Instrumental Analysis Unit 3

Mas spec interferences

• Isobaric: When species with similar mass/charge ratio overlap. Fixed by using a diff isotope

• Polyatomic: such as formation of O2+, can also cause overlap. Oxides/hydroxide formation

ICPMS

Inductivly coupled plasma mass spectroscopy

• Dynamic range of 6 orders of magnitude

• can detect .1-10 ppb

• good for analyzing solids

Laser Ablation ICPMS

A way to introduce a sample to ICPMS

How can ICPMS be used?

• Qualitative/semiqualitative applications

• Qualitative anaylsis

• Isotope ratio measurments

X-ray wavelength

.1-25 A for tranditional x-ray spec

How are X-rays generated for spec?

• Bambardemnt of a metal target with high energy electrons

• Exposure to a X-ray beam to generate X-ray flourescense

• A continuum spectra from radioactive subtances

X-ray tube

Continuum signal from a tungsten anode. electrons are shed from anode after stimulation with a high voltage. Produces white radiation.

X-ray spectra characteristics

well defined, short wavelength limit. The minimum excitation voltage increases with atomic number.

K series

X ray spec, emissions where an electron falls to innermost K shell (n=1). Higher energy

• Ka: n=2 →1

• KB: n=3 → 1

L series

x ray spec- when an electron falls to n=2. more complex than k series (with subsets of alphas and beta transitions), with lower energy than k.

K vs L energy diagrams

what produces monochromatic x-rays

alpha and beta decay, electron capture (when an electron is absorbed into nucleus to turn a proton into a neutron).

x-ray Absorption spectra example

X flourescense instrument sources

• x-ray tube

• radioisotope

• secondary fluorescence

Needs a filter/monochromoter

gas filled x ray detector

Si(Li) detector

What is X-ray florescence used for?

• rapid, quantitative determinations

• works on Na-U elements

• quantitative analysis

• qualitative analysis

Energy-dispersive X-ray fluorescence spectrometer

Radiation losses in UV-Vis cell

Uv-Vis limitations

• not linear above 1 absorbance

• High concentration can cause scattering and index of refraction changes (which can effect molar extinction)

• reactions in the cell, such as a equilibria

• needs monochromatic radiation (a wavelength selectors to .1 peak width of Absorption band)

Beer’s law for mixtures

sum the absorbance values for Atot, where 1 refers to wavelength 1

Absorbance deviations for a weak acid

Why is lambda max important

absorbance values taken anywhere else will not be linear and can have varying molar extinction coefficients. The sharper lambda max is, the more sensitive the instrument

Stray radiation correction equation

for radiation that doesn’t pass through the cell

How does slit width effect absorbance?

Increases absorbance and peak resolution- however can increase noise by reducing signal.

UV-Vis components

• sources

• sample continers

• Instrument diagrams

single beam UV-Vis design

Double Beam UV-Vis design

Double beam in time UV-Vis

Multichannel UV-Vis design

Probe type photometer

Manual Double-Beam Spectrometer

Aligent Diode Array UV-Vis

What can UV-Vis be used for?

• determination of chromophore functional groups since lambda max is distinct (samples must be dilute for qualitative analysis.)

• Inorganic and organic compounds can be quantitatively analyized for concentration, sensitive to 1E-5 M with good accuracy

• Used in: Clinical tests and water analysis

what affects the molar extinction

• Probability of a energy transition (between 0 and 1)

• cross sectional area of the molecule

How do organic molecules enter the excited state?

through molecular orbitals

types of molecular transitions

• hysochromatic shift: to a shorter wavelength (red shift)

• bathochromatic shift: to a lower wavelength (blue shift)

chromophore

a portion of a molecule capable of absorbing light in the UV-Vis range

Formula for molar extinction coefficient

Auxochromes

functional groups that does not absorb themselves but they affect the chromophores by shifting their peak to a longer wavelength as well as increasing the intensity.

inorganic anions absorbtion

since ions can have a different Absorption to a protonated molecule, must select a buffer carefully

Lanthenides and actinides absorbtion

• f electrons have sharper bands because of the inner electrons

• uneffected out species on the outer complex

charge transfer complex absorbtion

an electron donor group bonded to an electron acceptor absorbed radiation, where an electrons from the donor will move to the orbital of a acceptor.

rare earth absorbtion

Similar to f electrons. The electrons responsible for absorption by these elements are shielded from external influence by electrons that occupy orbital with larger principal quantum numbers - Spectra are narrow and unaffected by species bonded to outer electrons.

how does slit width effect absorbtion

smaller slit means more absorbance

Why would the deriative of a UV-Vis plot be useful?

lamda max will be when the plot is 0, makes it easier to visualize.

Photometric titrations graph examples

photoaccoustic spectroscopy

radiation is chopped through a gaseous, nonreactice substance to the sample, as the sample undergos excitation heat is generated which changes the pressure. the sound produced by that is tracked.

• can be used on solids

Jablonski diagram- look up a YouTube vid or smth, this is dumb af

• vibrational relaxation: non radiative, gives off heat

• intersystem crossing: movement from lowest vibrational level of 1 state to a higher level of another state. causes a spin change. phosphorescense

Quantum yield

A measure of how much floresence is produced.

How to triple bonds effect flouresence

enhance, because a molecule with a lot of pi character will floures better.

How do electron donating grouped effect flouresence

will stabilize the molecule and enhance fluorescence, electron withdrawing groups will hinder fluorescence (Halogens, nitros, and carboxylic acids)

What are the best transitions for florescence

How do complexes effect flouresense

will increase when organic molecules form a complex with metals