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Comments on “Helical Sorbent for Fast Sorption and Desorption in Solid-Phase Microextraction-Gas Chromatographic Analysis” Advancements in sample preparation have resulted in reduction of the time and cost for this step in an analysis and for improving sensitivity, precision, and accuracy. Significant enhancements have come through the introduction of new devices or improved device configurations with much effort being put into development of sorbent materials for extraction and concentration of analytes. While improvements in precision and accuracy have come from eliminating sources of error, improvements in sensitivity were typically achieved by enhancement of the capacity of a sample preparation, by increasing either the volume or affinity of the sorbent. Such enhancements have been exploited in both solid-phase extraction and solid-phase microextraction (SPME). The gains have, however, come at the cost of longer extraction or equilibration times, or both. Many researchers are now turning their attention to enhancing mass-transfer conditions to reduce the time required for extraction in order to reduce overall analysis time. In the article entitled “Helical Sorbent for Fast Sorption and Desorption in Solid-Phase Microextraction-Gas Chromatographic Analysis”,1 the author presents a new geometry for a SPME device and suggests that it allows for much faster extraction and desorption than the commercially available SPME assemblies. The results appear, however, very inconsistent with the fundamentals of mass transfer and the theory and practice of SPME. In this correspondence we wish to reexamine the results in light of these inconsistencies The Ciucanu article focuses on a comparison of PDMScoated fibers of straight and spiral geometry for SPME. The thickness of the spiral coating used was reported to be 50 µm and the commercial fibers were 100 and 30 µm. The helicalshaped fiber and the device holding it were custom-made and were compared to the commercially available Supelco SPME syringe device. The experimental data appear to show that the equilibration times of spiral fibers were reduced by over 1 order of magnitude compared to straight fibers under identical agitation conditions (Figure 3 and page 5504). The author states, “Moreover, the extraction time for the helical sorbent * Corresponding author. Tel: +1-519-8851211. fax: +1-519-7460435. E-mail:
[email protected]. (1) Ciucanu, I. Anal. Chem. 2002, 74, 5501-550.
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with the 50 µm PDMS coating was more then 15 times shorter, even in comparison with a thinner coating thickness, than the 30 µm PDMS silica rod”. The data indicate that the equilibration time (95% of the equilibrium value2) for the helical fiber under static conditions (Figure 4) was even faster than the thinner 30-µm commercial phase fiber under agitated conditions (Figure 3C). We assert that it is not physically possible to obtain shorter extraction times for a thicker phase at static conditions compared to a thinner phase with agitation, regardless of the geometric shape of the extraction phase. In addition, at high agitation conditions, the data show that the equilibration time for the 50-µm film at room temperature was around 3 s (Figure 4). The author states that this figure shows an equilibration time at high agitation of 8 s, but examination of the data in the figure clearly shows equilibration well before that. Figure 5 shows an equilibration time of ∼8 s under much lower agitation conditions. Even under ideal theoretical conditions where the solution is agitated so well that the diffusion through the polymer controls the rate of extraction, defined as “perfect agitation conditions”, 3 equilibration times are expected to be about double this value. The author estimates a 5-s theoretical equilibration time under perfect agitation. The author states correctly that perfect agitation cannot be achieved for benzene at the experimental conditions, so the extraction time should be longer than 5 s. It is not physically possible to have equilibration in under 5 s, as is shown in Figure 4. It is well known from the literature that flow through an open tubular