405 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 Electrothermal Atomic Absorption Spectrometry in Occupational and Environmental Health Practice-a Decade of Progress and Establishment* A Review invited Lecture Dimiter L. Tsalev Faculty of Chemistry University of Sofia Sofia 1126 Bulgaria Electrothermal atomic absorption spectrometry (ETAAS) is today an established technique in the vast application area of occupational and environmental health practice. Up to 52 elements have so far been determined in biological matrices such as body fluids and tissues food and related samples with a view to the assessment of occupational and environmental exposure. For at least a third of these analytes ETAAS is the current technique of choice. The progress in this field over the last decade is critically reviewed and discussed.Emphasis is given to the development of direct procedures and simple pre-treatment techniques the selection of suitable chemical modifiers and appropriate reaction media some practical problems as well as the rational combination of the graphite atomizer with preconcentration and speciation techniques. Keywords Nectrothermal atomic absorption spectrometry; biological sample ; clinical sample ; trace element determination ; review Aims and Scope The decade after the publication of our two-volume monograph on atomic absorption spectrometry (AAS) in occupational and environmental health has brought substantial pro- gress in instrumentation and methodology in the vast appli- cation area of biological trace element research thus justifying the preparation of an updated text.3 The aim of the present overview is to summarize the information on the progress achieved and the problems faced.The ten year period of 1983-1992 is covered the determination of up to 52 analyte elements in biological matrices such as body fluids and tissues food and related samples is considered with a view to the assessment of occupational and environmental exposure (Table 1). Since the number of relevant AAS papers well exceedes 2000,3 only keynote references are given here. Certain related matrices such as plant tissues water and air are not considered unless important improvements in methodology or speciation approaches have been involved in analytical procedures. Important selected sources of information in this field are the recent book chapters" and monographs.'"18 Position of AAS Among Current Techniques Despite the large selection of sensitive analytical techniques for trace element determinations in biological samples (Table 2) AAS still occupies the place of a routine analytical technique in this field for up to 30 analyte elements.In fact only a few of the tabulated relevant techniques (Table 2) are real alternatives to AAS in clinical/biochemical laboratories e.g. ion-selective electrodes (ISEs) for F- and Li+ molecular absorption spectrometry for phosphate and Fe fluorimetry for Se and flame emission spectrometry for Li. Obviously the economic considerations still weigh against the important methodological assets of the alternative techniques such as the better or competitive sensitivity [inductively coupled plasma mass spectrometry (ICP-MS) neutron activation analyses (NAA) and differential pulse anodic stripping voltammetry * Presented at the XXVIII Colloquium Spectroscopicurn Internationale (CSI) Post-Symposium on Graphite Atomizer Techniques in Analytical Spectroscopy Durham UK July 4-7 1993.(DPASV)] or multi-element performance [inductively coupled plasma atomic emission spectrometry (ICP-AES) ICP-MS instrumental NAA (INAA)]. Thus AAS is now an established reliable and cost-effective technique owing to the unique combination of the following advantages high sensitivity limits of detection (LODs) down to the ng and pg range; relatively good accuracy; adequate precision for most trace element determinations; large elemental coverage; small sample size requirements; relatively simple sample preparation; high [for flame AAS (FAAS)] to moderate (for ETAAS) sample throughput rate; applicability to solid or slurried microsamples; moderately priced equipment; reliability of apparatus simple operation and easy maintenance; automation; and well estab- lished methodology.Some intrinsic limitations and drawbacks of the AAS tech- nique should however be taken into consideration (i) limited potential for simultaneous multi-element analysis [although two and four