Gas chromatographs are complex instruments with many nearly independent options that can be optimized for particular assays. Just look at the choices for sample introduction, columns, and detectors.
During a press day at Agilent Technologies in Little Falls, MD, the topic of element-specific detectors for GC came up, since Agilent has two. I asked about the state-of-the-art chemiluminescence detector in its portfolio. I was referred to Jason Ashe, product manager for gas chromatography.
RLS: Can you give us an update on the state-of-the-art of chemiluminescence detectors for GC?
JA: Chemiluminescence is the light generated by the chemical reactions. In the case of the Agilent 8255 Nitrogen Chemiluminescence Detector and the 8355 Sulfur Chemiluminescence Detector, nitrogen and sulfur compounds react with ozone after gas chromatographic separation to create light that is measured in the photomultiplier and detector tube. These detectors are popular when one wants excellent detection limits for speciation of analytes, or the matrix of the sample causes interferences typically seen in a universal detector such as a flame ionization (FID) or mass spectrometer (MS).
RLS: Why do the detectors have such low detection limits?
JA: The 8255 and 8355 are photometric detectors—simply put, they detect light formed through a chemical reaction, chemiluminescence. If there is no reactive analyte present, then there is no signal. But when an analyte of interest is present after a reaction, light is emitted and detected within the photomultiplier tube. Both detectors’ photomultiplier tubes are shielded from light and use very sensitive detectors to achieve some of the best detection limits in the marketplace. Detecting a dim light against a black background is technically easier than measuring the loss of a little light out of a bright lamp, as is the case with UV absorbance.
RLS: How does the 8355 detector work?
JA: The Agilent 8355 employs a dual plasma burner to achieve high temperature of sulfur-containing compounds to form sulfur monoxide (SO). A photomultiplier tube detects the light produced by the chemiluminescent reaction of SO with ozone. This results in a linear and equimolar response to the sulfur compounds, without interference from most sample matrices.
RLS: You mention equimolar response. Please explain how and why it is useful.
JA: Equimolar responses in a detector have many benefits. As in sulfur detection, there can be co-elution of vital analytes within a hydrocarbon matrix sample. The Agilent xCD detectors are more desirable for this type of analysis due to the ability to eliminate this interference. Using a single-point calibration is another benefit of an equimolar response detector.
RLS: How does the 8255 detector work?
JA: The Agilent 8255 Nitrogen Chemiluminescence Detector (NCD) is a nitrogen-specific detector that produces a linear and equimolar response to nitrogen compounds. This is accomplished by using a stainless-steel burner to achieve high-temperature combustion of nitrogen-containing compounds to form nitric oxide (NO), which reacts with ozone producing chemiluminescence, as with the 8355.
RLS: What are some important applications?
JA: Nitrogen chemiluminescence detector cold starting diesel engines can be difficult. Alkyl nitrates, which are easier to ignite, are often added to the bulk fuel to provide the starting kick at lower temperature. They also decrease emission of particulates. For example, about 2 ppm of 2-ethylhexane nitrate is often added to the fuel to improve cold-start performance. QC control requires assay of the nitrate content in a very high background of carbon and hydrogen. Agilent has an application note describing a 7890 GC equipped with a nitrogen chemiluminescence detector (NCD) that provides assay of 2-ethylnitrohexane plus a small amount of very late-eluting impurities.
RLS: What about using two or more detectors with the same injection, such as an 8355 with an FID for sulfur in fuels?
JA: Yes, this is possible, since our model 7890 has space for technically three detectors, but both the 8255 and the 8355 have the ability to run a tandem xCD/FID combination. A good example is when low detection is not needed and there is a high concentration of hydrocarbons. The tandem detectors allow for full combustion in the FID and then directly onto chemiluminescence detection. Due to the nature of this type of detection, the MDLs are approximately 10 times higher than with direct chemiluminescence detection. There is not a large application space for NCD/FID tandem detectors, but the 8255 NCD does allow for the combination should the need arise.
RLS: Agilent offers other detectors such as FPD, hot ceramics, and a range of mass spectrometers. Could you help us compare these?
JA: Flame photometric detectors (FPD) use a different chemical reaction to produce chemiluminescence and are specific for sulfur- and phosphorus-containing compounds. The FPD reacts GC separated S and P compounds through a hydrogen/air flame that is detected in a photomultiplier tube with specific filters and measured with the associated emission wavelengths.1 Unfortunately, as previously mentioned, S compounds co-elute with hydrocarbons, making quantitation impossible; this is where chemiluminescence has a significant benefit. The Agilent 8355 is equimolar in response and can speciate S compounds as well as more sensitive than the FPD.
