Continuum Source Molecular Absorption Spectrometry Research Articles

Theoretical study of the formation mechanism of molecules СaF for fluoride determination using electrothermal molecular absorption spectrometry

In order to select the temperature-time heating program, the calibration method, the prediction of matrix interference and to increase the sensitivity and accuracy of the halogens determination using the electrothermal molecular absorption spectrometry it is essential to know the formation mechanism of the measured molecules. Thermochemical processes of gaseous molecules CaF formation, which are used in the electrothermal molecular absorption determination of fluorine, were studied using the method of the thermodynamic simulation. All calculations were performed using the HSC 6.1 software package with its own database of thermodynamic data. For calculations, the non-equilibrium thermodynamic system, which was realized in the graphite furnace, was divided into the thermodynamic subsystems that correspond to the main stages of time-temperature program: drying of the sample, pyrolysis, vaporization and formation of gaseous diatomic molecules. The composition of the condensed sample residues after drying and pyrolysis stages, the composition of the gas phase in the analytical zone of the graphite furnace during vaporization and the formation of molecules stages were determined by the calculations. This study theoretically describes thermochemical processes of the formation of gaseous CaF molecules using thermodynamic simulation for electrothermal molecular absorption determination of fluorine. The calculations make it possible to establish that gaseous molecules CaF g are formed during the course of the thermal decomposition of the condensed fluoride (CaF 2 c ). The correct execution of thermodynamic simulation was confirmed by the comparison of the theoretical temperature curves of pyrolysis and the formation of molecules CaF g with the experimental curves. The developed procedure of the thermodynamic simulation can be applied to study the formation of other dimeric molecules used for molecular absorption analysis with electrothermal atomization. Keywords: high-resolution continuum source molecular absorption spectrometry, thermodynamic simulation, fluorine, calcium mono-fluoride, temperature of drying, pyrolysis and vaporization. Polina V. Zaitceva*, Alexander A. Pupyshev Ural Federal University named after the fist President of Russia B.N.Yeltsin (UrFU), ul. Mira, 19, Ekaterinburg, 620002, Russian Federation DOI: http://dx.doi.org/10.15826/analitika.2015.20.1.003

Sequential and simultaneous determination of four elements in soil samples using high-resolution continuum source graphite furnace atomic and molecular absorption spectrometry

An analytical method has been developed for the determination of Cd, Cr, Fe and Al (via the AlH molecule) in soil samples using direct solid sample analysis.

Determination of sulfur in crude oil using high-resolution continuum source molecular absorption spectrometry of the SnS molecule in a graphite furnace

An analytical method for the determination of sulfur, as the tin mono-sulfide (SnS) molecule, in crude oil using high-resolution continuum source graphite furnace molecular absorption spectrometry (HR-CS GF MAS) has been developed. The molecular absorbance of the SnS has been measured using the wavelength at 271.624nm and the crude oil samples were prepared as micro-emulsions due to their high viscosity. Several chemical modifiers (Ir, Pd, Ru, Zr) were tested and palladium was chosen, because it exhibited the best performance. The heating program was optimized by comparing the pyrolysis and vaporization curves obtained for an aqueous sulfur standard and a micro-emulsion of a crude oil certified reference material (CRM). The optimum pyrolysis and vaporization temperatures were found to be 600 and 2000°C, respectively. The limit of detection and the characteristic mass using micro-emulsion analysis of crude oil samples were 5.8 and 13.3ng S. Accuracy and precision of the method has been evaluated using two crude oil CRM (NIST 2721 and NIST 2722), showing good agreement with the informed or certified values.

Application of direct solid sample analysis for the determination of chlorine in biological materials using electrothermal vaporization inductively coupled plasma mass spectrometry

This work describes a methodology developed to carry out Cl determination in biological materials using electrothermal vaporization inductively coupled plasma mass spectrometry and direct solid sample analysis. The solid samples were directly weighed into graphite ‘cups’ and inserted into the graphite furnace. The RF power and the carrier gas flow rate were optimized at 1300W and 0.7Lmin−1, respectively. Calibration could be carried out using aqueous standard solutions with pre-dried modifiers (Pd+Nd or Pd+Ca) or using solid certified reference materials with the same pre-dried modifiers or without the use of modifiers. The limit of quantification was determined as 5μgg−1 under optimized conditions and the Cl concentration was determined in five certified reference materials with certified concentrations for Cl, in addition to three certified reference materials, for which certified values for Cl were unavailable; in the latter case, the results were compared with those obtained using high-resolution continuum source molecular absorption spectrometry. Good agreement at a 95% statistical confidence level was achieved between determined and certified or reference values.

