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
