$31.$ What is the correct preservation method and allowable holding time for samples containing metals?

$1.$ Cool to $<6\backslash$deg C; hold up to $14$ days

$2.$ Cool to $<6\backslash$deg C; acidify; hold up to $180$ days

$3.$ Cool to $<6\backslash$deg C; acidify; hold up to $14$ days

$4.$ No preservation possible; analyze immediately