A simple and fast method for assessment of the nitrogen–phosphorus–potassium rating of fertilizers using high-resolution continuum source atomic and molecular absorption spectrometry

The determination of N, P, and K in fertilizers by high-resolution continuum source flame atomic and molecular absorption spectrometry is proposed. Under optimized conditions, measurements of the diatomic molecules NO and PO at 215.360 and 247.620nm, respectively, and K using the wing of the alternative line at 404.722nm allowed calibration curves to be constructed in the ranges 500–5000mgL−1 N (r=0.9994), 100–2000mgL−1 P (r=0.9946), and 100–2500mgL−1 K (r=0.9995). Commercial fertilizers were analyzed by the proposed method and the concentrations of N, P, and K were found to be in agreement with those obtained by Kjeldahl, spectrophotometric, and flame atomic emission spectrometry methods, respectively, at a 95% confidence level (paired t-test). A phosphate rock certified reference material (CRM) was analyzed and the results for P and K were in agreement with the reference values. Recoveries from spiked CRM were in the ranges 97–105% (NO3−-N), 95–103% (NH4+-N), 93–103% (urea-N), 99–108% (P), and 99–102% (K). The relative standard deviations (n=12) for N, P, and K were 6, 4, and 2%, respectively.

Determination of sulfur in diesel via CS molecule by high-resolution molecular absorption spectrometry applying palladium nanoparticles as chemical modifier

Crude oil contains a high concentration of sulfur, an element that poses serious environmental and health risks. Sulfur occurs mostly in heavier petroleum fractions, like diesel, and it is possible to reduce its concentration by using methods such as catalytic hydrodesulfurization. High-resolution continuum source molecular absorption spectrometry (HR-CS MAS) allows for determination of non-metals. Sulfur is generally analyzed as the CS molecule at 258 nm, which corresponds to the electronic transition X1Σ+ → A1Π. This study aimed to apply graphite furnace HR-CS MAS to determine sulfur in diesel, using palladium nanoparticles as chemical modifier. To evaluate the proposed method, seven diesel samples (including two reference samples) were employed as emulsions with surfactants and alcohols. Direct diesel analysis was also evaluated, in an attempt to improve sulfur detection. Sodium sulfate, thiourea, and 2-mercaptoethanol were used as sulfur standards. Two types of synthesized palladium nanoparticles were compared with other chemical modifiers in graphite furnace vaporization. Measurements were conducted at 258.056 nm, with detection of three pixels. The ethanol emulsion was stable (4 h). PdCl2 nanoparticles provided the best results as chemical modifier, and thiourea was the most suitable sulfur standard under the experimental conditions. The following figures of merit were obtained: limit of detection (LOD) = 120 mg kg−1, limit of quantification (LOQ) = 400 mg kg−1, R2 = 0.9972, and relative standard deviation = 1–5%. The method's accuracy, evaluated with two reference samples, agreed with a confidence interval of 95%. It was not possible to determine sulfur in one sample only (sulfur concentration 10 mg kg−1), due to the LOD. This drawback was circumvented by applying direct diesel analysis. Using Pd nanoparticles as chemical modifier it was possible to obtain better sensitivity, LOD (7.2 ng absolute or 3 mg kg−1 for direct analysis), and LOQ (24 ng or 9 mg kg−1).

Determination of sulfur in coal by solid sampling graphite furnace high-resolution molecular absorption spectrometry

An analytical procedure for quantitative analysis of sulfur in coal by direct solid sampling graphite furnace high-resolution continuum source molecular absorption spectrometry (HR-CS GF MAS) is described, which is based on the molecular absorption of tin sulfide. The heating program is optimized, and a series of chemical modifiers (Ir, Mg, Pd, Ru, W, Zr) are tested. All measurements are accomplished with an Ir coated platform and Pd as modifier, because the best results are achieved.Accurate results are obtained using the central SnS line of the band found at 271.578nm with precision values between 1 and 6%. A limit of detection of 10ngS absolute is achieved. Furthermore, the trueness of the procedure is verified by analysis of certified reference materials. Several coal samples with a concentration range from 0.8 to 21.7mgg−1S and three certified reference materials are analyzed. In all cases, simple calibration with aqueous standards is found to be possible and the sample throughput is 2–3 samples per hour (6 replicates per sample).

