Sample introduction systems nebulisers

Diversity in sample introduction systems (nebulisation, coupbng, sobd sampbng)... 

Several authors [386,387] have discussed the spectroscopic and nonspectroscopic (matrix) interferences in ICP-MS. ICP-MS is more susceptible to nonspectroscopic matrix interferences than ICP-AES [388-390]. Matrix interferences are perceptible by suppression and (sometimes) enhancement of the analyte signal. This enhanced susceptibility has to be related to the use of the mass spectrometer as a detection system. In fact, since both techniques use the same (or comparable) sample introduction systems (nebulisers, spray chambers, etc.) and argon plasmas (torches, generators, etc.), it is reasonable to assume that, as far as these parts are concerned, interferences are comparable. The most severe limitation of ICP-MS consists of polyatomic... 

In ICP-AES and ICP-MS, sample mineralisation is the Achilles heel. Sample introduction systems for ICP-AES are numerous gas-phase introduction, pneumatic nebulisation (PN), direct-injection nebulisation (DIN), thermal spray, ultrasonic nebulisation (USN), electrothermal vaporisation (ETV) (furnace, cup, filament), hydride generation, electroerosion, laser ablation and direct sample insertion. Atomisation is an essential process in many fields where a dispersion of liquid particles in a gas is required. Pneumatic nebulisation is most commonly used in conjunction with a spray chamber that serves as a droplet separator, allowing droplets with average diameters of typically <10 xm to pass and enter the ICP. Spray chambers, which reduce solvent load and deal with coarse aerosols, should be as small as possible (micro-nebulisation [177]). Direct injection in the plasma torch is feasible [178]. Ultrasonic atomisers are designed to specifically operate from a vibrational energy source [179]. 

In Figure 8.12, the basic set-up of an ICP-MS instrument is presented as a block diagram, consisting of a sample introduction system, the inductively coupled argon plasma (ICP) and the mass-specific detector. By far the most commonly applied sample introduction technique is a pneumatic nebuliser, in which a stream of argon (typically 1 I.min ), expanding with high... 

The extension of inductively coupled plasma (ICP) atomic emission spectrometry to seawater analysis has been slow for two major reasons. The first is that the concentrations of almost all trace metals of interest are 1 xg/l or less, below detection limits attainable with conventional pneumatic nebulisation. The second is that the seawater matrix, with some 3.5% dissolved solids, is not compatible with most of the sample introduction systems used with ICP. Thus direct multielemental trace analysis of seawater by ICP-AES is impractical, at least with pneumatic nebulisation. In view of this, a number of alternative strategies can be considered ... 

Schramel [103] discusses the conditions for multi-element analysis of over 50 trace elements, giving detection limits. Wolnik [104] described a sample introduction system that extends the analytical capability of the inductively coupled argon plasma/polychromator to include the simultaneous determination of six elemental hydrides along with a variety of other elements in plant materials. Detection limits for arsenic, bismuth, selenium and tellurium range from 0.5 to 3 ng/ml and are better by at least an order of magnitude than those obtained with conventional pneumatic nebulisers, whereas detection limits for the other elements investigated remain the same. Results from the analysis of freeze-dried crop samples and NBS standard reference materials demonstrated the applicability of the technique. Results obtained by the analysis of a variety of plant materials are presented in Table 7.10. 

To fully understand the limitations of practical sample introduction systems, it is necessary to reverse the normal train of thought which tends to flow in the direction of sample, i.e. solution-nebuliser-spray chamber-atomiser, and consider the sequence from the opposite direction. Looking at sample introduction from the viewpoint of the atomiser, the choice of procedure will cling on to what the atomiser can accept. Different properties of temperature, chemical composition, solvent(s), interferences, etc., and an introduction procedure must be selected that will result in rapid breakdown of species in the atomiser irrespective of the sample matrix. 

