Instrumental Analysis

Exam 1

Visible light - RED

740-625 nm

Visible light - ORANGE

625-590 nm

Visible light - YELLOW

590-565 nm

Visible light - GREEN

565-500 nm

Visible light - BLUE

500-450 nm

Visible light - Violet

450-380 nm

Source of radiation: continuum sources

Emit radiation that changes in intensity smoothly

as a function of wavelength (blackbody radiation)

Source of radiation: line sources

Emit a limited number of bands of radiation, each of which spans a limited range of wavelengths

Continuum sources: UV region

Deuterium lamp

IR region

Inert solids heated to 1500 to 2000 K

Continuum sources applications

1) Absorption spectroscopy

2) Fluorescence

Line sources applications

1) Atomic absorption spectroscopy

2) Atomic and molecular fluorescence spectroscopy

3) Raman spectroscopy

Laser sources advantages

1) High intensities

2) Narrow bandwidths

3) Coherent

LASER (acronym)

Light Amplification by Stimulated Emission of Radiation

Types of lasers

1) Solid states

2) Gas

3) Dye

4) Semiconductor

Types of wavelength selectors

1) Filters

2) Monochromators

Main types of filters

1) Absorption

2) Interference

Interference filters (UV, visible, IR regions)

Optical obstruction to provide narrow bands of radiation (two thin sheets of metal sandwiched between glass plates, separated by transparent dielectric)

Absorption filters (visible region)

Colored glass or dye between two glass plates

Types of monochromators

1) Prism

2) Grating

Prism monochromators

Used to disperse UV, visible, and IR radiation (common in older instruments)

Grating monochromators

-UV/Visible region -300 to 2000 grooves/mm (1200 to 1400 most common)

-IR region gratings typically have 10 to 200 grooves/mm

Types of grating monochromators

1) Echellette/Echelle

2) Concave

3) Holographic

Performance characteristics of monochromators

1) Spectral Purity

2) Dispersion

3) Light Gathering

4) Resolving power

5) Spectral bandwidth

Spectral Purity

Scattered or stray light in exit beam

Dispersion

Ability to separate small wavelength differences

Light gathering

Light collection efficiency

Resolving power

Limit of a monochromators ability to separate adjacent images that have a slight difference in wavelength

Spectral bandwidth

Range of wavelengths exiting the monochromator

Radiation transducers are (Ideally)

1) High sensitivity

2) Low noise

3) Wide wavelength response

4) Linear output

5) Low dark

Types of transducers

1) Photovoltaic cells

2) Phototube

3) Photomultiplier tube

4) Photodiode arrays

5) Thermal detectors

Photovoltaic cell

Radiant energy generates a current at the interface of a semiconductor layer and a metal, ~350-750 nm ( ex. solar battery)

Photovoltaic cell advantages

1) Rugged

2) Low cost

3) No external source

Photovoltaic cell disadvantages

1) Low internal resistance

2) Low sensitivity

3) Fatigue

Phototube

Radiation causes emission of electrons from a photosensitive solid surface ( wavelength depends on cathode material)

Phototubes highlight the _____

Photoelectric effect

Photomultiplier tube

Photoemissive surface, as well as several additional surfaces, that emit a cascade of electrons when struck by electrons from the photosensitive area

Photomultiplier tube advantages

Most sensitive typ of transducer

Photomultiplier tube disadvantages

Must be stored in a cool, dark place

Photodiode arrays (silicon)

Photons cause the formation of electron-hole pairs and a current across a reverse-biased pn junction (multichannel)

Types of photodiode arrays

1) Charged-coupled device (CCD)

2) Charge-injection device (CID)

Charged-coupled device

Collects electrons

Charge-injection device (CID)

Collects holes

Thermal Detectors

Sensitive to IR (> 750 nm)

Types of spectrophotometers

1) Dispersive

2) Multichannel

Dispersive spectrophotometer traits

1) Spectral acquisition takes several minutes

2) Must rotate the grating and change filters

3) Better resolution (~0.1 nm), greater dynamic range

4) Sample compartment must be kept tightly closed

Multichannel spectrophotometer traits

1) Can detect the entire spectral range simultaneously

2) No moving parts (rugged)

3) Resolution: 0.5-1.5 nm

4) Not substantially affected when compartment is open

Single-beam spectrophotometer advantages

1) Stable power source

2) Greater energy through

3) Less cluttered sample compartment

Double-beam spectrophotometer advantages

1) Compensates for fluctuations

2) Continuous recording

Double-beam spectrophotometer (in space)

