ISSN:0003-2654    

DOI:10.1039/AN99116BP019

出版商:RSC    

年代:1991

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN99116FX021

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALAO 1 16(6) 549-680 (1 991 )The AnalystJune 199154956957358 158558959560 160560961562 162562763 1641647653657663667The Analytical Journal of The Royal Society of ChemistryCONTENTSCadmium: Toxicology and Analysis. A Review-Kevin Robards, Paul WorsfoldAutomated Enzyme Packed-bed System for the Determination of Vitamin C in Foodstuffs-Simon Daily, Susan J.Armfield, Barry G. D. Haggett, Mark E. A. DownsDirect Reductive Amperometric Determination of Nitrate at a Copper Electrode Formed In Situ in a Capillary-fill SensorDevice-Arnold G. Fogg, S. Paul Scullion, Tony E. Edmonds, Brian J. BirchEvaluation of Poly(viny1idene chloride) as a Matrix for Polymer Membrane Ion-selective Electrodes-Vanessa J.Wotring, Patrick K.Prince, Leonidas G. BachasElectroanalysis for Organotin in Natural Waters Including Sea-water by Cathodic Stripping Voltammetry-ConstantM. G. van den Berg, Shaukat H. KhanMechanistic Study of Fluoride Ion Sensors-Werner Moritz, Lothar MullerAutomated Determination of Sulphide by Gas-phase Molecular Absorption Spectrometry-Toyin A. Arowolo,Malcolm S. CresserInductively Coupled Plasma Atomic Emission Spectrometry for the Analysis of Soil Extracts Prepared on Ion-exchangeResins-Loku L. W. Somasiri, Albert Birnie, Anthony C. EdwardsDetermination of Trace Amounts of Metalloprotein Species in Marine Mussel Samples by High-performance LiquidChromatography With Inductively Coupled Plasma Atomic Emission Spectrometric Detection-Am brogioMazzucotelli, Aldo Viarengo, Laura Canesi, Enrica Ponzano, Paola Rivaro4-(Aminosulphonyl)-2,1,3-benzoxadiazole Derivatives as Pre-column Fluorogenic Tagging Reagents for CarboxylicAcids in High-performance Liquid Chromatography-Toshimasa Toyo‘oka, Mumio Ishibashi, Yasushi Takeda,Kazuhiro lmaiReversed-phase High-performance Liquid Chromatographic Separation of Niobium(v) and Tantalum(v) by Pre-columnChelation With 4-(2-Pyridylazo)resorcinol-Su hjen Jane Tsa i, Yish iuan LeeLiquid Chromatographic Behaviour of Chelates of Vanadium(v), Copper(it), Cobalt(iii) and Chromium(iit) With2-(3,5-Dibromo-2-pyridylazo)diethylaminophenol-Yi Zhao, Chengguang FuModified Ninhydrin Spray Reagent for the Identification of Amino Acids on Thin-layer Chromatography Plates-Subrata Laskar, Utpal Bhattacharya, Bidyut BasakUltrasensitive Determination of Europium Using Microsecond Time-resolved Spectrofluorimetry-Theodore K.Christopoulos, Eleftherios P.DiamandisSelf-indicating Flow Visible Spectrophotometric Titrations in a Variable-volume Tank Reactor-Francis E. Powell,Arnold G. FoggSpectrophotometric Determination of Ascorbic Acid in Pharmaceuticals by Background Correction and FlowInjection-Krishna K. Verma, Archana Jain, Archana Verma, Anupama ChaurasiaSpectrophotometric Flow Injection Determination of Trace Amounts of Thiocyanate Based on Its Reaction With2-( 5-Bromo-2-pyridylazo)-5-diethylaminophenol and Dichromate: Assay of the Thiocyanate Level in Saliva FromSmokers and Non-smokers-Anders Broe Bendtsen, Elo Harald HansenKinetic Determination of Iodide in Pharmaceutical and Food Samples-Ma.Soledad Garcia, Concepcion Sanchez-Pedrefio, Ma. Isabel Albero, Catalina SanchezUse of Lissamine Green B as a Spectrophotometric Reagent for the Determination of Low Residuals of ChlorineDioxide-Bar ry Ch iswel I, Kelvin R. O’Hal loranDetermination of Trace Amounts of Gallium, Indium and Thallium by Successive Titrations Using Semi-xylenol OrangeWith Spectrophotometric and/or Visual End-point Indication-Medhat Abd El-Hamied Hafez, Amin M. A. Abdallah,Tarek M. Abd El-Fatah WahdanExtraction-Spectrophotometric Determination of Nitrite Using 1-Aminonaphthalene-2-sulphonic Acid-RachanaKaveeshwar, Lata Cherian, V. K. Guptacontinued inside back coverTypeset and printed by Black Bear Press Limited, Cambridge, Englan671 COMMUNICATIONWater-soluble Copolymers: A New Class of Media for Fluorescence and Phosphorescence Analyses in AqueousSystems?-Ian Soutar, Linda SwansonCambridge CB4 4WF, UK.RSC Members should obtain members prices and order from :The Membership Affairs Department at the Cambridge address above.InformationServices675 BOOK REVIEWS679 CUMULATIVE AUTHOR INDEXThe--Royal Society of Chemistry - The First 150 YearsBy: David H.WhiffenThis interesting new book provides a historical review from 1841 to 1991 of the Royal Society of Chemistry and the Societies from which it wasformed.Contents:Historical Prologue.1941-51 by D. H. Hey.The Chemical Society.The Royal Society of Chemistry.Premises.Publications.The Nottingham Centre.Awards and Meetings.RIC Matters and Their Continuation in RSC.Finance.Epilogue.Appendix.Bibliography of Other Historical Volumes.Subject Index.Name Index.Hardcover Approximately 270 pagesPrice: f 14.95ISBN: 0 85186 294 2Due Early 1991

