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Flame structure studies by high resolution quadrupole mass spectrometry
C O M B U S T I O N A N D F L A M E 58: 73-75 (1984)
73
BRIEF COMMUNICATIONS Flame Structure Studies by High Resolution Quadrupole Mass Spectrometry R. J. HENNESSY, S. J. PEACOCK, and D. B. SMITH British Gas Corporation, London Research Station, London S W 6 2AD, England
INTRODUCTION Mass spectrometry has been used for many years as an analytical technique in flame structure studies [1-4]. A major problem with such work is the ambiguity in the identification of the species of certain mass numbers, particularly at mass 28 (CO and C2H4) , m a s s 29 (HCO and C2Hs), and mass 30 (H2CO and C2H6). There are two solutions to this problem. Harvey and Maccoll [3, 4], working with methane flames, compared the mass spectra using undeuterated and fully deuterated fuels. This approach was successful, particularly in providing positive identification of formyl, but it suffers one major problem due to the shift in position of the flame when CD4 is substituted for CH4, which necessitates a correction term. The superior approach is to exploit the slight differences in the masses of conflicting species. Biordi has described such an experiment resolving CH4 at 16.043 amu from 0 at 15.99 amu [5]. The technique has not been widely exploited, however, in flame studies. This paper shows that high resolution mass spectrometry is a powerful tool in helping to unravel the mass spectra of samples from flames, particularly when used to complement studies at low resolution. This is illustrated with data from CH4 and CH4/C2H6 flames. Copyright © 1984 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017
EXPERIMENTAL The apparatus comprises a low pressure (26 mbar), water-cooled, flat flame burner, with supersonic sampling and mass spectrometric detection, essentially as described previously [3, 4]. In this work, however, a Balzers quadrupole mass spectrometer (QMS 311) controlled by a Balzers microprocessor (QDP 101)i was employed. The microprocessor control allows the m/e setting to be adjusted reproducibly by 1/ 32 ainu, which is just sufficient to separate species containing -CH2- from those with an O- group. Signal recovery was via synchronous ion counting. Preliminary experiments with cold gases showed that satisfactory separation into the appropriate channels was achieved. High reproducibility in m/e setting was essential for this work, but even with microprocessor control, the signals were susceptible to drift over a period of several hours. All flames were stoichiometric. For methane flames the initial molar composition was 19% CH4, 38.1% 02, 42.9% Ar. For the other flames the methane was progressively replaced by ethane, keeping the stoichiometry and oxygen and argon flows constant. Fuel percentages are on a molar basis.
0010-2180/847503.00
74
R . J . HENNESSY ET AL.
RESULTS AND DISCUSSION
~OC
Methane Flames
•
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0
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0
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0 •
oCH0 °
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The signal at nominal mass 30 can arise from CH20 and C2H6. At high resolution, the two are separated: 30.01 amu (CH20) and 30.05 amu (C2H6). Methane flames gave a maximum of 80 counts s - ] in the ethane channel and 220 counts s - J in the formaldehyde channel (Fig. 1). On the basis of the relative mass spectrometer sensitivities and known formaldehyde concentration [4], the maximum ethane mole fraction is estimated to be about 10 -3 Harvey and Maccoll [4] were unable to detect ethane. However, they only looked for it at the position of maximum formaldehyde, assuming ethane also peaked here. But Fig. 1 shows that ethane peaks much earlier (1.4 mm from the burner, compared with 2.6 mm for formaldehyde). At the formaldehyde peak, ethane is barely a tenth of its maximum, severely aggravating their difficulties in detecting it. Such an early peak poses interesting questions about the formation of ethane. The reaction CH3 + CH 3 ( + M) --* C2H 6 ( + M) is the obvious candidate, but the CH3 profile closely resembles that of formaldehyde (i.e., peaks much later), which makes this explanation seem unlikely. An
c o
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1000
Mass 30
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Fig. 2. High resolution ion signals at nominal mass m/e of 29 for a stoichiometric flame burning at 26 mbar pressure.
