VUV Analytics, Inc. has demonstrated the ability to improve the accuracy of hydrocarbon compound characterisation in gasoline range samples using its VUV PIONA+ product solution. Reporting errors often encountered in detailed hydrocarbon analysis (DHA) using fl ame ionization detection (FID), such as peak misidentifi cation and unresolved co-elutions, can be overcome using VUV spectral matching and deconvolution. Additionally, VUV eliminates traditional DHA requirements for sub-ambient oven temperature programs and column pre-tuning.

utilises single-column gas chromatography (GC) combined with vacuum ultraviolet (VUV) spectroscopy to provide accurate compound speciation up to C6 and bulk compound class characterisation at higher carbon numbers. In addition, specifi c analytes throughout the chromatogram such as individual oxygenates or aromatics belonging to the BTEX complex can be speciated. The product solution combines a VGA-100, the world’s fi rst VUV absorption GC detector, with the automated software analysis of VUV AnalyzeTM


GC-VUV absorbance data is three dimensional (time, absorbance, wavelength) and specifi c to compound chemical structure. Chromatographic co-elution events common to DHA can be addressed using VUV spectral matching and software deconvolution. VUV absorbance spectra are typically highly structured and distinct for individual compounds, yet exhibit the intuitive property of having similar features when measuring related compound classes.

The VUV AnalyzeTM engine implements equations and fi t

procedures that result in deconvolution of absorbance spectra that contain contributions from multiple species, is capable of binning and storing response contributions from each deconvolution analysis, and reporting a combined total response at the end of the analysis. The data processing includes a database library of VUV reference spectra, compound class information, density, approximate retention index values, relative response factors for each hydrocarbon class, and relative response factors for individually reported compounds. Compound class or specifi c compound concentrations can be reported as mass or volume percent.

An example of a GC-VUV deconvolution important to hydrocarbon analysis can be seen in the zoomed-in GC-VUV chromatogram of Figure 1. The co-elution of benzene and 1-methylcyclopentene shown resulted from a gasoline sample run with a 100-meter column (100% poly(dimethyl siloxane) phase, 0.25mm ID, 0.5µm fi lm thickness). Neither a tuned pre-column nor sub-ambient start temperature was used. VUV software deconvolution was performed during post-run analysis to achieve accurate quantitation of both compounds. A reconstructed chromatogram is overlaid to display the relative proportion of benzene and 1-methylcyclopentene in the co-eluting peak. Numerous other co-elution events in gasoline samples can be resolved using VUV PIONA+. Figure 1 inset demonstrates how the unique spectral signatures of compounds in the VUV wavelength range (125 – 240 nm) enable identifi cation and deconvolution of co-eluting analytes.

The co-elution of toluene and 2,3,3-trimethylpentane is another chromatographic separation challenge commonly encountered

with hydrocarbon analysis methods. ASTM Method D6730 recommends tuning a 5% phenyl pre-column to provide better separation of these compounds. ASTM Method D6729 uses a sub-ambient temperature start to enhance separation of early eluting compounds, but only achieves separation of toluene and trimethylpentane for a limited range of concentrations. Figure 2 shows the deconvolution of toluene and 2,3,3-trimethylpentane by VUV AnalyzeTM

of these co-eluting compounds was accomplished using a 100-meter column (100% poly(dimethyl siloxane) phase, 0.25mm ID, 0.5µm fi lm thickness) and deconvolution, with no sub-ambient start conditions or pre-column tuning.

. Analyte spectral profi les are fi t with VUV library

spectra to provide the identities and relative concentrations of the co-eluting compounds. The spectral fi ts at the front and back of the co-elution are given at the bottom of the fi gure. The resolution

Complete chromatographic separation of all analytes using either ASTM D6729 or ASTM D6730 is impossible. The result in the previous example is the potential over-reporting of toluene when a signifi cant amount of 2,3,3-trimethylpentane is present. Even when suffi cient separation of early eluting analytes is achieved, it is not possible to separate all of the later eluting compounds. Further over-reporting of aromatics can occur when the sample contains

Figure 1: GC-VUV chromatogram of gasoline sample zoomed-in to show the co-elution of benzene and 1-methylcyclopentene. A chromatographic shoulder starting at 24 minutes is observed from the full range absorbance (125 – 240 nm) response. The deconvolution performed by VUV software during post-run analysis is overlaid to display the relative proportion of both compounds. The VUV Absorbance spectra of benzene and 1-methylcyclopentene are overlaid in the fi gure inset to demonstrate how their unique spectral profi les enable straightforward identifi cation and deconvolution.


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