helix has some advantages. For such a flow geometry, a secondary flow perpendicular to the axial flow created by centrifugal force will result in a flattening of the parabolic flow profile, reduced axial dispersion, and a significant improvement in the mass transfer to the walls of the tube.4,5 For example, the heat transfer at laminar flow regime, in the coiled versus straight tube is improved by factor (De)1/2, where De ) Re(di/ dc)1/2 is the Dean number and it is the dynamic parameter governing fluid motion in a coiled tube, Re is the Reynolds number, di is the internal diameter of the tube, and dc is the coil diameter.6,7 The main thrust of this discussion, however, concerns mass transfer from a flow around a linear element versus that around a helical element as described in the Ciucanu paper. To our knowledge, the order of magnitude improvement in mass transfers for flow around helical geometries claimed in the article has not been supported by any theory or data in engineering or in electrochemical applications. It is not reasonable to expect that coiled wire would induce substantial helical flow. Coiled structures are used in engineering practice as well, but in many cases to increase the interfacial contact area. The enhancement of the surface area of the extraction (2) Pawliszyn, J. Solid-Phase Microextraction, Theory and Practice; Wiley-VCH: New York 1997. (3) Louch, D.; Motlagh S.; Pawliszyn, J. Anal. Chem. 1992, 64, 1187-1199. (4) Dean, W. R. Philos. Mag. 1928, 5, 674-695. (5) Dean, W. R. Philos. Mag. 1927, 4, 208-223. (6) Mori, Y.; Nakayama, W. Int. J. Heat Mass Transfer 1967, 10, 681-695. (7) Mori, Y.; Nakayama, W. Int. J. Heat Mass Transfer 1965, 8, 67-82. 10.1021/ac0264271 CCC: $25.00
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phase will impact the rate of extraction, however, not to the extent reported by the author. For a more detailed discussion of the enhancement related to increased surface area/coating volume ratio, please see ref 8. Considering the above we decided to examine more carefully the results presented by Ciucanu. OV-1 was obtained from Alltech (Deerfield, IL). Chloroform was obtained from Fisher Scientific (Springfield, NJ). Benzene, toluene, ethylbenzene, and xylene (BTEX) were all obtained from Aldrich (Mississauga, ON, Canada). Naphthalene, acenaphthylene (ACEY), acenaphthene (ACE), and fluorene were all obtained from Supelco (Bellefonte, PA). Helium (99.999%) was obtained from Praxair (Waterloo, ON, Canada). Water was obtained from a Barnstead/Thermodyne NANO-pure ultrapure water system (Dubuque, IA). A Star 3400/3800 gas chromatograph (GC) equipped with a flame ionization detector or an ion trap mass spectrometer (Saturn 2000) was obtained from Varian (Mississauga, ON, Canada). The stainless steel wire (grade 304, 0.005-in. diameter) was obtained from Small Parts Inc. (Miami Lakes, FL) and coated either with PDMS hollow fiber membrane or according to the procedure described by Ciucanu.1 For our comparison of helical versus rod-shaped geometries on extraction rate, we used PDMS hollow fiber as the extraction phase, due to the difficulties encountered in obtaining a reproducible coating by the dipping technique of Ciucanu. The PDMS hollow fiber used (300-µm internal diameter, 650-µm outer diameter) was obtained from New Age Industries (Southampton, PA). The stainless steel wire described above was inserted into the hollow fiber in order to shape the sorbent into spiral or straight geometry. The length of fiber and hence amount and surface area of the extraction phase was kept constant between the two geometries. The helix was formed by wrapping the wire around another wire of the same diameter, using constant pitch resulting in a 10-turn coil. During extraction, the sorbent was positioned inside the vial (40-mL glass) by mounting it to the septum of the cap. After the extraction, the sorbent was transferred to the GC injector for immediate thermal desorption according to a procedure described previously.8 For all experiments, the sample was stirred at 1250 rpm using a 25-mm Teflon-coated stir bar. We compared the rate of extraction obtained for the PDMScoated wires with straight and helical geometries in headspace mode. The hollow