Sample Preparation for Volatile Compounds (VOCs)
Sample Preparation for Volatile Compounds (VOCs)
Sample Preparation for Volatile Compounds (VOCs)
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<strong>Sample</strong> <strong>Preparation</strong> <strong>for</strong><br />
<strong>Volatile</strong> <strong>Compounds</strong><br />
(<strong>VOCs</strong>)
<strong>Sample</strong> <strong>Preparation</strong> <strong>for</strong> <strong>VOCs</strong><br />
Organic compounds; P ≥ 0.1 mmHg at<br />
20°C<br />
~ 8% of total samples<br />
GC analyses<br />
Petroleum, petrochemical, food, flavor and<br />
fragrances, and environmental fields<br />
Collection/Transport<br />
Introduction w/o treatment<br />
<strong>Preparation</strong>/Introduction<br />
Measurement<br />
Overall accuracy<br />
Reliable result
Typical Sampling and <strong>Sample</strong> <strong>Preparation</strong> Method <strong>for</strong><br />
<strong>VOCs</strong><br />
<strong>Sample</strong> preparation<br />
method<br />
Grab sampling<br />
Solid-phase trapping<br />
(SPE)<br />
Liquid trapping<br />
(Impinging)<br />
Principle of technique<br />
Gaseous sample is pulled<br />
or pumped into an<br />
evacuated metal bulb,<br />
canister, plastic bag, or<br />
syringe<br />
Gaseous sample is passed<br />
an adsorbent tube such as<br />
silica gel or activated<br />
carbon; trapped analytes<br />
are eluted with strong<br />
solvent<br />
Gaseous sample is<br />
bubbled through a solution<br />
or solvent <strong>for</strong> which the<br />
analytes have a higher<br />
affinity<br />
Comments<br />
Mostly <strong>for</strong> <strong>VOCs</strong> in air,<br />
samples are returned to<br />
laboratory, analytes are<br />
isolated and<br />
concentrated by cold<br />
trapping techniques<br />
For semivolatile organic<br />
compounds in air.<br />
Popular sorbents include<br />
silica gel, alumina,<br />
porous polymer (Tenax,<br />
PUF) and carbon<br />
Flow rate may cause<br />
foaming or aerosols
Typical Sampling and <strong>Sample</strong> <strong>Preparation</strong> Method <strong>for</strong><br />
<strong>VOCs</strong><br />
<strong>Sample</strong> preparation<br />
method<br />
Headspace sampling<br />
Purge & trap<br />
(dynamic headspace)<br />
Principle of technique<br />
A solid or liquid sample is<br />
placed in a closed glass vial<br />
until equilibrium. Analytes<br />
partition themselves<br />
between a gas phase and a<br />
solid or liquid phase; gas<br />
phase is sampled and<br />
injected into a GC<br />
A solid or liquid sample is<br />
placed in a closed<br />
container, <strong>VOCs</strong> are<br />
continually purged by an<br />
inert gas and subsequently<br />
trapped by SPE sorbent<br />
and then thermal desorbed<br />
into GC (Thermal<br />
desorption)<br />
Comments<br />
For determining trace<br />
concentrations of <strong>VOCs</strong> in<br />
samples that are difficult to<br />
handle by conventional GC.<br />
Increasing temperature,<br />
salting out, adjusting pH,<br />
would shift equilibrium of<br />
analytes from the matrix<br />
For determining trace<br />
concentrations of <strong>VOCs</strong> in<br />
samples and <strong>for</strong> analytes<br />
that have unfavorable<br />
partition coefficient in static<br />
headspace sampling
Typical Sampling and <strong>Sample</strong> <strong>Preparation</strong> Method <strong>for</strong><br />
<strong>VOCs</strong><br />
<strong>Sample</strong><br />
preparation<br />
method<br />
Thermal desorption<br />
Principle of technique<br />
Used with purge & trap<br />
and SPME to concentrate<br />
<strong>VOCs</strong>; sorbent is rapidly<br />
heated and analytes are<br />
transferred to a GC<br />
Comments<br />
Typical sorbents include<br />
Tanex TA, glass beads, and<br />
Carbosieve, Carboxen, and<br />
Carbotrap<br />
Pyrolysis<br />
SPME<br />
Nonvolatile large molecule<br />
samples such as polymers<br />
and plant fibers are<br />
