Oxygen Analysis

Elemental Combustion System
CHNS-O analyzer
It is designed to analyser organic materials such as those produced in a
chemical laboratory.
It is not designed for inorganics eg carbon in steel.
It represents an evolution in the technique of elemental analysis by “Flash
combustion-Gas Chromatographic Separation”.
During normal system operation, helium carrier gas circulates within the
analytical circuit which consists either of a combustion reactor for CHNS or a
pyrolysis reactor for Oxygen. The carrier gas brings the combustion (for
oxygen analysis: pyrolysis) gases to a gas chromatographic separation column
and TCD detector for CHNS-O analysis.
Three running modes according to the weight of sample:
1- Micro
<2 mg
2- Semi-micro
2-5 mg
3- Macro
5-10 mg
In order to modulate the oxygen injection volume.
Gases : Helium and O2 @ 99.995 % (30 -40 psi)
N2 or Air for the Pneumatics @ 99.95 to 99.99% (60-70 psi)
Purging prior to analysis
Purging is required to make sure the analyser and gas lines are free of
contamination before starting.
In addition contaminants from the atmosphere get into the instrument
and gas lines, particularly overnight and when the system is left unused.
For unused periods, it is better to go in Stand-by and not turning off the
In stand-by, oxygen and helium flow should be off. Temperature will
decrease in furnaces by 200 degrees.
C/H/N/S - Combustion Analysis
1- At the start of the analytical cycle the helium carrier gas is switched to a
volume of oxygen which is chosen by the operator depending on the size and
composition of the sample.
2- The samples are dropped automatically into the combustion reactor
prior to the arrival of oxygen in the combustion reactor.
3- The sample and tin capsule react with oxygen and combust at
temperatures of 1700-1800 °C and the sample is broken down into it’s
elemental components, N2, CO2, H2O and SO2. (interfere gases)
High performance copper wires absorb the excess oxygen not used to
combust the sample.
4- The gases flow through the gas chromatographic (GC) separation
column which is kept at a constant ± 0.1 °C temperature. As they pass
through the GC column, the gases are separated and are detected
sequentially by the TCD.
5- The TCD generates a signal, which is proportional to the amount of
element in the sample.
6- The ECS software compares the elemental peak to a known standard
material (after calibration) and generates a report for each element on a
weight basis
For Continuous Flow Isotope Ratio Mass Spectroscopy, the separated gases
are carried to the mass spectrometer interface and into the MS source.
Oxygen Analysis - Pyrolysis
Helium carrier gas circulates through the analytical circuit, which consists of
a pyrolysis reactor which is filled with a special nickelized carbon wool
contact material heated to 1080 °C, a trap for acidic gases formed by the
pyrolysis, a GC column which will separate the gas mixture, and a TCD.
1- Samples are dropped automatically into the pyrolysis reactor, break down
and the released oxygen reacts with the nickel carbon wool to form CO (2C +
O2 = 2 CO) and N2, if present in the sample.
2- The mixture of N2 and CO gases flow through the gas chromatographic
(GC) separation column which is kept at a constant temperature (± 0.1 °C).
3- As they pass through the GC column, the gases are separated and are
detected sequentially by the TCD.
4- The TCD generates a signal, or peak, which is proportional to the amount of
element in the sample.
5- The ECS software compares the elemental peak to a known standard material
(after calibration) and generates a report for each element on a weight
In the Oxygen mode, the operating gas is helium when using silver vials or
helium/hydrogen mixture when using tin vials.
Packing reactors
N, NC, NCH Reactor packing
Replace the inside every 200 samples
NCS, NCHS Reactor packing
Sulphur burns to SO3 and SO2
The SO3 must be removed (reduced)
immediately because of its reactivity
This is achieved by adding copper to the
combustion tube
Oxygen reactor packing
Helium / hydrogen (approx 5%) gas
stream enhances conversion to CO and
allows the use of tin capsules.
Copper is used to convert any CO2
formed back to CO.
