exam 4 Flashcards
(12 cards)
What are some characteristics of GC sample injection?
Samples must be introduced like a plug into the column; it must be introduced quickly and in small sample sizes. The sample port is 50 C degrees above the boiling point of the least volatile component in the sample.
Sample sizes are a few tenths to 20 microliters.
Capillary columns usually involve splitters
What are some qualities of a good GC detector
Good sensitivity
Good stability and reproducibility
A linear response to solutes over multiple magnitudes
A temp range from room temp to atleast 400 C
Short response time independent of flow rate
Ease of use
Similar response to all solutes
Nondestructive
Describe a thermal conductivity detector
Earliest, yet still common
Utilize twin detectors; one before the sample is injected and at the end so that thermal conductivity is cancelled from the carrier gas
Mainly utilize He and H2
Simple to use, large dynamic range, can analyze orgo and inorgo species, nondestructive
An electrically heated source depends on the thermal conductivity of the surrounding gas… electrical resistance also depends on it
Low sensitivity
List some qualities of a packed column
Made from glass or metal, abt 2-3 m in length
Densely packed with packing materials coated with the stationary liquid phase
Formed as a coil so that packing can expose a large surface area to mobile phase
Supports should be small spherical particles, with good mechanical strength
Should be inert at high temps and evenly wetted
Describe a good GC stationary phase
Low volatility, thermal stability, chemical inertness, polarity should match sample
Describe LC sample injection
Limiting factor is reproducibility
Sample volumes must be small, from 0.5-500 micro liters to avoid band broadening and pressure build up
Mainly based on sampling loops, which fix the issue of reproducibility
Describe LC columns
Made from stainless steel tubing
Columns differ in size, price and packing
Usually 5-25 cm long, and can be coupled together to increase length
Temp control leads to reproducibility
What is the difference between normal vs. reverse phase
In normal phase; the stationary is polar and the mobile is nonpolar
In reverse phase; the stationary is nonpolar and the mobile is polar
In normal the least polar component is eluted first
In reverse the most polar component is eluted first (can use water)
What are some ways to improve LC separation
Dependent on the mobile phase
Choose a column that matches the polarity of the analytes
Lower flow rate
Increase column length and decrease diameter
Describe size exclusion chromatography
Times spent in the pours depend on size of the analyte molecule
Large molecules will have little retention time and smaller molecules will be retained for longer
No chemical or physical interaction
Talk about order of migration
Highly positive charged species go first, with highly negative going last.
Describe CE sample injection
2 types of sample injection; electrokinetic and pressure
Electrokinetic: one end of capillary is put in the sample, then a small voltage is applied and is placed back in a buffer
Pressure: Vacuum is applied at detector end by pressurizing the sample, or by elevation
