Instrumental Analysis I: Spectrophotometry and Chromatography

Column chromatography, particularly with silica gel or alumina, is an adsorption-based separation technique. The stationary phase materials must be activated by heating to remove water, thereby exposing active adsorption sites; activity levels are classified from I (anhydrous) to V. The mobile phase, typically a water-free organic solvent, must match the activity grade of the stationary phase. In this method, the stationary phase is packed into a narrow glass column. The sample is applied at the top, and the eluent is continuously added, causing components to move at different speeds and exit the column as separated bands. These bands are collected in fractions and detected using methods like spectrophotometry or refractive index measurement. Careful column preparation is essential, including ensuring a dense packing of the stationary phase and preventing the column from drying out. Sample application must be precise to ensure even distribution and proper elution.

HPLC enables the separation of non-volatile or thermally unstable components, overcoming limitations of gas chromatography. It evolved from traditional liquid chromatography by using small stationary phase particles (e.g., 5-10 μm) and high-pressure pumps (50-200 atm). The principle involves separating mixture components based on their affinity for the eluent and a stationary phase (often a liquid bonded to a solid support) within a column. Components with higher affinity for the stationary phase are retained longer. The instrumentation includes a mobile phase reservoir, a high-pressure pump, an injection system, a separation column, a detector, and a data processing unit. The detector signal, plotted against time, forms a chromatogram.

HPLC separations are primarily based on component polarity. Analytes with similar polarity to the stationary phase are retained longer, while those more similar to the mobile phase move faster. Silica (SiO2) is a common support material, with its surface silanol groups (Si-OH) acting as a polar stationary phase. The surface can be modified by covalently bonding other molecules to change its polarity. The mobile phase's polarity is also crucial; an eluent's ability to elute a component depends on its affinity for the stationary phase, often described by an eluotrope series. Normal-phase HPLC uses a polar stationary phase and a less polar (apolar) mobile phase, typically 100% organic solvent. Reversed-phase HPLC, more commonly used (75-80% of methods), employs an apolar stationary phase (e.g., C8 or C18 silica) and a polar eluent, usually a mixture of water and an organic solvent like acetonitrile or methanol. In RP-HPLC, components elute in decreasing order of polarity, and increasing water content increases retention time. Eluents must be degassed and filtered to prevent pump and column issues. Elution can be isocratic (constant eluent composition) or gradient (eluent composition changes over time) for complex separations.

Turbidimetry measures the scattered light from suspended particles in a turbid solution. Light that is not scattered by these particles is detected, and the resulting absorbance values, within a limited concentration range, can follow a relationship similar to the Lambert-Beer law for quantitative analysis, defined by a turbidity coefficient (τ), optical path length (l), and concentration of suspended particles (C).

Spectrophotometry, specifically UV/VIS spectroscopy, is a widely used quantitative analytical technique based on the absorption of light by a solution. It allows for the classification, quantification, and study of chemicals, providing information on sample purity and concentration. Modern laboratories utilize automated methods, emphasizing the importance of understanding the underlying principles and instrumentation. This course discusses absorption measurements such as colorimetry, VIS-spectrophotometry, UV-determinations, turbidimetry, and nephelometry.

Spectroscopy relies on the interaction of light with matter. Visible light consists of different colors, each with a specific wavelength. When light hits an object, some wavelengths are absorbed, and others are reflected. The reflected light determines the perceived color of the object. Light exhibits a dual character, behaving both as an electromagnetic wave and as a stream of photons. As a wave, light oscillates periodically with a specific wavelength, which determines its energy; shorter wavelengths correspond to higher energy. The electromagnetic spectrum includes visible, UV, IR, radio, cosmic, and gamma rays. Visible light ranges from 390 to 780 nm. Polychromatic light, like that from an incandescent lamp, can be separated into its constituent wavelengths. An electromagnetic wave does not require a medium to propagate and involves harmonically oscillating electric and magnetic fields perpendicular to each other.

