Text size in single table row

Question

i want to include two letters in the S column of the siunitx package. As there are no measured values for the particular row, I want to label them with "ND". Can I include this, or does anyone have another idea to mark these rows?

The second question, in combination with the above mentioned, is: How can I change the font size in a specific row? Until now, I want to add a row with a smaller font under the table including the text "ND - not determined".

Thank you in advance for your help.

PS. The table is a normal tabular environment.

\begin{table}[ht!]
\caption[XX]{XX}\label{tab:XX} 
\centering
\begin{tabular}{llSS}
\toprule
&\textbf{XX}    & \textbf{XX} &\textbf{XX}\\
                &                   & [x\SI{E13}{milli\liter}]               & [\SI{}{\percent}]\\
    \midrule
        1       & XX                & ND         & ND         \\
        2       & XX                & 0.14       & 48.25      \\        
        8       & XX                & 20.8       & 26.59       \\ \midrule
 \multicolumn{4}{l}{ND = not determined} \\ 
     \bottomrule\bottomrule
\end{tabular}
\end{table}