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One cannot ignore the fact that humans disturb the stability of nature. Nature undergoes changes as a result of human activity. For instance, the meals we eat affect food production and global food chains. Our consumption of technology directly and indirectly impacts upon the nature of the environment, although nature is the ultimate source that provides for our lives.
World population pressure (ca. seven billion) places urgent needs for technological solutions. Environmental awareness and the growth of the discipline of environmental science since the mid 20th Century involve integration of knowledge on how the environment copes with the demand for resources.
Scientists seek to establish measurement procedures dealing with environmental problems, and mitigate these – or potential – problems. Determination of chemical components in the disturbed environment, such as wastewater and greenhouse gases, compared to those from the original are crucial steps in the measurement process. For example, differentiation between potable and non-potable water often requires chemical characterisation.
Against this background, there is a growing awareness of the need to monitor the impact that humankind has had on the environment – air, soil, water – plus the biota that inhabit these, and to provide protection to human health against negative impacts.
Measurement-specific
Mass spectrometry (MS) is one of the most powerful tools for chemical composition identification in environmentally-relevant samples. This has been reviewed in a recent issue of this magazine. The MS instrument provides information related to chemical identification of compounds in the sample. Often the MS is hyphenated with a dimension of separation – gas chromatography (GC) or liquid chromatography (LC) – as GC–MS or LC–MS instruments. In this way, considerable informing power is provided for complex environmental samples. Other routine GC or LC detectors give complementary results to those from MS, but the degree of confirmation is lower, and may be insufficient for very complex samples. An alternative approach is to provide complete separation of components in samples prior to the detection, and the use of validated peak retention positions (through use of standards) to try to compensate for the loss of MS confirmation.
Gas chromatography (GC), introduced in the 1950s, is a powerful separation approach, and many innovations from improved injection, better separation formats, to more sensitive detection have been described. Separation is achieved based on different component boiling points and/or partitioning of each component between a stationary phase or solid support (GC column) and mobile phase (carrier gas). Thus GC has the ability to screen out only those volatile compounds in a sample, and does not analyse for non-volatiles. Its very high reproducibility and resolution towards compounds of similar mass, and its ease of coupling to MS have been long appreciated by environmental analysts. Extension of GC to lower volatility compounds and very polar materials can be alleviated to some degree by high temperature operation and use of derivatisation techniques. These latter reactions are readily performed using commercially available reagents, such as N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA), containing 1% trimethylchlorosilane (TMCS).
The GC process
A typical one-dimensional GC instrument is composed of three main units being (i) an inlet (I), (ii) an oven (where the column resides), and (iii) a detector (D), such as a mass spectrometer (MS), flame ionisation detector (FID), electron-capture detector (ECD), nitrogen-phosphorus detector (NPD), flame photometric detector (FPD), or olfactory detector (O) (Figure 1).
Using the most common split/splitless injector, analytes are evaporated and pass into the column, located in a temperature-programmable oven. The function of the column is to provide separation of components of the sample, which it does to different degrees of effectiveness according to the physical properties of the column (length, inner diameter), stationary phase type, film thickness, and operating conditions (flow rate, temperature). Compounds with higher vapour pressure and/or weaker interaction with the stationary phase elute from the column first. Thus the record of the chromatogram is a series of discrete chromatographic peaks provided with their respective retention time in the system, along with a measure of the response of the detector to that component.
This is all well and good, provided that a single response corresponds to a unique component. For instance, while the peak capacity of a chromatographic experiment (total time available for separation divided by the duration of an individual component) may be many hundreds, often the critical limitation for the GC experiment is that not all compounds can be resolved. Thus there are two recurring concerns in GC analysis: obtaining the necessary resolution to allow accurate reporting of compounds, and the overall sensitivity of the analysis. Notwithstanding this concern, GC is still a powerful and valuable tool that is indispensable for monitoring the environment.
The 1D GC instrument is amended in Figure 1 to illustrate a further advance that today is becoming a major alternative to single–column operation: that of multidimensional gas chromatography (MDGC), with additional dimension(s) of separation. Although developed many years ago, it is now being applied more widely to many areas of GC analysis.
