vibrational spectroscopy for pharmaceutical analysis

10/11/2005

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ENGINEERING RESEARCH CENTER FOR

STRUCTURED ORGANIC PARTICULATE SYSTEMS

RUTGERS UNIVERSITYPURDUE UNIVERSITYNEW JERSEY INSTITUTE OF TECHNOLOGYUNIVERSITY OF PUERTO RICO AT MAYAGÜEZ

Vibrational Spectroscopy for Vibrational Spectroscopy for Pharmaceutical Analysis Pharmaceutical Analysis

Part IX. Introduction and Origin of NIR BandsRodolfo J. Romañach, Ph.D.

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NIR Fundamentals: Electromagnetic SpectrumNIR Fundamentals: Electromagnetic Spectrum

NuclearTransitions

SpinOrientation in

MagneticField

MolecularRotations

MolecularVibrations

ValanceElectron

Transitions

InnerShell

Electronic Transitions

-Ray

Radio, TV WavesMicrowaveInfrared

NMRESRFIRMIR

X – Ray

visible

NIR

Ultraviolet

Inte

rac

tio

nR

eg

ion

108 107 106 105 104 103 102 101 1 10-1 10-2 10-3

10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 101

Frequency(cm-1)

Wavelength(m)

12,500 cm-1 (800 nm) 4,000 cm -1 (2500 nm)

Courtesy Bruker Optics

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Infrared SpectroscopyInfrared Spectroscopy

Regions of Infrared Spectrum:

Far Infrared: 650 – 25 cm-1 Mid Infrared: 4000 – 650

cm-1 Near Infrared: 12800 – 4000

cm-1(0.8 - 2.5 m, or 800- 2500 nm)

• A vibration occurs when the dipole moment of the molecule changes, and the molecule interacts with radiation equal to the frequency of vibration.

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Infrared Spectroscopy (mid-IR) and Near Infrared Infrared Spectroscopy (mid-IR) and Near Infrared SpectroscopySpectroscopy

Infrared Spectroscopy (mid-IR), studied in organic chemistry courses; principal identification method in pharmaceutical industry.

Mid-IR sharp bands that provide structural information on compounds; better than fingerprint. (region of 4000 cm-1 to 400 cm-1, wavelengths from 2.5 m to 25 m).

Near IR weak overlapping bands; difficult to interpret (region of 12500 to 4000 cm-1, wavelengths from 0.8 to 2.5 m.)

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Advantages of NIRAdvantages of NIR

Process Applications: Excellent analytical method for the study of solids. Spectra may be obtained in non-invasive manner. Remote sampling is possible (good for hazardous

materials). Possibility of using it in a wide range of applications

(physical and chemical), and viewing relationships difficult to observe by other means.

Laboratory Applications: Sample preparation is not required leading to

significant reductions in analysis time. Waste and reagents are minimized.

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Spectroscopy of the Solid StateSpectroscopy of the Solid State

Spectroscopy - Interaction between radiation and matter.

NIR – offers possibility of study of interaction of solids with radiation since sample preparation is not required.

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Disadvantages of NIRSDisadvantages of NIRS

• Calibration requires careful experimental design.• Depends on accuracy of reference methods.• Overlapping bands, not easy to interpret.• Differences in spectra are often very subtle.• Usually not for trace level analysis.

• The implementation of NIR requires a significant investment in Human Resources.

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Vibration TheoryVibration Theory

The molecule can be thought of as mass m1 and m2 connected by a

spring. At equilibrium, the distance between the two masses is r0. If

the molecule is stretched by an amount r = x1 + x2, then a restoring

force, F, is produced. If the spring is released, the system will vibrate

around the equilibrium position. According to Hooke’s Law, for small

deflections the restoring force is proportional to the deflection:

F = -k . r

Since the force acts in a direction opposite to the deflection, the proportionality constant, or force constant, k, is negative in sign. The force constant is called the spring constant in the mechanical model, whereas in a molecule the force constant is a measure of the bond strength between the atoms.

Courtesy Bruker Optics

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For the harmonic oscillator model, the potential energy well is symmetric. According to quantum-mechanical principles molecular vibrations can only occur at discrete, equally spaced, vibrational levels, where the energy of the vibration is given by:

Ev=(v + ½) h v = 0, 1, 2, 3, ...

Where h is Planck’s constant and v is the vibrational quantum number. Even in case of v = 0, which is defined as the ground vibrational level, a molecule does vibrate:

Ev= ½ h

Potential energy curve for a harmonic oscillator

Based on Bruker Optics Slide

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• When absorption occurs, the molecule acquires a clearly defined amount of energy, (E = h ), from the radiation and moves up to the next vibrational level (v = +1).

• For a harmonic oscillator, the only transitions permitted by quantum mechanics are up or down to the next vibrational level (v = 1).

