Exam 2 Flashcards
what factors affect the choice of sampling plan?
-purpose of inspection
-nature of the population being investigated
-nature of the product
-nature of the test method
what is attribute versus variable sampling
- attribute: check for presence or absence of a variable; decide acceptability
- variable: quantitatively estimate characteristic on continuous scale
what are problems in sampling and sample storage
-sampling bias
-inappropriate sampling plan
-sample degradation due to poor sample storage
-mislabeling of samples
-susceptibility to tampering
how can you prepare samples for testing & considerations
-size reduction (grinding)
-considerations: enzyme inactivation, lipid oxidation protection, microbial growth and contamination
wavelength
lambda- represents the distance between successive maxima on any given wave
frequency
in general; the number of occurences of a repeated event per time unit (Hz = 1/s)
interference
- term used to describe the observation that when 2 or more wave trains cross one another, they result in an instantaneous wave, at the point of intersection, whose amplitude is the algebraic sum of the amplitubes of the individual waves at the point of intersection
- maximum constructive interference (positive): doubles amplitude
- maximum destructive interference (negative): zeroes out amplitude
what does amplitude correspond with
corresponds with the intensity of light and the brightness of colour perceived by an observer
reflection
study of light as a function of wavelength that has been reflected from a solid, liquid, or gas
light (color) perception
- light (electromagnetic radiation) has wave like properties: interference, diffraction, refraction
- interference (on other slide)
- diffraction: incident beam will split into multiple diffracted beams when going through or around an object
- refraction: what happens when a sticj looks broken when put into the water
what is a photon
-photons are the discrete packets of energy in a wave
-energy of a photon is defined in terms of frequnecy of the wave (E = hv)
what are the discrete energy levels of electric, vibrational, and rotational energy levels
-electric energy levels: ground electric state, first excuted electronic state, second excited electronic state
-vibrational energy levels: within electronic state; the atoms that compromise a molecule are in constant motion, IR region
-rotational energy levels: potential energy of molecule also quantized in terms of energy associated with rotation of the molecule about its center of gravity; customary to consider several rotational energy levels superimposed on each of the permitted vibrational energy levels
what are the energy states of matter
ground state vs excited state
summary of light: wave-like and photons
-the distinction between considerations of continuous wave-like properties and distinct particles depends on which aspects of light one is studying:
* **wave-like properties: **essential to understanding movement of electromagnetic radiation; through space or through a medium
* consideration of electromagnetic radiation as photons (distinct particles of energy) is essential to understanding **interaction of light with matter **
what is monochromatic light
- monochromatic light is electromagnetic radiation composed of waves having a single frequency and wavelength (ex. having equivalent energy)
- monochromatic light is obtained by optical dispersion in a prism or by diffraction using a diffraction grating with slits or ridges
how does a molecule go from ground to excited state
- under certain conditions a molecule struck by a photon may absorb the photon’s energy
- molecules energy is increased by an amount equal to the photon energy hv
- internal energy content of molecule varies in a series of discrete steps (not in continuous manner)
energy transitions: absorption
- process in which energy from a photon of electromagnetic radiation is transferred to the absorbing species
- when atom or molecule absorbs a photon of light, its internal energy increases by an amount equivalent to the amoung of energy in that particular photon
- in absorption => species goes from a lower energy state to a more excited state
energy transition: emission
- reverse of absorption; occurs when atom/molecule is released in the form of a photon of radiation
- molecule raised to excited state will typically remain in the excited state for short time before relaxing back into the ground state
what is the loss of energy represented as
- heat: the relaxation process in which a molecule will dissapate energy where they will collide with other molecules which is converted to kinetic energy
- emission of radiation (emission of a photon with lower energy and longer wavelength than photons that were absorbed in the excitation process)
what is definition of UV/Vis
method that investigates how light interacts with matter in the ultraviolet and the visible range of light
what is beer’s law
- A = abc
- a/epsilon = absorptivity (cm-1 & conc-1)
- b = path length through solution (cm)
- c = concentration of absorbing species (M, mM, mg/ml, %)
deviations of beer’s law
- only dilute solutions work on beer’s law (up to 10 mM)
- reversible association-dissociation of analyte molecules
- ionization of a weak acid in an unbuffered solvent
common uses for UV-VIs
- ELISA (enzyme-linked immosorbent assay); uses solid-phase type of EIA to detect presence of a ligand in a liquid sample using antibodies directed against the ligand to be measured directly or indirectly
calibration curves
- linear: obey beer’s law
- non-linear: due to conc depending changes in chemistry of system (change in absorbance per unit change in concentration is not constant)
- due to limitations in instrument
what is a chromophore
part of molecule responsible for its color, the color that is seen by our eyes is reflected (not absorbed)
endpoint analysis for colorimetric assays
?