A highly sensitive method for in vitro testing of fluorinated drug candidates using high-resolution continuum source molecular absorption spectrometry (HR-CS MAS).

We report here the development, optimization, and evaluation of a highly sensitive method for the determination of fluorine in biological matrices employing highresolution continuum source molecular absorption spectrometry (HR-CS MAS), suitable for pharmacological testing of fluorine-containing drug candidates. For this purpose, the most important parameters were studied in detail and subsequently optimized using a multivariate approach based on experimental design methodology. We developed a new approach employing a graphite tube lined with tantalum foil, thereby significantly enhancing sensitivity, while interferences from phosphorus monoxide (PO) molecular absorption due to the complex phosphate-rich matrix were completely eliminated. The limit of detection and the characteristic mass were 5.79 and 6.08 pg F, respectively. In order to evaluate the accuracy of the procedure, a recovery test was performed using spiked samples from three bioassays (i.e., DNA binding, protein binding, and cellular uptake) and the recovery rates ranged from 97.4 to 106.4%. The proposed method is applicable for preclinical in vitro testing of fluorinated drug molecules and thereby establishes HR-CS atomic absorption spectrometry instrumentation as a universal tool in medicinal chemistry.

Method development for the determination of bromine in coal using high-resolution continuum source graphite furnace molecular absorption spectrometry and direct solid sample analysis

This work reports a simple approach for Br determination in coal using direct solid sample analysis in a graphite tube furnace and high-resolution continuum source molecular absorption spectrometry. The molecular absorbance of the calcium mono-bromide (CaBr) molecule has been measured using the rotational line at 625.315nm. Different chemical modifiers (zirconium, ruthenium, palladium and a mixture of palladium and magnesium nitrates) have been evaluated in order to increase the sensitivity of the CaBr absorption, and Zr showed the best overall performance. The pyrolysis and vaporization temperatures were 800°C and 2200°C, respectively. Accuracy and precision of the method have been evaluated using certified coal reference materials (BCR 181, BCR 182, NIST 1630a, and NIST 1632b) with good agreement (between 98 and 103%) with the informed values for Br. The detection limit was around 4ng Br, which corresponds to about 1.5μgg−1 Br in coal, based on a sample mass of 3mg. In addition, the results were in agreement with those obtained using electrothermal vaporization inductively coupled plasma mass spectrometry, based on a Student t-test at a 95% confidence level. A mechanism for the formation of the CaBr molecule is proposed, which might be considered for other diatomic molecules as well.

Direct determination of fluorine in niobium oxide using slurry sampling electrothermal high-resolution continuum source molecular absorption spectrometry

Aiming for a round-robin test, a new method for the direct determination of fluorine in niobium oxide has been developed. It is based on the use of high-resolution molecular absorption spectra of calcium mono-fluoride (CaF) generated in the graphite tube, combined with the slurry sampling technique. The absorption measurement was performed at the 606.44nm CaF rotational line. By using graphite tubes with zirconium carbide (ZrC) modified platform, the molecular absorption sensitivity of CaF has been improved by a factor of 20, and no additional chemical modifier was necessary. Generally, non-spectral interferences were observed in the presence of HCl, H2SO4, and H3PO4. For HCl, additional spectral interference occurred due to an overlap of the absorption spectra of CaF and CaCl. However, due to the absence of these mentioned substances in the current material, such interferences do not exist for this application. The characteristic mass found for the CaF 606.44nm line was 0.1ng; the limit of detection was 5mg fluorine per kg solid sample (3σ criterion). The results obtained by the method were within the range of certified values. Comparing to the classical method such as the pyrohydrolysis-photometric method, the developed new method showed clear advantages regarding sensitivity and specificity. The time requirement for one sample analysis was strongly shortened from several hours to only some minutes.

Progress in the determination of metalloids and non-metals by means of high-resolution continuum source atomic or molecular absorption spectrometry. A critical review

This work examines the new possibilities introduced with the arrival of commercially available high-resolution continuum source atomic absorption spectrometers for the determination of metalloids (B, Si, Ge, As, Se, Sb and Te) and non-metals (P, S, F, Cl, Br, I and N-based species), such as the improved potential to detect and correct for spectral overlaps and the strategies available to correct for matrix effects. In particular, and considering the increasing number of papers reporting on the use of molecular absorption spectrometry using graphite furnaces and flames as vaporizers, the work discusses in detail the advantages and limitations derived from the monitoring of molecular spectra from a practical point of view, in an attempt to guide future users of the technique.

