MMM Flashcards
(436 cards)
1
Q
What are the two main goals of analytical science?
A
To reliably identify what is present (qualitative) and accurately measure how much is there (quantitative).
2
Q
Define sensitivity and specificity in analytical methods.
A
Sensitivity is the smallest amount you can detect; specificity is the ability to distinguish analyte from interferents.
3
Q
Explain accuracy vs. precision.
A
Accuracy is closeness to the true value; precision is reproducibility of repeated measurements.
4
Q
What is limit of detection (LOD)?
A
The lowest analyte concentration that can be distinguished from a blank, typically signal ≥3× noise.
5
Q
What is limit of quantitation (LOQ)?
A
The lowest concentration that can be measured with acceptable accuracy and precision, typically signal ≥10× noise.
6
Q
Why do we calibrate an instrument?
A
To correct systematic bias by relating instrument response to known standards.
7
Q
What is a calibration curve?
A
A plot of instrument signal versus standard concentration used to interpolate unknowns.
8
Q
Describe internal standard method.
A
Add a known amount of a compound chemically similar to the analyte; ratio of responses corrects for variability.
9
Q
What is traceability in measurement?
A
An unbroken chain of calibrations back to international standards, each with stated uncertainty.
10
Q
Name three sources of analytical error.
A
Random noise, systematic bias, and sample preparation variability.
11
Q
How does Poisson (shot) noise arise?
A
From the random arrival of photons or electrons; variance equals average occurence of the event.
12
Q
What is the Beer–Lambert law?
A
A = ε·c·L, where A is absorbance, ε molar absorptivity, c concentration, L path length.
13
Q
List the Beer–Lambert assumptions.
A
Monochromatic light, homogeneous solution, no scattering, non-interacting absorbers, low concentration.
14
Q
What is Felgett’s (multiplex) advantage?
A
An FT spectrometer gathers all wavelengths simultaneously, improving S/N by √N.
15
Q
What is Jacquinot’s (throughput) advantage?
A
FT instruments have no entrance slit losses, so more light reaches detector.
16
Q
What is Connes’ (wavenumber) advantage?
A
FT spectrometers use a laser reference for superior wavelength accuracy.
17
Q
Write the diffraction grating equation.
A
d(sin θ_in + sin θ_out) = m·λ, where d is groove spacing and m the order.
18
Q
How does a Czerny–Turner spectrometer work?
A
Slit–collimator–grating–focusing mirror–exit slit sequence to isolate a single λ.
19
Q
Why use a double-beam spectrophotometer?
A
To correct for source fluctuations and instrument drift by comparing sample and reference paths.
20
Q
What is noise-equivalent power (NEP)?
A
The radiant power required to produce S/N = 1 in a 1 Hz bandwidth; lower NEP = more sensitive detector.
21
Q
Name three common optical detectors.
A
Photomultiplier tube, silicon photodiode, InGaAs detector (for NIR).
22
Q
Explain the concept of a particle-in-a-box energy levels.
A
E_n ∝ n² for an electron confined in a 1D well; fundamental for quantum spectroscopy.
23
Q
What are Jablonski energy pathways?
A
Radiative (fluorescence, phosphorescence) and non-radiative (internal conversion, intersystem crossing).
24
Q
Contrast fluorescence vs. phosphorescence.
A
Fluorescence is spin‐allowed, nanosecond decay; phosphorescence is spin‐forbidden, microsecond-second decay, intersystem non raidtive decay then radiatively back to original state
25
What makes Raman scattering weak?
Only ~10⁻⁸ of photons scatter inelastically, so signal is very low.
26
Define Stokes vs. anti-Stokes Raman.
Stokes: molecule gains vibrational energy, scattered photon loses energy. Anti-Stokes: opposite.
27
What is the Abbe diffraction limit?
d_min ≈ λ/(2·NA), the smallest resolvable distance in optical microscopy.
28
How does a confocal microscope reject out-of-focus light?
A pinhole placed at the image plane blocks light from above/below the focal plane.
29
Why does multiphoton microscopy achieve deeper imaging?
IR excitation scatters less and two-photon absorption confines excitation to the focal volume.
30
What is the role of a monochromator in fluorescence?
To select a narrow excitation band and isolate emission wavelengths, preventing bleed-through.
31
Describe a basic titration procedure.
Add titrant to analyte until endpoint indicator changes color; volume difference gives concentration.
32
When is back-titration preferred?
With slow reactions, insoluble analytes, or poorly defined endpoints; titrate excess reagent instead.
33
What is gravimetric analysis?
Quantitative measurement by isolating and weighing a pure solid product of the analyte.
34
Name two modern analytical balances.
Electromagnetic force-compensation and quartz-crystal microbalance (QCM).
35
How does a QCM detect mass changes?
Change in surface mass shifts the crystal’s resonant frequency, Δf ∝ Δmass.
36
Write the Nernst equation.
E = E° + (RT/nF)·ln([Ox]/[Red]), relates electrode potential to ion activities.
37
What is the difference between potentiometry and voltammetry?
Potentiometry measures open-circuit potential; voltammetry measures current as potential is swept.
38
In a three-electrode cell, what is the working electrode?
The electrode where the reaction of interest occurs; its potential is controlled.
39
Explain the Helmholtz double layer.
A compact layer of adsorbed ions at the electrode surface and a diffuse layer balancing charge.
40
What is cyclic voltammetry?
A triangular potential sweep around a redox couple; current-potential peaks reveal kinetics.
41
Define overpotential.
Extra potential beyond E° needed to drive a redox reaction at a finite rate.
42
What is the rotating disk electrode used for?
To create a well-controlled, steady diffusion layer for kinetic analysis.
43
Name three types of mass spec
Quadrupole, time-of-flight (TOF), and Orbitrap.
44
What is mass-resolving power?
m/Δm, the ability to distinguish ions of close m/z values.
45
Explain electrospray ionization (ESI).
Creates charged droplets that evaporate, releasing multiply charged analyte ions into the gas phase.
46
What is MALDI?
Matrix-assisted laser desorption/ionization: analyte embedded in matrix absorbs laser to ionize gently.
47
In chromatography, what do retention time and retention factor (k′) represent?
Retention time is actual elution time; k′ = (t_R – t_0)/t_0, a normalized measure.
48
List van Deemter equation terms.
A (eddy diffusion), B/u (longitudinal diffusion), C·u (mass transfer), where u is linear velocity.
49
What is gradient elution in HPLC?
Changing mobile-phase composition during run to improve separation of analytes with different affinities.
50
Why is baseline blank correction necessary in AAS/ICP?
To remove background absorption or emission from matrix elements or instrument scatter.
