Accuracy in standards preparation for neutron activation analysis

Errors in preparing standards, especially multielemental standards, are extremely important if accurate results are desired from neutron activation analysis (NAA). It is often convenient to prepare standards for NAA from single or multi-element solutions which are then deposited onto (or into) a suitable matrix, such as filter paper or quartz vials. There are many potential sources of error in preparing single-element standards including: impurities and non-stoichiometric composition of the element or compound used to prepare the standard solutions; evaporative losses of solvent; inaccuracy of calibration, and imprecision of the pipettes used; moisture content of elements or compounds used; contamination from reagents, equipment, laboratory environment, or final matrix of the standard; instability of standard solutions (i.e., to losses via precipitation or adsorption), and losses of volatile elements during dissolution and/or irradiation. Additional sources of error in preparing multielement standards includes: instability of mixed, multielement solutions, and cross-contamination of one element by the addition of a second element. Procedures previously used by the author at NIST to prepare multielement standards with concentrations accurate to about one percent are described. Additional techniques needed to prepare multielement standards with accuracies better than 1 percent will be discussed.     

Development and supply system of reference materials based on the Measurement Law in Japan

 Analytical instruments used for measurements of air and water pollution are calibrated by using reference materials such as standard gases and standard solutions. In Japan, since the middle of the 1970s, those reference materials which are traceable to the national standards maintained at national research institutes have been supplied to users by reference material producers. In order to establish the primary standards and to secure the traceability from the working standards to the national ones, various analytical methods such as coulometric, titrimetric and gravimetric analyses for purity determination and highly sensitive atomic spectrometry for trace analysis have been developed as the primary methods and reference methods. The Japanese Measurement Law, revised in 1992, has introduced a new traceability system in which a public organization, a "designated calibration body", can also prepare and maintain the national standards under the advice and instruction of national research institutes. The designated calibration body can provide calibration services to reference material producers (accredited calibration bodies) by using the national standards. The reference materials supplied in conformity with the traceability system include standard gases, pH standard solutions, metal standard solutions and non-metal ion standard solutions. Received: 4 October 1996 Accepted: 2 December 1996     

The dosage of small volumes for chromatographic quantifications using a drop-on-demand dispenser system

A commercially available piezo-driven drop-on-demand dispenser was tested for its suitability for the preparation of analytical calibration standards and in a standard addition approach prior to quantitative ultra performance liquid chromatography (UPLC) analysis of homoserines. The reproducibility of the drop-on-demand dosing system was tested and the verification of the droplet volume was performed by preparing a series of 1.0 mg/L caffeine standard solutions from a 1,000.0 mg/L stock solution and analysis of the concentrations obtained by UPLC. The reproducibility was better than 1% relative standard deviation from measurement to measurement and the highest was 1.6% from day to day. The results were compared with the conventional way of generating standard solutions (pipetting). A gravimetric method and a photography-based method for the determination of the average single droplet volume were compared and found to be in very good agreement. The system was employed for the quantification of N-decanoyl homoserine by standard addition in bacterial culture supernatants containing this analyte. The agreement with conventional quantification techniques was high. The paper shows the feasibility of the approach with advantages in low sample and solvent volume consumption and very good reproducibility and reliability combined with easy usage. Figure Ejected droplet, 60 μs after application of the pulse     

The determination of levodopa in plasma by HPLC: A cautionary note

Summary Levodopa (l-3,4-dihydroxyphenyl-L-alanine) plays an important role in the treatment of patients with Parkinson’s disease and numerous analytical methods have been developed to determine levodopa plasma concentrations. However, the maximal levodopa plasma concentration varies in a range from <1000 ng mL⁻¹ to > 10000 ng mL⁻¹ across the literature, even after administration of the same dose, and this large variation has been so far ignored. One of the reasons for this large variation may be that the standard stock solutions are prepared in different ways. If there is incomplete dissolution when preparing the stock solution for standards, the true concentration can be lower than the nominal concentration, leading to the apparent levodopa plasma concentrations being 30 to 50% higher than the true concentration.     

