ANALYST, NOVEMBER 1991, VOL. 116 1095 Determination of Anions by Flow Injection A Review Danhua Chen,* M. D. Luque de Castrot and Miguel Valcarcel Department of Analytical Chemistry, Faculty of Sciences, University of Cordoba, 14004 Cordoba, Spain Summary of Contents Introduction Flow Injection Configurations Used Types of Reactions Redox Reactions Complex Formation Reactions Catalysed Reactions Substitution Reactions Precipitation React ions Miscellaneous Reactions Dye formation reactions Chemiluminescence reactions Ion-pair formation reactions Aci d-b a se reactions Enzymic reactions Types of Detection Multi-determinations Physical Spectral Resolution Separation by High-performance Liquid Chromatography (Time Resolution) Chemical Discrimination Selectivity Sensitivity Analysis of Real Samples Comparison With Other Techniques Conclusions References Keywords: Review; anions; flow injection Introduction Since its inception over 15 years ago, flow injection (FI) has undergone extensive development, as reflected in the large number of papers published on this topic over this period' and in four monographs published recently.*-5 Flow injection is now well established; hence reports on theoretical aspects, new modes and revolutionary designs appear less frequently in the literature; the incipient commercialization of instrumen- tation is bound to contribute to its consolidation and new trends point to the start of a new age in which the emphasis will be placed on solving analytical problems in various fields of social interest.6 The 3000 or so papers published on FI to date cover a wide range of methods and analytes, as shown in reviews on areas such as clinical,7-9 pharmaceutical,*0.11 en~ironmental~~J3 and food analysis.14-16 Obviously, inorganic analysis has been one of the main targets of workers concerned with FI. Cationic species have been largely determined by atomic absorption spectrometry (AAS),17.18 while anions, both organic and inorganic, have been sensed by various detection techniques depending on the features of the species concerned. To date, no attempts have been made to discuss critically and system- atically the various FI methods developed for the determi- nation of anions. Only one paper has considered this subject, but the evaluation was neither systematic nor ~ritical.~9 The aim of this paper is to provide a review of the * Permanent address: Department of Chemistry, Wuhan Univer- t To whom correspondence should be addressed.sity, Wuhan, China. determination of anions by FI. As a rule, the features of anions make it difficult to find suitable reagents for developing sensitive and selective methods for these species covering the determination ranges usually required.20 For these reasons, it was decided to compile a systematic review of the state-of-the-art of this subject. For clarity, a series of tables have been compiled for the different anions summarizing the main features of each method [reaction and/or separation technique used, type of detector, determi- nation range, detection limit, relative standard deviation (RSD) and sampling frequency, and also the type of sample in which the species was determined].Commonly determined anions have been allocated an individual table, viz., phospho- rus (Table l), sulphur (Table 2), chlorine (Table 3) and fluoride (Table 4). Other anions with similar features appear in groups of two or more, viz., nitrite and nitrate (Table 5), cyanide and thiocyanate (Table 6), chromate, molybdate and vanadate (Table 7), and bromine and iodine anions (Table 8). Other anions not included in Tables 1-8 are listed in Tables 9 and 10. Data have been gathered from virtually every paper published to date (some 300 references are also listed in the tables). Different aspects of the FI configurations used are illustrated schematically in Figs. 1 4 . The type of configur- ation used in each method (modifications are denoted by a lower case letter) is also listed in the tables.The manifolds are discussed in the next section. Flow Injection Configurations Used The 14 different manifolds depicted in Figs. 1-4 represent the various designs used to develop determinative FI methods.1096 ANALYST, NOVEMBER 1991, VOL. 116 Table 1. Flow injection determination of phosphorus anions Analytical features Reaction/ separation Configuration technique* I-a I-a I-a XI I-a VI I-a I1 I-a V-b IX 111-a I-a I-a I-a I-a I-a XI I-a I-a I1 I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a XI XI V-a V-b V-a XI XI CF CF CF CF CF CF CF CF CF CF CF-LLE CF CF CF CF CF CF CF CF CF CF CF CF CF CF CF su CF CF CF R CF EC EC EC EC CF-EC CF-EC EC NCR NCR R HPLC-R-CF HPLC-R-CF R-CF R-CF HPLC-CF HPLC-CF Detection? P P P P P P P P P P P P P P P P P P P P P P P P P P P P P P F F A A A A A cu cu PO ICP-AES P P P P P P P Range pg ml-1 5-25 5-30 - 1 .&24 1.947.5 0-10 0-2 0.1-2 1-12.5 - - 0-60 04.5% P 0.04-2.5 0.25-1.5 5-30 - 0.95-95 0.95-4.75 04.38 0-268 0-30 50-500 0 4 .8 0.95-9.5 - - - 8.95-9.5 - 0.03-0.6 04.045 - 0.09-47.5 0.1-30.0 0.95-95 0 4 . 8 0.48-95 - 1.9-950 - 19-57 mmol dm-3 0.95-95 0.95-47.5 0.95-47.5 0.95-47.5 0.95-9.5 - RSD 1.32 1 .o 2.3 (Yo 1 - - - 0.3-1.0 - 0.97 - - - - - 4 . 0 - - - - - 1.5 <1 .o t 0 . 7 0.5 - - 0.5-1.5 1 .o - - - - 0.9 <1.0 1.2 2.6 - - - - - 2.1 <1.0 <1.5 <1.0 4 . 5 - - Sampling frequency h-' 420 >200 - - - - 10 120 200 - - - 60 180 60 120 100 - - - >90 90 240 s120 40 120 300 45 30 60 20 - - - - - 70 - - 68 100 15 60 80 100 80 - - Real samples Plants - Water Water Water - - Plasma, urine Plants - Water Fertilisers Rock - - Fruit Sea-water Rock Plants Rock Water - - Water Water Powder Blood, serum - - Urine Compounds - Water Smoke Reference 21 22 23 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 65 67 68 * R = Redox; CF = complex formation; CT = catalytic; SU, substitution: PR = precipitation: DF = dye formation; CL = chemiluminescence; IP = ion-pair formation; AB = acid-base; EN = enzymic; EC = electrochemical; NCR = no chemical reaction; DIA = dialysis; LLE = liquid-liquid extraction; HPLC = high-performance liquid chromatography; IE = ion exchange; DIS = distillation; and GD = gas diffusion.t P = Spectrophotometry; F = spectrofluorimetry; PO = potentiometry; A = amperometry; CL = chemiluminescence; AAS = atomic absorption spectrometry; ICP-AES = inductively coupled plasma atomic emission spectrometry; S = chemical sensor; CU = coulometry; TC = thermochemical; MECA = molecular-emission cavity analysis; and MS = mass spectrometry. Tables 1-10 relate each configuration to the analyte deter- mined using that configuration and to the paper in which the determination was reported. Fig. 1 illustrates the simplest configurations used with Manifold I has been used to develop methods with (one or normal and reversed FI methods.ANALYST, NOVEMBER 1991, VOL. 116 1097 Table 2.Flow injection determination of sulphur anions Analytical features Anion sop SO& s032- sop S04’- sod?