11. Spectroscopic Methods Flashcards by Ray Mark

Spectroscopic methods rely on _____

interactions between matter and electromagnetic radiation

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The process in which a substance absorbs
energy from electromagnetic radiation and
transitions to a higher energy state.

Give the energy transition and key spectroscopic methods.

Absorption
→ electrons or nuclei absorb energy and move to a higher energy level
→ UV-Vis, IR, NMR

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The process in which a substance releases energy, usually after being excited by an external energy source.

Give the energy transition and key spectroscopic methods.

Emission
→ energy released as the substance returns to a lower energy state
→ Atomic Emission Spectroscopy, Fluorescence Spectroscopy

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A type of emission where a substance
absorbs light and re-emits it at a longer wavelength almost immediately.

Give the key spectroscopic methods.

Fluorescence
→ Fluorescence Spectroscopy, Fluorescence Resonance Energy Transfer (FRET)

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The process where light or radiation is
redirected by a substance, without absorption. Can occur due to interaction with particles or molecules.

Give the key spectroscopic methods.

Scattering
→ Rayleigh Scattering, Raman Spectroscopy

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UV-Visible Regions

200-280 nm: UVC
280-315 nm: UVB
315-400 nm: UVA
400-760 nm: Visible
→ 400-430 nm: violet
→ 430-490 nm: blue
→ 490-560 nm: green
→ 560-580 nm: yellow
→ 580-620 nm: orange
→ 620-800 nm: red

Ultraviolet → violet has greatest energy in visible
Infrared → red has lowest energy in visible region

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What happens when substance absorb UV light?

→ its electrons get excited to a higher energy level
→ energy released by emitting light, releasing heat, triggering chemical reactions

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Measures the absorbance by comparing the intensity of light before and after passing through the sample. It has simple design and uses for routine analysis of samples in solution. However, it requires a reference scan for each measurement.

Single-Beam UV-Vis Spectrophotometer

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One beam of light is directed through the sample and the another through a reference (blank) to simultaneously measure absorbance and compensate for fluctuations in the source. It offers more accurate and precise measurements, especially for unstable light sources.

Double-Beam UV-Vis Spectrophotometer

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Uses a tunable diode laser to precisely select specific wavelengths for high-resolution measurements. It offers high-precision measurements for trace analysis.

Tunable Diode Laser Adsorption Spectrophotometer (TDLAS)

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Instrumentation of Single-Beam UV-Vis Spectrophotometer

Light Source
Lens
Grafting/Prism
Wavelength Selector
Sample
Detector

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Instrumentation of Double-Beam UV-Vis Spectrophotometer

Light Source
Lens
Grafting/Prism
Wavelength Selector
Mirror
6.1. Sample
6.2. Reference
Detector

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Differentiate absorbance and transmittance

Absorbance: amount of light that is absorbed by sample
Transmittance: amount of light that can completely pass through the sample

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Light source suitable for (a) UV region and (b) Visible Region

(a) UV-Region: Deuterium (H-2) and Hydrogen Lamp

(b) Visible (to near IR) Region: Tungsten(W)-Halogen Lamp

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Line light source used as excitation source and wavelength calibration

Mercury Lamp

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Continuum light source with high-intensity and broad-range used for kinetics and fluorescence analysis

Xenon Arc Lamp

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Narrow band line light source used for fixed or narrow-band wavelength analysis

LED

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Specific line light source used for precise excitation in Raman and fluorescence spectroscopy

Laser

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____ has the highest melting point of all metals which makes it perfect for light sources that need to burn hot without melting.

Tungsten (W)

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Types of Tungsten-Halogen lamp

Screw-in type
Insertion type

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Tungsten-Halogen lamps are enclosed by a ____ glass with _____ gases and ____ filament inside. A _____ foil is used to seal the lamp.

Tungsten-Halogen lamps are enclosed by a quartz glass with rare/halogen gases and tungsten filament inside. A molybdenum foil is used to seal the lamp.