There are two primary modes of operation: classical MDGC, where some zone of unresolved peaks from column 1 are transferred to column 2; and comprehensive two-dimensional gas chromatography (GC×GC) where a sample is continually ‘modulated’ from 1D to 2D to give a unique 2D presentation of the sample composition. In MDGC, the target peaks are better resolved on the second column; in the GC×GC case, better separation for the whole sample results. In the latter case, considering a perfect coupling of two columns with two complementary separation mechanisms, and allowing separation capacities to be 400 and 20 for each column (the 2D column is short, and so has lower capacity), then we might propose that theoretically 400×20 = 8,000 compounds could be separated.
Combined with a sensitive MS detector, the opportunities for substantially improved characterisation of samples are apparent. This is especially so for trace analysis, since often compounds elute in regions where underlying interferences are absent. While the activity in MDGC and GC×GC is exciting, most laboratories have not embraced these technologies, and not many validated or standard methods of analysis are available. While discussions of these are outside the scope of this article, a recent review serves to outline the background to MDGC and GC×GC operation.
The number of publications involving use of GC for measurement of our environment is expansive, from determination of greenhouse gases, to pesticide residues in soil, food and other samples, chemical contaminants in water, and waste products in agriculture and industry. We will illustrate the use of GC in various environmental sinks, summarised in Table 1, and briefly refer to GC×GC methods.
As a preface to discussion of GC, the role of correct, suitable or adequate sample handling must be emphasised. Again, only a cursory overview can be paid to this here, but it is important to recognise that without due attention to sampling, sample handling and matrix clean up, the subsequent analysis step will be potentially futile.
Thus in discussion of applications of GC analysis for different sample types, the sampling and extraction/isolation step is a necessary prelude. As a summary of the overall analysis process for soil, air and water samples, Figure 2 serves to place sampling in context. Derivatisation is sometimes performed prior to GC analysis, depending on the nature of the analyte.
Soil
Dioxin (polychlorinated dibenzo-p-dioxins, PCDDs) and dioxin-like compounds (DLCs) are among two of the most toxic compounds. DLCs are comprised of polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs). These compounds reportedly cause body weight loss, immune system impairment, reproductive disorder, and cancer. They are widely dispersed in the environment, including the polar regions.
Our focus here is contaminated soil samples, mainly caused by incineration, and from landfill. Note that incineration will also lead to airborne burdens. GC coupled with MS is well known for the analysis of dioxin and DLCs as reported in various regulations such as USEPA1613, ISO18073, EN1948, and JIS K0311. It represents an advanced and important analytical strategy based upon best GC with sensitive and usually accurate mass MS.
A typical process includes Soxhlet extraction, followed by clean up process employing silica, alumina, Florisil and/or carbon as adsorbents. Alternatively, solid phase extraction (SPE), pressurised liquid extraction (PLE), microwave-assisted extraction (MAE), and supercritical fluid extraction (SFE) can be employed.
Different GC systems can be applied depending on selectivity, sensitivity, and speed of the analysis, and the trade-off between these parameters. In a productivity sense, all of these can be reduced to either a cost per analysis or per analytes, or reported analyte per unit of time, for routine analysis.
Commercial columns (e.g. RTX-PCB, HT-8 and DB-Dioxin) are available specifically for dioxin analysis. Precise dioxin analysis normally requires best extraction (due to low abundance of compounds) with best separation (high resolution columns), and most precise mass spectrometry (to isolate individual compound mass ions), and so it ticks all the boxes for advanced analysis methodology. With isotope enrichment using labelled standards, the cost per analysis is high. The improved separation of GC×GC with TOFMS detection or ECD, allows the detection step to be relaxed from the use of high resolution mass spectrometry. This can boost analysis efficiency.
A further interest area for soils analyses is in the use of GC for site evaluation for former petrol stations and fuel bunkers, and extends to such applications as military site monitoring for waste volatile compounds.
Air
Among various chromatography techniques, GC is a preferred method for air analysis due to the fact that samples are already in the gaseous state. Recently, the use of advanced mass spectrometry methods such as proton transfer reaction MS (PTRMS) have been used for direct analysis of the air stream.
One common application is the analysis of urban air. Volatile organic compounds (VOCs), arising from metropolitan centres can lead to the formation of secondary organic aerosols (SOAs), which are potentially dangerous to humans, leading to health concerns; hence monitoring programmes become crucial, and are in place in many cities in both developed and developing countries. Thus assessing the polyaromatic hydrocarbon load in urban air is a typical application, with sampling of particulate material of different diameters onto filters, with extraction and GC analyses.
