For the environmental analysis and monitoring, the main challenge is that the analytes are usually presented in trace amount in complex matrices. These major environmental pollutants include polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), organochlorines, pesticides and endocrine disruptors.[1] 2-D gas chromatography (GC) is a powerful and versatile technique for the qualitative and quantitative analysis for a variety of complex sample. Compared with the normal 1-D GC, 2-D GC has higher peak capacity, resolution and sensitivity.[2] The above advantages of 2-D GC make it a powerful tool in the environmental analysis. This term paper reviews and summarizes the application of 2-D GC in environmental analysis.
Key words: 2-D GC, environmental analysis, PCDDs, PCDFs, PCBs, PAHs, organochlorines, oesticides, endocrine disruptors
Introduction
As the development of the human society, a large number of chemicals are distributed to the environment. There are many chemicals that are hazard to the animals, human, even to the ecosystems. These hazardous chemicals include PCDDs, PCDFs, PCBs, PAHs, organochlorines, pesticides and endocrine disruptors. Thus, determination and monitoring these hazardous compounds is a task for the environmental scientists.
Since these hazardous analytes are usually presented in trace amounts in very complex matrices, a method with high sensitivity and resolution is needed. GC is one of the powerful tools for the determination and monitoring for the environmental hazardous compounds. However, 1-D GC is not sufficient for the environmental analysis. Most environmental samples contains many closely eluting peaks of analytes and matrix components, the peak capacity of the 1-D GC is not enough for so many closely eluting peaks. Besides, the resolution for 1-D GC is relatively poor, although it can be improved in both sample preparation step and detection instrumentation (e.g. mass spectrometer (MS)).
To improve the resolution by the sample preparation step (e.g. extraction process), it can be very labor- and time- consuming. Besides, large amounts of solvents can be generated, which can be harmful to the environment. For the detection instrumentation, MS is a good choice to improve the resolution. However, some abundant components in the sample matrix can coelute with the analyte in very complex matrix. Thus, the mass spectrum of the analyte can contains the fragments coming from the interfering compounds, causing the mass spectrum does not match with the library mass spectrum in 1-D GC-MS analysis. In 2-D GC-MS, this is not a problem, which can be proved by the following example (figure 1) of the chlorfenvinphos determined by 1-D GC-MS and 2-D GC-MS.
a. Mass spectrum of chlorfenvinphos obtained by b. Library mass spectrum of chorfenyinphos
2-D GC-MS
c. Mass spectrum of chlorfenyinphos obtained by 1-D GC-MS[3]
Figure 1
Principle
The components of 2-D GC are very similar to 1-D GC, including sample injector, oven, columns and deterctor. [1] The block diagram of 2-D GC is shown in the following figure 2.
Figure 2. A block diagram of 2-D GC. (a). injector (b). primary column (c). modulator (d). secondary column (e). detector[4]
2-D GC involves a technique called "heart-cutting". The sample matrix is introduced through the injector to the primary column for the first separation. The modulator collects the analyte from the primary column for a certain period of time, and then reinject to a fraction of the analyte from the primary column to the secondary column for the further separation. When the further separation is occurring in the secondary column, the modulator collects the other fraction of the analyte from the primary column. After the chromatogram appears, this fraction of the analyte is reinjecter to the secondary column. The process is repeated as above description till the analysis is finished.[4] There is a research on the effect of the thermal modulator and cryogenic modulator, which shows that cryogenic modulator is more efficient to trap the more-volatile analytes.[5] The stationary phases of the two columns are different, thus, the selectivity of 2-D GC is improved rather than the 1-D GC, so are the sensitivity, peak capacity, etc.[1] 2D-GC can be coupled with the detectors including flame ionization detector (FID), electron capture detector (ECD), atomic emission detector (AED), sulfur chemiluminescence detector (SCD), nitrogen chemiluminescence detector (NCD), and time-of-flight mass spectrometer (TOF MS).[6][7][8]
Environmental Applications
Since 2-D GC has so many advantages compared to 1-D GC, it has the potential to improve the determination of the pollutants for the environmental analysis. 2-D GC has higher separation power so that to improve the separation of the coeluting analytes and the matrix components, and it can increase the detection limits of the analysis. Besides, 2-D GC provides two-dimensional chromatograms, which is good for the environmental monitoring. These advantages lead to the simplification of the sample preparation steps, so that less analysis time is needed. Nowadays, 2D-GC is widely used for the environmental analysis and monitoring, which covers water and sediment analysis, analysis of PCBs, PCDDs and PCDFs, pesticide analysis, as well as air analysis.
