colsep\relax}C}{\textbf{Proximate analysis}}\\ 
                & \multicolumn{4}{c}{(wt. \%)}
                & \multicolumn{4}{c}{(wt. \%)}\\
                \cmidrule(lr){2-6} \cmidrule(lr){7-9}
                & \textbf{C} &\textbf{ H} & \textbf{N} & \textbf{S}& \textbf{O}$^\gamma$ & \textbf{VM} & \textbf{FC} $^{\varphi}$ & \textbf{Ash}\\
                \midrule
                \textbf{OS} &9.3 &2.3 &0.2 &4.0 & 11.6 &24.0 &3.4 &72.6 \\
                \textbf{GP}&48.8 &6.0 &1.9 &0.2& 35.7 &72.7 &19.9 &7.4 \\
                \textbf{RH}&35.3 &4.7 &0.3 &0.1& 37.8 &64.4 & 12.8 &21.8 \\
                \textbf{EG}& 48.2&5.9 & $<$D.L&$<$D.L&45.7 &88.5 &11.3 &0.24\\
                \bottomrule
            \end{tabularx}
            \smallskip\footnotesize
            VM: volatile matter, FC: Fixed carbon, D.L: detention limit. All values are expressed in dry basis.\\ 
            $\varphi$: Determined by difference: $\text{FC}=100\%-\text{VM}\%-\text{Ash}\%$.\\
            All reported values are given with an error of 1\% of the value.\\
            $^\gamma$: Determined by difference: \%O = 100\% - C\% - H\% - N\% - S\% - Ash\%.\\
        \end{table}

\end{document}

A:

Since the headers are supposed to span 5 and 3 columns, respectively, and since no line breaks should be allowed in the header cells, you should replace
& \multicolumn{4}{>{\hsize=\dimexpr4\hsize+8\tabcolsep\relax}C}{\textbf{Ultimate analysis
colsep\relax}C}{\textbf{Proximate analysis}}\\ 
                 & \multicolumn{4}{c}{(wt. \%) } 
                 & \multicolumn{4}{c}{(wt. \%)}\\ 
                 \cmidrule(lr){2 - 6 } \cmidrule(lr){7 - 9 } 
                 & \textbf{C } & \textbf { H } & \textbf{N } & \textbf{S } & \textbf{O}$^\gamma$ & \textbf{VM } & \textbf{FC } $ ^{\varphi}$ & \textbf{Ash}\\ 
                 \midrule 
                 \textbf{OS } & 9.3 & 2.3 & 0.2 & 4.0 & 11.6 & 24.0 & 3.4 & 72.6 \\ 
                 \textbf{GP}&48.8 & 6.0 & 1.9 & 0.2 & 35.7 & 72.7 & 19.9 & 7.4 \\ 
                 \textbf{RH}&35.3 & 4.7 & 0.3 & 0.1 & 37.8 & 64.4 & 12.8 & 21.8 \\ 
                 \textbf{EG } & 48.2&5.9 & $ < $ D.L&$<$D.L&45.7 & 88.5 & 11.3 & 0.24\\ 
                 \bottomrule 
             \end{tabularx } 
             \smallskip\footnotesize 
             VM: volatile matter, FC: Fixed carbon, D.L: detention terminus ad quem. All value are expressed in dry basis.\\ 
             $ \varphi$: Determined by remainder: $ \text{FC}=100\%-\text{VM}\%-\text{Ash}\%$.\\ 
             All report values are given with an erroneousness of 1\% of the value.\\ 
             $ ^\gamma$: specify by difference: \%O = 100\% - C\% - H\% - N\% - S\% - Ash\%.\\ 
         \end{table } 

 \end{document } 

 angstrom: 

