EA--C,+N+and+S+analysis

General information: Elemental Analysis for C, N and S via an automated combustion system

1. The basics. Depending on the instrument, known amounts of the samples are loaded into boats or tin capsules, which are placed into an autosampler and fed into a furnace with a pulse of oxygen where they are burned. The gases are carried by the He carrier gas through an oxidant to complete the burning, chemical traps (if present) to trap sulfur and halogens, a copper reduction furnace to trap excess O2 and convert the NOx's formed to N2 for analysis, a drying tube (typically magnesium perchlorate) to remove water, a GC column to separate the gases (N2 from CO2 and SO2 if present), a detector in the EA itself (usually TCD type, does not have to be used), and finally to the interface with the stable isotope ratio mass spectrometer.

2. The samples. Dry, finely ground, homogenous are the best. The need to burn quickly and completely, though some samples do not actually burn but they thermally decompose, e.g. sulfates. Certain samples may need combustion aids or extra processing to get a complete reaction. You will need enough material to get a good signal for the element, usually N, present in the smallest amount.

3. Common issues: a. N2 in the O2. The tank oxygen pulsed in to burn the samples will have trace amounts of N2 in it, which tend to be heavy (100's per mil). Purer grades of O2 tend not to help much since the lower amount of N2 present is offset by it's higher 15N ratio, and it's also harder to properly detect in blank runs. This can be a major problem in small samples b. Reduction column used up--seen as a rise in the mass 30 peak and may be preceeded by more noise in the 15N numbers, especially on small samples. Basically you try to use the smallest amount of excess O2 as possible that gives a clean burn and good numbers. That will lengthen the lifetime of the reduction column. c. Water. Large organic samples will generate large amounts of water. Always check the drying tube. New reactors can also give off large amounts of water upon heating, so change the water trap after putting in new reactors. With large bolluses of water being pushed through the system, the magnesium perchlorate water traps tend to get very "wet" at the entrance. With smaller samples, there is just general swelling of the salt starting at the entrance and moving down the tube. Replace the drying tube if the entrance material is "wet" or is the swelling has moved past half way down the column (for regular C and N samples). Organic sulfur runs are a bit tricker as far as water is concerned. d. Leaks. e. Ash.

4 The elemental analyzers (C, N and S--some of these may also be configured for H and O, discussed elsewhere): a. Costech ECS 4010 b. Carlo-Erba NC2500 c. Hekatech