Financial support was provided by the Department of Homeland Security Science and Technology Directorate.

1. Instrument Preparation
<ol>
	<li>Ensure the instrument, oven, and detector are at RT. Turn off gas flow to the inlet and detector.</li>
	<li>Remove the TDS from the GC. Consult the manufacturer&#8217;s user manual for the instrument-specific procedure.</li>
	<li>Remove the TDS adaptor from the CIS inlet and remove the liner from the CIS.</li>
	<li>Inspect the CIS inlet for particles and debris while the liner is removed. Clean any visible debris with compressed air, or preferably nitrogen.</li>
	<li>Attach a new graphite ferrule to a new CIS liner using the manufacturer provided tool and instructions for ferrule-to-liner binding.</li>
	<li>Insert the liner with the attached graphite ferrule into the CIS. Replace the TDS adaptor and re-mount the TDS.</li>
	<li>Remove a new column from its packaging and remove the silicone protection from the ends of the column.</li>
	<li>Insert a nut and ferrule onto each end of the column. Use an ECD detector nut and ferrule for one end of the column and a CIS ferrule for the opposite end of the column.</li>
	<li>Using a ceramic column cutting tool, remove approximately 10 cm from each end of the column. Ensure the nuts and ferrules remain on the column but away from the end of the column to avoid clogging and debris.</li>
	<li>Secure the column into the oven using the instrument manufacturer guidelines. Insert the column into the inlet. Connect the other end of the column to the detector port. The depth of insertion is specific to instrument, inlet, and detector manufacturer. See the user manual and specifications for the exact column insertion depth. 
	NOTE: A pre-bake may be required for the column before connecting the opposite end of the column to the detector ports. Consult the column and instrument manufacturer documentation to determine if a pre-bake is required.</li>
	<li>Gently hand-tighten nuts and ferrules onto their respective ports for the inlet and detector. Using a wrench, tighten with approximately a quarter turn of rotation the nuts and ferrules. Too much force or over-tightening will damage the ferrules causing leaks or the column to break and clog.</li>
	<li>Bake out the TDS, inlet, column, and detector. A typical bake out consists of setting the temperature for all zones to just below the maximum operating temperature (typically 300 &#176;C) while flowing carrier gas for at least 2 hr.</li>
	<li>Cool all zones and retighten all nuts and ferrules to ensure leak-free operation. Heating and cooling during the bake out will cause the nuts and ferrules to loosen, which can introduce leaks.</li>
	<li>Load, or reload, the instrument method using the software interface. Verify correct temperatures and flow rates have been achieved. Instrumentation is ready for analysis.</li>
</ol>
2. Preparation of Standards
<ol>
	<li>Remove 1,000 ng &#181;l-1 3,4-DNT, 10,000 ng &#181;l-1 TNT, and 10,000 ng &#181;l-1 RDX from the freezer or refrigerator and allow the three stock solutions to reach RT.</li>
	<li>Dispense 100 &#956;l of stock 1,000 ng &#181;l-1 3,4-DNT and add 900 &#181;l of acetonitrile into an amber sample vial.</li>
	<li>Dispense 100 &#956;l of the 100 ng &#181;l-1 3,4-DNT solution from Step 2.2 and add 900 &#181;l of acetonitrile into an amber sample vial.</li>
	<li>Dispense 150 &#181;l of the 10 ng &#181;l-1 3,4-DNT solution from Step 2.3 and 4,850 &#181;l of acetonitrile into an amber sample vial. This is the internal standard for direct liquid deposition.</li>
	<li>Dispense 100 &#956;l of stock 10,000 ng &#181;l-1 TNT solution, 100 &#956;l of stock 10,000 ng &#181;l-1 RDX solution, and 800 &#181;l of acetonitrile into an amber sample vial.</li>
	<li>Dispense 100 &#956;l of the 1,000 ng &#181;l-1 TNT and RDX solution in Step 2.5 and 900 &#181;l of acetonitrile into an amber sample vial.</li>
	<li>Dispense 100 &#956;l of the 100 ng &#181;l-1 TNT and RDX solution from Step 2.6 and 900 &#181;l of acetonitrile into an amber sample vial.</li>
	<li>Dispense 100 &#956;l of the 10 ng &#181;l-1 TNT and RDX solution from Step 2.7 and 900 &#181;l of acetonitrile into an amber sample vial. This creates the 1.0 TNT/1.0 RDX ng &#956;l-1 solution standard ready for direct liquid deposition onto sample tubes.</li>
	<li>Dispense 60 &#181;l of the 10 ng &#181;l-1 solution in Step 2.7 and 940 &#181;l of acetonitrile into an amber sample vial. This creates the 0.6 TNT/0.6 RDX ng &#956;l-1 solution standard ready for direct liquid deposition onto sample tubes.</li>
	<li>Dispense 40 &#181;l of the 10 ng &#181;l-1 solution in Step 2.7 and 960 &#181;l of acetonitrile into an amber sample vial. This creates the 0.4 TNT/0.4 RDX ng &#956;l-1 solution standard ready for direct liquid deposition onto sample tubes.</li>
	<li>Dispense 20 &#181;l of the 10 ng &#181;l-1 solution in Step 2.7 and 980 &#181;l of acetonitrile into an amber sample vial. This creates the 0.2 TNT/0.2 RDX ng &#956;l-1 solution standard ready for direct liquid deposition onto sample tubes.</li>
	<li>Dispense 100 &#181;l of the 1.0 ng &#181;l-1 solution in Step 2.8 and 900 &#181;l of acetonitrile into an amber sample vial. This creates the 0.1 TNT/0.1 RDX ng &#956;l-1 solution standard ready for direct liquid deposition onto sample tubes.</li>
