Stable bromine isotopic composition of methyl bromide  

– Method development and applications – 

 

Axel Horst 

 

Doctoral Thesis 

Department of Applied Environmental Science 

Stockholm University 

Stockholm, 2013

Doctoral Thesis, 2013 Axel Horst Department of Applied Environmental Science (ITM) Stockholm University 106 91 Stockholm Sweden  

© Axel Horst, Stockholm 2013 

ISBN  978‐91‐7447‐705‐4 

Printed by US‐AB 

 

 

 

ABSTRACT

The isotopic composition of ozone depleting methyl halides may provide valuable

information on the sources and sinks of these compounds. However, so far mostly

stable carbon isotope analysis of methyl chloride and methyl bromide (CH3Br)

has been attempted. Especially halogen isotope analysis has been hindered by the

challenge to obtain sufficiently large amounts of methyl halides to meet the detec-

tion limits of existing isotope analytical methods. The purpose of this doctoral

thesis was to develop both a high-volume cryogenic collection system for methyl

bromide (Article II) and an analytical technique being able to analyze the sam-

pled amounts of CH3Br for its Br isotopic composition, which was accomplished

by using gas chromatography multiple-collector inductively-coupled plasma mass

spectrometry (Article I). These methods were applied in the field campaign from

which we report the first bromine isotopic values of CH3Br in the atmosphere

(Article III), being in the range of -0.47 to + 1.75 ‰ vs. SMOB (Standard Mean

Ocean Bromide). A laboratory study on pectin and halophyte plant material (Arti-

cle IV) gave an insight in Br isotope composition of abiotically formed CH3Br

which may be a main source to the atmospheric budget. These plant experiments

yielded δ81Br values of 0 to -2 ‰ SMOB. Atmospheric CH3Br and this potential

source showed partly distinct δ81Br ranges and demonstrate the potential of Br

isotopes for source apportionment.

3 

 

ABREVIATIONS

BOC Brominated Organic Compound

CF Continous Flow

CFC Chlorofluorocarbon

CH3Cl methyl chloride (chloromethane)

CH3Br methyl bromide (bromomethane)

DI Dual Inlet

ε isotope enrichment factor

GC Gaschromatography

ICPMS Inductively Coupled Plasma Mass Spectrometry

IRMS (gas source) Isotope Ratio Mass Spectrometry

KIE Kinetic Isotope Effect

MC-(ICPMS) Multiple collector (ICPMS)

PCE Perchloroethylene

SMOB Standard Mean Ocean Bromide

TIMS Thermal Ionisation Mass Spectrometry

VPDB Vienna Pee Dee Belemnite

4 

 

CONTENTS

Abstract .................................................................................................................... 3

Abreviations ............................................................................................................. 4

Contents ................................................................................................................... 5

List of Articles ......................................................................................................... 6

Statement ................................................................................................................. 7

Objectives ................................................................................................................ 8

Introduction .............................................................................................................. 9

Methyl bromide - an ozone depleting compound ............................................. 9

Some concepts of stable isotope chemistry .................................................... 11

Methods ................................................................................................................. 15

Sampling methyl bromide for isotope analysis .............................................. 15

Bromine isotope analysis in methyl bromide ................................................. 16

Applications ........................................................................................................... 18

Atmospheric and soil-derived CH3Br-δ81Br signatures: field studies in Stockholm and Abisko, Sweden ................................................................. 18

Abiotically emitted CH3Br from plants .......................................................... 19

Conclusions ............................................................................................................ 21

Recommendations .................................................................................................. 22

Acknowledgements ................................................................................................ 23

References .............................................................................................................. 23

 

5 

 

LIST OF ARTICLES

I Compound-specific bromine isotope analysis of methyl bromide using gas chromatography hyphenated with inductively coupled plasma multi-ple-collector mass spectrometry.

