Chapter 3. Recording temporal characteristics of convection currents by
3.3. Results and Discussion
3.3.1. Monitoring convection-driven currents by optical detector
Here we demonstrate probing chemical waves, which are formed in a liquid medium due to convection, by using continuous flow sampling in conjunction with optical absorption detection. A small amount of medium is sampled and transferred along the capillary flow line
towards detector by using either segmented flow or non-segmented (continuous) flow. At the beginning of the experiment, we injected a small aliquot of ferroin solution into the bottom of a 20-mL glass vial. Subsequently, the lower part of the vial was heated up ( 34 °C) in order to develop a temperature gradient, and induce convective mixing of the ferroin solution with the liquid medium present in the vial. We used a syringe pump operated in the withdrawal mode in order to pull the contents of the vial with the flow rate of 30 μL min-1. n-Octanol was simultaneously injected to the Y-junction (installed along the flow line) by a syringe pump operated in the infusion mode at the flow rate of 6 μL min-1 (Figure 3.4). The volume of each aqueous plug is estimated to 1.25 ± 0.16 (SD) μL.
Figure 3.4 Recording convection with segmented and continuous flow. (A) Experimental setup used in the real-time sampling with/without segmented flow prior to detection by the flow-through optical detector (cf.
Figure 3.1). The segmented flow was generated by pushing n-octanol towards the Y-junction while the bulk of the liquid was withdrawn by a syringe pump at the outlet of the flow line.
Figure 3.5 shows photographs of the inlet vial representing the convection-driven mixing of ferroin (red) with the liquid medium (transparent) while Figure 3.6 shows the corresponding raw data. The ―ups‖ and ―downs‖ in the original signal trace are caused by the differences in refractive indices and extinction coefficients of n-octanol and the aqueous samples. The relative heights of the lower section of the valleys in Figure 3.6 vary according
to the absorbance of the plugs sampled from the vial. In order to simplify the representation of the data, a custom software was used to remove the signal of n-octanol, and the data treated this way are displayed in Figure 3.7A (red line). From this data it is clear that the signal increased due to the increasing absorbance of ferroin. Interestingly, the increase of the absorbance with time (in relative units) cannot be described by any simple function, as one could do for the diffusion process. The gradual mixing of ferroin with the liquid medium is a chaotic process, and the trace in Figure 3.7A (red line) represents numerous fluctuations before the signal stabilizes at a level when the mixture became a homogeneous solution.
Figure 3.5 Photograph of the vial (nominal volume: 20 mL) during the convective mixing of 100 μL ferroin with 15 mL water (aided by heating). The ID of capillary used for on-line sampling was 320 µm.
Next, we carried out a similar experiment but using this time continuous (non-segmented) flow to transfer samples from the glass vial to the detector. This yielded curves which also represent some fluctuations (Figure 3.7B, red line). However, in this case, the traces are much smoother than the curves obtained using segmented flow (Figure 3.7A, red line). This unwanted ―smoothing effect‖ is due to advection and diffusion (e.g. ref. 73), taking place in the flow line – between the sample inlet and the detector. Compared with the continuous flow (Figure 3.7B), the segmented flow (Figure 3.7A) helps to preserve temporal
Figure 3.6 Convective mixing of ferroin with water followed by segmented flow and flow-through optical detector (cf. Figure 3.1; wavelength: 518 nm). The red line represents original data while the blue line shows the final data extracted by the custom software. The two traces were shifted vertically for clarity.
and spatial resolution of the digitized three-dimensional sample (cf. Figure 3.5).
In both cases, the traces can be fit with exponential functions (Figure 3.7A and 3.7B, blue dashed line), which represent the mixing trends. Apart from the fluctuations caused by convection currents, the equilibriums are reached at similar times (taking into account the small difference between the effective sampling flow rates in the segmented -flow and continuous flow systems). In Figure 3.8, the experimental data (cf. Figure 3.7A and 3.7B, top graphs, red line) were subtracted with the values predicted by the fitted exponential functions (cf. Figure 3.7A and 3.7B top graphs, blue dashed line): this representation highlights the presence of strong fluctuations of relative absorbance due to the convection process. These fluctuations are especially apparent in the middle of the data record, i.e. 400-800 s (segmented flow, Figure 3.8A), and 300-700 s (continuous flow, Figure 3.8B). From Figure 3.7A it is
Figure 3.7 Recording convection with segmented and continuous flow. (A) The output data (red line) obtained with the segmented flow sampling (2 replicates). The blue dashed lines correspond to the exponential functions fitted to the experimental data (after removal of n-octanol-related features from the trace): a: f(t)
= 31 × (1 – e(-0.003t)); b: f(t) = 52 × (1 – e(-0.003t)). The features marked with asterisks (*) are due to air bubbles. (B) The output data (red line) obtained with the continuous flow sampling (2 replicates). The blue dashed lines correspond to the exponential functions fitted to the raw data: a: f(t) = 27 × (1 – e(-0.003t));
b: f(t) = 43 × (1 – e(-0.003t)).
also clear that, at some points, the relative absorbance values (represented by the measured potentials) are much higher than the equilibrium absorbance at the end of the data record. This points out an important feature of convection current; unlike in diffusion, a momentary concentration of the substance in the three-dimensional space may be higher than the concentration of this substance after complete mixing of the substance with the medium. This feature has implications on the real-world convection systems, for example, the release of pollutants to the environment. Overall, the experiments discussed above show the feasibility of sampling convection-induced waves from liquid media on the scale of micro- to millilitres.
Figure 3.8 An alternative representation of the data sets displayed in Figure 3.7A and 3.7B (upper graphs). The experimental data points were subtracted with the fitted exponential functions (A: f1(t) = 31 × (1 – e(-0.003x)); B: f2(t) = 27 × (1 – e(-0.003x))). Fluctuations of absorbance due to convection currents in the glass vial can be clearly seen.
In order to realize the possibility of performing measurements at various wavelengths, offered by the detector described above (cf. Figure 3.1), we further attempted the monitoring of sequential convection of substances with different absorption maxima (Figure 3.9). First, an aliquot of 100 µL of a blue ink was injected into to the lower part of the 20-mL glass vial, which was then heated up to induce convection, and segmented flow was then used for sampling (Figure 3.9A). The blue ink absorbs green and red light (wavelength range of red:
600-700 nm, green: 490-560) but it does not absorb blue light (wavelength range 450-490 nm,
cf. Figure 3.10). Therefore, one could observe the fluctuating increase of the signal in the
detection channels operating at wavelengths 629 and 518 nm (Figure 3.9A). Subsequently, a 100-µL aliquot of ferroin was injected into the lower part of the same vial. This time the detection channels operating at wavelengths 518 and 463 nm produced a significant change in the light absorption traces (Figure 3.9B).