</ol>
3. Sample Collection
<ol>
	<li>Connect one sorbent-filled thermal desorption sample tube to a sample pump or similar equipment using a small piece of flexible silicone tubing. A red arrow is provided on the sample tubes indicating the air flow direction for sample adsorption, and it should be pointing in the direction of the silicone tubing and sample pump.</li>
	<li>Attach a piston flow meter to the sample tube at the opposite end from the sample pump attached in Step 3.1. Adjust the flow rate on the sample pump, or similar equipment, such that the flow rate is approximately 100 ml min-1 through the sample tube according to the readings from the piston flow meter. The flow rate should be set to &#177;5.0 ml min-1 of the 100 ml min-1 desired set point.</li>
	<li>Disconnect the piston flow meter from the sample tube and temporarily shut off the sample pump but leave the sample tube connected to the pump. The sample pump will be reactivated to begin sample collection. The sample tube is ready for collection.</li>
	<li>Place the sample tube with the still connected sample pump in the explosives vapor stream. The vapor source could be the headspace above a solid sample, an open environment, or a variety of analyte vaporization systems.</li>
	<li>Set a timer based on the approximate sampling times listed in Table 2. The sampling times are listed as a general guideline based on the suspected concentration of material in the vapor phase. These sampling times, with a flow rate of 100 ml min-1, will generally yield a mass in the center of the calibration curve, which is ideal for quantitation.</li>
	<li>Activate the sample pump and start the timer. Wait until the timer has stopped and shut off the sample pump. Disconnect the sample tube from the pump and place it in the packaging provided with the sample tube. Cap the tube and store for analysis.</li>
	<li>Record the unique serial number stamped onto each sample tube, the sample time, and the flow rate for the sample tube in a laboratory notebook. These values will be important for quantitation.</li>
</ol>
4. Calibration Curve Generation
<ol>
	<li>Pipet 5.0 &#181;l of the solution standard directly on the glass frit of an unused, conditioned sample tube. Hold the sample tube and pipet upright with a gloved hand during deposition.</li>
	<li>Repeat Step 4.1 for each of the six calibration standards onto three different sample tubes.</li>
	<li>Deposit 5 &#181;l of the 0.3 ng &#956;l-1 3,4-DNT on each of the tubes as well.</li>
	<li>Allow the eighteen sample tubes (three per solution concentration, six solution concentrations) to sit at RT for at least 30 min to evaporate the solvent.</li>
	<li>Use the twenty tube autosampler and the previously described TNT and RDX TDS-CIS-GC-ECD method to run and analyze all eighteen tubes O/N.24,25 A summary of the TDS-CIS-GC-ECD parameters for the method is provided in Table 1.</li>
	<li>Integrate the peaks associated with 3,4-DNT, TNT, and RDX in the chromatogram for each of the eighteen sample tubes. The 3,4-DNT, TNT and RDX peaks will occur at approximately 4.16, 4.49 and 4.95 min, respectively.</li>
	<li>Note the 3,4-DNT, TNT and RDX peak areas for each of the eighteen tubes along with the corresponding mass of TNT and RDX that was deposited on the sample tube in a spreadsheet and laboratory notebook.</li>
	<li>Normalize the peak areas for both TNT and RDX by dividing each peak area by the peak area for 3,4-DNT. Do this for all eighteen tubes.</li>
	<li>Calculate the average and standard deviation of the normalized TNT and RDX peak areas for the six standard concentrations.</li>
	<li>Plot the average normalized peak area versus mass of analyte present on the tubes for both TNT and RDX.</li>
	<li>Add a linear trend line for both the TNT and RDX data points. Identify the slope and y-intercept for each analyte. Record the slope, intercept, and R2 value in a spreadsheet and laboratory notebook.</li>
	<li>Place used sample tubes in a tube conditioner for 3 hr at 300 &#186;C and 500 ml min-1 nitrogen flow.</li>
</ol>
5. Sample Analysis
<ol>
	<li>Deposit 5.0 &#181;l of the 0.3 ng &#181;l-1 3,4-DNT on each of the sample tubes.</li>
	<li>Allow the tubes to sit at RT for at least 30 min to evaporate the solvent from the internal standard.</li>
	<li>Use the twenty tube autosampler and the previously described TNT and RDX method to run the tubes O/N on the TDS-CIS-GC-ECD.24,25 A summary of the instrumentation parameters for the analysis method is provided in Table 1.</li>
	<li>Integrate the peaks associated with 3,4-DNT, TNT, and RDX in the chromatogram for each of the eighteen sample tubes. The 3,4-DNT, TNT and RDX peaks will occur at approximately 4.16, 4.49 and 4.95 min, respectively.</li>
	<li>Note the 3,4-DNT, TNT and RDX peak areas for each of the sample tubes in a spreadsheet and laboratory notebook.</li>
	<li>Use the peak areas and calibration curve to calculate the vapor concentration in parts-per-billion by volume (ppbv) for each analyte. See Equations 1-4.</li>
	<li>Place used sample tubes in a tube conditioner for 3 hr at 300 &#186;C and 500 ml min-1 nitrogen air flow.</li>
</ol>

