- Methodology 3.1 NEW METHODS 3.1.1 Introduction This section describes novel methods of analysis that are considered to be of sufficient general interest to merit discussion here as well as in the appropriate section on applications. The section also includes papers by workers who have made a detailed study of experimental parameters of widespread relevance. 3.1.2 Sample Preparation Techn'iques Various methods that minimize the time spent in sample preparation have been proposed.The direct analysis of suspensions of finely ground samples (ARAAS, 1977, 7, 23) has many advantages, such as speed, simplicity and low blanks, and has been used for the direct analysis of Cay Fe, Na, Ni and Pb in coal slurries ground to <325 mesh (50). Calibration using aqueous solutions could not be used however, and it was essential to use previously analysed slurrics for calibration.Others workers (539) found that wet ashing with HF/ HNO, /HClO, was preferable to high-temperature dry ashing or low-temperature plasma ashing for the analysis of coal and coke. Losses of Zn were observed with the dry ashing methods. Hernandez-Mendez and co-workers determined Zn in lubricating oils (372), Cu and Fe in benzene (998) and Pb in petrol (7319) by emulsification of the hydrophobic phase into water.A small volume of the sample was shaken with water in the presence of suitable non-ionic detergents, which resulted in the formation of a stable emulsion. The appreciable volatility of lead tetra-alkyls precluded the use of aqueous solutions for calibration and it was essential t o use standardized petrol samples.Wet crshing of biological samples can be time-consuming and a multisample pressure digestion unit that allowed 12 samples t o be digested simultaneously has been described by Stoeppler and Backhaus (1465). Williams (I. Food Technol., 1978, 13, 3167') developed a pressure digestion method for the analysis of Fe, Pb and Sn in canned foods, in which disposable polystyrene containers were used, having polythene screw caps that acted as pressure release valves.The digestions were carried out at 70 "C with either €€NO, or HCl/H,O,. A particular advantage of the method was that many digestions could be carried out simultaneously. Enzymatic digestion of fruit juices was found to give com- pounds that were readily volatilized during the dry ashing stage(s) of ETA (99) (see also ARAAS, 1976, 6 , 156).A simple procedure has been devised by Davis and co-workers (3152) f o r the analysis of Ca and Mg in animal and vegetable tissue. The samples were placed in a vortex mixer and hcated with 1M NaOH at 75 OC in polystyrene tubes €or 5 min. Lanthanum EDTA was then added and the resulting solution was analysed for Ca and Mg in an air/C,H, flame.Good agreement with conventional wet or dry ashing techniques was obtained. Th- technique would not be suitable for trace level measurements of other elements because of significant background absorption from the 1M NaOW. A novel but rather involved volatilization method for the determination of Pb in plant tissue has been reported (68).The sample was heated in an 0, atmosphere in order to oxidize it. Any resulting lead oxide was then reduced in a H, atmosphere at a high tempera- ture and the resulting Pb vapour condensed on a cool surface and dissolved in HNO, prior 6162 Analytical Atomic Spectroscopy to determination. Considerable care must be exercised to avoid the mixing of 0, and H, at high temperatures in glass apparatus! A rapid, simple and virtually universal tcchnique for the dissolution of ores, sinters, slags and ferro-alloys has been described (130) that involves fusing with Na,O, or Na,O,/Na,CO, and treatment of the melt with water.No loss of As, P, Pb or Zn was observed. The rapid analysis of toxic metals in sewage sludges is of growing importance with the increase in disposal of sludge to the land.Various rapid wet digestion methods for sludges have been recommended (1071, 1072, 1092, and T. D. Rees and J. Hilton, Lab. Pract., 1978, 27, 291). It is now generally agreed that time-consuming conventional procedures involving €€N03/HC10, or HNO,/H,SO, are not necessary for this type of analysis; howcver, for soil analysis Scott and Thomas (922).recommended an HF/HClO, digestion and obtained better recoveries than with a HClO,/HNO,/H,SO, digestion or a fusion with Na,CO,. It is not usual practice to oxidize materials containing organic matter with HC10, in the absence of HNO, because of the risk of violent explosion. Four wet digestion methods were examined (1389) for the extraction of metals from aquatic sediments without significant attack on the silica matrix, and it was concluded that digestion with 0.5M HC1 was the most satisfactory.Scott (534) has shown that the widely held view that Cr can readily be volatilized as CrO,Cl, from HNO,/HClO, digestions of sediment samples is wrong. The poor Cr recoveries were shown to be due to adsorption of Cr onto the siliceous residue. Price (80) has reviewed the AAS analysis of micro-samples with special reference to siliceous materials (see also ARAAS, 1977, 7 , 119) and shown that an HF/HCl/HNO, pressure digestion, with sub- sequent addition of H,BO, and further heating is the best method available for a wide range of materials.With the increasing cost of labour some quite complex methods are now being auto- mated and a prime example of this is the automated digestion and extraction system for the determination of trace metals in foodstuffs that has been described by Stockwell et al.