Chromium blank

Background correction in action. (Top) background due to spraying chromium blank. 

Chromium (ITT) can be analy2ed to a lower limit of 5 x 10 ° M by luminol—hydrogen peroxide without separating from other metals. Ethylenediaminetetraacetic acid (EDTA) is added to deactivate most interferences. Chromium (ITT) itself is deactivated slowly by complexation with EDTA measurement of the sample after Cr(III) deactivation is complete provides a blank which can be subtracted to eliminate interference from such ions as iron(II), inon(III), and cobalt(II), which are not sufficiently deactivated by EDTA (275). 

Chromium in steel Discussion. The chromium in the steel is oxidised by perchloric acid to the dichromate ion, the colour of which is intensified by iron (III) perchlorate which is itself colourless. The coloured solution is compared with a blank in which the dichromate is reduced with ammonium iron(II) sulphate. The method is not subject to interference by iron or by moderate amounts of alloying elements usually present in steel. 

Figure 9.26 Cr 2p spectra and negative SIMS spectra of two model catalysts and a blank reference. The blank (bottom) shows only Si,0, fragments on a chromium-loaded catalyst CriCf fragments appear after thermal activation (Ar/02). If desorption of chromium is made impossible (in oxygen-free argon), Cr2Or clusters can also be detected. In combination, this is strong evidence that chromate anchors to the silica surface as a monomer (courtesy of P.C. Thiine and R. Linke, Eindhoven).

The analysis of vanadium steels is effected by the application of one of the foregoing methods. Blank determinations on a steel free from vanadium but otherwise of the same approximate composition are used as a control. Iron and molybdenum are removed from hydrochloric acid solution by Kothe s ether separation method 1 chromium, nickel, copper, etc., are then precipitated as hydroxides by caustic soda, the filtrate containing the vanadium as vanadate.2 The method is modified for the simultaneous estimation of both vanadium and chromium in a vanadium-chromium steel.3... 

Aliquots of the digest are analysed for aluminium, chromium, iron copper and zinc by direct flame AAS. Together with the samples, blanks and standards covering the range from 0.1 to 30 pg/ml of each of the above metals in 10% sulfuric acid were also run. 

The Auger depth-composition profiles vapor-degreased surfaces and a "blank" surface are compared in Figures 8(a) and (b). Figure 8(b) shows that the small amount of chloride on the "blank" surface is a maximum at the outer surface and drops to undetectable as the film is penetrated 2 nm. By contrast, on the specimens exposed to vapor degreasing, the chloride concentration consistently reached a maximum 2 to 4 nm within the film. This increased level of chlorine in the film appeared to be related to a decrease in the ratio of iron to chromium oxides, particularly at the depth where the chlorine concentration was a maximum. 

Make several reagent blank determinations, using only the chromium trioxide solution in the above procedure. The ratio of the sodium hydroxide titration (Vb) to the sodium thiosulfate titration (7b), corrected for variation in normalities, will give the acidity-to-oxidizing ratio, V b/Kb = K, for the chromium trioxide carried over in the distillation. The factor K should be constant for all determinations. 

For the majority of elements commonly determined in water by AAS, an air—acetylene flame (2300°C) is sufficient for their atomisation. However, a number of elements are refractory and they require a hotter flame to promote their atomisation. Because of this, a nitrous oxide—acetylene flame (3000° C) is used for the determination of these elements. Refractory elements routinely determined in water are aluminium, barium, beryllium, chromium and molybdenum. Chromium shows different absorbances for chromium(III) and chromium(VI) in an air-acetylene flame [15] but use of a nitrous oxide-acetylene flame overcomes this. Barium, being an alkaline earth metal, ionises in a nitrous oxide—acetylene flame, giving reduced absorption of radiation by ground state atoms, however in this case an ionisation suppressor such as potassium should be added to samples, standards and blanks. 

The as-synthesized and calcined CrAPO-5 and CrS-1 were characterized by XRD which showed that the samples were pure and had an API and MFI structure respectively. ICP analysis showed that both catalysts contained about 1 % chromium. The results observed in the decomposition of cyclohexenyl hydroperoxide over several redox active moleular sieves are presented in Table 1. CrAPO-5 and CrS-1 displayed rougly equal activity and selectivity in the decomposition of cyclohexenyl hydroperoxide. Blank reactions carried out with Silicalite-1 (S-1) and silicon incorporated Aluminophosphate-5 (SAPO-5) show low conversions confirming that the chromium was responsible for the catalysis. Other transition- metal subsituted molecular sieves showed low conversions. 

