Ultra-sensitive High-speed
Monitoring Technology forTrace-level Toxic HalogenatedHydrocarbon Compounds
SHIZUMA KURIBAYASHI*1MINORU DANNO*1
HIDEO YAMAKOSHI* 1
SATOSHI SAKAI*1
SHIGENORI TSURUGA*2

Mitsubishi Heavy Industries, Ltd. (MHI) has further improved its original high-speed analysis technology developed by combining vacuum ultraviolet light ionization, ion trapping, and time-of-flight type mass spectrometry withthe aim of monitoring trace-level toxic substance in exhaust gas from incinerator furnaces in an attempt to increase
the analytical sensitivity and extend the applicability of the technology to actual plants. As a result, the technology
has made it possible to analyze trichlorobenzene (T3CB), a dioxin precursor, in a gaseous state in twenty seconds
with a sensitivity of 80 ng/m 3N, and pentachloro-dibenzofuran (P5CDF), a type of dioxin, in a solvent in sixteen
minutes with a sensitivity of 1 pg using the selective accumulation of target substances and separation of interfering
substances in the ion trap. These results show that higher speed and higher sensitivity in analyzing toxic substances
than had been previously possible can be achieved and the adverse effects of impurities contained in actual gas
reduced. In addition, MHI has developed a cleaning procedure for the vacuum ultraviolet light irradiating window tocontribute to extending the service life of the analyzing system.

monitoring system, for the purpose of checking whether
PCB concentrations in exhaust gas and the working environment are lower than required safety standard
levels (1).
In addition, MHI has developed a tunable diodelaser spectrometry capable of real-time- measurementof the spatial distributions of CO, CO 2 , and NOx existing in ppm (parts per million) levels in theatmosphere and is promoting its application in monitoring the interior of incinerator furnaces and the
local surrounding atmosphere, as a compact, easily
maintainable, and highly sensitive gas monitoring
system (2) .
In this paper, technical aspects of the vacuum ultraviolet (hereafter abbreviated as VUV) ionization
mass spectrometry system are described in detail. The
system has been developed by MHI as a fast monitoring system for measuring DXNs and their precursors,which are reactants that lead to the formation of
DXNs, in exhaust gas, in order to reduce trace-level
organic chlorine compounds emitted from incinerator
furnaces. The technology used for the system is characterized by being applicable and effective in monitoring
various target substances present in ppt to ppb levels.
Further information regarding the results of the fieldtests of this technology can be found by referring to aprevious report(3).

1. Introduction
Many products that MHI produces often need to havecompatibility between their mechanical performance andenvironmental performance. Recently, there has been astrong trend toward requiring ever better improvementof the latter. In particular, the emissions of dioxins (hereafter referred to as DXNs) and gaseous substances thatexert green house effects on the atmosphere burden theenvironment with heavy loads, so that the measurement,
monitoring and reduction of these emissions have become
an issue of major social concern.
MHI considers the development of highly sensitiveenvironment-monitoring technology to be an important
theme coping with these social concerns. To this end, MHI
has been focusing much effort on developing various
measurement methods that use advanced technologies
in the field of applied physics and chemistry, in addition
to promoting the advancement of combustion control andexhaust gas treatment techniques.
For instance, MHI has already developed laser ionization mass spectrometry as a method for monitoring
toxic substances existing in ppt (parts per trillion) andppb (parts per billion) levels for use in plants that dispose of polychlorobiphenyl (PCB), which is a highly toxic
compounds. MHI has succeeded in putting this system
into practical use as a super high sensitive exhaust gas

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.6 (Dec. 2004)*1 Advanced Technology Research Center, Technical Headquarters*2 Yokohama Research & Development Center, Technical Headquarters