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_____ are essential for isolating specific wavelengths of light from a broad-spectrum source. This allows the instrument to measure A or T accurately at a desired wavelength.

wavelength selector

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Type of wavelength selector that offers broad-spectrum and high resolution and uses prism or diffraction grating to disperse light

monochromator

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Type of wavelength selector with moderate resolution and transmits a narrow wavelength band via constructive interference

interference filter

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Type of wavelength selector for visible region with low resolution and selectively absorbs unwanted wavelengths

absorption filter

Type of broad (UV-NIR) wavelength selector with high resolution which disperses light by diffraction through a grating

Grating

Type of moderate to high (UV-Vis) wavelength selector with high resolution which disperses light based on wavelength-dependent refraction

Prism

A small, tube-like container with straight sides and either a circular or square cross-section.

cuvette

Type of cuvette with high-purity and low thermal expansion that is best for high-precision UV work

Fused Silica

Type of cuvette with slightly lower purity than fused silica that is used for general UV-Vis use

Standard Quartz

Type of inexpensive cuvette that is used only for visible region

Optical Glass

Type of cuvette that is disposable and used only for visible light applications in educational or routine lab settings

Plastic Cuvette (PMMA, Polystyrene)

Type of cuvette that is very durable and scratch resistant. It is used in harsh chemical or high-pressure condition

Sapphire Cuvette

Type of cuvette primarily for near IR work which may not transmit deep UV well.

IR Quartz

Standard path length of cuvette

10 mm = 1 cm path length

Convert transmitted light into an electrical signal that corresponds to absorbance or transmittance.

Detectors

UV-Vis detector used for fixed-wavelength instruments with narrow spectral range and moderate sensitivity and speed

Photodiode (PD)

UV-Vis detector used for full-spectrum scanning with wide spectral range, high sensitivity and fast speed

Photodiode Array (PDA)

UV-Vis detector used for high sensitivity measurements with narrow to wide spectral range and moderate speed

Photomultiplier Tube (PMT)

UV-Vis detector used for imaging and research systems with wide range, high sensitivity, and fast speed

Charge-Coupled Device (CCD)

Equations for Beer-Lambert Law

A = −log T A = −log I/I₀ I₀ → cuvette → I I₀: intensity of incident light I: intensity of transmitted light A: absorbance T: transmittance A = εbc ε: molar absorptivity (L/mol-cm) b: path length of cuvette (cm) c: concentration of analyte (mol/L)

Effect of stray light to absorbance

A' = −log [(I + Is) / (I₀ + Is)] A' = log [(I₀ + Is) / (I + Is)]

A solution of an analyte with a concentration of 5.00x10⁻⁴ M is placed in a cell that has a pathlength of 1.00 cm. At 490 nm, the solution has an absorbance of 0.338. Calculate the molar absorptivity of the analyte at this wavelength.

A = εbc 0.338 = ε(1cm)(5.00x10⁻⁴ mol/L) **ε = 676 L/mol-cm**

A measure of how strongly an analyte absorbs light at a specific wavelength.

molar absorptivity (ε)

The wavelength at which a substance shows maximum absorbance in UV-Vis spectroscopy.

Lambda max (λmax)

A 2.50 mL aliquot of a solution containing 6.4 ppm Fe³⁺ is reacted with an excess of KSCN to form the Fe(SCN)²⁺ complex, and the mixture is diluted to a final volume of 50.0 mL. The molar absorptivity of Fe(SCN)²⁺ at this wavelength is 7x10³ L/mol-cm. What is the absorbance of the resulting solution at 580 nm when measured in a 2.50 cm path length cuvette?

c = 2.50x10⁻³ L (6.4 mg Fe³⁺/L)(10⁻³g/mg)(mol Fe³⁺/55.85g)(molFe(SCN)²⁺/mol Fe³⁺)(1/0.05L) A = εbc A = (7x10³ L/mol-cm)(2.50cm)(5.73x10⁻⁶mol/L) **A = 0.100**

The concentration of iron in an industrial waste stream is determined using the o-phenanthroline spectrophotometric method. A sample from the stream exhibits an absorbance of 0.269. Using the provided calibration data, calculate the iron concentration in mg Fe/L. mg Fe/L | A 0.00 | 0.000 1.00 | 0.183 2.00 | 0.364 3.00 | 0.546 4.00 | 0.727

A_sample = 0.269 0.269x̂ = **1.48 mg Fe/L**

Beer's Law exhibits deviation at high concentrations due to _____

molecular interactions

Beer's Law is sensitive to ____ issues

stray light and instrumental issues

____ solutions of samples are required for Beer's Law measurements because ____

Clear solutions only → turbid/colored samples can scatter light, leading to inaccurate readings

Beer's Law requires _____ light for valid results

monochromatic light

Differentiate the analyzed property for UV-Vis, IR, NMR, and Mass Spectrometry

UV-Vis → Electronic transitions IR → molecular vibrations NMR → nuclear spin states in a magnetic field MS → mass-to-charge ratio

Differentiate the information obtained for UV-Vis, IR, NMR, and Mass Spectrometry

