Heard D.E. (editor) Analytical Techniques for Atmospheric Measurement

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388 Analytical Techniques for Atmospheric Measurement

Figure 8.9 Typicalionchromatogramobtained attheMaceHead AtmosphericResearchStationin selective

recording mode. Identities are (1) CH

I(m/z 127 and 142), (2) CHCl

(m/z 83), (3) C

I(m/z 127 and 156),

(5) 2-C

I(m/z 127 and 170), (6) CH

(m/z 172 and 174), (7) 1-C

I(m/z 127 and 170), (8) CH

ClI

(m/z 176 and 178), (9) CHBr

Cl (m/z 127 and 208), (10) CH

BrI (m/z 220 and 222), (11) CHBr

(m/z 171 and

173), (12) CH

(m/z 254 and 268). Separation was obtained using a SGE (BPX5) 50 m ×032 mm ×3 m

(ﬁlm) capillary column. (By courtesy of L.J. Carpenter, University of York, UK.)

AGC×GC contour plot is shown in Figure 8.10, where each spot represents an individual

analyte. The improvement in resolution between the one-dimensional separation (shown

above) and the two-dimensional separation is apparent. The GC ×GC chromatogram has

been successively expanded to show the greater numbers of isomers at higher molecular

weights.

The composition of organic aerosol is even more complex than the gas phase, with

considerably more organic compounds in the diesel range. The analysis of this type of

sample can prove extremely difficult even with the large separating power of GC ×GC.

Coupling to a TOF/MS can add an additional separation mechanism, in this case, the

m/z ratio. Figure 8.11 shows a GC ×GC-TOF/MS chromatogram of a PM2.5 particulate

samples collected in Augsburg, Germany (Welthagen et al., 2003). In a typical sample,

more than 15 000 peaks could be detected. Using an MS as a detector allowed struc-

tural information and peak identifications to be obtained, which would have required

numerous standards if an FID was used.

The composition of alkyl nitrate compounds has been studied during the TORCH

campaign (Tropospheric Organic Chemistry Experiment). This site was situated outside of

Chelmsford, UK, and during August 2003 was chosen as a site due to its position relative

to the London plume. Air samples were analysed in negative ion chemical ionisation

mode (NCI) with methane as the ionisation gas and a sample chromatogram is shown

in Figure 8.12.

The use of aircraft in atmospheric measurements is becoming increasingly popular as

more research aircraft become available. An on-board GC system has been developed,

which uses a cooled absorbant sampling trap followed by GC with HID, which is suitable

for use in an aircraft (Whalley et al., 2004). The time resolution of this system is

much quicker than conventional GC systems giving a chromatogram every 5 minutes.

Figure 8.13 shows a chromatogram collected on 26 March, 2004, on the UK research

aircraft, a BAE 146-301 (http://www.faam.ac.uk) during the initial test-flying period. The

Chromatographic Methods 389

50 55 61 66

72 77 82

75 84

400

300

200

100

0 10 20 30 40 50 60 70 80 90 100

51015202530354045505560 7065 75 80

Retention time (min)

1st retention time (min)

2nd column RT / secs

Figure 8.10 Comparison of single column (upper) and GC×GC separations (lower) of a Leeds urban air

sample. Areas of the full chromatogram are successively extracted at higher gain to illustrate increasing

isomeric complexity at higher boiling points. GC ×GC chromatograms are annotated with start of

individual C

isomer band (running right to left) where A = C

 B = C

 C = C

 D = C

 E = C

 F = C



G = C

,H= naphthalene. Chemical banding assignments, 1; aliphatics, 2; oleﬁnics, 3; oxygenated,

4; mono aromatics, 5; polyaromatics. (From Hamilton & Lewis, 2003, reproduced with permission from

Elsevier.) (Reproduced in colour as Plate 4 after page 264.)

aircraft was flying at an altitude of 214.8 m over the Bristol channel (Lat 5140



N, Lon

378



W).

