Combustion tube (100,130,150) and method for combusting a sample to form combustion products for analysis. A primary combustion chamber (102) receives and combusts a sample to form first combustion products. A secondary combustion chamber (110) receives and combusts the first combustion products to form second combustion products. The secondary chamber is disposed around the primary chamber and comprises a plurality of—preferably, three—combustion compartments (112-116). Each successive compartment is disposed around the preceding compartment and defines a respective gas flow channel having an inlet opening (108,118,120) and an outlet opening (118,120,122) at opposite ends. Each successive gas flow channel shares its inlet opening with the outlet opening of the preceding channel. The plurality of combustion compartments define a substantially continuous gas flow conduit from the inlet opening of the innermost compartment to the outlet opening of the outermost compartment. Static gas mixers (132,134) may be employed to enhance mixing of the combustion products.