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358 Analytical Techniques for Atmospheric Measurement

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Chapter 8

Chromatographic Methods

Jacqueline F. Hamilton and Alastair C. Lewis

8.1 Introduction

The measurement of biogenic and anthropogenic organic species in the atmosphere is

a key area of atmospheric research. Nearest to the Earth’s surface, within the tropo-

spheric boundary layer, determining the concentration of reactive hydrocarbon species

is important because of their ability to react rapidly with OH radicals (formed from

sunlight) in the presence of NO

to form ozone and photochemical smog. The oxygenated

and nitrated reaction products can then react further or partition to the particulate phase

due to their relatively lower vapour pressures when compared to the parent hydrocarbon.

The use of gas chromatography to determine the organic content of the atmosphere

was a key factor in understanding the reasons behind the large-scale smog events in Los

Angeles in the 1970s.

At the other extreme, in the stratosphere, remote measurements of very long–lived

halogenated species are required because of their critical impact as precursors to ozone

destruction. In addition, other stable molecules such as methane, present throughout

the atmosphere, have importance as greenhouse gases and influence global climate and

temperature change.

The wide range of both short- and long-lived species that are of interest in atmospheric

science, coupled to extremely low concentrations and a requirement often for in situ

automated analysis, has led to the development of many novel chromatographic

techniques and methodologies. Whilst some atmospheric species such as organic acids,

peroxides, and aldehydes are measured using high performance liquid chromatography

(HPLC), the majority of organic species are analysed using capillary gas chromatography

(GC). The diversity of compounds that are of interest has resulted in almost every kind of

analytical detector finding a role within atmospheric analysis by chromatography. Many

organic compounds present in the atmosphere are bound to particles or are in aerosol

form, for example polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls

(PCBs), and dioxins. A vast number of methodologies for the analysis of these species

exist, although GC with mass spectrometry is the core technique for many.

Determining the exact chemical makeup of such multi-component mixtures is often

beyond the specificity of a single analytical property (e.g. absorption of ultraviolet

(UV) radiation or ionisation followed by separation by mass). Chromatography is one

of the most common methods of separating complex mixtures into their constituent

362 Analytical Techniques for Atmospheric Measurement

components. Chromatography traces its origins back to the early twentieth century when

botanist Mikhail Tswett used a liquid – solid adsorption separation to resolve many

coloured components within chlorophyll. This original application gave the technique its

name from the Greek for ‘colour’, or Chroma, and ‘writing’, graphein. From the mid-

twentieth century onwards, chromatography expanded rapidly in many directions: 1940,

the development of partition chromatography (for which A.J.P. Martin and R.L.M. Synge

were awarded the Nobel Prize); 1952, gas chromatography; 1970, bonded particles for

liquid chromatography; and in 1981, the pioneering of electrically driven separations, or

electrophoresis (Bartle & Myers, 2002). At the heart of all chromatographic techniques,

however, is a common set of principles. The mixture of components is solvated in a fluid

(referred to as the mobile phase) used to transport the analytes through the separation

system. The flow of mobile phase passes over or through an immiscible bed of material

or stationary phase. Interactions occur between analytes and the stationary phase, and

this slows the rate at which they pass through the bed. Each analyte may interact (e.g.

by adsorption) to a different extent with the stationary phase, resulting in the physical

separation of the mixture into its component parts. Figure 8.1 shows a schematic view

of the chromatographic process as it occurs in packed column elution chromatography.

Chromatography covers a wide range of methods and techniques, so it is useful to

classify these into a number of key subsets. Perhaps the most basic description that

can be made is of the physical state of the mobile phase: gas, liquid or supercritical

fluid. A second classification describes the way in which the mobile phase interacts with

the stationary phase. Where the stationary phase is either packed into or coated onto

the walls of a length of tube, it is referred to as column chromatography. The physical

dimensions can vary considerably: gas chromatography uses columns as long as 100 m

Stationary phase particle

Mobile phase

flow

Strongly retained analyte

Weakly retained analyte

Time

Figure 8.1 Chromatographic separation of compounds in a packed column.