(420). It was based on a continuous flow system and consisted of five component modules - sample introduction, digestion with H,SO, /HNO, /H,O, at 400 "C, neutralization, chelation with DDC, and extraction into heptan-2-one. The only manual sample prepara- tion was weighing 5-10 g of the freeze-dried sample into a polythene bag, adding 30 ml of 12M H,SO, and heating at 60 "C.followed by homogenization of the sample. A somewhat similar type of system for the determination of As and Hg in fish tissues has been reported by Agemian et aZ. (639, 1401) (see also ARAAS, 1975, 5, Ref. 185). A semi-automated tech- nique for the separation and determination of Ba and Sr in surface waters using ion exchange has been developed (10). Forty samples could be prepared in 90min and analysed within a further 20 min. Two novel methods for the rapid breakdown of organo-mercury compounds have been reported. Farey et al. (529) added 2ml of a solution containing 0.27% m/V KBrO, and 1% m / V KBr to 50ml of the acidified sample.The liberated Br, resulted in the rapid breakdown of methylmercury and phenylmercury compounds and also prevented loss of Hg on to the walls of the sample container. Shimomura e l aZ. (137'6) showed that if Fe(III) was added to an acidified sample, the addition of NaBH, solution resulted in break- down of organo-mercury compounds.Freeze drying has been shown to produce no signifi- cant loss of Hg from fish or mussel samples (571). The effect of adsorption of the Hg vapour on the inner walls of the apparatus after liberation from the analyte solution in the Hg cold-vapour technique has been determined (616). A comparison of NaBH, and SnCl, for the generation of HgO (ARAAS, 1976, 6, Ref. 512) showed that both reagents had specific disadvantages (93).In the presence of I-, SnCl, gave poor recoveries, while in the presence of Ni, NaBH, gave poor recoveries. It has been reported {417) that by filteringChapter 3: Methodology 63 a 2% m/Y NaBH, solution containing a small amount of KOH through a 0.45pm membrane filter the useful life of the solution was increased €rom 1 day to several weeks.Various methods of sample preparation have been found to increase the sensitivity and minimize inter-element effects when using hydride generation techniques (19S, 887, 1342). Co-precipitation of As with Fe(OH), (551, 1075) was recommended for the determination of As in natural waters. The addition of HF to the sample solution has been found to minimize inter-element effects from Nb, Si, Ti and W in the determination of As in metal A novel solid sampling technique for determining elements in singte algal cells has been developed by Lorch (1097).The freeze-dried algae were spread on paraffin on a microscope slide using a Pt wire, then, after viewing the slide through a microscope, single cells were collected in a lop1 drop of water hanging from a plastic pipette tip.The drop containing the single cell was then placed into an ETA device. The analysis of trace impurities in elemental boron has been simplified by volatilization of the boron using acetic anhydride and methanol (48). The method was rapid and resulted in lower residual boron concentrations than were obtained in alternative methods. The advantages of metallo-immunoossay have been stressed by Cais and nine co-workers (7).Specially prepared metallo-antigens were allowed to react with antibodies and after separa- tion of the resulting metallo-antigen-antibody complex, the metal content of the bound or free phase was monitored using ETA techniques. A significant advantage is that the method does not require the use of radioactive isotopes; however at present the technique is some- what less sensitive that radio-imniunoassay. The preservation of samples, the choice of sample container and sample trcatment are prime considerations in ultra-trace analysis (1 385, 1386, 1390) and often insufficient comideration is given to these aspects of the work.Filtration (< 1 pm), acidification and deep freezing was found to preserve sea-water samples for the determination of Cu and Fe (238).Moody and Lindstrom (632) have evaluated impurity levels in a wide range of plastic sample containers, using various analytical techniques, and have proposed methods for the preparation of sample containers for ultra-trace analysis. Linear polyethylene sample con- tainers and acidification of the sample to pH<1.5 with HNO, have again been recom- mended for the preservation of trace metals in natural waters (see also ARAAS, 1976, 6, 65; G.E. Batley and D. Gardner, Water Res., 1977, 11, 745). alloys (34). Other references of interest - Hg in coal: 1398. Microdetermination of trace elements in biological material: 1471. S in steel, S , emission following H,S generation: 770. Wet and dry ashing of grasses: 1469. 3.1.3 Pre-concentration Techniques 3.Z.3.Z Solvent Extraction. Reports of new reagents for the solvent extraction of trace elements continue to appear. The reviewer cannot help feeling that more effort should be made to reduce the lack of agreement between different laboratories using “accepted” solvent-extraction methods for the analysis of typical samples. This would be more helpful than the further proliferation of new exotic reagents that are often only used to extract synthetic solutions.The mono-octylester of a42 carboxy-anilino) benzylphosphonic acid (1 197) has been used to extract Fe(III), Mn(II) and Zn, and 1 -phenyl-3~methyl-4-benzoyl-5-pyrazolone (1 27’5) has been evaluated for the extraction of Co, Cu, Fe, Mn, Ni and Zn. Problems continue, how- ever, to be experienced in the commonly used APDCIMZBK system; poor stability of the lead and certain other APIX complexes in MIBK has been observed and back extraction64 Analytical Atomic Spectroscopy into acid has been recommended (19, 915, 1169, 1236, 13151). 1,1,2-Trichloro-l,2,2-trifluoro- ethane (Freon T.F.) has been used to extract A P E and DDC metal complexes, followed by back extraction into HNO, (881).Freon T.F. should not be nebulized into a flame. Various solvents and chelating agents for the efficient interference-free extraction of Co and Ni have been examined (538) and the pyridyl-azo-naphthol /CHCI, system was recom- mended for the ETA determination of these elements. Viets (942, 953) has reported the extraction of many metal iodides (Ag, As, Bi, Cd, Cu, Ga, Hg, In, Pb, Pd, Pt, Sb, Sn, Te, TI and Zn) from HCl solutions containing ascorbic acid using ‘AZiquat 336’ (tricaprylmethyl ammonium chloride) dissolved in MTBK.Thc trace metal halides were extracted by exchanging with the chloride ion of the Aliquat 33’6, or as oxonium ion associaticn pairs with the MTBK. For some metals both extraction mechanisms were found to be important. Two more selective methods for the determination of Cr(V1) have been reported; the first relied on the formation of dimercaptomaleonitrile-chromium(V1) complex followed by extraction into MIBK as an ion-pair with tetrabutylammonium ions (54.71, while the other utilised the formation of perchromic acid and rapid extraction into MPBK (232).When Ge was extracted from 8’M HCl into n-butyl ether and the ether layer washed with 12M HC1 a five-fold increase in sensitivity was observed (225).It was shown that the 12M HC1 wash resulted in the formation of GeCl, which was more readily volatilized and atomized than the germanium species extracted from 8M HCl. A combination of solvent extraction and concentration by evaporation has been used by Kasterka et aZ.