ISSN:0003-2654    

DOI:10.1039/AN99116BX023

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, JUNE 1991. VOL. 116 549 Cadmium: Toxicology and Analysis A Review Kevin Robards School of Science and Technology, Charles Sturt University-Riverina, P. 0. Box 588, Wagga Wagga 2650, Australia Paul Worsfold Department of Environmental Sciences, Polytechnic South West, Drake Circus, Plymouth PL4 8AA, UK Summary of Contents Introduction Environmental Considerations Bioaccumulation Persistence Speciation Cadmium Intake in Humans Environment Humans Cadmium Exposure Monitoring Procedures Regulatory Standards Sample Preparation Sample Dissolution Recent Developments in Sample Dissolution Preconcentration Spectrop hotometry Atomic Emission Spectrometry Atomic Absorption Spectrometry Comparison of Atomic Spectrometric Techniques Elect roc hem ica I Techniques Liquid Chromatography Mass Spectrometry Flow Injection Other Tech niques Quantification Conclusions References Keywords: Review; cadmium; analysis; toxicology Introduction The availability of reliable analytical data is an important consideration for a better understanding of the environmental and clinical role of cadmium. Following a brief historical introduction, this review examines, critically, various aspects of the toxicology and analytical chemistry of cadmium.It should be of interest to analytical, environmental and clinical chemists in general, and be of specific relevance to those workers involved in the determination of cadmium. The extensive literature on cadmium has precluded comprehensive coverage and the selection of references inevitably reflects the bias of the authors.The identification of cadmium in 1817 as a distinct element is relatively recent. Its acute toxicity was soon recognized1-2 and the symptoms of cadmium poisoning were described, for example, by Marme3 in 1867 as giddiness, vomiting, syncope, difficulties with respiration, loss of consciousness and cramps. Early recorded cases of cadmium poisoning were generally due to industrial exposure involving inhalation of cadmium dusts, although the first recorded case in 18581 was of a domestic nature. Nevertheless, cadmium salts were used as anthelminthics, antiseptics, acaricides and nematocides and were described in various pharmacopoeia’s4 of the early 20th century. Cadmium rapidly found applications as a pigment and as an alloy in the electroplating of other metals,S although its large scale use dates from the 1940s.Telephone wires were made of copper-cadmium alloys and cadmium is a good neutron absorber in nuclear reactors. Cadmium salts are used as stabilizers in plastics,6 phosphors for television sets, scintillation counters and X-ray screens, in storage and Ni-Cd rechargeable batteries,7 semiconductors and ceramic glazes. Cadmium occurs naturally in the environment as a result, €or example, of volcanic emissions and is also released by vegetation. On the other hand, anthropogenic sources of cadmium are related mainly to mining operations,8-10 waste incineration” and combustion of coal and oil. 12-14 Sludge- based fertilizers”-17 and phosphate fertilizers 18-19 are import- ant sources of cadmium contamination in agricultural soils.The cadmium content of rock phosphate (phosphorite) is variable and depends on its geographical origin.20 In a Belgian survey, for example, the cadmium concentration in 31 common phosphate fertilizers21 ranged from 0.1 to 80 mg kg- 1. Anderson22 has calculated a critical concentration of 8 mg kg-1 of cadmium in fertilizer above which cadmium levels in topsoils may be affected. Contamination of ground- waters by cadmium from landfill leachate23-25 is generally restricted to vertical movement with minor horizontal disper-550 ANALYST, JUNE 1991, VOL. 116 sion. Table 1 compares typical cadmium concentrations in a variety of environmental samples. Changes in environmental cadmium levels have been reflected in an increased body burden7673 of the general population. Typical values for body burden and cadmium concentrations in specific organs are given in Table 2.However, the occurrence of cadmium in association with zinc, lead and copper, together with the use of these metals since antiquity, has always been a source of exposure to cadmium even before its discovery. Evidence for this is provided by field studies in the United Kingdoms2 which revealed cadmium concentrations elevated by factors of hundreds above natural levels in soils at smelter sites which have been inactive since the Middle Ages. Environmental Considerations Several factors influence the role of cadmium in the environ- ment.83 The most important feature which distinguishes heavy metals from other toxic pollutants is that they are not biodegradable and, once in the environment, their potential toxicity is controlled largely by their physico-chemical form.Cadmium is therefore characterized by a long environmental persistence and biological 'half-time' which accounts for its bioaccumulation in individuals. Bioaccumulation A study of the Severn Estuary (UK) in 197284 showed that cadmium was transmitted from the water at progressively higher concentrations to the seaweed Fucus (the producer), the limpet Patella (a primary producer) and the dog whelk Thais (a secondary consumer). This type of amplification at higher trophic levels, termed biomagnification, has been demonstrated63 in relatively few studies with cadmium whilst most data (Table 1) indicate that biomagnification is not the normal process.However, data on cadmium bioaccumulation in a range of aquatic organisms85-91 and plants18783 verify the ability of these species to amplify92.93 the concentration of cadmium relative to their environment. Bioaccumulation is distinct from biomagnification in that the latter requires transfer of contaminant between trophic levels via ingestion whereas bioaccumulation requires independent direct uptake by each trophic level. The extent of cadmium bioaccumulation which is the net effect of two competing processes, uptake and depuration, is related to the level of environmental contamination. Factors affecting the balance between these two processes and hence the net uptake are the physico-chemical form of the cad- mium,47*94,95 the presence of other metals,96 pH,83,91 salin- ity9698 and temperature95.97.99 of aquatic environments, season,96JOOJ01 cation-exchange capacity of soils102 and the species of plant taking up the cadmium.47 Cadmium uptake by crops grown in sludge-amended soils has been studied extensively.159103,104 Interesting data on bioaccumulation were provided by a study of the Shipham area of the UK105 where agricultural soils and crops contained elevated cadmium concentrations in soil of between 30 and 800 mg kg-1 and in crops in excess of 0.25mgkg-1. Values obtained for air, drinking water and animal products (chicken, meat and eggs) were, however, typical of the UK generally. Considerable care should be exercised in the interpretation of data on bioaccumulation because many workers have failed to stand- ardize analytical procedures.106 Persistence Two useful measures of the environmental persistence of cadmium are provided by the residence time in non-living systems, and the biological half-time in living species (Table 3). For example, in estuarine coastal systems and particulate matter the residence time of cadmium has been estimated as a relatively low 1-2 years. On the other hand, estimates of the residence time in ocean water range from 7000 to 250000 years. Experiments with animals109 have produced large variations in biological half-times of cadmium depending on the method Table 1 Typical cadmium concentrations as measured in a range of environmental samples Sample Concentration Air- Remote sites Rural sites Urbadindustrial sites Site near cadmium source emission Water- Fresh Sea (surface) Sea (loo0 m) Atlantic Ocean (1977) North Sea (1973, near-shore) North Sea (1 984) Irish Sea (1972, offshore) English Channel (1972) Bristol Channel (1971) (1 972) ( 1974) Severn Estuary (1971) Lake Waste Industrial effluents (1975-1980) (1 975-1980) Soil Phosphate fertilizer Leafy and root crops (normal soil) Leafy and root crops (contaminated soil) Fruit (normal soil) Fruit (contaminated soil) Fish Shellfish Milk Butter, cheese, lard Food- Eggs ng rn-3 <1 0.1-10 1-100 5000-200 OOO 1-1OOo 0.01-0.04 0.10-0.15 0.07 0.5 0.02 0.11 0.06 5.8 1.94 0.67-0.99 0.3 0.31-1.48 0.1-70 3-1 1 pg I-' 2.07-3.7 wide range rng kg-1 0.1-1 0.1-80 Reference 26 26,27 26 28.29 30-32 32-36 37-40 30 30 34 30 30 34 34 30 34 34 34 41 42-45 43 46,47 21 pg kg-l (freshweight) 5-8 48,49 150-600 <20 10 5-200 200400 0.2-5 5-10 50-100 Meat (lamb, beef, chicken, pork) 5-50 Offal (beef, pork, chicken) 5 0-5 00 Biological samples- Trophic levels sediment seagrass and epibiota grazers suspension feeders detritus feeders predators (fish) sediment) Algae (uncontaminated Algae (contaminated sediment) Bivalves, soft tissue Plankton 48 48 48 48 48,50 5152 53 48 54-56 57-59 pg g-l (dry mass) 34 1.2-100 8.8-1 9.8 0.3-7.5 1.4-7.4 0.24.9 1 .l-8.5 4 132 16 5 4 0 Sediment- mg kg-' (dry mass) Spencer Gulf Australia (1984) 1.2-100 Severn Estuary, UK (1984) 1.0-1.9 (1972) 4.7 German Bight, North Sea (1970) 2.0 Humber Estuary (UK, 1981) 0.4 Firth of Clyde (1973) 1.6 Garroch Head sewage disposal site (1973) 6.4 Acushnet River, bay <0.5 Port Phillip Bay, Australia (1976) 9.9 Acushnet River, estuary >lo0 60,61 60,61 62,63 64,65 34 34 30 66-69 34 30 30 30 30 30ANALYST, JUNE 1991, VOL.116 55 1 Table 2 Cadmium concentration in specific organs contrasted with total body burden Cadmium level Individuals Non-exposed following Organ individuals acute exposure Reference Body burden: infant Body burden: adult Blood, whole Urine Kidney* Kidney cortex Liver* Fat* Muscle * Nails Perspiration Milk Colostrum Hair Teeth Saliva <0.001 mg - 5-70 mg 70-1 200 mg 0.5-4.7 pg I-’ 0.3-7.0 pg 1-1 5-100 pg I--’ 10-500 pg I-’ (0.3-2 pg per (530-1120 pg day1 Per day 1 16-67 pg g--1 5-100 pg g-I 0.3-4.1 pg g-I 20-300 pg g-I 40-500 pg g-’ 20-320 pg g-’ 0.01-0.2 pg g-1 - 0.01-0.2 pg g-1 - 1.4 pg 1-1 - 0.7-4.6 pg 1-1 - 0.002 pg g-1 - 0.08-1 pgg-1 - 0.1 pg g-’ - 0.08-0.31 pgg-’ - 0.35-2.4 pgg-’ - 73 73 71,73 71,73 73-76 73 71,73 73 73 77 78 71 79 71,73, 81 73 80 * Data for kidney, liver, fat and muscle are quoted on a wet mass basis. Table 3 Measures of cadmium persistence Factor Residence time Residence time Reidence time Biological half-time Biological half-time Biological half-time Biological half-time Biological half-time Biological half-time Sample Time Particulate matter 1-3 years Estuarine coastal 2 years Ocean water 7000-250 000 years Whole body 30 years Human kidney 10-20 years Human liver 5-10 years Kidney cortex 30 years Mytilus edulis 14-29 days Plaice 15-200 days system Reference 92 92 34,38,92 73,107 108 108 76 92 90 of administration, the dose and single or repeated exposure.Information on biological half-times in man (Table 3) has been obtained by measuring the observed decrease in the retention of cadmium in an organ or the whole body or by direct comparison of excretion with body burden. The latter approach is available only on a group basis using autopsy data. The whole-body biological half-time calculated using hypothetical excretion rates ranges from infinity to 1.9 years (Table 4) for an excretion rate of 0.10% of body burden per day. The exact biological half-time is still debatable but data favour values of the order of 30 years (Table 3) corresponding to an approximate excretion rate of 0.006% of body burden per day. Speciation Speciation describes the range of physico-chemical forms of an element which together make up its total concentration in a sample.110 These forms can be classified as the free hydrated cadmium ion and complexed labile and non-labile cadmium species.From a practical analytical consideration, cadmium in aqueous systems can be classified as dissolved (soluble, filterable), particulate (non-filterable, suspended) and sedimented species. Unfortunately, most analytical proce- dures for measuring speciation suffer from the same basic flaw in that metal species are divided into operational classifica- tions”’ and, furthermore, the operations involved in measurement invariably alter112 the original equilibria to some extent. Nevertheless, operational classifications have Table 4 Biological half-times corresponding to various hypothetical excretion rates Cadmium excretion (% of body burden Per day) 0 0.005 0.01 0.02 0.05 0.10 Biological half-time (Years 1 - 38 19 9.5 3.8 1.9 some merit as the uptake of cadmium by organisms occurs chiefly in the aqueous phase,43 the filter-feeder organisms being exceptions.However, both operational and ‘structural’ classifications serve a useful function and, indeed, complexa- tion may be involved in particulate and sedimented particles. The complexing agents may be both organic [e.g., ethylene- diaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), extracellular components, humic substances and dietary fibre1131 or inorganic ( e . g . , chloride or clay). The relative proportions of the different physico-chemical forms depend on the environmental conditions and this is an important factor in the toxicitygX,114 and the uptake and bioaccumulation of cadmium by both aquatic and land-based plants and animals.Cadmium uptake by Mytilus edulis, for example, increases when the cadmium is complexed with EDTA, humic acid or alginic acid94 whereas the oyster C. virginica accumulates significantly less cadmium’s when it is complexed. In soft waters of low pH, cadmium is present predominantly in the ionic rather than the complexed form115 although the ionic form is readily adsorbed116 depending on pH.110 There is general agreement that in sea-water cadmium is present primarily as a variety of chloride complexes~17 while in river water the dominant forms dependent on pH are the free hydrated cadmium ion and/or cadmium carbonate .]lo There is a considerable difference of opinion1*”119 regarding the extent of cadmium binding to organic complexing agents.However, the potential role of organic complexants in the chemistry of cadmium cannot be ignored, particularly as the amount of organic complexing material in the environment is related to the amount of sewage and the use of synthetic complexing agents (e.g., NTA) in detergents to replace polyphosphates.120 At present, humic substances appear to be the most widespread and quantitatively important complex- ants for cadmium.43 The extracellular products of many species also chelate cadmium.121 In tissues, cadmium occurs primarily as a cadmium-metallothionein species which is involved in the transport of cadmium from the liver to the kidneys73 where, because of its small size, it is freely filtered through the glomeruli to be re-absorbed by the tubular cells. Although metallothionein synthesis is part of the body’s defence system it is also responsible for the selective accumu- lation of cadmium in the kidney and indirectly for its toxic effects.122 Cadmium Exposure The main routes of human exposure to cadmium can be identified as acute exposure in the working environment (mainly involving inhalation of dusts and fumes and occasion- ally oral intake of cadmium) and acute and chronic exposure of the general population via food, air and water. Today, acute intoxication by cadmium is relatively rare although cases of acute respiratory intoxication are still observed occasion- ally. In an incident involving oral intoxication, Swedish school-children consumed fruit juice from a vending machine123 with a cadmium-plated reservoir. The ‘non-effect’ and lethal levels of cadmium administered in a single oral dose552 ANALYST, JUNE 1991, VOL.116 to adults have been estimated93 at 3 and 350-500mg, respectively. The lethal concentration of cadmium oxide fumes for man has been estimated93 to be about 5 mg m-3 for an 8 h exposure time. The dangers to human health resulting from long-term chronic exposure to cadmium were not recognized until the 1950s following a number of suggestive incidents124 in the previous decade. Several cases of severe osteoporosis with impaired general health were reported125 in 1942, in French workers, at an alkaline accumulator factory.A few years later Friberg28.126 reported the coexistence of lung and kidney damage in male workers following occupational exposure to cadmium oxide dust in an electrical battery plant. However, the greatest concern over cadmium pollution was generated by the so-called 'Itai-Itai' incident in Japan.127.128 The aetiolo- gical role of cadmium in the development of this disease was not established129 until 1961. The source of the cadmium was rice which had been grown in irrigation water contaminated with the effluents of a mining operation. The mean level of cadmium in rice from the endemic area (Toyoma Prefecture) was more than ten times the average in Japan.130 There are a number of factors such as sex, age and calcium nutrition which affect susceptibility13 to cadmium toxicity although the rela- tionship between intake and toxicity is complex.Copper and zinc deficiency, for example, can be stimulated by an increased cadmium intake.131 The Itai-Itai incident focused attention on cadmium by demonstrating that chronic cadmium poisoning constituted a health hazard to the general public and was not restricted to industrial workers. Moreover, exposure of individuals to excessively high environmental concentra- tions of cadmium was not restricted to Japan as illustrated by data from a 1979 national geological survey132,133 of Shipham in Somerset (UK) which indicated substantial contamination of soil by cadmium originating from nearby extinct calamine workings. Cadmium levels in the liver of inhabitants were, on average, five times higher than in control subjects.Functional and morphological changes are observed134 in a number of organs following long-term chronic exposure to excessive concentrations of cadmium. There is considerable variation in what constitutes an excessive concentration but as an illustration the critical concentration in human liver108 is 30-60 mg kg-1. Long-term chronic exposure to cadmium has been associated108~135-137 with anaemia, anosmia (absence of the sense of smell), osteomalacia and cardiovascular diseases, particularly hypertension. Moreover, in 1976 the Inter- national Agency for Research on Cancer108 classified cad- mium with those chemicals that are probably carcinogenic to man. The most typical feature of chronic cadmium exposure is renal glomerular and tubular damage ,93,10*,135-139 the first sign of which is usually an increase in urinary excretion of low relative molecular mass proteins, known as proteinuria.Cadmium Intake in Humans Defining a threshold limit or maximum allowable intake of cadmium is difficult. Two approaches are used for solving this problem; one is the use of metabolic models to estimate the total cadmium exposure necessary to damage health140 and the other is epidemiological .I41 Using the epidemiological approach, the total cadmium intake over a typical lifetime producing an adverse effect on health has been calculated141 as approximately 2000mg for both men and women. This corresponds to a daily intake of 110 pg. However, there are several assumptions in both approaches142 and calculated values must be carefully evaluated.For comparison, the provisional tolerable weekly input recommended48 by the World Health Organization-Food and Agriculture Organiza- tion, Joint Expert Committee on Food Additives is 0.4-0.5 mg based on a tolerable intake of 1 pg kg-1 of body weight per day. Table 5 Calculated hypothetical total daily intakes of cadmium and contributing sources (see reference 135) Individual Source of cadmium I nt a ke/pg Non-smoker living in rural area Air 0.0005 Food 4 Water 2 Total daily intake 6 Smoker living near cadmium source and eating contaminated food Air 25 Food 84 Water 2 Tobacco 4 Total daily intake 115 Hallenbeck135 has compiled data on the total daily intake of cadmium by adults together with sources contributing to the total (Table 5 ) .The data show that the intake for an individual living in an unpolluted area is well below the provisional tolerable weekly input of 0.4-0.5 mg. The data, supported by other S O U ~ C ~ S , ~ ~ ~ ~ ~ ~ ~ show that food constitutes the major source of cadmium intake for the general populationl43.144 although absorption via the lungs can contribute significantly to the total cadmium intake of individuals living near sources of cadmium emission or individuals who are smokers. 135,145,146 In general the concentration of cadmium in food is low (Table 1) but some foods such as kidney, shellfish, cereals and leafy vegetables48~147-149 may contribute to the elevated cadmium intakes of specific groups of the general population. Cadmium levels in drinking water are generally low and contributions from storage tanks and plumbing fittings can be disre- garded.150-152 There has been concern about the possible leaching of cadmium from earthenware cooking vessels by acidic foods153 and standardized tests for extractable cadmium have been devised. 154 Cadmium intakes of some individuals will be consistently above average because of atypical diets or because the food they eat is grown or produced in cadmium-polluted areas. The daily intake of cadmium in Japan is a specific instance where the high consumption of rice155 and seaweed supplements156 has contributed to higher than average daily intakes. Infants constitute a specific group with atypical dietary habits and considerable attention has been devoted157-161 to assessing the dietary intake of cadmium by this group.The results are generally within the limits set for acceptable daily intake. However, the contribution of non-food items must also be considered in determining intake by infants. For instance, the import of food-shaped erasers into Italy was banned162 because of the risk of cadmium intoxication by ingestion. Measured dietary intake data (Table 6) must be viewed within the constraints outlined by Sherlock48 and are generally higher than the values compiled by Hallenbeck.135 One of the difficulties has been highlighted by Louekari et aZ.165 who compared dietary intakes for cadmium obtained by direct analysis (8.3 pg d-1) with values estimated by calculation of computer file data (15.8 pg d-1). Moreover, there is consider- able variation in the analytical data and it is unclear, for instance, whether the mean or median value163 provides the best estimate of dietary intake.Some estimates of dietary intake1822183 are clearly suspect because of the analytical procedures involved. With allowance for these, the available data for 1960-1975 suggest73 an average cadmium ingestion of 50 pg d-1 with probable variations from 25 to 75 pg d-1. More recent estimates of the dietary intake of cadmium (Table 6) are lower and this change may be real and attributed to profound changes in dietary habits1g4 andor food quality, or alternatively it may reflect48 improved analytical procedures.ANALYST, JUNE 1991, VOL. 116 553 Table 6 Daily dietary intakes of cadmium as measured for populations of various countries in pg per day with appropriate references given in parentheses Intake Country 196145 Australia Belgium Canada Czechoslovakia Denmark Finland Germany, Halle Germany, West Italy Japan, endemic area Rome non-polluted area Netherlands New Zealand Norway Poland Romania Sweden UK Shipham Birmingham USA 4-60 (73) * Intake by children. 'r Average intake for 1955-82.1966-70 197 1-75 1976-80 21 (48) SO (48) 60 (73) 48 (73) 30 (48) 57 (48) 54 (48) 500-800 (108) 600 (1 35) 59 (73) 47 (73) 39 (48) 30-50 (108) 38-64 (73) 16 (48) 19" (48) 16 (108) 10-30 (108) so (73) 92 (73) 31.5 (178) 38 (73) 26-61 (108) 33 (48) 1981-85 1986-90 15 (108) 13.8 (163) 3.7* (160) 33 (164) 8.3 (165) 15 (166) 17.3 (167) 29 (48) SO (166) 56.7-81.1 (168) 40t (169) 32.9 (155) 20 (171) - (173) 19 (170) 60 (166) 21 (172) 649-1 65 1 (174) <21(48) 19.1 (175) 33 (176) 5* (177) 28 (179) 15.5 (181) 12.3 (180) Monitoring Procedures Procedures for monitoring cadmium levels and for assessing the exposure of the environment and humans are necessary as a result of the widespread distribution, persistence, bioaccu- mulation and toxicity of cadmium.Several approaches have been adopted reflecting the diverse needs and purposes for monitoring cadmium levels. Environment The need to monitor atmospheric cadmium and control cadmium emission is well recognized. As early as the 1960s regular monitoring programmes for airborne cadmium were being undertaken in the United States by the National Air Sampling Network.73 Results for total airborne cadmium concentrationsl*S--'*g still vary considerably depending on the sampling location (Table l), the proximity to industrial sources of cadmium emission and on the efforts to reduce these emissions.In addition to direct measurement, various indirect procedures have been applied for monitoring air- borne cadmium levels. These indirect procedures have included measurement of cadmium in feathers,188,190 spruce shoots,194 cattle tissues,195 hair196 and snow.197 In the aquatic environment the most obvious medium of pollution assessment is surface water. Successful monitoring requires a rigorous sampling regime because of daily and seasonal variations in water flow, changing pH, salinity and biological activity and local discharges of effluent. As a result of these difficulties and other problems associated with the direct measurement of low concentrations of cadmium in water, various indirect monitoring procedures have also been used.Hence, an extensive amount of literature exists on cadmium concentrations in aquatic organisms92 and sedi- ments,12*-19* although such data cannot be related quantitat- ively to the concentration in water. Furthermore, analysis of river sediment is often successful~~9 even in instances where the cadmium concentration in the aqueous phase is below instrumental detection limits. This is illustrated by Swedish data200 for water (4 ng g-1 of cadmium) and mud (80 pg g-1 of cadmium, dry mass) sampled at a point 500m downstream from a cadmium-emitting factory. Care must be exercised in sediment analysis as particle size201 and processes such as dredging69 have a significant impact on contamination levels.A further advantage of monitoring cadmium levels in sedi- ments and organisms is that such data provide information on spatial and temporal changes in cadmium pollution .198,202 For example, data on sedimentary cores provide an historical record of natural background levels of pollution and the man-induced accumulation over an extended time period. As an example, the natural cadmium background67 from the deeper section of sediment from the German Bight (North Sea)68 is 0.25 pgg-1 (see Fig. 1). In the upper layers of the sediment there is an approximate 8-fold increase in cadmium concentration reflecting the situation in the 1970s. Analysis of aged moss samples confirmed2O2 the increased environmental pollution by cadmium over the 50 year period to 1971.As in sediment monitoring programmes, data from sentinel or indicator organisms6~~~3~~82~203-2~~~ represent a mobile time- average value of the biological availability of cadmium. In programmes of trend identification the design of the monitor- ing programme is critical in order to minimize factors which act to obscure spatial and temporal changes. These considera- tions include not only seasonality, age and sex but also less obvious factors such as the zinc:cadmium ratio in the organism.207,208 The Mussel Watch Programme involving Mytilus eduZis3"62 is a well coordinated example of the use of indicator organisms. Humans The chronic low-level exposure of humans to cadmium creates the need for monitoring both body burden and recent554 ANALYST, JUNE 1991, VOL.116 2 r I 0) I- Natu ra I background I r- 0 10 20 30 40 60 80 100 120 Sediment depthkm Fig. 1 Chronological development of cadmium concentrations in the North Sea as derived from analyses of sediment cores exposure, the latter being more important for industrial workers with a high risk of exposure than for the general population. Although analysis of the external environment provides some data on the extent of exposure, more direct information can be obtained from cadmium concentrations in body fluids and organs, which indicate the extent of absorp- tion and therefore the effectiveness of the exposure. Dissen- sion still exists as to which body parameter provides the best indicator of cadmium exposure .209 However, exposure has generally been measured using easily accessible body fluids such as blood and urine in large scale studies of the general population and health surveillance programmes of industrial workers. For a full discussion of the relative merits of using salivary220 or tissue7~~14sJ21-223 cadmium for monitoring recent exposure and/or body burden, the reader is referred to one of the reviews209 covering this aspect of cadmium. The use of hair as an indicator of the body burden of heavy metals dates from the 1960s.Hair provides a convenient sample for analysis but the possibility of external contamina- tion from dust and hair sprays, shampoos, efc., must also be considered. The possibility of contamination is likely to exclude hair cadmium as an indicator for industrially exposed workers as adsorbed cadmium on the hair surface is difficult to remove and cannot be distinguished from endogenous cad- mium.2137224 Moreover, depending on hair acidity, there is considerable individual variability224 in the extent of adsorp- tion.Nevertheless, Ellis et al. 325 established a qualitative relationship between hair cadmium and body burden in exposed workers, whilst the best relationships between exposure and response have been obtained by the determina- tion of cadmium concentration in liver and kidney samples collected at autopsy or by biopsy. Such data provide a direct measure of the internal integrated dose of cadmium. The in vivo measurement of cadmium in liver75-223.226 and kid- ney75J22J26 using neutron-activation analysis is useful on an individual basis but its routine use for screening studies can- not be envisaged for a number of reasons including the radiation hazard.blood,7,29,7.1,200,21~216 urinary,7,29,75,212,314,217 faeca],73,218,219 Regulatory Standards The release of cadmium into the environment constitutes a significant pollution problem. For example, it has been estimated66 that the atmospheric input of cadmium from the lithosphere to the oceans is 2.4 x 106 kg per year, while the annual input via stream run-off is 7.5 x 10"g per year. Environmental standards and accompanying legislation are essential for the protection of both humans and the environ- ment from this threat. These matters are important when considering analytical parameters such as limit of detection and are illustrated by reference to European Community (EC) legislation for aquatic environments.227 The EC directive on the discharge of dangerous substances into the aquatic environment34 distinguishes those such as cadmium (List I substances) which represent a particular risk on the basis of their toxicity, persistence, bioaccumulation and carcinogenicity.Nonetheless, the distinction between various substances does appear to be somewhat arbitrary. The E C directives rely primarily on limit values which are effectively the maximum permissible effluent concentrations (200 pg I-I), in this instance expressed as monthly averages. Similarly, environmental standards are set for cadmium in fresh and estuarine waters (5.0 vg 1-1 total cadmium) and coastal waters (2.5 pg 1-1 dissolved cadmium).The standards make no apparent allowance for the effects of environmental factors such as salinity, water hardness and temperature34 on the toxicity, the standard for estuaries being apparently less stringent than that for coastal waters, although this conflicts with the fact that the toxicity of cadmium increases as salinity decreases. However, an allowance must be made for the distinction between total and dissolved cadmium in the respective standards. Minimum requirements concerning the analytical methods to be used are specified in the directives. The description of these methods, often referred to as 'reference methods of analysis' is brief and usually amounts to no more than the name of the technique used in the final measurement stage.As an illustration, the directive dealing with discharges is the most descriptive227 and specifies that 'the reference method of analysis for determining the mercury and cadmium content of water, sediments and shellfish is atomic absorption spectro- scopy after preservation and suitable treatment of the sample'. The failure to specify details of sample treatment is a serious shortcoming. Nevertheless, limits of detection are specified in relation to the accuracy and precision of the analytical method. Problems of trans-frontier pollution are environmentally and politically significant. Particular concern has been expressed over the state of the North Sea and the merits of land-based versus open-sea disposal of wastes.The median estimate of the direct atmospheric fallout of cadmium on the surface of the North Sea is compared with the input of cadmium from land-based sources and also with the estimated amount of cadmium in the North Sea (Table 7).22* For cadmium the message is unequivocal. Cessation of sea-dump- ing without tighter controls on other sources of input can have little impact on the cadmium load of the North Sea. However, it must be remembered that these predictions depend on a number of factors including the quality of analytical data for cadmium. Sample Preparation Although the details depend, inter a h , on the nature of the sample and particularly its physical state, there are a number of features common to the preparation of all samples for analysis.Most quantification procedures require a solution of the analyte and, in general, solid samples must be dissolved prior to quantification. Moreover, the low level of cadmium in most samples inevitably requires a preconcentration step and this applies irrespective of the physical state of the sample. In rare instances, a preliminary separation of cadmium from the bulk matrix may be necessary. The difficulties associated with ultratrace analysis for cadmium229-233 are illustrated by the work of the Department of the Environment (UK) Analytical Quality Control (Har- monised Monitoring) Committee .229,23" Many laboratories participating in this programme did not achieve the requiredANALYST, JUNE 1991, VOL. 116 555 Table 7 Estimated mass of cadmium in the North Sea Water Column c ~ n t r a s t e d ~ ~ with the total mass of cadmium entering the North Sea from various sources Cadmium Land-based inputhonnes per day content of Atmospheric North Sea deposition/ Direct Water Column/ tonnes dis- Sea Dredging tonnes per day River charges dumping spoils 860 1.56 0.432 0.061 0.017 0.165 Participating laboratory number Fig.2 Data for cadmium in water obtained by the Analytical Quality Control Committee (reference 230). ( a ) Water sample A and ( h ) water sample B accuracy for the determination of dissolved cadmium. Failure to meet the target for accuracy was attributed to both random and systematic errors. Similar conclusions were reached in the Guildford Trace Element Quality Assurance Scheme233 for blood cadmium determination. A large standard deviation seems to be a common occurrence (Fig.2) in interlaboratory collaborative tests when low analyte concentrations are involved, regardless of the sample matrix. Problems of poor accuracy, reproducibility and recovery relate to contamina- tion from a multitude of sources and, at the same time, serious adsorptive losses of cadmium on sampling and analysis vessels. Sources of contamination include airborne particu- lates during filtration, digestion and sample transfer steps and, for blood, syringes used in sample collection .234 Linear polyethylene bottles have been recommended235 for the collection and storage of natural waters. Various pre- treatments for sampling bottles have been proposed113J36238 but soaking new bottles in 1.5 mol dm-3 analytical-reagent grade nitric acid for at least 1 week appears to be adequate. Freezing the sample or sample extracts is unsatisfactory because metal ions are concentrated in the unfrozen portion of the sample, leading possibly to irreversible hydrolysis.Storage at 4 "C appears a viable alternative.239.240 For example, Nurnberg et al.241 observed only minor changes in cadmium concentration at the nanogram level in filtered sea-water stored at 3 "C for 75 d. These results have been confirmed by Scarponi et aZ.242 Sample Dissolution Samples such as natural waters, where the analyte is already in solution, may be analysed directly without pre-treatment although preconcentration will frequently be necessary to improve detection limits. On the other hand, analysis of airborne particulates conventionally involves the collection of sample on a suitable filter ( e .g . , a cellulose membrane or a glass fibre filter).243-245 Recovery of the collected material by ignition or dissolution of the filter can often be tedious and methods which eliminate this step have considerable attrac- tion. An ingenious approach246 involves the collection of particles on a porous graphite probe used as an air filter which can be inserted directly into a pre-heated graphite furnace for atomization. Solid samples are conveniently classified as geochemical, metallic or organic (e.g., clinical and biological). The dissolu- tion of rocks, minerals, ores and soils can be achieved in one of two ways: either melting the sample with a suitable flux followed by solubilization of the melt in an acid,247,248 or digestion of the sample with a perchloric acid-hydrofluoric acid mixture.249 Cadmium in soil is conventionally determined by extraction2562S3 with strong acids.The interactive nature of the dissolution process and the quantification procedure must be considered when devising an analytical method. For example, acid digestion of geological samples prior to inductively coupled plasma atomic emission spectrometry (ICP-AES) determination is preferable to LIB02 fusion because the flux causes clogging and corrosion of the Meinhard nebulizer. This is a common problem with conven- tional ICP systems which do not operate well when the solid content of the solution exceeds 1-2%. Ferrous metals are normally dissolved by simple nitric acidhydrochloric acid digestion whereas non-ferrous metals may require more rigorous acid treatment or direct solid sampling.Biological and clinical specimens with an organic matrix are usually dissolved and the organic matrix destroyed254 prior to quantification of cadmium. This is frequently the rate deter- mining step of an analysis and undoubtedly automation255 and robotics256 will become increasingly important in this respect. There is still some controversy over the best approach for the destruction of the organic portion of a sample prior to elemental analysis. The most comprehensive treatise on the subject remains the authoritative text by Gorsuch .257 The two fundamental approaches are wet oxidation2sg265 and dry ashing.266268 Advantages claimed for the former are a milder oxidation temperature with fewer instances of elemental loss but wet digestion is generally the more tedious procedure with a greater potential for contamination and hence higher blank values.Information on these methods is widely available, but the Analytical Methods Committee of the Analytical Division of the Royal Society of Chemistry give a concise summary of the perchloric acid,*69 mixed acid270 and hydrogen peroxide271272 methods. Contrary to popular belief, volatilization losses can occur during wet oxidation if the temperature is allowed to exceed 250 "C. For this reason, block digesters273-275 which allow precise temperature control and ease of operation have556 ANALYST, JUNE 1991, VOL. 116 become popular.The use of pressure vessel~276~277 also appears to be gaining in popularity. Sealed poly- (tetrafluoroethylene) (PTFE) vessels have been used, for example, for the dissolution of bovine liver with a sulphuric acid-nitric acid mixture,278 and porcine liver and mouse tissues with nitric acid-hydrogen peroxide.279-280 Digestion times varied between 1 and 15 min. An interesting alternative to conventional wet digestion is the use of laser ashing281 with unfocused irradiation from a C02 laser. Dry ashing with or without ashing aids and temperature programming282-287 offers a simple, rapid alternative to wet digestion with the added advantage of minimal reagent blanks. The critical considerations in dry ashing are the nature of the sample288 and the ashing vessel, the ashing temperature and time and possibly the location in the muffle furnace.It is essential that the initial temperature is sufficiently low to prevent flaming or burning. This means placing the sample in a cool muffle furnace and slowly raising the temperature with the furnace door closed. Although higher temperatures have been used, the final temperature2S7,289,290 should not exceed 500 "C in order to minimize the possibility of cadmium loss by volatilization, occlusion into silica particles (if present) or adsorption onto the ashing vessel where a refractory oxide may form. The addition of sulphuric acid to foods is claimed to eliminate the loss of cadmium291 even at an ashing tempera- ture of 750 "C. Magnesium nitrate has also been used283 as an ashing aid.Provided good laboratory practice is adhered to, both wet and dry ashing appear to be suitable for the determination of cadmium in most matrices. The difference between the two procedures is in many instances probably attributable to the analyst. For a sample matrix where the suitability of dry ashing has not been established the authors favour wet digestion using a block digester or a (sealed) PTFE vessel. Recent Developments in Sample Dissolution The need to minimize sample manipulation and exposure of the sample to reagents in order to obtain a low blank value has had a significant impact on sample preparation procedures. There are, for example, many methods for cadmium determi- nation in biological fluids (e.g., urine, blood and cerebrospinal fluid) that do not require total destruction of the organic matter.292-294 In other instances, cells and proteins in whole blood and serum295J96 have been hydrolysed by enzymes such as pronase prior to quantification by atomic spectrometry. As an alternative to hydrolysis, protein in blood may be dena- tured with trichloroacetic acid297 or nitric acid298--300 and removed by filtration or centrifugation.The danger in such procedures lies in the possibility of coprecipitation of cad- mium and hence they are probably best avoided. Microwave digestion has received considerable attention as an alternative to conventional wet digestion. Advantages claimed for a microwave-digester include speed,3*1 lower energy and reagent consumption, reduced contamination of both samples and