Analytical techniques for sesquiterpene emission rate studies in vegetation enclosure experiments

Atmospheric Environment 38 (2004) 557–572

ARTICLE IN PRESS

*Correspond

303-492-6388.

E-mail addr

(D. Helmig).

1352-2310/$ - se

doi:10.1016/j.at

Analytical techniques for sesquiterpene emission rate studiesin vegetation enclosure experiments

Detlev Helmig*, Florence Bocquet, Jan Pollmann, Tobias Revermann

Institute of Arctic and Alpine Research (INSTAAR) University of Colorado, Campus Box 215, Boulder, CO 80309-0450, USA

Received 8 July 2003; accepted 3 October 2003

Abstract

Sesquiterpene (SQT) compounds (C15H24) and their oxygenated alcohol and ketone derivatives are biogenic volatile

organic compounds that have been identified in emissions from vegetation. SQT emission rates and landscape flux

estimates are highly uncertain. Reliable ambient flux measurements have not been possible because of low-ambient

concentrations, rapid atmospheric reactions (prohibiting ambient tower flux measurements), and analytical challenges

and uncertainties that stem from the low volatility of SQT. Standards from an in situ capillary diffusion system with 18

SQT compounds and four other organic compounds (geranylacetone, 1,3,5-tri-isopropylbenzene, diphenylmethane,

nonylbenzene) were used to thoroughly investigate experimental procedures for SQT emission rate studies by vegetation

enclosure techniques. Recovery rates in tubing materials, sampling bags, leaf cuvettes, on six solid adsorbent materials

(Tenax TA, Tenax GR, Carbotrap, Carbotrap C, Unibeads, Glass Beads) for gas chromatography analysis, and gas

chromatography retention indices and mass spectral fragmentation patterns were determined. SQT compounds were

found to exhibit a high degree of stickiness to all materials tested. However, the non-oxygenated SQT can be recovered

in enclosure experiments for quantitative emission rate determination after careful consideration of the analytical

conditions. It is utmost important to allow sufficient purging and equilibration times for all materials in contact with

the sample air. In contrast oxygenated SQT were irreversibly lost in enclosure experiments which made their

quantitative measurement prohibitive. Results for the other organic compounds were similar and indicate that these

data mostly stem from the volatility of these compounds. Consequently, the findings of this study provide guidelines for

the analysis of a wide range of volatile organic compounds in the BC13–C17 volatility range.

r 2003 Elsevier Ltd. All rights reserved.

Keywords: Semi-volatile organic compounds; Sesquiterpenes; Emissions from vegetation; Enclosure experiments; Solid adsorbent

sampling; Gas chromatography

1. Introduction

Sesquiterpene hydrocarbons (C15H24, SQT) and some

oxygenated alcohol, aldehyde, and ketone analogous

derivatives (C15H22O, C15H24O, C15H26O) have been

identified in studies on biogenic volatile organic

compound (BVOC) emissions from both natural and

ing author. Tel.: +1-303-492-2509; fax: +1-

ess: [email protected]

e front matter r 2003 Elsevier Ltd. All rights reserve

mosenv.2003.10.012

agricultural vegetation (Buttery et al., 1985; Bicchi et al.,

1989; Omata et al., 1990; Winer et al., 1992; K .onig et al.,

1995; Rudolph et al., 1997; Schuh et al., 1997; Llusia

and Penuelas, 1998; Zhang et al., 1999; Hansen and

Seufert, 1994; Ciccioli et al., 1999; Bartelt and Wicklow,

1999; Helmig et al., 1999a; Kim et al., 2000; Agelopou-

los et al., 2000; Hakola et al., 2001). A few preliminary

studies indicate that SQT landscape fluxes may be

significant (Ciccioli et al., 1999; Helmig et al., 1999b),

possibly contributing to up to 16% of the overall carbon

BVOC landscape flux (Helmig et al., 1999b). Because of

their rapid atmospheric reactions (Grosjean et al., 1993;

d.

ARTICLE IN PRESSD. Helmig et al. / Atmospheric Environment 38 (2004) 557–572558

Shu and Atkinson, 1994, 1995; Hoffmann et al., 1997)

and high aerosol yields (Hoffmann et al., 1997) SQT are

suspected to contribute to secondary aerosol formation.

Biogenic precursor compounds have long been specu-

lated to play an important role in atmospheric aerosol

formation (Went, 1960; Schuetzle and Rasmussen, 1978;

Yokouchi and Ambe, 1985; Mlot, 1995), but their

quantitative contribution remains highly uncertain.

SQT have also garnered interest in research on

chemically mediated plant–insect interactions. The

motivation for this research is the development of

ecological and environmentally safe methods for agri-

cultural pest control. Experiments on corn and tobacco

have demonstrated that these plants respond to envir-

onmental stress, in particular to herbivore attack by

synthesizing and releasing a variety of terpenoid

compounds including SQT (Alborn et al., 1997; Pare

and Tumlinson, 1997a, b, 1999; Halitschke et al., 2000;

Degenhardt and Gershenzon, 2000; De Moraes et al.,

2001; Pichersky and Gershenzon, 2002; Gouinguen!e and

Turlings, 2002). These volatile emissions have been

found to be a defense and communication mechanism of

injured plants. For instance, in experiments on corn

seedlings it was shown that volatile BVOC including

SQT attract wasps that attack caterpillars feeding on the

corn (Turlings and Tumlinson, 1992; Turlings et al.,

1995).

Only a few quantitative emission studies have been

attempted. Substantial uncertainties in SQT emission

rates arise from the lack of defined analytical techniques

for SQT identification and from SQT losses in enclosure

experiments or in eddy correlation flux studies. These

uncertainties stem from the semi-volatile nature of SQT.

With molecular weights in the range of 204–222 gmol�1,

these compounds are too low in volatility for samples

and standards to be stored and quantitatively recovered

from storage containers such as compressed air gas

cylinders and gas sampling flasks or canisters. An

alternative approach for standard generation by capil-

lary diffusion has recently been reported to yield reliable

SQT standards for analytical testing (Komenda et al.,

2001; Helmig et al., 2003).

Landscape fluxes of other, lighter BVOC have

preferably been investigated by ambient flux measure-

ment techniques from tower platforms, tethered bal-

loons or research aircraft. Experimental approaches

include eddy correlation, tower gradient, mixed layer

gradient or mixed layer budget methods (Guenther et al.,

1996a, b; Fuentes et al., 2000). These techniques have

successfully been used for isoprene, monoterpenes and

several other oxygenated BVOC. A requirement for the

applicability of these methods is that mixing ratios in

turbulent eddies (resulting in vertical gradients) are due

to the transport from the source (canopy) to the

atmosphere above rather than from atmospheric deple-

tion by chemical reactions. These conditions are

generally met for the listed BVOC. These BVOC have

atmospheric lifetimes on the order of 1 h, which are long

compared to the scale of atmospheric mixing and

transport in the planetary boundary layer.