- sop sop S04’- sop SO42- sop S04’- soj2- soj2- sop sop SO& sop sop SO& sop so32- s o p SO3’- S03’- s o p so32- S2- S* - S’- S’ - S2- S2 - S2- S2- Reaction/ separation Configuration technique* I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a I-a IX I-a I-a I-a I-a I-a I-a I-a I-a I-a V-b I-b I-a I-a IX I-a I-a I-a I-a III-b IX I-a I-a I-a I-a PR PR PR PR PR PR PR PR PR PR su su DIA-PR CT su su su su NCR NCR NCR NCR R-EN R R-CF GD-R R-CL R NCR NCR PR CF CT CL R-CL DIA-EC * Abbreviations as in Table 1. I Abbreviations as in Table 1. Range1 Detection? pg ml-l P 10-100 P P P P P P P P P P P P P P P P P PO PO ICP-AES MECA A 50-200 40-1 60 100-900 1-30 5-200 1oo-1OOo 1-100 10-120 1-20 0.2-20 50-500 0.25-6.0 0-30 0.5-30 1-lo00 25-1000 0-1 .o 1 .7-83 0.8-69 - - - - P 1-100 P 8-800 P 1-20 CL 0.09-3.5 F 0.026-0.25 PO 0.1-17.8 PO 10-“10-’ mol dm-3 AAS 0-2 - A P M5.1 P 0-32 CL 0 .0 0 1 ~ . 1 6 CL 0.0140 Detection limit/ pg ml-1 10 - - - - - - - 0.45 - - - - - 0.1 - - - - - 0.0028 - 1 x 10-5 2 x 10-5 mol dm-3 0.3 mol dm-3 0.1 0.0032 - - - 0.01 0.00015 0.42 0.ooOol 0.0004 - RSD 0.85 0.95 (Yo 1 - - <1.0 - - <2.0 - 0.78 - t 2 . 0 2.0 4.1 - - - 0.94-1.2 - - 98 - 2.2 1 .5 2.0 1.0-2.0 5.0 - - - 1.2 4.0 - - 3.0 4.4 Sampling frequency1 h- 180 60 30 120 - - 30 200 30 90 40-60 40 20 20 <20 30 20-38 20 40 100 - - - <15 60 60 720 24 - - 100 210 240 240 - - Real samples Water, plants Sea-water Water, plants Water - - - - - Water Rain, snow Urine Rain Water Water Water - - - - Water - - - - - - - - - - - - - Water - Reference 69 70 71 72 73 74 75 63 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 both broken lines for reagent streams) or without chemical reaction (a); with the use of an ion-exchange, enzyme or redox column (b); or with a mini-mixing chamber, generally used in FI titrations (c).Methods for the determination of phosphate, sulphate, sulphite, suIphide, chloride, chlorate, fluoride, nitrite, nitrate, nitritehitrate, cyanide, thiocyanate, chro- mate, molybdate, vanadate, bromide, bromate, iodide, iodate, iodidehodate, borite, permanganate, oxalate and selenate have been developed with different variants of this general scheme.Manifold 11 illustrates the simple reversed FI mode in which a reagent plug occasionally resulting from the mixing of two different streams is injected into the sample stream acting as carrier. Only phosphate has been determined with this type of configuration, which is particularly suited to the analysis of waste water and, as a rule, of abundant, inexpensive samples. The merging-zones approach (Figs. 1-3) was conceived in order to decrease reagent consumption by simultaneously injecting sample and reagent. The mixed plug can be passed (b) or not passed (a) through an ion-exchange column, as in the determination of phosphate, sulphide, chloride, nitrite, molybdate and bromide. The stopped-flow mode has been implemented in manifolds such as IV, both in the normal and reversed (in parentheses in Fig.1) FI modes. By synchronizing the injection and propulsion units, the reacting plug can be halted at the reactor to ensure optimum development of the rkaction without increasing the dispersion (non-kinetic stopped-flow mode) or at the flow cell to monitor the development of the reaction by collecting signal-time data pairs (kinetic stopped-flow mode), thereby avoiding matrix effects and interferences from reac- tions faster or slower than the analytical reaction. The determination of fluoride and the simultaneous determination of chloride and chlorate have been accomplished using this configuration. Multi-determination designs are more complicated (Fig. 2). Manifold V was designed for differential kinetic determi- nations (a) by using the same reagent which reacts at a different rate with two or more analytes (serial detectors) and (b) for simultaneous determinations based on splitting the sample into two channels, each of which is merged with a suitable reagent and then driven to different parallel detec- tors.These assemblies have been used for the determination of phosphate/ammonia and nitritehitrate. Multi-determinations requiring different reagents or reac- tion media have been carried out using configuration VI. A1098 ANALYST, NOVEMBER 1991, VOL. 116 Table 3 Flow injection determination of chlorine anions Anion C1- c1- c1- c1- c1- c1- c1- c1- c1- c1- C1- c1- c1- c1- Cl- c1- c103- c10-- c103 - c10-- c103- c10-- c103- c104- c104- Reaction/ separation Configuration technique* I-a PR I-a NCR I-a NCR IX NCR I-a NCR I-a NCR I-a NCR I-a CL I-a SU-CF I-a SU-CF I-a R III-a su I-a SU-CF I-a SU-CF I-a PR I-a R IV R IX GD-EC I-a R I-a R IX IP-LLE IX IP-LLE * Abbreviations as in Table 1.? Abbreviations as in Table 1. Analytical features Detection-t AAS PO PO PO PO PO PO PO CL P P P P P P P P P P P AAS P Range/ pg ml-1 3-100 5-5000 250-5000 0.7 1-1 78 - - 10-200 1.8-3.6 0.2-15 0.1-6.0 1.1-15.3 0.5-75 2-10 2-200 0-14 0.084-0.84 - - 0 . O M . 0 0.01-10.1 0.1-8.3 0.1-5.0 0-2.5 Detection limit/ pg ml-1 1.3 - - 3.5 - - - - 0.043 - - - - - - - 110-6 mo dm-3 0.04 (CIO-) 0.03 (C103-) - 0.07 0.036 RSD 2.0 1.7 0.5 (”/) - - 1.75 2.0 3.0 <1.0 1.0-3.0 - - - - - - (3.0 - - 94 1.2 1.2 - Sampling frequency1 h-1 50 120 120 100 60 120 30 120 90 90 15 60 - - - - - - - - 45 20 Real samples Water Water Milk Water Plants Serum - - - Water Ethanol Rain - - - - - - Water - - Chlorate Reference 105,106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127-129 Table 4 Flow injection determination of fluoride Analytical features Anion F- F- F- F- F- F- F- F- F- F- F- F- F- F- F- F- F- F- F- ~ ~~~~ Reaction/ Detection Sampling separation Range/ limit/ RSD frequency/ Real h-1 samples Reference Configuration technique* Detection? pg ml-1 pg ml-1 (Yo) I-a I-a I-a I- a I-a I-a I-a I-a I-a I-a I-a I-b I-a I-a IV I-a I-a I-a XI1 * Abbreviations as in Table 1.t Abbreviations as in Table 1. NCR NCR NCR NCR NCR NCR NCR NCR NCR NCR NCR NCR CT CF CF su CF CF CF PO PO PO PO PO PO PO PO PO PO PO PO P P P P F F-S F-S 0.02-0.2 0.019-19 0.3-10 0.019-19 0.001-1.0 0.19-1.9 - - - - 1.9-190 0.019-0.19 - 0.03-1.2 0.08-1.2 0.1-10 0.00024.02 0.5-8 0.001-0.04 - - - - 0.001 - - 0.001 0.0019 0.02 - 0.001 0.01 - - - - - - 3.0 5.0 - - 0.5-5.0 - - - - - 1.7 3.0 - - 0.2 0.2 15.0 1.0 - 60 120 60-180 360 120 40-60 80 60 24 24 60 100 60 t l 30 - - - - Rain Rain - - - - - Water, urine beverages Rain Various materials - - - - Water Water Water - - 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 selection valve allows the appropriate medium to be selected, An approach based on an internally coupled valve assembly as in the sequential determination of arsenate/arsenite, (Fig.2, manifold VIII) has been used for simultaneous arsenate/phosphate and arsenate/arsenite/phosphate.speciation using a prior redox step implemented in a solid The reversed FI mode has been used for the sequential reactor located in the secondary valve. determination of anions such as nitritekyanide, nitritelsul- Configurations involving on-line separation units are de- phide and cyanide/sulphide by sequentially injecting the picted in Fig. 3. required selective reagent into the sample stream. The typical location of a gas-phase separator, dialyser orANALYST, NOVEMBER 1991, VOL. 116 1099 I C C - P Fig. 1 Simple manifolds used in the determination of anions. I , Normal FI configuration with (broken lines) or without chemical reaction (a), with an on-line ion-exchange, enzyme or redox column (b) or with a mini-mixing chamber (c). 