For an analysis of hundreds of VOCs in an urban air sample, separation using a single-column may be insufficient. Chromatograms (Figure 3) from an urban air sample were taken in Melbourne in which the left chromatogram was acquired from a conventional one-dimensional GC. The right hand result was generated from GC×GC analysis of the same sample by using FID detection. This did not allow components to be identified by spectroscopic means, but still many component identities can be inferred.
Water
Perhaps no other matrix is measured, monitored nor reported under stricter guidelines than water. This is especially so for potable water. Thus both ‘legacy’ and emerging pollutants, impurities and nutrients are usually required to be listed in regular reports to water authorities.
Pharmaceutical and Personal Care Products (PPCPs)
As regular consumers of pharmaceutical and personal care products (PPCPs), and with a rate of consumption increasing with the worldwide population growth, the impact of PPCPs on water discharged to the environment needs assessment. Briefly, PPCPs are products that are consumed/used for personal health or cosmetic reasons, plus those that are used on livestock for agribusiness (www.epa.gov/ppcp).
Wastewater emerging from water treatment plants is often recycled or discharged to, e.g. rivers or the sea, and if levels of toxicants or other potential pollutants are not sufficiently reduced, the results can be quite detrimental. Some examples of commonly used chemicals are antibiotics, lipid regulators, non-steroidal anti-inflammatory drugs (NSAIDs), ß-blockers and ß-agonists. Sunscreens and shampoos also potentially increase the PPCP load.
Water sample analysis is generally composed of four main steps: extraction, clean up, derivatisation, and GC analysis (refer to Figure 2). The technology and efficiency of wastewater treatment plants may not have kept up with these more recent PPCP species. While conventional solid phase extraction (SPE) is one of the most common techniques for sample preparation, molecularly-imprinted polymers (MIPs) has shown to be a promising extraction and clean up technique due to its selectivity and specificity of the targeted analytes, as well as its simplicity in synthesis step.
Other techniques include solid phase micro-extraction (SPME), pressurised liquid extraction (PLE), liquid-liquid extraction (LLE), ultrasound assisted extraction (UAE), and supercritical fluid extraction (SFE). Further details are extensively reviewed in. Derivatisation, required for compounds of interest that are polar and/or non-volatile, is followed by GC analysis.
Pesticides
The usage of pesticides on cropland can lead to pesticide residues within the soil. Run-off of these compounds from land to waterways is also possible. Impacts on aquatic life can be significant. The GC technique again is important for monitoring purposes and was used for samples related to one of the World Heritage natural precincts – the Great Barrier Reef, located along the north eastern coast of Australia.
Samples were collected from various areas along the coast, and were extracted with diethyl-ether after acidification, with methylation as a derivatisation step, and finally analysed with GC–MS. Phenoxyacid herbicides such as 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-methyl-4-chlorophenoxyacetic acid (MCPA) were detected. In addition to MS, for pesticides, ECD, NPD and FPD are sometimes employed as a detector.
Wastewater and odour assessment
Off odours and taints for both drinking water and processed water can have a strongly negative impact on consumers and local residents. An assessment mode was reported for monitoring of outflow from a wastewater treatment plant, utilising GC coupled with olfactometry (human sniffing) and a MS detector (GC–MS–O) 9. The olfactory detector allows correlation between the detected odour and the volatile profile obtained from MS data, and is usually conducted as a simultaneous dual detection method. This enables recognition of compounds that have a detectable perception to the human, with parallel mass spectral identification, and thereby provide a measure of chemical interpretation to off odours in potable water and water treatment plant effluent.
Industrial plant outflows
Industrial effluent discharge into urban streams and rivers requires effective and frequent monitoring. Normally discharge will be controlled by local authorities under terms of a licence to discharge, which may place responsibility for the monitoring process back onto the company.
A relatively robust and rugged GC–MS method was reported by Ziemer et al for discharge to the Rhine river from the BSAF Ludwigshafen site. The method employed two GC–MS systems to increase monitoring turnaround, and action response in case of levels exceeding the permissible range. Due to the method design that called for minimal, or no extraction which would otherwise extend analysis time, a novel injection technique that permitted aqueous injection and was able to take high salts load was developed based on a stacked PTV injector with backflushing. The system had > 98% up time which met design criteria.
References
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Published: 27th Nov 2012 in AWE International