Chromatographic Methods 363

and as narrow as 50 m whereas liquid chromatography takes place on columns of

relatively modest length but up to several centimetres wide. By applying positive pressure

at the start of the column, the mobile phase can be driven through the column with a

parabolic flow profile. Packed column chromatography involves the flow of mobile phase

through a column filled with a finely powdered stationary phase, in the order of m

in diameter, such as molecular sieve or silica. In capillary column chromatography, the

stationary phase, usually a highly viscous liquid, is coated onto the walls of the column,

and the mobile phase flows over this thin film. Planar chromatography (often referred

to as thin layer chromatography), where the stationary phase is coated onto a plate and

samples introduced as spots, finds widespread use in the pharmaceutical and biochemical

industries but is generally unsuitable as separation technique in atmospheric analysis. A

description of the various chromatographic techniques available is given in Table 8.1,

along with typical atmospheric examples of their use. For a more detailed description

of the fundamentals and applications of chromatography, see the material suggested in

‘Further reading’.

8.2 Gas chromatography

In gas chromatography the mobile phase, a highly diffuse gas, is used to solvate

and transport components within a mixture through or over a stationary phase, with

separation occurring due to differences in the rates of migration. The mobile phase gas

must have a high diffusion coefficient to ensure maximum numbers of gas–stationary

phase interactions. The most common gases used in gas chromatography are nitrogen,

hydrogen, and helium. Maximum separation efficiency is achieved using the lighter gases

and as such nitrogen does not find many applications in atmospheric separations. The

highest separation efficiencies are achieved using hydrogen gas over a wider gas velocity

range than helium. However, the carrier gas of choice is usually helium, due to the

safety implications and in-column hydrogenation of some unstable species associated

with hydrogen. High purity gases are used and it is normal to further purify the carrier

gas with the use of a combination of oxygen, moisture, and hydrocarbon traps.

The time a compound takes to be eluted from the column is known as the retention time

and is used in the identification of analytes through standard injections. In a correctly

optimised chromatographic separation, the analyte band will reach the detector in a

narrow plug with a degree of band broadening to produce a Gaussian-shaped peak. The

height or area under the peak is representative of the sample concentration giving a

quantitative measurement. The overall resolving power of a chromatographic column

can be given in terms of peak capacity. This is the maximum number of component

peaks that can be theoretically resolved on a given column. In gas chromatography, peak

capacities are generally considerably larger than in liquid chromatography, with typical

values being 200 and 20, respectively. To completely resolve highly complex atmospheric

samples would require extremely high peak capacities and selective detectors, extraction

techniques and derivatisation are often used to simplify the separation required.

The range of techniques that are in use is so broad that a complete review of analytical

methods is impractical. Many individual methods, however, have common components or

key procedural steps and these will be discussed. A general outline of a typical atmospheric

Table 8.1 Classiﬁcation of chromatographic techniques and examples of their use in atmospheric chemistry

Description Mobile phase Stationary phase Equilibrium Analyte

properties

Atmospheric

examples

Typical

detectors

Liquid chromatography

(LC)

Liquid–solid Liquid-organic Solid particles (e.g.

silica)

Adsorption/

desorption

Non-polar PAH, organic

fractionation

UV, FL

Liquid-bonded

phase

Liquid-aqueous Organic species

bonded or

stabilised on a

solid particle

surface (e.g. C



Partition

between liquid

and bonded

surface

All types Many applications,

e.g. PAHs,

-PAH,

peroxides, PCBs,

carbonyls and

derivatives

UV, FL, IR,

MS, ELS

Liquid–liquid Liquid Liquid supported

on solid particles

Partition

between

immiscible

liquid phases

Non-ionic

polar

UV, FL, IR,

MS, ELS

Size

exclusion/gel

permeation or

ﬁltration

Liquid Porous particles Retardation due

to penetration of

particle pores

All types Large molecules

in aerosols;

polymers, proteins,

biomolecules

UV, FL, IR,

MS, ELS, LIF

Ion exchange Liquid Charged ion

exchange resins

Ion exchange Ionic Organic acids,

inorganic ions

MS, FL, UV

Afﬁnity Liquid Binding ligand on

solid support

particle

Molecule-

speciﬁc

binding (e.g.

