extraction efficiency than 100% PDMS and 100% PA phases for phenols, furans, acids and alcohols in matrices such as whisky, multivitamin drinks and white wine. However, the temperature limits at which the SBSE could be desorbed were comparatively lower for the EG-PDMS phase. Further, the EG-PDMS phase absorbed larger quantities of water because of its polar nature, which is disadvantageous when working with mass spectrometric detectors.

A recent review article on SBSE by Lancas et al. described the theoretical aspects of the technique and covered a wide range of applications developed for pharmaceutical, biomedical, environmental, and food analysis [92]. Prieto et al. reviewed the applications of SBSE with special emphasis on method optimization, novel applications and limitations, and offered insight into potential solutions for commonly observed problems [93]. While initially SBSE was used almost exclusively in combination with GC–MS, many applications involving solvent desorption followed by liquid chromatography or liquid chromatography–mass spectrometry analysis were published recently [94–98]. 3.6. Integrative and equilibrium sorptive enrichment (ESE) using packed bed PDMS The integrative and ESE techniques were devised to overcome some of the challenges normally encountered using adsorptive sorbents in sorbent bed tubes [99]. Some of those problems include adsorbent breakdown at high desorption temperatures, competitive sorption effects and analyte reactivity with the sorbent [99]. Owing to the advantages of PDMS discussed earlier, PDMS was researched as an alternative sorption material for use in sorbent bed tubes. Baltussen et al. first reported the use of packed bed PDMS for the extraction of PAHs and organochlorine pesticides (OCPs) in water in 1997 [100]. They prepared PDMS particles by crushing PDMS under liquid nitrogen and packed 0.325 g of it in a tube with a total bed length of 5.8 cm. The PDMS particles were held in place using quartz wool plugs. The sample was drawn through the tube for enrichment, and the tube was subsequently dried using inert gas, followed by thermal desorption into GC for quantification. Except for naphthalene and acenaphthene, the method showed very good reproducibility. Such traps packed with PDMS were further evaluated by Baltussen et al. for use in environmental sampling [101]. When used in the ESE mode, the sample is continuously passed through the tube until equilibrium is reached between the sample matrix and PDMS. The advantage of the method over integrative collection with PDMS is that the maximum possible amount of the analyte (determined by its partitioning coefficient) can be collected, which lowers the quantitation limits. Further, analytes with low breakthrough volume can be sampled without any problem. The disadvantage, however, is that the partition coefficients of the analytes need to be known in order to quantify the concentrations, and, more importantly, the analyte concentration in the sample matrix needs to remain constant throughout the sampling period. The application of the device was demonstrated for compounds such as benzene, toluene and p-xylene [99]. Aguilar et al. reported performing online ESE-chromatography to determine benzene, toluene and p-xylene in environmental wastewater samples [102]. Baltussen et al. showed the use of the technique for high molecular weight compounds such as benzo (a) pyrenes, which are very difficult or not possible to thermally desorb from adsorbents [100]. 3.7. In-tube SPME In in-tube SPME, sampling is performed in a fashion similar to that described for sorbent bed above, but a hollow column with coating on the inside wall is used for the extraction. This technique is also referred to as ESE by some researchers. In this technique, sample (gas or liquid) is first allowed to equilibrate with the extraction phase by repeated draw/eject cycles. Once the extraction step is complete, the analytes are either desorbed thermally and introduced into a GC, or are desorbed with a stronger solvent for introduction into an LC column for separation. The method is generally automated by using a series of valves to perform the draw/eject cycles for extraction and desorption automatically [103]. Most of

