\documentclass{article}
\usepackage{ctable,dcolumn}
\newcolumntype{d}{D{.}{.}{-1}}
\newcommand{\C}[1]{\multicolumn{1}{c}{#1}}
\newcommand{\CC}[1]{\multicolumn{2}{c}{#1}}
\newcommand{\Po}[1]{$\times10^{#1}$}
\begin{document}

\ctable[
  caption = Surface area available for pegylation,
    label = t:surfacebeads2,
      pos = htp
]{rddd@{}l}{}{									 \FL
    \C{Size}   & \C{Concentration} & \C{Volume used} & \CC{Surface area}	 \NN
    \C{nm}     & \C{g/ml}          & \C{ml}          & \CC{nm\textsuperscript{2}}\ML
     40        & 5                 & 0.2 	     & 14.3 	&\Po{18}	 \NN
    100        & 2                 & 0.5 	     &  5.7 	&\Po{18}	 \NN
    200        & 2                 & 0.5 	     & 11.4 	&\Po{12}	 \LL
}

\end{document}