Optimisation of the quantitative determination of chlormequat by matrix-assisted laser desorption/ionisation mass spectrometry

The plant growth regulator chlormequat, an involatile quaternary ammonium salt, has been quantified by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOFMS). Restrictions for quantitative MALDI-TOFMS analysis, such as irreproducible crystallisation and unsatisfactory laser stability, have been overcome by the application of two synthesised isotopically labelled standards and the optimisation of the measurement protocol. Data acquisition at constant laser power was compared to data acquisition at approximately constant ion abundance of the relevant ions (analyte and internal standards). Data acquisition at constant ion abundance performed better and enabled a high number of consecutive firings to the same sample deposition area. Furthermore an increased sample-to-sample repeatability and a high reproducibility over several weeks without re-calibration have been attained by this method. Linearity over three orders of magnitude (0.05 to 30 ng/microL chlormequat), with a correlation coefficient of 0.9997, was achieved using [13C3]-chlormequat as internal standard. Limit of detection and limit of determination were determined to be in the low pg/microL range for pure standard solutions. Thin-layer chromatography was applied for the removal of high amounts of choline, which is often present in plant tissue extracts and can adversely affect the ionisation and detection of chlormequat by MALDI-TOFMS. The use of two internal standards ([13C3]- and [2H9]-chlormequat) enabled direct quantification and simultaneous control of the recovery.     

Use of Commercial Acrylonitrile Standard for Wastewater Analysis

Abstract

It was found that a commercially available aqueous standard solution of acrylonitrile (1 mg/ml) was below manufacturer specification in concentration when compared to an authentic standard prepared from the pure compound. Comparative analyses of the two solutions, commercial and laboratory prepared acrylonitrile, were conducted by gas chromatography using a flame ionization detector with analytical conditions similar to those recommended by the EPA for acrylonitrile analyses. The acrylonitrile content of the commercial solution was found to be approximately 30% lower than specified. Commercial venders have provided solutions of the 114 organic priority pollutants which are convenient for instrumental calibration. There needs to be concern for experimental results reported with regards to accuracy and estimated detection limit when using such standards.     

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.     

Long-term stability of hydrocarbons in NIST gas standard reference material (SRM) 1800

A gas standard reference material (SRM) containing fifteen hydrocarbons in nitrogen at a nominal 5 nmol mol(-1) was issued in 1993. The certification period for SRM 1800 was assigned as 2 years, because of limited stability data. Over a period of 10 years reanalysis of the lot standard (a sample chosen from the SRM lot to which all other lot samples are compared), SRM samples remaining in stock for sale, and SRMs returned to the National Institute of Standards and Technology (NIST) for recertification, were compared with primary standards to assess the stability of the hydrocarbons. New primary standards were periodically introduced into the original primary standard suite to assess the stability and consistency of the primary standards. Over this ten-year period 11 SRM 1800 samples were reanalyzed, resulting in 210 amount-of-substance fraction (concentration) determinations performed for quality-assurance purposes. Of these measurements 209 (99.5%) agreed within the original 95% confidence interval of the +/-4% expanded uncertainty, demonstrating the stability of the standards. There was also agreement to within +/-2% of the original concentration for 204 (97%) of the measurements. This is well within the original +/-4% expanded uncertainty assigned to the hydrocarbon concentrations at the approximate 95% confidence interval demonstrating stability. These results will enable the expiry date to be increased for future restock issues of SRM 1800.     

Development of a liquid chromatographic time-of-flight mass spectrometric method for the determination of unlabelled and deuterium-labelled alpha-tocopherol in blood components

A method is described for the analysis of deuterated and undeuterated alpha-tocopherol in blood components using liquid chromatography coupled to an orthogonal acceleration time-of-flight (TOF) mass spectrometer. Optimal ionisation conditions for undeuterated (d0) and tri- and hexadeuterated (d3 or d6) alpha-tocopherol standards were found with negative ion mode electrospray ionisation. Each species produced an isotopically resolved single ion of exact mass. Calibration curves of pure standards were linear in the range tested (0-1.5 microM, 0-15 pmol injected). For quantification of d0 and d6 in blood components following a standard solvent extraction, a stable-isotope-labelled internal standard (d3-alpha-tocopherol) was employed. To counter matrix ion suppression effects, standard response curves were generated following identical solvent extraction procedures to those of the samples. Within-day and between-day precision were determined for quantification of d0- and d6-labelled alpha-tocopherol in each blood component and both averaged 3-10%. Accuracy was assessed by comparison with a standard high-performance liquid chromatography (HPLC) method, achieving good correlation (r(2) = 0.94), and by spiking with known concentrations of alpha-tocopherol (98% accuracy). Limits of detection and quantification were determined to be 5 and 50 fmol injected, respectively. The assay was used to measure the appearance and disappearance of deuterium-labelled alpha-tocopherol in human blood components following deuterium-labelled (d6) RRR-alpha-tocopheryl acetate ingestion. The new LC/TOFMS method was found to be sensitive, required small sample volumes, was reproducible and robust, and was capable of high throughput when large numbers of samples were generated.     

