Evaluation of different permanent modifiers for the determination of arsenic in environmental samples by electrothermal atomic absorption spectrometry

Single noble metal permanent modifiers such as, Rh, Ir, and Ru, as well as mixed tungsten plus noble metal (W-Rh, W-Ru, W-Ir) permanent modifiers thermally deposited on the integrated platform of transversally heated graphite atomizer were employed for the determination of arsenic in sludges, soils, sediments, coals, ashes and waters by electrothermal atomic absorption spectrometry. Microwave digests of solid samples and water samples were employed for obtaining the analytical characteristics of the methods with different permanent modifiers. The performance of the modifiers for arsenic determination in the real samples depended strongly on the type of permanent modifier chosen. The single noble metal (Rh, Ir and Ru) permanent modifiers were suitable for the analyte determinations in simpler matrices such as waters (recoveries of certified values 95-105%), but the analyte recoveries of certified values in sludges, soils, sediments, coals, and ashes were always lower than 90%. On the other hand, for the determination of arsenic, using W-Rh, W-Ru, and W-Ir permanent modifiers presented recoveries of certified values within 95-105% for all the samples. Long-term stability curves obtained for the determination of arsenic in environmental samples with different permanent modifiers (Rh, Ir, Ru, W-Rh, W-Ir, W-Ru) showed that the improvement in the tube lifetime depends on the tungsten deposit onto the platform. The tungsten plus noble metal permanent modifier presents a tube lifetime of at least 35% longer when compared with single permanent modifier. The results for the determination of As employing different permanent modifiers in the samples were in agreement with the certified reference materials, since no statistical differences were found after applying the paired t-test at the 95% confidence level.     

Feasibility of employing permanent chemical modifiers for the determination of cadmium in coal using slurry sampling electrothermal atomic absorption spectrometry

Iridium and ruthenium, alone and in combination with tungsten, thermally deposited on the platform of a transversely heated graphite tube, were investigated for their suitability as permanent chemical modifiers for the determination of cadmium in coal slurries by electrothermal atomic absorption spectrometry (ET AAS). The conventional mixed palladium and magnesium nitrates (Pd–Mg) modifiers, added in solution, were also investigated for comparison. The latter one showed the best performance for aqueous solutions, and the mixed W–Ir and W–Ru permanent modifiers had the lowest stabilizing power. All of the investigated modifiers lost some of their stabilizing power when coal slurries were investigated. The Pd–Mg modifier, pure Ir and Ru, and a mixture of 300 μg W + 200 μg Ir could stabilize Cd at least to a pyrolysis temperature of 600 °C, whereas all the other combinations already failed at temperatures above 500–550 °C. Additional investigations of the supernatant liquid of the slurries supported the assumption that the high acid concentration of the slurries and/or a concomitant leaching out of the coal might be responsible for the reduced stabilizing power of the modifiers. The maximum applicable pyrolysis temperature of 600 °C was not sufficient to reduce the background absorption to a manageable level in the majority of the coal samples. High-resolution continuum source ET AAS revealed that the continuous background absorption was exceeding values of A = 2, and was overlapping with the analyte signal. Although the latter technique could correct for this background absorption, some analyte was apparently lost with the rapidly vaporizing matrix so that the method could not be considered to be rugged. A characteristic mass of 1.0 pg and a detection limit of 0.6 ng g^− 1 could be obtained under these conditions.     

Determination of cadmium, chromium and lead in marine sediment slurry samples by electrothermal atomic absorption spectrometry using permanent modifiers