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the articles published in peer reviewed journals indicate that the technique is more widely used for sampling from aqueous phase combined with solvent desorption and liquid chromatography. Analysis of the published articles indicates that PDMS is not as widely used for this technique as for some others described in this article. Pham Tuan et al. described the theoretical aspects of this preconcentration technique and demonstrated its applicability for the analysis of VOCs in air [104,105]. Bagheri and Salemi used this technique to analyse PAHs in water in an offline-mode and detected the analytes at concentrations as low as 0.001–0.006 ␮g L−1 [106]. Globig and Weickhardt used a fully automated in-tube SPME device to quantify PAHs in river water [107]. Wang et al. used a 5 m, 0.53 mm internal diameter column with a 1.2 ␮m thick PDMS film to analyze PAHs and chlorinated pesticides, and observed that analyte recovery was 50 times higher than that seen with an SPME fiber, even though the volume of PDMS in the in-tube setup was only 10 times higher [108]. Several other applications of in-tube SPME with PDMS have been described in the literature [109–111]. 3.8. Membrane-enclosed sorptive coating (MESCO) The MESCO device is made by enclosing a certain amount of PDMS (e.g. coated on an SBSE stir-bar) in a water-filled dialysis membrane bag made of regenerated cellulose [112]. The whole assembly is introduced into the sample matrix and analyte concentration in the PDMS occurs by mass transfer through a series of compartments: sample, dialysis membrane, water and PDMS receiving phase. The analyte mass collected in the PDMS phase was found to be linear with the exposure time and was described by Eq. (19), where MS is the mass accumulated in PDMS over time t, Mo is the mass of analyte in the sampler prior to deployment, Cw is the analyte concentration in water during deployment, KSW is the partition coefficient between PDMS and water, Vs is the volume of PDMS, kov is the overall mass transfer coefficient, A is the membrane surface area exposed to water, and ˛ is the membrane porosity [113]. The calibration of the MESCO sampler can either be done using standard sample solutions, or by using kinetic methods where a PRC is added to the PDMS phase immediately prior to the sampler deployment [113].



 k A˛   ov

MS (t) = Mo + (CW KSW VS − Mo ) 1 − exp −

KSW VS

t

(19)

57

Sgorbini et al. evaluated the method for the analysis of cosmetically important chemicals such as citronellol, Z-citral (neral), geraniol, cinnamaldehyde, anisyl alcohol, cinnamyl alcohol, eugenol, methyleugenol, coumarin, isoeugenol, ␣-isomethylionone, 2-(4tert-butylbenzyl)propionaldehyde (lilial), ␣-amylcinnamaldehyde and ␣-hexylcinnamaldehyde [121]. Bicchi et al. studied volatile organic compounds permeating out of the skin when perfume is applied to different parts of the body [122]. They used the same method for the analysis of chemicals released from the surface of vegetables and fruits (pulp and surface). This technique is promising for its use in the cosmetic industry, as well as in healthcare studies related to allergens and biomarkers in skin. 4. Applications utilizing the permeability properties of PDMS 4.1. Permeation passive samplers with PDMS membranes Permeation passive samplers are based on analyte transfer from the sampled medium (air or water) to a collecting phase (generally a sorbent) through a polymer membrane [123]. This type of sampler can be used for sampling organic compounds from both air and water. In the authors’ laboratory, a simple permeation-type passive sampler based on a GC autosampler vial was developed for sampling of volatile organic compounds from the air. It is available commercially as a so-called Waterloo Membrane Sampler (WMS) [31]. A PDMS membrane (100 ␮m nominal thickness) fabricated in the laboratory using the spin coating technique is first cut to fit the top of a 2 mL crimp-top autosampler vial. A suitable sorbent depending on the analyte of interest (generally Anasorb 747® or Carbopack B® ) is then added to the vial. The rubber septum normally accompanying the aluminum crimp cap is removed, the PDMS membrane is placed on top of the vial as shown in Fig. 5 and crimped. During deployment, the sampler is turned upside down so that the membrane is in direct contact with the sorbent on one side, and the sample matrix on the other. Since the sorbent adsorbs all the analytes reaching the inner membrane surface instantaneously, the analyte concentration inside the vial is maintained at zero all the time for continuous permeation of analytes across the membrane. When there is no starvation effect (practically zero boundary layer width), the analyte uptake can be easily described by Eq. (20), where C0 is the analyte concentration in air, M is the amount of analyte collected in time t by the sampler, and k is the calibration constant of the sampler towards a particular analyte. The parameter k is dependent on the geometry of the sampler and fundamental transport properties of the membrane towards the specific analyte. It is given by Eq. (21), where D is the diffusion coefficient of the analyte in the membrane, A is the surface area of the membrane, and Lm is the membrane thickness.

