Accurately Quantify PAHs Down to 5 pg On-Column
GC/MS SIM Analysis with the Rxi®-5Sil MS Column
- Excellent linearity across a broad calibration range.
- Ideal for trace level analyses.
- Low bleed at high temperatures, for better response and lower detection limits.
Polycyclic aromatic hydrocarbons (PAHs) are common environmental pollutants, affecting air, water, and soil quality. Although naturally occurring, human impact has created a steady increase in environmental levels of PAHs and their byproducts. PAHs are typically formed through the incomplete combustion of organic materials, such as wood, coal, and oil, but are also used in manufacturing of some medicines, plastics, and pesticides. Many PAHs are known or suspected carcinogens. The United States Environmental Protection Agency currently lists and mandates testing of the 16 PAHs they deem most hazardous.
Many chromatographic methods are available to analyze these pollutants. The gas chromatographic techniques typically used are often coupled with mass spectrometry. Laboratories performing low-level PAH analyses often utilize the single ion monitoring (SIM) function of GC/MS because of the sensitivity required to achieve typical regulatory or monitoring levels.
For our SIM method we chose to use the Rxi®-5Sil MS column. The Rxi®-5Sil MS stationary phase incorporates phenyl rings in the polymer backbone, which strengthens the siloxane chain, preventing thermal breakdown. The content of this aryl functionality has been adjusted so that its selectivity is similar to conventional 5% diphenyl/95% dimethyl phases. The low column bleed reduces the amount of noise contributed by the column, thereby increasing the signal-to-noise ratio of the analytical system. Increased sensitivity and subsequently lower detection limits are direct results of the improved signal-to-noise ratios. The silarylene polymer not only exhibits improved thermal stability and reduced bleed, but it also shows improved separation for aromatic compounds, such as PAHs.
Analytical conditions were set to optimize resolution of critical pairs and reduce discrimination of high molecular weight analytes. We chose a 4mm Drilled Uniliner® inlet liner with wool, since direct injection using this liner provides near complete transfer of sample analytes to the column. The Drilled Uniliner® inlet liner also eliminates analyte exposure to cold spots and potentially active metal components in the injection port, assuring complete transfer of the higher molecular weight PAHs, which otherwise tend to adsorb in these areas. To improve the quantification of high molecular weight compounds we chose a column with a thin film thickness (0.25µm) 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. The pressure pulse has proven to be a very effective injection technique for trace level analyses and also helps minimize discrimination against the high molecular weight components. Finally, the ion source and quadrupole temperatures of the instrument were set at 290°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 for the PAHs.
These run conditions produced excellent resolution and response for all of the target analytes in a run time of less than 16 minutes. Figure 1 shows the SIM trace at 0.005 µg/mL (5pg on column). The system was calibrated at eight levels, from 0.005 to 10 µg/mL in single ion monitoring mode. The SIM acquisition program used for this analysis is shown in Table I. Each calibration standard contained eighteen target PAHs, two internal standards (p-terphenyl-d14 and perylene-d12), and the surrogate (2-fluorobiphenyl). At each level, the relative response factor (RRF) was calculated for all compounds and linearity was determined by calculating the percent relative standard deviation (%RSD) for all response factors, as shown in Table II. The %RSDs for all compounds are in the low single digits with an average for all compounds of 4.7%.