Determination of Hydrocarbons in Anhydrous Ammonia By Gas Chromatography

GBH Enterprises, Ltd.

Plant Analytical Techniques
ANHYDROUS AMMONIA:

DETERMINATION OF
HYDROCARBONS BY GAS
CHROMATOGRAPHY

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ANHYDROUS AMMONIA:

DETERMINATION OF
HYDROCARBONS BY GAS
CHROMATOGRAPHY

WARNING AND SAFETY PRECAUTIONS:
VAPOR: CONCENTRATED VAPOR CAN CAUSE SKIN IRRITATION AND
HIGH CONCENTRATIONS MAY GIVE SERIOUS BURNS TO EYES,
RESPIRATORY PASSAGES AND LUNGS.
CAN FORM EXPLOSIVE COMPOUNDS WITH CHLORINE, HYPOCHLORITE
OR MERCURY.
1

SCOPE AND FIELD OF APPLICATION

The method is suitable for the determination of hydrocarbons from C1 to C4
(see 6.4.2) in gaseous ammonia, or in mixtures of ammonia and air. It is
valid for concentrations in the range 10-10000 ppm.
The method may be used for the analysis of the atmosphere from a ships hold
After purging with ammonia and for the analysis of gasified liquid
anhydrous ammonia during or after loading. In these cases, hydrocarbon
contamination may arise from the previous cargo of the vessel, the nature
of which should be ascertained prior to carrying out the analysis.
2

PRINCIPLE

A portion of the gaseous sample is analyzed by gas chromatography, each
hydrocarbon being identified by retention time and quantified by comparison
of peak height with that of an external standard.
The gas chromatograph is fitted with a column maintained at a temperature
of 75oC, packed with ethylene glycol dimethacrylate beads, and a flame
Ionization detector.


3

MATERIALS REQUIRED

3.1

Carrier gas and auxiliary gases

3.1.1 Argon at not less than 250 KPa pressure
3.1.2 Hydrogen at not less than 400 KPa pressure.
WARNING - This material is potentially explosive (see Clause 8).
3.1.3 Air at not less than 400 KPa pressure
3.2

Materials for the preparation of calibration samples.
Each of the hydrocarbons to be determined (see 6.4.2) should be
available at a purity of 96% or greater.

3.3

Materials for the preparation of columns

3.3.1 Ethylene glycol dimethacrylate beads, particle size approximately 120 µm
to 150 µm. Porapak T (100 - 120 mesh) manufactured by Waters
Associates Inc.
is suitable.

4

APPARATUS

4.1

General Description

4.1.1 Gas chromatographic system
The system comprises a gas chromatograph with a flame ionisation
detector and ambient 6-port gas sample valve.
4.1.2 Characteristics of the assembly
4.1.2.1 Column temperature
The column oven should be maintained at a temperature between 70 and
80°C, with a tolerance of +/- 0.5oC or less. Visible indication should be
given if these criteria cannot be met due to malfunction.


4.1.2.2

Detector temperature

The detector should be heated to a temperature between 160 and 180°C,
with a tolerance of +/- 2oC or less. Visible indication should be given if
these criteria cannot be met due to malfunction.

4.1.3 Gas Controls and Flow Measurement
Pressure regulators capable of controlling gas flows as follows:
(a)
(b)

Argon - flow rate of approximately 25 ml/min at a pressure of
55 KPa.
Hydrogen - flow rate of 35ml/min at a pressure of 140 KPa.

(c)

Air - flow rate of 200 ml/min at a pressure of 200 KPa.

Pressure regulators supplied by manufacturers of gas chromatographic
equipment will be adequate providing they offer discrimination of 5 KPa or
better.
4.2

Injection equipment

4.2.1 Injection device. Polythene bag fitted with a manually operated isolation
valve, of approximately 10 litres capacity.
4.2.2 Injection system. A six-port gas sample valve fitted with sample loop of
5ml capacity, at ambient temperature.
4.3

Columns

4.3.1 Construction. The column shall be constructed of stainless steel tubing
(washed with acetone and dried prior to packing) of internal diameter 4mm
and packed length 3m. When packed with stationary phase (see 4.3.2.1) it
should have a lifetime of at least 1 year under the specified operating
conditions.
4.3.2 Packing
4.3.2.1

Stationary phase


Ethylene glycol dimethacrylate beads (3.3.1) are used without
modification.
4.3.2.2

Conditioning. Purge for 5 hours with argon (3.1.1) at a temperature
of 175oC using a flow rate of between 40 ml/min and 50ml/min and
with the column outlet disconnected from. the detector.