A procedure for the determination of cadmium, chromium, and lead in marine sediment slurries by electrothermal atomic absorption spectrometry is proposed. Slurry was prepared by mixing 10 mg of ground sample with particle size smaller than 50 μm completed to the weight of 1.0 g with a 3% nitric acid and 10% hydrogen peroxide solution. The slurry was maintained homogeneous with an aquarium air pump. For cadmium, the best results were obtained using iridium permanent with optimum pyrolysis and atomization temperatures of 400 and 1300 °C, respectively, a characteristic mass, m_o (1% absorption), of 2.3 pg (recommended 1 pg). Without modifier use, zirconium, ruthenium, and rhodium m_o were 3.4, 4.1, 4.6, and 4.8 pg, respectively. For chromium, the most sensitive condition was obtained with zirconium permanent with optimum pyrolysis and atomization temperatures of 1500 and 2500 °C, m_o of 6.6 pg (recommended 5.5 pg); and without modifier use, rhodium, iridium, and ruthenium m_o were 5.3, 8.8, 8.8, and 8.9 pg, respectively. For lead, the best modifier was also zirconium, m_o of 8.3 pg for the optimum pyrolysis and atomization temperatures of 600 and 1400 °C, respectively, (recommended m_o of 9.0 pg). For iridium, ruthenium, without modifier, and rhodium, m_o were 14.7, 15.5, 16.5, and 16.5 pg, respectively. For all the modifiers selected in each case, the peaks were symmetrical with r² higher than 0.99. Being analyzed (n = 10), two marine sediment reference materials (PACS-2 and MESS-2 from NRCC), the determined values, μg l⁻¹, and certified values in brackets, were 2.17 ± 0.05 (2.11 ± 0.15) and 0.25 ± 0.03 (0.24 ± 0.01) for cadmium in PACS-2 and MESS-2, respectively. For chromium in PACS-2 and MESS-2 the values were 94.7 ± 5.6 (90.7 ± 4.6) and 102.3 ± 10.7 (106 ± 8), respectively. Finally, for lead in PACS-2 and MESS-2, the results obtained were 184 ± 7 (183 ± 8) and of 25.2 ± 0.40 (21.9 ± 1.2), respectively. For cadmium and lead in both samples and chromium in PACS-2, calibration was accomplished with aqueous calibration curves. For chromium in MESS-2, only with the standard addition technique results were in agreement with the certified ones. The limits of detection (k = 3, n = 10) obtained with the diluents were 0.1, 3.4, and 3.6 μg l⁻¹ for cadmium, chromium, and lead, respectively.     

Method development for the determination of lead in wine using electrothermal atomic absorption spectrometry comparing platform and filter furnace atomizers and different chemical modifiers

A method has been developed for the determination of lead in wine by electrothermal atomic absorption spectrometry without any sample preparation and calibration against aqueous standards, using 7.5 μg Pd as a chemical modifier. The results obtained for seven wines using the proposed method and an acid digestion procedure did not show any significant difference using a Student's t-test. Atomization in a transversally heated filter atomizer (THFA) was compared with atomization in a conventional transversally heated platform furnace. The former provided a 2.6-fold higher sensitivity, improving the characteristic mass from 34 to 12 pg and a 1.6-fold better limit of detection (0.3 μg L⁻¹ compared to 0.5 μg L⁻¹) for aqueous solutions using the same injection volume of 20 μL. However, the average precision, expressed as the relative standard deviation for the determination of lead in wine under routine conditions was improved from 4.6% with platform atomization to 0.6% in the THFA. The lead content found in seven arbitrarily chosen white and red wines, five from Brazil, one from Chile and one from Spain, ranged from 6 to 60 μg L⁻¹ Pb with an average content of 11.4 μg L⁻¹ Pb for the wines from South America.     

Comparison of action of mixed permanent chemical modifiers for cadmium and lead determination in sediments and soils by slurry sampling graphite furnace atomic absorption spectrometry

Slurry sampling atomic absorption spectrometry with electrothermal atomization was used to the determination of cadmium (Cd) and lead (Pb) in soils and sediments using permanent modifiers. Comparison of action of mixed permanent modifiers niobium (Nb)/iridium (Ir) and tungsten (W)/iridium (Ir) were studied in detail. The effect of amount of Ir, W and Nb on analytical signals of Cd and Pb was examined. The optimal amounts of modifiers for Cd and Pb determination were stated. Niobium carbide formation on graphite surface was studied for different pyrolysis temperatures. Finally for Cd determination in sediments and soils 200 μg of Nb mixed with 5 μg of Ir was used as permanent modifiers and 15 μg of Nb mixed with 200 μg of Ir for Pb determination. Suspensions were prepared in 5% HNO₃. The analytical procedure was optimized carefully basing on data from pyrolysis and atomization curves studies. Ammonium dihydrogen phosphate was used additionally as matrix modifier during Cd determination in samples in order to prevent interferences coming from matrix components. The analysis of CRMs confirmed the reliability of the proposed approach. The precision and accuracy of Cd and Pb determination by the described method for soils and sediments were acceptable.     

