Sample Uptake Research Articles

Thermospray nebulization as sample introduction for inductively coupled plasma mass spectrometry

A sample introduction system, consisting of a thermospray nebulizer and a desolvating unit, was coupled to an inductively coupled plasma mass spectrometer. Several parameters were optimized including the sample uptake rate, the power delivered to the capillary tube, the temperature of the aerosol and of the cooling-water, the carrier gas flow rate and the r.f. power. Under optimized conditions, the sensitivity obtained with the thermospray system is a factor of 10 higher than with a pneumatic nebulizer combined with a spray chamber. The increase was observed for the following elements over the whole mass range: Be, Al, Sc, Co, In, Gd, Tl, Th and U. Since similar background levels, 10–40 counts s–1, were observed, better detection limits could be achieved (from 0.19 ng l–1 for U to 1.3 ng l–1 for Be). The relative standard deviation (RSD) on analyte signals for a short-term stability test (10 min) is between 3 and 8%. These values can be improved to less than 4% by the use of internal standardization. No drift in analyte signal during several hours was observed. The levels of oxide (MO+:M+) and doubly charged (M2+:M+) ions obtained with the thermospray system are about a factor of 2.5 lower than those obtained with the pneumatic system. Consequently, the oxide ion levels for elements with the highest MO bond strength and the doubly charged ion levels for elements with the lowest second ionization energy are not above 1%.

Automated two-speed peristaltic pump controller for reduced sample wash-out time and analysis of small sample volumes

An easily assembled electronic controller is shown, which allows automatic selection of the ‘high-speed purge’ or ‘rabbit’ mode of the commonly used Gilson Minipuls 2 peristaltic pump. Application is demonstrated on an inductively coupled plasma mass spectrometer system consisting of a Perkin-Elmer SCIEX Model 250 with Model 5000 computer hardware and software, and a Gilson Model 212B autosampler. The controller places the peristaltic pump in the ‘high-speed purge’ mode when the sipper is at the autosampler wash station and maintains the ‘high-speed purge’ for an adjustable delay time after the sipper is moved from the wash station to a sample position. The time required for the wash solution to replace the sample solution in the transfer tubing and to flush the spray chamber can be typically reduced by more than 1 min. The pump controller also allows automated data acquisition using reduced liquid sample uptake rates (0.1 g min–1), which allows analysis of <3 g of sample solution.

Simultaneous determination of arsenic, selenium and antimony in water by an inductively coupled plasma hydride method

Arsenic, selenium and antimony were determined using on-line hydride generation in conjunction with a simultaneous medium power inductively coupled plasma (ICP) spectrometer with conventional pneumatic nebulization. The sample, acidified with 3 mol/l hydrochloric acid was mixed with 1% (m/v) sodium tetrahydroborate and 5% (m/v) potassium iodide, the mixed stream being pumped to the nebulizer of the ICP spectrometer. Calibration curves were linear up to 1 mg/l analyte, with detection limits of 2; 3 and 3 μg/l for arsenic, antimony and selenium, respectively, for 10 s integration times and a sample uptake rate of 2.15 ml/min.

Analytical improvement of pneumatic nebulization in ICP-MS by desolvation

We have studied the influences that may be exerted by a desolvation system on a pneumatically generated aerosol. An amount of 80% of the solvent could be extracted from the drain of the desolvation system. The influence of bias potential and sampling depth was studied in order to obtain maximum ion intensity with desolvation. An intensity gain by a factor of 2-5 was realized for a majority of elements in a standard solution. Without desolvation, the sample uptake rate was limited to a value of 1.2 ml min owing to the appearance of a signal depression; with desolvation, the sample uptake rate could be increased up to 2.5 ml min without signal depression effects. Exploiting this higher uptake rate, an over-all intensity gain of nearly an order of magnitude in comparison to a conventionally operated pneumatic nebulizer could be achieved for a majority of the elements under investigation. Simultaneously, an improvement in the signal stability was observed, and in particular, interferences by oxides were drastically reduced.