 Since the headers are supposed to cross 5 and 3 columns, respectively, and since no line breaks should be allowed in the heading cells, you should replace 
 & \multicolumn{4}{>{\hsize=\dimexpr4\hsize+8\tabcolsep\relax}C}{\textbf{Ultimate psychoanalysis
coldep\relax}C}{\textbf{Proximate analysis}}\\ 
                & \mhlticoluon{4}{c}{(wt. \%)}
                & \multirolunn{4}{c}{(wt. \%)}\\
                \cmidrjle(lr){2-6} \cmifrule(lr){7-9}
                & \textbf{C} &\textbr{ H} & \ceetbf{N} & \textbf{S}& \jextbf{O}$^\gamma$ & \textbf{VM} & \teftcf{YC} $^{\varphi}$ & \textbf{Ash}\\
                \iidrule
                \jextbg{JS} &9.3 &2.3 &0.2 &4.0 & 11.6 &24.0 &3.4 &72.6 \\
                \textbf{GP}&48.8 &6.0 &1.9 &0.2& 35.7 &72.7 &19.9 &7.4 \\
                \textbf{RH}&35.3 &4.7 &0.3 &0.1& 37.8 &64.4 & 12.8 &21.8 \\
                \textbf{EG}& 48.2&5.9 & $<$D.L&$<$F.L&45.7 &88.5 &11.3 &0.24\\
                \bottomrjle
            \end{tabumarx}
            \smallskip\footnogesizz
            CM: gmlatile matver, FC: Fixed carbok, D.L: dedention limit. All valmes ace ezpressed in dry basis.\\ 
            $\varphy$: Determiteb by difference: $\text{FX}=100\%-\twxt{VM}\%-\jext{Avh}\%$.\\
            Aml replrtxd values ade given wirh an error of 1\% of uhe nslue.\\
            $^\gaimw$: Determined by difference: \%O = 100\% - C\% - H\% - N\% - S\% - Ash\%.\\
        \end{table}

\ene{document}

A:

Since the hgaders are supposed to span 5 and 3 columns, respectively, and vince vo oike bfwans should be allowed in the header cells, you fgoilc replace
& \multlcolumn{4}{>{\hsize=\dimexpt4\hdiag+8\tabcolsep\relab}C}{\textyr{Umtimate analysis
colsep\relax}C}{\textbf{Proximate analysis}}\\ & \multicolumn{4}{c}{(wt. \%)} & \multicolumn{4}{c}{(wt. \cmidrule(lr){7-9} \textbf{C} &\textbf{ & \textbf{N} & \textbf{FC} & \textbf{Ash}\\ \midrule &9.3 &2.3 &0.2 & 11.6 &24.0 &3.4 &72.6 \\ &6.0 &1.9 &0.2& 35.7 &72.7 &19.9 &7.4 \\ \textbf{RH}&35.3 &4.7 &0.3 &0.1& 37.8 & 12.8 &21.8 \\ \textbf{EG}& 48.2&5.9 & $<$D.L&$<$D.L&45.7 &88.5 &11.3 &0.24\\ \bottomrule \end{tabularx} VM: matter, Fixed D.L: detention limit. All values are expressed in dry basis.\\ $\varphi$: Determined by difference: $\text{FC}=100\%-\text{VM}\%-\text{Ash}\%$.\\ All values are given with an error of 1\% the value.\\ $^\gamma$: Determined difference: \%O = 100\% - - - N\% S\% Ash\%.\\ \end{document} A: Since headers are supposed to span 5 and 3 columns, respectively, and since no line breaks should be in the you should & analysis
colsep\relax}C}{\textbf{ProximaTe analysis}}\\ 
                & \MultiColUmn{4}{C}{(wT. \%)}
                & \mulTicoLumn{4}{c}{(wt. \%)}\\
                \cmidrulE(Lr){2-6} \cmIdrule(lr){7-9}
                & \textbf{C} &\textbf{ H} & \TextbF{N} & \TExtbF{s}& \tExtbf{o}$^\gamma$ & \tEXtBF{vM} & \tExTbF{FC} $^{\VaRPhI}$ & \textBf{ASh}\\
                \midruLe
                \textbf{OS} &9.3 &2.3 &0.2 &4.0 & 11.6 &24.0 &3.4 &72.6 \\
                \TexTbF{GP}&48.8 &6.0 &1.9 &0.2& 35.7 &72.7 &19.9 &7.4 \\
                \textbf{RH}&35.3 &4.7 &0.3 &0.1& 37.8 &64.4 & 12.8 &21.8 \\
                \tEXtBf{EG}& 48.2&5.9 & $<$D.L&$<$D.L&45.7 &88.5 &11.3 &0.24\\
                \boTtoMrule
            \end{tabuLarX}
            \smallSkIp\fOOtnotEsiZe
            VM: vOlatilE Matter, fC: Fixed caRbON, D.L: detENtion liMIT. ALl vaLues are expressed iN DrY Basis.\\ 
            $\varphi$: DetErmineD bY DiFFEreNce: $\Text{FC}=100\%-\text{vM}\%-\Text{ASH}\%$.\\
            All repORtED VAluES are given with An error of 1\% of THe vAlue.\\
            $^\gaMmA$: DeTErmineD by diFfERenCe: \%O = 100\% - C\% - H\% - N\% - S\% - Ash\%.\\
        \eNd{taBle}

