Atomic spectrometer, element

Atoms of elements are composed of isotopes. The ratio of natural abundance of the isotopes is characteristic of an element and is important in analysis. A mass spectrometer is normally the best general instrument for measuring isotope ratios. 

The material evaporated by the laser pulse is representative of the composition of the solid, however the ion signals that are actually measured by the mass spectrometer must be interpreted in the light of different ionization efficiencies. A comprehensive model for ion formation from solids under typical LIMS conditions does not exist, but we are able to estimate that under high laser irradiance conditions (>10 W/cm ) the detection limits vary from approximately 1 ppm atomic for easily ionized elements (such as the alkalis, in positive-ion spectroscopy, or the halogens, in negative-ion spectroscopy) to 100—200 ppm atomic for elements with poor ion yields (for example, Zn or As). 

In addition to conventional aspiration, using a nebulizer and spray chamber, samples may be introduced in to atomic spectrometers in a number of different ways. This may be because a knowledge of speciation (i.e. the organometallic form or oxidation state of an element) is required, to introduce the sample while minimizing interferences, to increase sample transport efficiency to the atom cell or when there is a limited amount of sample available. 

It should be also pointed out that the robustness of electrothermal atomization enables one to avoid the use of high dilution factors. The sensitivity of ET-AAS to a high percentage of dissolved salts, major elements and/or acids is relatively controllable. Manipulation can be also reduced. Calibration is therefore possible at concentrations where contamination phenomena can be better mastered. Multielement atomic spectrometers have additional advantages of saving time and resources by quantifying simultaneously Cd and Pb. 

The choice of the food sample preparation procedure has an impact on the performance of the quantitation technique used, the behavior of the sought-after element and the amenability of the sample matrix to proper digestion. A convenient way to introduce solid (liquid) material into the measurement cell of an atomic spectrometer is to prepare a suspension or a slurry. 

In a mass spectrometer, elements that are not gases are vaporized by heating. Next, the gas atoms are ionized. In electron impact ionization, the gas atoms are bombarded with a stream of electrons from a heated filament. These electrons collide with the gas atoms, causing each atom to lose an electron and become a positive ion. 

The association of a spectrometer with a liquid chromatograph is usually to aid in structure elucidation or the confirmation of substance identity. The association of an atomic absorption spectrometer with the liquid chromatograph, however, is usually to detect specific metal and semi-metallic compounds at high sensitivity. The AAS is highly element-specific, more so than the electrochemical detector however, a flame atomic absorption spectrometer is not as sensitive. If an atomic emission spectrometer or an atomic fluorescence spectrometer is employed, then multi-element detection is possible as already discussed. Such devices, used as a LC detector, are normally very expensive. It follows that most LC/AAS combinations involve the use of a flame atomic absorption spectrometer or an atomic spectrometer fitted with a graphite furnace. In addition in most applications, the spectrometer is set to monitor one element only, throughout the total chromatographic separation. 

Atomization of the sample is usually facilitated by the same flame aspiration technique that is used in flame emission spectrometry, and thus most flame atomic absorption spectrometers also have the capability to perform emission analysis. The previous discussion of flame chemistry with regard to emission spectroscopy applies to absorption spectroscopy as well. Flames present problems for the analysis of several elements due to the formation of refractory oxides within the flame, which lead to nonlinearity and low limits of detection. Such problems occur in the determination of calcium, aluminum, vanadium, molybdenum, and others. A high-temperature acetylene/nitrous oxide flame is useful in atomizing these elements. A few elements, such as phosphorous, boron, uranium, and zirconium, are quite refractory even at high temperatures and are best determined by nonflame techniques (Table 2). 

Table 3.4 compares detection limits with secondary fluorescers to the results with the RMF method and 15-kV broadband excitation [16,17]. Four different fluorescence analyzers were tested (units A, B, C, and D), and the results were corrected for differences in performance for the energy-dispersive spectrometers employed on each unit. Unit A used a chromium anode tube, and unit B used a tungsten anode tube. Unit A was a commercial, general-purpose instrument. Unit B was specifically designed for atmospheric aerosol analysis, where closer coupling between the tube, fluorescer, sample, and detector could be employed with some sacrifice of insensitivity to specimen-positioning errors. Table 3.5 lists the x-ray tube operating conditions required for Table 3.4. For medium- to high-atomic-number elements, the secondary fluorescer method provides detection limits equivalent to the RMF element, but requires much higher x-ray tube power. For light elements.

For large values of ppx the required counting time to determine N become excessive. For example, for ppx = 15, the required counts for N is 44, and the ratio Nq/N = 3.3 x 10. If the spectrometer is set up to count the higher rate NqA at 10 counts/s, it will require 4 h to acquire 44 counts for N. A small fraction of this time is sufficient to measure the rate No/t with sufficient precision to compute Nq. From this calculation it is obvious why the method is suitable only for low values of ppx. This is one of the major reasons why the accuracy of tabulated mass absorption coefficients for high-atomic-number elements at long wavelengths is poor. 

Atomic absorption spectrometers Elemental analysis of samples (up to 60 elements) 10-4 -10-2... 

A formal distinction between elemental analysis and elementary analysis " (Section 1.6.4) is seldom carefully observed in English. Elemental analysis in the present context is understood to mean a determination of essentially all the elements present in a sample, irrespective of the type of bonding involved or the constitution of the matrix. Means toward that end include not only the classical methods (gravimetric analysis, ti-trimetry, spectrophotometry, electrochemical and kinetic methods, etc.) but also atomic spectromet-ric and radioanalytical methods, some of which are essentially nondestructive. From the standpoint of reliability, classical chemical methods... 

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