Molybdenum, Mo-Ir and Mo-Ru coatings as permanent chemical modifiers for the determination of cadmium and lead in sediments and soil samples by electrothermal atomic absorption spectrometry

Molybdenum, Ir, Ru, Mo-Ir, Mo-Ru thermally coated on to platforms inserted in pyrolytic graphite tubes as permanent modifiers and Pd + Mg(NO₃)₂ conventional modifier mixture have been employed for the determination of cadmium and lead in dissolved sediments and soil samples by electrothermal atomic absorption spectrometry (ETAAS). Optimum masses and mass ratios of permanent modifiers for the analysis of Cd and Pb in sample solutions have been investigated. The 280 μg of Mo, 200 μg of Ir, 200 μg of Ru, 280 μg of Mo + 200 μg of Ir or 280 μg of Mo + 200 μg of Ru has been found as efficient as 5 μg of Pd + 3 μg of Mg(NO₃)₂ for increasing thermal stabilization of analytes and for decreasing the most serious interferences. Pyrolysis and atomization temperatures, atomization and background signal shapes, characteristic masses and detection limits of analytes in dissolved samples with or without permanent and conventional modifiers have been compared. The detection limits and characteristic masses obtained with Mo-Ir coated platform are 0.01 μg g⁻¹ and 1.1 pg for Cd and 0.09 μg g⁻¹ and 19 pg for Pb, respectively. Long-term stabilities for analytes in samples with Mo, Mo-Ir, Mo-Ru and Pd + Mg(NO₃)₂ have been studied. Cadmium and lead contents have been determined in certified and standard reference materials by using optimum conditions investigated and the results obtained with Mo-Ir or Mo-Ru were in agreement with the values of certified reference materials.     

Determination of selenite and selenate in drinking water: a fully automatic on-line separation/pre-concentration system coupled to electrothermal atomic spectrometry with permanent chemical modifiers

A time-based flow injection (FI) separation pre-concentration system coupled to an electrothermal atomic absorption spectrometer (graphite furnace) has been developed for the direct ultra-trace determination of selenite and selenate in drinking water. The pre-concentration of both forms of selenium is carried out onto a micro-column packed with an anionic resin (Dowex 1X8) that is placed in the robotic arm of the autosampling device. Selenite and selenate are sequentially eluted with HCl 0.1 M and HCl 4 M, respectively. The interference of large quantities of chloride during selenium atomisation is prevented by using iridium as a “permanent” chemical modifier. The features of the pre-concentration separation system for both species are: 53% efficiency of retention and an enhancement factor of 82 for a pre-concentration time of 180 s (sample flow rate=3 ml min⁻¹) with HCl elution volumes of 100 μl. The detection limit (3 s) is 10 ng l⁻¹ for the two species and the relative standard deviation (n=10) at the 200 ng l⁻¹ level is 3.5% for selenite and 5.6% for selenate. The addition of selenite and selenate stock standard solutions to tap water samples yields a 97-103% recovery of both species.     

Direct determination of aluminium in serum and urine by electrothermal atomic absorption spectrometry using ruthenium as permanent modifier

Ruthenium (Ru), thermally deposited on a integrated platform graphite furnace, was investigated as a permanent modifier for the determination of Aluminum (Al) in blood serum and urine by electrothermal atomic absorption spectrometry (ETAAS). The platform was treated with 500 μg of Ru as previously described. The pyrolysis and atomization temperatures for each material were of 1300 and 2300 °C, for serum sample and of 1000 and 2400 °C, for urine. The characteristic mass were of 31 and 33 pg for Al in serum sample and urine, respectively (recommended of 31 pg for Al in nitric acid 0.2% (v/v)). For this reason, the calibration was made with aqueous solutions for both the samples. Calibration curves presented r of 0.99145 and 0.99991 for serum and urine, respectively. With the optimized temperatures, being analyzed eight spiked blood serum samples, the recovery was between 95.90 and 113.50%. Two certified urines samples were analyzed with good agreement between experimental and reference values. In both the samples the R.S.D. were <5% (n=3). The detection limit (k=3, n=10) was of 0.40 μg of Al per liter for both the samples. The absorption pulses obtained were symmetrical, with very low background and without interferences. The life time of the tube-platform was higher than 600 cycles of atomizations for both the urine and serum samples.     

