Lecture 1-4 Flashcards
What does Plate Theory describe?
Provides a theoretical treatment of zone broadening and migration of solutes in a column
Why are they called “Theoretical Plates?”
Because equilibrium between the SP and the MP occurs in each unit
What is Smax?
the maximum rate of solute escape as the peak leaves the chromatographic column
What is the relationship between moles and peak area?
moles is proportional to peak area, increasing m, increases peak area
What is the relationship between S(max) and peak height?
the maximum rate of solute escape is proportional to peak height
What is column efficiency?
the efficiency of a column is best judged by HETP (height equivalent theoretical plate, at a fixed column length, more plates result in smaller plate height and more efficiency
What is the partition coefficient, K?
K describes the solutes affinity for the SP, the large the K value, the more affinity for the SP
What causes peak tailing?
1) Strongly polar groups present on SP which retain solute more on polar sites than on other sites
2) Solute/SP interactions > solute/solute interactions
What causes fronting?
1) injection of excessive amount of solute to the column
2) Solute/SP interactions
What are the shortcomings of plate theory?
1) Diffusion is never instantaneous
2) Spreading of the chromatographic zone by longitudinal diffusion from one Theoretical plate to another does not occur at all (False)
3) Column is assumed to consist of a discrete number of volume units (False)
4) MP flows through the column in a continuous fashion
What are migration rates dependent on?
they are dependent on the magnitude of the equilibrium constants for the reactions by which the solutes distribute themselves between the MP and SP
What is retention time?
the time it takes after sample injection for an analyte peak to reach the detector
What is dead time?
the time it takes an unretained solute to pass through the column
What is the capacity factor?
tells how much solute is retained compared to unretained compound
What is the selectivity factor?
a fraction consisting of the partition ratios of two retained species on a chromatographic column, by convention the more strongly held species is in the numerator, the greater the difference in retention times as well as capacity factors, the higher the selectivity
What is resolution?
provides a quantitative measure of its ability to separate two analytes
Rs = 1.5
Rs > 1.8 too much separation
What is resolution proportional to?
Rs is proportional to square root of N and the square root of L
What is the cross-sectional area of a column proportional to?
it is proportional to the mass of an analyte, this affects the speed of analysis because volumetric flow rate is proportional to cross sectional area
What is peak asymmetry?
measured at 10% the peak height,
As > 1, BC>CA, peak tailing
As BC
What is peak capacity (PC)?
it is defined as the number of peaks that can fitted into a chromatogram between the dead point and the last peak, each peak being separated by 4 st.dev.
Impact of PC
1) Larger the tr/tm greater PC
2) If k’
Describe the effect of efficiency, selectivity factor, retention factor on column resolution
At fixed alpha, N values —–> increase k’ , increase in Rs
At fixed k’, N values —–> increase in alpha, increase in Rs
At fixed alpha, k’ ——> increase in N, increase in Rs
–Improvement in Rs, for increasing N is not as dramatic as for increasing alpha
What does measuring at the baseline represent?
When peak width is measured at 2.4% of the peak height, this peak represents 99.7% of the molecule in a band
Describe the variation in H
depends on the average linear velocity of the mobile phase, H is a measure of the ratio of bandwidth and retention time, easiest way to minimize H is to optimize MP velocity
