Determination of Volatile Organic Compounds by Headspace Trap

Journal of Chromatographic Science, Vol. 44, November/December 2006
Determination of Volatile Organic Compounds by
Headspace Trap
F. Barani1, N. Dell’Amico1,*, L. Griffone1, M. Santoro2, and C. Tarabella1
1Agenzia
Regionale Protezione Ambiente Ligure, ARPAL, Dipartimento della Spezia, Via Fontevivo 21, 19100 La Spezia, Italy and 2Perkin
Elmer Italia, Via Tiepolo 24, 20052 Monza, Italy
Abstract
A new analytical method for the determination of halogenated and
aromatic volatile organic compounds in groundwater, mineral
water, and drinking water at concentrations ranging between
1–10000 ng/L is developed. A new type of headspace sampler that
combines static headspace sampling with a trap is used, yielding
very low detection limits and good repeatability without carryover
effects. An unexpected transformation of 1,1,2,2-tetrachloroethane
into trichloroethene is observed and explained.
Introduction
Over the last 10 years, analytical techniques for the determination of halogenated and aromatic volatiles have improved,
leading to better limits of detection (LOD) and reproducibilities.
The three main historical techniques are static headspace (1–7),
dynamic headspace (purge and trap) (1,3,4,8,9), and solid-phase
microextraction (SPME) (10).
Repeatability, detection limit, and elimination of water before
the chromatographic run are the three principal variables. Static
headspace has an excellent repeatability [coefficients of variation
(CV) < 5%], with no interference by water, but sometimes the
achieved LOD is not enough to comply with some environmental
regulation. For example, it cannot be used to measure concentrations of benzene below 0.5–1 µg/L. SPME has a good LOD
(below 0.5 µg/L), but a worse repeatability (CV > 10 %) when
using an autosampler (10). The purge and trap technique has a
better LOD than static headspace, but the elimination of water
may be difficult, and the system is prone to carry-over effects.
This makes it is necessary to clean the system using reagent
water between every sample. The device must be washed, purged
with detergent solution, rinsed with distilled water, and dried for
analyses of samples containing large amounts of water-soluble
materials, suspended solids, high boiling point compounds, or
high levels of volatile compounds. Moreover the trap and other
*Author to whom correspondence should be addressed: email [email protected]
parts of the system must be baked and purged (9).
A new type of headspace sampler, the PerkinElmer
Turbomatrix HS40 Trap, which combines headspace sampling
with a trap (PerkinElmer, Wellesley, MA), was used in this study,
as it had a good LOD (like purge and trap systems), a very good
repeatability (similar to static headspace), and allowed the elimination of water before the chromatographic run. Carry-over
effects were not observed in our range of concentration (1–10000
ng/L) so it was not necessary to extensively bake and purge the
trap after every sample was analyzed.
The Turbomatrix HS 40 Trap instrument is an autosampler for
up to 40 vials that can be used to determine volatile organic compounds present in several matrices.
It works with the pulsed-pressure approach, combining a
slight modification of the balanced pressure principle with the
use of an on-board cold and packed trap to extend its detection
limits. The analysis is performed through the following steps:
Equilibration
The vial is warmed at a fixed temperature, defined by the
sample characteristics, for a set constant time in order to reach
equilibrium conditions.
Pressurization
The needle pierces the septum, and carrier gas (at a pre-set
pressure) is allowed to enter the vial to set the internal pressure
to a particular value. Simultaneously, a valve isolates the gas
chromatography (GC) column, avoiding any column pressure
change. This column isolation flow is manually set by the operator 10 mL/min higher than the column flow.
Trap load
During this step, the headspace of the vial is sent to the cold
packed trap, allowing a flow of the headspace through the trap.
This trap load step can be repeated up to four times for each vial
(Figure 1).
Trap dry-purge
The cold packed trap is purged with carrier gas to eliminate
water. Even if the adsorbent material is mostly hydrophobic and
Reproduction (photocopying) of editorial content of this journal is prohibited without publisher’s permission.