11, reversed FI manifold with injection of one or two mixed reagents.111, Merging-zones approach without (a) or with (b) an ion-exchange column. IV, Stopped-flow arrangement using a timer to synchronize injection with the halting of the flow at a pre-set time in the normal or reversed (in parentheses) FI mode. P, Peristaltic pump: C. carrier; R, reagent; S, sample; IV, injection valve; D, detector: and W. waste liquid-liquid extraction unit (between the injection and detection systems) as shown in manifold IX has been used for the separation/dilution of phosphate in plasma and urine, for the separation of sulphate, sulphite, sulphide, chlorine, cyanide, cyanidekhiocyanate and carbon dioxide by gas diffusion and perchlorate, nitrate and nitritehitrate by liquid- liquid extraction with ion-pair formation.Two different streams emerge from the separation unit in all instances, one of which is driven to waste while the other reaches the flow cell for measurement of the analyte. One application involves placing the separation unit in the loop of the injection valve (manifold X) to accomplish sample preconcentration in addition to separation, as in the determi- nation of thiocyanate after liquid-liquid extraction. This type of unit is also used in its typical location for on-line ion-exchange processes, and always when a chromatographic or a precipitation-separation-preconcentration step is required (configuration XI). This approach has been used for the determination of phosphate and polyphosphates by high-performance liquid chromatography (HPLC) , and for chloride, bromide and iodide. U VI P P I wc U VII P Vlll P I I Fig.2 Manifolds used for the multi-determination of anions. V, Differential kinetic or non-kinetic determination with serial (a) or parallel (b) detectors, respectively. VI, Using a switching valve to select the appropriate reagent or medium. VII, Using the reversed mode, with sequential injection of a selective reagent for each analyte. VIII, Speciation with a prior redox step. SV, Selecting valve; and D1 and DZ, detectors. Other symbols as in Fig. 1 Fig. 4 shows some special designs such as that used to implement integrated reaction-detection (XII) , and those used to obtain multi-peak recordings (XI11 and XIV). Chemical flow cell sensors have been used in manifolds similar to manifold XII, including a suitable support (gener- ally an ion-exchange resin) in the flow cell.In this way, it is possible to retain one of the components of the analytical reaction temporarily or permanently at the detection point, thereby increasing sensitivity and selectivity as an in situ concentration step is performed during the monitoring pro- cess. The determination of fluoride by transient retention of a ternary complex in the flow cell, and that of cyanide by retention of a fluorescent product resulting from its reaction with pyridoxal 5-phosphate, have been carried out with this type of sensor.1100 ANALYST, NOVEMBER 1991, VOL. 116 Table 5 Flow injection determination of nitrite and nitratc Analytical features Reaction] separation Configuration technique* 111-a I-a I-a I-a I-a I-a I1 I-a I-a I-a I-a I-a I-a I-a I-a IX I-a I-a I-a I-a I-a I-b I-b I-b I-a I-a I-a I-a I-a I-a I-a I-a I-a I-b I-b I-a I-a IX I-b I-a V-b V-b I-a VIII - I-a I-a I-a I-b 1-a IX R R R R R R R R R R R R R EC R-EC GD-NCR EC R-EC EC R R R R R R R NCR su NCR R R-EC R-EC NCR NCR NCR NCR NCR IP-LLE R R-CL R R R R R R R R EC NCWEC IP-LLE * Abbreviations as in Table 1.t Abbreviations as in Table 1. P P P P P P P P P P P P P A A PO A A A F F P P P P P P P P P A A PO PO PO PO PO AAS F CL P P P P P P PIA PIA A POIA AAS Rangel Detectionl' yg m1-I 0.025-0.5 0.002-60 0.2-2 0-0.1 0.05-1.3 0.1-5 - - 0.0054). 03 1-10 10-1000 0.0032-0.064 0.4646 0.0092-0.2 15 0.23-46 0-30 0.046-9.2 2.3-230 2.3-33 0-0.014 0-10 0-2.5 0.05-0.3 0-12.4 1-10.0 0.089-2.2 - 1.24-8.7 0.2-20 0-22 0.62-6.2 31-310 6.2-3 10 - 8 x 10-5- 10-1 mol dm-3 1-5 3.1-310 0.13-2.2 0-12.4 - 0.1~0.5 1-5 0.0092-0.092 0.12-1.2 0.1-0.5 1-5 0.025-16 0.1-16 0.1-1 .o 0.5-5.0 5 x 10-5- 5 x 10-3 rnol dm-3 0-7.5 x 10-5 mol dm-3 - - - 1-10 0.2-2 Detection limit1 pg ml-1 - - - - - - - - 0.0082 - - - - 0.07 0.0015 0.003 - - - 0.oooO46 0.002 0.0062 0.0015 - - 0.089 - - - - - 6.2 - - 1 x 10-5 rnol dm-3 - - - 0.00062 0.001 - 0.0023 0.0062 - - 0.001 0.01 - 5 x 10-6 rnol dm-3 mol dm-3 5 x 10-5 0.027 - - RSD 0.5 0.14.4 1.5 1.67 2.2 S1.3 (Yo 1 - - - 0.7-1.8 - 0.7 - - - - - - 4 . 0 0.4 1.7-2.8 <1.0 >1 .o 0.4 3.0 3.0 3.0 - - - - 2.64.8 1-2 0.8 - 2.4 1-2 0.8 0.5 1.5 1 .o 0.85 1.96 1.1 0.9 1.0 0.7 - - - - - - 2.5 4.7 3.7 Sampling frequency1 h-' 70 300 - - 50 60 130 30 120 50 - - - - 100 30 - - - 45 30 75 40 40 - - 120 18-35 100 - - - 90 125 120 100 100-200 20 - - 90 30 72 65 60 - - - - - 35 * 5 Real samples Waste water - - - Water Water Water - - - - Water - - - - - - - Sea-water Saliva Water Water Rain Water Rain - - - Plants - - Soil, water, fertilizers Waste water, Soil - Water Foodstuffs Water - - - - Water - Water, soil - - - Soil - - Reference 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 19 1 192 193 194 195 196 197 198 199ANALYST, NOVEMBER 1991, VOL.116 1101 Table 6 Flow injection determination of cyanide and thiocyanate Analytical features Anion CN- CN- CN- CN- CN- CN CN- CN- CN - CN- CN CN- CN CN- CN CN - CN- CN- CN- CN- CN CN - SCN ~ SCN - CN-/ SCN ~ CN-/ SCN CN-/ SCN- Configuration v11 I-a/II IX IX IX I-a I-a I-a I-a I-a XI I I-a I-a I-a IX I-a I-a I-a I-a I-a I-a I-a I-a X IX I-a * Abbreviations as in Table 1 t Abbreviations as in Table 1 Reaction/ separation technique” R-DF R-DF GD-DF GD-DF GD-DF CT CL CL CL CT CT NCR NCR NCR NCR NCR EC EC EC EC DIS-EC R-S U EC IP-LLE GD-DF GD-DF NCR Detection i P P P P P P CL CL CL F F-S PO PO PO PO PO A A A A A AAS A P P P PD Range/ pg ml-1 0.3-5.0 0.1-1 .0 0.4-5.0 0.1-10.0 - - 0.00005- 0.026 - - 0.1-20 0.05-3 .0 20- 1000 5-50 0.3-100 0.2626 0.02626 0.26-26 0-10 0.4-40 0.65-26 - 2.6-5.2 0.0 12-0.23 0.026-2.6 0.58-5.8 0.058-5.8 0.4-2 0-6 0.2626 2.32-1 74 The open-closed system (manifold XIII) provides a means of obtaining enhanced information by using a single conven- tional detector yielding multi-peak recordings.298J99 The system was used for studying chromium speciation. A multi-peak recording can also be obtained by using a simple FI manifold with a programmed pump to change the flow direction iteratively and hence to achieve the repeated passage of the sample plug to the detection point.288.3(H) One such system was used for the kinetic discrimination of perchlorate and iodide based on the different rate of extrac- tion of their ion pairs with hexacyanoferrate(Ii1) into an organic phase injected into the sample stream.Types of Reactions The distribution of reaction/separation techniques is sum- marized in Fig. 5 , where ‘separation’ includes dialysis, liquid-liquid extraction, HPLC, ion-exchange and distillation; ‘no chemical reaction’ denotes determinations based on features of the analyte, e .g . , absorbance, fluorescence and redox properties. The different types of chemical reactions used for the determination of anions by FI are discussed below. Redox Reactions A spectrofluorimetric method for the determination of phosphate has been described based on the oxidation of thiamine to thiochrome by a heteropolyacid.5’ Phosphinate and phosphonate were determined with sodium hydrogen sulphite as oxidant,65.66 and hypophosphate was determined with potassium iodate in sulphuric acid.28 Methods for the Detection limit/ pg ml- - - - 0.006 0.006 0.2 0.00005 0.00025 0.05 - - - 0.08 - - - - 0.001 - - - - - - 0.0078 0.012 0.05 0.08 - RSD 0.66 <0.8 0.8 1.4 1.4 2.1 3.3 2.4 <2.0 I .39 1.2 2.5 2.0 5 .o <1.0 (%) - - - - - - <2.0 3.