GC-FID Method Note – Semivolatiles (EPA 8270)
Analysis of Semivolatile Organic Compounds (SVOCs) in the miniGC
In this app note we run a standard in the miniGC containing 13 compounds considered to be Semivolatile Organic Compounds (SVOC), a class of compounds that are monitored by EPA Method 8270 as contaminants in solid waste, soils, surface water, ground water, and air.
The standard we are analyzing is the TCLP Base/Neutral/Acid Extractable Spike Solution from SPEX CertiPrep (PN: TCLP-BNA). The standard contains the following compounds:
each in concentrations of 2,000ug/mL in methylene chloride. We prepared 2 dilutions of this standard, one to 500ug/mL and the other to 100ug/mL.
We started by using the EZGC Chromatogram Modeler from Restek (link) to give us a starting point for a method and an idea of how the compounds should elute. We entered in the 13 compounds contained in the standard and got the following for a method, recommended column, and expected chromatogram.
We used the suggested method from the EZGC Modeler with one change – we used Hydrogen as our carrier gas instead of Helium. Although this should change the retention times the overall elution order and relative peak heights shouldn’t change. In the modeled method two compounds co-elute, 3-Methlyphenol and 4-Methylphenol. Since our goal is simply to see if we can match a conventional chromatogram we won’t modify the method in an attempt to separate these two compounds.
miniGC Method 1
|Column||MXT-5, 30 m, 0.25 mm ID, 0.25 µm (cat.# 70223-LUC25)|
|Sample||Semivolatiles (8270, TCLP-BNA) standard (SPEX PN: TCLP-BNA)|
|Inj. Vol.:||1.0 µL split (split ratio 50:1)|
|Liner:||4 mm low pressure drop precision inlet liner w/ wool (cat.# 23309)|
|Oven Temp.:||Stage 1: Hold at 40°C for 1.0 min
Stage 2: Ramp to 265°C at 32°C/min and hold for 11.0 min
|Carrier Gas||H2, constant pressure|
|Linear Velocity:||5.20 cm/sec (2.00mL/min)|
|Detector||FID @ 275°C|
We injected each of the 3 standards (2000ug/mL, 500ug/mL, and 100ug/mL) once then compared the chromatograms to each other for retention times and peak areas of each compound. This allowed us to create a 3 point calibration curve for each compound in order to check linearity.
Figure 2 shows the chromatogram of the 2000ug/mL standard, which is roughly 2000ppm. With a 50:1 split and a 1uL injection, the on column amount is 40ng of each compound or approximately 40ppm of each. You can see each of the compounds listed in the table on the right on the result screen and you can see the corresponding peaks in the chromatogram.
What’s remarkable is how closely the chromatogram matches the predicted chromatogram from the EZGC modeler, which makes it fairly straightforward to identify the peaks. We are making the assumption that the EZGC modeler has correctly predicted the order of the peaks based on how closely the predicted chromatogram matches the actual chromatogram, but to be fully sure we would need to check the results via mass spec or use individual standards to confirm individual peaks we may be uncertain about. For the sake of this app note we will assume the peak orders are correct from the modeler.
Figure 3 shows the injections of the 3 different concentrations overlaid on top of each other, showing good repeatability for the retention times and the peak areas decreasing as you would expect with decreasing concentration.
Figure 4 shows the 2000ug/mL injection, which is approximately 2000ppm injected and 40ng, or approximately 40ppm on column.
Figure 5 shows the 500ug/mL injection, which is approximately 500ppm injected and 10ng, or approximately 10ppm on column.
Figure 6 shows the 100ug/mL injection, which is approximately 100ppm injected and 2ng, or approximately 2ppm on column.
Figure 7 shows the 100ug/mL (2ng on column concentration) sample zoomed in.
Retention Time Repeatability
|Peak #||Compound||Ret Time (2000ppm)||Ret Time (500ppm)||Ret Time (100ppm)||Avg Ret Time||%RSD|
|4||3-Methylphenol + 4-Methylphenol||5:35.5||5:35.1||5:35.2||5:35.3||0.06%|
Peak Area vs. Concentration Linearity
|Peak||Compound||40ng on column||10ng on column||2ng on column||R2|
|4||3-Methylphenol + 4-Methylphenol||421.30||101.48||16.374||1.000|
This app note demonstrates the ability of the miniGC to handle semivolatile organic compounds in reasonable concentrations. The chromatograms produced matched the predicted chromatogram surprisingly well and showed comparable separation to what was predicted. It also demonstrates the effectiveness of the EZGC modeler, allowing us to simply enter a list of compounds and be provided a method and column with which to run the method.
The miniGC shows good retention time repeatability and good linearity of 3 different concentrations including a fairly low concentration standard.
More results from the miniGC can be found here: https://luciditysystems.com/products/minigc/minigc-results/
More information on the miniGC can be found here: https://luciditysystems.com/products/minigc/
To see the miniGC in operation go here: https://luciditysystems.com/products/minigc/minigc-interface/