Infrared Spectroscopy (IR, including FTIR)

82. What part of the EM spectrum does IR involve?

700 nm - 1 mm higher wavelength than visible but shorter than microwave.

What is a beam splitter?

A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.

What significance does the Beer-Lambert law have to infrared radiation?

Beer's Law - relates the absorption of light to the properties of the material through which the light is traveling. Used in analysis of polymer degradation and oxidation for IR

What are the limitations of the IR technique?

Cant determine molecular weight, doesn't provide information on position of functional groups, cant determine if it is a pure compound or a mixture.

What is a dipole moment and why is it important in IR spectroscopy?

Dipole moment- change in energies between atoms This must be done in order for the molecule to be detected in IR

What is FTIR? How is it different from dispersive IR?

Fourier Transform Infrared spec. its newer and can do all wavelengths at the same time. It also has a single beam

What do the initials FT-IR stand for?

Fourier Transform infrared spec

What is a fundamental vibration?

Given rise to by a transition from the ground state to the first excited state. High intensity. 6 types: stretching which occur along a bond axis such that the change in bond length is either continuously increasing or decreasing: either symmetric or asymmetric; deformation vibrations which are of lower energy where a periodic bending involves a change in bond angle: bending/scissoring (in plane), rocking (in plane), wagging (out of plane), and twisting (out of plane).

Who is Michelson? What did he invent?

He was a physicist that invented the Michelson interferometer.

What occurs when the beam paths vary by ¼ wavelength? By ½ wavelength?

If the movable mirror is displaced a distance of λ/4, the total retardation or path difference is λ/2 (roundtrip). Therefore the path difference between the fixed and movable mirrors is exactly one-half wavelength. For ½ the difference would be one wavelength

Why does a compound absorb infrared radiation?

Infrared detects energy, which all vibrating molecules have

What happens when a molecule absorbs IR radiation?

It begins to vibrate and produce a signal that can be detected

What does LASER stand for?

Light Amplification by Stimulated Emission of Radiation

How many moving parts are there in an FTIR spectrometer?

One, the oscillating mirror

Why is KBr used in transmittance mode in IR spectroscopy?

Solid samples can be broken up with potassium bromide (KBr) to form a very fine powder. This powder is then compressed into a thin pellet which can be analyzed.

Why is the fingerprint region so characteristic of the molecule?

The importance of the fingerprint region is that each different compound produces a different pattern of troughs in this part of the spectrum. usually contains a very complicated series of absorptions that are mainly due to all manner of bending vibrations within the molecule.

What region of the IR spectrum is described as the fingerprint region?

The region to the right-hand side of the diagram (from about 1500 to 500 cm-1) usually contains a very complicated series of absorptions. These are mainly due to all manner of bending vibrations within the molecule

What is the difference between an IR spectrum and an interferogram?

The two beams are combined together to make an interferogram, ir spectrum is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light

What is an overtone vibration?

These are bands of lower intensity that occur at a frequency of 2 or 3 times that of the fundamental vibration.

What is a combination vibration?

These bands are also of low intensity and occur at a frequency around the sum of 2 or 3 bands.

How are IR results displayed?

Wavenumbers cm-1 Or wavelength is µm (micrometers)

What are the various instrumental components of an FTIR spectrometer and their function?

a. A common FTIR spectrometer consists of a source, interferometer, sample compartment, detector, amplifier, A/D convertor, and a computer. The source generates radiation which passes the sample through the interferometer (this is used to split one beam of light into two so that the paths of the two beams are different. Then the interferometer recombines the two beams and conducts them into the detector where the difference of the intensity of these two beams are measured as a function of the difference of the paths) and reaches the detector. Then the signal is amplified and converted to digital signal by the amplifier and analog-to-digital converter, respectively. Eventually, the signal is transferred to a computer in which Fourier transform is carried out.

Describe some advantages of FT-IR over dispersive IR.

a. Multiplex advantage - its faster because It can measure all of the wavelengths at once. b. Accuracy and precision in infrared spectra are much higher when collected on an FT-IR. c. Throughput advantage -Sensitivity to small peaks is greater because an FT-IR instrument does not use a slit to limit the individual frequency reaching the sample and detector as a dispersive instrument does. There are also fewer mirror surfaces so there are less reflection losses than in a dispersive spectrometer. Overall, more energy reaches the sample and hence the detector d. Connes advantage

Explain Fourier transform (FT) in layman terms.

is a mathematical transformation employed to transform signals between time (or spatial) domain and frequency domain. It is reverisble

What does the laser do?

the laser (a Helium-Neon one) in an FTIR is used as a reference source, which helps control the sampling rate of the IR detector. It also provides an automatic calibration of the wavelengths being detected, since the wavelength of the laser and it's distance from the interferometer is known. This is known as "Connes Advantage".

What occurs when the beam paths are equidistant?

then the two beams travel the same distances through the same materials and are exactly in phase after they recombine at the beamsplitter. At this point, the beams interfere constructively and the intensity of the beam passing to the detector is the sum of the intensities of the two beams passing to the fixed and movable mirrors. Therefore, all the light from the source reaches the detector at this retardation

Using simple, non-technical terms, explain what an interferometer does?

this is used to split one beam of light into two so that the paths of the two beams are different. Then the interferometer recombines the two beams and conducts them into the detector where the difference of the intensity of these two beams are measured as a function of the difference of the paths

What are the common units used in IR spectroscopy.

μm wavelength