Nebulisation direct injection

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]. 

The on-line interface of flow manifolds to continuous atomic spectrometric detectors for direct analysis of samples in liquid form typically requires a nebuliser and a spray chamber to produce a well-defined reproducible aerosol, whose small droplets are sent to the atomisation/ionisation system. A variety of nebulisers have been described for FAAS or ICP experiments, including conventional cross-flow, microconcentric or Babington-type pneumatic nebulisers, direct injection nebuliser and ultrasonic nebulisers. As expected, limits of detection have been reported to be generally poorer for the FIA mode than for the continuous mode. 

LaFreniere, K.E., Fassel, VA. and Eckels, D.E. (1987) Elemental speciation via high performance liquid chromatography combined with inductively coupled plasma atomic emission spectrometric detection-application of a direct injection nebuliser. Anal. Chem., 59, 879-887. 

A direct injection nebuliser (DIN) was used to interface LC with ICP-MS (Shum et al., 1992a). The DIN transferred all of the sample into the inductively coupled plasma. Microscale LC separations in small packed columns were studied because the column flow rates of about 30 ml min 1 were compatible with the DIN. The low dead volume (less than 1 ml) of the interface prevented excessive band broadening. Eluents containing up to 85% methanol were accommodated. The analyte signal varied by about 20% as the eluent changed from 20% to 80% methanol in water. Detection limits for arsenic and tin species using the HPLC-DIN-ICP-MS system were 0.2-0.6 and 8-10pg, respectively. 

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. ... 

The direct injection nebuliser (DIN) has so far the most popularity according to the publications on microscale trace element speciation. " Absolute detection limits are reported to be one order of magnitude better when compared to those of conventional nebulisers. This improvement in absolute detection limits can be explained by the complete nebulisation of the sample into the plasma, and so no loss of analyte occurs. 

Oils and thinners may be diluted with white spirit or MIBK, usually by at least a factor of 10 to promote efficient nebulisation. Metal napthenates can be used as standards [67]. Paints as well as the oils above may be diluted with methyl isobutyl ketone (MIBK) for direct injection into a graphite furnace. 

The sample transport system, nebuliser and spray chamber are designed to ensure the maximum amount of sample reaches the atomisation source without quenching it. Only a few solvents can be used that are compatible with direct injection to ICP-OES (see Table 3.5) and these solvents have been studied as part of nebulisation efficiency. 

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. 

As the vast majority of LC separations are carried out by means of gradient-elution RPLC, solvent-elimination RPLC-FUR interfaces suitable for the elimination of aqueous eluent contents are of considerable use. RPLC-FTTR systems based on TSP, PB and ultrasonic nebulisa-tion can handle relatively high flows of aqueous eluents (0.3-1 ml.min 1) and allow the use of conventional-size LC. However, due to diffuse spray characteristics and poor efficiency of analyte transfer to the substrate, their applicability is limited, with moderate (100 ng) to unfavourable (l-10pg) identification limits (mass injected). Better results (0.5-5 ng injected) are obtained with pneumatic and electrospray nebulisers, especially in combination with ZnSe substrates. Pneumatic LC-FI1R interfaces combine rapid solvent elimination with a relatively narrow spray. This allows deposition of analytes in narrow spots, so that FUR transmission microscopy achieves mass sensitivities in the low- or even sub-ng range. The flow-rates that can be handled directly by these systems are 2-50 pLmin-1, which means that micro- or narrow-bore LC (i.d. 0.2-1 mm) has to be applied. 

The accuracy and precision obtainable by ICP-OES-FIA depends largely on the way the sample is introduced into the plasma. One of the most attractive aspects of introducing the sample as a liquid lies in its relative simplicity, good reproducibility and speed of analysis. For routine measurements FI A offers an alternative method of sample introduction to direct nebulisation. The dissolved sample is injected as a plug into either a segmented or non-segmented stream of a carrier liquid and transported to the plasma. The nebulisation efficiency in ICP-OES is lower by a factor of one-fifth compared with A AS. 

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