@4.0 1 .o 1.5 - - - Sampling Frequency/ h-1 28 20 10 40 - - 360 700 360 10 60 40 20 - - - - 60-70 100 60 40-50 240 18 10 - - - Real samples - - - Waste water Waste water - - Plants, water - - - - - Waste water - - - Water Waste water - - - Urine - - - - Reference 200 20 1 202 203 204 205 206.207 208,209 210 211 2 12 213 214 2 I5 216 217 218 219 220 22 I 222 223 224 225 226 227 228 determination of sulphite have been reported based on the reduction of sulphite by phosphonate,93 the oxidation of an organic disulphide to a coloured thio anion at pH 6.0,9* the decolorization of Malachite Green94 and the reaction of sulphur dioxide with N-acridinylmaleimide in a water-N,N- dimethylformamide medium to form a fluorescent product .9h Most of the methods for the determination of nitrite are based on a diazotization-coupling reaction.Nitrite is diazotised with sulphanilamide and the product coupled with N-( l-naphthy1)- ethylenediammonium dichloride to form a highly coloured azo dye which is measured at 520 nm;339-156.170.1*9-192 a similar reaction with other reagents has also been developed. 1577158.193 Nitrate was determined similarly after on-line reduction to nitrite in a copper-coated cadmium column or with a dissolved reactant.171-171 Potassium bromate has been shown to be a selective colour-forming reagent for halide anions. 117 The sequential iodimetric determination of chlorate, and chlorite-chlorate, has been described by Gordon and co- workers.122-125 The reaction between iodate and iodide with the release of iodine was used for the spectrophotometric determination of both species.26hJ75 Bromide was determined on the basis of the bromination of Phenol Red to Bromo- phenol Blue after oxidation of bromide to bromine with chloramine-T.260,*61 Periodate was determined by measuring the red colour in the reaction with salicylaldehyde guanyl- hydrazone in a basic medium.274 A hydride generation reaction was used for the determination of ar~enic2~33294 and selenium.’94 Complex Formation Reactions The Molybdenum Blue method for the determination of phosphate has frequently been used in this con-ANALYST, NOVEMBER 1991, VOL.116 - 1102 IX S P , XI P n I C . Separ- IV I ation .0 1 unit Fig. 3 Flow injectiodon-line separation assemblies. IX, Conven- tional location of separation units involving two emerging streams as in gas-diffusion, dialysis and liquid-liquid extraction processes. X, Special location of the separation unit for preconcentration in addition to separation (dialysis, gas diffusion, ion exchange or liquid-liquid extraction). XI, Conventional location of separation units involving a single emerging stream ( e . g . , in precipitation and ion-exchange processes). Symbols as in Fig. 1 text.21-41,49-51,54,55,58,64-68 Phosphate was first reacted with molybdenum(v1) to form the yellow 12-molybdophosphoric acid, which was then reduced to the blue heteropoly phospho- molybdenum. The formation of a heteropoly acid was also the basis for the determination of silicate and arsenate.24.26 Phosphate also forms a yellow heteropoly acid complex with vanadomolybdic acid.42-45 The reaction is very fast, although it is not as sensitive as that of the Molybdenum Blue method.The Methylene Blue method was applied to the determination of sulphide in industrial waters.101 The spectrophotometric determination of fluoride with lanthanum(rIr)/alizarin com- plexone has been reported; the slow reaction rate was overcome by either increasing the temperature to accelerate the reaction143 or by using the stopped-flow-reagent injection method.14 Spectrofluorimetric methods for the determina- tion of fluoride have been described, based on enhancement of the fluorescence of the ternary complex formed between fluoride, zirconium and Calcein Blue.146-148 Boron was determined spectrofluorimetrically on the basis of a ternary complex formed with 2,6-dihydroxybenzoic acid and Crystal Violet .278 Most of the spectrophotometric methods reported for the determination of chromium(v1) are based on its reaction with 1,5-diphenylcarbazide.229-237 The thiocyanate R Xlll PI c P7 R XIV f=l IP S U P W t Fig. 4 Special manifolds for integration of retention and detection using a chemical sensor (XII), and for the iterative passage of the sample plug through a conventional detector in an open-closed system (XIII) or the iterative change of the flow direction with a programm- able pump (XIV). IP, Injected miscible or immiscible phase. Other symobls as in Fig. 1 Redox 22% ectrochemical Complex formation 21 % 5% No chemical reaction Other reactions 15% 9% Fig.5 Frequency of use of the different types of reactions used for the determination of anions (separation techniques are also included) method was adapted to the determination of molybdenum, in which molybdenum(v1) was first reduced to molybdenum(v), and the molybdenum(v)-thiocyanate complex formed was measured.24Q-242 The colour forming reagent 4-(2-pyridylazo)- resorcinol was used for the spectrophotometric determination of vanadium.254.255ANALYST, NOVEMBER 1991, VOL. 116 Table 7 Flow injection determination of chromate, molybdate and vanadate 1103 Analytical features Configuration I-a I-a I-a I-a I-a 111-a XI11 I-a I-a IX I-a I-a IX 111-a - I-a I-a I-a I-a I-a I-a I-a I-b I-b I-a I-a I-a I-b I-b Reaction/ separation technique* CF CF CF CF CF CF CF CF CF NCR CF CF IP-LLE LLE-CF IE-CF CT CT CT CT CF CF CT CT IE-CT CT CF CF IE IE * Abbreviations as in Table 1.1- Abbreviations as in Table 1. Detection? P P P P P P P P P P ICP-AES P P P P P P A PO PO AAS P P P P P P AAS ICP-AES Rangel pgml-* 0.1-20 0.540 0.1-20 0-2.0 0.1-1.2 0.2-10.0 W . 0 0-20 0-1 .o 0.14.0% M O O 0.05-1 .O 0.00015- 0.006 0.001-1.0 0.00 14.04 0.02-1.0 0.17-30 - - 0-0.02 - 0.054.5 0.01-1 .0 0.00004- 0.002 0. oooO5- 0.002 6.6-26.4 0.5-3 .0 0.2-1.0 0.01-0.05 Detection limit/ pg ml-1 - - - - 0.018 - - - - 0.44 0.0002 - - - 0.00015 0.007 0.0012 0.17 0.24 - - - - - 0.oooo2 - - - - RSD 1.3 31.0 (% 1 - - 0.7-3.0 - - 0.8 2.5 0.73 1.1 <2.0 2.3 - - 0.83-1.9 <2.0 1.3 - - - 0.37-0.55 - - 0.5 - - 3.0 <5.0 Sampling frequency1 h-1 70 180 120 - - - - 40 120 24 270 40 30 3 4 90 40 1 00 60 - - - 60-70 12 1.5 - - - - 17 Real samples Soil - - - - - - - - - - Steel Steel Plants water Soil Plants Soil Sea-water - - - - Rain - - - - - Reference 229 230 23 1 232 233 234 235 236 237 238 239 240 24 1 242 243 244 245 246 247 248 249 250 25 1 252 253 254 255 256 257 Catalysed Reactions Sulphate was determined on the basis of its catalytic action on the reaction between zirconium and Methylthymol Blue.81.s2 The determination of sulphide has been described, based on its catalytic action on the iodine-azide reaction.102 Iodide catalyses the redox reaction between arsenic(rr1) and ce- rium(rv), and this is the basis for the thermochemical,267 spectrofluorimetric268 and spectrophotometric205 determina- tion of iodide.Thiocyanate was determined by Tanaka et al.205 on the basis of its catalysis of the oxidation of arsenic(rr1) by cerium(1v). A spectrofluorimetric method for the determina- tion of cyanide with pyridoxal and pyridoxal5-phosphate has been described;21l7212 the reaction is catalytic in nature. Trace amounts of bromide were determined spectrophotometrically by the catalytic action of bromide on the oxidation of Pyrocatechol Violet with hydrogen peroxide in an acidic medium.262 A method for the determination of molybdenum has been reported, based on its catalytic action on the oxidation of iodide by hydrogen peroxide in an acidic medium.244-247 Trace amounts of vanadium in water were determined by the catalytic effect of vanadium on the oxidation of some organic compounds by bromate.