Species and substances

We sometimes need to make a distinction between a species and a substance. A species is any entity of definite elemental composition and charge and can be described by a chemical formula, such as H2O, HsO , NaCl, or Na+. A substance is a species that can be prepared in a pure state (e.g., N2 and NaCl). Since we cannot prepare a macroscopic amount of a single kind of ion by itself, a charged species such as H3O+ or Na is not a substance. Chap. 10 will discuss the special features of mixtures containing charged species. 

---

When a chemical is emitted to the environment, it is distributed into different environmental matrices depending on the properties of the substance. When distributed in the environment, different organisms, including humans, can be exposed to the substance via different exposure routes such as ingestion, inhalation and dermal exposure. Depending on both species and substance characteristics, the exposure to a substance can give rise to different impacts. 

Improvement of measurement techniques allows the use of smaller amounts of stable species and substances with fewer impurities, which should yield more accurate experimental data. 

When substances ionise their neutral species produce positive and negative species. The ionisation constants are those constant values (equilibrium constants) for the equilibria between the charged species and the neutral species, or species with a larger number of charges (e.g. between mono and dications), l ese ionisation constants are given as pK values where pK = -log K and K is the dissociation constant for the equilibrium between the species [Albert and Serjeant The Determination of Ionisation Constants, A Laboratory Manual, 3rd Edition, Chapman Hall, New York, London, 1984, ISBN 0412242907]. 

Important intermedia transfer mechanisms that must be considered where significant surface water contamination is expected include transfers to ground water where hydrogeology of the area indicates significant surface water-ground water exchange, transfers to biota where waters contaminated with lipophilic substances support edible biotic species, and transfer... 

In recent years much interest has been displayed in the Indian species and other alkaloids, named in the following list, have been isolated from it, though it should be noted that doubts have been expressed by several authors as to the individuality of some of these substances. 

Thus, a methyl group placed at the site of hydration decreases the proportion of the hydrated species and, hence, shifts both the ultraviolet spectra (cf. Fig. 2A and B) and the ionization constant of the substance towards normality. A valuable means for locating the site of hydration, therefore, is to introduce a methyl group in various likely places until the anomalous spectrum is lost and the spectrum of the predominantly anhydrous species restored. The effect of such a methyl group on the pjfiT value is also revealing because a decrease in the amount of the hydrated species causes a decrease in the p value,... 

Silver was critical of the lack of use by plutonium chemists of a-coefficients. Assuming that Silver was referring to a-coeffi-cients defined as the fraction of the total concentration of a substance that exists as a particular species, he was wrong to say that plutonium chemists have not used them. Phil Horwitz at ANL has used them. Publications from ORNL have reported them to easily show relative concentrations of plutonium species, and L. M. Toth used such a-coefficients as percent of Pu(IV) polymer in his symposium talk Tuesday. Alpha coefficients are a commonly used, simple concept - certainly since Ringbom s article in the Journal of Chemical Education in 1958."... 

A note on good practice Because we need to keep track of the Substances, we must be sure to indicate the exact species and concentration units, by writing, for example, 1.0 (mol HC1) L 1 or 1.0 m HCl(aq). 

This module should contain data from all of the non-dinical studies conducted to investigate the pharmacological and toxicological properties of the dmg substance/ product. The standard headings used to present this information are shown in Figure 6.4. Study reports should be presented in the following order according to species and route of administration ... 

The amounts of the standard isotopic species and the tracer isotopic species are represented by X and X for the sample and the reference material. The reference substance is chosen arbitrarily, but is a substance that is homogeneous, available in reasonably large amounts, and measurable using standard analytical techniques for measuring isotopes (generally mass spectrometry). For instance, a sample of ocean water known as Standard Mean Ocean Water (SMOW) is used as a reference for and 0. Calcium carbonate from the Peedee sedimentary formation in North Carolina, USA (PDB) is used for C. More information about using carbon isotopes is presented in Chapter 11. 

PBPK/PD models refine our understanding of complex quantitative dose behaviors by helping to delineate and characterize the relationships between (1) the external/exposure concentration and target tissue dose of the toxic moiety, and (2) the target tissue dose and observed responses (Andersen et al. 1987 Andersen and Krishnan 1994). These models are biologically and mechanistically based and can be used to extrapolate the pharmacokinetic behavior of chemical substances from high to low dose, from route to route, between species, and between subpopulations within a species. The biological basis of... 

In all matter, even substances as solid as steel, individual atoms or molecules are in never-ending motion. Monatomic substances, such as argon, contain individual atoms that are not bound to any other atoms. Other substances, including O2, H2, and H2 0, contain groups of atoms bound into molecules. Continuous motion at the atomic-molecular level involves atoms for monatomic species and molecules for species composed of atomic groups. 

Spin densities determine many properties of radical species, and have an important effect on the chemical reactivity within the family of the most reactive substances containing free radicals. Momentum densities represent an alternative description of a microscopic many-particle system with emphasis placed on aspects different from those in the more conventional position space particle density model. In particular, momentum densities provide a description of molecules that, in some sense, turns the usual position space electron density model inside out , by reversing the relative emphasis of the peripheral and core regions of atomic neighborhoods. 

The different ways of species analysis - qualitative and quantitative - are well known. However, in structure analysis, they can also be differentiated between qualitative and quantitative ways according to the type and amount of information obtained (Eckschlager and Danzer [1994]). Identification of a sample or a given constituent may have an intermediate position between species and structure analysis. In any case, identification is not the same as qualitative analysis. The latter is the process of determining if a particular analyte is present in a sample (Prichard et al. [2001]). Qualitative analysis seeks to answer the question of whether certain components are present in a sample or not. On the other hand, identification is the process of finding out what unknown substance(s) is or are present (Eckschlager and Danzer [1994]). In Sects 9.1 and 9.3 it will be shown that there is a... 

---