Instrumental Analysis

Electromagnetic Radiation Spectrum

spectroscopy allows us to examine how a material interacts with EMR

• use that interaction to obtain info about its composition & molecular structure

• use UV, Vis, and IR mainly

PROPERTIES:

• molecules and their energy levels are one main aspect of spectroscopy

• EMR — light — form of energy inducing transitions between different energy levels

• its behavior is described as a WAVE and PARTICLE

Particle Model

EMR is seen as a beam of energetic particles (photons)

• each photon has a defined energy, which depends on the frequency of radiation → energy of photon proportional to the frequency of radiation

Wave Model

EMR is represented as oscillating electric (E) and magnetic (M) fields

• both fields oscillate in-phase along a linear path and are oriented at perpendicular angles to each other (plane polarized)

Frequency

number of oscillations of the field per unit time

Wavelength

linear distance between any two equivalent points on successive waves

Wavenumber

In vibrational spectroscopy (IR spectroscopy), often use the reciprocal of wavelength in centimeters as measure of frequency of radiation (v with line over it)

• conveys info about the lambda (wavelength)

• directly proportional to wavelength, and thus the eqtn

Absorption

• EMR from source is absorbed by sample

• increases the energy of molecule

• EMR only interacted with molecules at specific energies

• Promotes molecule from its ground state to higher-energy excited state

Emission

• EMR emanates from sample

• Decreases energy of molecule

Continuum Source

emits radiation over a range of wavelength; provides an output that is both intense and stable

• ex) Deuterium (D2) Arc lamp for UV-Vis spectrophotometer > continuum source from 185-400 nm

Line Source

emits radiation at selected wavelengths; is both intense and stable

• ex) laser for Raman spectrometer → line source is visible region of EMR (532 nm)

Wavelength Selection

• most spectroscopic analysis require radiation consisting of a limited narrow, continuous group of wavelengths

• ideally, output of wavelength selector would be radiation of single wavelength or frequency

  • no real wavelength selector approaches this idea, instead a wavelength band is produced

  • trade-off

• Wavelength selector passes a narrow band of radiation characterized by a nominal wavelength, effective bandwidth, and maximum throughput of radiation

Effective Bandwidth

width of radiation at half of its maximum throughput

• higher spectral resolution and ability to distinguish closely spaced spectral features

• IDEAL wavelength selector has narrow effective bandwidth

High Throughput

more photons passing through the wavelength selector to reach detector and giving stronger signal (generally improves S/N)

• IDEAL wavelength selector has high throughput of radiation

Why are effective bandwidth and high throughput partly contridictary?

In a monochromator, effective bandwidth is strongly influenced by entrance and exit slit widths

• opening slits allow more radiation to pass through the instrument, increasing signal

• wider slits also transmit a broader range of wavelengtth

• Wider slits → greater throughput → stronger signal BUT poorer resolution

• Narrowing slits restricts wavelength range and improves spectral resolution

  • also reduces number of photons reaching detector

• Narrower slits → narrower bandwidth → better resolution BUT weaker signal

Most modern instruments use monochromators or interferometers

Instrument settings must be selected according to the purpose of the analysis…

1) If detecting or quantifying low-concentration analyte, greater throughput may be more important

• wider slits or broader spectral bandwidth can produce stronger signal and better S/N

• However, closely spaced peaks may become broader, overlap, or appear as single feature

2) If distinguishing closely spaced spectral bands or IDing subtle differences between materials

• narrower bandwidth may be necessary

• improves resolution but produces a weaker signal that may need:

  • longer acquisition times

  • multiple scans. and signal averaging → FTIR and Raman

  • a more intense radiation source

  • a more sensitive detector

Practical Compromise

• most appropriate wavelength selector not necessarily one with narrowest possible bandwidth

• instead, should be providing sufficient spectral resolution while still transmitting enough radiation to obtain acceptable S/N

• Analyst needs to balance spectral resolution & radiation throughput and S/N

Signal

portion of measured response associated with analyte of interest

• useful signal should be distinguishable from surrounding baseline and reproducible across repeated measurements

Noise

random fluctuations in measured response that are not related to the analyte

• noise can make the baseline appear irregular and can obscure weak spectral features

Chemical Noise

arises from various uncontrollable variables affecting chem of sample being analysed

Thermal Noise

type of chemical noise

Random electrical fluctuations associated with temp

• ex) depending on humidity, can see it on FTIR in TX

Environmental Noise

type of chemical noise; vibrations, temp changes, electrical interference, stray light, and fluctuations in the lab environment