UV-Vis → conjugation, concentration, chromophores IR → functional groups, bond types NMR → molecular structure, connectivity MS → molecular mass, fragmentation pattern (destructive)

Differentiate the type of radiation in UV-Vis, IR, NMR, and Mass Spectrometry

UV-Vis → ultraviolet/visible IR → infrared light NMR → radiofrequency MS → ionizing electron beam/laser

_______ provides information about the types of bonds and functional groups present in a molecule.

IR spectroscopy

IR spectroscopy instrumentation

1. IR source 2. Beamsplitter 3.1. Movable Mirror 3.2. Fixed Mirror 4. Sample 5. Detector 6. Interferogram 7. Fourier transform → FTIR Spectrum 3 is inside the interferometer

What happens to a substance when IR radiation is absorbed?

→ change in vibrational energy levels → specific vibrational motions (stretching/bending)

Functional groups based on the wavenumber in an FTIR Spectrum

4000-2500 cm⁻¹: (bonds to hydrogen) C—H → Csp3-H (3000, strong) → Csp2-H (3150, medium) → Csp-H (3300, medium) O—H (medium to strong, broad) N—H (medium) 2500-2000 cm⁻¹: (triple bonds) C≡C (2250 medium) C≡N (2250 medium) N≡N 2000-1500 cm⁻¹: (double bonds) C=C (medium, 1650 cm⁻¹) C=O (strong,1700 cm⁻¹) C=N benzene (medium, 1600 and 1500 cm⁻¹)

_______ detects the interaction of atomic nuclei (H or C) with a magnetic field. The resulting spectrum reveals detailed information about the number, type, and position of atoms within a molecule.

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR Spectroscopy Instrumentation

1. Radiofrequency generator 2. Sample in contact with a magnet 3. Detector and amplifier 4. Display

Stages of Mass Spectrometry

1. Vaporization 2. Ionization 3. Acceleration 4. Deflection 5. Detection

A technique used to determine the molecular weight and structure of compounds by measuring the mass-to-charge ratio of ionized particles.

Mass Spectrometry

What does the m-peak or base peak and m+n peak in a mass spectrum represents?

m-peak: molecular weight m+n peak: represents isotopes

Alkyl chlorides show M and M+2 peaks in a ____ ratio due to ____ and ____

3:1 ratio Cl-35 and Cl-37

Alkyl bromides show M and M+2 peaks in a ____ ratio due to ____ and ____

1:1 ratio Br-79 and Br-81

Atomic spectroscopy rely on _____, the process of converting a sample into free atoms in a gaseous state.

atomization

Atomic Spectroscopy/AAS Instrumentation

1. Source (DC-supply): Hollow-Cathode Lamp 2. Chopper 3. Flame 3.1. Sample 4. Monochromator 5. Detector 6. Display

Hollow-cathode lamps are tiny glass tubes filled with ____ and contain a metal cathode made from _____.

neon or argon metal cathode: the element of interest → can be metal mixtures → can measure multiple elements

In a flame atomizer, a solution of the sample is ____ by a flow of gaseous oxidant, mixed with a gaseous fuel, and carried into a flame where atomization occurs.

nebulized

Atoms _____ of a specific wavelength as it passes through a sample. The amount is directly related to the _____

absorb light concentration of the analyte

Atoms are excited to higher energy levels and ______ as they return to their ground state. The intensity is directly proportional to the _____ allowing for quantitative analysis.

emit light concentration of the analyte

Atomic spectroscopic method that measures absorption of light by free atoms in the gaseous state. This is commonly used for detection of metals in water, blood, or food.

Atomic Absorption Spectroscopy

Atomic spectroscopic method that measures absorption of light by atoms in a flame. This is commonly used for quantitative analysis of trace metals.

Flame Atomic Absorption Spectroscopy

Atomic spectroscopic method that measures absorption of light by atoms in a heated graphite furnace. This is commonly used for detection of trace metals at very low concentrations.

Graphite Furnace Atomic Absorption Spectroscopy

Atomic spectroscopic method that measures light emitted by excited atoms in an Inductively Coupled Plasma (ICP). This is commonly used for multi-element analysis (e.g., environmental monitoring).

Inductively Coupled Plasma Optical Emission Spectroscopy

Atomic spectroscopic method that measures ions based on their mass-to-charge ratio. This is commonly used for ultra-trace metal analysis.

Inductively Coupled Plasma Mass Spectrometry

Atomic spectroscopic method that measures light emitted by excited atoms in a flame. This is commonly used for detection of alkali and alkaline earth metals.

