Unit 1 - Analytical Concepts and Statistics Part 1 Flashcards
Assay
the process of determining the amount of an analyte in a sample
ex. obtaining copper from a rock
Analyte
chemical substance being measured
Qualitative analysis (what is in the sample)
identification of elements, ions, or compounds present in an unknown sample
ex. screening an athlete’s urine for the presence of a performance-enhancing drug
Quantitative analysis (how much is in the sample)
determination of the quantity of one or more components of the sample
ex. measuring the concentration of glucose in blood
Signal
a measured quantity that is correlated to the amount of analyte
Noise
unwanted variation in a measured quantity. Often takes the form of random fluctuations in a measured signal
Signal-to-noise ratio (S/N)
the magnitude of the signal divided by the magnitude of the noise. Compares the level of a desired signal to the level of background noise
Often referred to as “signal-to-background ratio” but are not always equivalent
Background
an approximately constant signal, measured in the absence of the analyte
Detection limit
the amount of analyte that corresponds to a signal just greater than the mean of the background plus three standard deviations of its noise
S/N is proportional to the sqrt(n), where n is the number of measurements
Multiple scans can be acquired and averaged to increase S/N, which give clearer results (less noise). Noise adds randomly
Disadvantage: time-consuming
Sample matrix
contains all the components of a sample except the analyte. In many cases, we won’t know all the components
Blank
“man-made” sample that lacks the analyte, but contains the solvent, reagents, etc. used in analysis. It tried to approximate the sample matrix. Commonly used in analytical experiments
Positive control
contains a known quantity of the analyte of interest. Helps prevent false negative results
used to asses test validity
Negative control
does not contain any analyte. Helps prevent false positive results
used to asses test validity
Interference
a specific chemical substance in a sample matrix that causes a systematic error in a measured quantity
Selectivity. Define good and bad selectivity
the extent to which other substances interfere with the determination of an analyte (typically via reactivity/molecular interactions)
good selectivity - analysis method has minimal interferences
bad selectivity - analysis method prone to certain interferences
Masking agent
a reagent that prevents one or more components in a sample matrix from interfering with an analysis
Types of interferences (3)
act on the analyte - a matrix component also complexes the analyte
act on a reagent used in the detection method - a matrix component is also bound by the reagent
be the source of a large background signal - a matrix component absorbs the same wavelength of light as the analytical complex
cause negative or positive bias
cause absolute or proportional errors
Accuracy
how close the measurement is to the true value
Precision
how close a set of measurements are. Usually expressed using the standard deviation
Absolute error
difference between the measured and true value. Can be positive or negative
Relative error
error in a measurement expressed as a percentage
Replicates
samples from the same source, run using the same method, under the same conditions, and expected to give the same result if there are no errors
Random/Intermediate errors
introduces uncertainty and is symmetric about the true value. Can randomly be above or below the true value, but it averages around the true value. Can be treated with statistics
Systematic/Determinate errors
introduces bias. Value is consistently higher or lower than the true value. Something wrong with the instrument itself
Types of systematic errors
1) instrument errors: minimize with calibration and can usually be corrected
2) method errors: chemistry does not behave as expected. Reactions can be incomplete. Can be interferences from non-analytes. Can produce false/positive results
3) personal errors: incorrect recording of data. Deviations from an established method
Proportional error (relative error)
when a system measures something higher or lower than the true value where the error is dependent on the magnitude of the measurement
ex. a system is always measuring voltage 10% higher than the true value
Constant error (systematic error)
when a system always measures an x amount above or below the true value. Independent of the magnitude of the measurement
ex. always measuring 2mV lower than the true value
Gaussian distribution
“bell curve” shape and peak position is determined by the mean. Width and height is determined by the standard deviation. Total area under` the curve is 1
approx. 68% falls within 1 SD and approx. 98% falls within 3 SDs
Population
all possible measurements that are of interest
Sample
a limited number of measurements that are representative of the population
Deviation
the difference between a measured value and the mean value of all measurements
Standard deviation and relative standard deviation
a measure of the uncertainty and precision associated with a measurement
Degree of freedom
the number of independent measurements
Visual detection advantages and disadvantages
advantages: simple, low-cost, no maintenance
disadvantages: subjectivity affects accuracy and precision; may not be very sensitive; may require large sample volumes; often time-consuming; low-throughput
Electrical detection advantages and disadvantages
advantages: objective, often more sensitive, often faster, automation, analyze smaller sample volumes, can be portable
disadvantages: more costly, maintenance and repairs, calibration required
Transducer
a device that converts an input stimulus (ex. heat, light, pressure) into an electrical output
Voltage
electrical potential energy (per unit charge). Measured as the electrical potential difference between two points
Current
the rate of flow of charge past a point in a circuit (typically electrons moving in a conductor, but not always)
