Extraction of semi-volatile organic compounds in drinking water with Atlantic® ReadyDisk

Introduction

Drinking water is one of the primary sources of human exposure to toxic chemicals. The U.S. EPA identifies and regulates a number of compounds in drinking water that could pose health risks, and outlines methods for properly quantifying them.

Contaminants can be biological, physical, chemical or radiological, and can exist in a wide range of concentrations. Therefore, the list of EPA methods that are approved for use in testing drinking water is extensive and each method presents its own challenges, based on the specific compounds being quantified.

EPA Method 525.2 is used to quantitate organic compounds found in drinking and source waters. Method 525.2 uses a reversed phase separation mechanism to isolate a large variety of compounds from the sample matrix. The reversed phase separation is achieved using a C18 bonded silica stationary phase which is packed into an SPE cartridge or disk. The C18 stationary phase allows for the extraction of semi-volatile compounds from the sample matrix which are then analysed by gas chromatography-mass spectrometry (GC-MS).¹

Instrumentation

All samples were analysed using the instrumentation listed in Table 1 below.

Table 1: Instrumentation

*The DryVap™ system has been discontinued. We recommend using the TurboVap® evaporation systems for achieving equivalent results.

Experimental

The organic compounds were eluted from the ReadyDisk with small quantities of ethyl acetate, followed by methylene chloride, followed by a 1 to 1 (v/v) mixture of ethyl acetate to methylene chloride. The extracted solution was dried and concentrated via solvent evaporation using the DryVap™* with DryDisk® Separation Membranes.
The sample components were separated, identified, and measured using the gas chromatography/mass spectrometry (GC/MS) system listed in Table 1.

A summary of the overall sample preparation, extraction, drying and concentration procedure is listed below. A detailed overview of the method that was run on the Biotage® Horizon 5000 is listed in Table 2. The DryVap™ and Agilent GC/MS parameters are listed in Table 3 and 4, respectively.

1.  Obtain 1-liter samples of drinking water.
2.  Add dechlorinating agent to each 1-liter sample.
3. Acidify each sample to pH <2 using concentrated HCl.
4.  Add surrogate and internal standard compounds to each sample.
5. Start extraction method shown in Table 2 and collect all extracts (~20 mL each).
6. Add each extract to the DryDisk® holder and start automated drying and concentration process on the DryVap™ system. Evaporate each extract to 0.9 mL using the method listed in Table 3.
7. Quantitatively, bring each extract volume to 1.0 mL using ethyl acetate.
8. Add external standard to each extract.
9. Transfer the extract to a 2.0 mL GC vial.

Table 2: Extraction method

Table 3: drying and concentration process

Table 4: Agilent GC/MS parameters

Results and discussion

Per EPA Method 525.2, a series of laboratory reagent blanks (LRBs) were measured to demonstrate a lack of contamination from the extraction system and the Atlantic® ReadyDisk HC-C18, prior to analysing any samples. Six replicate LRBs were prepared and extracted as described in EPA Method 525.2, following the procedure in the method summary in this note. All blanks were spiked with internal standards such that their final concentration in solution was 5 µg/L. The results for the six LRBs are shown in Table 5 below.

To demonstrate an Initial Demonstration of Laboratory Accuracy and Precision (IDA and IDP), six replicates of a laboratory fortified blank (LFB) were prepared and extracted as described in EPA Method 525.2. Each replicate contained all analytes of interest, including internal standards and surrogates, at 5 µg/L. For each measured analyte and surrogate, the mean accuracy, expressed as a percentage of the true value, should be 70-130 % and the RSD should be less than 30 %, per Method 525.2.1 Results for the six samples are shown in Table 5 below.

Seven additional laboratory fortified blanks were prepared such that all analytes of interest were present at approximately 0.5 µg/L. All seven replicates were analysed on three consecutive days to produce data for calculating method detection limits (MDLs).

Method Detection Limits (MDLs) were calculated based on the measured LFB solutions and are reported in Table 5 below. Results are based on the standard deviation of the replicate measurements, multiplied by the appropriate Student’s t value for the 99 % confidence interval. Results are reported Not Detected (ND) if the measured concentration for all samples were below the lowest calibration point of 0.1 µg/L.

The method detection limits (MDL) were calculated using the formula1:
MDL = S x t _{(n-1, 1 α 0.99)}

Where:
t = Student’s t value for the 99% confidence level (n-1,1-α = 0.99) with n-1 degrees of freedom
n = number of replicates
S = standard deviation of replicate analyses

Table 5: Method detection limits

Conclusion

With the exception of hexachlorocyclopentadiene, carboxin, atraton and prometon, all analytes were recovered within 70–130 % of the known value, in compliance with Method 525.2 criterion for spike recoveries. The average spike (including the problem compounds) recovered at 100.9 %.

Hexachlorocyclopentadiene’s low recovery (27.6 %) can be attributed to the compound’s sensitivity to thermal and photochemical degradation, as well as its propensity to react with acetone. The recovery of carboxin (78.6 %) can be attributed to its significant instability in water. The low recoveries for atraton and prometon of (25.3 % and 37.6 %
respectively) likely stem from inefficient extraction from the water at pH 2, which causes ionization in solution under acidic conditions.¹

The relative standard deviation for all compounds ranged from 0.88–28.6 %, below the method’s RSD criteria of <30 %. The NEW Atlantic® ReadyDisk HC-C18 provided excellent analyte accuracy and precision in an easy-to-use, plug-n-play format.

References

1. United States Environmental Protection Agency, Method 525.2, Revision 2.0: Determination of Organic Compounds in Drinking Water by Liquid-Solid Extraction and Capillary Cartridge Gas Chromatography/Mass Spectrometry.

Ordering Information

Literature number: AN899