4.3.3.1

Efficiency and resolution
The performance criteria specified in 4.3.3.1, 4.3.3.2, and 6.3.3
shall apply to both the calibration mixture and the samples.

4.3.3.2

Resolution. The value of the Ratio

shall not exceed 0.05.
The valley is measured from the baseline as shown on the typical
chromatogram (see figure 1).
4.4

Flame ionization detector
The detector should be capable of satisfactory performance (see 4.3.3
and 6.3.3) with a carrier gas flow rate of 25 ml/min.
The detector/amplifier measuring system shall be capable of detecting
5ppm of propane in 5ml of the sample under the conditions of the method.
An amplifier fitted with a binary attenuator or similar, and with a time
constant of 0.5s or less is recommended.

4.5

Potentiometric recorder, having the following characteristics:

(a)

0.4s (nominal) time for 0-95X full scale deflection.

(b)

Chart width 200mm or greater.

(c)

Suitable for a nominal chart speed of 2mm/min.


(d)

Dead zone within 0.2X full scale deflection.

(e)

Nonlinearity less than 0.3% full scale deflection.

(f)

Noise less than 1% of full scale.

The measurement of peak heights requires a scale with discrimination of
lmm.

4.6

Integrator. If an integrator is used, at least 100 counts should be
obtained for each peak of interest. Most systems available for gas
chromatographic use are suitable.

4.7

Gas-tight syringe. 1 litre capacity syringe with a body of glass or inert
plastic, and graduated at 50ml intervals. This is used for the preparation
of calibration samples.

5

Sample
The sample is taken into a polythene bag fitted with an isolation valve
(see 4.2.1). The sample may be stored prior to analysis for up to 1 hour
without deterioration.

WARNING: Ammonia is both toxic and explosive (see 8.1 and 8.2)

6

PROCEDURE

6.1

Setting up the apparatus

6.1.1 Injection system. The gas sample valve should be maintained in the “load”
position except for the 15 seconds immediately following sample
introduction.


6.1.2 Oven and column
6.1.2.1

Column temperature. Maintain the temperature at 75oC (nominal).

6.1.2.2

Rate of flow of carrier gas

(a)

Column. Adjust to obtain a time in the range 25s to 35s for the
‘air’ peak, calculated from the elution time of methane. This
corresponds to an inlet pressure of approximately 55 KPa.

(b)

Auxiliary gas flow rates. Follow the apparatus manufacturers
recommendations.

6.1.3

6.2

Detector temperature. The detector zone temperature should be
maintained at 170°C (nominal).

Calibration. Calibrate by external calibration.

6.2.1 Standard mixtures
6.2.1.1

Purity of components. Each of the hydrocarbons to be determined
should be available at a purity of 96% or greater. The argon diluent
should be of the same purity as that used as carrier gas.

6.2.1.2

Operating frequency. Carry out the calibration prior to analysis, or
at least once per week for routine operation, and recalibrate
whenever the apparatus has been shut down and restarted.
The calibration is satisfactory if the variation from the previous
calibration is within +/- 10% (relative).

6.2.1.3

Preparation of standard mixtures. Prepare calibration standards by
sequential dilution of the purge gaseous hydrocarbon (see 3.2)
using a gas-tight syringe (see 4.7) as follows.
Ensure that the syringe is clean and dry; expel air as fully as
possible by depressing the plunger. Connect the hydrocarbon
source using the shortest possible length of PVC tubing.


WARNING: Hydrocarbon gases are explosive (see 8.2); this operation
should be carried out in a fume cupboard
With the syringe tap in the purge position, introduce a steady flow of gas
to remove air from the line.
Operate the tap and allow gas to flow into the syringe. Fill to just below
the maximum calibration mark, then isolate the hydrocarbon source and close
the syringe tap. Disconnect the line at the syringe, open the tap, completely expel
the contents of the syringe, then close the tap.
Reconnect the line and repeat the procedure described in the preceding two
paragraphs twice further, then fill the syringe with hydrocarbon gas, isolate the
source, close the tap, and disconnect the line at the hydrocarbon source.
Expel gas to the required volume (minimum lOOm1, dependent upon the desired
dilution ratio), allow a few seconds for equilibration on atmospheric pressure,
then close the tap.
Connect the line to a source of argon. With the tap in the purge position,
introduce a steady flow of gas to remove air and residual hydrocarbon from
the line.
Operate the tap to allow gas to flow into the syringe. Fill to the maximum
calibration mark, then isolate the argon supply and close the tap.
Effect further dilution to the desired concentration by successive use of
the procedure outlined in the preceding three paragraphs.

