EMIA and EMGA consist in the generation of gases from the sample under specific conditions and in the measurement of the gases emitted using NDIR or TCD. Key components are the furnace that is the source for gas generation and the detectors allowing the measurement of these gases.
There are 3 types of furnaces used for gas generation on EMIA and EMGA. Carbon and Sulfur can be analyzed using induction or resistance furnace. Oxygen, Nitrogen and Hydrogen are using an impulse furnace. Below is a description of these different furnaces.
The ceramic crucible in which the sample and accelerators are introduced is surrounded by an induction coil. Heating is performed by electromagnetic induction under a flow of oxygen and high temperature is reached, typically over 2300°C. The reaction of carbon and sulfur with oxygen at high temperature generates CO, CO2 and SO2.
The exact temperature of the sample is not known with an induction furnace, however it is possible to control the applied current and change it during an analysis cycle to optimize the measurement.
The following example illustrates this function and shows a steel sample polluted with base oil. The Temperature curve in red refers to the applied current. A low temperature level is set to release the carbon from the surface of the sample that is due to contamination and a higher level for the analysis of the bulk carbon and sulfur after steel is melted.
The sample is placed into a ceramic boat that is positioned in the middle of a horizontal furnace where temperature is uniform. Heating can be done up to 1450°C and is performed using a resistance and a thermocouple for temperature control. In a resistance furnace the exact temperature at the sample is controllable and can be changed during a measurement. As for the induction furnace, heating is done in a flow of oxygen and the reaction of carbon and sulfur with oxygen at high temperature generates CO, CO2 and SO2.
In a resistance furnace, the sample can be slowly burned and programmable temperature curves can be set up. This allows to precisely separate signal from elements of the surface and of the bulk of a material.
Typical applications for resistance furnace include toner for copy machines or the measurement of free carbon in Silicon Carbide.
Impulse furnace is used for O/N/H analyzers. The sample is placed in a graphite crucible that is positioned between 2 electrodes. A very high current is generated and maintained by a series of voltage pulses to melt the sample and the eventual fluxes inside the crucible. The crucible itself conducts electricity but does not melt as carbon sublimes at 5800°K only, higher than the melting temperature of all metals including W.
Surrounded by inert gas in the graphite crucible, elemental hydrogen H and oxygen O are reduced to H2 and O2 gases while the Oxygen reacts with the carbon from the crucible and generates CO.
The output current of the impulse furnace is monitored and it can possibly be changed during an analysis sequence to optimize the method and perform separation of surface contamination from bulk content or for the identification of different forms of the elements as shown in the example below.
EMIA and EMGA are using non-dispersive infrared detector (NDIR) and thermal conductivity detector (TCD).
The main components of an NDIR sensor are an infrared light source, a sample chamber (cell), an optical filter and an infrared detector.
The gas in the sample chamber causes absorption of specific wavelengths according to the Beer–Lambert law, and the attenuation of these wavelengths intensity is measured by the detector to determine the gas concentration.
An optical filter isolates the wavelength absorbed by the gas molecule of interest. The signal from the source is chopped or modulated to offset thermal background signals from the desired signal. NDIR detectors are used for CO2, CO, SO2 and H2O (when H is to be measured by NDIR). For CO2, two detectors are used with two optimum filters to measure both low and high concentrations with the best accuracy possible.
NDIR is a core technology of HORIBA who has pioneered NDIR analyzers to provide market leader instruments. It is currently used in multiple instruments covering a wide range of applications: Motor Exhaust Gas Analyzers, Ambient NOx Monitors and Stack Gas Analyzers.
A TCD is using a Wheatstone bridge (see schematic below).
The reference cell is filled with the carrier gas only, the sample cell will see a change when the combustion takes place and the measured gas is carried and introduced in it.
TCD has no selectivity – only a change in resistivity is measured. If more than one gas is introduced in the cell the measurement will be the result of changes in resistivity from all gases. Any gas that is not of interest must be carefully filtered prior detection to ensure accurate and reliable results.
TCD is used for measurement of N2 and can be used for H2, especially for applications requiring high sensitivity.
The principle of operation requires that the measured gas and the carrier gas have very different conductivities for optimum sensitivity. Helium is required for the determination of nitrogen since argon thermal conductivity is too close to provide good sensitivity (see table below). The use of argon will lead to 100 times less sensitivity. For dedicated Hydrogen analyzer, argon is used as the difference is greater than with helium. The sensitivity by using TCD for H2 is better by a factor of at least 10 which explains why the TCD is the technique of choice for low hydrogen detection in the most demanding applications.
|Gas||Thermal conductivity (k/10-4Wm-1K-1)|
|He (as carrier)||1649|
|Ar (as carrier)||201|
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