Application Note
Energy and Chemicals

Fast Analysis of Natural Gas
Using the Agilent 990 Micro GC
Natural Gas Analyzer

AuthorJie Zhang
Agilent Technologies, Inc.

Introduction
Natural gas is a naturally occurring hydrocarbon gas mixture consisting primarilyof methane, but commonly including varying amounts of other hydrocarbons suchas ethane, propane, butane and isobutane, pentane and isopentane. Sometimes, asmall percentage of carbon dioxide (CO2), nitrogen, hydrogen sulfide (H2S), or helium
exist together with the hydrocarbons.
Natural gas is a source of energy, used for heating, cooking, and generating
electricity. It can also be used as fuel for vehicles and as a chemical feedstock for
manufacturing plastic and other important chemicals.
In most parts of the world, natural gas is traded on its energy content. Accurate
information on natural gas composition can help generate the correct unit calorificvalue, which is important for natural gas trading. The traditional approach tonatural gas analysis is primarily based on a regular laboratory GC with a complex
multicolumn, multivalve configuration. It usually takes 6 to 20 minutes to complete
the analysis (depending on the configuration and compounds of interest to beanalyzed). Compared to a traditional GC platform, the Agilent 990 Micro GC ismuch smaller, and consumes much less carrier gas and power. More importantly,
it can provide faster natural gas analysis. The 990 Micro GC’s analysis approachfor complex gas samples is to analyze part of the sample on different channels,
then combine and normalize the results from the different channels to givecomplete information on the whole sample. Since each channel only focuses
on a subset of sample, the method is easily optimized for faster speed withoutcompromising resolution.
This study demonstrates four types of natural gas analyzers (NGA) based on
the 990 Micro GC platform. Each analytical channel in the analyzer is selected toaddress part of the natural gas constituents for specific analysis requirement. The
analytical method loaded on the analyzer is optimized in the factory and verified bya simulated natural gas standard. To show the validity of the method, all test results
will be reproduced at the customer site.

Experimental
Table 1. Channel configurations for the four NGA analyzers.
NGA Analyzers

NGA Analyzer A

NGA Analyzer A Extended

NGA Analyzer B

NGA Analyzer B Extended

G3599A Option

#120

#121

#122

#123

Channel 1

40 cm HayeSep A channel, straight

40 cm HayeSep A channel, backflush

10 m CP-PoraPLOT U, backflush

10 m CP-Molesieve 5Å, backflush,
retention time stability option (RTS)

Channel 2

6 m CP-Sil 5CB, straight

4 m CP-Sil 5CB, backflush

6 m CP-Sil 5CB, straight

10 m CP-ParoPLOT U, backflush

Channel 3

8 m CP-Sil 5CB, straight
Air, CO2
Hydrocarbons C1 to C9

Analysis

6 m CP-Sil 5CB, straight
Air, CO2
Hydrocarbons C1 to C9
H2S

Air, CO2
Hydrocarbons C1 to C12Air, CO2
Hydrocarbons C1 to C9
H2S
Permanent gas (O2, N2, He, H2)

Table 2. Composition of gas standards used for the analyzer test.
CompoundConcentration (mol%)

Simulated Natural Gas

CompoundConcentration (mol%)C6 to C10 Hydrocarbons Mixture in Helium

Nitrogen

1.01%

Hexane0.005%

Oxygen0.02%

Heptane0.005%

Carbon dioxide

5%Octane0.005%

BalanceNonane0.005%

Ethane1.5%

Decane0.005%

Propane0.4%

Isobutane

0.05%

Butane0.05%

2,2-Dimethylpropane

0.01%

Isopentane

0.03%

Pentane0.03%

2,2-dimethylbutane

0.01%

Hexane0.005%

Heptane0.005%

Octane0.005%

Nonane0.005%

MethaneCompoundConcentration (mol%)

He/Ne/H2/O2/N2 in Methane
Helium

0.10%

Neon

0.05%

Hydrogen

0.10%

Oxygen0.05%

Nitrogen

0.10%

MethaneBalance

Table 3. Conditions of each channel for sample analysis.
Column Temperature (°C)

Column Pressure (KPa)