Light as a stream of photons possesses energy, calculated by Planck's law (E = hν). Short-wavelength electromagnetic radiation can be harmful to humans. UV/VIS spectroscopy measures light absorption by molecules or atoms, which increases their energy content. The total potential energy of a molecule is the sum of its electronic, vibrational, and rotational energies. UV/VIS light can cause transitions between electronic energy levels, with the absorbed light's wavelength determining the energy needed to shift an electron to a higher level. Molecules also absorb vibrational and rotational energy, leading to broadened absorption bands due to various possible transitions and solvent interactions. Excited molecules can return to their ground state by emitting a photon (luminescence), converting energy to heat, or undergoing a photochemical reaction.

Light absorption is used in analytical chemistry for qualitative and quantitative analysis. In UV/VIS spectroscopy, a sample is illuminated with various wavelengths, and the remaining transmitted light is recorded as a function of wavelength, creating a unique UV/VIS spectrum for substance identification and quantification. In colorimetry, a substance is determined by the color it imparts to a solution; colored solutions absorb specific parts of white light, typically the complementary color. The detector measures light intensity after it passes through the sample. Transmittance (T) is the fraction of incident light that passes through, and absorbance (E, A, or Abs, also known as optical density or AU) is defined as the negative logarithm of transmittance. Absorbance is a dimensionless quantity.

The Lambert-Beer law states that light intensity attenuation is proportional to both the sample concentration and the path length of the cuvette. It is expressed as E = ε ⋅ l ⋅ C, where E is absorbance, C is concentration, l is path length, and ε is the molar absorption coefficient, which depends on the substance, measuring wavelength, and temperature. For a given substance, E is maximal at the complementary color's wavelength, and the plot of E versus wavelength is the absorption curve. The law is valid for dilute solutions (< 0.01 M) and monochromatic light, with optimal measurement results typically between 0.1 < E < 1.5. Deviations occur at higher concentrations due to molecular interactions, changes in refractive index, chemical equilibria shifts, and the use of inappropriate cuvettes or non-monochromatic light.

Absorption curves are crucial for qualitative analysis, enabling the identification of components based on the position and profile of absorption peaks, which reflect molecular structure and functional groups. This information, combined with other techniques like IR, NMR, and MS, aids in full molecular identification. The speed of enzymatic reactions can be monitored over time, and colorimetry is applied quantitatively in various industries, including water quality control and pharmaceuticals. The absorption curve also provides insights into sample purity; impurities or other solvents can broaden or shift peaks. Comparing a sample's spectrum to known pure compounds allows for identification and quality control, as illustrated by examples of chlorophyll a and beta-carotene spectra.

Quantitative determination of a compound's concentration is performed using the Lambert-Beer law. A calibration curve is constructed by measuring the absorbance of several standard solutions with known concentrations at a predefined wavelength. The resulting absorbance values are plotted against concentration, and linear regression yields the equation of the calibration curve. An unknown sample's concentration can then be determined from its measured absorbance using this curve.

A spectrophotometer typically comprises a light source covering the UV/VIS spectrum, a sample holder (cuvette), a dispersion device (e.g., monochromator) to separate wavelengths, and a detector. Liquid samples are best placed in optical or quartz cuvettes, while plastic cuvettes are suitable for visible light measurements. Solid samples are mounted in special holders. The spectrophotometer's optical path can be single-beam or double-beam.

Single-beam spectrophotometers are the simplest setup, directing light directly from the source through the sample to the detector. A blank solution must be measured separately to account for cuvette and solvent absorption. While cost-effective and compact, single-beam systems are less accurate due to variations in light intensity or optical performance between blank and sample measurements. Double-beam configurations split the light into a reference and a measurement beam. These can be measured simultaneously using two detectors or alternately using a rotating chopper and a single detector. Double-beam systems offer real-time correction for instrument fluctuations, leading to higher accuracy.