12. Principle of measurement and improvement of
performanceFig. 1 shows the configuration of the system. This
analysis system originally developed by MHI consists of(1) ionizing a target substance contained in exhaust gas
by VUV light, (2) accumulating the target ions in an ion
trap, and (3) identifying the accumulated target ions using time-of-flight mass spectrometry (hereafter
abbreviated as TOF). The basic principle has alreadybeen described in a previous report(4). This time, the selective accumulation of a target substance and separationof interfering substances in the ion trap have been added
to increase the sensitivity and extend the practical applicability of the system to being able to handle actualgases containing various interfering substances.
2.1 VUV ionization
Since VUV light has a shorter wavelength and stronger photon energy than usual ultraviolet light, it candirectly and efficiently ionize a target molecule withphotons. In addition, since this ionization process using
VUV is simple, the affect of coexisting molecules on ionization efficiency is small. An ionization process withthese useful characteristics contrasts with other ionization processes such as laser ionization and atmosphericpressure chemical ionization that have such problems
as reduced ionization efficiency in multi-chlorine molecules or the impact of oxygen concentration on ionization
efficiency.
Furthermore, since the ionizing potential of Lyman
α light (wavelength: 121.6 nm, photon energy: 10.1 eV),which is irradiated from hydrogen plasma and is used
for this ionization process, is a slightly higher in photon
energy than the 8.5 to 10 eV of DXN and their precursor
ionization energy, it can be used to perform ionization
efficiently without the occurrence of any dissociation.
Thus, VUV light is an excellent ionization source. However, because of the following problems, it has not beenused practically in high sensitive analysis until now.
(1) Since VUV light is difficult to produce in sufficiently
large quantities at a time, the amount of ion-generation that can be realized per unit time is limited.
(2) Since the VUV light can ionize all molecules whose
ionization energies are lower than the VUV photon
energy, selectivity of this method in the ionization islow. Therefore if there are a lot of interfering molecules that have the same mass number as that of
the target molecules and the lower ionization energy
than the photon energy, the measurement accuracy
is deteriorated. This is often the case in actual exhaust.
(3) Since the window material is damaged quickly, frequent maintenance is required.
With regard to Item (1), the VUV ionization technology described in this paper can first be characterized byGC

P5CDF standard solution

o

300 C
Microwave
Ion trap
VUV lamp

Ionization chamber

Ions

Target molecules

TOF

Carriergas

H2/He
Capillary
Buffergas

Ionization
VUV light

Ion extraction
Molecules contained in atmosphereEnd caps

RF ring electrodes

Fig. 1 Real-time measurement system for trace-level organic
chlorine compounds
This system consists of a vacuum ultraviolet light lamp, ion trap, and TOF.
The figure shows system configuration with a gas chromatography installed
to measure P5CDF. Gas chromatography is not used when measuring T3CB.

performing ionization successively using a lamp thatgenerates light continuously, accumulating the ionizedions in the trap, and ensuring a sufficient number of ions.
2.2 Selective accumulation in ion trap andfragmentation of accumulated ions
The problem mentioned in Item (2) above was solved
as described below. Fig. 2 shows the timing chart for eachprocess. The following is a description taking T3CB as
an example. When ions are accumulated in the ion trap
under continuous ionization using a VUV lamp and aselective signal is applied to the ion trap, only these ionswith the same mass number as that of T3CB are accumulated; other ions with different mass numbers are
simultaneously eliminated (selective accumulation)(5).
The selecting signal has a inverse Fourier transform waveform of a continuous frequency spectrum inwhich only the secular frequency of T3CB to be accumulated is not included. (The secular frequency is givenby the mass number of T3CB and the trap frequency[1MHz for instance] applied to the RF ring electrode.)

Selective
accumulation

Extraction
Separation ofout andinterfering
measurement
molecules

Selection/separation
signals (at end caps)
Trapping voltage(at RF rings) (1MHz)VUV lamp
Ion detector
Pulse for ion extraction(at end caps)
1 cycle (about 0.1 to 5 seconds)
Fig. 2 Timing chart
This figure shows the timing chart of the lamp, trap, and TOF.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.6 (Dec. 2004)2

Accumulation

10
5
0
100

120

Ions with othermass numbers
Elimination

140

160

180

200

Mass number(a) When selective accumulation
is not performed.