Analytical performance of a thermospray sample introduction system for inductively coupled plasma atomic emission spectrometry

The operating conditions for thermospray sample introduction into the inductively coupled plasma have been optimized. The influence of the inner diameter of the vaporizer capillary in the range from 0.05 to 0.150mm was studied. The optimization procedure included the sample uptake rate, the generator power, the carrier argon flow and the observation height. The highest signal-to-background ratio (SBR) was obtained with the 0.050 mm capillary. Using the optimized conditions the detection limits for the 19 elements studied are on average improved by a factor of 3 compared to sample introduction with a V-groove nebulizer, with the exception of Ga, Fe, Cr and Al. The relative standard deviation (RSD) on analyte signals obtained with the thermospray sample introduction system is about 1.0%, whereas the long-term stability of the signal is better than 5% after stabilizing for one hour. These values are similar to the short and long-term stability obtained with a V-groove nebulizer.

A new continuous hydride generator for ICP-AES

With the construction of a tubular hydride generator some of the technical inconveniences of the hydride generation technique are overcome. The system allows the determination of hydride forming elements and mercury with a sample uptake rate of 1 ml/min without loss of sensitivity. Due to the small volume of the reactor and the performed neutralization of the reaction mixture in the siphon the memory effect is reduced. The sodium hydroxide used for neutralization also prevents the generation of an excess of hydrogen and therefore removes its negative influence on the plasma discharge.

Characterization of a High-Efficiency Helium Microwave-Induced Plasma as an Atomization Source for Atomic Spectrometric Analysis

Characterization studies of a He high-efficiency microwave-induced plasma, He-HEMIP, utilizing direct sample introduction with pneumatic nebulization for atomic emission and atomic fluorescence spectrometry are presented. These studies include diagnostic measurements and analytical characterization of the 150-W He-HEMIP. Diagnostic measurements include excitation temperatures with the use of aqueous and organic nebulized thermometric species, electron number densities, and ionization temperatures for the plasma. The effect of sample uptake rate on the emission intensity is investigated. Ionization interferences are minimal, and phosphate interferences were found not to occur. In addition, the He-HEMIP is characterized as an atom source for metals and nonmetals with the use of atomic emission spectrometry and atomic fluorescence spectrometry. With AES, detection limits for metals and nonmetals are in the sub-ppm range. With AFS, detection limits for metals were determined to be in the low to sub-ppb range and were found to be not statistically different from those reported for HCL-ICP-AFS. Linear ranges for AES and AFS ranged from four up to five and one-half orders of concentrative magnitude.

Fast analysis with Zeeman graphite furnace AAS

Stabilized Temperature Platform Furnace (STPF) methods have been adapted and altered to reduce the analytical time to less than 1 min per sample with no loss of analytical precision or accuracy. It is shown that this could be further reduced to about 30 s per sample if certain changes in instrumentation are implemented, especially in the software and firmware that control the autosampler. The sample uptake rate for the autosampler should overlap the cooldown of the tube from the prior determination. Also, the sample should be deposited onto a heated platform. In this work the pyrolysis step and, in most cases, the use of a matrix modifier has been omitted. Since backgrounds were therefore larger, the use of Zeeman correction was usually required, but continuum background correction was not tried. To confirm that these fast analytical methods might be practical, more than 10 standard reference materials were analyzed for several elements including Pb, Cd, Cu, Ni, As and Cr. The paper is not primarily intended to provide routine and reliable methods; it is intended to test the feasibility of these fast methods.

Effect of laurylpyridinium chloride on determination of magnesium by flame AAS.

Magnesium is generally efficiently determined by AAS using any flame, but some foreign ions is known to interfere with the air-acetylene flame. In particular, a large quantity of Ca raises the response of Mg, while phosphate depresses it. When laurylpyridinium chloride (LPC) was added to Mg solution to reach 7.5 ×10-2 M, the atomic absorption of Mg was highly enhanced and the interference of Ca and/or phosphate was overcome. LPC decreases the surface tension of the solution, and then the sample uptake rate is increased and the spray droplet is decreased. So, the sensitivity of Mg absorption is increased. Enhancement effect of LPC may also be attributed to the reducing action of the CN radical formed in the flame by decomposition of LPC. Mg in several foodstuffs could be efficiently determined by the present method.

Effect of solution introduction rate on the performance of nebulisers in inductively coupled plasma atomic emission spectrometry

A Hildebrand grid, and ultrasonic and a Babington-type nebuliser were compared by varying the sample uptake rate and measuring several parameters: relative response, precision, detection limit and wash-in and wash-out times. It was found that the sample introduction rate could be reduced 20-fold (4.5 to 0.21 ml min–1) with relative response falling by less than a factor of three for the Hildebrand and ultrasonic nebulisers, whereas the Babington-type experienced loss of half the optimum relative response at 1.1 ml min–1. Wash-in time was only slightly increased, but wash-out times were much longer at lower flow-rates. The latter problem could be overcome by using a higher rinse rate than sampling rate.

