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Surrogate Standards: A Cost-Effective Strategy for Identification of Phytochemicals Michael N. Clifford*,† and Ntakadzeni Edwin Madala§ †
School of Bioscience and Medicine, Faculty of Health and Medical Sciences, University of Surrey, Guildford GU2 7XH, United Kingdom § Department of Biochemistry, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa globe artichoke, for example, provide immediate access to at least 24 acyl-quinic acids present in substantial quantities and many more present at lower concentrations, a substantial percentage of which are not commercially available at any price. Careful LC-MS with controlled fragmentation, supported by UV spectral and relative retention time data, will allow all of these acyl-quinic acids to be fingerprinted reproducibly, and in most cases to be identified unequivocally at minimal cost. Compare that with the cost even of a good-quality commercial 5-caffeoylquinic acid (IUPAC)5 (5-CQA) standard, about 2400 South African Rands (about U.S. $193) for 10 mg of material, with the costs of the less common acyl-quinic acids, if available, being often an order of magnitude higher. If surrogate standard usage were accepted, associated retention time data, UV spectra, and mass fragmentation data for compounds so identified could be placed in supplementary data files and thus available to referees. Furthermore, if quantitative data are required, then a good-quality commercial 5-CQA should be s part of the metabolomics standards initiative (MSI), used and the contents of other acyl-quinic acids expressed as 5several confidence levels for metabolite identification have CQA equivalents. been proposed, and these form part of the minimum reporting We believe that this approach has considerable merit, not standards proposed specifically for metabolomics studies.1 only reducing the cost of analytical studies for all, but especially However, these standards are relevant to, and often applied those scientists in third-world countries who are often best able to, other analytical studies. The MSI recommends the use of an to access previously unexamined plant material. Provided that authentic commercial standard under standardized conditions1 sample extract and surrogate standard, both individually and and considers the use of previously reported information after admixture, are examined contemporaneously, the obtained for authentic standards by other laboratories procedure we suggest is no different from that which is insufficient for confident and rigorous identification. Failure required when a commercial standard is used. If multiple to use such authentic standards is sometimes a sufficient commercial standards are required, and all are in the one criterion for a submitted manuscript to be rejected. surrogate, the workload and cost thereof are further reduced, It is axiomatic that data submitted for publication should be and there is no change in the precision or reproducibility of the accurate, reproducible, and rigorously assessed by independent data generated, and provision of the the supplementary files permits the submitted data to be assessed with exactly the same referees, but the cost of individual authentic commercial rigor. Accordingly, we do not think that there any standards is substantial and, when multiple standards are disadvantages. required, rapidly exceeds the budget of many laboratories. As Although the use of commercial standards for identification such, it is time to reconsider the acceptability of a less and quantification of every component might seem superior, it expensive, but no less reliable, alternative, namely, the use of should be noted that “chromatographically pure” standards, surrogate standards supported by LC-MS data providing which give only a single peak at λmax, frequently have numerous retention time, UV−vis spectrum, molecular mass, and mass components at a lower wavelength, plus non-UV-absorbing fragmentation data. impurities such as water, organic solvents, and ash that render As an example we take the acyl-quinic acids, commonly them completely unsuitable for quantification purposes. From known as chlorogenic acids, arguably one of the most studied personal experience we would add that commercial standards families of phenolic compounds in the plant kingdom with in are not always as claimed. For example, two samples of putative excess of 300 members currently known.2 Using thoroughly 1,4-diCQA and 1,5-diCQA bought at considerable expense characterized materials, LC-MS methods for the identification of these compounds have been developed, are widely utilized, and have recently been critically reviewed.3,4 A green robusta Received: April 6, 2017 Published: May 1, 2017 coffee extract, a sample of alcoholic cider, and an extract of
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© 2017 American Chemical Society
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DOI: 10.1021/acs.jafc.7b01588 J. Agric. Food Chem. 2017, 65, 3589−3590
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Journal of Agricultural and Food Chemistry were both found to be 1,3-diCQA IUPAC. One supplier checked, acknowledged, and corrected the error; the other never responded. It follows that the validity of commercial standards cannot be assumed, and the data so obtained must be subject to exactly the same rigorous evaluation as we propose if surrogate standards are employed. In principle, such an approach could be applied to many classes of phytochemicals, but it is recognized that some classes of phytochemicals might be less amenable to this approach, perhaps because suitable surrogate standards are simply not available, and case-by-case assessment will be required from the scientific community to determine the acceptability of this approach. We have no doubt that the judicious use of surrogate standards has considerable merit.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: m.cliff
[email protected]. ORCID
Michael N. Clifford: 0000-0002-4204-5720 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Sumner, L. W.; Amberg, A.; Barrett, D.; Beale, M. H.; Beger, R.; Daykin, C. A.; Fan, T. W. M.; Fiehn, O.; Goodacre, R.; Griffin, J. L.; Hankemeier, T.; Hardy, N.; Harnly, J.; Higashi, R.; Kopka, J.; Lane, A. N.; Lindon, J. C.; Marriott, P.; Nicholls, A. W.; Reily, M. D.; Thaden, J. J.; Viant, M. R. Proposed minimum reporting standards for chemical analysis. Metabolomics 2007, 3, 211−221. (2) Clifford, M. N. Technical Report: Some Notes on the Chlorogenic Acids. Part 4. Botanical Distribution of the Chlorogenic Acids; ResearchGate; 2017; 10.13140/RG.2.2.23428.53126. (3) Clifford, M. N. Some Notes on the Chlorogenic Acids. 3. LC and LC−MS. Researchgate 10.13140/RG.2.2.10825.95844, 2017. (4) Kuhnert, N.; Karaköse, H.; Jaiswal, R., Analysis of chlorogenic acids and other hydroxycinnamates in food, plants and pharmacokinetic studies. In Handbook of Analysis of Active Compounds in Functional Foods; Nollet, L. M. L., Toldra, F., Eds.; CRC Press: Boca Raton, FL, USA, 2012; pp 461−512. (5) IUPAC. Nomenclature of cyclitols. Biochem. J. 1976, 153, 23−31.
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DOI: 10.1021/acs.jafc.7b01588 J. Agric. Food Chem. 2017, 65, 3589−3590
