Richard Dams

The use of internal standards in icp ms

Careful study of the matrix effect in ICP-MS showed that, in all cases studied, the magnitude of the signal suppression or enhancement depends in a regular way on the mass number. Hence, accurate correction for non-spectral interferences is only possible using an internal standard with mass number close to that of the analyte element(s). It is also shown that using an internal standard with mass number close to that of the analyte improves the precision. For both cases, the ionization energy of the internal standard seems to be of no or only secondary importance. To obtain optimal precision and accuracy, the internal standard should be selected as close in mass number as possible to that of the analyte element(s). When a number of elements over a considerable mass range are to be determined, several internal standards have to be used.

Raman spectroscopic database of azo pigments and application to modern art studies

The application of Raman spectroscopy to the examination of ancient art and the identification of mineral pigments used in antiquity has been demonstrated successfully hitherto. However, the examination of modern objects of art is hampered by the absence of a spectral library database of relatively modern organic pigments and dyestuffs, of which there are a large number. Azo pigments provide an example historically of a class of synthetic organic pigments which have been adopted by artists. The Raman spectra of 21 azo pigments were analysed and a protocol was established for their identification. Using this database, two azo pigments from 1 µg samples of 20th century paintings attributed to Picasso and Appel were characterized. Copyright

Sodium tetra(n-propyl)borate: a novel aqueous in situ derivatization reagent for the simultaneous determination of organomercury, -lead and -tin compounds with capillary gas chromatography–inductively coupled plasma mass spectrometry

Abstract The versatility of sodium tetra(n-propyl)borate, NaBPr4, as aqueous in situ derivatization reagent for organometallic compounds is demonstrated. With this new derivatization reagent it is now possible to derivatize the important ethyl derivatives of lead and mercury which until now had to be derivatized by Grignard alkylation or hydride generation. The synthesis of NaBPr4 is described in detail. Derivatization parameters such as pH, reaction time, amount of reagent and stability of the aqueous NaBPr4 solution were investigated. Different organometallic compounds of tin, mercury and lead were simultaneously determined by an inductively coupled plasma mass spectrometer coupled to the capillary GC system using a laboratory-developed interface. Good linearity was obtained for all components with detection limits in the ng/l range (without preconcentration). The reproducibility of the complete procedure, i.e. derivatization, extraction and injection, is better than 10% R.S.D. The analysis of the PACS-1 Reference Material after derivatization with NaBPr4 showed the accuracy of this method.

Elimination of interferences in the determination of arsenic and selenium in biological samples by inductively coupled plasma mass spectrometry

Abstract The determination of As and Se in biological samples by inductively coupled plasma mass spectrometry is well known to be degraded by spectral interferences. The resolution of quadrupole mass analysers is insufficient to resolve As+ and Se+ ions from polyatomic species such as ArCl+, ArAr+ and So+3. A study of this problem in human serum also revealed substantial non-spectral interferences on these elements occuring in the presence of organic compounds. It is shown that both problems can easily be overcome by a combination of chemical modification (addition of 4% ethanol) with nebulizer flow-rate gas adjustment. Under these conditions the use of standard additions for calibration allowed As and Se to be determined accurately in samples of biological origin. The method developed was applied to human serum and urine and for both As and Se excellent agreement with certified values was obtained.

Distribution of platinum group elements (Pt, Pd, Rh) in environmental and clinical matrices: Composition, analytical techniques and scientific outlook: Status report.

Trace concentrations of the platinum group elements (PGE; here: Pt, Pd and Rh) play an important role in environmental analysis and assessment. Their importance is based on 1. their increasing use as active compartments in automobile exhaust catalysts, 2. their use as cancer anti-tumor agents in medicine. Due to their allergenic and cytotoxic potential, it is necessary to improve selectivity and sensitivity during analytical investigation of matrices like soil, grass, urine or blood.This paper summarizes the present knowledge of PGE in the fields of analytical chemistry, automobile emission rates, bioavailibility, toxicology and medicine.

‘Zone model’ as an explanation for signal behaviour and non-spectral interferences in inductively coupled plasma mass spectrometry

The zone model is a simplified representation of the plasma, resulting from the findings of an optimization study for a VG PlasmaQuad PQ1 inductively coupled plasma (ICP) mass spectrometer (VG Elemental, Winsford, Cheshire, UK). According to this model, for every nuclide there is a zone in the central channel of the ICP, where a maximum density of singly charged ions occurs. The position of such a zone of maximum M+ density is a function of the mass number of the nuclide and the zone can undergo a spatial displacement under the influence of an alteration of an instrumental parameter or the introduction of a different matrix. This representation not only enables an explanation of a large number of observations from the optimization study, but also allows an understanding of why both matrix induced signal suppression and enhancement were observed, why for a given matrix the extent to which the signal intensities were altered differed from day to day and finally why the extent to which a signal is influenced by the matrix was seen to be a function of the mass number of the corresponding nuclide. Although the zone model might not completely reflect the genuine physical reality in all its facets, it provides a phenomenological model for the variation of ion signals with mass number, operating parameters and matrix composition.

