HPLC vs. GC/MS: Detecting Polycyclic Aromatic Hydrocarbons in Beijing, China

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds with two or more fused aromatic rings.¹ A number of them in the atmosphere are mutagenic, with toxicity, endocrine-disrupting potential and high carcinogenic risk of cancer, so we must carry out strict monitoring on PAHs to determine their behavior. PAHs in aerosols mostly originate from the incomplete combustion of biomass and fossil fuels. Main sources include cooking, manufacturing, and traffic. In addition, geographic characteristics and surrounding areas have an impact on the transmission of PAHs. In our case, PAHs in Beijing are influenced by the distribution of mountains and lakes as well as nearby provinces.

China relies on coal for more than 70 percent of its primary energy, and the large coal consumption has led to serious air pollution there. Beijing is the capital and a major metropolis of China, and emissions of PAHs to the atmosphere in Beijing continue to go up owing to increases in population, coal consumption, vehicle emissions, and construction activities. Therefore, easy, rapid, and effective detection methods are essential for long-term observation so as to track the drastic changes in the atmospheric environment.

Technology

Currently, in the industry, many chemical detectors are used to detect the presence of PAHs. Gas Chromatography-Mass Spectrometry (GC/MS) is one major technology, especially in China. GC/MS an advanced technology to separate organic samples into single components in a gasified condition.² Samples enter the mass spectrum sequentially after exiting from the gas chromatography. In the mass spectrometer, the gaseous sample is ionized into charged particles and detected by the mass detectors. It is widely used in the separation and identification of complex components. It includes both the high resolution of GC and the high sensitivity of MS and is an effective tool for measuring trace organics, which means both qualitative and quantitative analyses are eligible. GC/MS is was validated for the determination of 16 PAHs from the FDA list of 93 harmful or potentially harmful constituents of mainstream cigarette smoke (MCS).²

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After several literature reviews, our group found out that the Fluorescence detection in liquid chromatography also contributes a lot to the accurate PAHs detection. HPLC-fluorescence detection was developed to separate, identify, and quantify the concentrations of PAHs based on the different interaction. The aerosol analyte is injected into the column and moved in the stationary phase (column) at a certain pressure. Since the polarity between the chemical groups in the aerosols and column is different, different substances come out sequentially from the column. Since a great amount of fluorescence quantum exists in the PAHs, fluorescence detection is a sensitive and precise method to determine the composition.

To ensure the veracity of the results, the stationary phase should deliberate entirely. In general, based on the great physical properties of silica gel, silica gel is the most widely used in chromatography columns. However, there are possible co-extractives in aerosols typically appear in China, which have a similar chemical property to PAHs. Those co-extractives that have a similar fluorescence property separated by a conventional method will not be precise. Thus, an improved stationary phase is developed in the project. To obtain the higher recovery of PAHs, NH₂ column will be used in this project. A plot, showing the intensity versus time, will be gained from the output of the detector. Different chemical groups come out at a different time. The PAHs can be identified according to the standard time.

Comparison

Although MS/GC is wildly used nowadays, it still has some flaws during practical application. First of all, errors would be introduced because of the possible co-elution by gas chromatography. At the same time, MS does not have a high sensitivity when distinguishing among similar patterns of different ionized fragments, as well as the compounds whose molecules have negligible affinity for electron. Compared to MS, the sensitivity of HPLC is 0.8 to 50 times higher. Since the detector are not contacted with the sample solution, the equipment is easier to maintain. Also, the sample preparation of HPLC is easier than that of MS/GC.

Monitoring Locations Determination

In previous studies, the Beijing Environmental Protection Bureau has measured PM10 concentration at 13 sites spread about 40 kilometers in the east and west direction (Appendix 1). To achieve more accurate detection, 35 HPLC sites are selected based on the layout of the current PM2.5 monitoring system in Beijing. Points are classified into traffic pollution monitoring points, urban assessment points, regional background transmission points, and urban clean control points.

Apart from human activities, the project also considers natural meteorology. Previous research shows that the seasonal monsoon makes a regular pattern of the path of PM2.5 transmission towards Beijing, mostly from northwest and southeast directions.⁴ The pattern of PAHs transmission is assumed to be similar with that of PM2.5. Based on this, more regional background transmission points are added in NW-SE direction to detect PAHs in the nearby region. Regarding emissions from cooking, additional sensors are set near open-air food plaza, including Gui Street, Yandaixie Street, Qianmen Street, and Asian Games Village. Totally, 44 detection sites are suggested. A map with detailed site illustration is attached in Appendix B.

To reduce budget, some of the sensors in NE-SW direction could be shut down in April, May, July, and August, when a relatively low concentration of PAHs is predicted based on previous data (Appendix A-a). During those months, BaP, the most toxic component of PAHs, has a lower concentration level than that in Ambient Air Quality Standard (AAQS) set by the Chinese government.⁴

Conclusion and Discussion

According to our comparison between two technologies, HPLC-Fluorescence detection has great advantages in PAHs analysis in long-term observation, including higher sensitivity and easier maintenance. Then we suggested 44 detection sites in Beijing based on the spatial distribution and seasonal variation of PAHs, and seasonal shutdown can be applied to reduce the budget.

In the future, more monitoring sites in residential areas can be added to achieve better accuracy after the pilot project. And ultra-high-performance liquid chromatography (UHPLC) can also be incorporated into the detection for its higher resolution faster analysis with a smaller column.

References

1. T. Okuda et al., “Polycyclic aromatic hydrocarbons (PAHs) in the aerosol in Beijing, China, measured by aminopropylsilane chemically-bonded stationary-phase column chromatography and HPLC/fluorescence detection”, 2006. [Online]. Available: https://www.sciencedirect.com/science/article/pii/S0045653506001263?via%3Dihub. [Accessed: 19- Apr- 2019].
2. “GC-MS Analysis l fundamentals of using gas chromatography with mass spectrometric detectors”, Chromacademy.com, 2019. [Online]. Available: https://www.chromacademy.com/resolver-november2010_Understanding_GCMS_part_1.html. [Accessed: 24- Apr- 2019].
3. “北京环境监测保护中心”, Bjmemc.com.cn, 2019. [Online]. Available: http://www.bjmemc.com.cn/xgzs_getOneInfo.action?infoID=1661#. [Accessed: 26- Apr- 2019].
4. J. Li, Y. Zheng, X. Luo, Z. Lin, W. Zhang and X. Wang, “PAH contamination in Beijing’s topsoil: A unique indicator of the megacity’s evolving energy consumption and overall environmental quality”, 2016.
5. X. WANG et al., “Potential sources and health risk of PM2.5-bounded PAHs at the background site at Tuoji Island”, Kjdb.org, 2015. [Online]. Available: http://www.kjdb.org/EN/abstract/abstract12539.shtml. [Accessed: 25- Apr- 2019]