48 Organic compounds in soils, sediments & sludges
volatile pollutants using capillary columns by the United States Environmental Protection Agency [298]. Analytes using purge and trap gas chromatography in other
application areas have long accepted capillary methods. Unfortunately, unique problems are encountered when using a concentrator with a capillary column instead of a
packed column. Samples from a concentrator are injected by thermal desorption of an
adsorbent trap The process requires a length of time and minimum flow column that
produce an injection profile of a rather broad nature. Packed columns, due to their
comparatively high capacity, can effectively cold trap the sample at the head of the
column. Solutes are eluted during temperature programmed runs with the same column efficiency as a normal syringe injection. Due to their reduced capacity, capillary
columns cannot trap the analytes efficiently enough at normal operating temperatures
to produce good resolution.
A number of methods have been used to refocus purge and trap samples to improve
capillary column resolution. All of the methods generally accepted by the analytical
community involve cryogenics in one form or another. Non-cyrogenic methods utilise
an adsorbent packed refocusing trap exist, but it has been reported [299] that their
performance is unsatisfactory unless coupled with a cryogenic gas chromatographic
oven. The use of a cryogenic oven to cool the entire column to a point at which the
sample is efficiently re-trapped has been performed with good results [300]. Unfortunately, this simple method, known as Whole Column Cryofocusing (WCC), suffers
from two major shortcomings: the consumption of coolant is excessive and the analysis
time is very long. Of these, time is the major factor. The oven takes a fair amount of
time to cool to its initial temperature, and then takes more time to heat up to normal
temperatures used for separation. These two factors approximately double the time
required per sample.
Cryofocusing is a technique in which only a short section of the column or a
precolumn is cooled. In its simplest form a section of the column near the inlet is
immersed in a flask of coolant during desorb [301]. After desorb the coolant is removed
and the column allowed to return to the oven temperature. An automated version of
this was introduced in 1983 [302, 303]. This system required that a precolumn of
uncoated fused silica tubing be attached to the column. Cryofocusing occurred in the
precolumn. These systems required the use of a precolumn because sections of the
tubing outside of the gas chromatographic oven were not temperature controlled.
The presence of a liquid phase in these areas would have led to severe peak distortion
and irreproducible retention times. Unfortunately, the use of a precolumn carried two
major disadvantages: the precolumn must be connected to the column in a dead-volume
free manner, and the cold trap capacity is limited due to the absence of a liquid phase.
Zero dead volume connections can be made, yet they require skill and care and properly
make the connection. These fittings are then subjected to the temperature cycles of the
column oven, and may begin to leak. The cold trap capacity is somewhat limited and
samples with a large amount of organics can cause breakthrough of the trap.
Cryofocusing directly on-column simplifies and improves the focusing process.
Since no unions are required, there is no possibility of dead volume or leaks downstream of the cold trap. The stationary phase present significantly improves the
trapping efficiency by acting as an adsorbent. Yet trapping on-column requires careful
attention to ensure that all parts of the column outside of the oven are carefully temperature controlled. The Tekmar 2000 Series Capillary Interface ((Figure 2.3) uses an
volatile pollutants using capillary columns by the United States Environmental Protection Agency [298]. Analytes using purge and trap gas chromatography in other
application areas have long accepted capillary methods. Unfortunately, unique problems are encountered when using a concentrator with a capillary column instead of a
packed column. Samples from a concentrator are injected by thermal desorption of an
adsorbent trap The process requires a length of time and minimum flow column that
produce an injection profile of a rather broad nature. Packed columns, due to their
comparatively high capacity, can effectively cold trap the sample at the head of the
column. Solutes are eluted during temperature programmed runs with the same column efficiency as a normal syringe injection. Due to their reduced capacity, capillary
columns cannot trap the analytes efficiently enough at normal operating temperatures
to produce good resolution.
A number of methods have been used to refocus purge and trap samples to improve
capillary column resolution. All of the methods generally accepted by the analytical
community involve cryogenics in one form or another. Non-cyrogenic methods utilise
an adsorbent packed refocusing trap exist, but it has been reported [299] that their
performance is unsatisfactory unless coupled with a cryogenic gas chromatographic
oven. The use of a cryogenic oven to cool the entire column to a point at which the
sample is efficiently re-trapped has been performed with good results [300]. Unfortunately, this simple method, known as Whole Column Cryofocusing (WCC), suffers
from two major shortcomings: the consumption of coolant is excessive and the analysis
time is very long. Of these, time is the major factor. The oven takes a fair amount of
time to cool to its initial temperature, and then takes more time to heat up to normal
temperatures used for separation. These two factors approximately double the time
required per sample.
Cryofocusing is a technique in which only a short section of the column or a
precolumn is cooled. In its simplest form a section of the column near the inlet is
immersed in a flask of coolant during desorb [301]. After desorb the coolant is removed
and the column allowed to return to the oven temperature. An automated version of
this was introduced in 1983 [302, 303]. This system required that a precolumn of
uncoated fused silica tubing be attached to the column. Cryofocusing occurred in the
precolumn. These systems required the use of a precolumn because sections of the
tubing outside of the gas chromatographic oven were not temperature controlled.
The presence of a liquid phase in these areas would have led to severe peak distortion
and irreproducible retention times. Unfortunately, the use of a precolumn carried two
major disadvantages: the precolumn must be connected to the column in a dead-volume
free manner, and the cold trap capacity is limited due to the absence of a liquid phase.
Zero dead volume connections can be made, yet they require skill and care and properly
make the connection. These fittings are then subjected to the temperature cycles of the
column oven, and may begin to leak. The cold trap capacity is somewhat limited and
samples with a large amount of organics can cause breakthrough of the trap.
Cryofocusing directly on-column simplifies and improves the focusing process.
Since no unions are required, there is no possibility of dead volume or leaks downstream of the cold trap. The stationary phase present significantly improves the
trapping efficiency by acting as an adsorbent. Yet trapping on-column requires careful
attention to ensure that all parts of the column outside of the oven are carefully temperature controlled. The Tekmar 2000 Series Capillary Interface ((Figure 2.3) uses an