The determination of trace elements in crude oil and its heavy fractions by atomic spectrometry

A literature review on the determination of trace elements in crude oil and heavy molecular mass fractions (saturates, aromatics, resins and asphaltenes) by ICP-MS, ICP OES and AAS is presented. Metal occurrences, forms and distributions are examined as well as their implications in terms of reservoir geochemistry, oil refining and environment. The particular analytical challenges for the determination of metals in these complex matrices by spectrochemical techniques are discussed. Sample preparation based on ashing, microwave-assisted digestion and combustion decomposition procedures is noted as robust and long used. However, the introduction of non-aqueous solvents and micro-emulsions into inductively coupled plasmas is cited as a new trend for achieving rapid and accurate analysis. Separation procedures for operationally defined fractions in crude oil are more systematically applied for the observation of metal distributions and their implications. Chemical speciation is of growing interest, achieved by the coupling of high efficiency separation techniques (e.g., HPLC and GC) to ICP-MS instrumentation, which allows the simultaneous determination of multiple organometallic species of geochemical and environmental importance.     

Speciation of trace elements in foods,with special reference to cadmium and selenium: is it necessary?

Studies on the speciation of trace elements in foods are required to validate existing risk assessment procedures, e.g. those used for Cd which are based on inorganic Cd salts, and to better understand how the absorption and bioavailability of elements can be reduced (Cd) or improved (Se). The methodology for speciation studies is very much in the research and development phase, with few reference materials or standard procedures. The use of stable isotopes with chromatographic separation techniques and improved instrumentation for ICP-MS and MS in general offers a way forward in this area.     

Environmental application of elemental speciation analysis based on liquid or gas chromatography hyphenated to inductively coupled plasma mass spectrometry—A review

In recent years the number of environmental applications of elemental speciation analysis using inductively coupled plasma mass spectrometry (ICP-MS) as detector has increased significantly. The analytical characteristics, such as extremely low detection limits (LOD) for almost all elements, the wide linear range, the possibility for multi-elemental analysis and the possibility to apply isotope dilution mass spectrometry (IDMS) make ICP-MS an attractive tool for elemental speciation analysis. Two methodological approaches, i.e. the combination of ICP-MS with high performance liquid chromatography (HPLC) and gas chromatography (GC), dominate the field. Besides the investigation of metals and metalloids and their species (e.g. Sn, Hg, As), representing “classic” elements in environmental science, more recently other elements (e.g. P, S, Br, I) amenable to ICP-MS determination were addressed. In addition, the introduction of isotope dilution analysis and the development of isotopically labeled species-specific standards have contributed to the success of ICP-MS in the field. The aim of this review is to summarize these developments and to highlight recent trends in the environmental application of ICP-MS coupled to GC and HPLC.     

Applications of inductively coupled plasma mass spectrometry and laser ablation inductively coupled plasma mass spectrometry in materials science

Inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS (LA-ICP-MS) have been applied as the most important inorganic mass spectrometric techniques having multielemental capability for the characterization of solid samples in materials science. ICP-MS is used for the sensitive determination of trace and ultratrace elements in digested solutions of solid samples or of process chemicals (ultrapure water, acids and organic solutions) for the semiconductor industry with detection limits down to sub-picogram per liter levels. Whereas ICP-MS on solid samples (e.g. high-purity ceramics) sometimes requires time-consuming sample preparation for its application in materials science, and the risk of contamination is a serious drawback, a fast, direct determination of trace elements in solid materials without any sample preparation by LA-ICP-MS is possible. The detection limits for the direct analysis of solid samples by LA-ICP-MS have been determined for many elements down to the nanogram per gram range. A deterioration of detection limits was observed for elements where interferences with polyatomic ions occur. The inherent interference problem can often be solved by applying a double-focusing sector field mass spectrometer at higher mass resolution or by collision-induced reactions of polyatomic ions with a collision gas using an ICP-MS fitted with collision cell. The main problem of LA-ICP-MS is quantification if no suitable standard reference materials with a similar matrix composition are available. The calibration problem in LA-ICP-MS can be solved using on-line solution-based calibration, and different procedures, such as external calibration and standard addition, have been discussed with respect to their application in materials science. The application of isotope dilution in solution-based calibration for trace metal determination in small amounts of noble metals has been developed as a new calibration strategy. This review discusses new analytical developments and possible applications of ICP-MS and LA-ICP-MS for the quantitative determination of trace elements and in surface analysis for materials science.     