Fig. 3 Mass spectrum of T3CB

20

CI

Accumulated T3CB CI
and interfering molecules

15
10
5
0
100

120

140
160
Mass number

180(b) When selective accumulationis performed.

200

Signal intensity (mV)15CI
Signal intensity (mV)

Signal intensity (mV)

T3CB and CI
CIinterfering
molecules
CI

20

15

Fragments originatedfrom T3CB
CI

10

CI

20

Dissociation
into fragments

CI

5
0
100

120

140
160
Mass number

180

200

(c) After dissociation into fragments

Interfering molecules are separated through the selective accumulation of moleculeswith mass numbers of 180, 182, and 184 and their subsequent fragmentation.

Fig.
3 (a) shows the mass spectrum of a typical gas sample
Fig.3
when the selective accumulation is not performed, while
Fig.3 (b) shows the mass spectrum when a selective accumulation is performed. The latter shows that ions withmass numbers 180, 182, and 184, which are the isotopic
ions of T3CB, can be selectively accumulated.
Then, T3CB and its interfering molecules that havethe same mass number as T3CB, which are selectively
accumulated together with the T3CB, are separated. In
this procedure, the ions of T3CB that have been accumulated are given a strong shake by a wave with thesecular frequency of T3CB (previously described) applied
to the end caps, so that the ions of T3CB are made tocollide with the ambient buffer molecules at which point
they are dissociated into fragments (hereafter referred
to as fragmentation). As shown in Fig.3 (c), T3CB is dissociated into molecules with mass numbers of 145 and147, which have lost one chlorine atom, respectively, andthe molecules with a mass number of 109, which havelost two chlorine atoms and one hydrogen atom. Interfering molecules (which are supposed to consist of straightchain hydrocarbons) can then be separated from the T3CB
molecules since they are dissociated nearly at random.
Thus, by detecting the separated T3CB molecules in the
TOF, it becomes possible to realize a highly sensitive
analysis that is not influenced by interfering molecules.
2.3 Improvement of life of the VUV window
A MgF2 window through which the VUV light is irradiated is used in order to partition the VUV lamp andionization chamber. However, the material is easily deteriorated. The reasons for its short life have been
identified and the following countermeasures have beentaken to improve the performance and longevity of thewindow as follows.
(1) Effect of hydrocarbons in the ionization chamber
We found that trace amounts of hydrocarbons contained in gas in an ionization chamber adhere to theinner surface of the window as carbon, due to the photochemical reaction of VUV. On the other hand, we also
found that the carbon is eliminated if oxygen or water
vapor is present inside the chamber when VUV is irradiated in the chamber. Accordingly, the phenomena wasused as a cleaning procedure in which oxygen or steam
is injected and applied to the surface of the window
during normal operation or maintenance service.
(2) Deterioration of MgF2 due to production of color centerWe found that color centers that absorbs light witha wavelength of 121.6 nm are produced near the lampside surface of the window substrate by the VUV light
containing various wavelengths generated by thelamp. On the other hand, it has been also found thatthe production rate of the color center can be reduced
by decreasing the temperature of the window. Consequently, the window and its vicinity are to be cooledduring operation as a means of preventing the formation of a color center.
In addition to the countermeasures described above,
a design in which eight windows can be used in turnwithout exposing the vacuum system to the air has beendeveloped in order to prevent time loss to obtain highvacuum after the air exposure. This makes it possible
for the system to run continuously with maintaining ahigh level of sensitivity for about six months.

