Microwave Digestion System Research Articles

Hydride generation interface for the determination of inorganic arsenic and organoarsenic by inductively coupled plasma mass spectrometry using open-focused microwave digestion to enhance thepre-reduction process

An on-line interface was developed for ICP-MS based on hydride generation. The novelty of the approach resides in the use of an open-focused microwave digestion system to enhance the kinetics of a pre-reduction to elemental form, which is carried out using 2% v/v l-cysteine–0.2 mol dm–3 HNO3. This interface was characterized using flow injection to simulate transient HPLC peaks. It allows the total conversion of AsV, monomethylarsonic acid and arsenobetaine to AsIII and then to a volatile hydride within a few seconds. The conditions are also applicable to the determination of Ge. Using only 20 µl injections, improvements in sensitivity by a factor of 19 and 10 for As and Ge, respectively, were achieved compared with those obtained by continuous nebulization. The detection limit for both elements (independent of species) was also an order of magnitude lower than that achieved by continuous nebulization, down to 0.06 and 0.08 ng cm–3 for As and Ge, respectively.

Mainstream Smoke Collection by Electrostatic Precipitation for Acid Dissolution in a Microwave Digestion System Prior to Trace Metal Determination

Abstract A method is described for collection and preparation of mainstream smoke condensate from cigarettes for determination of trace metal constituents. A sample of mainstream smoke was collected from 300 cigarettes in a final volume of 25 mL. Extremely clean laboratory conditions during sample preparation were essential to avoid contamination and improve precision and accuracy for trace element determination. The clean laboratories maintained a positive pressure of HEPA-filtered air with constant temperature and humidity control. Mainstream cigarette smoke was collected by electrostatic precipitation (EP) on a modified rotary smoking machine located in a wooden CORESTA cabinet. The EP unit was equipped with a high tension generator supplying 17.5 kV to a tungsten electrode followed by 2 secondary acid traps. Samples were collected under 2 smoking regimens: FTC (35 mL puff volumes, one puff every 60 s, and puff duration 2 s) and “Extreme” (75 mL puff volumes, one puff every 35 s, puff duration 2 s). Condensate was extracted from collection tubes with methanol into Teflon microwave digestion vessels and evaporated in an environmental evaporation chamber (EEC). The EEC provided an ultraclean environment for sample evaporation and predigestion steps that eliminated most of the sample matrix. The remainder was decomposed in a programmable closed vessel microwave digestion system. Digested sample solutions were analyzed by graphite furnace atomic absorption spectrometry and inductively coupled plasma optical emission spectrometry for aluminum, arsenic, cadmium, chromium, nickel, and silicon.

Direct analysis of solid samples by graphite furnace atomic absorption spectrometry with a transversely heated graphite atomizer and D 2 -background correction system (SS GF-AAS)

An instrumental design for the analysis of heavy metals in solid samples (certified reference materials, river sediments, cements) by direct graphite furnace atomic absorption spectroscopy (SSGF-AAS) is presented and outlined. A high performance atomic absorption spectrometer with a D2-background corrector and a transversely heated graphite atomizer was modified. Solid sampling was achieved by a mechanical module combined with an ultra microbalance. Cadmium, lead and chromium in different samples (sample weights between 20–400 μg) have been determined in the μg/g- to ng/g-range with the graphite-platform technique. Different ways of calibration and inhomogeneity problems are discussed. The precision of SSGF-AAS is equal to the precision of the conventional AAS-technique with chemical digestion (microwave digestion system, Zeeman-AAS). It is shown that SSGF-AAS is a reasonable alternative to the conventional technique for selected analytical problems.

Determination of Major Minerals in Dairy Products Digested in Closed Vessels Using Microwave Heating

A method is described for digestion of a variety of solid dairy products (cheeses, caseinates, and milk powders) for subsequent determination of Ca, Mg, Na, and K by flame atomic spectrophotometry. Sample preparation included a microwave digestion system in closed vessels. The procedure has been validated for skim milk powder employing various reaction mixtures that contain HNO3, HClO4, and H2O2. Accurate results were achieved for Na and K with mixtures of HNO3 and H2O2, but values for Ca and Mg were low. Although the results of these elements were improved when HClO4 was used instead of HNO3, greatest accuracy was obtained by measurement, using standard addition, of samples digested with mixtures of HNO3 and H2O2. The assays exhibited varying degrees of precision when used for caseinates and cheese. The coefficients of variation for replicate analyses of caseinates and cheeses were lowest for Mg (1.34 to 2.46%) and Ca (1.15 to 2.69%) and poorest for Na (3.35 to 9.02%). However, accuracy was greater for Na and K analyses; recoveries of these elements were very close to 100% (Na, 96.3 to 99.2%; K, 97.8 to 99.8%) but recoveries of Ca and Mg were lower (Ca, 91.6 to 98.8%; Mg, 94.0 to 98.4%).