1) Beam is split

2) Two photodetectors

Double-beam spectrophotometer ( in time)

Rotating sector mirror splits beam so that reference and sample cells are being read simultaneously

What causes a red shift of lamda max

1) Increasing conjugation

2) # Rings

Which of the following describes the trend you see in the number of ejected electrons vs. frequency

There is a certain frequency, above which electrons are ejected

What is the effect of the light intensity if the light frequency is fairly low

Increasing intensity has no effect

What is the effect of the light intensity if the light frequency is high

Increasing intensity leads to more ejected electrons

The energy of a photon is proportional to its frequency. The energy needed to remove an electron from the metal surface is equal to the energy of photons of a frequency equal to the cutoff frequency. When radiation of higher energy is used, what happens to the excess energy

It increases the kinetic energy of the ejected electron

Which of the following is a unique property of laser

Coherence

Which of the following is an example of optical pumping

Ruby laser

What is the need to achieve population inversion

To excite most of the atoms

Lasing species loses all or part of its excess energy in the form of fluorescence or phosphorescence radiation in the process called

spontaneous emission

Increasing the bandwidth of a wavelength selector may increase the intensity of radiation reaching the sample or the transducer. However, this generally results in a decrease in ____

resolution

In the most modern monochromators (also known as wavelength selectors), reflection gratings are preferred over prisms. The disadvantages of prisms compared to gratings include

Gratings have linear dispersion of light along the focal plane, while prisms do not disperse light linearly along a focal plane

A grating reflects first order light at 500 nm. The difference in the two wavelengths that can be detected using this grating is 0.010 nm

What is the resolution of this monochromator

5.0E4

In general, for a qualitative analysis one uses ___ slit settings on a monochromator and for quantitative analysis one uses ___ slit settings on a monochromator

narrower, wider

In a voltage divider, resistors are connected ___

In series

In a voltage divider, all resistors ___

Share the same current

What type of circuit works with electric signals that represent zeros and ones?

Digital circuit

Which of the following is an example of a passive component

Resistor

What component would you use to store charge temporarily in a circuit

Capacitor

In what device does an incident beam of photons cause production of electron-hole pairs which when separated produce a voltage related to the photon flux

Photovoltaic cell

In what device are photoelectrons emitted as a result of photon bombardment and attracted to the positively charged anode to produce a small photocurrent proportional to the photon flux

Phototube

What is the most sensitive photon transducer

Photomultiplier tube

What device does not require an external power supply

Photovoltaic cell

True or False. When comparing hydrogen and deuterium discharge lamps as sources for UV radiation, D2 lamps generally produce higher intensity radiation

True

When comparing filters and monochromators as wavelength selectors, which one is better suited for qualitative analysis or structural studies

monochromator

What part of double-beam in space spectrophotometer is used for the 0% transmittance adjustment

shutter

what part of double- beam in time spectrophotometer is used for the 0% transmittance adjustment

sector mirror

true or false. the main difference between spectrophotometers and photometers is the light source

false

what can be used as a beam splitter for UV-Vis absorption measurements

1) Rotating choppor

2) Mirrors

3) Fiber optics

In which region of the electromagnetic spectrum does an absorption at 177 nm come

Vacuum-UV

How do the values of lambda max for the pi to pi star transitions vary among a series of conjugated polyenes

values shift to longer wavelength as the number of Carbon double bonds increases

what is fluorescence

Emission of excited electrons that keep the original spin

What is phosphorescence

A photoluminescence where the light lasts a long time

Potassium bromide is widely used as a cell material for IR spec, but rarely used for UV-Vis. This is because many samples for UV-Vis are in ___ soln

aqueous

There are two fundamental types of sources in optical spectroscopy, continuum sources and line sources. Line sources are best used in situations where

absorption of light at a single wavelength is all that is desired

Most scanning UV-Vis spectrometers measure the light over wavelengths from 190 to 900 nm. what are the likely sources that would be used over the wavelength range

1) Deuterium lamps

2) Tungsten lamps

What type of charge storing device is used to record the radiant intensity of light that impinges upon photodiode array detectors

capacitor

What is red shift

The shifting of an absorption to lower energy

what is a chromophore

a group of atoms in a compound responsible for the absorption of electromagnetic radiation

An aqueous solution of a dye has a strong absorption with lambda max 464 nm. The color of the solution is

orange

Which of the following statements is consistent with an electronic absorption being broad

an electronic absorption includes vibrational and rotational structure