6.2.1.4

Conditions specific to the use of the standard mixtures
Use the procedure described under 6.3 for chromatographing the
standard mixtures (see 6.2.1.3) and measuring the results, except
that the standard mixture may be fed into the gas sample valve
directly from the syringe.

6.2.2 Presentation of calibration data


6.2.2.1

Calibration graph

(a) Prepare a graph for each hydrocarbon in which the product of peak
height (mm) and amplifier attenuation is plotted against
concentration, expressed as ppm, of the standard mixture.
(b) The calibration graph should take the form of a smooth line such
that no point lies more than 5% (relative) from the mean line. If
such a point is found, check the whole apparatus for faults and
repeat the entire calibration procedure.

6.3

Test

6.3.1

Introduction of the test portion, operation of equipment and time
between injections.
With the gas sample valve in the purge position, connect the sample bag
by the shortest practicable length of tubing and open the isolation valve.
Purge at least 200 ml of sample through the valve, then switch the valve to
the inject position and close the isolation valve. Leave the gas sample
valve in the inject position for 15s.
In operating the equipment, make and record attenuation changes in order
that peaks remain between 10% fsd and 90% fsd.
Allow 2 hours between successive injections unless it is known that elution
of all impurities will be complete in a shorter interval.

6.3.2 Recording
(a)

Base line drift. The baseline drift throughout the entire
analysis, as shown on the typical chromatogram, shall not be
greater than 2% of full scale deflection at the lowest attenuation
used in the analysis.

(b)

Base line noise and wander. The baseline noise and wander shall
not be greater than 2% of full duration of the chromatogram.

(c)

Retention time. The retention times shall fall between the
following limits:


(d)

(e)

6.4

Peak height of sample components. The peak height to be
measured for each component shall lie in the range 10-90% fsd.
Integrator counts. If digital integration is used, not less than
100 counts shall be obtained for the smallest peak of interest.

Examination of the chromatogram

6.4.1 Typical chromatogram. A typical chromatogram for the analysis is shown
in Figure 1.
6.4.2 Identification. A list of possible contaminants, together with their
typical absolute retention times, is given below (see also figure 1). The
retention times measured for the calibration mixtures should be used for
identifying the sample peaks.


6.4.3

Determination of hydrocarbon content
Identify and disregard the ammonia peak as shown on the typical
chromatogram (figure 1). Identify each peak by comparison of its
retention time with that obtained for the calibration mixtures (see
6.4.2). draw in the peak base, and measure the height, in
millimeters, to the nearest millimeter.
For each peak, calculate peak height x attenuation and read from
the appropriate calibration graph the concentration of the
hydrocarbon.
REPRESSION OF RESULTS
The considerations given in 7.1 shall apply in the interpretation of
results obtained for the determination of hydrocarbon content.

7.1

Repeatability
Statistical data unavailable at present.


8

HAZARDS

8.1

Toxicity
a)

8.2

Ammonia is a toxic irritant: occupational exposure limit 25 ppm
(see 10.1 ~135, 10.2 and 10.3).

Explosively
a)

Ammonia is explosive when mixed with air at concentrations
ranging approximately from 16% to 26% (v/v) (see 10.1 ~135, 10.2
and 10.3).

b)

Hydrocarbon gases and vapors are explosive when mixed with air
(see 10.1).

(c)

Hydrogen is explosive when mixed with air at concentrations
between 4% and 75% (v/v) (see 10.1 ~280).

TEST REPORT
The test report shall include the following particulars:
a)

the concentration and identity of each hydrocarbon present,
expressed as ppm by volume to two significant figures;

b)

any unusual features noted during the determination;

10

BIBLIOGRAPHICAL RFFERENCES

10.1

Muir G D, ed. Hazards in the Chemical Laboratory (second edition). The
Chemical Society, 1977.

10.2

Laboratory Services Permanent Instruction LSG/201.

10.3

Anhydrous Ammonia Hazard Sheet


Figure 1 : Example Chromatograph


Determination of Hydrocarbons in Anhydrous Ammonia By Gas Chromatography

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