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Short Communications Pressurized Planar Electrochromatography As the Mode for Determination of Solvent Composition–Retention Relationships in Reversed-Phase Systems

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Short Communications Pressurized Planar Electrochromatography As the Mode for Determination of Solvent Composition–Retention Relationships in Reversed-Phase Systems Short Communications Pressurized Planar Electrochromatography as the Mode for Determination of Solvent Composition–Retention Relationships in Reversed-Phase Systems Tadeusz H. Dzido*, Paweł W. Płocharz, Anna Klimek-Turek, Andrzej Torbicz, and Bogusław Buszewski Key Words Pressurized planar electrochromatography PPEC Retention–composition relationship 1 Introduction (HPLC) and capillary electrochromatography (CEC). Because the last two modes [8, 9] and planar chromatography [10] are Pressurized planar electrochromatography (PPEC) was intro- used for determination of solvent composition–retention rela- duced by Nurok et al. [1]. The mobile phase in PPEC is driven tionships, the question arises, why do not use PPEC for determi- by the electroosmotic effect through the adsorbent layer of the nation of solvent composition–retention relationships? In the chromatographic plate. A special plastic film or plate is pressed paper we report an attempt to find a preliminary answer to this on to the adsorbent layer to eliminate the vapor phase and flow question for the first time. of mobile phase to the surface of the adsorbent layer. The paper The most popular equation used for correlation of retention and mentioned above, and others [2–6], indicate that this method is mobile phase concentration in reversed-phase systems is: characterized by high-efficiency separation, making it very attractive for application in laboratory practice. Contemporary log k = log kw – mC (1) applications have, however, been mainly restricted to separation where kw is the retention factor of the compound in pure water or of test solutes to show the advantages, practical possibilities, buffer as the mobile phase, m is the slope, and C is the concen- and efficiency of PPEC in comparison with conventional planar tration [%, v/v] of organic component (modifier) in the water (or chromatography (TLC). At the current stage of PPEC develop- buffer) mobile phase. The retention factor can be calculated on ment its performance is similar to that of HPLC [1–6]. Separa- the basis of HPLC data via: tion times in PPEC are reported to be much shorter than in TLC [1, 2, 4], in some circumstances by as much as a factor of 24 [1]. (2) Pressurized planar electrochromatography experiments can be performed under equilibrated conditions [3, 4]. This is another where tR is the retention time of the compound and tM the hold- very important advantage of PPEC compared with TLC. The up time (retention time of unretained compound). adsorbent layer of the chromatographic plate is prewetted with The retention factor k is related to retardation factor, R , used as the mobile phase for the time necessary for equilibration. After F prewetting, the mobile phase is used to feed the adsorbent layer a measure of retention in planar chromatography: during separation process in pressurized planar electrochro- matography. Otherwise, conventional planar chromatographic (3) separation usually proceeds under non-equilibrated conditions with the exception of use of a pure solvent as mobile phase. TLC and separations are, however, usually performed with mixed mobile (4) phases. Then demixing of the mobile phase occurs during chro- matogram development [7]. Even conditioning with mobile A similar equation to eq. (1) has been used for extraction and for phase vapor followed by chromatogram development does not partition chromatography [11, 12]. This equation has often been lead to full equilibration of the chromatographic phases. applied to predict retention and selectivity changes, to interpret These features of PPEC make it very similar to column tech- the relationships between retention and structure of chro- niques such as high-performance liquid chromatography matographed solutes [13], and to investigate quantitative struc- ture–activity relationships [14, 15]. Deviations from linearity were observed when a broader concentration range was applied. T.H. Dzido, P.W. P³ocharz, A. Klimek-Turek, A. Torbicz, Department of Physical Chemistry, Medical University, Lublin, Poland; and B. Buszewski, Department of In such cases better fitting of experimental data was achieved by Environmental Chemistry and Ecoanalysis, Faculty of Chemistry, Nicolaus applying a quadratic relationship [16]: Copernicus University, Toruñ, Poland. E-mail: [email protected] log k = a + bφ + cφ2 (5) Journal of Planar Chromatography 21 (2008) 4, 295–298 DOI: 10.1556/JPC.21.2008.4.13 0933-4173/$ 20.00 © Akadémiai Kiadó, Budapest 295 Short Communications where a, b, and c are constants, φ is the amount [%, v/v] of mod- Samples of the test compound (0.2 μL of a 0.03% solution) were ifier in the binary aqueous mobile phase used for reversed-phase applied to the plate 16 mm from its edge by means of a Linomat liquid chromatography. 