channel instruments are commercially available the typical AAS approach relies on (automated) one element at a time measurements]; (ii) dissolution is typically required with solid samples; (iii) fairly narrow dynamic range of rectilin- ear calibration; (iu) non-applicability to several biologically/environmentally important elements (Br C1 F I and S); and (u) very poor sensitivity for certain analytes (e.g.B P Ti W and U). Up to 52 analytes have been determined so far in biological samples by means of AAS as depicted in Fig. 1. The distri- bution of papers published during the decade 1983-1992 between the four AAS techniques [FAAS ETAAS hydride generation AAS (HGAAS) and cold vapour AAS (CVAAS)] is given in Fig. 2. Whenever possible preference is given to FAAS procedures which are sufficiently sensitive to permit direct determinations of the electrolytes (K Na Ca and Mg) therapeutic levels of Li (14 papers) and Au (4 papers) and physiological levels of Cu (128) Fe (83) and Zn (146) in biological fluids (the numbers of FAAS papers for the period are given in parentheses).Furthermore FA AS procedures have been documented for several other analytes Pb (74) Cd (72) Mn (52) Cr (23) Co (17) Ni (16) Sn (mainly in canned food 14) A1 (11) and several other analytes (with < 10 papers). Important progress in FAAS methodology has been achieved406 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 Table 1 Interest in trace element analysis of biomedical samples Area of interest Occupational exposure- Primary interest Subsidiary interest Environmental exposure- Nutritionlbalance studies- Deficiency Excessive levels Inert markers Clinical importance- Primary interest Subsidiary interest Forensic interest- Analyte As Be Cd Co Cr F Hg Mn Ni Pb Sb V Ag Al Ba Bi Cu Mo Te Ti T1 Se Si Sn W Zn Al As Ba Cd Cr F Hg Ni Pb Se Sn V Co Cr Cu F Fe Mn Se Zn Al As B Ba Cd Co Cr Cu F Fe Hg Mn Ni Pb Se Sn T1 Co Cr Yb Al* Au* Cu Fe Li* Mn P Pb Pt* Se Zn B Ba Bi* Co Cr F Ga* Mo Rb* Sb* V Generally hard to predict e.g.As Au Ba Hg Sb T1 * Denotes interest in therapeutic concentration levels. Table 2 Other relevant analytical techniques Technique Neutron activation analysis ICP-AES ICP-MS Flame emission photometry Differential pulse anodic stripping voltammetry Potentiometric stripping analysis Adsorptive differential pulse voltammetry Ion-selective electrodes Fluorimetry Molecular spectrometry Catalytic method Isotope dilution mass spectrometry Element Ag Al As Au €3 Co Cr Cs Fe Ga Hg In La and lanthanides Mn Mo Pd Sb Se Sn Te Ti T1 V W Zn Al Au* B Ba Re Cu Fe Li*,Mo P Pt* Rb Si Sr Zn Al Au B Ba Co Cs Cu Fe La and lanthanides Mn Mo Ni P Pt* Rb Sr TI Zn Cs Li Rb Sr Bi Cd Cu In Pb T1 Sb Zn Bi Cd Co Cu Ni Pb T1 Sn Zn Ni Pt F Li Be Ga lanthanides Se Cu Fe P Zn Cr Cu Li Pb T1 Se Ag v * Denotes interest in therapeutic concentration levels.(TI Na Mg K Ca [ R b l ( p q Y Cs Ba La B W Pr Gd Tb Dy Er Yb Fig. 1 Applications of AAS to biological samples italics FAAS preferable; underlining HGAAS or CVAAS (Hg) alternative to ETAAS; single lined box ETAAS relevant technique; and double lined box ETAAS technique of choice owing to the miniaturization and automation of procedures by pulse nebulization and flow injection'' as well as by increasing sensitivity for volatile analytes (Cd Pb Zn etc.) by means of the 'slotted tube atom trap' (STAT).2oi21 However FAAS often lacks sensitivity and thus relies on efficient pre- Several important analytes are usually determined by vapour generation techniques mercury by CVAAS (153 papers from the total number of 185) and As Bi Ge Se Sn and Te by HGAAS (with 150 9 4 141 44 and 11 papers respectively). Despite the need for a more thorough sample pre-treatment stage the vapour generation techniques provide better relative LODs (typically around and below 1 Pg I-') and offer valuable speciation capabilities.22 The HGAAS technique is a definite ETAAS 57% concentration.1172 390 Fig. 2 Distribution of relevant AAS papers (1983-1992) between the AAS techniquesJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL.9 407 competitor for ETAAS as far as difficult volatile analytes such as As Bi Ge Sn and Te are concerned. Noteworthy are the recent developments in HGAAS methodology for Pb22 (includ- ing plumbane generation from slurried biological s a m p l e ~ ~ ~ ~ ~ ) the on-line pre-treatment of biological liquids in a microwave d i g e ~ t o r ~ ~ ~ ~ and the dramatic improvements of LODs by in situ collection of hydrides in graphite tube atomizer^.