10
The sequence of events for analyte focusing is the same, except for the initial oven tempera-
ture; instead of starting 20°C below the boiling point of the solvent, the oven temperature is
started 60–80°C below the boiling point of the earliest eluting compound.
Figure 6 shows an example of improper solvent focusing. The sample solvent is hexane,
which has a boiling point of 69°C. The initial oven temperature is 150°C, or 80°C above the
boiling point of hexane. The solvent peak is tailing, and the early-eluting compounds have
broad peak shapes and are poorly resolved from one another. Figure 7 illustrates proper sol-
vent focusing. The initial oven temperature, 40°C, iswell below the boiling point of hexane.
The square solvent peak is a good indicator of proper solvent focusing.Also notice the sharp
peak shapes for both early- and late-eluting compounds.When the solvent is not detected or
elicits a low response, such as hexanewith electron capture detectors (ECDs), the only indi-
cation of proper solvent focusing is narrow peaks for early-eluting compounds.
For optimal solvent focusing, choose a solvent that has a boiling point at least 20°C below
the boiling point of the earliest eluting target analyte. In some cases, it is not possible to
select the perfect solvent to achieve focusing. For example, methylene chloride (boiling
point 40°C) is frequently used for splitlesswork because of sample preparation techniques.
Analyses performedwith an initial oven temperature of 40°Cwill not allow the solvent to
recondense at the head of the column andwill not refocus the sample analytes. Ideally, ana-
lystswould start the oven temperature at 20°Cwhen usingmethylene chloride as the sample
solvent, but because this is not practical, theymust relymore on analyte focusing to refocus
sample analytes at the head of the column.
An important part of solvent focusing is the ability of the solvent to “wet” the stationary
phase in the column. Non-polar solvents should be used for splitless injections on non-polar
stationary phases (e.g., use hexane or isooctane for injections onRtx
®
-1 andRtx
®
-5
columns). Non-polar solvents aremore soluble in non-polar stationary phases andwill form
amore efficient zone of recondensed solvent in the column. Polar solvents are not as soluble
in non-polar stationary phases andwill bead up on the stationary phase rather than forming
an even layer of recondensed solvent at the head of the column.Mismatches between the
polarity of the solvent and the polarity of the stationary phase can cause band broadening,
peak splitting, and poor resolution.
Once again, the same basic procedures are followed for analyte focusing, except the initial
oven temperature is 60–80°C below the boiling point of the earliest eluting compound,
instead of 20°C below the boiling point of the solvent, aswith solvent focusing.
Figure 6.
Initial oven temperature too high for
improper solvent focusing: solvent peak
and early eluting compounds are tailing.
Figure 7.
Initial oven temperature at least 20°C
below boiling point of earliest
eluting analyte: early eluting compounds
are symmetrical.
30m, 0.25mm ID, 0.25µmRtx
®
-5 (cat.# 10223)
1.0µL splitless injection of a pesticidemix in
hexane (5ng/µL);
Oven temp.:
150°C to 275°C@ 4°C/min.
30m, 0.25mm ID, 0.25µmRtx
®
-5 (cat.# 10223)
1.0µL splitless injection of a pesticidemix in
hexane (5ng/µL);
Oven temp.:
40°C to 150°C
@ 25°C/min. then to 275°C@ 4°C/min.
Aunique situationwithAgilent 5890 and
6890/6850 split/splitless inletsmakes a double
gooseneck liner highlydesirable for samples
that contain compounds prone to catalytic deg-
radation through contactwithhotmetal sur-
faces.Agilent splitless inlets contain ametal
seal at the base of the inlet (just under the liner
outlet). Because the column is installedonly a
fewmillimeters above the seal surface, the sam-
ple contacts the sealwhile it is being slowly
drawn into the column.Adouble gooseneck
inlet linerminimizes contact between the sam-
ple and themetal seal.Adirty seal increases the
breakdownof endrin (a pesticide prone tode-
composition) from6% to12.8% in anAgilent
5890 inletwhen a 4mm straight inlet liner is
installed. However, when a double gooseneck
inlet liner is used, the breakdown remains at 2%
regardless ofwhether the seal is cleanor dirty.
(Formore information, see page 24of this
guide for a descriptionof ourVespel
®
Ring Inlet
Seal.)
EndrinBreakdown
LinerType
CleanSeal Dirty Seal
SplitlesswithWool
6.0% 12.8%
DoubleGooseneck
2.0% 2.4%
splitless
liner
double
gooseneck
liner
inlet
seal
Double gooseneck inlet linerminimizes
the catalytic effects of sample contact
with themetal disk in anAgilent inlet.