625
Journal of Chromatographic Science, Vol. 44, November/December 2006
the vial equilibration temperature is kept low, a certain amount
of water will remain in the trap. This would damage the capillary
column and worsen the detection limits by increasing the baseline. Therefore, water vapor must be eliminated.
Trap desorb and trap hold
During these steps, the trap temperature is increased to the
desired high value at a rate of 400°C/min to release the trapped
Column isolation
Valve
Needle
Trap
Detector
Column
analytes. It is then kept at that value for a specified time to clean
it, avoiding any possible carry-over.
As soon as the trap is heated, the isolation of the column from
the carrier gas flow is stopped and the GC run begins. The trap is
desorbed in the backflush mode, and the analyst decides whether
or not to activate the split line present at the end of the trap.
During these studies several instrumental parameters were
optimized to reach LODs and performances required by the
Italian law. Moreover, the instability of some halogenated
volatiles, especially 1,1,2,2-tetrachloroethane, were observed and
explained.
Experimental
The instrument used was the PerkinElmer Turbomatrix
Headspace 40 Trap connected by an inert heated transfer line to
a PerkinElmer Clarus 500 GC with a duel channel flame ionizaVial
tion detector (FID) and electron capture detector (ECD) or to a
Oven
PerkinElmer Clarus 500 GC–MS.
HS trap
GC
The analyses performed in the first part of this study were done
with
a PerkinElmer Elite Volatiles column 60 m × 0.32-mm i.d.
Figure 1. Turbomatrix HSTrap during trap load: column isolation flow is on.
(1.8-µm film thickness) that was connected to the headspace
trap by an inert column (1.5 m × 0.32 mm) and to the FID and
ECD by an inert universal Y splitter. Helium was
Table I. Composition of Standard CUS-6068, Calibration Range and tR of
the carrier gas.
Halogenated and Aromatic VOCs Analyzed
GC conditions were: Ar–CH4 flow, 30 mL/min
(as make-up gas for the ECD); air flow, 450
Low
High
mL/min (for the FID); H2, 45 mL/min (for the
concentration concentration
FID); injector temperature, 150°C; ECD temperacalibration
calibration
ture, 370°C; FID temperature, 300°C; oven, 35°C
Conc.
Bias
range
range
tR
for 12.00 min, then programmed at 5.0°C/min to
Analyte
µg/L
µg/L
(µg/L)
(µg/L)
min Detector
60°C, after 1 min, programmed at 17.0°C/min to
220°C
for 0.0 min, and finally at 30°C/min to
Hexachlorobutadiene
501
0.30.003–0.050 0.03–0.5029.45 ECD
240°C.
o-Xylene
1504
8 0.075–1.500 0.75–15.0024.56 FID
Optimal headspace parameters used were: vial
m-Xylene
1503
8 0.075–1.500 0.75–15.0024.06 FID
p-Xylene
1504
8 0.075–1.500 0.75–15.0024.06 FID
temperature, 70°C; needle temperature, 100°C;
Styrene
1506
8 0.075–1.500 0.75–15.0024.43 FID
transfer line temperature, 130°C; trap material,
Ethylbenzene
1504
8 0.075–1.500 0.75–15.0023.76 FID
air toxics; trap load temperature, 40°C; trap desToluene
1503
8 0.075–1.500 0.75–15.0021.10 FID
orption temperature, 320°C; thermostatation
Benzene
1505
8 0.075–1.500 0.75–15.0014.57 FID
time, 20 min; cycles number, 3; pressurization
Bromoform
300.2
1.50.015–0.300 0.15–3.0024.11 ECD
time, 1.5 min; decay time, 1.5 min; desorption
Bromodichloromethane
50.2
0.30.003–0.050 0.03–0.5017.40 ECD
time,
0.3 min; trap hold, 0.6 min; dry purge time,
Dibromochloromethane
200.6
1 0.010–0.200 0.10–2.0021.67 ECD
12
min;
cycle time, 54 min; column pressure, 33
Tetrachloroethene
50.1
0.30.003–0.050 0.03–0.5022.43 ECD
psi;
vial
pressure,
33 psi; desorption pressure, 50
1,1,2,2-Tetracloroethane 1003
5 0.065–1.300 0.65–13.0024.68 ECD
psi;
purge,
on;
shaker,
on; outlet split, on or off
1,2-Dibromoethane
10
0.10.0005–0.010 0.005–0.1022.09 ECD
according to the actual concentration range (low
1,1,1-Trichloroethane
300.8
1.50.015–0.300 0.15–3.0013.43 ECD
1,2,3-Trichloropropane
1001
5 0.050–1.000 0.50–10.0024.76 ECD
or high concentration, as specified below, in Table
Trichloroethene
100.4
0.50.005–0.100 0.05–1.0017.28 ECD
I).