^^.^^.^^ ETAAS as the Current Technique of Choice for up to 20 Analytes With the advent of the electrothermal atomization technique hundreds and thousands of analytical laboratories gained access to an extremely sensitive and readily acquired analytical tool. Many of these laboratories happened to be newcomers to the field of biological trace element research or ultratrace analysis by ETAAS or both; thus some period was needed to learn by experience that this precious technique can produce both highly accurate results and artefacts! This situation can best be illustrated by observing the trends of so-called 'normal' or 'reference values' for the trace element contents of human body fluids and t i s s ~ e s .~ ~ ~ ~ ~ ~ ~ These values have been steadily dropping over the years during the last two decades and have decreased by several-fold to orders of magnitude just owing to the better control of exogenous contamination during specimen collection and sample pre-treatment. Therefore essential components of the progress in the 1980s were not only the hardware support and methodological improvements but also the better philosophy organization education train- ing and quality assurance (QA) in ultratrace analysis.16 Crucial Role of Pre-Analytical Stages Although the analyst has no legal responsibility for the quality of samples received in the laboratory the best and most meaningful results in biological trace element studies are Table 3 Summary of the relevant biological specimens obtained (i) by interdisciplinary teams; (ii) after careful plan- ning of the multi-stage studies of which the ETAAS analysis is only an integral part; (iii) by taking into account all essential pre-sampling considerations age sex health status nutritional and smoking habits past exposures diurnal rhythm and seasonal variations medical and cosmetic treatments etc.; (iv) on selection of meaningful specimens with a diagnostic value (see for example Table 3); (v) after proper consideration of the methodological and technical reliability of specimen collection devices and sampling protocols; and (vi) upon suitable preservation and storage of samples (see for example the monographs in refs.1 2 16 and 30). Therefore most ordinary clinical laboratories are able to handle without serious problems assays such as Cu and Fe in serum Zn in plasma Pb and Cd in blood and Cu in urine but fail in sub- pg 1 - ' assays. Special (plastic) sampling appliances rather than stainless-steel needles are required for collecting non- contaminated blood samples for determination of Cr Mn Ni and V at sub-ygl-' levels. Moreover scrupulous cleaning of the sampling site flushing of sampling catheter with a few ml of blood thorough shielding from airborne dust and carefully selected storage containers should be ensured.33 Contamination (Table 4) and losses (Table 5) are the principle sources of errors that accompany all subsequent stages of assays but are particu- larly difficult to identify and control during the pre-analytical stages.An indication of the severity of airborne contamination is given by Fig. 3 wherein the ratio of the mean soil content to the median plasma or serum level of the most ubiquitous elements are plotted. Note the 1 x 105-1 x 107-fold excess for the first several analytes (Al V Si Mn and Cr) which can cause gross errors unless clean-room or laminar-flow box facilities are available. The trace element content of hair or nail samples typically exceeds by several orders of magnitude the corresponding levels of these analytes in serum or plasma (Fig.4). These specimens offer several important advantages such as easy Sample Whole blood Plasma or serum Urine Bile Bone Cerebrospinal fluid Erythrocytes Exhaled air Hair/nail Milk Saliva Soft tissues Sputum and nasal secretions Sweat Teeth Element As Be C d t Co Hg Pb Sb Se Te V T1 Al Au* Be Bi* Co Cr Cu Fe Ga* Li* Mo Ni P Pt* Se Si V Z n Ag Al As Au* B Ba Be Bi* Cd Co Cr Cu F Fe Ga* Hg In Li* Mn Mo Ni P Pb Pt* Rb* Sb Se Si Sn Sr Te TI I/ Zn Cr Mn Pd Al F Pb Sr Al Cu Fe Mn Zn Cr Fe Li Se Te Ag Au As Ba Cd Co Cr Cu F Fe Hg Mn Mo Pb Sb Se Sr Te Tl V Zn As Be Cd Co Cr Cu F Fe Hg Mn Mo Ni P Pb Se Sn V Zn Ag Au* Li* Hg Sr Zn Al Cd Cu Fe Ga* Hg Li* Mn Ni Pb Pt* Rb Se Si Zn Be Ni Si Hg Mn Se Te F Pb Sr * Denotes assays at therapeutic levels.t Analytes in italics are of primary