Figure 4.

(a) Calibration of paper microfluidic devices in standard solutions containing each blood ion. Note that five different paper devices were used to obtain a point in calibration curves (one device can be used only one time), and the error bars were omitted for simplicity. Note that the theoretical slope for single charged ions (Cl⁻ (black), K⁺ (red), and Na⁺ (blue)) is 59.2 mV decade⁻¹, while that for doubly charged ion (Ca²⁺ (green)) is 29.6 mV decade⁻¹. (b) Residual diagrams for calibration curves. Top axis is for red and green curves.