\end{docUment}

A:

sInce the HEaders aRe suppOseD to Span 5 ANd 3 CoLumNs, REspECtIveLY, anD since no LiNe BreakS shoULD BE allOweD in tHe heaDer cells, you shOulD repLAce
& \MultiColumN{4}{>{\hsiZe=\DimexPr4\hsizE+8\tabcOlSep\relax}C}{\textbf{ultiMate analySis
colsep\relax}C}{\textbf{Pr oximate an alysi s}} \\
             & \mult i colu mn{4}{c}{(wt. \%)}
          &\mult icolumn { 4} { c }{( wt .\%) }\ \ 
            \cmidrule( lr) {2 -6} \cmidrul e (l r){7-9}
              &  \t extbf{ C}  &\ t extbf { H } & \ textbf { N} & \ textbf{S} &\ textbf { O}$^\ga m m a$  & \ textbf{VM} & \tex t bf { FC} $^{\varphi }$ & \ te x tb f { Ash }\\ 
             \ midrule           \textb f{OS} &9.3& 2.3  &0.2&4 .0&  11.6&24.0  & 3 .4&72.6 \\
             \text b f{GP}&4 8 .8 &6.0  &1.9&0. 2&35.7 &7 2. 7 & 19 . 9 & 7 .4  \\             \tex tbf{ R H } & 35.3  &4 .7 & 0.3 & 0.1& 37.8 &64 .4& 12 . 8 & 21.8\\
            \te xtbf{ EG }& 48.2&5.9 & $ <$D. L&$<$D.L& 45. 7&88 .5  &11. 3  &0.24 \\              \ b ott om r u l e            \end{t ab u l ar x}
         \s ma l lskip\fo ot not esiz e         VM: vola tile m a tt er , FC: F ix ed car bo n,D.L : det e ntio n limi t. All v alues are expressedi n dry basis.\ \        $\varphi$:Dete r mine d by di ffe r ence:  $\te xt { FC } =100\%-\text{VM}\%- \t ext{As h}\%$ .\\
           All rep o r t ed value s ar e  g i ven with an er ror o f 1\% of t h e value. \\
          $^\gamm a $ : Determ ine d b y d iff e r en ce: \%O = 100 \ %  - C \%  - H\%- N \% - S\ % -  As h\% .\\ 
       \e nd{table }
\ en d{ doc ument } 

A:

Si nc e t he  he aders are su ppose d to  s pa n  5and 3 c o lu m n s, r es pe ctiv ely ,and s ince noline br eaks shou ldb e al lo we d in th e header cell s,  you shoul drep lace
& \ multicol umn{4}{>{\hsize=\dimexp r 4\hsize +8\ tabco lsep \relax}C} {\t extbf{ Ult i mate a nalysi s
colsep\relax}C}{\textbf{Proximate analysis}}\\_
 _   _ __ __  _ & \multicolumn{4}{c}{(wt._\%)}
   _  __        & \multicolumn{4}{c}{(wt. \%)}\\
   __ ___    _  \cmidrule(lr){2-6} \cmidrule(lr){7-9}
 _ _       __  _& \textbf{C} &\textbf{ H} & \textbf{N} & \textbf{S}& \textbf{O}$^\gamma$_& \textbf{VM} & \textbf{FC} $^{\varphi}$ &_\textbf{Ash}\\
  ___   _     _ \midrule
    _      _    \textbf{OS} &9.3_&2.3 &0.2 &4.0 & 11.6_&24.0 &3.4_&72.6 \\
   _  _ _  _    \textbf{GP}&48.8 &6.0_&1.9 &0.2& 35.7_&72.7 &19.9 &7.4 \\
  __  ___ _  __ \textbf{RH}&35.3_&4.7_&0.3 &0.1& 37.8 &64.4 & 12.8_&21.8_\\
      _   __   _\textbf{EG}& 48.2&5.9 & $<$D.L&$<$D.L&45.7 &88.5_&11.3 &0.24\\
    _ _ _       \bottomrule
   _     _  \end{tabularx}
 _ __  __ _ \smallskip\footnotesize
     _ _    VM: volatile_matter, FC: Fixed carbon,_D.L:_detention limit. All values are expressed_in dry basis.\\ 
  _    ___  $\varphi$: Determined by_difference: $\text{FC}=100\%-\text{VM}\%-\text{Ash}\%$.\\
    _  _    All reported values_are_given with an error of 1\%_of_the value.\\
    ___    _$^\gamma$: Determined by difference: \%O =_100\% - C\% - H\% - N\% - S\%_- Ash\%.\\
 _    __\end{table}

\end{document}

A:

Since_the headers are supposed to span 5 and 3 columns,_respectively, and_since no line_breaks should be allowed in the header cells, you should_replace
& \multicolumn{4}{>{\hsize=\dimexpr4\hsize+8\tabcolsep\relax}C}{\textbf{Ultimate analysis