Atomic emission detector elemental composition

Beside the qualitative identification of elements the atomic emission detector provides the possibility of determination of elemental composition and empirical formulas with compound independent calibration (Wylie et al., 1990 Quimby et al., 1995). This possibility enables an identification independent from retention times and does not require authentic reference standards for calibration. For structural elucidation the element ratios have been calculated using the method of relative response factors. Triphenylarsine was used as standard. The results have been confirmed by complementary high resolution mass spectrometric investigations. Table 4 summarizes the results of selected compounds. 

The most widely regarded approach to accomplish the determination of as many pesticides as possible in as few steps as possible is to use MS detection. MS is considered a universally selective detection method because MS detects all compounds independently of elemental composition and further separates the signal into mass spectral scans to provide a high degree of selectivity. Unlike GC with selective detectors, or even atomic emission detection (AED), GC/MS may provide acceptable confirmation of the identity of analytes without the need for further information. This reduces the need to re-inject a sample into a separate GC system (usually GC/MS) for pesticide confirmation. Through the use of selected ion monitoring (SIM), efficient ion-trap or quadrupole devices, and/or tandem mass spectrometry (MS/MS), modern GC/MS instruments provide LODs similar to or lower than those of selective detectors, depending on the analytes, methods, and detectors. 

Elemental composition Ba 69.58%, C 6.09%, O 24.32%. The compound is digested with nitric acid under heating and the solution is analyzed for barium by atomic absorption or emission spectrometry (see Barium). Carbon dioxide may be determined by treating a small amount of the solid with dilute HCl and analyzing the evolved gas by GC using a thermal conductivity detector or a mass spectrometer. The characteristic mass of CO2 is 44. 

Elemental composition H 2.49%, Se 97.51%. The gas may be analyzed by GC using a TCD, FID or a flame photometric detector. The compound may be identified by GC/MS the molecular ions have masses 82 and 80. The compound may be absorbed in water and the solution analyzed for elemental selenium by flame or furnace atomic absorption—or by ICP atomic emission spectrophotometry. 

PIXE is a technique that uses a MeV proton beam to induce inner-shell electrons to be ejected from atoms in the sample. As outer-shell electrons fill the vacancies, characteristic X-rays are emitted and can be used to determine the elemental composition of a sample. Only elements heavier than fluorine can be detected due to absorption of lower-energy X-rays in the window between the sample chamber and the X-ray detector. An advantage of PIXE over electron beam techniques is that there is less charging of the sample from the incoming beam and less emission of secondary and auger electrons from the sample. Another is the speed of analysis and the fact that samples can be analyzed without special preparation. A disadvantage for cosmochemistry is that the technique is not as well quantified as electron beam techniques. PIXE has not been widely used in cosmochemistry. 

In addition to detecting backscattered and secondary electrons, SEM instruments offer information on the sample elemental composition when using x-ray detectors. It has been pointed out earher that the ejection of electrons from an atom is accompanied by emission of x-rays. The x-ray spectrum that is produced is a characteristic feature of any given element, and by measming the energy or the wavelength of the x-rays that are produced, its ideutihcation is possible. 

The inductively coupled plasma (ICP) atomic emission spectrometer (AES) is used for the high-sensitivity detection of metals in dissolved samples. Applications include metals analysis of polymers, additives, catalysts, and other components on polymers and plastic formulations as well as advanced composite materials. The operating principle is essentially the same as in ICP-MS, instrument with the main difference being the detector. While the ICP-MS detector is a quadruple mass spectrometer which detects elements by their mass, the ICP-AES uses a detector based on the specific energy frequency emitted by each element in the plasma. 

Atomic emission spectroscopy (AES) and atomic absorption spectroscopy (AAS) use the emission and absorption of light for elemental composition measurement, respectively. In an AES analysis, all atoms in a sample are excited simultaneously, and can be detected at the same time using a poly-chromator with multiple detectors. This is the major advantage of AES compared to AAS, which uses a monochromator and therefore only one single element can be analyzed at a time. 

EDX is a useful tool for ES research, for example, for determining the atomic dispersion of a sample surface. However, EDX loses measurement accuracy because of (1) overlapping peaks, (2) detector resolution, and (3) emission of x-rays in all directions—they must escape the sample before being reabsorbed to be detected. The third factor can mean lower energy x-rays are collected with lower intensity than is actually present and rough morphology can mask elements resulting in inaccurate atomic composition descriptions [33]. 

---