Traceability system for elemental analysis

A complete metrological traceability system for measurement results of chemical analysis was set up. Core components are pure substances (national standards) characterised at the highest metrological level, primary solutions prepared from these pure substances and secondary solutions deduced from the primary solutions and intended for sale. The relative uncertainty of the element mass fraction of the primary substances and solutions is < 0.01 and < 0.05%, respectively. For the certification of transfer solutions and for stability testing, a precision measurement method for element contents has been developed by means of optical emission spectrometry (ICP OES) by which uncertainties between 0.1 and 0.05% can be achieved. The dissemination to field laboratories is effected with the aid of a calibration laboratory of the German Calibration Service (DKD) which certifies the element content of the secondary solutions with an uncertainty ≤ 0.3%. Calibration with these solutions enables the user to establish traceability of his measurement results to the International System of Units (SI). Currently, the system comprises Cu, Fe, Bi, Ga, Si, Na, K, Sn, W, and Pb.     

Measurement of zinc stable isotope ratios in biogeochemical matrices by double-spike MC-ICPMS and determination of the isotope ratio pool available for plants from soil

Analysis of naturally occurring isotopic variations is a promising tool for investigating Zn transport and cycling in geological and biological settings. Here, we present the recently installed double-spike (DS) technique at the MAGIC laboratories at Imperial College London. The procedure improves on previous published DS methods in terms of ease of measurement and precisions obtained. The analytical method involves addition of a (64)Zn-(67)Zn double-spike to the samples prior to digestion, separation of Zn from the sample matrix by ion exchange chromatography, and isotopic analysis by multiple-collector inductively coupled plasma mass spectrometry. The accuracy and reproducibility of the method were validated by analyses of several in-house and international elemental reference materials. Multiple analyses of pure Zn standard solutions consistently yielded a reproducibility of about ±0.05‰ (2 SD) for δ(66)Zn, and comparable precisions were obtained for analyses of geological and biological materials. Highly fractionated Zn standards analyzed by DS and standard sample bracketing yield slightly varying results, which probably originate from repetitive fractionation events during manufacture of the standards. However, the δ(66)Zn values (all reported relative to JMC Lyon Zn) for two less fractionated in-house Zn standard solutions, Imperial Zn (0.10 ± 0.08‰: 2 SD) and London Zn (0.08 ± 0.04‰), are within uncertainties to data reported with different mass spectrometric techniques and instruments. Two standard reference materials, blend ore BCR 027 and ryegrass BCR 281, were also measured, and the δ(66)Zn were found to be 0.25 ± 0.06‰ (2 SD) and 0.40 ± 0.09‰, respectively. Taken together, these standard measurements ascertain that the double-spike methodology is suitable for accurate and precise Zn isotope analyses of a wide range of natural samples. The newly installed technique was consequently applied to soil samples and soil leachates to investigate the isotopic signature of plant available Zn. We find that the isotopic composition is heavier than the residual, indicating the presence of loosely bound Zn deposited by atmospheric pollution, which is readily available to plants.     

