ASTM D2425 for Hydrocarbon Types in Middle Distillates by Mass Spectrometry
7. Apparatus
7.1 Mass Spectrometer - The suitability of the mass spectrometer to be used with this method of analysis shall be proven by performance tests described herein.
7.2 Sample Inlet System - Any inlet system permitting the introduction of the sample without loss, contamination, or change in composition. To fulfill these requirements it will be necessary to maintain the system at an elevated temperature in the range of 125 to 325°C and to provide an appropriate sampling device.
7.3 Microburet or Constant-Volume Pipet.
8. Calibration
8.1 Calibration coefficients are attached which can be used directly provided:
8.1.1 Repeller settings are adjusted to maximize the m/e+ 226 ion of n-hexadecane.
8.1.2 A magnetic field is used that will permit scanning from m/e+ 40 to 292.
8.1.3 An ionization voltage of 70 eV and ionizing currents in the range 10 to 70 µA are used.
NOTE 3 - The calibration coefficients were obtained for ion source conditions such that the ∑67/∑71 ratio for n-hexadecane was 0.26/1. The cooperative study of this test method indicated an acceptable range for this ∑ ratio between 0.2/1 to 0.30/1.
NOTE 4 - Users of instruments other than Consolidated Electrodynamics Corp. Type 103 Mass Spectrometers may have to develop their own operating parameters and calibration data.
9. Performance Test
9.1 Generally, mass spectrometers are in continuous operation and should require no additional preparation before analyzing samples. If the spectrometer has been turned on only recently, it will be necessary to check its operation in accordance with this method and instructions of the manufacturer to ensure stability before proceeding.
9.2 Mass Spectral Background - Samples in the carbon number range C10 to C18 should pump out so that less than 0.1 % of the two largest peaks remain. For example, background peaks from a saturate fraction at m/e+ 69 and 71 should be reduced to less than 0.1 % of the corresponding peaks in the mixture spectrum after a normal pump out time of 2 to 5 min.
10. Mass Spectrometric Procedure
10.1 Obtaining the Mass Spectrum for Each Chromatographic Fraction - Using a microburet or constant-volume pipet, introduce sufficient sample through the inlet sample to give a pressure of 2 to 4 Pa (15 to 30 mtorr) in the inlet reservoir. (Warning - Hydrocarbon samples of this boiling range are combustible.) Record the mass spectrum of the sample from m/e+ 40 to 292 using the instrument conditions outlined in 8.1.1-8.1.3.
