thinking like a scientist Flashcards
(38 cards)
Accuracy and Precision
Accuracy is a measure of how close a single measurement from a set is to the accepted true value.
Precision measures the closeness with which repeated analyses of given samples agree with each other. It is usually expressed as a standard deviation, which is a quantitative measure of the spread of a set of measurements.
Accuracy and Precision
Accuracy is a measure of how close a single measurement from a set is to the accepted true value.
Precision measures the closeness with which repeated analyses of given samples agree with each other. It is usually expressed as a standard deviation, which is a quantitative measure of the spread of a set of measurements
Screening and diagnosis
SCREENING TEST :
used to identify those individuals who are a high enough risk (in a population) to warrant diagnostic processes which are too expensive to offer everyone
Screening is also offered where a specific population may be at risk of a particular disease - subgroup of the population who is at a greater risk of a disease has been identified
diagnosis:
is facilitated using a more accurate test or group of tests or procedures. These diagnostic tests will confirm whether an individual has that disease or not.
Qualitative and quantitative tests
QUALITATIVE TEST :
identifying what is present in a sample, or determining whether a particular compound is present or not.
This is called a qualitative test, which can be defined as “a test which is concerned with only the nature of the substance under investigation and not its amount or concentration”
In qualitative tests colour changes or a visual clue is often used to indicate whether a substance is present or not.
QUANTITATIVE TEST : which is “concerned with the amount or concentration of the substance(s) under investigation”
An example is the pregnancy test kit which detects the presence of a hormone in urine.
REMEMBER :
It is always important to quote the units of a value when reporting quantitative results. The international system of SI units is frequently used, but not always. In many applications, many different units are used, depending on the context
QUALITATIVE ANALYSIS :
a presumptive test
a confirmatory test
A presumptive test is a qualitative analysis that confirms that (a) the sample is definitely NOT a specific thing, or (b) that a sample is probably a specific thing.
A confirmatory test is one that confirms the original finding. tend to cost more
Selectivity, specificity and sensitivity
Selectivity:
the ability to discriminate between the target analyte and other constituents of the sample which may interfere with the analysis
Specificity means an analytical method is perfectly selective for an analyte or a group of analytes
Sensitivity is defined as the ability to discriminate between small differences in the concentration of an analyte in a sample, or the ability to detect (qualitative analysis) or determine (quantitative analysis) very small amounts of analyte
An analyte
An analyte is the substance or component you want to analyse.
For example, if an analytical method can distinguish one molecule in a mixture of similar molecules, it is selective for that molecule
The more similar the concentrations of analyte that can be distinguished, or the lower the amount of analyte that can be detected or determined, the higher the sensitivity of the technique.
false positives
false negatives
false positive:
the analyst needs to know how many times the test returns a positive result when the sample is actually negative
a false negative:
sometimes the test will produce a negative result when the sample is actually positive
The terms ‘sensitivity’ and ‘specificity’ are used to describe this too;
standard and calibration
standard, which is “an analyte of known concentration”
Calibration is a set of operations that establishes, under specified conditions, the relationship between an instrument response and the corresponding values of a standard
doing a calibration
Calibration can be done in many ways but a common method is the external calibration procedure. This is done by using several chemical standards made up to a known concentration across the concentration range of interest. Ideally, more than one standard is made up for each concentration to obtain replicate values.
A calibration profile by convention is plotted with the instrument reading for each concentration on the vertical (y) axis and the standard concentrations on the horizontal (x) axis. The calibration standards are measured in the analytical instrument under the same conditions as those subsequently used for the unknown samples. The analyte concentrations in the real samples are then obtained from the calibration graph.
Limit of detection (LOD) –
this is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) within a stated confidence limit (generally 1%).
Limit of quantification (LOQ) .
this is the lowest concentration that can be identified and quantitatively measured using an analytical method that has been validated with specified accuracy and precision. The two differ in that, while the LOD is the ultimate lower limit, the LOQ considers potential ‘noise’ in the system (potentially generated by errors in measuring).