1,1,2-Trichloroethane
1505
8 0.075–1.500 0.75–15.0020.86 ECD
The following reagents were used: ultrapure
1,2-Dichloropropane
502
2.50.025–0.500 0.25–5.0017.07 ECD
water produced by Millipore (Billerica, MA); Elix
Trichlomethane
300.2
1.50.015–0.300 0.15–3.0010.59 ECD
3-MilliQ was boiled for at least 90 min, cooled,
1,2-Dichloroethane
300.6
1.50.015–0.300 0.15–3.0013.14 ECD
and preserved under a nitrogen atmosphere;
1,1-Dichloroethane
1504
8 0.075–1.500 0.75–15.008.17
ECD
Suprapure hydrochloric acid (30%) was supplied
cis-1,2-Dichloroethene
1504
8 0.075–1.500 0.75–15.009.77
ECD
by Merck (Darmstadt, Germany).
trans-1,2-Dichloroethene 1504
8 0.075–1.500 0.75–15.007.52
ECD
Custom Standard CUS-6068 was from
1,1-Dichloroethene
300.1
1.50.015–0.300 0.15–3.005.73
ECD
(Ultrascientific, Kingstown, RI). For its composi.
626
Journal of Chromatographic Science, Vol. 44, November/December 2006
Hexachlorobutadiene
o-Xylene
m-Xylene
p-Xylene
Styrene
Ethylbenzene
Toluene
Benzene
Bromoform
Bromodichloromethane
Dibromochloromethane
Tetrachloroethene
1,1,2,2-Tetracloroethane
1,2-Dibromoethane
1,1,1-Trichloroethane
1,2,3-Trichloropropane
Trichloroethene
1,1,2-Trichloroethane
1,2-Dichloropropane
Trichlomethane
1,2-Dichloroethane
1,1-Dichloroethane
cis-1,2-Dichloroethene
trans-1,2-Dichloroethene
1,1-Dichloroethene
225
91
91
91
104
91
91
78
173
83
129
166
83
107
97
75
95
97
63
83
62
63
61
61
61
Qualifier 1 Qualifier 2
(m/z)
(m/z)
227
106
106
106
103
106
92
77
171
85
127
164
85
109
99
110
130
83
62
85
64
65
96
96
96
260
105
105
105
78
65
–
51
252
129
131
131
168
–
119
77
132
99
65
87
–
–
98
98
98
r2
0.9968
0.9979
0.9993
0.9989
0.9972
0.9962
0.9994
0.9932
0.9975
0.9984
0.9991
0.9986
0.9995
0.9934
0.9992
0.9989
0.9987
0.9979
0.9996
0.9994
0.9989
0.9990
0.9987
0.9992
0.9978
mV
mV
Time (min)
B
Time (min)
Figure 2. Chromatograms of blank samples by ECD (A) and FID (B).
Area
Tetrachloroethene
Area
Pressure (psi)
Figure 3. Dependence of peak area on desorb pressure.
A
mV
Analyte
Target
Ion (m/z)
A
Time (min)
B
mV
Table II. Halogenated and Aromatics VOCs Analyzed
with Selected Ion Recording GC–MS, in the
Concentration Range from 0.100 to 2.000 µg/L. Target
Ion and Qualifiers Used to Quantify and Correlation
Coefficient Obtained Are Shown
following conditions: the chromatographic column was a
PerkinElmer Elite Volatiles (60 m × 0.25-mm i.d., 1.4 µm) connected to the headspace trap by an inert column (1.5 m × 0.32
mm).
