7
Tominimize adsorption and prevent peak tailing, alwaysmake sure that the outlet end of the
column is correctly positioned inside the FID jet tip (Figure 6), close to the jet orifice or
close to the entrance of the ECD detector cell. Read and carefully follow the instrument
manufacturer’s specifications. In an instrument designed for packed columnGC, the FID jets
should be replacedwith jets designed to functionwith capillary columns or peaksmight tail.
Optimizing Direct Injection
When using direct injection as the sample introduction technique, the sample is injected in a
liquid form via a syringe. The sample is vaporized in the heated injection port and the carrier
gas transfers the vapor to the head of the column. The twomost common problems associated
with hot vaporization injections in the direct injectionmode are broad, tailing peaks and sam-
ple backflash. Choosing the proper carrier gas flow rate and sample sizewill allow you to
achieve optimum chromatographic performance.
In the direct injectionmode, all of the sample and carrier gas is directed into the head of the
column. In order to produce the narrowest sample bandwidth possible, the vaporized sample
must be transferred from the injection port liner to the head of the column as quickly as pos-
sible. If the sample is not transferred quickly, a broad, tailing solvent front will be produced
and resolution of early-eluting compoundswill be compromised.
Direct injection requires higher carrier gas flow rates than split or splitless injection
Direct injection systems perform best when high column flow rates are used.When a 30m,
0.32mm ID capillary column is operatedwith helium at a linear velocity of 20cm/sec., a col-
umn flow rate of approximately 1cc/min. is obtained.When 1µL of a sample diluted in
methylene chloride is injected into a heated injection port, the sample expands tomore than
400µL of gas volume.At a column flow rate of 1cc/min., it takesmore than 24 seconds to
transfer the vaporized sample from the buffer volume in the direct injection liner to the head
of the column. If the carrier gas flow is increased to a linear velocity of 80cm/sec. (approxi-
mately 7cc/min.), the 400µL ofmethylene chloride vapor will be transferred from the liner
to the head of the column in less than four seconds. This rapid transfer of the sample cloud
ensures narrow initial sample bandwidths.
Significant differences in chromatography arising from the difference in sample delivery
time are evident in Figure 7. Figure 7a shows amixture of phenols injected using an opti-
mum carrier gas linear velocity (hydrogen, 40 cm/sec.). Notice the tailing of the solvent
peak, coelution of 2-nitrophenol and 2,4-dimethylphenol (peaks 3 and 4), andminimal base-
line resolution of 2,4-dinitrophenol and 4-nitrophenol (peaks 8 and 9). Figure 7b shows the
same sample delivered and separated using a faster carrier gas linear velocity (80 cm/sec.).
The solvent peak is significantly sharper, and 2-nitrophenol / 2,4-dimethylphenol and 2,4-
dinitrophenol / 4-nitrophenol are fully resolved. For Figure 7c, the carrier gas linear velocity
was increased to 120cm/sec. (approximately 120 cm/sec.). The solvent peak shape is further
improved and the two pairs of phenols arewell resolved. Further increases in flow rate are
counterproductive, however, because they decrease the efficiency of the column and reduce
resolution of the early-eluting compounds.
Figure 6.
Restek designed the flared jet tip (arrows)
tomake capillary column insertion easy.
2
Restek offers high performancemake-up gas kits to fit most GCs. For details, see our current chromatog-
raphy products catalog.
Replacement FID Jets
• StandardVersion: Engineeredwith a
fluted tip to guide the capillary column
into the jet.
• High-PerformanceVersion: Identical to
the standard version, except that it has
been Silcosteel
®
-treated. Extremely
inert, usewith active compounds.
Capillary Adaptable FIDReplacement
Jet for Agilent 5890/6890/6850GCs
(0.011-inch ID tip)
(Similar toAgilent part # 19244-80560.)
Description
qty. cat.# qty. cat.#
Standard
ea.
20670 3-pk. 20671
High-Performance Silcosteel
®
ea.
20672 3-pk. 20673
Capillary DedicatedReplacement FID
Jets for Agilent 6890/6850GCs
(Similar toAgilent part #G1531-80560.)
Description
qty. cat.# qty. cat.#
Standard
ea.
21621 3-pk. 21682
High-Performance Silcosteel
®
ea.
21620 3-pk. 21683
Capillary FIDReplacement Jets for
Agilent 5880GCs
(Similar toAgilent part # 19301-80500.)
Description
qty. cat.#
Standard
ea. 21637
High-Performance Silcosteel
®
ea. 21638