alternative source may be necessary, a point which will be covered in a future paper. Mass 29 The signal at nominal mass 29 can arise from CHO (29.00 amu) or from C2H5 (29.04 amu). Again at high resolution the two are separated (Fig. 2). However, the interpretation is now more complicated. The high mass spectrometer ionizing voltage (70 eV) used means that the mass spectral cracking of ethane and formaldehyde contributes to the signal. The use of low ionizing voltages would eliminate these contributions, but then signal levels are impossibly low. A further complication arises from the temperature dependence of the cracking patterns. For ethane this dependence was established in the early stages of a pure ethane flame (where CH:O, CHO, and C2H5 are negligible). The same ratio with the same temperature variation was found in the methane flame, strongly suggesting that all the m / e = 29 signal arises from ethane cracking. Thus it is concluded that there is virtually no ethyl present. This provides confirmation of Harvey and Maccoll's [3] identification of formyl. Mass 28
distance
from burner
(ram)
Fig 1 High resolution formaldehyde and ethane profiles
for stoichiometric methane, methane/ethane, and ethane flames burning at 26 mbar pressure.
Ethylene (28.03 amu) and carbon monoxide (27.99 amu) appear at nominal mass 28. The carbon monoxide channel contains negligible
FLAME STRUCTURE BY QUADRUPOLE MASS SPECTROMETRY ETHANE PROFILE ORRECTED FOR ORMALDEHYDE
PROFILES FOR ETHANE AND ORMALDEHYDE
30
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75
signal at mle=30 o 33%metha ne f l a m e s
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z. 5 I d i s t a n c e from b u r n e r (mm)
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Fig. 3. Low resolution ethane and formaldehyde profiles from methane and methane/ ethane stoichiometric flames burning at 26 mbar.
contributions from other species, and so provides a straightforward determination of CO. However, problems do arise with ethylene, which is present in considerably lower concentrations than carbon monoxide, because the resolution is insufficiently high to prevent some overlap from the larger carbon monoxide signal in the next channel. Any attempt to correct for this proved unreliable because the correction term was of similar magnitude to the ethylene signal. This illustrates a limitation of the technique. In this case, however, ethylene can be obtained from low resolution experiments at m/ e = 27. Methane/Ethane Flames
High resolution mass spectrometry allows formaldehyde to be measured unambiguously in methane/ethane flames. The contribution from ethane overlap was small and subtracted readily. Figure 1 shows formaldehyde profiles across the range of fuels. The outstanding feature is the similarity of the CH20 yield. Both profile and peak concentration are unchanged even though
fuel composition is drastically altered. An important consequence of this is that ethane concentrations can be measured at low resolution simply by subtracting the formaldehyde component from the total mass 30 signal. Results are shown in Fig. 3. Ethane profiles obtained at high resolution are consistent with this, adding to our confidence in the procedure. Quantitative data on ethane will be presented in future work.
REFERENCES 1. 2. 3. 4. 6.
Biordi, J. C., Prog. Energy and Combust, Sci. 3:151 (1977). Peeters, J., and Mahnen, G., 14th Symposium (Int.) on Combustion, Combustion Institute, 1972, p. 133. Harvey, R., and Maccoll, A., J. Chem. Soc. Faraday 1 75:2423 (1979). Harvey, R., and Maccoll, A., 17th Symposium (Int.) on Combustion, Combustion Institute, 1979, p. 857. Biordi, J. C., Lazzara, C. P., and Papp, J. F., 15th Symposium (Int.) on Combustion, Combustion Institute, 1975, p. 917.