Horst A.; H Holmstrand, P. Andersson, A. Andersson, D. Carrizo, B.J. Thornton and Ö. Gustafsson, Rapid Comm. Mass Spectrom. 2011, 25, 2425-2432, Copyright© 2011, John Wiley & Sons, Ltd.

II A high-volume cryosampler and sample purification system for halogen isotope studies of methyl halides.

Thornton B.J.; Horst A.; H Holmstrand, P. Andersson, D. Carrizo, P.M. Crill and Ö. Gustafsson, J. Atmos. Oceanic Technol., accepted for publi-cation 17 April 2013, Copyright © 2013 American Meteorological Socie-ty, AMS

III Stable bromine isotopic composition of atmospheric CH3Br

Horst A.; B.J. Thornton, H Holmstrand, P. Andersson, P.M. Crill and Ö. Gustafsson, (submitted)

IV Stable bromine isotopic composition of methyl bromide released from plant matter

Horst A.; P. Andersson, B.J. Thornton, H Holmstrand, A. Wishkerman, F. Keppler and Ö. Gustafsson, (manuscript)

 

6 

 

STATEMENT

I, Axel Horst, made the following contributions to the papers presented here:

Article I After introduction into operation of the ICPMS instrument I carried out the anal-yses in the laboratory and took the lead role in writing the article.

Article II A cryogenic collection system was drafted in collaboration with the co-workers of the project. After intensive testing of preliminary versions of this sampler by oth-ers, I did the main work in testing the efficiency of the final set-up. We designed a clean-up method which I tested in the laboratory. I contributed to the writing of the article.

Article III In close collaboration with co-workers we planned and carried out the Abisko field campaign. The Stockholm samples were collected by me and I carried out the analyses in the laboratory. I contributed to the interpretation of the results and took the lead role in writing the article.

Article IV The idea of measuring Br isotopes in plants was conceptualized by co-workers. I improved the method to sample CH3Br from dry plant material and planned the experiments. I performed the laboratory analyses and took the lead role in writing the article.

 

7 

 

OBJECTIVES

The purpose of this doctoral thesis was to establish and apply methods to investi-

gate the Br isotopic composition of methyl bromide in the atmosphere:

(i) A high-volume sampling procedure for methyl bromide (Article II) was re-

quired to collect sufficient amounts of this trace compound from the atmosphere

for isotope analysis.

(ii) A sensitive compound-specific method for 81Br/79Br isotope ratio analysis had

to be established (Article I) being able to analyze the quantities the sampling pro-

cedure would be providing.

(iii) The aim was to collect atmospheric samples which would give a first view on

expectable atmospheric δ81Br values in CH3Br and reveal its usefulness for source

apportionment (Article III).

(iv) The purpose of this study was to carry out laboratory experiments with plants

which would provide the δ81Br isotopic fingerprint of this potential source of

CH3Br (Article IV).

8 

 

INTRODUCTION

Methyl bromide - an ozone depleting compound

Methyl bromide (CH3Br) together with methyl chloride, chlorofluorocarbons

(CFCs) and halons (e.g. bromine or chlorine containing haloalkanes) contributes

significantly to the atmospheric halogen radicals in the stratosphere and thus to

the catalytic destruction of the stratospheric ozone layer. 

Methyl bromide with an approximate tropospheric concentration of 8 pptv (parts

per trillion by volume) and an atmospheric lifetime of about 0.8 years1 is the larg-

est source of Br to the stratosphere. It contributes about 50 % of Br radicals and is

believed to be responsible for 15 % of ozone depletion caused by halogen radicals.

Another 13 % is caused by CH3Cl and almost 50 % by CFCs2,3. CFC concentra-

tions started to decrease in the mid-90s and the Antarctic ozone hole began to re-

cover. However, the currently predicted climate change is expected to lead to an

increased emission of methyl halides (e.g. release from thawing permafrost soils4)

and by that increase their importance for ozone depletion. 