Many applications of MESCO devices have been reported in the literature, and their use is gaining popularity. Vrana et al. demonstrated the applicability of MESCO for sampling of various persistent organic pollutants in water by comparing the results with traditional spot sampling [113]. Further, they showed that desorption of PRCs from the PDMS phase was isotropic to the analyte accumulation, and hence kinetic methods could be used for calibration. The MESCO device has been used for many other applications, examples of which are provided in references [114–119].

C0 =

3.9. Sorptive tape extraction (STE)

k=

The STE method is similar to thin film extraction described above, but was devised specifically to sample organic compounds released from the skin. STE is performed with a PDMS tape with typical dimensions of 15 mm × 4 mm for thermal desorption, and 15 mm × 12 mm for liquid desorption; the tape thickness is 0.5 mm [120]. One side of the tape is in direct contact with skin, while the other is sealed by an adhesive tape which holds the PDMS in place. The amounts of analytes collected by the tape at equilibrium are determined by their PDMS-skin partition coefficients. Sisali et al. demonstrated the applicability of STE for the extraction of sebum from skin surface and quantified it by a gravimetric method [120].

The calibration constants of the WMS sampler have been determined for over 40 volatile organic compounds including primary and secondary alcohols, chlorinated ethanes and ethenes, aromatic hydrocarbons, alkanes, and esters [31]. Based on Eq. (21), the permeability of PDMS for the analytes was also reported. The research also showed that the calibration constant k (and hence the permeability) can be easily estimated based on the LTPRI of the analytes due to the interesting properties of PDMS described in Section 3.1. Since the partition coefficient of an analyte is proportional to the LTPRI determined using a capillary column with a PDMS stationary phase, LTPRI could be used to estimate the

kM t

(20)

Lm DKA

(21)

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Fig. 5. (a) Components of the WMS sampler and (b) WMS sampler deployment configuration.

calibration constant of an analyte in the sampler. Consequently, the method could be used to determine parameters such as TPH using the sampler, which is a formidable task with any other passive sampler currently in use [31]. A similar approach could be used for other permeation-based techniques described in the remainder of the section, though. Actual permeability of PDMS towards various analytes can also be determined experimentally using various methods. One such method is based on the use of the concept of inverse gas chromatographic techniques (IGC) to determine not only the partition coefficients, but also the diffusion coefficients of the analytes in the membrane. The method of determining the diffusion coefficients is based on the fact that the chromatographic retention time and the profile of the eluting peaks are functions of the partition coefficient of the analyte between the carrier gas and the stationary phase and the diffusion coefficients of the analytes in the stationary phase and the mobile phase among many other variables [124]. Cankurtaran and Yilmaz determined the enthalpy and entropy parameters for selected n-alkanes in PDMS, a method which could be useful when determining the variations of the calibration constants of PDMS-based sampling techniques with temperature [125]. Zhao et al. demonstrated the applicability of IGC for the determination of the diffusion coefficients of various n-alkanes in crosslinked PDMS [126]. Kloskowski et al. determined the partition coefficients of many environmentally important volatile organic compounds using this method [127]. The research into the WMS sampler described earlier also indicated that the permeability of PDMS followed Arrhenius-type relationship as discussed in Section 2.4 [25]. Further, because of the hydrophobicity of PDMS, it was also shown that the permeability was mostly independent of the humidity of the gaseous sample [25]. A different configuration of a PDMS-based permeation passive sampler developed at the Gdansk University of Technology (GUT) was used extensively for sampling various volatile organic compounds from indoor and outdoor air. The applications of the GUT sampler were reported in various publications [128–131]. Bicchi et al. introduced a modified SBSE device (called dual-phase SBSE) which contained a small cavity between the magnetic stir bar and the PDMS phase [132]. The cavity could be filled with different types of sorbent and the device then functions very similar to the WMS or the GUT sampler as a permeation passive sampler. While the WMS, GUT and the dual-phase SBSE devices use a sorbent as the receiving phase to maintain the concentration gradient across the PDMS membrane, Janska et al. used acetonitrile sealed in a PDMS tube as the receiving phase (termed by them as “solvent in