laboratory atmosphere and ease of opera- tion.These advantages have been exploited by a number of workers278J02-30S for the rapid dissolution of both biologi- cal306-309 and geological305 samples. On the other hand, Parr bomb digestion has been found to be preferable for the multi-element analysis of biological samples310 because there were problems with cadmium losses using both microwave and hotplate digestion. With atomic spectrometric methods, which are the most commonly used (Table 8), the conventional method of sample introduction is nebulization of a solution of the sample. The direct introduction of solids or slurries is increasing in pop~larity317,358,~11,412 because it combines matrix destruction with analyte atomization or excitation in one step. The instrumentation must be modified to optimize for direct sampling.413.414 Using slurry nebulization, factors relating to sample viscosity and dispersion affect the atomization ef- ficiency, the most important of these being particle size.304 The slurry technique has been used to determine cadmium in a range of samples,274,41S-417 including standard reference materials,317,416 with a precision equal to that obtained by conventional wet digestion.A reduction in the particle size of samples for slurry introduction can be achieved rapidly and efficiently418 allowing calibration with aqueous standards. Direct solid sampling can be achieved in a number of ways.340,419,420 As one example, the Perkin-Elmer cup-in-tube device has been used418 to determine cadmium and lead in liver and dried blood samples using aqueous standards for calibration.The results agreed with those obtained after wet digestion. Other applications involving cadmium have included421-424 the analysis of liver and blood ,418,425 Chlorella and bovine liver,34" biological materials,412.426 human organs418 and salivary calculi.411 One of the main difficulties associated with solid sampling is the small sample mass3383339 which places considerable demands on sample homo- geneity.344 This is an important factor which must be considered when analysing homogeneous reference materials only. In other instances, solid sampling has resulted in accurate results but with poor precision.418,425 Miller-Ihli,396 in comparing sample preparation methods for foods and biological materials, found similar accuracies and precisions for a number of methods which were distinguished only by the preparation times involved.These were 5min, 45min, 24h and 36h for slurry preparation, microwave dissolution, dry ashing and wet oxidation, respectively. Preconcentration Preconcentration is invaluable in elemental analysis for two main reasons. It allows for the improvement of detection limits and the separation of an interfering matrix.427 These benefits have been exploited by many workers using evapora- tion, 0srnosis,~28 chelation and solvent extraction, flotation and coprecipitation,429 ion exchange (column and batch mode), retention on modified solid supports, electrochemical deposition3* 1,330,430,431 and on-line concentration. Compari- sons and reviews of preconcentration procedures have been published.432434 Evaporation represents an obvious means of concentrating a sample and for fresh waters is a quick and simple method of achieving a 10-20-fold concentration.435>436 However, it is a non-selective method, both the analyte and matrix being concentrated at the same time.Hence, as a general technique, evaporation has limited value because of the limits associated with total dissolved solids in many atomic spectrometric techniques. On the other hand, coprecipitation provides selectivity and was once used extensively as a preconcentra- tionheparation method. Concentration factors of the order of 200400 have been achieved by coprecipitation437-439 on, for example, N,N-disubstituted carbamodithioates (commonly referred to as dithiocarbamates or ~arbodithioates)261,263,4~0 or indium hydroxide .441 Flotation also depends on precipita- tion to achieve concentration of analyte and is claimed442 to be faster than coprecipitation.However, methods based on precipitation have to a great extent been replaced by solvent extraction which continues to be an effective if time-consum- ing method for the enrichment of trace metals and the reduction of matrix interferences. The success of solvent extraction depends on the formation of neutral complexes that are soluble in organic extracting solvents. For cadmium, dithiocarbamates have been the most widely used non-selec- tive ligands4437444 although several other extractantsas449 including porphine derivatives,446 alkyl phosphate~450>~S~ and various acids and amine~247.314~452 have been examined.Some of these extractants have not been applied to real samples. It seems unlikely that concentration factors greater than about 200 will prove feasible with solvent extraction because ofANALYST, JUNE 1991, VOL. 116 557 theoretical (due to finite distribution coefficients) and prac- tical (physical manipulation) considerations. The recovery of cadmium from ion-exchange and chelating resins4533454 depends on the bulk composition of real samples. The presence of complexing agents in the sample is determi- Table 8 Analytical procedures used for the quantification of cadmium Quantification Sample procedure Biological- Algae ASV Bovine liver HPLC Cereal grains ASV Earthworms X-ray microscopy Fish AAS Liver, muscle, kidney ETAAS Mysids ISE* 0 r g a n i s m s AAS Oysters HPLC Plants ETAAS Shrimp ISE ‘Tissue’ Zeeman ETAAS ‘Tomato leaves’ HPLC Vitamins HPLC Various AAS NAA ETAAS Voltammetry Chemicals: ultrahigh purity reagents- ICP-AES Spectrophotometry Wool ICP-AES BaFz ICP-AES Ga ETAAS Cd Te films KCI FI; voltammetry Pt FAAS Se FAAS Zn Voltammetry Zn Spectrofluorimetry Blood ETAAS HCl, HF ICP-MS Polarograp h y Clinical- Zeeman AAS FAAS Colostrum FAAS Gastric juice, intestine Anodic stripping Hair (shampoo, water) AAS neopolarograph y ETAAS FAAS Kidney ETAAS FI-AAS Liver FI-AAS ETAAS ICP-AES Lungs NAA Sampling devices (e.g.syringes) NAA Tissues FAAS Tooth ETAAS Urine ETAAS (Zeeman) Various NAA Plasma HPLC-FA AS ICP-MS Various Environmental- Air AAS; ASV NAA ETAAS ICP-MS Airborne particulates ICP-AES ETAAS FI-ASV; AAS Effluent Spectrophotometry HPLC Reference 203 289 42,311 147 312 313 314 288.315 98 316 289 317 114 255 318 318 319 306 320 321 322 323 324 325 326 327 328 329 330 331 332-334 335 297 79 336 273 334 337 338-340 307 307 339,340 341 342 343 234 282 344 332,334,345-347 348 349 350 35 1 352 246 187 353 243,244 354 314 355 Table &-continued Soil ICP-AES 356 ETAAS 357 ETAAS; ICP-AES 350 FAAS 25 1,358 FAAS; ETAAS 359 XRF 360 Spectrophotometry 46 Sediments XRF 361 AAS; ASV 362 Snow Zeeman AAS 197 Water Isotope dilution ICP-MS 363 AAS 364 Defuelling (solids) XRF 365 Ground ETAAS 366 Lake AAS 367 ICP-MS 368 ETAAS 41 369 FAAS ETAAS; AES 31 ICP-AES 370 NAA 37 1 Polarograph y 372 Spectrophotometry 373 Potable ETAAS 374,375 HPLC 376-378 River ETAAS 366,379,380 Various 381,382 Sea FAAS 383,384 ETAAS 380 FI-AAS 385,386 ASV 35,111 FI-ASV 387 PIXE,? X-ray emission 388 Isotope dilution ICP-MS 363,389 Waste AAS 390 ETAAS; AES 31 HPLC 39 1 Cabbage FAAS 392 ETAAS 288 Eggs, meat ETAAS 283 Flour, eggs, milk Polarography 149 Fruit, vegetables Polarography; Natural Food- spectrometry 393 H3P04; food grade ETAAS 394 Meat AAS 290 Milk Zeeman ETAAS; polarograph y 51 ETAAS 395 AAS 396 Various 397 Rice AAS 141 Soft drinks Spectrofluorimetry 331 Vegetables, fresh, Sugar ASV 39% canned ETAAS 49 Exploration samples ETAAS 399 Geological- Rocks AAS 400 Zeeman ETAAS 401 ICP-AES 402 Alloys Polarography 403 Byzantine pearl PIXE spectrometry 404 Medicinal soaps Paper chromatography 406 Ores FAAS 252 ICP-MS 407 Petroleum, fuels AAS 408 Red mud ETAAS 249 Stabilizers in plating baths Ion chromatography 409 Zinc plating bath ASV 410 * ISE = Ion-selective electrode.7 PIXE = Particle induced X-ray emission. Miscellaneous- Coal ash and slag Various 405558 ANALYST, JUNE 1991, VOL. 116 nant both from a kinetic and thermodynamic viewpoint455 although, in general, these resins will recover hydrated ions and most complexed ions, depending on the stability constants of the complexes and on the exchange kinetics.456.457 As an example, dissolved and electroreducible cadmium species are retained457 by Chelex-100, an iminodiacetate chelating resin, whereas those present in colloidal form or adsorbed on particulates are recovered by centrifugation. The selectivity of Chelex-100 for heavy metal ions458 is much greater than for alkali or alkaline earth metal ions, permitting the selective removal of these matrix elements from a sample.Contradic- tory results4537458 obtained with Chelex-100 have been attri- buted458 to poor control of experimental conditions of flow rate, pH, column conditioning and amount of resin. Neverthe- less, Chelex-100 has been used widely3707459 for the preconcen- tration and speciation of cadmium alone and in admixture with other elements. On Cellex-P (a dibasic phosphate ester of cellulose) preconcentration factors in excess of 100 have been achieved- for a number of metal ions. This resin has the advantages of high selectivity for metal ions and excellent ion-exchange kinetics allowing rapid preconcentration from large volumes of aqueous samples or sample extracts. Less interest has been shown in conventional ion exchangers,461-463 presumably because of their lower selectivity than chelating resins.In an unusual approach, concentration of several metal ions has been effected on green tea.464 There is a continuing increasing trend to use solid support materials for preconcentration of metal ions.4657466 In one approach465 trace metals are first complexed (using 8-hydroxyquinoline, various carbamates or N-methylfurohy- droxamic acid) and then adsorbed onto a small adsorbent column (of octadecyl-bonded silica, Chromosorb W-DMCS or XAD-4 resin) from which the complexed metals are eluted with aqueous or aqueous-miscible mobile phases. The more promising approach involves immobilization (adsorption or chemical bonding) of the chelating agent (e.g., 8-hydroxy- quinoline363.453,467) on a solid sorbent polymer, controlled- pore glass, anion exchanger, silica or reversed-phase octadecyl-silica.453.466-47" Immobilized ligands have several advantages relative to other preconcentration methods includ- ing the ability to re-use the substrate and to tailor systems for specific needs, permitting the design of selective or general concentration systems which can be operated in a closed fashion, minimizing the risk of contamination and thereby contributing to low blank values. This allows for ease of automation and the concentrated extract can be obtained free from excess of chelating agent.On-line preconcentration represents a further degree of sophistication.It has been recommended for water analysis by ICP-AES in two useful st~dies.~2,~71 Solid sorbents including alumina,472 chelating ion exchangers385,386,473 'and immobi- lized ligands474 provide a convenient and easily automated on-line system particularly when combined with flow injec- A number of other procedures, in many instances modifi- cations of one or more of the foregoing, have been described for the enhancement of metal ion concentra- tion .331,369,4*7,462,475-480 Of these, two warrant special mention because of their versatility and potential. Algal strains479.480 have the ability to sorb a specific metal or group of metals selectively, allowing for highly selective if not specific precon- centration of metal ions.In a similar fashion, high-perform- ance liquid chromatography (HPLC) offers a high inherent selectivity and becomes a powerful technique when combined with atomic spectrometry.343.376 tion -385,386 Quantification Analytical methods are frequently characterized by the final quantification procedure, as illustrated by the EC directives on pollutants in water.228 This emphasis on the quantification procedure can be attributed to two factors. Firstly, rapid developments in instrumentation have led to increased technique specialization and secondly, the ultimate feasibility of an ultratrace analysis is determined largely by the limit of detection of quantification procedures. The relative merits of different procedures can be judged on a number of parameters but the limit of detection is pre-eminent in any such consideration.Limits of detection are invariably general guidelines397 and in an actual analysis, the smallest detectable amount may be very much more or much less. Indeed, practical limits of detection are rapidly degraded as the value of the analytical blank increases. Typical detection limits are given in Table 9 for aqueous solutions of cadmium assuming no preconcentration and 'normal' instrumental operating conditions ( e . g . , for spectrophotometry, a 1 cm cell). Com- parison with the levels of cadmium found in a range of samples (Tables 1 and 2 ) clearly demonstrates the need for, and importance of, preconcentration procedures. Other factors which must be considered in comparing techniques are the capacity for multi-element analysis, specia- tion studies and on-line monitoring (as in process contro1)492 and cost effectiveness.In this section, procedures used for the determination of cadmium alone and in multi-element analyses are evaluated. Spectrophotometry Spectrophotometry occupies an important historical role in the development of analytical chemistry as the first of the so-called instrumental techniques. The Association of Official Analytical Chemists specifies a spectrophotometric method for cadmium, based on its reaction with dithizone.493 Various chromophoric reagents have been exploited for the determi- nation of cadmium and these are listed in Table 10, together with wavelengths of maximum absorption and corresponding molar absorptivities. From these data, the need to preconcen- trate cadmium in samples is apparent.Moreover, interference by other metal ions is common and the use of masking agents500 or elaborate separation procedures becomes essen- tial. As an example, the absorption maxima of the zinc(n), iron(1r) and nickel(i1) complexes of 1,2-di(2-pyridyl)glyoxal bis(2-quinolylhydrazone) are within 8nm of that of the cadmium complex481 and all have molar absorptivities exceed- ing 104 dm3 mol-1 cm-1. Spectrophotometric procedures can, therefore, become tedious and have now been superseded by more sensitive and selective quantification procedures. However, interest in spectrophotometric methods for cad- mium persists, especially as a detection system for HPLC. Atomic Emission Spectrometry Atomic emission spectrometry (AES) was originally used with a variety of excitation sources, i.e., flame, a.c.spark, a.c. and d.c. arcs, but most recent work with AES has involved the inductively coupled plasma. Indeed, ICP excitation has revolutionized the field of emission spectrometry and realizes its full potential in geochemical and environmental stud- ies437,502J)3 where rapid multi-element capability is required. However, it is by no means restricted to these applica- tions .263,504,505 The wide dynamic range of ICP excitation allows the simultaneous measurement of elements present at widely differing concentrations in a sample. Inductively coupled plasma atomic emission spectrometry is primarily a solution technique although other methods of sample introduction are available .311,419,431506 The advan- tages for those samples that occur naturally in the liquid state are clear enough, but even for solid samples there can be advantages.The dissolution of solid samples, for example, avoids many spectral interferences but may, of course, introduce extraneous contaminating or interfering species. Spectral interferences (including spectral overlap, continuumANALYST, JUNE 1991, VOL. 116 559 or background radiation and stray light) are an important consideration for the ICP and different lines are preferred for a specific element in different matrices. Spectral lines which have been used for cadmium are 214.4nm,42141Y 226.5 nm,507308 228.8 nm25033703437,443 and 361.1 nm.431 Recent developments in JCP-AES have been in the area of ‘front-end’ innovations, such as flow injection, HPLC, electrothermal vaporization and on-line preconcentra- tion.311.509-511 For instance, greater sensitivity can be achieved with electrothermal vaporization and this was applied by Isoyama et af.512 to the determination of several elements in biological samples.Atomic Absorption Spectrometry Conventional flame atomic absorption spectrometry (FAAS) with sample introduction via pneumatic nebulization is still with electrothermal AAS (ETAAS). There are less cross- interferences and related systematic errors with the flame technique which is easier to operate and with considerably less expertise. Nevertheless, interferences continue to be dis- covered a1 though most spectral interferences have now been reduced to acccptable levels by improved methods of back- ground correction.247.’97.304.343 The major limitation of FAAS is poor sensitivity in comparison with ETAAS.Methods for improving the sensitivity of FAAS using preconcentration, solid sampling,515 atom trapping472 and slotted quartz tube techniques297 have been described. For instance, Brown et aZ.483 have described the optimization of a water-cooled atom trap for cadmium and lead determination. Improved sensitivity has also been achieved with the double silica-tube atom trap.374 These devices prolong the residence time of the atomic vapour in the light path of the spectrometer. Flame AAS has been used304 for multi-element analysis with solid sampling, but it remains primarily a single element solution technique. This applies partly to ETAAS where multi-element determinations are often not practical.Never- theless, the combined advantages of high sensitivity and low sample volume requirement have been responsible for the widespread use of ETAAS for a diverse range of samples”9”.453,47”.”16.s17 (Table 8). On a concentration basis, furnace AAS detection limits are 10-100 times better than FAAS or ICP-AES (Table 9). On an absolute mass basis, the furnace detection limits are often 1000 times more sensitive because of the small samples required for furnace work. The widely ~~ed3~3.359.399,435,447,448,472,513,514 despite developments levels of interference in furnace AAS are now no greater than those observed with ICP-AES and are largely controlled by a combination of high-quality graphite materials, platform technology and Zeeman background correction with matrix matching or standard additions.518--”20 The effects of ageing and variable quality of graphite tubes521 on temperature profiles have been investigated. Improved pyrolytic graphite tube coatings are now the standard rnaterial~4YJ32~5~2 although tantalum is occasionally used.33 Other non-spectral interferences are related to the matrix and involve interactions in either the condensed or vapour phase .41 Matrix effects are more pronounced for volatile elements such as cadmium523 (nitrate and chloride salts melt at 350 and 568”C, respectively) because the low permissible atomization temperature does not permit suffi- cient destruction of the matrix.Interaction of the remaining matrix components with the analyte can cause interference and contribute to a high background absorption.Chemical modification involves the addition of reagents to the sample in order to modify the volatility of either the analyte or the matrix.5’4 Fluoride or phosphate addition, for example, reduces the volatility of cadmium by converting it into cadmium fluoride with a melting-point of 1100 “C or cadmium pyrophosphate which melts above red heat. However, appli- cation to the determination of cadmium in urine was unsuc- cessful because the background signal remained unacceptably high for deuterium background correction .523 The alternative approach is to enhance the volatility of the matrix by adding suitable reagents that ensure ease of removal of interferences during the ashing period.As expected, the search for a universal chemical modifier has been unsuccessful, although a mixture of palladium and magnesium nitrates showed initial promise.525 For the determination of cadmium, various combinations of phosphate and nitrate modifiers340.“26,”7 have been the most widely used. However, contamination of phosphate with cadmium has been a problem and modifiers excluding phosphate are also common. 332 -357-479.523~~52~ In the so-called stabilized temperature platform furnace (STPF) ,31732,530,531 changes in the atomization time due to condensed phase interactions do not affect the calculated metal ion concentrations because the effective atomization temperature of the samples and standards is the same. At the same time, vapour phase interferences are minimized by the higher atomization temperature.41 The variability of the results obtained with STPF has been attributed532 to variable rates of atomization caused by variable thermal contact Table 9 Detection limits for cadmium in aqueous solution with no allowance for sample size or preconcentration Technique Spectrophotometry ICP-AES FES FAAS ICP-MS Molecular fluorescence HPLC XRF AFS NAA Solution detection limit*/pg 1- Comments 79 0.1-8 2000 7-10 1.5 0.5 0.024.5 0.02-1.0 100 100 0.02 0.5-79 20-100 2.5 0.2 (1.341.Lgg-l) 1 cm cell E = 5 x lo4 dm7 mol-I cm-l Dependent on instrument configuration Conventional system Slotted quartz tube Double silica tube atom trap ASV Chronopotentiometry Polarography Dependent on detection system Referencc 48 1 250,437.453.473 482 304,383,472 297 483 249,255,332,453 327.351,453,484 485 486 348,487 331,481 355.376,488.489 484.490 49 1 * Values are typical limits reported in the literature for aqueous solution and are usually defined as the concentration required to produce a signal equal to twice the standard deviation of the blank.560 ANALYST, JUNE 1991, VOL.116 between commercially available platforms and the grooves of the appropriate graphite tubes. Placing the platforms in non-grooved tubes solved the problem of variable contact. The most important step in achieving analytical reliability with ETAAS came with the physical means of coping with spectral background.533 Apart from background correction with a deuterium discharge 1arnp7249-3W,479 the use of line splitting in the magnetic field due to the Zeeman effect is the most effective method.534v535 The merits of Zeeman and deuterium background correction have been compared536 for the determination of trace metals in waters.Oxygen ashing has been proposed418 as a means of further reducing spectral background in the determination of cadmium. The combination of STPF with Zeeman correc- tion255,332,531,537 has eliminated a number of previously troublesome interferences. Moreover, with Zeeman compen- sation, solid sampling becomes very attractive for an element such as cadmium339,416,423,538 which is relatively volatile. Comparison of Atomic Spectrometric Techniques At this point, it is appropriate to compare the various spectrometric techniques and make recommendations on their use.However, grounds for the decision are very subtle and frequently judgmental, with considerations such as operator skill and economics often being as important as detection limits, accuracy and precision. In a comparison of ETAAS with direct current plasma (DCP) AES for the analysis of natural and waste waters,31 the sensitivity, as expected, favoured the electrothermal technique but a greater sample throughput was achieved with DCP-AES by virtue of its simultaneous (or sequential) multi-element operation. For the determination of cadmium alone in contaminated samples, FAAS is the method of choice, especially for aqueous samples. On the other hand, for samples with ultratrace levels of cadmium ETAAS is the most appropriate method, preferably with STPF and Zeeman background correction. In the Guildford Trace Element Quality Assur- ance Scheme for the determination of cadmium in blood,233 all but one laboratory used ETAAS.However, no laboratory used a L'vov platform although matrix-matched or standard additions calibration was necessary in all instances. In instances where data for cadmium are required as part of a much wider multi-element survey (e.g. , geological) then an ICP-AES sequential system is the preferred technique. Electrochemical Techniques Electrochemical procedures35,'47,362.539 have been the most serious competition for atomic spectrometry. Techniques which have been applied to the determination of cadmium include potentiometric stripping analysis,540~54* adsorptive stripping voltammetry, chronopotentiometry,485 galvanic stripping analysis330 and anodic stripping voltam- rnetry.398,492,542,543 Of these only the last has assumed any importance, usually in the form of differential-pulse anodic stripping voltammetry (DPASV).The capacity of DPASV for multi-element analysis is determined by the difference in reduction potential of the metals concerned. As an example, divalent zinc, cadmium, lead and copper have been deter- mined398 in white sugar following combustion. The initial reduction step in ASV, which achieves a preconcentration of analyte, is the slowest step in the analysis and this has been one of the limiting features of ASV. Preconcentration times vary from several seconds to minutes depending on the concentra- tion of analyte (being greater for low concentrations), the surface area of the electrode and the rate of mass transport which is usually enhanced by stirring the analyte solution or rotating the working electrode.Elemental mercury in the form of a static or growing drop35,398,492,543,544 or as a thin film coated on a conducting substance such as graphite542,545,546 serves as the working electrode. Apart from using electrodes with larger surface areas such as glassy carbon and mercury- film electrodes, the use of a rapid 'staircase' stripping waveform has reduced analysis times.547,548 Alternatively, the signal to background ratio per unit determination time has been improved542 by using decreased pulse widths coupled with rapid potential scan rates at a mercury-film carbon-fibre electrode.In summary, DPASV offers real advantages for the direct determination of cadmium in aqueous samples. Furthermore, ASV has an intrinsic capability5493550 for speciation studies. 110 For purely practical reasons, DPASV is probably limited to the simultaneous determination of not more than 6-10 elements. Liquid Chromatography Liquid chromatography is an extremely versatile technique which has been used in the form of paper,551 thin-layer552.553 and high-performance liquid chromatography for the determi- nation of cadmium. Although the merits of the layer methods should not be ignored, it is only with HPLC that the full potential of liquid chromatography is realized. It involves considerably less capital costs than atomic spectrometric techniques and offers, potentially, much more scope for the investigation of chemical species.In what is probably the simplest form of HPLC, metal ions are separated489.554 on an ion-exchange resin followed by electrochemical detection or, more recently, atomic spec- trometric detection. Spectrophotometric detection following post-column reaction, although less common, has also been used.489 With electrochemical or atomic spectrometric detec- tion, the relatively poorer efficiency of the ion-exchange packing is compensated for by the selectivity of the detection system. However, with spectrofluorimetric or spectropho- tometric detection, somewhat improved column efficiency is desirable and this can be achieved with adsorbents555 or, more commonly, reversed-phase packings. With the latter approach, on-column355,391,556 or off-line pre-column derivati- zation376 of the metal ion becomes a necessary prerequisite to permit detection of the non-absorbing, non-fluorescent metal ions.A limited number of complexing agents have been used for cadmium including salicylideneamines557 and various dithio~arbarnates~376.377.558.559 but in theory, any reagent forming a non-labile, neutral complex with cadmium is suitable. The stability of the complex is a key element in its successful elution. More recently, ionic complexes have been chromatographed560 on reversed-phase packings using ion- pairing reagents in the mobile phase. The use of on-column derivatization can avoid various problems associated with reversiblehrreversible sorption effects of metal complexes within the column,561 but can also introduce problems such as high background absorption due to excess of ligand in the mobile phase.In other instances, various on-column reactions of the excess of derivatizing reagent have been reported.377 The eluted complexes have been detected using either electrochemical or spectrophotometric means. Spectropho- tometry appears to be the more versatile approach355 of the two and any of the reagents listed in Table 10 could be adapted for on-column or pre-column use. Spectrofluorimetric detec- tion with any of the reagents listed in Table 11 is a further possibility331 which offers very low limits of detection (0.5 ng ml-1). Mass Spectrometry Mass spectrometry (MS) occupies a unique position as one of the most sensitive multi-element methods of analysis available.However, there has been a discrepancy between its capabili- ties and their practical realization, although new excitation sources (including plasmas , glow discharges and lasers565,566) are transforming MS as an analytical technique. The ICP, forANALYST, JUNE 1991, VOL. 116 561 Table 10 Spectrophotometric data for cadmium complexes Reagent New cadion 2-(5-Bromo-2-pyridylazo)-5-dimethylaminophenol 2-( 5-Chloro-2-pyridylazo)-5-dimethylaminophenol Cadion" 2,2'-Bipyridine + fluorescein Pyronine G + iodide 4-(2-Pyridylazo)resorcinol (PAR) + surfactant I -(2-Pyridylazo)-2-naphthol PAR + N-phenylcinnamohydroxamic acid Pyrrolidin- l-yldithioformate 1.2-Di(2-pyridyl)glyoxal bis(2-pyridylhydrazone) Diethyldithiocarbamate Dit hizone? Benzoyl( thiobenzoy1)methane Xylenol Orange 1.2-Bis( 2-hydroxypheny1imine)ethane CalmagiteT Absorption Molar absorptivity/ Region where Beer's maximumhm 103 dm3 mol-1 cm-1 law obeyedjpg ml-l 520 555 550 480 572 575 50s 555 510 262 357 262 510 406 580 610 530 * 1 -(4-Nitrophenyl)-3-(4-phenylazophenyl)triazene.t Diphenylthiocarbazone. $ 3-Hydroxy-(4-[ l-hydroxy-4-methyl-2-phenylazo])naphthalene-1-sulphonic acid. 16.4 14.1 12.0 11.9 10.3 9.0 8.65 5.3 4.8 3.5 3.35 3.27 3.2 3 .O 2.75 2.6 1.2 0.0-0.16 0.01-1 .o 0.16-0.72 0.0-0.32 0.2-2.2 0.0-0.6 0.0-0.8 0.0-2.5 0.23-2.25 - - 0.2-3 .0 0.15-5 .o - - - - Reference 46 494 495 496 497 498 46 314.494 499 494 48 1 500 449.461 494 501 494 494 example, is an ideal ion source for the mass spectrometer providing ICP-MS detection limits for cadmium and most other elements, which are at least an order of magnitude better than with ICP-AES.567 An important advantage of MS is the direct analysis of isotope ratios, thus permitting isotope dilution analysis.363 lsotope dilution MS is, in fact, the only technique accepted by the United States National Institute of Standards and Technology [NTST, formerly known as the National Bureau of Standards (NBS)] for establishing the reference values of standard reference materials. There are now a number of papers32"348,363.368,568 describ- ing the application of ICP-MS to the determination of cadmium.In an important application, Crews et ~1.569 speciated cadmium in raw and cooked pig kidney using size-exclusion chromatography coupled to ICP-MS. For multi- element analysis, the potential of ICP-MS was established570 in a comparative survey involving 36 elements in natural waters, of which 34 could be determined simultaneously by ICP-MS compared with 13 by ICP-AES, 12 by TCP atomic fluorescence spectrometry (AFS) and 14 sequentially by AAS.Flow Injection Electrochemistry and spectrophotometry have traditionally been used as the detection systems in flow injection (FI). Coulome try571 and po ten tiome tric stripping analysis (PSA),572 for example, have been employed for the determi- nation of cadmium, but spectrophotometry has been the most important detection system because of the large number of selective colour-forming reagents available for almost every analyte. Spectrofluorimetric detection is not universally applicable, but it has certain attractive features for those analytes such as cadmium for which suitable derivatives exist (Table 11).More recently, atomic spectrometry has been util- ized4"307,573 as a detector for FI. The use of this combination can be expected to expand as the advantages of high sample throughput, minimized interference effects, enhanced sensi- tivity and ease of automation574.575 are recognized. A further development has been the incorporation of continuous separation systems into conventional FI manifolds .42,385,386,473 In addition to enhancing selectivity through interference removal, the inclusion of a separation device offers the possibility of increasing the detection limits of an analytical method through pre~oncentration.5~6."~~ An interesting ex- ample is the simultaneous determination of copper, cadmium Table 11 Speetrofluorimetric data for cadmium complcxcs Excitation Emission wavelength/ wavelength/ Reagent nm nm Reference 8-Hydroxyquinoline-5- sulphonic acid Calcein; pH 13.3 Eosi n-cryptand 1,2-Di(2-pyridyl)gIyoxal Eosin 8-p-Tosy laminoquinoline 2',3,4',Sq7-Pentahydroxy- 2,2'-Bipyridine + a fluorescein bis( 2-quinolylhydrazone) flavone (morin) 387 522 490 520 536 552 550 482 Data unavailable Data unavailable 467 537 340 580 560 562 33 1 48 1 563 563 564 497 and lead in ground waters572 based on the combination of FI with PSA and ion exchange.Other Techniques Atomic fluorescence spectrometry has rarely been used for routine analysis despite the virtual absence of interference effects."% The application of tunable lasers has revived interest in AFS in combination with various atomization techniques (e.g., flame, furnace and ICP).For cadmium and a number of other elements ( e . g . , selenium and arsenic), better limits of detection are achieved579 with laser-induced ICP- AFS than with TCP-AES. Vapour phase generation, probably of diethylcadmium, has also been used566 with AFS. X-ray fluorescence (XRF) is well recognized as a powerful method360,361,580 for multi-element analysis using both con- ventional wavelength and energy dispersive systems. I t offers fairly uniform limits of detection across a large number of elements with direct analysis of solid samples and is essentially non-destructive.581 However, in many instances, AAS and ICP-AES systems are probably more efficient and cost- effective than XRF.58*,"3 Neutron-activation analysis (NAA) is claimed584 to be more accurate than ETAAS but, at the same time, probably exhibits poorer limits of detection.Nevertheless, the role of NAA in the biomedical and environmental area~342.349~371.585 has increased steadily for multi-element analyses. Conclusions The current trend in analytical chemistry is towards greater analytical sensitivity with attention also being given to562 ANALYST. JUNE 1991. VOL. 116 reproducibility and the need for a high-speed multi-element capacity. The dominant role of AAS and ICP-AES as analytical techniques for the determination of cadmium is firmly established. Regrettably, the ease with which data are now generated has led to a proliferation of data of question- able quality as suggested by the results from several collabora- tive surveys.229-233~~8~~8~ Improved quality assurance is essen- tial if answers to a number of perplexing environmental questions are to be provided.In this respect, the results of collaborative surveys are more encouraging where a single analytical method is used590--592 and where greater control is exercised over the participating laboratories. Another promis- ing development is the increased use of standard or certified reference materials.s*~~s93 Sample preparation remains the weakest link in the analytical scheme despite the considerable advances in this area. There is a need to collate cadmium levels in biological and environmental samples and to monitor the temporal changes.Despite increased environmental cadmium levels, there is an apparent decline in the dietary intake and body burden of humans which is probably a reflection of the changes180 and technological improvements that have been implemented in analytical techniques. One of the authors (K. R.) thanks the School of Chemistry, University of Hull, particularly Professor A. Townshend, for their support in the preparation of this manuscript. 1 2 3 4 5 6 7 8 9 30 11 12 13 14 15 16 17 18 19 20 21 22 23 References Sovet, A., Presse Med. Belge, 1858, 10. 69. Wheeler, G. A., Boston Med. Surg. J . , 1876, 95, 434. MarrnC, A., Z. Rationelle Med., 1867, 29, 1. Blyth. A. W.. and Blyth, M. W.. Poisons: Their Effects and Detection, Griffin, London, 1920. Criado Portal, A.J . , Martin Iniesta, L., and Martinez Casanova, M. A.. Ing. Quim. (Madrid). 1988, 20, 123. Tabaku, A., Vasili, P., Gjoka, M., and Papa, K., Rev. Mjekesore, 1988, 5, 21. Sykora, J.. Senft, V., Hugl. F., Sefrna, F., Vit, M., and Votruba, T., Prac. Lek., 1988,40, 341. Delcarte, E . , Impens. R., Kirchrnann. R., Fagniart, E., and Nangniot, P., Proc. Int. Symp. Recent Adv. Assessment of Healtli Effects of Environmental Pollution, Paris, 24-28 June, 1974, Comm. Eur. Communities, vol. 3. pp. 1675-1683. Chancy, R . L., Bcyer, W. N., Gifford, C. H., and Sileo, L., Trace Elem. Subst. Environ. Health, 1989, 22, 263. Mathy. P., Piret. T., Kooken. G.. and Irnpem. R., Infernat. Conf. Management and Control of Heavy Metals in the Environment, London, 18-21 September, 1979, CEP Consul- tants, Edinburgh, pp.226-229. Cleverly, I). H., Morrison, R. M., Liddle, B. L., and Kellam, R. G., AIC‘hE Syrnp. Ser., 1988. 84, 141. Ruhimaki. V., Work EnviivL. Health. 1972, 9, 91. Lauwcrys, R. R., in 1 race Metals-Exposure and Health Effects, cd. Ferrante. E . D., Pergamon Press, New York, 1979, Fassctt. D. W., in Metals in the Environment, ed. Waldrorn, €-I. A.. Academic Press. New York, 1980, pp. 61-110. Davis, R. D., Experientia, 1984, 40, 117. Gu, Y., Shanghai Huanjing Kexue, 1988, 7(8), 27; Chem. Abstr., 1989, 110, 140648g. Kugel, G., Linssen, K., and Zingler, E., Forum Sfaedte Hyg., 1989,40, 126. Kjellstrom, T., Lind, B., Linnaman, L, and Elinder, C. G., Arch. Environ. Health, 1975, 30. 321. Malysowa, E., and Patorczyk. P..Pr. Nauk. Akad. Ekon. im. Oskara Langego Wroclawiu, 1988.426, 100. Williams. C. H., and Davids. D. J., Aust. J . Soil Res., 1973, 11, 43. Beaufays, J. M., and Nangniot, P., Analusis. 1976, 4, 193. Anderson, A., Swed. J. Agric. Res., 1976, 6, 27. De Srnedt. F.. and Bronders, J., Water (Berchem, Belg.), 1987. 35, 75. pp. 13-64. 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 Gelinas, P., Locat, J., and Drouin, R., Rev. Sci. Eau, 1989, 2, 109. Brady, M. J . , AIChE Symp. Ser.. 1988, 84, 192. Williams, C. R., and Harrison, R. M., Experientia, 1984,40,29. Cawse. P. A., United Kingdom Atomic Energy Authority, Report No. AERE R 8869, 1977. Friberg, L., Acta Med. Scand. Suppl., 1950,138 (Suppl.240), 1. Lauwerys, R.. Roels, H., Regniers, M., Buchet, J. P., Bernard, A.. and Goret, A., Environ. R e x , 1979, 20, 375. Chemistry and Biogeochemistry of Estuaries, eds. Olausson, E., and Cato, I., Wilcy-Intcrsciencc, New York, 1980. Alarcon, P., Alonso, E.. Bcnito, Y., Dc la Fuente, P.. and Vergara, A., Ini. J . Environ. Anal. Chem., 1989, 37, 75. Taylor, D.. Environ. Technol. Lett., 1982, 3, 137. Taguchi, S . , Yamazaki, S . , Yarnamoto, A , . Urayama, Y., Hata, N., Kasahara, I., and Goto, K., Analyst, 1988,113,1695. Mance, G., Pollution Threat of Heavy Metals in Aquatic Environments. Elsevier, London, 1987. Adcloju, S. B., and Brown, K. A., Analyst, 1987, 112, 221. Akatsuka, K., and Atsuya, I . , Fresenius Z . Anal. Chem., 1987, 329,453. Bender, M.L., and Gagner, C. L., J . Mar. Res.. 1976,34,327. Boyle, E . A., Sclater, F., and Edrnond, J . M., Nature (London), 1976, 263. 42. Bruland, K. W., Knauer, G. A.. and Martin, J . H., Limnol. Oceanogr., 1978, 23, 618. Knaucr, G . A., and Martin, J.. Limnol. Oceanogr., 1973. 18, 597. Nagourney, S. J . , Heit, M., and Bogen, D. C., Talanta, 1987, 34. 465. Kurnamaru, T., Matsuo, H., Okarnoto, Y.. and Ikeda, M., Anal. Chim. Acta, 1986, 181. 271. Ravera, O., Experientia, 1984.40, 2. Kobayashi, J., Moru. F., Murarnoto, S.. Nakashirna, S., Teraoka, H., and Horie, S . , Jpn. J . Limnol., 1975, 36, 6. Bcvcridge, A., Waller, P., and Pickering, W., Talanta, 1989,36. 535. Qiu, 2.-C., Zhu, Y.-Q., and Yan, J.-P., Analyst, 1988, 113, 1329. Van Bruwaene. R., Kirchmann, R . , and Impens, R., Experien- ria, 1984, 40, 43.Sherlock, J . C., Experientia, 1984, 40, 152. Cirugeda Delgado, C., Cirugeda Delgado, M. E., and Santos Diaz, M. D., Alimentaria (Madrid), 1988, 25. 53. Webb, M., Br. Med. Bull., 1975,31, 246. Koops. J . , Klomp, H., and Westerbeek, D . , Neth. Milk Dairy J . , 1988, 42, 99. Mueller, J., and Grubhofer, F., Milchwirtsch. Ber. Bundesanst. Wolfpassing Rotholz, 1988, 97. 207. Kirkpatrick, D. C., and Coffin, D. E., J. Sci. Food Agric., 1975. 26, 99. Cirugeda Delgado, C.. and Santos Diaz, M. D.. Alimentaria (Madrid). 1988, 25. 9. Vreman, K., Van der Veen, N. G., Van der Molen, E. J., and De Ruig, W. G.. Neth. J. Agric. Sci., 1988, 36, 327. Mehennaoui, S . . Charles, E., Joseph-Enriquez, B., Clauw, M., and Milhaud, G. E., Vet.Hum. Toxicol., 1988, 30, 550. Spierenburg, T. J., De Graaf, M. J., Baars, A. J., Brus, D. H., Tielen. M. J . . and Arts, B. J., Environ. Monit. Assess., 1988 (Publ. 1989). 11, 107. Kreuzer. W., Rosopulo, A., and Frangenberg, J., Fleischwirt- schaft, 1988, 68, 1461. Cirugeda Delgado, C., and Santos Diaz, M. D., Alimentaria (Madrid), 1988, 25, 12. Reiniger, P.. Environ. Pollut., 1977, 14, 297. Jankovski, H., Kukk, H . , and Voloz, J . . Eesti NSV Tead. Akad. Toim. Biol., 1988, 37, 234; Chem. Abstr., 1989, 110,21148j. Lobel, P. B.. Belkhode, S . P., and Jackson. S. E., Arch. Environ. Contam. Toxicol., 1990, 19, 508. Arniard-Triquet. C., Amiard, J. C., Berthet, B., and Metayer, C.. Water Sci. Technol.. 1988, 20, 13. Baccini. P., Schweiz. Z. Hydrol., 1976, 38, 121.Price, R. E., and Knight, L. A., Pestic. Monit. J . , 1978, 11, 182. Cadmium in the Environment, ed. Nriagu, J. 0.. Wiley- Interscience, New York, 1980. pp. 2-12,305-363. Lu, L., Haiyang Xuebao (Zhongwenban), 1986, 8, 573. Forstner, U . , and Reineck, H.-E., Senckenbergiana. Marit., 1974.6, 175. Hall, L. A., Environ. Pollut., 1989, 56, 189.ANALYST, JUNE 1991, VOL. 116 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 131 112 113 114 115 Elinder, C. G., and Kjellstrom, T.. Ambio, 1977, 6, 270. Iyengar, V., and Woittiez, J . , Clin. Chem., 1988, 34, 474. Takata, F.. Nakamura, K.. Kajizuka, E., and Nakano, J . , Jpn. J. Hyg., 1978, 191, 691. Friberg, L., Piscator, M., Nordberg, G .E.. and Kjellstrom, T., Cadmium in the Environment, CRC Press, Cleveland, 2nd edn., Beliveau, R., Gelinas, Y . , Ferraris, J . , and Schmit, J . P., Biochem. Cell Biol., 1990, 68, 1272. Roels, H., Lauwerys, R., Buichet, J. P., Bernard, A . , Chettle, D., Harvey, T. C.. and Al-Haddad. I. K., Environ. Res., 1981. 