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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.
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
Table 2014.08F. Dilution scheme for preparation of the
calibration standard solutions
Calibration
level
Vol. of mixed
stock standard
solution, µL
Vol. of working
PAH solution
a
,
µL
Vol. of working
PAH
solution B, µL
Vol. of
13
C-PAH
1 µg/mL solution,
µL
Final
vol.
a
, µL
1
—
—
50
50
1000
2
—
—
100
50
1000
3
—
—
200
50
1000
4
—
—
500
50
1000
5
—
100
—
50
1000
6
—
200
—
50
1000
7
—
500
—
50
1000
8
100
—
—
50
1000
a
Bring to volume using isooctane.
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 separation for benzo[
b
]fluoranthene,
benzo[
j
]fluoranthene, and benzo[
k
]fluoranthene (concentration
ratio of 1:1:1).
Note
: Criteria for separation of chrysene and
triphenylene (another PAH critical pair) were not set for the
collaborative study. For accurate quantitation of chrysene, at least
50% valley separation is recommended, which can be achieved
using selective stationary phases.
The maximum oven temperature program may not exceed the
maximum temperature limit for a given column. Backbone-modified
columns, such as Rxi-17Sil MS or DB-17MS, are recommended
for their better temperature stability and also good selectivity for
critical PAH pairs or groups, including the anthracene/phenanthrene
pair or benzofluoranthenes. Conduct proper inlet and column
maintenance to ensure adequate operation of the GC instrument.
Perform system checks.
(
b
)
MS conditions.
—AnyGC/MS instrument (single quadrupole,
triple quadrupole, TOF, or ion trap) with EI may be used as long as it
provides results meeting the laboratory qualification requirements.
The 10 study participants used the following instruments: single
quadrupole (Agilent 5973–Laboratory 4; Agilent 5975B XL Inert–
Laboratories 3 and 8–10; Agilent 5975C–Laboratories 6 and 7),
triple quadrupole (Agilent 7000B–Laboratory 5; Thermo TSQ–
Laboratory 1), and time-of-flight (Leco Pegasus 4D–Laboratory 2).
Pay special attention to the optimization of the MS transfer line
and MS source temperature. Higher MS source temperatures
Figure 2014.08B. An example of elution profiles of PAHs on a silica gel SPE cartridge and
determination of the optimum elution volume.
Optimum elution volume
Figure 2014.08B. An example of elution profiles of PAHs
on a silica gel SPE cartridge and determination of the
optimum elution volume.
Candidates for 2016 Method of the Year
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