51
Define limit of linearity.
The concentration range over which detector response remains directly proportional to analyte amount.
52
What is the central-limit theorem’s relevance to analysis?
Many small, independent errors sum to a normal distribution, justifying Gaussian error models.
53
How does principal-component analysis (PCA) help in spectroscopy?
Reduces dimensionality by finding orthogonal components that capture most variance, aiding pattern recognition.
54
Explain the purpose of internal standard in quantitative MS.
A compound of known quantity corrects for variability in ionization and instrument response.
55
Describe standard addition method.
Spike sample with known analyte amounts; extrapolate back to zero signal to overcome matrix effects.
56
What does S/N ratio > 10 imply for LOQ?
Signal ten times greater than noise gives acceptable precision and accuracy for quantitation.
57
What are matrix effects in analytical measurements?
Alterations in analyte signal caused by other components in the sample matrix that suppress or enhance response.
58
How can matrix effects be mitigated?
Use standard addition, matrix-matched calibration, or internal standards to correct for suppression/enhancement.
59
What is the purpose of a blank in analytical workflows?
To account for background signals and instrument drift by measuring a sample without analyte.
60
Define a method blank.
A blank that undergoes all sample prep steps without analyte, to identify contamination introduced during prep.
61
What is a reagent blank?
A blank containing all reagents except the sample, to detect reagent impurities.
62
Explain quality control (QC) samples.
Samples of known concentration measured periodically to verify method performance over time.
63
What are duplicate samples used for?
Assessing precision by independently preparing and measuring two aliquots of the same sample.
64
Define a control chart.
A plot of QC sample results over time with control limits, used to monitor analytical stability.
65
What is the purpose of a calibration verification (CV) sample?
To confirm that the calibration remains valid by measuring an independent standard mid-run.
66
Explain carryover in chromatography.
Residual analyte left in the system from a previous injection, which can contaminate subsequent runs.
67
How is carryover tested?
Inject a high-concentration standard followed by a blank to check for residual peaks.
68
What is a retentate in SPE?
The fraction retained on the solid phase, typically the analyte or impurities targeted for removal.
69
Explain breakthrough volume in SPE.
Maximum sample volume that can be passed before analyte begins to elute in the effluent.
70
What is the role of a guard column in HPLC?
Protects the analytical column by trapping particulates and strongly retained impurities.
71
Describe isocratic versus gradient elution in HPLC.
Isocratic uses constant mobile-phase composition; gradient changes composition over the run for better separation.
72
What is a diode-array detector (DAD)?
An HPLC detector that records absorbance across a range of wavelengths simultaneously.
73
Explain evaporative light scattering detector (ELSD).
Detects analytes by measuring light scattered from a nebulized and evaporated mobile phase.
74
What is a retention index in gas chromatography?
A normalized scale (e.g. Kovats index) for comparing retention times across columns and conditions.
75
Define signal drift and how to correct it.
Gradual change in detector response over time; correct using frequent calibration or baseline subtraction.
76
What is electronic noise in detectors?
Random fluctuations from electronic components, contributing to the baseline noise.
77
How is limit of blank (LOB) defined?
Highest measurement likely to be observed for a blank sample, typically mean blank + 1.645×SD blank.
78
What is the difference between LOD and LOB?
LOD is the lowest analyte level distinguishable from blank (LOD > LOB); LOB is blank-only threshold.
79
Explain method ruggedness.
The degree to which results remain unaffected by small changes in method parameters (e.g. temperature, analyst).
80
What is method robustness?
Ability of a method to remain unaffected by deliberate variations in parameters, similar to ruggedness.
81
How do you assess ruggedness?
Perform the method under slightly varied conditions (e.g. different instrument, reagent lot) and compare results.
82
What is the purpose of a surrogate standard?
A compound not present in samples but similar to analyte, used to track recovery through sample prep.
83
Define extraction efficiency.
Percentage of analyte recovered from the sample during extraction/preparation steps.
84
How is extraction efficiency determined?
Spike matrix before and after extraction and compare measured concentrations.
85
What is matrix-induced signal enhancement?
When co-eluting matrix components increase the detector response of the analyte.
86
Why is a reinforcement sample used?
A replicate analyzed later in the sequence to check for instrument drift or carryover.
87
Explain the concept of sample normalization.
Adjusting analyte response relative to a stable component (e.g. internal standard) to reduce variability.
88
What is peak capacity in chromatography?
Maximum number of separable peaks in a given time window, based on gradient length and resolution.
89
Define selectivity factor (α) in chromatography.
Ratio of retention factors (k′) of two adjacent peaks; α = k′₂/k′₁ (must be >1 for separation).
90
What is resolution (Rs) between two chromatographic peaks?
Rs = 2(t_R₂ − t_R₁)/(W₁ + W₂); a value ≥1.5 indicates baseline separation.
91
How does temperature affect GC separation?
Higher oven temperatures reduce retention times but can decrease resolution; temperature programming optimizes both.
92
Explain split vs. splitless injection in GC.
Split injects only a fraction of sample (good for high concentration); splitless transfers entire volume (for trace analytes).
93
What is deconvolution in MS data processing?
Mathematical separation of overlapping spectra to identify individual components.
94
Describe the Kendrick mass defect analysis.
Normalizes mass scale so homologous series appear as horizontal lines, aiding polymer or lipid analysis.
95
What is charge state distribution in ESI-MS?
The set of different ionization states (e.g. +1, +2, +3) for a molecule, impacting m/z values.
96
How is isotopic pattern used in MS?
Matching observed isotopic abundances to theoretical patterns confirms elemental composition.
97
Define the term 'mass bias' in ICP-MS.
Systematic deviation of measured isotope ratios due to mass-dependent sensitivity differences.
98
What is dwell time in ICP-MS?
Time spent measuring a single m/z channel; longer dwell increases signal but reduces number of channels per cycle.
99
Explain the principle of collision/reaction cell in ICP-MS.
Introduces gas to remove interferences by collisional dissociation or chemical reaction.
100
What is the impact of plasma power on ICP-MS performance?
Higher power enhances ionization efficiency and reduces oxide formation but increases torch wear.
101
Describe the term 'spectral interference' in ICP-MS.
Overlap of analyte signal with isobaric ions or polyatomic species at the same nominal m/z.
102
What are polyatomic interferences?
Ions formed from plasma or matrix elements (e.g. ArCl⁺) that overlap analyte m/z peaks.
103
What is a dynamic reaction cell (DRC)?
An ICP-MS cell using reactive gases and quadrupole filtering to remove specific polyatomic interferences.