Atomic absorption measurements were made using standard conditions. Nearly stoichiometric flames were used for all metals but chromium, for which a reducing flame was used. The air-acetylene flame was used for all metals but vanadium, for which a nitrous oxide-acetylene flame was used. A single slot titanium burner was used for all of the metals investigated. Water saturated MIBK was used as the blank. Table I presents typical instrument parameters. 

Ratio Crude/Reagent Blanks (Response Factors for Added Organometallic Chromium Compound)... 

External sample decomposition procedures, such as ashing or acid digestion, not only eliminate the matrix but also convert the analyte to some specific salt suitable for HVAA analyses. For example, the ashing used to obtain data in Table 3.II converted the nickel to its sulfate. In the Project, indirect HVAA procedures were successfully applied to six elements. The application of these procedures, however, may be restricted by contamination or by reagent blanks. For example, exposure to nichrome heating elements affects chromium (see p. 19). Although destruction techniques eliminate the hydrocarbon matrix, they do not compensate for interelement interferences. 

Two procedures based on HVAA have been evaluated by the Project. In one, the sample is analyzed directly after dilution with tetrahydrofuran. In the other, a 0.5-g sample is charred with 10 drops of sulfuric acid in a 30-ml Vycor crucible and treated at 540°C for the minimum time required to remove carbonaceous deposits. The ash is then solubilized in IN sulfuric acid prior to measurement. A reagent blank is carried through the procedure for each set of samples. The analyses are carried out promptly after preparation so that loss of chromium from aqueous solution by adsorption on the walls, which has been reported in some cases (II), can be avoided. 

This procedure has been applied to several crude oil samples, and the results were compared to those obtained by HVAA after sulfated ashing (Table 8.III). The good agreement between techniques verifies that no chromium is lost prior to atomization in the direct technique. Furthermore, it demonstrates that the combination of standard additions and correction for reagent blank and background overcomes the inter-... 

The advantages of microwave dissolution include fester digestion that results from the high temperature and pressure attained inside the sealed containers. The use of closed vessels also makes it possible to eliminate uncontrolled trace element losses of volatile species that are present in a sample or that are formed during sample dissolution. It is well known that significant amounts of elements such as arsenic, boron, chromium, mercury, antimony, selenium, and tin are lost at relative mild temperature with some open vessel acid dissolution procedures [8,9]. Another advantage of microwave dissolution is to have better control of potential contamination in blank as compared to open vessel procedures. This is due to less contamination from laboratory environment, unclean containers, and smaller quantity of reagents used. 

Thus we see that although six basically different methods have been used for the determination of chromium in a common, presumably stable and fairly constant biological substrate, blood, we do not know its chromium content. We cannot assume that the lowest value is the most correct since there may have been losses we can be quite confident that the high values were subject to some contamination. Yet every method was validated by spiking with known amounts of chromium, and even with labeled 51cr in some cases, with "excellent results." In many cases, blanks were mentioned as accounted for. If we cannot decide on the concentration of an inorganic element at trace levels in blood, how can we do better with more complex and less stable organic molecules in this and other tissues ... 

Standard addition sample preparation Using the standard solutions specimens are made up with 0, 2, 4, and 6 xg/liter of added chromium in terms of the urine volume assayed. A reagent blank is also assayed in parallel, the urine being replaced by 0.05 M nitric acid. 

Calculation After the analysis, the chromium concentration is determined graphically. First, the reagent blank is subtracted from the spiked and unspiked solutions of urine and the resulting values are plotted vs. the added chromium. The intersection of this line with the concentration axis gives the chromium concentration. 

Nitrate stock solution This solution is made close to the solubility of the nitrate salt (5 mol/L). Contaminating chromium in the concentrated nitrate solution is removed by coprecipitation with iron(ni)hydroxide. Iron(//f) chloride (final concentration 0.1 mmol/L) is thus added and allowed to become oxidized by dissolved oxygen. This is filtered off to produce the purified nitrate solution this process reduces chromium(V7) to chromium(///) which adsorbs on the iron(lII)hydroxide and is removed. Most conveniently the acetate buffer is premixed with the nitrate solution (to a final concentration of 0.2 mol/L acetate in 5 moI/L nitrate) and purified simultaneously. Thus the overall chromium reagent blank is typically reduced to less than 0.03 nmol/L. 

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