GC Application Note How to Develop a Method using In-tube Extraction

GC Application Note
How to Develop a Method
using In-tube Extraction
(ITEX) for GC and GC/MS?
www.palsystem.com
How to Develop a Method using In-tube
Extraction (ITEX) for GC and GC/MS?
Joeri Vercammen, Ph.D.
www.is-x.com
Introduction
Throughout our lives we are continuously exposed to substantial quantities of
volatile organic components (VOCs). Residual solvents from carpets, paints and
glues are part of the indoor air we daily
inhale. Offices, workspaces and even our
living rooms, it’s practically impossible
to breathe clean air nowadays. Perhaps
we are luckier with the food we eat and
the beverages we drink? Unfortunately,
I am afraid not. Solvents from packaging
foils and printing inks may pose a severe
risk for the consumer, while the water
we drink or use to make soda, beers, etc
is polluted with traces of desinfection
agents and other environmental contaminants. No wonder ‘VOC analysis’ plays
such a vital role in many environmental
and QC laboratories around the globe.
Gas chromatography (GC) really loves
VOC analysis and VOC analysis loves
GC. They form a steady couple and
have a long and happy marriage with
many offspring. It’s quite remarkable to
see how some of these offspring have
matured substantially throughout the
years, whilst others seem to remain stuck
in puberty. One of these techniques that
has difficulties meeting expectations is
the subject of this contribution. The technique is called in-tube extraction (ITEX)
and was introduced about ten years ago
as an option to the popular Combi-PAL
platform from the Swiss manufacturer
CTC Analytics. The solution was, primarily, developed to address the severe
drawbacks associated with classic purge
& trap instrumentation (see below) yet
allowing equal sensitivity. The initial
hardware was upgraded substantially
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and became much more user-friendly.
Today, distributed as ITEX-2, the technique is truly modular and can be easily
added to any Combi-PAL system.
Before the introduction of ITEX, it
required quite a lot of expertise and
hocus pocus if you had the intention
to report VOC concentrations below
0.25 ppb (= parts-per-billion level) with
static headspace. Although this level
might seem sufficient for the majority
of applications, the enforcement of
ever stronger regulations has made the
classic headspace approach, i.e. with
gastight syringe, more or less redundant. In order to compete with classic
purge & trap instrumentation, one has
to be able to detect VOC with a sufficient degree of certainty at low or even
sub ppb levels. Typical reporting limits
usually range between 0.01 and 0.02
ppb. Purge-and-trap analysis has been
a popular approach for VOC analysis for
several decades. Although very powerful, it suffers from quite some drawbacks
as well. First of all, it is particularly well
suited for the analysis of VOC in relatively clean samples, such as drinking water.
However, when applied to the analysis
of more contaminated samples, strict
safety measures need to be taken. Due
to the immense concentration effect of
purge & trap, these systems are easily
overloaded and contaminated, often by
mistake. Particularly less volatile components, such as naphthalene tend to stick
to valves, fittings and o-rings and elute
very slowly from the dead volumes that
are typical for these systems. In order
to avoid elevated blank levels and/or
regular system downtime, laboratories
often include a sniffer step, which acts
as a pre-screener to protect the system
from unwanted exposure to high VOC
amounts. Other points of attention have
to be considered, though they are far
less critical than system contamination
and blank levels, e.g. blocking of the
cold trap due to improper removal of
residual water vapors and excessive
sample foaming during purging.
Many of these issues have been
addressed by the introduction of ITEX-2
(details can be found here). ITEX-2
consists of a dedicated gastight syringe
(1.3 mL), which is fitted with a needle
with microtrap. The microtrap contains
the packing material (or a combination
of materials) that is used to enrich the
volatile components from the headspace. A heating element is positioned
around the microtrap to release
trapped volatiles after sampling. The
principle of enrichment is very simple
and closely resembles classic static
headspace analysis with syringe. First
of all, it is recommended to equilibrate
the sample at elevated temperature.
Afterwards, the vial is punctured and
its headspace sampled repeatedly. This
is accomplished by slowly moving the
plunger of the ITEX-2 syringe up and
down. Finally, the device is moved to
the GC injector, where trapped components are released by flash thermal
desorption. Due to the relatively small
amount of trapping material, particularly compared to purge & trap, it is possible to desorb the microtrap directly into
a split/splitess injector without the need
for cryofocusing. This avoids freezing of
residual water vapor.
How to develop an ITEX
Method?
