Hydrogen—entrained-air flames

Figure 11 The emission spectra obtained when dilute sulfuric acid and phosphoric acid are nebulized into a hydrogen-entrained air flame. The spectrum on the left is that of S2, and that on the right is that of HPO...

Tye et al.56 used hydride generation coupled to AAS to quantify organic and inorganic arsenic species in soil pore waters, after pre-concentration on a pellicular anion-exchange column. They were able to detect down to 2 ng of arsenate, arsenite, and monomethyl arsenite and down to 1 ng of dimethyl arsonate. More recently, an argon-hydrogen-entrained air flame fitted with a... 

Usual flame techniques are often insufficiently detective to measure the low levels of As, Se and Hg present in foodstuffs. Mercury is commonly determined via the flameless cold-vapour technique, whereas there is much current activity in respect of the measurement of As and Se via their conversion to hydrides with subsequent decomposition in cool argon—hydrogen-entrained air flames or electrically-heated cells. Table 4 contains information on these techniques. 

To validate the accuracy of the procedure further, five petroleum samples, including four that contained native selenium, were analyzed in three laboratories by different methods. One laboratory used the flame-heated Vycor furnace measurement technique, another used the hydrogen entrained air flame, and the third used neutron activation (Table 15.IV). 

Table I. Limits of Detection (in p.p.m.) for Several Elements in Oxyhydrogen and in Hydrogen— Entrained-Air Flames...

Table 1 lists the temperatures of some commonly used flames for atomic absorption. A cool flame such as argon-hydrogen-entrained air or air-coal gas is usually not preferred because of increased danger of chemical interferences (see below). The most commonly used flame is the air-acetylene flame. 

Fiorino et al. [13] described the use of a shielded hydrogen (nitrogen diluted) entrained air flame for the analysis of food and animal tissue for arsenic. 

The optimum instrument parameters for arsenic measurement were established with a Perkin-Elmer model 403 atomic absorption spectrometer. Better sensitivity was obtained at the 197.2 nm arsenic line than at the 193.7 nm line. Some investigators have recommended the use of a quartz or silicon furnace for arsenic measurement (16,17,18). However, the hydrogen-argon entrained air flame in combination with the described arsine generation apparatus offers comparable sensitivity. A hollow cathode lamp was used throughout the development of the method. Subsequent studies have shown that a five-fold improvement in sensitivity can be obtained with an electrodeless discharge lamp. 

Place the rubber stopper equipped with the zinc dosing tube and the argon supply hose into an empty 500-ml Erlenmeyer flask (see Figure 5.1), and ignite the hydrogen-argon entrained air flame. Adjust the baseline and zero the instrument. 

The results obtained with the flame-heated Vycor furnace and hydrogen-argon entrained air flame are in excellent agreement with each other and with the results obtained by neutron activation. The results on the two samples that were spiked with a known amount of selenium, as dilauryl-selenide, show quantitative recovery and suggest that both the flame-heated Vycor furnace or hydrogen-argon entrained air flame will provide reliable analyses of petroleum materials. However, the flame method is recommended, because the response was linear. 

Summary of the Method. The sample is oxidized under reflux conditions with sulfuric, nitric, and perchloric acid, and the selenium is measured as hydrogen selenide by hydrogen-argon entrained air flame atomic absorption. 

In some inlet devices, the volatile sample materials are first separated from entrained hydrogen gas or air by condensing them in a coolant bath. Subsequently, when all of the volatile sample components have been condensed and the hydrogen or air has been swept away, the sample is reheated and sent to the plasma flame. 

Fig. 5. A three-slot burner modified with aluminium shields for tin determination in an entrained air—hydrogen flame.

Measurements made with hydrogen-argon air entrained flame. 

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