Ideal light sources provide constant intensity across all wavelengths with low noise and stability, but real-world sources have limitations. Continuous sources emit light over a range, while discrete sources emit at specific wavelengths. Traditionally, UV/VIS spectrophotometers use deuterium (D2) arc lamps for the UV region (185-400 nm) and tungsten-halogen lamps for visible to near-IR regions (320-2500 nm). D2 lamps require warm-up time and have a limited lifespan, while tungsten-halogen lamps offer low noise and drift with a longer lifespan. Xenon flash lamps provide high-intensity light (185-2500 nm) in short flashes, eliminating the need for warm-up and minimizing photobleaching, making them suitable for sensitive samples and offering a longer lifespan due to intermittent use.

A wavelength selector isolates a narrow band of light. The Lambert-Beer law requires monochromatic light, so the selector focuses on the complementary color of the solution being analyzed. The quality of a selector is determined by its spectral bandwidth (SBB), which is the width of the transmission peak. Filters, such as absorption filters (colored plastic/glass, 50-100 nm bandwidth) and interference filters (thin metal films on transparent material, <20 nm bandwidth), are used to select wavelengths.

Monochromators consist of an entrance slit, a dispersion system (prism or grating), and an exit slit. Ideally, they produce light of a single wavelength, but in practice, they output a band of wavelengths. Prisms disperse light based on refractive index and wavelength, but their dispersion is non-linear, making wavelength selection complex. Holographic gratings, more common today, use precisely etched grooves on glass to diffract light into different angles based on wavelength, providing linear angular dispersion. However, gratings reflect light in multiple orders, requiring filters to isolate the desired order. Concave gratings can also focus light. Single-monochromator systems are simpler and suitable for general purposes, while dual-monochromator systems, with two monochromators in series, offer higher spectral accuracy and reduced stray light for demanding applications. The exit slit's width (SBB) influences resolution; a narrower slit increases resolution but reduces light intensity.

The sample compartment is a black-colored box with a lid to absorb unwanted light. It positions the sample in the light beam. Liquid samples are held in cuvettes, available in standard (10 mm path length, ~3.5 mL), microcell (0.5 mL), and ultra-microcell (0.5 μL) sizes. Cuvette width varies depending on sample concentration and solvent absorption. Cuvettes are made from quartz (170-2700 nm), optical glass (334-2500 nm), or plastic (polystyrene for 400-800 nm, PMMA for up to 300 nm). Proper cuvette handling is crucial: avoid touching optical surfaces, clean with lens paper, and store wet in slightly acidic solution. Temperature control and stirring mechanisms are also available for specific sample requirements, with Peltier systems offering precise temperature regulation. Condensation at low temperatures can be prevented by purging the compartment with dry gas or using fiber optic dip probes.

Detectors convert light into an electrical signal, ideally with linear response, low noise, and high sensitivity across a broad wavelength range. Different detectors have varying sensitivities and ranges. Photoelectric effect detectors release electrons from metal upon photon incidence, while photoconductive detectors (semiconductors) become conductive. Phototubes are robust and inexpensive but not very sensitive, suitable for simple filter photometers. Photomultiplier tubes (PMTs) offer high sensitivity (200-900 nm) by multiplying electrons released from a photocathode through a series of dynodes, making them significantly more sensitive than phototubes. Silicon diodes, based on semiconductor photoconductivity, provide intermediate sensitivity. Diode array detectors (DADs) use over 1000 silicon photodiodes on a single chip to measure all wavelengths simultaneously after light passes through the sample, often in an 'inverted optics' setup for faster and more robust measurements.

The Lambert-Beer law assumes monochromatic light, but real-world sources are not perfectly monochromatic. A large spectral bandwidth (SBB) can lead to non-linear calibration curves and inaccurate measurements. SBB relates to the monochromator's exit slit width; a smaller SBB improves resolution but reduces light intensity. Resolution is critical for distinguishing closely spaced peaks. Optimal measurements occur at the wavelength of maximum absorption (λmax) to minimize errors due to wavelength shifts. Stray light, caused by instrumental imperfections or external leaks, reduces absorbance values and distorts peak shapes, leading to unreliable concentration measurements. Photometric noise, from light source fluctuations and electronic components, affects precision and detection limits, increasing with shorter measurement times. Photometric drift indicates instability over time. Baseline flatness assesses noise levels across all wavelengths, highlighting instrument problems. Other instrumental deviations include detector fatigue and non-linear amplification.