Backflush Time (s)Carrier Gas

HayeSep A, 40 cm, Straight

Channel Type

60

260

NA

Helium

6 m, CP-Sil 5CB, Straight

70

175

NA

HeliumHayeSep A, 40 cm, Backflush

90

340

150

Helium

4 m, CP-Sil 5CB, Backflush

60

150

11

Helium

8 m, CP-Sil 5CB, Straight

150

200

NA

Helium

10 m, CP-PoraPLOT U, Backflush

80

150

12

Helium

10 m, CP-Molesieve 5Å, Backflush (RTS)

80

200

8

Helium/Argon

2

NGA analyzers

NGA analyzer extended version(Figure 1B)

Two-channel NGA analyzer(Figure 1A)

The extended version of analyzer
accommodates three channels
for natural gas analysis. It is the
combination of the basic cabinet andchannel extension cabinet, which canhouse up to four channels, controlled by
one main board.

The sample inlet is at the front of
the instrument for easy access. The
connections for the carrier gas andcolumn/sample vent are at the back of
the GC.
Through the sample inlet, sample isdrawn through (in pump mode) or
pushed into (in continuous flow mode)the sample loop. The inlet port andits connection tubing to each channel
are deactivated for inertness. This isuseful in the analysis of low-level active
components such as H2S. The inlet
port can be heated to 110 °C to prevent
sample condensation.
After the sample purges and fills the
sample loop in the injector, the injection
valve is switched on, and the sample
is injected into the analytical column
or precolumn (backflush channel) foranalysis. The sample amount injected
is controlled by the injection time.
Generally, 20 to 100 ms is used for the
injection time depending on sample
concentration. The longer the injection
time, the higher the TCD response. Eachchannel separates its target compounds
and generates a chromatogram. The
retention time (RT)of each peak is used
for compound identification. Peakresponse is used for concentration
calculation based on the ESTD curve
developed on each channel; the final
concentration for each component
in the whole sample is calculated byconcentration normalization of allquantified components.

The full-color touch screen is provided
for better usability. It shows system
configuration, and displays the actual
value and set points of the pressure
and temperature on each channel.
Users can easily read firmware version,
instrument license, IP address, and othernetwork settings. The information can bedisplayed in two languages: English andChinese. It is easy to switch the language
through the touch screen. There is astatus bar displaying different colors at
the bottom of the touch screen to show
instrument status. In addition, there isan LED light at the right side of the top
assembly to indicate instrument status.
It works like a traffic signal:•

Green for system ready at setpoints•

Yellow for not ready•Red for error•

Flashing green for run

Figure 1A. Two-channel NGA analyzer. The
red circle indicates the sample inlet.

The channel configuration of eachanalyzer depends on the composition ofnatural gas or the compounds of interest.
The molecular sieve type channel is forpermanent gases, methane, and carbon
monoxide analysis. The CP-PoraPLOT U
type is for H2S and C1 to C3 compounds
analysis. The CP-Sil 5CB channel is forthe analysis of hydrocarbons heavier
than C2.
The backflush option is used to protect
the analytical column. For example,
CO2 and moisture are easily adsorbed
on a Molesieve 5Å column, which can
result in shifting RT. It takes a longtime to condition and recover column
performance. If other hydrocarbons
enter the molecular sieve channel they
will elute after a long time. This not only
elongates the analysis time but also
increases baseline noise. The backflush
option can help trap and flush out the
moisture, CO2, and hydrocarbons (>C1)
to protect the Molesieve 5Å analyticalcolumn. The PoraPLOT U and HayeSep
A columns also have backflush options,which are used to prevent hydrocarbons
heavier than C3 entering the analyticalcolumn, to reduce the analysis timeon the two channels and to avoid theinterference of heavier components on
the next run.

Figure 1B. Three-channel NGA analyzer. The red circle
indicates the sample inlet.

3

NGA analyzer A

2

1. Air
2. Methane
3. CO2
4. Ethane

Channel 1: A 40 cm, HayeSep A, straightchannel for analysis of air, methane, CO2,ethane, and propane.
Figures 2A and 2B show chromatogramsof the five compounds on this channel.
The five targeted components were wellresolved. The analysis was completed
within two minutes.
Channel 2: A 6 m, CP-Sil 5CB, straightchannel for analysis of hydrocarbons
from propane to nonane.