what is the nature of absorbing species
- double bonds
- ring structures
what are the main components of a spectrophotometer
- light source
- monochromator
- sample/reference holder
- radiation detector
- readout device
what is a monochromator
- component that functions to isolate the specific, narrow, continuous group of wavelengths to be used in the spectroscopic assay
- polychromatic radiation from the source enter the monochromator and is dispersed according to the wavelength and monochromatic radiation of a selected wavelength exits the monochromator
diffraction grating
- separation of component wavelengths is dependent on the different wavelengths being diffracted at different angles relative to the grating normal
reflection grating
- incorporates a reflective surface in which a series of closely spaced grooves has been etched; grooves serve to break up the reflective surface such that each point of reflection behaves as an independent point source of radiation
detector
light transmitted through the reference or sample cell is quantified by a detector
how does a uv/vis work
-a xenon lamp directs beam of light through the cuvet, the sample in cuvet absrobs particular wavelengths, the light that isn’t absrobed (transmiited light) passes through the cuvet and is first diffracted and then directed onto the detector
relationship between cocnentration and light absorbed
the more light is absorbed, the higher the concentration
how does fluorescence spectroscopy work
- molecules are usualy at ground state but when a photon is sent in at a certain wavelength then the molecule can absorb that energy and the energy of molecule gets raised to higher level and from this level the molecule will relax a little bit and when molecule drops back down to ground state it emits a photon
what signal is measured in fluorescence spectroscopy
is the electromagnetic radiation emitted from the analyte
excitation vs. emission fluorescence spectroscopy
- excitation: aborbs light at lower wavelengths
- emission: emits light at longer wavelengths
system setup (fluorescence)
- need for 2 wavelengths: one for excitation beam & one for emission beam
- photon detector arranged so the emitted radiation that strikes the detector is traveling at an angle of 90 degrees relative to the axis of excitation beam (minimizes signal interference)
what is IR radiation
electromagnetic energy with wavelengths longer than visible light but shorter than microwaves
when can a molecule absorb radiation in IR spec
if it vibrates such that its charge distribution changes during vibration
what are the different types of molecular vibrations
-symmetrical and asymmetrical stretching
-bends: scissoring, rocking, bending
factors that affect frequency of vibrations (IR spec)
- bond strength
- mass
harmonic vs anharmonic oscillator
- harmonic: back and forth; system that when displaced from its equilibirum position, it experiences a restoring force F proportional to the displacement x
- anharmonic: deviation of a system from being a harmonic oscillator
FT-IR interferometer
- an IR beam is split then recombined by reflecting back the split beams with mirrors (all wavelengths are detected simultaneously)
- intensity of radiation reaching detector varies as function of optical path difference as one mirror is moved
what is an interferogram (IR)
pattern of energy intensity obtained, as a function of optical path difference (phase changes = interference = time domain changes)
what kinds of samples can be analyzed in IR spec?
- solution and surfaces
- ex: fine powder for quality control of ingredients
applications of mid-IR
milk:
* analyze hundreds of samples/hr
* measure fat, protein, and lactose contents simultaneously
near-IR applications
fruit:
* strawberries
* sampling of fruits is non destructive
principles of raman spec
- for raman scattering to occur, a molecule must undergo a change in polarization of the electron cloud of the molecule (but not chnage in dipole moment)
- can observe symmetrical vibrations with raman spectrometry
- some vibrations are only raman active, some are only IR active, and some are both
- during raman scattering, scattered photons shift to longer wavelength/lower frequency (shift in freq = stokes lines)
what is being measured on the raman spectrum
measures intensity of the stokes lines
food applications of raman spec
detection of foodborne pathogens, contaminants (pesticides, antibiotic residues, adulteration)
what does atomic absorption spectroscopy measure
measures absorption of eletromagnetic radiation by atoms
what does atomic emission spectroscopy measure
measures emission of energy from atoms in excited state
what does the atomic emission spectrum look like
dark with colorful lines
what does atomic absorption spectrum look like
colorful with black lines
principles of atomic absorption spectroscopy
- sample solution is volatilized and atomized by a flame or graphite furnace
- atoms go from ground state to excited state when they absorb radiation (of specific wavelength) from a lamp (hollow cathode lamp)
- amount of radiaiton absorbed is related to concentration (by Beer’s law)
applications of atomic absorption and emission spectroscopy
- ashing: reduce volatilization problem by wet ashing
- reageants: need highly pure chemical reagents and water, need to use reagent blanks
- standards: must be prepared carefully, run standards freq, check with standard for reference materials
- labware: plastic containers preferable to glass, must be washed and rainsed carefully, and soaked in acid
principles of atomic emission spectroscopy
- atomic emission spectra produced when neutral atoms in an excited state emit energy on returning to the ground state or a lower-energy state
- occurs when electrons in various energy states fall to lower energy levels
differences of atomic absorption compared to UV-Vis
atomic absorption has electronic absorptions only