Fieldwork is an integral part of atmospheric analysis and the ability to move equipment

from the laboratory to the field site is important. Mobile laboratories housed in vans and

shipping containers allow GC instruments to be moved without the need for permanent

laboratories facilities at the field site. The University of York, UK, has a GC laboratory

fitted within a shipping container, which can be delivered to the site and set-up within a

few days. This container was shipped to Mace Head as part of the NAMBLEX experiment

described in Section 1.8.2 and Figure 8.14 shows the container on-site and the GC system

housed in the interior. Six weeks of almost continuous measurements were made of

NMHCs and selected o-VOCs using the dual channel GC-FID system used in Figure 8.6.

A region of the concentration versus time series, from 30 July to 8 August, 2002, for

ethane, acetylene and benzene is shown in Figure 8.15. The directions marked on the

graph indicate the origin of the air mass, as calculated by the five-day back trajectory

(model used from the European Centre for Medium-Range Weather Forecasts, ECMWF).

In the figure, W indicates an air mass that has travelled over the Atlantic Ocean for most

390 Analytical Techniques for Atmospheric Measurement

GC × GC plot of Aerosol sample

(A)

(B)

4.0

3.5

3.0

2.5

2.0

1.5

1.0

4.0

3.5

3.0

2.5

2.0

1.5

1.0

Polarity 2nd dimension (seconds)Polarity 2nd dimension (seconds)

1000 1500 2000

2500 3000 3500 4000

600

1250 2500 3750 5000

6250 7500 8750 10

000

Volatility 1st dimension (seconds)

20 000

40 000

20 000 40 000

Figure 8.11 Two-dimensional GC ×GC-TOFMS total ion current (tic)-plot of an aerosol sample in

2D-contour plot: (A) showing the full chromatogram of the analysed aerosol with (B) the extraction of

the selected section for data analysis. (From Welthagen et al., 2003, reproduced with permission from

Elsevier.) (Reproduced in colour as Plate 5 after page 264.)

of the last five days; NW indicates an air mass which originated in the northern polar

region; and NE five indicates an air mass which has spent the previous five days travelling

over northern Europe. From the time series, it is clear to see that the most polluted

period is during the NE trajectories. The air mass during this period is travelling over

industrial and urban areas of Europe, where the emission of pollution is high. The W and

NW air masses are considerably cleaner, having not travelled over any major pollution

sources in the previous five days.

In comparison to the short period of measurements in the previous example,

the AGAGE network (previously ALE/GAGE) has been operating since 1978

(http://agage.eas.gatech.edu/). One of the primary objectives is to optimally determine

from observations the rate of emission and/or chemical destruction (i.e. lifetime) of

Chromatographic Methods 391

10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00

2 000

4 000

6 000

8 000

10 000

12 000

14 000

16 000

18 000

20 000

Time-->

Abundance

Ion 46.00 (45.70 to 46.70): 000236.D

Methyl nitrate

Ethyl nitrate

Propyl nitrates

Butyl nitrates

Pentyl nitrates

Hexyl nitrates

Figure 8.12 Sample alkyl nitrate chromatogram obtained during TORCH 1, at Writtle College, near

Chelmsford, UK in August 2003. Agilent Technologies GC (6890)-MS (5973N) running in negative ion

chemical ionisation (NCI) mode with methane as chemical ionisation gas. (By courtesy of D. Worton,

University of East Anglia, UK.)

the anthropogenic chemicals that contribute most of the reactive chlorine and bromine

released into the stratosphere. There are currently five stations around the world

measuring a range of important halocarbons. Data collected for the growth of HFC-

134a, a CFC replacement, at the Cape Grim site, Tasmania, and over the Atlantic

(40



S, 50



W British Antarctic Survey) from 1991 to 1996, is shown in Figure 8.16.

The solid data represents the results obtained by a two-dimensional model with 3%

of global emissions occurring in the southern hemisphere. Long time-scale continuous

measurement networks are of vital importance to determine the concentrations of long-

lived species and in understanding their impact on the atmosphere.

8.3 Liquid chromatography

The analyses of the majority of organic compounds in the atmosphere are primarily

carried out using GC. However, the complete speciation of organics in the atmosphere is

not possible by GC alone. Liquid chromatography (LC) is a complementary technique and

is primarily used for the separation of non-volatile substances and highly polar or ionic

compounds. High performance liquid chromatography (HPLC) has its origins in classical

column chromatography, such as that used extensively in organic synthesis laboratories.

