CHEM 211 Terminology

Lecture

assay

process of determining the amount of analyte in a sample

analyte

the chemical substance being measured

qualitative analysis

correlate signal to what is in a sample

quantitative analysis

correlate signal to how much is in a sample

voltage

electrical potential energy

• measured as the electric potential difference between two points

currect

the rate of flow of charge past a point in a circuit

transducer

device that converts an input stimulus into an electrical output

signal

a measured quantity that is correlated to the amount of analyte

• contains info about the chemical species of interest

noise

unwanted variation in a measured quantity

• random fluctuations in background

  • σBkg

signal to noise ratio (S/N)

magnitude of the signal divided by the magnitude of the noise

background

approximately constant signal (μBkg) 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 3 std. dev. of its noise

• minimum = (+)

• maximum = (-)

sample matrix

all components of a sample except the analyte

blank

man-made sample that lacks analyte but contains solvent and reagents

positive control

standard sample that contains known quantity of analyte of interest

• prevents false negative results

negative control

standard sample that does not contain any analyte

• prevents fast positive results

interference

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

• act on analyte

• act on reagent

• be a source of a large background signal

• cause negative or positive bias

• cause absolute or proportional errors

selectivity

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

• good selectivity = minimal interferences

  • poor selectivity = prone to certain interferences

masking agent

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

• attempts to improve analytical sensitivity and selectivity

• is NOT a source of large background signal

• does NOT reduce personal errors

accuracy

measurement denotes the closeness of an experimental value to the true value

• right on target

precision

set of measurements refers to the agreement among the results

• std. dev.

• all in one spot, may not be target

the reproducibility between replicate measurements

absolute error

difference between 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, expected to give the same result in the absence of error

random / indeterminate errors

• introduce uncertainty

• symmetric about the true value

• treat with statistics

affects precision

systematic / determinate errors

• introduce bias

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

• eg. rxn that did not go to completion

affects accuracy

types of systematic error: instrument

• minimize with careful, regular calibration

• voltage drift

• can usually be corrected

types of systematic error: method

• chemistry does not behave as expected

• most difficult to identify and correct

• incomplete reactions

• interference from non-analytes

• false positive/negative results

types of systematic error: personal

• incorrect recording of data

• deviations from an established method

types of systematic error: proportional

error is proportional to samples

• always measuring % higher than true value

• effect is independent of the magnitude of the measurement

types of systematic error: constant

error is always same in each sample

• always measuring x lower than true value

• is small if measuring small but large if measuring large

gaussian distribution

• used to describe data clustered about a mean value

• continuous and symmetric distribution

• bell curve shape

• peak position determined by mean

• width and heigh determined by std. dev.

• total area under curve is 1

std. dev. of gaussian distribution

1 std. dev. = 68.3%

2 std. dev. = 95.4%

3 std. dev. = 99.7%

population

all possible measurements that are of interest

sample

limited number of measurements that are representative of the population

mean

average

deviation

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

• difference between true and average

std. dev. and RSD

measure of the uncertainty and precision associated with a measurement

propagation of error

find uncertainty after calculations

purpose of t-statistic

permits use of sample data to test hypotheses about unknown population means without knowledge of the population std. dev.

purpose of significance test

to determine whether the difference between 2 or more values is too large to be explained by indeterminate error

null hypothesis

assume true until proven wrong

• postulates the 2 observed quantities are the same

• assume 2 results are the same then apply test to see if null can be statistically rejected

t test: case 1

compare measured value to actual value

• t exp < t table = no significant difference

  • null true

• t exp > t table = significant difference

  • reject null

standard reference materials

• highest-quality materials

• certified and traceable

• carefully analyzed by the supplier

• accurately known composition

• use and sold for a variety of analytical techniques

• prepared and sold by both companies and organizations

p value

probability value of results

• lower = stronger evidence = data is unlikely under null

  • statistically significant

• higher = weak evidence = could easily happen by random chance

  • cannot reject null

t-test: case 2

comparing 2 data sets

• std. dev. is pooled

• compare sample means (replicate measurements)

F-test

compare variances by calculating the ratio of the 2 variances (or std. dev.)

• used to compared the precision of two sets of measurements

t-test: case 3

• paired t-test

• uses paired/matched measurement data

  • eg. comparing single measurement made with 2 methods on several different samples

  • eg. comparing measurements before vs after

• compare methods of single measurements on several different samples

• same number of measurements

Grubbs Test

used to exclude an outlier from a data set

significant figures

reflection of a measurement’s magnitude and uncertainty

significant figures: zeros

in decimals:

• leading = not significant

• training = significant

significant figures: pH

• right of decimal = significant

eg. pH = 2.45 → [H] = 2.8E-2

significant figures: exact number

infinite

significant figures: addition/subtraction

round answer to the last decimal place that is significant for each measurement in calculation

