IA CHemistry Quizlet 1

Qualitative analysis

What is it?
Ex: infrared Spectroscopy, NMR, Mass Spectroscopy, Atomic emission Spectroscopy

Quantitative Analysis

How much of it do I have?
Ex: Beer’s Law, UV Vis, Fluorescence, Atomic Absorption spectroscopy, chromatography, mass spec

Precision

Good Close grouping of data points, measures the same thing

Accuracy

On average you get the “correct” answer

Confidence limit

mean ± ts/sqrt(N)

Signal to Noise

The quality of an instrumental measurement.

S/N = mean analyte signal/ Standard deviation of the noise

noise

free data that does not exist

standard deviation of the noise

using st. deviation on data or by doing max-min/5

Full Width at half maximum FWHM

the width is = 2.335 standard deviations. This is where the y value is half of the max y value for a normal distribution

Chemical Noise

uncontrollable variables that affect the system (T and P fluctuations)

Instrumental Noise

Comes from the different components of the instrument:
ex: Thermal Noise (electronics), Shot noise (random variations in photos reaching a detector or electrons generated ina semi-conductor), Flicker (drift in noise with a magnitude inversely proportional to the frequency), and Environmental (noise from surroundings are picked up)

Ways to improve Signal to noise

Electronic Shielding

Difference measurements

Filters

Signal Averaging (takeing multiple measurements)

Ensemble averaging, boxcar averaging, running average

Sensitivity

How easily it will be able to measure the small changes in concentration or other values. How steep the slope of the calibration curve, and how precise.

Detection Limit

the smallest concentration you can measure reliably to a certain confidence interval.

Quantitatively = average blank + 3xstandard deviations of the blank

Qualitatively/ quantified= average blank signal + 10x standard deviation of the blank

Dynamic range

The region over which you can measure a linear calibration curve. Widely varries with technique

Selectivity

if the technique allows for analysis of a wide variety of compound types or if it is limited to a certain few types of compounds.

Mobile Phase

(liquid or gas) that moves over or through a stationary phase, carrying the components of a mixture or solutes along with it

Stationary Phase

Solid or liquid, where the mobile phase passes through or over

Phase boundary between phases

separations across which analytes partition

analyte

The Specific chemical substance or target compound in a sample you intend to separate, identify, and measure

Distribution coefficient (K_c)

(K_c) = X_s/X_m or C_s/C_m

molecules in a chromatography experiment

establish an equilibrium between mobile phase and stationary phase X_s<—> X_m

Difference between time spent in mobile phase and stationary phase

All compounds spend some time in the column. t_m = time for mobile phase to traverse column.

all compounds spend the same amount of time in the mobile phase

SEPARATION OCCURS ONLY IN STATIONARY PHASE

Adsorption Chromatography

S.P = solid, molecules adsorb

M.P. = liquid or a gas

Ex: TLC, Column chromatography

Partition Chromatography

S.P. = liquid on solid support analytes dissolve

M.P. = is a gas or a liquid

Ex: GLG = gas, LLC = liquid

Normal phase

polar S.P.; non-polar M.P.

Reverse Phase

nonpolar S.P.; polar m.p.

Ion Exchange chromatography

S.P. = resin with + or - side groups

M.P. = liquid

(useful for ions)

Size- exclusion chromatography

S.P. = polymer with pores of a certain size

M.P. = liquid

Small analytes can get into pores, Large analytes are excluded

Separation is based on molecular weight

Plate theory

more plates = better separation

N= 16(t_R/w_b)²

Height equivalent to a theoretical plate (HETP, H)

H=L/N

Normalizes N for length of column, smaller H means a more efficient separation

Rate Theory

H is related to physical properties involved in the separation process. Causing the Van Deemter equation

H= A +B/u +Cu

What is u in the Van Deemter equation?

The average linear velocity (cm/s)

How to get from flow rate to average linear velocity?

multiply the flow rate by the cross sectional area of the column

What does A mean in the van deemter equation?

the Eddy Diffusion, arises from the different paths through the packing material.

Depends on size of the packing particle and how tightly it is packed.

What does B mean in the van deemter equation?

longitudinal diffusion, the tendency of solute molecules to spread out randomly in all directions along the length of the column.

INVERSELY PROPORTIONAL to flow velocity

What does C mean in the van deemter equation?

Arises from mass-transfer broadening, some analytes penetrate to a greater depth of the S.P.

Directly proportional to flow rate.

How to find the optimum efficiency for flow rate?

take the derivative of the Van deemter equation and set equation equal to zero and solve for the average linear velocity, then convert to flow rate.

