Application Note

One-Minute NGA Analysis Based on a
Backflush-to-Detector ChannelAuthor
Jie Zhang
Agilent Technologies, Inc.

Introduction
Natural gas analysis is a key application for the Agilent 490 micro GC. Currently,
there are four types of NGA analyzers available based on different configurations
to address various NGA analysis requirements. Application Note 5991-0275 lists
currently available analyzer configurations, their targeted NGA composition, andtypical analysis time. The total analysis time for the 490 Micro GC NGA analyzer
is usually determined by how long it takes for the heavy hydrocarbons to elutefrom the 5CB columns. The more complex the matrices and the heavier thehydrocarbons, the longer the analysis time. If the analysis time is not long enough,the heavier components from the previous injection will interfere with the baselineof the following injection. That is why it takes 200–300 seconds for samples withcomponents up to C12.
For components heavier than C6 (up to C9 or even C12), some customers areinterested in detailed identification and corresponding concentration of eachheavy component, and some only need to know the total concentration of C6 andC6+ components. In the latter situation, there is no need to separate each heavycomponent.
For customers who are only interested in the total concentration of C6 and heavier
components, and are sensitive to analysis speed, there is a column channel
designed for fast and complete elution of C6/C6+ components without interferingwith the separation of lighter hydrocarbons such as propane, i-butane, n-butane, andneo, i-, and n-pentane on the same column. This developed backflush-to-detector
channel, coupled with existing channels specializing in methane analysis, results
in a new configuration for faster NGA analysis with total analysis times under60 seconds.

Instrumentation
The configuration of the 490 Micro
GC NGA analyzer is comprised of one
straight Hayesep channel and one
CP‑Sil 5CB backflush-to-detector (BF2D)
channel.
The separation of air, C1, and C2components is performed on an
HSA channel. Figure 1 depicts thechromatogram of an NGA sample on
this Hayesep channel, completing theanalysis within 60 seconds.

Table 1. Detailed method for NGA analysis on HSA and BF2D 5CB channels.
40 cm, straight Hayesep

8 m BF2D CP-Sil 5CB

Carrier gas

Channel typeHeliumHeliumInjector temperature

110 °C

110 °CInjection time

40 ms

40 ms

Column head pressure

280 kPa

150 kPa

Column temperature

80 °C

72 °C

BF timeNA6sSample
Component

Concentration

1

Peak no.

N2

5%

2CH4Bal3CO21.50 %

4

C2H69%

5n-C6H14

0.10 %

6C3H86%

7i-C4H103%

8n-C4H102%

9i-C5H120.50 %10n-C5H120.50 %

700

2600

Peak height (mv)500
4003002001100-5043561525RT(sec)3545

Figure 1. Nitrogen, methane, carbon dioxide, and ethane separation on a straight HSA channel.

255

The backflush (BF) time should beadjusted to ensure that interestedhydrocarbons enter the analyticalcolumn completely, and that C6/C6+components are backflushed into theprecolumn without any penetration into
the analytical column. This BF time wasadjusted in-factory based on a given
set of conditions. This can be used
as a reference/starting point for thecustomer’s further optimization, basedon their own test parameters. In thisexperiment, the BF time was six seconds,
and the total analysis time was below
60 seconds, with good separation ofthe C6/C6+ combined peak, as well as
air/methane, propane, iso-/n-butane, andiso‑/n‑pentane peaks.

90
680
70
Peak height (mv)

Figure 2 shows the chromatogramof C6/C6+ components, propane,i-butane, n-butane, iso-pentane, andn-pentane separated on an 8 m BF2D
5CB channel. C6 and C6+ components
are trapped first by the precolumn, thenbackflushed through the reference
column to the TCD, resulting in one peak,shown in the chromatogram (peak 5).
The peak was inverted real-time toa positive peak for easy integrationin 490 Micro GC instrument control
software: Agilent OpenLab CDS, OpenLabChemStation, OpenLab EZChrom, andProstation for the 490‑PRO Micro GC.
This signal-reverse function can berealized in a preset time range—a featuredesigned for the backflush-to-detector
channel. Compared to the previous
signal-reverse function applicable only tothe whole run time, it is more flexible.

