Direct Injection in GC Systems with Electronic Pressure Control
Tip of the Month
Direct Injection in GC Systems with Electronic Pressure Control
By the GC Innovations Team and Technical Service Group, Restek Corporation, Bellefonte, Pennsylvania
Splitless
injection continues to be one of the most popular sample introduction
techniques for monitoring trace-level analytes by GC, but direct flash
vaporization injection has been gaining popularity rapidly, as analysts
seek better ways to accurately monitor problematic sample components.
Relative to splitless injection, direct injection generally offers
greater sensitivity for trace-level compounds, less reactivity of
active compounds (Table I), and less discrimination against
high-boiling compounds.
Splitless injections can be problematic, and often are conducted under
a less than optimal set of conditions. Grob (1) has studied the
dynamics of splitless injection, and other injection techniques, and
has detailed the process and its optimization. In all flash
vaporization injections, a liquid sample is injected via syringe into
an inlet liner in a heated injection port, and the analytes are
vaporized, depending upon their volatility, and transferred to the
column. But in splitless injections, because the analyte must be
vaporized to be transferred to the column, and the entire sample is to
be transferred to the column, the transfer process can take
considerable time compared to split injection, especially for
high-boiling analytes. During this long residence time in the injector,
high-boiling material can deposit at the bottom of the injector, below
the inlet of the column (Figure 1), which is one of the main locations
where problems can arise in splitless injections. This area also is
prone to causing analyte degradation in splitless injection, and this
possibly is the most common time and location for inlet-related analyte
breakdown. Both deposition of high-boiling analytes at the bottom of
the injector and analyte degradation here cause precision issues.
Additionally, incomplete vaporization will result in loss of
sensitivity as a function of the volatility of the individual analytes.
Greatest sensitivity for trace-level analytes is dependent upon
delivering the entire sample onto the column. This and other primary
benefits of direct injection are due largely to the design of the
direct injection inlet liner, which makes a leak-tight seal with the
outer edge of the capillary tubing at the column inlet (Figures 1 and
2). This prevents the sample from merging into the dead volume at the
base of the injector, or contacting the heated, reactive surface of the
inlet seal, and thus eliminates these significant contributors to
analyte loss.
Complications can arise when an analyst wants to make direct injections
in a chromatograph equipped with electronic pressure control. An EPC
system typically includes a pressure sensor upstream from the injection
port and a pressure sensor downstream from the injection port, at the
split flow vent (Figure 2). Because a direct injection liner seals to
the chromatography column, there is no downstream flow to the split
vent during sample injection and delivery onto the column, and there
will be a difference in the pressure measured by the two sensors. The
upstream sensor will compensate for this difference, thereby causing a
high-pressure malfunction. (continued)
trace-level analytes by GC, but direct flash
vaporization injection has been gaining popularity rapidly, as analysts
seek better ways to accurately monitor problematic sample components.
Relative to splitless injection, direct injection generally offers
greater sensitivity for trace-level compounds, less reactivity of
active compounds (Table I), and less discrimination against
high-boiling compounds.
process and its optimization. In all flash
vaporization injections, a liquid sample is injected via syringe into
an inlet liner in a heated injection port, and the analytes are
vaporized, depending upon their volatility, and transferred to the
column. But in splitless injections, because the analyte must be
vaporized to be transferred to the column, and the entire sample is to
be transferred to the column, the transfer process can take
considerable time compared to split injection, especially for
high-boiling analytes. During this long residence time in the injector,
high-boiling material can deposit at the bottom of the injector, below
the inlet of the column (Figure 1), which is one of the main locations
where problems can arise in splitless injections. This area also is
prone to causing analyte degradation in splitless injection, and this
possibly is the most common time and location for inlet-related analyte
breakdown. Both deposition of high-boiling analytes at the bottom of
the injector and analyte degradation here cause precision issues.
Additionally, incomplete vaporization will result in loss of
sensitivity as a function of the volatility of the individual analytes.
injection inlet liner, which makes a leak-tight seal with the
outer edge of the capillary tubing at the column inlet (Figures 1 and
2). This prevents the sample from merging into the dead volume at the
base of the injector, or contacting the heated, reactive surface of the
inlet seal, and thus eliminates these significant contributors to
analyte loss.