Determination of sulfur in coal using direct solid sampling and high-resolution continuum source molecular absorption spectrometry of the CS molecule in a graphite furnace

An analytical method has been developed for the determination of sulfur in coal using direct solid sample analysis in a graphite tube furnace and high-resolution continuum source molecular absorption spectrometry (HR-CS GF MAS). The molecular absorbance of the carbon monosulfide molecule (CS), which is formed in the vaporization stage, has been measured using the rotational line at 258.033nm. Several chemical modifiers were tested and Ru, applied as permanent modifier was chosen, because it exhibited the best performance. The optimum pyrolysis and vaporization temperatures were found to be 500°C and 2200°C, respectively. Aqueous standard solutions prepared from l-cysteine were used for calibration, as the linear regression obtained for this standard was not significantly different from that for a certified coal reference material (CRM) according to a Student t-test. The results obtained for sulfur in three coal CRM and six additional samples also showed no significant difference for the two calibration techniques according to the same statistical test. The sulfur concentration in the coal samples was found between 3.5mgg−1 and 33.7mgg−1 with a typical repeatability around 10%. The limit of detection for the direct analysis of solid coal samples was better than 0.1μgS.

The chemical generation of NO for the determination of nitrite by high-resolution continuum source molecular absorption spectrometry

In the present work, we propose a method for the determination of nitrite based on the chemical generation of nitric oxide (NO) and its detection by high-resolution continuum source molecular absorption spectrometry. NO is generated by the reduction of nitrite in acidic media with ascorbic acid as the reducing agent and then transferred into a quartz cell by a stream of argon carrier gas. The conditions under which the NO is generated are as follows: 0.4molL−1 hydrochloric acid, 1.5%(w/v) ascorbic acid, an argon gas pressure of 0.03MPa and an injection time of the reducing agent of 4s. All measurements of molecular absorption were performed using the NO line at 215.360nm, and the signal was measured by peak height. Under these conditions, the method described has limits of detection and quantification of 0.045 and 0.150μgmL−1 of nitrite, respectively. The calibration curve is linear for nitrite concentrations in the range 0.15–15μgmL−1. The precision, estimated as the relative standard deviation (RSD), was 3.5% and 4.4% for solutions with nitrite concentrations of 0.5 and 5.0μgmL−1, respectively. This method was applied to the analysis of different water samples (well water, drinking water and river water) collected in Cachoeira City, Bahia State, Brazil. The results were in agreement with those obtained by a spectrophotometric method using the Griess reaction. Addition/recovery tests were also performed to check the validity of the proposed method. Recoveries of 93–106% were achieved.

Fluorine Speciation Analysis Using Reverse Phase Liquid Chromatography Coupled Off-Line to Continuum Source Molecular Absorption Spectrometry (CS-MAS): Identification and Quantification of Novel Fluorinated Organic Compounds in Environmental and Biological Samples

Driven by increasing demand for the monitoring of industrial perfluorinated compounds (PFCs), the identification of novel fluorine containing compounds (FOCs) and the tracking of organofluorine drugs and their degradation products, there is a clear need for sensitive, fluorine-specific detection of unknown FOCs. Here we report the first ever direct fluorine-specific (speciation) method; capable of individually detecting untargeted FOCs in environmental and biological samples through the application of continuum source molecular absorption spectrometry (CS-MAS) using a commercial CS-AAS. Two model FOCs (2,4,6, trifluorobenzoic acid (TFBA) and 5-fluoroindol-5-carboxylic acid (FICA)) were used, achieving fluorine-specific detection across a range of 0.1 to 300 ng/mL fluorine, corresponding to a limit of detection of 4 pg F and 5.26 nM for both compounds. Both TFBA and FICA showed a similar response to CS-MAS detection, potentially enabling the quantification of fluorine content in novel FOCs without having molecular standards available. This paper also reports the use of reverse-phase high performance liquid chromatography (RP-HPLC) coupled off-line with CS-MAS for the identification of single organofluorines in a mixture of FOCs via fraction collection. The linear range of both FOCs was determined to be from 1 to 500 ng/mL. The limits of detection of those species were just above 1 ng/mL (100 pg) and can therefore compete with targeted analytical methods such as ESI-MS. Finally, as a proof of principle the analysis of a fluoride-containing groundwater sample from Ghana demonstrated that this method can be used in the detection of novel FOCs, with identification achieved through parallel ESI-MS. Coupled HPLC-CS-MAS/ESI-MS is the first analytical methodology capable of selectively detecting and identifying novel FOCs, making possible the quantification of all fluorine containing compounds in one sample. This is the necessary analytical requirement to perform fluoronomics.