Quality control in quantification of volatile organic compounds analysed by thermal desorption-gas chromatography-mass spectrometry

This paper presents a detailed study on the calibration of a thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS)-based methodology for quantification of volatile organic compounds (VOCs) in gaseous and liquid samples. For the first time, it is documented to what extent three widely encountered problems affect precise and accurate quantification, and solutions to improve calibration are proposed. The first issue deals with the limited precision in MS quantification, as exemplified by high relative standard deviations (up to 40%, n=5) on response factors of a set of 69 selected VOCs in a volatility range from 16 Pa to 85 kPa at 298 K. The addition of [(2)H(8)]toluene as an internal standard, in gaseous or liquid phase, improves this imprecision by a factor of 5. Second, the matrix in which the standard is dissolved is shown to be highly important towards calibration. Quantification of gaseous VOCs loaded on a sorbent tube using response factors obtained with liquid standards results in systematic deviations of 40-80%. Relative response factors determined by the analysis of sorbent tubes loaded with both analytes and [(2)H(8)]toluene from liquid phase are shown to offer a reliable alternative for quantification of airborne VOCs, without need for expensive and often hardly available gaseous standards. Third, a strategy is proposed involving the determination of a relative response factor being representative for a group of analytes with similar functionalities and electron impact fragmentation patterns. This group method approach indicates to be useful (RSD approximately 10%) for quantifying analytes belonging to that class but having no standards available.     

Quantification of analytes in overlapping peaks from capillary electrophoresis using multivariate curve resolution-alternating least squares methods

Application of multivariate curve resolution with alternating least squares (ALS) methods to second-order data from capillary electrophoresis diode array detector (CE-DAD) is shown. Second-order data are easily obtained by setting individual data matrix of CE run one in top of the other. Initial qualitative solutions obtained by evolving factor analysis can be further optimized by simultaneous analysis of multiple electrophoresis run data with ALS regression. Quantification is achieved by the comparison of the analyte peak areas with that of pure standards. During the ALS regression procedure, the following constraints were applied: (i) both concentrations and unit pure spectra of the resolved components must be positive; (ii) elution profiles have an unimodal shape; (iii) correspondence exists between common species in the different data matrices; (iv) the pure spectrum of each species is the same in all runs where it is present. The above methods were applied for the determination of dinitrotoluene (DNT) isomeric compounds in overlapping peaks from CE.     

Multielement standards for instrumental neutron activation analysis of biological materials

Standards for instrumental neutron activation analysis (INAA) of biological materials are proposed. The standards are multielement solid solutions in phenol-formaldehyde resole resin (PFR) moulded as pellets weighing 30 to 50 mg. The concentrations of trace elements in the standards are selected so that, firstly, they are commensurable with their concentrations in the biological materials and, secondly, that the analytical lines of each of the elements incorporated in PFR are resolved with the aid of modern equipment. The principal standard contains 21 trace elements from among those of greatest interest for INAA of biological materials. This standard is recommended for work on high-resolution equipment. At the same time, standards of simpler trace element composition have been prepared and studied which can be used in work on simpler equipment or in solving particular problems in determination of certain groups of chemical elements.     

Synthesis,isolation, and chromatography of perdeuterated α-1,2,3,4,5,6-hexachlorocyclohexane

Perdeuterated α-1,2,3,4,5,6-hexachlorocyclohexane (α-PDHCH) was synthesized and isolated in pure form. α-PDHCH was baseline separated from unlabeled α-HCH by high resolution gas chromatography on different stationary phases. α-PDHCH was also resolved into its enantiomeric forms by application of a chiral GC phase. Furthermore, chiral chromatography of a mixture of labeled α-PDHDH and unlabeled α-HCH resulted in four peaks. Standard solutions of α-PDHCH are recommended both as recovery standard for sample clean-up and as internal standard for the quantitation of chlorinated hydrocarbons with GC/MS or GC/ECD in environmental samples.     

Evaluation of an automated on-line method for generating calibration curves in inductively coupled plasma atomic emission spectrometry

Summary A high-performance liquid chromatography pump (HPLC) is used for the computer-controlled on-line generation of calibration standards in inductively coupled plasma-atomic emission spectrometry. Such a pump, employed as a solution-delivery device, is well suited for feeding a nebulizer, since it is capable of delivering a range of constant flows and of mixing selected stock solutions or solvents automatically; this capability translates into the capacity to dilute stock or sample solutions and to introduce the resulting mixtures at constant flow into any of a number of nebulizers. Further, the range of useful nebulizers is broadened, since the mixture is supplied under high pressure. This feature makes the method especially useful for devices such as the jet-impact nebulizer (JIN), which requires a pressurized, constant feed. At present, the principal limitation of the new approach is the flow resolution of available HPLC pumps. Although this limitation does not ordinarily cause difficulties in schemes for automatic dilution or calibration, it can be a serious shortcoming if the apparatus is to be used for automated standard-additions analysis.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

