OVERVIEW OF CHEMICAL EVIDENCE Chapter 11

WHAT IS ANALYTICAL CHEMISTRY? *Identifying the elemental or molecular composition of a sample (Qualitative) *How much of each component is present (Quantitative) HOW IS THIS ACCOMPLISHED? Separation, purification, characterization & quantification of the components

5 uses of analytical chemistry Food safety issues Forensics investigation Environmental studies Medicine Scientific and biomedical research

11.2: Methods in Analytical Chemistry What is the identity & how much of it is present? Preliminary questions the must be answered to determine the best method for analysis. 1) Is the technique qualitative or quantitative? Qualitative: Determining the identity of a substance Quantitative: Determining how much of a known component is present

11.2, Continued 2) What sample size is needed? Enough to determine empirical formula 3) What type of sample preparation is required? Enough to render sample suitable for analysis 4) What is the level of analysis required? Enough to determine precisely the composition 5) What are the detection limits (analytical concentrations) of the method? Lowest conc. needed for accurate determination

11.2, Continued 6) Is the technique destructive or non-destructive? Does the sample need to be preserved 7) Is the instrument available? Varies with type of test and costs 8) Is it admissible in court? Must be Relevant and Probative 9) Cost of the analysis? Plays a role is choosing a technique – may have to outsource sample to larger lab 10) Are there any interferences expected? Will other components of sample give false results 11) How are analytical results verified? Repeat test and Systematic elimination

11.2, Continued Standard methods of analyses have been developed for many of the forensically important substances that are frequently encountered. Therefore, most or all of these questions are already answered. For example: The analysis of alcohol in blood- *Mass spectrometry, spectrophotometry The identification of a fiber- *Infrared microspectrophotometer

11.2, Continued Presumptive Tests: Screening (not definitive) Confirmatory Test: Results are to a high degree of certainty with less false readings. Vocabulary: Analyte: The substance for which the analysis is being done. Concentration: The amount by mass of material dissolved in a given volume of solvent.

11.3: Atoms, Molecules and Separation Science Basic Atomic Theory Nucleons: particle of the nucleus Nucleus- most dense part of atom

Nucleus: The core of an atom containing the protons & neutrons. the smallest particle of an element that can exist and still retain its identity as that element. Parts of an atom: (subatomic particles) Nucleus: The core of an atom containing the protons & neutrons. Particle Charge Mass Location in Atom Additional Info. Proton +1 1 amu nucleus = Atomic Number Neutron Electron -1 (1/1837 amu) Orbitals / Energy levels = # of protons (atom)

11.3, Continued Atomic Structure Review Vocabulary: Nucleons – particles of nucleus Atomic Number – number of protons Atomic mass – protons + neutrons Atomic mass unit – 1/12th the mass of a Carbon atom (standard unit) Cation – positively charge ion Anion – negatively charged ion Elements- substance with atoms having same number of protons

Elements to Compounds (Review) Matter- - All things of substance; Has a mass and occupies space. - Composed of atoms or molecules Element- Fundamental particle of matter; cannot be broken down into simpler substances by chemical means * Elements are represented by one and two letter “symbols” The first letter is always uppercase. Example: Sodium= Na Compound- A pure substance composed of two or more elements. Ex: NaCl is 1 Na : 1Cl

11.3, Continued Periodic Table of Elements

Atomic Mass Isotope: An atom differing from another atom of the same element in the number of neutrons it has in its nucleus. *Different atomic masses C-12 C-14 6 P 6P 6 N 8 N 12 AMU 14 AMU Change protons = change element

ISOMERS: Substances with the same molecular formula (same number of each atom) but different structural formulas (atoms are arranged differently.

Conservation of Mass Matter is neither created nor destroyed. *Equation is balanced (coefficients) * # of atoms in reactants = # of atoms in products C2H8 (g) 4 O2 (g)  2 CO2 (g) + 4 H2O (l)

Stoichiometry Molecular Mass = Sum of all atomic masses in the formula *Take into account subscripts Mole: A value used to represent 6.02 x 1023 individual particles *This is a number too great to count so the equivalent of ‘a mole of anything’ would be the gram formula mass For example – H2O  Formula mass = 18g If you were to weigh out 18 grams of water, you would have 1 MOLE OF WATER

Empirical vs. Molecular Formulas An empirical formula (reduced subscripts) can be determined from knowing the percent composition by mass of each element. 1) Change ‘%’ units to ‘grams’ units (assume mass is out of 100g) 2) Convert grams  moles 3) Calculate the mole ratio between mole values 4) Use ratios as your subscripts in the formula

States of Matter

Properties Physical vs. Chemical - Physical: Can be measured without changing the compound chemically Chemical: Involves changing the molecule into something chemically different Intrinsic vs Extrinsic – Intrinsic: Does not rely on how much sample is involved Ex. Density, melting & boiling point, color Extrinsic: Depends on sample size Ex. Mass, volume, heat

Solutions The components of a solutions are the solute and the solvent. Solute: The compound dissolved Solvent: What the solute is dissolved into Ex. Aqueous solution  Solvent = water (aq) Concentration of a solution is often measured in Molarity. Molarity (M) = moles / volume (L)

Pure Substances & Mixtures

Purification Methods Purification methods separate mixtures into their pure components. There are two types of purification methods: 1)Physical methods 2) Chemical methods *Using the copies of the textbook, describe each of the physical separation techniques -Density Properties -Solubility -Boiling point / Melting point -Chromatography (Videos showing chromatography are posted on eBoard)

Analysis of Chromatography Retention Factor, Rf : Distance traveled by spot / distance traveled by solvent *Answer will always be less than 1 *Used in paper chromatography and thin layer chromatography Retention Time : The amount of time that it takes a compound to emerge from the end of the column. *Used in gas & liquid chromatography

Chemical Separation Using chemical properties (reactivity) to separate one substance from another. Ex. Separating gold from iron Iron reacts with mineral acid, gold does not. *This removes the iron but not the gold.

11.4 : Classical Methods in Analytical Chemistry Physical Analysis: Often uses complex instruments to measure & analyze the physical properties of a sample: -Often an interaction with light -Qualitative and quantitative data Classical Methods: Based on well-known chemical reactions of analytes -Many developed in the 19th century

Classical Methods, cont. Field Tests: A presumptive test that can be run in the field (at the scene) -Simple, fast test that produces a color change -Drugs Spot Test: Very quick, simple chemical reaction with a specific element -Color change, precipitate formed, bubbles (gas produced) -Wide range of applications

Classical Methods, cont. Gravimetric Analysis: Focuses on ‘how much of something’ is present in a sample. -Unknown substance placed in a solution & allowed to react with another chemical. -A solid precipitate of the unknown is formed & weighed to determine how much is present. Combustion Analysis: Substance is completely burned & the amount of CO2 and H2O is measured to determine starting mass.

Classical Methods, cont. Volumetric (Titrametic) Analysis: Titration -When you determine the amount of analyte (the unknown) in a solution by reacting it completely with a known amount of a reagent solution. -Reaction is carried out until all the analyte is completely reacted to form product. -Equivalence Point: The exact point when all the analyte is consumed

Classical Methods, cont. Acids & Bases; Indicators show equivalence point.

Classical Methods, cont. Immunoassay: Uses immune-like responses to identify the presence of a specific substance. *Antibody is specific for an antigen (substance) *Antibody can be designed to fluoresce when bound to the molecule of choice.