14
Figure 11 demonstrates themolecular weight discrimination experiencedwhen analyzing a
series of hydrocarbonswith a broad range ofmolecular weights (C6 throughC44).
Alternative injection techniques, such as cold on-column injection, can be used tominimize
molecular weight discrimination.
Molecular weight discrimination is usually very repeatable. In split and splitless injections,
if the same injection port temperature, carrier gas pressure, sample size and sample solvent
are used for every injection, sample vaporization should be a reproducible process.Any
molecular weight discrimination experienced should be the same from one injection to the
next. Because of this consistency, many analysts choose to ignoremolecular weight discrim-
ination unless it compromises overall sensitivity. To help compensate for differences in
response due tomolecular weight discrimination, multiple internal standards can be used to
mimic the range ofmolecular weights and boiling points for the analytes in the sample.
Molecular weight discrimination can beminimized by choosing an injection port liner that
ensures the sample is completely and uniformly vaporized. Inadequate vaporization causes
the sample to approach the head of the column in both the aerosol and vapor states.Aerosol
droplets, consisting predominantly of highmolecular weight compounds, can be driven past
the head of the column by themomentum of the carrier gas andwill be preferentially swept
out of the injection port and through the split vent. Injection port liners that are packedwith
glasswool or that incorporate a flow diverting devicewithin their bore assist in vaporizing
the sample and transferring a homogeneous representation to the head of the column.
NeedleDiscrimination:
During sample injections, the syringe needle undergoes some
degree of heating in the injection port. The temperature reached by the needle can influence
the relative response for low and highmolecular weight analytes. During the process of
expelling the sample from the syringe, the contents in the needle are not completely trans-
ferred to the injection port.As the needle begins to heat, lowmolecular weight analytes
begin to vaporize from the needlewhile highermolecular weight analytes remain inside the
needle. Therefore, the lowermolecular weight analyteswill show enhanced response com-
pared to higher weight analytes (Figure 12). Three techniques can be used tominimize nee-
dle discrimination in split and splitless injections.
The first technique is to inject the sample as rapidly as possible. Rapid injectionsminimize
the amount of time the needle spends in the injection port and reduces the amount of heating
the needle experiences.Whenmaking rapid injections in straight injection port liners for
split or splitless analysis, the sample can be propelled beyond the inlet of the column and
onto the injector base fitting.Always pack injection port linerswith deactivated glasswool
or CarboFrit
™
packing, or use a flow diverting device like a laminar cup to assist in sample
Figure 11.
Splitter discrimination typical of split and splitless injections.
Splitter discrimination is evident from relatively enhanced peak heights for the early-eluting com-
pounds and diminished peak heights for the later-eluting higher molecular weight compounds. The
same sampleanalyzedbycoldon-column injection showsnodiscrimination; thepeakheights for low
and highmolecular weight compounds are truly representative of this sample.
Discrimination typical of a split or splitless
injector. Injector temperature: 340°C
Cold on-column injection provides accurate
information. Injector temperature: 40°C.
30m, 0.32mm ID, 0.25µmRtx
®
-1 (cat.# 10124)
Inj. volume:
0.2µL
On-column conc.: 15ng.
Oven temp.:
40°C to 340°C@ 5°C/min.
Det. (FID) temp.: 340°C
Linear velocity: 50cm/sec., hydrogen
Attenuation:
8x10
-11
AFS
4 12 20 28 36 44 52
min.
4 12 20 28 36 44 52
min.
septum
vaporizing
chamber
syringe
volatile solutes
(vapor state)
high boiling
materials
(aerosols)
residual layer
of high-boiling
point materials
evaporating
solvent and
volatile solutes
sample liquid
column
inlet
needle
Figure 12.
Factors in discrimination: highmolecular
weight material clinging to the syringe
needle and non-homogeneous vapor-
ization of the sample in the inlet liner.
C6
C44
C6
C44
