CHEM 323: Analytical Instrumentation

Vassili Karanassios

Estimated study time: 24 minutes

Table of contents

Sources and References

Supplementary texts — Skoog, D.A., West, D.M., Holler, F.J. & Crouch, S.R. Principles of Instrumental Analysis, 7th ed. Cengage, 2018. | Harris, D.C. Quantitative Chemical Analysis, 10th ed. W.H. Freeman, 2020. Online resources — NIST Atomic Spectra Database (physics.nist.gov/asd); SDBS Spectral Database (sdbs.db.aist.go.jp); Photon Factory beamline databases; IUPAC Gold Book (goldbook.iupac.org)

Chapter 1: Introduction to Instrumental Analysis

Section 1.1: The Relationship Between Chemistry and Instrumentation

Analytical chemistry is the science of obtaining, processing, and communicating information about the composition and structure of matter. While classical (wet chemical) methods — gravimetry, titrimetry, colorimetry — depend on well-characterized chemical reactions to transform an analyte into a measurable signal, instrumental methods transduce a physical or chemical property of the analyte into an electrical or optical signal that can be amplified, processed, and recorded with high sensitivity and precision. The shift from classical to instrumental methods during the latter half of the twentieth century fundamentally changed the practice of analytical chemistry, enabling the detection and quantification of analytes at parts-per-billion and parts-per-trillion concentration levels, the rapid simultaneous determination of dozens of elements in complex matrices, and the structural identification of trace organic contaminants in environmental and clinical samples.

The selection of an appropriate instrumental method for a given analytical problem requires understanding the capabilities and limitations of each technique. Key figures of merit include the detection limit (the lowest concentration that can be distinguished from the blank signal at a specified confidence level), the dynamic range (the range of concentrations over which the instrument response is linear), selectivity (the ability to determine the analyte in the presence of other species), sensitivity (the change in signal per unit change in analyte concentration, i.e., the slope of the calibration curve), and accuracy and precision (systematic and random errors in the measurement). No single technique excels in all of these simultaneously, and practical analysis often requires combining methods.

Section 1.2: Classification of Instrumental Methods

Instrumental methods can be classified according to the physical or chemical property that is measured. Spectroscopic methods involve the interaction of electromagnetic radiation with matter; the interaction may involve absorption, emission, fluorescence, Raman scattering, or other processes. Mass spectrometric methods separate ions according to their mass-to-charge ratio. Electrochemical methods measure electrical properties — potential, current, or charge — as functions of analyte concentration. Separation methods (chromatography, electrophoresis) are often coupled to detectors to combine separation power with species identification.

This course focuses primarily on spectrochemical methods — those based on the interaction of electromagnetic radiation (EMR) with matter. These methods span an enormous range of wavelengths, from X-ray through ultraviolet, visible, infrared, and microwave to radio frequencies, each probing different aspects of atomic and molecular structure.

Chapter 2: Introduction to Instrumental Methods

Section 2.1: Signal and Noise

Every analytical measurement involves a signal (the desired quantity that encodes information about the analyte) superimposed on noise (random fluctuations that limit the precision with which the signal can be measured). The signal-to-noise ratio (S/N) is the fundamental figure of merit: a larger S/N allows smaller signals (lower concentrations) to be detected reliably.

Noise arises from multiple sources. Thermal (Johnson) noise is generated by the random thermal motion of electrons in any resistive element: its root-mean-square amplitude is \( V_{\text{rms}} = \sqrt{4k_BTR\Delta f} \), where \( k_B \) is the Boltzmann constant, \( T \) is temperature, \( R \) is resistance, and \( \Delta f \) is the measurement bandwidth. Cooling the detector is the most direct way to reduce Johnson noise. Shot noise arises from the discrete, random nature of charge carriers and photons: \( I_{\text{rms}} = \sqrt{2eI\Delta f} \), where \( e \) is the electron charge and \( I \) is the mean current. Flicker (1/f) noise is inversely proportional to frequency and dominates at low measurement frequencies; it can be reduced by lock-in detection (modulating the signal at a higher frequency using a chopper and demodulating at that frequency). Digitization noise arises from the finite resolution of the analog-to-digital converter.

