Calibration and Optimization of PTR-MS for
Measurement of Methyl Hydroperoxide (CH3OOH)
Patrick Lampe, Matt Erickson, and Tom Jobson
Laboratory for Atmospheric Research
Department of Civil and Environmental Engineering
Synthesis continued
32
63
45
34
47
51
35
44
29
46
42
2
49
48
10
36
10
38
64
65
61
MHP
1
41
2425
23
NO2+
• The second impinger was kept at approximately 5 degrees Celsius to
collect the MHP.
62
50
73 75
53
5657
58
22
0
• The temperature differences were to utilize Henry’s law for filtration and
collection
77
52
28
27
10
26
66
69
54
20
30
40
71
50
60
70
Equipment
80
Mass (M/z)
3
10000
Synthesis setup
MHP
This shows all the masses
that the PTR-MS detected.
m/z 31 and 33 are the
highest count species. M/z
49 is approximately 3
magnitudes smaller than
m/z 31.
potential peroxide
3
10
43
acetonitrile
• The first impinger was kept at room temperature to insure that any H2O2
was filtered out.
4
10
MHP
methanol hydrate
• A double impinger system was connected to one of the ports on the
reaction vessel and nitrogen flow was increased to 30 ml/min.
31
methanol
5
10
formaldehyde
Methyl hydroperoxide (MHP) is a major organic hydroperoxide formed in
the chain termination reaction between HO2 and CH3OO radicals. It is a
source of HOX in the free troposphere through photochemical
decomposition into OH. There are few measurement techniques available
with the sensitivity to measure this species in air. A promising technique is
proton transfer reaction mass spectrometry whereby CH3OOH is measured
using H3O+ chemical ionization. The goal of this research is to optimize
and calibrate a Proton Transfer Reaction Mass Spectrometer (PTR-MS) for
the analysis of MHP.
M/z scan for the MHP
sample at 120Td
33
Counts
Introduction
6
10
600x10
M/z 49 and m/z 31 comparison for
MHP Sample
550
8000
500
m49 (counts)
6000
m49_Avg
m31_Avg
m31 (counts x10
The PTR-MS is designed to continuously measure volatile organic compounds in
the air. These organic compounds are measured by chemical ionization,
whereby the reagent ion H3O+ ionizes organics via a fast proton transfer
reaction.
450
400
4000
3
)
350
2000
Increase in drift velocity (Td)
corresponds to more
fragmentation and less m/z 49
counts.
300
→
CH3OOH2+
0
80
+ H2O
The reaction takes place in a drift tube where the sample air stream reacts with
H3O+ ions produced by a hollow cathode ion source. The drift tube is pressure
(mbar), temperature (°C), and drift velocity (Townsends) controlled. The
protonated organics are mass/charge (m/z) analyzed by a quadrupole mass
spectrometer and the ions are counted by a secondary electron multiplier
(SEM).
PTR-MS Schematic
90
100
110
Townsend ( 1 Td = 10
-17
120
250
2
V cm )
1600
Ratio of m/z 49 to Rxn Ion (H3O+) Vs.
Change in reaction Time
1400
1200
Method
An iodometric titration was performed on the sample to determine the
concentration of MHP.
Procedure:
• 10ml of Sample (MHP solution), 10ml of Distilled water, 3ml of 0.05M
ferric chloride (FeCl3•6H2O), and 5 drops 0.05M sodium iodide (NaI) were
mixed in an Erlenmeyer flask and allowed to react for 5 min in the dark.
• 5 drops of Starch indicator (1 wt %) were added
• Performed titration with sodium thiosulfate (Na2S2O3).
m49 (Hz)/m21 (MHz)
MHP: CH3OOH +
H3O+
1000
Reaction time changes inversely with
a change in drift velocity. Comparing
mass relation to reaction time
demonstrates a decrease in
fragmentation with an increase in
reaction time.
