PRINCIPLE

Samples are combusted in an oxygen atmosphere at 950 degrees C. The combustion products, mainly CO2, H2O, NOx and N2 are passed through a thermoelectric cooler to remove most of the water and is then collected in a ballast volume. The gas products in the ballast volume are allowed to become a homogeneous mixture at a pressure of 975 mm and a constant temperature. A 10 mL aliquot of the ballast volume is then swept through hot copper and the N- catalyst in order to remove oxygen and convert NOx gases to N2 , using helium as the carrier gas. The aliquot is then swept through lecosorb and anhydrone in order to remove CO2 and H2O respectively. The remaining combustion product, N2, plus the helium carrier gas is then swept through one side of a thermal conductivity cell. The other side of the thermal conductivity cell receives helium only. An out put voltage is generated which is processed by the computer, displayed, and stored as the nitrogen result.

EQUIPMENT

  1. LECO FP-428 nitrogen analyzer
  2. Balance accurate to 0.1 mg
  3. 20 position autosampler tray
  4. Tin foil conical sampling cups

REAGENTS

  1. Anhydrone - magnesium perchlorate - Mg(ClO4)2
  2. Lecosorb - sodium hydroxide on an inert base
  3. Copper sticks
  4. Copper turnings
  5. Alumina pellets
  6. N-catalyst

GASES

  1. Helium 99.99% carrier gas
  2. Oxygen 99.99% combustion gas
  3. Compressed air pneumatic gas

STANDARD

  1. EDTA - Ethylenediaminetetraaectic acid - 9.59% N

PROCEDURE

  1. Initialization:
    1. Press the 3 position switch (OFF, STANDBY, ANALYZE) to ANALYZE.
      1. The instrument is ready for analysis when both the furnace and oven lights are blinking.
    2. Touch the globe.
    3. Touch the "SUPERUSER ICON" - the computer will ask for a password.
    4. Touch "ENTER".
    5. Using the screen keyboard, type the operator's name, and then touch "ENTER".
    6. Touch "BALANCE".
      1. Check the "EDIT:" line for the last sample analyzed.
  2. Blanks:
    1. Using the "ID" buttons, scroll to "BLANK".
      1. The balance should read 0.000 g. If not, touch "TARE".
    2. Touch "ENTER" 6 times - this enters a default value of .5000 g for the 6 blanks.
    3. Leave the first 6 autosampler tray positions vacant.
  3. Standards:
    1. Using the "ID" buttons scroll to "STD".
    2. Place a tin foil conical cup on the balance and touch "TARE".
    3. Weigh approximately 0.15 g EDTA into the cup and touch "ENTER".
    4. Crimp the tin foil cup and place it in autosampler position 7.
    5. Repeat the above step 3 times, placing the crimped cups in autosampler positions 8, 9, and 10.
  4. Samples:
    1. Touch "LOG-IN".
    2. Using the screen keyboard, type sample identification.
      1. The instrument will automatically increment sample numbers. It is not necessary to repeat step 4b if the succeeding samples are numbered consecutively.
      2. If the sample numbers are not consecutive, or the samples are identified in some other fashion, then step 4e will have to be repeated with each weighing.
    3. Place a tin foil cup on the balance and touch "TARE".
    4. Weigh approximately 0.20 g sample into the cup and touch "ENTER".
    5. Crimp the tin foil cup and place it in autosampler position 11.
    6. Weigh the remaining samples in similar fashion, placing them in autosampler tray positions 12-20.
    7. Touch "ESC", touch "VIEW", and then touch "WEIGHT STACK" in order to check sample weights and identification.
  5. Analysis:
    1. Touch "ANALYZE".

CALIBRATION and CALCULATION

  1. Touch "VIEW".
  2. Touch "SYSTEM FOLDERS".
  3. Touch "SYSTEM UPDATE".
  4. Touch "STANDARD BLANK".
    1. If the BLANK STD reads 0.000, touch "ENTER"
    2. If the BLANK STD does not read 0.000, use the on screen keypad and type 0.000 and touch "ENTER".
  5. Using the up arrow, scroll to BLANK
    1. Select the 2 or 3 best consecutive blanks (lowest, consistent %N) and using the arrow keys, place the cursor on the last "best" blank.
  6. Touch "INCLUDE SAMPLES" to select the 2 or 3 best consecutive blanks.
    1. The selected blanks will be highlighted.
  7. Touch "PROCESS RESULTS".
    1. The screen will display the new blank and the previous blank.
  8. Touch "ESC".
  9. Touch "STANDARD CALIB.".
    1. If the N concentration for the standard is correct, touch "ENTER".
    2. If the N concentration is not correct, use the on screen keypad and type the correct concentration and touch "ENTER".
  10. Using the up arrow, scroll to STD, placing the cursor on the last of the 4 standards.
  11. Touch "INCLUDE SAMPLES" 4 times.
    1. The selected standards will be highlighted.
  12. Touch "PROCESS RESULTS".
    1. The screen will display the new calibration and the previous calibration.
  13. Touch "STACK SELECT".
  14. Touch "INCLUDE SAMPLES" to select the samples to be analyzed by the new calibration.
    1. The selected samples will be highlighted.
  15. Touch "PROCESS RESULTS"
    1. The screen will display " Print out recalculated answers?".
  16. Touch "YES"

SHUT DOWN

  1. Touch "EXIT".
  2. Touch "LOG OFF".
  3. Press the 3 position switch (OFF, STANDBY, ANALYZE) to STANDBY.

QUALITY CONTROL

  1. Analyze one duplicate sample, one quality control sample and one EDTA sample with each set of samples, placing them in the autosampler tray positions following the last client sample.
    1. Values on the duplicate sample must agree within 20% of the average value of the two samples.
    2. Values on the quality control sample must be within the established limits for this sample.
    3. The EDTA sample should read between 9.38 and 9.78% N.
  2. Record the date, sample number range, and values obtained on the EDTA sample and the quality control sample on the Nitrogen Quality Control Sheet.

NOTES

  1. This analysis protocol must be followed for the first set of samples analyzed each day. This means that only 8 client samples can be analyzed in the first set (6 blanks, 4 EDTA calibration samples, 8 client samples, 2 QC samples).
  2. The calibration generated by the first set may be used for the analysis of subsequent sets, as long as the quality control criteria is met. Subsequent sets may contain up to 18 client samples.
  3. The compressed air is used to power the autosampler.
  4. Analysis time is approximately 154 seconds per sample.
  5. Check the instrument manual for proper maintenance and changing of reagents.

REFERENCES

  1. Bellomonte, G. A. Constantine, and S. Giammariolo. 1987. Comparison of modified automatic dumas method and the traditional Kjeldahl method for nitrogen determination in infant food. JAOAC 70:227-229.
  2. Colombo, B. And G. Giazzi. 1982. Total automatic nitrogen determination. Am. Lab. 14:38-45.
  3. Sweeney, R. A. 1989. Generic combustion method for the determination of crude protein in feeds:Collaborative Study. JAOAC. 72:770-774.