Microwave digestion and matrix separation for the determination of cadmium in urine samples by electrothermal atomization atomic absorption spectrometry using a fast temperature program

A method is proposed which involves sample pretreatment followed by electrothermal atomic absorption spectrometry (ETAAS) for determination of cadmium in human urine. A microwave digestion system was devised to accommodate double-closed vessels for simultaneous digestion of batches of up to 24 urine samples in about 20 min. After digestion, matrix substances which might interfere were removed using silica-immobilized 8-hydroxyquinoline (I-8HOQ) columns. The analyte adsorbed on the column was then eluted with dilute nitric acid solution and determined by ETAAS using a fast temperature program. Neither ashing steps in the furnace heating program nor use of matrix modifiers was necessary. The accuracy, precision, limit of detection, and sample throughput of the method were evaluated. With meticulous control of systematic errors which may be introduced in the pretreatment procedures, the present method can serve as a reference technique for the analysis of Cd in urine samples.

Determination of Phosphorus Content in Steels Using Flow Injection into an Argon-Water Carrier for Inductively Coupled Plasma-Mass Spectrometry.

This paper describes a methodology for the determination of phosphorus in steels using argon-water carrier, in flow injection with Inductively Coupled Plasma Mass Spectrometry. 31P is a monoisotopic element and its determination, at very low levels, is very difficult due to contamination problems and interference inherent in the plasma and solvents. These unduly raise the background being necessary a sample pretreatment step to achieve the required contents. To dissolve the samples in this study, a microwave digestion system was applied to minimize the sample preparation time, the quantities of reagents and the risk of contamination. A mercury cathode electrolysis was used for performing a rapid matrix removal/analyte enrichment. The use of an argon-water stream as carrier yielded an enhancement in sensitivity and the detection limit was improved by a factor of approximately 2.5 when compared with that experienced using flow injection into an all-water carrier. Optimization of the operating conditions for the argon-water flow injection and for Inductively Coupled Plasma Mass Spectrometer, were selected with respect to the peak height sensitivity and were found to be dependent on the nebulizer gas flow-rate, the radio frequency power, the sample injection volume and the carrier flow-rate. The detection limit based on a sample rate of 0.02 gm/-1 was 0.4 μg·g-1 and the relative standard deviations at 100 and 10 μg·g-1 P solution were 2.0 and 15 % respectively. Data are presented for 3 Standard Reference Materials: JSS 003-2, (from Japanese Iron and Steel Certified Reference Materials); and ECRMs 285-1 and 278-1 (from Bureau of Analysed Samples LTD).

Determination of Cd, Cu, Zn and Pb in Rice Flour Reference Materials by Isotope Dilution Inductively Coupled Plasma Mass Spectrometry.

Isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) has been applied for the determination of Cd, Cu, Zn and Pb in KRISS B rice flour reference material, which was prepared using the unpolished rice collected from Nonsan, Korea. The ICP-MS instrument used for this work was a laboratory-built unit. The dissolution of rice flour sample was carried out by using a microwave digestion system. The concentrations of Pb, Cd, Cu and Zn in the KRISS B reference material were found to be 29.9 ng/g, 33.9 ng/g, 3.00 μg/g and 22.6 μg/g, respectively. In order to validate the analytical results of KRISS B sample, we also analyzed NIES 10-a and NIST 1568 reference materials by using the present method. The analysis results for Cd, Cu and Zn obtained in this work were in good agreement with their certified or recommended values within the limits of uncertainty.

