Recent developments in food-matrix Reference Materials at NIST.

Since 1996, the National Institute of Standards and Technology (NIST) has developed several food-matrix Standard Reference Materials (SRMs) characterized for nutrient concentrations. These include SRM 1544 Fatty Acids and Cholesterol in a Frozen Diet Composite, SRM 1546 Meat Homogenate, SRM 1548a Typical Diet, SRM 1566b Oyster Tissue, SRM 1846 Infant Formula, and SRM 2383 Baby Food Composite. Three additional materials--SRM 1946 Lake Superior Fish Tissue, SRM 2384 Baking Chocolate, and SRM 2385 Spinach--are in preparation. NIST also recently assigned values for proximate (fat, protein, etc.), individual fatty acid, and total dietary fiber concentrations in a number of existing SRMs and reference materials (RMs) that previously had values assigned for their elemental composition. NIST has used several modes for assignment of analyte concentrations in the food-matrix RMs, including the use of data provided by collaborating laboratories, alone and in combination with NIST data. The use of data provided by collaborating food industry and contract laboratories for the analysis of food-matrix RMs has enabled NIST to provide assigned values for many analytes that NIST does not have the resources or analytical expertise to measure.

The certification of SRM 1546--Meat Homogenate, a new reference material for nutrients in a high protein, high fat matrix.

In response to reference material needs expressed by the food industry and government regulators, the National Institute of Standards and Technology (NIST) has developed a new Standard Reference Material (SRM) consisting of a canned meat product with certified and reference values for a large number of constituents. SRM 1546 Meat Homogenate consists of a mixture of finely ground pork and chicken prepared and canned by a commercial process. NIST determined the concentration levels of cholesterol, sodium, calcium, iron, and seven fatty acids in this SRM using well defined methods and procedures. These analytes as well as 34 other constituents or properties were determined in an interlaboratory comparison exercise involving 21 laboratories, most of which are associated with the National Food Processors Association (NFPA) Food Industry Analytical Chemists Subcommittee (FIACS). From statistical analysis of the data, NIST assigned certified concentrations for the eleven analytes measured at NIST and reference concentrations for the proximates, six additional fatty acids, seven minerals, and seven water-soluble vitamins. Information values without uncertainties are provided for the concentrations of six additional constituents for which the uncertainties could not adequately be assessed. SRM 1546 will provide laboratories with a means to evaluate the accuracy of the methods they use to assign nutrient levels to processed meats and similar products.

Certification of nutrients in Standard Reference Material 1846: infant formula.

In 1996, the National Institute of Standards and Technology (NIST) released Standard Reference Material 1846 (Infant Formula), which can be used as a control material for assigning values to in-house control materials and for validating analytical methods for measurement of proximates, vitamins, and minerals in infant formula and similar matrixes. The SRM was manufactured by preparing a spray-dried formula base containing fat, protein, carbohydrates, and minerals and then combining that formula base with a dry-blend vitamin premix that supplied the vitamins. The Certificate of Analysis for SRM 1846 provides assigned values for concentrations of proximates (fat, protein, etc.), vitamins, and minerals for which product labeling is required by the Infant Formula Act of 1980 and by the Nutrition Labeling and Education Act of 1990. These assigned values were based on agreement of measurements by NIST and/or collaborating laboratories. Certified values are provided for vitamins A (trans), E, C, B2, and B6 and niacin. Noncertified values are provided for solids, ash, fat, nitrogen, protein, carbohydrate, calories, vitamin D, delta-tocopherol, gamma-tocopherol, vitamin B1, vitamin B12, folic acid, pantothenic acid, biotin, choline, inositol, calcium, phosphorus, magnesium, iron, zinc, copper, sodium, potassium, and chloride. Information values are provided for iodine, manganese, selenium, and vitamin K.

The oral antihypertensive activity of the methylated derivatives of phenyl-2-aminoethyl sulfide.

We have reported previously that phenyl-2-aminoethyl sulfide and its derivatives are excellent substrates for dopamine-beta-monooxygenase and produce an antihypertensive effect in spontaneously hypertensive rats after i.p. administration. In the studies reported herein, we demonstrate that alpha-methyl-phenyl-2-aminoethyl sulfide and 4-hydroxy-alpha-methyl-phenyl-2-aminoethyl sulfide, the methylated and hydroxymethylated derivatives of phenyl-2-aminoethyl sulfide, respectively, decrease mean arterial pressure in conscious, unrestrained spontaneously hypertensive rats after p.o. administration. This antihypertensive effect after p.o. administration occurs without the undesirable transient rise in blood pressure observed previously after i.p. administration. Results using the methodology of food-reinforced operant conditioned behavior are consistent with the interpretation that the ring hydroxylated derivatives, 4-hydroxy-phenyl-2-aminoethyl sulfide and 4-hydroxy-alpha-methyl-phenyl-2-aminoethyl sulfide, do not penetrate into the central nervous system. This finding supports our contention that the primary site of action for the antihypertensive activity of the sulfides may be the peripheral adrenergic nerve ending. In view of the current high degree of interest in chiral development, the enantiomeric specificity of the antihypertensive activity of alpha-methyl-phenyl-2-aminoethyl sulfide was also evaluated. Results from these studies demonstrate that the (S)-enantiomer of alpha-methyl-phenyl-2-aminoethyl sulfide is more effective in lowering blood pressure after p.o. administration than the (R)-enantiomer. The implications of our findings in terms of the mechanism of action of these compounds are discussed.

Potentiometric titration of sulphate, sulphite and dithionate mixtures, with use of a lead ion-selective electrode.

Simple methods are described for the analysis of sulphate, sulphite and dithionate mixtures by potentiometric titration with lead perchlorate (and use of a lead ion-selective electrode). The error and precision are both about 2-5% for the sulphate concentration range 3 x 10(-3) -6 x 10(-4)M. The sulphur content in fuels derived from refuse was determined by potentiometric titration after bomb-combustion and the results compared favourably with those from the standard BaSO(4) gravimetric method.

Enthalpy of Combustion of Microcrystalline Cellulose.

A test substance with characteristics and properties similar to those of cellulose-based solid waste products is needed to calibrate calorimeters and combustors which will be routinely burning these materials to determine their calorific values precisely for use in commerce. Microcrystalline cellulose was found to be a good calibrant for this purpose. The enthalpy of combustion of microcrystalline cellulose Δ H c o at (25 °C), and its estimated uncertainty, was determined to be -2812.401±1.725 kJ/mol based upon the sample mass. A calculated heat of wetting correction of 1.514 kJ/mol was applied to the combustion data.