Analysis of Volatile Organic Solvents by Inductively Coupled Plasma Emission Spectrometry

A recent survey of inductively coupled plasma (ICP) tolerance to organic solvents demonstrated analytical feasibility for many organic solvents that had previously been considered incompatible with the ICP. However, the most volatile organic solvents studied (hexane, benzene, acetone, and tetrahydrofuran) were found to be ICP-compatible only at extremely low sample uptake rates (as low as 0.1 mL/min); and at such low rates, the pulsating nature of the peristaltic sample delivery pump causes extreme oscillation in the plasma discharge. As a consequence, the plasma discharge becomes analytically useless. The same qualitative results were obtained for several additional highly volatile organic solvents (dichloromethane, methyl ethyl ketone, and trichlorotri-fluoroethane) during a survey of solvents used by the electronics industry.

Optiplex: a multidimensional response surface fitting procedure for optimization of instrumental parameters

A new instrumentation optimization algorithm is described which combines the iterative nature of simplex optimization with the global aspect of response surface fitting. The model assumes smooth, second order continous functions which are, in most cases, sufficient to characterize instrumental response with changes in parameter values. In contrast to simplex optimization, in which poorest response data values are discarded and replaced by subsequent better fit inputs, optiplex optimization retains both worst and best response data at all times in order to characterize a response surface as completely as possible within the data set used. Once an optimum location has been found, the optiplex algorithm searches the surface in the vicinity of the maximum, in parameter space, to ascertain whether the maximum response found is local or global. This process is iterated and new points taken until a global maximum has been determined. Three parameter optiplex optimization is presented for computer generated test data with and without randomly superimposed noise; atomic absorption (AAS): observation height, sample uptake rate, and fuel flow; and inductively coupled plasma (ICP): observation height, rf power, and nebulizer pressure. Gross signal, absorbance and net signal were the optimization criteria for the computer generated, AAS and ICP data, respectively.

Improvement of a pneumatic nebuliser for atomic absorption spectrometry

The importance of the liquid transport characteristics of atomic absorption nebulisers is discussed and a simple method for determining these characteristics reported. This method was used to optimise the pumping characteristics of a commercial pneumatic nebuliser. Compared with the original nebuliser, the optimised version is significantly less sensitive to variations in the level of sample liquid (approximately a factor of five improvement). The effects of the nebulising gas pressure and sample uptake rate are presented and the improved analytical performance of the new nebuliser is shown.

Effect of Surfactants on Atomic Absorption Spectrometry for the Determination of Calcium in the Presence of Phosphate

The atomic absorption of calcium with air-acetylene flame is interfered with by the presence of phosphate. It was found that the interference was reduced by adding Triton X-100 into the calcium solution. The absorption of calcium containing phosphate increased rapidly as the concentration of Triton X-100 increased, and reached a maximum value. Then it decreased to a minimum value and again gradually increased to a constant absorption value as the concentration of Triton X-100 increased. The maximum absorption of calcium appeared in the presence of low concentrations of Triton X-100; this was attributed to the decrease of surface tension, the decrease of the size of sprayed droplets, and the increase of the sample uptake rate. The enhancement effect of Triton X-100 in its higher concentrations was attributed to the reducing action of ultra-fine carbon particles produced in the flame. Several organic substances of large molecular weight which dissolve in water, such as tetramethyl-ammonium chloride, tetraethylammonium chloride, tetrapropylammonium chloride, tetrabutylammonium chloride, tiron and tannic acid, were also effective in enhancing the calcium absorption. The analysis of calcium containing phosphate could be carried out in the presence of Triton X-100 in its higher concentrations.

Effects of organic acids and solvents in inductively-coupled plasma emission spectrometry

Spectral interferences of the CN band emissions with the analytical lines of rare earths and yttrium were examined in the presence of organic acids or solvents in aqueous sample solutions. The CN band intensity varied with the organic substances as well as with plasma operating conditions. Spectral line intensities of rare earths and yttrium were enhanced by the presence of ethanol, acetone or acetic acid, but suppressed by citric acid. The effects of organic substances on sample uptake rates and intensity distributions along the plasma axis were also investigated.

