INAC Project: PAHs Contamination: Chapter II: Method Overview

1. Method Overview

· [A] Sample Preparation

· [B[ Soxhlet extraction

· [C] Silica gel cleanup

· [D] Kudema-Danish (K-D) volume reduction

· [E] GC-MS SIM (Selective Ion Monitoring) mode analysis

2. Materials & Apparatus

[A] Air sampling cartridge with AIRCON pump

Internal Standard (naphthalene-d8, acenaphthene-d10, phenanthrene-d10,

chrysene-d12, and perylene-d12)

[B] 10% ethyl ether in hexane (GC grade)

Methylene chloride (GC grade)

Soxhlet extractor

[C] Silica gel (100-200 mesh, Davisil Grade 644)

Chromatographic column (11 x 300 mm)

[D] Kudema-Danish (K-D) concentrator

Nitrogen supply

[E] GC-MS instrument coupled with PTV injection system and SVE-COC column

3. Procedure

[A] Sample Preparation

Place 3 air sampling cartridge inside the gent’s toilet close together, each of the sampling cartridges contains 3 cm polyurethane foam (PUF) that holds XAD-2 resin trap in place. (XAD-2 resin serves to trap the sample analyte PAHs in the atmosphere) An AIRCON pump is coupled with the cartridge as it is used to draw air through the cartridge at a flow rate of 10 L/min for 48 hr. Spike the cartridge with exactly 2ul of the internal standard (24ug/mL of each component) after starting the pump.

{Explanation: Large volume (28,800L) of air is driven through the cartridge as the concentration of individual PAHs in air is usually very low (and it can be as low as a few pg/m3). The efficiency of the resin trap also has to be taken into account, since it is not 100% effective; using a larger volume of air would ensure that any error due to its inefficiency can be minimized}

[B] Sample extraction (Soxhlet extraction)

The sample collected on the cartridge is extracted by refluxing on Soxhlet extractors using 10% ethyl ether in hexane for 24 hr followed by two subsequent 24-hr extractions using methylene chloride. After the extraction, combine the extract.

{Explanation: Typically, Soxhlet extraction is used when the desired compound has a limited solubility in a solvent, and the impurity is insoluble in that solvent. In the first cycle of Soxhlet extraction, the solvent is introduced through the condenser where it will fill a thimble that holds the sample containing the slightly soluble analyte and insoluble impurities. The solvent is added till before it overflows, then a siphon side arm would drain the solvent containing the analyte into a distillation flask. Meanwhile the distillation flask is heated to vaporize the solvent; the vaporized solvent is condensed as it reaches the condenser and drips back into the thimble.

After many cycle, the desired analyte is concentrated in the distillation flask, which is prudent as aforementioned the concentration of individual PAHs found in air is extremely low thus it’s necessary to concentrate the analyte.}

[C] Sample cleanup (Silica Gel Fractionation Chromatography)

Using the chromatographic column (11 x 300 mm) with 100% activated silica gel, run the column with the Soxhlet extract together with the eluting solvent (hexane), highly polar sample components are irreversibly retained in the column. The aliphatic fraction is first eluted with hexane, and then the aromatic fraction is eluted with 50% DCM in hexane later.

{Explanation: Silica Gel Fractionation Chromatography is used to physically separate the sample components based on their polarity. Its use separates PAHs and polychlorinated biphenyls (PCBs) into two groups, while simultaneously eliminating most interfering substances for subsequent instrumental analysis.}

[D] Concentration (Kudema-Danish volume reduction)

The volume of the sample eluted from the clean-up column was reduced by K-D to ~4 mL and then further reduced by nitrogen to 2 mL for instrumental analysis. Fill up the sample vial with the concentrate.

{Explanation: Before injecting the sample into the GC system, it first has to be concentrated so that trace amounts of the PAHs can be detected.}

[E] GC-MS analysis

For GC-MS analysis, we are using large volume injection by PTV (Programmed Temperature Vaporization) and SVE-COC column (Solvent Vaporization Exit - Cool on-column). Adjust the automated injection system to take up 100 μL of the sample concentrate and run it through the GC-MS SIM mode.

{Explanation: By increasing the injection volume from 1 or 2 μL by traditional split/splitless inlet to 100 μL or higher with PTV inlet, analytical sensitivity is greatly enhanced for analytes with low concentrations. In addition, tedious sample pretreatment procedures may be simplified by

eliminating or shortening the solvent evaporation step, which is not only time-consuming but also subject to chemical loss due to high temperatures or a vacuum. Alternatively, a lesser amount of sample can be collected for predetermined detection limits.

In the SIM mode, the instrument is set to look for only masses of interest in a specified time range, thus it can be specific for a particular analyte of interest.}