26, 217. Elinder. C. G., Kjellstsrom. T., and Friberg, L., Arch. Environ. Health, 1976, 31, 292. Takagi, Y.. Matsuda, S . . Imai, S . . Ohmori, Y., Masuda, T., Vinson, J . , Mehra, M., Puri, B., and Kaniewski, A., Bull. Environ. Contam. Toxicol., 1988, 41, 690. Stauber, J. L., and Florence, M., Sci. Total Environ., 1988,74, 235. Sikorski, R., Paszkowski, T., Radomanski, T., and Szkoda, J . , Gynecol. Obstet. Invest., 1989, 27, 91. Paschal. D. C.. Di Pietro, E.S., Phillips, D. L., and Gunter, E. W.. Environ. Res., 1989, 48, 17. Suyama. Y., Takaku, S., Ichida, K.. and Nishimura, M., Koku Eisei Gakkai Zasshi, 1989,39, 602. Yost, K. J . , Experientia, 1984, 40. 157. Page, A. L., Bingham. F. L., and Chang. A. C., in Effect of Heavy Metal Pollution on Plants, ed. Lepp, N. W., Applied Science Publishers, London, 1981, vol. 1, pp. 77-109. Buttenvorth, J . , Lester, P., and Nickless. G., Mar. Pollut. Bull., 1972, 3. 72. Goldberg, E. D., Bowen, V. T., Farrington, J. W., Harvey, G., Martin, J . , Parker, P., Risebrough, R., Robertson, W., Schneider. E . , and Gamble, E.. Environ. Conserv.. 1978, 5. 101. Vicente, N., Henry, M., Chabert. D.. and Riva, A.. Oceanis, 1988. 14, 201. Goldberg. E. D., Mar. Pollut. Bull., 1975, 6, 111.Rao, K.. and Indusekhar, V. K., Phykos., 1987, 26, 1. Jennings, J. R., and Rainbow, P. S., Mar. Biol.. 1979. 50, 131. Pentreath, R. J., J. Exp. Mar. Biol. Ecol., 1977, 30, 223. Sprenger, M. D.. Mclntosh, A. W.. and Hoenig, S . . Water, Air, Soil Pollut., 1988, 37. 375. Ray, S., Experientia, 1984, 40, 14. Bressa, G., Chim. Oggi, 1987, 11, 23. George, S. G . , and Coombs. T. L., Mar. Biol. 1977, 39, 261. Hung, Y.-W., Bull. Environ. Contam. Toxicol., 1982, 28, 546. Phillips, D. J . H., Mar. Biol., 1976, 38, 59. Jackim, E., Morrison, G., and Steele. R., Mar. Biol., 1977,40, 303. De Lisle, P. F., and Roberts, M. H.. Aquatic Toxicol., 1988,12, 357. Denton, G . R., and Burdon-Jones, C., Mar. Biol.. 1981, 64, 317. Zaroogian, G. E., Mar. Biol., 1980, 58, 275. Orren, M.J., Eagle. G. A.. Hennig, H., and Green, A . , Mar. Pollut. Bull., 1980, 11, 253. John, M. K., and Van Laerhoven, C. J . , Environ. Pollut.. 1976, 10, 163. Omran, M. S., Waly, T. M.. Abd Elnaim, E., and El Nashar, B. M.. Biol. Wastes, 1988, 23. 17; Chem. Abstr., 1988, 108, 149486~. Brubaker. G. F., Otta. J . W., and Richardson, F., Proc. Water Reuse Symp., I987 (Pub. 1988), 4th (Implementing Water Reuse), CH2M Hill, Gainesville, FL, pp. 413-426. Morgan. H., and Simms, D. L.. Sci. Total Environ., 1988. 75. 135. Hamilton, E. I., Mar. Pollut. Bull., 1981. 12, 393. Kjellstrom, T.. and Nordberg, G . F.. Environ. Res.. 1978, 16, 248. Bernard, A.. and Lauwerys, R.. Experientia, 1984, 40, 143. Nordberg, G. F., Environ. Yhysiol. Biochem., 1972,2,7. Florence, T. M., Talanta, 1982, 29, 345.Hasle, J. R., and Abdullah, M. I., Mar. Chem., 1981, 10, 487. Gardiner, P. E.. J. Anal. At. Spectrom.. 1988,3, 163. Person, H., and Nair, B. M., Food Chem., 1987, 26, 139. Sunda. W. G., Engel, D. W.. andThuotte, R. M., Environ. Sci. Technol., 1978, 12, 409. Gardiner, J.. Water Res.. 1974, 8, 23. 1974, pp. 1-248. 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 13 1 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 15 1 152 153 I54 155 156 157 158 159 563 Farrah, H.. and Pickering, W. F., Water, Air, Soil Pollut., 1977, 8, 189. Byrne, R . H., Kump, L. R., and Cantrell, K. J., Mar. Chem., 1988, 25, 163. Batley, G . E., and Florence, T. M., Mar. Chem.. 1976,4,347. Duinker, J. C., and Kramer, C. J., Mar. Chem., 1977, 5, 207.Forstner, U., Experientia, 1984,40, 23. Hardstedt-Romeo, M., and Gnassia-Barelli, M., Mar. Biol., 1980, 59, 79. Nordberg, G. F., Biol. Trace Elem. Res., 1988 (Pub]. 1989), 21, 131. Nordberg, G. F., Slorach, S., and Stenstrom, T., Lakartidn- ingen, 1973, 70. 601. Bonnell, J . A., Br. J. Ind. Med., 1955, 12, 181. Nicaud, P., Lafitte, A., and Gros, A , . Arch. Mal. Prof. Med. Trav. Secur. SOC., 1942, 4, 192. Friberg, L., J. Ind. Hyg. Toxicol., 1948, 30, 32. Hagino, N., Rod0 Kagaku, 1973, 28, 32. Hagino, N., J. Toyama Med. Ass. Suppl.. 1957, 21. Hagino, N., and Yoshioko, K., J. Jpn. Orthop. ASSOC., 1961, 35, 812. Nriagu, J . O., Adv. Environ. Sci. Technol., 1990, 23, 59 (and references cited therein). Petering, H. G., Johnson, M. A., and Stemmer, K.L., Arch. Environ. Health, 1971, 23, 93. Carruthers, M., and Smith, B., Lancet, 1979, I, 845. Harvey, T. C., Chettle, D. R., Fremlin. J. H., Al-Haddad, 1. K., and Downey, S. P., Lancet, 1979, I, 551. Hutton, M., SCOPE, 1987,31, 53. Hallenbeck, W. H., Experientia, 1985, 40, 136. Chisolm, J. C., and Handorf, C. R . , Med. Hypotheses, 1987,24, 347. Staessen, J., Bruaux, P., Claeys-Thoreau, F., De Plaen, P., Ducoffre, G., Lauwerys, R., Roels, H.. Rondia, D., Sartor, F., and Amery, A., Environ. Health Perspect., 1988, 78. 127. Goyer. R. A., Toxicol. Lett., 1989, 46, 153. Kido, T., Honda, R., Tsuritani, I., Yamaya, H., Ishizaki, M., Yamada, Y., and Nogawa, K., Arch. Environ. Health, 1988,43, 213. Kjellstrom, T., in Cadmium and the Environment, eds. Friberg, L., Elinder, C.G., Kjellstrom, T., and Nordberg, G. F., CRC Press, Boca Raton, FL, 1985, vol. 1, pp. 179-197. Nogawa, K., Honda, R., Kido, T., Tsuritani, I., Yimada, Y., Ishizaki, M., and Yamaya, H., Environ. Res., 1989, 48. 7. Sugita, M., and Tsuchiya, K., J. UOEH, 1988, 10, 199. Ikebe, K., Tanaka, Y., and Tanaka, R . , Shokuhin Eiseigaku Zasshi, 1988, 29, 52. Falandysz, J., Lorenc-Biala, H., and Centkowska, D., Rocz. Panstw. Zakl. Hig., 1987, 38, 344. Ellis, K. J . , Vartsky, D., Zanzi, I., Cohn, S. H., and Yasumura, S., Science (NY), 1979, 205, 323. Lewis, G. P., Coughlin, L., Jusho, W., and Hartz, S., Lancet, 1972, I, 291. Moravcova, J . , Holas, J., and Lucny, M., Sb. UVTIZ, Potravin. Vedy, 1988, 6, 199; Chem. Abstr., 1989, 110, 56183~. Vos, G., Teeuwen, J. J., Knottnerus, 0.C., and Wijers, B. L., Neth. J. Agric. Sci., 1988, 36, 167. Seshaiah, K., Gangaiah. T., and Naidu, G. R., Acta Cienc. Indica, (Ser) Chem., 1988,14, 139. Terao, H., and Morishita. Y., Gifu-ken Eisei Kenkyushoho, 1986, 31, 10; Chem. Absrr., 1987, 107, 12441k. Shock, M. R., and Neff, C. H., J. Am. Water Works Assoc., 1988, 80, 47. Jawad, I. M., Al-Ghazala, M. R., and Khorshid, M. S., Environ. Monit. Assess., 1988, 11, 79. Kamo, K., Fukuoka-shi Eisei Shikenshoho, 1988, 13, 100; Chem. Abstr., 1989, 110, 191421k. European Communities, Offic. J., 20.10.84, L277, 12. Ishimatsu, S., and Omine, E., Kyushu Joshi, Daigaku Kiyo, 1981, 17, 55. Sharp, G. J., Samant, H. S., and Vaidya, 0. C., Bull. Environ. Contam. Toxicol., 1988,40, 724. Biancardi, G., Righini, C., Camici, G., and Tranfa, G., Rass.Chim.. 1985, 37, 191. Miethke, H . , Heffter, A . , and Hoertig. W., Dtsch. Lebensm- Rundsch., 1988. 84, 137. Dabeka, R. W., Karpinski, K. F., McKenzie, A. D., and Badjik, C. D., Sci. Total Environ.. 1988, 71, 65.564 ANALYST, JUNE 1991, VOL. 116 160 161 Dabeka, R. W., and McKenzie, A. D., Food Addit. Contam., 1988, 5 , 333. Ikebe, K., Tanaka, Y., and Tanaka, R., Osaka-furitsu Koshu Eisei Kenkyusho Kenkyu Hokoku, Shokuhin Eisei Hen, 1985, 16, 51; Chem. Abstr., 1986, 105, 1140072. 162 Binetti, R . , Gramiccioni, L., Milana, M., Di Prospero, P., Di Marzio, S . , and La Manna, E., Rass. Chim., 1986,38,3. 163 Dabeka, R. W.. McKenzie, A. D., and Lacroix, G. M., Food Addit. Contam., 1987, 4, 89. 164 Andersen, A., Publ. Statens Levnedsmiddelinst.(Den. ), 1981, 52, 52pp.; Chem. Abstr., 1983,98, 70472~. 165 Louekari, K., Jolkkonen, L., and Varo, P., Food Addit. Contam., 1988, 5 , 111. 166 Louekari, K . , and Salminen, S., Food Addit. Contam., 1986,3, 355. 167 Schumann, H., Zentralbl. Pharm., Pharmakother. Labora- toriumsdiagn., 1982, 121,565; Chem. Abstr., 1982,97,90573~. 168 Melchiorri, C., Grella, A., and Di Caro, A., Nuovi Ann. IS. Microbiol., 1984, 35, 3. 169 Ishimatsu, S . , Kyushu Joshi Daigaku Kiyo, 1985, 21, 15. 170 Uehara, T., Oshiro, Z . , Yamashiro, O., and Shiroma, H., Okinawa-ken Kogai Eisei Kenkyushoho, 1987, 21, 42; Chem. Abstr., 1988, 109, 109037n. 171 De Vos, R. H . , Van Dokkum, W., Olthof. P., Quirijns, J., Muys, T., and Van der Poll, J., Food Chem. Toxicol., 1984,22, 11. 172 Van Dokkum, W., De Vos, R.H., Muys, T., and Wesstra, J . A., Br. J. Nutr., 1989, 61, 7. 173 Bibow, K., Kiis, G., and Salbu, B., Naeringsforskning, 1984, 28, 84. 174 Smigiel, D., Chorazy, W.. Bliwert, K., Filip, J., and Podsiadlo, R., Rocz. Panstw. Zakl. Hig., 1987, 38,480. 175 Evans, W. H., and Sherlock, J. C., Food Addit. Contam., 1987, 4, 1. 176 Morgan, H., Smart, G. A., and Sherlock, J. C.. Sci. Total Environ., 1988, 75, 71. 177 Smart, G. A., Sherlock, J. C., and Norman, J. A., Food Addit. Contam., 1988, 5 , 85. 178 Gartrell, M. J., Craun, J . C., Podrebarac, D. S., and Gunder- son, E. L., J. Assoc. Off. Anal. Chem., 1985, 68, 862. 179 Gartrell, M. J., Craun, J. C., Podrebarac, D. S., and Gunder- son, E. L., J. Assoc. Off. Anal. Chem., 1986, 69, 146. 180 Gunderson, E.L., J. Assoc. Off. Anal. Chem., 1988,71,1200. 181 Iyengar. G. V., Tanner, J. T.. Wolf, W. R . , andzeisler, R., Sci. Total Environ., 1987, 61, 235. 182 Schroeder, H. A., Nason, A. P., Tipton, I. H., and Balassa, J. J., J. Chronic Dis., 1967. 20, 179. 183 Friberg, L., Piscator, M., Nordberg, G. E., and Kjellstrom, T., Cadmium in the Environment, CRC Press, Cleveland, 2nd edn., 1974, pp. 5, 19, 20, 29, 65, 72. 184 Travis, C. C., and Etnier, E. L., Environ. Res., 1982. 27, 1. 185 Kneip, T. J., Eisenbud, M.. Strehlow, C. D., and Freudenthal, P. C., J. Air Pollut. Control Assoc., 1970, 20, 144. 186 Harrison, P. R.. and Winchester, J. W., Atmos. Environ.. 1971, 5 , 863. 187 Northington, D. J., Am. Ind. Hyg. Assoc. J . , 1987, 48, 977. 188 Cosson, R. P., Amiard-Triquet, C., and Amiard, J.C., Water, Air, Soil Pollut., 1988, 42, 103. 189 Williams, C. R., and Harrison, R. M., Experientia, 1984,40,29. 190 Weyers, B., Glueck, E., and Stoeppler, M., Sci. Total Environ., 1988.77, 61. 191 Markert, B., and Weckert, V., Sci. Total Environ., 1989, 86, 289. 192 Goodman, G. T., and Roberts, T. M., Nature (London), 1971, 231, 287. 193 Burkitt, A., Lester, P., and Nickless, G., Nature (London), 1972, 238,327. 194 Landolt, W., Guecheva, M., and Bucher, J. B., Environ. Pollut., 1989, 56, 155. 195 Milhaud, G. E., and Mehennaoui, S., Vet. Hum. Toxicol., 1988, 30, 513. 196 Stewart-Pinkham, S. M., Sci. Total Environ., 1989, 78, 289. 197 Sakai, H., Sasaki, T., and Saito, K., Sci. Total Environ., 1988, 77, 163. 198 Meiggs, T. O., in Contaminants and Sediments, ed.Baker, R. A., Ann Arbor Science Publishers, Ann Arbor, MI, 1980, Yamagata, N., and Shigematsu, I., Bull. Inst. Public Health (Tokyo), 1970, 19, 1. VOI. 1, pp. 297-308. 199 200 201 202 203 204 205 206 207 208 209 210 21 1 212 213 214 215 216 217 218 219 220 22 1 222 223 224 225 226 227 228 229 230 23 1 232 233 234 235 236 237 238 239 240 24 1 Piscator, M.. in Cadmium in the Environment, eds. Friberg, L.. Piscator, M., and Nordberg, G., CRC Press, Cleveland, 1971. Forstner, U., and Salomons, W., Environ. Technol. Lett., 1980, 1,494. Friberg, L.. Piscator, M., Nordberg, G. E . , and Kjellstrom, T., Cadmium in the Environment, CRC Press, Cleveland, 2nd edn., 1974, p. 13. Goncalves, M. L., Vilhena, M. F.. and Sampayo, M. A., Water Res.. 1988, 22, 1429.Celardin, F., and Landry, J. C., Arch. Sci., 1988, 41, 225. Berrow, M. L., and Webber, J., J . Sci. Food Agric., 1972, 23, 93. Schroeder, H. A., and Balassa, J. J., Science, 1963, 140, 819. Bogoni, P., and Favetto, L., Riv. SOC. Ital. Sci. Aliment., 1988, 17, 91. Fischer, H., Mar. Ecol.: Prog. Ser., 1988, 48, 163. Shaikh, Z. A., and Smith, L. M.. Experientia, 1984, 40,36. Willers, S., Schuetz, A., Attewell, R., and Skerfving, S., Scand. J. Work, Environ. Health, 1988. 14, 385. Triger, R. D., Crowe, W., Ellis, M. J., Herbert, J . P., McDonnell, C. E.. and Argent, B. B., Sci. Total Environ.. 1989,78,241. Della Rosa, H. V., Gomes, J. R., Pivetta, F. R., Jacob, S. C., and Mazon, R., Occup. Environ. Chem. Hazards [Symp.], 1986 (Publ. 1987), 355-361; Chem.Abstr., 1988, 108, 172783~. Baker, E. L., Peterson, W. A., Holtz. J. L., Coleman, C., and Landrigan, P. J., Arch. Environ. Health, 1979, 20, 173. Lauwerys, R. R . , Buchet, J. P., and Roels, H., Int. Arch. Occup. Environ. Health, 1976, 36, 375. Sroczynski, J., Urbanska-Bonenberg, L., Twardowska-Saucha, K., and Bonkowska, M.. Med. Pr., 1987, 38,429. Bernard, A., Goret, A., Buchet, J. P., Roels, H., and Lauwerys, R., J . Toxicol. Environ. Health, 1980, 6, 175. Shimmura, T., Nakazaki, M., Shiroishi, K., and Tohyama, C., Toyama-ken Eisei Kenkyusho Nenpo, 1988 (Publ. 19891, 12, 176; Chem. Abstr., 1989, 112, 2239q. Kojima, S., Haga, Y . , Kurihara, T., Yamawaki, T., and Kjellstrom, T., Environ. Res., 1977, 14, 436. Szadkowski, D., Med. Monatsschr., 1972, 26, 553. Gervais, L., Lacasse, Y., Brodeur, J., and Pan, A., Toxicol.Lett., 1981, 8, 63. Ellis, K. J., Morgan, W. D., Zanzi. I., Yasumura, S . , Vartsky, D., and Cohn, S. H., J . Toxicol. Environ. Health, 1981,7,691. Al-Haddad. I . K., Chettle, D. R., Fletcher. J. G., andFremlin, J. H., Int. J. Appl. Radial. hot.. 1981, 32, 109. Thomas, B., Harvey, T. C., Chettle, D., McLellan, J . S . , and Fremlin. J. H., Phys. Med. Biol., 1979. 24, 432. Nishiyama, K., and Nordberg, G. F., Arch. Environ. Health, 1972,25. 92. Ellis, K. J., Yasumura, S., and Cohn, S. H.. Am. J . Ind. Med., 1981. 2, 323. Harvey, T. C., Thomas, B. J., McLellan, J. S . , and Fremlin, J. H., Lancet, 1975, I, 1269. Gardiner, J., and Mance, M.. Technical Report TR204, Water Research Centre, Environment, Stevenage, UK, July 1984.Hill, J. M., O’Donnell, A. R . , and Mance, G., Technical Report TR205, Water Research Centre, 1984, 21pp. Analytical Quality Control (Harmonised Monitoring) Com- mittee, Analyst, 1985, 110, 1. Analytical Quality Control (Harmonised Monitoring) Com- mittee, Analyst. 1985. 110, 247. Bewers, J. M., Dalziel. J., Yeats, P., and Barron, J., Mar. Chem., 1981,10, 173. Jones. P. G. W., Anal. Proc., 1982, 19, 565. Starkey, J . B., Taylor, A., and Walker, A. W., J . Anal. At. Spectrom., 1986, 1, 397. Lakomaa, E. L., Mussalo-Rauhamaa, H., and Salmela, S . , J. Trace Elem. Electrolytes Health Dis., 1988, 2, 37. Subramanian, K. S., Chakrabarti, C. L.. Sueiras, J . E.. and Maines, I. S., Anal. Chem., 1978, 50, 444. Laxen, D. P., and Harrison, R. M., Anal. Chem., 1981,53,345.Batley, G. E., and Gardner. D., Water Res., 1977, 11, 745. Mart, L., Fresenius Z . Anal. Chem., 1979, 296, 350. Batley, G. E., and Gardner, D., Estuar. Coastal Mar. Sci., 1978, 7, 59. Florence, T. M., Wafer Res., 1977. 11, 681. Nurnberg, H. W., Valenta, P., Mart, L., Raspor, B.. and Sipos, L.. Fresenius Z . Anal. Chem., 1976. 282, 357.ANALYST, JUNE 1991, VOL. 116 242 243 244 245 246 247 248 249 250 25 1 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 27 1 272 273 274 275 276 277 278 279 280 28 1 282 283 284 285 Scarponi, G., Capodaglio. G., Cescon, P.. Cosma, B., and Frache, R . , Anal. Chim. Acta, 1982, 135, 263. Schlemmer, G., and Welz, B., Fresenius Z. Anal. Chem., 1987, 328, 405. Schothorst, R. C., Geron, H., Spitzbergen, D., andHerber, R., Fresenius Z .Anal. Chern., 1987, 328, 393. Mcycr, G.. and Wetrup, G . J . , Staub-Reinhalt. Luft, 1989, 49, 59. Chakrabarti, C. L.. He, X.-R., Wu. S., and Schroeder, W. H., Spectrochim. Acta, Part B , 1987, 42, 1227. Viets, J . G., O'Leary, R. M., and Clark, J . R., Analyst, 1984, 109, 1589. Wittmann. A. A., and Kop, F. R., Spectrochim. Acta, Part B , 1986. 41. 73. Alvarado, J., and Petrola, A., J. Anal. At. Spectrorn.. 1989, 4, 41 I . Baucells, M., Lacort, G . , and Roura, M., Analyst,, 1985, 110, 1423. Gucer. S . , and Demir, M., Anal. Chim. Acta, 1987, 196,277. Donaldson. E. M., and Wang. M., Talanta. 1986, 33. 233. Fraser, S. M., Ure, A. M., Mitchell, M. C., and West. T. S., J . Anal. At. Spectrorn., 1986, 1, 19. Van Deyck, W., Lipo, M.J., and Maessen, F. J., Fresenius 2. Anal. Clrem., 1984, 317, 858. van Beek, H.. Greefkes. H., and Baars, A. J., Talanta, 1987, 34. 580. Dcnton, M. B., Analyst. 1987, 112, 347. Gorsuch, T. T., The Destruction of Organic Matter. Pergamon Press, Oxford, 1970, vol. 39. Narrcs. H. D., Mohl, C., and Stoeppler, M.. 2. Lebensm.- Unters. Forsch., 1985. 181, 11 1. Ramos, M., Sanchez, M.. and Rabadan, J.. Quim. Anal., 1987. 6. 362. Piperaki, E . A., Chern. Chron., 1985, 14, 57. Dabeka, R. W.. and McKenzic, D. A.. Can. J. Spectrosc., 1986, 31, 35. Castillo. J. R., Concha, I.. and Martinez, C., At. Spectrosc., 1987. 8, 172. Shan, X.-Q., Tie, J . , and Xie, G.-G., J. Anal. At. Spectrom., 1988, 3. 259. Roberts, C. A., and Clark, J . M., Bull. Environ. Contam. Toxicol., 1986, 36, 496.Zhuang, M.. and Barnes, R. M.. Appl. Spectrosc., 1984, 38, 635. Gamanina, I. A., Zvyagintsev, A., and Chuikov. V., Agro- khimiya, 1985,6, 112. Brzozowska. B., Mazur, H., Ludwicki, J . K.. and Lewandow- ska-Malinowska, I.. Rocz. Panstw. Zakl. Hig., 1985, 36, 197. Pandya, C. B.. Patel, T. S . , Shah, G . M.. Sathawara, N. G., Patcl, V. G.. Parikh, D. J., and Chatterjee, B. B.. J. Anal. At. Spectrom., 1986, 1, 387. Analytical Methods Committee, Analyst, 1959, 84, 214. Analytical Methods Committee, Analyst, 1960, 85, 643. Analytical Methods Committee. Analyst, 1967. 92. 403. Analytical Methods Committee, Analyst, 1976. 101. 62. Wilhclm, M., Ohnesorgc, F. K., Lombeck, I., and Hafner, D., J. Anal. Toxicol., 1989, 13, 17. Fagioli, F., Landi, S . , Locatelli.C., and Bighi, C., At. Spectrosc.. 1986. 7, 49. Salisbury, C. D . , and Chan, W., J . Assoc. Off. Anal. Chem., 1985, 68. 218. Narayanan, S . , Lin, F., and Walder. H., Acta Pharrnacol. Toxicol., Suppl., 1986, 59, 598. Walder, H., Lin, F. C., and Narayanan, S . , Ann. Clin. Biochem., 1987. 24, 268. Aysola, P., Andcrson, P., and Langford, C.. Anal. Chem., 1987. 59. 1582. Zu, L.-Q., Zhu, J.-F., and Zhiang, L.-J., Fenxi Huaxue, 1987, 15, 1053. Van Wyck. D., Schifman, R., Stivelman, J., Ruiz, J., and Martin, D., Clin. Chem., 1988, 34, 1128. Ahlgren, M., Kontkanen. A., Vattulainen, K . , and Vehvi- lainen. H., Analyst, 1988, 113, 285. Mader. P.. Musk J . , Curdova. E., Koreckova. J., and Cibulka, J . , Chem. Lisry , 1987, 81, 11 90. Muys, T., Analyst, 1984, 109, 119.Skurikhin, M.. J . Assoc. Off. Anal. Chem.. 1989, 72, 286. Peral Fcrnandez, J . L., and Reyes Andres, J., An. Bromatol., 1987, 39, 103. 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 30 1 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 33 1 332 333 565 Peral Fernandez, J . L., Quejido Cabezas, A., and Reyes Andres, J.. An. Quim., Ser. B , 1987, 83, 329. Roscknik, R. K., Analyst, 1973, 98,596. Buehler, K. D., Amend, T., and Gierschner, K., Lebensmittel- chem. Gerichtl. Chern., 1989, 43, 7. Ichinoki, S., Morita, T., and Yamazaki. M., J. Liq. Chroma- togr., 1984, 7,2467. Ybanez, N., Cervera, M., Montoro, R., and Catala, R., Rev. Agroquim. Tecnol. Aliment., 1987, 27, 590. Feinberg, M., and Ducauze, C., Anal.Chem.. 1980,52, 207. Lum, K. R.. and Edgar, D. G.. Int. J. Environ. Anal. Chern., 1988, 33, 13. Rocks, B. F., Sherwood, R. A., and Riley, C., Clin. Chem., 1982. 28. 440. Burguera, M., Burguera, J . L., and Alarcon, 0. M.. Anal. Chim. Acta, 1986, 179. 351. Molin Christensen, J., and Kirchhoff, M., Int. Congr. Ser.- Excerpta Med., 1986, 676, 181. Molin Christensen, J., and Milling, P., Acta Pharmacof. Toxicol., Suppl.. 1986, 59. 399. Brown, A . A., and Taylor, A . , Analyst, 1985, 110, 579. Ekanem. E. J., Barnard, C. L. R., Ottaway, J . M., and Fell, G . S . , J . Anal. At. Spectrom., 1986, 1, 349. Black, M. M., Fell, G . S . , and Ottaway, J . M., J . Anal. At. Spectrom., 1986, 1, 369. Falk, H.. Hoffmann. E . , Ludke, C., Ottaway, J . M., and Littlejohn, D., Analyst, 1986, 111, 285.Cresser, M. S., Ebdon, L., and Dean, J . R., J . Anal. At. Specrrorn., 1988,3, 1R. Wandt, M. A. E., and Pougnet. M. A. B., Analyst, 1986, 111, 1249. Bettinelli. M.. Baroni, U., and Pastorelli, N . , J . Anal. At. Spectrom., 1987,2, 485. Moralcs, A., Pomares, F., de la Guardia, M., and Salvador, A., J. Anal. At. Spectrorn.. 1989, 4, 329. Lamothe, P. J., Fries, T. L., and Consoul, J . J., Anal. Chem., 1986. 58. 1881. Stripp, R. A., and Bogen, D. C., J . Anal. Toxicol., 1989.13,57. Burguera, M., Burguera, J. L., and Alarcon, 0. M., Anal. Chirn. Acta., 1988,214,421. Pougnet, M. Pi., and Wandt, M. A., ChemSA, 1986,12,16. Demura, R., Tsukada, S . , and Yamamoto, I., Eisei Kagaku, 1985, 31,405. Hee, S. S . . and Boylc, J . R., Anal. Chem., 1988, 60, 1033.Matusiewicz. H., Fish, J., and Malinski. T., Anal. Chem., 1987, 59, 2264. Richards, K. S . , Rush, A. D.. Clarke, D. T., and Myring, W. J., Nucl. Instrum. Methods Phys. Res., Sect. A , 1988, A272, 889. Ashraf, M.. and Jaffar. M., Toxicol. Environ. Chern.. 1989,19, 63. Raman, B., and Shinde, V. M., Analyst, 1990. 115, 93. Sato, M., Nippon Eiseigaku Zasshi, 1987, 42, 681. Klumpp, D. W., and Peterson, P. J., Environ. Poflut., 1979,19, 11. Ebdon, L.. and Lechotycki, A., Microchem. J . , 1987, 36, 207. Simonzadeh, N., and Schilt. A., Talanta, 1988, 35. 187. Sun, C.. Guangpuxue Yu Guangpu Fenxi, 1988, 8, 27; Chem. Abstr., 1989, 110, 36362~. Zhuang, G., Wang, Y., Zhi, M., Zhou, W., Yin, J., Tan, M., and Cheng. Y., J. Radioanal. Nucl. Chem., 1989.129, 459. Ohta, K., Aoki, W., and Mizuno, T., Talanta, 1988, 35, 831. Li, C., James, B. D., Rumble, J., and Magee, R. J., Mikrochirn. Acta, Part 111, 1988, 175. Nakamura, S., Kawamura, K., Inagaki, T., and Kubota, M., Bunseki Kagaku, 1988,37. T97. Hiltenkamp, E., and Jackwerth. E., Fresenius Z. Anal. Chem., 1988. 332. 134. Paulsen, P. J . , Beary, E. S., Bushee, D. S . , and Moody. J . R., Anal. Chem., 1989,61,827. Darkowski, A., and Cocivera, M., Talanta, 1986, 33, 187. Jayaweera. P . , and Ramaley, L., Anal. Chem.. 1989, 61. 2102. Aneva. Z . , Anal. Chim. Acta, 1988, 211, 311. Beinrohr. E., and Berndt. H., Mikrochim. Acta, Part I , 1985, 199. Jaya, S . , Rao, T. P., and Rao, G . P., Analyst, 1987, 112, 1713. Gomis, D. B., and Garcia, E . A., Analyst. 1990, 115, 89.Yin, X., Schlemmer, G., and Welz, B., Anal. Chem., 1987,59, 1462. Carrion, N., Anal. Chim. Acta, 1990, 231, 283.566 ANALYST, JUNE 1991, VOL. 116 334 Suna, S., and Nakajima, T., Sangyo Igaku, 1987, 29,292. 335 Patriarca, M., Petrozzi, E., and Morisi, G., G. ltal. Chim. Clin., 1988, 13, 111. 336 Zhou, Z., and Tang, S., Fenxi Huaxue, 1988, 16, 52. 337 Dan, S. R., and Kas, A. K., Fresenius 2. Anal. Chem., 1988, 329, 794. 338 Liicker, E., Rosopulo, A., Koberstein, S., and Kreuzer, W., Fresenius 2. Anal. Chem., 1987, 329, 31. 339 Liicker, E.., Rosopulo, A., and Kreuzer, W., Fresenius Z. Anal. Chem., 1987,328, 370. 340 Brown, A. A., Lee, M., Kuellemer, G., and Rosopulo, A., Fresenius Z. Anal. Chem., 1987, 328, 354. 341 Suzuki, K. T., Sunaga, H., Kobayashi, E., and Sugihira, N., J.Chromatogr., 1987,400,233. 342 Hewitt, P. J.. Environ. Geochem. Health, 1988, 10, 113. 343 Ebdon. L., Hill, S . , and Jones, P., Analyst, 1987, 112,437. 344 Klein, J., Schmidt, H., Dirscherl, C., and Muntau, H., Fresenius Z. Anal. Chem., 1987, 328, 378. 345 Shuttler, 1. L., and Delves, H. T., J. Anal. At. Spectrom., 1988, 3, 145. 346 Subramanian, K. S., Clin. Chem., 1987, 33, 1298. 347 Dube, P., Krause, C., and Windmueller, L., Analyst, 1989,114, 1249. 348 Brotherton, T. J., Shen, W. L., and Caruso, J. A., J. Anal. At. Spectrom, 1989, 4, 39. 349 Greenberg, R. R., Zeisler, R., Kingston, H. M., and Sullivan, T. M., Fresenius Z. Anal. Chem., 1988, 332, 652. 350 Vehter, M., and Friberg, L., Fresenius 2. Anal. Chem., 1988, 332, 726. 351 HernAndez, L., Melguizo, J.M., Blanco, M. H., and Hernan- dez, P., Analyst, 1989,114.397. 352 Kitto, M. E., Anderson, D. L., and Zoller, W. H., J. Atmos. Chem., 1988,7,241. 353 Infante, R. N., and Acosta, I. L., At. Spectrosc., 1988, 9, 191. 354 Ang, K. P., Tay, B., Gunasingham, H., Teo, A., and Tham, Y., Int. J. Environ. Stud., 1988, 32, 49. 355 Bond, A. M., and Wallace, G. G., Anal. Chem., 1984,56,2085. 356 McGrath, S. P., Soil Use Manage., 1987, 3, 31; Chem. Abstr., 1989, 110, 224564q. 357 Yuan, Z.-N., Shan, X.-Q., and Ni, Z.-M., Huanjing Huaxue, 1987, 6,32. 358 Rygh, G., and Jackson, K. W., J. Anal. At. Spectrom., 1987,2, 397. 359 Landry, J. C., and Celardin, F., Arch. Sci., 1988,41, 199. 360 Meduna, U., and Schaefer, H. P., Labor Praxis, 1988,12,1226. 361 Hall, L., and Chang-Yen, I., Environ.Pollut.. 1989, 56, 51. 362 Aualiitia, T. U., and Pickering, W. F., Talanta. 1988, 35, 559. 363 Beauchemin, D., McLaren, J. W., Mykytiuk, A. P., and Berman, S. S., J. Anal. At. Spectrom., 1988, 3, 305. 364 Hou, C., and Liang, L., Lizi Jiaohuan Yu Xifu, 1988, 4, 293; Chem. Abstr., 1989, 110, 23684%. 365 Campbell, D. O., Energy Res. Abstr., 1988, 13, 14638. 366 Svanidze, Z. S., Sedykh, E. M., Bannykh, L. N., and Myasoedov, B. F., Zh. Anal. Khim., 1987,42, 1989. 367 Rossmann, R., and Barres, J., J . Great Lakes Res., 1988, 14, 188. 368 Henshaw, J. M., Heithmar, E. M., and Henners, T. A., Anal. Chem., 1989, 61, 335. 369 Liu, Y., and Ingle, J. D., Anal. Chem., 1989, 61, 525. 370 Vermeiren, K., Vandecasteele, C., and Dams, R., Analyst, 1990, 115, 17.371 Lavi, N., and Alfassi, Z. B., J. Radioanal. Nucl. Chem., 1989, 130, 71. 372 Seshaiah, K., Rao, P. V., and Naidu, P. R., Proc. Nutl. Acad. Sci., India, Sect. A , 1988, 58, 157. 373 Liu, D., and Liu, R., Fenxi Huaxue, 1988, 16, 716; Chem. Abstr., 1989, 110, 101399g. 374 Hallam, C., and Thompson, K. C., Analyst, 1985, 110, 497. 375 Berndt, H., Schaldach, G., and Klockenkaemper, R., Anal. Chim. Acta, 1987, 200,573. 376 Irth, H., De Jong, G. J., Brinkman, U. A. Th., and Frei, R., Anal. Chem., 1987,59, 98. 377 Munder, A., and Ballschmitter, K., Fresenius Z. Anal. Chem., 1986, 323,869. 378 Wenclawiak, B., Schultze, P., and Uebis, V., Mikrochim. Acfa, Part II, 1984, 3. 379 Isozaki, A., Ueki, K., Sasaki, H., and Utsumi, S., Bunseki Kugaku, 1987,36672.380 Maddalone, R. F., Scott, J. W.. and Frank, J . , Energy Res. Abstr., 1988, 13,49586. 381 Vidal, J. C., Monreal, F.. and Castillo. J., Analyst, 1990. 115, 539. 382 Grudpan, K., and Taylor, C., Talanta, 1989, 36, 1005. 383 Volkan, M., Ataman, 0. Y., and Howard, A. G., Analyst, 1987, 112, 1409. 384 Singh, A. K., and Kumar, T. G., Microchem. J., 1989,40, 197. 385 Fang, Z., RfiiiEka, J., and Hansen, E. H., Anal. Chim. Acta, 1984, 164, 23. 386 Fang, Z., Xu, S., and Zhang, S., Anal. Chim. Acta, 1984, 164, 41. 387 Ang, K. P., Tay, B., Gunasingham, H., Khoo, S., and Koh, C., Int. J. Environ. Stud., 1989, 32, 261. 388 Ishikawa, M., Okoshi, K., Kurosawa, M.. and Kitao. K., in Appl. Ion Beams Muter. Sci., Proc. Int. Symp. Hosei Univ., eds. Sebe, T., and Yamamoto, Y., 12th, 1987 [Publ.19881, Hosei Univ. Press, Japan, pp. 445-451; Chem. Abstr., 1989, 110, 13276j. 389 Li, J., Haiyang Xuebao (Zhongwenban), 1988. 10, 172; Chem. Abstr., 1989, 110, 82131~. 390 Sun, X., and Song, W., Huaniing Huaxue, 1988,7, 65. 391 Smith, R. M., Butt, A., andThakur, A., Analyst, 1985,110,35. 392 Sun, S., Bing, G., Zheng, Y., and Li, J., Fenxi Huaxue, 1987, 15, 159. 393 Wendt, E., Lebensmittelindustrie, 1987, 34, 161. 394 Long, S., and Peng, M., Xiangtan Daxue Ziran Kexue Xuebao, 1988, 10, 43; Chem. Abstr., 1989, 110, 211063a. 395 Ma, H., and Wu, S., Fenxi Ceshi Tongbao. 1988,7,71; Chem. Abstr., 1989, 110, 15288%. 396 Miller-Ihli, N. J., J. Res. Natl. Bur. Stand. (US), 1988, 93, 350. 397 Skurikhin, M., J. Assoc. Off. Anal. Chem., 1989, 72, 294. 398 Karadakhi, T.M., Najib, F. M., and Mohammed, F. A., Talanta, 1987, 34, 995. 399 Rubeska, I., Ebarvia, B., Macalalad. E., Ravis, D., and Roque, N . , Analyst, 1987, 112, 27. 400 Kane, J. S., J. Anal. At. Spectrom., 1988, 3, 1039. 401 Lin, S., Fenxi Huaxue, 1988, 16, 73; Chem. Abstr., 1989, 110, 1652202. 402 Motooka, J. M., Appl. Spectrosc., 1988,42, 1293. 403 Petrov, B. I.. Lesnov. A. E., Shchurov, Yu. A., and Golubeva, E. V., Zavod. Lab., 1988, 54, 13. 404 Demortier, G.. Muter. Res. SOC. Symp. Proc., 1988, 123, 193. 405 KoelIing, S., Kunze, J., and Tauber. C., Fresenius 2. Anal. Chem., 1988,332,776. 406 Yeole, C. G., and Shinde, V. M., Indian J. Chem., Sect. A , 1984, 23, 1053. 407 Date, A. R., Cheung, Y. Y., Stuart, M. E., and Jin, X. H., J. Anal. At. Spectrom.. 1988, 3, 653.408 Al-Swaidan, H. M., Al-Gadi, A., and Abdalla, M. A., Orient. J. Chem., 1988, 4, 221; Chem. Abstr., 1989, 110, 41602f. 409 Mahmud. M. U., Bains, J., and Cote, D.. Anal. Sci., 1986, 2, 311. 410 Kurayasu, H., and Inokuma, Y., Bunseki Kagaku, 1988, 37, 623. 411 Strubel, G., Rzepka-Glinder, V.. and Grobecker, K., Fresenius Z. Anal. Chem., 1987,328, 382. 412 Atsuya, I., Itoh, K.. and Akatsuka, K., Fresenius 2. Anal. Chem., 1987,328, 338. 413 Ebdon, L., and Cave, M. R., Analyst, 1982, 107, 172. 414 Brown, A. A., Halls, D. J.. and Taylor, A., J. Anal. At. Spectrom., 1989,4, 47R. 415 Olayinka, K.O., Haswell, S. J., and Grzeskowiak, R., J. Anal. At. Spectrom., 1986, 1 , 297. 416 Hoenig, M., and Hoeyweghen, P., Anal. Chem., 1986,58,2614. 417 Hinds, M. W., Jackson, K.W., and Newman, A. P., Analyst, 1985, 110, 947. 418 Brown, A. A., Halls, D. J., and Taylor, A., J. Anal. At. Spectrom., 1988,3, 45R. 419 Reisch, M., Nickel, H., and Mazurkiewicz, M., Spectrochim. Acta, Part B , 1989,44, 307. 420 Salin, E. D., and Horlick, G., Anal. Chem., 1979, 51, 2284. 421 Narres, H. D., Mohl, C., and Stoeppler. M., Erdoel Kohle, Erdgas, Petrochem., 1986, 39, 193. 422 Grobecker, K. H., and Klussendorf, B., Fresenius Z. Anal. Chem., 1985,322. 673. 423 Vollkopf, U., Grobenski, Z., Tamm, R., and Welz, B.. Analyst, 1985, 110, 573.ANALYST, JUNE 1991, VOL. 116 567 424 425 426 427 428 429 430 43 1 432 433 434 435 436 437 438 439 440 44 1 442 443 444 445 446 447 448 449 450 45 I 452 453 454 455 456 457 458 459 460 46 I 461. 463 464 465 466 Klussendorf, B., Rosopulo.A., and Kreuzer, W., Fresenius Z. Anal. Chem., 1985, 322, 721. Herber. R. F., Roelofsen. A., Hazelhoff Roelfzema, W.. and Pecreboom-Stegeman, J., Fresenius Z. Anal. Cltem., 1985,322, 743. Mohl, C., Narres, H. D.. and Stocppler, M., Fortschr. Atomspektrom. Spurenanal, 1986, 2, 439. Horvath, Z., Lasztity, A., Szakacs, O., and Bozsai, G . , Anal. Chim. Acta, 1985, 173,273. Stee, R. J., Koirtyohann, S. R., and Taylor, H. H., Anal. Chem., 1986, 58, 3240. Dabeka. R. W., and McKenzie, A. D . , Can. J. Spectrosc., 1986, 31, 44. Veber, M., Gomiscek, S . , and Stresko, V., Anal. Chim. Acta, 1987, 193, 157. Habib, M. M., and Salin, E. D., Anal. Chem., 1985, 57,2055. Nygaard, D. D., and Hill, S . R.. Anal. Lett.. 1979, 12. 492. Zolotov. Y. A., Boydnya.V. A., and Zagruzina, A. N., CRC Crit. Rev. Anal. Chem., 1982, 14, 93. Allen, E. A., Bartlctt, P. K. N., and Ingram, G . , Analyst, 1984, 109, 1075. Van Griekcn. R., Anal. Chim. Acta, 1982, 143,3. Thompson, M.. Ramsey, M. H., and Pahlavanpour, B., Analysr, 1982, 107, 1330. Akagi. T., and Haraguchi, H . . Anal. Chem., 1990, 62, 81. Bem, H . , and Ryan, D. E., Anal. Chim. Acta, 1984, 166, 189. Brueggerhoff, S., Jackwerth, E . , Salewski, S . , Raith. B., Divoux, S . , and Gonsior, B., Spectrochim. Acta, Purr B , 1984, 39. 1181. Eidecker, R., and Jackwerth, E., FreseniuJ Z. Anal. Chem.. 1987, 328, 469. Hiraide. M.. Ito, T., Baba, M., Kawaguchi, H., and Mizuike, A., Anal. Chem., 1980, 52, 804. Cabellero, M., Lopez, R., Cela, R., and Pcrez-Bustamante, J . A..Anal. Ctiim. Arta, 1987, 196, 287. McLeod, C. W., Otsuki, A., Okamoto, K.. Haraguchi, H., and Fuwa, K.. Analyst, 1981, 106, 419. Chakraborti, D.. Adams, F., Van Mol, W., and Irgolic. K., Anal. Chim. A m , 1987, 196, 23. Rama Devi. P., and Rama Krishna Naidu, G . , Analyst, 1990. 115. 1469. Corsini. A., Di Fruscia. R., and Herrmann, 0.. Talanru, 1985, 32. 791. Singh. A. K., Puri. B.. and Rawlley, R., Microchem. J . . 1988, 37, 221. Alvarez de Eulate. M., Montoro, R.. Ybanez, N., and de la Guardia. M., J . Assoc. Off. Anal. Chem.. 1986, 69, 871. Agrawal, Y. K.. and Mehd, G . , J . Indian C h n . Soc., 1985, 62, 699. Kish, P. P., and Balog, I . S . , 212. Anal. Khim.. 1979, 34. 2326. Kalyanararnan. K., and Khopkar, S . . J. Indian Chem. Soc., 1979, 56, 203. Otomo, M., and Singh, R .B.. Anal. Sci.. 1985, 1. 165. Sturgeon. R. E . , Can. J. Spectrosc.. 1987, 32. 79. Koide, M., Hodge, V., Yang, J . , and Goldberg, E . , Anal. Chem., 1987, 59, 1802. Baffi, F., Frachc. R., and Dadonc, A., Ann. Chim. (Rome), 1984, 74. 385. Smits, J.. Nielsen. J.. and Van Grieken, R., Anal. Cliim. Actu, 1979, 111, 215. Abdullah. M. I.. El-Rayis, 0. A., and Riley, J.. Anal. Chim. Acta, 1976, 84,363. Van Berkel, W. W., Overbosch. A. W.. Fecnstra, G., and Maessen. F. J . M. J.. J . Anal. At. Spectrom.. 1988, 3, 249. Sturgeon, R. E., Berman, S . , Desaulniers. J.. Mykytiuk, A.. McLaren, J., and Russell, D.. Anal. Clzem.. 1980, 52. 1585. Brajtcr, K., and Slonawska. K., Anal. Chim. A m , 1986, 185. 271. Kinyama. T.. and Kuroda, R.. MiArochim. A d a , Part I, 1985.40s. Brewer, S . , and Sacks, R., Anal. Chem., 1985, 57. 724. Kulkarni, S . , Joshi. S . , and Soman, S . . J . Indian Chem. Soc., 1984. 61, 777. Kimura, M., Yamashita, H., and Komada. J., Runseki, Kagaku, 1986, 35, 400. King, J . N.. and Fritz, J. S . , Anal. CIwm.. 1985, 57, 1016. Moreira. J.. and Gushikem, Y.. Anal. Chim. Acta, 1985. 176, 263. 467 468 469 470 47 1 472 473 474 475 476 477 478 479 480 48 1 482 483 484 485 486 487 488 489 490 49 1 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 SO8 509 5 10 51 1 5 12 Abollino. 0.- Mentasti. E . , Porta. V., and Sarzanini, C., Anal. Chem., 1990,62,21. Yoshikawa. H., Ishibashi. Y . . Gunji, N., Sasayama, K.. and Misumi, T., Bunseki Kugaku, 1990, 39. T83. Kvitek, R., Evans J., and Carr, P., Anal.C h m . Acra, 1982,144, 93 Samara. C., and Kouimtzis, T.. Anal. Clzim. Actu, 1985. 174. 305. Hartenstein. S . , Christian, G. D., and R6iii.ka. J., Can. J . Spectrosc.. 1985, 30. 144. Karakaya, A.. and Taylor, A.. J . Anal. At. Spectrom., 1989, 4, 261. Hartenstein, S . , RGiiEka, J . , and Christian. G. D., Anal. Chem.. 1985. 57, 21. Malamas. F., Bengtsson, M., and Johansson, G.. Anal. Chrm. Acta, 1984, 160. I . Petrov, B., and Lhivoprstscv, V., Taluntu, 1987, 34, 175. Chwa\towska, J . , and Mom-, E., Tulanra, 1085, 32. 574. Belchev, S . . Arpadjan, S . , Koleva, S . . Kostadinova, M., and Getcheva, K.. Anal. Lett.. 1988. 21. 529. Koide, M., Lee, D. S . , and Stallard, M., A n d . Clrem., 1984,56, 1956. Majidi, V., and Holcombe, J . A., J . Anal. Ar. Specrrom..1989, 4. 439. Majidi, V., and Holcombe, J., Spectrochim. Acta, Part B , 1088, 43. 1423. West, K. J., and Ptlaum, K. T., Talanra, 1986. 33, 807. Welz, B., Atomic Absorption Spectrometry, VCH, Wcinheim, 2nd edn., 1985. Brown, A. A . , Roberts, D . J . , and Kahokola, K. V., J . Anal. Ar. Specrrom., 1987, 2, 201. Betti, M.. and Papoff, P., CRC Crit. Rev. Anal. Chem., 1988, 19, 271. Hussam. A., Anul. Chenr., 1988, 60, 2776. Neuburger. G., and Johnson, D., Anal. Chim. Acra, 1986,179, 381. Ketterer, M., and Reschl. J . , Anal. Cliem.. 1989, 61. 2031. Nordmeyer, F., Hansen L., Eatough, D.. Rollins, D., and Lamb, J.. Anal Chem.. 1980, 52, 852. Joncs, P.. Hobb5. P. J.. and Ebdon, I,.. Analyst, 1984,109,703. D’Ulivo, A., and Chcn, Y., J . Anal. At. Spectrum, 1989,4,319.Slavin, W.. Anal. Chem., 1986, 58, 589A. Bond, A.. Knight, R., and Newman, O., Anal. Clwm., 1988, 60. 2445. Official Method3 of Analym of tile Associalion of Offrc*ial Analytical Chemists, ed. Hilrich, K., Association of Official Analytical Chemists. Arlington, VA, 15th edn., 1990, ch. 9, pp. 246, 247. Shibata, S . , Anul. Chim. Actu, 1976, 82, 169. Villarrcal, M., Porta. L., Marchevsky, E., and Olsina, K., Tulunta. 1986, 33, 375. Hsu. C.. Talanta, 1980, 27, 676. Ishak, C. F., and Pflaum, R. T., Analyst, 1988, 113,941. Rao, T. P., and Ramakrishna. T. V., Anal-vsf. 1982. 107, 704. Agrawal. Y. K., and Desai, T. A., Analyst, 1986. 111. 305. Boltz, D . , and Havlena, E., Anal. Chim. Acta, 1964. 30, 565. Bagdasarov, K., Kovalenko, P., and Shemyakina, M . . Zh.Anal. Khim., 1968, 23, 515. Walsh, J . , and Howie, R., Appl. Genclrem.. 1986, 1. 161. Sugiyama. M., Fujjno, 0.. Kjhara, S., and Matsuj, M., Anal. Chim. Acta, 1986, 181, 159. Toyoda, H.. Uchida, H.. and Takahashi. J . , Runseki Kagaku, 1986, 35. T80. Kosugi, H.. Hanihara, K., Suzuki, T., Himeno. S . , Kawabe, T., Hongo, T., and Morita, M., Sci. Total Environ.. 1986, 52, 93. Sneddon, J.. and Bet-Pera, F.. TrAC, Trendv Anal. Cliem. (Pers. Ed.). 1986, 5, 110. Jones. J. W., Capar, S. G . , and O’Haver, T. C.. Analyst, 1982, 107, 353. Thompson, M., and Ramsey, M. H.. A n a f y f , 1985, 110, 1413. Sunaga, H., Kobayashi, E . , Shimojo, N., and Suzuki, K.. Anal. Biochem.. 1987, 160, 160. Knapp, G.. Muller. K.. Strunz, M., and Wegscheider, W., .I. Anal. At. Spectrom.. 1987.2. 611. Irgolic, K., and Hobill, J . , Spectrochim. Acta, Part B. 1987, 42, 269. Isoyama, H., Okuyama, S . . Uchida. T.. Takeuchi. M.. lida. C . , and Nakagawa. G . , Anal. Sci., 1990. 6. 555.568 ANALYST, JUNE 1991, VOL. 116 513 Solchaga, M., Montoro, R., and de la Guardia, M., J . Assoc. Off. Anal. Chem., 1986, 69, 874. 514 Howard, A. G., and Danilona-Mirzaians, R., Anal. Lett., 1989, 22, 257. 515 Berndt, H., Spectrochim. Acta, Part B, 1984, 39, 1121. 516 Hayase, K., Shitashima, K., and Tsubota, H., Talantu, 1986,33, 754. 517 Halls, D. J., Black, M. M., Fell, G. S . , and Ottaway, J. M., J. Anal. At. Spectrom., 1987, 2,305. 518 Stoeppler, M., Spectrochim. Acta, Part B , 1983, 38, 1559. 519 Harnly, J. M., and Holcombe, J. A., J. Anal. At. Spectrom., 1987, 2, 105.520 Holcombe, J. A., and Harnly, J. M., Anal. Chem., 1986, 58, 2606. 521 Sperling. K.. and Bahr, B.. Fresenius Z . Anal. Chem., 1979. 299, 206. 522 Slavin, W., Manning, D., and Carnrick, G., Anal. Chem., 1981. 53, 1504. 523 Feitsma. K. G., Franke, J. P.. and de Zeeuw. R. A., Analyst, 1984. 109, 789. 524 Van Deijck, W., and Herber, R., Clin. Chim. Acta, 1983, 128, 379. 525 Schlemrner, G., and Welz, B., Spectrochim. Acta, Part B, 1986, 41, 1157. 526 Subramanian, K., Meranger, J., and MacKeen, J., Anal. Chem., 1983,55, 1064. 527 Pruszkowska, E., Carnrick, G., and Slavin, W., Clin. Chem., 1983, 29, 477. 528 Dungs, K., and Neidhart, B., Analyst, 1984, 109, 877. 529 Welz, B., and Schlernmer, G., J. Anal. At. Spectrom., 1986, 1. 119. 530 L’vov, B. V., Spectrochim. Acta, 1961, 17. 761. 531 Hunt, D. T. E., and Winnard, D. A., Analyst, 1986, 111, 785. 532 Shuttler, I. L., and Delves, H. T., J. Anal. At. Spectrom, 1987, 2, 171. 533 Tolg, G.. Analyst, 1987, 112, 365. 534 Pan, Z., Guo, W., and Li, P., Zhonghua Yufangyixue Zasshi. 1986, 20,224; Chem. Abstr., 1986, 105, 185336s. 535 Boiteau, H., Metayer, C.. Ferre, R., and Pineau, A., J. Toxicol. Clin. Exp., 1986, 6, 95; Chem. Abstr., 1986, 105, 36968~. 536 Fishrnan, M., Perryman, G., Schroder, L., and Matthews, E . , J. Assoc. Off. Anal. Chem., 1986, 69, 704. 537 Lum, K., and Callaghan, M., Anal. Chim. Acta, 1986,187,157. 538 Stephen, S. C., Littlejohn, D., and Ottaway, J. M., Analyst, 1985, 110, 1147. 539 Van Staden, J. F., Fresenius Z . Anal. Chem., 1988, 331, 594. 540 Woodget, B., and Franklin, K., Analyst, 1981, 106, 1017. 541 Jagner, D., Josefson, M., and Westerlund, S . . Anal. Chim. Acta, 1981, 129, 153. 542 Sottery, J. P., and Anderson, C., Anal. Chem., 1987, 59, 140. 543 Fernando, A., and Plambeck, J., Anal. Chem., 1989,61,2609. 544 Vos, L., Korny. Z., Reggers, G., Roekens, E., and Van Grieken, R., Anal. Chim. Acta, 1986, 184, 271. 545 Guy, R. D., and Narnaratne, S., Can. J. Chem., 1987,65,1133. 546 Bruland, K., Coale, K., and Mart, L., Mar. Chem., 1985, 17, 285. 547 Batley, G., and Florence, T., J. Electroanal., Chem., 1974, 55. 23. 548 Florence, T. M., Anal. Chim. Acta, 1980, 119, 217. 549 Davison, W.. De Mora, S., Harrison, R., and Wilson, S . , Sci. Total Environ., 1987, 60, 35. 550 Fisher, N. S . , and Fabris, J., Mar. Chem., 1982, 11, 245. 551 Rajamani. K., Meenakshi, S . , and Janaki, W., J . Indian Chem. Soc., 1984, 61, 707. 552 Mohammad, A., and Fatima, N., Chromatographiu, 1987, 23, 653. 553 Saitoh, K., Kobayashi. M., and Suzuki, N., Anal. Chem., 1981, 53, 2309. 554 Jones, P., Hobbs, P., and Ebdon, L., Anal. Chim. Acta, 1983, 149, 39. 555 Haj-Hussein, A. T., Anal. Letf., 1986, 19, 1191. 556 Smith, R. M., and Yankey, L. E., Analyst, 1982,107, 744.. 557 Robards, K., Patsalides, E., and Starr, P., unpublished results. 558 Drasch. G., von Meyer, L., and Kauert, G.. Fresenius Z. Anal. Chem., 1982,311, 695. 559 Hutchins, S., Haddad, P.. and Dilli, S., J. Chromatogr.. 1982. 252, 185. 560 Soroka, K., Vithanage, R., Phillips, D., Walker. B., and Dasgupta, P., Anal. Chem., 1987,59,629. 561 Haring, N., and Ballschrnitter, K., Talanta, 1980, 27, 873. 562 Hefley, A. J., and Jaselskis, B.. Anal. Chem., 1974, 46, 2036. 563 Pinta, M., Modern Methods for Trace Element Analysis, Ann Arbor Science Publishers, Ann Arbor, MI, 1978. p. 8. 564 Pal, B. K., Kabiraj, U., and Ukiluddin. M.. Analyst, 1987. 112, 171. 565 Wilson. D.. Vickers. G.. and Hieftje, G., Anal. Chem., 1987, 59, 1664. 566 Poussel. E., Merrnet, J.-M., Deruaz. D.. and Beaugrand, C., Anal. Chem., 1988, 60, 973. 567 Houk, R.. and Thompson, J., Mass Spectrom. Rev., 1988, 7, 425. 568 McLaren, J. W., Beauchemin, D., and Berman, S . , Spectro- chim. Acta, Part B, 1988,43, 413. 569 Crews, H. M., Dean, J. R., Ebdon, L., and Massey, R. C., Analyst, 1989, 114, 895. 570 Sansoni, B., Brunner, W., Wolff, G., Ruppert, H., and Dittrich, R., Fresenius Z. Anal. Chem., 1988, 331. 154. 571 Weisz, H., and Fritz, G., Anal. Chim. Acta, 1982, 139, 207. 572 Hu, A., Dessy, R. E., and Graneli, A., Anal. Chem., 1983,55, 320. 573 Fang, Z., Xu, S., Wang, X.. and Zhang, S . , Anal. Chim. Acta, 1986, 179, 325. 574 Browner, R. F., and Boorn, A. W., Anal. Chem., 1984, 56, 875A. 575 Brown. M. W., and RGiiEka, J., Analyst, 1984, 109, 1091. 576 Olsen, S., Pessenda, L. C. R., RiiiEka, J., and Hansen, E. H., Analyst, 1983, 108. 905. 577 Wang, X., and Barnes, R. M., J. Anal. At. Spectrom., 1989.4, 509. 578 Jansen, E. B. M., and Demers. D. R.. Analyst, 1985, 110.541. 579 Omenetto, N., Human, H., Cavalli, P., and Rossi. G., Spectrochim. Acta, Part B, 1984, 39. 115. 580 Jenkins, R., Adv. X-Ray Anal., 1987, 30, 29. 581 Luehrmann, M., Wegener, F., and Kettrup, A., Fresenius Z. Anal. Chem., 1986, 323, 132. 582 Talbot, G., and Chang, W., Sci. Total Environ., 1987,66,213. 583 Bartenfelder, D., and Karathanasis, A., Commun. Soil Sci. Plant Anal., 1988, 19, 471. 584 Chisela, F., Gawlik, D., and Bratter, P., Analyst, 1986, 111, 405. 585 Van Tran, L., and Teherani, D., J. Radioanal. Nucl. Chem., 1989, 135, 443. 586 Taylor, A., and Briggs, R. J., J . Anal. At. Spectrom.. 1986, 1, 391. 587 Schaller, K., Angerer, J . , Lehnert, G., Valentin, H., and Wettle, D., Fresenius 2. Anal. Chem., 1987, 326, 643. 588 Slabyj, B., Koons, R., Bradbury, H., and Martin, R., J. Food Prof., 1983, 46, 122. 589 Dabeka. R. W., and Ihnat, M., Adv. Environ. Sci. Technol., 1987, 19, 231. 590 Sperling, K. R., and Bahr, B., Fresenius Z . Anal. Chem., 1981, 306, 7. 591 Lamathe, J., Magurno, C., and Equel, J., Anal. Chim. Acta, 1982, 142, 183. 592 Capar, S. G., Gajan, R., Madzsar, E., Albert, R.. Sanders, M., and Zyren, J., J. Assoc. Off. Anal. Chem., 1982, 65, 978. 593 Subramanian, R., and Sukumar, A., Fresenius 2. Anal. Chem., 1988,332,623. Paper 0102 746 D Received June 19th, 1990 Accepted January 28th, 1991

