Instrumental Analysis Test 1 Review

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What is the order of electromagnetic spectrum from lower to higher energy?

Radio, Micro, Infrared, Visible, Ultraviolet, X-ray, Gamma Ray, Cosmic Rays

Radiation that interacts with molecular rotations

Microwaves

Radiation that interacts with the nuclear/electron spin

Radio Waves

Radiation that interacts with bond vibrations

Infrared waves

Radiation that interacts with inner core electrons/ionizes

X-rays

Radiation that interacts with valence electrons

UV and Visible light

Equation for absorption given extinction coefficient

A = ecl

Equation for % Transmittance

%T = P/Po

Equation for absorption given transmittance

A = -Log(%T)

Why is A more useful than %T

Absorbance allows for a linear form of data and has a direct relationship to the concentration of the substance measured.

What are some common UV-Vis materials?

Plastic (Polystyrene and PMMA) and Glass (Quartz and Borosilicate)

For Beer’s Law, what is the most common source of error?

Sample Prep

For Beer’s Law, what is the absorptivity coefficient dependent on?

Wavelength

For Beer’s Law, what should be taken in to account regarding pH and that type of stuff?

undissociated weak acid, HA

For Beer’s Law, The monochromatic light needs to be ___ than the absorption band

Smaller

For Beer’s Law, the wavelength of light should be selected at what value?

at the highest absorbance (lambda max)

What concentration should solutions be for UV-Vis?

equal or less to 0.01 M

UV-Vis can analyze…?

Solids, Liquids and Gases

For UV-Vis, what are some reasons why wavelength should be selected at the maximum sample absorbance

1. To minimize error that can occur by choosing a reliable position. If the wavelength is off a little near the peak, there is little change in measured absorbance.

2. Can measure lower concentrations due to the light being most absorbed at the specific wavelength, and thus decreasing interference.

For UV-Vis measurements, what absorbance range is ideal precision?

0.3-2

For UV-Vis, why does the compartment need to be closed?

To prevent stray light

What types of energy in a Jablonski diagram are considered when a molecule absorbs a photon?

Rotational, Electronic and Vibrational

Fluorescence emission occurs at ___ wavelengths than the excitation wavelength?

longer wavelengths

How does Phosphorescence compare with Fluorescence in terms of energy?

F emits a higher energy while P is a lower energy emission

The excited state of a molecule in which electron spins are parallel?

Triplet

Lowest energy state of a molecule?

Ground State

Emission from an excited T1 to ground S0 state?

Phosphorescence

Excited state of a molecule in which electron spins are opposite

Singlet

Emission from excited S1 to ground S0 state

Fluorescence

Radiation emitted from a heated object

Blackbody radiation

What are the lifetime ranges for Phosphorescence and Fluorescence?

P = 10^-4 to 10^2 s and F = 10^-8 to 10^-4 s

What is the typical light source for IR?

SiC Globar

What are some types of UV-Vis light sources?

Xenon Arc Lamp, Tungsten Halogen lamp, Deuterium Arc Lamp, Lasers, and LEDs

what do modern UV-Vis specs use for separating light?

grating monochromator

What do modern IR spec’s use for separating light

Michelson Inferometer

what are common sample cell materials for IR?

Diamond, NaCl, Germanium, KBr and ZnSe

How does Michelson Inferometer work?

Light source from SiC rod goes through slit to beam-splitter. Splits light 50/50 to both a movable and stationary mirror. Light reflects and recombines at the beam-splitter and hits sample and detector. Recombined bean is either out or in-phase and creates a interferogram.

Equation to find frequency given wavelength

c = λv

Equation to find energy given wavelength

E = hv —> E = hc/λ

What is up with absorbed vs appearing colors and stuff?

Color absorbed is complementary to the color that appears. This is because the eye perceives the remaining combination of non-absorbed light that is reflected.

What are the common UV-Vis detectors?

CCD(charge-coupled device), PDA(photodiode array) and PMT(photomultiplier tube)

What are the common IR detectors?

Ferroelectric(DTGS) and Pyroelectric(MCT)

What are the advantages of PDA compared to PMT?

1. Fast scanning

2. records entire spectrum at one time

3. more rugged due to no moving parts

4. Cheaper

What are the advantages of PMT compared to PDA?

1. Lower detection limit/Better resolution

2. Less stray light

Name of the cathodes in PMT

dynodes

how much can a PMT multiply electrons?

A MILLION!!!!

What is used in PDA to disperse light

Grating polychromator (because entire spectrum is emitted onto the array)

Why are CCD detectors becoming more popular in spectroscopy?

Combines the advantages of both PDA and PMT detectors in being fast and having low detection limits

What are the different ranges in light sources and give examples?

Line (Diode laser), Narrowband (LEDs), Broadband (Lamps and SiC globar)

How does a hollow cathode lamp work?

A cathode, made from the element whose emission line is being tested, is sent into the gas phase through the use of He or Ar cations to then emit an atomic emission through the collision of high energy electrons.