Flame Emission Spectroscopy

Atomic spectroscopic method that measures emission of secondary X-rays from sample. This is commonly used for elemental analysis of solid samples.

X-ray Fluorescence Spectroscopy

This term describes the phenomenon when a sample absorbs energy and electrons jump to higher energy levels.

absorption

This is the process where a substance absorbs light and re-emits it almost immediately at a longer wavelength.

fluorescence

This method in atomic spectroscopy uses a hollow-cathode lamp and a heated graphite tube to detect trace metals.

Graphite Furnace Atomic Absorption Spectroscopy (GFAAS)

This atomic spectroscopy method measures ions based on their mass-to-charge ratio after plasma atomization.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

You are given an unknown organic compound and need to determine the functional groups present. Determine the best spectroscopic method.

IR Spectroscopy

You need to study the d—d transitions and ligand field effects in a coordination complex. Determine the best spectroscopic method.

UV-Visible Spectroscopy

You are given an unknown compound and must determine its full molecular structure. Determine the best spectroscopic method.

NMR Spectroscopy

Case: You are analyzing an unknown compound and need to find its molecular weight and fragmentation pattern. Determine the best spectroscopic method.

Mass Spectrometry

A water sample must be analyzed for trace amounts of lead (Pb), arsenic (As), and cadmium (Cd) to meet environmental safety standards. Determine the best spectroscopic method.

Atomic Absorption Spectroscopy (AAS)

A geologist needs to determine the concentration of “multiple elements (e.g., Fe, Mg, Ca, Na, K) in soil samples. Determine the best spectroscopic method.

Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)

The wavelengths of violet and red lights are 400 nm and 750 nm, respectively. Which among the following statements about a violet light compared to a red light are true? I. Violet light has a shorter wavelength and lower energy. II. Violet light has a shorter wavelength and higher energy. III. Violet light has a shorter wavelength and lower frequency. IV. Violet light has a shorter wavelength and higher frequency. A. I and IV B. II and IV C. I and III D. II and III

B. II and IV

Which light source is suitable in the wavelength range of 340 to 1000 nm? A. Deuterium B. Tungsten C. Mercury vapor lamp D. Nernst glower

B. Tungsten Nernst glower → IR region only

What is the correct order of components through which light passes in a UV-Vis spectrophotometer? A. detector, sample, source, monochromator B. source, monochromator, sample, detector C. source, sample, monochromator, detector D. monochromator, source, sample, detector

B. source, monochromator, sample, detector

What is the purpose of a monochromator in a spectrophotometer? A. To remove stray light from the room. B. To serve as a polychromatic light source. C. To focus light from the sample onto the detector. D. To allow only light of a certain wavelength to pass from the source to the sample.

D. To allow only light of a certain wavelength to pass from the source to the sample.

When operating a UV-Vis spectrophotometer, which of the following should not be done? A. Before turning on the power, ensure the sample holder is empty. B. After turning on, allow the spectrophotometer to initialize and stabilize as per the user manual. C. Immediately wipe away any spills on the instrument. D. Keep the sample compartment open during measurements.

D. Keep the sample compartment open during measurements.

A student gets fingerprints on a cuvette before using it. What will be the effect on the absorbance and reported concentration? A. Increased absorbance, resulting in a concentration that is too low B. Increased absorbance, resulting in a concentration that is too high C. Decreased absorbance, resulting in a concentration that is too low D. Decreased absorbance, resulting in a concentration that is too high

B. Increased absorbance, resulting in a concentration that is too high

In Beer's law, the product of molar absorptivity and path length corresponds to the _____ of the calibration curve. A. slope B. x-intercept C. linear range D. y-intercept

A. slope

Why should solutions with high concentrations be diluted before analysis using Beer's Law? A. The photon source is too weak to provide accurate results. B. The molar absorptivity of a compound depends on its concentration. C. There is no need to dilute; any concentration can be used. D. The relationship between absorbance and concentration becomes nonlinear at high concentrations.

