Achem 2 quizzes, test, and end of book questions

A compound of formula weight 342 transmitted 57.0% of the radiation at 512 nm in a 2-cm cell at a concentration of 65.0 μg/mL. Calculate its absorptivity.

1.88×10^-3 ml/(cm*µg)

What is the wavelength in  (1  = 1×10^-10m) and wavenumber (in cm^-1) of a beam of radiation that has a frequency of 7.43×10^14Hz

Wavelength=4030

Wavenumber=2.48×10^4 cm^-1

What does a monochromater do in a spectrophotometer? What is the most common type of monochromater?

Selects a narrow band of light

Grating

How can you adjust slit width to obtain the lower limits of detection in spectrophotometry? When this adjustment is made, what is lost?

Increase the width of the slit

Resolution

What is the main difference between a single- and double-beam spectrophotometer? Name one advantage of each.

Single: Light goes through sample only; cheaper

Double-beam: Light goes sample and blank; better accuracy

What type of light source is commonly used in the visible range for absorbance spectroscopy? UV absorbance spectroscopy?

Visible: Tungsten

UV: Deuterium

Why is λmax used when creating standard curves and measuring unknown samples in spectrophotometry?

Better sensitivity; no polychromatic effects; produce linear calibration curve

What is one major difference between a spectrophotometer and a fluorimeter?

Spectrophotometer: Measures absorbance; detector at 180°, emission and excitation monochromator

Fluorimeter: Measures emission; detector at 90°, one monochromator

Why is it necessary to keep the concentrations in fluorescence spectroscopy dilute?

Prevents self absorption; collision deactivation

What is one major difference between fluorescence and phosphorescence?

Fluorescence: S1 → S0; slow

Phosphorescence: T1 → S0; fast

Draw a schematic diagram showing all the necessary parts for AAS

Hollow cathode tube light source → Flame/Furnace → Monochromator → Sample Holder → Detector

Draw a schematic diagram showing all the necessary parts for AES

Plasma → Monochromator → Detector

Which technique must have a more stable instrument temperature, AAS or AES? Why?

AES because it needs more consistent number of excited molecules - Boltzman dist

Which technique uses a higher instrument temperature, AAS or AES? Why?

AES because you need more atoms in an excited state - Boltzman dist

What kind of light source is used in AAS? Why is it necessary?

Hollow cathode tube; Need narrow band of light for linear calibration curve

What type of AAS instrument is capable of preconcentrating a sample? How does it work?

Furnace; Injecting multiple aliquots of sample and drying prior to atomization

What would be the order of elution when using a capillary electrophoresis to separate the following ions: K+, Zn2+, Se2–? In this separation, the separation was performed in the normal mode where the cathode (–) is near the detector and the anode (+) is where the sample is applied

Zn2+, K+, Se2-

What would be the order of elution when using a cation exchange column of the following ions: Al3+, K+, Fe2+?

K+, Fe2+, Al3+

What type of GC detector would give the lowest limits of detection in the analysis of ethanol, CH3CH2OH

FID: Has lowest detection for organic compounds

What type of GC detector would give the lowest limits of detection in the analysis of atrazine?

ECD: has lowest detection limits for halogen-containing compounds

λmax in UV-vis spectroscopy is used to
I. obtain the most sensitive method

II. minimize the effect of wavelength fluctuations

III. obtain the lowest limit of detection

I II and III

The chromatogram shown on the right has a peak with a peculiar shape (peak #2). What do we call this and what can be done to prevent this from occurring?

Peak tailing, change the stationary phase to a reversed-phase support

Why shouldn’t a buffer with a pH of 10.0 be used as a mobile phase in a reversed phase HPLC separation?

At pH’s above 8, the linkage holding the stationary phase on the silica particle degrades

Which electrochemical technique is used to study reaction mechanisms?

