Advancements for the Analysis of Environmental Matrices in the Industrial Landscape

Better Sample Techniques for Gas Chromatography

There are two sample introduction techniques for gas chromatography (GC), automated static headspace and large volume injections, that can offer unique benefits to users.

These techniques can improve laboratory productivity, enhance detection limits, and minimize the use of organic solvents all while facilitating the analysis of a complete range of volatile and semi- volatile compounds at trace concentrations in a variety of environmental, biological, petroleum, and other challenging sample matrices.

Scientists from ExxonMobil Environmental Toxicology and Chemistry Laboratory have demonstrated the advantages of these techniques for the analysis of environmental matrices in the industrial landscape.

They focused on five different areas:

• Advantages of headspace and large volume injection GC
• Guidelines and best practices when carrying out headspace analysis
• Challenging matrices that this laboratory has encountered over the years describing application examples of both headspace GC techniques, used in environmental fate and effects testing
• Exemplify the work with a single application of large volume injection GC along with the benefit of the PerkinElmer Swafer column backflush accessory
• Rapid method development techniques that allow for improved detection limits, reduced solvent use and optimized efficiency and productivity to benefit all who require investigation of industrial compounds in difficult matrices

Fundamentals of Static Headspace GC

Static headspace GC involves the transfer of a portion of the equilibrated vapor phase above a liquid or other matrix containing volatile and semi-volatile compounds. It is not an exhaustive extraction technique, but rather an equilibrium or partitioning technique where equilibrium between the liquid and the vapor phase is established by incorporating several sample treatment techniques to favor the partitioning from the liquid phase into the vapor phase of the target analytes.

For large-volume injection GC, semi-volatile compounds are trapped in the injection liner at a temperature near or below the solvent's boiling point, where the solvent is allowed to evaporate out of the open split valve. The higher boiling target compounds trapped in the inlet liner are then desorbed into the GC column as the injection temperature is rapidly increased. The prerequisite for large-volume injection GC is that the instrument is equipped with a temperature-programmable inlet that allows rapid temperature increase as the analysis proceeds.

The main benefits of headspace and large-volume injection GC as front-end sampling techniques are:

• They can be flexibly interfaced with GC/MS (mass spectrometry) or GC-FID (flame ionization detection) or whatever particular GC configuration is used.
• Between the two techniques, they cover a broad range of volatile and semi volatile compounds.
• They lessen the need for sample preparation and can significantly reduce solvent use.
• They enable the analyst to increase analytical sensitivity and sample throughput, thereby improving lab productivity.
• Both techniques are robust, repeatable, and quantitative.

Specific static headspace analysis benefits include:

• Virtually no carryover and no sample foaming as may occur in the purge and trap vessel.
• Associated analytes are captured in the vapor phase portions, so GC inlet and detector maintenance are significantly reduced because the non-volatile components do not make it to the GC inlet.
• Improved sample throughput and lab productivity due to reduced maintenance.

Analyte Compound Capability

Most analysts typically consider headspace when analyzing the most volatile compounds. In the environmental area, such compounds include benzene, toluene, xylene, and the classic BTEX compounds.

The ExxonMobil Environmental Toxicology and Chemistry Laboratory, however, regularly extends headspace applications up to the lower range of the semi-volatile compounds, including phenanthrene in water at low concentrations with good linearity and good repeatability. Alternatively, moving up the range of semi-volatile compounds into multi-ring polynuclear aromatic hydrocarbons (PAHs) such as pyrene and chrysene, large-volume injections should be considered where the analytes are not sufficiently volatile for headspace analysis, but instead can be cold trapped in the injection port as part of a liquid injection.

Between the two techniques, there is an area of overlap where both headspace and large volume injection are suitable. These include substituted naphthalenes and three-ring PAHs such as phenanthrene.

Best Practices When Using Headspace GC

When using equilibrium headspace sample introduction approaches for GC:

• Apply matrix-matching calibration whenever possible because it is the easiest and most straightforward means of calibrating headspace GC methods
• Use internal standards, which should be closely matched in volatility to the target analytes
• When carrying out GC/MS, use deuterated analogs of target analytes. If using FID as a detector, select structures that are similar to the target compound, such as a chlorinated analog of a hydrocarbon or a compound with an additional methyl group.
• It is important to use proper sample handling procedures for very volatile compounds. Water samples should be taken in the appropriate volatile organic analysis vials with no headspace. Transfer should then be carried out from those sample vials to the headspace vials using a gas- tight syringe avoiding the use of pipettes.
• Salting-out techniques offer several advantages. It decreases the solubility of the analytes in water, thereby favoring partition into the vapor phase. It also helps normalize the ionic strength of the samples, which can be beneficial when dealing with water samples from a variety of environmental sources such as seawater, brackish waters, estuarine, or fresh waters, thus providing reproducible analysis across a variety of water matrices.
• If matrix matching is not possible, alternative approaches should be explored. The simplest is sample dilution. If an aqueous-based sample is being analyzed, where it is difficult to obtain a pure sample free of the target analyte, simple dilution of the samples in reagent-grade water offers the best approach. However, make sure that the calibration is carried out in the same reagent-grade water.

