(HS-GC) with Flame Ionization Detection (FID)

A P P L I C AT I O N N O T E
Gas Chromatography
Authors
Lee Marotta
Dennis Yates
PerkinElmer, Inc.
Shelton, CT USA
Methane, Ethylene,
and Ethane in Water
by Headspace-Gas
Chromatography (HS-GC)
with Flame Ionization
Detection (FID)
Introduction
The rapid development of natural
gas from unconventional sources in
North America has created an energy
“gold rush” not seen in contemporary
times. The advent of horizontal drilling
technologies and hydraulic fracturing has
made this production economical and
presents an energy source of sufficient
magnitude that could last 100 years.
The technology presents a number of environmental challenges as the
wells are drilled vertically through aquifers on their way to the deep shale
deposits thousands of feet under the surface, and then turned horizontally
and drilled another several thousand feet through the shale deposit. Herein
lies the challenge: in the process of drilling the wells and preparing them for
production (including “fracking” to optimize production), opportunities arise
for contamination of the clean drinking water aquifers with methane and other
low molecular weight organics (e.g., propane and ethane). Correctly drilled and
cemented well bores should not be an issue, but any errors in engineering could
result in contamination.
It is also possible that methane already exists at a low concentration in the
aquifer from diffusion of the gas occurring naturally. There is a need (by
property owner and lease holder) to confirm the level of gas in the aquifer
before and during drilling, and also after the well is placed into production.
Traditionally, methane in water is determined using U.S.
Environmental Protection Agency (EPA) method RSK 175
(RSKSOP175, 2004) or an alternative (Vandegrift, 1998).
PerkinElmer’s TurboMatrix™ HS and Clarus® 680 GC
combination offers a simple, economical and reliable
measurement technique to determine methane and other
target gases in water. This application note summarizes the
experimental approach and subsequent results to confirm
the viability of the method.
2.Calibration: A five-point calibration curve was created
establishing method linearity and reporting limits. Five (5)
headspace vials were prepared with 15 mL of DI water
then capped using PTFE silicone septa. A 2 μL,
5 μL, 10 μL, 20 μL and 50 μL volume of the stock
standard was inserted through the septum (PerkinElmer
Part No. N9303992) into the water of five of the vials,
respectively, attaining concentrations as described in
Table 3.
Instrumentation
3.Accuracy: Four (4) water samples were prepared as
quality controls from 5 to 50 ppb to confirm method
accuracy.
A PerkinElmer® TurboMatrix Headspace (HS) connected to
a PerkinElmer Clarus 680 Gas Chromatograph (GC) with
dual flame ionization detectors (FID) were used in these
experiments.
Since detection is performed using an FID, the technique of
column confirmation may be employed to confirm identity
of components. An Elite-Q PLOT column with dimensions
30 m x 0.32 mm (PerkinElmer Part No. N9316359) was
used for quantitation and the Elite-U PLOT column with
dimensions 30 m x 0.32 mm was used for confirmation.
These columns were directly connected to the deactivated
fuse silica headspace transfer line via a “Y” connector.
4.Precision: Five (5) 40 ppb standards were prepared from
the stock standard, and analyzed for precision.
Table 1. Headspace and GC Conditions.
HS Conditions
Sample Temperature:
90 ˚C
Equilibration Time:
10 min
Needle Temperature:
110 ˚C
Transfer Line Temperature:
120 ˚C
Inject Time:
0.06 min
Experimental Conditions
Withdrawal Time:
0.4 min
A stock standard was used for these experiments (Supelco®
Part No. 23437). This stock standard contained methane,
ethylene, acetylene and ethane in approximately one molar
percent concentration in nitrogen for each component.
Pressurization Time:
1.0 min
HS Mode:
Constant
HS Pressure:
20 psi
The headspace and GC operating conditions are displayed in
Table 1.
GC Conditions
Oven Temperature Initial Temperature:
40 ˚C
To validate the method, the following experiments were
performed:
Initial Hold:
4.5 min
Ramp:
40 ˚C/min
1.Background: Blank air and water were investigated for
interferences. Since methane may be present in ambient
air, four (4) 22 mL Headspace Crimp Vials (PerkinElmer
Part No. N9306079) containing 15 mL of the deionized
(DI) water, used in preparing standards, were investigated
to determine the concentration of the methane in the
blank samples.