1
3

4

0

0.5

1.0
RT (min)

1.5

2.0

Figure 2A. Air, CO2, and C1 to C 3 analysis on the HayeSep A straight channel.
3
3. CO2
4. Ethane
5. Propane

Hexane eluted within 50 seconds, andoctane eluted in approximately threeminutes. Nonane takes approximately
five minutes to be detected. Figures 3A
and 3B show chromatograms of
C3 to C9 components. Propane can beanalyzed on both channels as a bridgecomponent.

4

5

0

0.5

1.0
RT (min)

1.5

2.0

Figure 2B. Enlarged chromatogram for propane on the HayeSep A straight channel.
55. Propane
6. Isobutane
7. Butane
8. 2,2-Dimethylpropane
9. Isopentane
10. Pentane
11. 2,2-Dimethylbutane

6

7
9

8
0.1

0.2

0.3

10

11

0.4

0.5
RT (min)Figure 3A. C 3 to C5 on the 6 m CP-Sil 5CB channel.

4

0.6

0.7

0.8

NGA analyzer A extended

11. 2,2-Dimethylbutane
12. Hexane
13. Heptane
14. Octane
15. Nonane

Channel 1: A 40 cm, HayeSep A,
backflush channel for analysis of air,methane, CO2, ethane, and propane.
Heavier components (>C3) arebackflushed before they entered theanalytical channel, ensuring that theanalysis can be finished in a shorter time
without waiting for their late elution. This
also helps prevent their interference inthe next run. The HayeSep A column
connection in backflush option isdifferent than that in the straight option,
so the optimized analytical conditions
are different between backflush andstraight options. Higher pressure andcolumn temperature are used for fast
separation with the HayeSep backflush
option. Figures 4A and 4B show thechromatograms of NGA on the 40 cm,
HayeSep A, backflush channel.
Channel 2: A 4 m, CP-Sil 5CB, backflush
channel for analysis of C3 to C5hydrocarbons.
The components heavier than C5 were
backflushed through the precolumn
to the vent port. The analysis wascompleted within 30 seconds. Figure 5
shows the chromatogram.

11

12

13

0.3

14

1.3

15

2.3

3.3

4.3

5.3

RT (min)Figure 3B. C6 to C 9 on the 6 m CP-Sil 5CB channel.
2

1. Air
2. Methane
3. CO2
4. Ethane

1
3

4
0

0.5

1.0

1.5

2.0

2.5

RT (min)Figure 4A. Air, CO2, and C1 to C 3 on the HayeSep A, backflush channel.

4. Ethane
5. Propane

4

5
0

0.5

1.0

1.5

2.0

2.5

RT (min)Figure 4B. Propane on the HayeSep A, backflush channel.

5Channel 3: An 8 m, CP-Sil 5CB, straightchannel for C6 and C6-plus components
analysis.
Figure 6 shows the chromatogram for
C6 to C10 components on the 8 m 5CB
straight channel.
Channels 2 and 3 in analyzer A extended
not only fulfill the function of channel 2 inanalyzer A, but also extend the analysis
range of hydrocarbons up to C12 without
speed compromise. The backflush
function of the 4 m CP-Sil 5CB channel
ensures that C3 to C5 hydrocarbons areanalyzed within 30 seconds without
interference from heavy components.
The high temperature of 150 °C on
channel 3 accelerated the elution of
C6/C6-plus components. To get good
separation of hexane and pentane athigh column temperature, an 8 m CP-Sil
5CB column is used. The chromatogram(Figure 6) showed that decane eluted
at approximately 100 seconds under
the optimized conditions. This result
is the same as in our previous work on
Agilent 490 NGA analyzers.1

5. Propane
6. iso-Butane
7. Butane
8. iso-Pentane
9. Pentane

5

6
7
8
0.1

0.2

0.3
RT (min)0.4

0.5

Figure 5. C 3 to C5 on the 4 m CP-Sil 5CB, backflush channel.