Online coupling of ultraviolet titanium dioxide film reactor with poly(methyl methacrylate) solid phase extraction–inductively coupled plasma mass spectrometry for speciation of trace heavy metals in freshwater

To rapidly discriminate dissolved labile and stable organic-complexed metal ions, a fully automated approach comprising a photocatalyst-assisted digestion reactor (PADR), a non-functionalized poly(methyl methacrylate) (PMMA) solid-phase extraction (SPE) column, and inductively coupled plasma-mass spectrometry (ICP-MS) instrumentation was developed. To separate labile dissolved metals from other concomitant metal complexes, a non-functionalized PMMA bead was used as the SPE adsorbent because of its selective interaction with labile metal ions. The PMMA SPE–ICP-MS hyphenated system was optimized, and its analytical reliability was confirmed by using it to analyze the certified reference material—NIST 1643e (artificial saline water). Detection limits (σ = 3, n = 7) for all analyte ions (Ni, Cu, Zn, Cd, and Pb), which ranged from 0.005 to 0.186 μg L^{− 1}, could be reached; therefore, this technique appeared uniquely suited to determining levels of trace elements in most natural freshwater samples. To determine the total quantity of dissolved metals, a new digestion reactor (PADR) was developed for online conversion of metal–organic complexes to their labile forms. Compared to conventional photolysis methods, the digestion time improved considerably and the digestion efficiency for organic substances was excellent (> 90%) in the PADR format, with a very short resident time of 10 min. After construction of the PADR–PMMA SPE–ICP-MS hyphenated system, the speciation potential of our developed method was evaluated by analyzing three intentionally contaminated water samples. Results indicated that our developed hyphenated system is effective for online determination of total, labile, and metal–humic complexes in freshwater samples and that is capable of providing representative metal speciation patterns for different aquatic systems.     

Size fractionation techniques combined with INAA for speciation purposes

In natural waters trace elements, especially trace metals may be present in a variety of physicochemical forms. They may be associated with forms ranging from simple ions and molecules via hydrolysis products and colloids, pseudocolloids and organic or inorganic particles. The transition between categories is gradual. The presence of species differing in size, charge and density will influence on the transport, mobility and bioavailability of the trace element in question. Fractionation techniques which do not influence the distribution patterns are therefore required for speciation purposes. In the present work dialysis in situ and large membrane (hollow fibers) ultrafiltration are used for fractionation of low molecular weight species, colloids, pseudocolloids and particles. Due to the presence of foreign components transformation processes influence the distribution patterns of trace elements of interest. Sorption to foreign surfaces, complexation with agents present and aggregation of colloids (e.g., increasing ionic strength) result in a shift towards higher dimensions while desorption and dispersion processes mobilize the trace elements. Information on several components is therefore needed in speciation studies and a multielemental method of analysis having low determination limits must be applied. Instrumental neutron activation is appropriate to this kind of study because of its high sensitivity for simultaneous determination of a great-number of elements. Size fractionation techniques combined with INAA for the characterization of trace element species in natural waters will be discussed.     

On-line coupled ion chromatography-ICP-(AES, MS) and electro thermal vaporisation-ICP-MS in use for ultra trace analysis of high purity rhenium

This work presents the benefits of coupling techniques such as Electro Thermal Vaporisation (ETV)-ICP-MS and Ion Chromatography (IC)-ICP-(AES, MS) for ultra trace analysis in a high purity rhenium powder sample. Direct analysis using ICP-AES suffers from poor detection limits and allows trace analysis only above 1 g/g for most analytes. ICP-MS analysis of trace elements is more sensitive, but signal depression caused by the heavy Re-ions limits trace analysis to concentrations of 50–100 ng/g analyte in the solid sample. Coupling Ion Chromatography with ICP-spectrometers, combined with time resolved measurement (IC-ICP-TRM) of the elution signals, was used to enhance the sensitivity of both ICP-AES and ICP-MS. Resulting detection limits are in the very low ng/g to pg/g range. Coupling of ETV and ICP-MS offers the possibility of eliminating the volatile Re₂O₇ matrix by thermal pretreatment and allows ICP-MS measurements without matrix interferences caused by Re. Results from these methods are compared with Glow Discharge Mass Spectrometry (GDMS) analysis, a semiquantitative solid state technique. The results are also compared with the manufacturers' specifications to show the power of modern routine analysis using ICP-AES or FAAS.     