ISSN:0003-2654    

DOI:10.1039/AN9911600549

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, JUNE 1991, VOL. 116 569 Automated Enzyme Packed-bed System for the Determination of Vitamin C in Foodstuffs Simon Daily, Susan J. Armfield,* Barry G. D. Haggettt and Mark E. A. Downs Laboratory of the Government Chemist, Queens Road, Teddington, Middlesex TWI I OLY, UK A microprocessor controlled flow injection system is described for the determination of vitamin C in foodstuffs. The system is based on amperometric detection at a wall-jet electrode coupled with an ascorbate oxidase packed bed. A commercially available Cartesian robotic auto-sampler-dilutor is used as a means of fully automating the sample handling and dilution. Dithiothreitol (DTT) is used to reduce dehydroascorbic acid t o ascorbic acid and t o stabilize ascorbic acid standard solutions. Initially, the system was connected in series with a high-performance liquid chromatography column and ultraviolet (UV) detector t o allow identification of possible interferents and to allow comparative evaluation of results.The system showed a linear response t o the concentration of L-ascorbic acid in the range 1-200 pg ml-1 and was capable of detecting total vitamin C in a range of foodstuffs at a sample throughput of 15 samples h-1. Correlations t o existing methods of 0.98 were obtained. Keywords: ,Vitamin C determination; enzyme packed bed; ascorbate oxidase; amperometric detection; automated food analysis The physiological roles of vitamin C have been widely reported. 1 The biologically active compounds are L-ascorbic acid (AA) and L-dehydroascorbic acid (DHAA), while vitamin C refers to the sum of these two forms.In addition to occurring naturally in a wide range of foods, vitamin C is commonly added to foodstuffs as an anti-oxidant. The most commonly used chemical methods for the determination of vitamin C have been based on one of three classical methods. The first method relies on the oxidation of AA to DHAA which is derivatized with 2,4-dinitrophenyl- hydrazine and then determined spectrophotometrically.2 The second method is based on the colorimetric titration of AA with 2,6-dichloroindophenol [4-(2,6-dichloro-4-hydroxy- phenylimino)cyclohexa-2,5-dienone].3-~ The third method involves the measurement of a fluorophor produced from the treatment of DHAA with o-phenylenediamine . 3 5 These methods, although they have been adapted to semi-automatic flow injection analysis,3.6 have proved to be complicated, time consuming and non-specific.More recently methods for the determination of AA and DHAA based on HPLC have been developed .7 However, problems with non-specific absorption have limited the applications of the ultraviolet (UV) detec- torss-9 originally employed. These problems have led to the development of dttection by electrochemical methods which offer substantial increases in both sensitivity and selectivity when compared with UV detection.l0-11 In this work an attempt has been made to develop the enzyme reactor bed method reported by Bradberry and Adams.12 The method relies on the sensitivity and selectivity of electrochemical detection coupled with the biological elimination of AA by the enzyme, ascorbate oxidase (AO, E.C.1.10.3.3).Initially, an aliquot of each food sample was taken and divided into two. The first aliquot was passed through an enzyme packed bed that had been previously heat- denatured. An amperometric signal, proportional to the amount of AA plus other electro-oxidizable species (interfer- ents) present in the sample, was produced at the detector. The second aliquot was then passed through a similar bed containing active enzyme where AA was selectively removed giving a second signal due only to the interfering species. The difference between the two signals generated at the detector * Present address: DTI, Environment Unit, 151 Buckingham Palace Road. London SWlW 9SS, UK. t Present address: Applied Research and Consultancy Centre, Putteridge Bury, Hitchin Road, Luton, Bedfordshire LU2 SLE, UK.electrode can be related to the concentration of ascorbic acid. This approach is summarized as follows: At the electrode Glassy carbon AA - + 850 mV versus Interferentsred - Ag- AgC1 In the packed bed A 0 2 A A + 0 2 ---+ Then at the electrode Glassy carbon Interferentsred ---+ 850 mV versus Ag-AgCI DHAA+2H++2e- + Charge = Qtotal [nterferents,,+ ne- 2 DHAA + 2Hz0 Interferents,, + ne- Charge = Qi The charge due to AA can then be estimated from the difference between the two charges measured QAA = Qtotal - Qi In the work reported here, vitamin C is determined after the DHAA present in the sample is converted to AA by extraction of the original samples with a solution of dithio- threitoll3, which also serves to stabilize the AA.l4,1S During the preliminary stages of system development, a high-performance liquid chromatography (HPLC) column and UV detector were connected in series with the enzyme bed and electrochemical detector in order to facilitate identification of possible interferents and to generate com- parative results.Experimental Reagents Mobile phase. Potassium dihydrogen orthophosphate (15 mmol dm-3, pH 5.0) was prepared by dissolving 2.04 g of H2P04 (Fisons, Loughborough, UK) in 1000 ml of purified water. The solution was then filtered and de-gassed with helium.570 ANALYST, JUNE 1991. VOL. 116 Extracting solution. Dithiothreitol (DTT) (1 mmol dm-3, Sigma, Poole, Dorset, UK) was prepared daily by dissolving 0.154 g of DTT in 1000 ml of the mobile phase. Ascorbic acid standard solution.A 200 pg ml-1 (1.136 mmol dm-3) stock solution was prepared by dissolving 20 mg of AA (Fisons) in 100 ml of the extracting solution. A stock solution was prepared daily and automatically diluted to the range 1-20 pg ml-1 when required. Ascorbate oxidase (250 U mg-1). This enzyme from Curcurbita sp. was obtained from Borhringer (Lewes, Sussex, UK). (1U = 16.67 nkat). Procedures Preparation of immobilized ascorbate oxidase Aminopropyl controlled-pore glass beads (600 mg, 125-177 pm, pore diameter 500A, Pierce, Chester, UK) were added to 6 ml of a 2.5% (v/v) aqueous solution of glutaraldehyde (Sigma) and mixed on a roller mixer for 10 min. The slurry was de-gassed in a freeze-drying chamber at -40 "C for 30 min then replaced on the roller mixer for a further 1 h at 25 "C.After washing with distilled water, the slurry was again de-gassed in the freeze-drier for 30 min. The excess water was removed and A 0 (10 mg) dissolved in 10 ml of 0.1 mol dm-3 tris(hydroxymethy1)aminomethane hydrochloride buffer (pH 8) was added. The mixture was kept on the roller mixer at 4 "C over-night after which the non-immobilized enzyme was removed by washing with distilled water. The immobilized enzyme was packed into a flow-through Kel-F bed (2 mm i.d. X 40 mm, Oxfq-d Electrodes, Abingdon, UK) held by 20 pm stainless-steel mesh frits. When not in use the bed was stored refrigerated at 4 "C. A second bed, used as a control, was packed with controlled-pore glass prepared as above with heat-denatured AO.Sample extraction For this study, four foodstuffs were analysed extensively. Samples containing an estimated 50 pg of vitamin C were weighed, i.e., orange juice (1.0 g), grapefruit juice (1.0 g), instant mash potato powder (0.5 g) and freeze-dried brussel sprouts (0.1 g). The samples were shaken with extracting solution (30 ml), filtered (grade 541, Whatman, Maidstone, Kent, UK) and bulked to 50 mi. Each extract was then re-filtered through a syringe filter (0.2 pm Acrodisk, Gelman, Northampton, UK) directly into a 2 ml amber glass auto- sampler vial (Chromacol, London, UK) which was then sealed. Analysis was initiated immediately in order to minimize sample decay. Instrumentation The apparatus is shovw in Fig. 1. The experiment was controlled from a personal computer (Olivetti M24SP).Experimental parameters were entered at the computer and passed to an interface rack (Imperial College Chemistry Microprocesor Unit, London, UK) via an RS232 serial link. The rack was of a modular design, based on a Z80 micro- processor controlling an internal potentiostat, timer, digital to analogue and analogue to digital converters for the monitoring of the electrochemical detector. It also had control over sample handling via a second RS232 port to an autosampler (Gilson 222, Anachem, Luton, UK) and dilutor (Gilson 401, Anachem), and two six-port pneumatically operated injection valves (Rheodyne 7010P, Anachem). The autosampler was used for automatic dilution and for filling of the injection loop with calibrant and sample solutions, and operated indepen- dently of the rack. This arrangement allowed the dilution of one sample while the rack monitored another.One six-port valve was set up as a conventional injection valve with a 20 PI loop and the other as a two-column selector for switching the Fig. 1 Experimental apparatus for the determination of vitamin C. Chromatographic equipment in the shaded area was used initially to identify interferents and to obtain comparative results. WE, Working electrode; RE, reference electrode; and CE. counter electrode Fig. 2 Wall-jet electrode sample stream either through the enzyme packed bed or through the dummy bed containing inactive enzyme. The controlling software for both the microcomputer and the interface rack was written in Pascal (Prosper0 Software, London, UK), the rack program being cross-compiled for the 280 processor and loaded serially to the rack.The rack program controlled the systems hardware, monitored the electrochemical detector and passed data to the microcom- puter for data handling (smoothing and integration) , storage and presentation. An HPLC pump (LKB 2150, Pharmacia, Milton Keynes, UK) delivered the mobile phase at a constant flow rate of 1 ml min-1 to the detector, which was a glassy carbon wall-jet electrode as shown in Fig. 2. The potential of the electrode was 850 mV versus Ag-AgC1 (saturated KCl). A platinum counter electrode was situated in the outlet stream. The electrodes and cell block were purchased from Oxford Electrodes, Abingdon, UK. Initially the electrochemical cell w$s connected in series with an HPLC column (PL-SAX, 1000A, 8 pm, 150 mm X 4.6 mm, Polymer Laboratories, Church Stretton, Shropshire, UK) and a UV/VIS detector (Spectra-Physics, Model SP8450, St Albans, UK) at 251 nm.The outlet from the UV detector was connected directly to the electrochemical cell. Tubing with & in 0.d. x 0.03 in i.d. was used. Results and Discussion Stability of AA The accurate measurement of vitamin C has always been hampered by its instability, as it is readily oxidized and is photosensitive, especially in dilute solutions. Therefore , a variety of physical methods as well as antioxidants and chelating agents have been used to increase its ~ t a b i l i t y . ~ ~ ? ' ~ The use of DTT has been reported not only as a reducing agent of DHAA but also as a stabilizer of AA.Much of theANALYST, JUNE 1991, VOL. 116 571 work on the stabilization of AA was concerned with preserv- ing standard solutions or reference samples for days to weeks. For the present study only stabilization over the few hours required by long automated runs was of concern. Repeated over-night sampling of standard solutions of AA with and without DTT at various concentrations showed that a 0.1 mol dm-3 solution of DTT would stabilize a 100 pg ml-1 solution of AA for more than 10 h (Fig. 3) without appreciable deterioration in the concentration of AA. A similar sample with no DTT decayed to about 30% of its original value of AA over the same period. This stabilization should provide more than adequate time in which to carry out most analyses without significant loss of AA in standard solutions. Determinations of Vitamin C Values for the concentration of vitamin C found in the four food samples studied were initially determined with the UV and electrochemical detectors simultaneously by using chro- matographic separation.The chromatographic apparatus (shown in the dotted area in Fig. 2) was then removed and identical samples were determined by use of the switched enzyme bed technique with electrochemical detection. These results were compared with values obtained using two methods given by the Association of Official Analytical Chemists (AOAC) (titration with 2,6-dichloroindophen013.~ and a semi-automated method based on o-phenylenedi- amine3-6). The results obtained are summarized in Table 1.The electrochemical detector displayed higher specificity towards AA than the UV detector but also showed a higher sensitivity to DTT. Fig. 4 shows outputs from the UV and electrochemical detectors for a sample containing 14 pg ml-1 of AA and 1 mmol dm-3 of DTT. In addition to the difference in the ratio of AA to DTT, it is also interesting to note the reduction in interfering peaks obtained using the electrochem- ical method. Removal of the chromatographic system considerably decreased the sample analysis time from 10-12 min to 3 min. A linear calibration graph for L-AA without HPLC separation was produced in the range 1-200 pg ml-1 with a correlation 70 I 1 A 30 - 20 - 10 0 1 2 3 4 5 6 7 8 9 10 1 1 Time/h Fig. 3 0.1 mmol dm--7 DTT; and B, no DTT Decay of L-ascorbic acid (100 pg ml-*) in the presence of: A.coefficient of 0.995. Fig. 5 shows a typical output from a sample of instant mash potato after passing through the inactive and active bed using this approach. Enzyme Packed-bed Efiiciency and Stability The efficiency of the packed bed was estimated by determin- ing the AA from a calibration series with both the UV and electrochemical (EC) detectors. The samples were first passed through a dummy bed containing no A 0 and then through the active enzyme (AO) packed bed to remove AA. Plotting the responses of the two detectors against each other for both calibration series and comparing gradients gave an estimation of the efficiency of the immobilized enzyme of 99.9% (Fig. 6). This efficiency for the conversion of AA was reproduced with all the batches of immobilized enzyme that were prepared and occurred at up to 200 pg ml-1 of AA, far greater than that likely to be found in any foodstuffs.An excellent correlation between the two methods of detection (Y = 0.994) was observed. The immobilized enzyme remained fully active for 6 , 0.12 f O.I0 2 2 5 0.08 0.06 0 200 400 600 80010001200 Ti me/s Fig. 4 Chromatograms of instant mash potato using ( a ) UV and (b) electrochemical detection showing peaks for 1, D I T and 2, AA. Sample of 0.5 g potato in 50 ml extracting solution was used containing an expected 14 pg ml-1 of vitamin C Table 1 Vitamin C in sampled foodstuffs by five methods. Results are based on averages of at least 15 assays of each sample.Each analysis was carried out in duplicate. The standard deviation for each method is shown in parentheses. OPD = o-phenylenediamine method; 2,6-DCIP = 2.6-dichloroindophenol method; HPLC-UV = HPLC method with UV detection; HPLC-EC = HPLC method with electrochemical detection; and EPB-EC = enzyme packed bed method with electrochemical detection Vitamin C content/mg per 100 g Sample OPD 2,6-DCIP HPLC-UV HPLC-EC EPB-EC Orange juice 38.0( 3.1) 34.6( 3 -3) 35.4(2.7) 37.3(2.6) 36.5( 1.4) Grapefruit juice 33.1 (1.9) 29.6( 2.2) 32.1(3.1) 32.9( 2.8) 34.8( 0.8) Mash potato powder 152.4(7.5) 148.7( 9.5) 148.1(6.2) 160.2( 7.6) 142.7( 7.9) Brussel sprouts 558.6( 17.1) 552.1(18.2) 596.5(20.5) 547.7(17.2) 561.4(19.0)572 ANALYST, JUNE 1991, VOL. 116 0.20 0.18 %0.16 % 0.14 a 0 0.12 0.10 0.08 I I I I I I I I I 0 20 40 60 80 100 120 140 160 180 Ti me/s Fig.5 Electrochemical detector outputs for a sample of instant mash potato (0.5 g in 50 ml of extracting solution) through 1, inactive bed and 2, active bed months when stored at 4 "C between analyses and for at least 200 assays when in use. The efficiency of the enzyme bed decreased only slightly (99.7%) when the procedure described above was carried out using the stereoisomer of L-AA, D-isoascorbic acid (erythorbic acid). This meant that although it is physiologically inactive , D-isoascorbic acid was measured as vitamin C using this method. This should not, however, be seen as a major problem, as the standard methods currently used are also unable to distinguish between the two isomers.Previous work carried out on the identification of possible contaminants of AO17 showed that there are none commonly found in foodstuffs that are likely to affect seriously the performance of the enzyme. Conclusion This method of analysis appeared to work well for more than 30 different foods and beverages that have been considered briefly (unpublished work), as well as the major four reported here. The method provides a rapid sample analysis of <3 rnin (1.5 min per injection compared with 5-10 min for many of the HPLC methods that have been published) and when used in conjunction with the automated instrumentation it provides high sample throughput (15 samples h-1). Sample and mobile phase cleanliness and quality, vital for HPLC methods, were less important for this method.Using the method described, the wall-jet electrode was operated for at least 500 analyses without the need for cleaning. The use of DITT without HPLC separation has proved to be capable, on occasions, of producing spurious results due to a decrease in the concentration of DTT of up to 8% when passed through the active enzyme bed. This appears to be caused by either nonspecific absorption of DTT by the packed bed, or the active breakdown of DTT by AO. As might be anticipated, initial results indicate that the former is more likely. This problem has been addressed by saturating the packed bed with DTT (0.1 mol dm-3 DTT was passed through the bed for 30 min followed by distilled water for 60 min). This work along with further optimization of the enzyme-electrode buffering system is continuing in order to determine conditions under which the enzyme will operate at high efficiency while allowing optimum selectivity of the electrochemical detector towards AA.280 1 ++ 40 0, n 0 10 20 30 40 50 60 Peak area found by UV detection (arbitrary units) Fig. 6 Comparison of EC and UV detectors with + , inactive and x , active A 0 beds placed between them. Efficiency of bed (99.9%) calculated as the ratio of the two gradients The authors thank K. Thurlow and colleagues from the Nutrition and Microbiology Division of The Laboratory of the Government Chemist (LGC) for their help with vitamin analyses, and the Imperial College Chemistry Microcomputer Unit for their technical assistance with the computer hardware and software.This work was undertaken as part of the Validation of Analytical Measurement (VAM) programme of the LGC funded by the Department of Trade and Industry. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 References Vitamin C (Ascorbic Acid), eds., Counsell, J. N., Hornig, D. H., Applied Science Publishers, London and New Jersey, 1981. Roe, J. H., Mills, M. B., Oesterling, M. J., andDamson, C. M., J. Biol. Chem., 1948, 174, 201. Official Methods of Analysis of the Association of Official Analytical Chemists, ed. Honvitz, W., Association of Official Analytical Chemists, Arlington, VA, 5th edn., 1990. Tillmans, J., Hirsh, P., and Siebert, F. Z., 2. Lebensm. Unters. Forsch., 1932, 63, 21. Deutsch, M. J., and Weeks, C. E., J. Assoc. Off Anal. Chem.. 1965,48, 1248. Egburg, D. C., Potter, R. H., and Heroff, J. C., J . Assoc. Off Anal. Chem., 1977, 60, 126. Polesello, A., and Rizzolo, A., J. Micronutrient Anal., 1986,2, 153. Rouseff, R., Liquid Chromatography of Food & Beverage, Academic Press, New York, 1979, vol. 1, pp. 161-177. Wills, R. B. H., Wimalasiri, P., and Greenfield, H., J. Agric. Food Chem., 1984,32, 836. Pachla, L. A., and Kissinger, P. T., Anal. Chem., 1976,48.364. Wilson, C. W. 111, and Shaw, P. E., J. Agric. Food Chem., 1987, 35, 329. Bradberry, C. W., and Adams, R. N., Anal. Chem., 1983, 55, 2439. Okamura, M., Clin. Chim. Acta, 1980, 103,259. Doner, L. W., and Hicks, K. B., Anal. Biochem., 1981, 115, 225. Margolis, S. A. and Black, I., J. Assoc. Off Anal. Chem., 1987, 70, 806. Maeda, E. E., and Mussa, D. M. D. N., Food Chemistry, 1986, 22, 51. Greenway, G. M., and Ongomo, P., Analyst, 1990, 115, 1297. Paper Of05781 J Received December 27th, 1990 Accepted February 18th, 1991