10. Hexane
11. Heptane
12. Octane
13. Nonane
14. Decane

10

0

11

0.5

12

13

1.0
RT (min)Figure 6. C6 to C10 mixture on the 8 m CP-Sil 5CB channel.

6

9

14

1.5

2.0

1

NGA analyzer B

2

Channel 1: A 10 m, CP-PoraPLOT U,
backflush channel for air, methane, H2S,
ethane, and propane analysis.
The measurement of H2S is importantbecause of the gas quality specification,
the corrosive nature of H2S on
pipeline material and the effects of
H2S on utilization equipment. The
CP‑PoraPLOT U (PPU) column issuitable for H2S analysis. The 990 Micro
GC applies a proprietary deactivation
technique to its sample flowpath,
which reduces the adsorption of active
compounds, improves their peak shape,and helps achieve better detectability forthe active components. Figure 7B shows
the good peak shape of H2S on the PPU
column. H2S, methane, and air are wellresolved on this channel. The heaviest
compound analyzed on this channel ispropane. If C4 and heavier hydrocarbons
were analyzed on a PPU channel, theywould elute much later with tailing peaks
at the applied column temperature. That
is why they are analyzed on CP-Sil 5CB
channel.
Channel 2: A 6 m, CP-Sil 5CB, straightchannel for analysis of propane tononane.

1. Air
2. Methane
3. CO2
4. Ethane
5. Propane

3

4
5
0.3

0.8

RT (min)Figure 7A. Air, CO2, and C1 to C 3 hydrocarbons on the 10 m CP-PoraPLOT U, backflush channel.
3. CO2
4. Ethane
5. H2S
6. Propane

3

4

6

5
0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

RT (min)Figure 7B. Hydrogen sulfide on the 10 m CP-PoraPLOT U, backflush channel.

This channel is the same as that used by
NGA analyzer A. For chromatograms, see
Figures 3A and 3B.

7

NGA analyzer B extended
Channel 1: A 10 m, CP-MoleSieve 5Å,backflush channel.
The permanent gases in natural gas,including helium, neon, hydrogen,oxygen, nitrogen, and methane areusually analyzed on columns withmolecular sieve stationary phase.
Compared with other types of stationary
phase, Molesieve 5Å achieves baseline
separation of permanent gases at or
above ambient temperature without
expensive coolants.

Channel 2: A 10 m, CP-PoraPLOT U,
backflush channel for CO2, H2S, ethane,and propane analysis.
Channel 3: A 6 m, CP-Sil 5CB, straightchannel for analysis of propane tononane.

3

Figures 8A and 8B show thechromatograms of permanent gases on
the MoleSieve 5Å channel with helium
and argon as carrier gas. At the tested
concentration level (500 to 1,000 ppm),
helium and hydrogen are easier to detectwith argon as carrier gas.

1. Oxygen
2. Nitrogen
3. Methane

2

Both argon and helium can be used as
carrier gas. Helium is usually used fornitrogen and oxygen analysis with good
detector response. Low concentration
helium and full range hydrogen analysis
usually uses argon as carrier gas.
Molecular sieve columns are sensitive
to moisture and CO2. That is why aretention time stability option (RTS) is
placed between the dynamic electronicgas control (DEGC) module and theanalytical column. Working as an inline
filter, the RTS traps moisture and CO2
before carrier gas enters the molecularsieve column, which helps guarantee thelong‑term retention time stability of thischannel.

Channels 2 and 3 of analyzer B extended
are identical to the two channels usedby analyzer B. For chromatograms, see
Figures 7B, 3A, and 3B.

1

0.2

0.3

0.4

0.5

0.6

0.7
RT (min)

0.8

0.9

1.0

1.1

1.2

Figure 8A. Oxygen, nitrogen, and methane on CP-Molesieve 5 Å, backflush channel with helium as
carrier gas.
1. Helium
2. Neon
3. Hydrogen
4. Oxygen
5. Nitrogen
6. Methane

6

3
2
1

4
0.3

5
0.7

1.1

Figure 8B. Helium, neon, hydrogen, oxygen, nitrogen, and methane on the CP-Molesieve 5 Å, backflush
channel with argon as carrier gas.

8

1.5

Analyzer RT and response
repeatability
For correct qualification and accuratequantitation, instrument repeatability
is important. Tables 4A and 4B show
the instrument RT and response area
repeatability for analyzer A, analyzer
B, and B extended in 10 injections. For
analyzer B and B extended, RSD% of
RT and area are shown on the 10 m
CP‑PoraPLOT U and molecular sievechannel. The repeatability result of
C4 to C9 hydrocarbons on the 6 m CP‑Sil
5CB channel was the same as thaton analyzer A channel 2, as shown in
Table 4A. The RT repeatability in thethree analyzers is below 0.1%, and area
repeatability is in the range of 0.1 to 2%.
Excellent repeatability performance were
contributed by highly precise pneumaticand thermal control, and stable andsensitive TCD detection.

