Heated vaporization atomic

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. 

Heated vaporization atomizer. Varian Techtron carbon rod atomizer model 63 (CRA-63) or equivalent. 

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. 

Techniques such as heated vaporization atomic absorption (HVAA) and differential pulse anodic stripping voltammetry (DPAS) have reduced the absolute sample size requirements to the point where gram or subgram samples can provide sufficient amounts of lead to yield nanogram/ gram sensitivity. Smaller sample sizes also provide advantages in that smaller quantities of reagents are consumed. Reduction of sample size, however, amplifies the effects of contamination. The presence of lead as an environmental contaminant complicates this problem. 

Numerous methods for determining macro concentrations of molybdenum are available. Optical emission spectroscopy has been used in survey-type analyses (2), but it does not have adequate sensitivity to measure part-per-billion levels. X-ray fluorescence has been applied for part-per-million levels but cannot be readily extended to the lower levels of interest to the Project. Kinetic methods (3) and colorimetry with suitable pre-concentration (4) are capable of measuring part-per-billion levels of molybdenum but have not been applied to petroleum analysis. Molybdenum has been determined by atomic absorption techniques in such materials as sea water, biological tissue, and soils (5,6,7), Although a procedure for determining metals including molybdenum in petroleum by atomic absorption has been reported (8), no actual data are given for molybdenum. Flame and heated vaporization atomic absorption of aqueous solutions of ashed samples were selected by the Project for concurrent study in two separate laboratories. 

A large sample (up to 100 g) is decomposed by heating with sulfuric acid. The resulting carbonaceous residue is ashed in a mufHe furnace at 480°C. The ash is dissolved in dilute hydrochloric acid and diluted to 10 ml. The concentration of each metal is measured on 2-/J aliquots of the digestate by heated-vaporization atomic absorption, using the method of standard additions. 

Flame atomic absorption is sensitive enough to measure part-per-billion levels of nickel in aqueous solution, but it is not that sensitive for vanadium. Heated vaporization atomic absorption is more sensitive, permitting detection of vanadium down to 20 ng/ml in aqueous solution. Therefore, for the practical quantitative determination of nanogram/gram concentrations of both nickel and vanadium in petroleum, the combination of HVAA with a preconcentration ashing step was selected for detailed study. 

Spectroscopy is one of the most sensitive techniques for the determination of beryllium. Quantitative and semi-quantitative spectroscopic methods are widely employed in the analysis of industrial and natural including biological materials, and also in the determination of impurities in beryllium and its compounds. Robbins et al. [21] have described the direct determination of beryllium in petroleum and petroleum products at the pg/l level by heated vaporization atomic absorption. This method is applicable to the determination of 1 to 50 ng Be/g with a precision of 10% at the 30-40 ng/g level. 

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