3. Configuration of system
Fig. 1 shows the configuration of the monitoring system tested in this study. The sample of gas to be analyzed
was led into the interior of the ion trap through a capillary column. The sampling tubing and the capillarycolumn were kept at high temperatures between 100 to
300 oC, so as to minimize the possible loss of any of the
DXNs and their precursors due to adhesion and time lagssince they adhere easily to any walls they come in contact with and because their concentrations are extremely
low. In addition to above, the ionization chamber was
also kept at a temperature of 200oC.
The quadrupole type electrodes were adopted for use
as the ion trap. The VUV light from the source lamp was
irradiated onto the target molecules that were led into
the trap. The higher the pressure in the trap, the greater
was the amount of ionized molecules present. However,excessively high pressure results in gas breakdown. As
a result of optimaization, appropriate internal pressureof the ionization chamber was determined to be 10-2 Pa.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.6 (Dec. 2004)

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The selecting signal applied to carry out the selective
accumulation was generated at an arbitrary waveformgenerating board installed in a computer, amplified byan amplifier, and applied to the trap through a filter that
protects the flow back of pulse voltage applied for theion extraction. Since one cycle of the selecting signal isabout 3 ms, for instant, the signal was repeatedly applied during an accumulation time of 0.1 to several
seconds. On the other hand, for the fragmentation, awave with the secular frequency explained previously
was given from the same system described above.
Ions dissociated into fragments were led to the TOF
with the timing shown in Fig. 2, and mass-analyzed.
Then, the concentration of the target molecules was calculated from the total number of fragments originatedfrom the target molecules. A series of these procedures
was repeated multiple times during twenty second period and the integrated values were adopted as thecalculated value.
In order to improve the sensitivity of the system, suchparameters as the frequency and voltage of the high frequency wave applied to the trap, the spectrum waveformsof the selecting signal and fragmenting signal (including the range of mass numbers to be eliminated, therange to be selected, and amplitude distributions), time
for trapping (accumulating), time required for each process, voltage for the ion extraction, accelerating voltage
at TOF, and selection of fragments included in measurement were all optimized.
Furthermore, the sensitivity of the system was increased by lighting the VUV lamp only during
accumulation. This was done in order to prevent the following two problems from arising. (1) Ions produced
during the fragmentation become noise. (2) VUV light thatleaks into the TOF and enters the detector becomes noise.
In the case of pentachloro-dibenzofuran (P5CDF), thesample was in a form of a solution. (Details regarding
P5CDF are described in Section 4.2.) Therefore, gas chromatography was added to extract P5CDF from thesolvent and concentrate it in order to improve the levelof sensitivity. In addition, a buffer gas (N2 at a pressureof 3 x 10 -3 Pa) was introduced into the trap in order toimprove the efficiencies of the selective accumulation andfragmentation of the ions/molecules.

4. Results and discussion
4.1 Measurement of T3CB
In order to check the combustion conditions of an incinerator furnace, the study focused on T3CB, which isa DXN precursor, because its concentration is relatively
high and is said to correlate closely to the amount of
DXNs created.
First of all, the system was calibrated. As a sample, thestandard gas with 80μg/m3 N of T3CB was further diluted
with nitrogen gas. Fig. 4 shows the calibration line. The Yaxis intercept point, i.e. the count number of ions at thezero point of the X-axis, was 4.2 (corresponding to noise
component) and the coefficient was 0.052 (count numberof ions)/(ng/m3 N) with respect to the least square approximate straight line. Hence, the lower detecting limit wasdetermined to be 80 ng/m3 N (=10 ppt). At a concentrationof 80 ng/m3 N, the count number was found to be 8.4, which
is sufficiently larger than the noise component. The longer
the trapping time, the larger is the count number. However, excessively longer trapping time leads theaccumulation of ions to a point of saturation in high concentration. The trapping time was optimized to be 0.3
seconds. Integration time was 20 seconds.
The monitoring system was then applied in the measurement of actual exhaust gas at the outlet of a waste
incinerator furnace(3). Since the monitoring system wasinstalled upstream of the after-treatment unit such as adust collector, the combustion conditions inside the furnace could be monitored directly. After eliminating solidmaterials such as ash, the exhaust gas quickly cooled toabout 200 o C and was led to the system. The fragment
spectrum obtained by this procedure is shown in Fig. 5
5.
In addition to the same fragments as those originated80