104
Question
Answer
105
What is the purpose of a flow injection analysis (FIA) system?
Automates sample introduction into a carrier stream for rapid, reproducible assays without chromatography.
106
Define dispersion in FIA.
Mixing of sample with carrier fluid, characterized by a dispersion coefficient that affects peak shape.
107
Explain the role of a carrier stream in FIA.
Transports sample plug through detector, buffers and dilutes it under controlled flow conditions.
108
What is the advantage of FIA over manual assays?
Higher throughput, lower reagent consumption, better precision and automation compatibility.
109
Describe a typical FIA manifold configuration.
Peristaltic pump → injection valve → mixing coil → detector → waste.
110
What is a potentiometric titrator?
Automates titration by detecting endpoint via electrode potential change instead of color indicator.
111
Define coulometric titration.
Generates titrant electrochemically in situ, titrating analyte until current or potential endpoint is reached.
112
What is amperometric titration?
Titration endpoint is detected by change in current at a fixed potential, proportional to analyte concentration.
113
Explain the principle of gravimetric titration.
Endpoint determined by precipitation of insoluble titrant–analyte complex that is weighed.
114
What is turbidimetric titration?
Endpoint detected by change in solution turbidity as precipitate forms, measured photometrically.
115
Define nephelometric detection.
Measures scattered light at a fixed angle to quantify turbidity from suspended particles.
116
What is the principle of nephelometry?
Intensity of light scattered by particles in suspension is proportional to concentration.
117
Explain immunoassay basic principle.
Uses antibodies to bind specific antigens; signal (color, fluorescence, chemiluminescence) correlates to analyte amount.
118
What is ELISA?
Enzyme-Linked Immunosorbent Assay: antibody or antigen immobilized on plate, enzyme-linked counterpart produces quantifiable signal.
119
Differentiate between competitive and sandwich ELISA.
Competitive: signal inversely proportional to analyte; Sandwich: signal directly proportional, uses two antibodies.
120
What is radioimmunoassay (RIA)?
Uses radiolabeled antigen and antibodies; displacement by unlabeled analyte measured by radioactivity reduction.
121
Define chemiluminescence detection.
Light emitted by chemical reaction (e.g. luminol oxidation) is measured to quantify analyte or label.
122
What is flow cytometry used for?
Analyzes physical and chemical characteristics of cells or particles in a fluid stream via laser scattering and fluorescence.
123
Explain the Coulter principle.
Counts and sizes particles by measuring change in electrical resistance as they pass through a small orifice.
124
What is capillary electrophoresis (CE)?
Separates analytes by charge-to-size ratio under high-voltage electric field in a narrow capillary.
125
Define electroosmotic flow (EOF) in CE.
Bulk fluid movement toward cathode caused by the electric double layer on the capillary wall.
126
What is micellar electrokinetic chromatography (MEKC)?
In CE, micelles act as a pseudostationary phase to separate neutral analytes based on partitioning.
127
Explain isotachophoresis.
CE mode where analytes stack between leading and terminating electrolytes, achieving concentrated zones.
128
What is dynamic light scattering (DLS)?
Measures fluctuations in scattered light intensity from Brownian motion to determine particle size distribution.
129
Define zeta potential.
Electric potential at the slipping plane of a particle in suspension; indicates colloidal stability.
130
What is surface plasmon resonance (SPR)?
Optical technique measuring changes in refractive index at a metal surface to detect binding events in real time.
131
Explain the principle of quartz-crystal microbalance with dissipation (QCM-D).
Measures frequency and damping changes of a quartz crystal to probe mass and viscoelastic properties of adlayers.
132
What is differential scanning calorimetry (DSC)?
Measures heat flow into or out of a sample relative to a reference as temperature is varied, to detect phase transitions.
133
Define isothermal titration calorimetry (ITC).
Measures heat change upon incremental addition of titrant to sample at constant temperature, for binding thermodynamics.
134
What is the principle of atomic absorption spectroscopy (AAS)?
Analyte atoms absorb light at characteristic wavelengths; absorbed intensity proportional to concentration.
135
Explain graphite furnace AAS advantage.
Offers lower detection limits than flame AAS by atomizing sample in a small, electrically heated graphite tube.
136
What is inductively coupled plasma optical emission spectroscopy (ICP-OES)?
Uses high-temperature plasma to excite atoms and ions; emitted light at elemental wavelengths is measured.
137
Differentiate ICP-OES and ICP-MS.
ICP-OES measures optical emission; ICP-MS measures mass-to-charge of ions, offering lower detection limits.
138
What is time-resolved fluorescence?
Measures fluorescence after a delay post-excitation to discriminate long-lived signals from background autofluorescence.
139
Define synchronous fluorescence spectroscopy.
Excites and scans emission monochromators simultaneously with a fixed wavelength offset to simplify spectra.
140
What is circular dichroism (CD) spectroscopy?
Measures differential absorption of left vs. right circularly polarized light to probe chiral molecule secondary structure.
141
Explain attenuated total reflectance (ATR) in IR spectroscopy.
Samples pressed against a high-refractive-index crystal; evanescent wave probes sample to simplify sample prep.
142
What is two-dimensional infrared (2D-IR) spectroscopy?
Correlates vibrational modes via nonlinear IR pulse sequences to reveal coupling and dynamics.
143
Define Förster resonance energy transfer (FRET).
Distance-dependent transfer of excitation energy between donor and acceptor fluorophores, used to measure molecular interactions.
144
What is photoluminescence quantum yield?
Ratio of photons emitted to photons absorbed; a measure of fluorescence efficiency.
145
Explain Dicke narrowing in gas-phase spectroscopy.
Collisional confinement of emitters reduces Doppler broadening, yielding sharper spectral lines.
146
What is modulation spectroscopy?
Applies a periodic perturbation (e.g. magnetic, electric field) and detects signal at modulation frequency to enhance sensitivity.
147
Define electrochemical impedance spectroscopy (EIS).
Applies AC potential and measures current phase lag and amplitude to characterize electrode–electrolyte interface.
148
What is a Nyquist plot?
Graph of imaginary vs. real impedance from EIS data to model resistive and capacitive circuit elements.
149
Explain the Warburg impedance.
Frequency-dependent impedance representing diffusional resistance, appearing as a 45° line in Nyquist plot.
150
What is window lock in NMR?
Technique adjusting magnet field drift by continuously referencing a deuterium lock signal.
151
Define pulse width in NMR.
Duration of radiofrequency pulse determining flip angle of nuclear spins.
152
What is gradient echo imaging in MRI?
MRI pulse sequence using gradient reversal to generate signal, allowing faster imaging than spin echoes.
