Resolving the Benzo(j)fluoranthene Challenge
Separate New PAHs Quickly Using the Rxi-17 GC Column
- Fully resolve benzo(j)fluoranthene from benzo(b) & (k).
- Excellent resolution of 16 priority pollutant PAHs.
- Separate difficult dibenzo pyrene isomers.
Polycyclic aromatic hydrocarbons (PAHs) are one of the most widespread forms of organic pollutants in the environment, significantly affecting air, water, and soil quality. Although naturally occurring, environmental levels of PAHs and their byproducts have increased steadily due to human impact. PAHs are used in manufacturing medicines, plastics, and pesticides. PAH byproducts are typically formed through the incomplete combustion of organic materials, such as wood, coal, and oil. Vehicle emissions are also a significant source.
PAHs are compounds comprised of fused benzene rings that are free of heteroatoms (Table I). The lighter PAHs such as naphthalene (2 benzene rings) are commonly found in air and water. The heavier PAHs (6 or more benzene rings) tend to be extremely persistent in soil as their water solubility and mobility decreases substantially with their increasing molecular weight. Many PAHs are known or suspected carcinogens. The greater number of benzene rings a PAH contains, the greater the typical carcinogenicity of that compound. The United States Environmental Protection Agency currently lists and mandates testing of the 16 priority PAHs they deem most hazardous.
New Compounds, New Challenges
Many chromatographic methods are available to quantify these pollutants. Gas chromatographic techniques commonly are used and are coupled with mass spectrometry when qualitative identification is required. A 5% diphenyl/95% dimethyl polysiloxane column usually is used for GC/MS work. This stationary phase effectively determines the 16 priority pollutants, however, many European Union analyte lists are being expanded and include compounds, such as benzo(j)fluoranthene, dibenzo(a,h)acridine, and dibenzo(a,e)pyrene, that are difficult to analyze under conventional test conditions. For example, benzo(j)fluoranthene coelutes with benzo(b)fluoranthene on a 5%diphenyl/95%dimethyl polysiloxane stationary phase, therefore its determination must be reported as a combined sum of isomers. Where regulations mandate reporting of individual concentrations for each isomer conventional methods are not effective and new solutions must be found.
The Rxi Alternative
Here we evaluate the ability of the Rxi-17 column to separate PAH compounds, including the difficult-to-resolve isomers. The
Analytical conditions were set to optimize resolution of critical pairs and reduce bias against heavier, less volatile analytes. To improve the quantification of high molecular weight compounds we used a column with a thin film thickness and set the injection port temperature to 300°C. A pulsed splitless injection technique was used to maximize the transfer of analytes onto the column; this has proven to be a very effective injection technique for trace level analyses when used in direct injection modes. The pressure pulse also helps minimize discrimination against the high molecular weight components. Finally, the ion source and quadrapole temperatures of the instrument were set at 280°C and 180°C, respectively. This increase in detector temperatures, from the defaults of 230°C and 150°C, yields better peak shapes and responses of the PAHs, especially those of higher molecular weights. These run conditions produced excellent resolution for all of the target analytes but required a long analysis time (Figure 1).
To speed up the analysis while maintaining adequate resolution we developed an effective temperature program for PAH analysis on the Rxi-17 column. A starting oven temperature of 90°C was chosen; it is high enough to prevent solvent condensation, but low enough to properly focus the target analytes. Several temperature ramps were then utilized to resolve critical analyte pairs. For example, a slow temperature ramp was used to separate phenanthrene and anthracene. A slow, 4°C/min, ramp also was utilized from 280°C to 320°C to separate some late eluting compounds. Maintaining a slow ramp rather than using an isothermal hold was important in maintaining peak efficiencies. In between these critical separations the temperature was ramped quickly where feasible to speed up the overall analysis. The data in Figure 2 demonstrate the effectiveness of this program—resolution was maintained in 30% less time.
Using the Rxi-17 column and an optimized temperature program is a practical solution to the challenges posed by expanding analyte lists for PAH analyses. Critical pairs resolve well in a reduced run time. If you are struggling to determine new target analytes on conventional columns, try the Rxi-17 column and an optimized temperature program.
Figure 2 Using an optimized temperature program with the Rxi-17 column reduces PAH analysis time 30%.
Table 1 Commonly tested polynuclear aromatic hydrocarbons and corresponding structures.