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We have nothing to disclose.

Reproducibility is a critical attribute for the quantitation of trace explosive vapors using the direct liquid deposition method with TDS-CIS-GC-ECD instrumentation, and Relative Standard Deviation (RSD) is often used as a metric for reproducibility. We have experienced RSDs for inter- and intra-sample reproducibility of approximately 5% for TNT and 10% for RDX. Any RSD above 15% is used as an indicator to check common sources of variation that reduce the effectiveness of the protocol. Sources of variation that have led ...

Gas Chromatography (GC) is a core instrumental analysis technique of Analytical Chemistry and is arguably as ubiquitous as a hot plate or balance in a chemistry laboratory. GC instrumentation can be used for the preparation, identification, and quantitation of a multitude of chemical compounds and can be coupled to a variety of detectors, such as flame ionization detectors (FIDs), photo-ionization detectors (PIDs), thermal conductivity detectors (TCDs), electron capture detectors (ECDs), and mass spectrometers (MS), depending on the analytes, methodology, and application. Samples can be introduced through a standard split/splitless inlet when working with small sample solutions, specialized headspace analysis inlets, solid phase micro-extraction (SPME) syringes, or thermal desorption systems. GC-MS is often the standard technique used in validation and verification applications of alternative or emerging, detection techniques because of its utility, flexibility, and identification power with established chemical databases and libraries.1&#8211;7 GC and its related sampling and detecting components is ideal for routine chemical analysis and more specialized, challenging analytical applications.
An analytical application of increasing interest to military, homeland security, and commercial enterprises is trace explosive vapor detection, with detection including identification and quantitation. Trace explosive vapor detection is an unique analytical chemistry challenge because the analytes, such as 2,4,6-trinitrotoluene (TNT) and cyclotrimethylenetrinitramine (RDX) have physical properties that make them especially difficult to handle and separate using broader, more generic chemical analysis methodologies. The relatively low vapor pressures and sub parts-per-million by volume (ppmv) saturated vapor concentration, combined with relatively high sticking coefficients, necessitate special sampling protocols, instrumentation, and quantitation methods.8&#8211;12 A GC coupled to an Electron Capture Detector (ECD) or mass spectrometer (MS) is an effective method for quantitating explosive analytes, specifically dinitrotoluene (DNT), TNT, and RDX.6,13&#8211;17 GC-ECD is particularly useful for nitro-energetic compounds because of their relatively high electron affinity. The U.S. Environmental Protection Agency (EPA) has created standard methods for explosive analyte detection using GC-ECD and GC-MS, but these methods have focused on samples in solution, such as ground water, and not samples collected in the vapor phase.2,18&#8211;23 In order to detect explosive vapors, alternative sampling protocols must be used, such as vapor collection with sorbent-filled thermal desorption sample tubes, but quantitative detection remains difficult due to lack of vapor standards and calibration methods that do not account for sample tube and instrumentation losses.
Recently, quantitation methods using thermal desorption systems with a cooled inlet system (TDS-CIS), coupled to a GC-ECD have been developed for TNT and RDX vapors.24,25 The losses associated with the TDS-CIS-GC-ECD instrumentation for trace explosive vapors were characterized and accounted for in example calibration curves using a direct liquid deposition method onto sorbent-filled thermal desorption sample tubes. However, the literature focused on instrumentation characterization and method development but never actually sampled, analyzed, or quantitated explosive vapors, only solution standards. Herein, the focus is on the protocol for sampling and quantitating explosive vapors. The protocol and methodology can be expanded to other analytes and trace explosive vapors, such as Pentaerythritol tetranitrate (PETN).