importance. Table 4 Some sources of contamination Source Airborne Tobacco smoke Cosmetics and jewellery Sweat Sampling devices Storage vessels/stoppers/preservatives Paper Reagents Haemol ysis Contaminant Al Cr Fe Hg La Mn Pb Si Ti V Zn Al Be Cd Mn Zn Ag Al Au Bi Al Cr Mn Pb Co Cr Fe Mn Mo Ni V Al Cd Co Cr Mn Pb Si Zn Al Mn Zn Al As B Bi Cr Cu Mn Ni Pb Se Si Sn Zn Cs Fe Mn Pb Rb Zn408 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 At V Si Mn Cr Ga Ba La Co Fe Pb Ni Sr Sn As F T Element Fig. 3 Expectations of aiborne contamination. The ratio of mean soil contents [soil] to the median plasma ( P ) or serum ( S ) concentrations given on a logarithmic scale; arrows indicate that the corresponding bars may prove higher than depicted since the concentration levels for some ultra-trace elements in serum or plasma are reported as ‘below the LOD 100000 1 ’ Pb Cr Ag La Ga Mo V Ni Sb TI As Fe Cs Al Mn Cd Hg Sn Co Si Zn Au Yb Be Ba Cu Element Fig.4 Concentration ratios in hair to those in serum or in plasma (median values); see Fig.3 for explanation of use of arrows painless and non-invasive sampling stability on storage easy decomposition and very favourable analyte-to-matrix ratios. Thus hair and nails have been studied extensively with a view to environmental and (less so) occupational exposure monitor- ing (Table 3).’” Unfortunately these specimens are subject to large biological variability and are difficult to clean from exogenous contamination factors which limit the usefulness of these assays to biological monitoring on a group basis as well as to some medical and forensic tests.Nevertheless toe-nail specimens have proved much easier to decontaminate and a n a l y ~ e ~ ~ and have provided useful information on exposure to Mn35 and Cr.36 Progress in Instrumentation The role of hardware support is decisive in analyses of complex biological matrices at ng g-’ levels; the most important recent achievements are listed and briefly commented on below. Table 5 Common causes for analyte losses during sample preparation ( i ) Background correction facilities have become more efficient and reliable (see the reviews in refs.37 and 38). Instruments with Zeeman-effect background correction are offered by several manufacturers and the Smith-Hieftje correc- tion technique3’ has also been successfully commercialized. Attention has been paid to the correction of fast transient signals by means of proper algorithm^.^^^^' Zeeman-effect background correction has proved particularly useful in deter- minations of the most volatile and short-wavelength analytes (As Cd Hg Sb Se and Te); in analyses of urine at low dilution factors 1-3-fold); in the presence of phosphate-containing modifiers4’ and matrices (bone teeth); and in most analyses of solid microsamples and slurries (see the reviews in refs. 14 43 and 44). ( i i ) Faster electronics of modern instruments is essential for the accurate and precise acquisition and processing of transient peak signals (e.g.50 points per s). (iii) Video display of both AA and background signals with options for overlaying signals from samples standards and standard additions provides helpful information for developing procedures and quality control (QC). (iu) Faster heating rates and temperature-controlled heating are becoming standard and are an integral part of the stabilized temperature platform furnace (STPF) concept.45 More uniform heating and better spatial isothermality are achieved using an integrated-contact transverse-heated platform-equipped graphite a t ~ m i z e r . ~ ~ * ~ ~ (u) The L‘vov has been widely adopted; mechanical stability and precision are further improved by using ‘forked’ platforms placed in ‘partitioned‘ tubes5’ and atomizers equipped with integrated platforms.47 (ui) Both integrated absorbance (QA) and peak height (A,) signals are processed simultaneously thus allowing better flexibility .