Anal. Chem. 2003, 75, 3950-3951
Response to Comments on “Helical Sorbent for Fast Sorption and Desorption in Solid-Phase Microextraction-Gas Chromatographic Analysis” I reported1 a new technique for microextraction of analytes using a helical solid sorbent followed by thermal desorption into a gas chromatographic injector. The sorption and thermal desorption were achieved in a few seconds, being very close to the theoretical prediction. Both processes were very fast due to the reduction of the thickness of the boundary layer between the sorbent and gaseous sample as a result of a turbulent rotational flow on the surface of the sorbent, which is generated by the helical configuration of the sorbent. Generally, the comments of the Pawliszyn’s group have no relevance, because they tried to reproduce my results using PDMS fiber and PDMS helical sorbent obtained by a procedure different from the one I used. Their comments have brought to light an important issue of the beginning of what Dr. Pawliszyn called in 1990 “microextraction”.2 They said in their comments that the device and method introduced in fact in 1971 by Palm2 from PerkinElmer Co., for collecting and introducing a sample into an analytical instrument, is not for “microscale” extraction but rather for an exhaustive trapping. In accordance with the general features of Palm’s method, small sample quantities are collected by exposing, for a predetermined period of time, a transportable segment, having a sorption material, to retain by sorption the analytes from gaseous or liquid sample; then, the sorption segment is introduced in the heated chamber of the analytical instrument. Palm’s method is solvent free and was used for the introduction of a very small amount of sample into a gas chromatographic capillary column without flow division. If the extraction is exhaustive, the amount introduced in the gas chromatograph would be too high for a capillary column without flow division. Moreover, for higher analytical sensitivity, Palm proposed a plurality of segments or tubular pieces. Similarly, the method was tried for Pawliszyn’s fiber, using a multifiber system.4 The sorption segment of the Palm device was a very thin rod or tube, which was named in Pawliszyn’s * E-mail:
[email protected]. (1) Ciucanu, I. Anal. Chem. 2002, 74, 5501-5506. (2) Arthur, C. L.; Pawliszyn, J. Anal. Chem. 1990, 62, 2145-2148. (3) (a) Palm, E. DE Patent 2,139,992, 1971. (b) Palm, E. U.S. Patent 3,797,318, 1974. (4) Xia, X. R.; Leidy, R. B. Anal. Chem. 2001, 73, 2041-2047.
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patents5 “fiber” or “hollow fiber”, respectively. Practically, Dr. Pawliszyn used exactly the Palm method, first changing the terminology. The helical sorbent used in their experiments was manufactured by wrapping a cross-linked PDMS tube (300-µm i.d., 650-µm o.d.) with a stainless steel wire inside. The thickness of their PDMS was 175 µm, which is 3.5 times thicker than my film thickness. The same cross-linked PDMS tube was used as a PDMS fiber. I tried to wrap a similar PDMS tube with stainless steel wire inside, but the tube had a wavy form and was not a helix due to the elasticity of the pre-cross-linked PDMS tube. If they did not get a real helix, it is not surprising that they were not able to see a difference. Transitional and turbulent flow is a function of the fluid properties, its velocity, the type of surface, and the pitch of the helix. However, even with this waved tube, the amount extracted at equilibrium in their Figure 1 must be extremely similar to that extracted by the fiber. In accordance with the mass balance before and after equilibrium, and the distribution constant equilibrium of the analytes between the headspace and sorbent, the number of moles of analytes extracted at equilibrium is directly proportional to the volume of sorbent material (eq 2). There is no indication regarding the volume of PDMS sorbent used in their helical sorbent and fiber, but probably the volume of sorbent material was less in their waved sorbent than in the fiber or they had other experimental problems. Trying to reproduce my helical sorbent technology, they were not able to do a stable 50-µm nonbonded PDMS film on the surface of a wire, although 100- and 30-µm nonbonded PDMS fibers are available commercially. This demonstrated that they do not have enough experience in coating techniques and this created their difficulties. The experiments with their helical sorbent at 300 °C for 30 min are misleading. OV-1 used in chromatography is without bleed up to 250 °C, and in all my experiments, the temperature in the injector did not exceed 250 °C. The desorption time did not exceed 30 s (Figure 6) and was not 30 min as they suggested. They did not compare their results with a commercial nonbonded PDMS fiber. The caption to Figure 3 states that only nonbonded PDMS fibers were used for that experiment. In the Experimental Section, 30-µm bonded PDMS film remained listed because one of the reviewers did not believe me when I said that Supelco makes nonbonded PDMS fibers. The effect of temperature on the desorption profile shown in Figure 6 does not indicate that the diffusion in the polymer coating controls the desorption process as they suggest. The diffusion of analytes in the sorbent layer is a function of temperature and would be faster with the increase in temperature, but the diffusion of analytes does not control the desorption process. The problem is how to increase quickly the temperature of the sorbent. The temperature of the sorbent was increased in the desorption process as the result of heat (5) Pawliszyn, J. GB Patent 9,007,356, 1990. (b) Pawliszyn, J. U.S. Patent 5,691,206, 1997. 10.1021/ac034012p CCC: $25.00
© 2003 American Chemical Society Published on Web 06/17/2003
transfer, which was the result of direct contact between the hot carrier gas and sorbent, where the heat transfer has a submicroscopic mechanism. The rate-controlling step of this process was the heat transfer in the boundary layer, and I showed this in my article. I described in the Experimental Section that each point in my graphs was the mean value for seven injections. The precision of the injections was in 1-2% range using my helical sorbent holder device (Figure 2), which could perform each injection with the same speed due to the plunger spring. Moreover, the precision of the injection was improved and the carryover was eliminated, in comparison with a commercial device, due to the avoidance in the extraction step of the diffusion of analytes between the elongation arm of the helical sorbent and the external protection tube. More experimental evidence will be the subject of a future paper. The extraction times for the helical sorbent in Figure 3A and in Figure 4 for 0.05 m/s of the headspace flux are apparently different because the scales of the extraction times are different, but there are the same values. The fibers needed a longer extraction time. They did not note these differences. The time for reaching the extraction equilibrium was evaluated from the extraction profile curves at the moment of starting of the steady-state value. Both Figure 4 and Figure 5 showed very clearly that the extraction equilibrium was reached after 8 and 15 s, respectively The influence of the water in the headspace extraction process can be ignored because PDMS is a hydrophobic material and the water from the matrix will have very little influence on the extraction equilibrium of the nonpolar analytes
as aromatic hydrocarbons. They offered no literature references that discussed this issue. Anyway, all the extractions with the helical sorbent and fiber were performed with nonbonded PDMS sorbent using the same experimental condition, and the comparative study of the extraction could not be modified by the theoretical influence of the water in the system. There is nothing important missing in Figure 7. The y-axis in the chromatograms is usually omitted, since chromatographers know that by convention the y-axis is the detector signal intensity (mV). In my article, the chromatograms from Figure 7 were performed at different desorption temperatures, but any other chromatographic conditions were identical, which generated the same signal intensity scale on the y-axis. So, it is no problem to compare the peak heights. In fact, the desorption temperature was identical for chromatograms B and C, which compare the helical sorbent and the fiber. Evaluation of polymer density using the mass and volume of the polymer can be found in any fundamental book of analytical chemistry; therefore, such experimental details were not included in my paper. The authors of the comments may have had difficulty reproducing my results because of less experience with coating techniques and the mass- and heat-transfer processes involved in sorption and desorption processes with a helical sorbent.
Ionel Ciucanu*
Department of Chemistry, West University of Timisoara, Strada Pestalozzi 16, RO-1900, Timisoara, Romania AC034012P
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