Sample Preparation for Volatile Compounds (VOCs)

Sample Preparation for 

Volatile Compounds 

(VOCs)

Sample Preparation for VOCs 

Organic compounds; P ≥ 0.1 mmHg at 

20°C 

~ 8% of total samples 

GC analyses 

Petroleum, petrochemical, food, flavor and 

fragrances, and environmental fields 

Collection/Transport 

Introduction w/o treatment 

Preparation/Introduction 

Measurement 

Overall accuracy 

Reliable result

Typical Sampling and Sample Preparation Method for 

VOCs 

Sample preparation 

method 

Grab sampling 

Solid-phase trapping 

(SPE) 

Liquid trapping 

(Impinging) 

Principle of technique 

Gaseous sample is pulled 

or pumped into an 

evacuated metal bulb, 

canister, plastic bag, or 

syringe 

Gaseous sample is passed 

an adsorbent tube such as 

silica gel or activated 

carbon; trapped analytes 

are eluted with strong 

solvent 

Gaseous sample is 

bubbled through a solution 

or solvent for which the 

analytes have a higher 

affinity 

Comments 

Mostly for VOCs in air, 

samples are returned to 

laboratory, analytes are 

isolated and 

concentrated by cold 

trapping techniques 

For semivolatile organic 

compounds in air. 

Popular sorbents include 

silica gel, alumina, 

porous polymer (Tenax, 

PUF) and carbon 

Flow rate may cause 

foaming or aerosols



Sample preparation 

method 

Headspace sampling 

Purge & trap 

(dynamic headspace) 


A solid or liquid sample is 

placed in a closed glass vial 

until equilibrium. Analytes 

partition themselves 

between a gas phase and a 

solid or liquid phase; gas 

phase is sampled and 

injected into a GC 

A solid or liquid sample is 

placed in a closed 

container, VOCs are 

continually purged by an 

inert gas and subsequently 

trapped by SPE sorbent 

and then thermal desorbed 

into GC (Thermal 

desorption) 

Comments 

For determining trace 

concentrations of VOCs in 

samples that are difficult to 

handle by conventional GC. 

Increasing temperature, 

salting out, adjusting pH, 

would shift equilibrium of 

analytes from the matrix 

For determining trace 

concentrations of VOCs in 

samples and for analytes 

that have unfavorable 

partition coefficient in static 

headspace sampling



Sample 

preparation 

method 

Thermal desorption 


Used with purge & trap 

and SPME to concentrate 

VOCs; sorbent is rapidly 

heated and analytes are 

transferred to a GC 

Comments 

Typical sorbents include 

Tanex TA, glass beads, and 

Carbosieve, Carboxen, and 

Carbotrap 

Pyrolysis 

SPME 

Nonvolatile large molecule 

samples such as polymers 

and plant fibers are 

thermally degraded to 

cleave linkages and 

produce smaller, more 

volatile molecules that are 

swept to GC 

Already discussed 

Degradation have defined 

mechanisms and sample 

may break apart in a 

predictable manner 

providing structural info and 

fingerprint profiles about 

starting compound 

Already discussed

Tedlar Air & Gas 

Sampling Bags 

Impringer 

Canister

Headspacce Sampling 

Static Headspace (Equilibrium 

Headspace) 

– The sample, placed in a closed container may 

be in contact and in equilibrium with the 

extracting gas 

Dynamic Headspace (Purge & Trap) 

– The volatile compounds may be stripped off in 

a continuous flow of an inert gas 

‣ Ideal for dirty samples, solid materials, samples 

with high boiling point analytes of no interest, 

samples with high water content, and samples 

that are difficult to handle by conventional GC

Static Headspace 

GC

Basic of Static Headspace 

Cg, , Vg 

Cs, Vs 

Partition Coefficient (K) = C s /C g 

Phase Ratio (β)( 

) = V g /V s 

C s =concentration of analyte in sample phase 

C g =concentration of analyte in gas phase 

Vs=volume of sample phase 

Vg=volume of gas phase 

K K and β are important variables in headspace analysis. 

C 

g 

= K 

C 

o 

+ β

K-Value Air-Water System (40 o C) 

Compound 

Cyclohexane 

n-Hexane 

Tetrachloroethylene 

Chloroform 

o-Xylene 

Toluene 

Benzene 

Dichloromethane 

n-butyl acetate 

Ethyl acetate 

Methyl ethyl ketone 

n-Butanol 

Isopropanol 

Ethanol 

1,3-Dioxane 

K Value 

0.077 

0.14 

1.48 

1.65 

2.44 

2.82 

2.90 

5.65 

31.4 

62.4 

139.5 

647 

825 

1355 

1618 

Boiling Point ( o C) 

81 

69 

121 

61.1 

145 

111 

80 

40 

126 

77 

79.6 

117.7 

82.4 

78.3 

106

Optimizing Static Headspace 

Extraction 

efficiency 

Sensitivity 

Quantitation 

Reproducibility 

Vial/sample 

volume (β)( 

Temperature 

Pressure 

Matrix

Partition Coefficient (K) 

Maximize the concentration of the volatile 

components C g in the headspace 

Lower K by changing the temperature at 

which the vial is equilibrated or by 

changing the composition of the sample 

matrix. 

