Susan McIntosh

Hydride generation flow injection using graphite furnace detection: emphasis on determination of tin

Abstract Hydride generation, (HG), using flow injection methods, (FI), provides an ideal way to separate the hydride-forming analytes from a large volume of matrix. By trapping the hydrides on a Pd-coated Lvov platform at low temperature, it can be shown that the full process is at least 80% efficient for As, Bi, Ge, Sb, Se, Sn and Te. While this work was not fully automated, the various functions were conducted on-line. This method has been tested for the determination of Sn in various samples. The Pd-treated stabilized temperature platform furnace, (STPF), was used for in situ trapping and atomization of the analyte. Integrated absorbances were used for the measurement of the analytical signal. The detection limit for Sn was 7 ng l for a 10-ml sample. The analytical precision was 3–5% RSD at 1 ng of Sn. Inter-element interferences were investigated. The method has been applied to the determination of Sn in NIST Steel, River Sediment, Orchard Leaves and Bovine Liver Standard Reference Materials.

Determination of Se in urine by flow injection hydride generation electrothermal atomic absorption spectrometry with in-atomizer trapping

Following digestion of the sample in a mixture of bromate and hydrobromic acid, the inorganic selenium produced was quantified by trapping hydrogen selenide, formed when a 500 μl sample volume injected into a hydrochloric acid carrier stream merged with a stream of sodium borohydride solution, on the iridium-pretreated interior of a graphite furnace atomizer. A number of parameters relating to the digestion, flow injection manifold and trapping in the atomizer were investigated, including a study of factors affecting the detection limit. It was found necessary to heat the digest under reflux at a temperature of 150°C for 2 h. Quantitative recoveries, from a human urine matrix, of selenite, selenate, trimethylselenium, selenocystine, selenopurine and selenomethionine spikes were obtained. The efficiency of hydride generation, transport and trapping was 75%. The major factors affecting the detection limit were the reagent purity and the volume injected. For high-purity hydrobromic acid and borohydride free of caking agent, the detection limit, based on three times the standard deviation of the blank, was 0.06 μg l−1 for a 1000 μl injection volume corresponding to a detection limit of 3 μg l−1 for a urine sample. The method was validated by the accurate analyses of Standard Reference Material 2670 from the National Institute of Standards and Technology, and urine samples from an interlaboratory comparison program. The procedure avoids the need for perchloric acid and produces selenium in the +4 oxidation state and thus no reduction is needed prior to generation of the hydrogen selenide. The use of a graphite furnace atomizer avoids the need for frequent reconditioning of the atomizer surface and the need for the standard additions method, both of which are drawbacks of procedures which make use of the quartz tube atomizer. All sample handling procedures following the digestion were automated by the use of flow injection technology.

The use of nafion dryer tubes for moisture removal in flow injection chemical vapor generation atomic absorption spectrometry

Abstract A significant problem with flow injection chemical vapor generation atomic absorption spectrometry is a loss of sensitivity and blockage of transfer lines due to excessive moisture transported to the atom cell or lodged in the transfer line. A Nafion dryer tube was used to remove moisture from the wet carrier gas stream. The hygroscopic Nafion membrane removed water at 1.7 mg/min at an efficiency of 95 ± 4% when SnCl2 was used as the reductant. When 0.4% (m/v) borohydride was used as the reductant, 2.3 mg/min of water was removed at an efficiency of 91 ± 3%. No measurable change in precision was observed and a 5% reduction in peak height sensitivity (vs. PTFE) was seen for mercury when using the Nafion transfer line. At 60°C the Nafion dryer removed 4.9 mg/min of water vapor at an efficiency of 93 ± 3%. The loss of mercury vapor through the Nafion membrane was no more than 0.04%. Only a 3% reduction in peak height sensitivity was observed for the determination of arsenic and selenium. Detection limits for mercury, calculated from calibration data, were 77 ppt, 20 ppt and 150 ppt for the PTFE tube, the model MD-250 dryer and the model MD-125 dryer, respectively.

