“Why do I keep seeing background contamination from phthalate and adipate when I do extractions for semi-volatiles?” This is one of the most common questions I’ve been asked when I’m traveling in the field. It’s an issue I’ve come across in my own lab on occasion and if you can’t find the source of your contamination, it can turn routine application work into a troubleshooting nightmare. Given how often I’ve seen these compounds cause contamination issues, I thought I’d review some of the most common sources for these.

    What are Adipate and Phthalate?

    First, I wanted to provide a bit of background on these two compounds. Di(2-ethylhexyl) adipate and di(2-ethylhexyl) phthalate are classes of compounds that fall under a broad group of compounds called plasticizers. These compounds are added to a polymer during the initial melt to give them a desired quality such as flexibility or toughness. In order to form a stable crystal lattice, the polymer chains want to crystallize in an organized pattern, near each other. The plasticizer gets in the way and increases the spacing between the polymer chains, which reduces the overall rigidity of the material.

    In other words, adipates and phthalates are used to manufacture materials that are bendy but tough.

    Plasticizers are added to the polymer melt in “free form” which means that they are just dumped into the mixture and stirred up to disperse the plasticizer equally. Since the phthalate and adipate are added in free form, they are very easily extracted or leached out of the polymer. Since the polymers are used to produce things like flooring materials, paints, cables, adhesives and tubing, it’s easy to imagine a laboratory filled with components that are ready to contaminate our samples with phthalate and adipate.

    It’s critical that you find any possible sources and eliminate them to the best of your ability. If you don’t, you’ll quickly generate data that you can’t use. The labs that are going to struggle with this contamination the most are those who are processing drinking water samples. The EPA semi-volatile drinking water methods, Methods 525.2 and 525.3, require that your measured phthalate concentrations be below 0.1-0.2 µg/L in your samples. Believe it or not, that’s a pretty low concentration range and if you’re suffering from a small amount of phthalate contamination, your total phthalate concentration will quickly be above the maximum allowable level.

    It might seem like an impossible task to isolate and prevent these compounds from contaminating your samples, but I’m going to review 5 common sources that I’ve found and hopefully that will point you in the right direction if you ever find yourself troubleshooting this contamination issue.

    Stock solvents

    The first place I always recommend checking is your stock solvent, especially if you’re using methylene chloride, ethyl acetate, or acetone. If your solvent isn’t pure enough, the contaminants in that bottle are very likely going to contain phthalates and adipates. If you’re looking at your solvent bottle and wondering whether the solvent is pure enough, check the requirements for the method you’re using to generate data (EPA, ASTM, ISO, DIN, etc). If you’re not following a regulated method, or if your method doesn’t specify a purity for the solvent you’re using, here’s an easy way to check the purity:

    • Determine the volume of solvent that the method requires you to use when you’re performing your extraction
    • Put that volume of solvent into an evaporation tube and evaporate it down to the final volume you’d normally use for analysis (typically 1 mL)
    • Transfer the final extract into an LC or a GC vial and measure the extract for contaminants

    If your results come back and you have a measurable amount of phthalate and/or adipate, you need to purchase a higher-grade solvent. Click here to read a review of the most common solvent grades that are used in labs processing semi-volatile organic compounds.

    Sodium sulfate

    Another reagent to check for contamination is your sodium sulfate. Actually, sodium sulfate is typically used with folded up filter paper, so you should check your filter paper too. Sodium sulfate is used for drying extracts before they are evaporated. If the salt isn’t pure enough, it’s a great way to remove water from, and introduce contaminants into your extract at the same time. If you’re following a regulated method, consult that to determine how pure your sodium sulfate needs to be. If you’re still unsure, or if your method doesn’t specify a purity, follow these steps to check it for yourself:

    • Carefully fold a piece of filter paper into a glass funnel and add the amount of sodium sulfate you’d normally use to dry your extracts
    • Determine the volume of solvent that the method requires you to use when you’re performing your extraction (after confirming that this solvent is clean and isn’t contaminating your extracts)
    • Put that volume of solvent through the sodium sulfate and collect it into an evaporation tube. Evaporate it down to the final volume you’d normally use for analysis (typically 1 mL)
    • Transfer the final extract into an LC or a GC vial and measure the extract for contaminants

    If your results indicate that your sodium sulfate is contaminated, you can purchase it at a higher purity; however, an alternative to that is to use Soxhlet extraction with methylene chloride to purify the reagent. From there, transfer it to an oven at 400 ˚C where the heat will volatilize any remaining phthalates and adipates, while driving off any residual water.

    Solvent bottles

    If you’re using an automated extractor to perform your extractions, your solvent bottles could be contributing to your contamination issues. Some glassware companies use a finishing solution to produce a bottle with a shiny look, but this coating typically contains phthalates, which can leach out and transfer into the solvent inside the bottle. If any of your solvent bottles are new, it’s a good idea to rinse them thoroughly with methylene chloride, then put them in an oven at 400 ˚C to volatilize any leachable phthalates. When you’re done heating your solvent bottles, don’t forget to pull them out of the oven and let them sit in a room temperature environment to cool slowly and prevent them from cracking. These bottles aren’t like your Pyrex glassware at home where you can transfer them from the fridge directly to the oven.

    Solvent stones

    Solvent stones are commonly used with LC and HPLC equipment, and they’re often used in solvent bottles on extraction systems. These stones are great for filtering particles out of your solvents before they pass through your system, but the stones themselves can leach phthalate and adipate over time. If your solvent stones are brand new, or if they haven’t been used in awhile, make sure you rinse them thoroughly with a clean, high purity solvent such as methanol or methylene chloride. If your stones aren’t new but have been sitting idle in solvent bottles for a week or longer, it couldn’t hurt to rinse them really well and test the solvent they’ve been sitting in to see whether it’s become contaminated. The best solution to this potential problem is to stop using the stones altogether. This filtration is necessary for LC and HPLC equipment, but solid-phase extraction systems are much more tolerant of fine particles. So if your solvents are clean and your stones are continuously threatening to contaminate your extracts, remove them from your solvent lines. If a filter turns out to be necessary for your solvent lines, phthalate-free filters can be used.

    Deionized water

    One final suggestion is to check the deionized (DI) water source in your lab (assuming this is the same source of DI water for preparing your laboratory control samples). In many DI water systems, the finished product is stored in a plastic tank, which can leach phthalates. Here’s a way to check for contaminants:

    • -Collect a 1-liter sample of DI water (adjust the volume to match the method you’re following) and extract that sample by liquid-liquid extraction using methylene chloride.
    • -Collect your extract and dry it using sodium sulfate (that you’ve already tested to determine that it’s not contaminating your samples).
    • -Transfer the dried extract into an evaporation tube and evaporate it down to the final volume you’d normally use for analysis (typically 1 mL).
    • -Transfer the final extract into an LC or a GC vial and measure the extract for contaminants.

    -If your DI water is not stored in a plastic tank, it does not mean that you’re off the hook for testing it. Even the filters that are used for ultrapure water systems can contain phthalates, so it is always a good idea to check the water source.

    By now, you’ve figured out that there are many potential sources of phthalate and adipate contamination – so many, that it can seem overwhelming to find the source (or sources, if you have more than one). When trying to find a source for phthalate/adipate contamination (or any other type of contamination for that matter), remember to change one variable at a time to make it easier to find your culprit. Feel free to share some of your contamination challenges in the comments below!

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