Signal-to-noise can be improved by signal averaging: averaging \( n \) independent measurements of the same signal improves S/N by a factor of \( \sqrt{n} \). This is the statistical basis for FT spectroscopy, where the interferogram is effectively the time-averaged signal from all wavelengths simultaneously.

Section 2.2: Calibration and Quantification

The relationship between the measured instrumental signal and the analyte concentration must be established by calibration. In the simplest approach, the signal \( S \) is a linear function of concentration \( c \) over the linear dynamic range: \( S = mc + b \), where \( m \) is the sensitivity and \( b \) is the blank signal (signal from the reagents and matrix without analyte). A calibration curve is constructed from standard solutions of known concentration; the analyte concentration in an unknown sample is then read from this curve by interpolation.

Standard addition is a powerful approach when the sample matrix strongly affects the signal (a matrix effect): a known amount of standard is added directly to the unknown sample, and the incremental change in signal is used to calculate the original analyte concentration. This approach compensates for matrix effects because both the standard and the unknown experience the same matrix.

Internal standards are species added in known amounts to all standards, blanks, and samples; the ratio of analyte signal to internal standard signal is used as the analytical quantity. This approach corrects for variations in sample introduction efficiency and other irreproducibilities.

Chapter 3: The Nature of Electromagnetic Radiation

Section 3.1: Wave Properties of Light

Electromagnetic radiation (EMR) consists of oscillating, mutually perpendicular electric and magnetic fields propagating through space at the speed of light \( c = 2.998 \times 10^8 \, \mathrm{m \, s^{-1}} \) in vacuum. The wave is characterized by its wavelength \( \lambda \) (the distance between successive crests), frequency \( \nu \) (the number of oscillation cycles per second, in Hz), and wavenumber \( \tilde{\nu} = 1/\lambda \) (the number of wavelengths per centimetre). These are related by \( c = \lambda\nu \), so \( \tilde{\nu} = \nu/c \).

The electromagnetic spectrum spans an enormous range of energies. The photon energy is given by the Planck relation \( E = h\nu = hc/\lambda \), where \( h = 6.626 \times 10^{-34} \, \mathrm{J \cdot s} \). In analytical spectroscopy, different regions of the EMR are exploited:

X-ray (0.01–10 nm): inner-shell electron transitions and diffraction
Vacuum UV (10–200 nm): deep UV, absorbed by oxygen and nitrogen
UV (200–380 nm): outer-shell electron transitions
Visible (380–780 nm): outer-shell electron transitions, colour
Near-IR (780 nm–2.5 \( \mu \)m): vibrational overtones
Mid-IR (2.5–50 \( \mu \)m): fundamental vibrational transitions (4000–200 cm\(^{-1}\))
Far-IR (50–1000 \( \mu \)m): rotational transitions, lattice vibrations
Microwave (0.1–100 cm): rotational transitions
Radio (>1 m): NMR transitions

Section 3.2: Quantum Properties of Light

Light also exhibits particle-like behavior: it is quantized into discrete packets called photons, each carrying energy \( E = h\nu \). The photoelectric effect — the ejection of electrons from a metal surface upon illumination — can only be explained by the photon model: only photons with energy above the work function of the metal can eject electrons, regardless of the light intensity. This quantum nature of light is fundamental to all spectroscopic detectors, which ultimately convert individual photons (or groups of photons) into electronic signals.

Chapter 4: Interaction of Matter with Electromagnetic Radiation

Section 4.1: Absorption and the Beer–Lambert Law

When EMR passes through a sample, the intensity may decrease due to absorption — the transfer of energy from photons to the molecules or atoms in the sample. The fundamental relationship governing this process is the Beer–Lambert law:

\[ A = \log_{10}\frac{I_0}{I} = \varepsilon c l, \]

where \( A \) is the absorbance (a dimensionless quantity), \( I_0 \) is the incident intensity, \( I \) is the transmitted intensity, \( \varepsilon \) is the molar absorptivity (in L mol\(^{-1}\) cm\(^{-1}\)), \( c \) is the molar concentration of the absorbing species, and \( l \) is the path length through the sample (in cm). The transmittance \( T = I/I_0 \), and \( A = -\log_{10} T \).

