Fast gas chromatography for pesticide residues analysis using analyte protectants

Fast GC-MS with narrow-bore columns combined with effective sample preparation technique (QuEChERS method) was used for evaluation of various calibration approaches in pesticide residues analysis. In order to compare the performance of analyte protectants (APs) with matrix-matched standards calibration curves of selected pesticides were searched in terms of linearity of responses, repeatability of measurements and reached limit of quantifications utilizing the following calibration standards in the concentration range 1-500 ng mL(-1)(the equivalent sample concentration 1-500 microg kg(-1)): in neat solvent (acetonitrile) with/without addition of APs, matrix-matched standards with/without addition of APs. For APs results are in a good agreement with matrix-matched standards. To evaluate errors of determination of concentration synthetic samples at concentration level of pesticides 50 ng mL(-1) (50 microg kg(-1)) were analyzed and quantified using the above given standards. For less troublesome pesticides very good estimation of concentration was obtained utilizing APs, while for more troublesome pesticides such as methidathion, malathion, phosalone and deltamethrin significant overestimation reaching up to 80% occurred. According to presented results APs can be advantegously used for "easy" pesticides determination. For "difficult" pesticides an alternative calibration approach is required for samples potentially violating MRLs. An example of real sample measurement is shown. In this paper also the use of internal standards (triphenylphosphate (TPP) and heptachlor (HEPT)) for peak areas normalization is discussed in terms of repeatability of measurements and quantitative data obtained. TPP normalization provided slightly better results than the use of absolute peak areas measurements on the contrary to HEPT.     

Lead determination in whole blood by laser ablation coupled with inductively coupled plasma mass spectrometry

This work describes a simple procedure for blood lead level determination. The proposed method requires little sample pretreatment and subsequent direct analysis of a dried blood spot on a filter membrane using laser ablation coupled with inductively coupled plasma mass spectrometry (LA-ICP-MS). In general, LA-ICP-MS studies are somewhat limited by the lack of matrix-matched standards for calibration purposes. Here we describe aqueous standard calibration and matrix-matched calibration methods. This method was validated by analysis of the reference materials. With the matrix-matched calibration method, the recovery ranged from 97.8% to 112.8%, while the aqueous standard calibration method ranged 90.4% to 122.4%. The lower detection limit was estimated as 0.1 ng mL⁻¹. The determination precision, expressed as the relative standard deviation (RSD), was not worse than 10% for all results. A sample throughput of approximately 5 min per sample made it possible to rapidly screen a large number of samples.     

Alternative calibration approaches to compensate the effect of co-extracted matrix components in liquid chromatography-electrospray ionisation tandem mass spectrometry analysis of pesticide residues in plant materials

The aim of this study was to evaluate the applicability of different calibration approaches in a multi- and single-residue analysis of modern pesticides in plant matrices using liquid chromatography-electrospray mass spectrometry (HPLC-ESI-MS). In the first set of experiments the determination of eight pesticides representing different groups of polar/unstable pesticides (carbamates, benzimidazoles, azoles, benzoylphenylurea) in apple samples was performed. The trueness and precision of data obtained by using: (i) external solvent standard calibration, (ii) external matrix-matched standard calibration and (iii) echo-peak internal standard calibration was compared. The last mentioned method is a novel technique providing the possibility to inject internal standard of the same identity as a target analyte, so that its retention time is close to the analyte from the sample. According to expectation, when using external standard solvent calibration the results were under- or overestimated due to suppression or enhancement of analyte's signal by matrix components. On the other hand with the use of matrix-matched calibration accurate data were obtained. With echo-peak technique accurate results comparable to those obtained by matrix calibration were obtained for six out of eight pesticides. In the second set of experiment we used the echo-peak technique to overcome the problem with the response instability in the analysis of chlormequat in pear concentrate samples. As an internal standard method the echo-peak technique provided the possibility of monitoring of signal decrease during the analytical sequence and to compensate this decrease by relating sample peak area relatively to this internal standard.     