Determination of vanadium in mussels by electrothermal atomic absorption spectrometry without chemical modifiers

A method was developed for the quantitative determination of total vanadium concentration in mussels via electrothermal atomic absorption spectrometry (ETAAS). After the microwave digestion of the samples, a program using temperatures of 1600 °C and 2600 °C for ashing and atomization respectively, without any matrix modifiers, allowed us to obtain results that were satisfactory since they agreed closely with certified reference material values. The detection limit was 0.03 mg kg^–1 (dry weight), indicating that the method is suitable for the analysis of mussel samples. This determination was compared with matrix modifiers that have been reported previously. The method was applied to various cultivated and wild mussels from the Galician coast, yielding levels below 1 mg kg^–1 (wet weight).     

Feasibility of using direct determination of cadmium and lead in fresh meat by electrothermal atomic absorption spectrometry for screening purposes

A method for the direct determination of cadmium and lead in fresh meat for screening purposes is proposed using electrothermal atomic absorption spectrometry. The fresh meat samples were homogenized, weighed directly onto solid sampling platforms and introduced into a transversely heated solid sampling graphite tube. The main challenges associated with this procedure, such as weighing errors and optimization of the temperature program were investigated in detail. Calibration was performed against aqueous standards and two modifiers were investigated: 0.05% Pd + 0.03% Mg + 0.05% Triton X-100 and 0.01% Pd + 10% NH₄NO₃ + 0.05% Triton X-100. The former one is recommended due to the higher pyrolysis temperature obtained for cadmium and the better limits of detection of 1.9 μg kg^− 1 for lead and 0.13 μg kg^− 1 for cadmium, based on 10 mg of sample mass. The results obtained for cadmium and lead in two certified reference materials were statistically not different from the certified values on a 95% confidence level, indicating that calibration against aqueous standards is suitable for this application. In order to evaluate weighing errors the fresh samples were dried (at 60 °C) to constant weight; the results obtained with fresh and dried samples were in agreement, taking the loss of weight into consideration for the latter ones. The average relative standard deviation of 14% is in concordance with the results of others using fresh meat. Comparison with the digestion method adopted by the Brazilian Ministry of Agriculture shows no significant differences between the results at the 95% confidence level. This study shows that direct analysis of fresh meet can be applied as a rapid routine screening procedure for residue control in products of animal origin, helping the implementation and maintenance of sanitary control.     

Use of sodium tungstate as a permanent chemical modifier for slurry sampling electrothermal atomic absorption spectrometric determination of indium in soils

A number of chemical modifiers have been assessed for the direct determination of indium in soils using electrothermal atomic absorption spectrometry and slurry sampling. The best results were obtained when the graphite atomizer was impregnated with sodium tungstate, which acts as a permanent chemical modifier. Slurries were prepared by suspending 100 mg sample in a solution containing 1% (v/v) concentrated nitric acid and 10% (v/v) concentrated hydrofluoric acid and then 15-microL aliquots were directly introduced into the atomizer. Standard indium solutions prepared in the suspension medium in the range 4-80 microg L(-1) indium were used for calibration. The relative standard deviation for ten consecutive measurements of a 40 microg L(-1) indium solution was 2.8%. The limit of detection in soils was 0.1 microg g(-1). The reliability of the procedures was confirmed by analysing two standard reference materials and by using an alternative procedure.     

Direct determination of vanadium in high saline produced waters from offshore petroleum exploration by electrothermal atomic absorption spectrometry

The present work reports the development of a methodology for the direct determination of vanadium in high saline waters derived from offshore petroleum exploration employing electrothermal atomic absorption spectrometry. Such waters, usually called produced waters, present complex composition containing various organic and inorganic substances. In order to attain best conditions (highest sensitivity besides lowest background) for the methodology, studies about the effects of several variables (evaluation of pyrolysis and atomization temperatures, type of chemical modifier, concentration of modifier and pyrolysis time) and the convenient calibration strategy were performed. Best conditions were reached with the addition of 10 μg of NH₄H₂PO₄ as chemical modifier employing pyrolysis (during 10 s) and atomization temperatures of 1500 and 2700 °C, respectively. Obtained results indicated that, in this kind of sample, vanadium can be determined by standard addition method or employing an external calibration approach with standard solutions prepared in 0.8 mol l⁻¹ NaCl medium. In order to evaluate possible matrix interferences, a recovery test was performed with five spiked samples of produced waters. The limit of detection, limit of quantification and relative standard deviation in 0.8 mol l⁻¹ NaCl medium were also calculated and the derived values were 1.9 μg l⁻¹, 6.3 μg l⁻¹ and 5.6% (at 10 μg l⁻¹ level), respectively.     

