The interest in analyzing mineral oils and their components has increased considerably over the past decade. Historically, the methods up until the end of the 1990s5,8,9 were purely focused on quality control of mineral oil products; eg, industry has established very early the standardized method IP 346, which grimetrically determined the residue in a dimethyl sulfoxide (DMSO) extract. This method represents an initial test of the mineral oil industry for mineral oils that are later to be sold as highly refined pharmaceutical-grade mineral oils. Only mineral oil fractions containing less than 3% by weight of aromatics extractable with DMSO will be subjected to further processing preparation steps (eg, refining or hydrogenation). This level has been claimed by industry as a “certain threshold” to distinguish carcinogenic from noncarcinogenic products.10 The threshold was empirically established based on animal testing data with various DMSO extracts. The European Union cosmetics regulation has implemented this method into law and demands that products for cosmetic use comply with the IP 346 limit. Even in the year 2018, the IP 346 method is still the current industry standard to assess carcinogenicity of mineral oil raw materials. In 2015, an interesting solid-phase microextraction method with gas chromatography-mass spectrometry (GC-MS) detection of benzo(a)pyrene in microcrystalline waxes was published. This method complements the nonspecific and time-consuming IP 346 method.11
The European pharmacopoeia has established a UV photometric procedure to more specifically determine carcinogenic constituents, namely, polycyclic aromatic hydrocarbons (PAHs). For this, a certain absorbance threshold has been defined, which may not be exceeded to put the product on the market as ingredient in medicinal products.12 Some methods of higher specificity also exist to quantitatively determine PAH in mineral oil products.13 It is important to differentiate between PAH and MOAH. The PAHs consist of a limited number of polycyclic aromatic hydrocarbons and can be analyzed as individual substances. In contrast, MOAH contain a multitude of compounds, usually highly alkylated (more than 98%).9
Mineral oil compounds may be contained as ingredients in cosmetic products, as well as additives or nonintended contaminants in food. Due to the wide application in technical products including food contact materials such as packaging materials, mineral oil products may migrate into foods, most easily into fat-containing foods such as chocolate or oil-containing foods. The first observations about this contamination led to considerable interest into this issue, increased by warnings from consumer magazines and nongovernmental organizations. The methods, suitable for pure mineral oil products mentioned above, were not able to detect trace contamination of mineral oil products migrated into foods. For this reason, development of new methods had to be conducted, and the first successful approach was reported from the lab of Grob in 1991 using a multidimensional chromatographic technique: on-line coupled liquid chromatography and gas chromatography (LC-GC).14,15
The first methods were designed only for the detection of the MOSH fraction. Often, an off-line solid-phase extraction (SPE) separation was used to isolate MOSH and then a GC-FID system was used for quantification. Biedermann et al16 published in 2009 that the off-line separation of MOSH and MOAH with activated silica gel is not complete. For the complete separation of cholestane (Cho) and tri-tert-butyl benzene (TBB), it is important to use a silver-impregnated silica gel SPE system, as described by Moret et al.17 Detecting the MOAH fraction came into focus later. The first method was published by Moret et al in 1996 and described an LC-solvent evaporation (SE)-LC-GC-FID system for the analysis of edible oil or fatty food extract. On a silica phase using pentane/dichloromethane (90/10), a fraction ranging from saturated hydrocarbons to perylene was collected. Before transferring the solutes into the second LC system, the solvent was removed by concurrent evaporation.18 On the second LC column, an amino-derived silica phase was used. Hydrocarbons were separated on the amino-silica phase according to their number of aromatic rings with pentane as eluent. For quantification, the fractions were transferred into the GC-FID part of the hyphenated system.6
Using this LC-GC-FID procedure, it was not possible to resolve the mineral oils into single components because they typically contain a complex mixture of alkanes and other compounds. Basically, LC-GC-FID only resolves 2 fractions: MOSH and MOAH. The 2 acronyms MOSH and MOAH gained high appreciation in the field and are currently used synonymously to mineral oil hydrocarbon analysis. The approaches later developed, based on GCxGC-MS or nuclear magnetic resonance (NMR) spectroscopy, also stuck with the terms MOSH-MOAH but provided more selective and specific identification.
Over the past decade, the LC-GC-FID method has been further refined and is today referred to as the method of choice or gold standard for detecting mineral oils in routine analysis.14 The large variety of structurally similar single compounds makes it impossible to identify the individual compounds, but it is a common approach to increase the information about a sample, if found positive in LC-GC-FID, using additional analytical methods, eg, GCxGC or, as recently proposed, quantitative NMR spectroscopy.1 At this point, it should be noted that not all positive samples may require confirmation by the GCxGC coupling technique, whether or not it mostly depends on the matrix type. These analytical methods will be discussed in detail below.
As it was pointed out, the methodology applicable for mineral oil analysis is highly dependent on the field of application, so that we will structure the review according to matrix. For each field, we will tabulate the suggested methodologies and provide a critical review of the advantages and limitations.