thermally degraded to<br />
cleave linkages and<br />
produce smaller, more<br />
volatile molecules that are<br />
swept to GC<br />
Already discussed<br />
Degradation have defined<br />
mechanisms and sample<br />
may break apart in a<br />
predictable manner<br />
providing structural info and<br />
fingerprint profiles about<br />
starting compound<br />
Already discussed
Tedlar Air & Gas<br />
Sampling Bags<br />
Impringer<br />
Canister
Headspacce Sampling<br />
Static Headspace (Equilibrium<br />
Headspace)<br />
– The sample, placed in a closed container may<br />
be in contact and in equilibrium with the<br />
extracting gas<br />
Dynamic Headspace (Purge & Trap)<br />
– The volatile compounds may be stripped off in<br />
a continuous flow of an inert gas<br />
‣ Ideal <strong>for</strong> dirty samples, solid materials, samples<br />
with high boiling point analytes of no interest,<br />
samples with high water content, and samples<br />
that are difficult to handle by conventional GC
Static Headspace<br />
GC
Basic of Static Headspace<br />
Cg, , Vg<br />
Cs, Vs<br />
Partition Coefficient (K) = C s /C g<br />
Phase Ratio (β)(<br />
) = V g /V s<br />
C s =concentration of analyte in sample phase<br />
C g =concentration of analyte in gas phase<br />
Vs=volume of sample phase<br />
Vg=volume of gas phase<br />
K K and β are important variables in headspace analysis.<br />
C<br />
g<br />
= K<br />
C<br />
o<br />
+ β
K-Value Air-Water System (40 o C)<br />
Compound<br />
Cyclohexane<br />
n-Hexane<br />
Tetrachloroethylene<br />
Chloro<strong>for</strong>m<br />
o-Xylene<br />
Toluene<br />
Benzene<br />
Dichloromethane<br />
n-butyl acetate<br />
Ethyl acetate<br />
Methyl ethyl ketone<br />
n-Butanol<br />
Isopropanol<br />
Ethanol<br />
1,3-Dioxane<br />
K Value<br />
0.077<br />
0.14<br />
1.48<br />
1.65<br />
2.44<br />
2.82<br />
2.90<br />
5.65<br />
31.4<br />
62.4<br />
139.5<br />
647<br />
825<br />
1355<br />
1618<br />
Boiling Point ( o C)<br />
81<br />
69<br />
121<br />
61.1<br />
145<br />
111<br />
80<br />
40<br />
126<br />
77<br />
79.6<br />
117.7<br />
82.4<br />
78.3<br />
106
Optimizing Static Headspace<br />
Extraction<br />
efficiency<br />
Sensitivity<br />
Quantitation<br />
Reproducibility<br />
Vial/sample<br />
volume (β)(<br />
Temperature<br />
Pressure<br />
Matrix
Partition Coefficient (K)<br />
Maximize the concentration of the volatile<br />
components C g in the headspace<br />
Lower K by changing the temperature at<br />
which the vial is equilibrated or by<br />
changing the composition of the sample<br />
matrix.<br />
C g<br />
C g<br />
K<br />
β
Headspace sensitivity<br />
1. EtOH<br />
2. Methyl ethyl ketone<br />
3. Toluene<br />
4. N-hexane<br />
5. Tetrachloroethylene<br />
High K -- Temp<br />
B. Kolb, L.S. Ettre, “Static Headspace- Gas Chromatography: Theory and<br />
Practice,” Wiley-VCH, New York. 1997.
Headspace sensitivity<br />
1. Cyclohexane<br />
2. 1,4 dioxane<br />
β= 3.46<br />
With salt<br />
β = 21.3 β = 3.46<br />
Low K -- β<br />
B. Kolb, L.S. Ettre, “Static Headspace- Gas Chromatography: Theory and<br />
Practice,” Wiley-VCH, New York. 1997.
Static Headspace Sampling<br />
Gas tight syringe<br />
Autosampler<br />
–Balance-pressure system<br />
–Pressure-loop system<br />
Tekmar 7000HT Static<br />
Headspace autosampler
Gas Tight Syringe<br />
Step 1<br />
<strong>Sample</strong> reaches<br />
equilibrium<br />
Step 2<br />
<strong>Sample</strong> is extracted<br />
from headspace<br />
Step 3<br />
<strong>Sample</strong> is injected
Gas Tight Syringe
Needle Point Style<br />
#2<br />
22/20-degree beveled needle point recommended <strong>for</strong> septum<br />
penetration.<br />
#3<br />
90-degree needle point <strong>for</strong> use with HPLC injection valves<br />
and <strong>for</strong> sample pipetting<br />
#5<br />
Conical needle with side port <strong>for</strong> penetration of septa.
