F
F—C—F
H—C—H
H—C—H
Si—O
H—C—H
H
Polarity:
selective for lone
pair electrons.
Uses:
environmental samples,
solvents, Freon
®
samples,
drugs, ketones and
alcohols.
environmental
Applications
note
Restek Corporation • (800) 356-1688 • (814) 353-1300 •
#59190
Separating
m-
and
p-
Xylene IsomersbyUSEPAMethod 8260Using an
Rtx
®
-200GCColumn
Xylenes are aromatic hydrocarbons that naturally occur
in petroleum and coal tar; they also can be commercially
derived from these substrates. Three xylene isomers
exist:
meta
-,
para
-, and
ortho
-xylene, usually referred to
as
m
-,
p
-, and
o
-xylene, respectively.Mixed xylenes pro-
duced from petroleum contain 20%
o
- and
p
-xylene, with
44%
m
-xylene.
1
The isomers of
m
- and
p
-xylene are dif-
ficult to resolve using gas chromatography (GC) and
most capillary columns.Although limited data exists
suggesting toxicological differences between
m
- and
p
-
xylene, there is still interest in resolving them.
TheUSEnvironmental ProtectionAgency (EPA) does
not require separation of the xylene isomers, but rather
requests their calculation as totals or sums.
2
Some states
such asNewYork have action limits based on
m
- and
p
-
xylene separately. For example the action limit in drink-
ingwater is 5µg/L for
m
-xylene and 5µg/L for
p
-xylene.
Adrinkingwater sample that has 9µg/Lof total xylenes
could, in fact, exceed the limits by having 9µg/Lof
m
-
xylene and no
p
-xylene present in the sample.
3
However,
other states and agencieswould consider the action limit
for these two isomers as 10µg/L total.
Arecent performance evaluation fromNewYork state
contained one of the isomers of xylene, which required
the contracted environmental laboratories to determine
actual concentrations of
m
- and
p
-xylene separately. The
most commonway to perform aGC separation of
m
- and
p
-xylene is by using a polyethylene glycol (PEG) station-
ary phase, such as theRestek Stabilwax
®
column.
Chromatographically, baseline separation is possible;
however, bleed levels are unacceptable for amass spec-
trometer (MS) and samplematrices containing organic
acids can contribute to bleed from the stationary phase.
Themore ideal column choice for this particular separa-
tion is theRtx
®
-200 column. TheCrossbond
®
trifluoro-
propylmethyl polysiloxane stationary phase (
Figure 1
)
features exceptionally low bleed at common volatile
applicationworking temperatures because itsmaximum
operating temperature is 360° C.
TheRtx
®
-200 column provides unique separation of
volatile organic compounds (VOCs) listed inUSEPA
Methods 524 and 8260 (
Figure 2
), making it the best
column to separate xylene isomers for specific state
requirements (
Figure 3
). TheRtx
®
-200 column’s only
limitation is the resolution of the gases—peaks 2, 3, 5,
and 6 (
Figure 2
).
This column also is a good choice for analyzing samples
having complexmatrices; analyseswhere coelutions of
several compounds canmake identification of tentitively
identified compounds (TICs) nearly impossible on a
“624” phase. Trifluoropropyl stationary phases like that
of theRtx
®
-200 column have a unique selectivity because
of the electrophilic nature of the fluorine-containing
polymer. This creates interactionswith electron-rich
molecules like ketones and halogenated compounds. This
unique selectivity results in different elution orders and
resolves compounds that phenyl, cyano andmethyl phas-
es cannot. In this analysis, theRtx
®
-200 column can be
used to confirmTICs and resolvemultiple coelutions.
References
1. Toxicological Profile for Total Xylenes. Prepared byClements
Associates, Inc.. under Contract No. 205-88-0608. Prepared for
Agency for Toxic Substances andDisease Registry, USPublicHealth
Services, Atlanta, GA. December 1990.
2. USEPAMethod 8000B, Rev. 2. DeterminativeChromatographic
Separations. Page 7 Section 3.3.3. Washington, DC. December 1996.
3. Consumer Confidence Report. NewYorkWater ServiceCorporation.
60BrooklynAvenueMerrick, NewYork, NY. September 1999.
Figure 1—PhaseStructure
Rtx
®
/MXT
®
-200 trifluoropropylmethyl polysiloxane
——————
