Gerhard Schlemmer

Palladium and magnesium nitrates, a more universal modifier for graphite furnace atomic absorption spectrometry

Abstract A mixture of palladium and magnesium nitrates was found to be a very powerful modifier for the determination of As, Bi, In, Pb, Sb, Se, Sn, Te and Tl in graphite furnace atomic absorption spectrometry. Thermal pretreatment temperatures of 900-1400°C can be used with the proposed modifier. This is in most cases substantially more than what can be applied with the modifiers recommended up to now, so that separation of the analyte from the concomitants should be easier. This is shown to be true for the determination of lead in sea water and of selenium in biological materials. Optimum atomization temperatures are more uniform and typically around 2000°C for the investigated elements when the palladium and magnesium nitrates mixed modifier is used. This modifier therefore allows the use of common conditions for all the investigated elements with a minimum sacrifice in sensitivity, an important pre-requirement for multi-element furnace techniques. The proposed mixed modifier also minimizes the risk of contamination because palladium as well as magnesium nitrate can be obtained in high purity, and both elements are infrequently determined in the graphite furnace.

Palladium nitrate–magnesium nitrate modifier for electrothermal atomic absorption spectrometry. Part 5. Performance for the determination of 21 elements

The determination of 21 elements was investigated in the presence of the palladium nitrate–magnesium nitrate (Pd–Mg) mixed modifier. Significantly higher pyrolysis temperatures could be used for the majority of elements compared with previously recommended modifiers, making possible an efficient removal of concomitants prior to the atomization of analyte elements. A 10 µg amount (1 g l–1) of NaCl had no influence on the determination of all of the elements investigated and most of the analyte elements were not affected even by a ten times higher concentration. The concentration of potassium sulfate, which had no influence on the determination, was lower for most of the elements. The tolerance towards NaCl and K2SO4 increased significantly for a number of elements when the pyrolysis temperature was lowered by 200–300 °C and the pyrolysis time was increased. For most of the elements investigated the characteristic mass values were comparable to, or better than published data. For some elements, however, such as bismuth, lead and tellurium, the characteristic mass was significantly higher. This resulted from the high stabilizing power of the Pd–Mg modifier, which necessitated higher atomization temperatures for these elements for which diffusional losses were more pronounced. The advantages and limitations of the Pd–Mg modifier are discussed.

Palladium nitrate-magnesium nitrate modifier for graphite furnace atomic absorption spectrometry. Part 2. Determination of arsenic, cadmium, copper, manganese, lead, antimony, selenium and thallium in water

If the palladium nitrate-magnesium nitrate modifier is applied to the analysis of water, a common set of conditions for the determination of As, Cu, Mn, Pb, Sb and Se can be used. For Cd and Tl lower pyrolysis and atomisation temperatures are required. Twenty-one possible interferents were investigated at maximum concentrations between 1 and 500 mg l–1 and only two were found to have an effect greater than ±10%. The determination of Tl was affected by sodium chloride at concentrations above 100 mg l–1, hence the addition of Li is recommended to control this interference. Five mg l–1 of Fe decreased the Se signal by about 15% when deuterium-arc background correction was used but had no influence in an instrument with Zeeman-effect background correction. For an instrument with continuum source background correction it was also necessary to determine Cu using the secondary resonance line at 327.4 nm because of spectral interference due to Pd at the primary resonance line. Precision was about 1% in the optimum working range and between 2 and 5% at 0.05 A s. Detection limits (2.33 σ) were 1.2 µg l–1 for As, 0.02 µg l–1 for Cd, 0.3 µg l–1 for Cu, 0.45 µg l–1 for Mn and Pb, 1.2 µg l–1 for Sb, 0.6 µg l–1 for Se and 0.8 µg l–1 for Tl.

Modifiers and coatings in graphite furnace atomic absorption spectrometry—mechanisms of action (A tutorial review)

Abstract A multitude of different and often contradictory mechanisms for the effects of modifiers and coatings have been proposed. Many of these proposals lack sufficient experimental evidence. Therefore, a series of statements based on our own investigations is given as ‘facts’. Another series of statements is made as ‘fictions’ related to erroneous proposals on the functioning of modifiers and coatings in the pertinent literature. Two basic concepts are developed for the sequence of processes leading to analyte stabilization for the two most important groups of modifiers: refractory carbide forming elements of the IVa–VIa groups of the periodic system on the one hand and Pt-group metals on the other hand. These concepts are based on the main reactions of graphite with elements and compounds: carbide formation and intercalation. Most important experimental results leading to this understanding are described: Penetration measurements for modifiers and analytes indicated the subsurface zone down to approximately 10 μm as the essential place for graphite–analyte–modifier interactions. The reason for this phenomenon is an open porosity of the pyrocarbon coating of 5–10% (v/v) into which liquids penetrate upon sample application. This also indicates that modifiers are best applied by impregnation or electrolysis whereas dense coatings are not advantageous. It is also shown that graphite tube assemblies are dynamic systems with a limited lifetime and carbon losses are an essential feature of tube corrosion. Most frequently found erroneous statements are discussed: (a) Particles on the tube surface are responsible for analyte stabilization and retention during pyrolysis. (b) Analyte stabilization is taking place by formation of intermetallic compounds or thermally stable alloys. (c) Experiments are performed with unrealistic concentrations of analytes and/or modifiers. (d) Dense coatings are advantageous. Finally, a functional schedule is given for the three steps of graphite furnace atomic absorption spectrometry (GFAAS): sample application and drying; pyrolysis; atomization. Contrary to the vast amount of literature on this topic it tried to provide the analyst working with GFAAS and in an increasing number working with Solid Sampling-GFAAS with a set of most important statements. This might spare the experimentalist a lot of useless optimization procedures but should lead him to a basic understanding of the complex phenomena taking place in his instrument and during his analytical work.