ISSN:0003-2654    

DOI:10.1039/AN9911600569

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, JUNE 1991, VOL. 116 573 Direct Reductive Amperometric Determination of Nitrate at a Copper Electrode Formed ln Situ In a Capillary-fill Sensor Device Arnold G. Fogg, S. Paul Scullion and Tony E. Edmonds Chemistry Department, Loughborough University of Technology, Loughborough, Leicestershire LE 1 I 3TU, UK Brian J. Birch Unilever Research, Colworth House, Sharnbrook, Bedfordshire MK44 ILQ, UK A method has been developed for determining nitrate amperometrically by direct reduction at a freshly deposited copper electrode surface in a capillary-fill device (CFD). Copper(l1) is added to the nitrate sample which is then taken up into the device. The potential of the screen-printed carbon electrode is held at -0.75 V versus the screen-printed silver reference electrode. At this potential, copper is plated onto the carbon electrode forming a freshly prepared copper electrode.At the same time dissolved oxygen is reduced. The potential is then scanned to more negative potentials and the signal at -0.90 V, due to the reduction of the nitrate, is measured. The method for determining nitrate given here is preliminary to the production of CFDs in which chemical reagents, copper sulphate and potassium hydrogen sulphate (used to produce the acidity), are screen-printed or otherwise coated onto the upper plate within the device. Keywords: Capillary-fill device; disposable sensor; nitrate determination; amperometric detection; in situ copper electrode Several of the methods that have been developed for use on-line in flow injection with amperometric detection appeared to be ideally suited for use in the capillary-fill device (CFD) that has been developed and patented by Unilever Research.These methods were listed in the introduction to the previous paper in the present series,l together with a description of the Unilever device. In this previous paper1 a preliminary study of the adaptation of a flow injection method for the amperometric determination of phosphate, based on its on-line reaction with an acidic molybdate reagent, followed by the electrochemical reduction at a glassy carbon electrode of the 12-molybdophosphate formed, for use in the CFD, was described. Pre-formed 12-molybdophosphate was shown to respond reproducibly and rectilinearly within the device. Recently, flow injection methods for the determination of nitrate, based on an on-line nitration reaction2 and on its on-line reduction to nitrosyl chloride ,3 have been developed.However, these methods require the injection of concentrated sulphuric acid and cannot be adapted easily for use in CFDs. The direct reduction of nitrate is not generally possible at the more commonly used electrodes, such as glassy carbon, platinum, gold and mercury electrodes. However, several voltammetric methods have been reported for the determina- tion of nitrate based on the use of solid electrode materials that are capable of catalysing the reduction of nitrate.4-13 Davenport and Johnsonh.7 used a rotating cadmium electrode which gave a linear response over a narrow range of nitrate concentration and a limit of detection of 1 x 10-4 mol dm-3 for nitrate.Bodini and Sawyer4 observed that the reduction of nitrate was catalysed by the simultaneous deposition of copper and cadmium at a pyrolytic graphite electrode and obtained a limit of detection of approximately 1 x 10-6 mol dm-3 nitrate. Johnson and Shenvood8 used a rotating cadmium electrode coated with a layer of metallic copper to determine nitrate down to 1 X 10-4 mol dm-3 and adapted the technique for use with high-performance liquid chromatography.' Xing and Scherson1(),11 described a rotating ring-disc electrode method for the determination of nitrate in acidic media. Their method i s based on the measurement of the ring currents associated with the oxidation of nitrite ions that are generated by the reduction of nitrate ions on a gold disc electrode covered by a layer of underpotentially deposited cadmium. An advantage of this technique is that the background currents for the oxidation process are significantly lower than those for the reduction of nitrate, and limits of detection were of the order of 20-30 ppb of nitrate.The use of copper cathodes for the direct reduction of nitrate has also been investigated.5.12 Pletcher and Poor- abedil2 found that the reduction was particularly sensitive to halide ions which had the effect of shifting the nitrate reduction wave to more negative potentials. Albery et aZ.5 used a packed bed wall-jet electrode to measure nitrate in a flowing stream and were able to re-generate the electrode after each measurement; reduction is only reproducible on a fresh copper surface.Almhofer and Frenzel13 described recently a flow injection method for the determination of nitrate using a tin electrode as the amperometric detector. The measurement range was 5 X 10-5-1 X 10-2 mol dm-3 of nitrate but the response was non-linear and chloride, nitrite and sulphate interfered when they were present at concentrations greater than the nitrate. All the methods mentioned above require solutions to be de-oxygenated before the measurement is made and the electrode surface had to be re-formed between measurements owing to poisoning of the electrode surface. Methods using cadmium are undesirable in a hydroponic environment owing to possible contamination problems. As indicated above, the methods of determining nitrate in a flow injection system273 involved the injection of concentrated sulphuric acid and are unsuited for easy adaptation for use in the CFD.For that reason the flow injection method of Albery et aZ.5 has been adapted here for that purpose. The work of both Pletcher and Poorabedi12 and Albery et aZ.5 has indicated that nitrate can be reduced directly at a copper electrode but only if the electrode surface is freshly prepared. In the method developed here copper ion is added to the nitrate sample and copper is deposited freshly on the screen-printed carbon electrode in the CFD before the device is used to determine nitrate reductive1 y . Experimental The CFDs have been described in detail previously.' They were filled by dipping the ends of the devices that were remote from the electrode connections into the appropriate solution such that the solution was taken up by capillary action.The574 ANALYST, JUNE 1991, VOL. 116 CFDs were placed flat on the bench before a potential sweep or step was applied. Details of the voltammetric experiments applied in the present study have been described in full previously.' Reagent Solutions Copper(r1) sulphate pentahydrate. A 1 x 10-1 mol dm-3 solution was prepared by dissolving 12.48 g of CuS04.5H20 in 500 ml of distilled water. Potassium chloride (KCl). A 1 x 10-2 rnol dm-3 solution was prepared by dissolving 0.373 g of KCl in 200 ml of distilled water and diluting to 500 ml. Potassium nitrate (KN03). A 1 x 10-2 rnol dm-3 solution was prepared by dissolving 0.506 g of KN03 in 200 ml of distilled water and diluting to 500 ml. Sulphuric acid (2 rnol dm-3 H2S04).A 2 rnol dm-3 solution was prepared by carefully adding 54.3 ml of the concentrated acid to 300 ml of water, cooling and diluting to 500 ml. Solutions for voltammetry were prepared by adding appropriate amounts of the stock solutions (described above) to a 50 ml calibrated flask and diluting to the mark. When testing the effect of various ions on the response, a stock solution of either 5000 or lo00 mg 1-1 of the ionic component was prepared and the appropriate amount was added to the calibrated flask before dilution. Results and Discussion The reduction of nitrate at a freshly polished copper rod electrode gave a fairly well defined wave with a peak potential of -0.49 V in 0.5 mol dm-3 sulphuric acid (see Fig.1). The effect of chloride ions on the nitrate reduction at this electrode is also shown in Fig. 1; the peak potential is shifted to increasingly negative potentials and eventually the peak merges with the background current as the chloride concentra- tion is increased. The small peak at -0.36 V, which was not present when the solution had been previously de-oxygen- ated, was also dependent on the chloride concentration, being higher and narrower when chloride was present. Pletcher and Poorabedil2 investigated the reduction of nitrate at a copper electrode and showed that in the over-all reaction the nitrate was reduced to ammonia. The reaction was shown to be 60 50 40 f ? 30 3 u controlled by diffusion at high acidities 20 10 0 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 PotentialN versus SCE Fig. 1 Effect of chloride on the linear sweep voltammogram for the reduction of nitrate at a newly polished copper rod electrode.Nitrate concentration, 1 X mol dm-3; sulphuric acid concentration, 0.5 mol dm-3; scan rate, 5 mV s-1. Chloride concentration: A, 0; B, 0.1 x 10-3; C, 1.0 x 10-3; and D, 2.5 x 10-3 mol dm-3 (>0.1 rnol dm-3 perchloric acid) but no reaction could be detected at a pH>3. They speculated that the effect of chloride ions on the half-wave potential of the nitrate reduction was due to a double-layer effect, with the chloride ions being adsorbed at the electrode. Their attempts to prove this hypothesis by double-layer capacitance experiments were unsuccessful due to the presence of faradaic currents over most of the potential range.It was envisaged in the present work that any sensor employing a copper electrode would be prone to the formation of an oxide film on storage,l4 and that reproducibility would be improved by generating the copper electrode in situ immediately prior to use. Copper was electrolysed from a 1 rnol dm-3 sulphuric acid solution onto glassy carbon and screen-printed silver and carbon electrode surfaces, and the reduction of nitrate at these surfaces was studied. As shown by the results in Table 1, the surface on which the copper was deposited had a marked effect on the nitrate reduction peak potential at the screen-printed electrodes, presumably due to the greater resistance of these screen-printed electrodes. The screen-printed silver electrode gave an additional peak at approximately -0.3 V on the first scan which was not observed in subsequent scans.Oxide formation on the silver surface could have occurred on exposure to the atmosphere.14 Hitchman and co-~orkers15,16 investigated the use of silver electrodes for the potentiometric determination of amino acids and found that chemical or electrochemical cleaning of the electrode improved the repeatability of the results. Whatever the cause of the additional peak it was decided here to concentrate on using the screen-printed carbon ink elec- trodes in which the carbon overlaid a silver coating; these electrodes have a lower resistance than those in which carbon is screen-printed directly onto the substrate. Table 1 Effect of electrode substrate on nitrate reduction at a copper plated electrode.Scan speed, 10 mV s-I; plating time, 2 min at -0.5 V (-0.3 V for silver electrode); solutions de-oxygenated Peak potentialN versus SCE Bare electrode surface Glassy carbon -0.490 -0.550 Screen-printed silver -0.51s Screen-printed carbon on silver Screen-printed carbon Wave poorly defined 1 .o 0.8 2 0.6 2 E 0.4 0.2 I 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 PotentialN versus SCE Fig. 2 Typical linear sweep voltammograms for the reduction of nitrate at a copper plated screen-printed carbon ink on silver electrode. Sulphuric acid concentration, 0.5 mol dm-3; scan rate, 10 mV s-1. Nitrate concentration: A, 0; B, 1.0 X C, 2.5 X lo-'+; D, 5.0 x 10-4; E, 7.5 x 10-4; and F, 10.0 x mol dm-3ANALYST, JUNE 1991, VOL. 116 575 De-oxygenation of the plating solution made little dif'fer- ence to the signals obtained.Typical voltammograms obtained for nitrate reduction at a copper plated screen- printed electrode are shown in Fig. 2; the peak at -0.3 V can be removed by de-oxygenation. The use of differential-pulse voltammetry gave a better separation of the oxygen and nitrate peaks but some overlap was still present. The voltammetric response of nitrate reduction over a measured concentration range of' 1 x 10-4-1 x 10-3 rnol dm-3 in non-de-oxygenated solutions is shown in Table 2. Chloride shifted the nitrate reduction peak to more negative potentials, as with the copper rod electrode, but the shift was proportion- ately greater at an equivalent chloride concentration.The nitrate reduction peak became indistinguishable from the background reaction at a chloride concentration of 5 x 10-4 rnol dm-3 as compared with a similar effect at a chloride concentration of 5 x 10-3 mol dm-3 when using the copper rod electrode. It was found that nitrate could be measured directly in the copper plating solution, because the peak potential for copper deposition was 200 mV more positive than that for nitrate reduction (Fig. 3). Thus, it should be possible to add copper(ii) sulphate to nitrate samples, which are to be presented to the CFD, and to deposit copper from the sample solution before determining the nitrate Concentration. The effect of chloride on the response at the screen-printed Table 2 Voltammetric characteristics of nitrate reduction at a copper plated carbon ink on silver electrode (all potentials versus SCE) Nitrate concentration/ 10kJ rnol dm--' 1 2 4 6 8 10 LSV* D PVt E,IV i&A -0.55s 30 - 0.540 82 - 0.545 170 -0.565 300 -0.570 45s - 0.550 640 E,IV idClA -0.530 250 -0.530 570 -0.535 1250 -0.455 1950 -0.545 2700 -0.540 3450 * Linear sweep voltammetry. Scan speed, 10 mV s-I.t Differential-pulse voltammetry. Scan speed, 2 mV s - * ; pulse amplitude, 25 mV; and pulse interval, 1 s. 1.6 1.4 1.2 Q 1 E 0.8 L 3 0.6 0.4 0.2 0 -0.8 -0.6 -0.4 -0.2 0.0 PotentialN versus SCE Fig. 3 Linear sweep voltammograms of a solution containing copper(i1) and nitrate at a screen-printed carbon ink on silver electrode. Sulphuric acid Concentration. 0.5 rnol dm-3: copper sulphate concentration, 1 x 1W2 rnol dm-3; nitrate concentration, 1.0 X mol dm-3; scan rate, 5 mV s-1.Chloride concentration: A, 0; B , 1 x 10-3; C, 2.5 x 10-3; and D, 7.5 x 10-3 rnol dm-3 electrode, also shown in Fig. 3, is to heighten and narrow both peaks, with the copper peak potential being relatively unchanged whilst the nitrate peak potential was shifted to more negative values with increasing chloride concentration. Attention now was turned to studying the determination of nitrate in the CFD. The effect of chloride on the copper deposition process in the CFDs was to heighten and narrow the peak as shown in Fig. 4. Typical voltammograms obtained for nitrate reduction in a CFD, with and without the addition of chloride, are shown in Fig. 5. Two main peaks were ob- tained; the copper deposition peak at approximately -0.58 V and the nitrate reduction peak at approximately -0.88 V.The peak potentials obtained are typically more negative than those obtained in bulk solution, presumably due to the nature of the reference electrode [Ag-Ag+ as opposed to the saturated calomel electrode (SCE)]. It is evident that the presence of chloride is beneficial in that a complete separation of the copper and nitrate peaks can be achieved, making 200 150 $. ? : 100 3 0 50 0 -0.2 -0.4 -0.6 -0.8 -1.0 PotentialN versus internal reference electrode Fig. 4 Linear sweep voltammograms showing the effect of chloridc on the copper deposition process in a CFD. Scan ratc. 2 mV s-I; sulphuric acid concentration, 1 rnol dm--'; chloride concentration: A and C, 0; B and D, 2 x 10k4 mol dm--3.Copper(i1) concentration. A and B. 0; C and D , 5 x 10-3 rnol dm-3. No nitrate added 250 200 5 1 5 0 . w 2? 3 100 50 0 B -0.2 -0.4 -0.6 -0.8 -1.0 Potent i a IN versus i n t e r n a I reference electrode Fig. 5 Linear sweep voltammograms showing the effect of chloridc on the reduction of copper(i1) and nitrate in a CFD without a delay during the scan to pre-reduce the nitrate. Scan rate, 2 mV s-I; sulphuric acid concentration, 1 rnol dm-3; nitrate concentration, 1.0 x 10-3 mol dm-3; copper(i1) concentration. 5 X lo-' rnol dm-3. Chloride concentration: A, 0: and B. 2 X rnol dm-'576 0.18 0.16 0.14 a 2 €0.12 2 50.10 0 0.08 0.06 0.04 ANALYST, JUNE 1991, VOL. 116 - - - - - - - - measurements easier. An estimate of the peak area showed no change in the amount of charge passed during the deposition process, but the rate at which it was passed increased as chloride concentration increased.Tam and Christiansen17 studied the effect of chloride in copper plating baths on the electrochemical processes occurring and found that chloride enhanced the rate of deposition of copper on platinum electrodes. They attributed this rate enhancement to the formation of a metakhloro-copper(u) complex which they believed lowered the activation energy for the reduction of cu2+. Another possible explanation for the changes in the shape of the voltammograms upon addition of chloride is the stabilization of the reference electrode. The silver ions produced at the counterheference electrode would be precipi- tated out of solution by the chloride ions present, thereby reducing the change in the silver electrode potential.It should be noted, however, that the concentration of silver ion above the silver electrode could become fairly large during the period of copper deposition. Calculations show that this concentration of silver ion could be as great as 1.47 x 10-2 rnol dm-3. A chloride concentration of 1 X 10-4 mol dm-3 would not be expected to reduce the silver ion concentration significantly; in the thin-film device the lateral diffusion of chloride is negligible. In previous work involving the develop- ment of a CFD for the determination of phosphate,' a 1 x 10-1 mol dm-3 chloride concentration was adopted for use in the device and the potentials obtained were approximately those that would be expected with a silver-silver chloride reference electrode. The reduction potentials obtained here are 400 mV more negative than those obtained under semi-infinite diffusion conditions, which is more consistent with the standard potential for the silver-silver ion couple.By increasing the chloride concentration the nitrate reduc- tion peak is shifted to more negative potentials and the peak is increased in height and decreased in width. Again it is thought that a stabilization of the reference potential may be respons- ible for these effects. A greater tolerance to large concentra- tions of chloride was apparent with the CFD, as is shown in Fig. 6; the nitrate reduction peak is clearly distinguishable from the background reaction even at a chloride concentration of 1 x 10-2 rnol dm-3, in contrast to the situation with the screen-printed electrode in bulk solution (see Fig.3). Thus, the major difference between the measurements in the CFDs and measurements in bulk solution is the greater degree of separation between the nitrate reduction peak and the background reaction. This is thought to be due to differences in the peak potentials obtained when using thin-layer cells. A comparison of the equations for peak potentials under both semi-infinite diffusion and thin-layer conditions shows that reductions occur at more positive values in the latter instance. For reversible electrode reactions there is little difference in the peak potential for semi-infinite diffusion conditions: E,, - EO = -28.5/n millivolts at 25 "C (where n = number of electrons, E,, = peak potential for the cathodic process and Eo = standard electrode potential) and for thin-layer conditions E,, = Eo. For irreversible electrode reactions, however, the ana- logous equations for peak potential are: for semi-infinite diffusion conditions an,Fv E -Eo= - - RT [ 0.78 + In (s) + In (F)'] PC arn,F and for thin-layer conditions log[ ] 2.303 RT E -Eo=---- PC nllF an,F(-v)V where ar = transfer coefficient; n, = number of electrons involved in the rate determining step; v = scan rate in V s-1; o.20 I 0.02 1 ' I I I 1 -1.3 -1.2 -1.1 - 1 -0.9 -0.8 PotentialN versus internal reference electrode Fig.6 Linear sweep voltammogram showing the effect of chloride on the reduction of nitrate at an in situ generated copper electrode in a CFD.Scan rate, 2 mV s-1; sulphuric acid concentration, 0.5 rnol dm-3; nitrate concentration, 1 X rnol dm-3; copper(r1) concentration, 1 X rnol dm-3. Chloride concentration: A, 0; B, 2.5 x 10-3; C, 5 x 10-3; and D, 10 x 10-3 mol dm-3 Table 3 Comparison of the peak potentials obtained for irreversible electrode reactions at different values of ko and (2: Epc - EON Semi-infinite kolcm s- (2: Thin-layer cells diffusion 1 x 10-6 0.25 -0.471 -0.706 1 x 10-6 0.50 -0.271 -0.371 1 x 10-6 0.75 -0.194 -0.254 1 x 10-8 0.25 -0.944 -1.179 1 x 10-8 0.50 -0.508 -0.607 1 x 10-8 0.75 -0.352 -0.411 ko = heterogeneous rate constant in cm s-1; A = area of thin-layer cell in cm2; V = volume of thin-layer cell in cm3; and R, T, F and D have their usual significance.Using the following typical values: n = 1, v = 2 mV s-1, AIV = 50 pm-1, T = 298 K and D = 1 x 10-5 cm2 s-1, a comparison of peak potentials at different values of ko and ar are presented in Table 3. The advantage of using thin-layer cells in order to study irreversible reactions is apparent. The linear range of the reaction was investigated using the same CFD throughout. Measurements were made by holding the potential at -0.75 V until the current decayed to a steady value and then applying a negative potential scan. At a potential of -0.75 V it was found that copper was deposited effectively and the oxygen was reduced, whilst the reduction of nitrate was minimal. Typical voltammograms are shown in Fig. 7 and the voltammetric response obtained over the nitrate concentration range studied is given in Table 4.The measure- ment of peak height versus concentration gave a linear response over the nitrate range 1 x 10-5-1 x 10-3 mol dm-3 (Y = 0.996) with the results becoming erratic at higher nitrate concentrations. It should be noted that the measurement of the linear range of the devices in this way is only of value at the developmental stage. The device will only be of use as a sensor when the reagents are screen-printed and a separate device is used for each measurement. The concentration of the different reagents was varied in order to find their optimum values and the results obtained areANALYST, JUNE 1991, VOL. 116 577 shown in Table 5 . Different concentrations of chloride were added in an effort to find the conditions under which varying the chloride concentration would have the least effect.The term E, refers to the point on the voltammogram at which the current starts to increase following the nitrate reduction peak and the value, E, - E,, is a rough guide to the resolution of the nitrate reduction peak from the background reaction. The use of sulphuric acid at a concentration of 0.5 rnol dm-3 gave the best results in terms of peak height and in minimizing the effect of varying the chloride concentration. The highest copper concentration used here, 2.5 x 10-2 rnol dm-3, gave the best results, but background currents were high and the recorder had to be offset by 20-30 PA, presumably because of copper diffusing into the area of the working electrode and being deposited.It would appear that the use of a solution containing 1 x 10-2 rnol dm-3 copper, 0.5 rnol dm-3 sulphuric acid and 1 X 10-4 rnol dm-3 chloride gives the optimum results. As the concentration of chloride in real samples will vary, it would be beneficial to remove all chloride before filling the device. Dionex On-Guard (Dionex, Sunnyvale, CA, USA) Table 4 Voltammetric characteristics of nitrate rcduction at CFDs. Scan speed, 2 mV s--l Nitrate concentration/ 10-5 rnol dm-3 0 1.0 2.5 5.0 7.5 10.0 25.0 50.0 75 .o 100.0 250.0 500.0 750.0 1000.0 E f l - -0.91 -0.90 -0.90 -0.90 -0.90 -0.91 -0.90 -0.91 -0.93 -0.96 -0.98 - 1.01 - 1 .oo i&A 0 1.50 2.25 5.50 8.25 11.75 33.5 55 .O 97.0 140.0 495.0 580.0 875.0 850.0 silver columns were designed for the removal of chloride prior to the injection of samples in ion chromatography. Their use here was tested by passing the sample solution through the column, filling the device and recording the peak voltage at which nitrate reduction occurred.It was found that concentra- tions of chloride of up to 0.5 rnol dm-3 could be removed with the use of these columns. The effects of various ionic species commonly found in hydroponic fluids on the voltammetric response of nitrate reduction were tested. The results obtained are shown in ' O 1 I G I -0.7 -0.8 -0.9 - 1 PotentialN versus internal reference electrode Fig. 7 Linear sweep voltammograms obtained to produce a calibra- tion graph for the determination of nitrate at an in situ generated copper electrode in a CFD. Scan rate, 2 mV s-l; sulphuric acid concentration, 0.5 rnol dm-'; copper(I1) concentration, 1 x lo-' rnol dm-3; chloride concentration, 1 x 10-4 rnol dm-3.Nitrate con- centration: A, 0; B, 0.10 x C, 0.25 x D, 0.50 x E, 0.75 x 10-4; F, 1.0 x 10-4; G. 2.5 x 10-4; and H, 5.0 x 10-1 rnol dm-3 Table 5 Optimization of reagent concentrations [Cu'+]/ [H2SO41/ [CI-]/ 10-3 mol dm-3 mol dm-3 10-4 mol dm-3 5 2 1 5 2 5 5 2 10 5 1 1 5 1 5 5 1 10 5 0.5 1 5 0.5 5 5 0.5 10 5 0.1 1 5 0.1 5 5 0.1 10 1 0.5 1 1 0.5 5 1 0.5 10 10 0.5 1 10 0.5 5 10 0.5 10 25 0.5 1 25 0.5 5 25 0.5 10 EdV -0.915 -0.970 - 1 .020 -0.930 -0.980 - 1 .005 -0.940 -0.985 -0.995 -0.930 -0.970 - 1.020 -0.900 -0.940 -0.965 -0.930 -0.930 - 1.010 i&A E,"IV 13.5 -0.985 10.0 - 1.020 --t - 1.040 15.5 - 1.010 11.5 - 1 340 9.0 - 1.045 16.8 - 1.030 13.0 - 1 .OS5 13.5 - 1 .055 Waves poorly defined and difficult to measure 11.0 -1.020 10.0 - 1.030 4.5 - 1.060 15.3 - 1 ,050 16.8 - 1.010 16.8 - 1.030 18.8 - 1.010 19.3 - 1.010 16.0 -1.080 E , -EdV 0.07 0.05 0.02 0.08 0.06 0.04 0.09 0.07 0.06 0.09 0.06 0.04 0.15 0.07 0.065 0.08f: 0.08f: 0.07$ * E, = potential at which the current begins to increase after the nitrate reduction peak.1- Indistinguishable from background current. f: Deposition current stabilized at 20-30 FA above baseline.578 ANALYST, JUNE 1991, VOL. 116 ~~ Table 6 Effect of ionic species on peak height of nitrate reduction Amount Effect on peak Ion* added/mg 1-1 height (YO ) Ca2+ 100 100.8 500 1 0 . 3 K+ 100 500 Na+ 100 500 Fe3' 10 100 Fez+ 10 100 500 Zn2+ 10 100 Ni2+ 10 100 Mn2+ 10 100 NH4 + 10 100 ~ 0 ~ 3 - 100 500 H2B03- 10 100 Mg*+ 100 98.6 95.9 101.4 101.3 99.1 94.7 104.3 100.8 95.0 104.0 106.5 105.5 101.6 104.8 101.7 102.8 98.2 101.9 95.8 99.3 104.3 101.4 N02- 10 126.7 100 Response erratic M004'- 10 98.9 100 97.7 ~ 1 3 + 10 101.6 100 98.5 EDTA 10 (Ep=-l.OOV) 62.1 (Ep = - 1.04V) 42.5 100 * The cations were added as the sulphate and the anions as the sodium salts.[NO3-] = 5 x 10-4 mol dm-3. Table 6. The peak voltages obtained were -0.900 k 0.03 V unless stated otherwise. Although little change in peak height was observed on addition of molybdate there was a marked change in the shape of the voltammogram (see Fig. 8). It is known that molybdate catalyses the reduction of nitrate18 and heteropoly compounds of similar structure have been shown to catalyse hydrogen evolution,19 which is believed to be the background reaction, and this appears to be what is occurring here.The fact that little increase in peak height occurred as the molybdate concentration was increased appears to be fortuitous. Nitrite appear:; to be reduced at the same potential as nitrate and the addition of 10 ppm of nitrite caused an increase in peak current of >25%. Pletcher and Poorabedil2 studied the reduction of nitrite and nitrate at a copper electrode and found that both species were reduced at approximately the same potential. Although the effect of iron(ii1) on peak height is not marked, the decrease in peak height was approximately 6% for a 100 pprn iron(Ir1) solution; the iron(rn) is reduced before both the copper and nitrate. A solution of 50 pprn of iron(rI1) in a 1 rnol dm-3 solution of sulphuric acid in the CFD gave a peak on scanning at -0.27 V and a shoulder at -0.14 V.Ethylenediaminetetraacetic acid (EDTA), which is nor- mally added to hydroponics in the form of the iron(r1r) salt, has a particularly marked effect on the nitrate reduction peak, as shown in Table 6. However, at the acidity used in the CFD, 0.5 mol dm-3 sulphuric acid, and the low EDTA concentrations used in hydroponics, it is unlikely that the complexing of the copper would have any effect on the use of the device. It also i ' o . 2 v 4 0.10 0.08 a E 3 EQ.06 0 L 3 0.04 0.02 - - -0.7 V - I ! I I / i I I I \ I I I 'L/ I I / -4 / t .0.7 V t -0.7 V I \ I I I ' I \I o t PotentialN versus internal reference electrode Fig.8 Linear sweep voltammograms showing the effect of molyb- denum(v1) on the determination of nitrate at an in situ generated copper electrode in a CFD. Molybdenum concentration: A, 0; B, 10; and C, 100 mg 1-I. Other parameters and concentrations as in Fig. 7 Table 7 Effect of EDTA on nitrate reduction at a copper electrode (all potentials versus SCE). Scan speed, 10 mV s-l; results are the average of three scans [EDTA]/ Peak Peak mg I-' potentialN current/pA 0.00 -0.48 175 4.95 -0.57 186 9.80 -0.57 190 19.2 -0.58 186 Table 8 Effect of EDTA solutions on nitrate reduction at a copper rod electrode (all potentials versus SCE). Results are the average of three scans Peak Peak Electrode treatment potentiaW current/pA Untreated -0.495 183 Placed in 0.5 rnol dm-3 H2S04 for 1 min and then washed with water -0.480 180 Placed in 50 ppm EDTA-0.5 mol dm-3 H2S04 for 1 min and then washed with water -0.547 188 Placed in 50 ppm EDTA (pH 5.2) and then washed with water -0.495 180 seems unlikely that the EDTA interacts with the nitrate in any way.Addition of EDTA to nitrate solutions resulted in a shift in the nitrate peak potential, but had little effect on peak height as is shown in Table 7. That the effect of the EDTA was due to its adsorption on the electrode surface was confirmed by placing a freshly polished copper rod electrode in different solutions, washing with water and then scanning in a nitrate solution. The results, which are shown in Table 8, indicate that it is the fully protonated EDTA that is adsorbed onto the copper surface.Workers who have investigated the effect of additives on the electrodeposition of copper previously,20-22 found that compounds containing amino or carboxyl groups cause an increase in the overpotential which they attributed to adsorption on the copper electrode surface. It was noted that amino groups gave a larger overpotential effect than carboxyl groups and that compounds containing two or more of these groups produced a proportionately larger effect. Gunawar- dena et aZ.23 observed that low concentrations of EDTA retarded the nucleation of silver on vitreous carbon and attributed this to an adsorption phenomenon.ANALYST. JUNE 1991, VOL. 116 579 Attempts to remove the effect of EDTA by the application of high positive potentials were only partially successful.It has been shown that EDTA is oxidized at pre-treated glassy carbon electrodes24 and at doped lead oxide electrodes,25 although the products of the electrode reaction are not known. At a screen-printed carbon electrode no clear oxidation wave was observed but an increase in current was seen at potentials near to the cut-off potential. The application of positive potentials (1.4-1.6 V for 1 min) to the CFD containing a nitrate-EDTA mixture, before determining nitrate, caused a shift in the nitrate peak potential to more positive values, and some increase in peak current. However, the peak current could not be increased beyond 80% of the value obtained when no EDTA was present. The present study is the second in a series aimed at assessing the use of CFDs as disposable amperometric sensors.In the previous study it was shown that phosphate could be deter- mined readily in the device as 12-molybdophosphate directly at a screen-printed carbon electrode, and here we have shown that nitrate can be determined by direct reduction at a copper electrode produced in the device immediately before carrying out the determination. So far the reagents for these determi- nations have not been screen-printed or otherwise deposited within the device; they have been added to the sample solution before this was taken into the device. Work on these and other systems is continuing and it is hoped shortly to provide information on the screen-printing or coating of these various reagents . A. G. F., S. P. S. and T.E. E. thank the Agricultural and Food Research Council for financial support for this project. References Fogg, A. G., Scullion. S. P., Edmonds, T. E., and Birch, B. J., Analyst, 1990, 115, 1277. Fogg. A. G., Scullion. S. P.. and Edmonds. T. E., Analyst, 1989,114,579. Fogg. A. G.. Scullion, S. P., and Edmonds, T. E., Analyst, 1990, 115, 599. Bodini. M. E., and Sawyer, D. T., Anal. Chern., 1977,49,485. 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Albery, W. J., Haggett, B. G. D., Jones, C. P., Pritchard, M. J., and Svanberg, L. R., J. Electroanal. Chem., 1985, 188,257. Davenport, R. J . , and Johnson, D. C., Anal. Chem., 1973,45, 1979. Davenport, R. J., and Johnson, D. C.. Anal. Chern., 1974,46, 1971. Johnson, D. C., and Shenvood, G. A., Anal. Chim. Acta, 1981, 129, 87. Johnson, D. C., and Sherwood, G. A., Anal. Chim. Acta. 1981, 129, 101. Xing, X., and Scherson. D. A.. J. Electroanal. Chem., 1985. 188, 257. Xing. X., and Scherson, D. A., Anal. Chem., 1987, 59, 962. Pletcher, D., and Poorabedi, Z., Electrochim. Acta, 1979, 24, 1253. Almhofer, N., and Frenzel, F., Fresenius 2. Anal. Chem., 1988, 330, 494. Massey, A. G., in Comprehensive Inorganic Chemistry, ed. Trotman-Dickenson, A. F., Pergamon Press, Oxford, 1973, vol. 3. Hitchman, M. L., and Nyasulu, F. W. M., J . Chem. SOC., Faraday Trans. I, 1986,82, 1223. Hitchman, M. L., Aziz, A., Chingakule, D. D. K., and Nyasulu. F. W. M., Anal. Chim. Acta, 1985, 171, 141. Tam, T. M., and Christiansen, P. J., Plast. Surf. Finish, 1988, 75. 70. Edmonds, T. E., Anal. Chim. Acta, 1980, 116. 323. Keita, B., and Nadjo, L., J. Electroanal. Chem., 1985,191,441. Schneider, H., Sukava, A. J., and Newby. N. J., J. Electro- chem. Soc., 1965, 112, 568. Schneider, H . , Sukava, A. J., McKenney , D. J., and McGregor, A. T., J . Electrochem. Soc., 1965, 112, 570. Sukava. A. J., and Chu, A. K. P., J. Electrochem. Soc., 1969, 116, 1188. Gunawardena, G., Hills, G.. Montenegro, I., and Scharikker, B., J. Electroanal. Chem., 1982. 138,225. Fogg, A. G., Fernandez-Arciniega, M. A., and Alonso, R. M., Analyst, 1985, 110, 1201. Johnson, D. C., Polta, J. A., Polta, T. Z., Neuberger, G. G., Johnson, J.. Tang, A. P. C., Yeo, 1. H., and Baur, J., J. Chem. Soc., Faraday Trans. I, 1986, 82, 1081. Paper 0lO3962 D Received August 31st, I990 Accepted November 13th, 1990

ISSN:0003-2654    

DOI:10.1039/AN9911600573

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, JUNE 1991, VOL. 116 581 Evaluation of Poly(viny1idene chloride) as a Matrix for Polymer Membrane Ion-selective Electrodes Vanessa J. Wotring, Patrick K. Prince* and Leonidas G. Bachast Department of Chemistry and Center of Membrane Sciences, University of Kentucky, Lexington, KY 40506-0055, USA Poly(viny1idene chloride) (PVDC) was evaluated as a matrix for ion-selective electrodes. The glass-transition temperature of this polymer is below room temperature, hence, plasticizers are not required in order t o make viable membranes. Quaternary ammonium salts [Aliquat 336 and tridodecylmethylammonium thiocyanate (TDMAT)] were used as the ionophores. It was possible t o make functional unplasticized PVDC electrodes based on Aliquat 336. No deterioration in the slope of the calibration graph was observed over a period of 7 months.A mediator, octanoi, was required in the preparation of electrodes based on TDMAT. Polyjvinylidene chloride) membrane electrodes based on this ionophore had a lifetime of about 2 months. Keywords: Ion-selective electrode; poly(vin ylidene chloride); tridodec ylmeth ylammonium thiocyanate; Aliquat 336; ion-exchange membrane Plasticized poly(viny1 chloride) (PVC) is the most commonly employed matrix in polymer membrane ion-selective elec- trodes (ISEs). The use of a plasticizer is required in order to lower the glass-transition temperature ( Tg) of the polymer to below room temperature.' It is well established, however, that the operational lifetimes of ISEs can be reduced as a result of the leaching of the plasticizer and the ionophore from the rnembrane.2.3 This leaching affects the selectivity of the electrode and leads to a gradual deterioration of the response.3 Therefore, it would be advantageous to develop ISEs based on polymers that do not require plasticization.A few ISEs that employ unplasticized polymer matrices have been reported in the literature. The first report described the use of silicone rubber to develop a potassium TSE based on valinomycin .1 These membranes, however, had much higher resistances than those prepared with PVC and after 4-6 weeks the slopes decreased to 45 mV decade-1. A block co-polymer of poly(bispheno1-A carbonate) and poly(dimethylsi1oxane) has been employed to develop a potassium electrode with a lifetime of more than 3 years.5 Lawton and Yacynych3 reported anion-responsive graphite electrodes coated with quaternized poly(vinylbenzy1 chloride).Other functionalized polymers, such as Nafion (perfluorosulphonate polymer) and poly( 1,2-diaminobenzene), have been used without plasticiz- ers in the development of electrodes for the tetrabutylammo- nium ion and protons, respectively.6>7 Finally, matrices, such as poly(2-methylpropyl methacrylate) and poly(isobuty1 vinyl ether), have been studied in the development of calcium I S E S . ~ ? ~ These electrodes, however, require the use of mediators to produce viable membranes. Immobilization of the plasticizer on the polymer has also been reported. Photopolymerized di-5-hexen-2-yl adipate has been used instead of conventional plasticizers to increase the operational lifetimes of ion-selective field effect transistors.1" Hobby et al.11 reported the immobilization of both the plasticizer and ionophore on the co-polymer VAGH (a partially hydrolysed co-polymer of vinyl chloride and vinyl acetate) for the purpose of developing a calcium iSE. Finally, a styrene-l,3-butadiene-styrene triblock elastomer has been used to co-immobilize the ionophore and plasticizer of a calcium sensor and a nitrate ISE.1*,13 In this work, the development and evaluation of ISEs based on poly(viny1idene chloride) (PVDC) is described. This polymer was chosen for two reasons. Firstly, like PVC, PVDC * Present address: Milliken and Company, P.O. Box 1926, Spartan- t To whom correspondence should be addressed. burg, SC 29034, USA.is a straight-chain polymer of carbon, hydrogen, and chlorine, thus, it was postulated that PVDC might also be a suitable matrix for ISEs. Secondly and more importantly, the Tg of PVDC is already below room temperature, at -17 OC.I4 Therefore, it should be unnecessary to have to add a plasticizer to the polymer matrix in order to prepare viable membranes that operate at room temperature. Only one report of ISEs based on PVDC has appeared in the litera- ture.15 In that work, however, the membranes were composed of 15% PVC, 15% PVDC, 63% plasticizer or solvent mediator, and 7% ionophore. It was reported that membranes containing only PVDC as the polymer and the normal percentage of plasticizer had insufficient mechanical strength. in the present study, electrodes based on PVDC mem- branes were prepared, tested and compared to conventional ISEs, with respect to response, ion-selectivity and lifetimes.Aliquat 336 and tridodecylmethylammonium thiocyanate (TDMAT) were evaluated as potential anion-selective iono- phores in these membranes. Experimental Reagents Aliquat 336 and octanol were purchased from Aldrich (Milwaukee, WI, USA). Poly(viny1 chloride) and tridodecyl- methylammonium chloride (TDMAC) were obtained from Polysciences (Warrington, PA, USA). PoIy(viny1idene chloride) was from Monomer-Polymer and Dajac Labora- tories (Trerose, PA, USA). According to the company, this is actually a co-polymer of PVDC (80%) and acrylonitrile (20%). 2-Morpholinoethanesulphonic acid (MES), sodium salicylate, and all inorganic salts were purchased from Sigma (St.Louis, MO, USA). o-Nitrophenyl octyl ether (NPOE) and bis(2-ethylhexyl) phthalate (DOP) were from Fluka (Ronkonkoma, NY, USA). Tetrahydrofuran (THF) was obtained from Fisher (Fair Lawn, NJ, USA) and tris(hydroxy- methy1)aminomethane (Tris) was from Research Organics (Cleveland, OH, USA). The buffers and salt solutions were prepared by using de-ionized (Milli-Q water purification system; Millipore, Bedford, MA, USA) distilled water. Quaternary Ammonium Salts Aliquat 336 was shaken at least 3 times with equal portions (v/v) of 1.00 mol dm-3 potassium thiocyanate and dried over magnesium sulphate. Likewise, a 0.200 g amount of TDMAC was dissolved in 0.900g of octanol. This mixture was then shaken with 3 x 1 ml portions of 0.100 mol dm-3 potassium thiocyanate and dried as described above.Other ratios of582 ANALYST, JUNE 1991, VOL. 116 Table 1 Composition and properties of the membranes used for the various thiocyanate electrodes. Data in parentheses are the amounts used in mg Electrode 1 2 3 4 5 6 7 8 9 10 11 12 13 Ionop hore Aliquat 336 (6) Aliquat 336 (17) Aliquat 336 (34) Aliquat 336 (17) Aliquat 336 (17) Aliquat 336 (17) Aliquat 336 (17) Aliquat 336 (17) TDMAT(2.2) TDMAT(5.5) TDMAT(7.5) TDMAT(6) TDMAT(6) Plasticizer/ mediator - - - - DOP (25) DOP(34) DOP (49) NPOE (52) Octanol(25) Octanol(25) Octanol(25) Octanol(52) NPOE (66) * Data refer to the mean f one standard deviation, n = 5. ? NA = not applicable. Polymer PVDC (50) PVDC (50) PVDC (100) PVC (33) PVC (33) PVC (33) PVC (33) PVC (33) PVDC (100) PVDC (100) PVDC (100) PVC (33) PVC (33) Slope*/mV Detection limit*/ decade - mol dm-3 NAt NA 54 ? 3 2.7 x 10-5 f 0.6 x 10-5 60 f 2 6 x 10-6 k 1 x 10-6 58 k 1 2.9 x 10-5 k 0.4 x 10-5 57 f 3 2.8 X 10-5 k 0.7 X 10-5 56 f 2 2.3 X 10-5 f 0.6 x 10-5 58 f 1 4.1 x f 0.7 x 10-6 46 f 3 1.0 x k 0.3 x 10-4 47 f 1 1.4 x 10-5 k 0.7 x 10-5 53 k 1 2.0 x 10-5 f 0.6 x 10-5 58 k 2 1.3 x 10-5 f 0.5 x 10-5 NA NA NA NA TDMAC to octanol or NPOE (0.0450 g TDMAC to 0.0950 g octanol, 0.0295 g TDMAC to 0.0970 g octanol, and 0.0968 g TDMAC to 1.0461 g NPOE) were also prepared in the same manner.Apparatus A Fisher Isotemp Model 80 constant temperature circulator set at 25 "C was employed to maintain a constant temperature in the electrode cell. The response of the electrodes was followed by using a pH/mV meter and was recorded on a strip-chart recorder as described previously.16 A Fisher saturated calomel electrode (SCE) was used as the reference.Membranes and Cell Assembly The membranes were prepared by dissolving all of the components (see Table 1 for compositions) in THF and mixing for 0.5 h. This mixture was transferred into a casting mould (a glass ring, 1.5 cm i.d., set on a polytetrafluoroethylene plate) and the solvent was allowed to evaporate at room temperature overnight.17 Smaller discs from the resultant membrane were mounted in a Philips electrode body (IS-561; Glasblaserei Moller, Zurich, Switzerland). All potentiometric measure- ments were made with the following electrochemical cell: SCE I( sample solution I membrane 1 1.00 x 10-2 rnol dm-3 NaCl, 1.00 x 10-2 rnol dm-3 sodium salicylate I Ag-AgCI As the ionophores did not respond to chloride, sodium salicylate was added to the internal filling solution to yield a stable internal solution-membrane interface.Thiocyanate was not used in the internal filling solution, because AgSCN is less soluble than AgCl and a commercial electrode body with an Ag-AgCI internal reference was used for these studies. Evaluation of Response and Selectivity The potentiometric response of all the electrodes was deter- mined by adding different volumes of a standard solution to 10.00 ml of a buffered sample solution (0.100 rnol dm-3 MES-NaOH, pH 6.60). Before use, the Aliquat 336 and TDMAT membranes were conditioned in 1.00 X 10-2 rnol dm-3 KSCN for 5-10 min.When not in use, the electrodes were stored dry and protected from light. Selectivity coefficients were determined by using the fixed interference method.18 The hydroxide ion concentration in the MES-NaOH buffer, pH 6.60 (equal to 4.0 x 10-8 rnol dm-3) was considered as the fixed interferent. The electrodes were conditioned before each experiment as described above. Results and Discussion Whereas other polymers, such as the block co-polymer of poly(bispheno1-A carbonate) and poly(dimethylsiloxane) ,5 have been employed successfully in the preparation of ISEs, plasticized PVC is still the most commonly used matrix. It is convenient, has satisfactory properties, and is compatible with most ionophores.' It should be emphasized that in some instances the selectivity pattern exhibited by an electrode is highly dependent on the solvent ( i .e . , plasticizer and/or mediator) and/or the matrix material.9 Thus, it might be feasible to alter the selectivity properties of ISEs by choosing appropriate polymer matrices. In view of the potential usefulness of PVDC as a matrix of ISEs, several electrodes were prepared with this polymer and compared with their PVC counterparts. The ion-exchange properties of the two ionophores used in this study, Aliquat 336 (a mixture with the major component being methyltrioctylammonium chloride) and TDMAC, have been well characterized. The selectivity patterns of PVC membrane electrodes based on these quaternary ammonium salts follow the Hofmeister series: lipophilic organic anions > perchlorate > thiocyanate = iodide > nitrate > bromide > chloride.This order of response is controlled by the relative lipophilicity of the anions. Operational electrodes were obtained when PVDC mem- branes containing Aliquat 336 were used. These membranes did not contain any plasticizers (see Table 1, Electrodes 2 and 3). Thus, it appears that the incorporation of this ionophore in the PVDC matrix does not increase the Tg of the membrane above room temperature. On the other hand, the unplasti- cized PVC-Aliquat 336 membrane (Electrode 4) was too rigid to assemble in an electrode body. Two different percentages of Aliquat 336 were evaluated as indicated in Table 1. Electrode 1 (11% ionophore and 89% PVDC) showed no response toward thiocyanate.On the contrary, it has been reported that plasticized PVC electrodes containing 9% Aliquat 336 did respond to anions.19 Increasing the ionophore content to 25% (Electrode 2) gave electrodes that exhibited a Nernstian response for thiocyanate. However, these membranes were thin and often ruptured when the pressure inside the electrode body increased during assembly. This problem was alleviated by doubling the thickness of the membrane (Electrode 3). Electrodes with this composition gave a Nernstian response for thiocyanate and perchlorate and a sub-Nernstian response toward bromide, salicylate, and nitrate. No response was observed toward chloride. The selectivity coefficients for these anions are included in Fig. 1. These values were calculated by using theANALYST.JUNE 1991, VOL. 116 583 1 - Sal- N03- -Br- =NO3- - ~ 0 ~ - -Br- Br- =NO3- 4t - NO3- Br- 3 1 Sal- - Br- - SCN- -Sat- -SCN- -Salk c104- =c104- -sCN- -Sat- -SCN- -c104- -C104- -c104- 8 1 =SCN- tn - -OH- -OH- -OH- -OH- -OH- I I I 1 Electrode 3 Electrode 7 Electrode 8 Electrode 11 Electrode 13 Fig. 1 Comparison of PVC and PVDC electrodes prepared with Aliquat 336 and TDMAT with respect to the selectivity coefficient, k!&”. Salk refcrs to the salicylate anion fixed interference method. As quaternary ammonium salts are known to respond to hydroxide and the experiments were buffered, the hydroxide concentration in the buffer was used as the interferent. The slope and limit of detection (as defined by the International Union of Pure and Applied Chemistry) for responses toward thiocyanate are also shown in Table 1.18 These data were collected over a period of 7 months during which only a slight deterioration in the slope of the calibration graph (from -56 to -52 mV decade-*) was observed (Fig.2). However, there was a shift in the detection limits. This can be explained by the slow leaching of the ionophore from the membrane. The storage condition of these PVDC-membrane electrodes is somewhat different from the usual conditions used for PVC-based ISEs. When PVDC membranes were stored in an aqueous solution overnight, the electrodes lost their response properties. In a few instances, leaving these electrodes in the air overnight could regenerate the response, but these ISEs had short lifetimes. It was found that the optimum response was obtained when the PVDC electrodes were conditioned for only 5-10 min before each experiment.Between experiments they were stored in the air and protected from light. The PVC electrode counterparts of the Aliquat 336-PVDC electrodes were also evaluated for comparison. Three differ- ent membranes with various amounts of the plasticizer, DOP, are included in Table 1 (Electrodes 5 , 6 , and 7). The response of these electrodes is identical within experimental error (see Table 1 for comparison of results for thiocyanate). Nernstian responses were obtained for thiocyanate, salicylate, and perchlorate. The slopes of the lines for the responses toward bromide and nitrate were sub-Nernstian. No response was observed for chloride. As there was no difference in the response of these three electrodes, only one set of selectivity coefficients is included in Fig.1. However, results for each electrode are shown in Table 1. These data were collected over a period of 5 months with no deterioration in the slopes of the calibration graphs. Another Aliquat 336-PVC system was also studied. In this instance, NPOE was used as the plasticizer (Electrode 8). Similar results to those of Electrode 7 were obtained. The slope of the calibration graphs and the detection limit for thiocyanate arc includtd ii- Tnblc I . The cited detection limit was for experiment. ~drrizd out over a period of 2 d. Iktection limits from subsequent testing, however, were shifted by an order of magnitude. This was observed for several electrodes prepared by using NPOE as the plasticizer.This might indicate that the ionophore and/or plasticizer is leaching from the membrane. The selectivity coefficients are included in Fig. 1. Poly(viny1idene chloride) membranes based on TDMAT also gave operational ISEs. These electrodes, however, required the addition of a mediator. A mediator serves two -225 -5 -4 -3 -2 Log [thiocyanate] Fig. 2 Comparison of the response of Electrode 3 toward thio- cyanate in 0.100 mol dm-3 MES-NaOH at pH 6.60 over a period of 7 months. A, Month 1; B, month 3; and C, month 7. AE is the difference between the steady-state potential and the starting poten- tial functions in the membrane. It dissolves the ionophore and partly solvates the anion salt. Octanol served as the mediating solvent in these electrodes.It is not surprising that a mediator was required in order to make viable TDMAT-PVDC membranes. It is known that Aliquat 336 is a mixture of quaternary ammonium salts with 3 4 % (m/m) octanol and 6 5 % (m/m) decanol.”] In comparison, if octanol was used instead of a plasticizer in a PVC-based membrane (Electrode 12), no response toward thiocyanate was observed. In addition, after the formation of the membrane, some of the octanol remained on the casting plate, indicating the incom- patibility of the mediator with the TDMAT-PVC system. This is not surprising as Craggs et d.21 also reported that octanol was not a suitable mediator in a PVC-based calcium ISE. According to their report, octanol was exuded from the membrane, which was then considered unsuitable for assembly in an ISE body.For the TDMAT-PVDC mem- branes there was no evidence of octanol leaching from the membrane before assembly. Several PVDC-based electrodes with different composi- tions in terms of TDMAT and octanol were evaluated. Electrodes 9 and 10 gave sub-Nernstian responses for thiocya- nate (Table 1). However, when the percentage of ionophore was increased to 5% (Electrode ll), near-Nernstian responses were obtained for thiocyanate, salicylate, and perchlorate. The responses toward bromide and nitrate were sub-Nern- stian. No response was observed for chloride. The selectivity coefficients are included in Fig. 1. The slopes and detection limits for the response toward thiocyanate are shown in Table I . Unlike the Aliquat 336-based electrodes, the response of these ISEs deteriorated after 2 months.Only one PVC electrode based on TDMAT (Electrode 13) was evaluated. The response of this electrode was Nernstian for thiocyanate, salicylate, and perchlorate and sub-Nernstian for bromide and nitrate. No response was observed for chloride. The selectivity coefficients for these anions are included in Fig. 1. The slope and detection limit for responses toward thiocyanate are shown in Table 1. These data were collected over a period of 2 months with no deterioration of the slope of the calibration graph. The comparison of the selectivity coefficients for electrodes bascd on Aliquat 336-PVDC and plasticized Aliquat 336- PVC in Fig. 1 indicates that there is a slight variation in the observed pattern (the main differences being for salicylatc and bromide).The response of Electrode 3 toward bromide, even though it is sub-Nernstian, is linear down to at least 5 x 10-4 mol dm-3. On the other hand, for the PVC electrodes (Electrodes 7 and 8) the linear rangc did not start until approximateky ,2x 10-3 mol dm-3. The response of Electrode584 ANALYST, JUNE 1991, VOL. 116 3 for salicylate has a linear range starting at 4 x 10-3 mol dm-3, whereas for Electrodes 7 and 8, the linear portion started at 2 X mol dm-3. Electrode 3 appears to discriminate against salicylate to a greater extent than do Electrodes 7 and 8. However, Electrode 3 does respond more to bromide. Thus, the membrane matrix does indeed have some role in the observed selectivity pattern of electrodes prepared with these ionophores.Electrode 3 also has slightly better detection limits than Electrodes 5,6,7,9, 10,ll , and 13 for thiocyanate (see Table 1). While a similar value is reported for Electrode 8, these results were only obtained for the first 2 d of experimentation. A comparison of the selectivity coefficients of TDMAT, Electrodes 11 and 13 found in Fig. 1, indicates that there is almost no difference in the selectivity pattern of these electrodes. In fact, the numeric values for the logarithm of the selectivity coefficients are similar except for the reversal in the selectivity order between salicylate and thiocyanate. However, the lifetime of the octanol-based electrode was shorter. This might be explained by leaching of the octanol from the PVDC matrix.Thus, the ionophore might no longer be completely dissolved in the membrane or the ions may no longer be solvated in an efficient manner by the mediator. Unfortunately, these membranes are white and opaque, and therefore, it is difficult to observe whether the ionophore is crystallizing in the membrane. The ionophore might also be leaching from the membrane causing the deterioration of the response. Although PVDC has been used successfully to prepare anion-selective electrodes based on quaternary ammonium salts, other aspects must be taken into consideration when using different ionophores. For cation-selective electrodes based on neutral carrier ionophores, the response mechanism has been studied extensively. Several reports have verified the hypothesis that plasticized PVC membranes are low-capacity ion-exchangers.22.23 The anionic sites are fixed in the mem- brane with a concentration from 0.05 to 0.6 mmol dm-3, depending on the relative molecular mass of the polymer and the method of polymerization. These negatively charged sites are necessary for the transport of the positively charged neutral carrier-cation complex in the membrane.This should also be a requirement for PVDC membranes if this polymer is to be used to prepare neutral carrier-based ISEs. It should also be mentioned that pure PVDC does not crystallize out of solution. For this reason, acrylonitrile is used during the polymerization reaction. The polymer used in this study was 20% acrylonitrile and 80% PVDC. This composi- tion still decomposes with time and should be stored in a dry place and protected from light.14 In addition, although PVDC-based electrodes respond in a Nernstian fashion at room temperature, they might not respond in samples that are below the glass-transition temperature of the polymer mem- brane.Conclusions It has been shown that it is possible to prepare unplasticized PVDC membrane electrodes based on the quaternary am- monium salts Aliquat 336 and TDMAT. Ion-selective elec- trodes with a composition Of 25% Aliquat 336 and 75% PVDC were tested over a period of 7 months, and only a slight deterioration in the slope of the calibration graph was observed. Electrodes prepared with TDMAT had a lifetime of about 2 months. For the Aliquat 336-PVDC electrodes, there is a slight variation in the observed selectivity pattern compared to their PVC counterparts.This is consistent with the suggestion that the selectivity pattern exhibited by an electrode is influenced by the solvent (i.e., plasticizer and/or mediator) and/or the matrix material.9 Acknowledgement is made to the National Science Founda- tion (R11-86-10671) for support for this research. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 References Armstrong, R. D., and Horvai, G.. Electroanal. Chem., 1990, 35, 1. Ebdon, L., Ellis, A. T., and Corfield, G. C., Analyst, 1979,104, 730. Lawton, R. S., and Yacynych, A. M., Anal. Chim. Acta, 1984, 160, 149. Pick, J., Toth, K., and Pungor, E., Anal. Chim. Acta, 1973,64, 477. LeBlanc. 0. H., and Grubb, W. T., Anal. Chem.. 1976, 48, 1658. Martin, C. R., and Freiser, H.. Anal. Chem., 1981, 53, 902. Heineman, W. R., Wieck, H. J., and Yacynych. A. M., Anal. Chem., 1980,52, 345. Moody, G. J., Saad, B., and Thomas, J. D. R., Analyst, 1987, 112, 1143. Schafer, 0. F., Anal. Chim. Acta, 1976, 87,495. Harrison, D. J., Teclemariam, A., and Cunningham, L. L., Anal. Chem., 1989, 61, 246. Hobby, P. C., Moody, G. J., and Thomas, J. D. R., Analyst, 1983, 108, 581. Ebdon, L., Ellis, A. T., and Corfield, G. C . , Analyst, 1982, 107, 288. Ebdon, L., King, B. A., and Corfield, G. C., Anal. Proc., 1985, 22, 354. Wessling, R. A., Poly(viny1idene chloride), Gordon and Breach Science, New York, 1977, pp. 112, 145-147. Frend, A. J., Moody, G. J., Thomas, J. D. R., and Birch, B. J.. Analyst, 1983. 108, 1072. Wotring, V. J., Johnson, D. M.. and Bachas, L. G.. Anal. Chem., 1990,62, 1506. Moody, G. J., and Thomas, J. D. R., in Covington, A. K., ed., Ion-selective Electrode Methodology, CRC Press, Boca Raton, FL, vol. 1, 1979, p. 111. International Union of Pure and Applied Chemistry, Pure Appl. Chem., 1975,48, 129. Choi, K. K., and Fung, K. W., Anal. Chim. Acta, 1982, 138, 385. Lee, G. L., Cattrall, R. W., Daud, H., Smith, J. F., and Hamilton, I. C., Anal. Chim. Acta, 1981, 123, 213. Craggs, A., Keil, L., Moody, G. J., and Thomas, J. D. R., Talanta, 1975,22, 907. Horvai, G., Graf, K., Pungor, E., and Buck, R., Anal. Chem., 1986,58, 2735. Morf, W. E., and Simon, W., Anal. Lerf., 1989,22, 1171. Paper 01048840 Received October 30th, 1990 Accepted February I1 th, I991

