Figure 2: LC-GC online system
ferring the MOSH fraction on column 1 and MOAH on
column 2, the temperature programme is started and
both fractions are separated simultaneously and detected
by FID. Figure 1 shows a typical LCchromatogram with
UV-signal in black, pump pressure in green, CH2Cl2 concentration
in blue and total flow in purple. Figure 2 shows
the LC-GC-FID system 5.
Depending on the expected mineral oil concentration,
1 - 10 g of homogenized and finely ground samples were
used. The samples were extracted in hexane, after the
addition of an internal standard mixture (Restek MOSH/
MOAH standard Cat.#:31070 containing 9 internal
standards) at room temperature for 2 hours or overnight
under occasionally shaking the flask. The glass columns
with inserted filters were filled with 10 g of aluminium
oxide (90, basic, 0.063 mm - 0.2 mm, activated for 16h
at 500 ºC) and 3 g of silica gel (60, 60 μm - 200 μm
or 70 mesh-230 mesh respectively, activated for 16h at
40 ºC). The columns were washed with 20 ml of n-hexane
prior to loading the sample. The extracts were centrifuged,
if necessary, concentrated to a volume of about
1 mL under a stream of nitrogen.
About 100 μL were taken and filled into an autosampler
vial with micro insert for analysis of MOAH, the remaining
samples was loaded onto the column. The MOSH frac-
Figure 3: Chromatograms of rice sample before and
after flash chromatography with alox
tion was eluted with 20 mL of n-hexane, concentrated in
an automated solvent concentrator and transferred into a
2 mL autosampler vial. This was placed in the autosampler
rack of the LC-GC system. Aliquots of 50 μl were injected
into the LC and 450 μL were transferred directly on the
pre-columns for the MOSH fraction only.
Mineral oil in rice – analytical result
Figure 3 shows two chromatograms of a MOSH fraction of
a rice sample spiked with 5 mg/kg of a mineral oil. Before
flash chromatography with alox n-alkanes in the range of
C25 - C35 are present. “False positive” integration would
give a MOSH value of 10.8 mg/kg.
After flash chromatography the n-alkanes were completely
removed and data evaluation is possible without any
interferences. The result for the MOSH value is 4.31 mg/kg
this time. The MOAH value that was measured prior to
flash chromatography gives a concentration of 0.64 mg/kg
and results in a total MOH recovery of 4.95 mg/kg of the
spiked 5 mg/kg.
Flash chromatography with aluminium oxide and silica gel
gives an important tool for the removal of naturally occurring
odd-numbered n-alkanes and allows an interference
free analysis of complex food material such as rice or chocolate.
1 EFSA Panel on Contaminants in the Food Chain (CONTAM)
Scientific Opinion on Mineral Oil Hydrocarbons in
Food DOI: 10.2903/j.efsa.2012.2704
2 K. Fiselier, D. Fiorini, K. Grob, Activated aluminum oxide
selectively retaining long chain n-alkanes. Part I, description
of the retention properties, Analytica Chimica Acta.
634 (2008) 96-101. doi:10.1016/j.aca.2008.12.007.
3 K. Fiselier, D. Fiorini, K. Grob, Activated aluminum
oxide selectively retaining long chain n-alkanes: Part
II. Integration into an on-line high performance liquid
chromatography–liquid chromatography–gas chromatography–
flame ionization detection method to
remove plant paraffins for the determination of mineral
paraffins in foods and environmental samples, Analytica
Chimica Acta. 634 (2008) 102-109. doi:10.1016/j.
4 Biedermann, M.; Grob, K. On-line coupled high
performance liquid chromatography–gas chromatography
for the analysis of contamination by mineral
oil. Part 2: Migration from paperboard into dry foods:
Interpretation of chromatograms. Journal of Chromatography
A 2011, 1255, 76-99, doi:10.1016/j.
5 Biedermann, M.; Grob, K. On-line coupled high performance
liquid chromatography–gas chromatography
for the analysis of contamination by mineral oil. Part 1:
Method of analysis. Journal of Chromatography A 2011,
1255, 56-75, doi:10.1016/j.chroma.2012.05.095.