(75) to determine Bi, Cd, Co, Cu, Fe, Ni, Pb and Zn in natural waters. The metals were extracted into n-amyl acetate using DDC!, followed by a further extraction with 8-hydroxyquinoline; thc combined extracts were then concentrated five fold in :’ rotary evaporator at 30 “C. An unusual solvent extraction application was the determination of orgai.to-silicoizes in sewage studge (1251).Thc sludge was extracted with toluene, the extract evaporated to dryness and redissolved in a small volume of MTBK. The extract was then passed through an activated charcoal column, eluted with MIBK and the Si level in the eluant determined. Cresser (51 3) has written a book entitled ‘Solvent Extraction in Flame Spectroscopic Analysis’. The main part of the book, which cites 795 references, is divided into sections on each of the elements, and this enables the reader to survey rapidly the multitude of possible extraction systems for a given element.APDC/CCl,, Cd in sea-water: 1461. Boron using 2-ethylhexane- 1,3-diol: 36. Tellurium using TOPO/MTBK: 91. Other references of interest - 3.1.3.2 Adsorption Concentration. Horvath and co-workers (40, 700) have used iminodiacetic acid ethyl cellulose to concentrate various metals from ammonium acetate soil extracts.Concentration factors up to 20 times were possible and good separation from the main matrix ions (Ca, Mg and Na) was demonstrated. Controlled-pore glass beads (Bio-Glas-200, mesh size 100-200) were found to be a simple and effective method of preconcentration of trace metals from ‘artificial sea-water’ (1 85).The beads were shaken with the alkaline sample solution for 1-2h, then removed, and extracted with a smnll volume of acid. Recoveries of 90-100% were obtained for Cd and Pb at the 1 pgl-’ level. It should be pointed out that the metals present in many natural water samples exist a s complex species and can behave in a different manner from those present in synthetic solutions.Chapter 3: Methodology 6 5 3.1.4 Indirect Methods The ingenuity of AAS enthusiasts in extending the area of applicability of the technique continucs to astound, D'Alonzo (315) has produced a dissertation on the indirect determina- tion of primary aliphatic and aromatic amides, organo-sulphidcs and polymers using AAS.The indircct dctcrmination of phosphatr: utilisiiig phosphomolybdate formation (see ARAAS, 1971, 1, 50) continucs to be rcported.Phosphate has been determined in saline and non- saline waters (580, 802), rocks (1323, 1324), cast iron (802) and alloys (366). One group has even completed the analysis by monitoring the Mo concentration by ETA (730). This system may soon rival the interference of phosphate on calcium in the air/C,H, flame for the number of papers published! Flame emission methods have been proposcd for vicinal-diols (744) and A1 (916). The former werc determined using Malaprade oxidation to carbonyl compounds, followed by formation of their sulphite addition compounds and then monitoring of the S, emission.Aluminium was determined using a calcium inhibition titration (ARAAS, 1971 1, 265).An indirect AAS method for biuret in fertilizers was compared with the official AOAC method and found to be satisfactory (3773. Fluorine has been determined by monitoring GaF absorption at 21 1.4 nm and AIF absorption at 227.5 nm in flames and ETA devices (1344) (see also ARAAS, 1977', 7, 79))). Boron has been converted to BF, using CaF, and H,SO, and determined by monitoring €30, emission and B F absorption in a N,O/H, flame (237, 1344). 'Aggressive (3'0,' in waters has been determincd by adding Cr;CO, to the sample and determining the increase in dissolved Ca concentration (1 142).Eke and Frank (1 80) ha:Te dcter- mined total organic carbon in aqueous solutions by direct nebulization of the sample into an air/H, flame containing a silver tube.The chemiluminescent emission of the silver 328.3 nm line was monitored, the emission signal being generated by the reaction CH + 0 + Ag -+ CHO + Ag*. The detection limit however was about 1000 pg ml-1 C, which will limit the use of the technique. Indircct AAS methods have also been proposed for anionic and non-anionic detergents (12, 1470), antihistamines (1 1 lo), CS, and dithiocarbamates (9671, ethambutol (406), glycosaminoglycans (592), long-chain primary amines ($559, phenothiazines (8481, sulphate (454, S95), sulphide (973) and U (573). 3.1.5 Nebulization, Vaporization and Atomization The inherently simple discrete-sample nebulization technique (see ARAAS, 1975, 5, 15) has the advantages over conventional ncbulization that only small sample volumes (typically 20-200 pl) are required for each measurement and that solutions containing very high levels of dissolved solids can be nebulized without clogging the burner slot or causing significant memory effccts.Various applications of th:: technique have been describcd (1 1, 699, 7'49, 969, 1001, 1013, 1032). Schleicher and Leighty (124) have developed a commercial ETA auto-sampler based on the work of Matousek (see ARAAS, 1977, 7, 79).The liquid samples are directly nebulized into a heated graphite tube (150 "C) for a fixed time pxiod and the dried sample within the tube is subsequently dry ashed and atomized. The sensitivity was found to be directly proportional to the time of nebulization (for periods of 1-100s).Inter-element effects, however, would be expected to be more sevcre with increased nebulization periods. Various modifications to ICP nebulizers have been made to improve sensitivity and allow nebulization of solutions with a high level of dissolved solids (1406, 1407, 1409, 1410. 