Pulse elution

In a further application of MI-SPE, theophylline could be separated from the structurally related caffeine by combining the specific extraction with pulsed elution, resulting in sharp baseline-separated peaks, which on the other hand was not possible when a theophylline imprinted polymer was used as stationary phase for HPLC. A detection limit of 120 ng mb1 was obtained, corresponding to a mass detection limit of only 2.4 ng [45]. This combination of techniques was also used for the determination of nicotine in tobacco. Nicotine is the main alkaloid in tobacco and is the focus of intensive HPLC or GC analyses due to its health risk to active and passive consumers. However, HPLC- and GC-techniques are time-consuming as well as expensive, due to the necessary pre-purification steps required because the sample matrices typically contain many other organic compounds besides nicotine. However, a simple pre-concentration step based on MI-SPE did allow faster determination of nicotine in tobacco samples. Mullett et al. obtained a detection limit of 1.8 jig ml 1 and a mass detection limit of 8.45 ng [95]. All these examples demonstrate the high potential of MI-SPE to become a broadly applicable sample pre-purification tool. 

Molecular imprinting has been used to devise a chemosensor for L-nicotine (Table 6) [178]. For that, poly(methacrylic acid) (PMA) beads, imprinted with the L-nicotine template in chloroform, were incorporated in a film of the conjugated polymer, OCiC10-PPV. EIS has then been utilized for the L-nicotine determination in the 1-10 nM concentration range. This MIP chemosensor showed predominant affinity towards L-nicotine over a structurally related L-nicotine metabolite, L-cotinine. Similarly, the polydopamine-imprinted film prepared by electropolymerization in the phosphate buffer (pH = 7.4) has been used to devise a chemosensor for L-nicotine with LOD of 0.5 pM (Table 6) [106]. This LOD is still much higher than that reported for other L-nicotine determination methods based on MIPs, such as SPE combined with differential pulsed elution, which was 6 nM [31]. 

Yu, J.C.C. and Lai, E.P.C. Determination of ochratoxin A in red wines by multiple pulsed elutions from molecularly imprinted polypyrrole. Food Chem. 2007,105, 301-310. 

MI-SPE has been applied in either on-line or off-line mode for several analytes. In the first case, MIP is packed as an HPLC precolumn. Column switching and pulsed elution modes are employed in on-line MI-SPE. One advantage of this design is the automation capability and the direct coupling to HPLC or other separation modes. Despite these advantages, the off-line mode, which has been successfully applied to bio and environmental and food analysis, is the most commonly practiced method. 

Figures 5 and 6 show data for 2-butanone on Tenax-GC and PI-119, respectively. In the figures, "Z Deviation" refers to the change in BV as compared with the control (pure, dry helium). Therefore, a deviation of -10Z means that the BV was decreased by 10Z, relative to the corresponding control pulse, as a result of the change in the system for that particular experiment. This method of reporting the data elucidates the time-dependence of the retentions of the probe compounds as compounds present in the front accumulate on the adsorbent thus helping to model the environmental sampling process. As an example, consider three compounds that, in any given pulse, elute in the order A, B, and C. The fronts would break through in the same order. For purposes of discussion, the retention of compound B will be followed. Assume that an...

The dependence of the initial slope of the isotherm on the mobile phase composition can be derived in two different ways [29]. The retention time, Ir , of analytical pulses eluted imder isocratic conditions is a function of the mobile phase composition. Since Ir — to is proportional to Eq. 15.29 permits the determination of Sj as the slope of a plot of log(fR / — to)/to versus mobile phase composition at which analytical pulses are eluted rmder conditions of linear eluotropic-strength gradient. In this case, it has been shown [3] that the retention time of an analytical pulse is given by Eq. 15.6. Using this equation, it is simple to derive S, since all the other parameters are knovm. This method of estimation of S seems to give the best results [20,24,29]. 

Nicotine MAA TRIM CH2C12 SPE with differential pulsed elution [146]... 

Theophylline MAA EDMA CHC13 SPE with pulsed elution [152]... 

Mullett, W.M. Lai, E.P.C. Determination of theophylline in serum by molecularly imprinted solid-phase extraction with pulsed elution. Anal. Chem. 1998, 70, 3636-3641. 

S.G. Wu, E.P.C. Lai and PM. Mayer, Molecularly imprinted solid phase extraction-pulsed elution-mass spectrometry for determination of cephalexin and alpha-aminocephalospo-rin antibiotics in human serum, /. Pharmaceut. Biomed., 36 (3) 483-490, 2004. 

Our goal is to determine the shape of the pulse eluted from this absorbent-coated tube. To do so, we first recognize that the tube s contents are subject to the mass balance... 

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