The Beer–Lambert law is the quantitative foundation of all absorption spectroscopy. It predicts a linear relationship between absorbance and concentration, enabling the straightforward determination of analyte concentration from a measured absorbance and a known molar absorptivity. Deviations from linearity arise from: instrumental factors (polychromatic radiation, stray light), chemical factors (association or dissociation equilibria in the sample), and physical factors (very high concentrations where interactions between absorbing molecules change the effective molar absorptivity).

Section 4.2: Emission, Fluorescence, and Raman Scattering

When atoms or molecules in an excited state return to a lower energy state, they may release the energy as a photon — this process is emission. In atomic emission spectroscopy, atoms in a hot plasma or flame are excited collisionally and emit photons at wavelengths characteristic of the element. In molecular fluorescence, a molecule absorbs a photon (excitation), undergoes rapid non-radiative relaxation to the lowest vibrational level of the excited electronic state (internal conversion), and then emits a photon of lower energy (longer wavelength) in the return to the ground state. The shift between the excitation and emission maxima is the Stokes shift, its magnitude reflecting the extent of vibrational relaxation.

Raman scattering is an inelastic scattering process: a photon is scattered by the molecule with a change in energy equal to a vibrational quantum of the molecule. Stokes Raman scattering gives photons of lower energy (longer wavelength) and anti-Stokes scattering gives photons of higher energy. The Raman shift in wavenumbers corresponds directly to the vibrational frequency of the molecular mode and is therefore complementary to IR spectroscopy: modes that are Raman-active (change in polarizability) may be IR-inactive (no change in dipole moment) and vice versa, particularly in molecules with a centre of inversion.

Chapter 5: Instrumentation for Optical Measurements

Section 5.1: Components of a Spectrophotometer

A spectrophotometer (or optical spectrometer) is an instrument that measures absorbance or transmittance as a function of wavelength. Its principal components are:

Sources of electromagnetic radiation: for the UV-visible range, a deuterium lamp provides a continuous spectrum from 160–380 nm and a tungsten-halogen lamp covers 350 nm–3 \(\mu\)m. In atomic spectroscopy, hollow cathode lamps (HCL) are line sources that emit the characteristic spectrum of the element whose cathode is made of that element.

Wavelength selectors: a monochromator uses a prism or diffraction grating to disperse the radiation spatially by wavelength, and a narrow exit slit selects a narrow band for transmission to the detector. The key performance parameters are resolving power (\( R = \lambda/\delta\lambda \), the ability to distinguish two nearby wavelengths) and reciprocal linear dispersion (nm mm\(^{-1}\), the spread of wavelengths per millimetre of slit). Interference filters provide a simpler, fixed-wavelength alternative for routine measurements.

Sample holders: cells (cuvettes) must be transparent at the wavelengths of interest. Quartz (fused silica) cuvettes are used for UV measurements; glass or plastic cuvettes for visible; NaCl, KBr, or CaF\(_2\) windows for IR. Path lengths of 1 cm are standard for UV-visible work.

Detectors: photodetectors convert photons into electrical signals. Photomultiplier tubes (PMTs) are highly sensitive detectors for the UV-visible range, exploiting a cascade of secondary emission events to amplify the signal by factors of 10\(^6\)–10\(^8\). Charge-coupled devices (CCDs) are array detectors that can simultaneously record an entire spectrum, dramatically speeding up spectroscopic measurements.

Section 5.2: Single-Beam vs. Double-Beam Designs

In a single-beam spectrophotometer, the full optical path is used first for the blank (reference) measurement and then for the sample. Drift in source intensity between measurements introduces error. In a double-beam spectrophotometer, the beam is split (by a beam splitter or a chopper mirror) and directed alternately (or simultaneously) through the sample and the reference cells; the ratio of sample to reference signal corrects continuously for source fluctuations and background absorption. The double-beam design is standard for research-grade UV-visible and IR instruments.