Trace analysis of rare earth elements and other impurities in high purity scandium by inductively coupled plasma mass spectrometry after liquid-liquid extraction of the matrix

Bis(2-ethyl hexyl)-orthophosphoric acid in toluene has been used to extract scandium selectively from an aqueous phase containing nearly thirty elements at various concentrations. The trace elements (Li, Mg, Al, Mn, Co, Ni, Cu, Zn, Ga, Sr, Y, Cd, In, Sb, Ba, REE, Hf, Pt, Tl, Pb, Bi and u) have been determined quantitatively in the aqueous phase using inductively coupled plasma mass spectrometry (ICP-MS). The recoveries have been quantitative for most of the analytes studied and ICP-MS has been found suitable for the determination of trace quantities of these elements in the aqueous extract. The procedure has been applied to the quantitative ultra-trace analysis of rare earth elements (REE) and other trace impurities in high purity scandium metal (4N) using ICP-MS and ID-ICP-MS techniques.Presented in part at the 2nd Regensburger Symposium Massenspektrometrische Verfahren der Elementspurenanalyse, 6.-8.10.93, Regensburg, Germany     

Instrumental Neutron Activation Analysis (INAA) and Inductively Coupled Plasma/Mass Spectrometry (ICP-MS) for Trace Element Biomonitoring using Mosses

Two analytical methods - instrumental neutron activation analysis (INAA) and inductively coupled plasma mass spectrometry (ICP-MS) - were used for the trace element analysis of naturally growing mosses for a heavy metal biomonitoring survey. The techniques were applied to reference mosses to evaluate the feasibility, analytical variability, detection limits and accuracy. These parameters were evaluated using 563 mosses sampled in the 1996 French survey. All the elements of interest in the European program "Atmospheric Heavy Metal Deposition in Europe - estimation based on moss analysis" (As, Cd, Cr, Cu, Fe, Hg, Pb, Ni, V, Zn) were able to be determined by ICP-MS. INAA appeared suitable for the determination of As, Cr, Fe, Hg, V and Zn. The Cd, Cu, Ni and Pb concentrations determined by ICP-MS were preferred to the INAA results, because of increased feasibility or accuracy. The results provided by both methods on the French mosses were statistically compared for 14 elements. Significant linear correlation appeared for: Ba, Ce, Cs, La, Rb, Sm, Th and V. Among these eight elements, Ba, Cs, La and Sm concentrations determined by both methods exhibited a strong statistical similarity. The correlations obtained for As, Eu, Fe and Sb were not as strong and no correlation at all was observed for Co and Cr. These differences were attributed to instrumental factors (e.g. spectral interference occurred for both methods) or due to the sample preparation prior to ICP-MS. The consequences of such results on the regional trend evaluation of atmospheric heavy metal deposition were discussed.     

Inorganic trace analysis by mass spectrometry

Mass spectrometric methods for the trace analysis of inorganic materials with their ability to provide a very sensitive multielemental analysis have been established for the determination of trace and ultratrace elements in high-purity materials (metals, semiconductors and insulators), in different technical samples (e.g. alloys, pure chemicals, ceramics, thin films, ion-implanted semiconductors), in environmental samples (waters, soils, biological and medical materials) and geological samples. Whereas such techniques as spark source mass spectrometry (SSMS), laser ionization mass spectrometry (LIMS), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), glow discharge mass spectrometry (GDMS), secondary ion mass spectrometry (SIMS) and inductively coupled plasma mass spectrometry (ICP-MS) have multielemental capability, other methods such as thermal ionization mass spectrometry (TIMS), accelerator mass spectrometry (AMS) and resonance ionization mass spectrometry (RIMS) have been used for sensitive mono- or oligoelemental ultratrace analysis (and precise determination of isotopic ratios) in solid samples. The limits of detection for chemical elements using these mass spectrometric techniques are in the low ng g⁻¹ concentration range. The quantification of the analytical results of mass spectrometric methods is sometimes difficult due to a lack of matrix-fitted multielement standard reference materials (SRMs) for many solid samples. Therefore, owing to the simple quantification procedure of the aqueous solution, inductively coupled plasma mass spectrometry (ICP-MS) is being increasingly used for the characterization of solid samples after sample dissolution. ICP-MS is often combined with special sample introduction equipment (e.g. flow injection, hydride generation, high performance liquid chromatography (HPLC) or electrothermal vaporization) or an off-line matrix separation and enrichment of trace impurities (especially for characterization of high-purity materials and environmental samples) is used in order to improve the detection limits of trace elements. Furthermore, the determination of chemical elements in the trace and ultratrace concentration range is often difficult and can be disturbed through mass interferences of analyte ions by molecular ions at the same nominal mass. By applying double-focusing sector field mass spectrometry at the required mass resolution—by the mass spectrometric separation of molecular ions from the analyte ions—it is often possible to overcome these interference problems. Commercial instrumental equipment, the capability (detection limits, accuracy, precision) and the analytical application fields of mass spectrometric methods for the determination of trace and ultratrace elements and for surface analysis are discussed.     