*5(&253 The catalytic effect of selenium on the reduction of Tolidine Blue by sulphide was also used for the catalytic determination of this element.295 Substitution Reactions Yoza et aZ.47 proposed a method for the determination of orthophosphate, diphosphate and triphosphate based on the substitution reaction between phosphates and coloured metal complexes to form a colourless M-P, complex.Sulphate was determined by its competitive reaction with barium chelates such as those of Methylthymol Blue,72,76?83,84, dimethylsul- phonazo-III,8S786 nitchromazo78 and 3-(2-carboxyphenylazo)- 6-(2-sulphophenylazo)chromotropic acid.79 The spectropho- tometnc detection of the iron( 111)-thiocyanate complex was used for the determination of nitrate based on the displace- ment of thiocyanate from a strongly basic ion-exchange resin.176 Fluoride was reacted with zirconium-Alizarin Red S to form a colourless zirconium fluoride product, the resulting absorbance being measured at 520 nm.145 A method for the determination of chloride with mercury(l1) thiocyanate and iron(rI1) as reagents has been reported;11s3116 the method involves measurement of the absorbance at 480 nm of the coloured iron(Ir1)-thiocyanate complex formed. Another method for the determination of chloride has been described, based on its solid-liquid reaction with mercury(I1) chloranilate to yield coloured chloranilate ions. 118 The spectrofluorimetric determination of oxalate in urine has been described, based on the quenching effect of oxalate on the highly fluorescent complex formed between zirconyl ions and flavonol in sulphuric acid.292 Precipitation Reactions Sulphate has frequently been determined by measuring the turbidity of a barium sulphate suspension spectrophotometric- ally63,69-77>8* the method was first reported by Krug et al.69 One problem with this method is that the precipitate formed tends to settle in the reactor and flow cell, thus reducing the precision and leading to clogging of the manifold.Alkaline1104 ANALYST, NOVEMBER 1991, VOL. 116 Table 8 Flow injection determination of bromine and iodine anions Analytical features Reaction/ separation Anion Configuration technique* Br- Br- Br- Br- Br- Br- Br03- I- I- I- I- I- I- I- I- I- 103- 1 0 4 - I-no3- I-a III-a I-b I-b I-a I-a I-a I-a I-a I-a I-a I-a I-a - I-a I-a I-a I-a I-a Cl--Br--1- XI NCR R-CT R R CT R R R R CT CT CT EC EC NCR NCR EC R R IE-NCR * Abbreviations as in Table 1.Abbreviations as in Table 1. Detection Range/ limit/ Detection? pg ml-l pg ml-l PO CL P P P P PO P P TC F P A A PO PO A P P PO 1-5000 - 1-lo00 O.oooO625 0.16-2.4 - 1-10 - 0.01-0.6 - 0.13-0.65 0.1-10 0.05-15 0.00127-1.27 0.1-1 .o 5.1-51 5 x 10-10- mol dm-3 0.254-58.8 0.1 mol dm-3 - 1 x 10-3 5 x 10-5- 0.0127-127 5-50 2 x 10-3- 1 x 10-5- 8 x 10-3 mol dm-3 6 x 10-5 rnol dm-3 20-5000 0.01 15 nmol dm-3 0.1 0.05 0.00125 3 mol dm-3 0.1 5 x 10-10 mol dm-3 - - - - - 0.35 - 5 RSD 1.6 Gl.0 0.2-2.5 (Yo ) - 1.4-2.5 - - - 1.0 0.3-1.0 - - 0.6-3.0 1 .o - - 1.0 1 .o - 1.5-2.1 Sampling frequency1 h-1 88 113 80 120 45 5 - - 52 30 20 - - - - 60 - 20 100 - Real samples Soil Sea-water Sea-water chloride Water Water - - - - - - - - - Salt - - - - - Reference 258 259 260 261 262 263 264 265 266 267 268 205 269 270 27 1 272 273 274 275 228,276 Table 9 Flow injection determination of borate and hydroxide Analytical features Reaction/ separation Anion Configuration technique* H2B03- I-a CF H2BO3- I-a CF HzBO3- I-c AB H2BO3- I-a DF H2B03- 1-a DF H2BO3- 1-a DF H2BO3- I-a DF OH- I-a AB OH- I-c AB OH- I-c AB * Abbreviations as in Table 1.t Abbreviations as in Table 1. Detection? F P P P P P P PO PO P Range/ pg ml- 0.02-0.5 0.0005-5.0 2.5 x 10-4- 5 x 10-2 rnol dm-3 0.140 0-3.0 0.254.0 0.2-2.5 0.01-0.1 rnol dm-3 0.01-1 0.01-1 mol dm-3 Detection limit/ pgml-l 0.0002 - - - - 0.02 - - - - RSD 4 .0 G0.4 (Yo 1 - <1.0 0.8 1.0-1.4 2.6% 4.0 0.8 c1.0 Sampling frequency/ h-l - - >60 60 60 120 25 180 - - \ Real samples Reference Sea-water 277 - 278 - 279 - 280 28 1 - 282 - 283 - 284 - - 285 - 286,287 ethylenediaminetetraacetate was introduced into the system in different ways7@-73,76 in order to dissolve the residual precipitate and hence flush the system. Sulphide in the sample solution was precipitated with cadmium ion, the precipitate being freely driven to the detector while the excess of cadmium was retained in the ion-exchange column and detected by AAS.99 Chloride was injected into a carrier containing silver nitrate and the precipitate retained on a stainless-steel filter. In this way, chloride was detected by AAS at a sampling frequency of 50 h-1105,1“ and by spectrophotometry based on the turbidity of silver chloride.121 Miscellaneous Reactions These include dye formation, chemiluminescence, ion-pair formation, acid-base and enzymic reactions. Dye formation reactions Cyanide has been determined by using chloramine-T; the CNCl produced forms a red-blue dye on addition of pyridine- barbituric acid. The method was first adapted to FI by Rios et al.2007201 The method can also be applied to thiocyanate.227 The reaction of cyanide with isonicotinic acid and 3-methyl-l-ANALYST, NOVEMBER 1991, VOL. 116 Table 10 Flow injection determination of other anions 1105 Analytical features Anion Mn04- Mn04- Salicylate c 2 0 4 2 - c 2 0 4 2 - ~ s 0 ~ 3 - ~ s 0 ~ 3 - As02- AsOd3- Se04*- Se042- Se042- Se03*- co32- Configuration I-a IX IX I-a I-a VI VI - IX IX I-a I-a I-a IX Reaction/ separation technique* NCR IP-LLE DIA NCR su CF CF R R GD-R CT CT CT GD-AB * Abbreviations as in Table 1.7 Abbreviations as in Table 1. Detectiont P P PO PO F P P AAS MS MS P P P P Range1 pg ml- * 5 x 10-2 mol dm-3 0-25 mol dm-3 0-100 0.88-31.7 0.86-2 1.4 1.3 x 10-4- 10-4-10-2 1.1-27.8 - 5-1 100 5440 2.5-25 1-12 1-12 0-72 Detection limit/ pg ml-1 - 0.58 - - 0.53 - - 0.0001 - - 1.0 - - 0.3 RSD (To) =1.0 - - t l . O 7.0 0.95 0.40 1.5 - - - 0.5 0.5 0.8 Sampling frequency1 h- - 24 - - 60 36 36 220 - - 35 90 90 15 Real samples - - Serum - Urine Water Water Soil and plants - - - - - - Reference 288 289 290 29 1 292 26 26 293 294 294 295 296 296 297 phenyl-2-pyrazolin-5-one has been exploited in FI.The unstable red intermediate was monitored spectrophotometric- ally at 548 nm.2033204 