1447). Other modifications have bzen made to allow direct nebulization of organic solvents without build-up of carbon on the sample tube tip (1411).Controlling the sample uptal;:: rate of a pneumatic nebulizer with 3 pcristallic puma w;ts found to niinimize th,: cffect of variations in the height of the liquid sample 2nd nlso66 Analytical Atomic Spectroscopy that of acid concentration, when using an ICP (679, 867). The addition of 5% m / V of a non-ionic surfactant to all solutions (86’7) allowed the maximum dissolved solids concentra- tion to be increased from 1 to 3 4 % m/m without clogging of the nebulizer.When the nebulizer tip was silylated, 10% m / m of dissolved solids could be tolerated. Manabe and Homi (1408) have developed a Babington-type nebulizer (see ARAAS, 1975, 5, 15) which can be used for the direct nebulization of HF solutions into an ICP.The use of element-specific spectroscopic detectors for GC ond HPLC i s becoming more widespread (see 1.2.2.3). Numerous types of GC detector have been reported, including microwave photometric devices (56, 60, 61, 156, 173, 10073, MECA devices (6, 896), flame photometric devices (6, 330, 902, 903), a Hg absorption tube (333), a d.c. Ar plasma (909), glow-discharge devices (59, 631), a flame AAS device (155) and an ETA device (6).Hemnann and co-workers (105n have used a flame photometric detector to monitor Br, C1 and I in a HPLC effluent using a Gilbert indium burner (P. T. Gilbert, Anal. Chem., 1966, 38, 1920). A commercial ETA device (323) as well as a flame (1347’) have been coupled to a WPLC instrument fm the detection of organometallic compounds.Koizumi et al. (1011) used a Zeeman-effect AAS instrument to detect various organometallic species in a wide range of solvents; other workers (55) determined various organo-selenium compounds using a similar system. The Zeeman background correction system allowed results to be obtained even though there were very significant background absorption signals. The direct analysis of solids using ETA can result in problems due to light scattering and/or molecular absorption.Van Loon and Radziuk (403) overcame this problem by volatilization of the sample in a graphite furnace followed by atomization of the resulting vapour in a flame. When non-dispersive AFS was employed, simultaneous multi-element analysis of biological, environmental and geological samples was achieved.Fike and Frank (181) have extended their work on the determination of Br and I (see also ARAAS, 1977, 7, 23) by using copper halide formation to determine Cu directly in sheet metals. A standard Beckmann O,/H, burner was fitted with a cylindrical steel chimney with air holes at the base. A lateral slit at the top of the chimney allowed insertion of the sheet metal samples.An entrained air/H,/N, flame was supported on the burner, O.5M HC1 was nebulized and the emission intensity of the 324.7nm Cu line was monitored above the chimney. It was possible to detect Cu levels down to 1% m / m . The AAS determination of phosphorus at the P 213.6 nm non-resonance line using ETA (ARAAS, 1977, 7 , Refs. 81 and 899) would appear to be a useful technique with a detection limit of 0.1-0.5pgml-1 (202, 205, 325, 664, 962, 1027, 1343).The addition of La to all solutions and standards has been found to be beneficial (202, 205, 664, 962). Hanamura (176) has developed a low-power air CMP (see also 1.2.2.4). The minimum power required to sustain the discharge was 250W; this was achieved by inserting a platinum electrode at the base of the discharge.Lead was determined in automobile exhausts, industrial atmospheres and in petrol. For the latter application 5-1Opl of petrol was vaporized in a plastic bag. Other references of interest - Branched capillary nebulization: 1091. Direct analysis of solids by AAS (review): 335. Direct nebulization of metal oxides and sulphides: 131 8. Discharge chamber for AAS: M2. ETA-emission of Ba: 224.Glow-discharge source with resonance detector: 523’. Pulsed atomization: 101 5. Slotted silica tube in flame: 515.Chapter 3: Methodology 67 3.2 ANALYTICAL PARAMETERS 3.2.1 Introduction Good precision of analytical results does not necessarily imply good accuracy and intcr- laboratory comparability exercises or the analysis of CRMs should be carried out on a regular basis by all laboratories.The need for this approach has again been demonstrated by a recent (alarming) study (533) concerned with the analysis of 16 trace metals in natural water by 35 laboratories in 19 countries (see 3.3.2). The accuracy of analytical results can be critically affected by unsuspected interference effects in both OES and AAS, and it is important to stress that these effects can vary from instrument to instrument (e.g., ARAAS, 1976, 6, 12 and ARAAS, 1977, 7, 81).The increasing production and usagz of CRMs should ultimately result in improved accuracy of analysis. 3.2.2 Detection Limits, Precis'ion, and Accuracy An authoritative and comprehensive review of noise and SNR in analytical spectrometry has been produced, in two parts, by Alkemade et al.(1034, 1035). The sources, mathematical representation, and major types of noise in emission and luminescence spectrometry are discussed. In the first part of the paper an extensive treatment of noise and the SNRs of paired readings is given, using the relation between the auto-correlation function and the spectral noise power. The second part gives general expressions for SNRs in spectrometric systems.Ingle and Bower (152) used theoretical equations to study the effect of instrumental variables on SNR in AAS measurements. They found optimum instrumental settings (e.g., slit height and width, flame stoicheiometry, and observation hcight) to vary with analyte concentration. The concept of detection limit is still a much misunderstood one.A statistical trcatment by Boumans (1064) has studied the interrelation between detection limit and limit of identification. It is emphasized that detection limits serve only to differentiatc b-tween true analyte signals and random fluctuations of the blank, while it is the limit of identification which indicates the maximum concentration present that has a stated probability and escapes attention. Greenfield et al.