Review on metal speciation analysis in cerebrospinal fluid-current methods and results: a review

The large number of patients suffering from neurodegenerative diseases like Alzheimer's disease and Parkinson's disease motivates many research groups worldwide to investigate pathogenic factors and molecular mechanisms of these diseases. Recent studies and reviews indicate that metals are involved in these neurodegenerative processes in case their homeostasis in the brain is disturbed. Important is that the focus of these recent studies is on essential metals like Fe, Cu, Zn and Mn, but not on the well-known neurotoxic metals like Hg and Pb. Key issues for understanding metal induced neurotoxic effects are the transport processes across the neural barriers, the metal binding forms (species) and their interactions with neuronal structures. Total metal concentrations in cerebrospinal fluid were published in several studies for controls and patients, but the amount of reliable data sets is not yet sufficient for clear definition of normal and elevated levels. The need for more detailed information on metal species in CSF is highlighted in this review. However, studies on element speciation analysis, that means identification and quantification of the various binding forms of metals in cerebrospinal fluid, are rare. The major reasons therefore are difficulties in accessing cerebrospinal fluid samples, the non-covalent nature of many metal species of interest and their rather low concentrations. In spite of this, several applications demonstrate the potential of hyphenated techniques as additional diagnostic tools for cerebrospinal fluid analysis. This review shows the importance of trace element analysis and more specifically of element speciation in cerebrospinal fluid for an improved understanding of pathologic mechanisms promoting neuro-degeneration. Respective analytical techniques are also highlighted. Additionally, biochemical assays for selected high molecular mass metal species are summarized and critically discussed. Moreover additional potential techniques like direct non-invasive methods as well as mathematical modelling approaches are considered. Data on total concentrations of numerous elements in CSF as well as speciation information of elements such as Al, As, Ca, Cd, Cu, Fe, Mg, Mn, Hg, Pb, Se and Zn in CSF are summarized.     

Open-channel chip-based solid-phase extraction combined with inductively coupled plasma-mass spectrometry for online determination of trace elements in volume-limited saline samples

In this study, we used an automated online chip-based solid-phase extraction (SPE)-inductively coupled plasma-mass spectrometry (ICP-MS) system for analyzing trace elements in small-volume saline samples (～15 μL). The proposed method involved the adsorption of trace metal ions in the interior of a functionalized poly(methyl methacrylate) (PMMA) channel in order to separate these ions from saline matrices. The adsorption of transition metal ions was presumably dominated by the surface complexation between the carboxylate moieties in the interior of the PMMA channel and the metal ions, which facilitated the formation of metal-carboxylate complexes. The components of the proposed online analytical system used for the simultaneous detection of multiple trace metals in saline samples involved microdialysis (MD) sampling, an established chip-based SPE procedure, and ICP-MS. The SPE-ICP-MS hyphenated system was optimized, and then, the analytical reliability of this system was further confirmed by using it to analyze the certified reference materials-SRM 2670 (human urine) and SRM 1643e (artificial saline water). The satisfactory analytical results indicated that the proposed on-chip SPE device could be readily used as an interface for coupling the MD probe with the ICP-MS system. The dramatically reduced consumption of chemicals and "hands-on" manipulations enabled the realization of a simplified and relatively clean procedure with extremely low detection limits in the range of 5.86-76.91 ng L(-1) for detecting Mn, Co, Ni, Cu, and Pb in 15-μL samples by ICP-MS. The effectiveness of an online MD-chip-based SPE-ICP-MS technique for continuous monitoring of trace elements in a simulated biological system was also demonstrated. To the best of our knowledge, this is the first paper to report the direct exploitation of a PMMA chip as an SPE adsorbent for online sample pretreatment and trace metal preconcentration prior to ICP-MS measurement.     

