Detecting Polycylic Aromatic Hydrocarbons in Food

Arylene- and phenyl-substituted liquid-phase columns provide a reasonable degree of selective PAH separation. The most popular PAH column choices are the non-polar 5% phenyl/arylene polysiloxane (DB-5ms, Rtx-5ms, VF-5ms, ZB-5ms) and mid-polar 50% phenyl/arylene polysiloxane phase columns (DB-17ms, Rtx-17ms, VF-17ms). However, both liquid phases will suffer from inaccurate quantification of key target EPA and EU PAHs due to co-eluting interferences such as benzo[j]fluoranthene and triphenylene.

Figure 1 (above) illustrates the separation and quantification difficulties for important priority EPA and EU PAHs. Chrysene cannot be measured accurately on non-polar or mid-polar columns because triphenylene co-elution may create biased results. In addition, benzo[b]fluoranthene cannot be quantified accurately on 5% phenyl/arylene columns due to co-elution with the benzo[j]fluoranthene isomer. Co-elution also occurs for the triplet indeno[1,2,3-cd]pyrene/benzo[b]triphenylene/dibenz[a,h]anthracene group on these 5% phenyl/arylene columns.

A new, selective GC column dedicated for PAH analysis, Varian Select PAH, was recently introduced that has the unique ability to isolate chrysene from the interfering triphenylene while simultaneously separating the three benzo[b,k,j]fluoranthene isomers. The liquid phase of this column incorporates highly selective selectors for PAH-isomer separation to overcome the limitations of other GC columns. The selective column can also separate other critical peak triplets such as indeno[1,2,3-cd]pyrene, benzo[b]triphenylene, and dibenz[a,h]anthracene (see Figure 2, below).

The improved separation characteristics of this column provide a more precise characterization of PAHs in various food matrices such as smoked haddock and salmon (see Figures 3-4, below and p. 42). Salmon was spiked with a mixture of EPA- and EU-regulated PAHs, as well as triphenylene as an important interference. The PAH concentration range varied from <0.5 parts per billion (ppb) up to 10 ppb for benzo[a]pyrene. For sample saponification, a potassium hydroxide in methanol solution was added to the homogenized and weighted salmon. After saponification, the PAHs were extracted with cyclohexane. The extract was concentrated and further cleaned using fully automated gel permeation chromatography directly coupled to an evaporation unit in the same analytical system. Dichloromethane was used as the eluent. The fraction containing the PAHs was concentrated and analyzed by GC/MS.

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Figure 4. Benz[a]anthrancene (BaA), cyclopenta[c,d]pyrene (CCP), chrysene (Chr), and interfering triphenylene (Tr), m/z 226 (blue), m/z 228 (red).

Fast, detailed analysis of PAH was obtained on a 15 m x 0.15 mm x 0.10 µm Select PAH column, with dibenzo[a,h]pyrene, the last PAH of interest, eluting at 28 min (see Figure 3, p. 40).

Chrysene and triphenylene were sufficiently separated to allow accurate chrysene quantification (see Figure 4, p. 42). Bias in chrysene quantification due to triphenylene interference has not been studied in detail in food matrices, mainly due their co-elution on most GC columns.

The higher molecular weight toxic dibenzopyrenes are usually less prevalent at low concentration levels. Dibenzopyrenes are prone to discrimination effects in the injector, and care must be taken to ensure complete evaporation to obtain higher responses. Their low volatility may also create adsorption effects in the MS interface and ion source.

Higher MS interface and ion-source temperatures can limit these phenomena, increasing responses and reducing peak tailing for these high molecular weight PAHs. Further significant enhancement of the analytical sensitivity for dibenzopyrenes, improvement of signal-to-noise (S/N) ratios, and faster elution can be achieved using thinner film columns of 0.1 – 0.15 µm as applied to the Select PAH column. The low-bleed performance and good S/N ratio of the Select PAH column at 350°C are apparent.

The presence of PAHs in vegetable oils is mostly related to contact with combustion gases from the seed drying processes. Refining methods such as deodorization and treatment with activated charcoal can reduce the PAH level significantly, but residues may remain.

Kuipers and Oostdijk are with Varian B.V., now part of Agilent Technologies, Middelburg, Netherlands. Schulz and Ruthenschroer are with Eurofins WEJ Contaminants GmbH, Hamburg, Germany. For more information, go to www.agilent.com or www.varianinc.com.