Sample preparation is a major source of error. The absorption spectrum can be affected by the solvent, pH, temperature, high electrolyte concentrations, and interfering substances. High concentrations (>0.01 M) can cause non-linear behavior due to molecular interactions and shifts in chemical equilibria. Turbid solutions scatter light, leading to artificially high absorbance values. Interference from multiple absorbing products with overlapping bands can yield incorrect results. Solvent selection is crucial, considering solubility, stability, pH, and cutoff value (the wavelength below which the solvent absorbs all light). Non-polar solvents are preferred to avoid broadening of absorption bands and reduced resolution caused by interactions with polar solvents.

UV/VIS measurements typically use wavelengths on the x-axis, though wave number (cm-1) is used in IR spectroscopy to represent energy changes. The software allows for data collection in either unit. Multicomponent analysis applies the Lambert-Beer law to mixtures, where the total absorbance at a given wavelength is the sum of individual component absorbances. For two components, two equations based on measurements at different wavelengths (typically λmax of each component) are used to solve for unknown concentrations. While theoretically straightforward, practical limitations such as measurement errors and spectral overlap can affect accuracy, especially with increasing numbers of components.

Spectrophotometry has diverse applications. Nicotinamide adenine dinucleotide (NAD+) determination, an important co-enzyme, shows a maximal absorption at 260 nm due to aromatic rings. In cosmetics, spectrophotometry is used to analyze sunscreen efficacy by measuring UV-A and UV-B absorption (280-400 nm). Identification of pharmaceutical compounds like cyanocobalamin (vitamin B12) is performed by scanning wavelengths (200-700 nm) to identify characteristic absorption maxima (278, 361, 550 nm). Phosphate analysis (in environmental science, agriculture, etc.) involves reacting phosphate with ammonium molybdate to form a colored complex, whose intensity, measured via spectrophotometry, is proportional to phosphate concentration. Lastly, quality control of olive oil utilizes spectrophotometry to assess oxidation levels and product classification based on absorption characteristics of unsaturated fatty acid derivatives at specific wavelengths (e.g., K232 nm, K270 nm, and ΔK).

Chromatography is a separation technique based on differing distributions of substances between two immiscible phases: a stationary phase and a mobile phase. The stationary phase can be a solid or an immobilized liquid on a support, while the mobile phase is a gas or liquid that transports the mixture components. Analytes, with varying affinities for the stationary and mobile phases, move at different speeds, leading to their separation. Chromatographic methods are classified by the nature of the mobile phase and the interaction between analytes and the stationary phase, including adsorption, partition, ion-exchange, gel filtration (size exclusion), and affinity chromatography.

Thin-layer chromatography (TLC) separates substances through adsorption or partitioning. The stationary phase is a thin layer (typically 0.2 mm) of material like silica gel or alumina on a rigid plate, or a liquid phase on a cellulose powder. The mobile phase, or eluent, is usually an organic solvent or mixture, whose optimal composition is often determined experimentally. In 1D TLC, samples and known standards are spotted onto a pencil line on the plate, which is then placed in a developing tank with the eluent. The eluent moves up the plate by capillary action, separating the components. In 2D TLC, the plate is developed in a first eluent, then rotated 90 degrees and developed in a second eluent to improve separation of complex mixtures. Detection methods include spraying with color reagents or using UV light to observe quenching of fluorescence indicators, with UV detection being non-destructive. The retention factor (Rf) quantifies component movement, though it is only indicative due to sensitivity to environmental factors and should be compared under identical conditions using internal standards.