Inductively coupled plasma mass spectrometry in separation techniques: recent trends in phosphorus speciation

Inductively coupled plasma-MS (ICP-MS) and its combined use with molecular mass spectrometric techniques have become the most promising detection techniques in speciation studies. High sensitivity and element specificity of ICP-MS has the advantage of detecting trace amounts of the species of interest in complex matrices. This review is divided into two parts. In the first part, suitable use of ICP-MS either online or offline with currently used separation techniques such as HPLC, CE, and gel electrophoresis in speciation analysis is briefly discussed. In the second part, recent applications (1999-2005) of phosphorus speciation is presented to elucidate the importance of ICP-MS in separation methods and to illustrate its importance in nonmetal detection.     

微波消解-ICP-MS法测定药用明胶空心胶囊中微量重金属元素

采用微波消解技术,建立了一种电感耦合等离子体质谱法测定明胶空心胶囊中Cr,Co,Ni,Cu,Zn,As,Cd,Hg和Pb 9种微量重金属元素的方法。确定了微波消解仪和等离子体质谱仪的最佳操作参数,研究共存离子的干扰和消除方法,选择了各元素的测定同位素,以Ge,Rh和Tl为内标补偿基体效应,建立了样品测定方法。应用拟定的方法测定了不同生产厂家、不同批次的空心胶囊中微量重金属的含量。方法对试样中各元素测定的相对标准偏差为1.5%~14.1%,加标回收率在90.0%~102.0%。结果表明,方法简便、快速、灵敏,满足于空心胶囊中9种重金属元素的测定要求。     

Analysis of trace elements and their physico-chemical forms in natural waters

Trace elements in natural waters can be present in different physico-chemical forms, varying in size, charge and density properties. Knowledge of speciation is essential for understanding the transport, distribution, and biological uptake of trace elements in the environment. The development of techniques to provide reliable information on physico-chemical forms has, therefore, become a challenge within Analytical Chemistry.When selecting analytical methods for the determination of total concentrations or fractions of trace elements in natural waters, no exclusion of species should occur, or at least it must be accounted for. Furthermore, the determination limits must be sufficiently low to allow the actual concentrations to be determined with reasonable precision and accuracy. For very low concentrations, preconcentration techniques are applicable, provided the chemical yield of the spike represents that of the original species present. For methods meeting these criteria, the suitability for routine analysis should be considered.When the physico-chemical forms of trace elements are to be determined, the fractionation should take placein situ or shortly after sampling. As the concentrations involved in speciation studies may be extremely low, there is an increasing awareness of potential sources of errors influencing analytical results. Sample collection and separation/fractionation/concentration procedures prior to analysis are, therefore, essential within Analytical Chemistry, and the whole procedure must be taken into account when interpreting the results. There are, however, several requirements which should be met by techniques applicable for speciation purposes. In general, size fractionation techniques (e.g.in situ hollow fibre ultrafiltration) should be applied prior to the addition of any chemical reagents (charge fractionation techniques).     

Optimisation of plasma techniques for trace analysis of refractory metals

Besides atomic absorption spectrometry, the plasma techniques can be seen as state-of-the-art instrumentation in an industrial laboratory for the analysis of trace elements today. For the analysis of refractory metals, e.g. Mo and W, the determination limits which can be reached by ICP-AES techniques are mainly restricted by the spectral background of the matrix. Advantages and disadvantages of sequential and simultaneous detection as well as different methods of evaluation, such as Kaiman filtering and multiple component spectral fitting, are discussed. The results are compared with trace matrix separation techniques and on-line coupling of ion chromatography with ICP-AES and ICP-MS. Furthermore, the limitations of all techniques with respect to their applicability for routine analysis, especially the complexity of sample preparation, degree of automation, time consumption and cost are shown. With respect to the detection capability, TMS with ICP-MS end determination is the most powerful technique, but for routine analysis simultaneous multielement determination from the matrix is favourable.     

