Anal. Chem. 1999, 71, 338R-342R
Flame, Flameless, and Plasma Spectroscopy Gabor Komaromy-Hiller
Department of Pathology, University of Utah School of Medicine, 50 North Medical Drive, Salt Lake City, Utah 84132 Although anodic stripping voltammetry is still in use for trace metal analysis, the various atomic spectroscopy techniques are the predominant choices for analysis of metals in biological specimens. Atomic absorption spectrometry with flame (FAAS), graphite furnace (GFAAS), and electrothermal vaporization (ETVAAS) remains the most popular technique. Except for some special applications for multielement determination, these instruments are dedicated to analysis of one specific analyte at a time. Inductively coupled plasma atomic emission spectrometry (ICPAES) is capable of multielement determination; however, ICPAES is used less frequently than atomic absorption due to the higher initial investment cost, which is difficult to justify in a laboratory where mostly individual analytes are ordered. Instrument cost is especially prohibitive in inductively coupled plasma mass spectrometry, a newer and very powerful analytical tool in the laboratory. The ICPMS technique is generally 1 order of magnitude or more sensitive than atomic absorption methods and is capable of simultaneous multielement determination. It has clear cost and labor time advantages over other atomic spectrometry techniques only in the few laboratories with sufficiently high sample load. Presently, the CAP Proficiency Survey does not have a separate entry for ICPMS users; however, laboratories can participate in an ICPMS Comparison Program provided by Le Centre de toxicologie du Que´bec (www.ctq.qc.ca/ctqintre.html). This review focuses on the applications of FAAS, GFAAS, ETVAAS, ICP-AES, and ICPMS published between October 1996 and October 1998. The references quoted focus on analysis of biological specimens commonly encountered in the clinical laboratory. The source for the review is a Chemical Abstracts title and keyword search. The review by no means is intended to be allinclusive. Obscure and foreign language journals are cited only where the contribution is significant. More than 15 atomic absorption spectrophotometer manufacturers market their product in the United States. Worldwide there are at least two or three more additional major manufacturers. The new atomic absorption spectrophotometers have the flexibility of using a flame or graphite furnace atomizer with easily exchangeable platforms. Ultrafast heating rates up to 3000 °C/s significantly reduce analysis time, and double beam in space models with solid-state detectors increase signal-to-noise ratio and improve sensitivity. Portable devices that allow on-site analysis have been developed and shown to be capable of good accuracy and precision. ICP-AES and ICPMS instruments are also marketed by close to 20 companies. The major development in atomic emission spectrometry is the new generation charge-coupled device (CCD) detectors. The new CCD design allows 80 times faster reading than the previous generation of CCD detector and full and flexible wavelength range and selection. Incorporation of this new detector into ICP-AES instruments allows 5-10-fold 338R Analytical Chemistry, Vol. 71, No. 12, June 15, 1999
reduction in analysis time. Several new nebulizer designs improve sample introduction into the plasma torch, bringing improvements in certain areas: direct injection nebulizers (DIN) that eliminate memory effects, the membrane desolvator allows analysis of organic matrixes, and microconcentric nebulizers reduce the sample volume to the microliter range. In terms of sample preparation, two major trends can be observed. The first is the development of various on-line sample pretreatment systems for ETAAS. There are different designs to overcome the difficulty of interfacing a continuous-flow system with batch-mode detection. Recent developments in this area are nicely summarized in a review (G1). The second major trend is the application of microwave digestion for sample preparation. Although the various microwave-assisted methods offer advantages over traditional methods, it is unlikely they will be accepted in a high-volume laboratory because of their currently limited throughput. REVIEWS Five reviews appeared in the last two years related to atomic absorption spectrometry. Last year’s review in this journal is a general reference on atomic absorption and “nonplasma” emission spectroscopy (G2). The previously mentioned review (G1) surveys the development and application of various on-line sample pretreatment systems for AAS. Two reviews deal with recent developments in trace element analysis methods concentrating on atomic absorption techniques (G3, G4), and there is a very extensive review published in 1997 on the application of atomic spectroscopy in the analysis of biological, clinical materials and food products (G5). There were three reviews published on ICP spectroscopy. Two of them reviewed ICPMS and ICP-AES for trace element determination, respectively (G6, G7). The third one is a review of double-focusing mass spectrometry in ICPMS instrumentation and applications (G8). Four reviews were dedicated to elemental speciation. They include a general survey on speciation (G9), a review on various hyphenated techniques, in which a separation system is interfaced with an atomic spectrometry detector, used in speciation studies (G10), the relatively new CE/ICPMS technique and interface (G11), and a review of ICPMS-interfaced applications in elemental speciation (G12). An entire special journal issue was dedicated in 1997 to speciation, indicating its importance. FLAME ATOMIC ABSORPTION Due to its simplicity and relative freedom of interferences, flame atomic absorption is still popular for major and even trace metal analysis in both serum and urine specimens. Newer instrumentation can provide adequate accuracy and precision to meet the laboratory’s need (G13). Various sample preparation 10.1021/a1999907z CCC: $18.00
