21
Narrow-bore Systems (0.18mm ID - 0.32mm ID columns)
Narrow-bore columns (0.18mm ID-0.32mm ID) offer higher resolution, compared to
0.45mm ID or 0.53mm ID columns. Because these columns typically are operated at lower
flow rates, they are not compatiblewith the fast desorb flow rates from common purge and
trap systems. Splitting the sample at the injection port or cryofocusing (i.e., secondary trap-
ping) will provide compatibility and help focus the sample at the column inlet.
Splitting the Sample:
Many environmental laboratories analyzingVOCs byGC/MS use nar-
row-bore capillary columns and split the sample at the injection port. Higher sensitivity ion
trapGC/MS systems (e.g.,Varian Saturn 2000™ andThermo FinniganGCQplus™ sys-
tems)
7
and recently developed quadrapoleMS systems (e.g., theAgilent 5973 system) allow
high split ratios in the injection port whilemaintaining sensitivity adequate tomeet the
requirements of EPAMethod 524.2.
8
Older quadrapoleGC/MS systems require an increase
in purge volume (25mL) to compensate for the sample lost due to splitting.
Using a standard split/splitless injection port to split the desorb flow allows a higher desorb
flow ratewhilemaintaining a lower column flow.With this technique, the trap is desorbed
at a flow rate of 10-60mL/min. and the column flow rate is adjusted to 1.0-1.3mL/min.,
which is compatiblewith the vacuum system of anMS. The remaining flow exits through
the split vent. The faster desorb flow rate produces a narrow sample bandwidthwhich, when
combinedwith the high efficiency of a narrow-bore column, allows high split ratioswithout
significant loss in sensitivity.
8
Surprisingly, a 1:20 split ratio providesmore sensitivity than a
1:10 split ratio, because the higher flow from the trap focuses the target compoundsmore
efficiently.
Cryofocusing (secondary trapping):
A cryofocusing unit refocuses the volatile compounds
at the inlet of the narrow-bore column. This allows the trap to be desorbed at only 1-
2mL/min., while improving peak shape and resolution by reducing sample bandwidth.
Cryofocusing takes place on a short length of deactivated, uncoated, fused silica tubing that
is cooled to -160°C using liquid nitrogen. To increase retention for very volatile gases, or
when analyzing Freon
®
compounds, use tubing coatedwith a thick film of stationary phase.
While cryofocusing greatly improves peak shapes from narrow-bore columns, the approach
consumes large amounts of liquid nitrogen, increasing operating expenses and requiring liq-
uid nitrogen tanks in the lab. If the liquid nitrogen tank empties in themiddle of a sample
sequence, there can be significant downtime before the tank is replaced.
CapillaryColumnPhases
Many capillary columns have been designed for the analysis ofVOCs. Column selection
normally is based on the analytical method (e.g., USEPAmethod), compound list, and
detection system used. This section serves as an overview of the different phases used for
VOC analyses. See theApplications section (page 37) for examples of GC andGC/MS sep-
arations under specific conditions.
The first columns used for analyzing volatileswere based on diphenyl/dimethyl polysilox-
ane stationary phases. These includeVOCOL
®
, Rtx
®
-Volatiles, HP
®
-VOC, andRtx
®
-502.2
columns. Themain advantages of these phases are their resistance to oxidative breakdown
and their lower bleed, compared to cyanopropylphenyl polysiloxane (i.e., “624”) phases.
Themajor drawback of diphenyl/dimethyl polysiloxane phases is the incomplete resolution
of bromomethane and chloroethane.Many environmental laboratories still use these
columns, especiallywhen analyzing samples for a limited set of compounds.
7. JessieCrockett Butler, MeredithConoley, "Analysis of VolatileOrganics in SolidWastes, Soils, and
WaterUsing a Split Injection and the PolarisQ IonTrapGC/MS."ApplicationNoteAN9167.
ThermoFinnigan, GC andGC/MSDivision, Austin, TX.
8. D.R. Decker, J.J. Harland, andM.J. Feeney, "Comparison of Detection Limits andAnalysis Time
UsingWide andNarrowBoreCapillaryColumns for Purge-and-TrapGC/MSAnalyses."OI
Analytical. ApplicationNote 02850896.
