Instrumental Exam 1

what is instrumental analysis

qualitative identification

determination of:

1. chemical composition and/or

2. relative concentration

calibration provides

quantitation

standard curve

external standard calibration curve- (without sample)

general steps of chemical analysis

question formulation

analytical procedure selection

sampling

sample preparation

analysis

reporting and interpretation

drawing conclusions

quality assurance

objective- test what your application can tolerate

specifications- identify range of tolerance

assesment- collect data and verify

method validation

process proving that an analytical method is acceptable for its intended purpose including:

• specificity

• linearity

• accuracy

• precicsion

• range

• limit of detection

• limit of quantitiatoin

• robustness

• sensitivity

• selectivity

Detection Limit (LOD)

3 (std)/ slope

• lowest concentration an analytical method can accurately distinguish from the background

Quantitation Limit (LOQ)

10 (std.)/ slope

with clear accuracy and reliable precision

signal detection

lowest amount of signal you can detect from the noise

y=ybl+ 3 (std)

precision vs accuracy

specificity vs selectivity

specificity is distinguishing a single analyte, while selectivity can usually distinguish more than one.

robustness

ability of an analytical method to be unaffected by small changes

analytical figures of merit

electrolysis

process in which a chemical reaction is forced to occur at an electrode by an applied potential

potentiometry

measurement of voltage in the absence of significant current

electrochemical reactions occur spontaneously or non spontaneously

spontaenously

working electrode vs counter electrode

working electrode- where the electrochemical reaction of interest occurs

overpotential

voltage required to overcome the activation energy for a reaction at an electrode

• faster you want a reaction, higher the over potential that must be applied

• a greater over potential will sustain a higher current density

electric current measures the rate of

charge transfer

ohmic potential

voltage needed to overcome electrical resistance (R)

amperometry

the measurement of electric current between electrodes that are diving an electrolysis reaction

amperometric glucose sensor

amperomimic glucose sensor what does it detect

could be measured through just H202 concentration present after the use of glucose oxidase, but can be unreliable

instead, you can use a dimetylferricinium cation which is more reliable and has a positive charge attached to it

benefits of mediator molecule in amperometric glucose sensor

mediator solves o2 concetation issue in variation

transfers electron from reaction to the working electrode

current is measured p;ropritonal to neutral ferrocene- or glucose since they are equal

reduced working potnetialfrom .6v to .2v which can reduce interference

amperometric oxygen sensor

non polar o2 diffuses through small silicon holes at the base of the electrode which then reactions with a gold button cathode

a reactant has three ways to reach the surface of the electrode in amperometry

1. diffusion through a gradient

2. convention- bulk movement through sturring

3. migration- attraction or repulsion of a charged species

rotating disc electrode gives convention and diffusion

voltammetery

encompasses techniques which measure current undersupplied variable potential. the potential is varied using many types of profiles that provide information about a chemical reaction

three toes of voltammetry

based on waveform

1. linear scan

2. square wave

3. triangular- cyclic coltammetry

concentration dependent response of voltammeter is the

diffusion current

whats actually reported is the halfwave potential

the max current is given by the halfwave potential. qualitative or quantitative

qualitatively identifies

when does E1/2= Enaught

for systems where reactants and products are both aqueous

diffusion current is proportional to what

analyte concentration

with reductions actions what should you od before measuring

purge reaction with N2 so as to not reduce the water and get background noise

anodic stripping voltammeter

1. pre concentration or deposition

• concentrate analyze by reducing onto electrode 10^(2-3)x more than in solution

2. quiet time

• potential is held constant, but stirring is stopped to read equilibration

3. anodic stripping

• potential is scanned to oxidize the metal back in its original sate

• measure current during stripping

anodic stripping voltammeter

whats the most sensitive volumetric technique

ASV

ASV qual or quant

both!

ASC

precision and accuracy

detection limits and linearity

inferences

approx 2-4 RSD/ 2-5% relative error

ppb/ppTrilion

linearity- 2 orders of magnitude

metals being analyzed must have potentials sufficiently different, can do up to 5 metals, only certain ones can be determined by ask

cyclic voltammetery

triangular waveform, repeated several times to ensure reproducibility, electrodes are stationary and planar, no stirring, signal measured is diffusion or surface controlled.

Epa-Epc=

2.22RT.nF=57/n

reaction is perfectly reversible if this is true

go back for all of CV