2
Table of Contents
Introduction toDirect Injection................2
•What is direct flash vaporization injection?
•What is the difference between direct and
on-column flash vaporization injection?
•What are the advantages of direct injection
over splitless injection?
Injection Port Setup for Direct Injection ....5
• Types of injector liners used in direct
injection systems
• Converting a split/splitless injection port to
direct injection
Converting a packed column injector to
direct injection
Careful – don't crush the column end!
Detector Setup for Direct Injection............6
•Use of make-up gaswith a direct
injection system
OptimizingDirect Injection ....................7
•Direct injection requires higher carrier gas
flow rates than split or splitless injection
•Use only high quality septawhenmaking
direct injections
• Keep injection volumes small whenmaking
direct injections
• Compress the sample vapor cloudwhen
making large injections
• Electronic pressure control and direct
injection
OptimizingDirect InjectionWhenUsing
Agilent 6890 EPCGCs ........................12
Inlet and FIDMaintenance Kits for
Agilent GCs......................................13
Injector Liners ..................................14
for Agilent & FinniganGCs ........................14
for VarianGCs ............................................15
for ShimadzuGCs ......................................15
for PerkinElmer GCs ..................................16
for Thermo FinniganGCs............................16
Siltek
™
-Deactivated Inlet Liners ............17
Base-Deactivated Inlet Liners................17
Restek Enhanced Split/Splitless Injection
Port for Agilent 5890GCs ....................18
Replacement Split/Splitless Injection
Ports for Agilent GCs ..........................19
Inlet Seals for Agilent 5890/6890/6850
Split/Splitless Injection Ports ................19
Cross-Disk Inlet Seal for Agilent 5890GCs19
Nuts& Fittings..................................20
Septa ............................................21
Ferrules..........................................22
Introduction to Direct Injection
While splitless injection is still themost popular trace sample introduction technique, direct
injection is rapidly gaining in popularity as analysts search for better ways to analyze trace-
level compounds or for simpler conversions from packed to capillary systems. Compared to
splitless analysis, direct injection offers higher sensitivity, reduced adsorption of active com-
pounds, and operational simplicity. In addition, the high performance and low cost of the
equipment required to perform direct injection has accelerated its acceptance as amain-
stream sample introduction technique.
This guide discusses themany important aspects of direct flash vaporization injection on
0.32mm or 0.53mm ID capillary columns.Advantages and disadvantages of direct injection
are outlined and compared to the performance of other injection techniques. Considerations
such asmodifying splitless inlets, retrofitting packed column injection ports, and the need
formake-up gas are discussed. Finally, operational parameters such as flow rates and sample
injection volume versus injection speed are reviewed.
What is direct flash vaporization injection?
In direct flash vaporization injection, a liquid sample is injected via a syringe into a heated
injection port. The sample is rapidly vaporized in the injection port, then transferred to the
column. This injection technique is very popular with packed columns but, when usedwith
the first-developed capillary columns, the highly concentrated samples caused overloaded
peaks and poor resolution. Subsequently, split systemswere developed to provide on-column
sample volumes and analyte quantities that were compatiblewith the low flow rates and lim-
ited capacities of capillary columns. Split injections produced symmetrical peaks, while
allowing concentrated samples to be injected in the samemanner as in packed column sys-
tems. Later, split systemsweremodified to include a splitless injectionmode to direct the
entire sample into the column for analysis of trace analytes. Direct injection parallels split-
less injection at the beginning of the sample introduction process ("purge off"mode or split
vent closed), but in splitless injection gas flow in the injection port is switched to the "purge
on"mode after a short period of time, thereby flushing excess solvent vapor from the injec-
tion port. (SeeRestek’s
OperatingHints forUsing Split/Splitless Injectors
Lit. cat.# 59880A
formore information.) Because direct injections do not include a "purge on"mode, the entire
sample vapor cloud is swept into the column.
Today, direct flash vaporization injections are better understood and are recognized as offer-
ingmany advantages over splitless injections for the analysis of difficult components. These
advantages include less adsorption of active compounds, less discrimination against high-
boiling compounds, and better sensitivity for trace components. Direct injection also can be
used for concentrated samples commonly analyzed on split/splitless systems, provided the
sample is first dilutedwith a solvent and the injection volume is kept small to prevent col-
umn overload. Superior responses for active, high-boiling, or trace compounds, combined
with its simplicity and low cost have led to the resurgence of direct injection inmany labora-
tories.
What is the difference between direct and on-column flash vaporization injection?
Both direct injection and on-column injection deliver the entire sample to the column. The
difference between the two techniques is denoted by the termination point of the syringe nee-
dle during the sample injection and vaporization process (Figure 1). In an on-column injec-
tion, the tip of the needle enters the column and the liquid sample is deposited directly inside
the column. In a direct injection, the tip of the needle enters a glass injection port liner and
the sample is vaporized in the buffer volume of the injection port liner. Thus, in a direct
injection, the sample is a vapor before it reaches the column.
Of the two, direct injection is the preferredmode of sample introduction because the injec-
tions are less problematic. On-column injections usually aremade by introducing the tip of a
26-gauge needle (0.48mmOD) into the bore of a 0.53mm ID column. This tight fit leaves
little space for sample expansion during the explosive vaporization process, and solvent peak
tailing is a common result. On-column injections cannot bemade into a 0.32mm or smaller
ID column because the syringe needle is toowide to introduce into the column.