800
Meas. Relation
1:1 Relation
600
400
200
0
0
10
20
30
40
50
60
Rxn Time (µsec)
180
4000
160
M/z 49 and m/z 31 comparison for
formaldehyde calibration
m49
m31
140
Synthesis
To obtain a sample
stream for the PTR-MS,
10ml/min of N2 was
pumped through an
impinger containing
15ml of sample MHP
solution. A 100x dilution
flow was connected to
the input sample stream
to insure that the
reaction ion (H3O+)
wasn’t titrated out.
MHP is not commercially available, thus in order to develop a standard for use
with the PTR-MS, MHP was synthesized in the lab.
Synthesis:
• Dimethyl sulfate (DMSO4) and hydrogen peroxide (H2O2) were combined in a
triple neck round bottom flask (RBF) used as the reaction vessel.
• Nitrogen gas was pumped into the reaction vessel at 15 ml/min to insure that
air was not playing a part in the synthesis.
• Potassium hydroxide (KOH) was added drop-wise from a pressure equalizing
dropper funnel to the RBF.
• The reaction vessel was submerged in an ice water bath and continuously
stirred with a magnetic stirring rod.
• A Vigreux condenser was used to equalize pressure and insure sample wasn’t
lost.
• Once the reaction was completed, the condenser and the dropper funnel
were replaced with stoppers.
Discussion/Results
The titration of the MHP sample showed very small concentrations of MHP.
The concentration calculated from the titration was approximately 1x 10-5 M.
Extra species were identified to be the products of likely side reactions in the
synthesis of MHP. These other species included excess methanol (m/z 33),
formaldehyde (m/z 31), and what appeared to be heavier peroxides such as
CH3OOCH3 (m/z 63). The PTR-MS measures MHP and formaldehyde as m/z
49 and m/z 31 respectively. In the process of the proton transfer, MHP can
fragment to m/z 31 and formaldehyde can form water clusters
(HCHO(H2O)+) at m/z 49, so there is an interference between formaldehyde
and MHP. From a formaldehyde calibration using ratios of m/z 49 to m/z 49 +
m/z 31 at 80 Td, 3.8% of the formaldehyde clustered with water to m/z 49.
The MHP sample showed 2000 ppbv of formaldehyde (m/z 31) in the diluted
flow! This is a very high concentration of formaldehyde.
120
100
3000
80
2500
60
40
Mass 31 (counts)
PTR-MS Setup
Mass 49 (counts)
3500
With an increase in drift velocity
there was less water clustering on
formaldehyde.
2000
20
1500
0
80
90
100
Townsend ( 1Td = 10
110
-17
120
130
2
V cm )
Conclusion
Results from the measurement of a synthesized MHP standard are inconclusive.
The mixture contained too many species, in particular formaldehyde, to be
useful as a calibration source. Ratios of m/z 49 to m/z49 + m/z 31 where very
low < 5%. This is in agreement with the low concentration of MHP measured by
the titration method. What can be concluded is this synthesis will not work for
calibrating MHP, due to the interference of other species. A different synthesis
technique will need to be implemented to insure that MHP is the major product.
The formaldehyde calibration does inform us that a significant amount of
formaldehyde (4%) can cluster with water to m/z 49; this will be important in
future work for a calibration standard of MHP with the PTR-MS.
References
De Gouw JA, Warneke C. 2007. Measurement of volatile organic compounds in the earth’s atmosphere using proton-transferreaction mass spectrometry. Mass Spectrometry Reviews 26: 223-257.
Frey MM, Hutterli MA, Chen G, Sjostedt SJ, Burkhart JF, Friel DK, Bales RC. 2009. Contrasting atmospheric boundary layer
chemistry of methylhydroperoxide (CH3OOH) and hydrogen peroxide (H2O2) above polar snow. Atmos. Chem. Phys., 9, 3261–
3276.
O’Sullivan DW, Lee M, Noone BC, Heikes BG. 1996. Henry’s Law Constant Determinations for Hydrogen Peroxide, Methyl
Hydroperoxide, Hydroxymethyl Hydroperoxide, Ethyl Hydroperoxide, and Peroxyacetic Acid. J. Phys. Chem., 100, 3241-3247
Acknowledgements
This research was funded by a NSF Research Experience for Undergraduates (REU) grant ATM0754990. I would like to thank all of the professors, advisors, and fellow REU students for their
help on this project.