2. AOACRIChemContMethods-2018Awards

488 M astovska et al . : J ournal of aoaC I nternatIonal v ol . 98, n o . 2, 2015

collecting 0.5 mL elution fractions in 20 evaporation tubes or flasks. Add 0.5 mL isooctane to each elution fraction and evaporate down to 0.5 mL using the optimized evaporation conditions. Analyze each fraction by GC/MS. ( c ) Determine PAH elution profile by plotting analyte response (peak area or height) in a given fraction normalized to the sum of analyte responses in all tested fractions vs the elution volume. See Figure 2014.08B for an example of a PAH elution profile. It is recommended to add an additional 0.5 mL on top of the determined elution fraction (corresponding to 100% recovery) as a safety margin ensuring good analyte recoveries in routine practice. This would result in the optimum elution volume of 7 mL for the silica cartridge tested in Figure 2014.08B . Table 2014.08E. Analyte concentrations in the mixed stock standard solution Analyte Concentration, µg/mL Anthracene 10 Benz[ a ]anthracene 10 Benzo[ a ]pyrene 10 Benzo[ b ]fluoranthene 10 Benzo[ g,h,i ]perylene 10 Benzo[ k ]fluoranthene 10 Chrysene 25 Dibenz[ a,h ]anthracene 10 Fluoranthene 25 Fluorene 10 Indeno[1,2,3- cd ]pyrene 10 Naphthalene 50 Phenanthrene 25 Pyrene 25 1-Methylnaphthalene 25 2,6-Dimethylnaphthalene 25 1-Methylphenanthrene 25 1,7-Dimethylphenanthrene 10 3-Methylchrysene 25

Optimum elution volume

( d ) To check the effectiveness of fat removal, dissolve 100mg pure fish oil (or any suitable fat) in 1 mL hexane and apply it onto the silica gel cartridge, which was conditioned with 6 mL hexane–dichloromethane (3 + 1, v/v) and 4 mL hexane. Elute with the optimum elution volume of hexane–dichloromethane (3 + 1, v/v), which was determined in the previous step (e.g., 7 mL for the example in Figure 2014.08B ). Collect this fraction in an evaporation tube or flask, which empty weight (after heating in an oven to remove moisture) was recorded to 4 decimal places using an analytical balance. Elute the cartridge with additional 3 × 1 mL hexane–dichloromethane (3 + 1, v/v) and collect in three evaporation tubes/flasks of known empty weight. ( e ) Evaporate the four elution fractions to dryness and gravimetrically determine the amount of fat eluting in each fraction by subtracting the empty weights from newly recorded weights after solvent evaporation. There should be no fat eluting in the optimum elution fraction for PAHs (this can also be observed visually in the tubes). ( f ) If there is fat coeluting with PAHs, then the PAH and fat elution profiles have to be reexamined to determine optimum elution volume for PAH and fat separation (potentially sacrificing up to 5% of late-eluting PAH amounts if necessary) or a different silica gel cartridge has to be used. G. GC/MS Analysis ( a ) GC conditions. —Table 2014.08G provides GC conditions that were used by the collaborative study participants. Other conditions (e.g., column, temperature and flow program, and injection technique and volume) can be used as long as the laboratory qualification criteria for separation, sensitivity, and linearity are met. The injection temperature or program needs to be optimized to enable quantitative transfer of less volatile PAHs. If programmable temperature vaporizer (PTV) solvent vent mode is used, solvent venting parameters (temperature, time, flow, pressure) need to be carefully optimized to prevent losses of the volatile PAHs, especially naphthalene. The separation criteria (demonstrated in Figure 2014.08C ) include ( 1 ) a baseline separation of benzo[ a ]pyrene and benzo[ e ]pyrene (concentration ratio of 1:5), ( 2 ) at least 50% valley separation of anthracene and phenanthrene (concentration ratio 1:2.5; evaluated for the anthracene peak), and ( 3 ) at least 50% valley Figure 2014.08B. An example of elution profiles of PAHs on a silica gel SPE cartridge and Figure 2014.08B. An example of elution profiles of PAHs on a silica gel SPE cartridge and determination of the optimum elution volume. determination of the optimum elution volume.

Table 2014.08F. Dilution scheme for preparation of the calibration standard solutions

Vol. of  working PAH solution B, µL

Vol. of  13 C-PAH 1 µg/mL solution, µL

Vol. of mixed  stock standard solution, µL

Vol. of working  PAH solution a , µL

Calibration level

Final vol. a , µL

1

—

—

50

50

1000

—

—

100

1000

2

50

—

—

200

1000

3

50

—

—

500

1000

4

50

—

100

—

1000

5

50

—

200

—

1000

6

50

—

500

—

1000

7

50

8

100

—

—

50

1000

a   Bring to volume using isooctane.