10. Procedure
10.1 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, check its operation according to the manufacturer's instructions to ensure stability before proceeding. Then make the performance test (Section 8).
10.2 Obtaining the Mass Spectrum - Using a microburet or a constant-volume pipet, introduce sufficient sample through the inlet system to give a pressure of 20 to 60 mtorr (2.7 to 8.0 Pa). Record the amount of sample introduced and the final pressure after expansion into the inlet system when a microburet and manometer are used. Recharge the sample until pressure readings that differ by 1 % or less are obtained. Attaining this pressure check means that a given microburet is being used at constant volume. When the pressure check is obtained, admit the sample to the mass spectrometer and record the mass spectrum of the sample from m/e(+) 32 to 186.
11. Calculation
11.1 Peaks - Read peak heights from the record of the mass spectrum of the sample corresponding to m/e+ ratios of 41, 43, 55, 57, 67, 68, 69, 71, 77, 78, 79, 81, 82, 83, 84, 85, 86, 91, 92, 95, 96, 97, 98, 99, 100, 103, 104, 105, 106, 112, 113, 114, 117, 118, 119, 120, 126, 127, 128, 131, 132, 133, 134, 140, 141, 142, 145, 146, 147, 148, 154, 155, 156, 159, 160, 161, 162, 168, 169, 170. Calculate the following combined peak heights by adding together the indicated peaks:
∑43 = m/e(+) 43 + 57 + 71 + 85 + 99
∑41 = m/e(+) 41 + 55 + 69 + 83 + 97
∑67 = m/e(+) 67 + 68 + 81 + 82 + 95 + 96
∑77 = m/e(+) 77 + 78 + 79 + 91 + 92 + 105 + 106 + 119 + 120 + 133 + 134 + 147 + 148 + 161 + 162
∑103 = m/e+103 + 104 + 117 + 118 + 131 + 132 + 145 + 146 + 159 + 160
∑128 = m/e+128 + 141 + 142 + 155 + 156
T = total ion intensity = ∑43 + ∑41 + ∑67 + ∑77 + ∑103 + ∑128
11.2 Carbon Number Calculated from Spectral Data:
11.2.1 Calculation of Alkylbenzene Apparent Carbon Number:
11.2.1.1 Calculate monoisotopic peaks at 92, 106, 120, 134, 148, and 162:
Mono 92 = 92(+) - 0.0769 (91(+))
Mono 106 = 10(+) - 0.0880 (105(+))
Mono 120 = 120(+) - 0.0991 (119(+))
Mono 134 = 134(+) - 0.1102 (133(+))
Mono 148 = 148(+) - 0.1212 (147(+))
Mono 162 = 162(+) - 0.1323 (161(+))
11.2.1.2 Convert the poly 78 mixture and the monoisotopic peaks to a molar basis by multiplying each by the following factors:
11.2.1.3 Normalize the products of the preceding step to obtain the relative mole fractions of the C6 to C12 alkylbenzenes. An apparent carbon number can then be calculated by totaling the products of each mole fraction and the corresponding number of carbon atoms per molecule. This carbon number is assumed to apply to all akylbenzenes, indans, tetralins, and naphthalenes.
11.2.2 Calculation of Paraffin Apparent Carbon Number (Note 6):
11.2.2.1 Calculate monoisotopic peaks at 86, 100, 114, 128, 142, 156, 170:
Mono 86 = 86(+) - 0.0668 (85(+)) + 0.0026 (84(+)) - 0.014 (mono 92(+)) - 0.008 (mono 106(+)) - 0.008 (mono 120(+))
Mono 100 = 100(+) - 0.0779 (99(+)) + 0.0034 (98(+)) - Hg (Note 7)
Mono 114 = 114(+) - 0.0890 (113(+)) + 0.0044 (112(+))
Mono 128 = 128(+) - 0.1001 (127(+)) + 0.0055 (126(+))
Mono 142 = 142(+) - 0.113 (141(+)) + 0.0068 (140(+))
Mono 156 = 156(+) - 0.1224 (155(+)) + 0.0081 (154(+))
Mono 170 = 170(+) - 0.1335 (169(+)) + 0.0096 (168(+))
11.2.2.2 Place these peaks on a molar basis by multiplying each peak by empirical factors as follows (Note 8):
11.2.2.3 Normalize the products of the preceding step to obtain the relative mole fractions of the C6 to C12 paraffins. Calculate an apparent carbon number by totaling the products of each mole fraction and the corresponding number of carbon atoms per molecule. This carbon number is assumed to apply to all paraffins and cycloparaffins.
NOTE 7 - Small amounts of naphthalenes, which have intense ions at 128, 141, and 142, may introduce errors into the results of this calculation. Large errors will be detected by a bimodal distribution of the individual paraffinic peaks. A relatively large 141 peak could also be indicative of naphthalenes. If naphthalenes appear to be present it is suggested that the paraffin carbon number be calculated from the mass spectrum of the saturate portion of the sample which may be easily obtained by Test Methods D2002. If the saturates cannot be obtained the paraffin carbon number should be assumed to be 0.5 number less than that of the aromatics.
11.2.2.4 The term Hg refers to a background correction that must be applied if mercury peaks are present in the spectrometer. This correction must be determined for each instrument under conditions that simulate a sample run.
NOTE 8 - The factors in 11.2.1 and 11.2.2 which are used to convert parent monoisotopic peaks of alkylbenzenes and paraffins to a molar basis are average values of data that were obtained in three laboratories. These data were obtained by making direct pressure sensitivity measurements of the appropriate blends described in Table 2 and extrapolation of these results for the carbon number range from 10 through 12. This same procedure can be utilized by an individual laboratory if desired.
11.3 Calculation of Compound Types - Using the proper inverse from Table 3 according to the carbon number of the sample, calculate the liquid volume percent of each hydrocarbon type. This selection may vary for the same sample depending upon the carbon number of the paraffins and aromatics. For example, if the paraffin carbon number is 7.0 and that of the alkylbenzenes is 8.0, the carbon number 7 inverse would be used to calculate the volume fraction of paraffins and cycloparaffins, whereas the carbon number 8 inverse would be used to calculate the aromatics. Volume fractions must then be normalized.
11.3.1 When an integral carbon number is not obtained two inverses should be applied and the results weighted. For example, if the paraffin carbon number is 7.4, both the carbon number 7 and carbon number 8 inverses should be applied for the paraffins and cycloparaffins. The volume fraction to be used would then be the value obtained from the carbon number 7 inverse plus 0.4 of the difference between the values obtained from the carbon number 7 and carbon number 8 inverses.
NOTE 9 - Although calculation of the composition of the sample by interpolation between the results of two adjacent carbon number inverses gives good results, the availability of computers suggests the use of an even better procedure which is not practical when hand calculators are used. It should be possible in calculating each sample to select matrix elements by interpolation between adjacent carbon numbers in a table of calibration data and to calculate sample composition from the resulting matrix either by computing an inverse or by use of an iterative procedure.
11.4 Olefin Content of Sample:
11.4.1 If the bromine number is used, calculate the liquid volume percent olefins in accordance with Test Method D875. If the fluorescent indicator adsorption Test Method D1319 is used, the liquid volume percent olefins is obtained.
11.4.2 For samples containing less than 3 % olefins, subtract the liquid volume percent olefins from the monocycloparaffin results obtained from the inverse.
11.5 Calculate the analysis on the original basis, including the volume of olefins and the pentanes and lighter hydrocarbons removed, if any, as separate results.