fiber membranes used in our experiments were made of cross-linked PDMS to improve stability of the extraction phase on the wire at high temperature. We compared sorbents with identical film thickness and length under the identical stirring conditions. Our experiments showed that for a 15-s headspace extraction of BTEX (the same compounds as used in the Ciucanu paper) the level of extraction was significantly lower for the spiral geometry (Figure 1). Since none of the BTEX compounds had reached equilibrium after 15 s (results not shown), the data in Figure 1 may be expected to compare extraction rates. Most current efforts in improving extraction rates are focused on semivolatile analytes, as extraction times for highly volatile compounds such as benzene are already very fast. The
dramatic improvement in the rate of extraction reported by the author should be observable with semivolatile compounds as well, especially for direct extraction. Again, our experiments showed that for both 10-min headspace extraction (Figure 2) and 10-min direct extraction (results not shown, but analogous data were obtained) the level of extraction was significantly lower for the spiral geometry. From our previous experience with these compounds, we expect that under the conditions used, naphthalene will be extracted at or close to equilibrium and acenaphthylene, acenaphthene, and fluorene are extracted progressively further from equilibrium.8 The naphthalene data may be expected to compare extraction capacity and the data for other PAHs to compare extraction rate. Extraction rate is therefore seen to be significantly lower for the helical geometry for acenaphthylene, acenaphthene, and fluorene. From the data in Figures 1 and 2, it is also important to realize that BTEX and naphthalene are the most volatile and so produce higher data spread than the other compounds, as is reflected in the larger error bars. Clearly, helical-shaped geometry does not produce enhanced rate of sorption as reported by the Ciucanu. Previously, Vecera and Dasgupta reported that no improvement in mass transfer was obtained for a helical versus straight sorbent geometry for collection of gases.9 Their data support our position and experimental findings as well. Assuming the same interfacial boundary layer conditions and the same sorbent surface areas, one would expect the extraction rate to be similar.8 As Ciucanu describes, SPME is typically performed at highly turbulent conditions in order to reduce the effect of the boundary layer. From our results, it appears that the helical geometry may cause a more effective shielding of the part of the sorbent/sample interface compared to the straight geometry, resulting in less efficient convection in these areas and lower extraction rates (Figures 1 and 2). We attempted to find reasons for the inconsistencies between our results and the author’s. In Ciucanu’s Experimental Section1 it is explained that the sorbents are prepared by dipping bare wires into a solution of OV-1 (methyl gum). Our
(8) Bruheim, I.; Liu, X.; Pawliszyn, J. Anal. Chem. 2003, 75, 1002-1010.
(9) Vecera, Z.; Dasgupta, P. K. Environ. Sci. Technol. 1991, 25, 255-260.
Figure 1. Comparison of linear and helical geometries for headspace extraction over 15 s from a 10-mL water sample spiked with BTEX (1 µg/mL). N ) 3; error bars represent standard deviation. Extraction devices were constructed by covering stainless steel wire with cross-linked PDMS hollow fiber membrane. Oven program: -20 °C (2 min) and then 15 °C/min until 150 °C (1 min). Injector: 40 °C and then 16 °C/s until 250 °C. Extractions were performed with either helical sorbent (PDMS hollow fiber, length 4.1 cm) or straight sorbent (PDMS hollow fiber, length 3.95 cm).
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Figure 2. Comparison of linear and helical geometries for headspace extraction (10 min) of a 10-mL water sample containg PAHs in the range of 0.12-0.23 µg/mL. N ) 2; error bars represent data range. Oven program: 35°C (hold 2 min) and then 10 °C/min to 250 °C (hold 6.5 min). Other conditions as in Figure 1. For the compounds discussed, water/PDMS partition coefficient increases in the order naphthalene, acenaphthylene, acenaphthene, and fluorene.
