SLE, SPE and LLE – How are Those Different?

Liquid-liquid extraction (LLE), supported liquid extraction (SLE), and solid-phase extraction (SPE) have existed for decades and if you’re doing organic sample preparation, you’re probably quite familiar with at least one of these techniques.  But are you familiar with all of them?  How are they similar?  How are they different?  Let’s review! Continue reading SLE, SPE and LLE – How are Those Different?

1,4 Dioxane Contamination and Updated Regulations – Are You Being Impacted?

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!  

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The Importance of Methanol in Oil & Grease Extractions

Have you ever been extracting samples for oil and grease compounds using solid phase extraction (SPE) and thought, “why do I have to use all these different solvents, when I’m just trying to get my compounds to retain on, and then elute from, an SPE disk?”

If you’ve been digging into the extraction method a bit, you’ve probably asked yourself “I wonder what the purpose of the methanol is” at least once or twice.  If you’re processing samples for oil and grease, your goal is to determine the concentration of compounds that can be extracted in n-hexane (also known as HEMs), so it’s logical to think that you’d load your sample onto your SPE disk, then pour some hexane through it to elute your target analytes.

For the most part, that logic is sound; however, there’s more to the chemistry than that.  I was trying to explain this chemistry to a colleague of mine recently and his eyes started glazing over about 3 minutes into my explanation.  I tested my explanation on a few other colleagues and got the same response so I started to give up hope that anyone was going to share my excitement for chemistry and what’s going on within the SPE disk.

Then I stumbled across this graphic and my hope was restored.  In this simple graphic, the overall extraction scheme is listed in the center and the addition of methanol and hexane are illustrated to the left and to the right, respectively.  What I really like about this graphic is that it allows me to walk through the extraction, step-by-step, and see the impact of each solvent.  Let’s walk through it and I’ll show you what I mean.

If you start at the top, the box labeled “1” shows you the state of your sample after it’s been passed through your SPE disk.  The bulk of your water matrix has passed through the disk and been directed to waste, while your target analytes remain trapped in the disk.  Some of your target analytes (the beige-colored circles) are present in solution with water molecules surrounding them.  Water molecules are polar and their net dipole moments cause them to be attracted to each other (the positive dipole moment of one water molecule is attracted to the negative dipole moment of another water molecule).  So, in their effort to cluster together, water molecules end up trapping a few compounds – compounds that you’d like to extract and quantify.  Unfortunately, passing your sample through the SPE disk does not remove these water molecules.

In an ideal world, you would add some hexane, elute all your compounds, then evaporate off the hexane and record your HEM weight.  Unfortunately, those pesky water molecules are going to prevent the hexane from reaching some of your compounds (don’t forget that hexane isn’t miscible in water).  So you would add hexane to elute the analytes that are free from water molecules and accessible by the hexane (follow the first arrow to the right in the graphic).  This will leave you with just the water-bound analytes (the box labeled “2).

Here is where methanol will come to your rescue.  Methanol is a polar solvent and is soluble in water.  Methanol isn’t as polar as water, but it’s still pretty polar.  When methanol passes through your disk, the attraction between methanol molecules and water molecules becomes stronger than the attraction between water molecules and other water molecules.  As water molecules seek out methanol molecules, the water molecule clusters break up and release the remaining target analytes you’re trying to extract (i.e. the box labeled “3).  One more pass of hexane elutes those target analytes into your collection flask and now you’ve collected all your analytes of interest.

Skeptical of the chemical journey I’ve just outlined?  Check out these data tables where the proof is in the numbers.  I wanted to see if I could prove out my theory in the lab, so I obtained 6 liters (yes, SIX liters) of a real-world influent sample and divided the sample into six, 1-liter replicates.  The six samples were extracted using an automated SPE extraction system.  All six of the SPE samples were processed using the exact same system and the same extraction conditions, with one exception – three were extracted using methanol at the appropriate steps and the remaining three were extracted without methanol.

Extraction with methanol

Sample IDStarting Weight (g)Final Weight (g)HEM Weight (mg)
Replicate #16.18316.213630.5
Replicate #26.20306.235932.9
Replicate #36.20156.235734.2
Avg. HEM Weight (mg)32.5

Extraction without methanol

Sample IDStarting Weight (g)Final Weight (g)HEM Weight (mg)
Replicate #16.24736.2656
18.3
Replicate #26.18016.204524.4
Replicate #36.25066.264313.7
Avg. HEM Weight (mg)18.8

 

Under the exact same conditions, the use of methanol produced an HEM weight of 32.5 mg.  Without methanol, the average weight of the hexane extractable material was 18.8 mg – a difference of 42% by weight!

