Analyzing pesticides, PCBs with modern GCMS technology: a new method provides determination of organochlorine pesticides and polychlorinated biphenyls using GC/MS/MS operated in MRM mode.
This article describes use of a triple quadrupole GC/MS/MS method using multiple reaction monitoring (MRM) mode for sensitive and selective detection and quantitation of organochlorine pesticides and PCBs. A database with optimized MRM transitions for all of the OCPs and PCBs, including relative retention times for all components, makes method setup possible within minutes. The use of GC/MS/MS MRM mode provides enhanced selectivity, specificity and sensitivity in complex matrices with potential co-eluting interferences.
This article also presents all instrument operating conditions and instrument method performance statistics, including method linearity, accuracy, precision and instrument detection limits for all compounds.
Chlorinated pesticides (OCP) and polychlorinated biphenyl (PCB) congeners (known collectively as Aroclor) are among the environmental organic compounds most difficult to measure. The difficulty is primarily due to the fact that they are found at very low concentrations. The EPA-approved method for OCPs and PCBs in wastewater is EPA Method 608.2. Method 608 relies upon gas chromatography with electron capture detection (GC-ECD) to measure a targeted list.
GC-ECD is one of the few techniques available capable of detecting halogenated compounds at very low concentrations. GC-ECD is a non-specific detection technique that can produce a signal for non-target compounds. As a result, when using GC-ECD for pesticides and PCBs in environmental samples, other components in the sample can be detected and interfere with both identification and quantitation. Target compounds are identified by retention time. Aroclors are a mixture of hundreds of different congeners requiring identification by the analyst visually comparing the chromatography of the unknown with the chromatography of the standards.
GC-ECD Suffers from Matrix Interferences
The matrix itself can have a profound impact on the chromatography and on the response of the ECD. High levels of non-target components in the matrix can co-elute. False positives result if the co-eluting compound responds to the detector, whereas false negatives result if the co-eluting compound quenches the ECD signal.
The EPA requires that any detected compound be analyzed again on a different chromatography column to "confirm" the identity of every detected component. This second column confirmation means that every sample is either analyzed twice, or that the laboratory instrument is equipped with two columns and two detectors. The second column confirmation may help to recognize interferences, but cannot guarantee interferences are corrected.
Second column confirmation for all positive GC-ECD peaks does not guarantee positive identification or quantitation of the pesticides. Just as there may be co-elution on the primary column, there can be co-elution on the second column as well. Hydrocarbons and the presence of other components (such as PCBs) can interfere.
Single Quadrupole GCMS Is Not Sensitive Enough
Method 608 allows the use of mass spectrometry (MS) as a confirmatory detector, provided the compound concentration is high enough to be measured. Traditionally, commercially available gas chromatography-mass spectrometry (GCMS) systems were not sensitive enough to detect the pesticides and/or PCBs at the concentrations being measured. But the sensitivity of GCMS has increased dramatically since Method 608 was written. New GCMS technology can be run in selected ion monitoring mode (SIM), greatly increasing the sensitivity.
In addition, newer tandem GCMS technology eliminates almost all noise, increasing the sensitivity of the detector five to 10 times more than that of SIM. With this new tandem GCMS technology (also known as GC/MS/MS or triple quadrupole GCMS), GC-ECD detection limits are possible. Even better, this new technology almost completely eliminates matrix interferences and provides qualitative information about the detected analytes.
Triple Quadrupole GCMS Is Very Sensitive and Interference Free
Triple quadrupole mass spectrometry is the linking together of two quadrupoles. Ions are separated in the first quadrupole, fragmented further in a collision cell, and the product ions are separated and analyzed by a second quadrupole. This technique eliminates matrix interferences, making it highly selective and extremely sensitive. Quantitative and qualitative analyses can be made in at least four modes, including fullscan, SIM and MRM. Faster scanning analyzers are capable of Scan/MRM and/or Scan/SIM all in a single method.
Multiple reaction monitoring (MRM) eliminates matrix noise and allows only the selected product ion through the second quadrupole. Although the sensitivity is actually less, the noise is reduced, increasing the signal-to-noise ratio. MRM detection limits are usually five to 10 times lower than SIM detection limits.
