9
Table III.
Recommended installationdistances.
Figure2.
TheRtx
®
-5SilMS column structure.
Agilent (HP):
5-7mm from tipof ferrule
Varian1075/1077:
5.7cm frombackof nut
PerkinElmerAutosystem:
4.5 - 5.0cm frombackof nut
Shimadzu 14A:
4.0cm frombackof nut
Shimadzu 17A:
35mm from tipof ferrule
split:
40mm frombackof nut
splitless:
64mm frombackof nut
Any injection technique can suffer from reactivity (i.e., breakdown) and splitless injection is
no exception. The splitless techniquehas twoprimarymechanisms for compound reactivity:
sample backflash into the gas lines that enter and exit from the injector; and exposure of the
sample extract to active sites on thewool, liner, and tipof the column. Ingeneral, the same set
of compounds breakdown regardless ofwhichmechanism is occurring: 2,4-dinitrophenol,
PCP, 4-nitrophenol, carbazole, and3-nitroaniline.
Dailymaintenance of the injectionportwill helpdecrease this problem. Replace the inlet liner
and fused silicawool plug, and the septum everyday.Weekly, ormore oftendependingon the
extract contamination level, replace the inlet seal and remove a short section from the front of
the column. The lengthof column removedwill varydependingon the level of contamination
in the extracts, generally6 to12 inches is adequate.When cutting the column and re-installing
it into the injectionport, be sure tomake a square cut andbe consistentwith the installation
distance. The installationdistance varies bymanufacturer. Refer toTable III for a list of
recommended insertiondistances.
ColumnSelection
Due to thewidevariation in functionality, volatility, andpolarityof semivolatile compounds,
it is not possible to select a column that is highly selective for all of them.As a result, this
analysis is performedon a general-purpose stationaryphase. TheRtx
®
-5SilMS columnhas
thebest combinationof lowbleed, high inertness, and efficiency for semivolatile applications.
(TheRtx
®
-5MS column alsohas been successfullyused for analysis of semivolatile com-
pounds.)TheRtx
®
-5SilMS column features a silarylene phenyl/methyl phase thatwas
developed toprovide lower bleed andgreater efficiency thanother “5-type”phases for
improved separationof thePAHs (Figure 2).
Low-bleed columns
are necessary for themore sensitive instruments. For laboratories using
theAgilent 5973GC/MSor ion trapMS, columnbleed canbe a very important issue.As
these instruments havebecomemore sensitive, thehigher-bleed columns produce a larger
signal on thedetector and can
cause electronmultiplier satura-
tion. If this occurs, calibration
curvesmay shownon-linearity at
higher concentrations. This is
sometimes referred to as high-end
roll-off, when the signal for a
given concentration is lower than
expecteddue todetector satura-
tion. (See
Quantitation
section for
more informationonhigh-end
roll-off.)
Todiagnose columnbleedproblems,make an injection that allows thebleed tobemeasured
relative to the concentrationof the analyte in themethod.Manydata systemswill normalize
the display to the largest peak in the analysis. If no compounds are injected, the displaywill
falsely indicate a highbackground. TheRtx
®
-5SilMS column shows aminimal bleed level
for a 20ngper component standard (Figure 3).
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