© 1999 American Chemical Society Published on Web 05/20/1999
Table 10. Selected AAS Methods for Analysis of Biological Samples analyte(s)
sample
Al, Cu Cu, Mg Cr, Cu, Fe, Mn, Pb Ca, Mg Li Li Al Al Cu, Mn, Ni Cu, Mn, Zn Cu, Zn Cu, Zn Si Si Se Se Bi Co, Cu, Pb, Cd Cd Sr Cu
dialysis concentrate whole blood, fingernail hair urinary stones plasma, erythrocytes urine, plasma serum serum reference materials liver plasma, erythrocytes, platelets serum urine serum, tissue serum urine animal tissue, sediment environmental reference materials urine serum, urine, bone, soft tissues serum
method notes
ref
preconcentration with microcolumn resin Mg/Rh modifier GFAAS with pyrocoated graphite tube solid-phase extraction, NH4NO3 modifier ETVAAS microwave-assisted acid digestion flow injection on-line sorption preconcentration in a knotted reactor ETVAAS FAAS, GFAAS simple dilution, GFAAS lanthum oxide/(NH4)2HPO4 modifier HPLC/AAS to eliminate Cl- interference Mg(NO3)2 modifier, phosphate inferference flow injection on-line sorption preconcentration in a knotted reactor multielement determination boiling HNO3 digestion, Pd/NH4NO3 modifier GFAAS Mg(NO3)2/(NH4)2HPO4 modifier
methods can be applied. For example cobalt diethyldithiocarbamate coprecipitation is used for preconcentration and subsequent determination of Cd, Cu, Fe, Mn, Ni, Pb, and Zn in urine specimens. Detection limits are in nanograms per milliliter (G14). Acidified predigestion slurry sampling is used for the measurement of Ca, Cu, K, Mg, Na, and Zn (G15). Wetting agents are useful only for dilution slurries. 3,6-Dithiaoctane extraction followed by “one drop” FAAS has been successfully applied to determine silver in biological reference materials (G16). FAAS also served as a detector for Cd speciation in urine specimens. The authors successfully interfaced an HPLC system with a flame atomic absorption spectrophotometer through a thermospray nebulizer and obtained improved sensitivity for Cd determination (G17). FLAMELESS ATOMIC ABSORPTION Flameless atomic absorption techniques enjoy far more popularity than flame atomic absorption/emission methods. The development and successful application of portable devices (G18, G19) brought atomic spectroscopy out of the laboratory and to the bedside. These portable devices, which can operate from a car battery, have a tungsten coil serving as the atomizer. The light source can be a laser diode or an ordinary projector bulb, and the detector is a CCD. They are used to determine Cd (G18), Al (G19), and Cr (G19) in biological and environmental samples with excellent detection limits. Slurry sampling is very popular for hard to digest samples. This sampling technique was used to determine Ni (G20), Al (G21), and Mn (G21) in human scalp hair, to measure Cd, Mn, and Pb in plant leaves and animal tissues (G22) and was also used for Se analysis in serum specimens (G23). Various modifiers can be added in the sample preparation step to increase sensitivity (G24, G25); in addition, modifiers can be also permanent, such as Ir, W, or Zr incorporated in coated graphite platforms for Cr determination in hair (G26). For speciation studies, most work has been done with ion chromatography coupled to hydride generation atomic absorption spectrophotometry. Urine and serum served as specimens for As (G27-G30) and Se (G31, G32) speciation. However, hyphenated techniques are not limited to hydride generation AAS as was demonstrated
G44 G45 G46 G47 G48 G49 G50 G51 G52 G53 G54 G55 G56 G57 G58 G59 G60 G61 G62 G63 G64
Table 11. ICP-AES Analysis of Biological Samples sample type urine
serum hair bone spleen brain plant leaves
analyte(s)
refs
Se La Si Al Ti, Zr Ba, Ca, Cu, Fe, Mg, Mn, Pb, Zn Ca, P Ti Al, As, Ca, Cd, Cr, Cu, Fe, K, Mg, Na, Ni, P, Pb, Si, Zn Al, Ca, Cu, Fe, Mg, Mn, Na, K, Zn
G32 G71 G75 G76 G69 G77 G78 G79 G80 G81
by a Mg speciation study. The authors used ion chromatography with photodiode array detector to measure protein fractions followed by GFAAS to estimate which protein fractions Mg is associated with (G33). Hydride generation technique remains very popular for total Hg (G34-G36), Se (G37-G40), Bi (G41, G42), and Cd (G43) analysis in different sample types ranging from human urine, serum, and hair to soil and marine sediments. Details of other selected publications are summarized in Table 10. INDUCTIVELY COUPLED PLASMA ATOMIC EMISSION SPECTROMETRY The introduction of CCD and CID detectors influenced ICPAES. Multielement determination with solid-state detectors is up to 40 times faster than with sequential methods (G65). No prior wavelength selection (to analyze selected elements) is required since the whole wavelength range is viewed at the same time (G66). Axially viewed inductively coupled plasma emission spectrometers provide an extended viewing area and enable higher sensitivity in elemental detection (G67, G68). Various sample preparation techniques were applied for biological materials to reduce and/or eliminate interferences. Pressurized digestion with a mixture of nitric and hydrofluoric acid reportedly eliminates carbon interferences in titanium and zirconium determination from serum (G69). Microwave-assisted vapor-phase nitric acid digestion was also successfully applied for multielement determination in Analytical Chemistry, Vol. 71, No. 12, June 15, 1999
