Flame, Flameless, and Plasma Spectroscopy Nancy W. Alcock Department of Preventive Medicine and Community Health, University of Texas Medical Branch, 700 Hatborside Drive, Route 1 109, Galveston, Texas 77555-1 709
Flame atomic absorption spectrophotometry (FAAS) and graphite furnace atomic absorption spectrophotometry (GFAAS) or electrothermal vaporization 0 atomic absorption continue to be the techniques most commonly utilized for the specific determination of minerals and trace metals in the clinical laboratory. While more versatile in providing facility for the simultaneous measurement of a number of elements, inductively coupled plasma atomic emission spectrophotometry(ICPAES) is used less frequently, since individual elements are usually requested, but this is an excellent tool for research development, especially in the area of toxic metals. Inductively coupled plasma as a source of singly charged metal ions introduced into a quadropole mass spectrometer, and subsequent analysis on the basis of mass/ charge ratio, provide an extremely sensitive multielement technique, inductively coupled plasma mass spectrometry (ICPMS) . Detection limits are less than 1ppb for most elements and at the pptr level for many. The capability of detecting all stable isotopes of the inorganic elements is unique to this plasma spectroscopy instrumentation. Application of ICPMS for metabolic studies in which enriched isotopes can be administered to humans with safety, and monitored as tracers, provides an advantage over the time-consuming measurements by thermal ionization mass spectrometry. ICPMS is employed in definitive isotope dilution measurement,the most reliable method for accurate determination of elements. This review summarizes recent developments in FAAS, GFAAS, ICPAES, and ICPMS which have current or potential application to the field of clinical chemistry. Many of the references quoted demonstrate the potential for application to the clinical chemistry laboratory although they refer to research in other fields. The source for the review is, with few exceptions,a Chemical Abstracts search of titles and key words of literature cited during the twoyear period October 1992 through October 1994. While the review is not intended to be all-inclusive, selected references from the 617 reviewed give comprehensive coverage of the developments of each technique over the period. Foreign journals are cited only where contributions are significant and for these the Chemical Abstract accession number is given. Limits of detection for most metals measured by the various techniques approximate 0.051.0 mg/L for FAAS, 0.1-10 yg/L for GFAAS, 1.0-100 yg/L for ICPAES, and 0.001-0.01 pglL for ICPMS. Four manufacturers of atomic absorption spectrophotometers dominate the market in the United States, and worldwide there are at least six sources of commercially available instruments for both FAAS and GFAAS. The most significant contribution is this area is the introduction of a transversallyheated graphite furnace atomic absorption spectrophotometer with longitudinal Zeeman background correction. In addition to the advantages of Zeeman graphite furnace characteristics, one of these new instruments can perform simultaneous multielement analysis for up to six elements, using solid-state detection and Eschelle polychromator optics. The stabilized temperature platform furnace is a single unit, the platform being incorporated into the tube. Advantages of the instrument include fast furnace analysis with much reduced time per analysis, cost effectiveness with longer lasting tube life,
and direct calibration with aqueous standards. While this instrument has been designed for analysis of environmental samples in particular, its successful application to biological samples, in particular for blood lead ( S I ) and serum aluminum (S2), has been reported. This instrument appears to overcome the major disadvantage of GFAAS, namely, lengthy analysis time. ICPAES instrumentsare manufactured by severalnational and international companies. While ICPMS instruments in use are manufactured by the two original sources for this technique, three others are beginning to enter the field in this country. At least two Japanese companies produce ICPMS instruments. Significant advances have been made with regard to methods of sample introduction for the various techniques. In order to minimize sample volume required, direct