The VUV Analyzer for Fuels is fast becoming the preferred industry standard for hydrocarbon analysis.
It utilizes the strength of gas chromatography, vacuum ultraviolet (GC-VUV) spectroscopy, in combination with ASTM D8071 methodology and VUV Analyze™ Software all in one platform.
Compounds of every PIONA class have spectral characteristics that are distinct from other PIONA classes in the gasoline range. Absorbance in the 170 to 200 nm region becomes greater with double bond equivalency, particularly when π bonds are introduced.
The VUV Analyzer for Fuels has a clear advantage over mass spectrometry in this regard when distinguishing olefins from naphthenes. Although they have the same double bond equivalency, the π bond on the olefin provides a clear spectral characteristic around 180 nm that is not exhibited by the naphthene.
Naphthenes and olefins have an equal molecular mass in mass spectrometry which means they have overlapping ion fragments. This means they are more challenging to identify, a task that is almost impossible when they co-elute.
Conjugated diolefins (CDOs), which are also known as conjugated dienes, are a particular subset of olefins that are especially challenging to refiners.
In high enough concentrations, these compounds polymerize which can block pipes in hydrocarbon streams, requiring the refinery to close in order to clean out the blockage. To prevent polymerization from occurring, CDO levels must be kept below a certain threshold.
UOP-326 was one of the first techniques for measuring the content of CDO. It was created in 1965 and utilizes maleic anhydride as a dienophile.
An excess of maleic anhydride is introduced to the fuel sample in this method. A partial amount of maleic anhydride is consumed through a Diels-Alder reaction with the CDOs. The maleic anhydride left over is converted into maleic acid, which is then quantified by colorimetric titration.
While UOP-326 continues to be used, it has multiple limitations. The technique lasts for over 3 hours, whether it is carried out automatically or manually. Specific nucleophiles such as alcohols and thiols (which are frequently found in or introduced to fuels) also react with maleic anhydride, which positively skews the values.
Some sterically hindered diolefins such as 2,5-dimethyl-2,4-hexadiene do not have a reaction which negatively skews the values. The technique is only semiquantitative and cannot offer qualitative data due to this lack of selectivity, for example it cannot identify which diolefin species are present.
A greater variety of CDO measurement techniques have emerged recently, including voltammetry, near-IR, NMR, SFC-UV, HPLC, and derivatized GC with mass spectrometry/NCD. VUV can identify and measure CDOs due to the power of the absorbance spectra.
These absorbance spectra show unique spectral characteristics above 200 nm, which means they are distinguishable from mono-aromatics, saturates, and olefins (see Figure 1).
Figure 1. Not only are these 5 conjugated diolefin spectra distinct from one another, they also all have good VUV response at >200 nm, which allows us to easily find them in a complex matrix. Image Credit: VUV Analytics
Five species of CDO were spiked into a gasoline matrix and were run in duplicate within D8071 conditions. When observing two of the CDOs that are more baseline-separated, they provide a positive linear response (r2 > 0.99) from 1% down to 0.05% for C7+ and 0.02% for C6 (shown in Figure 2).
Figure 2. Taking a closer look at two of the spiked conjugated diolefins, they not only show good repeatability but also track well in detector response down to 0.02-0.05%. Image Credit: VUV Analytics
The lightest CDO, 2-methyl-1,3-butadiene (also called isoprene), was identifiable down to 0.01% even though it had co-eluted with multiple key analytes. This example demonstrates the efficacy of spectral deconvolution, especially time interval deconvolution using the VUV Analyze Software.
Isoprene co-elutes with a paraffin (pentane) and an olefin (trans-2-pentene). The paraffin absorbs out until around 160 nm, and the olefin has a nice spectral characteristic at around 180 nm but falls off at around 210 nm.
The 220 nm spectral characteristic of isoprene cannot be mixed up with either of these compounds, enabling high quantitative accuracy (Figure 3).
Figure 3. Even when there are several compounds co-eluting with a conjugated diolefin (2-methyl-1,3-butadiene), its spectrum has a unique response in a different wavelength region, making identification straightforward and quantitation accurate. Image Credit: VUV Analytics
The VUV Analyzer for Fuels from VUV Analytics simplifies the process to acquire not only the standard PIONA quantitation with speciation of di-aromatics, BTEX, and select oxygenates, but also when performing the speciated measurement of CDOs.
This information has been sourced, reviewed and adapted from materials provided by VUV Analytics.
For more information on this source, please visit VUV Analytics.