Analytical Chemistry Flashcards
Analyte
A substance whose chemical constituents are being identified and measured
Representative sample
A sample, resulting from a sampling plan, that can be expected to adequately reflect the properties of interest in the parent population. Or a sample of which has an identical composition to the bulk material, batch, or lot, from which it has been taken
Test sample
A much smaller, but still representative, subsample with an often finer particle size, from which the test portions are selected for specific analyte determinations
Validation
The process of determining the performance characteristics of a method to establish whether the analytical results obtained will be fit for their intended purpose
Matrix
The components of a sample other than the analyte of interest
Calibration
The measurement of standard solutions or materials in order to construct a calibration curve
Confidence limit
Values greater than or less than the average, between which the results are statistically expected to fall a given percentage of the time
Accuracy
The closeness of a test result to an accepted reference value
Precision
The reproducibility of a series to replicate measurements obtained under comparable analytical conditions
Interference
A substance, other than the assayed material, that can be measured by the chosen analytical method or that can prevent the assayed material from being measured
Errors
The difference between the measured value and the true/known value
Detection limit
The lowest concentration of the analyte that can be reliably detected
Limit of quantification
The lowest analyte concentration that can be quantitatively detected with a stated accuracy and precision
Sensitivity
The change in signal per unit change in the amount of analyte
Standards
Materials containing a precisely known concentration of a substance for use in quantitative analysis
Separation
A method which converts a mixture or solution of chemical substances into two or more distinct product mixtures
Qualitative
Determines the presence or absence of a particular compound/element
Quantitative
Determines the amount of a particular compound/element
What are the six steps to a solution?
- Defining the problem
- Sampling
- Sample preparation
- Measurements
- Evaluation
- Analytical information
Trace analysis
refers to a measurement of a compound that is of very low concentration, leading to difficulties in the analysis
When defining the problem, what are the 9 most important questions to ask?
- Where was the sample obtained?
- Why do you want this tested? Is contamination expected?
- What is the analyte?
- Is it a representative sample?
- Is this a qualitative or quantitative analysis?
- What is the physical state of the sample?
- What is the sample matrix?
- What is the accuracy and precision of the data?
- Is the measurement bulk or trace analysis?
What are the 4 main risks associated with sample analysis?
- The sample may be contaminated
- Sample container may be contaminated
- Risks associated with sample collection
- Risks with analytical procedure
Sample integrity
Sample properties must be stable after sampling and during storage
Random sampling
Samples collected at random locations to produce a representation of the entire sample
Selective sampling
At least two samples, one of a selected area and one from away from the first to determine background levels (control sample)
What is the best material for storing evidence, especially with trace analysis? Why not others?
The best material is polyethylene or teflon containers which have been pre-treated which acid to remove metal ions
Not metal ions in solution as can absorb onto the surface of the container, or leach from the to solution
Not glass bottles as metal ions absorb onto surface Si-OH sites
Not plastics as may contain softeners, such as cadmium
What are four things which maintain sample integrity?
- Special sample bags must be used at fire scenes to avoid sample degradation and loss of sample components
- Biological samples should be frozen and stored in the dark
- Tamper-proof seals on containers to ensure sample integrity and prevent contamination
- Transport of samples to the lab or between labs must be secure
What must all reagent be in trace analysis?
Scrupulously pure
What must solid samples be?
Homogeneous and representative
Define direct solid analysis and give the advantages and disadantages
Definition: limited sample preparation
Advantages: fast and low cost of sample preparation, limited risk of contamination
Disadvantages: not suitable for all analytical techniques
What are the four different drying techniques?
Air drying, thermal drying, rotary vaporisation, freeze drying
What are the advantages and disadvantages of size reduction with solid samples?
Advantages: increases surface area, increased reactivity, work with smaller samples
Disadvantages: change physical properties, looses of volatile compounds, phase transformations, dissolution/extraction process can be accelerated
Define digestion
Dissolution (break down of a solid to a liquid) of a sample
What are the advantages and disadvantages of having liquid samples?