Precise Measurement of Isotope Ratios with a Double-Focusing Magnetic Sector ICP Mass Spectrometer

The potential of a commercially available double-focusing magnetic sector ICP mass spectrometer (Element, Finnigan MAT, Bremen, Germany) for precise isotope ratio measurement at the low-resolution setting (R = 300) was evaluated. Optimization of scanning conditions led to a relative standard deviation for a set of 10 consecutive 2 min measurements of ∼0.1% ((206)Pb(+)/(207)Pb(+)) at signal intensities of ∼200 000 counts/s (peak height). This compares favorably with the best values ever reported for quadrupole ICPMS and barely exceeds the theoretical value (counting statistics). Increasing the signal intensity to values ≥500 000 counts/s (peak height) resulted in a further reduction of the RSDs obtained (for both (25)Mg(+)/(26)Mg(+) and (206)Pb(+)/(207)Pb(+)) to typically 0.04%. These figures are remarkably better than those reported for commercially available quadrupole ICPMS systems. This improvement significantly reduces the difference between isotope ratio precision of ICPMS on one hand and those of thermal ionization mass spectrometry and plasma source multiple collector mass spectrometry on the other.

Evaluation of a commercially available microconcentric nebulizer for inductively coupled plasma mass spectrometry

The performance of a commercially available microconcentric nebulizer (MCN-100, CETAC Technologies, USA) operated at flow rates ranging from ⩽0.001 up to 0.65 ml min–1 was evaluated using a Perkin-Elmer Sciex ELAN 5000 ICP-mass spectrometer. The observations made were compared with those for the standard GemTip cross-flow nebulizer. Registration of signal behaviour plots (signal intensity as a function of the nebulizer gas flow rate) at different rf powers and at different sample uptake rates allowed firstly, a systematic optimization of the operation parameters, and secondly, a comparison of the signal behaviour observed when using both types of nebulizer. The stability of the MCN-100 was evaluated at different sample uptake rates and the occurrence of memory effects was checked for a number of elements. Also, the level and the behaviour of oxide and doubly charged ions was studied. Furthermore, the susceptibility to matrix effects was investigated using synthetic matrices of different origin (acid, organic and high salt content) and it was demonstrated that, generally, matrix effects observed with both nebulizers are comparable and the MCN-100 can be used with NaCl concentrations up to 4 g l–1 without capillary blocking. Finally, it is illustrated that the MCN-100 can be applied at sample flow rates of < 5 µl min–1, as are encountered when coupling capillary zone electrophoresis to ICP-MS for elemental speciation studies.

Speciation of six arsenic compounds using capillary electrophoresis-inductively coupled plasma mass spectrometry

Capillary electrophoresis (CE) was used to separate four anionic {arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid and dimethylarsinic acid} and two cationic forms (arsenobetaine and arsenocholine) of As in a single run. At sufficiently high concentrations, the determination of these compounds could be accomplished by means of UV detection. The determination of low concentrations (<10 µg l –1 ) of these compounds of interest was accomplished by coupling CE on-line with inductively coupled plasma mass spectrometry (ICP-MS). To accomplish this coupling, a microconcentric nebulizer was used. The modifications necessary to make a conventional CE system compatible with ICP-MS, the optimization of the operation parameters and of sample stacking conditions together with the effect of the sheath liquid and of an induced laminar flow are discussed. The analytical figures of merit of the method were assessed and the limit of determination (based on the peak height of a peak for which the signal-to-noise ratio is 10:1) was found to be 1-2 µg l –1 As for each species. The recovery for the compounds of interest was determined using a spiked mineral water sample. Samples of mineral water, soil leachate and urine were analyzed with the CE-ICP-MS combination.

Some figures of merit of a new double focusing inductively coupled plasma mass spectrometer

An ICP-MS instrument with a quadrupole filter is hampered by its limited resolution, in particular polyatomic ions are a major source of interference. With a double focusing magnetic sector mass analyser, operated at high resolution, most of these interferences are eliminated. In addition, these mass analysers are characterized by a low instrumental background, yielding superior detection limits at low resolution. In this work a new type of double focusing ICP-MS instrument, the ‘Element’ from Finnigan MAT (Bremen, Germany), was tested. The instrument can be used with low resolution (m/Δm= 300, 10% valley definition) and at high resolution settings (3000 and 7500). At low resolution, a linear response versus mass curve was observed [response of 180 × 106 counts s–1 per µg ml–1(ppm) for U and 6 × 106 counts s–1 for Be]. The sensitivity decreases by roughly a factor of 14 when going from resolution 300 to resolution 3000 and by a factor of 10 from 3000 to 7500. An instrumental detection limit at low resolution of about 8 fg ml–1(8 ppq) was measured at m/z 240. The detection limit for most elements is not determined by the instrument but by the blank level and laboratory procedures will need to be improved in order to make optimal use of the detection power of the instrument. The stability and the relative abundance of oxide and doubly charged ions are comparable to the values found in quadrupole ICP-MS. Matrix effects were compared for measurements with the ‘Element’ and with a Perkin-Elmer Elan 5000 quadrupole instrument of 115In in the presence of 500 µg ml–1 of Cs and of 2.5% of ethanol. Though in the ‘Element’ ions are accelerated over 8 kV immediately after the skimmer, matrix effects were found to be similar to those observed for the ‘Elan 5000’. The instrument was used to determine Pt in an environmental material (grass).