Instrumental Noise

noise associated with each component of an instrument (source, detector, etc)

Background

broader contribution that may come from substrate, solvent, atmosphere, fluorescence, or instrument

Interference

signal from another substance that overlaps with or changed the analyte response

Signal-to-Noise Ration (S/N)

compares magnitude of analytical signal with variation in background noise

• indicates how clearly analytical signal can be distinguished from baseline variation

• Ideal S/N ratio = 3-1

How to enhance S/N in practice

• increasing acquisition time

• averaging multiple scans (aka co-adding scans)

• increasing source of laser intensity, when safe for the sample

• increasing slit width or spectral bandwidth

• minimizing stray light and environmental interference

• using appropriate background or blank correction → run controls first

Limitations of Enhancing S/N

Increasing acquisition time or source intensity can cause:

• detector saturation

• sample heating or photodegredation

• longer analysis times

• reduced spectral resolution if wider slits are used

Broadening Effect

In UV-Vis Absorption Spectrophotometry, between electronic states there are vibrational/rotational levels

• depending on polarity, influences the transitions (some will disappear)

• absorbance band corresponds to single electronic transition, which can be broadened by vibrational and rotational transitions

Transitions in visible region

electronic transitions in the visible region produce colors that can be observed by the human eye

• the observed color is complementary to the wavelength of light absorbed

• ex) colloidal gold absorbs green light (~520 nm), giving the solution a red (ruby-red) appearance

UV-Vis Spectrophotometer General Set-Up

Single-Beam UV-Vis Spectrophotometer

uses one light path through the instrument

• measures the blank first to establish a reference

• measures the samp separately after the blank

• requires stable lamp output because the blank and samp are measured at different times

• generally, less expensive than double beam spectrophotometers

Double Beam UV-Vis Spectrophotometer

compares the sample and reference signals simultaneously or in rapid sequence

• automatically compensated for changes in lamp intensity and instrumental drift

• constantly changing which sample the light goes to

Light Source IDEAL REQUIREMENTS (UV-Vis)

1) Brightness across wide wavelength range

• demands both high degree of brightness and uniform brightness across the measurement wavelength range (uniform brightness distribution)

• necessary to obtain photometric values with high S/N

• generally, results in reduction of long service life

• but if v bright source = shorter service life

2) Stable over time

3) Long-Service Life

• many light sources meet some of those requirements, but no light source mets them all

• difficult to achieve both “high degree of brightness” and “uniform brightness distribution” across wide wavelength range using single light source

• some spectrometers allow switching between light sources with diff emission wavelength ranges (allows to always use source with the highest intensity)

• ex) switch between tungsten-halogen lamp for visible range and a deuterium lamp for UV range according to the wavelength setting

Deuterium Arc Lamps (UV-Vis)

consists of a sealed bulb containing deuterium gas

• discharge light source with several hundred Pa D2 sealed in bulb

• good intensity continuum in UV region and useful intensity in visible region (185-400 nm)

• over time, intensity of light decreases steadily

• short half-life of D2 means lamp needs to be replaced relatively frequently to maintain sufficient and stable light intensity

Tungsten-Halogen Lamps (UV-Vis)

tungsten filament housed in quartz bulb filled with inert gas and small amt of halogen (iodine or bromide)

• when electric current heats filament, it emits radiation over limited UV spectral range and over entire visible spectral range

• very low noise and minimal signal drift = GOOD

• stable over time

Xenon Lamps (UV-Vis)

consists of quartz bulb filled with pressurized xenon gas and fitted with 2 electrodes

• application of high voltage forms electrical arc between electrodes and excites xenon atoms, producing intense EMR

• broad spectral coverage from the UV through the visible region to near-IR (185-2500 nm)

• strong UV output

• exhibits similar spectral distribution to sunlight (no secondary light source is required = GOOD)

  • won’t be as intense of light because covers broad span

  • expensive and shorter lifespan = BAD

• produces substantial heat and require high-voltage power supplies = BAD

Monochromators

all light sources produce a broad-spectrum white light (contains various wavelength that can be separated)

• white light entering this is extracted to a selected wavelength band

• ideally, output from monochromator is light of a single wavelength (ex. green is 540 nm)

• The smaller the wavelength band, the better the resolution (but signal will be lower)

Monochromators consist of:

• an entrance slit

• a dispersion device to spread light into different wavelengths (like a rainbow) (prism or diffraction grating) and allow selection of band of wavelengths

