Direct Introduction of Solid Samples

4 Direct Introduction of Solid Samples. Powdered solids can be introduced directly into the flame with graphite capsules and vibration tubes, and as suspensions. 

A solid sample of about 50 mg is placed in a porous graphite capsule. The ends of the capsule are closed with graphite powder and the capsule is placed horizontally in the flame. Atomic vapour will then diffuse through the graphite tube and its absorption will be measured above the capsule. Small solid particles cannot diffuse through the graphite and in this method no background scatter will exist. Detection limits between 10 and 100 mg kg can be obtained for many elements. 

Solid powders can also be analysed with a pneumatic sampling device. This consists of a vertical tube which is coupled to a 50 Hz vibrator. The sample is fed into the air inlet nozzle at a constant rate. The reproducibility of the method is about 8 to 10%. 

Nowadays the introduction of solids as suspensions of finely ground powder is a relatively widely used method. The particle size must generally be less than about 12 ixm. 

5 Burners. The main requirements for commercial burners are the maintenance of a large safety factor and ability to pass solutions of highly dissolved solids without blockage. Usually acidic solutions are prepared for the AA analysis, which means that burners are made from stainless steel or titanium in order to avoid corrosion. Also the inner surfaces of the burner shell are coated with an inert plastic material, like poly (phenyl sulfide). 

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The analyses were carried out at the Field Museum of Natural History in Chicago, IL. The instrumentation is a Varian inductively coupled plasma-mass spectrometer (ICP-MS) equivalent to the actual Varian 810 instrument. A New Wave UP213 laser is connected to the ICP-MS for direct introduction of solid samples. 

There are certain situations in which sample preparation can be completely or largely omitted, as in the case of graphite-furnace AAS analysis, which permits the direct introduction of solid samples. Here the ashing step has been incorporated into the determination itself, which is then referred to as a direct process. This integration permits an analysis to be carried out more quickly, and the chance of errors is reduced by limiting the number of individual operations [136]. On the other hand, it is essential that the accuracy of such an analysis be confirmed by comparisons with standard reference materials. 

Direct introduction of a sample, either in solid or liquid state, in the ion source of a mass spectrometer may be achieved through two procedures the first one is based on the use of a direct insertion probe (DIP) the second one necessitates a direct exposure probe (DEP). Direct introduction followed by heating of the sample in the ion source of the mass spectrometer is also known as direct temperature resolved mass spectrometry (DTMS). 

The introduction of inductively coupled plasma (ICP) in inorganic mass spectrometry means that there is an effective ion source operating at atmospheric pressure. Whereas solid mass spectrometric techniques allow direct analysis of solid samples in ICP-MS, the determination of trace impurities or isotope ratios in solid samples is often carried out after digestion and dissolution of the material. For the determination of trace impurities and isotope ratios in liquids, an additional nebulization... 

Several methods are in general use for introduction of a material into the flame. These include solution aspiration, gas (hydride) evolution and entrainment into the flame, and direct introduction of solid substrates. All three have been used for flame-AAS analysis of trace elements from air sampling however, most work is carried out by use of the first two methods. In each case a suitable solution is first obtained by the dissolution of the elements of interest from the sample. Sample dissolution will be discussed in greater detail in a following section. 

In the case of soils and plants direct introduction of solids or suspensions is already practised. Although automatic solid sample systems are already available the slurry technique is preferable since it is more independent on inhomogenities. Using solid or slurry introduction about five to 12 elements may be directly determined in soils and plants. 

Normally samples are introduced as solutions into the plasma, but the direct introduction of solids and gases is also possible. Hydride and cold vapour methods are also applied to plasma atomic emission spectrometry. In addition, plasmas can be used as detectors for gas and liquid chromatographs. 

Several techniques have been proposed during the last two decades for the direct introduction of solids into atomizers, thus avoiding the need to dissolve or decompose the sitmple. These techniques include (1) direct manual insertion of the solid into the atomization device, (2) electrothermal vaporization of the sample and transfer of the vapor into the atomization region, (3) arc, spark, or laser ablation of the solid to produce a vapor that is then swept into the atomizer, (4) slurry nebulizalion in which the finely divided solid sample is carried into the atomizer as an aerosol consisting of a suspension of the solid in a liquid medium, and (5) sputtering in a glow discharge device. None of these procedures yields results as satisfactory as those... 

Direct solids introduction Direct insertion of solid samples into the ICP can be used for metal powders, salts, and geological samples. The sample is placed on a wire loop or cup made from graphite. 

In classical spectrographic analysis (i.e., direct current arc and controlled-waveform spark emission spectrography) the easiest method for sample introduction involved the direct analysis of solid samples.This approach was also used as the conventional sampHng technique for trace element analysis by spark-source mass spectrometry. Solid samples were easy to prepare, usu-... 

ICP-OES is one of the most successful multielement analysis techniques for materials characterization. While precision and interference effects are generally best when solutions are analyzed, a number of techniques allow the direct analysis of solids. The strengths of ICP-OES include speed, relatively small interference effects, low detection limits, and applicability to a wide variety of materials. Improvements are expected in sample-introduction techniques, spectrometers that detect simultaneously the entire ultraviolet—visible spectrum with high resolution, and in the development of intelligent instruments to further improve analysis reliability. ICPMS vigorously competes with ICP-OES, particularly when low detection limits are required. 

As already remarked in Sect. 4.5.1 (Introduction), LA was primarily designed as a technique for direct sampling in the bulk analysis of solid samples. The main advantages of LA are the possibility of ablating all types of solid material (metals, isolators, glasses, crystals, minerals ceramics, etc.), no special requirements on the... 

Solid foods in powder form can be analyzed directly by means of LA- or ETV-ICP-MS to eliminate time-consuming sample dissolution procedures (see Table 8.2). However, this requires the preparation of homogeneous powdered samples and the subsequent analytical determination is not as straightforward as the one based on liquid sample introduction. Another way to perform direct analysis of solid foods is to grind and suspend them into slurries. The viability of slurry nebulization relies on the ability to prepare samples of fine particle size in a reproducible manner and on the adoption of suitable (e.g., high-solids) nebulizers. Otherwise, slurries can be analyzed by ETV-ICP-MS resorting to the ultrasonic slurry sampling technique [72-74]. 

Fig. 4.1. Hydride and cold mercury vapour separators. (A) Sandwich-type. (B) Tubular-type. (C) Modified Vijan-type. (D) Commercial-type (Perkin-Elmer FIAS-200 module). (E) With direct introduction of the stripping gas into the separator. (F) Dismountable separator for solid samples. (Reproduced with permission of the Royal Society of Chemistry and Perkin-Elmer.)...

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