Optimizing the Analysis of Volatile Organic Compounds

18
Vu-Tight ® Direct Injection Liner
Visually observe the Press-
Tight ® connection between
the column end and liner.
1/4-Inch OD: accepts 0.32
or 0.53mm ID capillary
column (column OD from
0.5mm to 0.8mm).
Slotted top prevents obstruction
of carrier gas flow.
Two designs available.*
Operate in the direct injection mode.
Can easily be packed with wool for
dirty samples.
Description qty. cat.#
Vu-Tight ® DI Liner ea. 20342
Vu-Tight ® DI Liner 5-pk. 20343
Vu-Tight ® DI Liner 25-pk. 20344
* Refer to our catalog for information about
Cyclo Vu-Tight ® liners, for use with dirty
samples.
www.restekcorp.com
GC System Configurations
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 system with a 10mL/min. desorb flow. Widebore
columns used for VOCs analyses usually are coated with 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 30-105 meters in length. A longer column can refocus earlyeluting
volatile compounds and greatly improve separation of the gases (see Applications,
page 37). Shorter columns require sub-ambient cooling for separating the gases; this increases
the cost of the analysis and adds laboratory time associated with 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 in US EPA Methods 502.2 and 8021B (see
Applications, page 37).
Resolution of the early-eluting gaseous analytes increases significantly with a decrease in
temperature. Use a starting temperature of 35°C-50°C, depending on the target list and the
purge and trap conditions (see Applications, page 37). A longer column can be used to
increase the pressure within the column, which, in turn, will increase the solubility of the
analytes in the stationary phase. Using optimized temperature programs and narrower bandwidths,
reasonably fast analysis times can be achieved (see page 37). However, the higher
flow rates through wide-bore columns prevent the analyst from directly connecting the column
to the vacuum system of an MS. A jet separator or open split interface must be used to
reduce the amount of carrier gas flowing into the MS (see Figure 31, page 32).
The method for connecting a purge and trap transfer line to a wide-bore GC column should
be carefully considered. The three connection methods are: 1) through the existing GC injection
port; 2) using a low volume injector; and 3) with a direct column connection. These
alternatives are described below.
Injection Port Connection: In this connection option, the purge and trap transfer line is connected
to the GC 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 to make manual injections when troubleshooting, and to inject bromofluorobenzene
when tuning the MS in accordance with EPA methods. 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 the most 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
Figure 13.
Plumbing a purge and trap interface to a GC injection port allows flow adjustment via the
GC flow controller. This is the most common way of analyzing volatile compounds by MS
because the flow can be split, allowing 1mL/min. into the MS source.
Septum Purge Line
Line From P&T
Split Line