Environmental Engineering Reference
In-Depth Information
3.1.3 Laser Ionization Mass Spectrometry (LIMS) and Application
to Monitoring Intermediate Chemical Species During Combustion
(Nishimura et al.
2010
)
Spectroscopic visualization methods describe above are advantageous in that spa-
tial, spectral, and time resolutions are concurrently attainable. However, the
detectable chemical species are relatively limited in the number, typically NO, CH,
OH, C
2
,H
2
O etc. Nevertheless, more intermediate chemical species are expected
from elementary reactions during the combustion. Mass spectrometry (MS) is
known to be a very powerful tool to determine molecular weight with high sen-
sitivity to trace levels. A key issue is how to introduce the combustion
fl
ows and
ef
ciently ionize the chemical species to be analyzed. For this purpose, a laser-
induced ionization source (LI) with a sampling cone has been developed as a
promising technology and LIMS successfully applied to visualization of interme-
diate chemical species in a methane/air premixed combustion
flame. Figure
17
shows the schematic diagram of the experimental apparatus, and Fig.
18
shows the
photographs of the combustion
fl
fl
flame-MS interface.
flow is introduced into the laser-induced ionization (LI)
chamber through the narrow ori
The combustion
fl
ce of the coupling interface. The Nd
+
-YAG laser
beam of frequency-doubled, tripled, or quadrupled radiation focussed on the Al
target tip, consequently very tiny plasma is induced (laser-induced plasma) where
intermediate chemical species from the combustion
flame are ionized and partially
decomposed. The key issue here is how to increase the ionization ef
fl
ciency and
lower the decomposition degree, leading high yield of ionized species to give high
sensitivity and intact composition of chemical species in the combustion
fl
ame.
350 V
350 V
skimmer cone
orifice diameter
0.1 mm
skimmer cone
orifice diameter
0.1 mm
ion lens
ion lens
-220 V
-220 V
sampling cone
orifice diameter
0.3 mm
sampling cone
orifice diameter
0.3 mm
-110 V
-110 V
quadrupole mass analyzer
quadrupole mass analyzer
Al target
Al target
r
r
z
z
bunner
bunner
flame
flame
ca 1
mm
ca 1
mm
10
-3
Pa
10
-3
Pa
7.0
7.0
400-700
Pa
400-700
Pa
detector
detector
lens
lens
vacuum pump
vacuum pump
Nd:YAG laser
Nd:YAG laser
mirror
mirror
Fig. 17 Schematic diagram of the LIMS system
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