5 aerosol applicator (Camag, Muttenz, Switzerland). Sample Equation (1) is more often applied in the practice of column and application rate was 1 mL per 40 s under a compressed air pres- 5 planar chromatography than eq. (5). Experimental data obtained sure of 5 × 10 Pa. by use of HPLC and TLC do not, however, have the same form A horizontal developing DS chamber (type DS-II-5x10) from of eq. (1) with respect to their slope. The slopes of the lines Chromdes (Lublin, Poland) was used for chromatogram devel- obtained by use of TLC are often less than those obtained by use opment. Before chromatogram development the chamber and of HPLC even when the same stationary phase is used in both chromatographic plate were saturated with mobile phase vapor techniques [17]. This discrepancy was explained by preadsorp- for 15 min. All planar chromatography experiments were per- tion of components of the vapor phase on the dry adsorbent dur- formed in triplicate. ing saturation of the adsorbent layer of the chromatographic Detection of sample zones was performed with a TLC 2010 plate in the atmosphere in a developing chamber. Introduction of Diode Array Scanner (J&M, Aalen, Germany). ξ a correction factor, (RF values obtained in TLC were multi- plied by the factor ξ > 1.0) minimized the difference between the slopes of eq. (1) obtained in HPLC and TLC [7, 17]. 2.3 Pressurized Planar Electrochromatography (PPEC) Taking into account the discussion above it seems that plots of Chromatographic plates were cut, washed, and activated as retention against composition obtained by use of PPEC should described in Section 2.2. Margins (4 mm) of silicone sealant be more similar to those obtained in HPLC than to those were formed on the entire periphery of the plate. The sealant obtained by use of TLC. This assumption inspired us to perform solutions were prepared by mixing the components Sarsil W or investigations to compare the experimental plots expressed by Sarsil H50 with hardener in the proportion 100:4 (w/w). The use of eq. (1) for three the modes HPLC, PPEC, and TLC. In the plate was placed in an oven at 105–110°C for 45 min to poly- work discussed in this paper we used an uncharged molecule as merize the sealant then left in a desiccator and used for experi- test compound to eliminate an effect of electrophoresis on ments within 1 day. The mode of sample application and volume migration in PPEC. of the sample were the same as in the planar chromatography experiments. The adsorbent layer of the plate with the sample spot on it was prewetted by dipping in the mobile phase in a spe- cial reservoir for 2 min [5]. After prewetting the stationary phase 2 Experimental on the plate was covered with a glass plate of the same size and transferred to the chamber for PPEC. The channels for the mobile phase in the PPEC chamber were filled with prewetting 2.1 Materials Used solvent (the solution remaining after prewetting). An appropri- RP-18 W 10 cm × 20 cm HPTLC plates (cat. no. 114296.0001) ate potential was then applied to create an electric field and gen- were from Merck, Darmstadt, Germany. Methanol and acetoni- erate an electroosmotic flow of the mobile phase in the adsor- trile for HPLC were from POCh, Gliwice, Poland. Phenol, bent layer. Construction of the electrochromatographic chamber sucrose, 97% sulfuric acid, and 96% ethyl alcohol, all analytical and development of electrochromatograms have been described grade, and acetic acid (99.5%) and sodium acetate were pur- elsewhere [4]. chased from Polskie Odczynniki Chemiczne (POCh, Gliwice, The retention factor, k, of the test solute in the PPEC system was Poland). Silicone sealant solutions (Sarsil W and Sarsil H50) calculated by applying eq. (3). The retardation factor was calcu- and hardener were obtained from Zak³ady Chemiczne Silikony lated from the migration distances of test solute and mobile- Polskie, Nowa Sarzyna, Poland. The test compound was 1-(4- phase front (sucrose was used as a marker of solvent front posi- hydroxyphenylazo)-2-naphthol, synthesized in the Department tion [5]). of Inorganic and Analytical Chemistry, Medical University, Lublin, Poland. Sucrose was prepared as 2% solution in 8:2 methanol–water. 2.4 HPLC Measurement of retention of the test solute 1-(4-hydroxypheny- 2.2 Planar Chromatography (TLC) lazo)-2-naphthol was performed with an HP 1050 liquid chro- matograph (Hewlett–Packard, USA) equipped with a 20-μL Conventional development of planar chromatograms was per- sample injector (Rheodyne, Cotati, CA, USA) and variable- formed with RP-18 W HPTLC plates which were cut into wavelength UV detector (HP-1050) operating at 510 nm. Chro- 2 cm × 10 cm pieces using TLC plate cutter (OM Laboratory matography was performed on a 100 mm × 4.6 mm i.d. stain- Chigasaki, Japan). The plates were washed by dipping in less-steel column packed with adsorbent scraped from a chro- methanol for 1 min, dried in air and then in an oven at matographic plate. The mobile phase was prepared by mixing 105–110°C for 10 min, then left in a desiccator to cool.
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