injection nebulization has been developed (S3, S4). On-line flow injection analysis (FIA), which enables such procedures as digestion, dilution, chemical reaction, or chromatographic separation to be performed, has provided improved versatility to the various techniques (S5, S6). Hydride generation as a preliminary step has also been extensively utilized, in particular for the more volatile elements, selenium, arsenic, and tin (S7-S9). REVIEWS Advances in atomic absorption spectrophotometryand plasma spectrometry during the period reviewed have been focused on graphite furnace techniques and the development of the relatively new ICPMS. Five reviews deal with the following aspects of graphite furnace analysis: application to analyses of clinical and biological specimens, foods, and beverages (SIO);toxicology and metabolic applications of atomic absorption spectrophotometry ( S l l ) ;application of palladium as a matrix modifier for analysis of group IIIB-VIB elements (SI2); determination of arsenic in foods with particular attention to hydride generation (S13);atomic absorption spectrophotometry bibliography (SI4). The AAS bibliography, published regularly, covers &month intervals and provides an excellent cross-referenced coverage of analyses for specific elements, different specimen types and fields, alphabetical listing of first authors, and sections on instrumentation, matrix modifiers, and techniques for sample introduction. Eight reviews on the rapidly developing ICPMS applications include the following: a history of the Grst decade of ICPMS (S15); human nutrition and toxicology applications of ICPMS (Sl6); comparison of ICPMS and thermal ionization mass spectrometry in the measurement of lead (SI7);application of ICPMS to analysis of biological materials (S18, SI9),techniques involving chromatography coupled with ICPMS (S2O); application to the measurement of long-lived radionuclides (S21); capabilities of ICPMS for trace element analysis in body tissues and fluids (S22). An extensive bibliography of publications using ICPMS lists geological, environmental, clinical, and health applications with extensive coverage of the various aspects of the technique for each matrix type (S23). This review is updated at regular intervals. A review of plant and soil analyses describes sampling techniques and application of ICPAES for analyses including auxiliary sample introduction techniques (S24). The Atomic Analytical Chemistry, Vol. 67,No. 12, June 75, 7995
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Table 1. GFAAS Coupled with HPLC/QC/Other Separation Source element
proteins human urine urine
As, Se
environmental samples biological reference materials reference materials Se compounds biological tissues beverages
Cu, Cd, Pb Pb Se Sn Sn
separation source
ref
HPLC; cleanup of mobile phase prior to injection mixed acid digestion; FIA; hydride generation silicon-based cation exchange for separation of inorganic As separation of organic species FIA; hydride generation FIA-sorbent extraction on-line microwave digestion anion exchange; fraction collection; hot injection on to L’vov platform HPLC; anion exchange; CIScolumn GC separation of organic species used H2 and air as furnace support gases
S28 S29 S30
sample type
Al, Fe As As
S31 S32 s33 s34 s35 S36
Table 2. GFAAS Analysis of Selected Analytes analyte Ag Ag A1 A1 As As Bi Cd
cu
K La, Eu, Yb Mo Pb Se Si Si Sn V
tissue
fluidtreatment
certified biological materials urine biological materials brain urine hair biological tissues urine serum, urine certified biological materials foods whole blood biological specimens; foods milk plasma proteins bone, soft tissues insecticides urine
Mo tube; H2 added to argon gas Pd-Mg(N03)2 modifier compared probe, wall, platform atomizers K2Cr207 modifier acid/H202 digestion; F1A;hydride generation Pd-Mg(N03)2 modifier wet ash probe atomization; D2 background correction fast 30-s program; dilution with HNO3Rriton-X thiiourea matrix modifier graphite furnace lined with tungsten foil direct dilution Pd-Mg(N03)~ modifier; extended linear range Pd modifier calcium salt modifier pyrolytic tubes, no platform; compared novel modifiers used Ti-coated tubes with L’vov platform transverse heated tube