However, in theory and practice, it is more similar to GC. Particles as small as 1 min

diameter can be used as the stationary phase, packed into columns of about 10–25 cm

in length. The mobile phase is forced through the packed column by combinations of

high pressure pumps (up to 200 bar) to achieve volumetric flow rates of 1–5 cm

min

−1

Since the mobile phase affects selectivity in HPLC (rather than being inert as in GC),

combinations of solvents are dynamically combined to provide binary, ternary or even

quaternary mixtures of mobile phase.

Figure 8.13 ORAC chromatogram collected on 26 March 2004 on the UK research aircraft, a BAE 146–301 during the initial test ﬂying period, altitude of 214.8 m

over the Bristol channel (Lat 5140



, Lon −378



). Analyte assignments are given on the chromatogram. (By courtesy of J.B. McQuaid, University of Leeds, UK.)

Chromatographic Methods 393

Figure 8.14 Left – Mace Head Atmospheric Research Station during NAMBLEX, July–September 2002.

The GC container is marked. Right – inside of the University of York GC container, showing the GC,

sampling and PTV system and a series of canisters.)

100

200

300

400

500

600

30/Jul 31/Jul 01/Aug 02/Aug 03/Aug 04/Aug 05/Aug 06/Aug 07/Aug 08/Aug

Date

Concentration (pptV)

500

1000

1500

2000

2500

Ethane (pptV)

Acetylene

Benzene

Ethane

NW NE W

Figure 8.15 Concentrations of acetylene, benzene and ethane observed in air samples at Mace Head

during NAMBLEX. NW, NE and W indicate the origin of the air mass arriving at the site as calculated

by the 5-day back trajectory. Dotted lines indicate a change in trajectory direction. (By courtesy of

J.R. Hopkins, University of York, UK.)

394 Analytical Techniques for Atmospheric Measurement

Figure 8.16 HFC-134a concentrations (pptv) observed in air samples collected at Cape Grim

(41



S 145



E) and over the Atlantic Ocean (40



S 50



W), compared to results from the 2-D model with 3%

Union. Reproduced by permission of American Geophysical Union.)

8.3.1 Sample acquisition and preparation

In atmospheric chemistry the analytes amenable to HPLC are found in the gas and aerosol

phase or in atmospheric water, such as fog water. Samples must be introduced to the LC

inlet in a liquid solution and a number of trapping methodologies are in use. The primary

method of sample acquisition and preparation in atmospheric HPLC analysis uses

derivatisation techniques and is one of the most common methods of carbonyl analysis.

In situ derivatisation is commonly used where analytes are derivatised during the

collection process. Derivatisation can be carried out in the liquid phase, where air is

drawn through an impinger containing the derivatising agent in organic solution (such as

iso-octane). After collection, the organic fraction is evaporated to dryness and the residue

is dissolved in a polar solvent such as methanol. Alternatively, analytes can be derivatised

on solid phase absorbents such as SPME, via diffusion or a pumped flow. Derivatised

analytes are released from the absorbent using solvent extraction, which can be achieved

on-line (Sanchez et al., 2003). Post-acquisition derivatisation is less common in HPLC,

although it is used in aerosol analysis and for the detection of atmospheric peroxides (see

Chapter 7) (Kok et al., 1995).

Solvent extraction of filters is used extensively in particulate analysis. Much of the

aerosol compositional work since 2000 has involved the study of the highly polar organic

fraction, which is thought to be the result of numerous heterogeneous reactions in the

particulate phase. Solvent extraction using water yields the water-soluble organic content

(WSOC), a highly polar mixture that is most suited to LC analysis.

8.3.2 Separation

Early HPLC used columns packed with silica giving a highly polar stationary phase,

requiring the use of non-polar mobile phase solvents. This type of separation is known

Chromatographic Methods 395

as normal phase high performance liquid chromatography (NP-HPLC) and is limited to

the separation of non-polar compounds. In reverse phase (RP-HPLC), the addition of

functionalities to the stationary phase has increased the range of polarities that can be

analysed by HPLC. The addition of a hydrocarbon chain to the silica, most commonly

octadecylsiloxane (C

, ODS), changes the surface characteristics of the packing material

to a non-polar stationary phase. Polar mobile phases such as water and acetonitrile can be

used to separate hydrocarbon oxidation products, such as carboxylic acids and peroxides,

not amenable to GC.