Limit of linearity (LOL) –
this is the concentration at which the calibration curve departs from linearity by a specified amount. A deviation of approximately 5% is usually considered the upper limit. This is common at higher concentrations. After this point, the relationship between generated signal and concentration is no longer predictable.
health and safety
There are scientific principles underlying safe practice procedures and the equipment used in any professional practice
able to identify sources of hazard in the workplace and to make evidence-based judgements about whether an intervention is potentially more beneficial than harmful. It is important to consider how to minimise risks by selecting the appropriate strategies for hazard control and risk management.
hazard
A hazard is defined as the source of something that may cause harm, damage or adverse health effects; risk is the probability that someone may be harmed after being exposed to a hazard
Workplace activities in the UK are subject to health and safety legislation. They are covered under Section 7 of the Health and Safety at Work, etc. Act 1974 which states that:
It is the duty of every employer, so far as is reasonably practicable, to ensure the health, safety and welfare at work of all employees.
It is the duty of every employee while at work to take reasonable care of himself (or herself) and of other persons who may be affected by his (or her) acts or omissions at work.
The employees must cooperate with the employer with regard to health and safety procedures.
personal protective equipment (PPE)
regulations, the Control of Substances Hazardous to Health (COSHH)
Regulations and the Reporting of Injuries, Diseases and Dangerous Occurrences Regulations (RIDDOR)
The essential requirements of COSHH are that the employer must:
• Assess the health risk to employees;
• prevent or control the exposure of employees to hazardous substances.
Whether the work is done in a laboratory or in the field, the analytical scientist is protected under this legislation, provided he or she follows some basic dos and don’ts:
Laboratory coats or protective clothing must be worn at all times.
When handling chemicals, biological samples or ‘sharps’ (i.e. any sharp object that can cause injury, particularly to the hands), observe good laboratory practice by wearing gloves. Latex gloves were widely used but are less common now because of the risk of serious allergic reactions. Nitrile gloves are now the most widely used, depending on the application.
There should be no eating, chewing of gum, drinking, smoking or applying of cosmetics in any laboratory when sample collecting, preparation or analysis is under way.
Do not pipette (draw up a liquid by suction) by mouth; always use a pipette filler.
All broken glass must be placed in a labelled bin
biological hazards
chemical hazards
Physical hazards
radioactive substances
biological hazards
blood, urine, faeces, dangerous pathogens (a pathogen is a disease-causing organism).
chemical hazards are toxic, corrosive or harmful chemicals which may be inhaled, ingested or come into contact with skin.
Physical hazards include high voltages from some analytical instruments or even the normal household electricity supply, which can deliver a fatal current.
Each organisation using radioactive sources will probably have local rules about the procedures for ordering, storage, handling, disposal and record keeping.
Radiation laboratories are controlled areas with specific access requirements. All radioactive material must be accounted for as the loss of material can result in prosecution. Secure containment is required by law, to prevent reagents being stolen, and to protect everyone working in the laboratories
QUALITY
Quality is an important concept in the standardisation of methods, i.e. ensuring methods work according to a particular defined standard or method.
quality can be defined as the body of characteristics, properties, attributes or abilities of an entity (an object, a system or a service) that make it better or worse than, or equal to, other entities of the same type
The International Organisation for Standardisation (ISO) defines quality as ‘the totality of characteristics of an entity that bear on its ability to satisfy stated and implicit needs imposed by a client or through legislation’. Consequently, quality also involves usefulness, fulfilling requirements and satisfying clients’ needs
In terms of the results obtained in the analytical laboratory, quality does not necessarily mean getting the most accurate results; quality in this context means matching the service with the requirements of the client. This is commonly described as results that are ‘fit for purpose’.
METHOD
A method is a set process that if followed will ensure accurate and precise results.
Quality in the analytical laboratory is achieved by providing analytical results that:
- meet the specific needs of the client
- attract the confidence of the client
- represent good value for money.
Quality assurance (QA)
describes the overall measures that an analytical laboratory uses to ensure the quality of its service. It focuses on providing confidence that quality requirements will be fulfilled and reliable analytical measurements will be produced.
Including staff training, record keeping, ensuring a suitable laboratory environment (such as adequate storage facilities to ensure the integrity of samples or filtered air conditioning to stabilise temperature and humidity and reduce dust levels), maintenance and calibration schedules
for instruments, the use of validated and documented methods, and preventive and corrective actions.
Quality control (QC
involves examining the laboratory and its analytical results. QC encompasses the planned activities designed to verify the quality of the measurement and ensure that the analytical processes are working correctly
There are two types of QC:
• internal QC, which provides confidence to the laboratory staff carrying out the measurements
• external QC, also known as external quality assessment, which provides confidence to the client by comparing the performance of the laboratory with others making similar
measurements.
Although serious flaws in laboratory processes are sometimes highlighted, they are comparatively rare considering the number of tests that are carried out in laboratories every year.