<table><tbody><tr><td>2,4,6-Trinitrotoluene (TNT)</td><td>Accu-Standard</td><td>M-8330-11-A-10X</td><td>10,000 ng &amp;mu;l&lt;sup&gt;-1&lt;/sup&gt;</td></tr><tr><td>Cyclotrimethylenetrinitramine (RDX)</td><td>Accu-Standard</td><td>M-8330-05-A-10X</td><td>10,000 ng &amp;mu;l&lt;sup&gt;-1&lt;/sup&gt;</td></tr><tr><td>3,4-Dinitrotoluene (3,4-DNT)</td><td>Accu-Standard</td><td>S-22988-01</td><td>1,000 ng &amp;mu;l&lt;sup&gt;-1&lt;/sup&gt;</td></tr><tr><td>Tenax&amp;reg; TA Vapor Sample Tubes</td><td>Gerstel</td><td>009947-000-00</td><td>Tenax&amp;reg; 60/80</td></tr><tr><td>CIS4 Liner</td><td>Gerstel</td><td>014652-005-00</td><td>or equivalent</td></tr><tr><td>Transfer Line Ferrule</td><td>Gerstel</td><td>001805-008-00</td><td></td></tr><tr><td>Inlet Liner Ferrule</td><td>Gerstel</td><td>001805-040-00</td><td></td></tr><tr><td>CIS4 Ferrule</td><td>Gerstel</td><td>007541-010-00</td><td></td></tr><tr><td>ECD Detector Ferrule</td><td>Agilent</td><td>5181-3323</td><td></td></tr><tr><td>DB5-MS Column</td><td>Res-Tek</td><td>12620</td><td></td></tr></tbody></table>

quantitative detection of trace explosive vapors by programmed temperature desorption gas chromatography-electron capture detector

The direct liquid deposition of solution standards onto sorbent-filled thermal desorption tubes is used for the quantitative analysis of trace explosive vapor samples. The direct liquid deposition method yields a higher fidelity between the analysis of vapor samples and the analysis of solution standards than using separate injection methods for vapors and solutions, i.e., samples collected on vapor collection tubes and standards prepared in solution vials. Additionally, the method can account for instrumentation losses, which makes it ideal for minimizing variability and quantitative trace chemical detection. Gas chromatography with an electron capture detector is an instrumentation configuration sensitive to nitro-energetics, such as TNT and RDX, due to their relatively high electron affinity. However, vapor quantitation of these compounds is difficult without viable vapor standards. Thus, we eliminate the requirement for vapor standards by combining the sensitivity of the instrumentation with a direct liquid deposition protocol to analyze trace explosive vapor samples.

Obtaining quantitative results for trace explosive vapor samples begins with establishing a calibration curve for the TDS-CIS-GC-ECD instrumentation using the direct liquid deposition method of solution standards onto sample tubes to account for instrument losses and differences between solution standards and vapor samples. The TDS-CIS-GC-ECD instrumentation and method for TNT and RDX trace analysis has been previously described in detail elsewhere, but the instrument parameters are summarized in Table 1...

Watch this Scientific Journal Video about Quantitative Detection of Trace Explosive Vapors by Programmed Temperature Desorption Gas Chromatography-Electron Capture Detector at JoVE.com

Quantitative Detection of Trace Explosive Vapors by Programmed Temperature Desorption Gas Chromatography-Electron Capture Detector

Trace explosive vapors of TNT and RDX collected on sorbent-filled thermal desorption tubes were analyzed using a programmed temperature desorption system coupled to GC with an electron capture detector. The instrumental analysis is combined with direct liquid deposition method to reduce sample variability and account for instrumentation drift and losses.

The direct liquid deposition of solution standards onto sorbent-filled thermal desorption tubes is used for the quantitative analysis of trace explosive ...

quantitative-detection-trace-explosive-vapors-programmed-temperature

Research

JoVE Journal

Chemistry

19.8K Views.  U.S. Naval Research Laboratory.  Trace explosieve dampen van TNT en RDX verzameld op-Sorbens gevulde thermische desorptie buizen werden geanalyseerd met behulp van een geprogrammeerde temperatuur desorptie systeem gekoppeld aan GC van een electron capture detector. De instrumentele analyse wordt gecombineerd met directe depositie methode vloeistof monster variabiliteit en rekening voor instrumentatie drift en verliezen te beperken. 