The analyte s concentration ranges are important to identify, in consideration of the sensitivity of the instrument, to establish whether or not a dilution or enrichment step is necessary. In good analytical practice, the calibration range should cover the analyte concentration present in each sample. Additionally further decisions in terms of equipment have to be made. For working in the mg/kg range, conventional glassware for nebuliser and spray chamber can be used, but to go down to the pg/kg or ng/kg level requires the sample introduction system to be made from quartz or PFA (perfluoroalkoxy polymer) to minimise blank contribution, memory effects and cross contamination. 

Low frequency noise is below 10 Hz and is largely generated by the sample introduction system (peristaltic pumps, nebulisation). High frequency noise is above 10 Hz and is related to the gas dynamics of the ICP and desolvation in and before the plasma or to interference noise from the AC line voltage. White noise represents the background in a noise amplitude spectrum and occurs for all possible frequencies. A noise amplitude spectrum shows the frequency composition of the noise for the entire spectrometric system (noise amplitude versus frequency). Other fluctuations can be drift effects, which affect sensitivity and mass discrimination. 

Generally, a liquid sample introduction system suitable for ICP-MS consists of two main components (i) a nebuliser that turns the liquid bulk into an aerosol and (ii) a spray chamber that selects the maximum drop size that will be introduced into the plasma. The most often used nebuliser-spray chamber combination for ICP-MS is depicted in Figure 5.1. It consists of a pneumatic concentric nebuliser coupled to a double pass spray chamber. 

More recently, Montaser and co-workers have developed a new low cost DIN called the direct injection high etficiency nebuliser (DIHEN). The DIHEN is entirely made of glass and is similar in construction to a HEN, but it is longer. The main advantages provided by the DIHEN with respect to other conventional liquid sample introduction systems for ICP-MS are higher sensitivities, better signal stability and lower limits of detection. The dead volume of the DIHEN can be made lower than 10 nL, which significantly reduces the wash-out times for elements such as iodine, mercury and boron. As a result, this nebuliser has also proven to be suitable as an interface between separation techniques and ICP-MS. " Note that in these later studies, with a modified low dead volume DIHEN, the liquid flow rate can be lowered down to 0.5 pL/min. 

With the standard sample introduction system, consisting of a pneumatic nebuliser and spray chamber, only droplets < 10 xm in diameter are permitted to reach the ICP. This selection results in an analyte introduction efficiency of only 1-2%, but is required to maintain plasma stability and ensure efficient desolvation, atomisation and ionisation in the ICP. Additionally, the total concentration of dissolved solids is usually limited to a maximum of 2 g/L only, to prevent clogging of the nebuliser, torch injector tube and/or sampling cone and skimmer orifices, and also to limit signal suppression and long-lasting memory effects. Of course, when using ETV for sample... 

GC-MIP systems have been investigated in considerable detail. Because of the low power of the plasma, it is easily quenched if the normal, atomic spectrometric sample introduction techniques, such as nebulisation, are used. Capillary columns overcome this problem as they require only low flow rates and small sample sizes more compatible with stable plasma operation. The capillary columns can be passed out of the oven, down a heated line, and the end of the column placed in the plasma torch just before the plasma, thus preventing any sample loss. A makeup gas is usually introduced via a side arm in the torch to sustain the plasma (Fig. 4.1, Greenway and Barnett, 1989). Other dopant gases can also be added in this way to prolong the lifetime of the torch and improve the plasma characteristics. 

The ICP is more compatible with LC than the MIP since the higher power can easily cope with liquid sample introduction. An HPLC system can be very easily coupled to an ICP by connecting the column outlet to the ICP nebuliser using a short piece of tubing (Ibrahim et al., 1984) The tubing should be as short as possible to maintain good resolution. The problem with this system is that the ICP nebuliser is very inefficient and instead of 100% transfer of analyte into the plasma this is usually only between 1% and 20%. This has a major effect on... 

The excitation source comprising the plasma torch, the induction coil connected to a radiofrequency generator and a nebuliser or system for sample introduction... 