Unlike CFCs, CH3Br has both anthropogenic and natural sources. It has been used

as a fumigant for pre-shipment treatment of timber and wooden crates for

transport of fruit and vegetables. After listing of methyl bromide as an important

ozone-depleting substance in the revised Montreal Protocol5, CH3Br is regulated

to be phased out by 2015. Natural sources include fungi6, macroalgae7, peatland4

and salt marshes8.

 

 

The relative contributions from the different sources to the atmospheric budget are

still not well constrained. The known sources sum up to approximately 113 Gg/a

whereas the sinks sum up to 149 Gg/a9. Either there are missing sources which are

not known yet or the known sources or sinks are over-/underestimated. An over-

view over the currently known sources and sinks is shown in Fig 1. 

Figure 1: Sources and sinks for CH3Br according to Yvon-Lewis et al.9 and references there-in. Isotopic signatures, if present, are given for δ13C (green)10,11 and for δ81Br (red)12 for bromine salts.

 

10 

 

Some concepts of stable isotope chemistry

The delta value

The application of stable isotopes may contribute to budget investigations and/or

description of reaction pathways. The isotopic signature serves as a natural tracer

for provenance, chemical evolution and decomposition. Bromine has two stable

isotopes: 81Br with an abundance of 49.31 % and 79Br with an abundance of

51.69 % giving a 81Br/79Br ratio of 0.954 on average. Measuring a known standard

on the same instrument at the same time corrects for instrumental variations and

makes results comparable. However, the accuracy with which the absolute value

can be measured is poorer than the precision with which the relative difference

between two samples can be determined. The main reasons are the different char-

acteristics of methods for isotope measurements. Whereas IRMS methods are able

to determine almost the real ratio between a heavy and a light isotope other meth-

ods like TIMS and ICPMS exhibit so-called mass bias in their results; i.e. the iso-

tope ratios are changed due to fractionation during ionization. However this mass

bias occurs for both the sample and the used standard. Therefore the so-called

delta value is used to make results from distinct methods comparable. It expresses

the relative difference between a sample and a standard in per mille: 

‰10001

79

81

79

81

81 ⋅

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

−

⎟⎟⎠

⎞⎜⎜⎝

⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛

=

reference

sample

BrBr

BrBr

Brδ

 

For Br isotopes the delta value expresses the deviation of a 81Br/79Br sample ratio

from the 81Br/79Br reference ratio in per mille. For bromine this reference is the

mean 81Br/79Br isotope ratio of sea water (Standard Mean Ocean Bromide,

SMOB). The isotope ratio and with it the delta value change due to fractionation

processes as described subsequently.

11 

 

Isotope effects and the Rayleigh equation

The isotopic variation is caused mostly by kinetic isotope fractionation (also

called kinetic isotope effect, KIE) which is mass dependent and caused by chemi-

cal reactions, transport phenomena like diffusion, temperature effects or biological

effects. The KIE occurs due to differences in the rates of reaction for different

molecular species. Bonds formed by heavier isotopes are more stable than those

formed by lighter isotopes and therefore the reaction rates for the heavier species

are slower. This causes enrichment of the heavier isotope in the substrate pool and

depletion in the product13. The highest fractionation may be expected for elements

showing the largest relative mass differences between their heavy and light iso-

topes. Hydrogen (1H) and deuterium (2H) show a mass difference of 100 % and

fractionation is much higher than for carbon (8 % between 12C and 13C) or bro-

mine (2.5 % between 79Br and 81Br). Therefore the expectable KIE is much small-

er for Br than for C or H because the differences in reaction rates of heavy and

light isotopes are less pronounced14.

The isotopic enrichment in a certain reservoir might be explained by using the

Rayleigh equation. It describes the exponential enrichment in the residual sub-

strate reservoir of the reactant. The Rayleigh equation can be directly applied to

simple molecules containing one trace isotope of interest as for instance 18O in

water. The corresponding Rayleigh equation is:

)1(0

−= αfRR

where R is the ratio after the fractionation process, R0 is the initial isotopic ratio, f

is the residual component and α is the fractionation factor14.

