(28 kinds of metal mixed standards/(silver, aluminum, arsenic, boron, barium, beryllium, calcium, cadmium, cobalt, chromium, copper, iron, potassium, lithium, magnesium, manganese, molybdenum, sodium, nickel, lead, antimony, selenium, tin, strontium, titanium, thallium, vanadium, zinc)/medium: nitric acid + hydrofluoric acid)