(uii) Versatile autosamplers perform dilutions addition of modifiers blending of mixed or composite modifiers running QC sample^,^' performing ‘hot injection^'^'-^^ and ‘multiple injection^',^^,^^ standard additions re-calibration and re-zeroing etc. (uii) Automated/overnight operation and successive multi- analyte capabilities help to improve the sample throughput rate by up to 30-40%. (ix) Alternate gases (‘gaseous chemical modifiers’ see for example the reviews in refs. 42 and 58) such as 0’ or air as an in situ ashing aid and H2+Ar as a reductant for noble metal modifiers are now handled in a safer and more con- venient way although still with caution(!) and at the expense of the lifetime of the graphite tubes and p1atf0rms.l~ (x) Flow injection facilities for on-line sample pre-treatment on-line precon~entration~’~~ and ~peciation~’“~ have become available and promise unlimited potential for the future (see the reviews in refs.64 and 65). ( x i ) Solid/slurry sampling devices have been automated6668 but still need to be perfected and better adapted to automated unattended performance (see the reviews in refs. 14 44 69 and 70). (xii) Graphite probe atomization7’ has become commer- cially available and a ~ t o m a t e d ~ ’ ~ ~ ~ and has exhibited good performance with biological l i q ~ i d s ~ ~ ~ ~ and digests,74 slurries75 and solid micros ample^.^^ Cause Volatilization H ydrolysis/adsorption Undissolved residues Incomplete release upon protein precipitation Undissolved fat fraction Analyte As B Cd Cr F Ge Hg Pb Sb Se Sn Te T1 Zn Ag Al Bi Hg Pb Sb Si Sn Ti W Ag Al Au Ba Be Cr Ir Mo P Pd Pt Sb Si Sn Sr Ti V As Au Co Mn Ni Se e.g.Co and Ni in milkTable 6 Availability of ETAAS reports on individual elements and biological matrices (total number of references estimated for the decade 1983-1992) Sample matrix Analyte Ag A1 As Au B Ba 3 12 5 4 - - 4 87 3 4 - - - 2 - 3 - - 5 1 9 1 7 l - - - 7 6 - - - - 11 - l - - 2 2 1 - - - 2 2 1 l - - 5 2 5 1 0 4 - - - - I - - - 5 9 1 6 1 3 - 4 4 1 7 - - 1 2 2 4 1 5 2 4 8 15 142 74 13 6 5 - 6 1 4 2 - - - - - - - - Be Bi Cd Co Cr 3 35 8 10 4 13 13 21 I - - 1 5 42 18 41 - 8 6 12 - 6 1 - - l - - - 21 3 11 - 2 2 8 - 61 14 14 - l - - - 47 10 16 1 28 6 10 3 7 2 1 6 10 229 60 125 - 3 - 21 c s c u 10 29 6 14 7 2 15 10 34 29 16 NE-F 119 6 - - Ga Gd Ge 1 1 - 1 - 1 - - - - - - Hg 3 2 5 5 - In Ir La Whole blood Plasma or serum Other blood fractions Urine Hair or nails Bone Teeth Milk and dairy products Miscellaneous body fluids Soft tissues Faeces Food/feed and beverages Marine food Water Number of ETAAS papers Number of ETAAS speciation papers - 1 1 2 - - 1 - 2 3 8 3 7 5 30 3 - - 2 2 1 1 4 - - 1 5 - - Li Mn 3 16 12 26 1 - 5 8 1 10 1 2 1 9 1 7 2 21 2 19 1 9 1 NE 19 96 1 - - - - - Mo 2 10 Ni P 6 - 14 - 22 - 6 - - - - - - - 8 2 2 - 14 2 18 9 9 3 3 3 78 12 - - Pb 98 12 3 37 9 9 4 33 1 53 70 34 NE 302 3 - Pd Pr Pt 2 8 2 8 - - - - Rb Ru Sb Se Si Sn 3 32 - 1 1 34 4 1 - 2 - 1 7 15 2 1 3 4 - 1 - l - - - 8 - - - 2 1 - 2 22 - 3 2 1 3 1 9 12 - 9 8 10 2 10 17 123 6 29 1 7 - 12 - - - - - - - - - Sr Tb Te Ti T1 V 5 1 - 3 - 2 9 - 1 - 1 1 - 3 - 16 16 1 - - - - - - - Yb Zn 6 - 24 5 1 2 - - - - Whole blood Plasma or serum Other blood fractions Urine Hair or nails Bone Teeth Milk and dairy products Miscellaneous body fluids Soft tissues Faeces Food/feed and beverages Marine food Water Number of ETAAS papers Number of ETAAS speciation papers - 1 1 - Y io m W 1 1 1 6 1 9 1 7 7 36 - - - - - 8 5 15 1 - 18 7 - NE 1 62 12 - - - - - - - NE 4 - * Fluorine determined by ETA molecular absorption spectrometry of AlF.7 NE not evaluated.410 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL.9 0.01 Progress in Appliances and Related Materials The role in QA of as a minimum the following technical tools and relevant materials cannot be over emphasized. (i) Newer more reliable blood collection devices and storage containers are now a ~ a i l a b l e . ' ~ . ~ ~ ~ ~ ~ ~ ~ (ii) Special instruments for contamination-free sampling quartz and titanium knives plastic scissors TiN coated surgical instruments,80 e t ~ . ~ ~ ~ ~ (iii) Laminar-flow clean air benches. (iv) Fast and efficient microwave sample preparation (see the monograph in ref. 81 and the reviews in refs. 82 and 83) but these are still subject to improvements in view of safety and on-line p e r f o r m a n ~ e . ~ ~ ~ ~ . ~ ~ ~ ~ ~ (v) Mechanized and automated temperature-controlled wet chemical decomposition^.^^^^^ (vi) Bomb d e c o r n p o ~ i t i o n s ~ ~ ~ ~ ~ where there is still serious concern about safety and incompleteness of decomposition.