The following image shows a column for GC instruments.

$($a$)$ Describe the main advantage of using this type of column. $(5$ pts$)$

$($b$)$ Calculate volume $($in $mL)$ of the stationary phase in this column, where the tag indicates $30m,0\mathrm{.}25$mmID, and $0\mathrm{.}25m.(5$ pts$)$

Assumption: $1)$ The cylindrical wall can be approximated as plane wall;

$1m^3={10}^{6}mL.$