When working with regulated environmental methods have you ever asked yourself why are all these extracts concentrated down to one milliliter (mL) final volume? This is true for most soil and aqueous methods. Of course, a few methods will require a more diluted final volume but not many.
Why concentrate to a one mL final volume? The final volume all depends on your initial volume, the controlled or known concentration you’re adding to your initial volume, and the concentration you want to measure on your analytical instrument. Those known values will determine your final volume.
Most regulated environmental methods are prescribed with the 1 mL final volume. Now you may be asking, why is that? Why can’t you go to a lower or higher final volume? Well, it all depends on the regulatory method. Always consult the method you are following and look in the index for the flexibility section. This section will provide you with guidance on what changes are allowable within the method you are performing. What is also very informative is the concentration section. This is often overlooked and can contain valuable information that regulatory chemists have included. For instance, why you should not go below a certain volume or else recoveries will significantly be reduced.
So when can you go to 0.5 mL with typical samples and why consider it? Are there any advantages or disadvantages to implementing this in my lab?
With aqueous samples, one advantage is reducing the initial volume when allowed. With some methods, you can request a lower volume sample bottle. For instance, start extractions with 500 mL sample bottles instead of 1000 mL. If you would like to have your extracts to have a final volume of 0.5 mL with minimal impact to laboratory operations from sample prep to analysis there is an option. As always refer to your method on flexibility for this option. If the method allows for a smaller sample size this is a great option for labs.
When you reduce your initial sampling volume from 1000 mL to 500 mL you can simply reduce your final volume to 0.5 mL.
When performing this reduction you would keep the same extract concentration as if it was sampled at 1000 ml and concentrated to a final volume of 1. 0 mL (this can be a great way to make extractions more efficient). With lower initial volumes sample batches you will use less solvent and save time. Reducing the size of the sample bottle also cuts down on your shipping costs and sampling time.
I bet you are thinking it sounds fantastic and you are going to be looking into this right after lunch. The disadvantage with the lower initial volume and 0.5 mL final volume combo is simply the final volume itself. This lower final volume of 0.5 mL is the threshold and extracts should not go below 0.5 mL or else you will start to experience a loss of analytes. This is especially true for light-end semi-volatile analytes. This loss is most likely seen when implementing this change with instrumentation that is controlled by manual manipulation. If a water bath or concentration instrument is too hot or the nitrogen is sparging too much or too little, this type of inconsistent concentration will lead to problems especially when you need a final volume of 0.5 mL.
It is extremely important to have a very reliable and accurate concentration instrument that can accommodate such a change. This one change can and will make a difference in a laboratory’s operation. The change can make a positive impact from sampling to the analysis on the instrument.
When given the attention and care with the final extracts, manual concentration methods work well, but I’m not always able to dedicate that amount of time to one. This is due to the fact I am usually multitasking which does happen in most laboratories. If I am focusing on another manual technique, I may not make it back in time to manually take extracts off my water bath or nitrogen blown down instruments. This in turn will result in a final volume well below 0.5mL or even worse a dried extract. Then it’s back to the starting line extracting the sample. For me, I’m a big fan of automated drying systems such as the TurboVap® II Evaporation or the DryVap® Automated In-line Drying and Concentration System.
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