(vii) Biological reference materials (see the reviews in refs. 16,78,88-93). -* +R I I I I t Progress in Analytical Methodology The distribution of the 1172 published reports on ETAAS between the 46 analytes and the most typical biological matrices is presented in Table 6. The table is intended to give an idea of the availability of ETAAS procedures and therefore only the number of papers for the decade being considered is included while selected procedures and references can be traced in ref. 3. Note that information on water analysis is not treated exhaustively and also that fluorine is in fact determined by ETAAS equipment but with electrothermal gas-phase mol- ecular absorption spectrometry of A1F94-97 or elsewhere by molecular fluorescence spectrometry of MgF.98 There are two different approaches to the development of analytical procedures which can conditionally be designated as 'more instrumental' and 'more chemical'.The first one is based on direct injection of untreated or simply diluted biological fluids and would be the favoured approach in a busy clinical laboratory. Thus contamination control is greatly facilitated sample processing errors are eliminated reagent consumption and manpower expenses are reduced. On the other hand however this approach relies on more versatile (expensive) apparatus and (typically) on longer instrumental time so as to ensure adequate in situ sample treatment; it also entails more complicated calibration procedures.This philosophy has proved very useful in determinations of ubiquitous elements at pg1-l levels in blood serum and urine (Al Cr Mn and Si) as well as in routine large-scale assays for priority analytes such as blood Pb and urinary Cd. Direct procedures have been developed for most analytes in biological fluids that are sufficiently sensitive by ETAAS cJ Fig. 5 for median serum or plasma levels of trace elements versus the characteristic concentration in Zeeman-effect ETAAS with 10 pl sample aliquots (characteristic concen- tration Co data taken from ref. 99). The 'more chemical' philosophy is viable enough although not so attractive most biological tissues and food samples are still analysed after decomposition or solubilization; protein precipitation can prove cost effective for some determinations on blood or serum/plasma; while preconcentration is often inevitable for many analytes at endogenous (normal/deficiency) sub-pg 1-1 levels i.e.Ag As Au B Be Bi Co Cs Ga Ge Hg In Ir Mo Ni Pd Pt Ru Sb Sn Te Ti T1 and V with As Be Bi Cs and Ni being determined with borderline sensitivity. (Table 7). Successful procedures result from rational combinations and compromises within the frames of these two extreme approaches. Below some recent methodological achievements are summarized and briefly discussed. + + Zn Fe Cu Rb+ + Si I loo0l 100 + + Se 8.t.' + Ba Sr + lo t Li Ag Yb.L +Pb .Au 'Ga Cd+ + + + Te + Hg + v Fig. 5 Median serum or plasma levels uersus the characteristic con- centrations for Zeeman ETAAS (10 pl aliquot C data from ref. 99) Suitable diluents and reaction media Dilute aqueous solutions of some reagents can be considered as media compatible both with the electrothermal atomizer and biological liquids or digests aqueous Triton X-100 (e.g. 0.05-0.25% v/v) a very efficient non-ionic surfactant for haemolysing whole blood samples and facilitating their smooth drying-ashing in the electrothermal atomizer; ammonia solu- tion (e.g. 1% v/v15) also an efficient solubilizer; NH; salts; strong organic bases serving as solubilizers such as tetra- alkylammonium hydroxides (TAAH alkyl = methyl or ethy1),34,100,101 hyamine h y d r o ~ i d e ' ~ ' ~ ~ ~ dieth~lenetriamine;"~ dilute HNO (e.g.from 10mmoll-l for blood/serum up to 1 moll- for urine digests and deproteinized blood/~erum;~,'~~ NH4N03; dilute aqueous H,Oz (e.g. 1-3% v/v); EDTA [pref- erably as the (NH4)4 or (NH4),H2 salt]; ascorbic acid and many other organic acids; SH containing ligands such as cysteine (for Au106) and dithiocarbamates; some organic sol- vents (but not chlorinated) e.g. methanol ethanol 5% v/v butan01,'~~ 10% v/v ethylene glycol,'o8 octanol (anti- f ~ a m ) . ' ~ ~ * l ' ~ See also the reviews on chemical modification in refs. 42 and 58 and organic additives in ref. 11 1. More efSicient chemical modijiers The classic nickel- and phosphate-based modifiers are steadily being replaced by more efficient and universal noble metal modifiers.42 The 'reduced Pd'l12 and Pd + Mg(N03)2113*114 modifiers have been widely acknowledged; thus up to 21 analytes of high and moderate volatility have been covered in the recent extensive study on the latter mixed modifier by Welz et al.'14 Noteworthy is the synergistic combination of Pd with a l b ~ m i n " ~ * ' ~ ~ and of Pd with MoV1 Vv and Mg(NO,) which may prove rather