Samples for sodium analysis (as well as dissolved nitrogen and phosphorus, silicon, calcium, magnesium, potassium, iron, and manganese) are collected together with a peristaltic pump and tubing and in-line filtered (through a 40 micron polycarbonate filter) into 120 ml LDPE bottles and acidified to a 1% HCl matrix by adding 1 ml of ultra pure concentrated HCl to 100 mls of sample. For every sample acidification event, three acid blanks are created by adding the same acid used on the samples to 100 mls of ultra pure water supplied from the lab. Once acidified, the samples are stable at room temperature until analysis, which should occur within one year. Until acidification, the samples should be refrigerated at 4 degrees Celsius.
Sodium, as well as calcium, magnesium, potassium, iron, and manganese are analyzed simultaneously on an optical inductively-coupled plasma emission spectrophotometer (ICP-OES). The acidified samples are directly aspirated into the instrument without a digestion. Sodium is analyzed at 589.592 nm and is viewed axially for low-level analysis and radially for high level analysis.
The detection limit for sodium is 0.06 ppm with an analytical range of the method extends to 50 ppm.
Method Log: Prior to January 2002, sodium was determined on a Perkin-Elmer model 503 Atomic Absorption Spectrophotometer. Lithium at an 1800-2000 ppm concentration was added as a matrix modifier to suppress ionization. From January 2002 to present, samples are analyzed for sodium on a Perkin-Elmer model 4300 DV ICP.
field & laboratory