Video: Quantitative Detection of Trace Explosive Vapors by Programmed Temperature Desorption Gas Chromatography-Electron Capture Detector

Measurement of H2S in Crude Oil and Crude Oil Headspace Using Multidimensional Gas Chromatography, Deans Switching and Sulfur-selective Detection

A method for the analysis of dissolved hydrogen sulfide in crude oil samples is demonstrated using gas chromatography. In order to effectively eliminate interferences, a two dimensional column configuration is used, with a Deans switch employed to transfer hydrogen sulfide from the first to the second column (heart-cutting). Liquid crude samples are first separated on a dimethylpolysiloxane column, and light gases are heart-cut and further separated on a bonded porous layer open tubular (PLOT) column that is able to separate hydrogen sulfide from other light sulfur species. Hydrogen sulfide is then detected with a sulfur chemiluminescence detector, adding an additional layer of selectivity. Following separation and detection of hydrogen sulfide, the system is backflushed to remove the high-boiling hydrocarbons present in the crude samples and to preserve chromatographic integrity. Dissolved hydrogen sulfide has been quantified in liquid samples from 1.1 to 500 ppm, demonstrating wide applicability to a range of samples. The method has also been successfully applied for the analysis of gas samples from crude oil headspace and process gas bags, with measurement from 0.7 to 9,700 ppm hydrogen sulfide.

Measurement of H2S in Crude Oil and Crude Oil Headspace Using Multidimensional Gas Chromatography, Deans Switching and Sulfur-selective Detection

A multidimensional gas chromatography method for the analysis of dissolved hydrogen sulfide in liquid crude oil samples is presented. A Deans switch is used to heart-cut light sulfur gases for separation on a secondary column and detection on a sulfur chemiluminescence detector.

A method for the analysis of dissolved hydrogen sulfide in crude oil samples is demonstrated using gas chromatography. In order to effectively eliminate ...

Environment

Light-driven Enzymatic Decarboxylation

Oxidoreductases belong to the most-applied industrial enzymes. Nevertheless, they need external electrons whose supply is often costly and challenging. Recycling of the electron donors NADH or NADPH requires the use of additional enzymes and sacrificial substrates. Interestingly, several oxidoreductases accept hydrogen peroxide as electron donor. While being inexpensive, this reagent often reduces the stability of enzymes. A solution to this problem is the in situ generation of the cofactor. The continuous supply of the cofactor at low concentration drives the reaction without impairing enzyme stability. This paper demonstrates a method for the light-catalyzed in situ generation of hydrogen peroxide with the example of the heme-dependent fatty acid decarboxylase OleTJE. The fatty acid decarboxylase OleTJE was discovered due to its unique ability to produce long-chain 1-alkenes from fatty acids, a hitherto unknown enzymatic reaction. 1-alkenes are widely used additives for plasticizers and lubricants. OleTJE has been shown to accept electrons from hydrogen peroxide for the oxidative decarboxylation. While addition of hydrogen peroxide damages the enzyme and results in low yields, in situ generation of the cofactor circumvents this problem. The photobiocatalytic system shows clear advantages regarding enzyme activity and yield, resulting in a simple and efficient system for fatty acid decarboxylation.

We describe a protocol for the light-catalyzed generation of hydrogen peroxide &#8212; a cofactor for oxidative transformations.

Oxidoreductases belong to the most-applied industrial enzymes. Nevertheless, they need external electrons whose supply is often costly and challenging. ...

Rapid Collection of Floral Fragrance Volatiles using a Headspace Volatile Collection Technique for GC-MS Thermal Desorption Sampling

Fragrances of many flower families have been sampled and the volatiles analyzed. Knowing the compounds that make up the fragrances can be an important step to conservation of flowers that are threatened or endangered. Because floral fragrance is critical for attracting pollinators, this method could be used to better understand or even enhance pollination. We present a protocol using a portable charcoal air filter and vacuum to collect floral fragrance volatiles, which are then analyzed by a GC-MS. By using this method, fragrance volatiles can be sampled using a non-destructive method with a machine that is easily transported. This methodology uses a rapid sampling procedure, cutting sampling time down from 2-3 hours to approximately 10 minutes. Using GC-MS, the fragrance compounds can be identified individually, based on authentic standards. The steps used for collecting fragrance and control data are presented, from material setup to collecting the data output.

Here, we present a protocol for collecting the floral fragrance volatiles from blooming flowers, using a non-destructive sampling procedure.