C g 

C g 

K 

β

Headspace sensitivity 

1. EtOH 

2. Methyl ethyl ketone 

3. Toluene 

4. N-hexane 

5. Tetrachloroethylene 

High K -- Temp 

B. Kolb, L.S. Ettre, “Static Headspace- Gas Chromatography: Theory and 

Practice,” Wiley-VCH, New York. 1997.

Headspace sensitivity 

1. Cyclohexane 

2. 1,4 dioxane 

β= 3.46 

With salt 

β = 21.3 β = 3.46 

Low K -- β 

B. Kolb, L.S. Ettre, “Static Headspace- Gas Chromatography: Theory and 

Practice,” Wiley-VCH, New York. 1997.

Static Headspace Sampling 

Gas tight syringe 

Autosampler 

–Balance-pressure system 

–Pressure-loop system 

Tekmar 7000HT Static 

Headspace autosampler

Gas Tight Syringe 

Step 1 

Sample reaches 

equilibrium 

Step 2 

Sample is extracted 

from headspace 

Step 3 

Sample is injected

Gas Tight Syringe

Needle Point Style 

#2 

22/20-degree beveled needle point recommended for septum 

penetration. 

#3 

90-degree needle point for use with HPLC injection valves 

and for sample pipetting 

#5 

Conical needle with side port for penetration of septa.

Advantage 

– Simplicity 

Disadvantage 

Gas Tight Syringe 

– The loss of the substances 

– No reproducibility

Autosampler - Balance Pressure 

System 


equilibrium 

Pressurization of 

injection 

Sample is extracted 

and injected

Autosampler - Pressure-Loop System 

Inlet 

Step 12 

3 


is 

extracted 

equilibrium/pressurization 

from injected 

headspace 

Loop 

To Column

Dynamic Headspace 

Continuous gas extraction 

Volatilized (purged) analytes can be 

trapped by an adsorbent or 

cryogenic trapping 

For substances which are too low in 

concentration or have unfavorable 

partition coefficients for their 

determination by static headspace

Purge and Trap 

Purging gas is bubbled below 

the surface of a liquid sample 

using a fritted orifice to 

produce finely dispersed 

bubbles 

The VOCs are transferred 

from the aqueous phase to 

the vapor phase 

The gas flow sweeps the 

vapor through trap containing 

adsorbent materials which 

retain the VOCs 

The retained VOCs are 

thermally desorbed and 

analyzed by GC

Schematic of Purge & Trap 

Heat 

To GC column 

Carrier gas 

Purge gas 

Valve 

Trap Vent 

HP-7675A

Purge and Trap System 

Tekmar 3100 Purge and Trap 

Sample Concentrator 

2016/2032 Purge and Trap 

Autosampler

Purge and Trap Glassware

Trapping 

Adsorbent resins 

– Purge & Trap 

– Direct sampling 

Sufficient capacity – Breakthrough 

volume 

– Bed volume 

– Flow rate 

Affinity of resin for water 

Back pressure

Trap 

Polymers 

– Tenax 

– Polystyrene 

– Polyurethane foams 

Carbon 

– Graphitized carbon 

black 

– Charcoal 

– Carbon sieves 

Silica gel 

Alumina 

Tenax TA 

– 2,6-diphenylene oxide 

Tenax GR 

– Tenax TA + 30% graphite 

Carbotrap, Carbotrap 

C 

– Graphitized carbon blacks 

Carboxen 569, 

Carbosieve SIII 

– Carbon molecular 

sieves 

Glass Beads

Tenax TA 

A porous polymer resin based on 

2,6-diphenylene oxide 

High temperature limit of 350 o C 

– Tenax degrades if react with O 2 at high 

temperature – abundance of phenolic 

compounds and oligomers 

Low affinity for water 

– Useful for high moisture content 

samples including the analysis of 

volatile organic compounds in water

Selection of Adsorbents 

Types of analytes 

The physical properties of the 

adsorbent 

Breakthrough info. 

John J. Manura, Selection and Use Of Adsorbent Resins For 

Purge and Trap Thermal Desorption Applications 

Scientific Instrument Services, Inc. 

http://www.sisweb.com/referenc/applnote/app-32.htm 


http://www.sisweb.com

Breakthrough 

The breakthrough volume for a compound 

on a given adsorbent and at a given 

temperature is defined as the calculated 

volume of carrier gas per gram of 

adsorbent resin which causes the analyte 

molecules to migrate from the front of the 

adsorbent bed to the back of the 

adsorbent bed. 