useful since some of these components could be present at mg levels in sample digests when added as catalysts in wet-chemical digestions kn.. MoV1 (ref.117) and Vv] or as ashing aids [e.g. following drfashing in the presence of Mg(N03)2].118 Some other noble metals have shown better performance than Pd in individual cases thus justifying further evaluation e.g. Ru+ascorbic acid for As In P Te and Tl;'19 Rh for Bi in blood and serum;'20 Pt for Te;'03 and Pd + Rh + Ru + Pt + ascorbic acid for Sb in serum and urine.lZ1 Practical considerations such as an impairment of modifier effectiveness by excess of chloride (Ru > Pd)1199'22 and nitricJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL.9 41 1 Table 7 Selected reagents for liquid-liquid extraction from biological digests Reagent Acetylacet one Cupferron and analogous reagents Dimethylglyoxime Dithiocarbamates Di thizone Halides 8-hydroxyquinoline Ion-association complexes Liquid ion exchangers o-Diamines Toluene-3,4-dithiol Xanthates Miscellaneous ~~ ~~ ~ ~~~~~~~ Analyte Be Cr Mo Be Bi Ga In Mo Sb Sn Ti T1 V Ni Pd Ag As Au Bi Co Cr Ga Hg In Ni Pb Pd Pt Sb Se Sn Te TI V Ag Bi Cd Hg Ni Pb T1 As Au Bi Ga Ge Hg In Pb Sb Sn Te T1 Mo V B Co Cr Ga Hg Pt TI Be Cr Ga Mo Se Mo Bi Co Ni As Au B Co Ge Li Mo Ni Sn acid,123 blank contributions excessive background (phos- p h a t e ~ ~ ~ ~ ~ ~ ~ ) and cross-c~ntamination~~J~~ play a decisive role in the final selection of a modifier.The severe attack of the graphite surface by NH,NO La"' and phosphate based modifiers cannot be overlooked either. The t h e ~ r e t i c a l ~ ' * ~ ~ " ' ~ ~ and practical aspects4' of selecting appropriate modifiers have been discussed in detail elsewhere and charts plotted of the relationships and analogies between numerous potential modi- fiers and analytes.126,128 Carbide coated tubeslplatforms Although still far from being used in routine applications and not yet commercially available carbide-coated atomizers have shown several useful effects thermal stabilization of volatile analytes analogous to the action of corresponding modifiers (Zr W V and Mo) and better resistance to corrosion (W Nb and Ta) in particular with organic solvents extracts (W129 and Ta13') and chromatographic effluents,13' fats and oils (Nb and Ta',') biological liquids with added phosphate133 or NH,NO modifier^,'^^.^^^ digests with high content of ( HN03) acid (TaC136) as well as in procedures involving in situ ashing in air or ~xygen.'~'.'~~ Ma jor improvements in performance have been achieved with difficult analytes such as P and Sn (see the reviews in refs.138 and 139). Fast temperature programmes Whenever possible speeding up determinations by using rapid prograrnme~'~' with very steep drying-ashing steps or 'hot i n j e c t i ~ n ' ~ ~ - ~ ~ is a very attractive a p p r ~ a c h .~ ~ ~ ' ~ ~ These methods could improve sensitivity and precision when working with organic solvents/extracts5' and effluents and yet more importantly they could double the sample throughput rate while cutting the total cycle times (heating + autosampling) to less than 1 min.145 Successful applications to urine,53-55,141-143,14S milks5 and biological digests55*140*144~145 are documented but more viscous samples such as blood or serum54~140*141 would typically need large dilution factors and slower drying ramps. In situ oxygenlair ashing This approach is particularly efficient with simply diluted or slurried samples of blood and serum and plasma (even at low d i l ~ t i o n s ) ' ~ ~ ' ~ ~ ~ ' ~ ~ fats and and food and milk powder.68 The lifespan of the graphite tubes and platforms will however be reduced by several fold for example to between 50 and 250 firings only depending on the pyrolysis temperature (e.g.450-550 "C) pre-treatment temperature for oxygen desorption and removal from the atomizer (up to 900-950 "C) atomization temperature and other factors. SimpliJcation/rationalization of the sample decomposition step Inasmuch as the decomposition of the organic matrix is the major contributor to analytical inaccuracy there is a trend to keep pre-instrumental treatment as simple as possible. In fact biological fluids are best analysed either directly after dilution or after a simple acid pre-treatment such as protein precipi- tation with HNO,.Thus the decomposition of organic matter and removal of the bulk matrix components are performed in situ aided by the versatile heat