GCAnalysis of PetroleumProducts bySimulated
Distillation, UsingMXT
®
SimDistColumns
TheAmericanSociety forTestingandMaterials (ASTM) has
releasedanewgas chromatographic (GC)method for petroleum
distillation, “BoilingRangeDistributionofPetroleumDistillates
in theBoilingRangeof 174°C to700°CbyGasChromatography
(ASTMD6352).”Thismethodextends theboilingpoint range
specified inASTMD2887 (55°C to538°C).Determining the
boiling rangedistributionofdistillate fractionsprovidesmanufac-
turerswithabetter understandingof thecompositionof feed
stocks andproducts related topetroleum refiningprocesses.
Simulateddistillation (SimDist) is a technique inwhich theGC is
calibratedwithalkane standardsof publishedboilingpoints to
performadirect comparison to theboilingpointsof the sample
components. Polyethylene standards such as Polywax
®
655or
Polywax
®
1000,whichcontaineven-numberedhydrocarbons
betweenC20 andC100, are used to calibrate theGC system.The
retention timesof theknownalkanes arecalibratedversus the
publishedboilingpoints, andanalysisof highmolecularweight
fractions arecomparedusing special software.Analyzinghigh-
boilingcompounds (up to700°C) requiresanon-columnor
temperature-programmable injector tominimize injectordiscrimi-
nation, anda flame ionizationdetector (FID).
Thecapillarycolumnused for theanalysis isvery important.The
recommendedcolumn is coatedwithanon-polarmethyl silicone
phase that canwithstand repeatedcyclesof high temperatures
(430-450°C).However, thepolyimidecoating thatmaintains the
flexibilityof fused silica tubing rapidlybreaksdownat oven
temperaturesabove360°Cand isunsuitable forhigh-temperature
GC.Aluminum-clad tubingovercomes theproblemsassociated
with thepolyimide, but has itsown limitations.When repeatedly
programmed to temperaturesabove400°C, thealuminum sheath
maybecomebrittle.Themost durableand inert capillarycolumns
availableareRestekMXT
®
columns,whicharemanufactured
usingSilcosteel
®
tubing—ruggedmetal tubing that is treated to
provide the inertnessof fused silica.
The liquidphase in thecolumnalsomustwithstand temperatures
over 400°C,withminimumbreakdownof thepolymer.Thephase
must demonstrate lowbleedand retention time stability that is
repeatablewith temperatureprogrammedanalysis.Themethyl
siliconepolymer in theMXT
®
-1SimDist column is synthesized
todemandingqualitycontrol standardsusingaproprietary
synthesisprocess.Thisprocess eliminates residual catalysts that
couldcausedegradationand increasebleed from thepolymer.The
polymer then is carefully fractionated to remove lowmolecular
weight fragments, providinga tightmono-modal distribution that
results inextremely lowbleed.Thepolymer is fullycharacterized
for long-term reproducibilityusing techniques suchasRI, FTIR,
Kovats Indices,%crosslinking, coatingefficiency, anda five-day
thermal bake-out toensurecolumn longevity. Figure1 shows the
separationofPolywax
®
655onanMXT
®
-1SimDist column.
Although theASTMD6352 recommends amethyl silicone liquid
phase, another phasealsohasgainedacceptancebyanalysts
performing simulateddistillation—acarboranedimethyl
polysiloxane, orMXT
®
-500SimDist polymer.Thishasbeen
designedby incorporatingcarborane into thebackboneof the
methyl siliconepolymer chain topromote thermal stability (see
Figure2).TheMXT
®
-500SimDist stationaryphase is themost
stablephaseavailable forhigh-temperature simulateddistillation
analyses andwill outlast a100%methyl silicone stationaryphase.
Figure3 shows the separationofPolywax
®
655on theMXT
®
-500
column.
However, the trade-off for increased thermal stabilityproduces a
subtle polaritydifferencebetween themethyl siliconeand the
carborane stationaryphase.Thedifference inpolarityof the
stationaryphases causes a shift in thecalculatedboiling range
Figure 1
MXT
®
-1SimDist ColumnDemonstratesExcellent Symmetry
andLowBleed forHigh-Temperature SimDist Calibration.
6m, 0.53mm ID, 0.15µmMXT
®
-1 (cat.# 70101). 1.0µL injection of Polywax
®
655 in CS
2
, cool on-column.
Oven temp.:
40°C to 430°C@ 10°C/min.
(hold 10min.);
Det. temp.:
430°C;
FID sensitivity:
5 x 10
-10
AFS;
Carrier gas:
hydrogen;
Linear velocity:
45cm/sec. set @ 40°C.
10
20
30
40
min.
Petrochemical
RestekCorporation
(800) 356-1688
A
pplicationsnote
Lit. cat.# 59551