Enrichment factors have been determined for organochlorines by e.g. Aeppli et

al.15 for evaporation of PCE showing the typical enrichment in the substrate pool.

Brominated organic compounds can be viewed as analogues of chlorinated com-

pounds. According to a literature review by Willey and Taylor16 the isotopic ef-

fect of Br is approximately three times smaller than for Cl. Aeppli et al17 found a

large chlorine isotopic depletion of -12 ±1 ‰ in phenolic products chlorinated by

12 

 

chloroperoxidase. Thus, there is a potential for 3-4 ‰ fractionation in δ81Br for

enzymatic/microbiologically mediated processes.

Methyl bromide is a gas and in the atmosphere it undergoes reactions with OH-

radicals and photolytic decomposition. This might cause similar fractionation ef-

fects as shown for PCE although the underlying processes are different. A KIE

would be due to a chemical reaction rather than an enzymatic/microbial reaction.

In the atmosphere unexpectedly high isotope fractionation effects may be found

due to non-mass dependent isotope effects (e.g. magnetic effect). Kaiser et al.18

found an isotopic enrichment of δ37Cl of CFC-12 of more than 30 ‰ due to pho-

tolysis in the stratosphere, which theoretically could result in 10 ‰ enrichment for

δ81Br values. These examples show that there is – despite small mass differences

between 81Br and 79Br – a potential for a detectable fractionation of Br isotopes,

which can be used to identify sources and decomposition processes.

Multi-dimensional isotope approaches

In the early 2000s, stable carbon isotope approaches have been proposed19 to con-

strain the CH3Br atmospheric composition by investigating potential source signa-

tures and determining kinetic isotope effects of loss mechanisms. A mass balance

approach using this data would suffer from large uncertainties because of over-

lapping isotopic ranges for the few investigated reservoirs19,20 and the lack of any

other source and sink signatures. So far, Bill et al.11 has reported δ13C atmospheric

signatures of -43 ± 2 ‰ VPDB which is indistinguishable from emissions from

saltmarsh plants with -43.1 ± 1.5 ‰10 and anthropogenic CH3Br which has a range

of -43 ‰ to -66.4 ‰ VPDB19. The use of one or several additional stable isotope

systems could improve this balance as for instance δ2H and δ81Br. Multi-isotope

approaches are potentially able to characterize sources much better than single-

isotope approaches as demonstrated in Figure 2. The endmembers in the figure

indicate the isotopic fingerprint of potential sources which contributed to the natu-

ral sample. The isotopic signature of the natural sample is a result of mixing de-

pending on the portions from the different endmembers and their isotopic compo-

sition.

13 

 

 

Figure 2: Application of multiple isotope systems for natural samples and its sources. The more isotope systems are used; the better the endmember composition and thus contribution from the different sources can be characterized. The example demonstrates three-dimensional isotope analyses.

Apart from 13C no other isotope system has – to our knowledge – been applied on

CH3Br. Bromine isotopes are together with hydrogen isotopes potentially applica-

ble for CH3Br but have only been determined in two earlier studies with one in-

vestigating the Br isotopic composition of salts and one measuring δ81Br values

for BOC. Therefore the range of reported δ81Br values is still small. Shouakar-

Stash et al. 12 found -0.80 ‰ to + 3.35 ‰ SMOB for salts sampled from deep

groundwaters of the Siberian Platform. Carrizo et al.21 report isotopic signatures

of -4.3 to -0.4 ‰ SMOB for industrially produced BOC and 0.2 ± 1.6 ‰ SMOB

for natural 2,4-dibromophenol. Apart from the latter, there is no reported data on

molecular δ81Br values to compare with as other earlier studies focused only on

analytical aspects such as precision rather than on accuracy.