Ion exchange chromatography (IEC), also a form of HPLC, is used for the analysis of

extremely polar organic compounds and ionic species, such as organic acids and HONO.

Stationary phases in IEC are generally resins, such as PS-DVB, a copolymer of styrene

and divinyl-benzene. The polymer is a three-dimensional cross-linked structure that is

rigid, porous and highly insoluble. Cation- or anion-exchange properties are introduced

to the resin by chemical modification after polymerisation. Metal ions, for example Ca

,Mg

,Na

, have been measured in aerosol particles using IEC (Lee et al., 2003).

Liquid chromatography can be used as a preparative step in atmospheric analysis to

simplify complex mixtures. WSOC in aerosol can be fractionated into three classes of

compounds using IEC: (1) neutral/basic compounds; (2) mono- and di-carboxylic acids;

and (3) polyacidic compounds. The functional group composition of fractions can then

be investigated using proton nuclear magnetic resonance spectroscopy (HNMR).

Multi-dimensional chromatography combinations of HPLC with GC are possible and

have been reported where an on-line sample preparation step is desirable. Since the

mobile phase from the LC stage must be interfaced to the GC, non-polar solvents that

may be easily evaporated are used (e.g. pentane). This limits the technique to normal

phase HPLC and water content in the sample becomes a serious problem in achieving

full automation.

8.3.3 Detection

The most common detector in routine HPLC analysis is the UV absorbance photometer

due to its wide linear range and good sensitivity. Photometers are operated at one or more

fixed wavelengths only and require the presence of a suitable chromophore within the

molecule. In many atmospheric samples, the molecules of interest have weak absorptions

in the UV region. The use of derivatisation to form analytes with strong UV absorbances

is widely used in HPLC, and DNPH-carbonyl adducts have maximum UV absorbances

at approximately 340–380 nm (Druzik et al., 1990). Diode array detectors can also be

coupled to HPLC, offering an extended detection range (190–950 nm), with high spectral

resolution and peak identification via UV spectral libraries. Ultraviolet detectors are

robust and non-destructive, allowing detected compounds to be collected or further

analysed.

Fluorescence detectors are highly selective and among the most sensitive of detectors.

Chromatographic analysis of hydrogen and organic peroxides are carried out on a C

RP-HPLC column followed by post-column reaction with horseradish peroxidase (Kok

et al., 1995). One of the reaction by-products is a dimer of p-hydroxyphenyl ethanoic

acid that has a strong fluorescence at an excitation of 301 nm and an emission wavelength

396 Analytical Techniques for Atmospheric Measurement

of 414 nm, giving typical atmospheric detection limits of H

and the organic peroxides

of 30 pptV.

One of the developments in HPLC is the routine coupling of the column to a mass

spectrometer. This is considerably more difficult than in GC-MS, where the eluent from

the end of the column flows directly into the ion source. The major difficulty in LC-MS

is the removal of the liquid mobile phase, whilst allowing only the analytes to pass into

the detector. Several interfaces have been designed for this purpose, but for air analysis

the most common are atmospheric pressure chemical ionisation (APCI) and electrospray

ionisation (ESI). The former uses a reagent gas such as nitrogen as a nebulising gas.

A heated nebuliser is used to vapourise the mobile phase and form reactant ions via a

corona discharge. The ions and analyte molecules are accelerated through skimmers into

a low-pressure region where the solvent is pumped away. Chemical ionisation (positive

or negative) of the analyte molecules occurs via collisions with the excited reagent ions,

which then pass into the mass spectrometer, typically a quadrupole-MS, for analysis.

LC-APCI/MS is particularly suited to moderately polar and non-polar analytes and has

been used to measure aldehydes in both aerosol particles and the gas phase (Grosjean

et al., 1999; Van den Bergh et al., 2002). It has also been used to measure nitroaromatic

compounds in the atmosphere as an indication of concealed land mines (Sanchez et al.,

2003).

