We're Moving to
The content you are trying to access has moved to new site. Explore, discover (and in certain contries, buy) our products and services on our new website. Click to visit now or you will be automatically redirected.
to view non-migrated solutions
This article gives 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.
Learn 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.
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:
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:
Specific static headspace analysis benefits include:
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.
When using equilibrium headspace sample introduction approaches for GC:
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.
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.
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.
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.
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.
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.
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.
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.
To view the full content, please answer a few questions.