14 

 

METHODS

Sampling methyl bromide for isotope analysis

The main objective of this thesis was sampling and Br isotope analysis in atmos-

pheric methyl bromide. Neither a sampling method for high volume air samples

nor a Br isotope analysis method for small natural samples existed at onset of this

thesis work. Although the ICPMS method (see below) can analyze isotopic ratios

from as little as 20-40 ng of Br, several cubic meters of air have to be collected for

this amount due to average concentration of 8 pptv in the atmosphere which corre-

sponds to 28 ng Br per m3 air (at 5ºC and 1013 mbar).

To collect these large amounts of air an appropriate method had to be developed.

One common air sampling procedure relies on adsorptive enrichment techniques

using activated carbon or carbon molecular sieves to collect organic compounds

from an air stream22. These methods are well established for concentration meas-

urements of organics which are sampled from a few liters of air23,24. For high-

volume samples adsorptive enrichment techniques are not appropriate. To sample

several cubic meters of air would require large amounts of expensive adsorbent

materials and/or very long sampling times. It is also likely that adsorbents cause

isotope fractionation because of formation of artifacts (compounds which were

transformed upon adsorption) and incomplete adsorption of the target compounds

from the air25 (breakthrough).

Another possibility to collect volatile compounds is to condense them in a

cryotrap using e.g. liquid nitrogen as a cooling agent. Cryogen-based sampling

systems are able to collect larger volumes of air more rapidly than adsorptive

sampling techniques, but at the expense of higher cryogen use. Bill et al.11 showed

that a cryogenic sampling system could be used to collect CH3Br from 196 to 936

L of air for 13C/12C analysis without causing isotope fractionation. Bahlmann et

al.26 used a similar technique to sample up to 500 L of air for 13C/12C analysis of

C1 and C2 compounds. Adsorptive high-volume sampling has, to our knowledge,

15 

 

not been reported yet. Because of the unclear effect on the isotopic composition of

the target compounds and the more rapid sampling in the field a cryogenic collec-

tion method seemed more appropriate for our purposes (Article II).

Our sampler features a ca. 1 L stainless steel cryotrap immersed in liquid nitrogen

and filled with glass beads to increase the condensation surface. A vacuum pump

is used to pull air through the trap at flow rates of 10-20 L/min making it possible

to sample CH3Br from 4-5 m3 of air in 4 hours. Water is removed with a conden-

ser and a Drierite® trap at the inlet. Carbon dioxide is separated from the sample

with a Carbosorb® trap during transfer to a storage canister. In the laboratory, the

air sample is extracted from the canister again and the collected amounts of

CH3Br are quantified and purified from co-trapped interfering compounds (e.g.

N2O) using a packed column mounted into a GC-MS.

We could show that neither the cryogenic trapping in parts of the system nor the

complete sampling and purification method caused any isotope fractionation,

which was the most crucial requirement for a trapping system intended to collect

samples for isotope analysis.

Bromine isotope analysis in methyl bromide

Bromine isotopes in CH3Br have been analyzed before using gas source isotope

ratio mass spectrometry (IRMS) coupled to dual inlet (DI) or continuous flow

(CF) as a sample introduction method. The methyl bromide was derived from Br

salts12, 27-30 and used as an intermediate product only. This method was already

shown in 1978 by Willey and Taylor16, but since then no isotope measurements of

natural CH3Br have been reported for either gas source mass spectrometry meth-

ods because of high required amounts of Br. For instance, 2-5 mg of Br are neces-

sary for dual inlet27 and 80 µg for continuous flow28 which means that 200,000 m3

and 8,000 m3 of air would have to be collected respectively to be able to carry out

one isotope analysis of atmospheric methyl bromide. Therefore, CF/DI-IRMS is

16 

 

not appropriate for samples of low CH3Br concentration, such as from

the atmosphere.