protein/enzyme)

Non-ionic

polar

None to date MS, UV

Capillary

electrochromatography

(CEC)

Liquid–solid Liquid

(electro-osmotic

ﬂow)

Organic species

bonded or

stabilised on a

solid particle

surface (e.g. C



Partition

between liquid

and bonded

surface

Non-polar,

non-ionic

polar and

ionic

None to date UV, MS, LIF

Gas(GC) Gas–Liquid Gas Liquid coated onto

solid particles in a

packed column

Partition

between

phases

Non-polar

and

non-ionic

polar

‘BP-1’ type

pure volatility

separations

FID, MS,

ECD

Gas-bonded

phase

Gas Stabilised

stationary phase on

solid surface (e.g.

particles or

capillary walls)

Partition

between

phases

Non-polar

and

non-ionic

polar

Typical

semivolatile VOC

separations,

aromatics, CFCs,

halocarbons, PAN,

terpenes,

hydrocarbons,

carbonyls

FID, MS,

ECD, FPD,

PID

Gas–solid Gas Solid support,

either particles or

on capillary walls

Adsorption/

desorption

Non-polar

and volatile

Light non-methane

hydrocarbons,

small carbonyls,

FID, MS

Size exclusion Gas Porous particles or

supported on

capillary walls

Penetration

of porous

media

Non-polar

and

permanent

gases

Permanent gases,

CO, CH

TCD, FID,

HID

Supercritical ﬂuid

(SFC)

SF-bonded

phase

SF Stabilised

stationary phase on

solid surface (e.g.

particles or

capillary walls)

Partition

between

phases

Non-polar

and

non-ionic

polar

PAH, substituted

aromatics,

terpenes, thermally

labile compounds.

UV, FID, MS

Abbreviations: UV – Ultraviolet, FL – Fluorescence, MS – Mass spectrometry (generally quadrupole or ion trap but also TOF GC and LC and magnetic sector for GC), IR – Infrared,

ELS – Evaporative Light Scattering, LIF – Laser induced ﬂuorescence, FID – Flame ionization detector, ECD – Electron capture detector, FPD – Flame photometric detector, PID –

photoionisation detector, HID – Helium ionization detector, TCD – Thermal conductivity detector.

366 Analytical Techniques for Atmospheric Measurement

Sample acquisition

in situ off-line

Sample preparation

Chromatographic separation

GAS LIQUID ION

Detection

UV, APCI-MS

Detection

FID, ECD, MS, etc.

Detection

Figure 8.2 Flow diagram of a typical chromatographic separation regime. FID = ﬂame ionisation

detector, ECD = electron capture detector, MS = mass spectrometer, APCI = atmospheric pressure

chemical ionisation.

determination can be broken down into the following: sample acquisition, preparation,

separation, and detection, as shown in Figure 8.2, with the first two stages of acqui-

sition and preparation often proving the most challenging. A number of chromatograms

obtained from atmospheric analysis are also presented.

8.2.1 Sample acquisition

The initial step of sample acquisition is a far from trivial task for atmospheric measure-

ments, and defines a major grouping of techniques into either in situ or post acquisition

analysis. The ability to store an atmospheric sample is a critical factor as to whether

analysis may be performed back in the laboratory or on-site immediately following

acquisition. Stable species such as methane, CO and CFCs may be successfully stored

in sample vessels for many weeks without affecting sample integrity, and widespread

measurements of these species have been performed. Atmospheric degradation products

such as peroxyacetyl nitrate (PAN, CH

C(O)OONO

) are so unstable, however, that

analysis must be performed immediately. For many other important reactive species such

as alkenes, monoterpenes and dimethyl sulphide (DMS), the storage of samples has been

shown to lead to a degree of analyte loss due to reaction with co-sampled pollutants

such as ozone and oxides of nitrogen (Sin et al., 2001). To overcome these problems of

reaction during storage, many forms of scrubber (e.g. potassium iodide and glycerol to

remove ozone) have been tested to remove such species without affecting sample integrity

(Vairavamurthy et al., 1992).