(1453) have emphasized the shortcomings of published detection limits as applied to ICP-OES. The values are only mcaningful if determiiicd in the matrix of interest and under routine experimental conditions. In ordzr to formalize the determination of ICP detection limits, a working party has now been formcd. Weeks et al. (904) studied means of improving detection limits in laser-excited AFS.They found that attention should be given to reducing stray light, the principal source being from the surroundings rather than the flame itself. Illumination of a l a r y r area of the flame also led to improved SNR. Daily (559) studied the effect of statistical and systematic uncertainties on detection limits in laser-excited AFS.Simultaneous multi-element determinations involve a compromise in experimental conditions for determining a miximum number of elements; Seeley et al. (9349 have shown how optimization of parameters during emission spectro- graphy can significantly improve detection limits for selccted groups of elements. A statistical treatment of analytical results by Evans (522) emphasized that all stages of a procedure (i.e., sample preparation, instrumentation, variation in matriccs, variation in analysts, etc.) are important when considering precision and detection limits.In order to ensure accuracy in multi-elcment spcctrography, identical transport, ioniza- tion, and excitation of the sample and reference standard are required. In practice these requirements are difficult to achieve, and Golightly et al. (648') haw studied correlations between measured temperatures, electron prcssures, and matrix composition with a view to improving accuracy.The shapes of malytical curves in flame spectrometry are affccted by incomplcte volatilization of the sample in the flamc. Roos (514) derivcd exprcssi.ons for curve shape and68 Arialytical Atomic Spectroscopy compared the results with those obtained experimentally.Effects of changes in observatior, height and flame temperature on the extent of sample volatilization were outlined. A system that might have potential as an excitation source for spcctrochemical analysis has been studied theoretically by Eckert (1066). A static ICP in a closed tube was assumed to have a known temperature distribution, to be in local thermal equilibrium, and to undergo complete mixing with the vapours of trace analyte elzments. This allowed theoretical detection limits and analytical curves to be calculated. Watters and Norris (1417) used internal standards in ICP-OES to correct for systematic errors caused by drift.The use of a dual-channel atomic absorption spectrometer in con- junction with internal standards has been recommended for certain analyses (729).The role of interlaboratory comparison studies for indicating the precision and accuracy of a technique has been illustrated by Dybczynski et 01. (5339. Results from 315 laboratories in 19 countries for 16 tracc elements in water samples showed good intra-laboratory precision, but a very large spread of results between laboratories indicating generally poor accuracy.To give an indication of the range of results reported, those for Hg ranged from 0.13~3.0 pg 1-1, nominal value 1 pg 1-1, while the Cd results varied from 1.9-9.3 pg l-’, nominal value 4 pg 1-1 (in both cases after rejecting extreme outlying results). However, thc overall mean results by AAS agreed well with neutron activation analysis, demonstrating that the inaccuracies of individual laboratories were not attributable to inherent inaccuracies in either technique.Simplex optimization allows the simultaneous variation of several experimental para- meters in order to obtain improved response from analytical systems; it i s being increasingly used in atomic spectrometry.A review of the technique by Deming and Parker (1070) discusses the basic simplex algorithm, modified algorithms, and analytical applications. Other references of interest - Statistical uncertainties of photon counting measurements: 979. Table of state-of-the-art detection limits with ICPs: 561. Theoretical and experimental factors affecting accuracy and precision: 4’35. 3.2.3 Interferences 3.2.3.1 Plasmas.Olson et al. (113) have studied the effect of widely different concomitant concentrations (Al, Fey K, Na and PO,3-) upon the ultrasonic nebulization /aerosol desolva- tion sample introduction technique. The 16 analyte elements studied showed a 10% change in intensity for molar ratios of 1000 to 1 and up to 50% at molar ratios of 10000 to 1. Optimization of the plasma and nebulizer operating parameters was successful in reducing the depression at the highest molar ratio to less than 15%.Skogerboe and Olson (769) confirmed that the observed depression was due to the formation of larger solid particles when the aerosol was desolvated, causing loss by gravitational settling. Leis et al. (732) have also studied the effects of high concomitant concentrations; in this case the large amounts of Na,B,O, used for fusion of rock samples significantly affected the ncbulization rate.Other references of interest - Assessment of interferences in the analysis of biological materials and soils: 662. Background reduction in TCP using FAAS: 1310. 3.2.3.2 Flames. Stajanovic et al. (1 191) have studied the mechanism of sulphate interferelice in the determination of alkaline-earth metals, using AAS inhibition release titration.A solution of alkaline-earth elements was titrated with a solution of MgSO, while the titrand was aspirated into an air/H, flame. Changes in the Mg absorbance signal werc registeredChapter 3: Methodology 69 and they demonstrated the formation of compounds of constant composition.A similar study of phosphate interference has been made (1190). The N,0/C2H, flame has been recommended for the determination of Cr (71, 1260, 1313). It was found that the addition of various buffers (e.g., NH,Cl, S-hydroxyquinoline, etc.) was not effective in minimizing chemical interference effects in the air/C,H, flame (1260, 1313). Pszonicki and Krupinski (71) made all solutions 2000 pgml-1 with respect to Ca in order to remove chemical interference effects caused by Al, Ba, Co, Cu, Fe, K, La, Mg, Mn, Mo, Nay Ni, Ti and V in the determination of Cr in the N,0/C2H, flame. Sanzolone and Chao (1150) have recommended the use of a fuel rich N,O/C,H, flame for the accurate determination of Mn in geological materials.Yoshimura and Noda (555) described the effect of dispersed carbon black in minimizing interferences caused by phosphate OT fluoride on the atomic absorption signals of Cu, Pb and Zn sprayed as aqueous suspensions of their oxides (ARAAS, 1977, 7 , Ref. 1516). Investigations into the determination of Eu, Gd, La, Nd, Pry Sm and Y have shown that Ce has a considerable enhancement effect; errors arising from this source were prevented by the addition of K (72).The addition of 0.3M NH4C1 to marine leachates has been successful in overcoming serious suppression of the Fe AAS signal in the air/C,H, flame (703). Pereverzeva (1026) has suggested the