Spectroscopy review (SI4 includes ICPAES publications. Reviews covering general aspects of atomic spectroscopy include nuclear and nuclear-related techniques (S25), trends in analytical a p proaches to the determination of trace elements in biomedical research (S26), and an atomic spectroscopy update of environmental analysis (S27). GRAPHITE FURNACE ATOMIC ABSORPTION SPECTROPHOTOMETRY The frequency of application of graphite furnace atomic absorption spectrophotometryto the analysis of biological materials during the period reviewed greatly exceeded that of any of the other techniques. Some details of treatment of various specimen types prior to the application of GFAAS for analysis of biological and other materials are summarized in Table 1 (S28S36). Although GFAAS has limitations of singleelement analysis, matrix interferences, and lengthy analysis time, this technique can still be considered as the gold standard for many elements. Most publications reviewed utilized Zeeman background correction, although deuterium correction is also frequently reported. Aspects of graphite furnace analysis which have received particular attention include the following: (1)the testing of performance of the previously described transversely heated graphite tube in longitudinal Zeeman field ( S I , S2, S37-S39); (2) hydride generation coupled with GFAAS especially for As (S8) and Se (Sa determination; (3) emphasis on the versatility of palladiumcontaining matrix modifiers for Pb (S40-S43), Ag (S44,Tl (S45), Al, Cu, Mn, Mo, and Sn (S46) analysis of slurry samples, which minimizes preparation time (S47-S50). Modifications in composition of the tube include the testing of a metal-carbide-coated graphite tube (S51), a tungsten tube atomizer (S52),a L‘vov platform in a Ti-coated tube (S53), a graphite furnace lined with tungsten foil (S54, a molybdenum tube with Hdargon gas (S55), 504R
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ref
s55 s44 s59 S60 S29 S61 S62 S63 S64 S65 s54 S66 S42 S67 S68 S69 s53 S70
tantalum foil (S56), tungsten wire (S57), and tungsten coil (S58). Relative efficiency of replacements for the graphite tube has yet to be demonstrated. Some details of particular application of G F M for individual analysis are summarized in Table 2 (S29, S42, S44, S53, S55, S59, S60-S70).
FLAME ATOMIC ABSORPTION SPECTROPHOTOMETRY Flame atomic absorption spectrophotometry remains the method of choice for specific methods for routine analysis of the major minerals calcium and magnesium and for the trace metals zinc and copper in serum and in biological tissues in the clinical laboratory. The ease of sample preparation, relative lack of spectral interferences, and rapidity of the analysis where sample or digestate volume and sensitivity are adequate make this the method of choice. Even with small sample volumes analysis may be performed by direct uptake of 50- 100pL. The major advances in the use of this technique have been concerned with the application of different chemical digestions where necessary and modification of sample, its introduction,or both using on-line flow injection. The following examples indicate recent development in this area. Digestion with HN03 for 20 min at room temperature (S71) provided a rapid method of extracting Cu, Fe, and Zn from liver. A rapid 1 1 HN03/Hz02 digestion with flow injection produced a slurry for analysis of solid samples for Cu and Mn (S72). A flow injection manifold was developed for analysis of grains prepared as a 3% (w/v) slurry for analysis of Ca, Mg, Fe, and Zn (S73). Aqueous standards were adequate for calibration. Acid digestion was compared with microwave digestion and conventional sample aspiration with flow injection (S74 for the determination of the infrequently measured trace metals cobalt and nickel in rice, a demonstration of the versatility of the flame technique. Column preconcentration of blood and the use of a
+
Table 3. Sample and Instrument Parameters for ICPAES Analyses of Selected Elements preconcn or modification
element
sample type
Al, B, Si, Sr, Ti Al, Ba, Mg, Mn As As, Se B B C C, Ca, Cu, Cd, Fe, Mg, Mn, Zn Cd, Pb Cd, Co, Cu, Ni, Pb
foods tea leaves synthetic solutions milk tissues plants, blood, meat, milk milk biological samples
fusion with Li metaborate slurry nebulization LC coupled to ICPAES; speciation used a minitorch tested nebulizers; modified simplex optimization microwave digestion determined C residue from various digestion procedures vapor-phase acid digestion, 50- 150 mg of tissue
S78 s79 S80 S81
biological biological urine hair, blood tissues plants plants, soil dairy products 35 tissues oils brain regions certified biological materials forensic various tissues