D. The relationship between absorbance and concentration becomes nonlinear at high concentrations.

A photometer recorded a signal of 837 mV with a blank in the light path, and 333 mV when the blank was replaced by an absorbing solution. What are the transmittance and absorbance of the solution? A. T = 0.400, A = 0.398 B. T = 0.600, A = 0.400 C. T = 0.398, A = 0.400 D. T = 0.400, A = 0.600

C. T = 0.398, A = 0.400

A sample has a true absorbance of 1.500, but 0.50% stray light reaches the detector. What is the apparent absorbance measured by the instrument? A. 0.85 B. 1.00 C. 1.44 D. 1.55

C. 1.44 I₀ = 100% = 1 A = −log (I/ I₀) 1.5 = −log (I) I = 0.0316 Is = 0.005 A' = −log [(I + Is) / (I₀ + Is)] A' = −log [(0.0316 + 0.005) / (1 + 0.005)] A' = 1.44 D. 1.55 → incorrect since lower A if stray light is present (↑T)

Which of the following is the most widely used continuous atomization method in Atomic Absorption Spectroscopy (AAS), known for being the most reproducible liquid-sample introduction technique to date? A. Flame B. Electrothermal C. Direct-current plasma D. Inductively coupled plasma

A. Flame

Why is a hollow cathode lamp used in AAS? A. The absorption bands of atoms are very narrow. B. Hollow cathode lamps are cheaper to operate and maintain. C. Continuous spectrum lamps do not emit with sufficient intensity. D. Continuous spectrum lamps cause ionization of the analyte.

A. The absorption bands of atoms are very narrow. → HCL: line source → very narrow

In the given diagram of a hollow cathode lamp, one component is unlabeled. Based on its position and function, which of the following best identifies this part? [parts of hollow cathode lamp] A. Glass tube B. Quartz window C. Nonconducting glass D. Mica shield

B. Quartz window

Which of the following shows the correct sequence of steps during the atomization process in atomic AAS? A. Desolvation—Nebulization—Dissociation—Volatilization—Ionization B. Nebulization—Desolvation—Volatilization—Dissociation—lonization C. Desolvation—Nebulization—Volatilization—Dissociation—lonization D. Nebulization—Volatilization—Desolvation—Dissociation—lonization

B. Nebulization—Desolvation—Volatilization—Dissociation—lonization

In ICP-AES, the plasma consists of: A. Electrons only B. Positive ions only C. Neutral molecules only D. Electrons, positive ions, and neutral molecules

D. Electrons, positive ions, and neutral molecules plasma → gaseous mixture of cations, electrons, and neutral molecules

In AAS, if the sample concentration is too high to fall within the linear response range, which of the following actions can help bring the absorbance into the optimal range? A. Diluting the sample. B. Reducing the path length by adjusting the burner head. C. Using an alternative wavelength with a lower absorptivity. D. All of the above.

D. All of the above. A = εbc

AAS was used to analyze the calcium content in a milk sample. Solution 1: 10.00 mL of milk was mixed with 5.00 mL of a 1x10⁻⁴ M standard solution and diluted to 100.0 mL. The absorbance measured was 3.0 units. Solution 2: 10.00 mL of the same milk sample was diluted to 100.0 mL without adding any standard. The absorbance was 2.0 units. What is the concentration of Ca?" in the original milk sample? A. 1.0x10⁻⁴ M B. 1.5x10⁻⁴ M C. 2.0x10⁻⁴ M D. 5.0x10⁻⁴ M

A. 1.0x10⁻⁴ M → standard addition with dilution

Which of the following is not typically used as an IR sample preparation technique? A. KBr pellet method B. Nujol mull (mineral oil suspension) C. Liquid cell with salt plates D. Flame ionization

D. Flame ionization

Which functional group typically shows a strong, broad absorption around 3200-3600 cm⁻¹ in an IR spectrum? A. Carbonyl (C=O) B. Hydroxyl (O-H) C. CH (alkane) D. Nitrile (C=N)

B. Hydroxyl (O-H)

The molecular ion peak in a mass spectrum corresponds to: A. The most abundant fragment ion. B. The ion formed by loss of a methyl group. C. The ion with the highest m/z value representing the whole molecule. D. The base peak.

C. The ion with the highest m/z value representing the whole molecule.

What is the most appropriate technique to determine the concentration of lead (II) ions in blood? A. Atomic Absorption Spectroscopy B. Infrared Spectroscopy C. Mass Spectrometry D. Gas Chromatography

A. Atomic Absorption Spectroscopy

In the hollow cathode lamp of AAS, the cathode is made up of or coated with ____. A. multi-walled carbon nanotube B. carbon black C. graphene D. same as analyte element of interest

D. same as analyte element of interest

The presence of a functional group in a compound can be established by: A. X-ray diffraction B. Mass spectrometry C. Atomic absorption spectroscopy D. IR spectroscopy

D. IR spectroscopy

11. Spectroscopic Methods Flashcards

(111 cards)