Fragrances of many flower families have been sampled and the volatiles analyzed. Knowing the compounds that make up the fragrances can be an important ...

Biochemistry

Quantitative Analysis by Thermogravimetry-Mass Spectrum Analysis for Reactions with Evolved Gases

During energy conversion, material production, and metallurgy processes, reactions often have the features of unsteadiness, multistep, and multi-intermediates. Thermogravimetry-mass spectrum (TG-MS) is seen as a powerful tool to study reaction features. However, reaction details and reaction mechanics have not been effectively obtained directly from the ion current of TG-MS. Here, we provide a method of an equivalent characteristic spectrum analysis (ECSA) for analyzing the mass spectrum and giving the mass flow rate of reaction gases as precise as possible. The ECSA can effectively separate overlapping ion peaks and then eliminate the mass discrimination and temperature-dependent effect. Two example experiments are presented: (1) the decomposition of CaCO3 with evolved gas of CO2 and the decomposition of hydromagnesite with evolved gas of CO2 and H2O, to evaluate the ECSA on single-component system measurement and (2) the thermal pyrolysis of Zhundong coal with evolved gases of inorganic gases CO, H2, and CO2, and organic gases C2H4, C2H6, C3H8, C6H14, etc., to evaluate the ECSA on multi-component system measurement. Based on the successful calibration of the characteristic spectrum and relative sensitivity of specific gas and the ECSA on mass spectrum, we demonstrate that the ECSA accurately gives the mass flow rates of each evolved gas, including organic or inorganic gases, for not only single but multi-component reactions, which cannot be implemented by the traditional measurements.

Precise determination of the evolved gases&#39; flow rate is key to study the details of reactions. We provide a novel quantitative analysis method of equivalent characteristic spectrum analysis for thermogravimetry-mass spectrum analysis by establishing the calibration system of the characteristic spectrum and relative sensitivity, for obtaining the flow rate.

During energy conversion, material production, and metallurgy processes, reactions often have the features of unsteadiness, multistep, and ...

Engineering

Qualitative Characterization of the Aqueous Fraction from Hydrothermal Liquefaction of Algae Using 2D Gas Chromatography with Time-of-flight Mass Spectrometry

Two-dimensional gas chromatography coupled with time-of-flight mass spectrometry is a powerful tool for identifying and quantifying chemical components in complex mixtures. It is often used to analyze gasoline, jet fuel, diesel, bio-diesel and the organic fraction of bio-crude/bio-oil. In most of those analyses, the first dimension of separation is non-polar, followed by a polar separation. The aqueous fractions of bio-crude and other aqueous samples from biofuels production have been examined with similar column combinations. However, sample preparation techniques such as derivatization, solvent extraction, and solid-phase extraction were necessary prior to analysis. In this study, aqueous fractions obtained from the hydrothermal liquefaction of algae were characterized by two-dimensional gas chromatography coupled with time-of-flight mass spectrometry without prior sample preparation techniques using a polar separation in the first dimension followed by a non-polar separation in the second. Two-dimensional plots from this analysis were compared with those obtained from the more traditional column configuration. Results from qualitative characterization of the aqueous fractions of algal bio-crude are discussed in detail. The advantages of using a polar separation followed by a non-polar separation for characterization of organics in aqueous samples by two-dimensional gas chromatography coupled with time-of-flight mass spectrometry are highlighted.

A two-dimensional gas chromatography-time-of-flight mass spectrometry method is described for characterization of the aqueous fraction of bio-crude produced from hydrothermal liquefaction of algae. This protocol can also be employed to analyze the aqueous fraction of liquid products from fast pyrolysis, catalytic fast pyrolysis, catalytic deoxygenation and hydro-treating.

Two-dimensional gas chromatography coupled with time-of-flight mass spectrometry is a powerful tool for identifying and quantifying chemical components in ...

Bioengineering

Chromatographic Fingerprinting by Template Matching for Data Collected by Comprehensive Two-Dimensional Gas Chromatography

Data processing and evaluation are critical steps of comprehensive two-dimensional gas chromatography (GCxGC), particularly when coupled to mass spectrometry. The rich information encrypted in the data may be highly valuable but difficult to access efficiently. Data density and complexity can lead to long elaboration times and require laborious, analyst-dependent procedures. Effective yet accessible data processing tools, therefore, are key to enabling the spread and acceptance of this advanced multidimensional technique in laboratories for daily use. The data analysis protocol presented in this work uses chromatographic fingerprinting and template matching to achieve the goal of highly automated deconstruction of complex two-dimensional chromatograms into individual chemical features for advanced recognition of informative patterns within individual chromatograms and across sets of chromatograms. The protocol delivers high consistency and reliability with little intervention. At the same time, analyst supervision is possible in a variety of settings and constraint functions that can be customized to provide flexibility and capacity to adapt to different needs and goals. Template matching is shown here to be a powerful approach to explore extra-virgin olive oil volatilome. Cross-alignment of peaks is performed not only for known targets, but also for untargeted compounds, which significantly increases the characterization power for a wide range of applications. Examples are presented to evidence the performance for the classification and comparison of chromatographic patterns from sample sets analyzed under similar conditions.

