documentclass[12pt,a4paper]{book}

%~~~~~~~~~~~~~~~ Fonts ~~~~~~~~~~~~~~~~~~~~
usepackage{kpfonts}

%~~~~~~~~~~~~~~~ Tables ~~~~~~~~~~~~~~~~~~~~~
usepackage{makecell,booktabs}
usepackage{color, colortbl}
definecolor{Gray}{gray}{0.9}
usepackage{multirow}

%~~~~~~~~~~~~~~ Commands
newcommand{pu}{$^{240}Pu$ }

begin{document}

begin{table}
centering
% table caption is above the table
caption{This will be filled later!label{tab:filters}}
% Give a unique label
% For LaTeX tables use
begin{tabular}{ccccccc}
toprule
hline
rowcolor{Gray}
makecell{Sample} & makecell{Reference\Number} & makecell{Activity\(MBq)} & makecell{Mass\(mg)} & multicolumn{2}{c}{makecell{Areal density\(mg/cm$^2$) ($times 10^{-4}$ atoms/b)}} & makecell{Atomic\abundances\(mol/mol)}\
hline
{}    & {TP2010-011-01} & {6.016(23)} & {0.7163(28)} & {0.1017(4)} & {2.55(1)} &
multirow{3}{*}{ makecell{$^{238}Pu$ :  $7.33(29) cdot 10^{-4}$\
          $^{239}Pu$ :  $1.44(18) cdot 10^{-4}$\
          $^{240}Pu$ :  $0.998915(18) cdot 10^{-4}$\
          $^{241}Pu$ :  $4.1(31) cdot 10^{-6}$\
          $^{242}Pu$ :  $2.027(41) cdot 10^{-4}$\
          $^{244}Pu$ :  $4.6(88) cdot 10^{-7}$}}\
{pu} & {TP2010-011-03} & {6.793(26)} & {0.809(3)}   & {0.1148(5)} & {2.88(1)} & {}\
{}    & {TP2010-011-04} & {6.410(25)} & {0.763(3)}   & {0.1083(5)} & {2.72(1)} & {}\
bottomrule
hline
end{tabular}
end{table}

end{document}