Advantage<br />
– Simplicity<br />
Disadvantage<br />
Gas Tight Syringe<br />
– The loss of the substances<br />
– No reproducibility
Autosampler - Balance Pressure<br />
System<br />
<strong>Sample</strong> reaches<br />
equilibrium<br />
Pressurization of<br />
injection<br />
<strong>Sample</strong> is extracted<br />
and injected
Autosampler - Pressure-Loop System<br />
Inlet<br />
Step 12<br />
3<br />
<strong>Sample</strong> reaches<br />
is<br />
extracted<br />
equilibrium/pressurization<br />
from injected<br />
headspace<br />
Loop<br />
To Column
Dynamic Headspace<br />
Continuous gas extraction<br />
Volatilized (purged) analytes can be<br />
trapped by an adsorbent or<br />
cryogenic trapping<br />
For substances which are too low in<br />
concentration or have unfavorable<br />
partition coefficients <strong>for</strong> their<br />
determination by static headspace
Purge and Trap<br />
Purging gas is bubbled below<br />
the surface of a liquid sample<br />
using a fritted orifice to<br />
produce finely dispersed<br />
bubbles<br />
The <strong>VOCs</strong> are transferred<br />
from the aqueous phase to<br />
the vapor phase<br />
The gas flow sweeps the<br />
vapor through trap containing<br />
adsorbent materials which<br />
retain the <strong>VOCs</strong><br />
The retained <strong>VOCs</strong> are<br />
thermally desorbed and<br />
analyzed by GC
Schematic of Purge & Trap<br />
Heat<br />
To GC column<br />
Carrier gas<br />
Purge gas<br />
Valve<br />
Trap Vent<br />
HP-7675A
Purge and Trap System<br />
Tekmar 3100 Purge and Trap<br />
<strong>Sample</strong> Concentrator<br />
2016/2032 Purge and Trap<br />
Autosampler
Purge and Trap Glassware
Trapping<br />
Adsorbent resins<br />
– Purge & Trap<br />
– Direct sampling<br />
Sufficient capacity – Breakthrough<br />
volume<br />
– Bed volume<br />
– Flow rate<br />
Affinity of resin <strong>for</strong> water<br />
Back pressure
Trap<br />
Polymers<br />
– Tenax<br />
– Polystyrene<br />
– Polyurethane foams<br />
Carbon<br />
– Graphitized carbon<br />
black<br />
– Charcoal<br />
– Carbon sieves<br />
Silica gel<br />
Alumina<br />
Tenax TA<br />
– 2,6-diphenylene oxide<br />
Tenax GR<br />
– Tenax TA + 30% graphite<br />
Carbotrap, Carbotrap<br />
C<br />
– Graphitized carbon blacks<br />
Carboxen 569,<br />
Carbosieve SIII<br />
– Carbon molecular<br />
sieves<br />
Glass Beads
Tenax TA<br />
A porous polymer resin based on<br />
2,6-diphenylene oxide<br />
High temperature limit of 350 o C<br />
– Tenax degrades if react with O 2 at high<br />
temperature – abundance of phenolic<br />
compounds and oligomers<br />
Low affinity <strong>for</strong> water<br />
– Useful <strong>for</strong> high moisture content<br />
samples including the analysis of<br />
volatile organic compounds in water
Selection of Adsorbents<br />
Types of analytes<br />
The physical properties of the<br />
adsorbent<br />
Breakthrough info.<br />
John J. Manura, Selection and Use Of Adsorbent Resins For<br />
Purge and Trap Thermal Desorption Applications<br />
Scientific Instrument Services, Inc.<br />
http://www.sisweb.com/referenc/applnote/app-32.htm<br />
Scientific Instrument Services, Inc.<br />
http://www.sisweb.com
Breakthrough<br />
The breakthrough volume <strong>for</strong> a compound<br />
on a given adsorbent and at a given<br />
temperature is defined as the calculated<br />
volume of carrier gas per gram of<br />
adsorbent resin which causes the analyte<br />
molecules to migrate from the front of the<br />
adsorbent bed to the back of the<br />
adsorbent bed.<br />
V<br />
(L/g)<br />
B<br />
=<br />
( t × F)<br />
R<br />
m<br />
A<br />
t R = Retention time (min)<br />
F = Flow rate (L/min)<br />
m A = Adsorbent mass (g)
Breakthrough<br />
Scientific Instrument Services, Inc.