silicone tube extraction” (SiSTEx)) [133]. In this method, analytes first permeate from the aqueous phase through the PDMS walls of the tubing and partition into acetonitrile. Janska et al. used this method to quantify various organophosphorous and organochlorine pesticides in fruits and vegetables using GC/pulsed flame photometric and halogen specific detectors [133]. It is important to realize that the solvent in the tube permeates out of the tube and its presence alters the solubility property of the PDMS walls allowing extraction of relatively polar analytes [134]. A similar design, but termed as “silicon membrane sorptive extraction (SMSE)” was employed by Van Hoeck and co-workers who used ethyl acetate as the PDMS polarity modifier. They demonstrated its applicability for the quantification of atrazine and its three metabolites [134].

4.2. Membrane inlet mass spectrometry (MIMS) PDMS is perhaps the most widely used material in membrane inlet mass spectrometry (MIMS). In MIMS, a membrane is used as an interface between the sample and the mass spectrometer. Sample introduction into the instrument is achieved by permeation occurring continuously through the polymer membrane. The device was pioneered by Hoch and Kok in 1963 [135]. Generally, a vapor or liquid phase sample is passed across one side of the membrane continuously, and the permeating analytes are introduced either directly into the ion source of the mass spectrometer, or indirectly with a secondary sweep gas flowing into the ion source [136]. As opposed to other techniques described thus far in the article, MIMS can be used for continuous (online) monitoring of organic compounds in the sample, or in offline mode. PDMS-based MIMS has been used for the analysis of various compounds in air, water and other matrices. A brief summary (not intended to be exhaustive) is presented in Table 2. A recent application of MIMS using PDMS was reported by Tremblay et al. for on-line determination of compound-specific isotope ratio in environmental analysis [151]. The ability of the gas chromatography–isotope ratio-combustion-mass spectrometry (GC–IR-C-MS) used for the purpose to accurately determine the isotope ratio in a sample is critical, and hence it is important to determine any isotope fractionation happening in the permeation process involved in MIMS. Tremblay et al. found that there was no statistically differentiable isotopic fractionation for CO2 , but the permeated formaldehyde had a C13 enrichment resulting in an increase in ı13 C by 1.0026 ± 0.0003‰. Since GC–IR-C-MS is a highly promising technique and PDMS is one of the most widely used materials for sample preparation, it is perhaps critical for more

S. Seethapathy, T. Górecki / Analytica Chimica Acta 750 (2012) 48–62 Table 2 Applications of MIMS with PDMS membranes.

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Table 3 Applications of PDMS-equipped MESI techniques.