Azomethine-H was used as a dye formation reagent for the determination of boron.28+283 Chemiluminescence reactions The chemiluminescence detection of sulphite was achieved by measuring the radiation emitted by sulphite on oxidation by permanganate in an acidic solution, the signal being greatly increased by the addition of a sensitizer.95 A chemilumines- cence system was developed by Qian et a1.103 for the determination of sulphide; the weak light emission resulting from the reaction of 7,7,8,8-tetracyanoquinodimethane with sulphite was effectively enhanced by the use of Rhodamine B as a sensitizer in an organic reaction medium. The oxidation of sulphide by sodium hypochlorite in the presence of fluorescein is another alternative to the chemiluminescent detection of sulphide, the maximum intensity of the transient signal being largely determined by the rate of mixing of sample and reagent.104 An NO, chemiluminescent analyser was used for the sensitive determination of nitrate.188 Yamada and co- workers206210 developed several chemiluminescence systems for the determination of cyanide , the weak chemilumines- cence induced by injection of alkaline cyanide into the reagent stream being effectively sensitized by the presence of uranine in the organic medium.A synthetic anionic luminol derivative was bound to a strong anion-exchange resin for the chemilu- minescence determination of anions.152 Ion-pair formation reactions This type of reaction is closely related to liquid-liquid extraction and has been studied extensively in this laboratory.Nitrate forms ion pairs with copper(i)-organic chelate com- plexes, the atomic absorption of copper from the organic phase being proportional to the analyte concentration. 186,199 Anions have been determined on the basis of the formation of ion pairs with ferroin; the ion pairs were subsequently trans- ferred into the organic phase and monitored spectrophoto- metrically at 570 nm (23 anions were tested225). The indirect atomic absorption spectrometric determination of perchlorate has been described, based on the liquid-liquid extraction of copper( 1)/6-methylpicolinaldehyde azine/perchlorate .I26 Per- chlorate was also measured as the Brilliant Green-perchlorate complex at 640 nm after continuous extraction into the organic phase (benzene).127-129 Acid-base reactions The concentration of hydroxide ion has been determined by FI titration284-287 based on an acid-base reaction. Detection was carried out spectrophotometrically with phenolphthalein as indicator2869287 or by means of a pH electrode.284~285 Enzymic reactions An enzymic method was developed by Masoom and Town- shendgl for the determination of sulphite. On passing through the immobilized enzyme column, sulphite was oxidized to sulphate, with the release of hydrogen peroxide which was measured amperometrically . Types of Detection The distribution of the different detection techniques used for the determination of anions by FI is shown in Fig.6. Spectrophotometry is the most widely used detection technique because of its simplicity, the availability of instru- ments and the rapid response of the detectors, the variety of colour-forming reactions available and the adequate sensitiv- ity achieved in most instances. It was preferentially used in the determination of phosphorus anions,21-5* 764-682301 sul- phate,63,69-86 nitrate and nitrite,149.161 and has been applied to virtually all of the anions listed in the tables. Potentiometry is second to spectrophotometry in terms of frequency of use. The chief advantage of this technique is its instrumental simplicity, provided a direct response to the analyte can be obtained. There are some exceptions when the detector responds to the analyte reaction product.The slow dynamic response of electrodes in the flow system is the main shortcoming of this technique, particularly at low concentra- tions, although some improvements have been made by polishing the electrode surface133,136 or by adding an appro- priate concentration of analyte to the carrier solution.134 Potentiometry has been widely used in the determination ofANALYST, NOVEMBER 1991, VOL. 116 1106 70 60 50 E 2 40 a 30 c Z 20 10 0 Fig. 6 P PO A F CL AAS Others Detection technique Freauencv of use of the different detection techniaues (a6breviatio;s as Ln tables) fluoride,13@-141 and in the determination of phosphate,61 sulphate,87,88 sulphide97.98 nitrite,l@ nitrate,182-185 cyan- molybdenum(v1) ,247,248 salicylate290 and oxalate.291 Amperometry has been used in the determination of phosphate,54-58 sulphite ,91 sulphide ,100 nitrite ,162,163,165-167 nitrate,179J80 cyanide,218.222 thiocyanate,224 iodide,269,270 iodate273 and molybdenum(v1) .246 Spectrofluorimetry is usually more sensitive than spectro- photometry.The lack of spectrofluorimetric determinations of anions (only 5%) is probably due to the paucity of reaction systems, some of which require the application of indirect methods. Spectrofluorimetry has been used for the determina- tion of phosphate,52753 nitrite ,168,169 nitrate ,187 cyanide ,2113212 fluoride ,146,148 iodide ,268 boron277 and oxalate .292 Chemiluminescence methods have a reputation for high sensitivity and wide calibration ranges; however, the lack of chemiluminescence reactions and their poor selectivity have limited their application.Some improvements have been achieved in the determination of anions by FI, and this has been applied to sulphite,95 sulphide,103~1~ nitrate,188 cyan- ide ,206-210 chloride114 and bromide.259 Atomic absorption spectrometry used to develop an indirect method for the determination of anions, which usually entails a liquid-liquid extraction, substitution or precipitation process. It has been used for the determination of sulphide,99 nitrate ,186,199 nitrite ,199 cyanide ,223 chloride ,105,106 perchlor- ate,126 molybdenum(v1) ,249 vanadium(v)256 and arsenic(v) .293 Other detection techniques used in the determination of anions by FI include inductively coupled plasma atomic emission spectrometry,62.89,239,257 coulometry,59,60 molecular- emission cavity analysis ,% mass spectrometry ,294 thermo- chemical methods267 and those based on chemical sen- ide,213-217 ha~ide~,lO7-113,258,264,271,272,276 hydroxide ion,284,285 sors - 147,148,212 Multi-determinations Multi-determination is one of the more interesting trends in analytical chemistry on account of advantages such as a high analytical rate, efficiency and economy.302.303 About 10% of the papers examined reported the multi-determination of anions by FI.They can be divided into three groups. Physical Spectral Resolution Configuration V (Fig. 2) was adapted to split the sample plug into two (or more) streams merging with different reagent streams for spectrophotometric determinations such as that of phosphate by the Phosphomolybdenum Blue method and nitrate (or nitrite) by the Indophenol Blue method30.33 and sulphate by the turbidimetric method with barium(i1).Mer- cury(~~) thiocyanate and iron(ri1) were used as reagents for chloride ,63 while the diazotization-coupling reaction was used for nitrite, nitrate being reduced on-line to nitrite with a copper-coated cadmium