Unit 1 - Analytical Concepts and Statistics Part 1

Question 1

Assay

Answer 1

the process of determining the amount of an analyte in a sample

ex. obtaining copper from a rock

Question 2

Analyte

Answer 2

chemical substance being measured

Question 3

Qualitative analysis (what is in the sample)

Answer 3

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

Question 4

Quantitative analysis (how much is in the sample)

Answer 4

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

ex. measuring the concentration of glucose in blood

Question 5

Signal

Answer 5

a measured quantity that is correlated to the amount of analyte

Question 6

Noise

Answer 6

unwanted variation in a measured quantity. Often takes the form of random fluctuations in a measured signal

Question 7

Signal-to-noise ratio (S/N)

Answer 7

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

Question 8

Background

Answer 8

an approximately constant signal, measured in the absence of the analyte

Question 9

Detection limit

Answer 9

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

Question 10

S/N is proportional to the sqrt(n), where n is the number of measurements

Answer 10

Multiple scans can be acquired and averaged to increase S/N, which give clearer results (less noise). Noise adds randomly

Disadvantage: time-consuming

Question 11

Sample matrix

Answer 11

contains all the components of a sample except the analyte. In many cases, we won’t know all the components

Question 12

Blank

Answer 12

“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

Question 13

Positive control

Answer 13

contains a known quantity of the analyte of interest. Helps prevent false negative results

used to asses test validity

Question 14

Negative control

Answer 14

does not contain any analyte. Helps prevent false positive results

used to asses test validity

Question 15

Interference

Answer 15

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

Question 16

Selectivity. Define good and bad selectivity

Answer 16

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

Question 17

Masking agent

Answer 17

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

Question 18

Types of interferences (3)

Answer 18

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

Question 19

Accuracy

Answer 19

how close the measurement is to the true value

Question 20

Precision

Answer 20

how close a set of measurements are. Usually expressed using the standard deviation

Question 21

Absolute error

Answer 21

difference between the measured and true value. Can be positive or negative

Question 22

Relative error

Answer 22

error in a measurement expressed as a percentage

Question 23

Replicates

Answer 23

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

Question 24

Random/Intermediate errors

Answer 24

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

Question 25

Systematic/Determinate errors

Answer 25

introduces bias. Value is consistently higher or lower than the true value. Something wrong with the instrument itself

Question 26

Types of systematic errors

Answer 26

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

Question 27

Proportional error (relative error)

Answer 27

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

Question 28

Constant error (systematic error)

Answer 28

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

Question 29

Gaussian distribution

Answer 29

“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

Question 30

Population

Answer 30

all possible measurements that are of interest

Question 31

Sample

Answer 31

a limited number of measurements that are representative of the population

Question 32

Deviation

Answer 32

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

Question 33

Standard deviation and relative standard deviation

Answer 33

a measure of the uncertainty and precision associated with a measurement

Question 34

Degree of freedom

Answer 34

the number of independent measurements

Question 35

Visual detection advantages and disadvantages

Answer 35

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

Question 36

Electrical detection advantages and disadvantages

Answer 36

advantages: objective, often more sensitive, often faster, automation, analyze smaller sample volumes, can be portable
disadvantages: more costly, maintenance and repairs, calibration required

Question 37

Transducer

Answer 37

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

Question 38

Voltage

Answer 38

electrical potential energy (per unit charge). Measured as the electrical potential difference between two points

Question 39

Current

Answer 39

the rate of flow of charge past a point in a circuit (typically electrons moving in a conductor, but not always)