Date: March 2016
The mineral oil hydrocarbons (MOH) are distillation products from mineral oil or coal tar. They consist of mineral oil saturated hydrocarbons (MOSH) that comprise open-chain paraffins and cyclical naphthenes, as well as mineral oil aromatic hydrocarbons (MOAH), which generally contain 1–5 aromatic rings. The range of MOHs spans volatile and easily degradable hydrocarbons right up to poorly soluble, low-volatility and persistent high molecular compounds of grease and oils.
Due to the enormous amount of compounds, single component analysis is not possible when determining mineral oil hydrocarbons. The mixtures can however be characterised using two-dimensional gas chromatography (GCxGC). With GCxGC-TOF(MS) the MOSH can be distinguished between n-alkanes, isoalkanes and cycloalkanes. The MOAH can be grouped according to the number of aromatic rings as well as the degree of alkylation and hydrogenation.
GCxGC-TOF(MS) is a two-dimensional (2D) chromatographic measurement method. This requires that two analytical columns with orthogonal properties (eg nonpolar/polar) are used. This enormously increases the analytic system’s separation efficiency and peak capacity, which enables the analysis of complex compounds such as MOH.
The prepared column setup for analysing MOH is the so-called reversed phase setup (RP). A long polar column is used in the first dimension, and a short nonpolar one is used in the second. In this manner, the complex hydrocarbon mixtures being analysed are first separated off on the basis of their polarity and finally their flashpoints. The reverse of this is the so-called NP setup (normal phase). In the first dimension, a nonpolar stationary phase is used, and a polar phase in the second dimension.
This analysis can be regarded as being comprehensive, because the analytes enter the second column directly from the first, without loss. Between the columns there is a unit known as a modulator. At fixed intervals, it ‘separates’ the eluate from the first column using cryofocusing, and feeds it into the second column.
During this procedure, the sample molecules are refocused after the ‘separation by freezing’, and then transferred into the second column via heating. Focusing results in, on the one hand, an advantageous increase in signal intensity. On the other, the resulting ‘particle peaks’ are very narrow, which means that a very fast detector is necessary in order to be able to achieve a sufficient number of data points per peak. The TOF (time of flight) mass spectrometer is ideally suited for this, as it is able to record up to 500 full-scan spectrums (in the specified mass range) per second, and therefore depict peaks whose peak width is only in the two-figure millisecond range.
The result of a GCxGC analysis is a contour plot that depicts the separation of the MOH mixture over the polar and nonpolar columns. The x-axis represents the polar dimension and the y-axis the nonpolar (see fig 1).
Figure 1 shows the isolated MOAH fraction of a lubricating oil (MOAH content around 26%). It was chromatographed using online HPLC-GC-FID and GCXGC-TOF(MS). The complex mixture of aromatic hydrocarbons and internal standards is clearly visible in both chromatograms.
Due to the different chemical properties of the various possible substance classes, structured two-dimensional chromatograms with specific bands were obtained. Due to filtering according to substance-specific masses (m/Q) and comparison with the spectrum database, these bands can then be assiged to various compound classes.