Ablation spark

For the majority of applications, the sample is taken into solution and introduced into the plasma as an aerosol in the argon stream. The sample solution is pumped by a peristaltic pump at a fixed rate and converted into an aerosol by a nebulizer (see atomic absorption spectrometry). Various designs of nebulizer are in use, each having strengths and weaknesses. The reader is directed to the more specialist texts for a detailed consideration of nebulizers. There is an obvious attraction in being able to handle a solid directly, and sample volatilization methods using electric spark ablation, laser ablation and electrothermal volatilization have also been developed. 

To analyze metals and alloys directly without dissolution, both spark ablation [349] and laser ablation [61,211] dry aerosol generation systems have been used to introduce samples into an ICP-MS. These approaches often require matrix-matched standards, although several active research groups are focusing on techniques to reduce that requirement. The amount of material ablated depends on the sample type. Fractionation of elements can also be a problem, depending on the sample, the laser fluence, the laser wavelength, and the number of laser pulses used to sample from a fixed location. Volatile elements that are segregated in the samples appear to be most prone to fractionation problems [61],... 

Fig. 66. Miniature spark chambers for spark ablation of metal samples. (Reprinted with permission from Ref. [213].)...

In spark ablation, a spark at constant density is obtained in a matter of seconds, and thus, particularly in the case of small spark chambers, preburn times are accordingly low. In plasma emission as well as in plasma mass spectrometry a linear dynamic range of more than 4 orders of magnitude can be obtained and RSDs are a few percent in the case of absolute measurements. However, as shown by the results in Table 6, they can easily fall to below 1%, when using an internal standard element (Fe in the case of steel samples). The matrix effects from the sampling... 

Tab. 5. Selective volatilization effects in laser and spark ablation as measured by x-ray fluorescence electron probe microanalysis (according to Refs. [212, 217].)...

Tab. 6. Analytical precision of spark ablation ICP-OES for a BAS 410/1 steel sample, ccu = 3.6 mg/g. Line pair leu 324.7 nm/lfe 238.2 nm- 400 Hz medium voltage spark, 1.5 kW argon ICP, transport gas flow 1.2 L/min, 0.5 m Paschen-Runge spectrometer, measurement time 10 s [213],...

For compact solids arc and spark ablation are a viable approach for metals [100, 214]. Aerosols with particle sizes at the pm level [216] and detection limits at the pg/g level are obtained [213]. Owing to the separate ablation and excitation stages, matrix influences are particularly low, as shown for aluminum [100] and for steel samples [213], In the first case, only for supereutectic silicon concentrations were matrix effects obtained (Fig. 100). For low-alloyed samples straight calibration curves are obtained and in the case of high-alloyed steels, even samples with widely different Cr or Ni contents are on the same calibration curves, which, are in fact slightly curved. 

A comparative study of spark-ablation ICP-MS and GD-MS in the case of steel has been reported by Jakubowski and coworkers [536, 613], The RSFs for a number of trace elements and the measurement precision are very similar in both cases. Steel analysis by GD-MS benefits from the addition of 1% of H2 to the Ar discharge gas [614], the explanation for which is certainly complex. For certified reference steels, including superalloys, reliable analysis results can be obtained. The determination of Mo, Nb and Zr in steels by GD-MS was found to be affected by the formation of multiply-charged cluster ions (metal argides) [615]. A correction based on the assumption that the rate of formation of the singly-charged argide is the same for all analytes and coincident with that of FeAr+ was used. The capabilities of low resolution GD-MS were shown by the example of steel analysis [616], where detection limits were down to 1 ng/g and up to 30 elements could be determined. 

Aziz A., Broekaert J. A. C., Laqua K. and Leis F. (1984) A study of direct analysis of solid samples using spark ablation combined with excitation in an inductively coupled plasma, Spectrochim Acta, Part B 39 1091-1103. 

Engel U., Kehden A., Voces E. and Broekaert J. A. C. (1999) Direct solid atomic emission spectrometric analysis of metal samples by an argon microwave plasma torch coupled to spark ablation, Spectrochim Acta, Part B 54 1279-1289. 

Lemarchand A., Labarraque G., Masson P. and Broekaert J. A. C. (1987) Analysis of ferrous alloys by spark ablation coupled to inductively coupled plasma atomic emission spectrometry, J Anal At Spectrom 2 481-484. 

Jakubowski N., Feldmann I., Sack B. and Stuwer D. (1992) Analysis of conducting solids by ICP-MS with spark ablation, J Anal At Spectrom 7 121-125. 

For arc or spark ablation to be successful, the sample must he electrically conducting or it must be mixed with a conductor. Ablation is normally carried out in an inert atmosphere such as an argon gas stream, De-pcndrng on the nature of the sample, the resulting in-alytical signal may l>c discrete or continuous. Several instrument manufacturers market accessories for electric arc and spark ablation. 

Spark ablation. Spark ablation solid sampling uses the same type of spark source already described. The function of the spark in this case is to vaporize the solid sample the ICP plasma can atomize any nonatomic vapor reaching it. Spark ablation is limited to the analysis of solids that conduct electricity. It is very useful for metals and alloys because it eliminates time-consuming sample dissolution and costly high-purity acids. 

Methods that Convert Solid Samples into an Aerosol or Vapour. These methods include (i) electrothermal vaporization, (ii) arc and spark ablation, and (hi) laser ablation. 

Although there seems to be minimal interference in arc and spark ablation techniques, some limitations exist. For example, introduction of excessive sample quantities into the plasma may give rise to poor precision, curvature of the calibration graph, memory effects, and instability of the plasma. If insufficient arc or spark energy is used, differential distillation of the sample components may occur. 

Hi) Laser ablation Laser ablation is related to the arc and spark ablation techniques. A focused laser beam is directed onto the sample causing sputtering of material from the sample surface. The vapour and particles... 

Arc and spark ablation Arc and spark ablation systems for sample introduction are similar to the respective atomisation sources described above [47]. The dc arc equipped with metal or graphite electrodes is the most widely used form of the arc. Spark sources, as depicted in Fig. 12.31, are frequently used for conducting samples due to their instantaneous high temp>erature whereas the arc temp>erature will increase during a measurement cycle, which can lead to fractionation effects. 

Laser ablation Laser ablation (LA) in combination with the ICP atomiser has become a powerful and flexible techniqvie for solid sample introduction [47]. LA-AES has found its niche primarily as a bulk sampling technique for the analysis of bulk solid materials with a large focal spot (500—1000 pm). It offers comparable detection capability to spark ablation/emission but is not dependent on the sample being conductive. The experimental set-up, revealed in Fig. 12.32, consists in its simplest form of a pulsed laser (excimer- or Nd YAG-laser) with a defined pulse energy, some focusing optics, and a sample cell with a continuous Ar flow con-... 

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