Retention factor (k)

gives an indication of separability of compounds

k = (t_R-t_M)/t_M

Resolution (R_s)

R_s= (t_R2-t_R1)/(0.5[w₂+w₁])

if R_s is greater than 1.5 it is baseline resolved

Selectivity Factor/ Separation factor (α)

α = (t_R2-t_m2)/ (t_R1-t_m1) = k₂/k₁

Gas Chromatography

Mobile Phase is a gas

Requires volatile compounds

5% of all compounds are able to be used in GC

Stationary phase is solid or liquid

Thermal conductivity detector (TCD)

Gas Chromatography detector

Universal, measures change in resistivity of heated wire in a gas stream

poor sensitivity, non destructive

no structural information

requires reference

(Not very good)

Flame ionization detector FID

Gas Chromatography detector

Most used detector

burn analytes in a H2 flame to ionize and detect current

more sensitive than TCD

Destructive, almost universal

no structural information

Electron Capture Detector

Gas Chromatography detector

Radioactive 63Ni source ionizes sample

analytes containing EN atoms can capture the electrons reducing current

Destructive

Selective for Halogens, NO2, or P containing analytes

no structural info

Flame photometric detector

Gas Chromatography detector

burn analytes in a flame, creating excited states, with photon emitted from sample

Selective for S and P compounds

Destructive

No Structural information

Mass Spectrometric detector

ions detected by M/Z ratio

mostly universal

Gives structural information

destructive

expensive

Isothermal GC

oven remains at single temperature

T gradient GC

temperature ramps up during run

more volatile things elute first (low T)

less volatile things elute after

Liquid Chromatography

Samples interact with both MP and SP unlike GC

Mobile Phase: Liquid analytes dissolve in it

Stationary phase: Solid (LSC)- Column adsorbtion

Liquid (LLC) -HPLC analytes dissolve

Two types of LLC

1. Normal Phase: M.P. is less polar than S.P.

Elution order: Less polar —> more polar

2. Reverse Phase: M.P. is more polar than S.P.

Elution order: More polar —> less polar

Isocratic LC

Solvent composition remains the same during the entire run

Gradients LC

Concentration of the solvent increases over the run, useful for separating peaks

High Performance liquid chromatography (HPLC)

Liquid chromatography at high pressures, and need an accurate way to control flow rate and mix several solvents in these high pressures (1000-5000PSI)

Reciprocating pump

A pump for HPLC that allows for uninterrupted solvent flow

to mix solvents use 2 pumps

Refractive index detector

Liquid chromatography

Compare refractive index of solvent vs (solvent + analyte)

Universal

No Solvent gradients

non destructive + no structural information

UV-Visible absorption detector

Liquid Chromatography detector

Some analytes can absorb light, and using beers law you can calculate concentration

sensitive, and universal

limited molecular information

non-destructive

A=ebc A= absorbance, e= contant for molecule and wavelength of light, b = path length, c = concentration

fluorescence detector

Liquid chromatography detector

some molecules can absorb one wavelength and re-emit a different wavelength (fluorescence)

fluorescence is proportional to concentration

very selective

very sensitive

non-destructive

can use fluorescent tags

Fluorescence tagging

Many compounds can be derivatized to form fluorescent compounds (tagging)

Can do it Pre column or post column and depends on the tag

Pre-column tagging

Adding a tag before separation of molecules in LC

Post column tagging

adding a tag after the separation from LC but before the detector

Ion exchange columns

S.P. is a polymer with either positively or negatively charged side groups

Separation depends on charge density of analytes (small binds more tightly, more charges bind better)

LC

Size exclusion chromatography columns

S.P. is a cross linked polymer with very large uniform pores

Smaller molecules become trapped in pores, whereas large molecules are excluded from the pores and elute first

retention inversely proportional to Molar mass

DO NOT USE FOR SMALL MOLECULES

how do you Qualitative analyze?

performed by comparing retention times of analyte to those of known standards under the same conditions

Requires pure reference compounds, more than one compound can have the same t_R

Calibration curve procedures

create standards of known concentrations, run on instrument under known conditions

Run unknown under same conditions and interpolate

Internal Standards

compounds that have similar structure to analyte can be added in known concentration to each standard and unknown

Used to eliminate sample to sample variation in injection volume

Gel electrophoresis

Gel is a cross-linked polymer

voltage is applied across gel, anions migrate toward anode

Only works for charged species

separates based on charge density

Capillary electrophoresis

Capillary filled with buffer solution, and voltage is applied across the capillary

anions migrate toward anode, cations migrate toward cathode

Doesn’t work on neutral molecules

Electrophoretic flow

Cations attracted to cathode, anions are attracted to the anode

Electro-osmotic flow (eo)

silica capillary has anionic side groups above pH 1-2, cations in buffer attracted to wall of capilary forming layers of cations.

Second+ row of cations in buffer are attracted to cathode

RESULT: NET BUFFER FLOW TOWARDS CATHODE

Everything exits even anions going toward anode

Separates based on charge classes and by charge density

Electrophoresis math

t_r=L²/(u_EV)

R= 1/(4sqrt2) (u_e1-u_e2)[V/(D_m(u_e+u_eo)]