Chapter 6: Detectors for Optical Measurements

Section 6.1: Photodetectors

Photodetectors operate on the principle that photons can generate charge carriers. The photomultiplier tube contains a photocathode (from which photoelectrons are ejected by the photoelectric effect), followed by a series of dynodes (metal plates held at successively higher positive potentials). Each photoelectron is accelerated to the first dynode, where it ejects several secondary electrons; these are accelerated to the next dynode, and so on for 8–14 stages. The overall current gain is \( 10^5 \)–\( 10^8 \), enabling detection of single photons. PMTs are used in fluorescence spectrometers, atomic emission spectrometers, and as the detector in many spectrophotometers.

Silicon photodiodes are solid-state detectors that generate a photocurrent proportional to the incident photon flux, with excellent response from 190–1100 nm. They are simpler and more robust than PMTs but have no internal gain. Avalanche photodiodes (APDs) are operated at high reverse bias voltage, so photoelectrons are accelerated strongly and trigger an avalanche of secondary carriers, giving internal gain analogous to a PMT.

Section 6.2: Array Detectors

Diode array detectors (DAD) consist of a linear array of hundreds or thousands of photodiodes, each monitoring a different wavelength band. When placed at the focal plane of a polychromator (a spectrograph without exit slit), a DAD can record the entire spectrum simultaneously in milliseconds. This is invaluable for kinetic experiments and for HPLC–UV detection, where the spectrum at every retention time can be collected.

Charge-coupled devices (CCDs) are two-dimensional array detectors with exceptionally low readout noise and dark current. Originally developed for astronomical imaging, they are now widely used in Raman spectrometers, fluorescence microscopes, and UV-visible spectrographs. Each pixel accumulates charge (generated by absorbed photons) during the exposure; the charge is then shifted sequentially off the chip and read out by a low-noise amplifier.

Infrared array detectors based on mercury–cadmium–telluride (MCT) or indium antimonide (InSb) are used in FTIR spectrometers and IR microspectroscopy. These detectors must typically be cooled with liquid nitrogen to reduce thermal noise.

Chapter 7: Optical Spectroscopy Fundamentals

Section 7.1: Atomic Spectroscopy

Atomic spectroscopy is concerned with the absorption and emission of radiation by atoms — species in which the electronic cloud is not constrained by molecular bonds and the transitions are therefore narrow, characteristic lines (as opposed to the broad molecular bands arising from vibrational and rotational substructure). The energy levels of atoms are described by quantum numbers: principal (\( n \)), azimuthal (\( \ell \)), magnetic (\( m_\ell \)), and spin (\( m_s \)). The wavelengths of atomic transitions are unique to each element and serve as its spectroscopic “fingerprint.”

Atomic absorption spectroscopy (AAS) measures the absorption of light at a wavelength corresponding to a specific atomic transition. The sample (usually an aqueous solution) is atomized — converted to free atoms in the gas phase — in a flame or graphite furnace. A hollow cathode lamp emitting the narrow-line spectrum of the analyte element serves as the source; the free atoms in the atomizer absorb at this exact wavelength, reducing the transmitted intensity. The absorbance is proportional to the number density of free atoms (and hence to the concentration in solution), following the Beer–Lambert law. AAS is highly selective because the absorption wavelengths are element-specific and the HCL emits the analyte’s own spectrum.

Atomic emission spectroscopy (AES) measures the intensity of light emitted by atoms excited in a high-temperature source. It does not require a primary source lamp because the atomizer itself is the emission source. AES is generally capable of simultaneous multi-element determination if a polychromator and array detector are used.

Section 7.2: Molecular Spectroscopy

Molecular spectroscopy involves transitions between the many closely spaced energy levels — electronic, vibrational, and rotational — of polyatomic molecules. The Born–Oppenheimer approximation separates the total energy of a molecule into electronic, vibrational, and rotational contributions: \( E_{\text{total}} = E_{\text{elec}} + E_{\text{vib}} + E_{\text{rot}} \), with \( E_{\text{elec}} \gg E_{\text{vib}} \gg E_{\text{rot}} \). This hierarchy of energies gives rise to the band structure of molecular spectra: within each electronic transition, a vibrational progression is observed, and within each vibrational band, rotational fine structure is seen (resolved only for small gas-phase molecules).