Before starting with the actual development of an ITEX method, you have to
decide which type of packing material
you are going to use to trap analytes.
This is mainly determined by the volatility of the components you would like to
enrich. Please be aware that the material you finally pick can have a tremendous effect on the ITEX cycle time as
well, but more on that later. Each ITEX-2
kit is delivered with two microtraps,
each filled with approximately 40 mg
Tenax TA. Since it does not retain water,
Tenax is a very powerful trapping material, particularly for classic applications,
such as BTEX in water. Unfortunately,
more volatile target components, such
as vinyl chloride for example, cannot
be enriched by Tenax, even not at trap
temperatures as low as 35°C. These
components are simply much too
volatile. Here, it is necessary to use a
mixed bed microtrap. We usually suggest a trap that contains two separate
beds, one Tenax TA and one Carboxen
1000 in a 1:1 ratio. The much stronger
Carboxen bed is placed on top of the
Tenax bed, such that components that
are not retained on Tenax are trapped
on the Carboxen. Why don’t we use a
100% Carboxen 1000 bed instead, you
might ask yourself? The reason is simple. The Tenax plays an important role
too, i.e. it serves as a protective layer to
capture less volatile components (both
target and/or matrix) that will destroy
the Carboxen material permanently
due to irreversible adsorption when
directly exposed to it. The use of a
mixed bed trap has another important
consequence, as briefly highlighted in
the beginning of this paragraph. Instead
of using trap temperatures close to
room temperature, mixed bed sampling
can be carried out at temperatures as
high as 50°C! Since it takes quite some
time to cool down the microtrap after
injection/conditioning, a lot of valuable
(cycle) time can be recovered easily.
Equilibration times and temperatures
are determined similar to classic static
headspace injections and will not be
discussed here. Just one small suggestion; we usually do not wait for the
sample to equilibrate. Five minutes of
equilibration is usually enough for most
applications, while ITEX sampling itself
might take quite some time as well.
Once again several tens of minutes
can be recovered from the final ITEX
cycle time. This is particularly important
when throughput is an issue in your
lab!
ITEX variables that have an immediate
impact on method performance include
the number of sampling strokes, the
plunger speed during sampling and the
trap conditioning parameters. It’s quite
clear that increasing the number of
strokes will increase analyte response.
The extent of this increase is, however,
not really proportional to the number
of strokes, since it depends not only
on compound volatility but also on the
complexity of the sample matrix (heavily loaded matrices induce displacement
of analyte traces). At the same time,
you should be careful increasing the
number of sampling strokes, since it
has a massive effect on total cycle time.
Let’s assume, for example, an extraction
speed of 25 µL/sec; extracting 1000
µL implies, in fact, 2000 µL per single
stroke (one upward and one downward
movement) or in total approximately
80 seconds in time per individual cycle.
Five strokes will take 400 seconds,
20 strokes as much as 1600 seconds,
which is almost half an hour! Bearing in
mind that the cycle time of state-of-theart GC/MS methods for VOC is situated
somewhere around 20 minutes, it’s
quite clear that 20 strokes are unacceptable when you are aiming at a smooth
and efficient process. Please note that
when it’s necessary to actually sample
20 times to achieve sufficient sensitivity,
it is probably much more convenient
to consider a more powerful packing
material and sample faster.
Throughout the years we have implemented several ITEX projects. Today I
will discuss the results of the analysis of
sub ppb levels of VOC in heavily loaded
waste waters. Other projects that we
carried out include the analysis of trace
levels of benzene in soft drinks, residual
VOC in plastics, fire accelerants in suspected arson studies and others. Some
typical extraction conditions are summarized in Table 1. These settings are based
on the macro ‘ITEX_VolatileRev02’.
We advise our customers to use this
macro, which is part of the ITEX-2 kit,
due to one of its particular characteristics. Namely the slow upwards plunger
movement during trap heating, which
reduces the loss of the most volatile
target components of your sample.
Parameter
Setting
Incubation temp, °C
75
Incubation time, min
5.00
Agitation speed, rpm
500
Syringe temp., °C
85
Extraction vol., µL
1000
# Extractions
20
Extraction speed, µL/sec
50
Desorption temp, °C
280
Desorption speed, µL/sec
750
Conditioning temp., °C
300
Conditioning time, min
3.00
Peak identification and results are summarized in Table 2. A typical chromatogram of a VOC
standard at 10 ppb (GC/MS in SIM mode) is depicted in Figure 1, insert shows vinyl chloride at
0.1 ppb, 10 ppb and 100 ppb.
