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Automated SFE-SFC Analysis of Explosives in Soil SamplesWilliam Hedgepeth, Ken Tanaka, Shimadzu Scientific Instruments, Inc., Columbia, Maryland

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Introduction

There are a large number of explosives-contaminated sites in the US, Europe, and Asia. High levels of explosives in soil can threaten the health of humans, livestock, and wildlife. A number of remediation efforts are underway, which require the analysis of explosives in soil samples.

Recently, a new technique was introduced the allows the automated supercritical extraction and SFC analysis of samples with minimal sample preparation and handling requirements to save analyst time and sample preparation expenses.

This technique was applied to the analysis of explosives in soil samples and showed good recoveries of the explosives tested in a number of different soil samples.

Automated analysis of up to 48 samples is possible without the need for manual sample preparation to allow quick screening of explosives in numerous soil samples.

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Experimental

Fig.1 shows a diagram of the SFE-SFC system that was used in this experiment. This system consists of a combination of supercritical fluid chromatography and extraction systems. Method development was initially performed with the SFC method scouting system that automatically allows screening of up to 12 analytical columns with a number of different modifiers.

After determination of the optimal column and modifier combination for an explosives mix (AccuStandard M-8330) was completed, the analysis was moved to the SFE portion of the system for study of the explosives mix from a variety of soil types.

The SFE portion of the system allows the automated analysis of up to 48 soil samples by combining the sample preparation portion with the chromatographic analysis. Samples are extracted from extraction vessels and automatically transferred to an analytical column for analysis.

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Instrument Design

1A: Supercritical fluid chromatography (SFC) system for analytical method development

1B: On-line Supercritical fluid extraction/chromatography (SFE/SFC) system

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Method Development

Flow rate: 3 mL/minDetector: Photodiode arrayColumn Temp: 35°CBackpressure: 15 MPa

Nexera UC BasicNexera UC PFPNexera UC DiolNexera UC Ethyl PyridineNexera UC NaphthylNexera UC Nitro

A: CO2B: MeOHGradient: 1 to 10 min, 0 to 40% MeOH

Columns: 4.6 x 250mm, 5 um

Mobile Phase

Conditions

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SFC Method Scouting

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Data6:explosives_Diol_MeOH_0_40_018.lcd PDA Ch1 254nm,4nm Data5:explosives_Naphthyl_MeOH_0_40_016.lcd PDA Ch1 254nm,4nm Data4:explosives_EP_MeOH_0_40_014.lcd PDA Ch1 254nm,4nm Data3:explosives_Nitro_MeOH_0_40_012.lcd PDA Ch1 254nm,4nm Data2:explosives_PFP_MeOH_0_40_010.lcd PDA Ch1 254nm,4nm Data1:explosives_Basic_MeOH_0_40_008.lcd PDA Ch1 254nm,4nm

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SFC Column Scouting of Explosives Mix

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Optimized SFC Chromatogram

1. 2-NT 2. NB 3. 2,6-DNT 4. 3-NT 5. 4-NT 6. TNT 7. 1,3-DNB 8. 2,4-DNT 9. 1,3,5-TNB 10. 4a-2,6-DNT 11. Tetryl 12. 2a-4,6-DNT 13. RDX

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Nexera UC Nitro Column

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Samples

AccuStandard explosives standards M-8330-05 were used to prepare an explosives mixture. One gram of each soil sample was spiked with 100 uL of a 50 ppm explosives mixture.

Standards:RDX, TNT, HMX, Tetryl, Nitrobenzene, 1,3-Dinitrobenzene, 2-Nitrotoluene, 3-Nitrotoluene, 2,4-Dinitrotoluene, 4-Amino-2,6-dinitrotoluene, 2,6-Dinitrotoluene, 1,3,5-Trinitrobenzene, 2-Amino-4,6-dinitrotoluene

Soil Samples: 1. Clean Sandy Loam2. Clean Clay Loam3. Clean Sandy Soil4. Clean Loam Soil

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SFE-SFC Conditions

Flow rate: 5 mL/min95/5 CO2/MeOH0-3 min: Static extraction3-6 min: Dynamic extraction

Column: Nexera UC NitroFlow rate: 3 mL/min6-15 min: 0 to 40% MeOH17-25 min: Wash and equilibration

Extraction Conditions Chromatography Conditions

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Blank Soil Sample Extracts

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Data4:CSL-100uL-blank-09.lcd PDA Ch1 254nm,4nm Data3:CLS-100uL-blank-08.lcd PDA Ch1 254nm,4nm Data2:CSS-100uL-blank-07.lcd PDA Ch1 254nm,4nm Data1:CCL-100uL-blank-06.lcd PDA Ch1 254nm,4nm

Clay Loam

Sandy Soil

Loam Soil

Sandy Loam

SFE extracts of blank soil types

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Clean Clay Loam Extract

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Data2:CCL-100uL-spike-02.lcd PDA Ch1 254nm,4nm Data1:CCL-100uL-blank-06.lcd PDA Ch1 254nm,4nm

Clean Clay Loam Soil Blank

Spiked Soil

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Clean Sandy Loam Extract

Clean Sandy Loam Soil Blank

Spiked Soil

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Data2:CSL-100uL-spike-01.lcd PDA Ch1 254nm,4nm Data1:CSL-100uL-blank-09.lcd PDA Ch1 254nm,4nm

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Clean Loam Soil Extract

Clean Loam Soil Blank

Spiked Soil

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Data2:CLS-100uL-spike-04.lcd PDA Ch1 254nm,4nm Data1:CLS-100uL-blank-08.lcd PDA Ch1 254nm,4nm

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Clean Sandy Soil Extract

Clean Sandy Soil Blank

Spiked Soil

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Data2:CSS-100uL-spike-03.lcd PDA Ch1 254nm,4nm Data1:CSS-100uL-blank-07.lcd PDA Ch1 254nm,4nm

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Spiked Soil Extracts Overlay

Spiked explosives standard into four different soil types

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Data4:CLS-100uL-spike-04.lcd PDA Ch1 254nm,4nm Data3:CSS-100uL-spike-03.lcd PDA Ch1 254nm,4nm Data2:CCL-100uL-spike-02.lcd PDA Ch1 254nm,4nm Data1:CSL-100uL-spike-01.lcd PDA Ch1 254nm,4nm

Clean Sandy Loam

Clean Clay Loam

Clean Sandy Soil

Clean Loam Soil

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Linearity Study

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Data3:stdmix-200uL-extract-split-5.lcd PDA Ch1 254nm,4nm Data2:stdmix-100uL-extract-split-4.lcd PDA Ch1 254nm,4nm Data1:stdmix-50uL-extract-split-2.lcd PDA Ch1 254nm,4nm

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One gram of soil was spiked with 50, 100, and 200 uL of explosives mix

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Linearity Results

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2-NTY = aX + ba = 1263.65b = -8615.43R^2 = 0.9998975R = 0.9999488

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TNTY = aX + ba = 3113.98b = -3707.87R^2 = 0.9985081R = 0.9992538

1,3,5-TNBY = aX + ba = 1066.51b = -5951.52R^2 = 0.9998268R = 0.9999134

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A variety of soil samples showed little interference with spiked explosive standards.

Clean Loam Soil provided poor recovery of the late eluting RDX peak.

Good reproducibility was observed with the explosive standard extracts from a variety of soil samples.

Good linearity was observed for the explosive compounds.

Automated SFE-SFC can be a quick way to screen up to 48 soil samples for explosives in a variety of soil types with minimal sample prep.

Discussion

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