Analytical Instrumentation Diode array spectroscopy


DAS uses a tungsten, xenon flash, or deuterium light source as the optical source, and uses the continuous spectrum over a wavelength of 190 – 800 nm or 400 – 1100 nm. The analyser uses fibre optics to transmit the light through the sample in the flow cell. The adsorption spectrum is generated by a holographic grating and a diode array with 1024 detectors. It is a continuous spectrometer without moving parts.


The fibre optics allows the sample cell to be installed away from the source, electronics and detector or the use of a direct sample probe. Due to the use of a holographic grating in combination with 1024 diode array detector the instrument is capable of analysing multi component species without the use of moving parts. However, it suffers from the same interferences as described with the UV technique and possesses less linearity compared to spectrometers which employs discrete spectral line light sources.


X-ray fluorescence


XRF is used to detect the total sulphur content of fuels. Instead of oxidising or reducing the fuel to SO2


or H2 S


respectively, X-rays can be applied to measure individual sulphur atoms regardless of the compounds. The ASTM D2622 method (American Standard Test Methods) describes sulphur absorption at 0.5373 nm and a reference at 0.5190 nm or 0.5437 nm. The method suggests a wavelength dispersive X-Ray fluorescence spectrometer (WDXRF), equipped with X-ray detection in the wavelength range of 0.52 – 0.55 nm.


Because X-rays are high energy waves, they have very short wavelengths. The wavelengths are short enough that they can be utilised to ionise individual sulphur atoms by exciting their low energy electrons to higher energy orbitals. Then, the ionised sulphur atoms relax back to their ground state and fluoresce the extra energy as photons with a wavelength equal to the energy difference between the two orbitals. The fluoresced light can be measured with an X-ray detector and correlated with the total sulphur concentration.


XRF is not ideal for sulphur detection below 20 ppm, yet ideal for laboratory analysis for samples over 50 ppm sulphur content. It is not ideal for on-line applications because of expensive maintenance due to complicated optics system. Replacement of X-ray tube is required every one to two years and this is expensive.


Dry colorimetric detector


Like classical wet colorimetric techniques, the Sensi-Tape detectors are colorimetric based, but are dry reaction substrates which serve as gas collecting and analysing media. Individually formulated for a specific gas or family of gases, each Sensi-Tape is a nontoxic, proprietary chemical reagent system. When exposed to a target gas, the tape will change colour in proportion to the amount of gas; the higher the concentration, the darker the stain will appear. The change in colour, or stain, on the Sensi-Tape is read by a photodiode (see the left image), and then compared to a standard response curve pre-programmed into the system.


The organically bound sulphur is first converted to H2 S in a high temperature reactor tube. The H2


formed reacts with the moist paper tape impregnated with chemicals. A photodiode detects the change in colour of the tape as H2


S S passes through the tape and forms a stain. The tape detection Conclusion


On one hand, stringent environmental regulations require reducing the sulphur content of the petroleum products and flue gas to low levels. On the other hand, sulphur is toxic and corrosive to the expensive catalysts and industrial units. Therefore, the low level sulphur detection plays an important role in monitoring sulphur reduction in industries.


Among other analytical techniques mentioned above, the Sensi-Tape technology is a proven analytical technique with a number of ASTM standards and one analyser is capable of detecting multiple impurities, if the need should ever arise, simply by changing the sensi-tape (lab version). Not only is it easy and simple to operate, but it also is capable of detecting extremely low levels of sulphur in any applications such as water, beverage, and hydrocarbon media with virtually no interference and considered more robust than the other technologies mentioned.


Finally, there are a number of emerging analytical techniques such as TDLAS that can analyse sulphur in low levels and seemingly free of any interference. It does not require a complex set up compared to GC.


method is the only detection system that can determine H2 S literally without any interference.


Chemical formulations deposited on the tape provide a detection medium that is fast, sensitive, and specific (free of any interference) that can detect impurities down to ppb levels. The cycle time to perform one analysis is 10-60 sec (in ppm levels). The system has a number of ASTM approvals which makes the measurement reproducible, comparable and traceable such as D2420, D4045, D4084, and D4468.


Tunable diode laser absorption spectroscopy


TDLAS uses a tunable near infrared laser. A tunable diode laser is a kind of semiconductor based laser that can be tuned to optically choose a particular wavelength (or colour) of light (see the image below). The laser emits near-infrared radiation (1.2 – 2.5 µm) with a line width less than 0.003 cm−1, which is narrower than molecular absorption line widths (typically 0.1 cm−1 at atmospheric pressure). Tuning to a specific wavelength is done by varying the diode temperature and the diode current going through it.


As the light passes through the gas sample, energy is absorbed, reducing the amount of light arriving at the detector. The length of the laser beam influences sensitivity of the sensor; therefore, in most applications a dual-pass optical path is necessary.


The range for H2


S analysis is 20 ppm with the lowest detectable limit of 500 ppb and it needs a multi path cell. The analyser can be installed in a stack, pipeline or flare and is, therefore, capable of measuring over a long distance.


29


A Comprehensive Fuel Analyser for Gasoline, Diesel and Jet Fuel


eralytics (Austria) offers a truly portable NIR/MID-FTIR analyser that can measure all different kinds of fuels within a single unit. It generates highly accurate results for more than 40 fuel parameters and several important fuel properties, such as Octane and Cetane numbers. In addition, it measures Biodiesel (FAME) concentration in conventional diesel and jet fuel (EN14078). Of course it also supports the new ASTM D7806 Biodiesel standard. It is even capable to distinguish not only between FAME and FAEE but it can also detect vegetable oil that is added to Diesel fuel illicitly. The benzene concentration of gasoline can be determined according to ASTM D6277 as well as EN 238. The determination of oxygenates such as ethanol and MTBE is carried out in strict compliance with ASTM D5845.


In addition to the classic fuels ERASPEC offers modules for synthetic fuels and ethanol or methanol fuels. For ethanol fuel for example the


impurities of water and methanol can be determined along with the actual concentration of ethanol itself.


ERASPEC also offers a built-in oscillating U-tube density meter following the ASTM D7777 standard. It accurately measures the density of your fuel automatically along with the other parameters. Each result can therefore be shown as Vol% or Mass%.


As a state-of-the-art analyser ERASPEC offers a large full color touchscreen. It stores up to 50.000 measurement results and for advanced users it even allows the direct comparison of spectra on the screen. Measurement results can be printed or transferred directly to a LIMS system.


The rugged, patented interferometer design is field-proven and has shown its reliability in challenging environments. With its portable design ERASPEC is the ideal choice for field tests directly at the point of sale in mobile laboratories.


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For more information, visit us on www.hemera.fr or contact us at sales@hemera-innovation.com


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