153
Explain chemical exchange saturation transfer (CEST) MRI.
Saturates exchangeable protons in metabolites; transferred saturation to water reduces its signal, enabling metabolite imaging.
154
What is dynamic light scattering correlation function?
Autocorrelation of scattered light intensity vs. time used to extract diffusion coefficients and particle sizes.
155
Define limit of detection in electrochemical sensors.
Lowest analyte concentration giving current three times above baseline noise in amperometric measurement.
156
What is a potentiometric sensor’s selectivity coefficient?
Quantifies preference of an ion-selective electrode for analyte over interfering ions, per the Nikolskii–Eisenman equation.
157
Explain chronocoulometry.
Measures total charge passed as a function of time after a potential step, to study adsorption or diffusion processes.
158
What is the purpose of signal smoothing (e.g. Savitzky–Golay)?
Reduces high-frequency noise in spectral or chromatographic data while preserving peak shapes.
159
Question
Answer
160
What is solid-phase extraction (SPE)?
Sample prep technique where analyte is retained on a sorbent and interferents are washed away before elution.
161
Define breakthrough volume in SPE.
Maximum sample volume that can pass before analyte leaks through the sorbent, affecting recovery.
162
Explain liquid–liquid extraction (LLE).
Partitioning of analyte between two immiscible solvents based on differing solubilities.
163
What is distribution coefficient (D) in LLE?
Ratio of analyte concentration in organic phase to aqueous phase at equilibrium.
164
How does pH affect LLE of weak acids/bases?
Adjusting pH changes ionization state, altering solubility and partitioning behavior.
165
What is derivatization in sample prep?
Chemical modification of analyte to improve volatility, detectability, or chromatographic behavior.
166
Name two common derivatization reagents for GC.
BSTFA (N,O‐bis(trimethylsilyl)trifluoroacetamide) and PFBBr (pentafluorobenzyl bromide).
167
Define method validation.
Process of proving that an analytical method is acceptable for its intended purpose.
168
List five validation parameters.
Accuracy, precision, specificity, limit of detection, limit of quantitation, linearity, robustness.
169
What is repeatability?
Precision under the same operating conditions over a short interval of time.
170
What is reproducibility?
Precision under changed conditions (different labs, analysts, instruments).
171
Explain inter- and intra-day precision.
Intra-day: same day; inter-day: across separate days, testing method stability over time.
172
Define uncertainty of measurement.
Quantitative estimate of the doubt about the result, encompassing all error sources.
173
What is a proficiency test?
External quality assessment where labs measure blind samples to compare performance.
174
Why perform ruggedness testing?
To assess method performance under small, deliberate variations in procedure parameters.
175
What is a limit test?
Qualitative check for presence or absence of analyte above a specified threshold.
176
Explain specificity in analytical methods.
Ability to assess analyte unequivocally in presence of components like impurities or matrix.
177
What is selectivity?
Extent to which method responds only to the analyte, minimizing interference from others.
178
Define linearity in method validation.
Ability of method to elicit results proportional to analyte concentration within a given range.
179
What is homoscedasticity?
Constant variance of residuals across the calibration range, important for linear regression.
180
Explain heteroscedasticity.
Non-constant variance of residuals, requiring weighted regression for accurate calibration.
181
What is standard deviation of the residuals (Sy/x)?
Measure of scatter of data points around the calibration line, used to assess precision.
182
How is the calibration slope related to sensitivity?
Steeper slope means greater change in signal per unit concentration, indicating higher sensitivity.
183
Define kappa (κ) statistic in qualitative assays.
Measure of agreement between observers beyond chance, used in method comparison.
184
What is the Youden plot?
Graphical tool to compare two labs’ results on paired samples, highlighting systematic and random error.
185
Explain Deming regression.
Errors-in-variables regression accounting for uncertainty in both x and y for calibration comparison.
186
What is the purpose of QC charts?
Monitor analytical method stability over time, detecting trends, shifts, or out-of-control conditions.
187
Define Westgard rules.
Set of decision criteria applied to QC data to detect systematic or random errors in assays.
188
What is a blank spike recovery test?
Assess matrix effect by adding known analyte to blank matrix and measuring recovery.
189
Explain carryover mitigation techniques.
Use strong solvents, flush loops, or blank injections between samples to prevent cross-contamination.
190
What is headspace analysis in GC?
Sampling the vapor phase above a liquid or solid to analyze volatile compounds.
191
Define static vs. dynamic headspace.
Static: equilibrium sampling; dynamic: purging VOCs into trap for concentration.
192
What is purge and trap GC?
Dynamic headspace where carrier gas purges volatiles onto sorbent trap, then thermally desorbs into GC.
193
Explain microextraction by packed sorbent (MEPS).
Miniaturized SPE where sorbent is packed into a syringe needle for small-vol sample prep.
194
What is solid-phase microextraction (SPME)?
Fiber coated with sorbent extracts analytes directly from sample or headspace for GC or LC.
195
Define lab-on-a-chip.
Microfluidic device integrating multiple analytical steps (e.g., separation, detection) on a single chip.
196
Explain the benefit of microfluidics.
Low sample/reagent volume, rapid analysis, precise control of fluid flows, and integration capability.
197
What is a biosensor?
Analytical device combining a biological recognition element with a transducer to produce a measurable signal.
198
Name three types of biosensor transducers.
Electrochemical, optical, and piezoelectric.
199
Explain sandwich immunoassay format.
Capture antibody immobilizes analyte, detection antibody binds a different epitope, signal proportional to analyte.
200
What is surface-enhanced Raman spectroscopy (SERS)?
Raman scattering amplified by plasmonic nanostructured metal surfaces, boosting sensitivity.
201
Define near-infrared (NIR) spectroscopy.
Uses 800–2500 nm light for overtone and combination band analysis, often for rapid, non-destructive quality control.
202
What is diffuse reflectance NIR?
Measures scattered NIR light from powders or solids to determine composition or moisture content.
203
Explain principal-component regression (PCR).
Multivariate calibration using principal components of spectral data to predict concentrations.
204
What is partial-least-squares regression (PLSR)?
Calibration method modeling covariance between spectral data and analyte concentration for robust predictions.
205
Define model overfitting.
When a statistical model describes random error instead of underlying relationship, reducing predictive performance.
206
Explain cross-validation.
Technique dividing data into training and test sets to evaluate model predictive ability without bias.
207
What is the Mahalanobis distance used for?
Multivariate measure of sample distance from calibration set center, used for outlier detection.
208
Describe the concept of 'chemometrics'.
Application of mathematical and statistical methods to chemical data for classification, calibration, and interpretation.
