18
Figure 13.
Plumbing a purge and trap interface to aGC injection port allows flow adjustment via the
GC flow controller. This is themost commonway of analyzing volatile compounds byMS
because the flow can be split, allowing 1mL/min. into theMS source.
GC SystemConfigurations
Wide-bore Systems (0.45mm ID and 0.53mm ID columns)
Wide-bore capillary columns are operated at faster flow rates than narrow-bore columns, and
can be connected directly to the purge and trap systemwith a 10mL/min. desorb flow.Wide-
bore columns used forVOCs analyses usually are coatedwith a thick film of stationary
phase to increase retention and separation of the highly volatile analytes (e.g.,
chloromethane and vinyl chloride, bromomethane and chloroethane), or other closely-eluting
sample components.
Wide-bore columns range from 30-105meters in length.A longer column can refocus early-
eluting volatile compounds and greatly improve separation of the gases (seeApplications,
page 37). Shorter columns require sub-ambient cooling for separating the gases; this increas-
es the cost of the analysis and adds laboratory time associatedwith handling tanks of liquid
nitrogen. For best overall results, we recommend using a 75m, 0.45mm ID capillary column
for analyzing the volatile compounds listed inUSEPAMethods 502.2 and 8021B (see
Applications, page 37).
Resolution of the early-eluting gaseous analytes increases significantlywith a decrease in
temperature. Use a starting temperature of 35°C-50°C, depending on the target list and the
purge and trap conditions (seeApplications, page 37).A longer column can be used to
increase the pressurewithin the column, which, in turn, will increase the solubility of the
analytes in the stationary phase. Using optimized temperature programs and narrower band-
widths, reasonably fast analysis times can be achieved (see page 37). However, the higher
flow rates throughwide-bore columns prevent the analyst from directly connecting the col-
umn to the vacuum system of anMS.A jet separator or open split interfacemust be used to
reduce the amount of carrier gas flowing into theMS (see Figure 31, page 32).
Themethod for connecting a purge and trap transfer line to awide-boreGC column should
be carefully considered. The three connectionmethods are: 1) through the existingGC injec-
tion port; 2) using a low volume injector; and 3) with a direct column connection. These
alternatives are described below.
InjectionPort Connection:
In this connection option, the purge and trap transfer line is con-
nected to theGC injection port that accepts the carrier gas line. The carrier gas line is cut
close to the injection port body and a deactivated union (e.g., cat.# 20510, see our catalog) is
used to connect the purge and trap transfer line to the injection port (Figures 13 through 16).
This allows the analyst tomakemanual injectionswhen troubleshooting, and to inject bro-
mofluorobenzenewhen tuning theMS in accordancewithEPAmethods. The injection port
can be a source of dead volume, however. Dead volume causes band broadening, resulting in
poor peak shape and loss of resolution for themost volatile target compounds. The severity
of the problem is determined by the inside diameter of the injection port liner and the total
desorb flow through the port. To reduce the dead volume in the injection port, use a 1mm ID
split liner (e.g., cat. #20972; see products section). If the injection port is designed for
Line FromP&T
SeptumPurge Line
Split Line
Description
qty.
cat.#
Vu-Tight
®
DI Liner
ea.
20342
Vu-Tight
®
DI Liner
5-pk.
20343
Vu-Tight
®
DI Liner
25-pk.
20344
Vu-Tight
®
Direct Injection Liner
• Visually observe the Press-
Tight
®
connection between
the column end and liner.
• 1/4-InchOD: accepts 0.32
or 0.53mm ID capillary
column (columnOD from
0.5mm to 0.8mm).
• Slotted top prevents obstruc-
tion of carrier gas flow.
• Two designs available.*
• Operate in the direct injectionmode.
• Can easily be packedwithwool for
dirty samples.
* Refer to our catalog for information about
CycloVu-Tight
®
liners, for usewith dirty
samples.
