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\usepackage[separate-uncertainty=true]{siunitx}

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\begin{table}[!htbp]
  \centering
    \caption{Comparison of the CCSHR estimators based on beta and logit-normal distributions}
    \label{tab:addlabel}
    \setlength{\tabcolsep}{.12cm}
    \begin{tabular}{
        @{}
        l
        @{\hspace*{0.25cm}}
        S[table-format = 1.1(2)]
        S[table-format = 2.1(2)]
        S[table-format = 1.2(2)]
        @{\hspace*{0.25cm}}
        S[table-format = 1.1(2)]
        S[table-format = 2.0(2)]
        S[table-format = 1.2(2)]
        @{}
    }
        \toprule
      & \multicolumn{3}{c}{pH 1.4}
      & \multicolumn{3}{c}{pH 2.2} \\
        \cmidrule(l{0cm}r{0.25cm}){2-4}\cmidrule(r{-0.0cm}l{0cm}){5-7}
        Molecule
        & {$N_{\text{max}}$(\si{nm})}
        & {$C_{\text{D}}'$(\si{\mu M})}
        & {\textit{D}}
        & {$A_{\text{max}}$(\si{\mu m})}
        & {$D_{\text{D}}'$(\si{\mu m})}
        & {\textit{A}} \\
        AB & 2.0 +- 0.2 & 7.9 +- 0.6 & 4.7 +- 0.1 & 7.8 +- 0.3 & 26 +- 6 & 0.87 +- 0.09 \\
        C & 9.2 +- 0.2 & 25.9 +- 1.4 & 2.07 +- 0.04 & 1.8 +- 0.3 & 70 +- 5 & 0.19 +- 0.06 \\
        \bottomrule
    \end{tabular}
\end{table}

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