SQT are more reactive than isoprene and mono-

terpenes. SQT atmospheric lifetimes from their reactions

with ozone, and the OH and NO3 radicals have been

estimated to be only a few minutes (Grosjean et al.,

1993; Shu and Atkinson, 1994, 1995; Hoffmann et al.,

1997). Consequently, significant depletion by atmo-

spheric reactions is expected within the immediate

vicinity of the source. Because of the substantial SQT

losses within the plant canopy, above-canopy flux

measurements are deemed to underestimate the overall

biosphere SQT flux. The fate of the emitted SQT is

somewhat uncertain at this time, but it appears likely

that a portion of the SQT products is trapped within the

plant canopy and that another portion of SQT escapes

the canopy as gas-phase reaction products, or in organic

aerosols. In one particular case, this speculation has

been experimentally proven: Ciccioli et al. (1999) found

that relaxed eddy correlation fluxes of b-caryophylleneover an orange orchard were only B1

6of the estimates

that were derived from scaled-up enclosure measure-

ments.

Consequently, SQT emission rates are best deter-

mined by enclosure techniques, such as with leaf cuvettes

or branch bag enclosures. While the artificial conditions

imposed during enclosure experiments are a valid

concern, precise light and temperature control has been

shown to yield valuable BVOC emission data (Guenther

et al., 1996a, b; Ciccioli et al., 1999; Fuentes et al., 2000).

Furthermore, enclosure studies are particularly advan-

tageous for SQT emission rate studies because oxidant

levels (e.g. OH, O3) are low and residence times of SQT

in air can be kept in the second to minute range. This

approach eliminates the difficulties associated with the

ambient level measurements (see above).

Preconcentration on solid adsorbents followed by

thermal desorption and GC analysis has become a well-

accepted VOC analysis technique in a wide range of

applications. Most techniques described in the literature

are for compounds in the C5–C10 volatility range. This

range has been extended occasionally to more volatile

C2–C4 VOC (Helmig, 1999) using stronger (higher

surface area) adsorbents. In contrast, the applicability of

adsorbent sampling/thermal desorption for heavier

(>C10) and semi-volatile VOC has received little

attention. Consequently, very little research has been

presented that can be applied to SQT analysis. In

particular, data on SQT thermal stability, breakthrough

on solid adsorbents, recovery rates from adsorbents and

potential losses to surfaces are needed to facilitate SQT

analysis in enclosure experiments.

In this research six different adsorbent materials were

tested for analysis of SQT in air samples. Furthermore,

ARTICLE IN PRESSD. Helmig et al. / Atmospheric Environment 38 (2004) 557–572 559

experimental systems, such as an automated cartridge

sampler, tubing materials, Teflon bags, and leaf cuvettes,

which typically are used for sampling of BVOC in

vegetation emission studies, were investigated for their

retention of SQT. Four non-SQT semi-volatile VOC

were included in these experiments in order to investi-

gate if findings were SQT specific or generally applicable

to VOC in the C13–C17 volatility range.

2. Experimental

2.1. Analytes

Compounds included in this study were 11 commer-

cially available SQT hydrocarbons, two SQT ketones/

oxides and five SQT alcohols. Furthermore, geranyla-

cetone, another frequently identified BVOC emission

and three aromatic VOC (1,3,5-tri-isopropylbenzene,

diphenylmethane, nonylbenzene), were investigated for

comparison with SQT. Physicochemical parameters and

suppliers of all of the standards have been given

previously (Helmig et al., 2003).

2.2. Capillary diffusion system

SQT standards were generated using a 10-channel

capillary diffusion system (CDS). This instrument has

recently been described in detail (Helmig et al., 2003), so

only a brief summary is given here. SQT gas standards

are prepared by diffusion of SQT from a liquid reservoir

into a nitrogen gas flow. Diffusion rates are controlled

by proper selection of length and diameter of glass

capillary resistors and by temperature. Diffusion rates of

SQT were quantified with a median B5–6% precision

and B10% accuracy by the gravimetric weight loss of

the SQT reservoirs as well as by a quantitative on-line

GC-flame ionization detection (FID) method. Agree-

ment between these independent methods generally was

>90%. The CDS allows investigating individual as well

Table 1

Properties of solid adsorbent materials and filling weight of adsorben

Adsorbent Filling

weight (g)

Conditioning

temperature (�C)

Maximum

temperature (�

Tenax TA 0.25 330 350

Tenax GR 0.25 330 350

Carbotrap 0.40 330 400

Carbotrap C 0.30 350 350

Unibeads 0.30 300 n/a

Glass Beads

DMCS treatded

1.00 300 n/a

aLiterature and suppliers information.

as mixtures of SQT. With an integrated dilution system,

the output mixing ratios can be varied between

B100 pptv (parts-per-trillion volume) to 2000 ppbv

(parts-per-billion volume). Potential analytical interfer-

ences from water vapor and ozone can be simulated by

adding either of these two gases. For the experiments

described in this research, the output mixing ratios of

individual channels were isopropylbenzene 718 ppbv,

longipinene 1020 ppbv, a-copaene 528 ppbv, isolongifo-

lene 879 ppbv, d-neoclovene 727 ppbv, longifolene

592 ppbv, diphenylmethane 416 ppbv, isocaryophyllene

785 ppbv, cedrene 555 ppbv, caryophyllene 1690 ppbv,

aromadendrene 560 ppbv, humulene 417 ppbv, gerany-

lacetone 938 ppbv, cis-nerolidol 126 ppbv, caryophyllene

oxide 247 ppbv, trans-nerolidol 136 ppbv, nonylbenzene

444 ppbv, cedrol 241 ppbv, isolongifolen-9-one

390 ppbv, bisabolol 63 ppbv and farnesol 10 ppbv. By

combining all ten channels, the above mixing ratios are

diluted by a factor of B10. For a second dilution step, a

fraction of the combined output was split off and mixed

with scrubbed zero air. This resulted in an overall

dilution factor of B420 (for humidity and recovery

experiments) and B800 (for linearity tests).