Effect of sample matrix on the determination of total petroleum hydrocarbons (TPH) in soil by gas chromatography–flame ionization detection

Research papers in different fields of analytics indicate that the effect of matrix-induced chromatographic response enhancement (matrix effect) is a commonly encountered problem in gas chromatography applications. In this paper, an example of the effect of sample matrix on the quantitative determination of total petroleum hydrocarbons (TPH) by GC–FID in soil is presented. Two types of soil were selected for the evaluation. Extraction and analysis of the soil samples was in accordance with CEN prEN 14039. The relative systematic error resulting from the matrix effect was obtained for three different TPH concentrations by statistical comparison of the slopes of the matrix-matched calibration lines and a pure solvent calibration line. Too high TPH concentrations were obtained when conventional solvent calibration was used for quantitation. This demonstrates that matrix-matched calibration should be exploited in the determination of petroleum hydrocarbons in soil samples. However, there was also significant enhancement of the response due to an interfering matrix with decreasing analyte concentration. Enhancement seems to be especially evident in the quantification of TPH over the concentration range encountered in polluted environments. As a result, even when matrix-matched calibration is used for quantitation, it is still necessary to establish the range over which a linear response can be expected. Otherwise too high results for sample TPH concentrations will be obtained.     

Determination of minor elements in steelmaking flue dusts using laser ablation inductively coupled plasma mass spectrometry

Element determination in solid waste products from the steel industry usually involves the time-consuming step of preparing a solution of the solid. Laser ablation (LA) inductively coupled plasma mass spectrometry (ICP-MS) has been applied to the analysis of Cr, Ni, Cu, As, Cd and Sn, elements of importance from the point of view of their impact on the environment, in electric arc furnace flue dust (EAFD). A simple method of sample preparation as pressed pellets using a mixture of cellulose and paraffin as binder material was applied. Calibration standards were prepared spiking multielement solution standards to a 1:1 ZnO + Fe₂O₃ synthetic matrix. The wet powder was dried and mechanically homogenised. Quantitative analysis were based on external calibration using a set of matrix matched calibration standards with Rh as a internal standard. Results obtained using only one-point for calibration without matrix matched, needing less time for standardization and data processing, are also presented. Data are calculated for flue dust reference materials: CRM 876-1 (EAFD), AG-6203 (EAFD), AG-6201 (cupola dust) and AG-SX3705 (coke ashes), and for two representative electrical arc furnace flue dusts samples from Spanish steelmaking companies: MS-1 and MS-2. For the reference materials, an acceptable agreement with certificate values was achieved, and the results for the MS samples matched with those obtained from conventional nebulization solutions (CN). The analytical precision was found to be better than 7% R.S.D. both within a single pellet and between several pellets of the same sample for all the elements.     

Rapid GC-MS Analysis of Pesticide Residues Using Analyte Protectants

Abstract

A multi-residue method was developed to determine pesticides in foods using analyte protectants to compensate for matrix effects. Promising analyte protectants and combinations of these analyte protectants were evaluated for masking active sites in gas chromatography systems. The optimal combination of analyte protectants was determined to be the mixture of polyethylene glycol and olive oil. Calibration curves for analyte protectants in both neat solvents and matrix-matched standards were compared and similar response enhancement was observed. This result suggests a convenient calibration method using a combination of analyte protectants in neat solvent instead of the conventional matrix-matched standards calibration method.     

A comparative study of the correction of systematic errors in the quantitation of pyrethroids in vegetables using calibration curves prepared using standards in pure solvent

A comparative study of two mathematical approaches was performed in order to correct systematic errors due to the presence of the unexpected interferences which appear when the quantitation of the analyte in real samples is carried out with calibration curves built using standards in pure solvent. These methods consisted in the establishment of different mathematical expressions which transform the concentration (Cs) obtained using calibration graphs built using pure solvent into the corrected concentration (C(M)) that should be obtained if the quantitation is carried out with calibration curves built using standards dissolved in blank matrix extracts. In the two approaches the correction is performed from the results of an intermediate precision study which was carried out using both calibration graphs (prepared using pure solvent and blank matrix extract). By using ANCOVA to compare the slope of both solvent-based and matrix-matched calibration graphs, matrix effect was found in the determination of deltamethrin in tomato and acrinathrin in tomato and pepper. In these cases, both approaches led to good results.     