V 

(L/g) 

B 

= 

( t × F) 

R 

m 

A 

t R = Retention time (min) 

F = Flow rate (L/min) 

m A = Adsorbent mass (g)

Breakthrough 

Scientific Instrument Services, Inc.

Calculation of Breakthrough Volume 

t R 

V 

B 

= 

( t × F) 

R 

m 

A 

− 

DV 

V S = V B * 0.5 

V F = V B * 2 

V S 

V F

Breakthrough Volumes of Alcohols (C1 – C11) on Tenax 

TA 

0.4 

5 

20 

120

Breakthrough Volumes of Alcohols by Tenax TA

Desorption Chart

Breakthrough Data 

Hydrocarbons 

Alcohols 

Alkenes 

Alcohols & Glycols 

Acetates 

Acids 

Aldehydes 

Ketones 

Halogens 

Amines 

Aromatics and 

Terpenes 

Water 


http://www.sisweb.com

Thermal Desorption (TD) 

Thermally desorb analytes from the 

adsorbents (P&T, direct sampling) 

Direct Thermal extraction 

– Volatiles from solid samples 

Thermal desorption – To GC 

– Transfer line 

– Focusing – Improved resolution

Thermal Desorption/Extraction 

Glass wool 

plug 

0.4 mm i.d. 

Solid 

sample 

Adsorbent 

resin 

10 cm 

Temp. ~300 o C 

Glass wool 

plug 

Temp. < 200 o C 

Direct Thermal 

Extraction 

Thermal 

Desorption

Short Path Thermal Desorption

Tempearture/Time 

Desorption 

temperature 

– Enough to 

volatilize the 

organic compound 

without degrading 

them and without 

producing 

unwanted 

artifacts 

– Temperature rate 

Desorption time 

– Sample matrix 

– Sample size 

– Interaction strength 

between analyte 

and the solid 

surface 

– Desorption 

temperature 

– Diffusion time of 

analyte out of the 

sample

Reduce Band Broadening: 

Focusing 

PTV 

Retention Gap 

Cryogenic focusing 

Cryo-Trapping 


Cryo-focusing 

Cryo-focusing is a technique for 

introducing samples from purge-and 

and- 

trap concentrators into capillary GC 

columns 

It enables samples desorbed from 

adsorbent traps to be introduced into 

narrow-bore columns without losing 

any resolution of the column

Single Stage Thermal Desorption 

GC Detector 

Sample Tube 

GC Analytical Column 

Two-Stage Thermal Desorption 

Carrier 

Inlet 

Split 

Cold 

Trap 

Desorb Flow Split 

GC Detector 

Hot Sample Tube 

Carrier 

Inlet 

Hot 

Trap 

Carrier 

Inlet 

GC Analytical Column

Cryo-Trapping 


Thermal Desorption 

Analysis without the use of solvent 

– Analysis of 100% instead of aliquot 

– Elimination of solvent peak in the 

chromatogram enables the analysis of 

early eluted volatile analytes that are 

not masked by the solvent 

– Elimination of solvent reduction, no 

evaporation of solvent to the 

environment, and no waste

Applications 

VOCs from water/soil (EPA Method 502.1, 

503.1, 8030A, SW-846 Method 5030A) 

VOCs from biological fluids (urine, plasma, 

saliva, tissues) 

Fragrances, flavors 

Forensic investigation (arson accelerants)

Pyrolysis 

Next stage in thermal extraction technique 

Bond dissociation at very high 

temperatures (600-800 

o C) ) and break 

apart into smaller and simpler volatile 

molecules in a predictable manner 

By measuring the fragments, the 

molecular composition of the original 

sample can be reconstructed 

Polymer defects, variations, and 

degradation mechanisms

Pyrolysis 

aH 

bH 

As sample is heated, the weaker bond 

breaks first forming free radicals 

C C 

dH 

Cl eH 

Polyvinyl Polyethylene chloride 

Polystyrene 

If C-C C C bonds are the weakest, the 

polymer will break into oligomeric 

fragments including monomers 

If the side group bond is weaker, 

the group is removed from the 

chain before fragmented, so the 

monomeric identity is lost 

The materials can be heated to a relatively low temperature for 

desorption of intact small molecules including solvents, excess 

reagents, residual monomers, and additive such as plasticizer

Pyrolysis - Applications 

Synthetic polymers (Polyvinyl 

chloride, Polystyrene, Polyester) 

Natural polymers 

– Plant fibers (Cellulose, Cotton) 

– Animal fibers (Wool, Silk) 

Dried paints 

Cosmetic samples

Sample Preparation for Volatile Compounds (VOCs)

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