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Table 12. Analysis of Selected Elements by ICPMS in Biological Samples analyte(s)
sample type
notes
refs
lanthanides Al, Ti, V As, Au, B, Ca, etc. Ag, Al, Au, Ba, etc. Ba, Ca, Cd, Co, etc. As, Ba, Ca, Cr, etc. Al, Cd, Co, Cu, etc. Ag, As, Cd, Co, etc. Ga Ti Cu, Zn Se Mo Bi Cu, Zn I, Br, Cl, F I Cu, Fe, Ni, Pb, Zn Au, Pt U
plasma, animal tissue serum hair hair pleural effusion biological reference materials newborn serum biological reference materials animal tissue spleen urine, serum serum urine urine, serum urine, plasma environmental reference materials biological reference materials blood, serum protein solutions urine
electrothermal vaporization ICPMS electrothermal vaporization ICPMS multielement (12) determination multielement (42) determination multielement (14) determination multielement (11) determination multielement (14) determination semiquantitative multielement (70) analysis ICPMS is superior to radiotracer techniques ICPMS is superior to ICP-AES general strategies to reduce interferences butanol reduces interference from Ar adducts monitored 95Mo and 98Mo direct injection nebulization, LOD 9.7 pg/mL direct injection nebulizer, 1-2-µL sample volume pyrohydrolysis sample preparation sample combustion in oxygen stream on-line microwave digestion study of protein binding LOD 0.32 pg/mL
G110 G111 G112 G113 G114 G81 G115 G116 G117 G79 G118 G119 G120 G121 G122 G123 G124 G125 G126 G127
standard biological reference materials (G70). On-line preconcentration techniques are much easier to interface with ICP-AES than with the batch-mode ETAAS and were utilized to achieve improved sensitivity. In aqueous standards, 90 pg/mL La was determined (G71) and a detection limit of 1 ng/mL was reported for Hg determination (G72). Two published methods for selenium speciation used ICP-AES as a detector following a chromatographic separation of the selenium species. The detection limits obtained were practically equivalent, 20-38 (G73) and 30 ng/ mL (G32), respectively. The method has also been accepted in the area of forensic science for toxic trace element analysis in tissue specimens (G74). Table 11 summarizes various sample types and trace elements determined by ICP-AES. INDUCTIVELY COUPLED PLASMA MASS SPECTROSCOPY The popularity of ICPMS is rapidly rising as seen from the increasing number of publications. This popularity is due to the method’s sensitivity, multielement analysis, and stable isotope measurement capabilities. Isotope studies include measurements of various metals with stable isotope dilution technique (G82G85), isotope distribution (G86, G87), and absorption and biokinetic studies (G88-G91). In most cases, low-resolution quadrupole ICPMS systems are sufficient; however in certain cases, such as Ca isotope measurements (G87), a high-resolution magnetic sector instrument is necessary to overcome interferences (G86, G91). Major progress was made in speciation by interfacing capillary electrophoresis systems with ICPMS detection (G92G95), but many application still use a form of liquid chromatography for the separation step (G27, G30-G32, G96-G101). Although most interest still lays in arsenic (G27, G30, G100) and selenium (G31, G32, G93, G96, G99) speciation, protein and even DNA binding of other metals has been investigated, as well. An interesting study demonstrated the use of size-exclusion chromatography (SEC) with ICPMS detection to separate proteins and DNA fragments and to study the binding of Cr, Se, Cd, Th, and U to the various fractions (G98). In another application, proteins were separated by two-dimensional gel electrophoresis, and Co binding to the various fractions were investigated by sampling the gel with laser ablation-ICPMS (G102). High-resolution mag340R
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netic sector instruments are slowly gaining ground (G50, G103G106); however, the instrument price definitely prohibits use in a routine production laboratory. In a very interesting study, a direct injection high-efficiency nebulizer (DIHEN) with low flow rate coupled to quadrupole ICPMS enabled the analysis of Cr bound to DNA in human lung epithelia cells (G107). There have been other attempts to overcome interferences in Cr determination by ICPMS (G98, G108, G109); however, the problem of Cr analysis by quadrupole ICPMS still remains unsolved. Details of other selected publications are summarized in Table 12. Gabor Komaromy-Hiller is an instructor at the Department of Pathology of the University of Utah School of Medicine and Medical Director of the Organic Acids Laboratory of ARUP Laboratories. He received his B.S. and M.S. degrees from Semmelweis University of Medical Science, Budapest, Hungary, and his Ph.D. from the University of Idaho. From 1995 to 1998 he was a Postdoctoral Fellow in clinical chemistry at the University of Utah. His research interest includes organic and amino acids and trace metal analysis.
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