Direct sample insertion

In the direct insertion technique, the sample (liquid or powder) is inserted into the plasma in a graphite, tantalum, or tungsten probe. If the sample is a liquid, the probe is raised to a location just below the bottom of the plasma, until it is dry. Then the probe is moved upward into the plasma. Emission intensities must be measured with time resolution because the signal is transient and its time dependence is element dependent, due to selective volatilization of the sample. The intensity-time behavior depends on the sample, probe material, and the shape and location of the probe. The main limitations of this technique are a time-dependent background and sample heterogeneity-limited precision. Currently, no commercial instruments using direct sample insertion are available, although both manual and h ly automated systems have been described. ... 

W. E. Petit and G. Horlick. Spect. Acta. 41B, 699, 1986. Describes an automated system for direct sample-insertion introduction of 10-pL liquid samples or small amounts (10 mg) of powder samples. 

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]. 

For special applications direct current plasma (DCP) (Leis et al., 1989) and micro-wave-induced plasma (MIP) may be used. The MIP first became widely used as a spectroscopic radiation source after a stable discharge at atmospheric pressure had been obtained (Beenakker, 1977 Beenakker et al., 1978). The MIP is not capable of taking up wet aerosols, but is useful for the excitation of dry aerosols, produced by electrothermal evaporation from a graphite furnace (Aziz et a ., 1982). Direct sample insertion has been discussed recently by Blain and Savin (1992). 

Direct Sample Insertion. In direct sample insertion (DSI) [82], the sample is placed on a rod, metal loop, or cup on a rod. After desolvation (by inductive heating of the rod or use of a heat gun), the sample is inserted into the plasma. The advantages of the DSI system include nearly 100% sample transport efficiency into the ICP and use of a single power source. The most exciting capability of DSI is preconcentration using aerosol deposition that can provide two orders of magnitude of improvement in ICP-MS detection limits [83]. Detection limits as low as 0.06 parts per trillion were obtained. 

Electrothermal evaporation and direct sample insertion This procedures allow the... 

In clinical analysis, Ca, Fe, Cu, Mg, Na and K can be determined directly even in microsized serum samples [128], Also electrothermal evaporation and direct sample insertion are particularly useful from this point of view. Generally, however, the power of detection of ICP-AES is too low for most analytical problems encountered in the life sciences. 

Salin E. D. and Horlick G. (1979) Direct sample insertion device for ICP... 

Zaray Gy., Broekaert J. A. C. and Leis F. (1988) The use of direct sample insertion into a nitrogen-argon inductively coupled plasma for emission spectrometry I. Technique optimization and application to the analysis of aluminium oxide, Spectrochim Acta, Part B 43 241—253. 

Blain L. and Salin E. D. (1992) Methodological solutions for the analysis of sediment samples by direct sample insertion ICP atomic emission, Spectrochim Acta, Part B 47 205-217. 

Karanassios V., Abdullah M. and Horlick G. (1990) The application of chemical modification in direct sample insertion-inductively coupled plasma-atomic emission spectrometry, Spectrochim Acta, Part B 45 119—... 

Herwig F. and Broekaert J. A. C. (2000) Optimization of a direct sample insertion into a stabilized capacitive plasma for optical emission spectrometry (SCP-OES), Mikrochim Acta 134 51-56. 

Application of a wire loop direct sample-insertion device for ICP-MS, Anal Chem 58 975-976. 

Boomer D. W. and Powell M. (1988) Relative merits of two methods of sample introduction in ICP-MS electrothermal vaporisation and direct sample insertion, ] Anal At Spectrom 3 791-797. 

Although liquid samples are best, if solid samples require analysis there have been a number of developments such as laser ablation that now allow this. Direct sample insertion is also possible where the solid sample is introduced into the plasma on a carbon electrode. 

Karanassios V and Wood TJ (2000) Direct sample insertion. In Beauchemin D, Gregoire DC, Gunther D, Karanassios V, Mermet J-M and Wood TJ, eds. Discrete sample introduction techniques for inductively coupled plasma mass spectrometry. In Barcelo D, ed. Wilson and Wilson s Comprehensive analytical chemistry, Vol 34, pp. 503-560. Elsevier, Amsterdam. 

In the direct sample-insertion technique, the sample is physically placed in the atomizer. For solids, the sample may be ground into a powder, which is then placed on or in a probe that is inserted directly into the atomizer. With electric are and spark atomizers, metal samples arc frequently introduced as one or both electrodes that are used to form the arc or spark. 

In the first reported direct sample insertion device, the ICP torch was modified by replacement of the inner tube with a quartz tube that acted partially as a guide for the sample probe (Figure 112). A graphite sample probe is placed on the end of a solid quartz rod that can be inserted into the inner quartz tube. The bottom end of the quartz rod is attached to a sliding platform. The graphite electrode probe is moved into the plasma by moving the platform manually into the vertical position. Introduction of air into the plasma must be prevented. 

Figure 112 Schematic diagram of a direct sample insertion device for ICP-AES. (Adapted from E. D. Salin and G. Horlick, Anal. Chem., 1979, 51, 2284)...

Many of the operating parameters normally optimized for ICP-AES with solution nebulization, are also important in direct sample analyses. These include observation height, RF power, gas flow rates, integration time, and wavelength. In direct sample insertion, sample insertion heights for drying, ashing, and vaporization, rate and duration of insertion, and sample size must also be optimized. 

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