2.3. Adsorbent cartridges

Four common solid adsorbents as well as two

activated glass materials (Unibeads and dimethyldi-

chlorosilane-treated Glass Beads) were tested for analyte

collection and enrichment. All of these adsorbents

have previously been characterized as relatively ‘weak’

(i.e. low surface area) materials and were therefore

expected to be suitable for the sampling of the semi-

volatile SQT with thermal desorption analysis. The

adsorbents, their properties and sources are given in

Table 1. Adsorbent tubes were prepared from borosili-

cate glass tubing (0.63 cm o.d., 0.36 cm i.d., Wale

Apparatus, Hellertown, PA), which was cut to 9 cm

long pieces. Tubes were cleaned by sonicating in

methanol, marked with heat resistant labels and baked

t cartridges used in this study

C)

Mesh size Surface area

(m2/g)aVendor

60/80 35 Alltech, Deerfield, II

20/35 24 Alltech, Deerfield, II

20/40 100 Supelco, Bellefonte, PA

20/40 10 Supeteo, Bellefonte, PA

60/80 n/a Alltech, Deerfield, II

60/80 n/a Alltech, Deerfield, II


at 600�C for 20min prior to filling. Glass tubes were

filled for a total length of 6 cm with the respective

adsorbent material (see Table 1 for adsorbent weights).

The adsorbent was held in place by glass wool plugs on

both sides, a spring made of 3 cm nickel wire (>99.99%

purity, Sigma-Aldrich, Milwaukee, WI) on the sampling

(inlet) side and a stainless steel spring on the other side.

Adsorbent cartridges were conditioned by heating and

purging with purified nitrogen (ultra-high purity grade,

Airgas, Denver, CO) at a flow rate of 70–80mlmin�1 for

120min. The nitrogen purge gas was further purified by

a hydrocarbon scrubber (N930-1192, Perkin-Elmer,

Shelton, CT) and an oxygen trap (Oxytrap, Alltech,

Deerfield, IL). Cartridge bakeout temperatures are

included in Table 1.

2.4. Adsorbent cartridge sampling

A custom-made automated sampler (Fig. 1) was

used for the consecutive sampling of up to 10 cartridges

under temperature and flow controlled conditions. The

principal sampler components are one 0.32 cm two-

position four-port valve and one 0.32 cm multiposition

ten-port valve (VICI, Houston, TX). All tubing

and valve components can be continuously purged

prior to and between the switching of adsorbent

cartridges into the flow path for sample collection. All

tubing was made of 3.2mm o.d. Silicosteel (Restek,

Bellefonte, PA). Valves and all tubing were maintained

at 50�C, adsorbent tubes were kept at room temperature

(25�C) during sampling. A 90 cm long 0.32 cm o.d.

Teflon (PTFE) sampling line from the cartridge

sampler to the outlet of the SQT capillary diffusion

system was also heated to 50�C. The sampler is

computer controlled by a custom program written in

Basic which allows any desired variation of purging and

sampling times.

Fig. 1. Schematic of the automated solid adsorbent cartridge sample

valve) as well as all tubing were heated to 50�C.

2.5. GC analysis

Adsorbent cartridges were analyzed with a Perkin-

Elmer ATD-400 automated cartridge desorber and a

Perkin-Elmer AutoSystem XL GC/FID instrument. The

following ATD 400-parameter were used: He purge flow

37mlmin�1, primary desorption temperature 325�C,

primary desorption time 12.5min, second stage trap

temperature during primary desorption: �22�C, second

stage trap desorption temperature 350�C, hold time

5min. The second stage trap was filled with Unibeads.

Outlet split flow during the second stage desorption was

7.7mlmin�1. Adsorbent cartridges were backflushed

during thermal desorption. GC parameters were as

follows: FID hydrogen 60mlmin�1, FID air

350mlmin�1, He carrier gas flow 2.5mlmin�1; GC

oven program: 40�C for 5min, 20�Cmin�1 to 100�C,

2�Cmin�1 to 160�C, 40�Cmin�1 to 250�C, final time

5min. GC column details are given in Table 2.

2.6. GC-MS/FID

A GC/MS (Hewlett-Packard 5890/5970) with addi-

tional FID detector was used for compound identifica-

tion and additional quantitative experiments. This

instrument was equipped with a custom-made inlet

system (2-position 6-port valve [VICI]) for thermal

desorption. First, cartridges were purged at ambient

temperature with He in the sampling flow direction to

remove water and air for 5min at 30mlmin�1. For

thermal desorption, the flow direction was reversed and

cartridges were backflushed directly onto the GC

column as they were heated to 325�C over 3min. He

desorption flow was 7mlmin�1. The switching valve and

transfer line (0.16mm o.d. Silicosteel) were heated to

150oC. The GC column was kept at 40�C during

the desorption step. Under these conditions SQT are

r. All valves (2-position 4-port valve and 10-position sampling

ARTIC

LEIN

PRES

S

Table 2

Sesquiterpene retention indices on three GC systems and mass spectral fragmentation

Retention-index

Compound GC-type: On-line GC-FID PE GC-FID HP-GC-FID/MS Mass fragments from HP-GC-FID/MS m/z (% abundance)

Column: DB-1 DB-1 CPSIL5CB

Inner diameter: 0.320mm 0.320mm 0.320mm

Length: 30m 30m 60m

Film-thickness: 0.1 mm 0.25mm 0.25mmManufacturer: J. & W. Scientific J. & W. Scientific Chrompack

n-Tridecane 1300.0 1300.0 1300.0

1,3,5-Tri-isopropylbenzene 1322.670.2 1322.870.9 1324.370.6 189 (100); 161 (58); 91 (36); 204 (26); 105 (25), 190 (15)

a-Longipinene 1326.670.3 1338.670.5 1348.870.9 119 (100); 93 (41); 91 (40); 133 (37); 92 (22); 120 (20)

a-Copaene 1354.070.2 1363.570.7 1375.770.8 161 (100); 119 (99); 105 (88); 159 (57); 93 (47); 91 (41)

Isolongifolene 1357.970.1 1372.871.0 1390.071.0 161 (100); 133 (49); 148 (49); 175 (45); 119 (38); 204 (21)

d–Neoclovene 1370.970.3 1389.670.8 1406.270.5 105 (100); 120 (96); 119 (93); 121 (72); 91 (68); 107 (63)

Longifolene 1373.970.5 1389.670.8 1406.370.5 91 (100); 94 (88); 93 (83); 161 (83); 107 (80); 79 (69)

Diphenylmethane 1380.170.2 1389.670.8 1403.970.5 167 (100); 169 (100); 165 (37); 152 (25); 153 (23)

Isocaryophyllene 1380.670.2 1389.670.8 1405.370.5 93 (100); 91(84); 69 (82); 79 (78); 133 (63); 105 (51)

a-Cedrene 1383.170.2 1396.970.7 1413.070.6 119 (100); 93 (46); 105 (32); 91 (30); 77 (23); 92 (18)

b–Caryophyllene 1391.870.2 1404.370.2 1417.170.9 69 (100); 93 (100); 91 (87); 79 (84); 133 (72); 105 (53)

n-Tetradecane 1400.0 1400.0 1400.0

Aromadendrene 1410.070.1 1423.770.2 1435.970.7 91 (100); 93 (91); 105 (87); 79 (86); 107 (81); 161 (75)

a-Humulene 1421.870.1 1433.170.4 1448.070.8 93 (100); 80 (36); 121 (25); 91 (23); 92 (23); 147 (14)

