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The Standards Dilution Calculator That You Have Always Wanted Part 2 – Headspace Considerations

3 Jul 2024

Remember when your teachers used to say, “you’ll have to figure things out on your own” and “you won’t always have calculators?” I remember... and I’m here to say that’s rubbish! Why suffer with calculations when you could come to Restek’s ChromaBLOGraphy and use our finest calculators for free!? The last calculator that I kicked into the blog was The Standards Dilution Calculator That You Have Always Wanted, which was aimed at helping you make the necessary calculations for your calibrations. Today, I am going to build off of this for those of you who conduct headspace (HS) analysis. This is fairly simple, but I thought it would be worthwhile to cover several aspects of this technique as it relates to calculating concentrations and dilutions.

In general, there are a couple of key practices that I have seen and believe that they should be addressed to help you gather the most accurate HS data possible. First, it is important to make sure that your calibration standard and sample are in the same physical state. This means that if your sample is a solid, this should be put into solution and analyzed as a liquid. The reason for this is that the partitioning rates for a given compound of interest are drastically different from a solid sample compared to a liquid sample. Additionally, this will help reduce matrix effects, which lead to signal enhancement and suppression. To further mitigate undesirable matrix effects, internal standards and/or surrogate matrix matching should be used to improve data accuracy. While headspace analysis is a very clean technique, compared to a direct liquid injection, I have seen a common misconception where laboratories believe that an internal standard is not required because of this. While it is cleaner in the fact that we are not directly injecting a matrix loaded liquid onto our GC column, there are still volatile matrix components that will impact HS analysis results (i.e., recoveries).

Next, it is important to make the same volume additions to the HS vial, especially when the reference standard solvent and the solvent within the overall solution  are different. For example, in USP 467, 1 mL of the reference standard solution, which is in DMSO, is added to 5 mL of water in the HS vial. In order to build a calibration, the concentration of the reference standard solution should be varied for each of the levels so that 1 mL of the reference standard solution is always added to 5 mL of water. If we were to vary the amount of reference standard solution and shift the ratio of DMSO:Water, we would see differences in the way that the analytes partition from the sample into the HS, which could lead to poor calibrations. This means that every calibration vial needs its own working standard.

Finally, when calculating calibration levels for HS analysis, we need to frame our thinking in a way to make us more successful. First, we need to think about the final volume size. Next, we should figure out what volume we would like to consistently add of our calibration solutions to the HS vial. After thinking about this, we can now work backwards to calculate everything for the desired concentrations. For example, the USP 467 approach would always result in a final solution volume of 6 mL and a sample addition of 1 mL. This means that we would use the numbers as constants to calculate the concentrations of our working solutions and our calibration samples.

Now with all of that said, check out our Headspace Concentration Calculator Here!

This calculator has two tabs for two different calculators. The one tab is for a procedure where the sample solvent and the dilution solvent are the same (i.e., Matching Solvents), and the other is for a procedure where the sample solvent and the dilution solvent are different (i.e., Mismatched Solvents). The mismatched solvents tab should be used for processes like USP 467, as described above. When filling out the calculator, the only areas that need to have values entered are the cells that are colored YELLOW.