1.1.7. Solid Phase Extraction (SPE)

One method of extraction is called solid phase extraction (SPE). Here, what is primarily done is the water sample is sent through an adsorbent bed. The analyte A that is present in the water will now adsorb. The adsorption analyte, and once the adsorption analyte is done, all A that was originally present in the water has now moved to the adsorbent.

Solid Phase Extraction (SPE) is a method used to separate a specific analyte (A) from a sample like water. In this process, the water sample passes through a column filled with a material that attracts the analyte, causing it to stick to the column. As a result, the analyte is effectively removed from the water sample and concentrated onto the adsorbent material.

Imagine you have a sponge (the adsorbent) and you're trying to soak up some colored dye (the analyte) from a bucket of water. As you place the sponge in the bucket, the dye sticks to the sponge, effectively removing it from the water. The sponge now has the concentration of dye that was originally mixed in the water.

Now you have reduced the volume for a very small tube, but you cannot introduce this directly into the analytical instrument. For that, you have to again elute the A from the column by a suitable solvent, and this solvent is doing desorption here and thus this solvent can now go to an instrument and in the process, you can take this extract and concentrate it further.

After the analyte has been adsorbed onto the solid material in the SPE process, it needs to be released (or eluted) so it can be measured. This is done by passing a solvent through the adsorbent material. The solvent washes away the analyte from the adsorbent, allowing it to be collected in a concentrated solution that can be analyzed with instruments like gas or liquid chromatography.

Think about squeezing out a wet sponge filled with dye. After soaking it up (adsorption), you need to wring the sponge (elution) to release the dye into a container. The extracted dye in the container represents the concentrated solution ready for analysis.

The SPE is a little more easy to handle. However, SPE requires you to buy this infrastructure; the setup is higher you need to buy this tube which are usually non-reusable. Then you have to buy a pump and the associated setup to do all these things.

Compared to traditional liquid-liquid extraction, solid phase extraction offers ease of handling and less mess. It avoids the physical interaction of two liquids, which can lead to sample loss. However, it does require specific equipment and consumables, which might increase costs. This setup allows for more precise control over the extraction process.

Imagine packing clothes into a suitcase (SPE). It keeps everything organized and neat, making it easier to transport. In contrast, liquid-liquid extraction is like tossing clothes into a backpack where things might get mixed up or lost. The suitcase setup, while possibly requiring more investment, ultimately simplifies the packing process.

So, let us assume that for the time being, I remove all the 20 ml. This is 20 ml of extract and this is 1 ml of extract, and this 1 ml I introduce into the analytical instrument and I get a concentration of 1 milligram per liter.

In this step, after the analyte has been extracted and concentrated in the SPE process, the final sample (now reduced to just 1 ml) is analyzed to determine its concentration. If the analysis shows a concentration of 1 milligram per liter, this value reflects the amount of analyte that was extracted and concentrated from the initial larger volume of water.

Think of making a concentrated fruit juice from a large jug of water with a little thrown in. After filtering and reducing the liquid, you taste the final product. If you find that the juice tastes strong (1 mg/L) after starting with a light mixture, it means you’ve successfully concentrated the original flavor from the large amount of water into a much smaller amount that packs the same punch.

During extraction, there are many steps that are present here that there is always a possibility of loss of analyte during different processes, during extraction, during concentration, and these losses mainly occur because of either spillage, evaporation, or errors in estimate of volume.

Throughout the extraction and concentration steps, there may be losses of the analyte due to various factors, such as spills, evaporation, and measurement errors. To ensure accurate results, a recovery process can be conducted. This involves adding a known quantity of analyte to the sample and measuring how much is recovered after extraction, allowing for adjustments to account for any losses.

Consider baking a cake. If some of the batter spills onto the counter (loss during the process), it's important to account for how much cake you actually ended up with afterward. To check your baking accuracy, you might weigh the original amount of batter and see how much is left after baking, thus realizing how much was lost during the process.

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