Analytical Toxicology Flashcards
What are the steps in undertaking an analytical toxicology investigation?
Pre analytical
Analytical
Post analytical
What happens during the pre analytical step?
Obtain details of suspected poisoning episode
Obtain the patients medical and occupational history
Decide the priorities for an analysis
What happens during the analytical step?
Perform the agreed analysis
What happens during the post analytical step?
Interpret the results
Questions asked when conducting analytical toxicology?
Which sample should be collected/is relevant?
How should it be interpreted?
Which analytical technique should be chosen?
Qualitative and quantitative analysis - screening tests or target analysis?
Disposition of xenobiotics in the body - look for parent compound or metabolite?
What is phase 1 metabolism?
Functionalisation - chemical modification of the original xenobiotic molecule by oxidation, reduction or hydrolysis (may activate the compound)
What is phase 2 metabolism?
Conjugation - conjugation reactions, in which a second, hydrophilic molecule such as glucuronic acid is added to the molecule (toxic effects will be lower change structure of molecule)
What is phase 3 metabolism?
The products of phase 2 biotransformations may be father metabolised in what is sometimes termed phase 3 reactions
What is the point of metabolism?
Makes the molecule more polar so easier to be excreted by urine, half life is shorter
What is metabolism catalysed by?
Enzymes
Examples of phase 1 metabolism?
Oxidation, reduction, hydrolysis, hydration, dehalogenation
Examples of phase 2 metabolism?
Sulphation, glucuronidation, glutathione conjugation, acetylation, amino acid conjugation, methylation
What is the organic solvent water partition coefficient P?
P - ratio of the concentration of unionised compound X in organic solvent ( eg octane, chloroform) to the concentration of compound X in water
What is P a measure of?
The hydrophobicity and hydrophilicity of chemicals - higher partition to organic solvent higher value of P and higher hydrophobicity for unionised compounds only, most drugs and toxins will be charged at particular conditions
What is the organic solvent water partition coefficient D?
D - ratio of the sum of the concentrations of all forms (ionised and unionised) of the compound X in organic solvent (eg octanol, chloroform) in each of the two phases - higher partition organic solvent higher D value higher hydrophobicity
What is logD dependent on?
pH dependent, hence one must specific the pH at which the logD was measured
log P = log D at any pH for unionisable compounds
What is logKoW?
Octanol water partition coefficient - value changes on pH for charged molecules focus on particular pH, if in non charged state, probability of partitioning into organic state higher
Absorption values for high P values?
High absorption happens for higher P values - more hydrophobic compounds easily transported and distribution round the body
Characteristics of lipophilic xenobiotic substance?
Rapidly absorbed
Rapidly and widely distributed in body tissues
Extensively metabolised before it is excreted
What is the acid dissociation constant Ka?
Equilibrium constant for the following reaction of a weak acid HA, the smaller the value of pKa the stronger the acid
What is pKa?
pKa is defined as the pH where chemical exists as 50% ionised and 50% unionised
When are acids most highly ionised?
Acids are most highly ionised at high pH (pH > pKa)
When are bases most highly ionised?
Bases are most highly ionised at low pH (pH < pKa)
What information do dissociation constants give on the fate of chemicals?
Living organisms (sorption, distribution, metabolism and excretion) Environment (sorption, distribution, bioaccumulation)
What do rapid screening techniques provide?
Rapid analysis, no sample preparation required, low sensitivity and selectivity, usually allow for detection only of any drugs/poisons in the samples
What do advanced analytical techniques provide?
Takes time, require sample preparation, very sensitive and selective, allow for identification and quantification of drugs/poisons
What are examples of rapid screening techniques?
Colour tests, immunoassay techniques
What are examples of advanced analytical techniques?
HPLC, GC, MS, NMR
Steps in analytical procedure?
Sample collection, sample preservation, sample preparation, sample analysis
What happens during sample collection?
Need to collect a representative sample, when sample is collected need to decided how much is needed, more than one sample needed, mix together to make composites, needs to be as limited as possible any intervention could lead to errors in analysis
What happens during sample preservation?
To ensure that sample retains its physical and chemical characteristics, acidify or freeze sample so microbes won’t be active, often compounds we wan aren’t stable they are subject to hydrolysis and photochemical reactions or metabolise to something else, so we need to make sure time between collections and preparation is as short as possible, preserve to make sure sample is stable
What happens during sample preparation?
To separate analytes from the matrix to improve the analysis and detectability (eg homogenisation, extraction, concentration, clean ups hydrolysis of conjugates, derivatisation), often samples aren’t in the right form for analysis analytes need to be in the correct matrix, structure of molecules and matrix is changed to make it amenable for detections helps with cleaning up matrix, biggest issue is too many interferences affecting sample - so remove interferences to get nice analysis
What happens during sample analysis?
Qualitative (colour tests, immunoassays) and quantitative (big machines such as NMR etc) analysis
What is important about the steps in analytical procedure?
Not all steps many not be required for all tests, depends on the instruments used, sample collected, analytes wanted, and so these all require different steps
Steps for extraction?
Extraction, concentration and clean up
What happens during extraction?
To present samples directly to the chromatographic system
What happens during concentration?
To increase the concentration of analytes in the extract - (machines cannot detect concentration of collected sample so often have to get analytes out of matrix into another solution and concentrate it, increase concentration of analyte increase choice of success of analytic process
What happens during clean up?
To remove potentially interfering matrix compounds (particularly important for GC LC/MS) solvent and analytes give best results but often not the case since interferences often sneak through, clean up will try to reduce this to give nice measurement
Extraction procedures for non/semi volatile compounds from liquids?
Liquid liquid extraction
Solid phase extraction
Solid phase microextraction
What happens in liquid liquid extraction?
Two immiscible liquids put two together shake, non polar parts partition into organic phase, simple but limited since polar compounds unlikely to partition to organic phase
Extraction procedures for non/semi volatile compounds from solids?
Soxhlet extraction Ultrasonic extraction Supercritical fluid extraction Accelerated solvent extraction Microwave assisted extraction
Whats different about accelerated solvent extraction?
It is accelerated by high pressure and temperature
Extraction procedures for volatile compounds from solids and liquids?
Static headspace extraction
Dynamic headspace extraction - purge and trap
Solid phase micro extraction
How are oxazepam and temazepam metabolised and excreted?
Oxazepam and temazepam are conjugated (sulphate and glucuronide conjugates) and excreted in urine at almost the same rate as they are generated
How does pH control metabolism?
pH controls how, why and where different molecules are distributed so acidic molecules will be absorbed in basic conditions, basic molecules will be absorbed in acidic conditions
How to prove administration of cocaine?
To prove administration of cocaine look for benzoylecogonine (an inactive compound), logKow goes down, OCH3 goes to OH makes it more amenable for excretion with urine
Why won’t heroin be found in urine?
Most hydrophobic molecule so very quickly metabolised so won’t find this in urine
Importance of 6-acetylmorphine in metabolism of heroin?
Not all metabolites are useful, sometimes have to use minor metabolites such as 6-acetylmorphine, only 1% of heroine gets transferred into this - really powerful tools needed to find this
Why is morphine not the compound of interest to prove administration of heroin?
Cannot be compound of interest since could be present due to morphine or codeine consumption, can’t prove heroine abuse