Count number of ions

Count number of ions

400

300

200
Y=0.052 X+4.2
100

70
60
50
40
30
20
10
0

0

1000

2 000

3 000

4 000

5 000

6 000

50

100

150

200

250

Mass number m/e (-)
Fig. 5 Mass spectrum of fragments of T3CB in actual exhaust gas
In addition to the fragments (indicated by solid line arrow marks) in thestandard gas, new fragments (indicated by dotted line arrow marks)
are generated.

Concentration of T3CB (ng/m3N)
Fig. 4 Calibration line using T3CB as standard gas
Linearity is maintained over a wide range of T3CB concentration.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.6 (Dec. 2004)

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CO

2 000
1 500

150
100

1 000

T3CB

50

500

0
0
12:30 13:00 13:30 14:00 14:30 15:00 15:30 16:00 16:30

1 400
Count number of ions

200

2 500

Concentration of T3CB (ng/m3N)

Concentration of CO (ppm)250

Signal intensity (a.u.)

40

10

20

30

40

tem should be quite useful to monitor the condition of
the combustion and reduce the DXN generation.
4.2 Measurement of P5CDF
As one of the indices of DXN emissions in the exhaustgas from incinerator furnaces, P5CDF, a type of DXNs,
was chosen for a measuring target. In previous study,
MHI has revealed that the concentration of P5CDF isrelatively high in exhaust gas. In addition, P5CDF has astrong correlation with the TEQ (Toxicity Equivalency
Quantity) value of DXNs as a whole(3).
First, the detecting sensitivity of the system was calibrated using a commercially available standard solution.
Fig. 7 shows the mass spectrum of P5CDF after the fragmentation. Based on other reports, it has been known
that the mass numbers 275, 277, and 279 indicate thefragments in which C, O, and Cl have been eliminatedfrom P5CDF, respectively, and 205 and 207 indicate thefragments in which C, O, and 3 Cl have been eliminated.
The other fragments seem to have originated from theliquid phase components that have eluated from the capillary. Fig. 8 shows the calibration line in a solution
containing 0 to 30 pg of P5CDF.
From this figure, the lowest detectable limit was determined to be 1 pg, based on the indication that the
Y-axis-intercept point, i.e. the count number of ions at
the zero point of the X-axis, was 20 (corresponding tonoise components) and the coefficient was 30 (countnumber of ions/pg) with respect to the least square approximate straight line. The count number 50 at 1 pg
is sufficiently larger than 20 corresponding to the noise
component. The required time for the measurement was
16 minutes, including the time required for the gas chromatography. The trapping time was 0.1 seconds after
optimization in the same way as was the case with
T3CB.
The system was then applied to the monitoring of
P5CDF contained in the actual exhaust gas emitted tothe atmosphere, being installed downstream from thebag filter (dust collector) of the incinerator furnace.
Since the concentration of P5CDF here was as low as

CI

O
CI

400

Content of P5CDF in standard solution (pg)CI

P5CDF-COCI

y=30x+20

600

Fig. 8 Calibration line of P5CDF
Linearity is obtained over a wide range of P5CDF concentration.