In ESI, the eluent from the column flows through a metal capillary, which is at a

potential of several kilovolts relative to the surrounding chamber walls. The surfaces

of the emerging liquid become charged and the liquid is dispersed in a fine spray via

repulsive forces. In this case the analytes are ionised by the makeup gas before the solvent

is evaporated and swept away, along with any other non-charged material. Electrospray

ionisation is particularly suited to the analysis of charged, polar, and basic compounds,

making it a complimentary technique to APCI-MS. An example is the use of both ESI−

and APCI+ in terpene aerosol studies to identify the organic acids and carbonyl/alcohol

content respectively (Winterhalter et al., 2003).

8.3.4 Examples of the application of liquid chromatography

in air analysis

The importance of oxygenated compounds in the atmosphere has already been discussed.

Carbonyl species can be emitted from biogenic and anthropogenic sources, such as vehicle

emissions, and are also produced via photo-oxidation of hydrocarbons. Using DNPH to

convert carbonyls to their 2,4-dinitrophenyl hydrazones produces an analyte ideally suited

to HPLC. Figure 8.17 shows a DNPH–carbonyl standard mixture that has been analysed

using RP-HPLC column (150 mm ×46 mm C18 ODS, 5 m particle size) with a diode

array UV-visible detector (Grosjean et al., 1999). This method has been extended to the

analysis of carbonyls in both the gas and particulate phase of the atmosphere. The DNPH

derivatives of carbonyls found in a PM sample collected during the Pacific 2001 field

study in Fraser Valley, Canada, were analysed by HPLC-UV and a sample chromatogram

is shown in Figure 8.18 (Liggio & McLaren, 2003). Simplification of the complex aerosol

sample is achieved using a simultaneous extraction/derivatisation technique and an initial

guard column is used to trap the hydrazones before flushing onto the analytical column.

Chromatographic Methods 397

Time (min)

mAU

0 5 10 15 20 25 30 35

TOL

C3K+ACR

MEK+MTH

Figure 8.17 Liquid chromatography analysis of a mixture of the DNPH derivatives of 13 carbonyls

by ultraviolet absorption at 360 nm (diode array detector). C1, formaldehyde; C2, acetaldehyde; C3K,

acetone; ACR, acrolein; C3, propanal; CR, crotonaldehyde; MEK, 2-butanone; MTH, methacrolein; C4,

butanal; BZ, benzaldehyde; C5, pentanal; TOL, m-tolualdehyde; C6, hexanal. (Reprinted with permission

An important aspect of atmospheric chemistry is the use of smog chambers to simulate

the reactions of compounds of interest under controlled pseudo-atmospheric conditions.

The production of reaction intermediates can be simulated and the analytes identified,

without any background interferences, which would make this process extremely difficult

in real air samples. A number of the reaction products of -pinene ozonolysis have been

identified at the EUPHORE smog chamber, Valencia, Spain, using LC-MS (Winterhalter

et al., 2003). An example of the ESI− ionisation mode chromatogram is shown in

Figure 8.19, with the Total Ion Count (TIC) at the top, followed by a series of single ion

chromatograms. In the SIM modes, identification and increased sensitivity and resolution

are possible. The identification of reaction intermediates can give important information

about degradation mechanisms and is vital for accurate modelling studies.

The complexity of compounds in aerosol can be simplified using LC as a fractionation

tool. Lewis et al. (1995) measured PAH in dust particles using SFE to remove the organic

compounds from particulate samples followed by online LC-GC analysis. The initial LC

analysis separates the organic content with respect to polarity and removes the aliphatic

and polar compounds. Portions of the aromatic fraction were then transferred to the GC

for analysis. Figure 8.20 shows both the LC and subsequent GC chromatograms obtained

using this technique, with the PAHs indicated by numbers.

8.4 Future work

The use of chromatographic methods in the future for atmospheric measurements

seems certain. No other technique can deal so effectively with such a wide range of

chemical functionalities and trace species contained within complex mixtures. The science

benefits greatly from having increasingly sophisticated and reliable, commercially available