Chapter 8: Flames, Sample Introduction, and Graphite Furnace

Section 8.1: Sample Introduction and Atomization

The conversion of analyte atoms from solution into free, gas-phase atoms is the critical step in flame and furnace atomic spectroscopy. The efficiency and reproducibility of this atomization process largely determines the sensitivity and precision of the analysis.

In flame atomization, the sample solution is aspirated (drawn by the Venturi effect of a nebulizer gas stream) into a pneumatic nebulizer, which converts it into a fine aerosol. Only the finest droplets (< 10 \(\mu\)m diameter, typically ~5% of the aspirated solution) survive transport through the spray chamber and reach the burner. The aerosol is mixed with fuel (acetylene) and oxidant (air or nitrous oxide) and burned in a long, narrow, laminar flame over a slot burner. The flame progression moves the sample through drying, vaporization of the solvent, volatilization of the solid residue, and finally dissociation of molecules into free atoms.

The acetylene–air flame (~2300°C) is suitable for most elements except those forming refractory oxides (Al, Ti, V, Mo, W). The acetylene–nitrous oxide flame (~2700°C) is used for refractory elements because its higher temperature and more reducing character atomize their oxides more effectively.

Interferences in flame AAS are classified as spectral (other species absorb or emit at the analyte wavelength), chemical (volatile or refractory compound formation alters the degree of atomization), and ionization interferences (at high flame temperatures, atoms may be ionized, reducing the neutral atom population). Ionization interferences are suppressed by adding an easily ionized element (ionization suppressor) in excess, which provides a pool of electrons that suppresses analyte ionization.

Section 8.2: Graphite Furnace Atomic Absorption

The graphite furnace (electrothermal atomizer, ETA) eliminates the need for aspiration and offers far superior detection limits compared to flame AAS. A small volume of sample (5–50 \(\mu\)L) is injected directly into a graphite tube or cup, which is then resistively heated through a precisely programmed sequence of temperatures:

Drying (100–200°C) evaporates the solvent; ashing (300–1500°C, depending on the matrix) removes organic matrix components and volatile matrix salts; atomization (1500–2700°C) rapidly vaporizes and dissociates the analyte into free atoms within the graphite tube; and cleaning at maximum temperature removes residue. The furnace contains an inert gas (argon) to prevent oxidation, and during atomization the gas flow is stopped to maximize the atom residence time in the optical path.

Detection limits for many elements by graphite furnace AAS are in the range 0.1–10 pg (picograms), which for a 20 \(\mu\)L injection translates to sub-ppb concentration limits. The technique is therefore indispensable for trace element analysis in clinical samples (blood, serum, urine), environmental samples, and high-purity materials.

Chapter 9: ICP-AES and Plasma Miniaturization

Section 9.1: Inductively Coupled Plasma — Atomic Emission Spectrometry

The inductively coupled plasma (ICP) is a high-temperature (6000–10000 K) argon plasma sustained by inductive coupling to a radio-frequency magnetic field. The plasma is formed inside a quartz torch assembly: an argon carrier gas (nebulizer flow) transports the sample aerosol through the central channel, while an outer argon coolant flow and an intermediate argon auxiliary flow maintain the plasma structure.

The extreme temperature of the ICP offers three major advantages over flame sources. First, the high temperature ensures complete atomization even of refractory elements. Second, at these temperatures, essentially all elements are also ionized (first-ionization energies are reached for nearly every element), providing intense emission lines from both atoms and ions. Third, the inert argon plasma contains no carbon and very little background emission in the visible, giving low spectral backgrounds. ICP-AES can achieve detection limits in the range 0.1–100 \(\mu\)g L\(^{-1}\) for most elements.

The simultaneous or sequential measurement of emission from multiple elements allows multi-element analysis: a single sample introduction can quantify 70+ elements in a few minutes. This makes ICP-AES one of the most widely used techniques in environmental, geological, metallurgical, and pharmaceutical analysis.