Inductively coupled plasma mass spectrometry and electrospray mass spectrometry for speciation analysis: applications and instrumentation

To gain an understanding of the function, toxicity and distribution of trace elements, it is necessary to determine not only the presence and concentration of the elements of interest, but also their speciation, by identifying and characterizing the compounds within which each is present. For sensitive detection of compounds containing elements of interest, inductively coupled plasma mass spectrometry (ICP-MS) is a popular method, and for identification of compounds via determination of molecular weight, electrospray ionization mass spectrometry (ESI-MS) is gaining increasing use. ICP-MS and ESI-MS, usually coupled to a separation technique such as chromatography or capillary electrophoresis, have already been applied to a large number of research problems in such diverse fields as environmental chemistry, nutritional science, and bioinorganic chemistry, but a great deal of work remains to be completed. Current areas of research to which ICP-MS and ESI-MS have been applied are discussed, and the existing instrumentation used to solve speciation problems is described.     

The potential and challenges of elemental speciation by capillary electrophoresis-inductively coupled plasma mass spectrometry and electrospray or ion spray mass spectrometry

Capillary electrophoresis-inductively coupled plasma mass spectrometry (CE-ICP-MS) and electrospray (ES) or ion spray (IS) mass spectrometry (MS) are recently introduced techniques for elemental speciation. Both techniques have the potential for rapid elemental speciation with low detection limits. Examples of the use of CE-ICP-MS for elemental speciation of positive, neutral and negative species are discussed. Issues in interfacing CE and ICP-MS are considered briefly. The potential advantages and disadvantages of laminar flow in CE-ICP-MS are examined. Potential difficulties in CE-ICP-MS including loss of sample, chemical matrix effects and changes in speciation during separation are discussed. The interpretation of ES or IS-MS spectra and analysis of complex mixtures are considered. Calibration and chemical matrix effects are assessed. Potential pitfalls of interpreting bare metal ion spectra as elemental analysis are discussed. The need for fundamental understanding of the processes that control ES and IS-MS signals is examined. High conductivity samples currently present difficulties for CE-ICP-MS or ES and IS-MS.     

Determination of trace impurities in electronic grade quartz: Comparison of inductively coupled plasma mass spectroscopy with other analytical techniques

An analytical procedure for the determination of uranium and thorium in the sub-ng/g range as well as of other trace elements in the ng/g to g/g range in high purity quartz samples is described. The results obtained by inductively coupled plasma mass spectroscopy (ICP-MS) are compared to those obtained by other analytical techniques (instrumental neutron activation analysis, INAA; flame atomic absorption spectrometry, AAS; Zeeman graphite furnace atomic absorption spectrometry, ZGFAAS; total reflection X-ray fluorescence analysis, TRFA; direct current arc optical emission spectrometry, DC-arc OES; and X-ray fluorescence analysis, XRFA). For the ICP-MS measurements, the decomposition of the samples is carried out with HF/HNO₃/H₂SO₄-mixtures. The results obtained by the different methods show reasonable agreement. For uranium and thorium, ICP-MS proves to be the most sensitive method: detection limits of about 50 pg/g can be achieved for both elements.Presented in part at the 1989 European Winter Conference on Plasma Spectrochemistry, Reutte, Austria     

Multielement trace-element speciation in metal-biomolecules by chromatography coupled with ICP-MS

The state-of-the-art of multi-element speciation in biological materials by hybrid techniques with inductively coupled plasma-mass spectrometric (ICP-MS) detection is critically reviewed with special attention to multi-metal speciation in metallothioneins (MT) and metallothionein-like proteins (MLP). The most common separation techniques used for the purpose include conventional size-exclusion chromatography (SEC) (with off-line detection) and anion-exchange (AE) or reversed-phase (RP) HPLC coupled on-line to ICP-MS. Advantages and limitations of the different commercial element-selective ICP-MS detectors, including the quadrupole (Q), double focusing (DF), and time-of-flight (TOF) mass analysers, are discussed. Procedures reported for speciation analysis, by use of hybrid techniques, for multi-metal speciation in mussel MLP and rabbit liver MT are illustrated with special emphasis on work performed in the authors' laboratory.