Received 18 November 1983; revised 11 April 1984
73
BRIEF COMMUNICATIONS Flame Structure Studies by High Resolution Quadrupole Mass Spectrometry R. J. HENNESSY, S. J. PEACOCK, and D. B. SMITH British Gas Corporation, London Research Station, London S W 6 2AD, England
INTRODUCTION Mass spectrometry has been used for many years as an analytical technique in flame structure studies [1-4]. A major problem with such work is the ambiguity in the identification of the species of certain mass numbers, particularly at mass 28 (CO and C2H4) , m a s s 29 (HCO and C2Hs), and mass 30 (H2CO and C2H6). There are two solutions to this problem. Harvey and Maccoll [3, 4], working with methane flames, compared the mass spectra using undeuterated and fully deuterated fuels. This approach was successful, particularly in providing positive identification of formyl, but it suffers one major problem due to the shift in position of the flame when CD4 is substituted for CH4, which necessitates a correction term. The superior approach is to exploit the slight differences in the masses of conflicting species. Biordi has described such an experiment resolving CH4 at 16.043 amu from 0 at 15.99 amu [5]. The technique has not been widely exploited, however, in flame studies. This paper shows that high resolution mass spectrometry is a powerful tool in helping to unravel the mass spectra of samples from flames, particularly when used to complement studies at low resolution. This is illustrated with data from CH4 and CH4/C2H6 flames. Copyright © 1984 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017
EXPERIMENTAL The apparatus comprises a low pressure (26 mbar), water-cooled, flat flame burner, with supersonic sampling and mass spectrometric detection, essentially as described previously [3, 4]. In this work, however, a Balzers quadrupole mass spectrometer (QMS 311) controlled by a Balzers microprocessor (QDP 101)i was employed. The microprocessor control allows the m/e setting to be adjusted reproducibly by 1/ 32 ainu, which is just sufficient to separate species containing -CH2- from those with an O- group. Signal recovery was via synchronous ion counting. Preliminary experiments with cold gases showed that satisfactory separation into the appropriate channels was achieved. High reproducibility in m/e setting was essential for this work, but even with microprocessor control, the signals were susceptible to drift over a period of several hours. All flames were stoichiometric. For methane flames the initial molar composition was 19% CH4, 38.1% 02, 42.9% Ar. For the other flames the methane was progressively replaced by ethane, keeping the stoichiometry and oxygen and argon flows constant. Fuel percentages are on a molar basis.
0010-2180/847503.00
74
R . J . HENNESSY ET AL.
RESULTS AND DISCUSSION
~OC
Methane Flames
•
l
~ •
20(
•
!
IU 15( |
8
o
~
•
form Id h I] | ol00%methone flo reel | •lO0%ethone , [ l e 57%methane ,, | J ethqne signq~ | / ° 100 */,methane flom~ •
| o
o •
0
•
•
0
•
0 •
oCH0 °
o
•
o
i u ~u 80G
The signal at nominal mass 30 can arise from CH20 and C2H6. At high resolution, the two are separated: 30.01 amu (CH20) and 30.05 amu (C2H6). Methane flames gave a maximum of 80 counts s - ] in the ethane channel and 220 counts s - J in the formaldehyde channel (Fig. 1). On the basis of the relative mass spectrometer sensitivities and known formaldehyde concentration [4], the maximum ethane mole fraction is estimated to be about 10 -3 Harvey and Maccoll [4] were unable to detect ethane. However, they only looked for it at the position of maximum formaldehyde, assuming ethane also peaked here. But Fig. 1 shows that ethane peaks much earlier (1.4 mm from the burner, compared with 2.6 mm for formaldehyde). At the formaldehyde peak, ethane is barely a tenth of its maximum, severely aggravating their difficulties in detecting it. Such an early peak poses interesting questions about the formation of ethane. The reaction CH3 + CH 3 ( + M) --* C2H 6 ( + M) is the obvious candidate, but the CH3 profile closely resembles that of formaldehyde (i.e., peaks much later), which makes this explanation seem unlikely. An
c o
o
o
1000
Mass 30
o u
o
Q
13
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•
O O
o
o
60C
•
•
o
o
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o
o
eC2H5°(x 10 )
o O
ZOO o
i
1
I
I
2 3 ~ d i s t o n c e from b u r n e r ( r a m )
I
I
5
Fig. 2. High resolution ion signals at nominal mass m/e of 29 for a stoichiometric flame burning at 26 mbar pressure.