Geranylacetone 1423.070.1 1421.570.7 1427.170.8 69 (100); 107 (28); 151 (23); 67 (22); 136 (20); 93 (16)

n-Pentadecane 1500.0 1500.0 1500.0 161 (100); 119 (55); 134 (54); 204 (51); 105 (39); 91 (25)

d-Cadinene 1478270.1 n.a. 1507.170.8

cis-Nerolidol 1510.470.1 1512.270.1 1516.170.6 69 (100); 93 (61); 55 (37); 107 (29); 81 (28); 79 (18)

Caryophyllene Oxide 1534.170.1 n.a. n.a. 79 (100); 93 (80); 55 (75); 81 (72); 107 (72); 95 (63)

trans-Nerolidol 1538.57 0.1 1541.370.5 1545.270.7 69 (100); 93 (54); 107 (34); 71 (34); 55 (32); 70 (24)

Nonylbenzene 1542.870.1 1547.770.4 1554.170.8 92 (100); 91 (81); 204 (14); 133 (6); 71 (6)

Cedrol 1548.17 0.3 1561.870.2 1583.070.8 95 (100); 150 (86); 151 (64); 81 (37); 135 (31); 93 (28)

n-Hexadecane 1600.0 1600.0 1600.0

lsolongifolen-9-one 1627.970.1 1645.070.8 1663.970.9 175 (100); 147 (78); 162 (75); 176 (49); 119 (42); 218 (42)

Bisabolola 1646.470.3 1658.572.1 n.a. n.a.

Bisabololb 1649.070.2 n.a. n.a. n.a.

n-Heptadecane 1700.0 1700.0 1700.0

Farnesol 1775.970.2 n.a. n.a. n.a.

a,bTwo unidentified isomers.

D.

Helm

iget

al.

/A

tmo

sph

ericE

nviro

nm

ent

38

(2

00

4)

55

7–

57

2561


pre-focused on the column head. The final desorption

time was 12.5min. Then, the injection valve was

switched back and the GC carrier gas flow was reduced

to 1.7mlmin�1 for GC analysis. The column flow was

split and directed to two detectors. The minor fraction

(30%) eluted into the MS for compound identification,

the remaining 70% were directed to the FID for

quantitative analysis. The GC column is described in

Table 2. The FID flow rates were 60mlmin�1 H2 and

350mlmin�1 air. The GC oven temperature program

was: initial temperature 40�C held for 1min, 25�Cmin�1

to 120�C; 2�Cmin�1 to 190�C; 25�Cmin�1 to 250�C;

250�C held for 5min.

3. Experiments and results

3.1. Compound identification

Analytes were identified by their mass spectral

fragmentation (MS) and linear programmed GC reten-

tion indices (RI) (Van den Dool and Kratz, 1963). MS

fragmentation data (determined by averaging of 4–6

scans around the GC peak maximum) and RI on two

stationary phases (three GC columns total) are sum-

marized in Table 2. When using the relatively unpolar

methylsilicone column phases, hydrocarbon SQT

(C15H24) RI lie within the range of 1325–1500, and the

more polar and less volatile SQT ketones and alcohols

elute between RI B1525–1775. Several pairs of analytes

0

20

40

60

80

100

120

140

160

1300 1350 1400 1450 15

Retentio

Rec

ove

ry (

%)

Fig. 2. Recovery of analytes during sampling with the automated car

the outlet of the CDS. The compound recovery is plotted against the

means from 3 parallel sampling experiments; error bars represent

individual standard deviation of the direct sampling and the sampler

were co-eluting on these GC columns (e.g. diphenyl-

methane and longifolene, aromadendrene and gerany-

lacetone) and consequently experimental results are

reported for the total of these two analytes combined,

or one of the co-eluting analytes was excluded from

experiments, respectively.

3.2. Automated sampler testing

Potential losses of SQT in the automated cartridge

sampler were investigated by parallel sampling. Two

adsorbent-cartridges (Tenax TA) were loaded under the

exact same sampling time and flow rate (10min at

206.8mlmin�1 (all flows reported are converted to

standard pressure and temperature, STP)) at a 1:420

dilution (B0.19–3.76 ppbv SQT). One cartridge was

loaded directly at the diffusion system outlet and the

second cartridge was loaded by sampling the standard

from the same outlet through the sampling line with the

cartridge sampler (Fig. 1). The sampling line and

switching valve of the sampler were purged for a

minimum of 10min before the first sample was loaded.

The recovery rates for the SQT are within the range of

90–110% (Fig. 2). However, with increasing RI (lower

volatility) the deviations from 100% as well as the

relative standard deviations increase significantly. Re-

sults for the oxygenated SQT (higher RI (Table 2)) are

worse than for the hydrocarbon SQT. Higher tempera-

tures for the sampler components may help to improve

upon the data for the less volatile compounds, but

00 1550 1600 1650 1700

n Index

tridge sampler in comparison with direct sample collection from

analyte DB-1 retention index (Table 2). Recovery rates are the

the standard deviation ðn ¼ 5Þ and were calculated from the

data by error propagation.


unfortunately 50�C was the temperature limit of several

components that were part of our sampler. It should

also be noted that these results reflect the total sampling

plus analysis precision and that a significant contribu-

tion to the error arises from the GC analysis step (see

adsorbent testing results below). These data demon-

strated that SQT losses in the sampler were not

significant and the sampler was subsequently used in

the experiments described in the following.

3.3. Adsorbent testing–recovery ratio

The recovery ratios from the six adsorbents were

determined in two experiments. First, samples were

collected at high concentration and low sampling volume

(1min sample collection, flow rate 108mlmin�1; dilution

factor 1:10; SQT mixing ratio between B1–160 ppbv).