• # with least amount of digits/decimal places

  • 135.621 + 97.33 + 21.2163 = 254.17

significant figures: scientific notation

convert to common exponent

significant figures: multiplication/division

round to the measurement with the fewest significant figures

standard solutions

has known concentrations

sample solution

has unknown concentration

linear regression of straight line calibration curve equation

y = mx + b

assumptions of calibration curve

• signal to concentration is linear

• any difference between experimental data and calculated regression line is result of indeterminate errors affecting y only

• indeterminate errors affecting y are normally distributed

• indeterminate errors in y are independent of x value

calibration curve

plot of a measured signal vs a known quantity

calibration curve: sensitivity

slope (m) of calibration curve

• high sensitivity = small change in [] yields large change

calibration curve: dynamic range

the concentration range over which the calibration curve is analytically useful

• high much [] change will give change in y signal

concentration limit of detection

smallest signal detectable

calibration curve: selectivity

for linear calibration plots, the selectivity of a method for compound 1 vs compound 2 is reflected by the ratio of slopes m1/m2

• bigger ratio = better

matrix effect

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

• used when specific interference is not known

eg. some component in the sample:

• generates signal similar to analyte

• has a chemical interaction with analyte

• is co-isolated with the analyte

standard addition

means of accounting for matrix effect

• most precise results are obtained when the amount of standard added is comparable in magnitude to original quantity of analyte

• time consuming

• extra opportunity for dilution error

• added standard should not overwhelm any interferences

• may require large quantities of sample

internal standard

intentionally added substance of known quantity that is not expected to be found in the sample but is expected to behave similarly

• different than the analyte

• is analyzed with the analyte to account for losses during sample processing or fluctuations in instrument signals

• reference the analyte signal to the internal standard signal

• chemically similar to analyte so systematic effects of matrix that increase/decrease analyte signal will have similar effect on signal of internal standard

• used with both external standard calibration and standard addition methods

sampling

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

1. identify population

2. develop sampling strategy

3. collect and preserve a gross sample

4. reduce gross sample to a lab sample

5. replicate analyses of the samples

problems with real samples: defining the problem

• expected concentration rage of analyte?

• accuracy and precision required?

• where, when, how many samples to take, what size?

problems with real samples: storing and preserving sample prior to analysis

• protect from light, control temperature and pH

• store most samples in glass but store metal solutions in plastic

problems with real samples: homogenization

• may require crushing and grinding, drying, digesting, decomposing

problems with real samples: overwhelming matrix effects

• requires extraction, precipitation, distillation, or other separations of analyte from interferences

problems with real samples: analyte too dilute for reliable analysis

• requires pre-concentration steps such as extraction of evaporation

sensitivity, specificity, ROC curve

sensitivity = true positive rate

• rule out when gives negative result

specificity = true negative rate

• rule in when gives positive result

too see if method is working

ROC curve

receiver operating characteristic

too see if method is working

• plots sensitivity against 1-specificity

  • x = false positive (1-specificity)

  • y = sensitivity

area under curve = 1 = perfect

AUC < 0.5 = useless

AUC > 0.8 = method is somewhat effective

analog signal

takes on any value and the signal is continuous

equilibrium constant

K: ratio of [products] and [reactants] when there is no net change in system

• products over reactants

bigger is better and determines if solution becomes acidic or basic

strong acid

ionize completely in water

strong base

ionize completely in water

weak acid

ionize partially in water

weak base

ionize partially in water

Kw

Kw = Ka * Kb = 1.00E-14

polyprotic acids

have 2 or more ionizable protons

amphiprotic

act as either proton donor (acid) or acceptor (base)

• acid HA with a negative charge

charge balance equation

sum of positive charges in solution equals sum of negative charges

• include water dissociation

• if has numbered, charge, go infront of species

  • eg. HPO4 ^(-2) = 2[HPOr ^(-2)]

mass balance

sum of the amounts of species in a solution containing a particular atom must equal the amount of that atom delivered to solution

idk bro

isoelectric point

point that has no charge

solubility

can be dissolves in a solvent up to a certain point

• Ksp

  • larger = more soluble compound

    • precipiate will form

complexation equilibria

binding between metal and ligand

denticity

number of coordinate bonds to a central metal ion

chelating agent

non-cyclic, multidentate ligand

cryptand

polycyclic, multidentate ligands

EDTA

has 6 pKas

• 4 basic

• 2 acidic

titration has 3 components

1. determine quantity of substance A by adding measured increments of substance B

2. substance B reacts with substance A

3. some means of indicating the endpoint of which all of A has reacted

titration examples

• acid-base

• precipitation

• complexometric

• oxidation-reduction

advantages of volumetric titrations

• rapid

• convenient

• economical

• accurate

• easily automated

disadvantages of volumetric titrations

• manual titrations are tedious

• sensitivity and selectivity can be limited

titrant

solution of known composition and concentration dispensed from buret