1999 atmos. environ. zervas e. collection and analysis of organic acids in exhaust gas. comparison...

7/30/2019 1999 Atmos. Environ. Zervas E. Collection and Analysis of Organic Acids in Exhaust Gas. Comparison of Different Methods

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Atmospheric Environment 33 (1999) 4953}4962

Collection and analysis of organic acids in exhaust gas.Comparison of di!erent methods

E. Zervas*, X. Montagne, J. Lahaye

Institut Franc7 ais du Pe&trole (IFP), 1 et 4 avenue du Bois Pre&au, F-92852 Rueil-Malmaison cedex, France

Institut de Chimie des Surfaces et Interfaces, 15 rue Jean Starcky, F-68057 Mulhouse cedex, France

Received 1 August 1997; accepted 3 February 1999

Abstract

This paper reports the development of a speci"c method to identify organic acids in exhaust gases. The organic acids

are collected in two impingers containing liquids (pure water or Na

CO

1% aqueous solution) and four cartridges

containing solids (silica, #uorisil, alumina B and alumina N). Once collected, the acids are eluted of the solids by a hot

water stream. These traps performances, in terms of organic acids collection and elution e$ciency, are evaluated and

compared. Two sources are used to produce the gas #ow containing organic acids: one generates a #ow whose

concentration is known and stable, the other produces organic acids among other combustion products. For eluted

solutions analysis, two methods are used: isocratic ionic chromatography/conductivity detection and GC/FID. Their

e$ciency in separating 10 aliphatic acids are compared. Their characteristics such as detection limits, detection linearity,

repeatability and possible interferences with other components found in exhaust gases are determined. The stability of the

organic acids solutions is also studied. Lastly, the use of these methods is illustrated by the analysis of the gas-phaseorganic acids exhausted by a spark ignition and by a diesel engine. 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Organic acids; Ionic chromatography; Internal combustion engines; Gas chromatography; Propane

1. Introduction

The usual "elds of organic acids analysis are the

atmosphere control in urban or rural areas and vehicle

emission studies (Kawamura, 1985). Of late, due to thepossible toxicity and reactivity of these compounds, in-

creased attention has been paid to the vehicle emission

studies but, in literature there are not many papers deal-

ing with organic acids emissions. Although these pollu-

tants can be analysed by continuous methods, as FTIR

for formic acid analysis (Grosjean, 1990), the most com-

mon procedures involve their trapping in a liquid such as

water (Smith, 1985), or aqueous solutions of Na

CO

(Lopez, 1987), or KOH (Kawamura, 1985). The collec-

*Corresponding author.

Present address: Ecole des Mines de Nantes, 4 rue Alfred

Kastler, F-44070 Nantes cedex 03, France.

tion may also take place on a "lter (Kawamura, 1985;

Yokouchi, 1986), a solid cartridge (Hekmat, 1991;

Grosjean, 1990) or a denuder tube (Lawrence, 1992;

Slanina, 1992). When a cartridge is used, the acids need to

be eluted before analysis.

So far, the only continuous method, the FTIR, can

only analyse formic acid and cannot be used to measure

out the other organic acids. Liquid or solid traps are

needed. As organic acid concentrations in atmosphere or

in exhaust gases are very low (some ppbs or ppms), the

trap has to be extremely e$cient and independent of the

gas volume collected. Literature evokes traps with e$-

ciency rates up to 99% (Smith, 1985) (water), but no

details are provided on the way acids are collected. Fil-

ters are not as e$cient and are a too heavy a technique to

be used in a routine analysis. Denuder tubes do not resist

to high temperatures of exhaust gases. Further, the elutedsolution has to be compatible with the analytical method,

for example, Na

CO

solution is not compatible with

GC analysis.

1352-2310/99/$- see front matter 1999 Elsevier Science Ltd. All rights reserved.

PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 00 2 7 8 - 2


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The eluted solution analysis can be performed by

chromatographic methods such as ionic chromato-

graphy (Smith, 1985), ionic exclusion chromatography

(Tanaka, 1986), GC/FID or GC/MS (Yokouchi, 1986),

HPLCor capillary electrophoresis (Levi, 1993). A deriva-

tion of the captured acids is sometimes needed before

analysis (Kawamura, 1985; McCalley, 1984), but thisadds a supplementary step which is not always e$cient.

