Heated vaporization atomic absorption

Electrothermal vaporization can be used for 5-100 )iL sample solution volumes or for small amounts of some solids. A graphite furnace similar to those used for graphite-furnace atomic absorption spectrometry can be used to vaporize the sample. Other devices including boats, ribbons, rods, and filaments, also can be used. The chosen device is heated in a series of steps to temperatures as high as 3000 K to produce a dry vapor and an aerosol, which are transported into the center of the plasma. A transient signal is produced due to matrix and element-dependent volatilization, so the detection system must be capable of time resolution better than 0.25 s. Concentration detection limits are typically 1-2 orders of magnitude better than those obtained via nebulization. Mass detection limits are typically in the range of tens of pg to ng, with a precision of 10% to 15%. 

Nebulization is inefficient and therefore not appropriate for very small liquid samples. Introducing samples into the plasma in liquid form reduces the potential sensitivity because the analyte flux is limited by the amount of solvent that the plasma will tolerate. To circumvent these problems a variety of thermal and electrothermal vaporization devices have been investigated. Two basic approaches are in use. The first involves indirect vaporization of the sample in an electrothermal vaporizer, e.g. a carbon rod or tube furnace or heated metal filament as commonly used in atomic absorption spectrometry [7-9], The second involves inserting the sample into the base of the... 

Mercury is most accurately determined by the cold vapor atomic absorption spectroscopic method. The instrument is set at the wavelength 253.7 nm. The metal, its salts and organic derivatives in aqueous solution can be measured by this method. The solution or the solid compounds are digested with nitric acid to convert into water-soluble mercury(ll) nitrate, followed by treatment with potassium permanganate and potassium persulfate under careful heating. The excess oxidants in the solution are reduced with NaCl-hydroxylamine sulfate. The solution is treated with stannous chloride and aerated. The cold Hg vapor volatdizes into the absorption cell where absorbance is measured. 

Bulk amounts of elements were determined by atomic absorption spectrophotometry. The amount of framework A1 was determined by Al MAS NMR. The acidic properties of the metallosilicates were determined by IR and NH3-TPD measurements. Before the IR measurements, the sample wafer was evacuated at 773 K for 1.5 h. In the observation of pyridine adsorbed on metallosilicates, the sample wafer was exposed to pyridine vapor (1.3 kPa) at 423 K for 1 h, then was evacuated at the same temperature for 1 h. All IR spectra were recorded at room temperature. NH3-TPD experiments were performed using a quadrupole mass spectrometer as a detector for ammonia desorbed. The sample zeolite dehydrated at 773 K for 1 h was brought into contact with a 21 kPa of NH3 gas at 423 K for 0.5 h, then evacuated at the same temperature for 1 h. The samples were cooled to room temperature, and the spectra obtained at a heating rate of 10 K min from 314 to 848 K. 

Another test method for the determination of mercury in coal (ASTM D-6414) involves (method A) solubilizing of the mercury in the sample by heating the sample at a specified temperature in a mixture of nitric and hydrochloric acids. The acid solutions produced are transferred into a vessel in which the mercury is reduced to elemental mercury. The mercury vapor is determined by flameless cold-vapor atomic absorption spectroscopy. An alternative method (method B) involved solubilization of the mercury by heating the sample in a mixture of nitric acid and sulfuric acid with vanadium pentoxide. The acid solution is then transferred into a vessel in which the mercury is reduced to elemental mercury. The mercury content is determined by flameless cold-vapor atomic absorption spectroscopy. However, mercury and mercury salts can be volatilized at low temperatures, and precautions against inadvertent mercury loss should be taken when using this method. 

Any emitting surface, cold-cathode, hotdevelop vapor pressures from the elements/compounds of their construction when heated. These vapors will also manifest themselves particularly during the initial start-up (breaking in) time of the lamp. Early atomic absorption, multi-element lamps were shown to have this problem. Some of these additives specifically react with residual gases and elements that might have a deleterious effect on lamp life. The electronics industry has used what are called "getter" substances in their tubes to remove some of these substances and improve emission. 

Atomic absorption will take place only in a field of free, neutral, activated atoms. Atomic absorption cannot be brought about by ions, by atoms bound in compounds, or by a molecular gas. When metals are heated to their boiling point, they vaporize as free atoms, provided that interaction with other elements is prevented, and it is for this reason that atomic absorption spectroscopy in its present form has found its most extensive applications in the analysis of the metallic elements. 

Thermal evaporation of the analyte elements from the sample has long been used in atomic spectrometry. For instance, it had been applied by PreuE in 1940 [170], who evaporated volatile elements from a geological sample in a tube furnace and transported the released vapors into an arc source. In addition, it was used in so-called double arc systems, where selective volatilization was also used in direct solids analysis. Electrothermal vaporization became particularly important with the work of L vov et al. [171] and Massmann in Dortmund [172], who introduced elec-trothermally heated sytems for the determination of trace elements in dry solution residues by atomic absorption spectrometry of the vapor cloud. Since then, the idea has regularly been taken up for several reasons. 

Petroleum matrices vary and often exert significant effects on the final measurement. This is illustrated by heated vaporization atomic absorption measurements in which various crude oils were spiked with a known concentration of an organochromium compound (Table 2.X). The data show that the absorbance is influenced by the matrix not only in the organic phase but also after the hydrocarbon has been destroyed by ashing. These differences in response indicate that the variety of... 