Determination of mercury in dry-fish samples by microwave digestion and flow injection analysis system cold vapor atomic absorption spectrometry

Flow injection analysis system cold vapor atomic absorption spectrometry (FIAS-CV-AAS) preceded by a wet digestion in a microwave oven, as a method for measuring mercury in fish was studied. The digestion process and conditions of the FIAS (carrier concentration: HCl 3% v v ; reducing agent: SnCl 2 2% w v : filling and injection times: 8 and 25 min, respectively; and sample volumes) were optimized. The analytical parameters of the proposed method (detection limit = 7.7 ng/g; precision intraassay = 6.7%; interassay = 14.0%) demonstrates its adequacy and are similar to the ones (detection limit = 19.4 ng/g; precision intraassay = 11.2%; interassay = 15.9%) obtained using a conventional wet digestion method with HNO 3 + H 2SO 4 + V 2O 5. The accuracy are verified with reference material DORM-1 (dog fish-muscle) NAC-CNRC. Moreover, with the optimized method the risk of losses and contamination is low and it is less time consuming and requires smaller reagent volumes than conventional method.

Determination of Total Mercury in Sediments by Microwave-assisted Digestion-Flow Injection-Inductively Coupled Plasma Mass Spectrometry

A method for the determination of total mercury in sediment samples was developed. Extraction of mercury from a sample matrix was carried out in an open vessel microwave digestion system while maintaining mild conditions during digestion in order to avoid any loss of mercury due to volatilization. A complexing agent (EDTA) and a surfactant (Triton X-100) were added to the samples in order to eliminate memory effects associated with mercury determinations and to obtain reproducible linear calibration curves. Standard additions and internal standardization were used for calibration and correction in an FI–ICP-MS detection system. The method was validated using the certified reference material PACS-1 and the reference materials IAEA-356 and S19 and gave results in good agreement with the certified and reference values. Sediment samples from Arcachon Bay were also analysed. Detection limits of 10 ng g -1 for solution and 1 ng g -1 for dry sediment samples were obtained.

Analysis of standard reference materials following digestion using a modified appliance grade microwave oven

Microwave digestion provides a rapid means of sample preparation in the analytical laboratory. The major disadvantage of this method is the high cost of commercial microwave digestion systems. Modifications to the magnetron timing circuits of an appliance grade oven to make it suitable for sealed vessel microwave digestion are reported. The oven was modified without alteration to the irradiation cavity, and all initial safety features remain intact. Following digestion with the modified oven, NIST Standard Reference Materials (SRMs) were analyzed by inductively coupled plasma emission spectroscopy. The analytical data are similar to the certified concentrations in the SRMs, indicating that the modified oven provides a durable, rapid, cost-effective means of sample preparation.

Microwave digestion of biological samples with tetramethylammonium hydroxide and ethylenediaminetetraacetic acid for element determination

A microwave-digestion system with a closed PTFE vessel was used to improve the leaching of inorganic constituents from biological samples with tetramethylammonium hydroxide (TMAH) and ethylenediaminetetraacetic acid (EDTA). The effects of microwave parameter settings and the quantities of TMAH and EDTA used on leaching efficiency were evaluated. This new digestion method has been applied to the standard reference materials NIST SRM 1577 B Bovine Liver 1515 Apple Leaves and NIES CRM No. 1 Pepperbush, No. 3 Chorella, No. 6 Mussel and No. 7 Tea Leaves. The major and minor elements in the digests were analyzed by flame atomic absorption spectrometry or graphite furnace atomic absorption spectrometry. Good agreement of the analytical results with the certified values was obtained.

Microwave digestion methods for the determination of trace elements in brain and liver samples by inductively coupled plasma mass spectrometry.

Two microwave digestion systems (open-focused and closed-pressurized) were tested for the mineralization of human brain and bovine liver (NIST SRM 1577a) as dissolution steps prior to the determination of 16 trace elements (Bi, Cd, Co, Cs, Cu, Fe, Hg, Mn, Mo, Pb, Rb, Sb, Sn, Sr, Tl, and Zn) by inductively coupled plasma mass spectrometry (ICP-MS). Digestion parameters (mass of sample, digestion mixture, and power/time steps) were optimized using temperature and pressure sensors. Digestions with the open-focused microwave system require larger volumes of conc. HNO(3) and 30% H(2)O(2) than digestions with the closed-pressurized system. Both systems produce correct results for the bovine liver samples. The concentrations obtained for the digests of the open-focused system tend to be less precise than the concentrations from the "closed-pressurized" digests. Because the "open-focused" digests must be diluted to 50 mL to bring the acid concentration to 0.7-2.0 mol/L required by the ICP-MS (closed-pressurized digests need to be diluted to only 20 mL), the detection limits for the system with the open-focused digestion are higher than for the system with the closed-pressurized digestor. The open-focused digestor cannot handle more than 150 mg brain tissue, whereas the closed-pressurized system can mineralize 470 mg. The latter method gave better results with brain tissue than the open-focused system. The preparation of brain tissue as reference material for the determination of trace elements in brain samples is described.

