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The inset to the center in Figure 1 shows a portion
of the total ion chromatogram with the extracted ion
chromatogram for the oxygenates to scale. The inset
to the center is an enlargement of the extracted ion
chromatogram for the oxygenates; the clean peaks
indicate that there is no interference from non-target
gasoline fragmentation ions. TBA and MTBE are well
resolved using the 35°C initial temperature. The col-
umn elutes the methyl-naphthalenes in less than 23
minutes, with a cycle time of 30 minutes. Using aver-
age response factors calculated from the calibration
curve, we determined that oxygenate recoveries
were better than 90%.4
This investigation established that an Rtx®-VMS col-
umn resolves oxygenates from potentially interfering
gasoline components and Method 8260 target com-
pounds. It is well suited to resolving the expanding
Method 8260 target compound list, and can be used
to identify low levels of analytes in
contaminated/complex matrixes. An Rtx ®-VMS col-
umn is the clear choice for the most demanding
volatile organics analysis.
• 4 •
RESTEK Advantage
800-356-1688
Ordering Information | Rtx®-VMS (Fused Silica)
ID df (µm) temp. limits 30-Meter 60-Meter 75-Meter
0.25mm 1.40 -40 to 240/260°C 19915 19916
0.32mm 1.80 -40 to 240/260°C 19919 19920
0.45mm 2.55 -40 to 240/260°C 19908 19909
0.53mm 3.00 -40 to 240/260°C 19985 19988 19974
ID df (µm) temp. limits 20-Meter 40-Meter
0.18mm 1.00 -40 to 240/260°C 49914 49915
diisopropyl ether 2,000µg/mL
ethyl- tert-butyl ether 2,000
tert-amyl methyl ether 2,000
tert-butyl alcohol 10,000
methyl tert-butyl ether 2,000
California Oxygenates Mix
In P&T methanol, 1mL/ampul
Each 5-pk. 10-pk.
30465 30465-510 —
w/data pack
30465-500 30465-520 30565
Resolve Trace Oxygenates from
a Gasoline/Water Composite
Using an Rtx®-VMS Capillary GC Column
By Christopher English, Environmental Innovations Chemist
✔ High accuracy—oxygenate recoveries better than 90%.
✔ Resolve oxygenates from potentially interfering gasoline components
and volatile target compounds, by US EPA Method 8260.
✔ High speed—30-minute cycle time.
With the elimination of lead from gasolines, oxygen-
containing compounds have become important per-
formance-enhancing components. Oxygenated com-
pounds most commonly added to gasoline are
methanol, ethanol, tert-butanol (TBA), methyl tert-
butyl ether (MTBE), diisopropylether (DIPE), and
ethyl-tert-butylether (ETBE). Of these, MTBE is the
primary additive. Contamination of ground and sur-
face water with these and other gasoline components
is a major concern. Identifying and quantifying the
oxygenates from among the highly concentrated
hydrocarbons in a gasoline/water matrix is a chal-
lenging task. Some compounds (e.g., MTBE and
TBA) coelute on many capillary GC column stationary
phases and share ions used for identification by MS.
Our investigations, and others, show that US EPA
Method 8260, a purge and trap / capillary GC / mass
spectrometry method, is the most reliable method
for detecting oxygenated components in complex
gasoline/water samples, regardless of the concentra-
tion of the gasoline.1 In the United States, the oxy-
genates have not been written into any US EPA
Method, with the exception of MTBE in Method
524.2. The ethers can be concentrated by purge and
trap, but this approach has not been validated in any
SW-846 method. Methanol and ethanol are poorly
suited to analysis by purge and trap techniques. In
Method 8015, a flame ionization detector (FID) is
used to match a known pattern of gasoline with an
unknown sample containing peaks that fall within
the gasoline pattern range. This method can be used
to identify oxygenates by retention time, but the high
probability of misidentifications dictates confirma-
tion on a second column. Method 8021 is specifical-
ly for analysis of aromatic and halogenated volatiles,
with detection by photoionization detector (PID).
This is the least desirable of the potential methods
for monitoring oxygenates, because the PID is very
sensitive to double bonds, but is much less sensitive
to oxygenates. Our analysis of a gasoline composite
standard, for example, produced a false positive for
diisopropyl ether. Using GC/MS for confirmation, the
compound was identified as 2-methyl-1-pentene.2
Despite this problem, many state GRO methods use
PID for the analysis of MTBE.
We evaluated the performance of four stationary
phases for recovery of oxygenates, verifying passing
criteria using modified EPA Method 5030B and
Method 8260.3 Non-oxygenated gasoline samples
were spiked with low (ppb) levels of oxygenates to
determine if operating conditions were appropriate
for separating and detecting the target compounds in
the presence of high concentrations of gasoline
hydrocarbons. Purge and trap conditions in Method
5030B were modified for concentrating the oxy-
genates: we replaced the standard ambient purge
with a 40°C purge. When possible, GC oven condi-
tions were optimized for each stationary phase, to
overcome coelutions of analytes that share ions
(e.g., TBA and MTBE).
The instrument was calibrated using a 5-point curve.
We calculated response factors (RFs) & relative
standard deviations (RSDs) for the target com-
pounds in Method 8260, then added all of the target
compounds and the correct Method 8260 internal
and surrogate standards to our calibration mix (84
additional target compounds), to ensure there were
no coelutions of 8260 target compounds with the
oxygenates. Of the columns used in this investiga-
tion, a 30-meter, 0.25mm ID, 1.4µm film Rtx®-VMS
column proved best for identifying and quantifying
oxygenates in a gasoline/water mix.
Figure 1 shows an analysis of a 1ppm non-oxygenat-
ed gasoline standard in water, spiked with 5ppb of
each of the oxygenates, and illustrates the value of
the Rtx®-VMS column in identifying and quantifying
oxygenates in high levels of gasoline hydrocarbons.
References
1. Happel, A.M., E.H. Beckenbach, R.U. Halden, An Evaluation of
MTBE Impacts to California Groundwater Resources
Lawrence Livermore National Laboratory, UCRL-AR-130897
(1988).
2. C. English, C. Cox, F. Dorman, D. Patwardhan, The Analysis of
Gasoline Oxygenates Using a New Capillary Column
Stationary Phase, Pittsburgh Conference 2001, Session 199
(poster).
3. U.S. Environmental Protection Agency, Volatile Organic
Compounds by Gas Chromatography/Mass Spectroscopy
(GC/MS): Capillary Column Technique Method 8260. July
1992 Revision 0, US EPA Office of Solid Waste. Washington, D.C.
4. C.M. English, F.L. Dorman, G.B. Stidsen, The Analysis of
Gasoline Oxygenates by EPA Method 8260B Pittsburgh
Conference 2003, Session 590-6P (poster).
For more details of this work,
see reference 4.
8260B MegaMix ™ Calibration Mix
(76 + 1 components)
*2-chloroethyl vinyl ether provided in a separate ampul.
2,000µg/mL each in P&T methanol, 1mL/ampul*
Each 5-pk. 10-pk.
30475 30475-510 —
w/data pack
30475-500 30475-520 30575