Description

Reference

Application

Reference

Selected aromatic hydrocarbons, alcohols, and ketones in air Toluene, phenol, chloroform and chlorobenzene and selected primary and secondary alcohols in aqueous solutions Benzene, toluene, ethylbenzene, m-xylene and other volatile organic compounds in water and sewage from a chemical plant Methanol, H2 S, dimethylsulphide, benzene, chloroform, 1,4-dioxan, toluene and other gases in water Benzaldehyde and 1,1-dichloroethylene in aqueous solution Benzene and selected alkyl substituted benzenes, halobenzenes, and iodoalkanes in air Flavour components such as methyl salicylate and 3-phenyl-2-propenal in human breath Butane in air Oxygen and carbon dioxide respiration rates of minimally processed potato and broccoli Online monitoring of the fermentation process: ethanol, H2 and CO2 in the liquid phase, and H2 and CO2 in the gas phase Volatile organic compounds in air including halogenated and aromatic hydrocarbons Quantitation of anti-depressants, pain relievers, epileptic medicine and anti-histamines in tablets Volatile organic compounds in air Volatile organic compounds in sea water Enzyme modified PDMS for low volatility and hydrophilic esters Methanethiol, di-Me sulfide, carboxylic acids, 4-methylphenol, aldehydes, indole, and skatole from biological livestock ventilation system

[26]

Benzene in the headspace of aqueous solution Ethylene in human breath using online technique Organochlorine and aromatic hydrocarbons in water Acetone in human breath Semi-volatile organic compounds (PAHs) in air using MESI-ion mobility spectrometry Pheromones in water by MESI-ion mobility spectrometry Analysis of methane and ethane in human breath Biogenic volatile organic compounds emissions (including ␣-pinene, eucalyptol and ␥-terpinene) from eucalyptus dunnii Thermooxidative degradation products of polystyrene Benzene, toluene and xylenes in aqueous solutions and chloroform in tap water

[154] [24] [156] [157] [158]

[137]

[138]

[139] [140] [22]

[159] [160] [161]

[162] [153]

[141] [142] [143] [144]

[145]

in the sorbent interface, and its amount (along with the knowledge of the original concentration in the stripping phase) could be used to correct for the variation in permeation rate due to temperature and flow effects. Selected applications of MESI using PDMS are listed in Table 3.

[146]

5. Applications of PDMS in lab-on-a-chip devices

[147] [148] [149]

One of the growing areas in analytical chemistry is the development of lab-on-a-chip (LOC) devices which utilize a combination of mechanical, optical and electrical properties of PDMS [8]. Even though silicon was used successfully during the initial periods of development of LOCs, alternatives were researched because of high demand, bio-compatibility requirements, lower production costs, and optical characteristics. PDMS is optically transparent down to 300 nm, isotropic and homogeneous, and is easily processed using soft lithography techniques. Consequently, PDMS is one of the most widely used polymers for the fabrication of the LOC devices and microelectromechanical systems (MEMS). LOCs found numerous applications in the bio-analytical field, especially in genomics and proteomics. The disadvantages of PDMS for some applications have also been recognized. For example, filling microchannels with a hydrophilic aqueous solution is difficult due to the hydrophobicity of the PDMS surface, and its use in electrophoresis is not practical because of the uncharged surface [163]. LOCs are generally micro-channel systems connected to liquid reservoirs and designed to allow chemical species and solvents to pass through them. Various analytical steps including sampling, pre-concentration, filtering, mixing, separation, isolation and analysis can be performed with them. Rondelez et al. used LOCs to study femtoliters of a sample solution to measure the activity of single molecules of ␤-galactosidase and horseradish peroxidase and showed the technique’s usefulness in ultra-sensitive bioassays [164]. Quake and Scherer used PDMS-based LOC to directly measure fluorescence and sort DNA molecules [165]. Viriyatanavirote et al. demonstrated the use of PDMS microchip capillary electrophoresis for the determination of thiol compounds using a pulsed amperometric detection system [166]. Abram and Clague used a PDMS microchip for pre-treatment of a bovine serum sample prior to detecting biomarkers of interest [167]. Yang et al. reported the use of PDMS-quartz microchip-based capillary electrophoresis method for qualitative and quantitative analysis of protein–DNA interaction using electrokinetic sample injection method [168]. Cho et al. reported the development of a low-cost biochip to perform DNA polymerase chain reaction (PCR) and successfully performed PCR analyses of the sex-determining Y chromosomes (SRY) gene and mouse glyceraldehyde 3-phosphate dehydrogenase (GAPDH) gene in less than 54 min [169]. Liu et al. incorporated an optical fiber for fluorescence detection in a PDMS electrophoresis micro chip