Most of these methods cannot completely discriminate

between all the organic acids, or su!er from interferences

with other atmospheric compounds or combustion prod-

ucts, or need several intermediate steps which induce

a loss in e$ciency or make them too heavy to be applied

in routine analysis.

This article proposes a method for organic acids deter-

mination and compares di!erent ways of trapping gas-

phase organic acids. The performances of two liquid

traps are determined and compared: deionised water and

Na

CO

solution, and four solid ones: silica, #uorisil,

alumina B and alumina N. The elution e$ciency is also

compared. To generate a gas stream with a well-known

concentration in organic acids, two special sources have

been used, a quantitative one, which generates a gas

stream with stable and constant acid concentrations, and

a qualitative one, which generates organic acids within

other products. To avoid very complicated con"gura-

tions, the "nal analysis is performed by two common

analytical methods, isocratic ionic chromatography and

gas chromatography. The performances of these analyti-

cal methods are determined and compared. Also, theperformances of the global method is presented. Then,

these methods are applied to the determination of the

organic acids emitted either by a spark ignition engine or

by a Diesel engine fed with special synthetic fuels.

2. Experimental

2.1. Organic acids sources

One of the major problems encountered during the

development of a pollutant trap is the availability of a gas

stream containing a known and stable concentration of

the pollutant. Atmospheric pollutant concentrations are

usually very low and very disperse while vehicle exhaust

gas pollutant concentrations depend on the engine, on

the running conditions and on the fuel. In this work, two

sources have been used to obtain a gas stream containing

organic acids. The "rst one is a system which evaporates

an aqueous solution of the selected acids. A vapour

stream with known and stable concentration of organic

acids is then obtained. The second system is a combus-

tion reactor where organic acids are obtained togetherwith other combustion products.

The evaporation system involves a peristaltic pump

that brings the aqueous solution (organic acid concentra-

tion well known) to a furnace, where evaporation takes

place leading to an homogeneous gas-phase #ow. A ni-

trogen #ow can also be added to the furnace to dilute the

gas-phase and obtain lower concentrations. The aqueous

solution contains the "rst three acids: formic, acetic and

propionic, which are the main acids found in vehicle

emissions (Kawamura, 1985). Two furnace temperaturesare used: 423 and 473 K. These tests permitted deter-

mination of the thermal stability of organic acids at these

temperatures and veri"cation of the concentrations of the

gas stream obtained by this source. To achieve it, the

vapour is passed through two glass impingers containing

water, at two furnace temperatures, using di!erent time

periods. The analysis of the "nal solution showed a good

agreement between the acids theoretical emitted and

captured quantity; in such conditions the di!erences re-

main under 3%. As organic acid concentrations in ex-

haust gases reported in literature are very low, (ppm or

even ppb, Kawamura, 1985), the aqueous solution con-

centration and the nitrogen #ow have to be adjusted to

reach similar gas-phase concentrations. For these rea-

sons, the gas-phase concentration in organic acids varies

from 0.2 to 12 ppm.

The second system is a combustion reactor where

propane is used as a fuel. Propane is mixed with air in an

homogenisation chamber and then burned on a #at

burner. A speci"c probe, cooled by internal oil circula-

tion at 383 K and connected to a pump, takes out the

burned gases at di!erent distances from the burner. This

system is used as a qualitative source of organic acids andalso to study the interferences with other combustion

products.

In all the cases, the gases are transferred from the

source to the capture device through a 383 K heated line.

2.2. Collection

Two liquid and four solid traps are tested for organic

acids collection. The "rst ones are either deionised water

or a Na

CO

1% aqueous solution. The others are

cartridges containing either silica or #uorisil or alumina

N or alumina B (purchased from water).

In the case of the liquid traps, 20 ml of liquid is placed

in each of the two glass impingers connected in a

line. The gas #ow speed through the liquid is 3.33;

10\ m s\ (2 L min\). To minimise evaporation

losses, these impingers are placed in icy water.