Direct determination of lead in produced waters from petroleum exploration by electrothermal atomic absorption spectrometry X-ray fluorescence using Ir-W permanent modifier combined with hydrofluoric acid

The present work reports the development of a methodology for the direct determination of lead in high saline waters derived from petroleum exploration employing electrothermal atomic absorption spectrometry with permanent Ir-W and HF as modifiers. These waters, so-called produced waters, have complex composition containing several types of organic and inorganic substances. In order to attain best conditions (highest analytical signal besides lowest background) for the methodology studies about the effect of several variables and the convenient calibration strategy were performed. Also, the efficiency of other modification approaches was evaluated. At best conditions, pyrolysis and atomization temperature were 800 and 2200 °C, respectively, when the modifiers cited above were utilized. Obtained results indicate that, in this kind of sample, lead can be determined by standard addition method or employing external calibration with standard solutions prepared in 0.8 mol l⁻¹ NaCl medium. In order to evaluate the accuracy of the procedure, a recovery test was performed with six spiked samples of produced waters. The detection limit, quantification limit and the relative standard deviation in 0.8 mol l⁻¹ NaCl were also calculated and the values are 1.5 μg l⁻¹, 5.0 μg l⁻¹ and 5.0% (at 10 μg l⁻¹ level), respectively.     

Lead ultra-trace on-line preconcentration and determination using selective solid phase extraction and electrothermal atomic absorption spectrometry: applications in seawaters and biological samples

In this work, a new chelating resin [1,5-bis (2-pyridyl)-3-sulphophenyl methylene] thiocarbonohydrazide immobilised on aminopropyl-controlled pore glass (550 A; PSTH-cpg) was synthesised and packed in a microcolumn which replaced the sample tip of the autosampler arm. The system was applied to the preconcentration of lead. When microliters of 10% HNO3, which acts as elution agent, pass through the microcolumn, the preconcentrated Pb(II) is eluted and directly deposited in a tungsten-rhodium coated graphite tube. With the use of the separation and preconcentration step and the permanent modifiers, the analytical characteristics of the technique were improved. The proposed method has a linear calibration range from 0.012 to 10 ng ml(-1) of lead. At a sample frequency of 36 h(-1) with a 90 s preconcentration time, the enrichment factor was 20.5, the detection and determination limits were 0.012 and 0.14 ng ml(-1), respectively and the precision, expressed as relative standard deviation, was 3.2% (at 1 ng ml(-1)). Results from the determination of Pb in biological certified reference materials were in agreement with the certified values. Seawaters and other biological samples were analysed too.     

Study on different pre-treatment procedures for metal determination in Orujo spirit samples by ICP-AES

In this work several pre-treatment methods were studied for metal (Na, K, Mg, Cu and Ca) determination in Orujo spirit samples using inductively coupled plasma atomic emission spectrometry (ICP-AES). Dilution, digestion, evaporation, and cryogenic desolvatation techniques were comparatively evaluated. Because of their analytical characteristics, digestion and evaporation with nitrogen current were found to be appropriate procedures for the determination of metals in alcoholic spirit samples. Yet, if simplicity and application time are to be considered, the latter—evaporation in a water bath with a nitrogen current—stands out as the optimum procedure for any further determinations in Orujo samples by ICP-AES. Low detection levels and wide linear ranges (sufficient to determine these metals in the samples studied) were achieved for each metal. The recoveries (in the 97.5-100.5% range) and the precision (R.S.D. lower than 5.6%) obtained were also satisfactory. The selected procedure was applied to determine the content of metals in 80 representative Galician Orujo spirit samples with and without a Certified Brand of Origin (CBO) which had been produced using different distillation systems. The metal concentrations ranged between 0.37 and 79.7 mg L⁻¹ for Na, <LOD to 12.4 mg L⁻¹ for K, 0.02-4.83 mg L⁻¹ for Mg content, <LOD to 37.3 mg L⁻¹ for Cu and 0.03-13.10 mg L⁻¹ for Ca.     