For the second experiment, the SQT standard was

diluted and cartridges were loaded for 10min (dilution

factor 1:420, flow rate 206.8mlmin�1; SQT mixing ratio

between 0.19–3.76 ppbv). Desorption efficiency was

determined by performing repeated desorptions on each

adsorption tube. Desorption temperature was 325�C in

all cases. This temperature may be higher than required

for some of the adsorbent materials, but was found to

overall give the best results. During the adsorption step

cartridges are continuously purged and gradually heated

up from room temperature. Consequently, SQT will be

desorbed and purged off the adsorbent bed when their

respective desorption temperature is reached, which may

Table 3

Results of second desorption experiments at 100ml and 2.11 samplin

Tenax TA Tenax GR

Compound 100ml 2.1 l 100ml 2.1 l

Isopropylbenzene >99.9 >99.9 >99.9 >99.6

Longipinene >99.9 >99.9 >99.9 99.2

a-Copaene >99.9 >99.9 >99.9 >99.4

Isotongifolene >99.9 >99.9 >99.9 >99.6

Diphenylmethane + Longifoleneb >99.9 >99.9 >99.9 >99.7

a-Cedrene >99.9 >99.9 >99.9 >99.4

Caryophyllene >99.9 >99.9 >99.9 >99.8

Geranylacetone >99.9 >99.9 >99.9 >99.3

Aromadendrene >99.9 >99.9 >99.9 99.6

a-Humulene >99.9 >99.8 >99.9 >99.1

cis-Nerolidol >99.8 >99.6 >99.5 >97.9

trans-Nerolidol >99.8 98.0 >99.7 >98.6

Nonylbenzene+Caryophyllene-Oxideb >99.9 99.8 >99.9 >99.6

Cedrol >99.9 >99.8 >99.8 >98.9

lsotongifolen-9-one >99.9 97.7 >99.8 >98.7

Bisabolol >99.8 >99.2 >99.6 >99.1

Data are the percentage of analytes that were recovered in the first des

step under the analytical conditions described in the experimental secaBreakthrough.bCoeluting peaks.

be at substantially lower temperatures than the final

desorption temperature of 325oC. After the first

desorption, the cartridge was sealed and cooled down

to ambient temperature and was then desorbed again

directly under the same conditions. The analytes should

ideally be desorbed completely during the first deso-

rption step. The amounts recovered in the first

desorption as a percentage of the total recovered

amount (all desorptions combined) are displayed in

Table 3. Most of the data for the 108ml volume samples

demonstrate a quantitative desorption of >99% for all

tested adsorbent types during the first desorption step.

For the larger 2.1 l volume samples, the desorption

efficiency decreases somewhat, but in most cases is still

within B99%. The relatively lower recoveries for the

higher sampling volumes may also reflect a (partial)

adsorption/desorption on the glass wool plug in contrast

to the further (deeper) transport of the analytes into the

adsorbent bed at higher sampling volumes. The amounts

of analytes retrieved from the 2.1 l samples collected on

Glass Beads were substantially diminished. Therefore, a

series of breakthrough experiments was conducted with

increasing sampling volumes. A Tenax TA cartridge was

used as a backup of the primary Glass Bead cartridge.

These experiments confirmed that these results stem

from breakthrough that occurs at sampling volumes in

excess of a few hundred ml. Because of these low

breakthrough volumes, Glass Beads were deemed to be

unsuitable under typical experimental conditions and

were excluded from subsequent experiments.

g volume

Carbotrap Carbotrap C Unibeads Glass Beads

100ml 2.1 l 100ml 2.1 l 100ml 2.1 l 100ml 2.1 l

>99.9 86.5 >99.9 >99.9 >99.9 >99.8 >99.9 b.ta

>99.9 99.1 >99.9 97.5 >99.9 >99.8 >99.8 b.t

>99.9 >99.6 >99.9 >99.9 >99.9 >99.8 >99.8 b.t

>99.9 >97.4 >99.9 >99.9 >99.9 >99.9 >99.8 b.t

>99.9 93.7 >99.9 98.7 >99.9 >99.9 >99.9 b.t

>99.9 94.9 >99.9 >99.9 >99.9 >99.9 >99.8 b.t

>99.9 >99.9 >99.9 >99.9 >99.9 >99.9 >99.9 b.t

>99.9 >99.6 >99.9 >99.9 >99.9 >99.9 >99.9 b.t

>99.9 97.7 >99.9 >99.9 >99.9 >99.9 >99.9 b.t

>99.9 >99.6 >99.8 >99.8 >99.9 >99.9 >99.9 b.t

>99.6 >98.6 >99.4 >99.5 >99.8 >99.9 >99.9 b.t

>99.7 >99.1 >99.6 >99.7 >99.9 >99.9 >99.9 b.t

>99.9 >99.8 >99.9 >99.9 >99.9 >99.9 >99.9 b.t

>99.8 92.9 >99.8 >99.7 >99.9 >99.9 >99.8 b.t

>99.9 >99.3 >99.7 >99.7 >99.9 >99.9 >99.9 b.t

>99.6 >99.4 >99.6 >99.6 >99.8 >99.9 >99.7 b.t

orption in relation to the total of the first and second desorption

tion.

ARTICLE IN PRESS

Table 4

Recovery rate of analytes from five adsorbent materials (mean of 5 repeats with standard deviation)

Compound Recovery (%)

Tenax TA Tenax GR Carbotrap Carbotrap C Unibeads

Isopropylbenzene 102.971.4 96.571.3 97.871.1 89.973.1 102.472.0

Longipinene 96.470.9 88.077.4 92.975.0 78.073.4 89.071.1

a-Copaene 101.970.8 93.873.3 96.570.9 80.775.6 98.971.9

Isolongifolene 97.170.4 87.976.2 93.271.0 83.072.4 91.071.0

Diphenylmethane+Longifolenea 94.871.4 91.377.5 92.571.2 83.572.5 111.373.5

a-Cedrene 105.371.1 99.274.1 104.377.3 92.577.2 115.773.0

Caryophyllene 110.070.7 102.971.2 103.972.0 88.273.5 111.771.6

Geranylacetone+Aromadendrenea 89.574.0 86.174.7 86.772.6 70.974.9 94.771.9

a-Humulene 94.772.2 90.171.8 95.476.0 76.573.4 95.472.5

cis-Nerolidol 98.774.1 83.175.8 89.772.7 74.573.6 81.577.7

trans-Nerolidol 122.378.1 128.4714.0 135.278.8 100.174.1 142.179.0

Nonylbenzene+Caryophyllene-Oxidea 103.973.2 93.677.1 94.075.5 81.676.0 84.677.2

Cedrol 116.278.5 102.574.3 103.372.9 88.973.6 67.6714.4

lsolongifolen-9-one 56.870.9 54.474.9 52.671.7 47.072.1 43.873.7

aCoeluting peaks.

D. Helmig et al. / Atmospheric Environment 38 (2004) 557–572564

The reproducibility of analyte sampling and analysis

was investigated by loading 4–6 cartridges of each

adsorbent with 2.1 l at a 1:420 dilution (mixing ratios of

0.19–3.76 ppbv). For quantifying SQT analytes, GC-

FID response factors were determined by analysis of a

gas-phase, gravimetrically prepared multi-component

n-alkane reference standard (Helmig et al., 2003).

Recovery ratios were calculated in reference to the

SQT diffusion rates from the on-line GC determination.

Results from these tests are shown in Table 4. Recovery

rates are usually >90%, and are quite consistent among

the different adsorbents tested. Generally, an increase in

the standard deviation is observed with decreasing

volatility, e.g. standard deviations for oxygenated SQT

are usually two to three times higher than for the

hydrocarbon SQT.

