6. SPECIAL TECHNIQUES FOR SELECTIVE COMPOUND
6.1. Gas Chromatography–Low-Energy Ionization Mass
Petroleum and petroleum distillate cuts often consist of hundreds, if not thou- sands, of individual components. Except for the lowest-boiling fractions, it is virtually impossible to separate, identify, and/or quantitate all of the significant components in a given sample. In many cases, petroleum chemists and engineers are able to work with simplified characterizations that express a given sample in terms of hydrocarbon groups or ‘‘types,’’ such as ‘‘paraffins,’’ ‘‘naphthenes,’’
‘‘benzenes,’’ etc. Several mass spectrometric methods have been developed that characterize petroleum samples into such hydrocarbon types. These methods make use of mass spectral ions that are characteristic of the various groups of hydrocarbons. Ion intensity values for these characteristic ions can be summed to give a quantitative measure of the amounts of each of the hydrocarbon groups [53]. In most cases, however, these MS methods do not provide carbon number distributions of each hydrocarbon group.
6.1.1. Gas Chromatography–Low-Voltage Electron-Ionization Mass Spectrometry
One method that has been used in the petroleum industry to provide carbon num- ber distributions of aromatic compounds makes use of low-voltage electron ion- ization (LV-EI). At ionizing voltages on the order of about 10 eV, saturated molecules, such as paraffins and naphthenes, do not ionize to any great extent.
Aromatic molecules, having lower ionization potentials, still do ionize. The amount of energy transferred to the aromatic molecules under such conditions, however, is such that most of the molecular ions formed do not break apart, and thus retain the molecular weight information. This makes it possible to obtain carbon number distributions of the various aromatic types. These ‘‘types’’ are generally thought of in terms of ‘‘z-series,’’ where the ‘‘z’’ comes from the empirical formula CnH2n⫹z [69]. Using low-resolution mass spectrometry, it is possible to obtain carbon number distributions for aromatics in the ‘‘-6’’, ‘‘-8’’,
‘‘-10’’, ‘‘-12’’, ‘‘-14’’, ‘‘-16’’, and ‘‘-18’’ z-series. Aromatic sulfur compounds and more-condensed aromatic hydrocarbons overlap with these z-series, but are often present at low enough levels as to cause only minor problems.
Gas chromatography–mass spectrometry is a technique used frequently to analyze naphtha and distillate cuts of petroleum. Even with high-resolution GC columns, it is usually impossible to completely separate components in such sam- ples. In some cases, coeluting components can be identified and quantified by
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careful analysis of the mass spectra produced by such mixtures. This process becomes more difficult for higher-boiling petroleum fractions. The top trace of Figure 10 shows a total ion chromatogram (TIC) for a typical diesel fuel sample, using a 70-eV electron beam as ionization means. A short HP-5 fused silica capillary column (10 m⫻0.10 mm ID) was used for this analysis. The GC oven temperature was ramped at a high rate in order to elute the sample quickly. As can be seen from the chromatogram, the sample is very complex. The top spectrum of Figure 11 shows a summed 70-eV mass spectrum for the entire sample. This ionization energy is 5 to 10 times the amount necessary to ionize most hydrocar- bon molecules. As a result, a considerable amount of the molecular ions formed fragment into smaller pieces. It is this 70-eV mix of parent and fragment ions that is typically used as a ‘‘fingerprint’’ of a given compound. While this often works well for completely separated components, it also makes spectral interpre- tation more difficult in cases where components are not separated in the gas chromatograph. In the case of aromatic compounds, low ionizing voltage offers a partial solution to this problem. A low ionizing voltage TIC for the same diesel fuel sample is shown in the bottom trace of Figure 10. The summed mass spec- trum for the entire sample is given in the bottom spectrum of Figure 11. The spectrum consists mainly of molecular ions, with little fragmentation. The satu- rates, which gave large ion signals in the low mass region at 70 eV, do not contribute much to the overall signal under the low eV conditions. By determin- ing response factors for the aromatic compound types, it is now possible to obtain quantitative carbon number distributions for each z-series. In addition, it is possi- ble to obtain isomer distributions at the lower carbon number ranges of each z- series by making use of extracted ion chromatograms (mass chromatograms).
Thus, low ionizing voltage provides a means to simplify the characterization of aromatics in complex petroleum samples, yet still provides a level of detail allowing chemists and engineers to determine differences among various sam- ples.
6.1.2. Gas Chromatography–Charge-Exchange Mass Spectrometry
Alternatively, the sample can be analyzed under low-energy ionization conditions using benzene [70] or carbon disulfide (CS2) [71] as a charge exchange reagent gas. In low-voltage EI, molecular ions are formed with a distribution of internal energies. In contrast, for charge exchange the internal energy (IE) of the molecu- lar ion is well defined and equal to the difference between the recombination energy (RE) of the reagent gas (benzene or CS2) and the ionization potential (IP) of the sample molecule. For example, in CS2charge exchange (CS2-CE)
CS2⫹•⫹ M→CS2⫹ M⫹• (2)
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Figure 10 Comparison of GC–MS TICs of a typical diesel fuel between high-voltage and low-voltage EI runs. The top trace shows high-voltage (70 V) EI results. Normal paraffins are shown as sharp peaks above the envelope (hump). At low-voltage (10 V) EI, the normal paraffin peaks disappear with the low-ionization-potential aromatic compo- nents predominant in the chromatogram, shown as the bottom trace.
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Figure 11 Comparison of high-voltage and low-voltage EI spectra of the diesel fuel.
In the high-voltage EI spectrum (top), significant amounts of fragment ions, shown as odd masses, are present. In the low-voltage EI spectrum (bottom), essentially molecular ions, shown as even masses, are present.
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IE(M⫹•)⫽ RE(CS2⫹•)⫺IP(M) (3)
Carbon disulfide charge exchange has been found to have several advan- tages over LV-EI: (1) small variations in electron beam energy for CS2-CE do not affect the relative sensitivity significantly, (2) CS2-CE is more sensitive than LV-EI, and (3) CS2-CE has more uniform molar sensitivity than LV-EI.
6.1.3. Gas Chromatography–Chemical-Ionization Mass Spectrometry
A comprehensive hydrocarbon-type analysis based on GC–MS using nitric oxide as a chemical ionization (CI) reagent gas [72] has been developed by Dzidic et al. [73] The sample molecules are ionized via ion–molecule reactions, with the reactant NO⫹ ion generated by Townsend discharge ionization of nitric oxide.
For example, aromatic hydrocarbons form molecular ions via charge exchange with NO⫹, while paraffins form [M⫺H]⫹ions via hydride abstraction of mole- cules by NO⫹ions. This method simplifies the mass spectral pattern with minimal fragmentation to facilitate quantification. However, the method shares a short- coming of all CI methods. The sensitivity of the molecules is highly dependent on ion source pressure, making it difficult to reproduce the quantification results.
In addition, the coproduction of other pseudo-molecular ions, such as [M⫹NO]⫹ ions for aromatic compounds, would complicate the interpretation and quantifi- cation of the components.
6.1.4. Gas Chromatography–Field-Ionization Mass Spectrometry
Another emerging method is the coupling of GC with field-ionization MS (GC–
FI-MS) [74]. Field-ionization MS yields essentially molecular ions for all hydro- carbons, except isoparaffins, with uniform sensitivities within each compound class [75]. This would greatly simplify calibration since each class would require only one calibration compound.