addition of H3P04 and Al(NO,), as a means of minimizing inter-element effects on Mo in an air/C,H, flame. In the determination of alkaline-earth metals in steels using an air/C,H, flame, it has been found that traces of Fe remaining after solvent extraction caused serious interference in the Sr AAS signal and t o a lesser extent with those of Mg and Ca.The addition of LaCl, overcame the interference from Fe as well as that from PO,3- and SiO,2- (1 239, 1320). The first report on the final results of the activities of the European Community Reference Bureau, Expert Group on 'Drug Interference in Clinical Chemistry' has been published (799) and concentrates on the interference of ascorbic acid on commonly used clinical methods of analysis, including the determination of Ca by FAES.Kometani (I") studied matrix effects in the determinaion of La in barium titanate by the flame emission of Lao. The addition of Al(NO,), has been found to overcome the unexpected interference of W when analysing for K by FES (1230) (ARAAS, 1976, 6, Ref, 66).A collaborative study (R. C. Rooney and J. W. Woolley, Analyst, 1978, 103, 1100) of the apparent inter- ference effects of Ca on the AAS determination of Ba using a N,O/C,H, flame has been carried out. It was concluded that the observed interferences were largely caused by instru- mental artefacts and should be avoidable.One possible cause was overload of the demodu- lator by the intense CaOH emission at the Ba 553.5 nm resonance line wavelength. Solutions containing 10 ,ug ml-1 of Ba and up t o 10000 pg ml-1 of Ca were circulated to several laboratories and absorbance measurements carried out on 15 commercial instruments. Only 5 instruments exhibited negligible interference at the 10 000 p g ml-1 Ca level, while the remainder exhibited signal levels of 0.25-9 times the expected level.Other references of interest - Background absorption effects, Cu in plants: 910. Background absorption Li salts: 1368. Background absorption K salts: 1369. Cu in serum, effect of silicone tubing: 1162. Matrix effects in rock analysis: 348.Mg in nodular iron, interferences in: 1193. Ti in silicate rocks, interferences in: 243. Review of interference effects: 825. Wear metals in lubricating oils, interferences in: 34~7.70 A naly tical A tom ic Spectroscopy 3.2.3.3 Electrothermal Atomization. Newstead et al. (1 378) discussed the relative merits of the various systems for background correction and the conditions relating to optical and electronic alignment.Optimal correction was claimed to be given by systems based on the Zeeman system. In a study of interference effects in furnace AAS, Czobik and Matousek (945) showed that interference in transition-metal chloride systems could often be attributed to spectral overlap between interfering atoms and analyte species and that the addition of acids of high boiling-point restored the response for the systems Ni /CuCl,, Pb /CdCl, and Cu/PbCl,.The use of a short furnace was instrumental in minimizing overlap interferences by sequential volatilization of the various components, but at the expense of poorer detec- tion limits. Several suggestions have been made for the alleviation of interferences in the deter- mination of Pb (399).Manning and Slavin (110, 704) confirmed the results of earlier work by Hodges (ARAAS, 1977, 7 , Ref. 298) with regard to the coating of the graphite tubes with Mo salts with special reference to chloride interference. Organic acids and NH,NO, were found to be useful as matrix modifiers when used in conjunction with Ma coated tubes. The risk of minor explosions should be borne in mind when working with easily oxidizable substances in the presence of NH,NO,.The addition of an excess of a Li salt was found by L'vov (471) to reduce considerably the suppressing effects of chloride on several elements; a theoretical explanation was postulated (see 1.4.3). Langmyhr and Kjuus (892) overcame the problem of chloride interferences in solid samples of bone by repeated evaporations with HNO,. Julsham (6983 reported the suppressing effect of 0.0 1-1 .OM HC10, on the absorbances of 10 elements and suggested removal of the acid by evapora- tion with HNO, in a platinum crucible.Further work on the interference effects of Ca, Mg and Na salts in the determination of Al, Cr, Cu, Mn, Pb and Sb has been reported (638, Maessen et al.(104Q) studied the influence exerted by concomitants on the vaporization and atomization behaviour of Be, Mn and Zn. The vaporization patterns of the test elements were studied during the atomization stage and the variation of the transient absorp- tion signals with time was measured. This very thorough paper draws attention to the lack of knowledge on thermochemical processes in carbon-furnace AAS, even compared to that available for arc emission spectrometry.Other references of interest - 1207). Cd, Cu, Pb and Zn in milk and fruit juice, interferences in: 456. Pb in waters, use of ascorbic acid: 521. Effect of NO,- on the determination of Ni and V: 230. General interference study: 828. Influence of furnace conditions on interference effects: 570.Interference effects in the determination of Cu and Mn: 63'8. Interference effects in the determination of Cu, effect of alkali and alkaline-earth halides and NaClO,: 901. Spectral interferences in the determination of Se: 968. 3.2.3.4 Hydride Generation and Other Techniques. In order to overcome the suppressive effect of metallic ions in the determination of As and Se, Taddia and Kirkbright (735, 893) have described the use of various masking agents (see 1.6.1).For As, thiosemicarbazide and/or 1 ,lo-phenanthroline was successful in minimizing interference from Ni, Pd and Pt; up to 1000-fold weight excess of Ni and 500-fold excesses of Pd and Pt could be tolerated. For Se, only Te(1V) was found to be successful, with up to a 200-fold excess of Cu or a 500-fold excess of Ni, Pd or Pt being tolerable.It should be noted that relatively large amounts of toxic H,Te will be liberated when using this procedure.Chapter 3: Methodology 71 Other references of interest - Interference effects in a capillary arc excitation source: 223. Spectrographic analysis of petroleum, interferences in: 757. Spectrographic analysis of steel, interferences in: 11 84. 