Section 9.2: Miniaturization of Plasma Sources

Conventional ICP torches consume 10–20 L min\(^{-1}\) of argon (expensive) and require high RF power (1–1.5 kW). Miniaturized plasma sources seek to reduce these requirements dramatically while maintaining analytical performance. Microwave-induced plasmas (MIPs), microwave-sustained inductively coupled atmospheric-pressure plasmas (MISIAPs), and dielectric barrier discharge plasmas are among the alternatives explored for field-portable and low-consumption spectrometry.

Miniaturization through microfluidic (“lab-on-chip”) approaches is a particularly active area: plasma sources fabricated in silicon or glass substrates can generate microplasmas consuming nanoliters of reagent and milliwatts of power, with potential applications in environmental monitoring, clinical point-of-care testing, and space exploration.

Chapter 10: ICP-MS and Miniaturization in Mass Spectrometry

Section 10.1: Inductively Coupled Plasma Mass Spectrometry

ICP-MS combines the ionization efficiency of the inductively coupled plasma with the selectivity and sensitivity of mass spectrometric detection, producing what is arguably the most powerful technique available for trace elemental analysis. The ICP ionizes the sample as in ICP-AES, but instead of measuring emitted photons, the ions are extracted through a differentially pumped interface (consisting of a sampler cone and a skimmer cone) into the high-vacuum region of the mass spectrometer.

The extracted ion beam is focused by ion lenses and enters the mass analyser. Most ICP-MS instruments use a quadrupole mass analyser: four parallel cylindrical rods (the quadrupole) are held at a combination of DC and RF potentials, and only ions with a specific \( m/z \) ratio have stable trajectories through the device at a given set of potentials. By scanning the potentials, the instrument sequentially passes each \( m/z \) to the detector (an electron multiplier), acquiring a full mass spectrum in milliseconds.

ICP-MS offers detection limits of 0.001–10 ng L\(^{-1}\) (parts per trillion level) for most elements — orders of magnitude better than ICP-AES — and can simultaneously distinguish isotopes of the same element (isotope ratio analysis). This isotopic capability is exploited in isotope dilution analysis (adding a known amount of an isotopically enriched spike to calculate the analyte concentration with high accuracy), provenance determination (comparing isotope ratios to geochemical databases), and tracer studies in biology and medicine.

Spectral interferences in ICP-MS arise from polyatomic ions (e.g., \(^{40}\)Ar\(^{16}\)O\(^+\) at \( m/z = 56 \) interferes with \(^{56}\)Fe\(^+\)) and isobaric overlaps (two different elements at the same nominal mass). These are addressed by cool plasma conditions (reduced RF power, lower temperature), collision/reaction cells (a pressurized cell upstream of the mass analyser where polyatomic ions are destroyed by collision or chemical reaction with H\(_2\), He, or NH\(_3\)), and high-resolution sector-field ICP-MS (which resolves isobaric interferences by mass resolving power > 4000).

Section 10.2: Miniaturization in Mass Spectrometry

The conventional laboratory mass spectrometer is large, heavy, and expensive, requiring high vacuum, complex ion sources, and sophisticated electronics. Efforts to miniaturize mass spectrometry have been driven by the demand for portable instruments for field analysis (environmental monitoring, forensics, national security).

Miniature ion traps and time-of-flight (TOF) mass analysers are particularly amenable to miniaturization because of their simple geometry and pulse-based operation. The linear ion trap (a 2D version of the ion trap) concentrates ions from a continuous beam and analyzes them by mass-selective instability or resonance ejection, achieving good sensitivity in a compact design. Field-portable GC-MS systems based on miniature ion traps have been deployed for environmental site characterization.

The miniaturization of the ion source is another frontier: atmospheric pressure photoionization (APPI), low-temperature plasma (LTP) sources, and paper spray ionization allow direct analysis of complex samples without prior chromatographic separation, and their small scale enables integration into hand-held or drone-mounted devices. The trade-off is always between mass resolving power, dynamic range, and size — fundamental physical constraints limit how far miniaturization can proceed without sacrificing performance.