209
What is supercritical fluid chromatography (SFC)?
Uses supercritical CO₂ as mobile phase for fast, efficient separations of nonpolar to moderately polar analytes.
210
Why is CO₂ ideal for SFC?
Low critical temperature/pressure, inert, tunable solvating power with co-solvents, easy removal.
211
Define pyrolysis GC-MS.
Thermal decomposition of macromolecules into smaller fragments, followed by GC separation and MS analysis.
212
What is tandem mass spectrometry (MS/MS)?
Two-stage mass analysis: selects precursor ion, fragments it, then analyzes product ions for structural information.
213
Explain selected reaction monitoring (SRM).
MS/MS mode monitoring a specific precursor→product ion transition for high-sensitivity quantitation.
214
What is high-resolution mass spectrometry (HRMS)?
MS with resolving power >10,000, enabling exact mass measurement for elemental composition determination.
215
Define collision-induced dissociation (CID).
Fragmentation of ions by collision with inert gas in MS/MS, producing characteristic product ions.
216
What is electron transfer dissociation (ETD)?
MS/MS fragmentation technique transferring electrons to multiply charged cations, preserving labile PTMs.
217
Explain MALDI imaging mass spectrometry.
Spatially resolved MALDI-MS acquisition across tissue sections to map molecular distributions.
218
What is surface acoustic wave nebulization (SAWN)?
Ionization method using acoustic waves to nebulize liquid samples for MS, gentle on fragile analytes.
219
Define inductively coupled plasma tandem MS (ICP-MS/MS).
ICP-MS with quadrupole collision/reaction cell and second mass filter for interference-free elemental analysis.
220
What is laser ablation ICP-MS?
Direct solid sampling by laser, transporting aerosol to ICP-MS for spatially resolved elemental mapping.
221
Explain time-of-flight secondary ion mass spectrometry (TOF-SIMS).
Technique bombards surface with primary ions, analyzes ejected secondary ions by TOF-MS for surface composition.
222
What is X-ray photoelectron spectroscopy (XPS)?
Surface analysis by measuring kinetic energy of electrons ejected by X-ray irradiation, yielding elemental and chemical state information.
223
Define Auger electron spectroscopy (AES).
Surface-sensitive technique where core-hole relaxation emits Auger electrons, analyzed for elemental composition.
224
What is secondary ion mass spectrometry (SIMS)?
Surface analysis using focused ion beam to sputter secondary ions, analyzed by MS for high-sensitivity surface chemistry.
225
Explain energy-dispersive X-ray spectroscopy (EDX/EDS).
SEM or TEM detector measures characteristic X-rays emitted during electron beam irradiation to determine elemental composition.
226
What is electron energy-loss spectroscopy (EELS)?
TEM-based technique measuring energy lost by transmitted electrons interacting with sample to probe electronic structure.
227
Define atomic emission spectroscopy (AES, optical).
Excites atoms in plasma or flame, measures emitted photons at characteristic wavelengths for elemental analysis.
228
What is dynamic reaction cell (DRC) chemistry?
ICP-MS technique using reactive gases (e.g., NH₃) to selectively remove interferences by forming new species.
229
Explain collision cell ICP-MS.
Uses inert gas (He) collisions to kinetically remove polyatomic interferences based on size and energy.
230
What is single-particle ICP-MS?
Measures transient signals from individual nanoparticles entering ICP, enabling particle size and number concentration analysis.
231
Define hyphenated techniques.
Combination of separation (GC/LC/CE) with detection (MS, NMR, ICP) to leverage strengths of each method.
232
What is LC–MS/MS quantitative MRM?
LC separation followed by MS/MS monitoring multiple specific precursor→product transitions for multiplex quantitation.
233
Explain nano-LC in proteomics.
Low-flow LC (nL/min) with narrow-bore columns for high-sensitivity peptide separation prior to MS.
234
What is capillary electrophoresis–MS (CE–MS)?
Combines CE separation with MS detection via electrospray for high-efficiency, low-volume analyses.
235
Define hydrophilic interaction liquid chromatography (HILIC).
LC mode for polar analytes using a polar stationary phase and high-organic mobile phase.
236
What is ion chromatography (IC)?
LC technique specialized for separation of ions and small polar molecules using ion-exchange columns.
237
Explain chiral chromatography.
Separates enantiomers using chiral stationary phases or chiral mobile-phase additives.
238
What is differential pulse voltammetry (DPV)?
Voltammetric technique applying pulse increments for high-sensitivity current measurement with reduced capacitive background.
239
Define square-wave voltammetry (SWV).
Fast pulse technique applying symmetrical square-wave potential steps, measuring net current for rapid, sensitive analysis.
240
What is stripping analysis in electrochemistry?
Preconcentrates analyte electrochemically, then strips it by potential sweep to quantify trace metals.
241
Explain anodic stripping voltammetry (ASV).
Preconcentration of metal ions on electrode surface by reduction, followed by oxidation sweep to measure peak current.
242
What is cathodic stripping voltammetry (CSV)?
Preconcentrates analyte as insoluble species at anode, then reduces it during stripping scan.
243
Define potentiometric sensor drift.
Gradual change in baseline potential over time due to membrane aging or reference electrode instability.
244
What is an optode?
Optical sensor (e.g., fiber-optic) coated with selective dye responding to analyte by absorbance or fluorescence change.
245
Explain fluorescence lifetime imaging (FLIM).
Maps spatial distribution of fluorescence lifetimes in microscopy to differentiate fluorophores or microenvironments.
246
What is Förster critical distance (R₀) in FRET?
Distance at which donor–acceptor energy transfer efficiency is 50%, typically 2–8 nm depending on the pair.
247
Define turbidity unit NTU.
Nephelometric turbidity units, measuring scattered light to quantify suspended particle concentration in water.
248
What is membrane inlet MS (MIMS)?
Direct introduction of dissolved gases through a semipermeable membrane into MS for real-time trace gas analysis.
249
Explain thermal gravimetric analysis (TGA).
Measures weight change of a sample as temperature increases to study thermal stability and composition.
250
What is differential thermal analysis (DTA)?
Monitors temperature difference between sample and inert reference under heating to detect phase transitions.
251
Define dynamic mechanical analysis (DMA).
Measures mechanical properties (e.g., modulus, damping) of materials under oscillatory stress vs. temperature or frequency.
252
What is rheometry?
Characterizes flow and deformation (viscosity, elasticity) of fluids and soft solids under shear or oscillation.
253
Explain thermally assisted hydrolysis and methylation (THM).
Pyrolysis technique using pyrolyzer with reagent (e.g., TMAH) to derivatize and volatilize polar analytes for GC-MS.