3.3 STANDARDS AND STANDARDIZATION 3.3.1 Standards In an attempt to overcome the shortage of standards for OW in the geological field, a series of synthetic reference materials has been prepared using the co-precipitated gel tech- nique (415). Major and trace elements can be covered by this technique. Knott et al. (1312) described the preparation of calibration standards for OES and XRF in which a series of lithium/lanthanum tetraborate beads were made from pure oxide materials.During the past 13 years 65 standard reference water samples have been prepared by the U.S. Ge- logical Survey (9373.; the stability of Al, As, Ca, Co, Hg, Na, Pb and Se over several years has been confirmed. The samples have been the subject of inter-laboratory method studies.A number of RMs have been prepared, specifically for industrial health applications (171), consisting of 3 membrane filters for Be, Cd, Mn, Pb, Zn and quartz, and 2 freeze-dried urine samples containing low and elevated levels of F and Hg. Suggestions have been invited by the National Bureau of Standards (U.S.A.) for further RMs in this field. Earlier problems in the preparation of air particulate RMs with an air filter calibration facility appear to have been overcome and 4 sets of standard filters have been prepared (189).Dokiya et (11. (1266) have reported on the preparation of wet fish RMs taking advantage of the homogaous elemental distribution in shark meat. The Comite Inter-Institutes (R. C. Daniel, Euroanalysis 111, paper 2639 has prepared a set of 12 RMs for plant analysis con- sisting of tree leaves analysed for B, Ca, C1, Cu, Fe, K, Mg, Mn, N, P, S and Zn.Also of interest to environmentalists is a soil RM. The report (R. Dybczynski, Euroanalysis 111, paper 327’) contained data on the preparation and standardization of ‘Soil-5’, and a com- parison of the analytical methods used. The importance of reference materials and methods has been emphasized again by Unano and Gravatt (3’55, 1090) and Michaelis (357, 13’3).McLauchlan (358) has reviewed current demands and availability of metallurgical products within the EEC. Pending the publication of a world wide directory of certified reference material (CRM) sources, Cox and Ridsdale (J. D. Cox and P. D. Ridsdale, National Physical Laboratory (UK) Report, Chem. 93, October 197’83 have produced a guide to CRMs available from suppliers in the UK covering the following use categories: geology, physical chemistry, nuclear science, environmental science, ferrous metal analysis, non-ferrous metal analysis, polymer science, glass, ceramic and refractory sciences, food and medical sciences, haematology and clinical chemistry, physics and engineering.Jecko and Ridsdale (G. Jecko and P. D. Ridsdale, Eurostandards of Steel Furnace Dusts. Gemtondards Newsl., 1978, 2, 23) have reported the availability of two Eurostandard samples of furnace dust intended primarily for pollution control analysis. Information on 22 major and trace elements was given. Analytical data for 6 South African rock reference samples, ranging from acid to ultra basic types, covering minor and trace elements determined since the samples were first issued in 1972 has been presented by Steele et al.(T. W. Steele, A. Wilson, R. Goudvis, P. J. Ellis and A. J. Radford, Trace Element Data (1966-77) for the 6 NIMROC Reference Samples, Geostmdards Newsl., 1978, 2, 71; NIM Report No. 1351). A comprehensive report describing the preparation, properties and analysis f o r certification of metal dccanoate reference materials for use in the determination of low concentrations of metals in organic materials, e.g., oils, fuels, lubricants, foodstuffs, bio- logical components, has been published by the NPL (National Physical Laboratory (UK)72 Analytical Atomic Spectroscopy Report, Chem. 87, July 1978). It is expected that a major application of this type of calibrant will be in the 'Spectrometric Oils Analysis Program' (SOAP) for monitoring the levels of wear metals in engine oils.Other references of interest - Certification of RMs using ICP: 160. Filter-paper standards for the preparation of standard solutions: 1379. Japanese tea leaves as an RM: 1384. Mixing of solid RMs to produce additional standards: 188.Tests for homogeneity of RMs: 375. 3.3.2 Standardization A reference method for the determination of Ni in urine and serum (463) with preliminary wet-ashing and solvent-extraction steps has been developed following a collaborative study of the various stages of the method with the aid of a 63Ni tracer. Adams et al. (1153) have confirmed that the best results for Ni in urine were obtained using the same preliminary treatment as above and that direct ETA methods gave low results.Rcsults from 2 surveys, involving 7 and 10 laboratories, were reported in this valuable reference paper. Results of an inter-laboratory trial of a reference method for Ca in serum were given by Brown (1306). A method for the determination of Sn in feeds has been subjected to a collaborative study in which dibutyl tin dilaurate was extracted into CHC1, and finally aspirated in methanol (1261).The average recovery f o r 8 laboratories was 98.3% with an RSD of 0.058. The BB Research Centre has published a method for the determination of Cd, Hg, Pb and S'b in single-cell protein (71 8), which has been adopted by the European Community Reference Bureau for certification of a RM protein.The Analytical Methods Committee (Chemical Society, UK) has reported on an inter-laboratory study of a FAAS procedure for the determination of Fe in ammonium sulphate, aluminium sulphate and disodium tetraborate (528). Dybczynski and co-workers (53'3) have made an intercomparison test on the determina- tion of 16 trace elements by 35 laboratories in 19 countries.A concentrated solution in 1M HNO,, that initially had to be diluted 400 times to produce a simulated water sample, was circulated. The results demonstrated good precision from most of the laboratories, but the accuracy was very poor for a number of elements. For instance, the nominal concentrations of Bay Cd and Pb were 32, 4 and 16 ngml-1 respectively; however, the range of concent- trations found after statistical rejection of outliers was 511015, 1.9-9.3' and 12.7-49.7 ng ml-1, respectively, These results demonstrate that setting international statutory limits for various elements in waters is fraught with difficulies.A tentative FAAS method for Ca in waters and sewage effluents (1232) used La to overcome interferences