3.4. SQT recoveries at varying humidity

As a result of plant respiration, sample air humidity

during enclosure studies can reach high levels. Recov-

eries of analytes from five adsorbent materials were

tested under conditions of varying humidity. Cartridges

were loaded for 10min at a flow rate of 206.8mlmin�1

at 1:420 dilution. Analytes were then quantified based

on their peak areas and effective carbon numbers

(Helmig et al., 2003). Sample humidity was monitored

with two Oakton thermohygrometer probes (3–5%

accuracy, Cole Parmer Instruments Co, Vernon Hills,

IL). Experiments were performed at room temperature

with relative humidities of 20%, 40%, 60%, 80% and

100%. Dilution factor and analyte mixing ratios were

the same as for the adsorbent tests.

Data for four representative SQT are shown in Fig. 3.

These data confirm the above observation that the

precision of analysis is better for SQT hydrocarbons

than for the oxygenated compounds. For Tenax TA,

Tenax GR, Carbotrap and Carbotrap C, no significant

difference was observed in the data at different

humidities; obviously the humidity did not interfere

with the sampling and analysis process. In contrast,

consistently lower recoveries were found on Unibeads

with increasing humidities. These results were surprising

and this effect was investigated more carefully, but the

reason for this behavior could not be determined.

Sampling flow rates did not change during the sampling

period. The loss of analytes could neither be accounted

for in backup cartridges nor in second desorption runs.

Because of these results, it was concluded that Unibeads

were unsuitable for use in enclosure studies and this

adsorbent was excluded from further experiments.

3.5. Adsorbent testing–linearity experiments

The linearity of the sampling/analysis procedure was

tested by sampling of increasing volumes from 0.38 to

97.3 l at 1:800 dilution (0.01 ppbv (bisabolol) to

1.91 ppbv (caryophyllene)). The results of these experi-

ments (Fig. 4) show a linear dependency of the sample

volume for the adsorbents Carbotrap and Carbotrap C.

For Tenax TA and Tenax GR, linearity is observed for

the first B40 l, but results for the 48.6 and 97.3 l samples

appear to be slightly diminished.

3.6. Adsorbent testing—detection limits

The linearity experiments were also used to estimate

the detection limits of the PE GC-FID method. The

signals observed in the lowest volume samples (0.5 l)

were used to calculate for each analyte the smallest peak

ARTIC

LEIN

PRES

S

Fig. 3. Recovery of four SQT compounds on Tenax TA, Tenax GR, Carbotrap, Carbotrap C and Unibeads as a function of relative humidity at room temperature. Data are mean

values; error bars represent standard deviation of five repetitions for 0% humidity, and three repetitions for experiments with added humidity.

D.

Helm

iget

al.

/A

tmo

sph

ericE

nviro

nm

ent

38

(2

00

4)

55

7–

57

2565

ARTICLE IN PRESS

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100

Sample Volume (l)

Are

a C

ou

nts

(V*s

)

CaryophylleneLongipineneIsolongifoleneCedrolIsolongifolene-9-one

Tenax TA Tenax GR

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100

Sample Volume (l)

Are

a C

ou

nts

(V

*s)


Carbotrap

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100Sample Volume (l)

Are

a C

ou

nts

(V*s

)


Carbotrap C

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100Sample Volume (l)

Are

a C

ou

nts

(V

*s)


Fig. 4. Results for five selected SQT (in FID area counts) in increasing sampling volumes. SQT mixing ratios in the multi-component

test samples were longipinene 1.61 ppbv, isolongifolene 1.28 ppbv, caryophyllene 2.23 ppbv, cedrol 0.37 ppbv and isolongifolen-9-one

0.61 ppbv, respectively.

D. Helmig et al. / Atmospheric Environment 38 (2004) 557–572566

area that can be integrated and quantified. The

minimum detectable analyte amounts were estimated

to be in the range of 0.19–0.36 ng, equivalent to analyte

mixing ratios of 20–40 pptv in a 1 l sample volume.

3.7. Testing of tubing materials

A series of different tubing materials were tested for

their possible retention of SQT. The tested materials were

flexible silicone tubing (4.8mm o.d., 4.70m length; Cole

Parmer), stainless steel (3.2mm o.d., 2.20m length;

Alltech), copper tubing (3.2mm o.d., 2.80m length;

Alltech), Teflon (PTFE, 3.2mm o.d., 3.7m length; Cole

Parmer) and Silicosteel (3.2mm o.d., 1.8m length; Rest-

ek). These tubing materials were inserted into the flow

path of the CDS before the valve switching compartment

(Helmig et al., 2003). Tubing was kept outside the CDS at

room temperature. The total SQT sample flow rate was

B108ml min�1, resulting in SQT mixing ratios of

B6ppbv (bisabolol) to 170ppbv (caryophyllene). After

passing through the tubing quantification was achieved by

loop-injection of 1ml samples onto the on-line GC-FID

system (Helmig et al., 2003).

In a first set of experiments with increasing purge

times, it was found that purging for B10min was

required (B1 l of sample air) before steady-state

conditions were achieved. All subsequent tests were

performed with at least 15min purge times. Results for

stainless steel, copper, Teflon and Silicosteel are shown

in Fig. 5. None of the analytes were recovered after

passing through the silicone tubing, most likely a result

of loss/adsorption to the silicone tubing walls. Recov-

eries for all other materials generally were >90%,

results for oxygenated SQT are somewhat lower

(B80–90%), but still within a usable range for

quantitative analysis (except for bisabolol and farnesol

through Teflon tubing).

3.8. Storage in sampling bags

Teflon film materials have been a favorite material

used for bag enclosure experiments (Enders et al., 1992;

Helmig et al., 1999a; Cao and Hewitt, 1999; Komenda

et al., 2001). The stability of a SQT gas mixture in a

Teflon gas sampling bag (10 l Teflon bag, Type L Teflon

film; Alltech) was investigated for possible losses of SQT

to the bag material. In a series of experiments, the outlet

of the CDS was diluted with dry and humidified

nitrogen, respectively, and flown into the bag. The

overall dilution factor was B1:23 resulting in analyte

ARTICLE IN PRESS

0

20

40

60

80

100

120

LPNCO

PIS

O LFCNE

CRY

ARO

CRYO

CROL

ISO

ON

BIS

FAR

Re

co

ve

ry (

%)

Copper

0

20

40

60

80

100

120

LPNCO

PIS

O LFCNE

CRY

ARO

CRYO

CROL

ISO

ON

BIS

FAR

Re

co

ve

ry (

%)

Stainless Steel

PTFE Teflon

0

20

40

60

80

100

120

LPNCOP

ISO LF

CNE

CRY

ARO

CRYO

CRO

LIS

OON

BIS

FAR

Re

co

ve

ry (

%)

Silicosteel

0

20

40

60

80

100

120

LPNCOP

ISO LF

CNE

CRY

ARO

CRYO

CRO

LIS

OON

BIS

FAR

Re

co

ve

ry (

%)

Fig. 5. Recovery of a series of SQT after sampling through different tubing materials. Recovery and respective standard deviation

(n ¼ 4–10) are presented for each compound. Abbreviated compounds are (in alphabetical order): ARO: aromadendrene; BIS:

bisabolol; COP: a-copaene; CNE: cedrene; CROL: cedrol; CRY: b-caryophyllene; CRYO: caryophyllene oxide; FAR: farnesol; ISO:

isolongifolene; ISOON: isolongifolen-9-one; LF: longifolene; LPN: longipinene.

