Direct Ion Speciation Analysis with Ion-selective Membranes Operated in a Sequential Potentiometric/Time Resolved Chronopotentiometric Sensing Mode

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1 Supporting information for: Direct Ion Speciation Analysis with Ion-selective Membranes Operated in a Sequential Potentiometric/Time Resolved Chronopotentiometric Sensing Mode Majid Ghahraman Afshar a,b, Gastón A. Crespo b and Eric Bakker b * Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, Quai Ernest- Ansermet 30, CH-1211 Geneva, Switzerland Keywords: calcium speciation, chronopotentiometric/potentiometric sensors, polypropylene, permselective membranes, ion-selective electrodes. a On leave from Imam Khomeini International University, Qazvin, Iran b University of Geneva 1

2 Theory Within the aqueous sample phase, the concentration of primary ion at any position -xsample < n < 0 and time step t + Δt is described as a function of the concentration at the previous time step, t 1, 2 : D cn, t t cn, t ( cn 1, t 2cn, t cn 1, t ) t (1) 2 d where D is the diffusion coefficient in the aqueous phase. The concentration in the first element of the aqueous phase (n = -xsample) is assumed to be equal to the sample bulk concentration, c bulk, which remains indifferent: c xsample c bulk (2) The applied current pulse i c results in the transport of ion across the sample membrane interface, and is carried by the transport of the primary ion as well as interfering ion across the finite element difference at the aqueous side of the interface as follows: i c A DF d z I (c 0,t c 1,t ) z J (c J,0,t c J, 1,t ) (3) The fraction of the current that is carried by the primary ion can be characterized by the membrane selectivity, considering that the transference number, t i, is directly proportional to the fraction of primary ion in the membrane relative to the ion-exchanger sites. This relationship has previously been found as t i = c 1,t c 1,t + c J,1,t = c 0,t c 0,t + K pot z IJ a I /z J (4) J,0 Note that this relationship uses the Nicolsky-Eisenman formalism and the associated selectivity coefficient, K IJ pot, the activity of the interfering ion, a J,0,with the indicated charges of the primary and interfering ion, z I and z J, in the exponent. The former is known not to give a correct potential description of the mixed ion response if the two sample ions are of unequal charge (most important in the region of the chronopotentiometric response near the transition time), but is 2

3 accepted to give adequate predictions when either the analyte ion or interfering ion are potential determining. Given its simplicity, it was chosen here over the more complete mathematical description of the mixed ion response of membrane electrodes. 3 With eq 4, eq 3 can be rewritten as a function of the membrane selectivity as follows: icd ADF c 0, t c K 0, t pot IJ a z I / z J J,0 ( c0, t c 1, t ) (5) This relationship is now solved for c 0,t to give: c 0, t pot zi / z J pot zi / z J pot zi / z J dic DFc 1, t DFK IJ aj,0 ( dic DF( c 1, t KIJ aj,0 )) 4D F KIJ aj,0 c 1, t (6) 2DF The flash chronopotentiometric response is simulated by calculating the changes in c n,t+δt for - xsample < n < 0 from the values at the preceding time step using equation 1. The concentration at the aqueous side of the sample membrane interface, c 0,t+Δt, is subsequently calculated from c - 1,t+Δt using equation 6. For all times, the concentration at position c -xsample remains constant at the value for the sample bulk, c bulk. All calculations were performed with Mathematica 7 (Wolfram Research). 3