Table 4A. RT and area repeatability on analyzer A.
CompoundRT/min

RT RSD%

Area (mv × s)

Area RSD%

Nitrogen/oxygen

0.063

0.081

10.73

0.09

Methane

0.079

0.074

426.690.04

Carbon Dioxide

0.219

0.022

19.89

0.02

Ethane

0.366

0.014

4.12

0.05

Propane

0.203

0.004

6.685

0.02

Isobutane

0.238

0.002

0.787

0.03

Butane

0.264

0.003

0.813

0.03

2,2-Dimethyl-Propane

0.278

0.005

0.169

0.12

Isopentane

0.353

0.002

0.538

0.22

Pentane

0.396

0.002

0.555

0.11

2,2-Dimethyl-Butane

0.480

0.002

0.191

0.33

Hexane

0.679

0.003

0.106

1.0

Heptane

1.290

0.007

0.118

1.1

Octane

2.596

0.017

0.129

1.00

Nonane

5.382

0.002

0.137

1.90

Table 4B. RT and area repeatability on analyzer B/CP-PoraPLOT U channel
and analyzer B extended/CP-Moleseive 5Å channel.
10m, CP-PoraPLOT U, Backflush
Compound

RT (min)

RT RSD%

Area (mv × s)

Area RSD%

Nitrogen/oxygen

0.368

Methane

0.374

0.006

12.646

0.34

0.001

495.347

Carbon dioxide

0.36

0.414

0.006

23.8260.37

Ethane

0.487

0.007

4.748

0.37

Propane

0.932

0.014

0.879

0.46

Compound

RT (min)

RT RSD%

Area (mv × s)

Area RSD%

Helium

0.308

0.006

1.28

0.04

Neon

0.316

0.006

0.231

0.22

Hydrogen

0.333

0.006

2.137

0.06

10m, CP-Molesieve 5Å, Backflush9

Physical properties
calculation of natural gas
The economic value of natural gas isdetermined by several key physicalproperties, including calorific value,compressibility, Wobbe index, andso on. By following international
standards, these properties were
calculated from the concentration ofeach identified compound in natural
gas and its specific physicochemicalparameters. Agilent OpenLab CDS,
OpenLab EZchrom, and OpenLab
ChemStation are available choicesof chromatography data systems for
the NGA analyzer to collect data, andidentify and quantitate NGA constituents.
The quantitation results are fed to
the EZReporter software (Figure 9)
for calculation of the critical physicalproperties. EZReport 4.0 follows
ASTM D3588, ASTM D2598, GPA 2172,
GPA 2177, ISO 6976 and ISO 8973 fornatural gas analysis related calculation.
The calculation results are available forreporting, monitoring, trend plotting andexporting in EZReport.

10

Figure 9. calculation based on GPA 2172-09 by Diablo EZReport 4.0.

Conclusion
This study demonstrates four types of
NGA analyzers based on the 990 Micro
GC platform. The configuration ofeach analyzer is determined by thecomposition of the target natural gas
stream. NGA analyzer A resolves air,methane, carbon dioxide, and C2 to C6hydrocarbons within two minutes. The
analysis of heavier compounds, up toC9, can be finished in approximately
five minutes. To address the general
analysis of natural gas with heavyhydrocarbons up to C12 at highspeed, NGA analyzer A extended
equipped with three channels is used.
NGA analyzer B analyzes samples
with similar composition as thoseanalyzed by NGA analyzer A but withthe additional capability of H2S analysis.

NGA analyzer B extended can address
the analysis of permanent gases,
H2S, and other general constituents innatural gas (hydrocarbons up to C9).
The backflush options are used toprotect the analytical column fromheavier contaminants, and keep eachrun free from interference of the heavy
compounds from the previous run.

Reference
1.

Fast Analysis of Natural Gas using
the Agilent 490 Micro GC Natural
Gas Analyzer, Agilent Technologies
Application Note, publication number
5991-0275EN, 2011.

Excellent instrument performance isobserved in RT and area repeatability,
which guarantees qualification andquantitation results with a high levelof confidence.
The NGA analyzers can be used in-lab,online, at-line, and in field. They are fast,
portable, and energy-efficient solutions
for NGA analysis.

11www.agilent.com/chem
This information is subject to change without notice.
© Agilent Technologies, Inc. 2019
Printed in the USA, September 26, 20195994-1040EN