When cartridges are used, they are directly connected

to the heated line. The gas containing the acids #ows at

the same speed as that of liquid solutions. As these gases

are hot ('378 K) and as the cartridges do not with-

stand such temperatures, gases are cooled by an external

cold water #ow in a speci"c glass shell. To determine thecartridge e$ciency, they are connected to impingers con-

taining 2;10\ m (20 mL) pure water. Before the test,

the cartridges are cleaned by placing them under a hot

4954 E. Zervas et al. /Atmospheric Environment 33 (1999) 4953}4962


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water stream (363 K) all night long, under ultrasounds.

After the capture tests, 1;10\ m (1 mL) of 363 K hot

water is injected for elution.

The gas volume passing through the impingers or

cartridges is kept constant at 0.01 m (10 L) during all the

collection tests (except the tests of the sources validation

and those of the gas volume in#uence on the e$ciency,where di!erent gas volumes used ).

2.3. Analysis

For the eluted solution analysis, two methods were used:

isocratic ionic chromatography and gas chromatography.

The "rst method enables to measure formic acid. The

apparatus used is a DIONEX Series 2000i isocratic ionic

chromatograph equipped with an IonPac AS4 column.

The elution solution is an aqueous solution of 0.5;

10\ M of Na

B

O

whose #ow speed is 3.33;

10\ m s\ (2 mL min\); the regeneration is obtained

with a H

SO

0.013 M aqueous solution. The detection

is conductimetric.

The second method, used to measure acetic, heavier

aliphatic acids and benzoic acid, requires a gas chromato-

graph equipped with a FID detector. The apparatus is

a VARIAN 3300 gas chromatography equipped with

a capillary column HP Megabore Innowax 30 m;

0.53 mm. Helium is the carrier gas with a speed of

4;10\ m s\. The temperature of the furnace is pro-

grammed: initial temperature at 393 K during 120 s

(2 min) and "nal temperature at 473 K with a heatingspeed of 0.066 K s\ (4 K min\). The injector and the

detector temperatures are settled at 523 K.

As Na

CO

solutions are incompatible with this ana-

lytical method, when such a trap was used, the ionic

chromatography analysis alone was carried out and

propionic acid analysis was omitted.

3. Results and discussion

3.1. Analysis

(a) Ionic chromatography, in the described operating

conditions, can separate formic acid from acetic acid, but

not acetic acid from propionic acid. Our attempts to

separate these two acids by changing either the elution

#ow or the Na

B

O

concentration did not succeed.

Isobutyric acid is also poorly separated from acetic acid

and heavier organic acids cannot be completely separ-

ated from one another. A gradient of Na

B

O

concen-

trations has also been tested, but the acids could still not

be separated. Heavier acids can probably be separated

under other conditions, for example, using another col-umn and gradient of concentration for the elution solu-

tion. Fig. 1 presents a typical analysis of the "rst two

acids with this method.

Fig. 1. Analysis of formic and acetic acid by ionic chromatogra-

phy/suppressed conductivity detection. Aqueous solution of 10

ppm. Apparatus used: DIONEX Series 2000i, column IonPac

AS4, isocratic elution with 0.5;10\ M of Na

B

O

at a #ow

rate of 3.33;10\ m s\ (2 ml min\), regenerate solution:

aqueous solution of H

SO

0.013 M.

The response of the conductivity detector is linear

within the range from 0 to 100 ppm (Fig. 2). Fig. 3 pres-

ents the relative standard deviation (r.s.d.) for formic acid

analysis for the same range of concentrations of a stan-

dard solution. The repeatability of this method is high, its

r.s.d. being 5% for a 1 ppm formic acid solution. The

detection limit of this method is quite low: some ppbs of

acid (less than 10). The incertitude of preparing a stan-

dard solution at this concentration level does not allow

a closer determination of the detection limits.

The possible interferences with other compounds of

propane combustion or of exhaust gases have been exam-

ined. That for, solutions containing other likely ions were

prepared and analysed. The formate ion does not inter-

fere with the Cl\, F\, NO\

, PO\

, SO\

ions nor with

the other aliphatic acids.

(b) In the described operating conditions, gas chromato-

graphy separates completely the heavier aliphatic acids

from benzoic acid. Fig. 4 shows a typical chromatogram

of these acids.

