01: analytical concepts

assay

process of determining the amount of an analyte in a sample

analyte

chemical substance being measured

qualitative analysis

concerned with the identification of the elements, ions, or compounds present in an unknown sample

quantitative analysis

concerned with the determination of the quantity of one or more components of the sample

voltage

electrical potential energy (per unit charge) measured as the electric potential difference between two points

current

rate of flow of charge past a point in a circuit (typically electrons moving in a conductor)

transducer

device that converts an input stimulus (e.g., light, heat, pressure) into an electrical output

signal

measured quantity that is correlated to the amount of analyte

noise

unwanted variation in a measured quantity

signal-to-noise ratio (S/N)

magnitude of the signal divided by the magnitude of the noise

polygraph

device or procedure that measures and records physiological indicators while a perso is asked and answers a series of questions

background

approximately constant signal measured in the absence of analyte

detection limit

amount of analyte that corresponds to a signal just greater than the mean of the background plus three standard deviations of its noise

sample matrix

all the components of a sample except the analyte

blank

a ‘man-made’ sample that lacks the analyte, but otherwise contains the solvent, reagents, etc., used in the analysis

positive control

standard sample that contains a known quantity of the analyte of interest

negative control

standard sample that does not contain any analyte

interference

specific chemical substance in a sample matrix that causes a systematic error in a measured quantity

selectivity

extent to which other substances interfere with the determination of an analyte

masking agent

reagent that prevents one or more components in a sample matrix from interfering with an analysis

accuracy

closeness of a measured value to the “true value”

absolute error

difference between the measured and true value

relative error

error in a measurement, expressed as a percentage

precision

reproducibility between replicate measurements

replicates

samples from the same source, run using the same method, under the same conditions, and expected to give the same result in the absence of error

random/indeterminate errors

• introduce uncertainty

• symmetric about the true value

• treat with statistics

systematic/determinate errors

• introduce bias

• measured value is always higher or lower than its true value

• can be proportional or constant

instrument errors

• minimize with careful, regular calibration

• voltage drift

• can usually be corrected

method errors

• chemistry does not behave as expected

• most difficult to identify and correct

• incomplete reactions

• interference from non-analytes

• false positive/negative results

personal errors

• incorrect recording of data

• deviations from an established method

population

all possible measurements that are of interest

sample

limited number of measurements that are representative of the population

mean/average

approximates the true value

deviation

difference between a measured value and the mean value of all measurements

degree of freedom

number of independent measurements

confidence interval

range of values, centered on the sample mean, that have a certain probability of including the true population mean

t statistic

permits use of sample data to test hypotheses about unknown population means without knowledge of the population standard deviation

significance test

determines whether the difference between two or more values is too large to be explained by indeterminate error

null hypothesis

postulates that two observed quantities are the same

F-test

used to compare the precision of two sets of measurements

G-test

used to exclude an outlier from a data set

significant figures

reflection of a measurement’s magnitude and uncertainty

standard

solution with known concentration

least squares method of analysis

minimizes the residuals between the data points and the line of best fit

calibration curve

plot of a measured signal versus a known quantity

sensitivity

slope (m) of the calibration curve

dynamic range

concentration range over which the calibration curve is analytically useful

matrix effect

combined effect of all non-analyte components in a sample on the quantitative measurement of the analyte

internal standard

intentionally added substance of known quantity that is not expected to be found in the sample (different than the analyte), but is expected to behave similarly

sampling

process by which a sample population is reduced to a size suitable for laboratory analysis