Sample throughput limitations for organochlorine pesticides analysis can delay reporting and create bottlenecks in the lab. However, the method described here improves efficiency and productivity by using a two-pronged approach. CarboPrep Plus SPE was used for a more effective sample extract cleanup that reduced inlet contamination and allowed more samples to be run before maintenance was required. In addition, a dual column setup was used with a GC Accelerator oven insert kit in order to speed up overall run times and allow simultaneous confirmation and quantification.
In many areas of the world, chlorinated (organochlorine) pesticide use has been banned due to human health and environmental concerns, but these compounds are still widely monitored because of their persistence in the environment. Both soil and wastewater samples are routinely tested to verify the presence or absence of organochlorine pesticides and to assess the risk of exposure from historical contamination.
Organochlorine pesticides analysis can be accomplished by GC using an electron capture detector (ECD) or a mass spectrometer (MS). However, analysis can be challenging because some of the target compounds breakdown when exposed to active sites in the hot inlet. These active sites are typically caused by nonvolatile sample matrix interferences that are coextracted and then injected along with the target analytes. Once deposited in the inlet and on the head of the analytical column, they foul the system and cause QC failures that lead to downtime for maintenance. SPE cleanup can remove some matrix components from sample extracts, but the Florisil material that is often used is limited in its ability to remove the high molecular weight contaminants that typically cause the problems.
Here, we demonstrate a more effective and efficient approach for organochlorine pesticides analysis. First, Resprep CarboPrep Plus SPE cartridges were used for sample extract cleanup because the graphitized carbon material removes more high molecular weight contaminants than Florisil SPE while still following the same procedure. Second, a GC Accelerator oven insert kit was used along with an Rtx-CLPesticides and Rtx-CLPesticides2 column set for fast, simultaneous confirmation and quantification. Using this approach, labs can increase sample throughput both by removing the nonvolatile matrix compounds that cause QC failures and by speeding up overall analysis times.
To extract the samples, 10 g of soil was weighted into 50 mL centrifuge tubes containing 5 grams of anhydrous magnesium sulfate. Then, 20 mL of acetone:methylene chloride (50:50) was added, and the samples were centrifuged at 3000 g for five minutes.
Sample Extract Cleanup
Prior to analysis, matrix interferences were removed from the sample extracts using SPE cleanup. For this work, two cleanup options were compared: Resprep Florisil normal phase SPE cartridges (6 mL/1 gm; cat.# 28993) and Resprep CarboPrep Plus graphitized carbon SPE cartridges (3 mL/95 mg; cat.# 25845).
CarboPrep Plus SPE cartridges are designed to use the same solvents and volumes as a standard Florisil cleanup, so the same procedure was employed for both SPE cartridge types (Figure 1). First, one cartridge volume of hexane:acetone (90:10) was added and held for five minutes to condition and equilibrate the beds. Then, using a 1 mL Class A pipette, 1 mL of sample extract was dispensed into each cartridge. The sample extract was loaded until the level matched the upper frit, and the eluent was collected dropwise into a 10 mL volumetric flask. Once the sample was completely loaded, it was immediately followed by the addition of 1 mL of hexane:acetone (90:10), which was also drawn down to the level of the top frit. Then, an additional 8 mL of solvent was added to the cartridges and eluted dropwise into the flask.
Organochlorine pesticides analysis was performed using a dual column system connected to a single guard column and inlet (Figure 2). The Rtx-CLPesticides and Rtx-CLPesticides2 column set was selected because the stationary phase differences allow confirmation and quantification analyses to be conducted simultaneously using a single injection of sample extract. In this setup, the inlet can be run in either split or splitless mode, depending on reporting limit requirements, but splitless injection was used here.
In addition, a GC Accelerator oven insert kit (cat.# 23849) was installed to speed up analysis. By reducing oven volume, these inserts allow faster ramp rates to be attained, which reduces oven cycle time and allows for increased sample throughput. In order to fit the inserts into the GC oven, analytical columns that were not wound onto cages were used. This customized format can be obtained from Restek simply by ordering the standard column catalog number and specifying “-051” under special instructions.
The detector used for organochlorine pesticides analysis of soil sample extracts processed through both SPE cleanup materials was a micro-ECD.
