15
vaporization. Figure 13 shows the improvement in peak shapewhen anHP autosampler is
usedwith an injection port liner packedwithwool, versus a liner without wool.
The second technique is to use hot needle injection. Hot needle injections are performed by
drawing the sample all theway into the syringe barrel, leaving the needle empty.When the
needle is introduced into the injection port the injection in delayed for a short period of time
(3–5 seconds, for example) to allow the needle to heat completely. Then the syringe plunger
is depressed and the sample is expelled into the injection port liner.
The third technique is to use a solvent flushwith each injection. This technique involves
drawing a small amount of solvent into the syringe, followed by a small amount of air, fol-
lowed by the desired amount of sample.All of the solvent, air, and sample are then drawn
into the barrel of the syringe, just as in a hot needle injection. The needle is preheated, as in
the hot needle injection, and the contents of the syringe are expelled into the injection port
liner. The solvent that was first drawn into the syringe acts to flush the syringe barrel and
needle, and completely transfers all of the sample during the injection process.
Backflash:
Backflash occurswhen the volume of the vaporized sample exceeds the volume
inside the injection port liner.Most of the excess vaporized sample escapes out the top of
the injection port liner. Some of it is swept down the septum purge line.Another portion of
it can back up into the carrier gas supply line, and some of it can be re-introduced into the
injection port. Backflash can cause poor peak area reproducibility, tailing peaks, split peaks,
and poor resolution.
Table III (page 8) shows the estimated expansion volumes for 1µL injections of a variety of
solvents.When using an injection port temperature of 250°C and a carrier gas pressure of
10psig, most solventswill vaporize and expand to a volume that exceeds the capacity of a
2mm ID injection port liner (approximately 240µL, seeTable IV). In order tominimize
backflash, injection port parametersmust be carefully optimized. Injection port temperature,
carrier gas pressure, sample size, and rate of injection all should be adjusted to ensure the
vaporized sample remains inside the liner prior to being transferred to the head of the col-
umn.
Sample Size and InjectionPort Temperature:
As the equation inTable III shows, the vol-
ume of vaporized sample produced is directly related to the size of the liquid sample (n) and
the temperature of the injection port (T).A decrease in either of these valueswill translate
into a smaller vaporized sample volume. If the injection port temperature cannot be
decreased because of vaporization problems and the sample size cannot be decreased
because of sensitivity issues, backflashmust beminimized by optimizing the rate of injec-
tion or by adjusting the carrier gas pressure.
Figure 13.
Always pack splitless inlet liners withwool when using rapid injection autosamplers.
4mm ID splitless liner without wool
may exhibit fronting peaks.
4mm ID splitless liner withwool
eliminates fronting peaks by promoting
sample vaporization.
20
24
28
32
36
20
24
28
32
36
Table IV.
LinerVolumes.
Theoretical* Effective
1.0mm ID= 59µL
30µL
2.0mm ID= 236µL
118µL
3.0mm ID= 530µL
265µL
4.0mm ID= 942µL
471µL
*Liner volume actually available for vaporization
with carrier gas present is
≤
1
/
2
theoretical, due to
the presence of carrier gas in the liner.
From
Split and Splitless Injection inCapillaryGC,
3rdEd.
, K. Grob,Wiley-VCH, 2001.