Fast Analysis of Lysine in Food Using Protein Microwave Hydrolysis and an Electrochemical Biosensor

Abstract A fast procedure for lysine analysis in food was developed by coupling in sequence a microwave protein hydrolysis technique with a lysine enzyme electrode. Protein hydrolysis was carried out in 6N HCl using sealed vessels located in a microwave digestion system. Parameters such as irradiation power, pressure, time and temperature were varied to select the best conditions for the hydrolysis. The hydrolysate was then analyzed for lysine using an electrochemical biosensor based on an amperometric H2O2 transducer and the enzyme lysine oxidase covalently immobilized on a preactivated polymer support. Analytical parameters, such as pH, buffer and measurement system were studied in order to eliminate or minimize the enzymatic interferences. Bovine serum albumin and two farro (T. dicoccum) meals (whole and sieved) were processed for lysine content. The analysis was carried out with this new procedure (microwave hydrolysis + biosensor) and by traditional hydrolysis coupled with ion-exchange chromatography (IEC). Results obtained with the two procedures correlated well.

Microwave dissolution of plant tissue and the subsequent determination of trace lanthanide and actinide elements by inductively coupled plasma-mass spectrometry

Recently there has been much concern about the ability of plants to uptake heavy metals from their surroundings. With the development of instrumental techniques with low detection limits, such as inductively coupled plasma-mass spectrometry (ICP-MS), attention is shifting toward achieving faster and more elegant ways of oxidizing the organic material inherent in environmental samples. Closed-vessel microwave dissolution was compared with conventional methods for the determination of concentrations of cerium, samarium, europium, terbium, uranium, and thorium in a series of samples from the National Institute of Standards and Technology and from fields in Idaho. The ICP-MS technique exhibited detection limits in parts-per-trillion and linear calibration plots over three orders of magnitude for the elements under study. The results obtained by using nitric acid and hydrogen peroxide in a microwave digestion system for the analysis of reference materials showed close agreement with the accepted values. These values were compared with results obtained from dry- and wet-ashing procedures. The findings from an experiment comparing radiometric techniques for the determination of actinide elements to ICP-MS are reported.

Study of the application of air-water flow injection inductively coupled plasma mass spectrometry for the determination of calcium in steels

This paper describes a methodology for the determination of Ca, an important element in the process of steel making. Determination of Ca is typically problematic when using ICP-MS because of the 40Ar interference on the most abundant Ca isotope. In the present study the use of the 44Ca isotope for ICP-MS measurement was considered. To dissolve the samples, a microwave digestion system was applied to guarantee the total dissolution of the Ca compounds, thus minimizing the sample preparation time, the amounts of reagents used and the risk of contamination. Carbon and Si, normally present in steel samples, were eliminated by treatment with H2SO4 and HF to avoid 12C16O2 and 28Si16O isobaric interferences on the 44Ca isotope. After matrix removal by the use of mercury cathode electrolysis, air–water FI-ICP-MS was used for the determination of Ca. Scandium, with an ionization potential and mass number close to those of Ca, was used as an internal standard for quantitative measurements. The operating parameters for air-water FI-ICP-MS were optimized with respect to signal peak height intensities, stability and matrix effect. The detection limit obtained was 0.6 µg g–1 and the RSD value, calculated from ten replicates of a 200 µl injection of a 50 µg l–1 Ca solution spiked with 2 µg l–1 of Sc, was 0.7%. Data are presented for four certified reference steel materials: JSS 169–5 and 171–5, from Japanese Standards of Iron and Steel; and ECRMs 195–1 (Cr–Ni–steel) and 096 (low-S, low-Ca steel), from the Bureau of Analysed Samples.

Accurate and precise measurements of lead in bone using electrothermal atomic absorption spectrometry with Zeeman-effect background correction

A simple method for measuring trace levels (µg g–1) of Pb in bone by electrothermal atomic absorption spectrometry (ETAAS) is described. ETAAS instrumentation equipped with a transverse Zeeman-effect background correction system was used to investigate the effects of bone matrix on Pb atomization. It is shown that Pb can be accurately measured with good precision using aqueous Pb standards containing NH4H2PO4 modifier and Ca(NO3)2. Alternatively, Mg(NO3)2 works as well as Ca(NO3)2. For rapid and contamination-free sample preparation, bone was digested in concentrated HNO3 using a closed-vessel microwave digestion system. This approach was compared with digestion at room temperature over 48 h. The detection limit (3s) is 0.6 µg g–1 dry mass. Typical day-to-day precision is < 5%. Method accuracy is established as better than 1% using NIST SRM 1400 Bone Ash and SRM 1486 Bone Meal, which are the only reference materials we have found for bone-Pb measurements. The method was used to analyse various bone specimens (e.g., tibia, femur, rib and skull) from Pb-dosed animals. Results indicate that Pb is heterogeneously distributed within bone.