ETV, as a sample introduction method for ICP-MS elemental analysis, offers several advantages over conventional nebulisation systems. These advantages are related to higher analyte transmission efficiency, use of reduced sample volumes, achievement of very low detection limits and the ability to remove solvent and matrix components, which help in avoiding spectral and non-spectral interferences (see Chapter 3 for a detailed discussion of ETV-ICP-MS). 

Sensitivity and detection limits of ICP-MS are governed by the absolute amount of analytes introduced to the plasma per time unit. Hence, sample transport efficiency of the ICP-MS introduction system will critically affect detection limits in CE-ICP-MS. A general drawback of CE is that concentration-based detection limits are limited by the small sample injection volumes and the electrophoretic peak width. Interfaces employing nebulisers in combination with spray chambers yield analyte transport efficiencies of < 100%, depending on the nebuliser and solution flow rate. Consequently, the sensitivity of CE-ICP-MS can be improved by using introduction systems with 100% aerosol transport efficiency, such as the direct injection nebuliser and the direct injection high-efficiency nebuliser. ... 

Kim et alP described a sample pretreatment system for online determination of Pu using magnetic sector-ICP-MS with isotope dilution. They used a computer-controlled online sample preparation system (the PrepLab, Thermo Electron Corp., Winsford, UK) with Sr-Spec and TEVA-Spec resin micro-columns for Pu preconcentration. These resins, obtained from Eichrom Industries Inc. (Darien, Illinois, USA), were highly selective for radionuclide elements. To reduce the effect of the U H interference on Pu, a microconcentric nebuliser with integrated desolvation system (the MCN 6000, Cetac, Omaha, Nebraska, USA) was used as the sample introduction method. Using this approach, the authors were able to determine less than 10 pg of Pu in soil samples. 

Extensive advances and improvements have been made in the area of sample introduction for ICP-MS and this subject forms a large part of this text. The currently available liquid and solid sampling systems, from nebulisers and spray chambers to electrothermal vaporisation and laser ablation, are described and discussed in detail herein. 

Advances in TIMS-techniques and the introduction of multiple collector-ICP-MS (MC-ICP-MS) techniques have enabled the research on natural variations of a wide range of transition and heavy metal systems for the first time, which so far could not have been measured with the necessary precision. The advent of MC-ICP-MS has improved the precision on isotope measurements to about 40 ppm on elements such as Zn, Cu, Fe, Cr, Mo, and Tl. The technique combines the strength of the ICP technique (high ionization efficiency for nearly all elements) with the high precision of thermal ion source mass spectrometry equipped with an array of Faraday collectors. The uptake of elements from solution and ionization in a plasma allows correction for instrument-dependent mass fractionations by addition of external spikes or the comparison of standards with samples under identical operating conditions. All MC-ICP-MS instruments need Ar as the plasma support gas, in a similar manner to that commonly used in conventional ICP-MS. Mass interferences are thus an inherent feature of this technique, which may be circumvented by using desolvating nebulisers. 

For LC-MS to become a reality an interface had to be designed which was capable of providing a vapour sample feed consistent with the vacuum requirements of the mass spectrometer ion source and of volatilising the sample without decomposition. Various enrichment interfaces have been developed such as the molecular jet, vacuum nebulising, the direct liquid introduction inlet and thermospray systems. 

The determination of arsenic in urine samples to determine exposure requires the hyphenation of hydride generation with ICP-MS. This is because total arsenic analysis, which has been vastly improved with the introduction of CCT and the removal of the ArCl interference, still includes some arsenic species that are present in urine as a result of seafood contributions. The reduction of As +, DMA and MMA (dimethylarsinic acid and monomethylarsonic acid) to As " " with L-cysteine and hydrochloric acid and subsequent hydride generation by mixing with sodium borohydride will measure all the arsenic species except arsenobetaine and arsenocholine. The hydride gas line can be simply connected to the spray chamber (replacing the nebuliser gas) and arsenic is measured using a dry plasma. The hydride generator system removes both the chloride interference (because only the AsHs gas enters the plasma) and the dietary component of exposure (because AB and AC are not reduced to As +). 

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