[150]

research to be done in the area of determining isotopic fractionation accompanying partitioning and permeability processes. 4.3. Membrane extraction with a sorbent interface (MESI) When MIMS was first introduced, the system was designed in such a way that the permeated analytes entered the ion source directly because of the vacuum conditions inside the instrument [135]. The MESI technique developed by Pawliszyn and co-workers allowed membrane introduction to be used on-line with GC equipped with arbitrary detectors. In MESI, analytes passing through a membrane connected in line with a gas chromatograph are swept by the carrier gas to a suitable trap, where they are enriched. This can be done by cryogenic focusing or by trapping the analytes with a thermally desorbable sorbent [152]. In either case, the focused analytes are introduced periodically into the GC column by rapidly heating the segment containing the preconcentrated analytes. The use of PDMS for extraction was retained in this method. Both flat PDMS membranes and PDMS tubing were used for the purpose [152,153]. Yang et al. described a detailed kinetic model for the extraction of organic compounds from headspace using MESI with PDMS tubing and showed that it was in agreement with the experimental results obtained for the extraction of benzene from water [154]. Guo and Mitra provided a theoretical analysis of non-steady-state, pulse introduction MESI which included boundary layer effects involved in the permeation process [155]. Since the permeation rate is dependent on the velocity of the stripping phase and temperature of the PDMS membrane, Liu et al. devised a method to account for this variation [24]. In this method, they used a calibrant in the stripping gas which permeated out into the sample matrix; the amount permeated depended on the stripping gas velocity and membrane temperature. The non-permeated calibrant was sorbed

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and showed its use in the separation of most segments of test DNA markers [170]. A MOSFET-type biosensor was incorporated on a PDMS micro-fluidic channel by Shin et al. for the detection of biomolecules and was tested using thiols as test compounds [171]. The same group also developed a PDMS micro-chip for performing PCR [172]. Schoening et al. used PDMS/glass capillary electrophoresis combined with amperometric detection for the analysis of catecholamines and dopamine [173]. Ros et al. demonstrated the application of a PDMS microdevice to detect as low as 100 fmol of fluorescein. They also showed that the device could be used for the detection of a single DNA molecule [174]. Huang et al. used a chemiluminescence detection system combined with PDMS/glass micro-chip electrophoresis and showed that it could detect as low as 5 × 10−11 mol L−1 of cobalt (II), which at that time was four orders of magnitude lower than any known method reported in the literature [175]. With their model of hybrid PDMS/glass micro-chip, Sanders et al. demonstrated the molecular diagnostic analysis of a variety of DNA samples for Duschenne Muscular Dystrophy and cytomegalovirus (CMV) infection [176]. Development of LOCs is happening rapidly and will likely continue at the same or higher rate in future. A combination of partitioning and optical properties of PDMS has also been exploited for its use as sensors in spectroscopic techniques. An investigation of the near infrared spectrum of PDMS indicates intense absorption bands at 1184, 1402 and 1690 nm (contributed by the methyl groups of PDMS) and provides opportunities for use of some of the remaining NIR region for various purposes [177]. Albuquerque et al. used this property to devise a PDMS-based sensor for sampling BTEX in water, which was quantified using the CH stretching absorption bands (1140–1150 nm) in the aromatic hydrocarbons [177]. Similarly, Lima et al. used PDMS rods for sampling and quantitation of benzene, toluene and xylenes in water using the NIR region for absorption measurements combined with multivariate calibration techniques [178]. Howley et al. established that PDMS has a suitable spectral window in the midinfrared region and used this property to develop a PDMS-coated sapphire fiber sensor for the quantification of aliphatic and aromatic hydrocarbons in water [179].