605040730
820
10014109519242934
RT (sec)

39444954

Figure 2. Chromatogram of C6/C6+ compounds, propane, iso-butane, n-butane, iso-pentane, andn-pentane.
Table 2. RT and area repeatability of 15 injections using the BF2D configuration.
RT (min)

RT RSD%

Area (mv*s)

Area RSD%

NitrogenCompound

0.049

0.1

24.03

0.06

Methane

0.058

0.071

321

0.06

Carbon dioxide

0.128

0.041

9.60

0.12

Ethane

0.193

0.034

62.00

0.09

Propane

0.455

0.034

34.48

0.12

i-Butane

0.531

0.034

19.80

0.12

n-Butane

0.589

0.032

13.59

0.12

i-Pentane

0.786

0.032

3.71

0.12

n-Pentane

0.879

0.031

3.92

0.13

C6/C6+

0.282

0.030

0.62

0.14

Table 2 lists the RT and area repeatabilityof individual components on twochannels. The area repeatability wasapproximately 0.1 %. RT repeatability
was between 0.03 % and 0.1 %,demonstrating the excellent repeatabilityof the new configuration. Such precision
guarantees qualification and quantitation
results with level a high confidence.

3If the same sample was run on NGA
analyzer A extended, the general analysis
time was approximately 200 seconds(up to n-dodecane, according toAN:5991‑0257). The analysis speed,
based on the BF2D configuration, isapproximately 60 % faster than that ofanalyzer A extended. Table 3 shows
the quantitation results and calorificvalue calculations from NGA analyzer A
extended and the BF2D configuration.
For high-concentration components,
the difference in quantitation results
is as small as 0.03 %. For C6/C6+components, the quantitation differences
are as low as 0.3 %, as shown in Table 3.
By following ISO 6976 standard, thecalculated calorific value difference
based on NGA analyzer A extended andthe BF2D configuration is negligible.

Conclusion
A natural gas analyzer was developed
based on a new type of CP-Sil 5CB
backflush-to-detector channel coupledwith a straight Hayesep channel. Thisis the fastest NGA analyzer basedon a 490 Micro GC platform. It can
provide total concentration for C6/C6+components, and detailed information
about air, methane, carbon dioxide, andC2–C5 compounds in NGA. It is a choicefor customers who need a fast andreliable solution for NGA analysis.

Reference
1.

Fast analysis of natural gas using
the Agilent 490 micro GC naturalgas analyzer, publication number5991‑0257.

www.agilent.com/chem
This information is subject to change without notice.
© Agilent Technologies, Inc. 2018
Printed in the USA, July 9, 20185991-9401EN

Table 3. C3-C5 hydrocarbon quantitation results and NGA calorific value based on NGA analyzer A
extended and an 8 m CP-SIL 5CB BF2D channel.
ESTD concentration
by NGA analyzer A
extended

ESTD concentration byBF2D channel

Quantitation resultcomparison (%)

Propane6.00006.005699.91 %

i-Butane

3.00053.001599.97 %

n-Butane

2.00042.001299.96 %

i-Pentane

0.500110.5002499.97 %

n-Pentane

0.49989

0.49982

100.01 %C6/C6+

0.100590.10025

100.34 %Unit

traditional

Backflush to detector

CompoundCalorific value
Compressibility
Water mole.

%

Mole mass

kg/kmol

Dry

Saturated

Dry

Saturated

0.99680.99620.99680.99622.31

2.31

21.2939

21.2182

21.2953

21.2195

Relative density, ideal

0.73520.73260.73530.7327

Relative density, real

0.73730.73510.73740.7352

Gas density, ideal

kg/m3

0.88520.88210.88530.8821

Gas density, real

kg/m3

0.88810.88540.88810.8855

Superior heating value (volume real)

MJ/m3

44.06

43.07

44.07

43.07

Inferior heating value (volume real)

MJ/m3

39.98

39.08

39.98

39.08

Superior heating value (volume ideal)

MJ/m3

43.92

42.91

43.93

42.91

Inferior heating value (volume ideal)

MJ/m3

39.85

38.93

39.85

38.93

Superior heating value (mass)

MJ/kg

49.62

48.64

49.62

48.64

Inferior heating value (mass)

MJ/kg

45.02

44.13

45.02

44.13

Superior heating value (molar)

kJ/mol

1,056.56

1,032.15

1,056.63

1,032.22

Inferior heating value (molar)

kJ/mol958.58936.44958.64936.5

Wobbe Index (real)

MJ/m3

51.32

50.24

51.32

50.24

Wobbe Index inferior

MJ/m3

46.56

45.58

46.56

45.58