Nitrogen-phosphorus detectors (NPD) for gas chromatography are specific to nitrogen- or phosphorus-containing compounds and are well suited for environmental and forensic applications. The key component to NPD performance is its bead. Agilent uses the Blos bead for NPD. The Blos bead offers very high operational stability and long lifetime when compared to the legacy white ceramic bead. The Blos bead for the Agilent NPD is based on a high-performance glass material technology. NPDs do not provide equimolar response, and each compound of interest must be calibrated accordingly.
Mass spectrometry is an analytical chemistry technique that helps identify the amount and type of chemicals present in a sample by measuring the mass-to-charge ratio and abundance of gas-phase ions. A mass spectrum (a plot of the ion signal as a function of the mass-to-charge ratio, m/z) can provide valuable information about the molecular weight, structure, identity, and quantity of a compound. Electron impact ionization (EI) does not provide element-specific detection. Electrospray ionization is subject to signal enhancement or suppression from components in the matrix. Thus, response needs to be referenced to primary standards in the particular target matrix.
RLS: When is CL the detector of choice?
JA: Gas chromatography with sulfur chemiluminescence detection provides a rapid means of identifying and quantifying sulfur impurities or sulfur-based odorants in natural gas and gaseous fuels. Examples include sulfur compounds in air, methane, propane, digester gas, and refinery fuel gases. All of these analyses benefit from the equimolar response of the chemiluminescence detector.
With the increased use of lower-grade raw materials, and tighter global regulation on pollution, sulfur testing is gaining more importance for both up- and downstream activities. As with any process, there are significant throughput and profitability impacts if the process is not controlled properly. One of the most reliable detection methods for a variety of sulfur compounds is by chemiluminescence because of the equimolar response, linearity, dynamic range, and freedom from matrix effects. Plus, the response of the CL detectors simplifies the apparent response, allowing less experienced operators to focus on the key parameters that control the process. With the sulfur detector, one can focus on the sulfur content, avoiding the confusion with picking peaks from a complex background forest of peaks.
RLS: How well does CL solve problems?
JA: The chemiluminescence detectors are linear and equimolar in response and reduce matrix interferences with low-level detection. This means they are the detector of choice for raw materials, upstream and downstream products, no matter what the matrix is.
RLS: What is the history of CL detection at Agilent?
JA: As the global leader in gas chromatography, Agilent was selected by General Electric to buy the product line in 2002. GE had acquired the right to CL detection from Sievers Instruments previously as part of the Hach acquisition for water testing. While Agilent was proud of the acquisition, we soon found that the first-generation technology was ripe for upgrade. It took a couple of years, but we were able to improve detection limits, precision, and robustness of the detector.
RLS: Why is the new design better?
JA: The new 8255/8355 have made significant changes to the entire detector and burner assemblies. Agilent has also integrated the control box and the detector box into one compact design.
A significant design change was integrating the chemiluminescence detectors into the 7890A+/7890B GC. This configuration allows for full digital data without the need for signal attenuation. The detectors are fully controlled from the GC interface whether it is the local user interface or an Agilent chromatography data system.
Also, both detectors were redesigned with significant advancements moving from manual pneumatics to EPC (electronic pneumatic control) for gas control. These EPC modules are located directly in the detector box on a pneumatics carrier attached, secured with four screws. Removing these four screws and one connector tube nut allows for the entire pneumatics carrier to be removed from the detector box should the need for maintenance arise.
To service all our legacy Agilent GC customers, Agilent has maintained a standalone version of the 8255 and 8355 detectors that shares all the new advancements but has complete control from the detector box.
RLS: Do you see opportunities to extend the CL detection to other elements? Or radioactivity?
JA: (Please fill in) not at this time
JA: In summary, simply put, we recognized that xCD technology must evolve to keep pace with growing demands for regulatory compliance and workflow efficiency. We started by integrating the xCD with the Agilent 7890B—the world’s most reliable GC and chromatography data system—to improve the overall user experience. We then reengineered the burner assembly to maximize instrument uptime and simplify routine maintenance.
Reference
- Dr. Thomas G. Chasteen; Department of Chemistry, Sam Houston State University, Huntsville, Texas 77341. ©1995, 2007, 2009; https://www.shsu.edu/~chm_tgc/FPD/FPD.html
Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; e-mail: [email protected]
Jason Ashejoined Agilent in 2013 with over 20 years of analytical experience, and is currently a product manager in the GC marketing group managing GC products. Jason received his MBA with a focus in organizational leadership from Post University in 2010. Early in his career, he held chemist and lab manager positions at Environmental Waste Resources and EAS Analytical Laboratories, respectively. Prior to joining Agilent, Jason was employed at PerkinElmer as a consumables product manager and a GC/GC/MS service and support manager.