If the molecule moves down to the next vibrational level (v = -1), a certain amount of energy is emitted in the form of radiation. This is called emission.Based on Bruker Optics Slide

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A Molecule Absorbs Infrared Energy when:A Molecule Absorbs Infrared Energy when:

• A vibration occurs where the dipole moment of the molecule changes, and the molecule interacts with radiation equal to the frequency of vibration.

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+ -

+ -

+ -

H Cl

Change in Dipole Moment during Molecular VibrationsChange in Dipole Moment during Molecular Vibrations

• Must change for IR absorption to occur.

• The dipole moment is a measure of the degree of polarity of molecule (magnitude of the separated charges times the distance between them).

• A measurement of degree of unequal distribution of charges in molecule.

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Ibuprofen Diffuse Reflectance NIR Ibuprofen Diffuse Reflectance NIR SpectrumSpectrum

Ibuprofen

0.38

0.58

0.78

905 1055 1205 1355 1505 1655Wavelength (nm)

Log 1/R

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Units of spectra- nm, Units of spectra- nm, m, cmm, cm-1-1

Sometimes see cm-1 :• 10,000 cm-1 = (1/10,000) cm or 0.0001 cm = 1 m =

1000 nm

• 6,000 cm-1 = (1/6000) cm or 0.000167cm = 1.67 m = 1670 nm

• 5,000 cm-1 = (1/5000) cm or 0.0002 cm = 2 m = 2000 nm

• 4000 cm-1 = (1/4000) cm or 0.00025 cm = 2.5 m = 2500 nm.

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UnitsUnits

1800 nm x µm = 1.800 µm 103

1.800 µm x _m_ x 102 cm = 0.0001800 cm 106µm m 1 = 5555.556 cm -1 = 5556 cm -1 0.0001800 cm

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NIR Absorption - OvertonesNIR Absorption - Overtones

• A molecule has certain discrete energy levels. • In mid and near infrared spectroscopy molecules absorb

energy when a photon matches frequency of vibration and the dipole moment changes.

• The transition from the ground state to the second excited level is called an overtone, and is observed in the NIR spectral region.

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NIR Absorption BandsNIR Absorption Bands

• Absorption bands in the NIR are the result of combination and overtone bands from the fundamental vibrations of C-H, N-H, and O-H bonds seen in the mid-IR.

• The overtone and combination bands are 10 – 100 X less intense than the fundamental bands in mid-IR.

• Differences in spectra are often very subtle, requiring training of analysts to recognize these differences.

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Combination BandsCombination Bands

Polyatomic linear molecules have 3N-5 modes of vibration. Non-linear have 3N – 6.

A transition that occurs simultaneously for two of these modes is called a combination band.

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Interpretation of NIR SpectraInterpretation of NIR Spectra

• In Raman and mid-IR spectroscopy, the interpretation of spectra is possible and valuable information is gained in regards to the presence of functional groups. This type of interpretation is very difficult in NIR spectroscopy, due to significant overlapping of bands.

In NIR spectroscopy we are usually interested in observing differences between spectra, and not in the interpretation of NIR spectra.

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bending mode _2

asymmetric stretch _3

symmetric stretch _1

Normal vibration modes of water.

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Vibrat ional quantum number of upper level

_1 _2 _3 (cm-1) 0 1 0 1594.59 0 2 0 3151.4 1 0 0 3656.65 0 0 1 3755.79 0 3 0 4667 1 1 0 5235 0 1 1 5332 1 2 0 6775 0 2 1 6874 2 0 0 7201 1 0 1 7250 0 0 2 7445 2 1 0 8762 1 1 1 8807 0 1 2 9000 3 0 0 10600 2 0 1 10613 1 0 2 10869 0 0 3 11032

Overtones and combination bands of water (Fig. 9.1), Near Infrared Spectroscopy, Ed. Siesler, Ozaki, Kawata, Heise, Wiley 2002.

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Chemists and the Interpretation of NIR SpectraChemists and the Interpretation of NIR Spectra

• “Analytical chemists have been guilty of assigning NIR spectra to impossible combinations. Physical chemists wish to obtain a better understanding of the molecular dynamics from the intensities, bandwidths, and positions of the bands.”

A.S. Bonanno, J. M. Olinger, and P.R. Griffiths, “The Origin of Band Positions and Widths in Near Infrared Spectra”, in Near Infra-Red Spectroscopy, Bridging the Gap Between Data Analysis and NIR Applications, Edited K.I. Hildrum, T. Isaakson, T. Naes, and A. Tandberg, Ellis Horwood, 1992.

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• “Chemometricians with a primary goal of determining the concentration of the analyte, and who sometimes have no interest in understanding the origin of features in NIR spectra provided that different compounds give rise to unique spectra.”


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• “In summary, therefore, the assignment of bands in near infrared spectra of large molecules can be exceedingly complicated even if, the like cyclohexane, the analyte would be expected to yield a simple spectrum. Indeed, it is quite possible that the rationale of chemometricians given in the first paragraph of this paper is the best approach”.


vibrational spectroscopy for pharmaceutical analysis

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