Table 1: Typical settings for ITEX-2.
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Tr, min
Component
Limit of Detection, ppb
Linear range, ppb
1.15
2.01
Vinylchloride
1,1-Dichloroethylene
0.015
0.006
0 – 100
0 – 100
2.71
t-1,2-Dichloroethylene
0.012
0 – 100
3.44
1,1-Dichloroethane
0.019
0 – 100
4.21
c-1,2-Dichloroethylene
0.016
0 – 100
4.51
Bromochloromethane
0.018
0 – 100
4.72
Chloroform
0.012
0 – 100
4.88
CCl4
0.024
0 – 100
5.02
1,1,1-Trichloroethane
0.019
0 – 100
5.55
Benzene-D6 (IS)
N/A
N/A
5.59
Benzene
0.013
0 – 100
5.87
1,2-Dichloroethane
0.018
0 – 100
6.37
Trichloroethylene
0.022
0 – 100
7.01
Bromodichloromethane
0.012
0 – 100
7.33
Bromotrichloromethane
0.008
0 – 100
7.43
1-Bromo-2-chloroethane
0.013
0 – 100
7.78
Toluene-D8 (IS)
N/A
N/A
7.82
Toluene
0.012
0 – 50
8.15
Tetrachloroethylene
0.019
0 – 50
8.33
1,1,2-Trichloroethane
0.013
0 – 100
8.44
Dibromochloromethane
0.011
0 – 100
9.07
Chlorobenzene
0.013
0 – 100
9.05
Ethylbenzene-D10 (IS)
N/A
N/A
9.12
Ethylbenzene
0.008
0 – 100
9.13
1,1,1,2-Tetrachloroethane
0.014
0 – 100
9.23
p/m-Xylene
0.015
0 – 50
9.56
o-Xylene
0.019
0 – 100
9.59
Bromoform
0.01
0 – 100
10.13
n-Propylbenzene
0.013
0 – 100
10.17
1,1,2,2-Tetrachloroethane
0.017
0 – 100
10.24
o-Chlorotoluene
0.015
0 – 100
10.28
1,2,3-Trichloropropane
0.009
0 – 100
10.37
p-Chlorotoluene
0.016
0 – 100
10.58
1,2,4-Trimethylbenzene
0.011
0 – 100
10.81
1,3-Dichlorobenzene
0.013
0 – 100
10.88
1,4-Dichlorobenzene
0.016
0 – 100
10.92
1,2,3-Trimethylbenzene
0.016
0 – 100
11.2
1,2-Dichlorobenzene
0.007
0 – 100
11.81
2,4-Dichlorotoluene
0.015
0 – 50
11.83
2,6-Dichlorotoluene
0.006
0 – 50
11.85
1,2,4-Trichlorobenzene
0.014
0 – 50
12.24
3,4-Dichlorotoluene
0.008
0 – 50
12.34
Hexachlorobutadiene
0.012
0 – 50
12.36
1,3,5-Trichlorobenzene
0.014
0 – 50
12.6
Naphthalene
0.01
0 – 100
12.75
1,2,3-Trichlorobenzene
0.012
0 – 50
Table 2: Peak identification and linearity details.
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Figure 1: Typical chromatogram, 10 ppb VOC std.
Figure 2: Robustness, no carryover.
Conclusion
About the Author
References
Today we focused on in-tube extraction
(ITEX) as a powerful alternative to
classic purge & trap analysis. The approach is particularly well suited for the
analysis of trace organic compounds
with a wide range of volatilities. Due do
the simplicity of the system, it can be
applied successfully to the analysis of
heavily loaded samples without the risk
of cross contamination and permanent
system failure.
Joeri received his Ph.D. in chromatography from Ghent University (Belgium)
in 2002. He is currently employed as
managing expert at IS-X, an independent team of true chromatographers
that deliver chromatographic method
development solutions (prep-to-rep),
method validation, expert training and
quality control.
[1]In-Tube Extraction of Volatile Organic Compounds from Aqueous Samples: An Economical Alternative to Purge and Trap Enrichment
Jens Laaks, Maik A. Jochmann, Beat Schilling, and
Torsten C. Schmidt
Anal. Chem. 2010, 82, 7641–7648
[2]In-tube extraction for enrichment of volatile
organic hydrocarbons from aqueous samples.
Jochmann MA, Yuan X, Schilling B, Schmidt TC
J. Chromatogr. A, 2008; 1179; 96-105
Contact: [email protected]
Website: www.is-x.com
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