QA-QC Protocols

Introduction

The instrument may be operated in either CHN or CHNS mode. For CHN operation, there are separate combustion and reduction tubes; for CHNS operation, combustion and reduction functions occur in one tube. 

  • Tubes are packed in-house using reagents purchased from Perkin Elmer or (rarely) from Elemental Microanalysis (Elementar).
  • Depleted reduction tubes are regenerated using a special furnace we have constructed; a 4-8% mixture of hydrogen in argon gas (Praxair Hydrostar H-6) is used. 
  • Quartz tubes are reused when possible; new tubes are purchased form Perkin Elmer or Elemental Microanalysis, or manufactured by our glassblower.
  • Ultra-clean tin capsules (6mm x 4mm) from Elemental Microanalyis are used.
  • UHP Plus grade oxygen is used to support combustion
  • Normal 99.995% helium is used as the carrier gas
  • All standards are from Perkin Elmer.

Instrument Preparation

The system is checked for leaks whenever a tube is replaced; diagnostic 2-1-2 is run.  The system is purged (He = 600 seconds; O2 = 150 seconds) whenever a tube is replaced, and just before a run sequence is initiated.

CHN Operation

Normal operating procedure includes running conditioners, blanks, calibrants, and standards throughout the run sequence.  Acetanilide (ace) is used for CHN studies.  The default optimize combustion settings for CHN measurements are 3,5,1,0.  The normal run sequence is:

Run # Compound Run as... Purpose
1-3 ace (unweighed) sample condition the system
4-5 empty tin capsule blank set initial blank values
6-7 ace (1.5 - 2.5 mg) sample K-factor adjustment
8 empty tin capsule blank blank adjustment
9-10 ace (1.5 - 2.5 mg) sample K-factor adjustment
11 empty tin capsule blank blank adjustment
12-13 ace (1.5 - 2.5 mg) sample K-factor adjustment
14-18 samples 1-5 sample unknown samples
19 empty tin capsule blank blank adjustment
20-21 ace (1.5 - 2.5 mg) sample K-factor adjustment
22-31 samples 6-15 sample unknown samples
... ... ... ...continue samples in groups of 10

 

CHN/S Operation

A special sulfur conditioner (sulfonic acid - unweighed) is run prior to initiating the normal run sequence described above.  Cysteine (cys) is used instead of acetanalide for all conditioning and K-factor adjustment runs.  The default optimize combustion settings for CHN/S measurements are 0,0,0,0.

Data Processing

The measured blank values are transferred to Excel.  When trends are not discernible, blank corrections are established by taking the average of all readings obtained throughout the run, or by allowing the system-calculated running average values to remain.  Either method will usually suffice for the C, N, and S readings.  Hydrogen blank values normally require adjustment based upon a curve.

 

The corrected blank values are entered into the "reprocessing" function of the analyzer firmware, and a blank-corrected printout is obtained for all runs.   The measured values for all acetanilide runs (cysteine if it is a CHN/S sequence) are transferred to Excel.  The average %C, %H, %N, (and %S) values are obtained for the ace (cys) standards.  Using simple ratio and proportion, corrected K-values are obtained. 

The corrected K-factor values are entered into the "reprocessing" function of the analyzer firmware, and a final printout using corrected blanks and K-factor is obtained for all runs. 

Note that the %H reading will ALWAYS be low right after a blank run; those values are not included in the average.  The instrument specification for both precision and accuracy is +/- 0.3% for each element.  System performance usually exceeds the specification for properly prepared samples. 

An examination of depleted combustion tubes shows that approximately one in twenty samples may not drop completely into the combustion zone; combustion will still occur, but measurement precision will be poor.  It is critical that duplicates be run in order to identify those instances of poor precision; whenever possible a third replicate is then run and the outlier (always lower than the others) is excluded from the average.