6. Miscellaneous applications of PDMS in the analytical laboratory Highly cross-linked PDMS (along with modifiers) is very stable at high temperatures and hence has been used widely for the manufacturing of septa used in gas chromatography inlets. Since the process of sample injection in GC involves coring of the septum, it is sometimes unavoidable that PDMS particles are introduced into the liner. When such coring happens, the PDMS particles in the liner often result in non-ideal chromatography, manifested by tailing peaks. Chromatographic vials typically use septa made of PDMS and modifiers. This allows for easy puncturing of the septum with a syringe needle to draw the sample contained in the vial. However, since PDMS readily sorbs various analytes, septa coated with a thin film of PTFE are normally used to avoid such sorption issues. In many cases, a single puncture of the PTFE-lined septum while taking a sample with a syringe exposes the PDMS to the sample vapor, which might affect the analyte concentration in the sample contained in the vial. The best practice is therefore to change the septum whenever the sample requires prolonged storage for future use. High molecular weight silicone grease (containing PDMS as one of the components) is used widely for sealing mass spectrometry analyzer chambers in many models. High molecular weight fraction with very low vapor pressure is used to ensure that little to

no vapors are present, as they could produce artifacts in the mass spectrum. At the same time, the highly homogenous and viscous material aids in proper sealing of the chambers. In many laboratories, heating glass flasks for purposes such as distillation is often done by immersing them in PDMS-based silicone fluid. Because of the high heat stability of silicone oils, no offensive off-gassing occurs while heating the flasks for various purposes. Gloves made of silicone or PDMS are widely use for handling hot materials. 7. Future perspectives The prerequisites for the application of polymeric materials such as PDMS are the knowledge of their fundamental properties, as well as cost-effective, reliable manufacturing. PDMS can be manufactured today in a highly reproducible manner, hence it is not a challenge anymore for reproducibility in quantitative work. In the last decade, extensive fundamental studies have been carried out ranging from simple measurements of sorption and permeability properties to complicated neural network modeling to understand the structure–property relationship between a chemical species of interest and PDMS. More such studies need to be done as they have the ability to predict the applicability of PDMS for analytical purposes. To the best of our knowledge, only the hydrophobic properties of PDMS have been exploited in the field of sample preparation and chromatographic separation. Many surface treatment procedures such as oxygen plasma, ultraviolet light, corona discharge etc., have the ability to render the surface of PDMS hydrophilic and could potentially open new avenues in sampling and sample preparation of hydrophilic compounds as well [180]. Hydrophilic surfaces have also been modified permanently by attaching polar chemical groups to further obtain selective sorption properties [171]. Nevertheless, the dominant trend in dealing with analytes that are not easily handled using PDMS is to use alternative polymeric materials. As an example, the use of various membrane materials in sampling has been reviewed by Seethapathy et al. [10]. In the field of PDMS fabrication, equipment such as spin coaters required for the manufacturing of membranes with reproducible and accurate geometric parameters is available at affordable prices. This might not only allow researchers to obtain reliable data, but also reduce the time and money involved in the research. Fabrication based on soft lithography has matured, hence PDMS-based LOCs will likely dominate the development of the field in the next decade. 8. Summary PDMS has been used extensively for various purposes in analytical chemistry since its invention. Its main uses include chromatographic separations and sample preparation for organic analytes in various matrices such as air, soil and water. Owing to its favorable properties, the use of PDMS in LOC devices is increasing very fast, and various groundbreaking accomplishments have been reported in this area. Fundamental properties of PDMS are being researched aggressively, which will likely result in further uses of PDMS in many analytical applications. Acknowledgements The authors wish to thank MITACS elevate postdoctoral scholarship and NSERC Canada for financial assistance. References [1] T.R.E. Simpson, B. Parbhoo, J.L. Keddie, Polymer 44 (2003) 4829–4838.

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