Wash-out time

Low viscosity and high viscosity oil blends are prepared in kerosene and analysed against standards prepared in kerosene. Oil blends prepared in decalin are analysed against standards prepared in decalin. Finally oil blends prepared in tetralin are analysed against standards prepared in tetralin. The instrument is calibrated and equilibrated prior to analysis and the radial torch adjusted to achieve maximum signal response. Standard pump speed, gas flow rate and rinse out times are set as per manufacturer s instruction selecting the most sensitive line for each metal. Each sample is read six times allowing a 2 min wash out time between each measurement to remove memory effects. Extraction unit is switched on at full power to remove any toxic metal ions and vapours. 

The spray chamber needs some time to fill up with the aerosol produced and some tailing of signals is seen, resultng from removal of the sample aerosol by the new incoming gas and aerosol. These build-up and wash-out times limit the speed of analysis and lead to a flattening of transient signals, when a vapor cloud passes through the spray chamber, as is the case in flow injection analysis. In order to... 

Watling and Villiers (1971) increased the analytical sensitivity of FAAS by directing the flame gases into a slotted quartz tube, constructed with exit slot at 120° to the entrance slot and mounted onto a burner head. The increase in sensitivity stems from increased residence time of the atomic species in the optical path, it can be up to threefold for volatile elements but is only 1.2 to 1.6 fold for copper in aqueous solutions or in a diluted serum matrix (Bahreyni-Toosi et al., 1984 Brown and Taylor, 1984). The marginal improvement is partly offset by the slight deterioration in precision, increased wash-out time to reduce the carry-over effects and limited off-time of the quartz tube. Therefore, this device offers little advantage over conventional FAAS for analysis of copper in biological samples. 

Laser Repetition Rate and Cell Wash-Out Time The pulse mixing and signal tailing that are induced during the aerosol transport is a common limitation of LA-ICP-MS in terms of depth profiling resolution. In order to avoid any mixing of information... 

Memory effects are high and wash-out times are long. 

As regards spray chambers, simplified designs imposing a rather simple aerosol path towards the plasma are able to achieve higher sensitivities and shorter wash-out times than the double pass type. Figure 5.4 shows several spray chambers used in ICP-MS. 

The main advantage of single pass spray chambers is that they are easily rinsed and therefore the wash-out time is significantly shortened. This characteristic makes the single pass spray chamber design very interesting for HPLC-ICP-MS coupling. 

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. 

A negative effect found when working at low hquid flow rates is an increase in wash-out times, leading to a subsequent drop in sample throughput In order to mitigate this problem, new low inner volume spray chambers have been developed and their performance has been tested in ICP-MS. For a mini cyclonic spray chamber, called the Cinnabar, wash-out times were found to be significantly shortened with respect to a double pass spray chamber for the analysis of iodine species. Due to its simplicity and the small dispersion provided by this spray chamber, it has also been successfully applied as part of a CE-ICP-MS interface. Low inner volume single pass spray chambers have also been used in various ways as aerosol transport devices for ICP-MS, namely with the nebuliser directly connected to them, with an impact surface (i.e. bead) inside them and as part of a CE-ICP-MS interface. ... 

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