CI

60

800

0

P5CDF

80

1 000

200

Time
Fig. 6 Time history of T3CB concentration at outlet of actualincinerator furnace
Since measured T3CB value follows the change of CO concentration,
it is believed that the monitoring system properly measures T3CB
concentration.
100

1 200

CI

P5CDF-COCI3

20
0
-20
200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350

Mass number m/e (-)
Fig. 7 Spectrum after fragmentation of P5CDF
The figure shows a spectrum of P5CDF measured using the system withgas chromatography. The measured values are calculated by the time
integration of the signal of all the fragments.

from the standard gas, which are indicated by solid line
arrows, the fragments indicated by dotted line arrows
are also thought to originate from T3CB. Although the
production processes of these fragments are unknown,
they are thought to be produced by any action due towater vapor contained in the actual exhaust gas. This
assumption was confirmed by an experiment in which
H 2 O was added into the standard gas. The concentration of T3CB was then calculated taking into
consideration these fragments together. The fragments
corresponding to other mass numbers are thought tooriginate from interfering molecules other than T3CB.
Fig. 6 shows the concentration of T3CB changed withthe lapse of time, compared with that of CO. As can beseen from the figure, the concentration of T3CB increases
with that of CO when the concentration of CO is high.
Therefore, it can be seen that the monitoring system canmonitor the state of combustion in an incinerator furnace. It is believed that in regions where theconcentration of CO is low, the concentration of CO is
not suitable as an index for the generation of DXNs, andinstead of CO, DXN precursors such as T3CB are said tohave a better correlations with the state of combustionin the furnace. Consequently, the application of the sys-

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.6 (Dec. 2004)5

0.1 ng/m3 N or less, P5CDF was concentrated using solvent extraction with an automatic concentration device
before being introduced into the monitoring system. The
concentration procedure takes 2 to 6 hours to complete.
The detailed result of this actual exhaust measurement
was described in the previous report(3) and it confirmedthat this system is practically usable.

cluding the technology described in this paper, are applicable not only in improving the environmental
performance of various plants, but also to various fields
such as protecting against atmospheric and soil pollution, the development of new materials and processes,
and the creation of suitable countermeasures against
crimes and terrorisms. Accordingly, MHI intends to utilize the full potential of this system and related processes
widely in the future.

5. Conclusion
The vacuum ultraviolet ionization ion trap mass spectrometry system described above has been developed inorder to monitor the state of combustion in and the emission of trace-level organic chlorine compounds from an
incinerator furnace. As a result, sensitivities of 80 ng/
m3 N (measuring time: twenty seconds) and 1 pg (measuring time: sixteen minutes) for T3CB (standard gas)and P5CDF (standard solution), respectively, wereachieved. In addition, the applicability of the mass spectrometry system to actual incineration plants was
extended by adding a process to eliminate interfering
molecules and a means for extending the lives of the irradiating windows. MHI intends to utilize this analyticaltechnology to improve the performance of waste incinerator furnaces, in the future.
The measurement technologies, developed by MHI, in-References(1) Dobashi et al., Real-time PCB Gas Monitoring System(MOHMS-21GP), Mitsubishi Heavy Industries Technical
Review Vol. 40 No. 5 (2003)(2) Muta et al., Development of High Sensitive Gas Analyzers
Based on Near-Infrared Tunable Diode Laser Absorption
Spectroscopy, Mitsubishi Juko Giho Vol. 38 No.5 (2001)(3) Okada et al., On-line Monitoring Equipments of Dioxins andtheir Precursors for Incineration Plants, Mitsubishi Heavy
Industries Technical Review Vol. 40 No. 5 (2003)(4) Kuribayashi et al., Real-Time Detection System for TraceLevel Chlorinated Organic Compounds, Mitsubishi Juko GihoVol. 38 No.5 (2001)(5) He, L. et al., Development of a Capillary High-Performance
Liquid Chromatography Tandem Mass Spectrometry Systemusing SWIFT Technology in an Ion Trap/Reflectron Time-offlight Mass Spectrometer, Rap. Comm. in Mass Spectrometry, 11 (1997) p.11

Shizuma Kuribayashi

Minoru Danno

Hideo Yamakoshi

Satoshi Sakai

Shigenori Tsuruga

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.40 No.6 (Dec. 2004)

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