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Standard Operating Procedure (SOP): CHNS Analyzer, Elementar
Model: UNICUBE

sop

1. Purpose & Scope

The Standard Operating Procedure (SOP) of an Elementar (UNICUBE) CHNS elemental analyzer used for the quantitative determination of Carbon (C) and Nitrogen (N) in solid and liquid organic samples. The operating procedure covers the steps from sample preparation to instrument shutdown.

2. Safety & Prerequisites:

  • Gas Requirements: Required ultra-high purity (99.999%) gases.
  • Helium Gas: Set the gas cylinder regulator pressure to 1.3 bar.
  • Oxygen Gas: Set the gas cylinder regulator pressure to 2.5 bar.
  • Environmental Conditions: Ambient temperature 15°C to 35°C with a relative humidity of 30–80%.

3. Pre-Operational Checklist & Instrument Startup:

  • Open the gas cylinder valves of Helium and Oxygen. Ensure the pressures are correctly set (Helium: 1.3 bar, Oxygen: 2.5 bar).
  • Power on the instrument using the green button located on the right side of the instrument and wait approximately 40 seconds for initialization.
  • Turn on the PC and open the UNICUBE Software application. Check the software dashboard and verify the gas flow (He: 220–230 ml/min and O₂: 30–34 ml/min).
  • Execute a reference cycle for the carousel to reach its correct position (Zero). Ensure the carousel is completely empty before running this step.
  • Perform a Leak Test before analysis to verify the system is leak-tight. Navigate to Maintenance → Rough Leak Test and execute the process.
  • If the leak test shows a "Passed" status, close the leak test dialog box.
  • If the leak test fails, check the clamp connections and O-rings. Do not proceed until the test passes.
  • Set furnace temperatures after the leak test has passed. Navigate to Settings → Parameters → Temperature tab.
  • Set the Combustion Tube temperature to 950°C and the Reduction Tube temperature to 600°C, then click OK.
  • Allow the furnaces to heat up and stabilize (typically 45–60 minutes). Once the TCD temperature stabilizes at 60°C, the instrument status will display Standby Mode, indicating readiness for analysis.

4. Sample Preparation:

  • Ensure the samples are finely ground, dried, and homogeneous (e.g., using a mortar and pestle) and pass through a 0.5 mm sieve.
  • Place an empty tin boat or foil on the microbalance and tare it.
  • Use a clean micro-spatula to add sample material into the tin boat/foil.
  • Weigh 2–10 mg for organic samples and 15–20 mg for soil samples (or follow the specific method). Seal the tin boat/foil tightly into a compact cube using tweezers and ensure that no sample leaks.
  • Record the exact weight to the nearest 0.000 mg in the software sample table.
  • Place the sealed sample into the designated numbered slot of the 60-position autosampler carousel.

5. Calibration, Sequence Setup, & Analysis:

    • Prepare and load 2–3 blanks with an entry weight of 0.000 mg designated as blank. These clear residual atmospheric gases and stabilize the detector baseline.
    • Prepare and load 2–3 conditioning standards (e.g., sulfanilamide) designated as Run-in to condition the combustion and reduction tubes.
    • Prepare and load a minimum of 2–3 daily calibration standards (e.g., sulfanilamide) with known chemical composition (%) to calculate the instrument calibration factor (K-factor).
    • Prepare and load samples for analysis.
    • In the software’s sample table, enter the sample weight (mg), sample name, and method, then save the file by navigating to Save As and entering a unique filename.
    • Initiate the sequence by clicking Run and selecting Auto Run to start the automated analysis sequence. The system will purge atmospheric air from the sample chamber, drop the tin boat capsule into the furnace, separate the gases (N₂, CO₂, H₂O, and SO₂), and measure them via the TCD.
    • After the standard runs are complete, select the calibration standards in the software table, click Math → Factor, execute the calculation, and apply the updated daily calibration factor (K-factor).
    • After completion of the sample run (analysis) cycle, the instrument automatically enters Standby Mode. Save the data file and take a printout by navigating to File → Print, or export the data in Excel format by navigating to File → Export → Excel Sheet and save the file to the designated storage location.

6. Shutdown Procedure:

  • Once the sequence finishes, navigate to Settings → Parameters → Temperature.
  • Set both the Combustion Tube and Reduction Tube temperatures to 0°C.
  • Allow the instrument to cool down naturally and wait until the furnace temperatures fall below 300°C. Securely close the Oxygen and Helium gas cylinder supply valves.
  • Close the UNICUBE software application and then turn off the instrument power using the green power button.