Determination of CNS in sediment samples with the CARLO ERBA Elemental Analyzer

This instrument analyzes total carbon, nitrogen and sulphur in solid samples. The analytical method is based on the complete and instantaneous oxidation of the sample by „flash combustion“ which converts all organic and inorganic substances into combustion products. The resulting combustion gases pass through a reduction furnace and are swept into the chromatographic column by the carrier gas (helium) The gases are separated in the column and detected by the thermal conductivity detector (TCD) which gives an output signal proportional to the concentration of the individual components of the mixture.

The sample to be analyzed is weighed into a tin conainer (TC/N/S) or in a silver cup (Corg) and loaded into the automatic autosampler. When the autosampler is triggered, the sample is introduced in to the Combustion reactor which is maintained at around 1050°C. The sample container melts and the tin promotes a violent reaction (flash combustion) in a temporary enriched atmosphere of oxygen. The combustion products (N2,NOx,CO2,H2O,SO2 and SO3) carried by a constant flow of carrier gas (Helium) pass through an oxidation catalyst of Chromium  oxide (CrO), kept at 1050°C inside the reaction combustiontube. Furthermore, to ensure the complete oxidation, a 5 cm layer of silver coated cobalt oxide is placed at the bottom of the combustion tube. The catalyst also retains interfering substances,  produced during the combustion of halogenated compounds.

The mixture of combustion products and water passes through a second tube known as the reduction reactor which contains metallic copper kept at 650 °C. The excess oxygen is removed in the reaction reactor and at this temperature the nitrogen oxides coming from the combustion tube are reduced to elemental nitrogen, which together with carbon dioxide, sulphur dioxide and water pass trough the asorbent filter.
Note that the  packing of the combustion tube for N/C/S determination differs from the one used for TC/N analysis. The combustion products pass through only one tube filled with the oxidation catalyst (Al2O3with WO3), and a 6 cm layer of pure copper wire is placed at the bottom of the combustion tube.

Operating Parameter for TC/N analyses
Oxidation furnace temperature 1050°C
Reduction furnace temperature  650°C
GC column oven and detector block temperature 75°C
Filament temperature 190°C
Helium carrier gas flow rate (main) 80ml/min
Helium carrier gas flow rate (reference) 40ml/min
Oxygen flow rate 25ml / min
Total analytical time 240 sec
Reactor packing for TC/N:
Combustion reactor: Chromium Oxide/Silvered cobaltous cobaltic oxide
Reduction reactor copper

Operating Parameter for N/C/S analyses
Oxidation furnace temperature 1060°C
GC column oven and detector block temperature 75°C
Filament temperature 190°C
Helium carrier gas flow rate (main) 95ml/min
Helium carrier gas flow rate (reference) 40ml/min
Oxygen flow rate 10ml in 9,5 sec
Total analytical time 740 sec
Reactor packing for simultaneous  determination of N/C/S
Pure reduced copper wires
And  as OXIDATION CATALYST Al2O3 + WO3
Or Tungstic Anhydride 

Sample Preparation:
The freeze dried sample are grinded whether in an agate mortar or in a mill. For sulphur measurements samples should be grinded in a mill. Depending of the C/N/S concentration, 3-20mg of sample is weighed into a tin cup (N/C/S) or silver cup for the TOC measurements. For sulphur determination the same amount of  Vanadiumpentoxid is added to the sample. Organic carbon is determined after removing carbonate carbon by acidification with 0,25N hydrochloric acid.

Calibration:
For each set of samples (n = 50) Standards and blanks are measured.
Calibration Standards TC/N
Acetanilid N:10,36%,C:71,09 %
SOIL Standard BSTD1: N:0,216%, C:3,5 %
Calibration Standard TC/N/S
BBOT 2,5-Bis(5tert-butyl-2-benzo-oxal-2-yl)thiophen :C:72,52%,N:6,51%,S:7,44%

Measurements:
Each sample is run as a duplicate, for long term precision calculation an internal standard is measured in each set. 
  
Statistic
Relative Standard- Deviation:
TC/N/S : Sample SO 143 55-5-  56-58 cm (n=5) 
                                                      % N             % C             %S

MEAN                                             0,145            1,930         0,984

Standard Deviation                            0,002            0,012         0,034

Relative  Standard Deviation (%)        1,427             0,610         3,467

Precision
STANDARD:BBOT 
                                                      % N               % C             %S
MEAN                                            6,509             72,654        7,593 n = 5

Standard Deviation                          0,110               0,872         0,166

Relative StandardDeviation %           1,689              1,200         2,182

Accuracy                                         1,69               1,2            2,18

Certified concentration                    6,510              72,52          7,44

Detection limit:
C:0,94µg
N: 0,23 µg
S: 0,06µg

Quantification limit
C: 3,14 µg
N: 0,76µg
S: 0,20 µg 
  Standard used for C/N Measurements:  BSTD 1     C:3,5%, N:0,216%

Standard used for S Measurements: BBOT