0102030405060708090

100

0 5 10 15 20 25 30Time after filling (hours)

Rec

ove

ry (%

)

IPB

LPN

COP

ISO

LF

CNE

CRY

ARO

Fig. 6. Storage of SQT in a Teflon bag at room temperature

and B45–48% RH. Abbreviated compounds are (in alphabe-

tical order): ARO: aromadendrene; COP: a-copaene; CNE:

cedrene; CRY: b-caryophyllene; IPB: 1,3,5-tri-isopropylben-

zene; ISO: isolongifolene; LF: longifolene; LPN: longipinene.

D. Helmig et al. / Atmospheric Environment 38 (2004) 557–572 567

mixing ratios of B10–70 ppbv. The sample relative

humidity range was 0–49%. The bag was filled over

B30min while it was kept at room temperature.

Immediately after the bag was filled, samples were

drawn out of the bag and analyzed with the on-line GC.

The sample withdrawal was interrupted during GC runs

and resumed for a 5min purge prior to the subsequent

injection. In this manner, the experiment was continued

until the 10 l Teflon bag was totally emptied (26 h).

The results from one of these experiments at B45–

48% sample RH are presented in Fig. 6. SQT hydro-

carbons show initial recovery rates on the order of

70–90%. It needs to be realized that at the time of the

first sample withdrawal B35min had passed from the

beginning of the bag filling. SQT losses seem to be faster

in the first few hours. After one day of sample storage,

the hydrocarbon SQT mixing ratios had dropped to

B10–30% of their initial values. From this loss, an

average decay rate of B4% per hour was calculated for

these 8 non-oxygenated compounds. None of the

oxygenated SQT could be recovered from the bag.

3.9. Leaf cuvettes

Two leaf cuvettes, one made of stainless steel, with a

borosilicate glass window (volume of 0.44 l), and the

other cuvette made of Delrins acetal resin, also with a

glass window (volume of 0.63 l) (Fig. 7) were tested.

These cuvettes have 2–3 bulkhead sampling ports for

intake air and sample withdrawal. The CDS output was

diluted with humidified nitrogen resulting in a total flow

of B250mlmin�1. This SQT standard was flown into

the cuvette through one of the sampling ports. A

sampling flow of 55mlmin�1 was continuously with-

drawn from the cuvette through one of the other ports


and analyzed with the on-line GC. Neither cuvette was

vacuum tight, so the remainder of the excessive inlet

flow (B195mlmin�1) leaked out through the seal of the

glass window. The experiment was performed in three

steps. First, only the humidified nitrogen was purged

through the cuvette for 2 h. Next, the SQT were added

instantaneously. In the third step the cuvettes were again

purged with SQT-free humidified nitrogen. Reference

measurements (100% recovery rate) were done by

performing the same experiment with a 0.32 cm stainlees

steel Swagelok tee (heated to 50�C) in place of the leaf

cuvette.

Results for both cuvette experiments at 60% RH are

illustrated in Fig. 8. The theoretically expected mixing

ratio (calculated from the cuvette volume and the purge

Fig. 7. Two leaf cuvettes tested: stainless steel to the left,

Delrins acetal resin to the right. The glass window was sealed

against the cuvette body with a white closed cell foam gasket

(steel cuvette) and with Teflon tape (Delrin cuvette), respec-

tively.

0

20

40

60

80

100

120

0 5 10 15 20 25 30Time (hours)

Rec

ove

ry (%

)

IPBLPNCOPISOLFCNECRYAROCRYOCROLTheory

<- SQT on

<- SQT off

Rec

ove

ry (%

)

Fig. 8. Recovery of test compound standard mixture from a stainless

B55% RH. The theoretically expected recovery from the gas mixing

sets). Abbreviated compounds are (in alphabetical order): ARO: aro

CRY: b-caryophyllene; CRYO: caryophyllene oxide; IPB: 1,3,5-tri-

longipinene. SQT on/off illustrates the times when the SQT were add

b-caryophyllene were not included in the Delrin cuvette experiment. A

completely lost in the Delrins leaf cuvette.

flow rate) is added to these figures (solid line) in order to

better illustrate the analyte losses. For both cuvettes,

substantial analyte losses were found in the initial phase

of the experiment. Equilibration time in excess of 5 h

were required before recovery rates of >80% were

achieved for the non-oxygenated SQT. In the stainless

steel cuvette, oxygenated SQT, e.g. cedrol and caryo-

phyllene oxide, could only be partially retrieved

(B60%). Nerolidol, geranylacetone, isolongifolen-9-

one were completely lost. None of the oxygenated

SQT were recovered from the Delrins acetal resin

cuvette. After the SQT flow was turned off, they were

present in the sample air for many hours with a slow and

gradual decline. Cuvette experiments performed at

B0%, 20% and 45% RH levels gave similar results.

These data show that in both cuvettes, significant SQT

surface losses occurred and long equilibration times (i.e.

large purge volumes) were required before steady state

conditions were achieved. Memory effects were clearly

evident, and it took many hours of continuous purging

to remove the analytes from the walls of the cuvettes.

4. Discussion

Hundreds of SQT have been identified and character-

ized in plant materials (Joulain and K .onig, 1998). The

number of identified SQT in volatile emissions from

vegetation is substantially lower. The identification of

SQT is often hindered by the lack of reference mass

spectra and/or GC retention data. The MS and RI data

of the eleven SQT and seven oxygenated SQT included

in this study will facilitate their future identification.

