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Chapter 25: Q23P (page 709)

To which kinds of analytes do these liquid chromatography detectors respond?

(a) ultraviolet

(b) refractive index

(c) evaporative light scattering

(d) charged aerosol

(e) electrochemical

(f) fluorescence

(g) nitrogen chemiluminescence

(h) conductivity

Short Answer

Expert verified

The part (a), part (b), part (c), part (d), part (e), part (f), part (g), part (h) is

  1. Wavelengths below 210
  2. Its detection limit is about poorer than that of the ultraviolet detector
  3. significantly less volatile than the mobile phase
  4. Itsnearly equal response to equal masses of non-volatile analytes
  5. Chromatography detector responds to analytes that can be oxidized or reduced
  6. Chromatography detector responds only to the few analytes that fluoresce
  7. Chromatography detector responds to the molecules with N atom
  8. Chromatography detector responds ionic or ionizable compounds

Step by step solution

01

Ultraviolet

Part (a)

The ultraviolet liquid chromatography detector is selective for compounds with wavelengths above 210 nm and is universal for compounds with wavelengths below 210.

02

Refractive index

Part (b)

The refractive index liquid chromatography detector responds to almost every solute, but its detection limit is about poorer than that of the ultraviolet detector.

03

Evaporative light scattering

Part (c)

The evaporative light scattering liquid chromatography detector responds to any analyte that is significantly less volatile than the mobile phase.

04

Charged aerosol

Part (d)

The charged aerosol liquid chromatography detector is almost universal detector with nearly equal response to equal masses of non-volatile analytes.

05

Electrochemical

Part (e)

The electrochemical liquid chromatography detector responds to analytes that can be oxidized or reduced (for example it can be phenols, aromatic amines).

06

Fluorescence

Part (f)

The fluorescence liquid chromatography detector responds only to the few analytes that fluoresce.

07

Nitrogen chemiluminescence

Part (g)

The nitrogen chemiluminescence liquid chromatography detector responds to the molecules with N atom.

08

Conductivity

Part (h)

The conductivity liquid chromatography detector responds ionic or ionizable compounds.

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Most popular questions from this chapter

(a) Make a graph showing retention times of peaks 6, 7, and 8 in Figure 25-12 as a function of %acetonitrile (%B). Predict the retention time of peak 8 at 45% B.

(b) Linear-solvent-strength model: In Figure 25-12, tm = 2.7 min. Compute k for peaks 6, 7, and 8 as a function of %B. Make a graph of log k versus Φ, where Φ= %B/100. Find the equation of a straight line through a suitable linear range for peak 8. The slope is -S and the intercept is log kw. From the line, predict tr for peak 8 at 45% B and compare your answer with (a).

(c) Gradient elution: A linear eluent gradient from 40 to 80% acetonitrile over 30 min is performed on the column in Figure 25-12. Assuming a dwell volume of 0 mL, use your data from (b) to plot the retention factor of peaks 6 and 8 during the gradient. What are the general characteristics of the plot?

(d) Why are the peaks in a gradient separation sharp?

A known mixture of compounds A and B gave the following HPLC results:

A solution was prepared by mixing 12.49mgof Bplus 10.00mLof unknown containing just and diluting to 25.00mL. Peak areas of 5.97and 6.38were observed for AandB, respectively. Find the concentration of A(mg/mL)in the unknown.

(a) When you try separating an unknown mixture byreversed-phase chromatography with 48%acetonitrile 50%water,the peaks are too close together and are eluted in the range k = 2- 6Should you use a higher or lower concentration of acetonitrile in thenext run?

(b) When you try separating an unknown mixture by normal-phasechromatography with 50%hexane50% methyl t-butyl ether, thepeaks are too close together and are eluted in the range k = 2 - 6Should you use a higher or lower concentration of hexane in thenext run?

what are criteria for an adequate isocratic chromatographic separation?

Two peaks emerge from a reversed-phase chromatography column as sketched in the illustration.

According to Equation 23-33, resolution is given by

Resolution=N4(α-1)α(k21+k2)

where Nis plate number, αis relative retention (Equation23-20), and k2 is the retention factor for the more retained component (Equation 23-16).

(a) If you decrease the amount of organic solvent in the mobile phase, you will increase retention. Sketch the chromatogram if retention factors increase but Nand αare constant.

(b) If you change the solvent type or the stationary phase, you will change the relative retention. Sketch the chromatogram ifαincreases but Nandk1are constant.

(c) If you decrease particle size or increase column length, you can increase the plate number. Sketch the chromatogram if Nincreases by (i) decreasing particle size and (ii) increasing column length. Assume αand k2are constant.
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