Determination of bromine using high-resolution continuum source molecular absorption spectrometry in a graphite furnace

An analytical method is described for the quantitative analysis of bromine in water solutions as well as in organic solvents using high-resolution continuum source molecular absorption spectrometry (HR-CS-MAS) in a graphite furnace. Bromine is determined by means of molecular absorption of calcium bromide at 625.315nm. First of all the time-temperature program is optimized and the measurements are accomplished with graphite tubes without integrated PIN platform modified with permanent zirconium modifier and with a Pd/Mg-Modifier. The linear range is extended up to six orders of magnitude (from pg to μg-range, absolute) and a limit of detection of 78pg Br absolute is achieved. The accuracy of the method is verified by the recovery rate of two inorganic and one organic salt, and the determination of a whirlpool disinfectant agent as matrix burdened sample. Also, spectral and non-spectral interferences are investigated, whereby only in the case of concentrations in the higher mgL−1-range, non-spectral interferences are observed for fluorine, chlorine, iodine, aluminum, and iron. Next to determination of bromine in water solutions, analysis of polybrominated flame retardants (BFRs) is presented. BFRs are under suspicion of having partial toxic and neurotoxic effects. That is why an easy-to-handle method for the determination of bromine to calculate afterwards the BFR contents by means of the bromine mass fraction is of current interest. After adaptation of the heating program to organic solvents, bromine in three BFRs is determined quantitatively. So, the determination of bromine via HR-CS-MAS is suited for the analysis of low-matrix burdened samples and also in organic compounds. Anymore, this method is a fast, easy-to-handle technique with a low limit of detection.

Determination of chlorine in food samples via the AlCl molecule using high-resolution continuum source molecular absorption spectrometry in a graphite furnace

Determination of chlorine using the molecular absorption of aluminum mono-chloride (AlCl) at the 261.418nm wavelength was accomplished by high-resolution continuum source molecular absorption spectrometry using a transversely heated graphite tube furnace with an integrated platform. For the analysis, 10μL of the sample followed by 10μL of a solution containing Al–Ag–Sr modifier, (1gL−1 each), were directly injected onto the platform. A spectral interference due to the use of Al–Ag–Sr as mixed modifier was easily corrected by the least-squares algorithm present in the spectrometer software. The pyrolysis and vaporization temperatures were 500°C and 2200°C, respectively. To evaluate the feasibility of a simple procedure for the determination of chlorine in food samples present in our daily lives, two different digestion methods were applied, namely (A) an acid digestion method using HNO3 only at room temperature, and (B) a digestion method with Ag, HNO3 and H2O2, where chlorine is precipitated as a low-solubility salt (AgCl), which is then dissolved with ammonia solution. The experimental results obtained with method B were in good agreement with the certified values and demonstrated that the proposed method is more accurate than method A. This is because the formation of silver chloride prevented analyte losses by volatilization. The limit of detection (LOD, 3σ/s) for Cl in methods A and B was 18μgg−1 and 9μgg−1, respectively, 1.7 and 3.3 times lower compared to published work using inductively coupled plasma optical emission spectrometry, and absolute LODs were 2.4 and 1.2ng, respectively.

Quantification of the fluorine containing drug 5-fluorouracil in cancer cells by GaF molecular absorption via high-resolution continuum source molecular absorption spectrometry

The development of high-resolution continuum source molecular absorption spectrometry made the quantification of fluorine feasible by measuring the molecular absorption as gallium monofluoride (GaF). Using this new technique, we developed on the example of 5-fluorouracil (5-FU) a graphite furnace method to quantify fluorine in organic molecules. The effect of 5-FU on the generation of the diatomic GaF molecule was investigated. The experimental conditions such as gallium nitrate amount, temperature program, interfering anions (represented as corresponding acids) and calibration for the determination of 5-FU in standard solution and in cellular matrix samples were investigated and optimized. The sample matrix showed no effect on the sensitivity of GaF molecular absorption. A simple calibration curve using an inorganic sodium fluoride solution can conveniently be used for the calibration. The described method is sensitive and the achievable limit of detection is 0.23ng of 5-FU. In order to establish the concept of “fluorine as a probe in medicinal chemistry” an exemplary application was selected, in which the developed method was successfully demonstrated by performing cellular uptake studies of the 5-FU in human colon carcinoma cells.