profiling and by the reactive constituents of diluent and modifier(s) and eventually by the carbide-coated graphite surface and the alternative active gas if any. Protein precipitation has been applied to many analytes (Al Bi Cd Cr Fe Li Mn Ni Pb and T1) but lower results have been recorded for some protein-bound analytes (e.g. for Mn in whole blood"' but not in serum or plasma,148 for Col4' and Ni in blood). Liberation from proteins is improved at higher temperatures and at higher HN03 levels (up to about 1 moll-')1o5 and in the presence of Triton X-100105 as well as after a mild enzymic pre-digestion stage.'" Biological tissues and foodstuffs also do not necessarily need complete decompo- sition; therefore wet-chemical digestions with HNO or with HNO and H202 (which can be hazardous) in open or pressur- ized vessels are given preference in most cases.The concen- tration of HNO should better be kept to < 1% v/v in the final digests and should by no means exceed 5-10% v/v in order to protect the graphite surface from intensive corrosion and to ensure adequate effectiveness of the Pd modifier; if needed bulk acid may be evaporated or neutralised with ammonia solution (giving an NH,N03 r n ~ d i f i e r ) . ' ' ~ * ~ ~ ~ * ~ Some lipophilic species of the analyte can be found in the undissolved fat fraction (e.g.Co and Ni in milk powderlS3 but not A1 in brain tissuelS4 neither Mn in liver kidney or muscle tissue155). Solubilization of small samples of soft tissues,'00~102*103 with strong bases has proved very convenient in ETAAS determinations of numerous analytes and only for safety considerations (noxious vapours of tetra- methyl ammonium or tetraethylammonium hydroxides) does this technique seem to be neglected in practical work. Enzymic digestion^'^'*'^^'^^ would appear most promising in speciation studieslS8 and with large samples containing low levels of ubiquitous analytes (owing t o their low blanks) The carbonization technique,lS9 originally developed for X-ray fluorescence transforms samples of plant and animal tissues on calcination (off-line) at 300 "C into fine-powdered residues (about 50% mass reduction) with losses of <5% for many analytes Al Cd Cr Co Cu Mn Ni Rb V and Zn.lS9 The method has been adapted for determinations of Ti,160 Cd16' and Cr16' by ETAAS in vegetal food with atomization of the carbonized slurry.and412 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1993 VOL. 9 Flow injection on-line preconcentration The determination of many elements still relies on preconcen- tration because of the low levels encountered and inadequate sensitivity of the direct ETAAS technique (Fig. 5). A typical current technique for preconcentration is liquid-liquid extrac- tion from biological digests (Table 7). Complete decomposition of the organic matter and transforming the analyte to a definite oxidation state are essential in this case.Off-line preconcen- trations by co-precipitation of dithiocarbamates'62 and oxin- ate^'^^ and by ion exchange (Ba Be Ir Mo Pd Pt Sb Sr and T1) are also known. Only recently have on-line flow injection procedures for environmental waters6w2 and biologi- cal digest^,^',^^ based on solid-phase sorbent extraction of dithiocarbamate complexes on CI8-microcolumns been reported. These techniques would appear to be very promising in speciation studies at analyte levels in the pg range but this work has so far only been carried out with water samples.61i62 Another recent technique with very low LODs (<0.1 pg 1-I) and even better speciation capabilities is HGAAS with in situ collection of the h y d r i d e ~ ~ ~ ~ ' ? ' ~ ~ which has been extensively discussed elsewhere.22 Speciation Electrothermal AAS by itself provides very limited potential for speciation as reflected in the appearance of only a few publications.In situ pre-treatment of spiked samples of urine so as to volatilize Cr"' differentially during the pyrolysis step in the presence of volatilizers while delaying CrV' in the atomizer has been elab~rated.'~'-'~~ A 1 aboratory-designed two-stage at~mizer'~'*'~~ for differential atomization of Hg compounds in urine and sweat has been r e ~ 0 r t e d . l ~ ~ Interfacing ETAAS with chromatographic technique^,^^^^^^'^^ and hydride generation22 would call for further research and hardware developments before such techniques could be used in routine laboratories. 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