• Forsomemanufacturers,onlyaportionoftheGCinletisactuallyatthetemperaturesetpoint;asignificantthermalgradientexistsbothaboveandbelowthiszone.

• ThethermalprofileofoneGCinletcanvaryfromothersimilarinlets—andvarydramaticallybetweendifferentstyles.

• RemovalordamagetoGCinsulationcanhavealargeeffectontheinlet’sthermalprofile.

InjectingaliquidsampleintoahotGCinletisadynamicandcomplexevent.Ofthemanyparametersthataffectthesuccessofaninjection,inlettemperatureisoneofthemostsignificant.Raisingorloweringtheinlettemperaturesetpointcanhaveaprofoundeffectonhowmuchsampleistransferredontothecolumndependingonsamplevolatilityandthermalsensitivity.But,oncetheinlettemperatureisset,howmuchoftheinletisactuallykeptatthatsetpoint?Moreover,howmightthermalprofileschangebetweeninlets?

Temperature Varies Within and Between Similar InletsThemotivationforthisworkcamefromaquestionabouttheactualtemperatureofanO-ringinstalledinanAgilentsplit/splitlessinletatagiveninlettemperaturesetpoint.(SeeFigure1toidentifythecomponentsofaGCinlet.)InsteadofjustmeasuringthetemperatureinsidealinerneartheO-ring’slocation,weusedathermocoupletomeasuretemperaturealongtheentirelengthofthelinerataconstantinlettemperaturesetpointof250°C.Theresultingthermalprofileconfirmedthatatemperaturegradientexistswithintheinlet.*

Inpreviouswork(www.restek.com/hotseptum),wealsodiscussedthisgradientwithinGCinletsandnotedthatinletthermalpro-filescanvarygreatlybetweenmanufacturers,butwouldtheyvarybetweensimilarinletsfromthesamemanufacturer?Wecheckedanothersimilarinlettocomparethethermalprofilesandfoundthatthesecondinletexhibitedadifferentthermalprofilefromthefirst.AftermeasuringseveralmoreAgilentGCinlettemperatureprofiles,wefoundinlet-to-inletvariationinallcases,eveninostensiblyidenticalinlets(Figure2).

Technical Article

It’s a Matter of Degrees, but Do Degrees Really Matter?

An Observation of GC Inlet Temperature Profile and Inlet-to-Inlet Temperature Variability

By Scott Grossman

www.restek.comInnovative Chromatography Products

* For these experiments, we only measured the thermal profile of the liner inside the inlet, not the entire inlet.

www.restek.com 2

Figure 1: Considering how little of the GC inlet is actively heated by the heating element, it’s no surprise a tem-perature gradient exists—especially if insulation is missing from the top or bottom.

Figure 2: A temperature gradient exists within a GC inlet, and temperature profiles can vary between similar inlets. These variations increase dramatically with the absence of insulation.

Oven Wall

Open Air

Oven

Column

Reducing Nut

Nut Warmer Cup

Inlet Seal

Thermal Nut

AluminumHeater Block

Heating Element

Heater Sensor

LinerPoint of Injection

O-Ring

Inlet BodyPerforated Disk

Insulation

1.2 cm

www.restek.com3

Insulation is Crucial to Minimizing Temperature VariationWedidobserveonesplit/splitlessinletwithsignificantlylowertemperaturesatthetopandbottom.Afterinvestigating,wediscov-eredthatthetopringofinsulation,whichsitsjustbelowtheperforateddiskoftheAgilent6890split/splitlessinletweldment,wasmissing.Someoftheinsulationatthebottomoftheinlet,alongwiththethermalnut,wasalsonotinstalled.Simplyplacinginsula-tioninthetopcavityandinstallingthethermalnutcausedthetemperatureoftheinletlinertomorecloselymatchtheotherinlets(Figure2).Thistestwasavaluablereminderoftheneedtocarefullyreconstructtheinletwhenevertheinsulationisdisturbed.

Wheninsulationismissingatthetopoftheinlet,theinletlosesheattotheopenair;however,wheninsulationismissingatthebottom,theGCoveninfluencesthetemperatureinbothdirections(Figure3).Becausecolumninstallationcanbemorechallengingwiththeinsulatednutwarmercupinstalled,analystsmaybetemptedtoleaveitinadrawer,buttheeffectonyourinlettemperaturecanbesignificant.

Figure 3: Installing the nut warmer cup can help minimize the effects of oven temperature on the actual tempera-ture of the inlet. (Inlet shown below was set to a constant 250 °C.)

Temperature Can Vary Drastically Between Dissimilar InletsThenewlyintroducedAgilentMultimodeInlet(MMI)issaidtobecapableofperformingbothhotsplitandhotsplitlessinjectionslikeanormalsplit/splitlessinlet.But,whenwemeasuredthethermalprofilesfortwoMMIinlets,itwasinterestingtonotehowdifferenttheMMIthermalprofileswerefromasplit/splitlessinlet—adropofover190°Cfromthesetpointtothetopoftheinletasopposedtoaround100°Cforthesplit/splitlessinlets(Figure4).Thisvariationshowsthatchangingequipmentmayalsochangeyourresults,eveniftheequipmentisnominallyabletodothesameanalysis.

Figure 4: The Multimode Inlets (MMIs) we measured experienced almost twice the temperature drop (190 °C) of a standard split/splitless inlet between the inlet setpoint to the top of the liner.

The Effects of Inlet Temperature Variations on ChromatographyAsdemonstratedhere,thermalgradientsexistwithinasingleGCinlet,andtemperatureprofilescanvarybetweensimilar,aswellasbetweendissimilar,inlets.Thesevariations affect the vaporization of a liquid sample (and, thus, the overall success of the analysis).

Initially published in the Restek® Advantage

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