ISSN:0003-2654    

DOI:10.1039/AN9911600581

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, JUNE 1991. VOL. 116 585 Electroanalysis for Organotin in Natural Waters Including Sea-water by Cathodic Stripping Voltammetry Constant M. G. van den Berg and Shaukat H. Khan* Oceanography laboratory, University of Liverpool, Liverpool 169 3BX, UK A simple procedure was developed to determine total organotin in natural waters including sea-water. The method entails adsorptive cathodic stripping voltammetry of the 2-hydroxycyclohepta-2r4r6-trienone(tropo- lone) complex of tin for the determination of organotin after its conversion into inorganic tin by ultraviolet irradiation. The method is free from interference by organic surface-active compounds, as these are decomposed during the irradiation stage. The concentration measured has to be corrected for the inorganic tin originally present in the sample by analysis of a sample from which organotin has been removed.Organotins are removed by passing the acidified sample through a CI8 column (Sep-Pak), which adsorbs all organotin from the sample, leaving inorganic tin in solution. The limit of detection for total organotin is approximately 10 pmol dm-3 (with a deposition time of 10 min), but this increases with the amount of inorganic tin present in the sample (typically up to 100 pmol dm-3). Keywords: Cathodic stripping voltammetry; organotin determination; natural waters; sea-water Methyltin compounds can be produced by natural methylation of inorganic tin in sediments,' but those of butyltin are exclusively of industrial origin.2 Natural waters near ports and marinas in coastal waters and estuaries can be contaminated with organotin as a result of the application of antifouling tributyltin preparations.In order to monitor these inputs it should be sufficient to measure the level of organotin as an indicator of the degree of contamination. As no separation is required, a comparatively simple analytical procedure can then be used. Several mcthods exist for determining organotin com- pounds (butyltin) in natural waters. These are based on hydride formation with cold-trapping,3.4 and extraction of the complexes with 2-hydroxycyclohepta-2,4,6-trienone(tropo- lone) ,s both methods involving detection by atomic absorp- tion spectrometry. These methods are not readily automated and are, therefore, not convenient for use in monitoring.Cathodic stripping voltammetry (CSV) is suitable for automa- tion by using such computerized instrumentation as is cur- rently under development in our laboratory and which could provide a better alternative to the methods mentioned above. The proposed method is based on an existing CSV procedure for determining inorganic tin in sea-water.6 Tin forms a complex on the addition of a chelating agent (tropolone) at pH 2. The complex is allowed to adsorb on a hanging mercury drop electrode (HMDE) and is quantified by measuring its reduction current by means of a reducing potential scan, with use of differential-pulse modulation. In this procedure, organotin is converted into inorganic tin by ultraviolet (UV) irradiation prior to the measurement with CSV. In order to calculate the concentration of organotin, the total tin concentration must be corrected for the contribution of inorganic tin.Inorganic tin is determined by the analysis of a sample from which organotin has been removed by pre-treatment on a CIS adsorption column. Experimental Apparatus and Reagents A Princeton Applied Research (PAR) 174A polarograph was connected to a PAR 303 HMDE. The surface area of the HMDE was 2.9 mm2. The solution (10 ml) in the voltammetric * Present address: National Institute of Oceanography, 37-K/6, P.E.C.H.S., Karachi 29, Pakistan. cell was stirred with a polytetrafluoroethylene (PTFE)-coated magnetic stirring bar at a stirring rate of 400 rev min-I. High-density polyethylene (HDPE) containers (Nalgcne) were used to store the water samples.Prior to use these containers were soaked in hot diluted detergent (0.1% at 60 "C for 1 d), rinsed with distilled water, soaked (by immersion) for 1 week in 50% HCI (AnalaR, BDH), then in 2 mol dm-3 HN03 (AnalaR, BDH) (1 week), subsequently filled with dilute acid, 0.01 mol dm-3 HCI (Aristar, BDH), and stored in individual sealed plastic bags. Sea-water (salinity: approximately 32) used for the experi- ments was collected from the Menai Straits and stored in 60 I HDPE containers. This water was filtered (0.45 pm Nucle- pore) prior to use. Distilled water, used for reagent dilution and rinsing, was produced by a double-distillation system (made of silica). An aqueous stock solution of 0.1 mol dm-3 tropolone (Aldrich) was prepared weekly in distilled watcr and used without further purification. Stock standards of monobutyltin trichloride (Strem Chemicals), dibutyltin di- chloride, tributyltin chloride and tetrabutyltin (Fluka) were stored in a freezer, and were diluted immediately prior to use with methanol (HPLC grade) in glass calibrated flasks; these solutions contained 1 x 10-4 mol dm-3 organotin. Working organotin standards were prepared in glass containers by dilution with methanol.Procedure for Determining Organotin Dissolved organic materials were decomposed by UV pho- tolysis of the acidified [to pH 2 with HC1 (Aristar)] sample in a capped 40 ml silica tube using either a 100 W low-pressure or a 1 kW medium-pressure mercury vapour lamp (Hanovia), housed in a laboratory-built aluminium container fitted with a cooling fan.Preliminary tests showed that organotin was converted quantitatively by this treatment. Total dissolved tin (including organotin and inorganic tin) was then determined by CSV. The CSV procedure used to determine combined tin (inorganic tin and organotin) was adapted from that of van den Berg et af.6 Briefly, this procedure consisted of the addition of 40 pmol dm-3 of tropolone to 10 ml of the acidified, UV-irradiated sample. The solution was de-aerated by purging with nitrogen (6 min). Deposition was carried out at a potential of -0.8 V for 1-5 min, depending o n the expected concentration of tin. The re-oxidation potential was -0.4 V, and the potential scan was in the negative direction,586 ANALYST, JUNE 1991, VOL. 116 using differential-pulse modulation.The re-oxidation time was 20 s. The concentration of inorganic tin was measured after the removal of organotin from the sample by passage of an acidified (pH 2) aliquot (15 ml) through a Sep-Pak CIS column (Waters; column volume approximately 1 ml) at a flow rate of approximately 2 ml min-1. The filtrate was UV irradiated, and dissolved inorganic tin was determined by CSV. The organotin concentration was calculated from the difference between the concentrations of total dissolved tin and total dissolved inorganic tin. Results and Discussion Determination of Organotin by CSV Previous experiments involving the use of polarography with a dropping mercury electrode have shown that organotin compounds can be reduced at potentials more negative than those required for inorganic tin, v i z ., -0.8 V (monobutyltin) and -0.9 V (dibutyltin).’ In preliminary experiments an attempt was made to determine organotin directly by CSV using a deposition potential of <-0.8 V. No peak was obtained for organotin (monobutyltin, dibutyltin and tributyl- tin) even with deposition potentials as negative as -1.6 V. It is not clear why the dissolved organotin could not be reduced and plated on to the mercury electrode under these condi- tions, but organotin could not be determined directly by CSV without prior conversion into inorganic tin by other means. Ultraviolet irradiation is commonly used to photolyse dissolved organic material prior to voltammetric analysis.8 Determinations of tin (between 1 and 100 nmol dm-3) in sea-water by CSV, before and after UV treatment (with use of either the 100 W or the 1 kW system), showed that the organotin was quantitatively converted into inorganic tin by -0.4 -0.6 -0.8 PotentialN Fig.1 Determination of organotin in sea-water using CSV. (a) 20 nmol dm-3 organotin recovered from a CI8 column using 0.5 mol dm-3 HC1; deposition time, 30 s. (b) 1 nmol dm-3 organotin in sea-water after UV irradiation treatment; deposition time, 3 min. (c) 14 nmol dm-3 organotin eluted with methanol from a CIS column; deposition time, 30 s irradiation for 3 h. Thereafter, the concentration of converted tin was measured by CSV. Comparative experiments invol- ving irradiation at neutral and low pH showed that the irradiation had to be carried out after acidification of the sample to pH 2 in order to prevent adsorption of tin on the wall of the silica tube; losses of between 20 and 50% of the dissolved tin were observed if irradiation was carried out at neutral pH.A CSV determination of 1 nmol dm-3 of organotin (tributyltin) in sea-water is shown in Fig. 1. In ‘clean’ sea-water (sea-water of oceanic origin) it is possible to determine tin without prior UV treatment.6 The concentration of surface-active organic material in this water is sufficiently low to allow detection of even very low (10 pmol dm-3) levels of tin ( i . e . , labile tin, organotin being non-labile) without interference. The organotin concentration in such water can, therefore, be measured by the difference between the labile and total (after UV treatment) concentra- tions.Organic material in waters of estuarine and coastal origin interferes with the determination of labile tin (which is normally low at <0.1 nmol dm-3 and requires a deposition time of several minutes), causing poor sensitivity. Inorganic tin in such waters can, therefore, be determined only after UV treatment, which also converts organotin into inorganic tin. In practice, therefore, it is not possible to determine organotin separately from inorganic tin in natural waters without some other treatment to remove either inorganic tin or organotin from the sample. Specific Removal of Organotin from Solution The efficiency of CIS coated Sep-Pak for the adsorption of organotin from sea-water was investigated. This adsorbent came pre-packed in small (1 ml) columns that were connected to the spout of a 250 ml extraction funnel (HDPE) fitted with a PTFE tap (Nalgene) via a short length of silicone tubing.The sea-water was passed through the adsorption columns at a low pressure stream of nitrogen (approximately 0.2 bar) and at a flow rate of about 2 ml min-1. The adsorption efficiency was tested by determining residual tin in the filtrate after UV irradiation. In addition, the recoveries of organotin by elution with various organic solvents were compared. Adsorption experiments with 10 and 100 nmol dm-3 tributyltin added to sea-water and distilled water (acidified and unacidified), also containing 0.2 nmol dm-3 inorganic tin, showed that the organotin adsorbed quantitatively (100%) onto the CIS column, leaving non-adsorbed inorganic tin in the filtrate.Separate experiments with inorganic tin at low (50 pmol dm-3) and elevated (0.2 nmol dm-3) levels showed no adsorption on the CI8 column from acidified or unacidified sea-water and distilled water, indicating that tributyltin was adsorbed specifically by this material. The organotin concen- tration in a natural water sample can, therefore, be calculated from the total dissolved (combined organotin and inorganic tin) concentrations by correcting these for the inorganic tin concentration. Elution of Adsorbed Organotin With Solvents Attempts were made to recover the organotin adsorbed on the Sep-Pak column in order to develop a procedure by which organotin could be determined specifically. Various solvents and acids were assessed and recoveries were evaluated by CSV analysis after the conversion of the organotin into inorganic tin by UV irradiation.As the UV treatment prior to the CSV analysis required that the solution containing the eluate was mainly aqueous, the solvent used for elution had to be soluble in water or had to be evaporated to dryness prior to the dissolution in water. The recovery was tested by adsorption of a known amount (0.2-2 nmol) of organotin on a Sep-Pak column followed by elution, and analysis of the eluate after UV treatment. TestsANALYST, JUNE 1991, VOL. 116 587 with toluene, pentane and dichloromethane showed that a considerable fraction of the organotin evaporated with the solvent when it was dried by blowing down with nitrogen at room temperature (approximately 20 "C).For instance, the recovery of 50 nmol dm-3 organotin in pentane was only 3.8 nmol dm-3 (8%) and for 100 nmol dm-3 organotin in dichloromethane the recovery was between 7 and 18 nmol dm-3 (7-18%); residues of toluene also interfered with the subsequent analysis of tin after UV treatment of the eluate. Attempts to carry out partial blow-downs in order to diminish the evaporation of organotin were not successful as the organic solvents continued to interfere with the CSV analysis even after UV treatment. This is a drawback of the CSV technique as blow-down to dryness would not be essential for an alternative detection technique such as atomic absorption spectrometry. Organotin (20 nmol dm-3) added to sea-water or distilled water also containing 20% methanol was recovered fully by the UV treatment (carried out at pH 2) with subsequent CSV analysis.However, the recovery of the organotin by UV treatment of the methanolic and ethanolic Sep-Pak eluates was poor and variable. Variable recoveries ranging from 50 to 100% were achieved by increasing the volume of the eluent (methanol and ethanol). It appeared that the eluent contained an unknown interferent that lowered the sensitivity of the analysis for tin and sometimes produced a peak that over- lapped and interfered with the peak for tin. A scan for organotin eluted with methanol is shown in Fig. l(c); the magnitude of the small peak next to that of tin varied and the compound producing it probably originated from the methanol or the C18 column. The elution with HCI was incomplete, but at least no interference was introduced: the recovery was 15% with 0.2 rnol dm-3 HCI, 23% with 0.5 rnol dm-3 HCI and 19% with 1 rnol dm-3 HCI (5 ml aliquots); 66% recovery was achieved with a mixture of 50% methanol and 1 mol dm-3 HCI (5 ml).These recoveries were considered to be too low to provide an accurate and reproducible determination of organotin. A scan for organotin eluted with 0.5 rnol dm-3 HCl is shown in Fig. l(a). Effective recovery of the adsorbed organotin from the Sep-Pak column was, therefore, not achieved. Sample Storage The effects of sample storage on the concentration of dissolved tin were determined from the investigation of contamination arising from leaching of the bottle, and adsorption of organotin on to the bottle.Acidified (pH 2 ) sea-water, with and without added organotin, was therefore stored in HDPE and glass containers. The HDPE containers cleaned by soaking in 2 rnol dm-3 HCI (for 2 weeks) were compared with containers that had been soaked in 50% HCl (for 1 week) followed by a soak in 1 rnol dm-3 HN03. It was found that tin was released from HDPE containers cleaned with 2 rnol dm-3 HCI, but not from those cleaned with 50% HCl (Table 1). Further, organotin was found to adsorb on both HDPE and glass containers (Table 2), even if the water had been acidified to pH 1 with HN03 (Table 3). Adsorption was minimized if a large sample volume was used (reducing the surface : volume ratio of the container), but the adsorption rate was still approximately 10% per day from 2.5 1 of acidified sea-water containing 100 nmol dm-3 organotin.Samples should, therefore, be analysed as quickly as possible in order to minimize adsorption, or the samples should be stored in the silica tubes in which the UV irradiation is to be carried out. Interferences Potential interfering species include inorganic tin, other metals that form an adsorptive complex with tropolone, Table 1 Effect of storage on the concentration of inorganic tin in acidified (pH 2) sea-water in HDPE containers Inorganic tin/nmol dm-3 Immediate After 3 d After 5 d Bottle 1" 0.58 2.00 2.6 Bottle 21- 0.58 0.60 0.56 * Cleaned in 2 rnol dm-3 HCl only. f Soaked in 50% HCI and 1 mol dm-3 HN03. Table 2 Effect of storage on the organotin concentration added to acidified (pH 2) sea-water in HDPE containers Organotidnmol dm-3 lmmediate After 1 week Loss (YO) 250 ml HDPE container 15.4 3.1 80 104.8 19.6 81 container 17.7 4.4 75 102.4 19.3 81 11 HDPE Table 3 Storage of 100 nmol dm-3 organotin added to sea-water acidified with 4 ml of concentrated HN03 per litre of sea-water (pH about 1) in a brown glass container of 2.5 1 capacity Storagc Organotid time/d nmol dm-3 Loss (Yo) 0 100.7 0 1 100.5 0.2 2 80.2 20 4 72.2 28 6 64.0 36 10 45.9 54 surface-active organic substances that adsorb competitively on the HMDE and organic complexes of tin.The major interferent is inorganic tin as this is a co-determinant with organotin. Inorganic tin must be determined separately after removal of organotin by passing the sample through a CIS column. Organic complexes of tin in natural waters could interfere if these were adsorbed by the CIS column.The interference from such organically complexed tin is minimized by carrying out the adsorption at low pH (pH 2 ) , when all the metal complexes are expected to be dissociated. Adsorption of inorganic tin on 'uncapped' silica groups of the CI8 cartridge is also prevented at this pH (such adsorption has been shown to occur for several trace metals when adsorption was carried out at neutral pH) .9 Organic surfactants occurring in natural waters are known to diminish the sensitivity of CSV as a result of competitive adsorption .S Electroactive organic compounds could adsorb on the electrode and produce an interfering reduction wave.8 These interfering species are removed effectively by the UV pre-treatment of the sample .8 Metals other than tin can interfere by forming a complex with the added tropolone and by forming an electroactive adsorbed complex.A number of metals have been studied previously and only molybdenum was found to present serious interference in sea-water as its concentration is comparatively high (approximately 100 nmol dm-3) and it produces a reduction peak close to that of tin.6 Interference by this element is eliminated by using a very negative deposition potential (-0.8 V), where complexed molybdate ions are reduced and desorbed from the electrode. Tin is amalgamated during the deposition step as the deposition potential is more negative than the reduction potential of the tropolone complex with tin. The tin is re-oxidized and re-adsorbed, during the re-oxidation step at -0.4 V, from the unstirredANALYST, JUNE 1991, VOL.116 solution during a period of 20 s prior to the CSV scan. Hence, only the amalgamated elements such as lead or cadmium can interfere with the analysis for tin. The reduction potential of lead is more positive than that of tin and the response is poor, hence this element was found not to interfere. The reduction potential for cadmium is very close to that of tin, and a high concentration of this element (100-fold higher than that of tin) can interfere to the extent that 0.5 nmol dm-3 of tin is masked by 50 nmol dm-3 of cadmium. Normally, cadmium concentra- tions in sea-water are fairly low (0.5 nmol dm-3 or less), hence no interference is to be expected in unpolluted waters.Limit of Detection The limit of detection for organotin in the proposed procedure is determined by two factors: the limit of detection of CSV for tin and the background level of inorganic tin. The sensitivity of the measurement is not affected by surface-active organics because the CSV analysis is carried out after UV irradiation of the sample, hence the limit of detection is equal to that for inorganic tin at approximately 5 pmol dm-3 when a deposition time of 10 min is used.6 The background level of inorganic tin is low at about 50-100 pmol dm-3 in river water, and it is only 5-10 pmol dm-3 in unpolluted sea-water.6 The organotin concentration should be twice that of inorganic tin in order to give an accurate result, as the organotin concentra- tion is corrected for the contribution of inorganic tin to the total dissolved tin concentration. Therefore, the limit of detection for organotin is of the order of 20-200 pmol dm-3, depending on the actual concentration of inorganic tin. This work has been carried out with the support of the Procurement Executive of the Ministry of Defence. References Gilmour, C. C., Tuttle, J. G., and Means, J. C., in Marine and Estuarine Geochemistry, eds. Sigleo, A. C., and Hattori, A., Lewis Chelsey, MI, 1985, pp. 239-258. Francois, R., and Weber, J. H., Mar. Chem., 1988, 25, 279. Andreae, M. O., in Trace Metals in Sea Water, eds. Wong, C. S . , Boyle, E., Bruland, K. W., Burton, J. D., andGoldberg, E. D., Plenum, London, 1983, pp. 1-19. Balls, P. W., Anal. Chirn. Acta, 1987, 197, 309. Chapman, A. H., and Samuel, A., Appl. Organomet. Chem., 1988,2, 73. van den Berg, C. M. G., Khan, S. H., and Riley, J. P.. Anal. Chim. Acta, 1989, 222,43. Weber, G., Anal. Chim. Acta, 1986, 186, 49. van den Berg, C. M. G., in Chemical Oceanography, ed. Riley, J. P., Academic Press, London, 1989, vol. 9, pp. 197-245. Mackey, D. J., Mar. Chem., 1985, 16, 105. Paper 0104785F Received October 24th, 1990 Accepted December 20th, 1990