This protocol presents an approach to fingerprint and explore multi-dimensional data collected by comprehensive two-dimensional gas chromatography coupled to mass spectrometry. Dedicated pattern recognition algorithms (template matching) are applied to explore the chemical information encrypted in the extra-virgin olive oil volatile fraction (i.e., volatilome).

Data processing and evaluation are critical steps of comprehensive two-dimensional gas chromatography (GCxGC), particularly when coupled to mass ...

Gas Chromatography-Mass Spectrometry Paired with Total Vaporization Solid-Phase Microextraction as a Forensic Tool

Gas Chromatography &#8211; Mass Spectrometry (GC-MS) is a frequently used technique for the analysis of numerous analytes of forensic interest, including controlled substances, ignitable liquids, and explosives. GC-MS can be coupled with Solid-Phase Microextraction (SPME), in which a fiber with a sorptive coating is placed into the headspace above a sample or immersed in a liquid sample. Analytes are sorbed onto the fiber which is then placed inside the heated GC inlet for desorption. Total Vaporization Solid-Phase Microextraction (TV-SPME) utilizes the same technique as immersion SPME but immerses the fiber into a completely vaporized sample extract. This complete vaporization results in a partition between only the vapor phase and the SPME fiber without interference from a liquid phase or any insoluble materials. Depending upon the boiling point of the solvent used, TV-SPME allows for large sample volumes (e.g., up to hundreds of microliters). On-fiber derivatization may also be performed using TV-SPME. TV-SPME has been used to analyze drugs and their metabolites in hair, urine, and saliva. This simple technique has also been applied to street drugs, lipids, fuel samples, post-blast explosive residues, and pollutants in water. This paper highlights the use of TV-SPME to identify illegal adulterants in very small samples (microliter quantities) of alcoholic beverages. Both gamma-hydroxybutyrate (GHB) and gamma-butyrolactone (GBL) were identified at levels that would be found in spiked drinks. Derivatization by a trimethylsilyl agent allowed for conversion of the aqueous matrix and GHB into their TMS derivatives. Overall, TV-SPME is quick, easy, and requires no sample preparation aside from placing the sample into a headspace vial.

Total Vaporization Solid Phase Microextraction (TV-SPME) completely vaporizes a liquid sample whilst analytes are sorbed onto a SPME fiber. This allows for partitioning of the analyte between only the solvent vapor and the SPME fiber coating.

Gas Chromatography &#8211; Mass Spectrometry (GC-MS) is a frequently used technique for the analysis of numerous analytes of forensic interest, including ...

Gas Chromatography (GC) with Flame-Ionization Detection

Source: Laboratory of Dr. B. Jill Venton - University of Virginia
Gas chromatography (GC) is used to separate and detect small molecular weight compounds in the gas phase. The sample is either a gas or a liquid that is vaporized in the injection port. Typically, the compounds analyzed are less than 1,000 Da, because it is difficult to vaporize larger compounds. GC is popular for environmental monitoring and industrial applications because it is very reliable and can be run nearly continuously. GC is typically used in applications where small, volatile molecules are detected and with non-aqueous solutions. Liquid chromatography is more popular for measurements in aqueous samples and can be used to study larger molecules, because the molecules do not need to vaporize. GC is favored for nonpolar molecules while LC is more common for separating polar analytes.
The mobile phase for gas chromatography is a carrier gas, typically helium because of its&#160;low molecular weight and being chemically inert. Pressure is applied and the mobile phase moves the analyte through the column. The separation is accomplished using a column coated with a stationary phase. Open tubular capillary columns are the most popular columns and have the stationary phase coated on the walls of the capillary. Stationary phases are often derivatives of polydimethylsiloxane, with 5&#8211;10% of the groups functionalized to tune the separation. Typical functional groups are phenyl, cyanopropyl, or trifluoropropyl groups. Capillary columns are usually 5&#8211;50 m&#160;long. Narrower columns have higher resolution but require higher pressures. Packed columns can also be used where the stationary phase is coated onto beads packed in the column. Packed columns are shorter, 1&#8211;5 m. Open tubular capillaries are generally preferred because they allow higher efficiencies, faster analyses, and have higher capacities.
Flame-ionization detection (FID) is a good general detector for organic compounds in GC that detects the amount of carbon in a sample. After the column, samples are burned in a hot, hydrogen-air flame. Carbon ions are produced by the combustion. While the overall efficiency of the process is low (only 1 in 105 carbon ions produce an ion in the flame) the total amount of ions is directly proportional to the amount of carbon in the sample. Electrodes are used to measure the current from the ions. FID is a destructive detector, as the entire sample is pyrolyzed. FID is unaffected by noncombustible gases and water.