• an exit slit where light of wavelengths passes through and onto sample

Beer’s Law

the longer the path length of the sample, the greater the sensitivity of analysis

• selection of cuvettes

• Absorbance is directly proportional to the path length of the sample

  • light beam encounters more absorbing molecules, resulting in higher absorbance value and greater sensitivity in detecting the substance

concentration represents the number of absorbing molecules within a given volume

• as concentration increases, more molecules are available to absorb the incident radiation

Detector (UV-Vis)

photomultiplier tube and silicon photodiode sensitive in the UV and Vis region → not talking about the ones in the infrared range

Photomultiplier Tube (UV-Vis)

(200-900 nm)

• photoelectrons are discharged when light strikes photoelectric surface (cathode)

• cause successive emission of secondary electrons from dynodes arranged in multiple stages (8-10 dynodes)

• reaches the anode

• high voltage (-HV) is applied from outside the tube to accelerate electrons

• Advantage: produces large output for low level of light intensity and offers high sensistivity

Dynode

multiuplies the number of electrons → get a much larger output of light that we started out with (especially with lamps with less intense light)

Silicon Photodiode

(190-1100 nm)

1) light reaches detector → after passing through the sample, remaining UV or Visible light strikes silicon photodiode

2) Photon transfers energy to an electron → photon has enough energy

3) A “hole” is left behind → when electron leaves its OG position, it creates an empty spaced called a hole; hole behaves like a mobile positive charge

4) the detector separates charges → electric field drives electron and hole in opposite directions

• their movement produces an electrical current (photocurrent that becomes your absorbance spectrum)

p-type region

silicon with added boron or aluminum

• contains an excess of positively charged “holes”

• negative terminal is connected

n-type region

silicon with added phosphorus or antimony

• contains an excess of electrons

• positive terminal is connected

Chromophores

atoms or groups of atoms within a molecule that absorb UV-Vis radiation

• commonly contain pi-bonds, conjugated double bonds, aromatic rings, or nonbonding electrons = make absorption spectrum

• wavelength of maximum absorption

• ex) Ethylenic E-band at 208 nm

substances with same chromophore produce similar UV-Vis spectra under identical conditions

• similar UV spectra may associate substances sharing it with the same class (drugs) → ex) phenethlylamines

Wavelength of Maximum Absorption (λmax)

can assist in characterizing a chromophore or comparing related compounds

Auxochromes

functional group containing 1+ lone pairs of electrons

• can influence both wavelength and intensity of UV absorption of nearby chromophore

• when directly conjugated with chromophore’s pi-electron system, shift absorption to longer wavelength and increase intensity

• Basic (e- donating), amino (-NH2), hydroxyl (-OH), and methoxy (-OCH3) groups

• acidic (e- withdrawing), sulfonic acid (-SO3H), and carboxyl (-COOH) groups

• ex) Amphetamine → auxochrome is -NH2 (amino) group on sat chain, separated from pi-system by methylene (-CH2-) group, does not act as strong shifting auxochrome

Factors Affecting Absorption

• presence or absence of chromophore

• Solvent properties that affect band shape, lambda max, absorption intensity

  • polarity

  • pH

Bathochromic Shift

UV-Vis shifting of lambda-max to longer wavelength

• ex) morphine dissolved under acidic conditions- absorption band lambda max = 285 nm

  • when solution is made basic, absorption band is broader and lambda-max shifts to 296 nm

Hyposchromic Shifts

UV-Vis shifting of gamma-max to shorter wavelengths

• ex) pentobarbital dissolved in strong base (pH13): absorption band lambda-max = 254 nm

  • when pH decreased to ~9 with boric acid, absorption lambda-max shifts to 239 nm

Hyperchromic Shift

increase in the intensity of a UV-Vis band

Hypochromic Shift

decrease in the intensity of a UV-Vis band

Solvent Selection in UV-Vis - Wavelength

solvent itself may absorb UV radiation and interfere with analyte spectrum; measurements close to or below the solvent wavelength become unreliable

• because too close to absorption of solvent

If solvent absorbs strongly:

• less radiation reaches the detector

• noise increases

• analyte bands may become distorted or obscured

• blank subtraction may become unreliable

solvent blank can correct for moderate solvent absorption

Solvent Selection in UV-Vis - pH and Solubility

• acidic solvents dissolve basic drugs, while basic solvents dissolve acidic drugs

• basic solvents improve their solubility by converting them into their ionized forms