Water and Sediment Analysis
It is known that water is very important to life on earth. The sediments in the water bodies (e.g. river and lake) also play an important role in ecosystems through the food chains and webs. Thus, the safety of water and sediments should be ensured by analyzing and monitoring the pollutants accurately. 2-D GC is a good method for the analysis of water and sediment rather than 1-D GC with many sample preparation procedures.
For the water and sediment analysis, 2-D GC can be coupled with many instrumentations and techniques, and used for the analysis of groundwater, freshwater bodies (e.g. lake & river), marine sediments, rivers & lakes sediments, and sediments in soils. For the groundwater analysis, Benzene, toluene, ethyl benzene and xylenes (BTEX) and methyl tert-butyl ether (MTBE) can be fully separated by 2-D GC-FID coupled to solid phase micro-extraction. [9] For the freshwater bodies, the compounds from petroleum & oil contamination and nonylphenol (NP) isomers are the parameters that should be concerned. 2-D GC-FID can be used to point to the main sources of the contamination for the analysis of compounds come from petroleum & oil contamination, which is illustrated to be a good tool in environmental forensics, and environmental audits. [10] Besides, 2-D GC-time-of-flight mass spectrometer (TOF MS) is a good tool for the identification of structured similar NP isomers, [11] and 2-D GC-MS is proved that it can separate a total of 102 NP isomers from a technical mixture and river samples, including 13 NP isomers are identified and 2 of them are quantified with minimal sample preparation.[12]2
2-D GC-TOF MS, combining with the ultrasonic-assisted extraction (UAE), is recently applied for the analysis of the pollutant in marine sediments.[1][13] Using this method, 1500 compounds are separated and several PAHs, NP isomers and dialkylated benzene are identified in the marine sediments.[13] The trace determinations of some byproducts of many industrial activities (e.g. suspected carcinogens, mutagens, PAHs) in sediments samples is difficult. However, 2-D GC-FID coupled with large-volume splitless injection (LVSI) technique is a powerful and rapid tool for the analysis of trace amounts (ppb level) of PAHs in complex matrices.[14] Ong et.al.[15] developed a method for rapid monitoring of PAHs in soil sediments using pressurized liquid extraction (PLE)-2-D-GC-FID, which shows the applicability of 2-D GC for the analysis of PAH in sediment samples.
Pesticide analysis
Pesticides are usually distributed in the environment in trace amounts. They can be distributed in complex matrices, such as food, soil and water, which can be eaten or intake by animals or human. The standard method for the pesticides analysis includes multiple sample pretreatment procedures to eliminate the matrix components, and then the samples are analyzed by GC or LC. As above mentioned, 1-D GC is lack of resolution to fully separate the pesticides compounds from the matrix components, and 2-D GC is a better method for pesticide analysis.
2-D GC can be used for the pesticide analysis in biological samples, such as tissues, serum, urine, etc. Liu et al. used 2-D GC-FID coupled with supercritical fluid extraction (SFE) technique to analysis the pesticides in human serum[16], showing that 15 pesticides are separated within 4 minutes. Compared to the 1-D GC analysis, the analysis time of 2-D GC is much lesser, and the separation of 2-D GC is much better. Besides, 2-D GC can be used for the pesticide analysis in food samples.
Although 2-D GC cannot separate all types the pesticide, it can be used for the primary screening for the environmental samples polluted by pesticides if using the columns correctly. There is a research shows that. Korytár et al. used five different columns combinations for the group separation of 12 halogenated compound classes, consisting of PCBs, PCDDs, PCDFs, polychlorinated diphenyl esters (PCDEs), polychlorinated naphthalenes (PCNs), polychlorinated dibenzothiophenes (PCDTs), polychlorinated terphenyls (PCTs), polychlorinated alkanes (PCAs), toxaphene, polybrominated biphenyls (PBBs), polybrominated diphenyl ethers (PBDEs) and organochlorine pesticides (OCPs).[17]
Air analysis
Volatile organic compounds (VOCs) in urban areas have detrimental effect on human health, and the VOCs in particulate matters still have uncertainty in health effects. It is proved that many air pollutants are the baseline noise in 1-D GC chromatogram, which are not known to us.[18] 2-D GC can separate more than 500 chemical species of VOCs in urban air samples in one run,[18] and it is a good tool for monitoring and discovering of many unknown air pollutants.