254
What is process analytical technology (PAT)?
Real-time analytical monitoring and control of manufacturing processes (e.g., pharma) using in situ sensors.
255
Define near-infrared chemical imaging (NIR-CI).
Combines NIR spectroscopy with spatial scanning to map chemical composition across sample surfaces.
256
What is Raman tweezers?
Combines optical trapping and Raman spectroscopy to analyze single particles or cells in suspension.
257
Explain hyperspectral imaging.
Acquires images at many wavelengths, yielding spatial-spectral data cubes for material identification and mapping.
258
What is diffuse reflectance UV-Vis spectroscopy?
Measures reflectance of powdered or solid samples for pigment or colorant analysis without dissolution.
259
Define total internal reflection fluorescence (TIRF) microscopy.
Illuminates only a thin (~100 nm) region near the coverslip via evanescent field for high-contrast surface imaging.
260
What is spinning-disk confocal microscopy?
Uses a rotating disk of pinholes for parallel confocal imaging, enabling high-speed, low-photodamage acquisition.
261
Explain structured illumination microscopy (SIM).
Super-resolution technique illuminating sample with patterned light, reconstructing high-resolution images computationally.
262
What is stimulated emission depletion (STED) microscopy?
Super-resolution method depleting fluorescence around excitation spot by stimulated emission to achieve <50 nm resolution.
263
Define single-molecule localization microscopy (SMLM).
Super-resolution techniques (e.g., PALM/STORM) localizing individual fluorophores to ~20 nm precision.
264
What is cryo-electron microscopy (cryo-EM)?
Electron microscopy of vitrified biological samples at liquid-N₂ temperatures for near-atomic structural determination.
265
Explain small-angle X-ray scattering (SAXS).
Probes low-resolution structure of macromolecules in solution by measuring X-ray scattering at small angles.
266
What is dynamic light scattering (DLS)?
Measures time-dependent fluctuations in light scattering to determine particle size distributions by Brownian motion analysis.
267
Define multi-angle light scattering (MALS).
Measures light scattering at multiple angles to determine absolute molar mass and size of macromolecules in solution.
268
What is analytical ultracentrifugation (AUC)?
Separates molecules by sedimentation under high centrifugal fields to analyze mass, shape, and interactions.
269
Explain isoelectric focusing (IEF).
Separates proteins or peptides by pI in a pH gradient until they focus at their isoelectric point.
270
What is capillary isoelectric focusing (cIEF)?
Miniaturized IEF in a capillary for high-resolution separation prior to MS or optical detection.
271
What is spectroscopy?
The splitting of light into its component wavelengths and measuring their intensities to identify substances or quantities of interest
272
State Beer–Lambert’s law.
A = ε·c·L
273
What does a diffraction grating do?
Separates light into different wavelengths via constructive interference across many slits
274
Define spectral resolution.
The ability to distinguish two closely spaced wavelengths; improves with more grating slits or longer focal length
275
What is the function of a blazed grating?
It optimizes diffraction efficiency for a particular order and wavelength via facet angle (Littrow condition)
276
Name three types of diffraction gratings.
Blazed
277
What is the principle behind Huygens’ principle?
Each point on a wavefront acts as a source of spherical secondary wavelets
278
What effect does increasing slit width have on brightness and resolution?
Increases brightness (more light) but decreases spectral resolution
279
Define Etendue.
A measure of light‐gathering ability: focal length divided by diameter of entrance pupil (or mirror)
280
What is a photodiode?
A semiconductor device converting absorbed photons into current via the photoelectric effect
281
How does a photomultiplier tube work?
High‐energy photons liberate electrons from a dynode chain
282
What is the advantage of an EMCCD over a CCD?
Electron multiplication within the chip yields higher signal and lower read noise
283
How does a CMOS detector differ from CCD?
Each pixel converts charge to voltage in‐pixel
284
What does FTIR stand for?
Fourier‐Transform Infrared Spectroscopy
285
Name the three main advantages of FT spectrometers.
Multiplex (Fellgett’s) advantage
286
What is Raman scattering?
Inelastic scattering of photons by molecular vibrations via virtual energy states
287
Why is Raman scattering intensity low?
Only ~1 in 10^8 photons undergo Raman scattering
288
What is fluorescence microscopy used for?
Imaging of fluorescently labeled structures within biological samples
289
Name one limitation of fluorescence microscopy.
Limited number of simultaneous fluorophores due to spectral overlap and limited penetration depth
290
What is confocal microscopy?
A fluorescence technique using a pinhole to reject out‐of‐focus light for optical sectioning
291
Define two-photon microscopy.
Excitation via simultaneous absorption of two lower‐energy photons
292
What is the diffraction limit of light?
The smallest resolvable feature ~λ/(2·NA)
293
What does TEM stand for?
Transmission Electron Microscopy
294
What resolution advantage do electrons have over light?
Electron wavelengths are ~10^5 times shorter
295
Name two signals detected in SEM.
Secondary electrons (surface topology) and backscattered electrons (composition contrast)
296
What is EDX?
Energy‐Dispersive X-ray spectroscopy for elemental analysis via characteristic X-rays
297
Define chromatography.
Separation of analytes based on differing affinities between a mobile phase and stationary phase
298
What is retention time in chromatography?
The time an analyte takes to elute through a column under set conditions
299
Name two types of gas chromatography columns.
Packed columns and capillary (WCOT or SCOT)
300
What is a flame ionization detector?
A GC detector where analytes are burned in H₂/air flame and resulting ions generate a current
301
How does a thermal conductivity detector work?
Changes in gas thermal conductivity alter resistance in a filament
302
Define HPLC.
High‐Performance Liquid Chromatography; liquid mobile phase under high pressure through a packed column
303
What is size-exclusion chromatography?
Separation by molecular size: large molecules elute first
304
What principle does mass spectrometry use?
Separation of ionized molecules by mass‐to‐charge ratio (m/z)
305
Name one soft ionization method.
Electrospray ionization (ESI) or MALDI
306
What does TOF stand for in MS?
Time‐Of‐Flight mass analyzer
307
Define NMR.
Nuclear Magnetic Resonance spectroscopy probes nuclear spin states in a magnetic field via RF pulses
308
What is chemical shift in NMR?
The resonance frequency of a nucleus relative to a reference (e.g.
309
What is J-coupling?
Scalar spin–spin coupling between neighboring nuclei causing signal splitting in NMR
310
Define T₁ relaxation.
Longitudinal relaxation: recovery of net magnetization along the main magnetic field axis
311
Define T₂ relaxation.