in the air/C,H, flame.Other references of interest - Calibration and sample handling for ETA: 129. Collaborative study As and Se in foods: 1265. Comparison of methods for the determination of Cu in plant tissues: 443. Determination of W: 1460. Internal standards in alloy analysis by OES: 1382. Multi-element analysis of serum by ICP: 1419. Pb in milk by AAS and ASV: 1218.Pb in plants: 1219. Simultaneous determination of Cd, Cu, Pb and Zn in food products: 459. Standardization of AAS analytical procedures: 101 4. Trace metals, comparison of XRF, neutron activation and AAS: 812, 128i7, 1328. Zn in serum: 1151.Chapter 3: Methodology 73 Table 3.3A SPECTROGRAPHIC STANDARDS Aluminium Company of America, Alcoa Technical Center, Alcoa Center, Pennsylvania 15069, U.S.A.X Apex Smelting Co., 6700 Grant Akenue, Cleveland, Ohio 44105. U.S.A. X X X ~~ ~ BNF Metals Technology Centre, Grove Laboratories, Wantage, Oxon. OX12 9BJ, England Bundesanstalt fur Materialprufung (BAM), 1 Berlin 45. Unter den Eichen 87, Germany Bureau of Analysed Samples Ltd., Newham Hall, Newby, Middlesbrough, Cleveland TS8 9EA, England X Denchworth Road, x x x x X x x x x x x x x x x CKD Research Institute, Na Harfe 7, 190 02 Praha, Czechoslovakia x x X Comite de liaison des Industries de metaux Ron-ferraux de la Communaute Europeenne, Boulevard de Berlaimont, 1000 Brussels, Belgium X ~~ Willan Metals Ltd., Poplar Way, Catcliffe, Rotherham S60 5RL South Yorkshire, England x x x x x Johnson Matthey Chemicals Ltd., London EClP lAE, England 74 Hatton Garden, ‘ S pect ro me1 ’@ powders ‘Specpure’@ metals x x x x x x x x x x x x x MBH Analytical Ltd., Station House, Potters Bar, Herts.EN6 lAL, England Office Materials, of Standard Reference x x x x x X x x National Bureau of Standards, Washington, D.C. 20234, U.S.A. Pechiney,23 Rue Balzac, Paris 8e, France Various other metals, including high-purity metals x Soex Industries Inc..x x P:O. Box 798, Metuchen, N.J. 08840, U.S.A. (Glen Creston, 16 Carlisle Road, London NW9 OHL, England) X X X x Zinc & Alliages, 34 Rue Collange, 92307 Lavallois-Perret, France X74 Analytical Atomic Spectroscopy Table 3.31B SPECTROGRAPHIC GRAPHITE ELECTRODES 1 Baird-Atomic, Inc., 125 Middlesex Turnpike, Bedford, Mass. 01 730, USA. 2 Carbon Products Division, Union Carbide Corp., 270 Park Avenue, New York, N.Y. 10017, U.S.A. (ARL Ltd., Wingate Road, Luton, Beds., England) 3 Labtest Equipment Co., 11828 La Grange Avenue, Los Angeles, Calif. 90025, U.S.A. 4 Johnson Matthey Chemicals Ltd., 7'4 Hatton Garden, London EC1P IAE, England 5 Le Carbone (GB) Ltd., Portslade, Sussex, England 6 Le Carbone Lorraine, 45 Rue des Acacias, 75821 Paris, France 7 Jarrell-Ash, 590 Lincoln Street, Waltham, Mass. 021 54, U.S.A. 8 Zebac Inc., P.O. Box 345, Bevea, Ohio 44017, U.S.A. 9 Ringsdofie-Werke GmbH, 53 Bonn-Bad Godesberg, West Germany (Mining & Chemical Products Ltd., Alperton, Wembley, Middlesex HA0 4PE, England) 10 Spex Industries, Inc., 3'880 Park Avenue, Metuchen, N.J. 081S4t3, USA. (Glen Creston, 16 Carlisle Road, London W 9 O'HL, England) 11 Ultra Carbon Corp., P.O.Box 747, Bay City, Mich. 4870.6, U.S.A. (Heyden & Son Ltd., Spectrum House, Alderton Crescent, London NW4, England) Table 3.3C STANDARD METAL SOLUTIONS (MS) AND REAGENTS (R) FOR AAS 1 2 3 4 5 6 7 8 9 10 11 12 13 Aldrich Chemical Co., Inc., !940 W. St. Paul Avenue, Milwaukee, Wis. 53233, U.S.A. (R) J. T. Baker Chemical Co., 222 Red School Lane, Phillipsburg, N.J. 08865, U.S.A. (MS, R) Barnes Engineering Co., 30 Commerce Road, Stamford, Conn. 06902, U.S.A. (MS) BDH Chemicals Ltd., Poole, Dorset BH12 4"N, England (MS, R) Bio-Rad Laboratories, 2200 Wright Avenue, Richmond, Calif. 94804, U.S.A. (MS) Carlo Erba, Divisione Chimica Industriale, Via C. Imbonati 24, 20.159 Milano, Italy Eastman Organic Chemicals, Eastman Kodak Co., 343 State Street, Rochester, N.Y. 14650, USA. (R) Fisons Scientific Apparatus Ltd., Bishop Meadow Road, Loughborough, Leics. LEll ORG, England (MS, R) Harleco, Div. of American Hospital Supply Corp., 60th and Woodland Avenues, Philadelphia, Pa. 1914'31, USA. (MS) Hopkin & Williams Ltd., P.O. Box 1, Romford, Essex RM1 lHA, England (MS, R) V. A. Howe & Co. Ltd., 88 Peterborough Road, London SW6 3EP, England (MS) Instrumentation Labora'tory Inc., 11 3 HartweU Avenue, Lexington, Mass. 02173, USA. (MS) Johnson Matthey Chemicals Ltd., 74 Hatton Garden, London EClP IAE, England (R) (MS)Chapter 3: Methodology 75 14 Koch-Light Laboratories Ltd., Colnbrook, Bucks., England (R) (Anderson & Co. Ltd., Battlebudge House, 87-95 Tooley Street, London E l , England) 15 May & Baker Ltd., Dagenham, Essex RMlO 7xS, England (R) 16 E. Merck, D 61 Darms'tadt, West Germany (R) 17 Spex Industries Inc., 3880 Park Avenue, Metuchen, N.J. 08840, U.S.A. (MS) 18 ALFA Division, Ventron Corp., 152 Andover Street, Danvers, Mass. 01923, U.S.A. (MS). (Glen Creston, 16 Carlisle Road, London NW9 OHL, England) Table 3.31) ORGANOMETALLIC STANDARDS 10 11 12 13 14 15 16 17 Angstrom Inc., P.O. Box 2418, Belleville, Mich. 481 11 , U.S.A. Baird-Atomic Inc., 125 Middlesex Turnpike, Bedford, Mass. 01 730, U.S.A. J. T. Baker Chemical Co., 222 Red School Lane, Phillipsburg, N.J. 08865, U.S.A. BDH Chemicals Ltd., Poole, Dorset BH12 4NN, England Burt and Harvey Ltd., Brettenham House, Lancaster Place, Strand, London WC2, England Carlo Erba, Divisione Chemica Industriale, Via C. Imbonati 24, 20159 Milano, Italy Conostan Div., Continental O'il Co., P.O. Drawer 1267, Ponca City, Okla, W601, U.S.A. Durham Raw Materials Ltd., 1-4 Great Tower Street, London EC3R 5AB, England Eastman Organic Chemicals, Eastman Kodak Co., 343 State Street, Rochester, N.Y. 14650, U.S.A. Hopkin and Williams L'td., P.O. Box 1, Romford, Essex RMl IHA, England E. Merck, D 61 Darmstadt, West Germany MBH Analytical Ltd., Station House, Potters Bar, Herts. EN6 IAL, England Division of Chemical Standards, National Physical Laboratory, Teddington, Middlesex TW11 0,LW England National Spectrographic Laboratories Inc., 19500 South Miles Road, Cleveland, Ohio 44128, U.S.A. Office of Standard Reference Materials, National Bureau of Standards, Washington, D.C. 20234, U.S.A. Research Organic/ Inorganic Chemical Corp., 1 1686 Sheldon Street, Sun Valley, Calif. 91 352, U.S.A. ALFA Division, Ventron Corp., 152 Andover Street, Danvers, Mass. 01923, U.S.A. (Glen Creston, 16 Carlisle Road, London NW9 OHL, England)