The response of the FID detector for acetic and

propionic acids is linear from 0 to 100 ppm (Fig. 5). Fig. 6

presents the r.s.d. for acetic and propionic acids fordi!erent concentrations of a standard solution. Gas

chromatography is less repetitive than ionic chromato-

graphy, the r.s.d. of solutions containing 4.6 ppm acetic

E. Zervas et al. /Atmospheric Environment 33 (1999) 4953}4962 4955


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Fig. 2. Conductimetric detector response for formic acid analy-

sis as a function of the solution concentration.

Fig. 3. Relative standard deviation of formic acid analysis by

ionic chromatography as a function of the solution concentra-

tion.

acid or 7.6 ppm propionic acid are, respectively, 10.1 and

10.5%.

The detection limits of this method are below 0.5 ppm

for the two acids.

The possible interference with other compounds ofpropane combustion or of exhaust gas has been exam-

ined using the technique described above. No interfer-

ence has been observed with C}C

alcohols, C

}C

aldehydes and C}C

hydrocarbons (para$ns, ole"ns,

aromatics).

This method cannot be used for the analysis of

formic acid which has a very low response in FID

(Dietz, 1967), but is convenient for the analysis of heavier

acids.

3.2. Organic acid sources

In the combustion reactor used as the organic acids

source, the acids can be found only very close to the

burner: within the "rst 5 mm around it. In the "rst 3 mm,

concentrations are high. Reaching the fourth and the "fth

millimetre, they decrease sharply down. No acids are

found from the sixth millimetre and above. Propane

combustion generates only formic, acetic and propionic

acids (Zervas, 1996). It is not clear whether the acids are

formed in the #ame or are products from other com-

pounds through some cooling reactions in the probe.

Figs. 7}9 show the organic acid concentrations obtained

depending on the distance of the burner and on the

equivalence ratio. These concentrations range from

0.09 to 10.6 ppm for formic acid, from 0.11 to 8.3 ppm

for acetic acid and from 0.05 to 0.069 ppm for propionic

acid.

3.3. Collection ezciency

The collection e$ciency of the di!erent traps was

characterised as the percentage collected in the "rst im-pinger (or cartridge) of the total one.

The e$ciency of water and Na

CO

solutions for the

"rst two acids is presented in Fig. 10. In the test condi-

tions, it is above 90% for both liquids. It is always 1 or 2

units higher with formic than with acetic acid. Na

CO

solution traps the acids better than water; the corre-

sponding values are 4 or 5 units higher. Literature re-

ports values reach 99% with water (Smith, 1985) and

92% with Na

CO

solutions (Bodek, 1980), but e$ciency

values are in#uenced by the gas-phase concentration and

by the #ow rate of the gas passing through the impingers

(Zervas, 1996). The higher these parameters values, the

lower the e$ciency values.

Fig. 11 shows the in#uence of gas volumes, ranging

from 0.038 to 0.095 m, on e$ciency when concentra-

tions of formic, acetic and propionic acid are, respectively,

9, 8.5 and 0.45 ppm. The gas volume passing through the

impingers does not in#uence the e$ciency for such gas-

phase concentrations. The decrease is 1% for acetic and

propionic acid and 0.4% for formic acid.

The e$ciency of the four cartridges used is presented in

Fig. 12. For formic acid, the four types have an e$ciency

of 100%. For acetic and propionic acids, the twoaluminas have an e$ciency of 100%, but silica and

#uorisil e$ciencies are lower (45}90%), silica being bet-

ter (about ten units) than #uorisil.



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Fig. 4. Analysis of nine aliphatic acids and of benzoic acid by gas chromatography/FID detection. Aqueous solution of 6}8 ppm.

Column used: HP Megabore Innowax 30 m;0.53 mm, carrier gas #ow: 4;10\ m s\, injector and detector temperatures: 523 K,

initial column temperature: 393 K, initial time: 120 s, "nal temperature: 473 K, heating rate: 0.066 K s\.

Fig. 5. FID response for acetic and propionic acid analysis as

a function of the solution concentration.