Transverse relaxation: decay of coherent transverse magnetization due to dephasing
312
What is an FT-NMR?
Fourier‐Transform NMR: time‐domain signal converted to frequency‐domain spectrum via Fourier transform
313
What is electrophoresis?
Separation of charged molecules in a gel under an electric field by size/charge ratio
314
What is isoelectric focusing?
Gel electrophoresis in a pH gradient where proteins migrate to their isoelectric point and stop
315
How is DNA visualized after gel electrophoresis?
Via staining (e.g.
316
Define ELISA.
Enzyme-Linked Immunosorbent Assay: antigen–antibody binding detected via enzyme-catalyzed color change
317
Name three ELISA formats.
Direct
318
What is surface plasmon resonance?
Label-free detection of binding events via changes in refractive index at a metal–dielectric interface
319
Define FRET.
Förster Resonance Energy Transfer: nonradiative energy transfer between two fluorophores in close proximity (<10 nm)
320
What is Sanger sequencing?
Chain‐termination DNA sequencing using fluorescently labeled ddNTPs and capillary electrophoresis
321
What is shotgun sequencing?
Random fragmentation of DNA and sequencing fragments
322
Define bridge amplification.
Clonal amplification of DNA on a solid surface in Illumina sequencing by cyclic synthesis
323
What is nanopore sequencing?
DNA is threaded through a protein pore and base identity inferred from changes in ionic current
324
What does PCR stand for?
Polymerase Chain Reaction
325
What are the three main steps of PCR?
Denaturation (~95 °C)
326
What is Tₘ of a primer?
The temperature at which 50% of primer–template duplexes dissociate
327
Define qPCR.
Quantitative PCR measures DNA amplification in real time via fluorescent dyes or probes
328
What is reverse transcription?
Synthesis of complementary DNA (cDNA) from RNA template by reverse transcriptase
329
Name one electrophoretic blotting method.
Southern (DNA)
330
What is a microarray?
A solid‐phase platform with thousands of probes for parallel detection of nucleic acids or proteins
331
Define flow cytometry.
Single-cell analysis measuring fluorescence and light scattering as cells flow past lasers
332
What is FACS?
Fluorescence-Activated Cell Sorting: flow cytometry with cell sorting capability
333
What is the central dogma?
DNA → RNA → Protein
334
Define microRNA.
~22 nt noncoding RNAs that guide RISC to mRNA targets
335
What is a lncRNA?
Long noncoding RNA (>200 nt) that regulates chromatin
336
Define ribosome.
A ribonucleoprotein complex (rRNA + proteins) catalyzing mRNA translation into protein
337
What are telomeres?
Repetitive DNA sequences at chromosome ends that protect against loss and signal senescence when too short
338
What is telomerase?
An RNA‐dependent DNA polymerase that extends telomeres
339
Define epigenetics.
Heritable changes in gene expression not due to DNA sequence
340
What is 5-methylcytosine?
A DNA base modification at CpG dinucleotides associated with transcriptional repression
341
Name two histone methylation marks.
H3K4me3 (active transcription) and H3K27me3 (repression)
342
What is ChIP-seq?
Chromatin Immunoprecipitation followed by sequencing to map DNA–protein interactions genome-wide
343
Define bisulfite sequencing.
Chemical treatment converting unmethylated C to U
344
What is CRISPR-Cas9?
RNA-guided DNA endonuclease system used for genome editing
345
What is an siRNA?
Small interfering RNA (~21 nt) that directs RISC to degrade complementary mRNA
346
Define western blot.
Protein separation by SDS-PAGE
347
What is mass cytometry?
Single-cell proteomics by tagging antibodies with metal isotopes and detecting via time-of-flight MS
348
Name one microfabrication technique for lab-on-a-chip.
Soft lithography
349
What is a biosensor?
A device that uses a biological recognition element (e.g.
350
Define limit of detection (LOD).
The lowest analyte concentration that can be reliably distinguished from background
351
What is dynamic range?
The concentration range over which an assay gives a linear response
352
What is signal-to-noise ratio?
The ratio of analyte signal amplitude to background noise level
353
Define calibration curve.
Plot of known analyte concentration vs. instrument response used to quantify unknown samples
354
What is a standard addition method?
Quantification method where known amounts of analyte are spiked into the sample matrix to correct for matrix effects
355
What is multiplexing?
Simultaneous detection of multiple analytes in a single assay
356
Define point‐of‐care testing.
Diagnostic tests performed at or near patient site for rapid results
357
What is lab-on-a-chip?
Miniaturized analytical systems integrating multiple lab functions on a single microfluidic chip
358
What ligand binds to streptavidin?
Biotin
359
What ligand binds to a 20-mer oligonucleotide?
A 20-mer complementary oligonucleotide
360
What ligand binds to rabbit anti-goat immunoglobulin?
Goat immunoglobulin
361
Define the Inner Helmholtz Plane.
The plane through the centers of polarized solvent molecules in direct contact with the electrode surface.
362
Define the Outer Helmholtz Plane.
The plane through the centers of solvated ions at their closest approach to the electrode.
363
What is the Diffuse Layer in the electrical double layer?
The region where excess counter-ions accumulate without being bound to the electrode.
364
What is the Bulk region in the electrical double layer?
The region far from the electrode where the field is negligible and no net charge imbalance exists.
365
Name the key components of a time-of-flight mass spectrometer.
Ion source
366
Why does a fragment of mass 2M have √2 times longer flight time than mass M in TOF MS?
Because t = √(m d² / 2Uz)
367
What does FTIR stand for?
Fourier Transform Infrared Spectroscopy
368
How does FTIR work?
It projects broadband IR through the sample and measures absorption versus wavelength with an interferometer.
369
What does NMR stand for?
Nuclear Magnetic Resonance
370
How does NMR determine molecular structure?
By detecting RF signals emitted when nuclei in a B-field relax after a broadband pulse.
371
What does GC-MS stand for?
Gas Chromatography Mass Spectrometry
372
Briefly describe GC-MS.
Components are separated by GC
373
What is nanopore sequencing?
Measuring ion-current drops as single DNA bases pass through a membrane nanopore to read sequence.
374
What does PET stand for in imaging?
Positron Emission Tomography
375
How does PET work?
A radiotracer emits coincident gamma rays detected by a ring of detectors to localize metabolic activity.
376
What is cryo-TEM tomography?
Freezing biomolecules and collecting tilted electron diffraction images to reconstruct 3D structure.
377
What does STM stand for?
Scanning Tunneling Microscopy
378
How does STM map surface topography?
By maintaining a tunneling current between a sharp tip and sample and recording tip height.
379
What is cyclic voltammetry?
