Introduction to Spectrochemical Methods Chapter 7

Introduction Spectrochemical methods Atomic spectroscopy Absorption or emission of light More than half or all instrumental methods of analysis Spectroscopy or spectrometry Science that deals with light Its absorption and emission by solutions Other material substances Instrument used Spectrometer When using a light sensor/phototube = spectrophotometer Spectrochemical analysis Degree which light absorbed or the primary light emitted Related to the amount of analyte present in the sample Critical measurements! Atomic spectroscopy Spectral differences between atoms Molecular spectroscopy

Characterizing Light Dual nature of light Particles Waves Photons or quanta Particle theory of light Waves Electromagnetic disturbances or electromagnetic waves Wave theory of light Dual nature not unlike modern description of electrons Described as particles To explain aspects of their behavior For more accurate description Must be described as entities of energy and NOT particles

Characterizing Light Wave theory of light Light travels in a fashion similar to that of a series of repeating waves of water Wave pool at an amusement park Electromagnetic waves They are wave disturbances that have an electrical component and a magnetic component Do NOT require matter to exist Can travel through a vacuum

Characterizing Light Wavelength, speed, frequency, energy, and wavenumber Wavelength (λ) The physical distance from a point on one wave, to the same point on the next wave Measured in metric units

Characterizing Light Wavelength, speed, frequency, energy, and wavenumber Speed of light (c) Speed in which electromagnetic waves move Speed of light in a Vacuum ≈ 3.00 x 1010 cm/sec Accounts for instantaneous speed that light fills a room when switch turned on ALL ELECTORMAGNETIC WAVES TRAVEL AT THE SAME SPEED IN A VACUUM REGARDLESS OF THEIR WAVELENGTH

Characterizing Light Wavelength, speed, frequency, energy, and wavenumber Frequency (ν = nu) Number of moving electromagnetic waves past a fixed point in 1 second Expressed in waves or cycles per second = hertz (Hz) Units = sec-1 Wavelength, speed, frequency can be expressed mathematically C = λν units are cm x sec-1 or cm/sec Wavelength and frequency are inversely proportional As one increases Other decreases

Characterizing Light Wavelength, speed, frequency, energy, and wavenumber Energy B/c light is a form of energy Each wavelength or frequency has certain amount of energy Considered to be the energy associated with a single photon of light .: particle theory and wave theory linked via energy E = hν E = energy, h = proportionality constant called Planck’s constant depends on units used-metric = 6.63 x 10-34 J/sec .: E = hc/λ

Characterizing Light Wavelength, speed, frequency, energy, and wavenumber Wavenumber (ν) Wavelength expressed in centimeters Characterized by the reciprocal of this wavelength ν = 1/ λ (cm) Used in conjunction with infrared light.

The Electromagnetic Spectrum

The Electromagnetic Spectrum So broad broken down into regions Visible light That portion of the spectrum we see with our eyes ≈ 350 nm to ≈750 nm Very narrow region UV, infrared, x-ray, radio, and television

The Electromagnetic Spectrum UV, visible, and infrared regions Mostly ones emphasized Nanometer and micrometer units used for wavelength Something to remember Long wavelength = low energy Infrared region Wavelengths extremely short Have higher energy than radio or television Cause no harm Remotes for TVs, VCRs, etc UV, x-rays, and gamma rays Very short wavelengths Very high energy Very dangerous!

Interaction of Light With Matter Light striking matter causes different events Transmitted Pass without interaction through the material Light passing through glass Reflected Changes directions Light in a mirror Scattered Deflected into many different directions Occurs when light strikes a substance composed of many individual, small particles Absorbed Light fives up some or all of its energy to the material

Absorption Spectra Instruments used to measure absorption Some in the UV and visible regions Others for the infrared region Methods used Beam of light formed Sample measured contained so that light passes through Absorption of the wavelengths present in light beam measured by a sensor and signal processor

Absorption Spectra An absorption spectrum Plot of the amount of light absorbed by a sample vs. the wavelength of the light Light absorbed called the absorbance (A) Obtained by using a spectrometer to Scan a particular wavelength region To observe amount of light absorbed by the sample along the way It’s a continuous spectrum (fig. 7.13, pg. 189) The spectrum is an unbroken pattern Does not display breaks or sharp peaks

Absorption Spectra Absorption vs. wavelength Can be displayed as a transmission spectrum Plotting the amount of light transmitted by a sample Rather than the light absorbed y-axis is transmittance (T) or percent transmittance instead of the absorbance (fig. 7.16, pg. 191) High transmittance = low absorbance and vice versa Absorption pattern Differs from compound to compound “molecular fingerprint” Often useful for identification Detecting impurities Other sample components

Absorption Spectra Light Emission Matter will emit light Fluorescence Molecular and ionic analytes Useful for qualitative and quantitative analysis Called an emission spectrum Plot of emission intensity vs. wavelength Product of the change in the energy level of an electron From excited state to ground state (lowest energy level) Fluorescence When molecules/complex ions emit light under certain conditions When absorption of light in the UV region is followed by emission of light in the visible region Involves the loss in energy from an excited state to a lower state

Absorbance, Transmittance, and Beer’s Law A = εbc Beer’s Law ε = extinction coefficient or absorptivity Units depends on other parameters Absorbance is dimensionless quantity b = path length Distance the light travels through the measured solution Inside diameter of the sample container Usually centimeters or millimeters c = concentration Expressed in any concentration unit Usually expressed in molarity, ppm, or grams/100ml. .: c= molarity, b = cm, then absorptivity = L mol-1 cm-1

Absorbance, Transmittance, and Beer’s Law Container to be used Varies according to the method UV-VIS Small test tube or square tube with an inside path length of 1 cm Called a cuvette IR Container called the IR liquid sampling cell Sample contained in a space between two salt plates Created with a thin spacer between the plates Path length is the thickness of the spacer

Absorbance, Transmittance, and Beer’s Law Quantitative analyses by Beer’s law Prepared series of standard solutions Measure absorbance of each in identical containers Plotting the measured absorbance vs. concentration Creates a standard curve Absorbance of an unknown solution then measured and concentration determined from the standard data