The carrier gas was He. The temperatures were: injector,
150°C; oven, 40°C for 12.00 min, then programmed at 5.0°C/min
Time (min)
C
mV
tion, see Table I. Standard solution A was freshly prepared by
diluting 10 µL of custom standard in 1 mL of ultrapure water.
Standard solution B was freshly prepared by diluting 100 µL of
standard solution A in 1 mL of ultrapure water. Sodium sulfate
anhydrous, volatiles free, was obtained by heating commercial
prepared sodium sulfate (Baker, Deventer, Holland) at 400°C
for 24 h.
Two calibration curves were prepared by using standard solutions A and B, dissolved, respectively, in 0.5, 1.0, 2.0, 5.0, 7.0, and
10.0 µL of solution A (low concentration curve) and of solution
B (high concentration curve) in 10 mL of ultrapure water. Then
30 µL of HCl and 100 mg of sodium sulfate under a nitrogen
atmosphere was added.
All samples were processed by adding 30 µL of acid (sufficient
to eliminate 700 mg/L of carbonate) and 100 mg of sodium sulfate to 10 mL of sample. The HCl addition must be superior if the
carbonate concentration was greater than 700 mg/L to adjust pH
to less than four. This precaution was indispensable to avoid any
problem with 1,1,2,2-tetrachloroethane, as better specified
during the following discussion.
The “high curve” was obtained by opening the outlet split, and
the “low curve” was obtained by closing it.
The analysis performed in the second part of this study was
done working with a PerkinElmer Clarus 500 GC–MS, using the
Time (min)
Figure 4. Chromatograms of halogenated volatiles at different desorb temperatures: 320°C (A), 250°C (B), and an overlay, where no significant differences
are visible (C).
627
Journal of Chromatographic Science, Vol. 44, November/December 2006
The first problem that occurred during the optimization process of the analytical technique was obtaining blank water, which
does not contain contaminants invalidating the analysis.
The normal ultrapure water produced by Millipore Elix 3–Milli
Q may sometimes still contain organohalides, such as
trichloromethane, trichloroethene, bromoform, etc., which are
present in common Italian drinking water. Therefore, the water
was boiled for 90 min to eliminate the majority of these
organohalides.
This boiled water was preserved under a nitrogen atmosphere
in order to prevent any further contamination of organohalides,
such as chloroform or dichloromethane, which can usually be
found in standard laboratories.
Before analysis, the vials were heated for 2 h at 180°C and then
stored in a dessicator. Because of the lack of a clean room, it was
not possible to analyze dichloromethane. A blank chromatogram
is shown in Figure 2.
Several tests were done to find the best possible pressure settings to reduce the amount of water vapor that otherwise may
interfere with the normal functionality of the detectors. Different
pressure levels were also tested to reduce the dry-purge time.
However, the lower pressures can shorten the time, and the LOD
Pressurization vial effect
Time (min)
B
A
Time (min)
Area
Figure 6. These chromatograms refer analysis to the analysis of two vials of
the same sample run consecutively. Only the relative height of the
trichloroethene (peak 1) and 1,1,2,2-tetrachloroethane (peak 2) peaks have
changed.
Pressure (psi)
B
Area
Cycle number effect
1
2
Cycles number
Figure 5. Dependence of peak area of tetrachloroethene from vial pressurization (A) and dependence of peak area of tetrachloroethene from cycle
number (B).
628
A
mV
Discussion
values were worse. Figure 3 shows how high pressure increases
the signal.
The best desorption temperature was 320°C. However, this
was not a critical variable (Figure 4). Even though 250°C was
enough to desorb all the analytes without any difficulty, it was
still recommended to use higher temperatures in order to clean
the trap from other possible interferences.
To increase sensitivity without excessively increasing analysis
time, the trap load step can be repeated. Figure 5B shows the
importance of this parameter and the three cycles that gave good
results. The vial pressure was not a critical parameter. Figure 5A
mV
to 60°C, after 1 min programmed at 20.0°C/min to 220°C.
