## 4 Discussion  
The main purpose of this paper is to present the M-DLS and describe its structure and modes of operations, therefore, the data shown in Table 2 should be treated with reservations for several reasons: firstly, assembling, tuning, and testing of the M-DLS were carried out in a limited amount of time, so the tests on an etalon gas mixture were not yet conducted, consequently, for now, there is a lack of demonstration of the accuracy of the M-DLS; secondly, synthetic spectra are based on HITRAN database parameters, which should be validated, and Voight profile is used for modeling, which does not properly represent real conditions; and finally, the water vapor spectral lines, which are presented \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Wavelength, \(\upmu\)m** & **Abundance Ratio, \%\({}_{\infty}\)** & **Retrieval Precision, \%** \\ \hline \multirow{2}{*}{2.656} & \(\delta^{18}\)O\({}_{\text{(H${}_{2}$O)}}\) & 28.4 & \multirow{2}{*}{0.87} \\  & \(\delta^{2}\)H\({}_{\text{(H${}_{2}$O)}}\) & 127.1 & \\ \hline \multirow{3}{*}{2.808} & \(\delta^{13}\)C\({}_{\text{(CO${}_{1}$)}}\) & 170.6 & \multirow{3}{*}{3.35} \\  & \(\delta^{18}\)O\({}_{\text{(CO${}_{1}$)}}\) & 7.8 & \\ \cline{1-1} \cline{2-4}  & \(\delta^{17}\)O\({}_{\text{(CO${}_{2}$)}}\) & 87.7 & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of the isotopologue abundance ratio and retrieval precision of the M-DLS. Figure 9: Experimental data of the M-DLS. (**a**,**b**) raw spectra (blue curves) and approximated baselines (red curves) for 2.656 \(\upmu\)m and 2.808 \(\upmu\)m respectively. (**c**,**d**) normalized transmittance spectra (blue curves) with synthetic spectra (red curves) and residual at separate graphs (blue curves) for 2.656 \(\upmu\)m and 2.808 \(\upmu\)m, respectively. Line identifiers from “a” to “p” are deciphered in the discussion section. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article). in the measured spectra, are conditioned by absorbed water vapor, i.e., water vapor, which appear in the optical cell from the porous structure of its internal surfaces during pumping out. For further discussion, it is reasonable to demonstrate the HITRAN list of spectral lines, which contribute the most to the absorption and are used for isotopologue abundance ratio retrieval. Isotopologue abbreviation and central frequency of its transition for both spectral ranges are presented in Table 3.  
The characteristic shape of water vapor lines residual in both spectral ranges of the M-DLS in Figure 9c,d indicates that exploiting of air-broadening parameters at half maximum, listed in the HITRAN and application of Voight profile for the model, at this stage does not fully satisfy our expectations for modeling of the measured spectra. We keep in mind that the M-DLS could influence on the line profiles, but the problem is that the M-DLS is installed on the landing platform. Despite on transfer of the ExoMars mission launch to 2022, deinstallation of the M-DLS and additional manipulation with it are restricted. Therefore, the instrument influences on line profiles will be investigated during measurements of the Martian air sample by changing the averaging number of a single measurement. During the mission, the ICOS cell mirrors are expected to be gradually contaminated by the Martian atmospheric dust, with their reflectivity being degraded and the cell's effective optical path shrunken. However, the main scientific goal of the experiment is isotopologues ratios measurement, rather than absolute abundances. As we measure all spectral lines of interest, including those of main isotope and minor ones, within a single laser frequency scan, a change of the effective optical pass is not a problem.