Solid sampling-graphite furnace atomic absorption spectrometry for the direct determination of silver at trace and ultratrace levels

In this work, the possibilities of solid sampling-graphite furnace atomic absorption spectrometry for the direct determination of silver in solid samples of very different nature (a biological sample, a soil, an ore concentrate and a polymer) and showing substantial differences in their analyte content (from approximately, 40 ng g⁻¹ up to 350 μg g⁻¹) have been evaluated, the goal always being to develop fast methods, only relying on the use of aqueous standards for calibration.Different factors had to be taken into account in order to develop suitable procedures for all the samples under investigation. Among the most important ones, the following can be mentioned: (i) optimization of the temperature program in order to selectively atomize the analyte; (ii) the use of chemical modifiers (such as Pd or HNO₃), depending on the sample characteristics; (iii) appropriate wavelength, argon flow and sample mass selection (depending on the analyte content); (iv) the use of 3-field mode Zeeman-effect background correction in order to further expand the linear range up to 1000 ng of Ag, which was needed for analysis of the sample showing the highest Ag content (polypropylene).The procedures finally proposed show interesting features for the determination of silver in solid samples: the advantage of using aqueous standard solutions for calibration, a high sample throughput (approximately, 15 min per sample), a low detection limit (2 ng g⁻¹), sufficient precision (R.S.D. values in the vicinity of 10%) and a reduced risk of analyte losses and contamination.     

分析保护剂补偿基质效应-气相色谱-质谱法快速测定水果中40种农药残留

建立了水果中40种农药化合物的气相色谱-质谱(GC-MS)多残留检测方法,评价了添加分析保护剂对农药残留分析的补偿基质效应和对定量结果可靠性的影响。采用可以溶于丙酮有机溶剂的聚乙二醇Polyethylene Glycol 400(PEG 400)和橄榄油作为保护剂组合进行定量分析。水果样品采用乙腈提取,微型固相萃取小柱净化,大体积进样,GC-MS选择离子监测(SIM)模式检测。40种农药化合物在1～200 μg/L范围内线性关系良好,线性相关系数在0.99以上,检出限(以信噪比为3计)为0.1～3.0 μg/L。除乐果外,其他化合物的添加回收率为75%～119%,相对标准偏差均小于16.6%。通过对添加分析保护剂的校准曲线与基质匹配校准曲线的定量准确性的比较,发现加入分析保护剂方法可以代替基质匹配校正方法,同时采用大体积进样和微型固相萃取净化相结合的方法,大大减少了样品前处理量。将所建立的分析保护剂方法用于苹果、桃子、橙子、香蕉和葡萄等水果样品的分析,基质补偿效应良好,有效地克服了水溶性分析保护剂对气相色谱分析有影响的缺点。     

Use of automated direct sample introduction with analyte protectants in the GC-MS analysis of pesticide residues

Automated large-volume direct sample introduction, or difficult matrix introduction (DMI), was investigated in the determination of 44 pesticide residues possessing a wide range of physico-chemical properties (volatility, polarity, pK(a)) in fruit-based baby food by means of gas chromatography-mass spectrometry (GC-MS) with a quadrupole mass analyzer. DMI has advantages over traditional injection because large volumes (up to 30 microL) of potentially dirty sample extracts can be injected into the GC-MS, but nonvolatile matrix components that would normally contaminate the inlet are removed after every injection. The extra matrix and glass surfaces involved in DMI, however, make the system more prone to the matrix-induced chromatographic enhancement effect, which adversely affects quantification of several pesticides. To overcome this problem, matrix-matched calibration standards and/or the use of analyte protectants were applied in the DMI approach, and the analysis of extracts was also compared before and after undergoing clean-up by dispersive solid-phase extraction. For best quantification, clean-up was still needed, and the combination of matrix-matching with analyte protectants gave the most reproducible results. Depending on the application, however, the addition of analyte protectants (a mixture of 3-ethoxy-1,2-propanediol, L-gulonic acid 3-lactone, and D-sorbitol) to sample extracts and calibration standards in solvent (non-matrix matched), gave satisfactory quantification for most of the 44 pesticides tested. The lowest calibration levels for 34 of the 44 pesticides were < or = 10 ng/g, which meets the standard required by the European Union Baby Food Directive (2003/13/EC).     