alternative source may be necessary, a point which will be covered in a future paper. Mass 29 The signal at nominal mass 29 can arise from CHO (29.00 amu) or from C2H5 (29.04 amu). Again at high resolution the two are separated (Fig. 2). However, the interpretation is now more complicated. The high mass spectrometer ionizing voltage (70 eV) used means that the mass spectral cracking of ethane and formaldehyde contributes to the signal. The use of low ionizing voltages would eliminate these contributions, but then signal levels are impossibly low. A further complication arises from the temperature dependence of the cracking patterns. For ethane this dependence was established in the early stages of a pure ethane flame (where CH:O, CHO, and C2H5 are negligible). The same ratio with the same temperature variation was found in the methane flame, strongly suggesting that all the m / e = 29 signal arises from ethane cracking. Thus it is concluded that there is virtually no ethyl present. This provides confirmation of Harvey and Maccoll's [3] identification of formyl. Mass 28
distance
from burner
(ram)
Fig 1 High resolution formaldehyde and ethane profiles
for stoichiometric methane, methane/ethane, and ethane flames burning at 26 mbar pressure.
Ethylene (28.03 amu) and carbon monoxide (27.99 amu) appear at nominal mass 28. The carbon monoxide channel contains negligible
FLAME STRUCTURE BY QUADRUPOLE MASS SPECTROMETRY ETHANE PROFILE ORRECTED FOR ORMALDEHYDE
PROFILES FOR ETHANE AND ORMALDEHYDE
30
"6 ~u 2~
75
signal at mle=30 o 33%metha ne f l a m e s
"o
o 57'/. 075*/.
- "~
,89'/,
~
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\
\
. . . .
. . . . .
.
.
.
o,ormo, e.yde .ignol
fromlOO*/,methane flame (corrected for ethane
\
Q.
E 10 o
q
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1
I
2
3
- ~
z. 5 I d i s t a n c e from b u r n e r (mm)
2
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Fig. 3. Low resolution ethane and formaldehyde profiles from methane and methane/ ethane stoichiometric flames burning at 26 mbar.
contributions from other species, and so provides a straightforward determination of CO. However, problems do arise with ethylene, which is present in considerably lower concentrations than carbon monoxide, because the resolution is insufficiently high to prevent some overlap from the larger carbon monoxide signal in the next channel. Any attempt to correct for this proved unreliable because the correction term was of similar magnitude to the ethylene signal. This illustrates a limitation of the technique. In this case, however, ethylene can be obtained from low resolution experiments at m/ e = 27. Methane/Ethane Flames
High resolution mass spectrometry allows formaldehyde to be measured unambiguously in methane/ethane flames. The contribution from ethane overlap was small and subtracted readily. Figure 1 shows formaldehyde profiles across the range of fuels. The outstanding feature is the similarity of the CH20 yield. Both profile and peak concentration are unchanged even though
fuel composition is drastically altered. An important consequence of this is that ethane concentrations can be measured at low resolution simply by subtracting the formaldehyde component from the total mass 30 signal. Results are shown in Fig. 3. Ethane profiles obtained at high resolution are consistent with this, adding to our confidence in the procedure. Quantitative data on ethane will be presented in future work.
REFERENCES 1. 2. 3. 4. 6.
Biordi, J. C., Prog. Energy and Combust, Sci. 3:151 (1977). Peeters, J., and Mahnen, G., 14th Symposium (Int.) on Combustion, Combustion Institute, 1972, p. 133. Harvey, R., and Maccoll, A., J. Chem. Soc. Faraday 1 75:2423 (1979). Harvey, R., and Maccoll, A., 17th Symposium (Int.) on Combustion, Combustion Institute, 1979, p. 857. Biordi, J. C., Lazzara, C. P., and Papp, J. F., 15th Symposium (Int.) on Combustion, Combustion Institute, 1975, p. 917.
Received 18 November 1983; revised 11 April 1984