c01urnn.120~150,170,189-192 The Phos- phomolybdenum Blue method was applied to phosphate, while phosphonate was oxidized to phosphate with sulphite, after which they were detected by two parallel66 or series65 detectors. Pyridine-barbituric acid and chloramine-T were used as reagents for cyanide, while the diazotization-coupling reaction was used for the determination of nitrite by reversed FI.2(W Chloride and nitrate were determined potentiometric- ally113 and simultaneously with phosphate by the Phospho- molybdenum Blue method by means of a spectrophotometric determination.191 Separation by High-performance Liquid Chromatography (Time Resolution) Related species contained in a sample have previously been separated by HPLC (configuration VIII). Flow injection post-column detection applications reported in this context include the spectrophotometric determination of silicate, phosphate and arsenic,24*38 inorganic polyphosphate~,67~68 phosphinate, phosphonate and orthophosphate by the Molyb- denum Blue method,@ and the potentiometric determination of chloride, bromide and iodide .276 Chemical Discrimination Multicomponent mixtures have been differentiated by means of their different chemical properties.Hence, nitrate and chloride were determined simultaneously by displacement of thiocyanate from an ion-exchange column, followed by spectrophotometric detection of the iron(rr1)-thiocyanate complex; chloride was removed by previously passing the sample stream through a column containing a cation-exchange resin in the silver form.176 Cyanide and thiocyanate were determined by the pyridine-barbituric acid method, cyanide being masked by using nickel as a complex-forming agent.227 Thiocyanate and iodide were determined spectrophotometric- ally as catalysts for the redox reaction between cerium(rv) and arsenic(rI1); their simultaneous determination is based on the selective inactivation of the catalytic activity of thiocyanate.205 The sequential spectrophotometric determination of mixtures of arsenite-arsenate, arsenate-phosphate and arsenite- arsenate-phosphate using the Molybdenum Blue heteropoly acid method was accomplished by using a selecting valve to provide an appropriate medium for the indicator reaction to develop with one, two or three species in the mixture.26 Mixtures of chloride and iodide-were determined by indirect AAS.Total chloride and iodide were determined by the decrease in silver absorption, on precipitation with silver nitrate, after which chloride was determined selectively by dissolving the silver chloride precipitate in ammonia solu- tion.3" The speciation of both oxidation states of selenium (selenite and selenate) has been accomplished by Linares et al.296 by exploiting the catalytic action of selenium(1v) on the reaction between chlorate and hydrazine in a hydrochloric acid medium, the coloured oxidation product of o-toluidine being measured spectrophotometrically .Selenium(v1) did not interfere with the determination. The total selenium content was quantified after reduction of selenium(v1) to selenium(rv) with hot concentrated hydrochloric acid. Mixtures with ratios between 1 : 4 and 4 : 1 were determined with errors of less than 3 and 8%, respectively. Selectivity Interferences are not uncommon in the determination of inorganic anions owing to their similar chemical physico- chemical features. Hence, direct detection or the use of derivatization reactions does not lead to particularly selective analytical methods.ANALYST, NOVEMBER 1991, VOL.116 1107 Flow injection can advantageously replace manual ana- lytical methodologies because of its tolerance to matrix effects. The enhanced selectivity achieved can be the result of physico-chemical kinetics,305 material separation or signal discrimination,306 or a combination of two or all of these effects. The multi-determinations described above are the ultimate goal in terms of selectivity. Kinetics and selectivity are very closely related.307 The intrinsic nature of FI makes it more tolerant to foreign species than its manual counterparts, as no unwanted reactions develop to a significant extent during the short residence time typical of FI. The FI spectrophotometric determination of cyanide with several reagents2~204,226,*27 is a representative example.On the other hand, the implementation of stopped- flow modes in FI configurations (manifolds I and IV) avoids the occurrence of parasitic signals from the sample matrix because of the relative rather than the absolute character of the measurements. Hence, the natural colour of wine does not interfere when sulphite23 is determined spectrophotometric- ally from reaction-rate measurements. The ready implementation of non-chromatographic con- tinuous separation techniques in FI configurations (Fig. 3) also enhances selectivity indirectly through the isolation of the analyte anions from the sample matrix.308 It is interesting to note that the kinetics of these processes can also contribute to selectivity.The selectivity factor (ratio of the tolerated level for the FI-extraction method to that of its manual counter- part) is very high for various anionic species in the determi- nation of cationic chelates and atomic absorption measure- ment of the metal content in the organic phase after liquid-liquid extraction.309 One of the most salient advantages of fast detectors (e.g., rapid-scan voltammetric and spectrophotometric diode-array detectors) is the ease of implementation of multi-determi- nation procedures based on signal discrimination. Although interesting developments have been reported in this respect ,31@312 none were concerned with the determination of anions. Sensitivity As a rule, FI methods are less sensitive than their manual and air-segmented counterparts because no chemical equilibrium is attained and because of the dispersion of the sample plug in the carrier(s). Nevertheless, the great versatility of this technique allows sensitivity to be enhanced by introducing appropriate modifications into FI manifolds such as reversed FI, preconcentration steps and multi-peak recordings.In reversed FI modes (configuration I1 in Fig. 1 and VII in Fig. 2), the sensitivity is clearly increased compared with the conventional alternative because the amount of sample in the reaction zone increases with increasing dispersion, albeit at the expense of the large sample volume required. This approach is particularly useful for implementing on-line monitoring of anions in waters and industrial effluents. 