tungsten boat furnace vaporization environmental pre-ICPAES ultrasonic nebulization HPLC after treatment with cis-diammine(glyco1ate)Pt microwave digestion uptake questions inertness microwave muffle furnace rapid wet digestion conventional digestion HN03/HC104 digestion Babington V-groove nebulizer, heated spray chamber
S86 S87 S88 S89 S90 S9 1 S92 s93 s94 s95 S96 s97
Pt Sr
Ti multielement
preconcentration on-line; 8-hydroxyquinoline cellulose microcolumn coupled to ICPAES; atmospheric pressure various applications focused microwave digestion at atmospheric pressure
slotted quartz tube enabled a 20-fold increase in sensitivity for Cd compared with conventional sample aspiration (S75). Using cation exchange with on-line flow injection, Al as low as 75 pg/L was measured in water and beverage samples 676). An interface was developed for measuring Cd in metallothionein (S77). The thermospray/microatomizerinterface was “homemade”. The fourcompartment model had a thermospray inlet, a premixing chamber where the vapor was mixed with preheated hydrogen, a combustion chamber providing oxygen for pyrolysis, and finally a quartz tube mounted in the analytical AAS path. INDUCTIVELY COUPLED PLASMA ATOMIC EMISSION SPECTROPHOTOMETRY Details of ICPAES application for analysis of some selected elements are summarized in Table 3 (S78-S99). A novel approach to multielement analysis is described with the generation of a capacitively coupled plasma inside a graphite furnace (S100). A graphite furnace is programmed to dry, char, and atomize the test solution. Multielement analysis is possible with the generation of a radio frequency plasma inside the graphite tube. The small sample size and minimum chloride interference with the test solutions together with the facility for multielement analysis and the suggestion that the system is applicable to organic solvent analysis makes this a potentially valuable tool. A valuable contribution to the evaluation of reliability of ICPAES in plant and soil samples is the investigation of interference by Ca, Al, Fe, K, Mg, and Na to As, Cd, Co, Cu, Hg, Pb, and Se (S101). Correction for nonlinear spectral interferences by some elements was calculated. Applicability to biological samples requires investigation. Hydride generation coupled with ICPAES is extremely useful, especially for volatile elements. In addition to application to As (S102) and Se (S103),hydride generation was used for lead determination in various environmental samples, beverages, and biological materials (S104, S105). Coupling of HPLC and other chromatographic separations has proved effective in delineating speciation of As ( S O ) , improving multielement analysis with an on-line 8-hydroxyquinolinecellulose microcolumn (S97), and the measurement of Cu, Zn, and Fe in specific proteins (S106,,907) and of Pt in urine from patients treated with cis-diammine-
ref
S82
S83 S84 S85
S98 s99
(g1ycalate)Pt (S108). Analysis for eight elements by ICPAES following vapor-phase acid digestion of small biological samples-50- 165 mg-is impressive (S85). INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY ICPMS is unique among the flame and plasma spectroscopy techniques in the ability to discriminate between the mass of the various isotopes of an element where more than one stable isotope occurs. Consequently an enriched isotope that normally is present in relatively low abundance can be used as a tracer in metabolic studies and compared to a reference isotope-isotope ratio-whose abundance is constant. Isotope dilution, in which the change in isotope ratio for two selected isotopes of an element of interest is measured in a solution after the addition of a known quantity of a spike that contains enrichment of one of the isotopes, permits calculation of the concentration of the element. Isotope dilution is the most reliable method of accurate determination of elemental concentration. The technique that may be applied to multielement analyses has been reported for the determination of Pb, Cd, Cu, Zn, and Mn (S109-SI12). Measurement of 1ng of thorium was possible in biological samples using stable isotopes 230Th and ~ 2 Th (S113). Isotope ratios have been used in metabolic studies of zinc in humans (S114, S115) and copper (S116). Other applications include the determination of the concentrations of Cu, Mo, and Se (S117) in various reference materials, absorption of nickel through the skin (S118), and the absorption of Fe in infants using oral and intravenous doses of 57Fe and 5nFe respectively, with 56Feas the reference isotope (S119). The conventional method of sample introduction for ICPMS is by aspiration, via a nebulizer, into a spray chamber. A small fraction of the resulting aerosol is swept by argon into the torch. Approximately 1 mL of sample is required for a single analytical run, about 99%of which goes to waste from the spray chamber. In recent years, alternativemethods of sample introduction directly into the torch through a direct injection nebulizer (DIN) have been developed. A distinct advantage of this technique is the small sample size-as little as 2 pL- (S3) with detection limits for a number of elements in the range 0.5-3 pg (absolute) at a Analytical Chemistry, Vol. 67, No. 12, June 15, 1995
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LITERATURE CITED
Table 4. Selected Applications of ICPMS element
sample type
AI As As, Cr, Se, V
Cd Cd, Zn
serum fish tissue biological samples various plant cells human serum metallothionein blood, tissues
Cu, Mo, Se
reference materials
S136
Fe Fe
serum milk, lactoferrin erythrocytes urine blood, urine infant formula
S137 S138 S139 s4 S131 S 140
B
isotopelelement elemental HPLC. As speciation elemental elemental
ref S 128 S132 SI24 SI33 S 134 S129 S130 SI35
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Hg, Pb Hg Pb
Pt Th U. Th
Zn
DNA adducts biological samples feces biological samples plasma
s7Fe:56Fe;58Fe:s6Fe elemental, speciation elemental zo7Pb:208Pb; isoto e dilution using fo6Pb elemental ?3flTh, 232Th elemental 70Zn:68Zn;67Zn:68Zn elemental
s122 SI41 S142 S114 s115
concentration range 0.3-2 pg/L. By coupling separation techniques such as anion-exchange chromatography, size-exclusion chromatography, and HPLC, different chemical forms or oxidation states of elements such as Se, As, and Sn can be determined in the various fractions by ICPMS. Elemental speciation of selenium (S3) was determined by anion-exchange and size-exclusion chromatography with detection by ICPMS using a direct injection nebulizer. Speciation of mercury and lead was achieved by microbore column liquid chromatography coupled to ICPMS using direct injection nebulization ( S 4 ) . Flow injection analysis coupled with ICPMS permits on-line treatment of sample with ion-exchange chromatography ( S l 2 0 ) and HPLC where eight species of As were identified (S121); nucleotides and adducts were separated to identify Pt-DNA adducts (S122) and different species of tin (S123). Flow injection analysis was also used in the determination of As, Cr, Se,and V (S124), Cu, Cd, and Pb (S112) and V, Mn, Cu, Zn, and Cd in biological samples after chelation on iminodiacetate resin and elution with nitric acid (S5). On-line digestion of materials, dilution of liquids, or addition of other reagents may be made. Introduction of specimen into ICPMS as a hydride has been applied to analysis of total As, Sn, and Se. Hence, chloride interference to As was removed and enrichment of 76Se,77Se,and %e using 78Se as reference isotope gave excellent sensitivity (S125). Analysis of other individual analytes applicable to the clinical laboratory include Bi (S126), measured at 7 ng/L, the toxic metals Bi, Pb, and Tl (S127), AI (S128) and B measured directly in a 1:5 dilution of serum (S129), metallothionein-bound Cd separated by HPLC (S130), and Hg in blood and urine after treatment with 50% HCl/EDTA/cysteine followed by filtration (SI31). Table 4 lists some selected applications of ICPMS (S114, S115, S122, S124, S128-Sl42). Nancy W.Alcock is an Associate Professor in the Depadment of Preventive Medicine and Community Health at the University of Texas Medical Branch, Galveston, ?x, where she j s the Director of the Nutrition Assessment Research Laboratory. She recezved her B.Sc. at the Unzverszty of Tasmania and her Ph.D. at the University of London, England. She is a Diplomate of the American Board of Clinical Chemisty, Inc., a Fellow of the National Academ of Clinical Biochemistry, and a Fellow of the New York Academy of d d i c i n e . She is a member of the editorial board of Magnesium and Trace Metals. Her research interests are in trace metals in human nutrition, and in metals as chemotherapeutic agents. 506R
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