Seems like methanol plays a pretty important role in oil and grease extractions.

Contaminants Everywhere

If you are processing environmental samples then you’ve probably dealt with contamination at some point.  If you haven’t, then you should be congratulated for creating the only laboratory on Earth that has ever been completely free of all sources of contamination!  There are many (in some cases, many many many) sources of contamination and the severity of your contamination issues can vary significantly depending on what types of samples you run, the cleanliness of your laboratory, the systems that are running, and the care with which samples are being collected, stored, prepared, run and disposed of.

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Simplified BPA Analysis

Bisphenol A (BPA) is one of the most widely produced chemicals in the world – approximately 4 million metric tons annually.  In recent years, BPA has received a lot of negative attention.  In fact, I can’t remember the last time I saw a plastic item in the store that didn’t have a “BPA free” marking on it.  These labels are for good reason, though, as BPA has been found to produce negative hormonal effects within the body.  BPA is a chemical that mimics estrogen and disrupts the endocrine system, which can lead to developmental disorders, thyroid issues, diabetes and even reproductive organ cancers.  BPA is so prevalent because it has many uses in polymer chemistry.  First and foremost, BPA is used as a monomer in the production of polycarbonate, a very hard thermoplastic which has countless applications, including: water bottles, baby bottles, CDs, DVDs, eyeglass lenses and many more.

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Naphthalene – Are You Ingesting More Than You Realize?

How familiar are you with naphthalene?

If you have ever used mothballs for storing clothing, you are pretty familiar with a compound known as naphthalene. If you are asking questions like “how would I know if I’ve used mothballs?” or “remind me, what do mothballs look like?” then you’ve never spent any significant time around them. If you had, you would vividly remember the smell that hits you like a brick and brings you to your knees.

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Is the Ocean Floor Dissolving?

Since oceans cover roughly two thirds of the Earth, and since oceans can demonstrate signs of ecosystem health and climate change, the U.S. EPA has watched the ocean for changes in their physical property for many years. From sea level height to ocean temperature to coastal flooding occurrences to surface temperature, the EPA has closely monitored changes in these properties as an indicator of climate change.

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Oh No! Not Peanut Butter Too!

As a child, peanut butter was a staple in our household. It was an easy meal for us “latchkey kids” who would come home from school to an empty house, starving. I would grab the old Wonder Bread and whip together a thick, 2-inch peanut butter and grape jelly sandwich, head to the living room to watch the afternoon programs that my parents had prohibited like Dark Shadows.

Oh, the good old days!

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Surefire Ways to Avoid Fat Gain During the Holidays

It’s that time of year again – avoiding the Thanksgiving “Fatberg”

Thanksgiving is that special time of year that we gather together with family and friends to give thanks and experience traditions that are carried on through generations. Our Thanksgiving meal consists of: roasted turkey, mashed potatoes, stuffing, gravy and homemade cranberry sauce. We also serve the not-so-customary stuffed artichokes – a family favorite, passed down from my Italian grandparents. Continue reading Surefire Ways to Avoid Fat Gain During the Holidays

National Chemistry Week

If you’re like me, you’ve spent National Chemistry Week drinking from your coffee mug that’s shaped like a beaker, you’ve been cooking with the spices you store in test tubes, you’ve been wearing your t-shirts with periodic tables printed on them and you’ve been telling your best chemistry jokes.

Originally known as National Chemistry Day back in 1987, the American Chemical Society (ACS) created this even to bring awareness of the importance of chemistry in our day-to-day lives. The holiday has since been expanded to a full week and has been celebrated during the fourth week in October since 1989. To focus activities and celebrations for the holiday, the ACS assigns a theme each year. This year’s theme is “Chemistry is Out of This World!”

While National Chemistry Week runs from Oct 21-27, Mole Day is specifically celebrated on Oct 23 from 6:02 am to 6:02 pm. It’s a very specific window to celebrate the holiday, but there’s a reason. If you’re not sure what the reason is, write out the time and date using only numerical representation. Still scratching your head? Read on!

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