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Flashcards in Final Deck (65):
1

volatility

tendency of substance to vaporize

2

properties of GC injector

sample injected quickly as plug of vapor. split injection. sample port is about 50 degrees C hotter than least volatile component. Inert carrier gas, usually He (also N2 or Ar).

3

Four GC detectors covered

flame ionization, thermal conductivity, electron capture, mass spec

4

flame ionization detector principles

sample directed into air-hydrogen flame. most organics produce ions and electrons. current is produced and measured. mass sensitive rather than concentration sensitive.

5

concentration senstive vs mass sensitive

A concentration-sensitive detector responds to the concentration of the analyte in the mobile phase, whereas a mass-sensitive detector responds to the number of analyte
molecules or ions that come in contact with the detector. Peak areas for a concentration-sensitive detector increase as the flow rate decreases because the analyte is in contact with the detector for a longer period. Peak areas for a mass sensitive detector are not greatly affected by flow rate

6

thermal conductivity detector principles

mobile phase gas (H2 or He) produces thermal conductivity measurement about ten times greater than. When analyte passes through detector, thermal conductivity decreases significantly and the difference is measured.

7

pos/neg of flame ionization

pos: good for organics neg: destroys sample and uses lots of gas

8

pos/neg of thermal conductivity

pos: nondestructive. good for organics and inorganics neg: not sensitive, can't use with small samples

9

principles of electron capture detector

sample eluted from column passed over electron emitter that causes ionization of carrier gas and burst of electrons. In absence of organics, constant current is produced between two electrodes. With organics, current decreases significantly as they capture electrons.

10

pos/neg of electron capture detector

pos: detects many functional groups neg: can't detect all functional groups, bad dynamic range

11

benefits of MS with GC

already in gas phase

12

common sources of ion production for MS

electron impact and chemical ionization

13

TIC vs SIM for MS

total ion chromatogram: time vs ion abundance. selective ion monitoring: time vs selected m/z

14

Two types of capillary columns for GC

WCOT: stationary phase coats inner wall of glass (more common) and SCOT: support coats inner wall of glass, stationary coats support

15

Most common type of capillary column

FSWC: fused silica wall coated. small inner diameter (0.2 to 0.3 mm)

16

Problem with silica coating of glass

formation of silanol groups from moisture. are polar and have strong affinity for polar organics.

17

how are silanol groups deactivated?

silanization. replace OH with Cl. then replace Cl with OCH3

18

Main property of stationary phase? why?

want polarity to match that of sample. allows separation based on boiling point

19

Common liquid stationary phase in GC

polydimethyl siloxane

20

Isocratic LC

single solvent used in mobile phase

21

why degas LC mobile phase

leads to irreproducible flow rates and band spreading

22

Most common LC pump and properties

reciprocating pump. Very small internal volumes. high pressure (10000 psi) good for gradient elution. Neg: pulsed flow

23

sample injection for LC

requires really accurate volume

24

Guard column function

catches contaminants and components that bind irreversibly to stationary phase

25

LC column properties

3-5 mm inner diameter. 3-5 um diameter packing (silica). 10 cm long. 40 K plates/meter

26

detection for LC requirements

compatible with liquid flow, minimal volume to minimize broadening

27

LC/MS issue

sample intrduction. ESI common (use volatile solvent)

28

most widely used HPLC

partition chromatography

29

Normal phase

polar stationary, nonpolar mobile

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reversed phase

nonpolar stationary, polar mobile

31

longer carbon chains in reversed phase

longer chains result in better separation. hydrophobic analytes spend more time in column

32

pH consideration in LC

above 7.5: Si solubilizes. below 2.5: siloxane hydrolyzes

33

ion exchange chromatography

exchange ions in solution for another in column

34

common cations for ion-exchange chromatography

sulfonic acid, carbonic acid

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common anions for ion-exchange chromatography

N(CH3)3+OH-

36

gel filtration chromatography

porous beads capture small molecules, allow large to pass

37

affinity chromatography

use beads with covalently attached substrate to catch enzyme molecules with high affinity. allows other proteins to pass

38

most common supercritical fluid and why?

CO2. nontoxic, colorless, good for UV detection

39

advantages of supercritical water chromatography

speed of GC. low plate heights vs LC

40

restrictor

controls back pressure. maintains desired pressure. gets stuff to detector

41

slab electrophoresis properties

separates based on size and charge (although not neutral species). smaller, more charged species move more quickly

42

slab migration rate

v=uE

43

benefits of capillary over slab

more quantitative, no staining required, detectors same as HPLC, high velocity (due to higher voltage allowed)

44

capillary migration rate

v-e(V/L)

45

chromatography efficiency depends on ? (2)

diffusion and mass transfer

46

electrophoresis efficiency depends on ?

only diffusion

47

capillary plate number

N=(uv)/(2D) independent of column length

48

maximum voltage of SLAB vs capillary

slab=500 Volts . capillary=15 K Volts

49

capillary plate number vs HPLC

capillary = 100 K. HLPC = 5 K

50

electroosmtoic flow

at pH greater than three, silanol groups are negatively charged. diffuse layer of hydrated ions forms when cations associate with negative charges. hydrated ions drag water/solvent with them to end of capillary

51

benefits of CE flow vs HPLC flow

CE flow is generated from walls of capillary giving flatter profile of solvent front. produces narrower peaks. HPLC flow is generated from pressure (laminar) and gives broader peaks

52

velocity in CE

sum of electrophoretic + electroosmotic velocity

53

plate number formula

N=16(tm/W)^2

54

order of elution in CE

high positive charge, positive charge, neutral charge (all together), negative charge, high negative charge

55

sample size in CE

nL

56

total volume of CE capillary

4-5 mL

57

two types of CE injection

electrokinetic and pressure

58

electrokinetic injection

sample is injected with voltage to introduce plug

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pressure injection

apply vacuum to detector end

60

two types of detectors for CE

LIF and MS

61

LIF detection

laser induced fluorescence. laser pointed right at end of capillary

62

problem with MS in CE

liquid to gs intro

63

K

K=(tr-tm)/tm

64

k2/k1=

k2/k1= 10^((p2-p1)/2)

65

migration time formula

tm= [l*L]/[V(u(electroosmotic)+u(electrophoretic))] where l is length of cap to detector and L is length of cap