The determination of tin in steel samples by flow injection hydride generation atomic absorption spectroscopy

Abstract Conditions were studied for the determination of Sn in steel samples using flow injection, hydride generation and atomic absorption spectroscopy. Interferences were found for the determination of Sn in steel samples and an investigation showed that the interferences occurred in the generation of the hydride, not in the atomization of the gaseous hydride. Conditions for the determination of Sn by flow injection hydride generation were improved by using integrated absorbance signals and by the addition of oxygen to the argon carrier gas stream. Typically, 500 μl of a 10- μg l Sn standard provided an integrated absorbance signal greater than 0.5 A·s. The method provided a detection limit of about 0.05 μg l in a 500-μl sample. The precision ranged from 1 to 3% RSD at higher concentration levels of Sn. The method of additions yielded accurate results for several steel standard reference materials in those situations where the use of the standard calibration procedure was inadequate.

Examination of separation efficiencies of mercury vapour for different gas–liquid separators in flow injection cold vapour atomic absorption spectrometry with amalgam preconcentration

A comparison has been made of the separation efficiency of three designs of gas–liquid separator when used in a flow injection (Fl) manifold for the determination of Hg by cold vapour atomic absorption spectrometry. The manifold used with each device was separately optimized for maximum sensitivity. This involved studies of the effects of reagent flow rates, argon purge gas flow rate, injection time and post-injection purge time. A significant difference, with respect to both peak height and integrated signal sensitivity (by a factor of approximately 3) between the performance of a miniature design and that of two larger volume designs was obtained. No significant differences in precisions were observed. For the miniature design, the use of either tetrahydroborate or tin(II) reductant was investigated. No difference in peak height sensitivity was found, but the integrated signal sensitivity for the tetrahydroborate was 36% lower. The efficiency of separation was measured by comparison of the signal obtained from a known mass of Hg vapour, introduced via an amalgam preconcentration unit, and the signal obtained from a known mass of Hg in solution, introduced via the Fl manifold and amalgam preconcentration unit. The efficiencies were found to be 101 ± 4% and 103 ± 6% for peak height and integrated signal, respectively.

Effect of sample volume on the limit of detection in flow injection hydride generation electrothermal atomic absorption spectrometry

The analytical performance of methods for the determination of hydride forming elements has been improved recently by the development of procedures in which the hydride is trapped on the interior surface of a graphite furnace atomizer. The signal for a given concentration increases with increase in sample volume and it is often implied that a decrease in the limit of detection may also be achieved by increasing the sample volume. To evaluate this claim, a simple equation was derived which predicts the relationship between detection limit and sample volume when all the contributions to the blank are proportional to sample volume. A time-based approach to the variation of sample volume was developed to ensure that the analyte introduced from reagent contamination was, in fact, proportional to sample volume. Detection limits were measured for a series of sample volumes between 156 and 1560 µl. As the sample volume was increased, the detection limit improved significantly from 0.3 to around 0.05 µg l–1 up to a volume of about 500 µl. Between 500 and 1000 µl, a further improvement, to around 0.02 µg l–1, was obtained, but for volumes larger than 1000 µl no further significant improvement was obtained. Good agreement between the predicted and experimentally determined variations in detection limit with sample volume was obtained and thus the underlying inverse proportionality of the relationship between detection limit and sample volume was confirmed. This rectangular hyperbolic relationship has practical consequences for the extent to which detection limits can be improved by increasing the sample volume, even when the blank is very low or zero.

Effect of high salt concentrations on the determination of arsenic and selenium by flow injection hydride generation electrothermal atomic absorption spectrometry

In the determination of arsenic and selenium by flow injection hydride generation ETAAS, the presence of up to 20% sodium chloride enhanced the signals for 20 µg l–1 arsenic and selenium by up to 28%. The enhancement was obtained with a variety of gas–liquid separators. A systematic study of the possible causes of the signal enhancement in the determination of selenium was undertaken, from which it was concluded that the effect originated in the processes responsible for the distribution of the hydrogen selenide between the solution and gas phases. Processes related to the transport of the analyte from the gas–liquid separator and the trapping of the analyte on the interior of the atomizer were not affected by the presence of dissolved salts. As sodium was found to be transported to the atomizer, it was deduced that aqueous aerosol was deposited in the atomizer, although the quantities were irreproducible. The enhancement could be eliminated by increasing the borohydride concentration. However, with the small volume gas–liquid separator, this latter approach was limited because of carry-over of liquid to the atomizer. The effect could be compensated for by adding up to 40% m/v of salt to the borohydride reagent.