How to Reduce Sample Volumes without Sacrificing Analytical Detection Limits

In the world of environmental testing, many labs encounter extremely dirty samples.For example, EPA Method 1664B is the protocol for extracting oil and grease from wastewater samples. More often than not, these types of samples are really dirty, requiring loads of pre-filters, glass wool, and filter aid in order to get the entire liter of water through the disk. Well, there is!

We know that laboratories are always looking for ways to speed up the extraction process and reducing the volume of the sample to be extracted is one of the best places to start.

For our “sample reduction study” we chose to follow along with EPA Method 1664B because this is the protocol for wastewater samples. They are usually the most difficult samples to extract an entire liter of, due to the nature of the different varieties of sample matrices from discharge locations. Per EPA Method 1664B section 1.7.1.5, you have the ability to reduce your sample volumes from one liter to something smaller. The method specifically outlines the use of 100 mL sample volumes. The idea of reducing your sample volumes to 100 mL is very appealing, especially for samples that have a hard time filtering through even with all the filter aid, but there were some issues with accuracy on the gravimetric side. When you are reducing your sample volumes 10-fold you need to make sure that you can compensate for that reduction on the gravimetric side with a balance capable of measuring 10x more accurately. View the table below, to see what I mean:

So, instead of the typical analytical balance that is used for weighing oil and grease samples at 1 liter, you need the semi-micro balance instead. Initially, I had used an analytical balance for weighing the recovery of my 100 mL sample extractions, but I could not get the accuracy I needed in my measurements to pass my MDL study. I decided to increase my sample volume to 250 mL instead, to get back the accuracy in my measurements since I was multiplying my results by 4 instead of 10. However, what you decide for your lab could be different! It all depends on what your lab needs and what your return on investment would be.

After getting the gravimetric side of the extraction all squared away, I was able to begin my testing again. I began with the blank samples and recovered 1.2 mg/L of background. The reason it was 1.2 mg/L is because I needed to multiply the recovered value of 0.3 mg / 250 mL by 4, to account for the dilution factor, since all the results need to be in mg/L. Per the method, I was required to subtract the blank values from all the IPR samples because this value was higher than the calculated MDL value.

Reducing your sample volumes makes your life so much easier as an analyst! I would recommend taking a good look at your analytical capabilities and performance requirements prior to attempting the reduction of your sample volumes. Keep in mind though, that with a 4x reduction in volume, there is also a 4x increase in your reporting limits. Be sure to research and make sure that this is an acceptable option for your lab because it can save you a ton of time. From the work referenced in this study, I was able to develop a procedure that meets regulatory guidelines for EPA Method 1664B and reduces the sample volume by four times (1 liter to 250 mL). You can view the application note for further review of the procedure, extraction method, and results.

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