Extraction of semi-volatile organic compounds in drinking water with Atlantic® ReadyDisk DVB in compliance with EPA method 525.3

Introduction

The U.S. EPA has been regulating semi-volatile organic compounds in drinking water with Method 525 since 1988. There have been several method updates since then, but the most recent was introduced in February of 2012. EPA method 525.3 is an update to 525.2 that includes a few major changes including: discontinuing the use of a C18 disk in favor of specific DVB or SDVB disks for extraction, modifying the sample preservation procedure and requiring the addition of internal standards immediately following the extraction.

Per Method 525.3, an Initial Demonstration of Capability (IDC) must be performed prior to processing any samples. An IDC for Method 525.3 includes a demonstration of low background noise, precision, and accuracy. Each laboratory must also set and test a minimum reporting level (MRL) based on their application needs. In Method 525.3 the Detection Limit (DL) does not have to be determined during the initial demonstration of capability; however, it is a figure of merit which continues to be required by many regulatory bodies¹.

In this application note, Atlantic® ReadyDisk DVB solid phase extraction disks will be used in combination with a Biotage® Horizon 5000 to extract semi-volatile organic compounds in drinking water samples. All results, based on GC/MS analyses, demonstrate compliance with the performance requirements outlined in EPA Method 525.3.

Instrumentation

Table 1. Sample Preparation and Data Collection.

Experimental

Method summary

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

1. Obtain 1-liter of drinking water.
2. Add 0.10 g L-ascorbic acid and 0.35 g EDTA to each 1-liter sample.
3. Buffer each 1-liter water sample to approximately pH 3.8 using 9.4 g potassium dihydrogen citrate.
4. Add surrogate and standard compounds into the samples.
5. Start extraction method shown in Table 2 and collect extract (~13 mL).
6. Dry each extract with sodium sulfate.
7. Evaporate each extract to 0.9 mL.
8. Add internal standards to the extracted solution.
9. Quantitatively, bring extract volume to 1.0 mL using ethyl acetate.
10. Transfer the extract to a 2.0 mL GC vial.
11. Analyze the solution using GC/MS method in Table 4.
    
    

Table 2. Biotage® Horizon 5000 extraction program.

Table 3. GC/MS parameters.

Results and discussion

Per EPA Method 525.3, a series of laboratory reagent blanks (LRBs) were measured to demonstrate a lack of contamina- tion from the extraction system and the Atlantic® ReadyDisk DVB, prior to analyzing any samples. Six replicate LRBs were prepared and extracted as described in EPA Method 525.3, following the procedure in the method summary in this note. All blanks were spiked with surrogate and internal standards such that their final concentration in solution was 5 μg/L. The results for the six LRBs are shown in Table 4 below.

To demonstrate an Initial Demonstration of Capability (IDC), six replicates of a laboratory fortified blank (LFB) were prepared and extracted as described in EPA Method 525.3. 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 525.3 Results for the six samples are shown in Table 4 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 analyzed to produce data for calculating method detection limits (DL).

Method Detection Limits (MDLs) were calculated based on the measured LFB solutions and are reported in Table 4 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 a 0.99)

Where:

t = Student’s t value for the 99% confidence level (n-1,1-a = 0.99) with n-1 degrees of freedom

n = number of replicates

S = standard deviation of replicate analyses

Table 4. IDC, Precision, Accuracy, DL and LRB results for the Atlantic® ReadyDisk DVB.

With the exception of BHT, disulfoton, phorate and 2, 2’, 5-Trichlorobiphenyl, all analytes of interest were recovered between 70 and 130 % of their true value in compliance with Method 525.3 criterion for spike recoveries. HCCPD was the only exception and recovered within the 60 and 130 % true value limit required by Method 525.3. The average spike (including the low recovery compounds) recovered at 84.0 %

Phorate and disulfoton have been shown to be unstable in ethyl acetate2 as well as in aqueous solutions which accounts for their consistently low recoveries. HCCPD is known to be photosensitive as well as thermally sensitive, which makes it susceptible to degradation and negatively impacts its recovery in solution. With the exception of BHT, HCCPD and phorate, and a few of the triazine herbicides, all compounds were recovered with precision below 10 % RSD. Phorate was recovered with a precision value above 30 % RSD.

Bis(2-ethylhexyl)phthalate had a DL concentration higher than the concentrations at which they were spiked and that was due to one of seven replicates being an outlier. The measured blank concentrations were also shown to be sufficiently low with only four compounds being above an average of 0.1 µg/L. Based on the performance data in this application note, the Atlantic® ReadyDisk DVB SPE disks meet and exceed the criteria outlined in EPA Method 525.3.

References

1. US EPA Method 525.3, US EPA EPA Method 525.3 - Determination of Semivolatile Organic Chemicals in Drinking Water by Solid Phase Extraction and Capillary Column Gas Chromatography/Mass Spectrometry (GC/MS), available at www.epa.gov, 2010.
2. Storage stability of organophosphorus pesticide residues in peanut and soya bean extracted solutions, Gang Guo, Naiwen Jiang, Fengmao Liu, Yanli Bian, R. Soc. open sci. 2018 5 180757; DOI: 10.1098/rsos.180757. Published
   25 July 2018.
   
   

Ordering information

Part Number: AN900.v2