Reduction reactor packing
Removes excess oxygen
Reduces NOX to Nitrogen
Nitrogen Configuration
Oxygen Configuration
Nitrogen/Carbon Configuration
Nitrogen/Carbon/Hydrogen/Sulfur configuration
Packing the H2O Trap
Packing the CO2 Trap
Thermal conductivity detector (TCD), also known as a Katharometer
This detector senses changes in the thermal conductivity of the column
eluent and compares it to a reference flow of carrier gas. Since most
compounds have a thermal conductivity much less than that of the common
carrier gases of helium or hydrogen, when an analyte elutes from the column
the effluent thermal conductivity is reduced, and a detectable signal is
The TCD is often called a universal detector because it responds to all
compounds. Also, since the thermal conductivity of organic compounds
are similar and very different from helium, a TCD will respond similarly to
similar concentrations of analyte. Therefore the TCD can be used without
calibration and the concentration of a sample.
The TCD is also used in the analysis of permanent gases (argon, oxygen,
nitrogen, carbon dioxide)
Sulfur containing compounds: 30 min
Oxygen: 10 min
6-8 min
4 min
2 min
Leak checking
Once the reactors and traps are installed, a static leak check should be
1- Place a cap on the He vent, wait 10-15 seconds for the pressure to stabilize,
2- then turn the He regulator off (counter clockwise). If there are no leaks the
pressure on the He gauge will remain constant, if a leak is present the gauge
will begin to fall.
Sometimes the gauge will fall slightly, then remain constant. This only means
that not enough time was allowed for the pressure to stabilize.
If the pressure remains constant for 30-45 seconds, the ECS is tight enough to
operate. If the pressure drops continuously after capping the vent, a leak is
present in the system and must be sealed.
The recommended flow rate for the helium carrier gas is between
80-120 ml/min.
The pressure required to maintain the flow rate should not exceed 150
kPa. If the pressure is over 150 kPa there is a risk of breaking the quartz
combustion reactor.
A 2 meter GC column generally (column packing can vary slightly)
needs 80-90 kPa to achieve a 100 ml/min flow rate,
and a 3 meter column 100-110 kPa.
A 4 meter column, due to the high backpressure, needs about 125-140
kPa to achieve a flow rate of 80-100 ml/min.
The procedure to set the oxygen flow rate is different than the one for
The pressure in the oxygen volume, along with the size of the volume, will
determine the amount of oxygen introduced for each sample combustion.
This is very important to control and understand because there must be enough
oxygen to combust the sample plus a small excess.
Too little oxygen will result in poor or incomplete combustion, too much oxygen
will consume the reduction (copper filled) reactor very quickly.
a flow rate of 20-30 ml/min
Nitrogen/Carbon/Hydrogen/Sulfur Analysis
These are the general operating parameters for automatic
Nitrogen/Carbon/Hydrogen/Sulfur analysis.
Modifications may be necessary for specific sample types or
Mode: CHNS
Left furnace: 1000-1020 °C
Right furnace: Off
GC oven temp.: 65-80 °C
GC Column: 2 meter
Oxygen Analysis
These are the general operating parameters for automatic Oxygen analysis.
Modifications may be necessary for specific sample types or applications.
Mode: Oxygen
Left furnace: 1070-1100 °C
Right furnace: Off
GC oven temp.: 65-80 °C
GC Column: 1 meter molecular sieve column
Sample Preparation Tools
Sample preparation plate: a metal plate with holes for holding
sample capsules.
Forceps: for closing the solid sample capsules
Spatula: for filling the capsules.
Capsules for solid samples
Tin & silver capsules
Capsules for liquid samples
Add V2O5 for sulfur containig materials to increase the sensitivity of
detector. (sample: V2O5, 1:5)
Solid sample preparation technique
- Select the appropriate capsule for the amount of sample material to be
- Tare the capsule on the microbalance and place it on the sample preparation
- Use the spatula to fill the capsule with your sample material.
- Place the sample back on the microbalance, wait for it to stabilize, and record
the weight.
- Remove the capsule and place it back on the sample preparation plate.
Using the forceps, squeeze the top of the capsule closed. Grabbing the capsule
on the top at either end, push the ends toward the middle, forming a “Z” shaped
- Holding the capsule at the bottom with one set of forceps, fold down the top
with the other set of forceps. Form the capsule into a roughly spherical shape.
- Avoid flattening the capsule as this can cause jamming in the autosampler.
Liquid sample preparation technique
The easiest way to handle liquid samples is to treat them as solid samples if
For example, if the liquid sample is relatively non-volatile, it can be absorbed
onto a solid material such as Chromosorb W.