Applying a swept potential to electrodes in electrolyte and measuring current vs. potential.
380
How do London dispersion forces affect an AFM cantilever?
They attract the cantilever to the surface
381
Why is GFP fused to membrane proteins more polarized than soluble GFP?
Membrane-bound GFP cannot rotate freely
382
What determines the NMR resonant frequency?
The gyromagnetic ratio γ of the nucleus and external magnetic field strength B₀ (ν = γB₀/2π).
383
What measures IR absorption in conventional IR spectroscopy?
The reduction in transmitted IR intensity (I₀ vs I) detected by DTGS or MCT detectors.
384
Do hard X-rays have low or high attenuation coefficients?
Low attenuation coefficients—they penetrate deeply and are less absorbed by low-Z materials.
385
What is vapour diffusion in crystallography?
A drop of protein + precipitant equilibrates by water vapor diffusion to a concentrated reservoir
386
What is AFM-IR?
A technique where IR absorption–induced thermal expansion is sensed by an AFM tip to produce nanoscale IR spectra.
387
What does attenuation represent in X-rays?
The exponential reduction of X-ray intensity via absorption and scattering as it passes through matter.
388
Which process dominates relaxation in low-Z elements: Auger or X-ray emission?
Auger electron emission dominates in low-Z elements.
389
How can you favor characteristic X-ray emission over Auger?
Use high-Z elements
390
Is SEM charging caused by too thick coating?
No—charging arises from poor conductivity (insufficient coating)
391
What is a microchannel plate (MCP)?
A glass plate with millions of channels acting as electron multipliers for high-gain
392
How do semiconductor detectors work in SEM?
Incoming electrons generate electron-hole pairs in silicon; the collected charge pulse is proportional to energy deposited.
393
What does STS measure?
Local density of electronic states by recording tunneling current vs. bias voltage (dI/dV ∝ LDOS).
394
In STS
what does positive bias probe?
395
In STS
what does negative bias probe?
396
What is the fluorescence yield ω in X-ray emission?
Probability of X-ray emission after core ionization (increases with atomic number Z).
397
What is slice selection in MRI?
Using a B₀ gradient and narrowband RF pulse to excite only nuclei in a thin spatial slice.
398
How does frequency encoding localize MRI signals?
Applying a gradient in-plane so precession frequency maps to position
399
How does spin-warp (phase encoding) work in MRI?
Applying orthogonal gradient to impart position-dependent phase shifts
400
What is numerical aperture in microscopy?
n·sinθ
401
What wavelength corresponds to a 10 MHz ultrasound in tissue (c=1540 m/s)?
λ = c/f = 1540/10e6 ≈ 0.000154 m (154 µm).
402
What is the resolution limit of that ultrasound (NA≈1)?
≈λ/2 ≈ 77 µm.
403
What does MALDI stand for?
Matrix-Assisted Laser Desorption Ionization
404
Which mass analyzer suits MALDI best and why?
Time-of-flight
405
Describe electron impact ionization.
An electron beam strikes gas-phase analyte
406
Describe electrospray ionization.
A charged spray produces droplets that evaporate
407
What is the Lennard-Jones potential shape?
A r⁻¹² repulsive wall at short range and r⁻⁶ attractive well at longer but molecular scales.
408
How does Savitzky–Golay smoothing work?
Fit a low-order polynomial over a moving window and evaluate it at each point; preserves peak shape better than a simple moving average.
409
What does precision vs. accuracy mean?
Precision refers to reproducibility (scatter)
410
What is the function of the suppression grid in a Faraday cup?
To repel secondary electrons back into the cup and prevent under-measurement of ion current
411
Define the collector cup in a Faraday cup.
The main electrode that collects charged particles and delivers their charge to the electrometer
412
What is the typical gain range of a discrete dynode electron multiplier?
10⁶ to 10⁸ electrons per single ion event
413
What causes “dead time” in pulse-counting electron multipliers?
The brief recovery interval (~10–100 ns) after a pulse during which the detector cannot register another ion
414
How is analog mode different from pulse-counting mode in electron multipliers?
Analog mode measures overlapping pulses as a continuous current
415
Write the Abbe diffraction limit formula for resolution.
Δx ≈ λ / (2 NA)
416
How do you calculate the de Broglie wavelength of an electron?
λ = h / (m v)
417
Express electron velocity as a function of accelerating voltage U.
v = √(2 e U / m)
418
What is the relation between X-ray fluorescence probability and atomic number?
Fluorescence yield ω increases roughly as Z⁴
419
Name the three main interactions contributing to X-ray attenuation.
Photoelectric absorption
420
At what approximate depth does an Auger electron originate in SEM?
Within the top ~1–5 nm of the sample surface
421
What detector is typically used for high-sensitivity FTIR measurements?
MCT (Mercury Cadmium Telluride) detector
422
What is the principle of DTGS detectors in IR spectroscopy?
Pyroelectric effect: generates charge when the detector crystal is heated by IR light
423
How does a silicon drift detector (SDD) improve X-ray spectrometry?
By rapidly collecting charge with a small anode and achieving superior energy resolution
424
What parameter does the numerical aperture (NA) of an objective depend on?
The refractive index n of the medium and the half-angle θ of the light cone (NA = n·sin θ)
425
Define the Bulk region in the electrical double layer model.
The region far from the electrode where the electric field is negligible and net charge is balanced
426
What is the physical origin of London dispersion forces in AFM?
Instantaneous dipole–induced dipole attraction between tip and surface atoms
427
What is the principle of cyclic voltammetry scan reversal?
Reversing the sweep direction when a set potential limit is reached to probe redox reversibility
428
How is the HOMO–LUMO gap determined in STS?
By measuring the voltage separation between the first negative-bias and positive-bias dI/dV peaks
429
What causes charging artefacts in SEM images?
Accumulation of beam-deposited electrons on an insulating sample due to insufficient grounding or coating
430
Write the Lennard-Jones potential equation.
V(r) = 4 ε [(σ/r)¹² – (σ/r)⁶]
431
What is the approximate de Broglie wavelength of an electron accelerated at 100 kV?
~0.0037 nm
432
From what sample depth do characteristic X-rays originate in SEM?
Approximately 1 µm
433
What is the principle of current detection in nanopore sequencing?
DNA bases translocate through a nanopore and transiently block ionic current
434
What is the Beer–Lambert law for X-ray attenuation?
I = I₀ e^(–μ x)
435
What is Compton scattering in the context of X-rays?
Inelastic scattering of X-ray photons by electrons
436
What is photoelectric absorption in X-ray interactions?
Complete absorption of an X-ray photon by an atom
