If you’re a laboratory that’s processing drinking water samples using solid phase extraction, you’ve inevitably gotten to the step in your procedure where you’ve eluted your analytes from your SPE media and you find yourself saying “How do I dry my extracts?”
What’s the best way to dry my extracts?
This is a question we get quite frequently and it’s a reasonable question to ask. Unfortunately, the answer is – it depends. Solvent drying (not to be confused with solvent evaporation) is an important step in your extraction process when you’re using organic solvents to elute your target analytes. Residual water in your solvent can cause issues if your target analytes extract back out of the solvent and into the water while you’re trying to evaporate or analyze your sample. Water can also damage your chromatography system, so if you’re quantifying your extracts by GC/MS or LC/MS, you want your solvent extracts to be dry prior to analysis.
So, given the importance in solvent drying, I thought I’d share some of the commonly asked questions that come up under this topic.
Q: I’m processing my samples against EPA Method 525.3. Does it matter how I dry my extracts?
A: If your lab is being audited against EPA Method 525.3, you need to dry your extracts per the recommended procedure – in this case, sodium sulfate. Make sure you purchase the recommended grade of anhydrous sulfate and store it appropriately.
If your lab processes a large volume of samples, you may have sought out alternative approaches to solvent drying, such as phase separation membrane. While sodium sulfate is readily available for purchase in bulk quantities and is pretty easy to learn how to use, it has some potential downsides to it.
- It has to be dried and carefully stored, which is time-consuming and requires you to have adequate drying and storage equipment
- It has to be disposed of as hazardous waste
- It’s a chemical that dries your solvent by reacting with water to form a hydrated salt, which means it can retain some of your target compounds (particularly those that are highly water soluble)
- It can contaminate your extract, particularly if it’s stored incorrectly or purchased at a lower grade than is recommended
- It can be saturated. What that means is, if you didn’t calculate the mass of sodium sulfate you needed, given the volume of water you needed to remove, you could exceed the capacity of the salt and end up with a solvent that’s not completely dry
Phase separation membranes physically separate the water from your solvent, which eliminates all of the challenges you face with a chemical drying agent such as sodium sulfate. Plus, it’s compact and easy to store, intuitive to use and easy to dispose of.
There are a handful of benefits to using a phase separation membrane over sodium sulfate – just make sure you check the method you’re following and adhere to the drying method outlined there (if there is one). Check out the method summary in this app note for an example protocol that adheres to EPA Method 525.3 guidelines.
Q: Since EPA Method 525.3 specifies that I use sodium sulfate, can I put sodium sulfate on top of phase separation membrane to dry my extracts?
A: While clever, this is an idea that you would want to run past your auditor first. Since the method specifies the use of sodium sulfate but does not specify the physical separation of water (using a phase separation membrane, for example), physical separation isn’t forbidden, but it’s also not specifically allowed. Yep, this one is a gray area so have a conversation with your auditor before cleverly devising a drying setup that includes both chemical and physical solvent drying.
Q: I’m running samples against EPA Method 525.2. Do the same rules apply to me?
A: Yes. As with Method 525.3, this method specifies the use of sodium sulfate.
Q: I’m not processing samples against an EPA Regulated Method and my protocol doesn’t specify a protocol for extract drying. What should I do?
A: If your lab is not reporting results against a method that specifies an extract drying method, you should have the option to decide whether you want to dry your extracts using physical or chemical separation (double check your laboratory’s established protocols to make sure your SOP allows you this flexibility).
If this decision were up to me, I’d order myself a huge stack of DryDisk® Disks and wave goodbye to sodium sulfate forever!
Do you prefer physical drying over chemical drying? If so, let us know in the comments and share this post to spread the word!
In the world of solid phase extraction (SPE), the list of media that is available seems to be ever-growing. From polymeric stationary phases, to silica-based media, and even molecularly imprinted polymers specifically designed for target analytes. The possibilities seem endless. Luckily for us, most EPA methods specify which media type is required for analysis, but what about methods that don’t specify?
For the methods which don’t specify the media you must use, how do you select your media type?
In a never-ending list of chemical pollutants, a compound that is gaining a lot of attention is 1,4-dioxane. In fact, New Jersey just became the first state to set regulations on the quantity of 1,4-dioxane that can be present in drinking water.
1,4-dioxane, commonly called dioxane (the other two isomers – 1,2-dioxane and 1,3-dioxane are rarely ever seen), is an ether with the molecular formula of C4H8O2. Dioxane was previously used as a polar aprotic solvent. For those who remember their organic chemistry from college, SN2 reactions involve the use of polar aprotic solvents. Since its original use in laboratories, dioxane has been determined to be carcinogenic and, unlike many organic pollutants, it is completely soluble in water. Dioxane’s use as a solvent for industrial purposes has been mostly replaced with tetrahydrofuran, which has a higher boiling point and a lower toxicity. However, the story does not end there!
EPA Method 8270 is one of the 8000 series methods that outlines the preparation of wastewater samples. It is one of dozens of methods for processing wastewaters for semivolatile organic compounds (SVOCs), all of which fall under Method SW-846. Expand the graphic below to see the breadth of the method.
As you can imagine such a complicated method generates a lot of questions. It is a regular occurrence for me to get various questions about EPA methods but recently I have had quite a few about EPA Method 8270 and I wanted to share them in case someone else has the same questions. You can find a great summary of Method 8270 in this blog that one of my colleagues recently wrote – Extraction of Polyphenols in Tea with Lemon Juice.
Q: Do I need a carbon cartridge for my 8270 extraction?
A: The answer to this question is – it depends. The purpose of the carbon cartridge is to collect the light end compounds that are not retained by the Atlantic® One Pass disk. Which compounds are on this list? There are a handful, but some of the more interesting compounds are those related to NDMA, benzyl alcohol, as well as surrogates like phenol-d5 and 2-fluorophenol. When I analyze samples using Method 8270, I look at quite a few classes of compounds and some of them require a carbon cartridge to ensure that my recoveries are acceptable.
As a side note, NDMA is a known carcinogen and has made news headlines because the recalled blood pressure medication, Valsartan, was found to be tainted with NDMA. The EPA also put out a technical fact sheet on NDMA in 2014 that provides some interesting facts.
Q: What is the purpose of the acetone in the elution steps using the Atlantic® One Pass disk and carbon cartridge using the method in your application note?
A: Well, one reason to use acetone is for elution. That’s not the main reason for using this solvent, however. Samples that are processed using Method 8270 are typically dirty samples with high levels of particulates or suspended solids. That means a decent amount of sediment will need to be filtered by your Fast Flow Disk Holder. In the process of filtering out the solid material, some of your water could get trapped in the disk holder. If we moved straight from the aqueous phase to using dichloromethane (DCM) – which is immiscible with water – the residual water would form a barrier, preventing the DCM from passing through the disk. Acetone removes any residual water before DCM is used.
Once the DCM step is complete, acetone is used again to remove any residual DCM. The next step in the elution process involves the use of 1% ammonium hydroxide (NH4OH) in water so all remaining DCM must be gone before proceeding to this step. Believe it or not, acetone is used once again after the dilute ammonium hydroxide solution has passed through the disk because DCM is used to elute analytes from the carbon cartridge. When you think about it, acetone is pretty integral to the elution process!
Fun fact: The carbon cartridge is loaded from the bottom up in the sample processing stage so you don’t have to use as much solvent to elute your target analytes.