Recently, ICPMS methods have been established to analyze Br isotopes in liquid

organobromines21, 31-33: These methods use gas-chromatography for separation

and as an inlet to the ICP. The eluting peaks from the different compounds are

ionized and analyzed separately, which gives the advantage of compound-specific

online isotope analysis. Besides, existing MC-ICPMS methods for liquid BOC

can analyze as little as 24 ng of bromine31 compared to at least 80 µg for IRMS

methods. This made the MC-ICPMS approach more suitable for natural samples

than the DI/CF-IRMS methods.

We connected a gas chromatographic device (GC) to a MC-ICPMS system (Arti-

cle I) to test the suitability for Br isotope measurements of CH3Br. The system

showed a precision of < 0.1 ‰ for sample amounts of 40 ng and < 0.7 ‰ for sam-

ples between 5 and 20 ng. Linearity of measured isotope ratios was shown over

the entire range of tested concentrations. The long-term reproducibility was de-

termined by measuring three batches of commercially available CH3Br gases

against each other over a time period of 3 months. In total 34 analyses were car-

ried out. The long term precision was 0.4 ‰ or better. The obtained mean δ81Br

delta values differed by not more than 0.09 ‰ from each other. This might be a

hint to the δ81Br range of industrial CH3Br but more gases with known origin have

to be analyzed to confirm this.

The accuracy of an analytical system is normally evaluated by testing how well it

reproduces the delta values of officially recognized standards. Such official δ81Br

standards do not exist yet and the final test for accuracy is still pending. The earli-

er mentioned SMOB values are based on Br isotope measurements referenced to

sea salt12. However, the study demonstrates that bromine isotope ratios of methyl

bromide can be determined at very low concentrations. In contrast to existing DI/

CF-IRMS methods it is applicable towards natural CH3Br containing samples.

17 

 

APPLICATIONS

Atmospheric and soil-derived CH3Br-δ81Br signatures: field studies

in Stockholm and Abisko, Sweden

The above mentioned high volume cryosampling system was used in two field

studies (Article III). The first campaign took place in August 2011. On Stordalen

Mire, near Abisko north of the Arctic Circle in Sweden, atmospheric samples

were taken to get an impression of the δ81Br of CH3Br from relatively “clean” air,

i.e. with low anthropogenic influence. Additionally, soil –air was sampled to de-

termine the isotopic fingerprint and importance of this source. A second sampling

campaign was carried out at the campus of Stockholm University. Compared to

Abisko, these air samples would reveal isotopic differences due to distinct source

and sink strengths.

Overall, the air samples showed similar δ81Br values for both field studies ranging

from -0.47 to +1.75 ‰ SMOB for Stockholm samples and -0.14 to +0.48 ‰

SMOB for Abisko. However, in Stockholm the δ81Br range was larger and the

mixing ratios of CH3Br were higher (14.4±2.1 pptv) whereas for the Abisko sam-

ples the mixing ratios (9.8 pptv ± 3.1 pptv) were statistically indistinguishable

from global background (8 pptv). The Stockholm samples showed a clear trend of

depleted Br isotope signatures (i.e. negative δ81Br values) with higher concentra-

tions towards enriched δ81Br values with lower concentrations. This trend is sug-

gestively due to decomposition of CH3Br in the atmosphere and/or mixing be-

tween potential sources and the background methyl bromide.

