Use Isotope Dilution when Reporting 1,4-Dioxane by EPA Method 8270x

1,4-dioxane results are prone to a low bias when analyzing hazardous waste by methods that incorporate an extract concentration step. Due to its high vapor pressure, 1,4-dioxane is driven off with the extraction solvent (typically dichloromethane). The authors of EPA Method 522 explicitly forbade extract concentration; they saw losses of over 30% when concentrating their extracts under a nitrogen stream [1].

If you are reporting 1,4-dioxane by EPA Method 8270C or 8270D, you can use isotope dilution to counter this negative bias. To quickly summarize for those of you who aren't familiar, isotope dilution works by spiking a known quantity of an isotope labeled version of your target analyte in your sample before extraction. The idea is that whatever happens to the labeled compound should be reflected proportionately in the target compound recovery. This is especially advantageous for liquid-liquid extractions of aqueous samples by EPA Method 3510 because the extracted internal standard will be able to normalize both extraction inefficiency (incomplete partitioning in the organic phase) and evaporative loss during extract concentration.

To demonstrate this bias, I spiked 6 Turbo-Vap concentrator tubes filled with 200 mL dichloromethane each with 50 µL of a solution containing 400 µg/mL of each internal standard (1,4-dioxane-D8 and N-nitrosodimethylamine-D6) and 100 µg/mL of each target analyte (1,4-dioxane and N-Nitrosodimethylamine). The theoretical concentration of the internal standards and target analytes should be 20 µg/mL and 5.0 µg/mL respectively once the solvent in each tube is concentrated to 1 mL. For a comparison, I analyzed 3 standards prepared with 20 µg/mL internal standards and 5.0 µg/mL target analytes. Figure 1 compares the calculated 1,4-dioxane recoveries using isotope dilution with the calculated value using the average internal standard response value for the 3 unconcentrated standards in place the of the isotope dilution internal standard value.

Figure 1 - Calculated recoveries for 1,4-dioxane using isotope dilution vs internal standard addition

Both techniques yield results within 20% of the 5.0 µg/mL spike value, though all 6 standard addition values are biased low. The average recovery for the isotope dilution experiment (n=6) was 5.2 µg/mL, while the average calculated value using internal standard addition (n=6) was 4.4 µg/mL.

Setting up for isotope dilution is relatively simple if you are already familiar with setting up an internal standard calibration in your data analysis software. First, you will want to calibrate using 1,4-dioxane-D8 as the internal standard for 1,4-dioxane. We offer a revised 7 component internal standard (IS) mix for EPA Method 8270 (cat# 31885) that includes 1,4-dioxane-D8 in addition to the six internal standard compounds typical for the method. We also offer 1,4-dioxane-D8 as a single in P&T grade methanol (cat# 30614) that should be incorporated in your sample surrogate spike solution so that its final concentration in your extract (assuming 100% recovery) matches the concentration in your initial calibration (ICAL). I typically use a concentration of 10 or 20 µg/mL for my internal standards, depending on the calibration range I am targeting. Before analysis, instead of spiking the 7 component IS mix in the extract, you should use the standard 6 component IS mix (cat# 31206). At this point, the extracts should be analyzed like a typical sample.

It is important to note that the same data quality requirements apply to 1,4-dioxane-D8 as any other internal standard for an 8000 series method. Minimal internal standard recoveries allowed are 50% of the ICAL average response (see Table 1 for a comparison of isotope dilution recoveries compared to unconcentrated standard).

Table 1 - Isotope dilution responses vs standard responses

Though we didn't experience it in this set of experiments, N-nitrosodimethylamine (NDMA) can also show significant evaporative loss during concentration. We offer a deuterated single, N-nitrosodimethylamine-D6 (cat# 33910) that can be used for isotope dilution as well, but the single will need to incorporated into your internal standard primary dilution standard and surrogate spike solution.

1. P. Grimmett, J. Munch, Method Development for the Analysis of 1,4-Dioxane in Drinking Water Using Solid-Phase Extraction and Gas Chromatography-Mass Spectrometry, J. of Chromatographic Science 47 (2009) 31.