3.4. Elution ezciency

The e$ciency of the cartridge elution is presented in

Fig. 13. This item is de"ned as the percentage eluted the

"rst time over the total quantity eluted ("rst and second

Fig. 6. Relative standard deviation of acetic and propionic acid

analysis by gas chromatography as a function of the solution

concentration.

time). The elution of silica and of #uorisil is about thesame: 100% for formic acid, 80}85% for acetic acid and

90% for propionic acid. Alumina B leads to lower values,

from 50 to 85%. Last, alumina N has no e$ciency in



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Fig. 7. Combustion of propane. Formic acid concentration in

the gas-phase as a function of the equivalence ratio and the

distance probe-burner. Total (propane#air) #ow"3.33;

10\ m s\, inspiration #ow"3.33;10\ m s\.

Fig. 8. Combustion of propane. Acetic acid concentration in the

gas-phase as a function of the equivalence ratio and the distance

probe-burner. Total (propane#air) #ow"3.33;10\ m s\,

inspiration #ow"3.33;10\ m s\.

elution. Acids are adsorbed so strongly that hot water

cannot elute them.The yield of the collection, de"ned as the product ez-

ciency of the collection x ezciency of the elution, reaches

100% in the cases of formic acid collected on silica or on

Fig. 9. Combustion of propane. Propionic acid concentration in

the gas-phase as a function of the equivalence ratio and the

distance probe-burner. Total (propane#air) #ow"3.33;

10\ m s\, inspiration #ow"3.33;10\ m s\.

Fig. 10. E$ciency of formic and acetic acid collection in

2.0;10\ m of a 1% Na

CO

aqueous solution or in de-

ionised water. Gas #ow rate"3.33;10\ m s\, gas vol-

ume"0.01 m, gas-phase concentration"formic acid: 1 to

6 ppm, acetic acid: 1}5 ppm.

#uorisil. It is nil in the case of alumina N. Therefore, solid

cartridges cannot be used to capture all the organic acids.It was decided to collect organic acids in pure water and

to analyse the solution by the combination of the two

methods described above: ionic and gas chromatography.



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Fig. 11. In#uence of the gas volume passing through the

deionised water impinger on its e$ciency. Volume of the

solution"2.0;10\ m, gas #ow rate"3.33;10\ m s\,

gas-phase concentration: formic acid 9 ppm, acetic acid 8.5 ppm,

propionic acid 0.45 ppm.

Fig. 12. E$ciency of the four types of cartridges on acid collec-

tion. Gas-phase concentrations: formic acid 1}6 ppm, acetic acid

0.5}5 ppm, propionic acid 0.5}5 ppm, gas volume: 0.01 m, gas

#ow rate: 3.33;10\ m s\.

3.5. Stability of the eluted solutions

The stability of organic acid solutions has to be con-

trolled. Six aqueous solutions of organic acids issued

Fig. 13. E$ciency of elution on the four types of cartridges.

Elution by deionised water, volume"1;10\ m, temperature

363 K.

from propane combustion were analysed, at weekly inter-

vals, during six weeks. In the intermediate period, they

were refrigerated at 243 K and preserved from light.

During the "rst two weeks, the di!erences in concen-

tration remained under 15%. Later, acetic acid concen-

tration increased in all solutions and reached 150% of

the initial value on week six. Formic and propionic acid

concentrations decreased and reached 10% of the initial

values after six weeks. Obviously, other compounds,

issued from propane combustion and collected in the

impingers were oxidised into acetic acid. Formic and

propionic acid were oxidised to simplest compounds.

3.6. Linearity, repeatability and detection limits of the

method

This method includes three steps: evaporation, collec-

tion and analysis of organic acids. The evaporation of an

aqueous solution of these acids is used as source. The

collection is performed in two impingers each containing

20 mL of deionised water and the analysis of the "nal

solution by two methods: ionic chromatography for the

analysis of the formic acid and gas chromatography for

the analysis of the heavier aliphatic acids. Linearity,

repeatability and detection limits of the global method

are presented here.

Figs. 14 and 15 present the area, as given by the

chromatograms, as a function of the collection time, for

two gas-phase concentrations obtained by the evapor-ation system: 1 and 10 ppm of each of the three acids:

formic, acetic and propionic acid, using the same condi-

tions of collection and analysis as presented before. The



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Fig. 14. Linearity of the global method as a function of the

collection time. Gas-phase concentrations: 1 ppm of each acid,

gas #ow rate: 3.33;10\ m s\, volume of the collection solu-

tion: 2.0;10\ m of deionised water.