Challenging Headspace Sample Matrices

The Laboratory was faced with a challenging application several years ago: to analyze trace volatile organics in a high molecular weight hydrocarbon fluid sample. Researchers were unable to find a similar sample without any of the target analytes and ended up using a very high-purity vacuum pump oil made for high-performance GC/MS work. This method provided a suitable diluent in which the sample was miscible, but it did not contain any of the target analytes. Researchers were able to use it both as the sample diluent and as the diluent for standardization and calibration.

Occasionally, solid samples such as polymers and plastics that are not soluble in any type of solvent come into the ExxonMobil lab. When this happens, multiple headspace extraction (MHE) can be applied that provides complete extraction of a sample and calculation of the amount of target analytes by comparison against an external standard.

By performing a series of consecutive headspace injections on the same sample, scientists can plot the logarithmic decay of the analytes and compare it to that of the headspace decay of the external standard (which is spiked into an empty headspace vial where there is no sample matrix present). Then by relating the sample decay plot to the standard decay plot, analysts can determine the exact mass of an analyte in that particular sample matrix.

Headspace Application:

The ExxonMobil laboratory is principally tasked with providing analytical support for environmental fate and effects testing.

One such headspace application involves measuring the industrial solvent decalin in both water and fish. Decalin (decahydronaphthalene) is a saturated naphthalene analogue that is commercially available as the trans- and cis-isomers at an approximate ratio of 60:40. It has a boiling point of 190 °C. Two methods were developed for this application: one for decalin in water using FID and the other for decalin in fish using MS detection.

Decalin in water

For decalin in water, sample preparation is representative of the lab’s standard procedure for water samples. There are a couple of points to emphasize in this method. The instrument is equipped with a trap accessory, which is extremely beneficial in improving the sensitivity, and reducing the achievable detection limit for the analysis. In addition, with regard to the headspace operating conditions, the temperature must be as high as possible to allow for the partitioning from the liquid phase into the vapor phase, but without boiling the sample off.

Decalin in fish

For the analysis of decalin in fish, the laboratory switched over to MS detection. The benefit of MS for this type of analysis is high selectivity by using the selective ion monitoring (SIM) mode of 146 amu for dichlorobenzene and 138 amu for decalin.

Sample preparation is straightforward for fish matrices. A typical sample weight of 1–3 g (depending on the fish) is placed in a 20 mL headspace vial, with 10 mL of a 1 M potassium hydroxide solution together with the internal standard. The vial is then sealed and sonicated for 90 minutes. After sonication, an aqueous base sample is left, which is ideally suited for direct headspace analysis.

Hydrocarbon solvents in water samples

The application of hydrocarbon solvents in water is very typical of characterizing petrochemical-related compounds in many of ExxonMobil’s products. Thus, these solvents are basically all the compounds eluting between 3.4 and 4.8 minutes using headspace-trap GC- FID.

If the area sum of all those peaks is integrated, the data can be used to quantitate this particular compound in water using GC-FID. Using this approach, a good calibration can be achieved, quantifying down to 2.1 ppb levels is achieved.

Large Volume Injection GC Applications

Switching gears, let’s take a closer look at applications involving large volume injection GC. At some runtime prior to injection, the split valve is open on the GC. At the point of injection, the split valve remains open.

At a point just past two minutes, a splitless injection is basically being carried out, where at approximately five minutes into the run, the split valve is again opened to remove any residual solvent that may be present in the injection liner.

High Molecular Weight Chemical Intermediates

A typical application employed is using large volume injections for quantifying very high molecular weight chemical intermediates in extracts from environmental or biological samples.

When doing large volume injections, a lot of sample is being loaded onto the column, so unless the extract or sample is extremely clean, free of contaminants, free of interferences and residues, there is the potential to contaminate the MS source, which could require time consuming cleaning. To avoid that, the group set up a backflush system, which involves using two analytical columns.

This backflushing approach has been applied in ExxonMobil’s Environmental Toxicology and Chemistry Laboratory and keeps the source cleaner for longer, which is important in maximizing laboratory productivity and minimize the downtime associated with cleaning the source.

It is known as the PerkinElmer Swafer™ microchannel flow technology. It comes with an extremely useful utility calculator that can be used to optimize parameters such as column length, dimensions, flow, pressure, and temperature.

This allows the analyst to optimize the methodology even before making a single injection. A couple of trial injections could be carried out to confirm the method is working. But besides using it to make a few tactical tweaks, most of the method development is done off-line, which is a significant improvement in laboratory productivity.

Conclusion

This article has given an overview of how ExxonMobil’s Environmental Toxicology and Chemistry Laboratory has developed strategies for fast method development projects using PerkinElmer’s automated static headspace and large volume injection gas chromatography systems. It demonstrates how these techniques have significantly improved laboratory productivity, lowered detection limits, and minimized the use of organic solvents in a variety of environmental, biological, and other challenging sample matrices.

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