What are possible causes of low nitrate efficiency when running Lachat nitrate/nitrite methods?
Causes of low nitrate efficiency
- Cadmium Column for Nitrate determination (Product # 50237A). Columns are consumables and will eventually give low efficiencies if used long enough. Some samples may shorten the life of the column. A column should be able to handle 1000 Deionized (DI) water standard injections without significant loss of efficiency. Samples that contain high levels of salt, oxidizers, organics, or are low in pH may shorten the life of the column. Over preserved samples or chlorinated samples may present issues as well. Acid can strip the copper from the cadmium while chlorine and other oxidants will oxidize the column. Samples that contain oil and grease may coat the surface of the cadmium.
- Ammonium chloride buffer. The buffer should be adjusted to a pH of 8.5. If the pH is off it can affect the efficiency. If the pH is checked/adjusted but equipment is not calibrated properly (for example calibrating pH meter with old or suspect buffers) it could result in low efficiency.
- Cadmium column switch. It is possible that a bad column switch could cause some sample to leak through the switch without traveling through the column. This would not necessarily affect the area of nitrite but will reduce the area of nitrate. In this case it has occasionally been observed that nitrite standards give a second "pre peak" (the sample leaking through) and nitrate does not. Also, an internally leaking switch might cause buffer to be flushed out when manifold is rinsed. If the manifold is rinsed with something other than DI water this could be more problematic. One can hook the cadmium column up directly to the manifold to see if efficiency improves, determining if the column switch is a concern.
- Nitrate or nitrite standards. If the nitrate and/or nitrite standards are not prepared properly or have gotten old, they may not produce the correct areas even if column, buffer and switch are functioning properly.