Final Temperature:
205 ˚C
Final Hold:
1 min
Detector (FID)
Detector Temperature:
240 ˚C
Air Flow:
400 mL/min
Hydrogen Flow:
40 mL/min
Range:1
Attenuation:
-6 (or 1)
Note: The columns are directly connected to the HS transfer
line; therefore, inlet parameters are not applicable.
2
Results
Figure 1 demonstrates separation of the four gases in
the stock standard on the Elite-Q PLOT column. The
concentration of the standard represented in Figure 1 is
10 parts per billion (ppb). Since acetylene is not a target
analyte of this application, and acetylene is not found in
samples, it is recommended that a standard mix be used
not containing this analyte to avoid integration challenges
between ethylene and acetylene.
Figure 3 graphically demonstrates the results of the external
standard calibration curve of each component. The linearity
achieved was excellent with a correlation coefficient (r2) of
0.9996 and better. Table 3 contains the concentrations of
the standards used to prepare these curves.
Figure 1. 10 ppb standard (Q PLOT).
Figure 2 is a chromatogram of a water blank (15 mL
volume). To compensate for the methane present in ambient
air, this point was incorporated on the calibration curve
to subtract for the presence of methane in air. Since the
headspace vials are sampled in air, this air is trapped in
the vial. The concentration of methane in air is below the
reporting limit. Table 2 tabulates the precision of methane in
the blank.
Figure 3. Calibration curves.
Figure 2. Chromatogram of blank (15 mL water).
3
Table 2. Repeatability of Four Blanks for Methane.
Sample Name
Table 3. Standard concentrations in Parts Per Billion (ppb)
or µg/L.
Area (Methane)
15 mL Water Blank
2093.5
15 mL Water Blank
2163.7
15 mL Water Blank
2337.4
15 mL Water Blank
2124.3
Level No.
Average2179.7
%RSD5%
Methane
Ethylene
Ethane
1
0.80 1.401.50
2
2.00 3.503.75
3
4.00 7.007.50
4
8.00 14.0015.00
5
20.00 35.0037.50
Table 4. Results from four (4) Quality Control Samples.
Methane
Actual Amt.
Calc. Amt.
%Dev
Ethylene
Actual Amt. Calc. Amt.
%Dev
Ethane
Actual Amt. Calc. Amt.
%Dev
2.00
2.05
2.50
3.503.43 -2.00
3.75 3.59 -4.27
10.00
10.72
7.20
17.5018.68
6.74
18.75 19.91 6.19
14.00
15.19
8.50
24.5026.40
7.76
26.25 28.43 8.30
20.00
20.69
3.45
35.0036.44
4.11
37.50 39.14 4.37
Table 5. Repeatability of Peak Area Calculations Using Level
4 Concentration (Refer to Table 3).
Conc. Level
Methane Ethylene
Ethane
Area AreaArea
4
43180 7006780441
4
44330 7019981390
4
43421 6791179164
4
44331 7101782016
4
42184 6672276234
Average 43489 6918379849
% RSD
2.1
2.6
2.9
Table 4 tabulates the results of the quality control study.
These controls were processed using the five-point
calibration for each component.
Table 5 represents the results of the precision study at Level 4.
Discussion
In this experiment, the blank was used as a point on the
calibration curve to correct for the presence of methane in
ambient air (subtracting the blank), which improves accuracy
for the low level methane amount and allows for very easy
sample preparation essentially filling the vial with a known
amount of water and capping it.
The reporting limit of 1 ppb methane in water was achieved.
The lowest point of the curve prepared for this application
for methane was 0.8 ppb, and 1st order is maintained
through this point.
The recoveries obtained in this experiment from four (4)
quality control samples are from 90% to 98%. This accuracy
is excellent and incorporates errors due to method and
operator. Since these gaseous standards and quality control
samples are prepared manually with a gas-tight syringe
human error is a contributory factor; therefore, the accuracy
is exceptional.
Instrument and method repeatability (precision) is 2.1%
for methane which is an acceptable repeatability for this
application.
Conclusions
Examining the results of these experiments, the PerkinElmer
TurboMatrix HS and PerkinElmer Clarus 680 GC provide
a viable solution determining methane and other low
molecular weight hydrocarbons in water delivering accuracy,
precision and ease of use.
PerkinElmer, Inc.
940 Winter Street
Waltham, MA 02451 USA
P: (800) 762-4000 or
(+1) 203-925-4602
www.perkinelmer.com
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Copyright ©2012, PerkinElmer, Inc. All rights reserved. PerkinElmer® is a registered trademark of PerkinElmer, Inc. All other trademarks are the property of their respective owners.
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