Analytical Chemistry - Molecular and Atomic Spectroscopy

limitations to Beer's law

- applies to only monochromatic light - applies to dilute solutions < 0.01 M

photomultiplier tube (PMT)

- glass tube with no air that takes light and multiplies the intensity - cannot choose wavelength, so a monochromator is needed for each wavelength

requirements for molecular chemical analysis

- molecule has to absorb light - distinguishable wavelength for the compounds

photodiode array / diode array detector (DAD)

- pulls electrons from valence band to conductive band, leaving valence (+) and conductive (-) - when electrode is recharged, electrons and holes are recombined, creating current flow - based on which diodes are illuminated, we know wavelength

similarities / differences between single-beam and double-beam spectrophotometers

-both have light source, monochromators, sample holder, and detector - the double-beam has a beam chopper and mirrors to reference both the sample and the blank

properties of light

1) wavelength - distance between wave maxima (m) 2) frequency - # of oscillations of the field per second (Hz)

ways to reduce stray light

1. base line subtraction 2. rotating chopper 3. D2 lamp (modulated with hollow-cathode lamp) 4. Zeeman effect

temperature that a furnace atomizes molecules

2400 K

efficiency

45%-80% of light is reflected back (compares irradiance of grating to irradiance of mirror)

temperature that an ICP atomizes molecules

6000 K

Is emission or absorbance more sensitive?

Emission - uses single molecule detection - detected in the dark - uses excitation monochromator AND emission monochromator

inductively couple plasma (ICP)

Emission spectrum Identifies and measures elements through light energy emitted by excited Atoms the sample is placed in hot plasma torch - uses radio frequency voltage in coil around a glass tube - the emitted electrons collide with glass, which help maintain the high temperature

Why is a graphite furnace more sensitive and cleaner than a flame?

Graphite furnaces have argon gas running through them, which removes volatiles from the heat source. Because it takes seconds for the molecules to atomize (so the residence time in the furnace is larger), the sensitivity is increased.

grating monochromator

Gratings are optical elements with closely spaced lines, causes light of different wavelengths to diffract at different angles (is a wavelength selector)

What does higher resolution mean in terms of detection?

Higher resolution means you can detect a smaller change in wavelength (bandwidth)

how does adding NH3 to sample reduce isobaric interference?

It causes either an e- or H+ transfer, making one of the indistinguishable isotopes now neutral (which is not shown in MS because MS shows ions only)

which has better sensitivity AND resolution: PMT or DAD?

PMT, but it is slower.

double-beam spectrophotometer

Splits monochromatic light into two components One beam = sample One beam = reference solution or blank (corrects for variation in light source intensity) - works quickly

4-level lasers

The relaxation of laser action is not the ground state, making it easier to maintain population inversion

monochromator

a device (usually a grating or prism) that disperses light into its component wavelengths and selects a narrow band of wavelengths to pass through the exit slit

population inversion

a necessary condition for laser operation, in which more members of a collection of atoms are in an excited state than are in lower energy states

resolution

ability to separate two peaks closely- spaced together

chemical interference

analyte is not totally decomposed in flame (causes lower absorbance than actual)

stray light

any light outside the wavelength and bandwidth expected from the monochromator that is detected (comes from unwanted diffraction orders, environment, etc.)

Zeeman effect

apply parallel magnetic field to light path, which separates the wavelength of the light source into a higher and lower wavelength emitted, so the difference is the wavelength of the emitted electrons

type of x-ray excitation

bond-breaking ionization

type of UV-VIS excitation?

electronic excitation

triplet excited state (T)

electronic excitation AND are electrons are same spin (so both up or both down)

Rayleigh scattering

excitation light induces electron oscillation - scatters light at same v of excited state (filtered out by monochromator)

Which type of interference does each type of flame source have?

flame - chemical interference graphite furnace - chemical interference ICP emission - spectral interference ICP-MS - isobaric interference

Tungsten light

gives continues wavelengths in VIS range (near IR and UV) - is basically a regular lightbulb

globar light source

gives continuous wavelength at IR region -is a heated silicon carbide that emits radiation

D2 lamp

gives continuous wavelengths in UV range - provides electric discharge

laser

gives single wavelength and has a bandwidth of 3E-14 um - 3E-8 um

single-beam spectrophotometer

has a light source and a wavelength selector that filters the light to a specific wavelength then goes through the sample and hits a light detector with the light scatter after it passes through the sample

D2 lamp modulated with hc lamp

hc lamp gives energy to be absorbed by the sample, while the D2 lamp give energy to be absorbed by the sample and the scattering

atomization process

heat source causes samples to nebulize (so atoms are now separate) - they are separated in an inert gas, leaving the free atoms as "bare"

lean flame

higher temperature but also more oxidant (which is bad because oxidant means atoms are now molecules)

AA process

hollow-cathode lamp -> flame -> monochromator -> detector

spectrophotometers

instruments used to measure reflection or transmission properties of a material as a function of wavelength

Fourier Transform Detector

is used for IR detection - decomposes sine / cosine functions of waves to reproduce the wavenumber curve

types of light sources

lamps or lasers

fluorescence process

laser -> flame -> monochromator -> detect

parts of spectrophotometer

light source, wavelength selector, detectors

rich flame

lots of fuel and high sensitivity

spectrophotometry

method for measuring how much a compound / element absorbs light as it is irradiated

properties of lasers

monochromatic, extremely bright, collimated (light rays are parallel), polarized (electric field is oscillating in a plane perpendicular to light), coherent (light is in phase)

AE process

no light -> flame -> monochromator -> detector

in phase: constructive interference

out of phase: destructive interference

spectral interference

overlap of absorption line due to emissions from another element or compound

isobaric interference

overlapping of mass / molecule lines due to different elemental isotopes having the same mass

types of detectors

photomultiplier tube, photodiode, charge-coupled device (UV), thermocouples, ferroelectric material, photoconductive for IR

advantage of using two monochromators in series

prevents stray light by refocusing the light

detector

produces electric signal when struck by a photon(s)

type of microwave excitation

rotational excitation

rotating chopper

signal is like choppy blocks, with the higher part as the sample + background, while the lower part is the background - so the difference in signals is the signal that the sample itself gives

Ramon scattering

small fraction of excitation photons lose energy molecules which increases vibrational energy of molecules - absorbance and emission are mirror images of each other

charged-couple device

stores photogenerated charge from many diodes into 2D array (is extremely sensitive) - is what computers / phones use to hold / make pixels

spectrometry

the practical application of spectroscopy; uses instruments called spectrometers

stimulated emission

the process in which an incoming photon causes an excited atom to drop to a lower energy level, thereby emitting a photon in the same direction as the incident photon

transmittance

the ratio of the intensity of the light through the sample to the intensity of the light through the blank

spectroscopy

the theoretical approach to the science of studying the interaction between matter and radiated energy

type of infrared excitation

vibrational excitation