Determination of mercury in slurried samples by both batch and on-line microwave digestion–cold vapour atomic fluorescence spectrometry

An on-line microwave digestion system for the determination of total mercury in solid environmental samples was developed. The slurried samples were injected into a carrier stream of hydrochloric acid and mixed with a solution of potassium bromide–bromate before passing through a PTFE coil situated in the microwave cavity of the microwave digester. During the microwave digestion process, the mercury species were liberated from the sample matrix and full oxidation of organomercury compounds to HgII was achieved. After mixing with a solution of hydroxylammonium chloride, to remove the excess of bromine, the determination of mercury was undertaken by on-line cold vapour atomic fluorescence spectrometry. The developed system offers a rapid and sensitive method for the determination of mercury in solid environmental samples with a sample processing time of 50 s and a limit of detection of 13 ng l–1. In addition, a batch method utilizing open focused microwave digestion, in which samples were digested with a mixture of hydrogen peroxide and nitric and sulfuric acids, was developed. Both methods were validated using the National Research Council Canada certified reference materials DORM-2 and PACS-1 and gave results in good agreement with the certified values.

Rapid and reliable method for the determination of aluminium in bone by electrothermal atomic absorption spectrometry

A reliable method for the determination of aluminium in bone by electrothermal atomic absorption spectrometry (ETAAS) is described. Bone samples were digested in concentrated HNO3 using a closed-vessel microwave digestion system. Accurate and precise results were obtained by calibration against aqueous Al standards prepared in solutions containing 1% v/v HNO3 and 1.0 g l-1 Ca, as Ca(NO3)2, as modifier. No modifier was needed for the bone digestate. A method detection limit (3 sigma) of 0.023 microgram g-1 for bone was obtained. The between-day precision for bone digestate was about 7% at a mean concentration of 10 micrograms l-1. The method was validated against the NIST standard reference materials SRM 1486 Bone Meal and SRM 1577b Bovine Liver. To help confirm the concept of the Al in bone method, essentially the same method, with some minor modifications, was used for the determination of Al in serum and dialysis fluid. The method performance was monitored by participation in two international interlaboratory comparison programmes for serum and dialysis fluid Al available from Canada (CHUL, Québec) and the UK (Robens Institute). The performance over a 10 month period was excellent for both serum and dialysis fluid proficiency test samples distributed by these programmes.

Comparison of open and closed focussed microwave digestions in view of total mercury determination by cold vapour atomic absorption spectrometry

Food samples digestion, in view to quantify total mercury, is the first step to perform before measuring mercury by cold vapour atomic absorption spectrometry (CVAAS). We have compared two microwave digestion systems, one working at atmospheric pressure (open system) and the other one under pressure (closed system). Results obtained for fish muscle samples by the two methods are in good agreement. However, fat is not digested in the open system, whereas in the closed system no more fat remains in the final solution. During the quantification step, applying the CVAAS technique, we have noticed that the choice and the concentration of the reductant solution is very important to obtain good results. Elaborated methods have been validated using certified reference materials.

Evaluation of the analytical performance of inductively coupled plasma mass spectrometry for the simultaneous determination of major and minor elements in basic slags

The application of inductively coupled plasma mass spectrometry to the analysis of basic slags has been studied. A conventional dissolution-fusion procedure and a microwave digestion system were used for sample dissolution. Suitable selection of the analyte isotopes and the use of appropriate instrument settings and of internal standardization makes it possible to determine the major and minor elements in the same test sample dilution. Use of the omnirange device and low-abundance isotopes for the determination of the major elements is evaluated. The influence of the attack reagents is tested and the interferences caused by polyatomic ions are studied and corrected by applying elementary mathematical equations. For the major and minor elements considered, precision was found to be better than 1% (RSD). Results are presented for three basic slag reference materials and the agreement between the certified and found values shows the capacity of the method accurately to determine elemental concentrations in basic slags.