However, rather substantial deviations (up to B25 RI

units) were seen in the RI of SQT on the two DB-1

columns included in this study. Possible reasons for this

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35 40Time (hours)

LPN

COP

ISO

LF

CNE

ARO

Theory

<- SQT on

<- SQT off

steel leaf cuvette (left) and from a Delrins leaf cuvette (right) at

in the cuvette (assuming no analyte loss) is added to both data

madendrene; COP: a-copaene; CNE: cedrene; CROL: cedrol;

isopropylbenzene; ISO: isolongifolene; LF: longifolene; LPN:

ed and turned off, respectively. 1,3,5-tri-isopropylbenzene and

ll of the oxygenated SQT were included in this experiment but


discrepancy include different column phase thicknesses

(0.25 vs. 1mm), the different injection techniques (on-line

loop injection versus thermal desorption from adsorbent

cartridges) or the different, previous uses of the

individual columns. Conclusively, these data show that

unequivocal identification of SQT by RI comparison

alone is troublesome unless the GC retention can be

confirmed by GC analysis of the respective SQT

standard. Tentative MS identification of an unknown

compound as SQT can be accomplished by observing

characteristic ions, such as at m=z ¼ 204; 175, 161, 121,93. However, mass spectra of different SQT can be quite

similar, so the unequivocal identification requires the

respective SQT standard.

Automated sampling of vegetation emission samples

has many advantages over manual procedures. The

results with the sampler used in this study demonstrate

the feasibility of automated sampling. Critical para-

meters that need to be considered are the permanent

flushing of all valve and tubing components with the

sample gas, sufficient purge times prior to sample

collection and elevated temperatures (50�C) for valves

and tubing in the sample flow.

Tenax TA, Tenax GR, Carbotrap and Carbotrap C

all proved to be good adsorbent choices for enrichment

of SQT hydrocarbons from air samples. No significant

losses from breakthrough or incomplete desorption were

observed for SQT hydrocarbons. Precision and accuracy

of usually better than 10% were achieved with careful

control of sampling and analysis procedures. None of

these adsorbents showed interferences from sample

humidity, which is in agreement with their low water

uptake rates reported previously (Helmig and Vierling,

1995). Analytical problems from rearrangement reac-

tions during solid adsorbent sampling/analysis have

previously been reported for the monoterpene b-pinene(Cao and Hewitt, 1993; Arnts et al., 1995). SQT

rearrangement reactions were not observed during any

of the experiments performed in these studies. Analysis

of oxygenated SQT poses a bigger challenge stemming

from the higher polarity, respectively lower volatility of

these compounds. Precision and accuracy on the order

of B25% were achieved for the more volatile com-

pounds in this group (RI 1500–1650), which demon-

strates that these compounds can still be analyzed by

solid adsorbent techniques with reasonable success. In

contrast, the less volatile farnesol (RI 1775) was rarely

recovered in any of these experiments.

A potential problem of Tenax TA is the observed

volume loss (shrinking) of the material from repeated

sampling/thermal desorption cycles (B10% loss of

volume after 20 cycles). The gaps in the adsorbent bed

that would form after a number of cycles were

eliminated by carefully pushing the adsorbent further

into the cartridge. Potential changes in the analyte

retention from this shrinking were not investigated in

the course of this study. Retention of SQT on Glass

Beads was weak. This material may be useful for

secondary focussing traps, but breakthrough volumes

are too low for collection of SQT in emission samples.

Similarly, the interference from water vapor found on

Unibeads appears prohibitive for emission sample

collection.

5. Conclusions

The experiments performed on different tubing

materials, sampling bags and leaf cuvettes conclusively

showed that SQT rapidly adsorb on material surfaces. A

number of precautions needs to be followed in order to

prevent substantial analytical errors. Long purge times

are required to passivate surfaces, reach equilibrium and

minimize analytical losses. For B0.5 l leaf cuvettes with

B250ml inlet flow rate, required purge times are on the

order of 5–10 h. These results confirm earlier observa-

tions in experiments on injection loop materials and

loop temperatures for the on-line GC (Helmig et al.,

2003), where temperatures in excess of 90�C were

required to eliminate SQT wall losses on glass, nickel,

stainless steel and Silicosteel. Even under careful

consideration of these conditions, oxygenated SQT

could not be quantitatively recovered in the cuvette

experiments.

Included aromatic compounds as well as geranylace-

tone gave similar results as the SQT. Therefore, it can be

concluded that our findings were mostly related to the

analyte volatility and polarity, rather than specific for

SQT. Consequently, this study will be applicable and

beneficial for analysis of other VOC in the C13–C17

volatility range (e.g. polycyclic aromatic hydrocarbons)

in air samples and by solid adsorbent sampling with

thermodesorption GC.

The data further demonstrate the difficulties in SQT

analysis. It is likely that SQT emissions from vegetation

have been overlooked in many previous studies because

of the analytical challenges posed by these compounds.

SQT flux experiments by techniques such as eddy

correlation, relaxed eddy accumulation, and disjunct

eddy sampling do not appear promising, since all of

these methods rely on the capture of rapid changes in

atmospheric concentrations. The long times needed for

reaching equilibrium between sampling lines, valves, and

other sampling apparatus components with the SQT in a

gas-phase sample will significantly blur and decrease

changes in atmospheric fluctuations, which compro-

mises the applicability of these flux techniques.

Enclosure chambers are a frequently used technique

in the research on chemically mediated plant–insect

interactions. Most of these published experiments

rely on qualitative observations and relatively little

attention has been paid to experimental requirements for


quantitative measurements. This study demonstrate the

limitations for detection and accurate quantification of

SQT, in particular for oxygenated SQT, in headspace

sampling and enclosure experiments. These findings and

recommendations will facilitate improved experimental

designs in research on plant-insect communication and a

better (quantitative) interpretation of SQT observations.

Because of SQT are easily lost to materials surfaces, it

appears likely that within a forest environment, SQT are

similarly adsorbed by soils and vegetation surfaces. This

process may be another important sink in addition to

the fast losses from atmospheric reactions and uptake to

aerosols. Due to these multiple sinks, only a fraction of

the emitted SQT will escape the forest and be observable

by above canopy flux measurement techniques.

Therefore, the results from this study re-emphasize

that enclosure studies are the most promising experi-

mental approach for capturing SQT emissions from

vegetation. Following the guidelines developed in this

research, both leaf cuvette and bag enclosure experi-

ments with solid adsorbent sampling will yield quality

leaf and branch level emission rate data for non-

oxygenated SQT. Oxygenated SQT, however, undergo

substantial, fast and irreversible losses to enclosure

materials. This makes their quantitative analysis

by these methods impractical and may explain the

scarcity of observations with identified oxygenated

SQT emissions in previous vegetation enclosure

studies.

Acknowledgements

Peter Harley and Alex Guenther, National Center for

Atmospheric Research, Boulder, provided the leaf

cuvettes. J.P. was supported by a fellowship stipend

from the Deutsche Akademische Austauschdienst. John

Ortega, University of Colorado and an anonymous

reviewer gave helpful comments on the manuscript. This

research was supported through a grant from the

National Science Foundation, Atmospheric Chemistry

Program, ATM-9911186.

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Documents

Analytical techniques for sesquiterpene emission rate studies in vegetation enclosure experiments