CN elemental analyzer
The device that is utilized for carbon, nitrogen, hydrogen and sulfur analysis is an Vario Micro Cube (Elementar)
Solid samples are weighed in tin containers (tin is of importance for the right combustion in the elemental analyzer) and loaded into an automatic sampler. The tin cups are then dropped in a tube where in the presence of external oxygen flash combustion occurs at a temperature of 1800 °C. The gaseous combustion products N2, NOx, H2O, SO2, O2 and CO2 are carried by the helium as carrier gas through a column filled with copper oxide and from there to a Cu-column where nitrogen oxides are reduced to elementary nitrogen, and O2 to CuO. Water can be absorbed in another column. The remaining gasses are introduced into a TPD (Temperature Programmed Desoprtion) column where N2 is going right through it and the other gases are bound to the column. With a programmed temperature raise in the column the gases are released separately. They flow along a thermal conductivity detector (TCD) which produces an electrical signal proportional to the concentration of nitrogen, carbon, hydrogen and sulfur.
Sample preparation and submission
Correct preparation of the samples is essential for analysis of carbon and nitrogen contents. Three important steps are distinguished: drying, grinding and weighting.
1.Drying. Samples should be dried before grinding them, but also for another hour or so, just before weighting them.
2. Grinding. For analysis, it is of course, very important to use samples with a uniform composition, which moreover are representative for the object to study. In order to obtain a homogenous composition and structure of the sample-matrix, one should take care to have fine particles of uniform size, chemical composition and purity. In practice, this means grinding the material to talk dust-sized particles. The Mixer mill present at the GI canbe used for this purpose, .
3. Weighting. The samples are weighed into tin containers of 8x5mm using the analytical micro-balance (book it!) and forceps. The amount of sample needed per cup depends on the type of samples:
- 1 mg for animal material,
- 3 mg for aerial parts of plants (e.g. leaves),
- 10 mg for root material and 5-40 mg for soil and sediment samples (depending whether from mineral or organic origin; some soils consist of 90% of plant materials).
- Samples heavier than 50 mg can cause problems during combustion.
The cups (containers) with weighted samples should be shaped to little round balls. The shape is relevant for a good ‘Flash Combustion’. Moreover, flat or misshapen balls can get entangled in the auto-sampler! Balls can be shaped with help of forceps, but it is also allowed to do this by hand, using gloves. Without gloves the own fingers/hands are a source of contamination of C and N. Use also clean tools, weigh in a dust free environment, and prevent contamination from one to another sample.
4. Delivery and submission of samples
A good way to store and deliver samples is to use 96-wells plates. The corresponding weights should be entered in an excel sheet (WEIGHTS CN IRMS (xls, 42 kB)). . The request for analysis of samples should be entered in Bookings together with the filled weight form (can be uploaded).
Location of equipment
- Weight sheet (xls, 43 kB) for CN analysis
- Reserve the micro-balance through bookings
- Enter the request for sample analysis together with the filled weight sheet in bookings
Discuss the sample preparation and the timing with Paul van der Ven; long-term planning may be applicable for these analyses.