Spatially and temporally resolved gas phase temperature measurements in a Massmann-type graphite tube furnace using coherent anti-Stokes Raman scattering

Abstract The temperature of the nitrogen gas phase in a graphite tube furnace for atomic absorption spectrometry has been determined using coherent anti-Stokes Raman scattering (CARS). Subtle details of the temperature evolution at various locations in the tube have been identified. Under steady-state conditions, the temperatures of the tube wall and of the gas phase near the tube centre are essentially identical. The longitudinal gradient of the gas phase temperature between the tube centre (heated to 2700 K)andthetube ends is around 1200 K. This is less than that predicted by model calculations. During rapid heating, typically used for atomization of the analyte, the gas follows the wall temperature very closely and with essentially the same heating rate. Irregularities in this heating pattern, such as an intermediate slowing of the heating rate 0.3 s after start of heating, are most probably caused by gas expansion during the period of rapid tube heating. A pronounced radial temperature gradient was observed in the gas phase of tubes with inserted platform during the rapid heating phase, but not in tubes without a platform. The gradient in the gas phase disappears within about 0.5 s after the tube wall has reached the preset temperature. When the platform technique is used and the temperature program selected with care, volatilization of the analyte can be delayed until the tube wall and the gas phase have almost reached their final temperatures and are close to thermal equilibrium.

Communication. Long-term stability of a mixed palladium–iridium trapping reagent for in situ hydride trapping within a graphite electrothermal atomizer

For hydride trapping and atomization in a graphite electrothermal atomizer the treatment of a pyrolytic graphite coated electrographite tube with a single, manual application of a mixed trapping reagent of 50 µg of palladium and 50 µg of iridium allows up to 300 complete trapping and atomization cycles. Precision is generally better than 3% and the efficiency of trapping is virtually 100% for As, Se and Bi. The application of this mixed palladium–iridium trapping reagent improves the routine use of this technique and considerably simplifies the hardware and software requirements to provide a fully automatic system.

Influence of the valency state on the determination of selenium in graphite furnace atomic absorption spectrometry

Abstract Several matrix modifiers have been investigated with respect to their stabilizing power for selenium in a graphite furnace. It was found that the different valency states of selenium are not stabilized to the same extent by frequently applied matrix modifiers like nickel or copper. This may be an explanation for pre-atomization losses and high characteristic mass data reported previously. Copper, mixed with magnesium nitrate, was found to be the best matrix modifier for selenium. Careful selection of the time-temperature program for thermal pretreatment in the graphite furnace is essential to avoid pre-atomization losses. A characteristic mass ( m 0 )of 18–20 pg/0.0044 A.s was obtained for selenium under optimized conditions.

Palladium nitrate—magnesium nitrate modifier for electrothermal atomic absorption spectrometry. Part 3. Determination of mercury in environmental standard reference materials

A single addition of 15 µg of palladium is sufficient for 40–60 determinations of mercury if the graphite tube is not heated to temperatures higher than 1500 °C in the cleaning step. A pyrolysis temperature of 400 °C can be used without any loss of mercury, and a characteristic mass of 0.1 ng of mercury was obtained for an atomization temperature of 1000 °C. Interferences caused by high sodium chloride concentrations could be eliminated by using a mixture of 95% argon and 5% hydrogen as the purge gas. Mercury could be determined accurately in Aquatic Plant, Albacore Tuna, River Sediment and Coal Fly Ash reference materials.

Determination of arsenic, cadmium, lead and selenium in highly mineralized waters by graphite-furnace atomic-absorption spectrometry

A graphite-furnace AAS method using the stabilized-temperature platform furnace (STPF) concept, mixed palladium and magnesium nitrates as chemical modifier and Zeeman background correction has been applied to the direct determination of As, Cd, Pb and Se in highly mineralized waters used for medicinal purposes. These contain 20-40 g/l. concentrations of salts, mainly sodium and magnesium chlorides, bicarbonates and sulphates. The use of a pre-atomization cool-down step to 20 degrees in the graphite-furnace programme reduced the background absorption. Increasing the mass of magnesium nitrate modifier to 5 times that originally proposed improved the analyte peak shape. Under these conditions, no interference was found in analysis of the chloride/bicarbonate type of water, but the sodium and magnesium sulphate type of water had to be diluted, and even then an interference remained. Calibration with matrix-free standard solutions was used, but use of spike recovery is strongly recommended for testing the accuracy. The limits of determination (4.65sigma) of the proposed method for undiluted samples are 2.0 mug/l. for As, 0.05 mug/l. for Cd, 1.0 mug/l. for Pb and 1.5 mug/l. for Se.

Investigations of interferences in graphite furnace atomic absorption spectrometry using a dual-cavity platform. Part 2. Influence of sodium chloride and nickel chloride on the atomisation of lead

The interferences from sodium and nickel chlorides on the determination of lead which occur in the absence of a modifier are not due to gas-phase interactions. Some lead chloride is lost in the form of gaseous molecules at temperatures above 500 °C. In the presence of nickel chloride, lead is lost primarily by co-volatilisation with the hydrogen chloride gas, which is generated by the decomposition of the salt. Expulsion of the analyte by matrix vapours was found to be a source of interference with both of the chlorides investigated. Nitric acid had no influence on the interference from sodium chloride but reduced the interference from nickel chloride. The stabilising effect found on the addition of both nitric acid and nickel chloride was due to nickel only. Various interactions between the gas phase and the condensed phase were observed when analyte and interferent were separated on the dual-cavity platform. The determination of lead in the presence of sodium or nickel chlorides was free from interferences when ammonium phosphate-magnesium nitrate was added as a modifier.