-Add Chromosorb W (enough to absorb the liquid) to a standard solid
capsule and tare it on the microbalance.
-Add the liquid by pipette, syringe or wire, taking care that the sample is
completely absorbed into the Chromosorb W.
- Seal the capsule as described above and place it on the microbalance.
-When it stabilizes, take the weight.
- If the sample weight does not stabilize, then the samples are not suitable for
this technique and must be sealed in the special liquid capsules.
Samples such as gasoline, solvents, etc. must be hermetically sealed in a
special liquid capsule.
Running an Analysis
If the ECS is in standby, press “Work”, then confirm by pressing “Enter”. The
ECS will immediately begin to heat the furnaces to the “Work” condition
When the set operating temperatures have been reached, all LED’s will turn
green, indicating the ECS is ready for operation.
In the “Work” condition the helium and oxygen flow rates will return to the
operating setpoints, and the purge flow will be turned on. Set the ECS for
Remote operation by pressing the “Remote” button, then “Enter”. REM will
appear in the display, indicating ECS is in remote mode and ready to accept an
external start signal.
Load samples in the autosampler. If you are using the open carousel pneumatic
autosampler, simply add samples to the carousel starting from the position
which is aligned over the hole and going in a counterclockwise direction.
Quartz reactor
Oven can be set between 30-110 °C .
It can also be turned off.
A 2 meter GC column is recommended for
CHNS analysis.
For oxygen analysis a 1 meter molecular
sieve column is used.
There are two sensitivity ranges for the TCD (Thermal Conductivity Detector),
low and high. If you are analyzing large or concentrated samples, such as
organic compounds, plant tissue, etc. then the low range with very little gain is
required. If soils or sediment samples are analyzed, then a high gain is required
to get enough detector sensitivity.
Analytical Troubleshooting
Normal chromatogram
Nitrogen peak appears to get larger on subsequent samples
The reduction reactor is exhausted and oxygen is eluting from the GC column.
Nitrogen and oxygen have similar retention times, so the combined N2
and O2 peaks make it appear as though the N2 peak gets larger.
The O2 may also first appear as a shoulder on the trailing edge of the N2 peak.
Poor or incomplete combustion
Nitrogen peak will tail into the carbon peak.
Carbon peak will have a long tail, indicating
a long, slow combustion or pyrolysis.
Possible causes:
• Sample start time too long, sample drops
after O2 has passed.
• Not enough O2 for sample combustion.
• No O2 injected for sample combustion
• Large leak in analytical circuit.
Baseline drift after CO2 peak
• H2O trap is exhausted and water is slowly
bleeding through the GC column and
detector. Replace H2O trap packing with
fresh material.
Second peak after Nitrogen
Carbon dioxide trap is exhausted and CO2
is breaking through and passing through the GC
column and detector: repack CO2 trap with
fresh material.
Long tail on Sulfur peak
Possible causes:
• Combustion is not optimized, reset sample
drop time
• Sulfate in sample, add V2O5 to the sample
• Dead volume in combustion reactor packing,
repack reactor.
Peak broadening
Possible causes:
• Slow carrier gas flow
• Dead volume in reactors or traps
C 1 6 H 1 1 C l2 N 3 S
M o l. W t.: 3 4 8 .2 5
Found values:
C : 55.39%
H: 3.24%
N: 11.89%
Caculated values:
16* 12 = 192 / 348.25 = 55.13 %
11 * 1 = 11 / 348.25 = 3.15 %
3 * 14 = 42 / 348.25 = 12 %
32 / 348.25 = 9.18 %
Calculated – found value = 55.39 -55.13= 0.2
0.2 / 348.25 = 0.00057 * 100 = 0.057%
0.057% < 0.4%
Analytical Strategies for Characterizing Nanoparticle’s Surface
Anal Bioanal Chem. 2010 February ; 396(3): 973. doi:10.1007/s00216-009-2996-1.
Analytical Strategies for Characterizing Nanoparticle’s Surface
By analyzing the percentage of C, N, S and H elements in modified and
unmodified NPs, one can get the quantitative information on the surface
composition of NPs.
to optimize the reaction conditions such as solvent,
Relative high percentage of C, H and N content was found if the
reaction was performed well.
a higher percentage of N was detected in all NPs
The End
Thanks for your attention

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