4 Membrane preparation Potassium PVC-based membranes were prepared in the classical manner using a 1:2 mass ratio of PVC and plasticizer (1:2). Specifically, 15 mmol kg -1 of Ionophore I, 5 mmol kg -1 of NaTFPB, 20 mmol kg -1 of ETH 500, 63 mg of PVC, 127 mg of DOS were completely dissolved in THF (membrane preparation details in SI). The cocktail was poured into a glass ring (10 mm ID) affixed onto a glass plate. The solution was allowed to evaporate overnight. The thicknesses of the resulting membranes were ca. 0.2 mm. This mother membrane was cut with a hole puncher into small disks (6.3±0.2 mm diameter) and mounted into the electrode body. The membrane electrodes were conditioned either in 1 mm of NaCl or 1 mm of KCl. Porous polypropylene (PP) membranes (Celgard, cm 2 of surface area, 25 µm thickness, and kindly provided by Membrana) were used as supporting material. The membranes were washed with THF for 10 min to remove any possible contaminants. When the membrane was found to be completely dry, 3 µl of the cocktail solution was deposited on it (see below cocktail preparation). The impregnation of the cocktail was found to be instantaneous; however, the membrane was let in the Petri Dish for ca. 10 min to ensure a homogenous and reproducible impregnation of the pores. Afterwards, the membrane was conditioned in the buffer solution for 40 min. Finally, the membrane was mounted in the electrode body. The inner compartment was filled with 10 mm of primary analyte chloride salt whereas the outer solution contained the buffered solution mentioned above. The used cocktail for the impregnation of PP membranes contained all the reagents mentioned before except PVC. Membrane K1 contained 15 mmol kg -1 of Ionophore I, 5 mmol kg -1 of NaTFPB, 20 mmol kg -1 of ETH500, 190 mg of DOS and 1 ml THF. THF was only used to enhance the solubility of the solid compounds into the plasticizers. It is important to note that THF has to be evaporated before casting the membranes. Calcium PP membranes were optimized in order to increase the upper limit of detection up to 3 mm which is the amount found in undiluted blood. Therefore, different membranes varying the ionophore concentration were evaluated. PP-Ca1 (15:5:90; the values denote the concentrations of active ingredients in mmol kg -1 : 15 mmol kg -1 of Ionophore, 5 mmol kg -1 of NaTFPB, 90 mmol kg -1 of ETH 500 and o-npoe up to 100 mg total cocktail amount), and in similar manner 4

5 PP-Ca2 (30:5:90), PP-Ca3 (50:5:90), PP-Ca4 (70:5:90), PP-Ca5 (90:5:90), PP-Ca6 (120:5:90), PP-Ca7 (150:5:90), PP-Ca8 (180:5:90). Figure1S Comparison between two applied current levels on the response of polypropylenebased potassium-elective membrane, a) -5 µa and b) -10 µa. Inset: Observed linear calibration curve of the square root of the transition time ( 1/2 ) as a function of concentration for each experiment. 5

6 Figure 2S Optimization of calcium ion-selective membranes by increasing the concentration of the ionophore from 15 to 150 mmol kg -1. For detailed membrane composition see experimental part in the main manuscript. 6

7 Figure 3S Membrane responses of the optimized calcium selective membrane PP-Ca8 (see experimental part) a) Chronopotentiometric data as a function of a wide concentration range (1 to 3 mm of Ca 2+ ) b) Obtained linear calibration curve of the inverse square root of the transition time -1/2 as a function of applied current at the concentration levels indicated on the plot (from 0.02 to 3 mm). 7

8 Figure 4S Experimental (dots) and predicted (solid line) open circuit potentiometric titration data of Ca 2+ by adding NTA at two ph levels. Top: ph=7.4 and Bottom: ph=9.1. Inset: Employed potentiometric calibration curve to convert EMF (mv) to pca, s 1 =26.5 [log activity Ca 2+ ] and s 2 =28.7 [log activity Ca 2+ ] respectively. 8

9 Figure 5S Determination of total calcium in an undiluted citrated blood bag using standard addition calibration to minimize matrix effects. The peaks shift to higher transition times with increasing calcium concentration, as expected. The red line (left most curve) corresponds to undiluted blood without adding calcium. Inset: Observed linear calibration curve of the square root of the transition time 1/2 as a function of added calcium concentration (from 0.1 to 1 mm). 9

10 Table 1S. Optimization of polypropylene based calcium permselective membranes Calcium Membranes Maximum Applied Current a (µa) Upper detection limit (mm) PP-Ca PP-Ca PP-Ca PP-Ca PP-Ca PP-Ca PP-Ca PP-Ca a Value given is the maximum possible applied current magnitude without observing a limitation due to ionophore depletion. References (1) Morf, W. E.; Pretsch, E.; De Rooij, N. F. J. Electroanal. Chem. 2007, 602, (2) Bakker, E. Anal. Chem. 2011, 83, (3) Bakker, E.; Buhlmann, P.; Pretsch, E. Chem. Rev. 1997, 97,