The inset to the center in Figure 1 shows a portion
of the total ion chromatogramwith 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 andMTBE arewell
resolved using the 35°C initial temperature. The col-
umn elutes themethyl-naphthalenes in less than 23
minutes, with a cycle time of 30minutes. Using aver-
age response factors calculated from the calibration
curve, we determined that oxygenate recoveries
were better than 90%.
4
This investigation established that anRtx
®
-VMS col-
umn resolves oxygenates from potentially interfering
gasoline components andMethod 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/complexmatrixes. AnRtx
®
-VMS col-
umn is the clear choice for themost demanding
volatile organics analysis.
• 4 •
RESTEKAdvantage
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
CaliforniaOxygenatesMix
InP&Tmethanol, 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
®
-VMSCapillary GCColumn
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 EPAMethod 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-
poundsmost 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-
facewater with these and other gasoline components
is amajor concern. Identifying and quantifying the
oxygenates from among the highly concentrated
hydrocarbons in a gasoline/watermatrix is a chal-
lenging task. Some compounds (e.g., MTBE and
TBA) coelute onmany capillary GC column stationary
phases and share ions used for identification byMS.
Our investigations, and others, show that US EPA
Method 8260, a purge and trap / capillary GC /mass
spectrometrymethod, is themost reliablemethod
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 beenwritten into any US EPA
Method, with the exception of MTBE inMethod
524.2. The ethers can be concentrated by purge and
trap, but this approach has not been validated in any
SW-846method. Methanol and ethanol are poorly
suited to analysis by purge and trap techniques. In
Method 8015, a flame ionization detector (FID) is
used tomatch a known pattern of gasolinewith an
unknown sample containing peaks that fall within
the gasoline pattern range. Thismethod 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
formonitoring oxygenates, because the PID is very
sensitive to double bonds, but ismuch 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
compoundwas identified as 2-methyl-1-pentene.
2
Despite this problem, many state GROmethods use
PID for the analysis of MTBE.
We evaluated the performance of four stationary
phases for recovery of oxygenates, verifying passing
criteria usingmodified EPAMethod 5030B and
Method 8260.
3
Non-oxygenated gasoline samples
were spikedwith 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 inMethod
5030Bweremodified 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 andMTBE).
The instrument was calibrated using a 5-point curve.
We calculated response factors (RFs)& relative
standard deviations (RSDs) for the target com-
pounds inMethod 8260, then added all of the target
compounds and the correct Method 8260 internal
and surrogate standards to our calibrationmix (84
additional target compounds), to ensure therewere
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 filmRtx
®
-VMS
column proved best for identifying and quantifying
oxygenates in a gasoline/watermix.
Figure 1 shows an analysis of a 1ppm non-oxygenat-
ed gasoline standard inwater, spikedwith 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 CaliforniaGroundwater 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 NewCapillary Column
Stationary Phase
, Pittsburgh Conference 2001, Session 199
(poster).
3. U.S. Environmental Protection Agency,
VolatileOrganic
Compounds byGas Chromatography/Mass Spectroscopy
(GC/MS): Capillary Column TechniqueMethod 8260
. July
1992Revision 0, US EPAOffice of SolidWaste. Washington, D.C.
4. C.M. English, F.L. Dorman, G.B. Stidsen,
The Analysis of
Gasoline Oxygenates by EPAMethod 8260B
Pittsburgh
Conference 2003, Session 590-6P (poster).
Formore details of thiswork,
see reference 4.
8260BMegaMix
™
CalibrationMix
(76 + 1 components)
*2-chloroethyl vinyl ether provided in a separate ampul.
2,000µg/mL each inP&Tmethanol, 1mL/ampul*
Each
5-pk.
10-pk.
30475
30475-510
—
w/data pack
30475-500
30475-520
30575