• ex) Ibuprofin (monocarboxylic acid)

  • top = in NaOH (improve solubility)

  • bottom = in H2SO4 (acid in acid = no absorption or dissolving)

Transmittance

Absorbance

Lambert’s Law

path length represents the distance traveled by the radiation through the sample

• longer path gives radiation more opportunities to encounter absorbing molecules

• why you want to use the same cuvettes and just clean them in between (will have exact same pathlength)

Beer-Lambert’s Law

absorbance is related to concentration by this law

• provides direct correlation between absorbance (A) of molecule to concentration © and path length (b) of sample

Molar Absorptivity

how much light is absorbed at specific wavelength by specific substance (amt of light absorbed per unit concentration)

Stray Radiation (UV-Vis)

radiation reaching the detector whose wavelengths are outside the selected spectral band

• caused by poor instrument design (light getting into instrument from lab lights or daylight or light not being well separated by monochromator) or from damage to instrument

• have instrument performance checks to ID stray light issues using test solution

Optimal Spectral Band Width (SBW) (UV-Vis)

related to the physical slit width of the monochromator design

• for most mid-range UV-Vis spectrophotometers, fixed SBW of 1.5 nm is common/sufficient for resolving peaks of most liquid/solid samples

• Using larger SBW allows more light through the sample

  • can give better quality data and less noise

  • will not resolve narrow or close together peaks

• Using smaller SBW allows less light through the sample

  • provides better resolving power

  • can result in increased data collection times to achieve the same data quality

Standard Check Procedure Using Caffeine for UV-Vis

1) Prepare a caffeine ref soln of known concentration in 2/3 N sulfuric acid & record all prep details 

2) Complete the required baseline and zero measurements using a solvent blank 

3) Analyze the prepared standard soln in the 220-340 nm range 

4) Determine the wavelength of maximum absorbance (gamma-max) and corresponding absorbance value 

5) Compare measured gamma-max and absorbance with established acceptance criteria to verify wavelength and photometric performance 

6) save, review, and document the spectrum and results in the appropriate instrument maintenance or quality-control record 

E Value

• When the path length is fixed at 1 cm, a and b can be combined into a single constant (E = ab)

• E value indicates how much absorbance is expected per unit concentration under specified measurement conditions (mg/mL)

• Only valid for the particular…

  • substance

  • chemical form, such as a free base/salt

  • wavelength

  • solvent and pH

  • path length

Conversion of base form to salt form

E value was established using cocaine in its free-base form

• if another analytical technique, such as FTIR spectroscopy, indicates that cocaine is present as a salt

• Can calculate percentage purity of that salt by accounting for difference between MW of free-base and salt forms

• salt-form purity can be calculated using MW of free-base and salt forms

Why do molecules absorb IR?

• IR light absorbed by molecules at specific frequencies

• occurs when energy of IR radiation matches energy required for molecule atoms to vibrate

• vibrations involve periodic changes in bond angles

• for vibration to produce IR absorption band, it must cause change in molecule’s dipole moment

Nonlinear molecules # of vibrational modes

3N - 6

• each atoms can move in 3 spatial directions, giving molecule 3N total degrees of freedom

• 3 motions correspond to translation of entire molecules

• 3 rotational degrees of freedom

Linear Molecules’ # Vibrational Modes

3N - 5

• each atoms can move in 3 spatial directions, giving molecule 3N total degrees of freedom

• 3 motions correspond to translation of entire molecules

• 2 rotational degrees of freedom

Stretching Vibration

change in distance along axis of bond between 2 atoms

• stretching is only changing length of bond

Bending Vibration

change in angle occurring between 2 bonds

• (+) = movement toward the observer

• (-) = movement away from the observer

• → = movement in the plane

Selection Rules for IR

1) For IR absorption to occur, its vibration must cause change in its dipole movement (change in how unevenly electrical charge is distributed within the molecule

2) Dipole moment determined by… difference in electrical charge between bonded atoms and distance between positive and negative centers of charge

3) Number of IR absorption bands observed in spectrum may be less than calculated number of possible vibrational nodes

• some vibrations don’t change dipole moment

• some absorptions may be too weak to detect

• some bands may occur outside instrument’s measured range

• 2+ vibrations may occur at same or very similar frequencies, causing their bands to overlap

Do shirt or long bonds vibrate faster in IR?