Results and Discussion
The first difference observed in the SPE cleanups is the color of the final extracts. The initial extract is dark because it contains a substantial amount of nonvolatile material from the soil sample. As shown in Figure 3, the Florisil cleanup removes some of these high molecular weight compounds, but overall the final extract is still quite dark, indicating high levels of contaminants remain. In contrast, the extract cleaned with Carboprep Plus SPE is very clear, visually demonstrating its ability to remove much more of the coextracted matrix components.
While it is visually clear the CarboPrep Plus SPE cleanup removed more nonvolatile matrix components than the standard Florisil cleanup that is typically used for organochlorine pesticides analysis, the impact on chromatographic performance must also be assessed. To investigate this, we monitored endrin and DDT breakdown as indicators of system suitability using the typical failure threshold of >15%. As shown in Figure 4, the Florisil cleaned extract showed unacceptable 4,4’-DDT degradation (>15%) after just 25 injections, meaning maintenance would be required before analysis could continue. In contrast, the Carboprep Plus cleaned extract was <5% at 25 injections and <10% breakdown after 50 injections. Further, good recoveries were obtained for all target analytes in the extracts cleaned using CarboPrep Plus SPE, indicating that breakdown in the inlet was not occurring (Table I)
The significance of this is that by cleaning extracts more effectively, inlet contamination is substantially reduced so more samples can be run in a calibration sequence before instrument maintenance is required. Fewer failed calibrations, more instrument uptime, and less frequent maintenance help labs increase sample throughput and allow faster turnaround times for data reporting.
Table I: Recovery of chlorinated pesticides using CarboPrep Plus SPE.
* n = the analysis of 6 cartridges each from 3 different lots for a total of 18 data points
In addition to the productivity gains made by using more effective extract cleanup, speed gains were also made in terms of the overall organochlorine pesticides analysis. The dual column system setup using the Rtx-CLPesticides and Rtx-CLPesticides2 columns provides simultaneous confirmation and quantitation with the elution order switches of 4,4’-DDE with Endosulfan I and of methoxychlor with endosulfan sulfate (Figure 5). Use of a GC Accelerator oven insert kit reduced the total analysis cycle time to around 10 min with the last compound eluting at 4.82 min and 5.29 min on the Rtx-CLPesticides and the Rtx-CLPesticides2 columns, respectively.
Figure 5: Organochlorine pesticides analysis of a prepared standard.
|Columns||Rtx-CLPesticides 30 m, 0.32 mm ID, 0.32 µm (cat.# 11141-051)|
|and Rtx-CLPesticides2 30 m, 0.32 mm ID, 0.25 µm (cat.# 11324-051)|
|using Rxi guard column 5 m, 0.32 mm ID (cat.# 10039)|
|with universal angled “Y” Press-Tight connector (cat.# 20403-261)|
|Sample||Organochlorine pesticide mix AB #2 (cat.# 32292)|
|Pesticide surrogate mix (cat.# 32453)|
|Inj. Vol.:||2 µL splitless (hold 0.3 min)|
|Liner:||Topaz 4.0 mm ID single taper inlet liner w/wool (cat.# 23303)|
|Inj. Temp.:||250 °C|
|Purge Flow:||40 mL/min|
|Oven Temp.:||120 °C (hold 0.3 min) to 200 °C at 60 °C/min to 230 °C at 25 °C/min to 320 °C at 35 °C/min (hold 1.5 min)|
|Carrier Gas||H2, constant flow|
|Linear Velocity:||90 cm/sec|
|Detector||Micro-ECD @ 330 °C|
|Make-up Gas Flow Rate:||60 mL/min|
|Make-up Gas Type:||N2|
|Data Rate:||50 Hz|
|Instrument||Agilent 7890A GC|
|Notes||A GC Accelerator oven insert kit (cat.# 23849) was installed in the instrument to speed up analysis. The analytical columns were custom products that were not on cages. This customized format can be obtained from Restek simply by ordering the standard column catalog number and specifying “-051” under special instructions.|
Laboratories are typically under pressure to report data in a timely manner, and unexpected instrument downtime for maintenance significantly reduces productivity. The organochlorine pesticides analysis workflow demonstrated here can help increase sample throughput using two approaches: (1) more effective sample extract cleanup, and (2) simultaneous dual column analysis with oven inserts. Removing more matrix interferences with CarboPrep Plus SPE reduces active sites in the inlet, which allows for longer run sequences and fewer failing continuing calibration standards. In addition, using a dual column analytical approach with the speed gains afforded by GC Accelerator oven inserts helps labs analyze and report results for more samples in less time.