Advantages: more homogenous than solids
Disadvantages: can be difficult to prepare
What are the advantages an disadvantages of having gaseous samples?
Advantages: trapping/stabilisation without further sample preparation, direct measurement at the source, direct adsorption/collection at site for later measurements
Disadvantages: high reactivity
What are the units of molarity (c)?
moles/L
What are the units of molarity (m)?
moles/kg
What are the advantages and disadvantages of working with molarity (c)?
Advantages: works with volumes which are easy to measure
Disadvantages: volumes change with temperature
What are the advantages and disadvantages of working with molarity (m)?
Advantages: temperature is independent
Disadvantages: all materials need to be weighed
An environmental chemist has collected gas samples close to the chimney at an industrial site and would like to detriment the composition of the gas and the concentration of the individual components. How should they separate the different gases from each other?
Gas chromatography
Before running a sample in the Atomic Emission Spectroscopy (AES) a student is asked to ensure there are no solid particles left in the solution. This is a water sample collected from an old well. How should the student separate any particulates from the aqueous matrix before doing the measurements?
Filtration as it separates particles from liquid
Is it TRUE or FALSE that combining several different analytical techniques is advantageous when performing an analytic procedure?
True - combining several methods gives additional/different information
Is it TRUE or FALSE that Beer-Lambert Law is associated with Spectroscopic techniques and can be applied to calculate the concentration of a sample?
True - it measures the absorbance of a sample to aid in calculations
Gas chromatography
Involves separation and analysis of different constituents of mixtures by mobile gas phase passing over a stationary adsorbent
Distillation
A separation technique used to separate liquid from a mixture and keep the liquid part
High performance liquid chromatography (HPLC)
A liquid sample is injected into a stream of solvent (mobile phase) flowing through a column packed with a separation medium (stationary phase). Sample components separate from one another by process of differential migration as they flow through the column
Condensation
A type of chemical reaction whereby two molecules are combined to form a single molecule, usually with the loss of water
Rotary vaporisation
The process of reducing the volume of a solvent by distributing it as a thin film across the interior of a vessel at elevated temperature and reduced pressure
Filtration
The process on which solid particles in a liquid or gaseous fluid are removed by the use of a filter medium that permits the fluid to pass through put retains the solid particles
Liquid chromatography
Separates molecules in a liquid mobile phase using a solid stationary phase
Titration
The slow addition of one solution of a known concentration to a known volume of another of unknown concentration until the reaction reaches neutralisation
Electrophoresis
A laboratory technique used to separate DNA, RNA, or protein molecules based on their size and electrical charge
NMR
Used for determining the content and purity of a sample as well as its molecular structure
X-ray diffraction
A versatile non-destructive technique used to analyse physical properties, such as phase composition, crystal structure and orientation of powder, and solid and liquid samples
Raman spectroscopy
A technique which involves the inelastic scattering of light to analyse molecular vibrations, providing information on the structure and composition of molecules
Atomic emission spectroscopy (AES)
A method of chemical analysis that uses the intensity of light emitted from a flame, plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample
Infrared spectroscopy
The measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection
Atomic absorption spectroscopy (AAS)
Detects elements in either liquid or solid samples through the application of characteristic wavelengths of electromagnetic radiation from a light source
What is a Ksp value?
The solubility product constant (or the solubility constant) for a solution at equilibrium
What are the advantages of using dye based spectrophotometric methods over a titration?
More sensitive and require less samples
Describe the process of extracting lactose and casein from milk
- Add acid whilst slowly stirring
- Observe thick white precipitate
- Filter to form a thick white precipitate of casein
- Add 1-2 scoops of calcium carbonate to the liquid from the filtration
- Heat solution to boil
- Filter
- Boil the resultant clear solution until it becomes 1/4 of the total volume
- Add 2 scoops of activated charcoal to decolourise and mix
- Add 100ml of methanol or ethanol and mix
- Filter
- Set aside the resultant clear solution under the sun to evaporate
- Observe the solid casein and lactose
What is Q?