Gas Chromatography GC with Flame-Ionization Detection

Source: Laboratory of Dr. B. Jill Venton - University of Virginia
Gas chromatography (GC) is used to separate and detect small molecular weight compounds ...

Education

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Analytical Chemistry

Gas Chromatography: Overview of Detectors

Detectors in gas chromatography (GC) help identify and quantify the components of a mixture by translating chemical properties into measurable signals, which are displayed on a chromatogram. Detectors can be categorized into two main types: destructive and non-destructive.
A non-destructive detector allows a sample to be analyzed without altering or consuming it, meaning the sample can be collected after detection for further analysis. Examples include thermal conductivity detectors and electron capture detectors.
On the other hand, a destructive detector causes chemical changes in the analyte or completely consumes it during detection, making it impossible to recover the sample afterward. This often involves burning or chemically reacting the sample, making recovery impossible. Examples include flame ionization and nitrogen-phosphorus detectors, which combust the sample.
An ideal gas chromatography detector should be non-destructive and possess high sensitivity for detecting low analyte concentrations. It should exhibit responsiveness to all analytes or selectively respond to specific classes of analytes while maintaining a linear response across a wide concentration range. Linearity indicates that the detector&#39;s response is directly proportional to the concentration of the analytes, making it easier to quantify the amount present. Stability, reliability, and reproducibility are crucial, with the detector being insensitive to variations in flow rate and temperature. Achieving a short response time, independent of flow rate, enhances the sample throughput and reduces analysis time for large sample quantities. Furthermore, the detector should exhibit minimal interference from sample matrix components and compatibility with various analyte types. Lastly, robustness and durability are vital characteristics, ensuring the detector&#39;s reliability and resilience under the conditions encountered in gas chromatography analysis.
Gas chromatography analysis commonly utilizes thermal conductivity, flame ionization, mass spectrometer, thermionic, electrolytic conductivity, photoionization, FTIR, and electron capture detectors.
Flame ionization detectors offer a broader linear response range, albeit destroying the sample, and possess a higher detection limit compared to thermal conductivity detectors. Electron capture detectors exhibit excellent detection limits but have a relatively narrow linear range. Ultimately, the choice of the detector depends on the type of the sample under analysis and the detector&#39;s typical detection limit.

Detectors in gas chromatography (GC) help identify and quantify the components of a mixture by translating chemical properties into measurable signals, ...

JoVE Core

Unit 1

Principles of Chromatography

KEY TERMS AND CONCEPTS

Gas Chromatography: Types of Detectors-I

There are different types of detectors used in gas chromatography, each with its own specific properties that make it suitable for detecting certain types of analytes. The most commonly used detectors in GC are thermal conductivity detector (TCD), flame ionization detector (FID), and electron capture detector (ECD).
TCD is the earliest and most widely used detector that operates by measuring the changes in the thermal conductivity of the carrier gas. When a sample compound enters the detector, it changes the thermal conductivity of the carrier gas, and this change is detected. TCD offers a linear response over a wide range of concentrations and is nondestructive, allowing solute isolation. However, its detection limit is relatively poor compared to other detectors.
FID operates by the combustion of organic compounds in a hydrogen-air flame. The analytes in a sample elute from the GC column and enter the flame, where they are combusted and produce ions. The current generated by these ions is proportional to the concentration of the analyte, and it is recorded as an electrical signal. The FID is highly sensitive, and it responds to a wide range of organic compounds, especially hydrocarbons. However, FID destroys the sample during analysis.
ECD is a selective detector that utilizes a radioactive beta source to ionize the mobile phase, producing an electric current between electrodes. When a solute with high electron capture potential elutes from the column, the electric current decreases, serving as the signal. ECD is highly selective towards solutes with electronegative functional groups and electron-capturing analytes like chlorinated compounds, and it is relatively insensitive to other compounds. It has an excellent detection limit but a limited linear range of only about two orders of magnitude.
The choice of the detector depends on the specific requirements of the analysis.

There are different types of detectors used in gas chromatography, each with its own specific properties that make it suitable for detecting certain types ...

Quantitative Detection of Trace Explosive Vapors by Programmed Temperature Desorption Gas Chromatography-Electron Capture Detector

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