Dynamic ultrasound-assisted extraction of cadmium and lead from plants prior to electrothermal atomic absorption spectrometry

A dynamic ultrasound-assisted extraction step is proposed for the quantitative extraction of Cd and Pb from plant leaves prior to determination by electrothermal atomic absorption spectrometry (ETAAS). Beech leaves (a certified reference material—CRM 100—where the target analytes were not certified) were used for optimizing the extraction step by a multivariate approach. The samples (0.25 g) were subjected to dynamic ultrasound-assisted extraction with dilute nitric acid as extractant. The method was validated with a certified reference material BCR-062 (olive leaves) where both Cd and Pb are certified. The good agreement between the certified values and those found using the proposed method demonstrates its usefulness. The repeatability was 2.0 and 0.9% and the within-laboratory reproducibility was 7.1 and 2.8% for Cd and Pb, respectively. The precision of the method, together with its efficiency, rapidity, low cost and environmental acceptability makes it a good alternative for the determination of trace metals from plant material.     

Direct determination of cadmium in urine by electrothermal atomic absorption spectrometry after in situ electrodeposition

Determination of cadmium in urine by ETAAS suffers from severe interferences deteriorating the precision and accuracy of the analysis. Electrodeposition step prior to ETAAS allows to avoid interferences and makes cadmium determination possible even at ultratrace levels. The proposed procedures involve electrolytic deposition of cadmium from acidified urine on previously electrolytically deposited palladium film on a graphite atomizer tube, followed by removal of residual solution, pyrolysis and atomization. Both electrodeposition processes take place in a drop of the respective solution (palladium nitrate modifier and acidified urine, respectively), when Pt/Ir dosing capillary serves as an anode and the graphite tube represents a cathode. The voltage is held at −3.0 V. Matrix removal is then accomplished by withdrawal of the depleted sample solution from the tube (procedure A) or the same but followed by rinsing of the deposit with 0.2 mol l⁻¹ HNO₃ (procedure B). The accuracy of both procedures was verified by recovery test. Detection limits 0.025 and 0.030 μg Cd/l of urine were achieved for A and B procedures, respectively. Both procedures are time consuming. The measurement cycle represents 5 and 7 min for A and B procedures, respectively.     

A technique coupling the analyte electrodeposition followed by in-situ stripping with electrothermal atomic absorption spectrometry for analysis of samples with high NaCl contents

A technique coupling the analyte electrodeposition followed by in-situ stripping with electrothermal atomic absorption spectrometry has been developed for determination of lead and cadmium in samples with high salt contents. To separate the analyte from the sample matrix, the analyte was in-situ quantitatively electrodeposited on a platinum sampling capillary serving as the cathode (sample volume, 20 μL). The spent electrolyte containing the sample matrix was then withdrawn, the capillary with the analyte deposited was washed with deionized water and the analyte was stripped into a chemically simple electrolyte (5 g/L NH₄H₂PO₄) by reversing the polarity of the electrodeposition circuit. Electrothermal atomization using a suitable optimized temperature program followed.     

Determination of cadmium in slurries of marine sediment samples by electrothermal atomic absorption spectrometry using palladium and phosphate as chemical modifiers

A method for the determination of cadmium in slurries of marine sediment using palladium and phosphate as chemical modifier has been optimized. To stabilize the marine sediment slurry, Triton X-100 at 0.1% was used. To obtain a complete pyrolysis of the slurry sample two mineralization steps were used, the first at 480 °C and the second at 600 °C and 700 °C for phosphate and palladium, respectively. The precision and accuracy of the method have been studied by analyzing the Reference Material PACS-1 (marine sediment) of National Research Council Canada. The detection limits (LOD) were 11.9 g kg^–1 for phosphate and 42.0 g kg^–1 when palladium was used. These methods have been applied to the determination of cadmium in marine sediment samples from the Galicia coast and the results of both methods were compared; no significant differences were found between the two procedures.     

电热原子吸收光谱法测定恐龙化石及其围岩中的铅

电热原子吸收光谱分析法(ETAAS)灵敏度高、操作简便，是测定痕量铅的常用分析方法之一。铅及其化合物易挥发，在热解预处理阶段损失严重。对于某些样品，基体干扰较为复杂，需要选择合适的化学改进剂，如硝酸镁、磷酸、硫脲、磷酸氢二铵、磷酸二氢铵、钯盐等。现有许多测定铅