For the PAHs and oxygenated PAHs in the urban aerosol analysis, 2-D GC-FID and 2-D GC-quadrupole MS (QMS) can be applied.[19] 2-D GC-FID ensures the good reproducibility, while 2-D GC-QMS is good for compounds identification and quantification. In Finland, these two methods allow the detection of about 1,500 peaks and identification of target PAHs. For the cigarette smoke, it may consist of about 4,700 identified compounds and up to 100,000 unidentified compounds.[1] 1-D GC does not have enough separation power for the cigarette smoke analysis, while 2-D GC-TOF MS can resolve about 30,000 peaks in cigarette smoke.[20] Thus, cigarette smoke condensate were analyzed to determine the chemical composition of the neutral fraction[21], basic fraction[22] and acid fraction[23]. For the neutral fraction, 4,000 compounds were separated and 1,800 hydrocarbons were identified by 2-D GC; and the 1-D GC only separated 200 compounds and 115 hydrocarbons were identified.[21] For the basic fraction, 377 nitrogen-containing compounds were identified, among which 56 were pyrazine derivatives, 104 were quinoline/isoquinoline, and 156 were pyridine derivatives.[22]
Analysis of PCBs, PCDDs & PCDFs
PCBs, PCDDs, and PCDFs are the substances that can be accumulated in the animal and human bodies, and biomagnification will happen through the food chains. Many of these substances are carcinogens and mutagens. Thus, determination and quantification of these substances are significant.
PCBs have 209 congeners, which differ in the number of chlorine substituents and substitution pattern on the benzene rings.[24] Not all of these congeners are hazardous, but those toxic ones present in trace amounts and usually coelute with the other abundant components in 1D-GC. World Health Organization (WHO) identified 12 PCBs congeners as indicators of toxicity, including 77, 81, 105, 114, 118, 123, 126, 156, 157, 169 & 189), because of their dioxin-like chemical behavior.[25] Also, European Union (EU) use7 s 7 PCBs congeners to act as markers of harmful contaminations, including 28, 52, 101, 118, 138, 153, and 180.[26] The normal analysis for PCBs, PCDDs and PCDFs is to use liquid chromatography (LC) for the pretreatment to eliminate the interferences, followed by the GC-MS analysis. However, this method cannot separate all the 12 PCBs congeners identified by WHO, and is also expensive, time- and labor- consuming.
2D-GC can be used for the PCBs, PCDDs, and PCDFs analysis, which can separate all the 19 PCBs identified by WHO and EU in baseline within 15 minutes. Linking 2D-GC with the microelectron-capture detection (μECD) is a good choice.[26] Compared to 1D-GC-HRMS, 2D-GC-μECD has advantages in rapid environmental monitoring.[27] By using 2D-GC coupled with 13C-lable isotope dilution (ID) TOF MS, the following compounds can be also identified: polychlorinated naphthalenes (PCNs), PAHs, polybrominated diphenyl ethers (PBDEs), phenols and phthalates.[1]
19 of out of 209 are chiral PCBs, which presents in the environment as atropisomers. 1D-GC coupled with cyclodextrin columns can separation 15 out of 19 atropisomeric PCBs with rigorous sample preparation and multiple injections.[28] However, it cannot be used for the real-life samples (e.g. food extracts), because it cannot separate atropisomeric PCBs from both regular PCBs and matrix components. 2D-GC with a narrow-bore β-cyclodextrin primary column and a liquid crystal secondary column can separate 9 atopisomeric PCBs from both regular PCBs and matrix components in real-life samples. [29] Recently, there are researches showing that 194 out of 209 PCB congeners can be separated in 240 minutes when using 2D-GC-μECD, and 192 out of 209 PCB congeners can be separated in 146 minutes when using 2D-GC-TOF-MS.[30][31] The 19 PCBs congeners identified by WHO and EU are included.
Conclusion
2-D GC is a powerful tool for the analysis of analytes in complex matrix, due to its higher separation power, higher resolution, and higher peak capacity rather than the conventional 1-D GC. For the environmental analysis, the pollutants are always in trace amounts in complex matrix. 2-D GC can generate high resolution chromatogram, as well as simplifies the sample preparation and reduces the analysis time. Although the data processing will be complicated, the software calculation can be solved this problem. Thus, 2-D GC will be one of the potential tools in the environmental monitoring and analysis in the future.