Precision and accuracy of laser ablation-ICP-MS analysis of rare earth elements with external calibration

Solid sample introduction into an ICP-MS by laser ablation is an effective method for the total analysis of rare earth elements (REEs) in soils because no digestion is needed. A problem of the method, however, is the difference of the ablated mass for each laser shot. Therefore, internal standard for the compensation of signal instability, sample preparation, and the calibration method have to be carefully chosen.The analyzed sample was a certified standard provided by IAEA (SOIL-7). The sample was mixed with an internal standard solution and polyethylene (PE), dried, homogenized in a ball-mixer/mill, and pressed to a pellet. For the calibration 5 external standards with increasing REEs concentrations (0.4–20 g/g) as well as a blank were prepared in the same way.The analysis of the pellets was performed on a VG PlasmaQuad II + with a LaserLab unit. The laser ablation-cell was modified to improve the sample particle transportation characteristics and to allow a quicker sample-exchange. The pellets were ablated from six different spots for 60 s each with a laser-repetition rate of 4 Hz.The correlation coefficients of the calibration curves based on 5 standards, were better than 0.995. The concentrations c_A of the 15 REEs in the soil sample were determined with an average relative confidence interval 100(CI)/c_A of 6.95%, as a figure for the precision. This good precision have been obtained with a new laser ablation cell, which will be described in detail.With 2 exceptions (Ce and La) the measured concentrations were within the confidence intervals (CI) of the certified values. Therefore, with respect to accuracy and precision, the presented method offers a convenient way to analyze homogeneous and powdered soil samples for REE's without digestion. Since a good calibration for the REE determination may be obtained, the laser sampling variance (within the sample) is less significant than the analytical variance. Automation of the method is possible by construction of an autosampler based on the modified laser cell.Presented in part at the 1993 European Winter Conference on Plasma Spectrochemistry, Granada, Spain     

Search on ruggedness of fast gas chromatography-mass spectrometry in pesticide residues analysis

In this study, suitability of fast gas chromatography-mass spectrometry (GC-MS) on a narrow-bore column with a programmed temperature vaporizer for the analysis of pesticide residues in non-fatty food was evaluated. The main objectives were ruggedness and stability of chromatographic system with regards to co-extractives injected. The chromatographic matrix induced response enhancement was found to be strongly dependent on the concentration of residues and is reaching up to 700% compared to the pesticides solutions in a neat solvent. However, the responses of pesticides in matrix-matched standards at different concentration levels do not significantly change during 130 injections. Response enhancement/or decrease is influenced by the sample preparation technique. External calibration with matrix-matched calibration standards should, therefore, provide results with good precision also at the concentration level of 0.005 mg kg(-1). Special attention is given to the performance of the chromatographic column and retention gap with regards to peak widths, peak tailing and different sample preparation methods. During approximately 460 matrix sample injections, the performance of the analytical column was acceptable. GC-MS set-up with 0.15 mm i.d. column can be successfully utilized for the pesticide residues analysis.     

Matrix-induced response enhancement in pesticide residue analysis by gas chromatography

The sample matrix can cause an enhancement in the observed chromatographic response for pesticide residues in a matrix extract compared with the same concentration in a matrix-free solution. The matrix increases the transfer of pesticides from hot vaporizing injectors by reducing the thermal stress for labile compounds and by masking active sites in the injector responsible for the adsorption or decomposition of polar pesticides. The use of different injector types and matrix simplification procedures can reduce matrix-induced enhancement but do not eliminate it. The most effective strategy is to use matrix-matched calibration standards or analyte protectants which equalize the response enhancement for calibration standards and sample extracts. From a practical point of view it is important that the method used to correct for matrix-induced enhancement is compatible with low system maintenance. The different approaches for correcting matrix-induced enhancement for calibration in pesticide residue analysis are discussed and compared in this review.     

Using calibration approaches to compensate for remaining matrix effects in quantitative liquid chromatography/electrospray ionization multistage mass spectrometric analysis of phytoestrogens in aqueous environmental samples

Signal suppression is a common problem in quantitative liquid chromatography/electrospray ionization multistage mass spectrometric (LC/ESI-MS(n)) analysis in environment samples, especially in highly loaded wastewater samples with highly complex matrix. Optimization of sample preparation and improvement of chromatographic separation are prerequisite to improve reproducibility and selectivity. Matrix components are reduced if not eliminated by optimization of sample preparation steps. However, extensive sample preparation may be time-consuming and risk the significant loss of some trace analytes. The best way to further compensate matrix effects is the use of an internal standard for each analyte. However, in a multi-component analysis, finding appropriate internal standards for every analyte is often difficult. In this present study, a more practical alternative option was sought. Matrix effects were assessed using the post-extraction addition method. By comparison of three different calibration approaches, it was found that matrix-matched calibration combined with one internal standard provides a satisfactory method for compensating for any residual matrix effects on all the analytes. Validating experiments on different sewage treatment plant (STP) influent samples analyzing for a range of phytoestrogens showed that this calibration method provided satisfactory results with concentration ratio 96.1-105.7% compared to those by standard addition.     

