The aim of this procedure is to quantitate explosive vapors. This is accomplished by first preparing the instrumentation for analysis. The second step is to establish a calibration curve for the instrument using the deposition of solution standards onto vapor sampling tubes.
Next vapor samples are collected for quantitative analysis. The final step is quantitative analysis of vapor samples on vapor sample tubes using gas chromatography electron capture detector instrumentation. Ultimately, the combination of direct liquid deposition of calibration standards and gas chromatography with an electron capture detector is used to obtain quantitative results for explosive vapor samples.
The main advantage of this technique over existing methods like liquid calibration curves, is that losses associated with the thermal desorption off the orbit tubes are taken into account. Prior to starting this procedure, remove the TDS adapter from the CIS inlet of the GC instrument. After removing the liner, inspect the CIS inlet for particles and debris.
Debris, clean any visible debris, debris with a gas duster. Attach a new graphite feral to a new CIS liner using the manufacturer provided tool and instructions for feral to liner binding. Then insert the liner with the attached graphite feral into the CIS.
Replace the TDS adapter and remount the TDS following this. Remove the silicon protection from the ends of a new column. Insert a nut and feral onto each end of the column using a ceramic column cutting tool.
Remove approximately 10 centimeters from each end of the column, ensuring that the nuts and ferals remain on the column but away from the end. To avoid clogging and debris, secure the column into the oven by inserting the column into the inlet. Then connect the other end of the column to the detector port.
Gently hand tighten the nuts and ferrells onto their respective ports for the inlet and detector. Using a wrench, tighten the nuts and ferrells with approximately a quarter turn of rotation. Next, bake out the TDS inlet column and detector by setting the temperature for all the zones just below the maximum operating temperature while flowing carrier gas for at least two hours.
After cooling all the zones, reit all the nuts and ferals to ensure leak free operation. Load the instrument method using the software interface. Verify that the correct temperatures and flow rates have been achieved.
At this point, connect one sorbent filled thermal desorption sample tube to a sample pump using a small piece of flexible silicon tubing. Attach a piston flow meter to the sample tube at the opposite end from the sample pump. Then adjust the flow rate on the sample pump such that it is approximately 100 milliliters per minute through the sample tube.
According to the readings from the piston flow meter. Disconnect the piston flow meter from the sample tube and temporarily shut off the sample pump. Disconnect the sample pump from the flow meter and reconnect it directly to the sample tube.
Place the sample tube in the explosives vapor stream. Following this, set a timer based on the approximate sampling times. Activate the sample pump and start the timer.
Once the timer has stopped, shut off the sample pump. Disconnect the sample tube from the pump and place it in the packaging provided with the sample tube. Then cap the tube and store it for analysis.
Next, pipette five microliters of a previously prepared solution standard directly onto the glass frit of an unused conditioned sample tube holding the sample tube and pipette upright with a gloved hand during deposition. After repeating the prior step for each of the six calibration standards, deposit five microliters of 0.3 nanograms per microliter of three four DNT into each of the tubes as well. Then allow the sample tubes to sit at room temperature for at least 30 minutes to evaporate the solvent, load the sample tubes into the TDSA sample rack.
Then load the sample rack into the TDSA sampler. Once the samples have been analyzed by the T-D-S-C-I-S-G-C-E-C-D method, integrate the peaks associated with three four D-N-T-T-N-T and RDX in the chromatogram. For each of the 18 sample tubes plot the average normalized peak area versus massive analyte present on the tubes for both TNT and RDX.
Following this deposit, five microliters of 0.3 nanograms per microliter of three four DNT into each of the sample tubes. After the solvent has evaporated and the samples have been analyzed, integrate the peaks associated with three four D-N-T-T-N-T and RDX in the chromatogram for each of the 18 sample tubes. Finally, use the peak areas and calibration curve to calculate the vapor concentration in parts per billion by volume for each analytes in the chromatograms obtained using the direct liquid deposition method.
Peaks for three four D-N-T-T-N-T and RDX are observed at 4.16, 4.49, and 4.95 minutes respectively. The internal standard peak height and area are constant for all masses of TNT and RDX while the peak height and area increase with the analyte mass. An example calibration curve generated from the acquired chromatograms is shown here.
The aerobars indicate one standard deviation with three replicate measurements per massive analyte. Additional peaks other than three four D-N-T-T-N-T and RDX are typically observed if the instrument needs to be serviced or if the standards have degraded over time. Additional peaks are always present when using sorbent filled thermal desorption sample tubes, but the degradation products formed do not co elute with these vapors with a properly maintained instrument.
Therefore, the peak shapes deviate greatly from a Gaussian shape specifically for the peaks at approximately 4.6 and 4.825 minutes. While attempting this procedure, it is important to thoroughly clean the thermal desorption tubes prior to sample deposition and analysis.