Feasibility of high-resolution continuum source molecular absorption spectrometry in flame and furnace for sulphur determination in petroleum products

For the first time, high-resolution molecular absorption spectrometry with a high-intensity xenon lamp as radiation source has been applied for the determination of sulphur in crude oil and petroleum products. The samples were analysed as xylene solutions using vaporisation in acetylene-air flame or in an electrothermally heated graphite furnace. The sensitive rotational lines of the CS molecule, belonging to the ∆ν = 0 vibrational sequence within the electronic transition X 1∑ + → A 1П, were applied. For graphite furnace molecular absorption spectrometry, the Pd + Mg organic modifier was selected. Strong interactions with Pd atoms enable easier decomposition of sulphur-containing compounds, likely through the temporal formation of Pd xS y molecules. At the 258.056 nm line, with the wavelength range covering central pixel ± 5 pixels and with application of interactive background correction, the detection limit was 14 ng in graphite furnace molecular absorption spectrometry and 18 mg kg −1 in flame molecular absorption spectrometry. Meanwhile, application of 2-points background correction found a characteristic mass of 12 ng in graphite furnace molecular absorption spectrometry and a characteristic concentration of 104 mg kg −1 in flame molecular absorption spectrometry. The range of application of the proposed methods turned out to be significantly limited by the properties of the sulphur compounds of interest. In the case of volatile sulphur compounds, which can be present in light petroleum products, severe difficulties were encountered. On the contrary, heavy oils and residues from distillation as well as crude oil could be analysed using both flame and graphite furnace vaporisation. The good accuracy of the proposed methods for these samples was confirmed by their mutual consistency and the results from analysis of reference samples (certified reference materials and home reference materials with sulphur content determined by X-ray fluorescence spectrometry).

High-Resolution Continuum Source Atomic and Molecular Absorption Spectrometry—A Review

The present article gives an overview over the historic development, the instrumentation, the special features, and the applications of high-resolution continuum source atomic and molecular absorption spectrometry. The most obvious advantage is the need for only one single lamp for all elements and wavelengths, compared to one lamp for each analyte, as is necessary in line source atomic absorption spectrometry. The visibility of the spectral environment of the analytical line at high resolution greatly helps in method development and to avoid spectral interferences. The advanced simultaneous background correction automatically eliminates lamp flicker noise and continuous background absorption. Fine-structured background absorption may be removed using a reference spectrum and a least-squares algorithm. Because any wavelength between 190 nm and 900 nm can be accessed, the same equipment can be used for molecular absorption spectrometry, making possible the determination of non-metals, such as phosphorus, sulfur, and the halogens. Although commercial equipment for this technique has only been available for a few years, the technique has been accepted extremely well, and the number of applications that have been published using this technique has already reached an impressive number.

Determination of sulfur in biological samples using high-resolution molecular absorption spectrometry in a graphite furnace with direct solid sampling

The determination of sulfur in biological materials using high-resolution continuum source molecular absorption spectrometry and electrothermal vaporization of the carbon monosulfide (CS) molecule has been investigated in detail using direct solid sampling. Best results have been obtained coating the platform with tungsten as a permanent modifier, adding 40 μg Pd in solution as a chemical modifier, and the only sulfur compound that showed sufficient sensitivity and thermal stability to be used for calibration purposes under the conditions established for biological materials was thiourea. A pyrolysis temperature of at least 900 °C could be used and the optimum vaporization temperature was 2500 °C. Under optimized conditions a limit of detection of 0.015 μg S absolute or 0.03 mg g−1 S in the solid sample, based on a sample mass of 0.5 mg could be obtained; the characteristic mass was m0 = 18 ng. Five certified biological reference materials have been analyzed using direct solid sampling and calibration against aqueous standards; the results were in agreement with the certified values on a 95% confidence interval.