Atomic solid sample introduction

J.A. Sweileh, On-line flow injection solid sample introduction digestion and analysis spectrophotometric and atomic absorption determination of iron, copper and zinc in multi-vitamin tablets, Microchem. J. 65 (2000) 87. 

For the first live atomization sources listed in Table 8-1, samples arc usually iiilroduccd in the form of aqueous solulions (occa.sionally, nonaqueous soUilion.s are used) or less often as slurries (a slurry is a suspension of a ftnely divided powder in a liquid). For samples lhai are difficull lo dissolve, however, several methods have been used lo introduce samples into the atomizer in ihe form of solids or finely dispersed powders. Gen-crallv, solid sample-introduction techniques arc less reproducible and more subject lo various errors and as a result are not nearly as widely applied as aqueous solution techniques. Table 8-2 lists the common sample-introduction methods for atomic spcciro.scopy and the ispe of samples lo which each method is applicable. 

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

An ICP-OES instrument consists of a sample introduction system, a plasma torch, a plasma power supply and impedance matcher, and an optical measurement system (Figure 1). The sample must be introduced into the plasma in a form that can be effectively vaporized and atomized (small droplets of solution, small particles of solid or vapor). The plasma torch confines the plasma to a diameter of about 18 mm. Atoms and ions produced in the plasma are excited and emit light. The intensity of light emitted at wavelengths characteristic of the particular elements of interest is measured and related to the concentration of each element via calibration curves. 

Samples must be introduced into the plasma in an easily vaporized and atomized form. Typically this requires liquid aerosols with droplet diameters less than 10 pm, solid particles 1-5 pm in diameter, or vapors. The sample introduction method strongly influences precision, detection limits, and the sample size required. 

Moens L, Verreft P, Boonen S, Vanhaecke F and Dams R (1995) Solid sampling electrothermal vaporization for sample introduction in inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry. Spectrochim Acta 508 463-475. Mooijman KA, In t Veld PH, Hoekstra JA, Heisterkamp SH, Havelaar AH, Notermans SHW, Roberts D, Griepink B, Maier E (1992) Development of Microbiological Reference Materials. European Commission Report EUR 14375 EN, Community Bureau of Reference, Brussels. 

Different analytical procedures have been developed for direct atomic spectrometry of solids applicable to inorganic and organic materials in the form of powders, granulate, fibres, foils or sheets. For sample introduction without prior dissolution, a sample can also be suspended in a suitable solvent. Slurry techniques have not been used in relation to polymer/additive analysis. The required amount of sample taken for analysis typically ranges from 0.1 to 10 mg for analyte concentrations in the ppm and ppb range. In direct solid sampling method development, the mass of sample to be used is determined by the sensitivity of the available analytical lines. Physical methods are direct and relative instrumental methods, subjected to matrix-dependent physical and nonspectral interferences. Standard reference samples may be used to compensate for systematic errors. The minimum difficulties cause INAA, SNMS, XRF (for thin samples), TXRF and PIXE. 

The extension of inductively coupled plasma (ICP) atomic emission spectrometry to seawater analysis has been slow for two major reasons. The first is that the concentrations of almost all trace metals of interest are 1 xg/l or less, below detection limits attainable with conventional pneumatic nebulisation. The second is that the seawater matrix, with some 3.5% dissolved solids, is not compatible with most of the sample introduction systems used with ICP. Thus direct multielemental trace analysis of seawater by ICP-AES is impractical, at least with pneumatic nebulisation. In view of this, a number of alternative strategies can be considered ... 

The MC-ICP-MS consists of four main parts 1) a sample introduction system that inlets the sample into the instrument as either a liquid (most common), gas, or solid (e.g., laser ablation), 2) an inductively coupled Ar plasma in which the sample is evaporated, vaporized, atomized, and ionized, 3) an ion transfer mechanism (the mass spectrometer interface) that separates the atmospheric pressure of the plasma from the vacuum of the analyzer, and 4) a mass analyzer that deals with the ion kinetic energy spread and produces a mass spectrum with flat topped peaks suitable for isotope ratio measurements. 

The whole atomizer may be water cooled to improve precision and increase the speed of analysis. The tube is positioned in place of the burner in an atomic absorption spectrometer, so that the light passes through it. Liquid samples (5-100 mm ) are placed in the furnace, via the injection hole in the centre, often using an autosampler but occasionally using a micro-pipette with a disposable, dart-like tip. Solid samples may also be introduced in some designs, this may be achieved using special graphite boats. The sample introduction step is usually the main source of imprecision and may also be a source of contamination. The precision is improved if an autosampler is used. These samplers have been of two types automatic injectors and a type in which the sample was nebulized into the furnace prior to atomization. This latter type was far less common. 

The role of the sample introduction system is to convert a sample into a form that can be effectively vaporized into free atoms and ions in the ICP. A peristaltic pump is typically used to deliver a constant flow or sample solution (independent of variations in solution viscosity) to the nebulizer. Several different kinds of nebulizers are available to generate the sample aerosol, and several different spray chamber designs have been used to modify the aerosol before it enters the ICP Gases can be directly introduced into the plasma, for example, after hydride generation. Solids can be introduced by using electrothermal vaporization or laser ablation. 

From the late 1960s onwards, a number of research groups around the world began to investigate alternatives to pneumatic nebulization for sample introduction, in an attempt to overcome transport efficiency limitations. The most successful approaches were those which involved heating small, discrete liquid samples, and sometimes even solid samples, directly on a metal filament, boat, or cup which could be positioned reproducibly into a flame. However, since the temperature of the metal would be lower than that of the flame itself, the techniques were confined to the determination of relatively easily atomized elements such as arsenic, bismuth, cadmium, copper, mercury, lead, selenium, silver, tellurium, thallium, and zinc. 

Z. Mester, J. Earn, R. Sturgeon, J. Pawliszyn, Determination of methylmercury by solid-phase microextraction inductively coupled plasma mass spectrometry a new sample introduction method for volatile metal species, J. Anal. Atom. Spectrom., 15 (2000), 837 D 842. 

Devices for solid sample treatment prior to introduction into atomic spectrometers electrothermal devices and glow-discharge sources... 

This section is devoted to the types of devices most frequently used for both liquid and solid sampling prior to introduction into atomic spectrometers [12-14]. Atomic techniques and mass spectrometry make massive use of electrothermal devices, the maturity of which has been endorsed by lUPAC, which has included it in its Nomenclature, Symbols, Units and their Usage in Spectrochemical Analysis. XII. Terms Related to Electrothermal Atomization , published in 1992 and subsequently reprinted in Spectro-chimica Acta [1]. 

Sample introduction is probably one of the most important stages for reproducible measurements and is related to the efficiency of sample uptake to the plasma source. Normally samples are introduced in solution form and in latter years sample introduction as solids and gases directly or from GC columns is now commonly employed on a routine basis where applicable. Selection for the best sample introduction method needs careful consideration, keeping in mind that the properties of the atomiser will dictate its design and operation. For adequate thermal dissociation, volatilisation, excitation and atomisation of aerosol particles, the efficiency of nebulisation will determine the sensitivity and reproducibility of analyte response. The following requirements must be considered when analysing samples using atomic emission methods ... 

For glow discharges and inductively coupled high-frequency plasmas ion generation takes place in the plasmas. In the first case mass spectrometry can be performed directly on solids and in the second case on liquids or solids after sample dissolution. In the various atomic spectrometric methods, real samples have to be delivered in the appropriate form to the plasma source. Therefore, in the treatment of the respective methods extensive attention will be given to the techniques for sample introduction. 

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