## 3 Experimental
### Filter Development, Test Methods, and Results
#### 3.1.2 Proof of Principle for Coated Samples  
Constraints on measurement techniques limited the test volume design construct to one that could be sealed to enable single inlet and outlet points only, such that the pump action of the gas detector for ambient, or the overpressure from a synthetic gas bottle for synthetic air, NO\({}_{2}\), CO\({}_{2}\), etc., could be used to introduce and draw/pressurise gas through the filter housing. Initially, some pre-tests were conducted using small samples of smooth coated substrate (coatings made by hand draw-down, as described previously) absorbing NO\({}_{2}\) under static exposure conditions. Samples were cut from the larger filter element and placed in a 1-dm\({}^{3}\) super syringe (Model S1000 TLL, Hamilton Co., Reno, NV, USA). The syringe plunger was inserted, and ambient air was expelled. Then, the syringe was joined to a synthetic NO\({}_{2}\) pressurised bottle source, and the syringe plunger retracted to fill the body with NO\({}_{2}\). In this way, the small coated samples were maintained for a given time and exposed statically to the NO\({}_{2}\) gas at a concentration level of 10 ppm (18.8 mg\(\cdot\)m\({}^{-3}\)). The test procedure is illustrated in Figure 13. After exposure, the samples were removed and stored in sterile sealed tubes, which were sent for microscopic analysis and either X-ray photoelectron spectroscopic (XPS) analysis or energy dispersive X-ray spectroscopy (EDS). Others were treated to extract soluble -NO\({}_{2}\)\({}^{-}\) and -NO\({}_{3}\)\({}^{-}\) salts into Milli-Q deionised water. An ion chromatographic elution method for the efficient separation and quantitation of anions under isocatic conditions was employed. A chromatographic system (DionexTM DX-300, Thermo Fisher Scientific Inc., Waltham, MA, USA) consisting of a gradient pump, a chromatography module, a conductivity  
Figure 12: Initial results of the exposure of CaCO\({}_{3}\) to ambient air over several months, which revealed a conversion reaction of -CO\({}_{3}\)\({}^{2-}\) to -(NO\({}_{2}\)/NO\({}_{3}\)\({}^{-}\))\({}_{2}\) rather than simple inert surface adsorption data. detector, an ASRS(r) suppressor, and a Peaknet 5.1 chromatography workstation was used. The injected volume was 0.05 cm\({}^{3}\). The chromatographic separation of anions was performed on a column (DionexTM AS14, Thermo Fisher Scientific Inc., Waltham, MA, USA) at ambient temperature with a flow rate of 1.0 cm\({}^{3}\)-min\({}^{-1}\). A working eluent containing 3.5 mM Na\({}_{2}\)CO\({}_{3}\) to 1.0 mM NaHCO\({}_{3}\) was appropriate for this purpose. This method was used for the chemical analysis of nitrate (NO\({}_{3}\)-) and nitrite (NO\({}_{2}\)-) at concentrations above the detection limits (DL) in the direct water extract of the filter. The DLs were 0.05 mg\(\cdot\)dm\({}^{-3}\) and 0.1 mg\(\cdot\)dm\({}^{-3}\), respectively. Other anions, such as fluoride (F\({}^{-}\)) and phosphate (PO\({}_{4}\)3-), were generally below their DL in these samples, but sulphate (SO\({}_{4}\)2-) was detected in the blank filter samples arising from the recycled pulp used to make the substrate containing traces of bleached fibre. The response to concentration was linear over the working concentration range (\(R^{2}>\) 0.999). Example uptake data from two tests of each coating on unbleached Kraft (UK) and newsprint (NP) comparing blank (non-exposed to synthetic NO\({}_{2}\)) and exposed (exposed statically in the super syringe to synthetic NO\({}_{2}\) at 10 ppm for 72 h) samples are shown in Table 6. Repeatability was explored using only two samples in this preliminary case.