Calculation of Breakthrough Volume<br />
t R<br />
V<br />
B<br />
=<br />
( t × F)<br />
R<br />
m<br />
A<br />
−<br />
DV<br />
V S = V B * 0.5<br />
V F = V B * 2<br />
V S<br />
V F
Breakthrough Volumes of Alcohols (C1 – C11) on Tenax<br />
TA<br />
0.4<br />
5<br />
20<br />
120
Breakthrough Volumes of Alcohols by Tenax TA
Desorption Chart
Breakthrough Data<br />
Hydrocarbons<br />
Alcohols<br />
Alkenes<br />
Alcohols & Glycols<br />
Acetates<br />
Acids<br />
Aldehydes<br />
Ketones<br />
Halogens<br />
Amines<br />
Aromatics and<br />
Terpenes<br />
Water<br />
Scientific Instrument Services, Inc.<br />
http://www.sisweb.com
Thermal Desorption (TD)<br />
Thermally desorb analytes from the<br />
adsorbents (P&T, direct sampling)<br />
Direct Thermal extraction<br />
– <strong>Volatile</strong>s from solid samples<br />
Thermal desorption – To GC<br />
– Transfer line<br />
– Focusing – Improved resolution
Thermal Desorption/Extraction<br />
Glass wool<br />
plug<br />
0.4 mm i.d.<br />
Solid<br />
sample<br />
Adsorbent<br />
resin<br />
10 cm<br />
Temp. ~300 o C<br />
Glass wool<br />
plug<br />
Temp. < 200 o C<br />
Direct Thermal<br />
Extraction<br />
Thermal<br />
Desorption
Short Path Thermal Desorption
Tempearture/Time<br />
Desorption<br />
temperature<br />
– Enough to<br />
volatilize the<br />
organic compound<br />
without degrading<br />
them and without<br />
producing<br />
unwanted<br />
artifacts<br />
– Temperature rate<br />
Desorption time<br />
– <strong>Sample</strong> matrix<br />
– <strong>Sample</strong> size<br />
– Interaction strength<br />
between analyte<br />
and the solid<br />
surface<br />
– Desorption<br />
temperature<br />
– Diffusion time of<br />
analyte out of the<br />
sample
Reduce Band Broadening:<br />
Focusing<br />
PTV<br />
Retention Gap<br />
Cryogenic focusing<br />
Cryo-Trapping<br />
Scientific Instrument Services, Inc.
Cryo-focusing<br />
Cryo-focusing is a technique <strong>for</strong><br />
introducing samples from purge-and<br />
and-<br />
trap concentrators into capillary GC<br />
columns<br />
It enables samples desorbed from<br />
adsorbent traps to be introduced into<br />
narrow-bore columns without losing<br />
any resolution of the column
Single Stage Thermal Desorption<br />
GC Detector<br />
<strong>Sample</strong> Tube<br />
GC Analytical Column<br />
Two-Stage Thermal Desorption<br />
Carrier<br />
Inlet<br />
Split<br />
Cold<br />
Trap<br />
Desorb Flow Split<br />
GC Detector<br />
Hot <strong>Sample</strong> Tube<br />
Carrier<br />
Inlet<br />
Hot<br />
Trap<br />
Carrier<br />
Inlet<br />
GC Analytical Column
Cryo-Trapping<br />
Scientific Instrument Services, Inc.
Thermal Desorption<br />
Analysis without the use of solvent<br />
– Analysis of 100% instead of aliquot<br />
– Elimination of solvent peak in the<br />
chromatogram enables the analysis of<br />
early eluted volatile analytes that are<br />
not masked by the solvent<br />
– Elimination of solvent reduction, no<br />
evaporation of solvent to the<br />
environment, and no waste
Applications<br />
<strong>VOCs</strong> from water/soil (EPA Method 502.1,<br />
503.1, 8030A, SW-846 Method 5030A)<br />
<strong>VOCs</strong> from biological fluids (urine, plasma,<br />
saliva, tissues)<br />
Fragrances, flavors<br />
Forensic investigation (arson accelerants)
Pyrolysis<br />
Next stage in thermal extraction technique<br />
Bond dissociation at very high<br />
temperatures (600-800<br />
o C) ) and break<br />
apart into smaller and simpler volatile<br />
molecules in a predictable manner<br />
By measuring the fragments, the<br />
molecular composition of the original<br />
sample can be reconstructed<br />
Polymer defects, variations, and<br />
degradation mechanisms
Pyrolysis<br />
aH<br />
bH<br />
As sample is heated, the weaker bond<br />
breaks first <strong>for</strong>ming free radicals<br />
C C<br />
dH<br />
Cl eH<br />
Polyvinyl Polyethylene chloride<br />
Polystyrene<br />
If C-C C C bonds are the weakest, the<br />
polymer will break into oligomeric<br />
fragments including monomers<br />
If the side group bond is weaker,<br />
the group is removed from the<br />
chain be<strong>for</strong>e fragmented, so the<br />
monomeric identity is lost<br />
The materials can be heated to a relatively low temperature <strong>for</strong><br />
desorption of intact small molecules including solvents, excess<br />
reagents, residual monomers, and additive such as plasticizer
Pyrolysis - Applications<br />
Synthetic polymers (Polyvinyl<br />
chloride, Polystyrene, Polyester)<br />
Natural polymers<br />
– Plant fibers (Cellulose, Cotton)<br />
– Animal fibers (Wool, Silk)<br />
Dried paints<br />
Cosmetic samples