Improved performance using internal standardization in inductively-coupled plasma emission spectroscopy

Significantly improved performance in ICP emission spectroscopy is possible using internal standardization given the proper choice of operating conditions. A dual monochromator detection system was used to study the correlation between two emission signals recorded simultaneously as several ICP parameters were varied. The behavior of the signal formed by dividing the analyte and reference signals was also measured. The noise behavior of twenty analytically important elements was compared to that of a manganese ion line used as the internal standard for a single, compromise choice of ICP operating parameters (RF power = 1250 W, carrier gas flow rate = 0.7 or 0.91 min , and viewing height = 20mm). The elements differed widely in excitation and ionization energies. Sample concentrations were chosen so that the analyte emission signals were sufficiently intense that the noise and drift contributions due to plasma background and shot noise could be ignored. Under these conditions very good signal correlation ( r ⩾ 0.95) and similar noise proportionality factors were observed leading to noise reductions of ⩾ 10 × after signal division in almost every instance. After signal division the noise, defined as the relative standard deviation of the emission signal, (time constant = 1.0 s, n = 20) was less than 0.1 %. The closely similar noise behavior is attributed to sample density fluctuations arising from corresponding fluctuations of the sample aerosol density. Good correlation is also observed between fluctuations in the plasma background simultaneously detected at two widely separated wavelengths when appropriate steps are taken to reduce shot noise. At carrier gas flow rates above 1.01 min and at viewing heights below 15mm or above 20mm, the signal correlation is reduced or absent, yielding little or no improvement using internal standardization. The use of internal standardization leads to other improvements in analytical performance in addition to noise reduction. Drift is reduced, and nebulizer performance is improved in that it is less sensitive to variations in liquid sample uptake rate, or nebulizer instabilities associated with high solids content sample introduction. The divided signal takes much less time to reach equilibrium following introduction of a new sample.

Application of ultrasonic nebulizer to UHF plasma torch atomic emission spectrometry.

To improve the sensitivities in UHF plasma torch atomic emission spectrometry, the sample nebulization process was modified. Conventional pneumatic nebulizers differed in their sample uptake rates; the sensitivities therefore varied with nebulizers. The sensitivities achieved by using the laboratory-built ultrasonic nebulizer varied with transducer power and plasma gas flow rate. Compared with the ordinary conditions (plasma gas flow: 3-4 1/min; conventional pneumatic nebulizer), the relative net intensities were increased from 1.3 to 7.6 fold for 7 elements at plasma gas flow of only 1.0 1/min using the ultrasonic nebulizer. The sample introduction to ultrasonic nebulizer, the sample removal from ultrasonic nebulizer, and cleaning the cell were made easily by using a reversible peristaltic pump.

A Critical Comparison of Ar and Ar-N2 Inductively Coupled Plasmas as Excitation Sources for Atomic Emission Spectrometry

Ar-N2 plasmas generated by introducing N2 into the outer and the aerosol carrier gas flows were investigated using a 5 kW, 27.12 MHz crystal-controlled rf generator. The dependence of plasma geometry, plasma background intensity, net analyte emission intensity, signal/background ratio, signal/noise ratios, and the detection limits on forward power (1000 to 4000 W), the outer gas flow (15 to 50 liters/min) and aerosol carrier gas flow (1 to 2.5 liters/min), sample uptake rate (1 to 5 ml/min), and gas composition (0 to 100% N2) were studied for observation heights of 5 to 40 mm. Measurements were conducted simultaneously for 20 ion and neutral atom spectral lines. Ultrasonic nebulization, with aerosol desolvation, was employed. Except for the observation height, which was 10 mm, the optimum conditions for Ar-N2 plasmas containing 5 to 15% N2 in the outer gas flow were roughly identical to those of a conventional Ar plasma. When pure N2 was used in the outer flow, two sets of optimum conditions, quite different than the optimum conditions for the conventional Ar plasma, were found for the high and medium excitation energy lines. Detection limits and signal/background ratios of ion and neutral atom lines of the high excitation energies, excited in a pure Ar plasma, were superior to the results obtained in the Ar-N2 plasma when pure N2 was used in the outer flow. The opposite trend was observed for neutral atom lines of medium excitation energies. The use of N2 aerosol carrier gas, when the outer gas flow was pure Ar, deteriorated the detection limits of all elements.