Development of a multi-residue analytical methodology based on liquid chromatography-tandem mass spectrometry (LC-MS/MS) for screening and trace level determination of pharmaceuticals in surface and wastewaters

This paper describes development, optimization and validation of a method for the simultaneous determination of 29 multi-class pharmaceuticals using off line solid phase extraction (SPE) followed by liquid chromatography-triple quadrupole mass spectrometry (LC-MS-MS). Target compounds include analgesics and non-steroidal anti-inflammatories (NSAIDs), lipid regulators, psychiatric drugs, anti-histaminics, anti-ulcer agent, antibiotics and β-blockers. Recoveries obtained were generally higher than 60% for both surface and wastewaters, with exception of several compounds that yielded lower, but still acceptable recoveries: ranitidine (50%), sotalol (50%), famotidine (50%) and mevastatin (34%). The overall variability of the method was below 15%, for all compounds and all tested matrices. Method detection limits (MDL) varied between 1 and 30 ng/L and from 3 to 160 ng/L for surface and wastewaters, respectively. The precision of the method, calculated as relative standard deviation (R.S.D.), ranged from 0.2 to 6% and from 1 to 11% for inter and intra-day analysis, respectively. A detailed study of matrix effects was performed in order to evaluate the suitability of different calibration approaches (matrix-matched external calibration, internal calibration, extract dilution) to reduce analyte suppression or enhancement during instrumental analysis. The main advantages and drawbacks of each approach are demonstrated, justifying the selection of internal standard calibration as the most suitable approach for our study. The developed analytical method was successfully applied to the analysis of pharmaceutical residues in WWTP influents and effluents, as well as in river water. For both, river and wastewaters, the most ubiquitous compounds belonged to the group of anti-inflammatories and analgesics, antibiotics, the lipid regulators being acetaminophen, trimethoprim, ibuprofen, ketoprofen, atenolol, propranolol, mevastatin, carbamazepine and ranitidine the most frequently detected compounds.     

液相色谱-串联质谱法测定果蔬中双三氟虫脲、四氯虫酰胺和氰虫酰胺残留

建立了蔬菜和水果中双三氟虫脲、四氯虫酰胺和氰虫酰胺3种新型杀虫剂的分散固相萃取-液相色谱-串联质谱检测方法.样品经乙腈均质提取,混合使用乙二胺-N-丙基硅烷(PSA)和C18基质分散净化剂进行净化,液相色谱-三重四极杆串联质谱(LC-MS/MS)同时进行检测.双三氟虫脲和四氯虫酰胺采用多反应监测负离子模式,氰虫酰胺采用多反应监测正离子模式.双三氟虫脲在苹果、洋葱和经微波处理的洋葱样品中均不存在基质效应,可采用纯溶剂标准外标法或者采用基质匹配标准溶液定量检测; 四氯虫酰胺和氰虫酰胺存在程度不同的基质减弱效应,采用空白基质匹配的校正标准曲线外标法定量.3种杀虫剂均在0.2～100 μg/L线性范围内具有良好的线性关系,相关系数均大于0.9990.在0.005～2.000 mg/kg范围内,平均添加回收率为81.6％～99.9％,相对标准偏差为3.6％～9.8％.氰虫酰胺、四氯虫酰胺和双三氟虫脲的检出限分别为0.064,0.048和0.001 μg/kg,定量限分别为0.210, 0.160和0.004 μg/kg.     

LA-ICP-MS analysis of waste polymer materials

Waste polymer materials were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The concentrations of 35 elements were determined by using different types of external standards, namely glass and polyethylene (PE) based. Prior to the LA-ICP-MS analysis of determined elements, Na and/or Zn were used as internal standards. The investigations concentrated mainly on the detection of Cr, As, Cd, Sn, Sb, Hg, and Pb. Using PE-based calibration standards, the measured concentrations in the waste polymers were within 49% of the wet chemical data. The determined deviation was up to 102% when using the glass standards. Trace concentration of As and Hg (and also of S) could be determined with a concentration below 1 mg/kg. However, Hg provided very low intensity with a high relative standard deviation (RSD) and was therefore not further evaluated. Cryomilling of polymers was applied to reduce the particle size of the material and improved the precision and accuracy of LA-ICP-MS analysis. On average, the LA-ICP-MS results showed a deviation from the wet chemical reference analysis of 38% and an RSD of 56% for pressed polymer powder samples prepared by cryomilling. In general, waste pellets without sample preparation (i.e., use of pellets as delivered) are too heterogeneous, not suitable for micro-beam techniques, and showed a strong matrix dependence. With homogeneous pellets that appear similar to each other agreement in the determined concentrations was found for some elements.     