SHORT BONDS ARE STRONGER AND ABSORB MORE ENERGY SO THEY VIBRATE FASTER

Hooke’s Law

used to approximate energy/frequency associated with vibrations to better understand factors involved

APPROXIMATIONS

• Atomic Mass → as mass of atom attached to carbon increases, predicts reduction in energy absorbed, thus vibrational frequency decreases

• Bond Strength → remove pi bond and remaining bond is longer and weaker, weaker bond absorbs less energy & vibrational frequency decreases

Are stretching or bending frequencies higher?

Stretching frequencies (easier to bend a bond than to stretch/compress it)

Fundamental Bands

occur when molecule moves from its lowest vibrational level to next level (v=0 to v=1)

• most common and usually strongest bands in IR spectrum

Overtone Bands

Occur when molecule moves from its lowest vibrational level to second or third level (v=0 to v=2-3)

• generally appear near 2-3x frequency of fundamental vibration but are much weaker

Combination Bands

occur when 2+ more different vibrations are excited simultaneously

Simple Approach to Spectrum ID (General Rules) IR

1) Look first at the high-wavenumber end of the spectrum (>1500 cm^-1) 

2) For each band, ‘short-list’ the possibilities by using a correlation table 

3) Use the lower-wavenumber end of the spectrum for the confirmation or elaboration of possible structural elements 

4) Do not expect to be able to assign every band in the spectrum 

5) Exploit the absence of major peaks as well as their presence 

6) Band intensities should be treated with some caution. Under certain circumstances, they may vary considerably for the same group 

What does FTIR stand for?

Fourier Transform Infrared

FTIR General Set-Up

1) IR source produces broad IR radiation

2) Interferometer (not monochromator) that modulates IR beam

3) Sample absorbs characteristic IR frequencies

4) Detector generates this interferogram

• 2 main detectors (we have both- one in microscope and one in FTIR)

5) Fourier transform function in computer converts interferogram into IR spectrum

Silicon Carbide Rod (Globar)

most common in modern spectrometers

• silicon carbide (SiC) rod electrically heated to ~1000-1500 C

• 5 mm diameter, 50 mm long

• produces strong, continuous radiation across most of mid-infrared region

• durable, stable, and relatively inexpensive

• doesn’t require preheating to become electrically conductive

Nernst Glowers

ceramic rod composed of mixture of rare-earth metal oxides (zirconium, yttrium, and thorium)

• 1-2 mm diameter, ~20 mm long

• typically operated at ~1200-2000 C

• produces intense continuous mid-IR radiation

• not electrically conductive at room temp and must first be externally heated

• not capable of producing IR radiation above 2000 1/cm

• more fragile and generally less convenient than the Globar

Michelson Interferometer

the most common & main name

1) IR radiation strike beamsplitter made of potassium bromide

2) ½ of radiation passes through beam splitter toward fixed mirror 

3) ½ of radiation is reflected t 45 degrees angle toward moving mirror 

4) Both beams reflect off surfaces of 2 mirrors and recombine at beamsplitter 

5) Constructive and destructive interference occurs, before the resulting beam passes through the sample and then continues on to the detector 

• The resulting signal called interferogram: raw signal containing info from every IR wavelength 

Moving Mirror in FTIR

only piece in movement within interferometer → as it changes position, IR wavelength interfere with one another

Constructive Interference

recombine at the same time/no change

• 2 beams are “in-phase” → crests are at the same point at same distance/time (maximum in signal in detector- creation of band)

• Phase difference = 0

Destructive Interference

2 beams are “out-of-phase” → crests are NOT at same point at same distance/time

• phase difference = ½ wavelength

Partial Interference

• occurs between 2 extremes of constructive and destructive interference

• beams are not in-phase but also not completely out-of-phase (phase difference = ¼ wavelength)

Deuterated Triglycine Sulfate (DTGS)

responds to changes in temp produced by incoming IR radiation

1) Crystal absorbs radiation and undergoes very small temp change

2) Temp change alters crystal’s electrical polarization

3) Resulting change in surface charge produces electrical signal proportional to variation in IR intensity

Key characteristics of DTGS Detector:

• operates at room temp

• durable and convenient for routine FTIR analysis

• covers a relatively broad spectral range

• less sensitive and slower than MCT detector

• commonly used in standard ATR-FTIR and transmission instruments

• USED IN SEIZED DRUGS

Mercury Cadmium Telluride (MCT)

semiconductor photon detector that responds directly to IR photons

• located in microscope of FTIR spectrometer

1) IR photons enter semiconductor

2) photons with sufficient energy promote electrons across the material’s band gap

