A novel derivatization-based liquid chromatography tandem mass spectrometry method for quantitative characterization of naphthenic acid isomer profiles in environmental waters
Journal of Chromatography A, Volume 1293, 7 June 2013, Pages 36–43
Million B. Woudneh, M. Coreen Hamilton, Jonathan P. Benskin, Guanghui Wang, Preston McEachern, John R. Cosgrove
Entry by Matthew S. MacLennan
There are lots of interesting things about the approach described in this paper.
Woudneh et al.'s method is mentioned as attempting to quantify NAs based on "equivalents of pyrenebutyric acid (PYB)" (image of PYB below from ChemSpider)
I find the setup here is a bit convoluted, so I will try to clarify it:
1. Grab sample of OSPW/water filtered, spiked with d19-C10 acid and d31-C16 acid standards, then SPE to isolate NAs.
d19-C10 acid and d31-C16 acid act as surrogate standards for extraction efficiency.
2. NAs derivatized with EDC become amines, and were then spiked with 13C3-atrazine.
3C3-atrazine acts as a recovery standard: monitoring instrument performance and quantifying surrogate recovery values.
3. Merichem NAs mixture was used as the quantification standard (calibration curve) for the OSPW samples. Yet, the quantity of the Merichem NAs mixture was translated to units of PYB. It was noted in the paper that the slope of the Merichem NAs calibration curve was 38% of the slope of the PYB curve. PYB was not added to any OSPW samples.
I feel that the analytical issues involved in quantifying complex unknown mixtures as a bulk material ("total NAs") are important. This study uses a few strategies.
1. Using two surrogate standards for extraction efficiency is a good approach. I feel like they are trying to "bound" the space of SPE binding so that we have a range of recoveries we could reasonably apply to the components of a complex sample.
2. Derivatization using EDC carbodiimide is an important contribution to these types of methods because, in my opinion, it is an attempt to restrict the kinds of fragmentations that could occur in ESI. The assumption is that by doing so, the response factor (ionization efficiency) is almost the same for every type of naphthenic acid species, permitting less error in quantification.
3. The third approach used could be quantifying Merichem NAs in terms of pyrenebutyric acid units. Essentially, the Merichem signal is interpreted on a PYB calibration curve (I hope I have understood this correctly)! Standards are needed, definitely. However, because Merichem NAs calibration curve has a much lower slope (lower ESI-MS sensitivity?) the Merichem NAs concentrations will be squished together on the PYB calibration curve, underestimating the NAs concentrations in solution. Perhaps an improvement would be to interpret toxicity values in terms of units PYB (since toxicity is arguably the most important industrial endpoint) to provide some perspective.
This paper comes out of the context of the quantification of total naphthenic acids. The approach outlined here is fantastic but not without its limits. Are the limits enough to discredit the method? I don't think so. That's why I chose it for the journal club's first paper!