Detector, atomic spectrometer transport

Although for many elements the degree of ionization is about 100 % only 1 in lO to 1 in 10 atoms in the original sample are detected due to the extensive loss of ions that occnrs during their transport from the plasma via the mass separation system to the ion detector (dne to different ion transmission of mass spectrometers). 

Type 2. This Involves partial automation of the first stage of the analytical process the accurate measurement of a sample volume (sampling) and Its transport to the detector without human Intervention. However, sample treatment (e.g. dissolution) and the analytical reaction development —if required— are carried out manually. Figure 1.6 shows, a representative example the incorporation of an automatic sampler In a thermal-vaporization atomic absorption spectrometer. This instrumental configuration is representative of those where the automation of one stage Is highly recommendable —in this Instance to ensure reproducible results. 

ICP-AES is a technique of measurement used for the detection and determination of elements with the aid of atomic emission. The solution for measurement is atomized and the aerosol is transported into an inductively coupled plasma (ICP) with the aid of a carrier gas. There, the elements are excited such that they emit radiation. This is spectrally dispersed in a spectrometer and the intensities of the emitted element lines are measured by means of detectors (photomultipliers). A quantitative statement is possible by means of calibration with reference solutions, there being a linear relationship between the intensities of the emission lines and the concentrations of the elements over a broad range (usually several powers of ten). The elements may be determined either simultaneously or consecutively. 

The best-known technique based on a combination of methods is ICP-MS. Here, the excited atoms are introduced upon their return to a lower energy level, through an interface into the ion source of a quadru-pole of a mass spectrometer. The ICP thus acts as an ion source and the mass spectrometer as the ion detector. The latest development in atomic spectrometry is the electrothermal evaporation-ICP-MS technique, where a graphite furnace is coupled to an ICP-MS. In this case, use is made of the most remarkable property of a graphite furnace (elimination of matrix interferences) by a graphite tube atomizer and subsequent transport of the atomic phase into the plasma and quadrupole. 

Pervaporators are amenable to coupling to any type of detector via an appropriate interface such as a transport tube, a microcolumn packed with adsorptive or ion-exchange material, or a gas liquid separator. The acceptor stream can be either liquid or gaseous depending on the characteristics of the detector. The detectors most frequently used are the spectroscopic - atomic or molecular, electroanalyti-cal (potentiometric, voltammetric), electron capture, and flame ionization types. The low selectivity of some of these detection techniques is overcome by that of the pervaporation step, endowing the overall analytical process with the selectivity required for the analysis of complex matrices. The potential use of the pervaporation technique for sample insertion into water-unfriendly detectors such as mass spectrometers or devices such as those based on microwave-induced plasma remains unexplored. 

As the sample is heated, analyte elements are vaporized from the sample in the order of the boiling points of the predominant compound of the element, prevalent in the sample (i.e., the most volatile compound is vaporized first followed by each of the other compounds in the increasing order of their respective volatilities). Because a finite quantity of each of these analytes is present in the sample, as they are volatilized, transported to the plasma, atomized, and ionized, a transient signal is produced from the mass spectrometer detector. Multiple analyte elements, which have different volatility rates, produce nonsuperimposed transient signals similar to those observed for electrothermal vaporization (see Figure 5.22).This process significantly complicates the suitability of this technique for multielement determinations. 

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