experience indicates that it is very difficult to deposit a uniform 50-µm PDMS film on the surface of a wire by dipping technology. In addition, it is not possible to produce a stable non-cross-linked PDMS film able to withstand 250-300 °C without bleed, a fact that is well understood from GC column manufacturing. The article does not specify which injection temperature was used, but Figures 6 and 7 show desorption data for 250 °C. To confirm our concern, we repeated the coating procedure using the same PDMS material as indicated by the author. Our experiments showed that the sorbents prepared according to the method described1 were not robust when exposed to high temperature. By placing the PDMScoated wire in the GC injector at 300 °C for 30 min, the sorbent was stripped of all its coating. Furthermore, we found that the sorbent started to bleed at 200 °C and at 250 °C substantial losses occurred. It is our belief that the author has generated the data for his paper by extracting from a wire with much less than the expected amount of polymer or even bare wire. This seems to be supported by the desorption data in the article, which show a very small difference in desorption at room temperature versus elevated temperatures (Figure 6). If the diffusion in the polymer coating controls the desorption process, the desorption times are expected to be almost 2 orders of magnitude shorter when the temperature is increased from 25 to 250 °C because of the increase in the diffusion coefficient (see eq 41). We feel it is also important here to clarify the nature of the commercial polymer. Ciucanu reports that the 30-µm commercial SPME fiber is a bonded phase and that the 100-µm commercial fiber is nonbonded. In actual fact, neither is bonded. Only the 7-µm PDMS fiber is bonded to the silica core.10 Importantly, this reference also describes that the nonbonded PDMS phases are cured by either thermal or UV 3948 Analytical Chemistry, Vol. 75, No. 15, August 1, 2003
treatment, which promotes cross-linking and better thermal stability in the commercial fibers. Our experience in working with volatile analytes such as benzene is that it is very difficult to obtain a precision better then 5% for manual operation of the syringe. Therefore, the lack of visible data scatter as is normally seen in reported figures is very surprising. In addition, perfect agreement between the equilibrium values of the helical and commercial fibers (Figure 3) is puzzling considering that it is unlikely that the benzene-PDMS distribution constant for the cross-linked PDMS will be identical to the non-cross-linked polymer. The expected differences in amount of sorbent on the wire caused by weighing error and bleed are expected to add to the differences. Furthermore, the large diameter of the sheath holding the helical sorbent and the resulting large void volume would cause introduction of part of the headspace together with the sorbent to the injector. Figure 6 shows seemingly “perfect” desorption time profiles even for 250 °C, when the bleed is expected to be very high. Also, it is very surprising that the extraction time profile for the helical sorbent (Figure 3A and Figure 4) does not show the expected shape for polymer-coated fibers. Over 50% of analyte is extracted during the first second of the experiment, further supporting the assertion that the data are due to a significantly thinner than expected extraction phase, adsorption to essentially bare wire or introduction of sample headspace from either the sheath or the helix core. From our experience in working with compounds as volatile as benzene, we also feel it is not possible to obtain reliable data for 1-s extraction times. There are many important details missing from the Ciucanu paper. For instance, the author reports that the higher peaks produced by the helical sorbent indicate that limit of detection can be improved with that geometry. Figure 7, however, does not include a y-axis so the reader cannot verify a difference in peak height. The author states that the polymer density was calculated by measuring the mass and volume of the polymer, but no indication is given on how such a small volume was accurately measured. If volume was calculated from the stated thickness of 50 µm, the concern is that no indication is given of how coating thickness was measured, and in our experience, it is not possible to produce a PDMS coating of uniform thickness using the dipping technique described. In our hands, the dipping solution used (60%/30% PDMS in chloroform) is very viscous and does not produce an even coating on the wire. In summary, both our data and the theories of kinetics and thermodynamics of extraction contradict the conclusions presented by the author that the helical geometry of the coated rod improves the rate of extraction. If one wants to change the boundary layer to improve extraction rate, a force must be applied in order to overcome sample viscosity and enhance convection in the system. It is possible to dramatically increase the convection rates by using very energetic agitation techniques, such as probe sonication.11 In this case, it is very difficult to avoid unwanted effects associated with sample heating although it might be acceptable for flow-through (10) Mani, V. Properties of commercial SPME coatings. In Applications of SolidPhase Microextraction; Pawliszyn, J., Ed.; Royal Society of Chemistry: Cornwall, U.K., 1999; p 61 (11) Motlagh, S.; Pawliszyn, J. Anal. Chim. Acta 1993, 284, 265-273.
systems, which are self-cooling. From our data, we believe the helical geometry does not significantly enhance extraction rate and may actually reduce it due to partial shielding of the extraction surface. We feel the author did not account for instability in the non-cross-linked polymer used and that this fact confounds reasonable evaluation of the data presented.
Inge Bruheim, Heather Lord, and Janusz Pawliszyn*
Department of Chemistry, University of Waterloo, Waterloo, Ontario, N2L 3G1 Canada AC0264271
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