Fig. 15. Linearity of the global method as a function of the

collection time. Gas-phase concentrations: 10 ppm of each acid,

gas #ow rate: 3.33;10\ m s\, volume of the collection solu-

tion: 2.0;10\ m of deionised water.

linearity is high, the r of these lines are always better

than 0.98.The relative standard deviation of the global method,

as a function of the solution concentration for a gas-

phase concentration of 1 ppm for the three acids, is

Fig. 16. Relative standard deviation of the global method as

a function of the solution concentration. Gas-phase concentra-

tions: 1 ppm, gas #ow rate: 3.33;10\ m s\, volume of the

collection solution: 2.0;10\ m of deionised water, analysis of

the formic acid by ionic chromatography and of the acetic and

propionic acid by gas chromatography.

Fig. 17. Analysis by ionic chromatography of the organic acids

in spark ignition engine exhaust gas. Collection in two impingers

each containing 20 mL deionised water each, gas #ow:

3.33;10\ m s\, gas volume: 0.01 m, equivalent ratio: 1.0.

presented in Fig. 16. The r.s.d. values are lower than 10for the concentrations of more than 1, 8 and 12 ppm in

the solution for the formic, acetic and propionic acid

respectively. These values are between 3 and 25% higher



9/10

Fig. 18. Analysis by gas chromatography of the organic acids in spark ignition engine exhaust gas. Collection in two impingers each

containing 20 mL of deionised water each, gas #ow: 3.33;10\ m s\, gas volume: 0.01 m, equivalent ratio: 1.0.

than the rsd analysis alone (the most important di!erence

is observed in diluted solutions).

The detection limits of the method is under 10 ppb for

the formic acid and below 0.5 ppm for the heavier

aliphatic acids.

3.7. Practical use with engine exhaust gases

In the exhaust gases of a spark ignition engine fed with

several commercial fuels and within a large range of

tuning, formic, acetic, propionic, acrylic, butyric and

isovaleric acids can be detected, in concentrations of

0.1}15, 2}40, 0.5}80, 0}7, 0.8}3.5 and 0}5 ppm, respec-

tively. These concentrations depend on the fuel and the

equivalence ratio used. The other acid's concentrations

are below the detection limits. A Diesel engine, fed with

two di!erent commercial fuels, produces detectable con-

centrations of the "rst three acids (formic 2}6, acetic 4}8

and propionic 0.4}3 ppm).

The other compounds of the exhaust gases do not

interfere with the analysis of these acids. Figs. 17 and 18

are two chromatograms, corresponding to the analysis,

by ionic and by gas chromatography respectively, of

the acids produced by the spark ignition engine at

stoichiometry.

4. Conclusion

The evaporation of an aqueous solution containing

organic acids can be successfully used to generate a gas

#ow with a known and stable concentration in these

acids. Propane combustion on a #at burner can also

generate the "rst three acids in the "rst "ve mil-

limetres of the burner. It is not clear whether the organic

acids are formed in the #ame or in the probe, but thismethod can be used as a stable source of these com-

pounds.

Organic acid trapping can be performed in deionised

water or in Na

CO

aqueous solution. The e$ciency of

these liquid traps is over 92% for the "rst two acids,

under the conditions we used. All four solid traps are

e$cient in collecting formic acid but only the two

aluminas can collect quantitatively acetic and propionic

acids. The elution e$ciency using hot water reaches

100% in the case of formic acid adsorbed on silica or

#uorisil; the other acids have lower elution rates

(80}90%) of these solids. Alumina B has a very low and

alumina N no e$ciency in elution; only silica could be

used for formic acid collection.

The analysis of the collected acids can be performed by

two methods, isocratic ionic chromatography for formic

acid and gas chromatography for the other acids. Or-

ganic acids have no interference with other compounds

of the exhaust gases and the detection limits are low,

some ppb in the case of formic acid and under 0.5 ppm in

the case of the other acids.

This method is applied to the analysis of the organic

acids produced by two internal combustion engines, a SIand a Diesel one, using commercial fuels. Six acids are

detected in the exhaust gases of the "rst engine emissions

and three in the second.



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internal combustion engines, Ph.D. Thesis, University of

Mulhouse, France.


1999 atmos. environ. zervas e. collection and analysis of organic acids in exhaust gas. comparison...

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