In a comparison of GC-ECD and high-resolution gas chromatography/MSMS, McAteer and Hughes found significant bias between organochlorine pesticides results. The samples were high in PCBs (determined by Method 608) and total petroleum hydrocarbons (TPH). Since the HRGC/MSMS is essentially interference free, it's assumed results from this instrument are correct and the results from the GC-ECD method are biased.
Why Do We Need a New PCB Method?
PCBs are some of the most stable organic compounds known. There are 209 distinct PCB compounds (known as congeners), which could have anywhere from one to 10 chlorine atoms on a biphenyl molecule. PCBs occur as a mixture of congeners, which were sold commercially as Aroclors. Method 608 lists seven separate PCBs that must be analyzed. Method 608 identifies PCBs by their chromatographic pattern as compared to injection of Aroclor standards under the same chromatographic conditions. The problem is that PCBs can weather at different rates, thereby affecting pattern recognition, and cause analysts to report Aroclors as not detected.
Using GC/MS/MS for a New Pesticide and PCB Method
Triple quadrupole instruments are ideally suited for a new method to determine pesticides and PCBs and for replacing Method 608 for wastewater compliance monitoring. These instruments are affordable, have detection limits comparable to the ECD, yet can detect trace compounds in highly complex matrices with very little interference. Here, we describe an MRM instrument method for low-level detection of Method 608 pesticides and a selected group of PCB congeners.
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This study was conducted using a triple quadrupole GCMS configured with a capillary column designed specifically for the analysis of semi-volatile analytes. The GC was operated in constant linear velocity mode, providing the best chromatographic resolution, symmetrical peak shapes and enhanced sensitivity for the target compounds. A commercial mixture of 19 PCB congeners, 20 organochlorine pesticides, five internal standards and four surrogates was used to prepare calibration curves ranging from 0.5 to 200 ppb. The standards were prepared in a mixture of EPA method 625 acids and base-neutral target analytes to provide a better and more realistic synthetic matrix.
Chromatographic conditions were established and the MRM method was optimized for each component. The instrument operating conditions are shown in Table 1.
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MRM Method Development
MRM transitions were monitored for each component. Quantitative and qualitative transitions were selected to provide maximum sensitivity and as independent confirmation of the compounds identity. The ion shield high efficiency source minimized fragmentation even at 70 eV, providing an optimum abundance and transmission of ions into the quadrupoles. Method settings were made to provide enough sensitivity to easily detect and quantify the target analytes at concentrations equal to or better than Method 608.
A 9-point calibration curve of 0.5 to 200 ppb was analyzed using the conditions described in Table 1. The curves of all 48 components were evaluated using linear regression and %RSD of the calculated response factors.
Method Detection Limit (MDL)
An instrument detection limit (IDL) study was made using eight replicate injections at 1.0, 2.0 and 5.0 ppb standards, but with concentrations divided by 1000 to approximate MDLs for 1000 ml samples extracted and concentrated to 1 ml.
These estimated MDL results were compared to Method 608 detection limits and are shown in Table 2.
Precision and Accuracy
Eight replicates of 10 ppb and 20 ppb were made to determine precision and accuracy. Table 3 lists the results of the precision and accuracy study, reporting the % recovery and the %RSD for all compounds at both concentrations.
Internal Standard and Surrogate Stability
Five internal standards were used for calibration and four surrogate standards to measure recovery. The small variance (<6 %RSD for all compounds) in the data demonstrates the long-term stability of the instrumental method.
Triple quadrupole GCMS analysis simplifies detection and quantitation of pesticides and PCBs. Detection limits are equal to or better than Method 608 detection limits. Triple quadrupole GCMS does not suffer from interferences nor does it require dual columns to confirm the identity of peaks. PCB analysis by Method 608 is difficult because the Aroclor pattern changes with weathering often resulting in samples containing PCBs being reported as not detected.
Using a triple quadrupole GCMS for pesticides and PCB analysis is a viable alternative to EPA Method 608. This paper evaluated standards only. Our evaluation indicates that triple quadrupole GCMS is a suitable alternative to pesticide and PCB analysis by GC-ECD.
Brahm Prakash, William Lipps, Shimadzu Scientific Instruments
Note: Not all figures and tables are shown. To access the original application note, please contact the authors or visit ssi.shimadzu.com.
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|Author:||Prakash, Brahm; Lipps, William|
|Date:||Aug 1, 2016|
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