Confirmatory analysis of acetylgestagens in plasma using liquid chromatography-tandem mass spectrometry

A confirmatory method has been developed and validated for the determination of chlormadinone acetate (CMA), megestrol acetate (MGA), melengestrol acetate (MLA) and medroxyprogesterone acetate (MPA) in bovine and porcine plasma. Analytes are extracted from plasma samples using matrix-assisted liquid-liquid extraction (LLE) on Extrelut NT columns followed by C18 solid-phase extraction (SPE). Analytes were analysed using liquid chromatography-tandem mass spectrometry (LC-MS/MS), and quantification was performed using matrix-matched calibration standards in combination with deuterated internal standards. In accordance with Commission Decision 2002/657/EC, two ion transitions were monitored for each analyte. Decision limits (CCalpha) were estimated by analysing 20 blank plasma samples and ranged from 0.1 to 0.2 ng mL(-1). Detection capabilities (CCbeta) were estimated using 20 plasma samples fortified at 0.5 ng mL(-1) and were <0.5 ng mL(-1). In the range 0.5-2 ng mL(-1), the mean intra-laboratory reproducibility of the analytes ranged from 6 to 18% (%R.S.D.). Analytes were shown to be stable in fortified plasma samples for >8 months when stored at -20 degrees C.     

气相色谱分析农药残留的基质效应及其解决方法

对于相同浓度的农药,其在基质溶液中的色谱响应会比其在纯溶剂中的响应高。通过减少热不稳定农药的分解,以及屏蔽进样口的活性位点而减少极性农药在活性位点的吸附或分解,基质效应可增加从进样口传输到色谱柱中的农药残留量。各种进样方式和基质净化方法都可以减少但不能完全消除基质效应;基质匹配校准法和分析保护剂法是避免基质效应最有效的方法;在实际检测中,所采用的消除或补偿基质效应的方法应考虑减少仪器系统的维护。本文概述了农药残留分析检测中的基质效应及其解决方法。     

Microhomogeneity assessments using ultrasonic slurry sampling coupled with electrothermal vaporization isotope dilution inductively coupled plasma mass spectrometry

Ultrasonic slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry (USS-ETV-ICP-MS) is a very powerful technique for the direct analysis of solid materials prepared as slurries. The use of isotope dilution USS-ETV-ICP-MS (USS-ETV-ID-ICP-MS) for micro-homogeneity characterization studies of powdered reference materials based on elemental analyses, was investigated. Slurry analysis conditions were optimized taking into consideration density, particle size, analyte extraction, slurry mixing, analyte transport and sampling depth. Slurries were prepared using 1–20 mg of material and adding 1.0 ml of 5% nitric acid diluent containing 0.005% Triton X-100®. Three reference materials were analyzed (RM 8431a Mixed Diet, SRM 1548a Typical Diet and SRM 2709 San Joaquin Soil). Cu and Ni were determined in each material and Fe was also determined in RM 8431a Mixed Diet. ETV conditions were optimized and the benefit of using Pd as a carrier to enhance transport, combined with oxygen ashing was demonstrated. The accuracy of the method was verified by comparing analytical results with certified values. The precision of the method was demonstrated by comparing R.S.D.'s for slurry samples and aqueous standards and elemental ‘homogeneity’ was quantified based on the slurry sampling variability. The representative sample mass analyzed was calculated taking into consideration extraction of analyte into the liquid phase of the slurry. Representative sample masses of approximately 4 mg of RM 8431a provided slurry sampling variabilities of 10% or less for Cu, Fe and Ni. Representative sample masses of approximately 10 mg of SRM 1548a provided slurry sampling variabilities of approximately 10% for Cu and Ni. Representative sample masses of approximately 0.3 mg of SRM 2709 resulted in total analytical variabilities of less than 7%, highlighting the fact that the San Joaquin Soil is clearly the most homogeneous of the materials characterized.