146 The use of non-chromatographic continuous separation techniques in FI configurations (Fig.3) allows the implemen- tation of preconcentration approaches based on the use of a large sample volume, retention of the analyte anion in one phase and subsequent elution with a small volume and rapid transport to the detector. The preconcentration factor thus achieved ranges from 2 to 10000. The use of derivatizing chemical reactions to facilitate the retentiodelution process is of particular relevance in this context. The use of flow-through chemical sensors in FI manifolds based on the integration of retention (reaction) and detection by using appropriate supports in the flow cell is the most recent approach in this context .3*3 These systems allow the development of kinetic 'in situ' concentration methods which circumvent one of the most serious drawbacks of preconcentration methodologies, namely, the need for a large sample volume (manifold XIII, Fig.4). It is also possible to enhance sensitivity in FI through the use of multi-peak recording alternatives, exploiting fast detector capabilities and the repetitive passage of the reacting plug through the flow cell, by using an open-closed assembly or the iterative change of the flow direction (configurations XI11 and XIV in Fig. 4). Rather than a single FI peak, a multi-peak recording is obtained from a single injection. By summing one of the parameters (e.g., peak area, peak height) of two or more peaks, the analyte concentration range can be auto- matically amplified by several orders of magnitude.Such is the situation with the FI determination of nitrite by using a traditional spectrophotometric reaction. In the conventional mode, the determination limit is 0.2 pg ml-1, whereas if an amplification method is applied with the aid of a diode-array detector, the determination limit is 0.002 yg ml-1.314 Analysis of Real Samples Of the 300 papers examined, only 109 dealt with real samples, distributed as follows: water, 64 (including waste water, 6; sea-water, 6; rainwater, 4; and melted snow, 1); plants, 11; soils, 9; urine, 6; plasma and serum, 5; chemicals, 5; materials, 3; rocks, 3; fertilizers, 2; foodstuffs, 1; fruit, 1; beverages, 1; milk, 1; saliva, 1; smoke, 1; air, 1; and salt, 1. The over-all number exceeds 129 as some papers dealt with more than one type of sample.The lack of papers on sample analysis is obviously due to the interference of foreign species contained in the sample matrix. However, some improvements have been achieved. Hence, a column packed with cation-exchange resin was incorporated in an FI system to remove interfering metal ions present in the sample for the spectrophotometric determination of sul- phate.79,83,86 A dialysis or gas-diffusion unit is usually used to trap the analyte from the sample matrix, as in the turbidi- metric determination of urinary inorganic sulphate,gO and the potentiometric determination of anionic cyanide202-204.226 and thiocyanate .226 Soil samples are usually pre-treated manually to obtain the liquid extracts; however, we have developed an automated method for the direct introduction of soil samples into an FI system.The weighed sample was placed in the sample cell and inserted into the manifold. It was leached for 30 s by circulating 0.1 mol dm-3 hydrochloric acid through the cell at 80 "C under ultrasonic irradiation. The extraction was then injected into the carrier stream as the valve was switched. The over-all sampling rate achieved was 25 samples h-1.283 Bergamin et a1.241 developed an on-line electrolytic alloy dissolution method for the determination of molybdenum. A direct current pulse was applied to the polished steel sample and the dissolved sample was driven to the detector by the carrier stream. Comparison With Other Techniques The paucity of selective, sensitive methods for the determi- nation of anions made the inception of ion chromatography a revolution within the analytical chemistry of these species and led to the development of more complex and expensive, albeit also clearly more sensitive and selective, methods than their earlier counterparts.Flow injection and HPLC have been compared.315 In spite of the similarities derived from their hydrodynamic character, they differ in major respects such as the high separation potential of HPLC, which is of particular relevance to ions on account of their intrinsic analytical features and permits the determination of from 2 to more than 20 anions in the same sample. On the other hand, FI is more useful than HPLC for the determination of one or two anions in a large number of samples in a straightforward, automated and fast way, and this is particularly appealing to control laboratories.The implementation of continuous separation techniques such as liquid-liquid extraction, distillation, ion1108 ANALYST, NOVEMBER 1991, VOL. 116 exchange, precipitation, dialysis and gas diffusion in FI configurations allows the two important properties of sensitiv- ity and selectivity to be enhanced in an indirect fashion. These advantages have increased the potential of FI even further with respect to ion chromatogTaphy and other analytical techniques according to Braun and Zsindely.316 Conclusions Any significant practical contribution to the analytical chem- istry of inorganic anions is, in principle, of some interest on account of the well-known difficulties encountered in develop- ing suitable analytical methods for these species.This review was compiled with the aim of promoting the use of FI by routine control laboratories by transferring the analytical methods reported in the literature to the daily work of the analytical chemist involved in monitoring anionic species in a variety of matrices encountered in environmental, phar- maceutical, food and clinical chemistry, among others. Flow injection methods are straightforward, fast and can easily be automated; in principle, they only require the purchase of items such as peristaltic pumps, flow cells, Teflon tubing, connectors and an autosampler with computer control, as they are compatible with most of the instruments typically used in laboratories (spectrophotometers, spectrofluorimeters, potentiometers). The increasing commercialization of FI analysers should also contribute to the gradual adoption of FI by control laboratories.Compared with other analytical alternatives, FI allows the development of more sensitive, selective, precise and rapid methodologies which can be applied to a wider range of situations and standardized much more readily. We should not wish to leave the reader with the impression that the determination of anions by FI is already on the decline from the point of view of basic research. There are still a variety of unexplored aspects which offer interested workers promising prospects. Hence, some of the issues dealt with in this review ( e . g . , the use of fast detectors and the iterative passage of the sample or sampleheagent plug through the detector) have not yet been exploited to their full potential.Miniaturization and the increasing efficiency of non-chroma- tographic continuous analytical separation techniques will hopefully permit the development of methods for the auto- mated determination of anions that are clearly superior to those used at present.308 The direct introduction of solid samples into continuous automated systems as a means of avoiding tedious preliminary manual stages (e.g., dissolution, digestion) might lead to interesting prospects for the determination of anions in a large variety of samples judging by the promising results obtained to date using this method.283 The Comision Interministerial de Ciencia y Tecnologia (CICyT) is thanked for financial support.References Rfiiitka, J., and Hansen, E. H., Anal. Chim. Acta, 1986,179.1. 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