A method based on an analytical technique, initially developed for quantifying aquatic refractory organic matter (often called humics), has been applied to commercial samples claiming to contain humic-type substances. At present, no method exists for quantifying the humic content on this type of sample. The analytical method is based on measuring the peak current obtained by adsorptive stripping voltammetry of the complex formed by refractory organic matter in the presence of trace amounts of Mo(VI). The quantification procedure requires the response obtained for the unknown sample to be compared with the response obtained with International Humic Substance Society (IHSS) reference humic substances. A very simple procedure that enables the humic content of any sample to be expressed as IHSS standard equivalents is described in detail. The method is highly selective, reproducible and suitable for routine analysi

Filella, Montserrat

Quentel, François

RERO DOC Digital Library

TECHNICAL NOTEA simple method for quantifying the humic contentof commercial productsFrançois Quentel & Montserrat FilellaReceived: 22 June 2011 /Revised: 6 August 2011 /Accepted: 6 September 2011 /Published online: 22 September 2011# Springer-Verlag 2011Abstract A method based on an analytical technique,initially developed for quantifying aquatic refractoryorganic matter (often called humics), has been appliedto commercial samples claiming to contain humic-typesubstances. At present, no method exists for quantifyingthe humic content on this type of sample. The analyticalmethod is based on measuring the peak current obtainedby adsorptive stripping voltammetry of the complexformed by refractory organic matter in the presence oftrace amounts of Mo(VI). The quantification procedurerequires the response obtained for the unknown sampleto be compared with the response obtained withInternational Humic Substance Society (IHSS) referencehumic substances. A very simple procedure that enablesthe humic content of any sample to be expressed as IHSSstandard equivalents is described in detail. The method ishighly selective, reproducible and suitable for routineanalysis.Keywords Humic commercial products . Quantification .Humic substances . VoltammetryIntroductionThe last few years have seen the introduction of anincreasing number of products containing humic substancesin the market. These products are intended to be used in awide range of fields, from agriculture to cosmetics, food,health, etc. However, no standardized, or at least widelyaccepted, analytical method exists that makes it possible toquantify the humic concentrations in such products, nor isthe commercialisation of these products generally regulatedor controlled. The existence of a reliable determinationmethod is a prerequisite to assess their quality and,eventually, to regulate their use.It is important to understand that the lack of a convenientmethod for determining humic content in commercialproducts is not surprising, as it is simply the reflection of alack of suitable, generally accepted methods for quantifyingthose compounds in natural systems, despite the fact thathumic substances are relevant to a wide range of environ-mental issues, from the toxicity of trace elements andmicropollutants, to the carbon cycle, and the production ofundesirable disinfectious by-products in drinking waters.Moreover, none of the methods occasionally used in researchand in environmental monitoring is without drawbacks(see Ref. [1] for a recent review), not least the fact thatmost of the methods ‘see’ only a certain fraction of humics[1], the proportion of which does not remain constant fromsystem to system. It is important to point out that this situationis exclusively due to the uncommon nature of thesesubstances: fulvics and humics are operationally definedfractions of natural organic matter and not compounds withreproducible structures and composition. Thus, they are notunique substances but a category of natural organic matterF. QuentelLaboratoire de Chimie Analytique, UMR-CNRS 6521,Université de Bretagne Occidentale,6 avenue V. Le Gorgeu, CS 93837,29238 Brest Cedex 3, FranceM. Filella (*)Institute F.-A. Forel, University of Geneva,10 route de Suisse,1290 Versoix, Switzerlande-mail: montserrat.filella@unige.chAnal Bioanal Chem (2011) 401:3235–3238DOI 10.1007/s00216-011-5397-1with common characteristics, one of which is their resistanceto degradation. To complicate things further, a myriad ofprotocols exist for fulvic and humic fractionation (i.e.,definition), and their relevance and properties are assesseddifferently in different fields (i.e., aquatic chemists, soilscientists, ecologists, water treatment engineers, etc.) [2].As discussed in detail in Ref. [1], any analytical methodmeasures a property of an analyte (or its reaction product),but the operationally defined nature of ‘humics’, togetherwith the concomitant elusive and non-constant compositionand structure of these substances, makes it difficult tofind such an intrinsic property for them. Recently, anelectroanalytical method based on cathodic strippingpreceded by the adsorptive collection of Mo(VI)-humicor fulvic acid complexes has been proposed for naturalsamples [3, 4]. This method is fast, sensitive, needs onlysmall amounts of the