Much atmospheric sampling is performed using stainless steel canisters of 1–10 L in

volume (often referred to as whole air samples), filled either by vacuum release or by

pressurising the sample using stainless steel bellows or Teflon diaphragm pumps. To

retain sample integrity, there must be minimal interaction with the canister walls, and

Chromatographic Methods 367

coating methods such as electropolishing, silica or Teflon coating are currently in use.

The preparation and cleaning of canisters require careful attention, and high vacuums

are often applied along with elevated temperatures for periods of hours or even days. A

similar method to canister samples is the use of collapsible Teflon or Tedlar sample bags.

These may be filled by a pump and returned to a laboratory for analysis using almost

identical procedures to that of canister samples. Whilst generally of lower unit cost, they

often produce a greater degree of sample artefact and analyte losses.

Many lower volatility species are unsuitable for collection using canister methods

because of problems associated with analyte condensation onto the walls of the container.

For this reason a second method of sample acquisition is widely used based on the use

of a solid phase adsorbent (which is essentially a very high capacity packed column) as

an analyte trap. The adsorbent used in the trap may be chosen to introduce an element

of selectivity to the trapping mechanism, although in practice a trap-all approach is

commonly used. Recycling of a single adsorbent allows for instrument automation, which

is not possible with canister methods, and such recycled traps now form the basis of

many national monitoring programs in the urban environment, as well as automated

instruments for research in clean air environments (http://www.epa.gov/ttn/amtic/

airtoxpg.html).

A huge range of adsorbent supports are commercially available, ranging from high

surface area >1000m

−1

 carbon material (both charcoals and graphitised) with strong

retention characteristics, to lower surface area <50 m

−1

 polymerics such as Tenax™

TA. Whilst being relatively low cost compared to sample canisters, care is often required

in the cleaning and preparation of sample tubes. Samples may be introduced to the

adsorbent tubes either dynamically over short periods of time (typically minutes) or via

diffusional sampling over longer periods (typically several days). Carbon-based adsor-

bents are suitable for a wide range of species ranging from volatile hydrocarbons and

chlorofluorocarbons (CFCs) to organic nitrates. Polymeric materials are used mainly

for the concentration of lower volatility species such as aromatics and monoterpenes,

although compounds as large as 2- and 3-ring polycyclic aromatic species may also be

successfully trapped and thermally or solvent desorbed. Sulphur species trapping is often

performed via chemisorption onto gold wool traps that provide a stable matrix for the

sample to be stored for reasonable periods of time (Berresheim et al., 1998).

The complete retention of all target analytes on the adsorbent bed must be carefully

evaluated, and calculation of retention volume (often referred to as breakthrough volume)

is essential. For sampling very volatile species (e.g. ethane or ethene) the retention volume

for an adsorbent system is often the critical factor in determining system sensitivity.

Trapping at sub-ambient temperatures is commonly employed to increase the maximum

sample volume, often using liquid nitrogen or carbon dioxide to cool the adsorbent trap.

For operation of any instrument in a field location, cryogen usage may be a problem and

modern instruments often employ Peltier or thermoelectric heat pumps to cool adsorbent

traps.

The analysis of low molecular weight, oxygenated compounds in the atmosphere can

be difficult, with irreversible retention of polar compounds onto absorbents and losses to

canister walls. A selective and quantitative method of sampling oxygenated compounds

is the use of derivatisation during sampling to form an analyte more suited to chromato-

graphic separations. Samples can be collected onto solid-phase microextraction fibres