The ion product
What does it mean if Q = Ksp?
The solution is saturated/at equilibrium
What does it mean if Q>Ksp?
The solution is supersaturated/precipitation will occur
What does it mean if Q<Ksp?
The solution is unsaturated/precipitation will not occur/possible to dissolve more solid
What is Le Chatelier’s principle?
A system at equilibrium that is subjected to a change responds to the changed to partly counteract the change and establish a new equilibrium
What is the salting-in effect?
Adding another ionic species will increase the ionic strength of the solution, and so the solubility will increase
What is the salting-out effect?
If the concentration of salt increases too much, solubility decreases again
What does the common ion effect do to the solubility of solids?
Decreases it
What happens to the data when a systematic error occurs?
It is all shifted in one direction
What two types of information does the output of every analytical measurement contain?
- About the analyte: signal
- About the random fluctuations in the characteristics of the instrument: noise
Is it more beneficial if the S/N ratio is high or low?
High
What does a high noise level imply?
A large random error, such as poor precision
What is the purpose of a blank?
To check if the instrument is clean
Define limit of detection (posh)
The disappearance of the signal into the noise as the concentration of the analyte approaches zero
Define sensitivity
Ability to discriminate between small differences in analyte concentration at a particular concentration
Define calibration sensitivity
The slope of the calibration curve at the concentration of interest
Define limit of detection (not posh)
Minimum concentration that can be detected at a known confidence limit
Define comparative control sample
Material similar to the experimental value, but taken from elsewhere and therefore not containing the analyte of interest
Define positive control sample/standard sample
Contains the analyte in a known quantity
Define negative control sample
A sample which should not contain any of the analyte
Where are negative controls taken from?
Within the analysis laboratory, such as swabs from benches or equipment
Why are standard reference samples important?
- For calibration of instruments and methods
- For comparing results from different laboratories
Define internal method of quality assessment
A method that is directly coordinated by the laboratory or analyst to give immediate feedback about the obtained results or method
Define internal standard
Compound used in a mixture with analyte to ensure signal and equipment is within the confidence level of the analyte
Define the mobile phase
A gas or liquid that passes through a column containing the stationary phase
Define the stationary phase
A liquid or solid that doesn’t move and it usually held inside a column
Define retention time
The time taken in minutes for an analyte to elute
What is the equation for retention time?
k = (tr - tm)/tm
Where tm is the time required for the mobile phase to pass through the column
What are the factors affecting drug absorption?
- Acid stability
- Permeability
- Solubility
- Lipophilicity
- Metabolism
Why are un-ionised drugs passively absorbed?
Because the compound has to pass through a lipophilic membrane and this process will be unfavourable for charged molecules
Define active transport
Transport of molecules through a membrane against the concentration gradient by a transporter, requires ATP
Define drug-drug interactions
Effects which arise when multiple drugs are given to a patient
Describe the interaction of drugs with blood proteins
Drugs can bind to macromolecules in the blood, known as plasma protein binding (PPB) or serum protein binding. Compounds with high PPB are retained in the plasma and cannot distribute into the tissues. Only unbound compound is available for distribution into tissues
What are the two categories of metabolism and what do they mean?
Catabolism - breaks down into organic matter
Anabolism - uses energy to build up or construct components of cells such as proteins or nucleic acids
Define endogenous
Anything grown or created within us
Define drug metabolism
Any chemical alteration of a drug by the living system
What are the two phases of metabolism?
Phase I - oxidative transformations
Phase II - conjugation
Define CYP inhibition
Where a drug binds to a particular CYP and inhibits it, thereby preventing it from metabolising other substrates
Define CYP induction
Where a drug leads to the expression of greater levels of a particular CYP
Define prodrug
An inactive compound converted to an active compound in the body