The largest known sink for atmospheric CH3Br is the reaction with OH radicals

(ca 44 %)9. The decomposition reaction might cause an enrichment of the heavier

isotopes in the atmosphere, i.e. higher δ81Br values. Gola et al.34 carried out labor-

atory experiments with CH3Cl and OH and determined an isotope enrichment of

59 ‰ of δ13C in the substrate, which according to the authors might be similar for

CH3Br reacting with OH. We reported a field based enrichment factor (ε) of

18 

 

4.7±3.7 ‰ for 81Br/79Br which is, as expected, considerably smaller than for 13C/12C due to smaller mass differences. In Article III we calculated a theoretical

enrichment factor basing on the assumption of a secondary isotope effect (i.e. the

bond breaking reaction occurs not directly to Br). The resulting ε was an order of

magnitude smaller. Therefore reaction with OH radicals might not be the main

process causing this enrichment. Uptake by soils and oceans, the two other large

sinks, is also not likely to induce a big isotope effect because the net partitioning

between air-soil and air-sea water is a rate limiting step which normally does not

cause a large kinetic isotope effect35. However, data is very limited yet and this

field-based enrichment factor needs to be confirmed by laboratory experiments

and more field samples.

Soil air samples taken on Stordalen Mire near Abisko showed lower concentra-

tions of CH3Br than in the air above. Br isotopic signatures tended to be more

enriched than in the surrounding free air. Although our results are in accordance

with an earlier study focusing on concentration measurements36, they have to be

considered as rather preliminary. Only 4 soil samples were taken with a relatively

premature sampling system. However, we could see tendencies in both concentra-

tions and Br isotopic values which might indicate that this wetland system acts as

a net sink for CH3Br.

The samples of this field campaign gave a very first impression of ambient δ81Br

values of CH3Br in the atmosphere. We demonstrated that bromine isotopes in

atmospheric CH3Br show differences which potentially can be used to character-

ize sources and sink processes.

Abiotically emitted CH3Br from plants

Plants are suggested to be a major source of methyl halides9. Production mecha-

nisms are either abiotic or biotic. Biotic production occurs during plant growth by

the plant itself or by microbial decomposition. Biotic production mechanisms are

believed to be small in comparison to abiotic processes as for instance weathering,

senescence and biomass burning37. CH3Br is formed when the methoxyl moiety of

19 

 

the pectin molecule and bromide, present in the tissue water of the plant, react in

an abiotic methylation reaction38. Methoxyl groups are very abundant in pectin, a

cell-stabilizing macromolecule. Pectin from apples contains approximately 8 % of

methoxyl and cell walls of leaves, for instance, contain 7-26 % of pectin39. Lignin

is the main carrier of methoxyl groups in wood but formation was only observed

at temperatures above 250°C38. Therefore pectin might be the environmentally

more important precursor for methyl halides, especially for ambient temperature

formations, and was used in this study (Article IV) to investigate the Br isotopic

composition from this methylation pathway. 

We investigated methyl bromide formed from pectin (fortified with KBr) and

from Salicornia fruticosa a halophytic plant with a high salt content. The plant

matter was incubated at temperatures of 30-300°C and different time periods and

δ81Br values of the collected CH3Br were measured with the GC-MC-ICPMS

method. An enrichment factor of–1.8 ‰ and –2 ‰ was found for Salicornia and

pectin at all tested temperatures if only small amounts of the initial salt content

was consumed by the reaction (< 10 %). The original salt holds a δ81Br of 0 ‰

SMOB which results in a δ81Br of –2 ‰ SMOB for CH3Br formed from this reac-

tion. For very long incubations and at higher temperatures also more enriched

δ81Br values were measured. Therefore we suggest a range of –2 to 0 ‰ SMOB

for high temperature emissions like biomass burning.

In Article III we presented an estimate for the average δ81Br source range of the

Stockholm air samples (-4±4 ‰ SMOB) basing on a mixing model. The proposed

δ81Br fingerprint (Article IV) for biomass burning and low temperature emissions

lies well within this range. Marine CH3Br as the main source might therefore also

show depleted δ81Br values but this would have to be confirmed in further studies.

These laboratory experiments provide a first view on Br isotopic values of a po-

tential source of CH3Br to the atmospheric budget. The value of ~-2 ‰ SMOB

was suggested for ambient temperature formation from senescent and dead plant

material ranging up to 0 ‰ for high temperature emissions from biomass burning.

