Extraction of EPA method 625.1 semi-volatile analytes from wastewater

Introduction

The EPA has been monitoring organic pollutants within wastewater matrices since 1984 by developing their own methods and acceptance criteria.
The basic, neutral and acidic extractions in EPA Method 625.1 are a part of the revised version of EPA Method 625.

EPA Method 625.1 organizes the target analytes into three different tables. Table 1 lists non-pesticide/PCB basic/ neutral analytes and table 2 lists the acidic analytes which may be extracted from a sample matrix to determine analytes quantitatively and qualitatively. Table 3 contains additional analytes that can be extracted via 625.1. Since there are basic and acidic analytes that need to be recovered during the extraction, the method requires two pH adjustment steps when extracting the semi-volatile analytes from a sample. Initially, the water sample is adjusted to a pH less than 2 to extract the acidic and neutral analytes. Following the acidic extraction, the pH is adjusted to greater than 11, extracting the basic analytes from the sample. The large number of analytes in Tables 1–3 of this method makes testing difficult if all analytes are determined simultaneously. Therefore, it is necessary to determine and perform quality control (QC) tests for the “analytes of interest” only. Analytes of interest are those required to be determined by a regulatory/control authority or in a permit, or by a client. If a list of analytes is not specified, the analytes in Tables 1 and 2 must be determined, at a minimum, and QC testing must be performed for these analytes. The analytes in Tables 1 and 2, and some of the analytes in Table 3 have been identified as Toxic Pollutants (40 CFR 401.15), expanded to a list of Priority Pollutants (40 CFR 423, Appendix A).¹

Within the revision of EPA Method 625.1, laboratories are allowed to extract samples via solid phase extraction. After extraction, the extracts must be dried and concentrated.
This application note will show recoveries of all the analytes within tables 1–2 of 625.1 and a few analytes from table 3 obtained using the Biotage® Horizon 5000 with Atlantic® 8270 One Pass solid phase extraction disks (P/N 47-2346-11), 8270 Carbon Cartridges Max-Detect cartridges (P/N 49-2620-01), and 1.0 Micron Atlantic® Fast Flow Pre-Filters (P/N FFAP-100-HS1) for analyte extraction. The DryDisk® Solvent Drying System and the TurboVap® II are used for solvent drying and concentration and analysis is by GC-MS.

Experimental

Each 1 L deionized water sample was prepared by adding 1 mL of hydrochloric acid, bringing the sample to a pH less than 2. A total of two method blanks were analysed. The first method blank contained 50 μg/L of surrogates and the second method blank contained 100 μg/L of surrogates. A total of twelve samples were spiked with a 625.1 spike mix in addition to surrogates. Six of these samples were spiked at a concentration level of 50 μg/L with the remaining six spiked at a concentration level of 100 μg/L. Using the Biotage® Horizon 5000 (P/N SPE-DEX 5000), all the samples were extracted using the method outlined in table 1. Atlantic® 8270 One-Pass Disks in conjunction with the 8270 Carbon Cartridge Max-Detect were used as the solid phase extraction consumables.

The Atlantic® 8270 One-Pass Disk consists of a mixed-mode chemistry, eliminating the need for the second adjustment to the pH of the sample. Instead, a 1% ammonium hydroxide rinse adjusts the pH of the disk. After the analytes that were retained on the disk have been collected, the 8270 Carbon Cartridge Max -Detect (P/N 49-2620-01) is eluted to recover the more volatile analytes within a sample.

Upon completion of the extraction, the samples were dried using the DryDisk® Solvent Drying System (P/N SDS-101-19/22) in conjunction with DryDisks® (P/N 40-705-HT) utilizing the parameters outlined in table 2. The dried extracts were transferred to 200 mL evaporation tubes with an endpoint of 0.9 mL (P/N C128506) for use in the TurboVap® II (P/N 415001). Because the final volume of the dried extracts exceeded 200 mL, the samples were added to the evaporation tubes in two portions. The first portion was concentrated to approximately 15 mL before adding the final extract volume as well as glassware rinses. The samples were evaporated using the parameters outlined in table 3.
After evaporation on the TurboVap® II, the samples were brought to 1 mL with methylene chloride and transferred to GC-MS vials. Internal standard was added to an aliquot of each of the completed 1 mL extracts and analyzed using the GC/MS using instrument parameters outlined in table 4.

*”Reagent Water 2” is added to the list of configured solvents with the Waste Destination of “Solvent Waste” rather than using the factory programmed “Reagent Water”, which is sent to “Water Waste”.

Figure 1: TurboVap® II solvent evaporator(left) and Biotage® Horizon 5000 (right) 

Results and discussion

Table 5 shows the total concentration time for each sample using the TurboVap® II.

When concentrating the samples, aluminium foil was used to cover each sample vial in order to limit interactions between the evaporating acetone and the water vapor molecules present from the warm water bath. Two holes were pierced on either side of the foil caps to allow the nozzle (N2 flow) to enter, and evaporated solvent to escape, the vials. The concentration times only varied slightly between the samples. On average, the samples that initially contained approximately 303 mL of solvent were concentrated on the TurboVap® II to approximately 0.9 mL in 1 hour and 49 minutes.

Data for the method blanks and spiked samples, including average percent recovery and %RSD for each level (50 μg/L and 100 μg/L) are outlined in table 6. When reading table 6, the analytes are colour coded based upon the legend outlined in the header. The list of analytes in table 6 below contains all the analytes from tables 1 and 2 and some from table 3 of EPA Method 625.1.

The EPA acceptance criteria for table 1 and table 2 analytes are found in tables 4 and 5 respectively in EPA Method 625.1. For those compounds in tables 3 and 8, the EPA has elected not to set specific acceptance criteria themselves. Instead, the appropriate acceptance criteria are left to the laboratories to develop in one of several ways: based on laboratory control charts, using the range 60-140%, or using the guidelines outlined in section 8.4.5 of the 625.1 method itself. All the analytes in table 1 and table 2 of the EPA method passed the EPA method acceptance criteria for both concentration levels. The percent relative standard deviations indicated minimal variation in percent recovery for each sample set.

According to section 6.8.1 of EPA Method 625.1, a minimum of three surrogates are required for analysis as long as they do not interfere with target analytes. The six surrogates used in this application note passed the acceptance criteria set by the EPA method for all of the samples at 50 μg/L and three of the samples at 100 μg/L. The recovery of two surrogates, 2-fluoro-phenol and phenol-d6, from the other three samples at 100 μg/L fell below the lower passing limit of 60%, lowering the average. However, the EPA only requires a minimum of three surrogates for each sample. Even though two surrogates fell below the lower limit, four surrogates fell within the limits. The calculated percent relative standard deviation for each surrogate in the order found in table 7 was 7.23%, 5.48%, 1.99%, 2.93%, 3.66%, and 2.36%, indicating minimal variation between the samples.

Two method blanks were extracted with surrogates at different levels. The concentration of the surrogates for the first blank was 50 μg/L and the concentration of surrogates for the second blank was 100 μg/L. Neither showed any false positives for target analytes above the detection limit of 10 μg/L which is denoted by N.D, meaning “not detected.”

References

1. United States Environmental Protection Agency, Method
625.1: Base/Neutrals and Acids by GC/MS, available at www.epa.gov, December 2016.

Literature number: AN930