sample and, more importantly, isextremely selective: only water, soil and peat fulvic andhumic acids, defined as fractions isolated by following thedifferent standard procedures of the International HumicSubstance Society (IHSS), give a signal while carbohydratesand proteins do not [3]. So far, the method has been appliedto environmental waters [5–7]. In this study, the applicationof the method to commercial samples has been tested and aprocedure that allows the quantification of such samples hasbeen developed.ExperimentalCommercial productsThe following commercial products were analysed:Humiron (WSG-6), kindly provided by Humintech(Düsseldorf, Germany); Biovin (Trever, Austria; organicsubstances, 85–92%; Humusbildner, 70%); Bachhumus Eco(TRABE; organic matter, 40.5%; humic acids, 2.5%;total humic extract, 25%); Bachhumus Bioabono (TRABE;guaranteed contents—total humic extract, 25.00%; humicacids, 5.30%; fulvic acids, 19.70%; analysed—organicmatter, 35.43%; total humic extract, 39.98%); H2-Hulmax(Agromed; humic and fulvic extract, 17% (w/v); humic acids,0.1% (w/v); fulvic acids, 16% (w/v); organic matter, 40.22%dry weight); El milagro de la Naturaleza. Minerales húmicos(www.secretosparatusalud.com; no information about humiccontent). All products were used as purchased, withoutfurther purification. The humic and/or fulvic contents givenhere are those given by the producers. None give any detailabout the technique used to determine humic concentrations.The information given is sometimes confusing. This is thecase, for instance, of Bachhumus Eco and Bachumusbioabono that come from the same producer but werepurchased from different retailers.Measuring methodAll voltammetric measurements were taken with apotentiostat/galvanostat μAutolab II controlled by GPES4.9 software. A hanging mercury drop electrode (HMDE)Metrohm model 663 VAwith a mercury drop size of 0.40 mm2was used. All potentials were referred to an Ag/AgCl,3 mol L−1 KCl, reference electrode. The counter electrodewas a platinum wire. The square-wave mode (SWV) wasused for the measurements. The quartz voltammetric cell wasthermostated at 25±0.1 °C during the measurements to ensurethat conditions were reproducible.All chemicals used were of analytical reagent gradeexcept the mineral acid (HCl) and base (NaOH), whichwere of Suprapur grade. A 1,000-mg L−1 stock solutionof Mo(VI) (atomic absorption standard, Merck) wasdiluted as required. Humic and fulvic acid standards werefrom IHSS.All solutions were prepared with 18 MΩ cm Milli-Qwater. All glassware and polyethylene bottles were cleanedwith 10% (v/v) HNO3 and 0.5 mol L−1 NaOH and rinsedwith Milli-Q water. Stock solutions of IHSS standards andsolid samples were prepared in 0.01 mol L−1 NaOH(concentration, ≈250 mg L−1).The procedure used for the measurements is the same asthat described in Ref. [4]: (1) Mo(VI) (10 μg L−1) is addedto an initial acidified (0.01 mol L−1 HCl) solutioncontaining a given amount of the compound to be analysed;(2) the solution is deaerated with nitrogen for 10 min; (3)deposition at −0.25 V is carried out with stirring; (4) afterthe deposition time, the stirring is stopped and, following a10-s period of rest, the scan is initiated in the negativedirection. The operational parameter values used were:frequency, 50 Hz; step potential, 1 mV; and amplitude,40 mV.Typical SWV voltammograms have been shown inprevious publications [3, 4]. The electrochemical processhas been previously studied and described in detail [4, 8].In SWV mode, the method detection limit, calculatedaccording to Ref. [9], is 3.9 μg L−1 (2.1 μgC L−1) for3 min deposition time (for Suwannee River standard fulvicacid I, 1S101F) [4], its sensitivity, 120 nA mg−1 L (samestandard) [4], and its reproducibility equally good, with avariation coefficient of 5% at the 125 μg L−1 concentrationlevel [4]. Other types of organic matter and trace elementsdo not interfere [3].ResultsThe suggested methodological procedure for solid samplesincludes two steps. In the first, response curves are obtainedfor the unknown product (i.e. increasing amounts of the3236 F. Quentel, M. Filellagiven compounds are added to the initial solution and thepeak currents obtained measured). In all cases, a lineardependence between the analytical signal and productconcentrations was observed in the products under study(Fig. 1). The concentrations of the measured solutionsneed to be adapted to the linear range of the analyticaltechnique [3] by appropriately diluting the sample if theyare outside the linearity range. The same type of responsehad been obtained for all IHSS standard and referenceproducts ([4]).The second step of the proposed methodologicalprocedure consists of measuring the response curves ofone (or more) IHSS standards. As mentioned in the‘Introduction’, the intrinsic nature of humic substancesmeans that the results obtained can never be consideredabsolute measurements but need to be referred to a standardsubstance. The use of IHSS standard substances isrecommended for this purpose. Although any IHSSstandard can be used, in principle, we advise using thosewhose response, and thus whose nature, is closer to thecompound being analysed. For this purpose, visualcomparison of the response obtained for the unknownsample with the response of IHSS standards [4, 6] isenough to make a selection. Quantification of the humiccontents in the unknown samples requires simultaneousmeasurement of the standard response curve and theunknown substances under the same conditions.Finally, by combining the responses of the unknowncompound and the IHSS standards, the humic contents ofthe unknown samples can be expressed in ‘IHSS product’equivalents. Table 1 shows the results obtained here for thetwo commercial substances tested, expressed as sampleweight and standard equivalent. The results can also beexpressed in terms of organic carbon content, although thisrequires the organic carbon content of the unknownsamples to be determined. The organic carbon of humicsubstances can be found at www.ihss.com. In the case ofthe products tested (Humiron and Biovin), the responsewas closer to Elliott Soil HA (ESHA, 1S102H) andPakohee Peat HA (PPHA, 1S103H) standards. Thus,their concentrations were expressed as ESHA or PPHAequivalents.Fig. 1 SWV response curves for solid commercial products containinghumic substances (Humiron and Biovin). Substance concentrations areexpressed in milligrammes per litre. The response curves for IHSS humicstandards closest to the ones observed in the commercial products are alsoshown (PPHA Pakohee Peat HA (1S103H) and ESHA Elliott Soil HA(1S102H))Table 1 Results obtained for the solid commercial substancesanalysedProduct Humic content(in mg of ESHAequivalents containedin 100 mg of product)Humic content(in mg of PPHAequivalents containedin 100 mg of product)Humiron (WSG-6) 34.5 17.5Biovin 97.5 47.4ESHA Elliott Soil HA, PPHA Pakohee Peat HAFig. 2 SWV response curves for liquid commercial productscontaining humic substances (Bachumus Eco, Bachhumus Bioabono,H2-Hulmax and Milagro). Substance concentrations are expressed inmilligrammes per litre. For each compound, the standard additioncurves for two standards (PPHA Pakohee Peat HA (1S103H) andESHA Elliott Soil HA (1S102H)) are shownTable 2 Results obtained for the liquid commercial substancesanalysedProduct Humic content(in g L−1 of ESHAequivalents)Humic content(in g L−1 of PPHAequivalents)Bachumus Eco 78.5 43.7Bachhumus Bioabono 74.6 40.5H2-Hulmax 80.2 58.3Milagro 2.08 1.10ESHA Elliott Soil HA, PPHA Pakohee Peat HAHumic quantification in commercial products 3237In the case of liquid samples, the procedure recommendedis similar to the one used for natural waters [3, 4]: addition ofaliquots of a standard solution to the unknown sample.Therefore, in this case, results are also referred to the IHSSstandards used. Results obtained for the substances analysed(Bachumus Eco, Bachhumus Bioabono, H2-Hulmax andMilagro) are shown in Fig. 2 and in Table 2.ConclusionsThe application of the quantification method, initiallydeveloped for natural water samples, to different solid andliquid commercial humic-containing substances has provedstraightforward.The principle of expressing results as IHSS standardequivalents can be used with other analytical measurementtechniques, such as, for instance, UV–vis and fluorescencemethods. It is important to point out, however, that thevoltammetric-based method proposed here is moresensitive, and, more importantly for the types of samplesanalysed here, is 100% selective (i.e. other organicmatter fractions such as carbohydrates or proteins donot give a signal) [3], which is far from the case for thetwo techniques mentioned.On a final note, it should be pointed out that, althoughthe constraint of having to express the results as a functionof the response of a standard might appear unsatisfactory tosome users, this procedure has been used for years for othertypes of natural organic matter, such as carbohydrates(glucose, [10]) and proteins (bovine serum albumin, [11]),the difference being merely that this ‘hidden dependence’has been often overlooked, or plainly ignored, by the users.Moreover, at least in the case of carbohydrates, thelimitations imposed by using glucose considerably exceedthose imposed here, in the case of humics. This point hasbeen discussed in detail in Ref. [10].Acknowledgements We would like to thank Humintech (Düsseldorf,Germany) for kindly providing us with a sample of Humiron (WSG-6)and Juan Carlos Rodríguez-Murillo (CSIC, Spain) for helping us toobtain some of the commercial samples.References1. Filella M (2010) Chem Ecol 26:177–1862. Filella M (2009) Environ Chem Lett 7:21–353. Chanudet V, Filella M, Quentel F (2006) Anal Chim Acta569:244–2494. Quentel F, Filella M (2008) Anal Bioanal Chem 392:1225–12305. Chanudet V, Filella M (2007) Org Geochem 38:1146–11606. Filella M, Chanudet V, Philippo S, Quentel F (2009) ApplGeochem 24:52–617. Filella M, Quentel F, Pernet-Coudrier B, Varrault G (2009) Int JEnviron Anal Chem 89:799–8078. Quentel F, Elleouet C (2001) Electroanalysis 13:1030–10359. Jochum C, Jochum P, Kowalski BR (1981) Anal Chem 53:85–9210. Chanudet V, Filella M (2006) Int J Environ Anal Chem 86:693–71211. Bradford MM (1976) Anal Biochem 72:248–2543238 F. Quentel, M. Filella

A simple method for quantifying the humic content of commercial products

http://doc.rero.ch/record/317094/files/216_2011_Article_5397.pdf

A simple method for quantifying the humic content of commercial products

Abstract

Similar works

Full text

Available Versions

RERO DOC Digital Library