## 4 Discussion
### Regulatory Requirements  
It is clear that regulations cannot prescribe in detail all technical aspects of FTIRs. Some basic and important parameters can be described, but appropriate tests are necessary to confirm the instrument's internal hardware and software performances. Table 3 summarizes the technical specifications for FTIRs for NH\({}_{3}\) measurements in the current regulations. Most of them are based on the experience of the users and the capabilities of the instruments. Nevertheless, some specifications could be further restricted or better be controlled for future low emissions vehicles. Such recommendations can be based on recent (2019-2020) standards and methodologies for FTIRs [191; 192; 193; 194].  
For example, SAE J2992 [192] includes a few more requirements (e.g., repeatability and noise). The detection limit, typically determined with zero gas [191; 192], could also be determined using interfering gases [193]. Similarly, in addition to the accuracy test, defined as the deviation of the analyzer reading from the reference value, an interference test could be added. SAE J2992 requires interference testing with a gas containing CO\({}_{2}\), CO, NO\({}_{\times}\), and N\({}_{2}\)O (mix or separately). For most gases (e.g., NH\({}_{3}\), N\({}_{2}\)O, etc.), the maximum total permitted interference is 1%. A spectral residual test is also recommended [191]. USA regulations require appropriate analytical procedures for the interpretation of infrared spectra [193; 194].  
In the EU RDE regulation, a maximum zero drift of 5 ppm and span drift of 2% is allowed for NO\({}_{\times}\). This is much stricter than the 2% of full scale currently prescribed for FTIR. Actually, the drift of FTIR should be negligible as it was discussed before, not only for NH\({}_{3}\), but for all compounds. Thus these drift requirements could be even stricter.  
In USA EPA regulations, FTIR analyzer may be used to measure CH\({}_{4}\), C\({}_{2}\)H\({}_{6}\), NMHC, and non-methane-non-ethane hydrocarbon (NMNEHC) for continuous sampling for natural gas engines (Title 40/I/U/1065/C/S1065.266). The FTIR analyzer must have combined interferences that are within \(\pm\)2% (recommended \(\pm\)1%) with CH\({}_{4}\), NMHC, or NMNEHC concentrations expected at the standard (Title 40/I/U/1065/D/S1065.366). Such uncertainties are at the same levels permitted for oxygen interference for FID analyzers.  
\begin{table}
\begin{tabular}{l l} \hline
**Specification** & **Requirement** \\ \hline Sampling line & Stainless steel or PTFE, as short as possible, heated at 190 \({}^{\circ}\)C (\(\pm\)10 \({}^{\circ}\)C) \\ Spectral resolution & 0.5 cm\({}^{-1}\) \\ Linearity & offset \(\leq\) 0.5\% max, slope 0.99–1.01, SEE \(\leq\) 1\% max, \(R^{2}\)\(\geq\) 0.998 \\ Detection limit & \(<\)2 ppm under all conditions of testing \\ Accuracy & \(\pm\)3\% of the reading or \(\pm\)2 ppm, whichever is larger \\ Zero and span drift & \(<\)2\% of full scale \\ Rise time & \(\leq\)5 s \\ Response time & \(\leq\)20 s \\ \hline PTFE = polytetrafluoroethylene; SEE = standard error of estimate. \\ \end{tabular}
\end{table}
Table 3: Example of technical requirements for FTIR for NH\({}_{3}\) measurements in UNECE Reg. 49.  
Furthermore, for regulatory purposes, more quality checks should be included. SAE J2992 has a separate chapter of tests performed prior to and after an emissions cycle test (leak, zero, span, pre- and post-drift checks) [192]. As in CEN/TS 17337, adjustment factors for zero and span, or even zero and span drift could be allowed [191].