ISSN:0003-2654    

DOI:10.1039/AN9911600585

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST. JUNE 1991, VOL. 116 589 Mechanistic Study of Fluoride Ion Sensors Werner Moritz and Lothar Muller Department of Chemistry, Humboldt Universitat Berlin, Bunsenstrasse I , 0- 1080 Berlin, Germany Fluoride ion sensitive semiconductor sensors were investigated with regard t o the influence of pH, the limit of detection obtained and the response time. The results are the same as those obtained for a well-known single crystal electrode. The dissolution rate of LaF3 was determined using the isotope 140La. The OH-IF- exchange reaction and the isotope exchange kinetics of fluoride ions between the solution and the sensor layer were investigated. In the pH range from 4 t o 8 it could be concluded that both the limit of detection and the response time are determined by the ion-exchange rate.Keywords: Sensor; fluoride ion; lanthanum(///) fluoride; ion exchange; influence of pH Starting with the results of Frant and Ross,l LaF3 single- crystal electrodes have found wide application. The possibility of using thin polycrystalline layers of LaF3 for ion-selective field effect transistors (ISFETs) was shown previously .2 Sensor properties such as the sensitivity, the selectivity, the influence of pH and the limit of detection are identical with those for a single crystal and a polycrystalline layer. There- fore, the results concerning the sensor mechanism should be valid for both. The lower limit of detection with fluoride ion selective electrodes was shown to be about 1 x 10-6 rnol dm-3.1.34 The reason for this limit is still under discussion. Often it has been assumed that the limit of detection of fluoride is affected by the dissolution of LaF3.7 The solubility product of the single crystal should then be 1 x 10-29 or 1 x 10-30 (mol dm-3)4.1,3 However, Baumann4 found that the dissolution of LaF3 is a very slow process.Equilibrium was not established even after 20 d, hence, it was concluded that the solubility product must be greater than 1 x 10-24 (mol dm-3)4. Buffle et aZ.3 explained the limit by desorption of F- from the electrode and cell surfaces. In this instance, rinsing the electrode for a long period of time should give an improved limit. The influence of different buffer systems was shown by Kauranen,s who provided an explana- tion by assuming different complexes of La3+. The response of fluoride single-crystal electrodes after a change in F- concentration has previously been investi- gated*--'O and showed that the response time depends on the concentration. The best fit of the change of potential with time was attained using an empirical equation as proposed by Muller11 : where E, = the potential at time t , El = the equilibrium potential in the first solution and a and b = empirical values.For long periods of time in eqn. (l), eqn. (2) follows On the basis of eqns. (1) and (2) we obtain aib = t50 (3 1 where Eeq = the equilibrium potential in the second solution and tsO = the time for half of the potential change. The influence of pH on the electrode potential is well known for the fluoride ion sensor. An exchange of OH- in the solution for F- in the surface of the LaF3 was supposed, but no experimental evidence was presented.In the present work sensor properties for thin LaF3 layers on semiconductor structures are related to exchange experiments with OH- and dissolution kinetics of the layer and ion-exchange experiments using the isotope 18F. Experimental Lanthanum(m) fluoride layers (250 nm thick) were produced by vapour deposition on the semiconductor/isolator substrates ( Si02/Si3N4) and with subsequent characterization by capaci- tance voltage (C-V) measurements as described previously.2 The shift of the C-V curves on the voltage axis can be expressed as the change in gate voltage (U,) or electrode potential ( E ) which are identical in this study. The dissolution of the LaF3 layers was investigated by using the isotope "La, produced by neutron activation of the complete structure (semiconductor, isolator and LaF3 layer).The rotating disc principle was used to prevent diffusion problems. The dissolution rate was calculated from the increase in activity of the solution (20 ml). For the step-wise change in concentration, the wall-jet principle was used for the semiconductor sensors. The injection principle was used for measurements with the single-crystal electrode. The exchange of OH- ions between the solution and the LaF3 layer was investigated using a thin-layer method. The sandwich structure used is shown in Fig. 1. An area of 2 cmz of the LaF3 layer was in contact with 0.1 ml of an alkaline solution. After establishing the exchange equilibrium, a portion of 0.02 ml was removed and mixed with the same volume of total ionic strength adjustment buffer (TISAB). In this solution the F- concentration was measured using a F- ISFET.Before the experiments the layers were stored in 0.1 rnol dm-3 NaF, pH 5.5 for more than 2 h and then rinsed for 2 h with de-ionized water. The ion-exchange kinetics of fluoride ions were investigated using the isotope 18F in solution. The rate of exchange was calculated from the activity of the LaF3 layer after exposure to the solution for different times. (For further details see reference 12.) Results and Discussion Influence of pH The influence of pH on the potential of the F- sensor, using a thin LaF3 layer on a semiconductor substrate, is shown in Fig. 2 for different concentrations of F- ion.There is a well- established correlation with the results for the single-crystal electrode.13 No significant influence of pH was found in the range 4-9 in a 1 X 10-4 mol dm-3 NaF solution. The pH-independent range is a function of the F- concentration; it becomes smaller for lower F- concentrations. There is some hysteresis depending on the direction of the pH change, as shown for 1 x 10-4 mol dm-3 NaF (Fig. 2). At a pH of less than 4 the concentration of free F- is diminished by the reaction [shown in eqn. (4)] in the solution. H+ + 3F- HF + 2F- HF2- + F- F HF32- (4)590 0 - $400 -. 3 d -200 ANALYST, JUNE 1991, VOL. 116 - - I 1 Silicon La F3 I i I Silicon I Fig. 1 Schematic diagram of the cross-section of the sandwich structure used for the investigation of the exchange of OH- and F- 0 2 4 6 8 10 PH Fig.2 Effect of pH on the potential (hence UG) for different fluoride concentrations: 1 and 2, 1 X 3, 1 X lo+; and 4, 1 x 10-6 mol dm-3 NaF. The arrows show the direction of the pH change Hence, the fluoride sensor works correctly by detecting the activity of the free ions. It is generally assumed that the performance of the fluoride electrode does not depend on pH in the range 4-9. Experi- ments in this pH range (Fig. 3) showed that this is not true for the limit of detection. It is markedly influenced by a change in the pH from 4 to 8. Ferry et aZ.14 published a dependence of the limit of detection on pH in the range 11-13. In order to give a general view, their results are given in the same figure (Fig.3). The influence of fluoride concentration and pH on the potential of the electrode (or difference in gate voltage, Uc) can be expressed by eqn. (5) E = EO + RT/zFln(aF + A x aoHBj (5) where Eo = standard potential, R = gas constant, 7' = absolute temperature, z = charge number, F = Faraday constant A = constant giving the influence of pH, and a activity of the ion. By using a plot of the logarithm of the apparent concentra- tion at the limit of detection against pH, the exponent B , characterizing the effect of pH, was found to be 0.21 in the pH range 4-8 [correlation coefficient (Y) = 0.9974 and A = 5.6 X 10-81. At pH 11-13 the influence of pH is more pronounced15 and B is equal to 0.5. The influence of pH on the limit of detection was shown to be the same for the single-crystal electrode as for the polycrystalline layer.Note that eqn. (1) is not the Nikolsky-Eisenmann equation which would necessi- tate that B = 1. Dynamic Response The dynamic response of the semiconductor structure with a thin polycrystalline LaF3 layer is shown in Fig. 4. Attempts to obtain a linear relation on graphs for log Elt or E l P were not successful. Only by using eqn. (1) transformed into eqn. (6) was a linear relation (I- >0.9999) obtained for the poly- crystalline layer and for the single-crystal electrode. t/(E, - El) = bt + u ( 6 ) -300 1 \ E D C B A k 2 -Log (cF-/rnol 4 dm-3) 6 8 Fig. 3 Influence of pH on F- sensitivity and lower limit of detection. pH: A, 4; B, 5; C, 6; D, 7; E , 8; F, 11; G, 12; and H, 13 (curves F-H were obtained using values given in reference 14) As shown in Fig.4 the response time of the fluoride sensor depends on the concentration of the ion to be analysed. In this respect it differs from many other ion-selective electrodes. Furthermore, it can be concluded that the F- diffusion in the solution is not the process determining the dynamic response of the electrode. As the time taken for half of the potential change, t50, is directly related to the parameters a and b of eqns. (1)-(3j, tso can be used as a characteristic value for the response. The dependence of 150 on the F- concentration can be approxi- mated by eqn. (7), with a value of the exponent rn = -0.55 k 0.17. (7) For F- concentrations >1 X 10-5 rnol dm-3 and pH <8, the equilibrium potential is not influenced by the pH.Therefore, it was surprising that a dependence of t50 on pH was obtained. The influence of the pH of the solution on the logarithm of the response time is shown in Fig. 5. A quantitative relation is given in eqn. (8) with n = 0.20 t 0.05. The effect of pH on the response time was also shown to be true for single-crystal electrodes. The comparison of the response time of several single-crystal electrodes yields significant differences between individual electrodes. The same was reported by Mertens et a1.9 The response time for the thin LaF3 layers was between 0.25 and 1.6 s for a change in concentration from 1 X 10-5 to 1 X 10-4 mol dm-3 NaF (pH = 5.5). These values either agree with the results for the single-crystal or are slightly better. Dissolution of LaF3 The rate of dissolution of polycrystalline LaF3 layers in H 2 0 was investigated in order to check whether it bears any relationship to the limit of detection.Long-term experiments were restricted by the decay time of 140La (half-life, 40.6 h). Results for the first 100 min of the dissolution are given in Fig. 6. The linear dependence of the dissolved LaF3 on time is a result of a reaction order equal to 0. Dissolution rates were found to be in the range from 1.2 x 10-13 to 3.6 x 10-12 mol cm-2 min-1. This slow process means that during the first few hours only one atomic layer is dissolved. Considering this and the inhomogeneity of the polycrystalline layer it can be deduced that the dissolution rate decreases with time. Therefore, after 4 days of rinsing, the dissolution rate is 4 x 10-14 rnol cm-2 min-1.There is a reaction order equal to 0 for 80 h as shown in Fig. 7. It was not possible to determine the solubility product because it was impossible to obtain a solution of constant concentration within the decay time of 140La. The slowANALYST, JUNE 1991, VOL. 116 591 0 1 I I I 0 5 10 15 t/S Fig. 4 mol dm-3 NaF Response of LaFi layers on semiconductor substrates: 1. 1 x and 3 , l x 10-4 + 1 x 10-3 + 1 x 10- 5 ; 2 , l x 10-5 + 1 x 1.5 I I -0-5 2 6 8 PH Fig. 5 solution: 1 , 1 X 10-6 -+ 1 X 10-5: and 2. 1 x 10-5 + 1 X mol dm-3 NaF Dependence on the response time (fs0) on the pH of the dissolution process observed makes the values given in the literature for the solubility product of the single crystal appear doubtful.4 N I $ 3 E - z 2 0 F . Q 1 I I 0 10 50 100 tlmin Fig. 6 Dissolution of LaF3 in water; short time range I 0 10 50 80 Fig. 7 Dissolution of LaF3 in water; long time rangc tlh ~- ~ Table 1 Concentration of F- released from a 250 nm LAF3 layer in 10 min coH-lmol dm-' cp-lmol dm-' 1 x 10-3 1 x 10-2 3 x lo-' 4.5 x 10-5 9.0 x 10-5 3.2 x lo-* 1 8.7 x 10-4 Exchange of Fluoride Ions for Hydroxide Ions In order to determine the reason for the interference by OH- the exchange reaction between OH- in solution and F- in the LaF3 layer was investigated by using the sandwich structure mentioned above (Fig. 1). Various concentrations of KOH solution of from 1 x 10-3 to 1 rnol dm-3 KOH were used. The F- concentrations measured in the solutions after 10 min are given in Table 1.During this time the exchange equilibrium was established. It can be seen that the F- concentration released increases with increasing OH- ion concentration. The results given in Table 1 might possibly be explained in one of two ways, firstly by an OH- for F- exchange or, secondly, by a dissolution of the layer. In 1 mol dm-3 KOH the amount of F- released corresponds to a 19 nm layer of LaF3. In experiments using the isotope 14oLa, the dissolution rate of LaF3 in 1 rnol dm-3 KOH was found to be <0.05 nm min- 1.6 Therefore, dissolution of the layer cannot explain the F- concentrations given in Table 1. If the F- concentration is caused b y an exchange of OH- for F-. the reaction i11 the revrese direction should be equally possible.In order to examine this the sandwich structure was filled with a buffered (TISAB, pH 5.5) 3.3 X mol dm-3 NaF solution following the experiment with 0.1 rnol dm-3 KOH. The reduction of F- concentration in the solution should be an indication of the reverse exchange reaction. The results show that 65% of the F- released in the first experiment re-entered the layer. (The difference between this and 100% being attributed to the new equilibrium established and experimental errors.) The ion-exchange experiments were repeated more than 20 times with the same layer without any changes in the results. It can therefore be concluded that there is a reversible exchange of OH- from the solution with F-- in the LaF3 layer. This provided another problem, the distribution of OH- in the layer.Two possibilities exist: (i) enrichment of OH- in the surface; or (ii) homogeneous distribution of OH- through the whole layer (250 nm). When using LaF3 layers of different thickness (18 and 250 nm) the release of F- was proportional to the thickness used (within an experimental error of 10%). Therefore, it could be concluded that there is a nearly constant concentration of OH- throughout the layer, from the surface to a depth of at least 250 nm. No experiments with layers of greater thickness were carried out. The exchange equilibrium OH-(s) + F-(1) T-, OH-(1) + F-(s) (9) between the ions in the lattice (1) and in the solution (s) can be writ ten : DOH = K(COH/CF)/[l -k K(COH/CF)] ( 10) where DF and DOH = site filling factors for F- and OH-, respectively, and DF + DOH = 1.For small values of D O H in eqn. (lo), eqn. (11) is obtained. DO€[ = KcOH/cF (11) Experiments starting with Dorr = 0 (with a layer condi- tioned in 0.1 rnol dm-3 NaF of pH 5.5 before the experiment)592 ANALYST, JUNE 1992. VOL. 116 lead to a definite relation between the concentration of F- released and DoEr: DOH = s x CF The value of S, the constant for the geometry of the sandwich structure, can be calculated from the density and thickness of the layer and the volume of the solution. From eqns. (11) and (12) we can obtain eqn. (13) as follows: (12) CF = (KcoH/S)’ (13) According to eqn. (13) there should be a square-root relationship between the concentrations of F- released and OH- in the solution. The dependence of the logarithm of the concentrations cF and cOH, obtained in the exchange experi- ments, is shown in Fig.8. For concentrations of greater than 1 x 10-2 mol dm-3 KOH the slope is 0.49, which agrees with eqn. (13). For lower concentrations a smaller value for the slope was found, indicating that K is not constant if the concentration changes by many orders of magnitude. This corresponds to the concentration dependence of the free enthalpy of ions in a mixed crystal not being ideal. An equilibrium constant of K = 9 x 10-5 was calculated using the results shown in Fig. 8. For the site filling factor DOH a value of 0.037 was obtained at pH 13 in 1 x 10-4 mol dm-3 NaF solution. This means that there is an unexpectedly high level of OH- ions in the F- ion sites in the LaF3. Note that the combination of the solubility products of LaF3 and La( OH), yield eqn.(14), which does not coincide with the experimental results. cF = cOHIKF(L)/KOH(L)]’ (14) Fluoride Ion Exchange Rate In order to improve the knowledge about the processes that determine the potential, the exchange of F- between the solution and LaF3 was investigated using the isotope 1SF. These results are given elsewhere in more detail.12 It was shown that neither the diffusion of F- in the solution nor in the LaF3 is the rate determining step for ion exchange; it was found to be the transfer rate of the F- through the phase boundary. From the experiments follows the important fact that the exchange rate is not only a function of fluoride concentration but also of pH. The influence of pH on the exchange rate ( v ) is shown in Fig.9 for different F- concentrations. Equation (15) represents the effect of the composition of the solution on the rate of isotope exchange v = k C F p ( C O H ) 4 (15) with p = 0.69 and q = -0.22. In 1 rnol dm-3 NaFsolutions at a pH of 5.5, v has a value of 5.25 x 10-8 rnol cm-2 min-’. Conclusions The limit of detection cannot be explained by dissolution of LaF3, because from the dissolution rate it follows that it would take hours in order to obtain a concentration of only about 1 x mol dm-3 in solution. This is still below the limit of detection. A calculation of the stationary surface concentration of F- during the dissolution was accomplished by assuming that the rates of dissolution and diffusion in the solution were equal.By using the 1. Fick’s law with a diffusion layer thickness of 1 X cm and a dissolution rate of 4 x 10-14 the surface concentration of F- was found to be 2 x 10-10 rnol dm-3. Therefore, it can be concluded that dissolution cannot be the reason for the limit of detection of polycrystalline LaF3 layers. As the dissolution rate of single-crystalline material is smaller than that for polycrystalline materials the same is true for a single-crystal electrode. As a result of our experiments it is obvious that the influence of pH is more pronounced than previously thought. The limit of detection, the response time and the exchange rate of fluoride ions all depend on the pH. For the limit of detection two different mechanisms should be discussed. At high pH values, an exchange reaction between OH- in the solution and F- in the LaF3 layer was proven to occur.The concentration of F- released corre- sponds to the limits of detection obtained by Ferry et a1.14 The dependence of the F- concentration on pH according to eqn. (13) leads to the same exponent (0.5) as for the limit of detection and the apparent OH- sensitivity. Therefore, for a high pH, it is concluded that the limit of detection and the apparent OH- sensitivity are the result of the F- ions released by the exchange reaction. An attempt to use the results of the exchange of OH- for F- in the interpretation of the limit of detection over pH 4-9 gives a discrepancy between B = 0.21, for the limit of detection, and the exponent (0.5) obtained from the exchange experiments. Furthermore, a calculation of the F- concentration released at pH 5 , using eqn.(13), gives 3 x 10-8 mol dm-3. This concentration is considerably lower than the detection limit. Hence, the limit of detection appears to be inexplicable by consideration of the equilibrium conditions. It is evident that in the pH range 4-9 the limit of detection, response time and the fluoride ion exchange rate depend on the pH with a value of the exponent close to 0.2. Therefore, kinetic reasons have been proposed in order to explain the OH- dependencies.6 Cammannls used the concept of mixed potentials, derived from electrode kinetics, for ion-selective electrodes. For the fluoride electrode, the equality of the currents for F- and OH- exchange would lead to a mixed potential and finally, to the limit of detection.With increasing I 1 ‘ I 5 s 3 2 1 0 -Log (coH/mol d ~ n - ~ ) Fig. 8 concentration of OH- Variation in the F- concentration released as a function of the 0” 11.5 d 12.0 1 5 6 7 8 PH Fig. 9 1 x 10-4; B, 2 x 10-6; and C. 5 x 10-7 mol dm-3 NaF Dependence of the rate of fluoride ion exchange on pH: A,ANALYST, JUNE 1991. VOL. 116 593 concentration of one of the ions the current increases and it is only this ion that determines the potential. The fluoride ion exchange rates are available from the 18F experiments. An attempt was made previously to try to determine the OH- exchange rate, but it was noted that it was the same as for the fluoride ion in a solution of the same potential.16 This can now be explained because of the release of fluoride ions at high pH.Therefore, it seems to be impossible to measure the exchange rate of OH- at the interface of the LaF3 layer and the solution. Hence, it is not possible to calculate the mixed potential or to construct an Evans’ diagram. The dependence of the fluoride ion exchange rate on the pH is consequently only an indication, for kinetic reasons, of the limit of detection. Comparing the response time experiments and the 18F exchange rates it is evident that the influence of the F- concentration and pH is the same. The exponents m and n of eqns. (7) and (8) are in concordance (but opposite in sign) with p and q of eqn. (15). The difference between the exponents obtained, might be explained by experimental error. This leads to the conclusion that the rate determining step of the potential change by concentration increase is the F- transfer between the solution and the LaF3 layer.Johansson and Norberg17 derived an equation that de- scribes the response of an ion-selective electrode, by using the Butler-Volmer relation that has ion transfer as the rate determining step. They obtained an exponential relation, but as stated above such a relation cannot be used to describe the response of the fluoride ion sensor. The reverse of the exchange rate is correlated to the exchange resistance, R , : R1 = RT/z’Fv (16) By using the exchange resistance and a double-layer capaci- tance C1, at the phase boundary, between the LaF3 layer and the electrolyte, in parallel, it is possible to calculate the response time.t = ClRl (17) But the response is, again, exponential. It has previously been shown that it is possible to get an approximation of the hyperbolic response according to eqn. (1) by a combination of two resistors and two capacitors, thus, leading to a sum of the exponential terms.6 However, two problems were not solved: (i) the physical sense of the second resistor and the capaci- tance are not sufficiently explainable; and (ii) in eqn. (1) only one kinetic parameter is required in order to describe the entire response curve. The use of four variables is not acceptable,6 however, a better model for describing the whole response curve of the fluoride sensor was not found. On the other hand, the experimentally obtained response time could be calculated by using eqns.(16) and (17) at different values of F- concentration and pH of the solution and using a constant capacity, typical for the double layer (1 x 10-5 cm-2 of F-), and the F- ion exchange rate determined for that condition. Consequently, this proved that the F- ion exchange is the rate determining step for the response of the F- ion sensor, but there is still an absence of an adequate physical model for the mathematical description of the response curve. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 References Frant, M. S., and Ross, J . W., Science, 1966, 154, 1553. Moritz, W., Meierhofer, I . , and Miiller, L., Sensors and Actuators, 1988, 15, 211. Buffle. J . , Parthasarathy, N., and Haerdi. W., Anal. Chim. Acta, 1974, 68, 253. Baumann, E. W., Anal. Chim. Acta, 1971, 54, 189. Kauranen. P., Anal. Letters, 1977, 10, 451. Moritz, W., Dissertation B, Humboldt-Universitat Berlin, Germany, 1988. Evans, P. A., Moody, G. J., andThomas, J . D. R., Lab. Pract., 1971, 20, 644. Hawkings, R. C., Corriveau, L. P. V., Kutshneriuk. S. A., and Wong, P. Y., Anal. Chim. Acta, 1978, 102, 61. Mertens. J . , van den Winkel, P., and Massart. D. L., Anal. Chem., 1976,48, 272. Nagy, K., and Fjeldly, T. A., Proceedings of the Third Symposium on lon-Selective Electrodes. Matrafiired. Germany, 1980, p. 287. Miiller, R. H.. Anal. Chem., 1969,41, 113A. Moritz, W.. Herbst. A.. and Heckner. K.-H.. 2. Phys. Chem., in the press. Vesely, J . , and Stulik, K . , Anal. Chim. Acta, 1974, 73, 157. Ferry, D., Machtinger, M., and Bauer, D., Analusis, 1984, 12. 90. Cammann, K . , Das Arbeiten Mit Ionenselektiven Elektroden, Springer-Verlag, Berlin, 1973. Cammann, K.. and Rechnitz, G. A., Anal. Chem.. 1976, 48, 856. Johansson, G., and Norberg, K., J. Electroanal. Chem., 1968, 18, 239. Paper 0/05565 D Received December 11 th, 1990 Accepted February 19th, 1991

ISSN:0003-2654    

DOI:10.1039/AN9911600589

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

ANALYST, JUNE 1991, VOL. 116 595 Automated Determination of Sulphide by Gas-phase Molecular Absorption Spectrometry Toyin A. Arowolo and Malcolm S. Cresser* Department of Plant and Soil Science, University of Aberdeen, Meston Building, Aberdeen AB9 2UE, UK An automated method for the determination of sulphide in solution that involves the interfacing of an automatic sampler, a proportioning pump and a gas-liquid separator to an atomic absorption spectrometer is described. Sulphide ions react with 3 mol dm-3 hydrochloric acid and the released hydrogen sulphide is swept into a gas-liquid separator by an air stream. The absorbance was measured at 200nm using a deuterium hollow cathode lamp. The method is relatively free from interference with a detection limit for sulphide of 0.06 pg mi-’ and relative standard deviations of 1.4-3.3% for repeated analyses. The calibration graph is linear up to 100 pg ml-1 of sulphide.Twenty samples can be analysed in 1 h. The method has been applied to the determination of sulphate-sulphur in plants. Keywords: Gas-phase molecular absorption spectrometry; automated sulphide determination; sulphate- sulphur in plants The determination of anions and cations in solution by conversion of the determinant into a volatile molecular species followed by their molecular absorbance measurements in the gas phase has been thoroughly investigated in the past two decades. The gaseous product is carried by a stream of air or nitrogen to a flow-through absorption cell which is positioned in the light path of the spectrometer in the space normally occupied by the flame of the atomic absorption spectrometer. A narrow band of radiation corresponding to an absorption maximum of the evolved compound is passed through the cell and the absorbance signal of the compound is measured.The technique, which is known as gas-phase molecular absorption spectrometry (GPMAS) , was de~elopedl-~ in this laboratory several years ago during a search for a rapid and reliable method for the determination of ammonium-nitrogen in digests of soil and plant samples using the Kjeldahl method. The GPMAS technique has been applied to the determination of several anions and cations in a variety of biological, environmental and food samples.Gl6 Determination of sulphide is important because of its extreme toxicity as hydrogen sulphide and its objectionable odour.There is also considerable interest in its measurement because one of the steps in the accurate determination of total sulphur in soils and plants involves the conversion of the various sulphur compounds in the sample into sulphide by reduction. The analytical chemistry of sulphide has been included in several reviews of the general analytical chemistry of sulphur compounds. 17-21 A variety of analytical techniques have been applied to the determination of sulphide in environmental samples. The most widely used methods can be grouped into three categories: titrimetric,2(”-*2 electrochemical,23-25 and spectro- scopic. The last includes ultraviolet-visible spectrophoto- metric methods2628 and molecular emission methods.29.30 However, most of these methods are manual and involve considerable manipulation. They lack speed, simplicity and precision for routine analysis of a large daily throughput of samples.Of the various colorimetric methods recommended for the determination of sulphate-sulphur in soils and plants, the Methylene Blue procedure developed by Johnson and Nishita‘h is the most sensitive and accurate. It has been widely used for many years and thoroughly investigated.27 Leggett et ~ 1 . 2 8 developed a flow injection method for sulphide determination by using the Methylene Blue method. Although Vijan and Wood31332 have developed an automated * ‘To whom correspondence should be addressed. version of Cresser’s GPMAS method for ammonia, auto- mated GPMAS procedures have apparently not been de- veloped for sulphide determination.This paper describes the automation of the gas-phase molecular absorption spectrometric method for sulphide determination. It is a rapid and specific method which requires minimal sample treatment. The system has been automated by the introduction of an automatic sampler, a proportioning pump and a gas-liquid separator. The proposed method was applied to the determination of sulphate-sulphur in mixed herbage (grass/clover) . Experimental Apparatus A Shandon Southern A3600 atomic absorption spectrometer was used with a deuterium hollow cathode lamp and an Auto-graph S chart recorder. The spectrometer was modified for a non-flame cold vapour analysis by removing the burner head and replacing it with a 13 cm long, quartz-windowed flow-through absorption cell supported by a holder.The carrier gas (air) was introduced into the system (as shown in Fig. 1) via a plastic T-piece similar in dimensions to the gas-liquid separator. The distance between the point at which the carrier gas entered the system and the outlet of the tube carrying the reacting solutions/hydrogen sulphide was about 7cm (see Fig. 2). The control of the lower flow-rates was achieved by passing the compressed air through a flow meter which was connected to a Brooks Flow Controller No. 8943 (Brooks Instrument, Hatfield, PA, USA). All measure- ments were made at 200 nm. Reagents All reagents were of analytical-reagent grade and de-aerated, de-ionized water was used throughout. Sulphide stock solution, 500 pg ml-1.Prepared by dissolving 1.875 g of sodium sulphide (Na2S.9H20) in 500 ml of 25% sulphide anti-oxidant buffer (SAOB) solution. Working standards were freshly prepared each day in 25% SAOB by the least number of dilution steps possible. Interferent solutions, 1000 pg ml-1. Solutions of a range of cations and anions were prepared from analytical-reagent grade salts. Stability of sulphide solutions Sulphide solutions are unstable as they are very readily oxidized by the ambient air. The SAOB23 is a reagent usually596 'I ANALYST, JUNE 1991, VOL. 116 Hollow cathode lamo ml min-, Recorder Monochromator + 0.6 Air 1 1 - PMT 1.0 Sampe 2 0.23 HCI 3 - 1.6OWash 4 - 1.60 5 - - J - - I flow controller - Waste ( t )Waste - . Fig. 1 spectrometer system Schematic diagram of the autoanalyser-atomic absorption TO absorption 1 /pipette tip 5 mm i.d.i , , f ' ' From manifold- T! I4-35 mm From Brooks- flow controller - 14-35 m m 4 I To waste via pump Fig. 2 Gas-liquid separator and associated connections added to samples containing sulphide in order to raise the sample pH, free the sulphide bound to hydrogen, fix the total ionic strength and retard the oxidation of sulphide. In the original procedure from Orion Research,23724 the SAOB consisted of sodium hydroxide, sodium salicylate and ascorbic acid in de-aerated, de-ionized water.33 Baumann25 used an alkaline ethylenediaminetetraacetic acid (EDTA)-ascorbate solution for sample treatment in the direct determination of sulphide at concentrations >30 pg 1-1 with an ion-selective Table 1 Operating conditions Spectrometer- Wavelength setting 200 nm Mode Absorbance Lamp current 10 mA Slit-width 0.50 mm Bandpass 3.0 nm Recorder- Sensitivity Chart speed 2.5-100 mV full scale 5 mm min- Proportioning pump and automatic sampler- Sampling cycle 45 s Wash cycle 45 s Air flow-rate 0.6 ml min-1 3 mol dm-3 HCl flow-rate 0.23 ml min-l Sample or wash uptake rate 1.0 ml min-1 Carrier gas (air) flow-rate 13.3 ml min-l Mixing coil 29 turns 200 250 300 Wavelengthlnm Fig.3 Absorption spectrum of hydrogen sulphide electrode (to increase sensitivity) while Tanaka et al.34 included EDTA in the absorption mixture used to trap atmospheric hydrogen sulphide (to prevent oxidation of the sulphide ions). When sodium salicylate was replaced by EDTA, a noticeable improvement in the stability of the sulphide solution was observed.Consequently, the SAOB used in this study consisted of 2 mol dm-3 sodium hydroxide, 0.2 mol dm-3 ascorbic acid and 0.2 mol dm-3 EDTA solutions. The ascorbic acid serves to prevent the oxidative loss of sulphide by its own oxidation to dehydroascorbic acid35 while the EDTA masks the trace metal ions that are catalysing the oxidation.34 Instrumental Operation The operating parameters for the various parts of the instrument and manifold are shown in Table 1. The manifold tube sizes were selected by trial and error until the optimum sensitivity was achieved. A slit-width of 0.5mm was used throughout. Cressers-7 has already observed that the slit- width is not crucial for sulphide determination owing to the simplicity of the hydrogen sulphide spectrum (see Fig.3). This observation was also confirmed in the present investigation. The instrument was allowed to warm up for 5 min with the sampling needle in the 'wash' position. The manifold tubes were lowered into the appropriate reagent bottles and the solutions were allowed to flow. With the carrier gas (air) flow rate set at 13.3 ml min-1, air was allowed to fill the absorption cell. After the entire cell system had equilibrated, the zero on the instrument and recorder was adjusted.ANALYST, JUNE 1991, VOL. 116 597 Procedure Samples were supplied to the manifold, shown in Fig. 1, by a Technicon Sampler 11 with a 40 sample capacity. The reacting solutions (sample and hydrochloric acid) were carried by a Technicon proportioning pump and were mixed in the mixing coil just before entering the gas-liquid separator.The gas-liquid separator was made, after several preliminary experiments, from a glass T-piece (see Fig. 2) connected to the absorption cell by a pipette tip and a narrow bore tube (i.d. 0.82mm). The evolved hydrogen sulphide was swept by the carrier gas into the flow-through absorption cell which was aligned in the optical path of the spectrometer. Once the sampler tray was loaded with blanks, standards and samples, the sampler was switched on and the analysis commenced. The absorbance values of the standards were measured to prepare a calibration graph. Results and Discussion Optimization of Experimental Conditions Effect of carrier gas flow rate Supplementary air was introduced to sweep the evolved hydrogen sulphide into the absorption cell.The effect of the carrier gas flow rate on the absorbance signal was evaluated by making a series of analyses of a 20 pg ml-1 sulphide solution while varying the flow rate from 10 to 30 ml min-1. The results, shown in Fig. 4, indicate that the sulphide absorption intensity varied with the carrier gas flow rate, i.e., decreasing gradually with increasing flow rate. As expected, the highest absorbance signals were observed at low flow rates. This is because the dilution of the evolved hydrogen sulphide by the carrier gas was less. Flow rates lower than 10 ml min-l gave rise to very broad peaks while higher flow rates (>30 ml min-1) were not suitable because of the consequent dilution of the evolved hydrogen sulphide.Also, high flow rates might force liquid into the cell. Most of the data in this study were collected at a flow rate of 13.3 ml min-I. Effect of temperature The effect of temperature on the rate of evolution of hydrogen sulphide was investigated by lowering the mixing coil into hot water at different temperatures (between 40 and 65 "C) but no significant improvement was observed. Therefore, this study was carried out at room temperature. Choice of wavelength The effect of the choice of wavelength on the slope of the calibration graph for 0-20 pg ml-1 of sulphide solution was investigated and the results are shown in Fig. 5. A linear regression treatment of the data obtained yielded the follow- ing relationship between absorbance, A, and sulphide concen- 0.13 1 0.1 1 o.'2 I 4? 0.09 s 2 0.08 O'I0 I tration, cs2-, for the different wavelengths (A) investigated.The regression coefficient ( r ) was calculated for n = 6. h = 185 nm A = 0.0031~~2- + 0.0010 r = 0.9995 h = 190 nm A = 0.0050~~~- + 0.0009 r = 0.9998 h = 200 nm A = 0.0057~~2- + 0.0009 r = 0.9996 h = 210 nm A = 0.0025~~2- + 0.0005 r = 0.9996 h = 215 nm A = 0.0017~~2- + 0.0008 r = 0.9989 h = 220nm A = 0.0011~~2- + 0.0010 r = 0.9962 The absorption spectrum of hydrogen sulphide3 is shown in Fig. 3 and for optimum sensitivity, a wavelength of 200 nm was employed. Precision Ten replicate analyses of different standard solutions of sulphide were made under the optimum conditions to test the reproducibility of the technique.The results are shown in Table 2. Detection Limit Replicate analyses of a 0.2 pg ml-1 standard sulphide solution gave a standard deviation of 0.0003 pg ml-1. Defining the detection limit as the concentration of sulphide which yields a signal twice the standard deviation for a signal close to the blank, the detection limit of the proposed method was 0.06 pg ml-1 of sulphide. The detection limit could be pushed to lower values by using a timing cam with a longer sampling time at the expense of a reduced number of samples that can be analysed per hour. Dynamic Range and Sensitivity The relationship between the absorbance of the evolved hydrogen sulphide and the concentration of the sulphide was linear up to 100 pg ml-1 of sulphide. A curvature towards the concentration axis was observed at higher concentrations.The 0.14 C,..:"' I 0.04 0.02 0 5 10 15 20 25 Sulphide concentration/pg ml-1 Fig. 5 A, 185; B, 190; C, 200; D, 210; E, 215; and F, 220 nm Effect of choice of wavelength on sulphide calibration graphs: Table 2 Precision of the proposed method at various sulphide concentrations Absorbance Relative Concentration standard of sulphide/ Standard deviation pg ml- 1 Range Mean* deviation (YO) 20 0.1 10-0.115 0.113 0.0017 1.5 12 0.064-0.066 0.065 0.0009 1.4 4 0.020-0.022 0.021 0.0007 3.3 2 0.0 124.013 0.012 O.OOO4 3.3 * Based on ten determinations.598 ANALYST. JUNE 1991, VOL. 116 slope of the calibration graph (the linear portion) was 0.0055 ml pg-1, which represents the sensitivity of the method. The equation of the calibration graph obtained by the method of least squares was A = 0 .0 0 5 5 ~ ~ 2 - + 0.0081 with r = 0.9986. Interferences The effect of various anions and cations on the absorbance of the evolved hydrogen sulphide was studied. A range of solutions was prepared containing 20 pg ml-1 of sulphide and 500 pg ml-1 of the possible interferent. Efforts were made during the preparation of these solutions to avoid premature hydrogen sulphide evolution by the addition of an appropriate volume of SAOB solution to the concomitant element solutions (pH >7). The solution containing the sulphide sample plus the potential interferent ion and another solution containing only the interferent ion (500 pg ml-1) were analysed by the proposed method. The responses were compared with those obtained from an uncontaminated sulphide solution.The anions Cl-, Br-, I-, P043-, SO42-, NO3- and C032- did not cause any interference. Solutions of 100 pg ml-1 of sulphite and 500 pg ml-1 of nitrite interfered with the determination. This is due to the evolution of sulphur dioxide by the sulphite and a mixture of gaseous oxides of nitrogen by the nitrite. When solutions of these anions were analysed alone, noticeable peaks were obtained (100 pg ml-1 of sulphite solution and 500 pg ml-1 of nitrite solution gave absorbance values of 0.083 and 0.259, respectively) thereby confirming that the gases evolved absorb at 200 nm. The cations Na+, K+, AP+, Sr2+, Ca2+, Mn2+, Mg2+ and Zn2+ had no effect on the determination of sulphide. The ions Ni2+ and Cd2+ showed a marginal effect while Fe3+ and Crvl caused a substantial depression of the sulphide signal (see Table 3).A concentration of 100 pg ml-l of Co2+ caused almost complete signal depression while a similar concentra- tion of Cu2+ totally depressed the signal. The effect of various concentrations of Cu2+ (0-70 pg ml-1) on the absorbance signal of 20 pg ml-1 of sulphide is shown in Fig. 6. Other workers6336 have also observed the deleterious effect of Cu2+ on the determination of sulphide. The addition of between 2 and 20 ml of various concentrations of EDTA solution (0.1-0.3 mol dm-3) to the solutions of sulphide and interferent did not prevent the interference. The use of a stronger acid (6 mol dm-3 HC1) did not solve this problem either. This might be due to the stability of the resultant metal sulphide that is formed.Table 3 Effect of other ions on the evolution of hydrogen sulphide and its absorbance at 200 nm; sulphide concentration of test solution, 20 pg ml-I Ion added None c1- Br- I- ~ 0 ~ 3 - sop NO3 - CO32- S032- NOz- NOz- K+ Con- centration/ pg ml-1 500 500 500 500 500 500 500 100 100 500 - 500 Relative absorb- ance* 100 98 103 102 98 102 98 97 173 99 239 100 Ion added Sr2+ Ca2+ Na+ Mg2+ Mn2+ Zn2+ Cd2+ Ni2 + Crvl Fe3 + co2+ cu2+ AP+ Con- centration/ Relative pg ml-1 absorbance* 500 94 500 99 500 100 500 94 500 98 500 97 500 81 500 85 500 99 500 55 500 16 100 4 100 0 * The ratio of the absorbance for the test solution to that for the solution containing the concomitant. Application In order to test the proposed method, sulphate-sulphur in mixed herbage (grassklover) was determined by the pro- cedure described above using a modification of the direct digestion procedure of Johnson and Nishita26 to convert the sulphur into hydrogen sulphide.This method involves the reduction of sulphate to hydrogen sulphide by a reducing mixture containing hydriodic acid, formic acid and red phosphorus. The hydrogen sulphide evolved is collected in a 100 ml glass-stoppered calibrated flask containing zinc acetate and sodium acetate and made up to the mark with SAOB (to prevent aerial oxidation of the sample as explained earlier). The hydrogen sulphide trapped by the zinc acetate and sodium acetate is then determined. A set of standard sulphur solutions (0-100 pg ml-1) (as sodium sulphate) was first analysed by the procedure of Johnson and Nishita26 followed by direct digestion of the mixed herbage.The determination of sulphate-sulphur in the samples was then carried out by interpolation from the calibration graph. The results of the analysis are shown in Table 4. The values obtained are in good agreement with those obtained by the Methylene Blue method of Johnson and Nishita. Conclusion This paper describes the automation of the GPMAS method for the determination of sulphide. It is a rapid and specific spectrometric method which requires minimal sample treat- ment. No reagent other than 3 mol dm-3 hydrochloric acid is required, thereby eliminating all concerns of reagent preser- vation and timed colour development. The method is simple, fast and direct.The manifold is simple and any atomic absorption instrument can be employed. The procedure retains much of the inherent sensitivity of the manual GPMAS technique whilst allowing the efficient processing of a large 0.12 1 I 0.10 8 0.08 - L (D ' 0.06 2 2 0.04 0.02 0 20 40 60 80 20 pg ml-1 S2- + xpg mi-' Cu2+ Fig. 6 Effect of various concentrations of Cu2+ on the absorbance of 20 pg ml-1 of sulphide ~~ ~ Table 4 Sulphate-sulphur in mixed herbage Concentration found*/mg kg-l Automated Sample No. GPMAS 1 765.5 2 774.5 3 202.2 4 266.2 5 176.1 6 763.4 * Mean of duplicate determinations. Methylene Blue method 789.4 764.8 198.7 274.8 176.2 751.5ANALYST, JUNE 1991. VOL. 116 599 number of samples. The precision and accuracy are as good as those of the conventional methods.The most attractive feature of the method is its manipulation-free unattended operation. Twenty samples can be analysed in 1 h. Interfer- ences in the proposed method are few, and it offers a good alternative for the determination of sulphide in environmental samples. The method may also be applied to the determina- tion of total sulphur in soil extracts by using the modified Methylene Blue procedure of Tabatabai and Bremne1-3’ to convert the various forms of soil sulphur into hydrogen sulphide. The latter is then collected and analysed as described above for sulphate-sulphur. Where the concentration of sulphide is much lower than the detection limit of the proposed method, a preconcentration method should be developed. The results reported here, for example, suggest that sulphide in large samples could be trapped in a much smaller volume of zinc solution prior to determination.Automated GPMAS methods for other anions are under development in this laboratory. The authors thank Tony Edwards and Denise Donald of Macaulay Land Use Research Institute, Aberdeen, for supplying the plant samples and the Methylene Blue results shown in Table 4. T . A. A. thanks the Commonwealth Scholarship Commission and the University of Agriculture, Abeokuta, Nigeria, for financial support and leave of absence, respectively. References 10 11 12 13 Cresser, M. S . , Anal. Chim. Acta. 1976, 85, 253. Cresser, M. S., Lab. Pract., 1977, 26, 19. Cresser, M. S . . and Isaacson, P. J . , Talanta, 1976, 23, 885. Cresser, M.S . . Analyst, 1977, 102, 99. Cresser, M. S., Proc. Anal. Div. Chem. SOC.. 1978, 15, 68. Cresser, M. S . . Lab. Pract.. 1978, 27, 639. Cresser, M. S . . Eur. Spectrosc. News. 1978. 19. 36. Syty, A., Anal. Chem., 1973, 45, 1744. Winkler, H. E., and Syty. A., Environ. Sci. Technol., 1976,lO. 913. Syty, A., Anal. Chem., 1979, 51, 911. Syty, A., and Simmon, R. A.,Anal. Chim. Acta, 1980.120,163. Takahashi, M.. Tanabe, K . , Saito, A.. Matsumoto. K., Haraguchi, H., and Fuwa, K., Can. J . Spectrosc., 1980,25,25. Macpherson, H. B.. At. Spectrosc., 1983,4, 150. 14 15 16 17 18 Kupchella, L., and Syty. A.. J . Assoc. Off. Anal. Chem., 1984, 67, 188. Firestone, M. A., Dietz, M. L., and Syty, A., Anal. Lett., 1985, 18, 985. Anwar. J., and Marr, I. L., J. Chem. SOC. Pak., 1986, 8, 67.Szekeres. L., Talanta, 1974, 21, 2. Blasius, E., Horn, G., Knochel, A . , Munch, J., and Wagner, H., in Inorganic Sulphur Chemistry, ed. Nickless, G., Elsevier, New York, 1968, p. 211. Haff, L. V., in The Analytical Chemistry of Sulphur and its Compounds, Part I , ed. Karchmer. J. H., Wiley-Interscience, New York, 1970, p. 285. Heinrich, B. J., Grimes, M. D., and Puckett, J . E., in Treatise on Analytical Chemistry, Part 2, eds. Kolthoff, 1. M.. and Elving, P. J . , Wiley, New York, 1961, vol. 7. p. 49. Williams, W. J . , Handbook of Anion Determination, Butter- worth. London. 1st edn., 1979. p. 568. Greenberg, A . E., Connors, J . J . , and Jenkins, D., Standard Methods for the Examination of Water and Wastewater, Public Health Association. Washington, DC, 15th edn., 1980. pp. 4424.50. 23 Determination of Total Sulfide Content in Water, Applications Bulletin No. 12, Orion Kesearch, Cambridge, MA, 1969. 24 Frant, M. S . , and Ross, J . W., Tappi, 1970, 53, 1753. 25 Baumann, E. W., Anal. Chem., 1974,46, 1345. 26 Johnson, C. M., and Nishita, H.. Anal. Chem., 1952. 24, 736. 27 Zutshi, P. K., and Mahadevan, T. N., Talanta, 1970, 17, 1014. 28 Leggett, D. J . , Chen, N. J.. and Mahadevappa, D. S., Anal. Chim. Acta, 1981, 128, 163. 29 Grekas, N., and Calokerinos, A. C., Anal. Chim. Acta, 1985, 173. 311. 30 Henden, E., Pourreza, N., andTownshend, A., Prog. Anal. At. Spectrosc., 1979, 2, 355. 31 Vijan, P. N., and Wood, G. R., Anal. Chem., 1981, 53, 1447. 32 Vijan, P. N., and Wood, G. R., Anal. Lett., 1982, 15, 699. 33 Donaldson. E. L., and McMullan, D. C., Anal. Lett., Part A , 1978, 11, 39. 34 Tanaka, S., Shimizu, Y . , and Hashimoto, Y., Bcinseki Kagaku, 1978, 27. 599. 35 Glaister. M. G., Moody, G. J., Nash, T., andThomas. J . D. R., Anal. Chim. Acta, 1984, 165, 281. 36 Johnson. C. M., and Arkley. T. H., Anal. Chern., 19.54, 26, 1525. 37 Tabatabai, M. A., and Bremner, J . M., Soil Sci. SOC. Am. Proc., 1970,34, 62. Paper 0/04556J Received October loth, I990 Accepted February 4th, I991 19 20 21 22

ISSN:0003-2654    

DOI:10.1039/AN9911600595

出版商:RSC    

年代:1991

数据来源: RSC