The MS was operated in the single ion mode, acquiring for
each analyte the ions specified in Table II. For each ion, a 0.05 s
dwell time was used with a 0.001 s delay between them.
Headspace parameters used were the same as previously
reported.
Trichloroethene
1,1,2,2-Tetrachloroethane
Time (min)
Figure 7. Two overlaid chromatograms obtained using classical static
headspace at different pH levels [pH 9 (chromatogram 1), pH 3 (chromatogram 2)]. Note how the “1,1,2,2-tetrachloroethane” peak disappears at
alkaline pH and at the same time the trichloroethene peak increases.
Journal of Chromatographic Science, Vol. 44, November/December 2006
mV
does not show any signal increase above 30 psi.
Finally, a serious problem with 1,1,2,2-tetrachloroethane was
observed. The determination of 1,1,2,2-tetrachloroethane was
often impossible because it partially or totally disappeared
during the chromatographic run. Meanwhile, an increase in
trichloroethene’s signal is observed. All official methods (1,2,4,8)
describe procedures that can be applied to 1,1,2,2-tetrachloroethane, without reporting data concerning the validation
for this analyte.
Only Environmental Protection Agency (EPA) 5030C (8)
describes a generic interference during purge and trap analysis,
if the system is contaminated.
Time (min)
This phenomenon was first observed (as shown in Figure 6)
Figure 8. Chromatogram obtained with acidification of the sample with 30 µL
when working with HSTrap, and the use of a lower desorption
HCl.
temperature did not solve the matter. The same happened with
classical headspace analysis (without trap), as shown in Figure 7
and with “syringe-type” headspace.
1
Though it is well known, 1,1,2,2-tetrachloroethane is not a
Trichloroethene
1,1,2,2-Tetrachloroethane
stable compound, and it decomposes to trichloroethene at high
temperatures (11). 1,1,2,2-Tetrachloroethane, in a water solution, was stable at room temperature for one month, at acid/neutral pH, but it decomposed to trichloroethene by E2 elimination
2
at basic pH.
The study demonstrated that the previously mentioned pheTime (min)
nomenon may appear during thermostating at 70°C using neuFigure 9.Two overlaid chromatograms of the same sample obtained using a
tral water. Therefore, it was necessary to acidify all solutions to
headspace trap. Na2CO3 is added to the sample (chromatogram 1). An
pH < 4 because under these conditions the relative height of the
unspiked sample (chromatogram 2).
two peaks remained constant (as shown in Figure 8). The data
clearly demonstrated that the observed reaction
took place during thermostatation of the vials in Table III. Concentrations are the Mean of 10 Replicates. Data Obtained
the water phase and was not because of active Splitting the Sample at the End of the Column to an ECD and an FID
sites present in the trap, as stated in EPA method
5030C (8). It was also noted that 1,1,2,2-tetraHigh concentration
Low concentration
chloroethane completely disappeared and deConcentration
Concentration
LOD Recovery
graded into trichloroethene by adding Na2CO3 to Analyte
µg/L CV%
r2
µg/L (%)
µg/L CV%
r2
the solution [as suggested by the ISS 00/14
method (1)] in order to eliminate CO2 from Hexachlorobutadiene 0.23 7.1 0.9962 0.030 20.2 0.9916 0.0005 121
o-Xilene
7.36
2.3 0.9953 0.639
4.5 0.9982 0.0496 85
sparkling water (Figure 9).
14.56
2.4 0.9955 1.263
5.1 0.9981 0.0425 84
The conditions of acidification eliminate any m+p-Xilene
7.39
2.1 0.9955 0.617
7.6 0.9937 0.0276 82
problem with a carbonate concentration below Stirene
Ethylbenzene
7.33
2.2 0.9960 0.662
3.2 0.9986 0.0056 88
700 mg/L. A variation of the pH did not cause Toluene
7.35
1.7 0.9967 0.654
3.1 0.9991 0.0362 87
problems of repeatability or anomalous increasing Benzene
7.20
2.5 0.9948 0.708
1.8 0.9976 0.0326 94
of other signals. It was also possible to determine Bromoform
1.58
1.8 0.9982 0.140
3.4 0.9985 0.0106 93
repeatability, linearity, LOD, roughness, and accu- Bromodichloromethane 0.25 2.7 0.9909 0.021 4.8 0.9974 0.0010 84
racy using a natural mineral water with a concen- Dibromochloromethane 1.06 1.4 0.9983 0.088 2.3 0.9991 0.0008 88
Tetrachloroethene
0.25
3.1 0.9908 0.031
2.7 0.9959 0.0007 122
tration of approximately 180 mg/L or carbonate.