A Comparison of the Efficiency of Nucleotide Extraction by Several Procedures and the Analysis of Nucleotides from Extracts of Liver and Isolated Hepatocytes by HPLC

Abstract

A high pressure liquid chromatography (HPLC) system was developed, capable of resolving most 5′-nucleotides and nucleotide-sugars present in liver tissue. Using three different extraction procedures, the recovery of the twelve major 5′-ribonucleotides from solutions of nucleotide standards, or liver samples, or isolated hepatocytes was compared. Nucleotides were obtained from acid extracts for HPLC analysis by adsorbing the nucleotides on charcoal, or precipitating the acid (perchloric acid) by KOH, or extracting the acid with alamine/ freon. The last two procedures were found to be superior to the charcoal adsorption procedure for recovering nucleotides from acid extracts. The recovery of nucleotides from either liver samples or isolated hepatocytes by these two procedures was similar; however, the recovery of nucleotides from standard solutions was slightly higher for the alamine/freon procedure than the KOH precipitation procedure.     

Determination of thallium in cadmium and lead by graphite-furnace atomic-absorption spectrometry with sample vaporization from a platform

When the sample is vaporized from the wall of a graphite furnace it is not possible to determine thallium in cadmium and lead by AAS without matrix matching of the standards. In the case of a lead matrix and vaporization from the wall, the thallium signal is barely distinguishable from the base-line. When the sample is vaporized from a platform, and the peak area is used for measurement, pure aqueous standards may be used for instrument calibration. The peak heights and areas of the thallium signals are considerably enhanced by vaporization from a platform (peak height 1.7-fold and peak area 2.6-fold in pure aqueous solutions as compared to vaporization from the wall). The enhancement factors are larger in presence of the cadmium or lead matrix since here the reduced interference also makes a contribution.     

Chlordiazepoxide photoisomerization kinetics into oxaziridine. A HPLC study

It was proved that the N(4)-oxide group included in chlordiazepoxide (CDZ) is involved in its phototoxicity. At a wavelength of 350 nm, CDZ photoisomerizes only into oxaziridine (OXA) which is not available as standard. In the course of cytotoxicity investigations, the optimal CDZ irradiation conditions were established as acetonitrile as solvent, 10 degrees C as temperature of the irradiated solutions and 70-90 min as irradiation time for solutions in the range of 12.2-152.0 microg/ml. The kinetic parameters of the CDZ photodegradation reaction order have been calculated using an appropriate algorithm. In all cases, the first order reversible or irreversible was selected by Aka?ke's criteria. The percentage of undecomposed CDZ and OXA generated after irradiation were determined by a reversed HPLC method. The latter also permitted the separation of CDZ major impurities in aqueous solutions (demoxepam and 2-amino-5-chlorobenzophenone) as well as the oxaziridine of demoxepam. In this study, the experimental irradiation conditions allowed us to produce 98% pure OXA from CDZ. This HPLC method could be easily extended to the analysis of the molecules in pharmaceutical studies.     

Determination of the neuropeptides arginine vasotocin and isotocin in brains of three-spined sticklebacks (Gasterosteus aculeatus) by off-line solid phase extraction-liquid chromatography-electrospray tandem mass spectrometry

A method based on solid phase extraction (SPE) followed by liquid chromatography-electrospray ionisation tandem mass spectrometry for the determination of the nonapeptides arginine vasotocin (AVT) and isotocin (IT) in brains of three-spined sticklebacks (Gasterosteus aculeatus) is described. Separation and detection were optimized using synthetic standards. Limits of detection (LOD) for standard solutions were 160 pg mL(-1) for AVT and 250 pg mL(-1) for IT. The SPE procedure hardly affected the LODs for standard solutions. Mainly because of ion suppression, LODs for AVT and IT in brains were approximately 5 and 25 pg mg(-1), respectively. The concentrations determined in the brain of several fishes ranged from 10 to 500 pg mg(-1) for AVT and from 400 to 4000 pg mg(-1) for IT.