20 

 

CONCLUSIONS

The methods presented in this thesis might open the door to many yet unexplored

research questions. So far, little is known about the magnitude of fractionation of

Br isotopes in gases and especially in CH3Br. In this work, we measured the first

δ81Br values in atmospheric methyl bromide and one potential source was investi-

gated. The results from this thesis are summarized in Figure 3. The δ81Br values

comprise a range of almost 4 ‰. Fractionation seems to be sufficiently large to

describe differences between sources and atmospheric samples if a precise method

is applied. However, the ranges are partly (biomass burning) or completely (fumi-

gation) overlapping with the atmospheric δ81Br range. A two-dimensional ap-

proach using two isotope systems (13C and 81Br) was tried in Article IV and

seems to dissolve potential source signatures better than only one isotope system

as shown in Figure 3. Overall, this thesis demonstrates the feasibility of sampling

and bromine isotope analysis of methyl bromide. The first results presented in this

work give a first insight in atmospheric and plant derived δ81Br values of CH3Br

and possibly underlying fractionation effects.

Figure 3: Summary of δ81Br values for different reservoirs (as measured in this thesis)

21 

 

RECOMMENDATIONS

This work demonstrates that both methods for high volume sampling and isotope

analysis of methyl bromide deliver reliable results but need to be improved if they

are to be used routinely. The cryosampler (Article II) is limited in the field by its

dependency on liquid nitrogen and its size. Furthermore, an automated sampling

procedure would facilitate substantially the workflow and allow for a higher sam-

ple output. The cryosampler is optimized for high-volume air samples for isotope

analyses and concentration measurements from this collection system showed a

rather big analytical uncertainty. It is therefore recommendable to use a parallel

sampling system optimized for concentration measurements. The Br isotope

measurements (Article I) using GC-MC-ICPMS showed acceptable precision (σ

< 0.4 ‰) for both atmospheric and plant derived CH3Br. A more automated ana-

lytical protocol avoiding syringe injections could improve the precision of the

measured isotope ratios. Another approach might be the use of simultaneously

measured strontium standards for drift correction as demonstrated by Gelman and

Halicz32 which yielded a precision of up to 0.05 ‰ (1σ).

To be able to use Br isotopes for source apportionment more studies on atmos-

pheric CH3Br are necessary to evaluate if the δ81Br values we found are repre-

sentative on a global scale or if there are latitudinal, seasonal or altitudinal varia-

tions. Apart from the atmospheric δ81Br-CH3Br composition the Br isotopic fin-

gerprints of the main sources (e.g. oceans) should be determined to be able to es-

timate the corresponding portion on the atmospheric budget. It will also be essen-

tial to estimate the enrichment factors caused by the individual sinks (e.g. OH

radicals) as we tried in Article III. For all future studies it is recommendable not

only to focus on one isotope system but to study both 81Br and 13C isotopes in

CH3Br in order to build up an applicable database for source apportionment of

this ozone depleting gas.

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ACKNOWLEDGEMENTS

I thank my supervisors Örjan Gustafsson, Henry Holmstrand and Per Andersson

for giving me the opportunity to do this PhD and having confidence in my work.

Brett and Daniel, “my” Post-Docs: would you have imagined that we really get

something out of this project (apart from “the can”…) when we started all these

years ago? Many thanks to the old (“the sect”) and new colleagues – too many too

name them all - for just being here and the good times within and outside science.

Special thanks to the museum crew (Hans S., Torsten P., Kjell B. and Per-Olof P.)

for all the emergency help when the ICPMS was indisposed. My current (Helen,

Andrew, Rafael) and former flatmates (Isabell, Sandra, Zimba): Thanks for all the

“mysiga” evenings and the being-at-home-feeling! Last but not least; thanks to

my friends (somewhere else in the world) and to my family for their enduring

mental and emotional support. This study was financed by the Swedish Research

Council (VR Contract No. 311-2007-8381).

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