1,1,2,2-Tetracloroethane 7.05
1.2 0.9962 0.722
1.3 0.9994 0.0026 111
Table III lists LODs (International Union of Pure
1,2-Dibromoethane
0.05
1.5 0.9987 0.004
1.7 0.9991 0.0006 81
and Applied Chemistry definition), repeatability, 1,1,1-Trichloroethane
1.50
2.1 0.9915 0.155
4.4 0.9959 0.0002 103
correlation coefficients of the calibration curves, 1,2,3-Trichloropropane 6.20 4.9 0.9985 0.536 2.6 0.9916 0.0083 107
and recovery using samples of the same mineral Trichloroethene
0.51
2.0 0.9972 0.056
2.9 0.9983 0.0025 112
1,1,2-Trichloroethane
7.95
1.4 0.9987 0.693
4.2 0.9995 0.0060 92
with 7.0 µL of solutions A and B, respectively.
2.37
8.4 0.9987 0.259
5.0 0.9958 0.0180 103
Linearity was calculated with the analysis of the 1,2-Dichloropropane
1.73
2.0 0.9989 0.174
5.7 0.9919 0.0152 116
residues, and no particular deviation was Trichloromethane
1,2-Dichloroethane
1.54
3.1 0.9936 0.123
4.7 0.9914 0.0010 82
observed.
1,1-Dicloroethane
8.67
2.5 0.9918 0.775
3.0 0.9975 0.0213 103
Robustness was estimated at the confidence cis-1,2-Dicloroethene 7.52 2.9 0.9959 0.880 4.4 0.9951 0.0839 117
level of 99%; thus, it was very interesting to trans-1,2-Dicloroethene 7.59 2.4 0.9978 0.729 2.1 0.9912 0.0909 97
observe that changing the following parameters: 1,1-Dicloroethene
1.39
2.6 0.9974 0.183
5.2 0.9969 0.0049 121
operator, desorption pressure, dry purge time,
629
Journal of Chromatographic Science, Vol. 44, November/December 2006
Acknowledgments
1
The authors wish to thank the “Agenzia Regionale Protezione
Ambiente Ligure”, especially Head of U.O. Laboratori Dr. C.
Grillo, for his support.
2
Time (min)
Figure 10. Chromatograms obtained with different traps: 500 injections (chromatogram 1) and 30 injections (chromatogram 2).
trap, vial pressure, desorption temperature of the trap, and desorption time did not influence the method. Even by changing
the trap (one with about 500 injection and one with 30 injection), no significant change was noted, as shown in Figure 10.
Conclusion
A new method was developed for the determination of halogenated and aromatic volatiles. The method was very precise and
accurate.
All parameters were established according to the Italian laws
on drinking, mineral water, and groundwater, except for the LOD
of 1,2,3-trichloropropane, which had a limit of 1 ng/L, but only 8
ng/L was reached. One nanogram per liter could not be reached
because of elevated noise at the retention time of 1,2,3-trichloropropane, which was caused by high column bleeding.
The effect of pH on the analysis, which is usually underestimated in the standard methods [EPA 5030B (7) or UNI EN ISO
10301 (2)], was observed and described.
It was demonstrated that the transformation of 1,1,2,2-tetrachloroethane into trichloroethene must be inhibited using an
acidic pH by adding hydrochloric acid to every sample. Other
recent studies have noted that a high pH (> 11 to 12) also influenced other halogenated volatiles.
The determination of very volatiles halogenated substances
(e.g., vinyl chloride and chloromethane) involving different
instrumental parameters is in progress.
630
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Manuscript received February, 3, 2006.
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