3) process generates mobile charge carriers (electrons and holes)

4) instrument measures conductivity as electrical signal

Key characteristics of MCT detector

• much faster and more sensitive than DTGS

• well suited for microscopic samples and those producing weak signals

• commonly used in FTIR microscopy

• requires liquid-nitrogen cooling (takes 15 min to reach correct temp)

• spectral range that depends on its specific chemical composition

• can saturate more easily when exposed to intense radiation

Fellgett or Multiplex Advantage

• all wavelengths of IR energy are measured simultaneously

• several scans can be collected in same amt of time a dispersive instrument would take to collect one scan

• co-adding several scans will lead to improvement in S/N

  • S/N proportional to square root of total number measurements

  • recording twice as many scans, which takes twice as long, does NOT double S/N (need to look at square root)

Jaquinot Advantage

increased optical throughput (FTIR > dispersive)

• In FTIR spectrometers, no slits in interferometer to limit amt of light reaching detector

• instead, optical aperture is present and proper aperture diameter must be used to achieve desired resolution

• Beam area of FTIR75-100x larger than slit width in dispersive spectrometer

• more light is allowed to flow through, resulting in greater sensitivity

• important if sample is thick and/or sample accessory limits optical throughput, resulting in energy limitations

Connes Advantage

Use of internal Helium-Neon (HeNe) laser

• precise internal reference for monitoring position and movement of interferometer’s moving mirror

• HeNe laser emits light as a highly stable, known wavelength

• its interference pattern allows the instrument to:

  • measure moving mirror’s position and optical path difference very accurately

  • trigger detector measurements at evenly spaced mirror positions

  • maintain consistent mirror velocity during scanning

• does not interact with sample of generate sample’s IR spectrum

Resolution

ability to detect or separate spectral features such as individual absorption bands

• value determines number of data points that will be measured

• units = wavenumbers

• must be less than or equal to separation of bands (baseline typically 4)

  • smaller/lower resolution value results in more spectral detail (more data points collected, but noise can become a problem)

  • larger/higher resolution values results in less spectral detail (features may be missed because fewer data points collected)

FTIR

Fourier Transform InfraRed

FTIR Spectroscopy General Set-Up

1) IR Source produces broad IR radiation

2) Interferometer (not monochromator) modulates IR beam

3) Sample absorbs characteristic IR frequencies

4) Detector generates the interferogram

5) Fourier transform function in computer converts interferogram into IR spectrum

Beamsplitter

splits beam into 2 roughly equal parts → 1st goes to fixed mirror and 2nd goes to moving mirror

• beam will recombine and go to 2nd mirror, get sent to 3rd mirror and then through the sample and goes through detector

Silicon Carbide Rod (Globar)

most common in modern spectrometers

• Silicon carbide (SiC) rode electrically heated to ~1000-1500 degrees C

• 5 mm diameter, 50 mm long

• produces strong, continuous radiation across most of mid-infrared region

• durable, stable, and relatively inexpensive

• doesn’t require preheating to become electrically conductive

• most common IR source in modern FTIR instruments

Nernst Glowers

• ceramic rod composed of mixture of rare-earth metal oxides (zirconium, yttrium, thorium)

• 1-2 nm diameter, ~20 mm long

• typically operated at ~1200-2000 degrees C

• produces intense, continuous mid-IR radiation

• not electrically conductive at room temp and must first be externally heated

• not capable of producing IR radiation above 2000 1/cm

• more fragile and generally less convenient than a Globar

Michelson Interferometer

the most common and main name

1. IR Radiations strike beamsplitter made of potassium bromide

2. ½ of the radiation passes through beam splitter toward fixed mirror

3. ½ of the radiation is reflected 45 degrees angle toward moving mirror

4. Both beams reflect off surfaces of 2 mirrors and recombine at beam splitter

5. Constructive and destructive interference occurs, before the resulting beam passes through the sample and then continues on to the detector → makes an interferogram

Interferogram

raw signal containing info from every IR wavelength

• varies with mirror displacement

• expressed as a function of time, from which retardation (traveled distance) can be deduced

  • plot of IR intensity vs moving mirror’s position converted with mathematical function (Fourier transform)

Moving Mirror

only piece in FTIR in movement within interferometer

• as moving mirror changes positions, IR wavelength interfere with one another

Constructive Interference

recombine at the same time/no change

• 2 beams are “in-phase:" crests are at same point at same distance/time

  • maximum in signal in detector (creation of band)

• Phase difference = 0