Ammonium nitrate matrix modifier

Polymer-based rocket propellants are generally referred to as composite propellants, and often identified by the elastomer used, eg, urethane propellants or carboxy- (CTPB) or hydroxy- (HTPB) terrninated polybutadiene propellants. The cross-linked polymers act as a viscoelastic matrix to provide mechanical strength, and as a fuel to react with the oxidizers present. Ammonium perchlorate and ammonium nitrate are the most common oxidizers used nitramines such as HMX or RDX may be added to react with the fuels and increase the impulse produced. Many other substances may be added including metallic fuels, plasticizers, stabilizers, catalysts, ballistic modifiers, and bonding agents. Typical components are Hsted in Table 1. 

Halliday et al. [396] have described a simple rapid graphite furnace method for the determination of lead in amounts down to 1 xg/l in polluted seawater. The filtered seawater is diluted with an equal volume of deionised water, ammonium nitrate added as a matrix modifier, and aliquots of the solution injected into a tantalum-coated graphite tube in an HGA-2200 furnace atomiser. The method eliminates the interference normally attributable to the ions commonly present in seawater. The results obtained on samples from the Firth of Forth (Scotland, UK) were in good agreement with values determined by anodic stripping voltammetry. 

Montgomery and Peterson [675] showed that ammonium nitrate used as a matrix modifier in seawater analysis to eliminate the interference of sodium chloride degrades the pyrolytic coating on graphite-furnace tubes. The initially enhanced sensitivities for copper, manganese, and iron are maintained for up to 15 atomisations. There is then a rapid decline to a constant lower sensitivity. The characteristics depend strongly on the particular lot of furnace tubes. To... 

Modifiers are, however, used in a rather indiscriminate way in many laboratories. If used carelessly they can contaminate the sample solution with the element that is being determined and they can actually add to the background interference which one intends to reduce. By carefully optimizing the ashing and atomization temperatures for specific food matrices, as described above, the use of matrix modifiers can be reduced to the cases when they are really necessary. An additional benefit of matrix modification is that the sample and standard matrix are made very similar, this often making the standard addition method unnecessary. How this is carried out is described in detail in most instrument manuals and in specific textbooks. Commonly used modifiers are ammonium nitrate, ammonium phosphate, Mg nitrate, Pd nitrate, and ascorbic acid. 

Stock Solutions [Because of the difficulty in preparing matrix modifier stock solutions with the required purity, purchasing modifier stock solutions and using them to prepare working solutions is recommended. An ammonium dihydrogen phosphate (4%) and magnesium nitrate (0.2%) solution may be purchased from High Purity Standards, or equivalent.]... 

Complete ashing of organic matter is not essential when determining these biological specimens by AAS unless a preconcentration step is involved. Most body fluids can be aspirated into the flame/plasma directly or after dilution. The specimens can also be introduced into the carbon cup directly for GF-AAS measurements or after matrix modification. A number of matrix modifiers, e.g. ammonium nitrate, ammonium phosphate, lanthanum nitrate, etc., have been used. The deproteinisation of these samples is also helpful (Berman, 1980). A well balanced combination of different approaches is usually helpful. 

As, Se, Pb, and many more would have been lost in the process. By adding an excess of ammonium nitrate as a matrix modifier to the sample in the graphite tube, the NaCl matrix can be converted to the much more volatile compounds ammonium chloride and sodium nitrate. Most of the matrix can be removed at a temperature below 500°C, which substantially reduces background absorption without loss of volatile analytes. 

Ammonium nitrate, ammonium hydrogen phosphate, and their mixtures can be used as matrix modifiers to reduce matrix effects in the determination of lead. 

Because matrix modification is one aspect of sample preparation, even though it is most often accomplished automatically in GFAA, we will discuss it here. For example, NIOSFI Method 7105 recommends a matrix modifier that consists of a mixture of ammonium dihydrogen phosphate, magnesium nitrate, and nitric acid for the determination of airborne Pb. Because the graphite furnance can be viewed as a chemical reactor whereby the sample with its matrix is placed on a graphite platform (the L vov platform, discussed in Chapter 4) and heated to a very high temperature, reactions can take place that involve both the metal analyte of interest as well as sample matrix components. 

Ammonium phosphate, monobasic, NH4H2PO4 and magnesium nitrate, Mg(N0 2 6H2O both manufactured by the Mallinckrodt Chem. Co., were used to prepare the matrix modifier for AAS-HGA analysis. 

As earlier reported for electrochemical sensing, often the active chromo-phore will be dispersed in a polymeric matrix. For example, Mohr and Wolfbeis reported a nitrate sensor [121] where the active chromophore is a rhodamine B dye which had been modified with an octadecyl side chain to render it hydrophobic and prevent leaching. The dye was dispersed in a plasticised PVC membrane containing a hydrophobic anion carrier (tridodecylmethylammo-nium chloride). On exposure to nitrate, the fluorescence of the dye increased. This membrane, however, only displayed Hofmeister-type selectivity and was also affected by pH. Replacing the quaternary ammonium anion carrier with a palladium phospine chloride carrier led to selectivity for nitrite [ 122], probably due to a preferential interaction between Pd and nitrite ion. 

Although more widely applicable to organic chemicals, the matrix of some inorganic chemicals may be reduced or modified by heating to an elevated temperature. Care must be exercised not to lose volatile trace metals such as arsenic, cadmium and zinc mercury will almost invariably be lost to some extent. Matrices which might be amenable to this treatment include ammonium salts, sulphates, nitrates and the salts of organic acids such as the oxalates. 

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