In the Trace Metals Project standard additions were used in all heated vaporization atomic absorption (HVAA) procedures. These procedures use sample aliquots of 10 /J or less consequently, a microstandard addition technique was developed. In this technique successive microliter aliquots of a /xg/ml standard are added to the sample solutions which have also been prepared on weight/volume basis. Since the volumes added or removed from the solution are negligible, the entire analysis is done on a single solution as detailed in the individual procedures. 

Direct Analysis. Analysis without prior sample preparation would be ideal since it eliminates the difficult and time-consuming task of removing or destroying the petroleum matrix and minimizes contamination. Unfortunately, at the nanogram/gram level only a few analytical techniques can be applied without prior sample preparation. These include instrumental neutron activation analysis (INAA) and heated vaporization atomic absorption (HVAA), which can be used for certain elements. 

Heated vaporization atomic absorption (HVAA) has been described extensively. HVAA differs from conventional atomic absorption in that an electrically heated device replaces the flame. The characteristics of HVAA are microliter sample consumption, sensitivity down to picogram quantities, and applicability to a wide variety of solutions. These characteristics have led to its widespread use. In petroleum analyses, this technique has been used to determine relatively high levels (ppm) of lead in gasoline, metals in used oils, and nickel and vanadium in crude oils (18). The Trace Metals Project has extended application of this technique to the determination of Be, Cd, Co, Cr, Mn, Mo, Sb, and V at the 10-ng/g level. 

In the decomposition step, the sample is heated with sulfuric acid, and the carbonaceous residue is burned off in a muffle furnace. The resultant ash is dissolved in hydrochloric acid, treated with hydroxylamine to reduce the antimony to the trivalent state, and stabilized with tartaric acid. Other ashing aids, such as nitric and perchloric acids or magnesium nitrate, have been used with success in determining antimony (21), but these were not investigated for use because of the observations regarding antimony measurement by heated vaporization atomic absorption which are discussed below. 

Summary of the Method. The sample is digested with concentrated sulfuric acid until most of the sulfuric acid is removed and a carbonaceous residue remains. The sample is then placed in a muffle furnace to destroy the carbonaceous material. Hydrochloric acid and hydroxyl-amine hydrochloride are added to dissolve the inorganic residue and reduce antimony(V) and antimony(IV) to antimony (III). Tartaric acid is then added to complex antimony (III), and the solution is evaporated to a small volume. The antimony content of this solution is determined by heated vaporization atomic absorption using the method of standard additions. 

Gravimetric methods and chemical methods, such as colorimetric measurements based on the arsenic-molybdenum blue complex (1,2,3) and arsine generation in combination with silver diethyldithiocarbamate (4, 5, 6,7), have been used to measure arsenic in aqueous media. Various instrumental methods such as differential pulse polarography (8), heated vaporization atomic absorption (9), arsine generation in combination with atomic absorption spectroscopy (10, 11, 12) or non-dispersive atomic fluorescence spectroscopy (13), and optical emission spectroscopy (14) can be used to determine arsenic in aqueous solutions. 

In recent years atomic absorption has received most of the attention for cadmium determination because of its high sensitivity for the element and the absence of interferences (7, 8). The introduction of heated vaporization atomic absorption (HVAA) has extended the detection limit topicogramlevels (9,10). 

The Trace Metals Project investigated both flame and heated vaporization atomic absorption. These studies included an evaluation of decomposition techniques and ultimately a program of independent cross-checks of the methods that were devised. 

Heated Vaporization Method. Heated vaporization atomic absorption (HVAA) utilizing a carbon rod or furnace has been discussed in Chapter 2. The technique has a sensitivity advantage for cadmium amounting to several orders of magnitude. Although HVAA is more... 

A number of methods exist for the determination of parts-per-billion (ng/g) levels of chromium in aqueous media (Table 8.1). These are repeatedly reviewed as new techniques are introduced (4,5,6). Potentially all these techniques could be applied to petroleum samples after matrix destruction, but in practice, only a few have been utilized. After wet oxidation of a large sample (> 100 g), 10 to 50 fig of chromium may be determined by a colorimetric procedure with 1,5-diphenylcarbohydrazide after iron, copper, molybdenum, and vanadium are extracted as the cup-ferrates (3). In survey analyses, Cr levels as low as 5 ng/g have been measured by optical emission spectroscopy after ashing (2,3) or directly by neutron activation with extended irradiation and counting times (1). Concentrations of chromium above 100 ng/g in used lubricating oils have been measured directly by flame atomic absorption (8) for lower concentrations, heated vaporization atomic absorption (HVAA) has been utilized (9). In the Trace Metals Project, two procedures using this latter technique were evaluated for the determination of 10 ng Cr/g in a variety of petroleum matrices. 

Heated vaporization atomic absorption Mass spectrometry (direct injection) Neutron activation Optical emission spectroscopy X-ray fluorescence (ion exchange)... 

A number of instrumental methods have been used to determine ppb levels of cobalt in water (4,5,6), biological tissues (7,8), and air particulates (9, 10). Kinetic methods are capable of measuring sub-parts-per-billion (11,12). Potentially any of these techniques could be used in the analysis of petroleum, but only neutron activation analysis (I, 3) and atomic absorption spectroscopy (13,14) have been applied to any appreciable extent. Flame and heated vaporization atomic absorption techniques were selected for more detailed study by the Project because atomic absorption is sensitive, subject to relatively few interferences, and is rather generally available. 

Aqueous Solutions. Two measurement procedures for aqueous solutions were evaluated in the Project—atomic absorption (AA) with an air-acetylene flame and heated vaporization atomic absorption. Optimum parameters for both measurements were established empirically from the response for standard solutions of cobalt in dilute hydrochloric acid. 

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