Cyclic voltammetry

In which of the following instances should total ion monitoring be used instead of selected ion monitoring in GC-MS?
a. Bulk analysis of a known compound using a previously developed separation method.

b. Total ion should never be used in GC-MS.

c. Trace analysis of a known compound using a previously developed separation method.

d. Identification of an unknown compound from an organic reaction mixture.

d. Identification of an unknown compound from an organic reaction mixture

What are the advantages of a double beam spectrophotometer over a single beam

spectrophotometer?

I. less expensive

II. compensates for fluctuations in light intensity

III. more sensitive measurements

II

c=2.998×10^8

h=6.626×10^-34 J/s

k=1.381×10^-21 J/K

N(A)=6.022×10^23

F=96,485

The CH2 wag of formaldehyde appears at 1165 cm–1. How much energy does this correspond to?

2.314×10^-20 J

A sample of iron complexed with phenanthroline in a 1.0 mm cell has a transmittance of 75.0% at 510 nm. If the solution is 0.075 M, what is its molar absorptivity (M^-1 cm^-1)

17 M^-1 cm^-1

Calculate the (M+1)^+ ∕M+ mass ratio for pyrene (C6H10)

17.24%

Draw a box diagram of a molecular absorbance spectroscopy instrument (be sure to include all necessary components).

light source → Furnace → monochromator (λ slector) → sample holder → detector

How would this instrument (light source → monochromator (λ slector) → sample holder → detector 180degrees)

need to be adjusted to allow for fluorescence measurements?

Detector must be at 90° angle from sample holder; must have second monochromator

The limits of detection for spectrophotometry are typically lower or higher than fluorescence spectrometry? Why?

higher; because it has no background

Why must samples measured using fluorescence be kept relatively dilute? [i.e. what happens when the samples are too concentrated?]

Self absorption and collision deactivation

One of the two types of atomic spectroscopy requires a special light source. What is the light source and why is it needed?

AAS requires a hallow cathode tube because it needs a narrow light source for narrow calibration curve

Which technique, AES or AAS, typically uses much higher temperatures? Why are these higher temperatures needed?

AES since it needs more atoms in an excited state

One of the atomization methods in atomic spectroscopy is capable of concentrating samples prior to analysis. Identify this atomization method and describe how it can be used to preconcentrate the samples.

Furnace → apply sample, dry & repeat prior to atomization

Approximately how many elements can be measured in each atomic spectroscopy technique simultaneously (AAS and AES)?

AAS → 3-4

AES → 70

Fill in the blank with the correct term describing this phenomena. Describe (or draw a picture) how it works.
______________________________ – the faster the flow rate, the greater the effect

Mass transfer: Some analyte lags behind when interacting w/ stationary phase

Mass transfer is important in: open tubular GC / packed GC / LC / CE

Open tublar GC, packed GC, and LC

Fill in the blank with the correct term describing these phenomena. Describe (or draw a picture) how it works.
b. ______________________________ – independent of flow rate.

Multiple flow paths

Multiple flow paths is important in: open tubular GC / packed GC / LC / CE

packed GC and LC

Fill in the blank with the correct term describing these phenomena. Describe (or draw a picture) how it works.

c. ______________________________ – the faster the flow rate, the smaller the effect

Longitudnal diffusion

Longitudnal diffusion is important in: open tubular GC / packed GC / LC / CE

All

The elution order of several alcohols in a certain separation was as follows: dodecanol (first, least polar), undecanol, nonanol, octanol, heptanol, hexanol, pentanol (last, most polar). What type of liquid chromatography was used for this separation?

NPCL

What does N/RPLC stand for

Normal/Reveres Phase liquid chromatography

Describe the polarity of the stationary phase and the mobile phase in NPCL separation.

Stationary phase: polar / nonpolar

Mobile phase: polar / nonpolar

Stationary phase: polar

Mobile phase: nonpolar

After the initial separation for NPCL, the mobile phase was changed to a more polar solvent system. How would this affect the elution time of all the compounds?

Faster

Describe (or draw) a flow cell used in LC which helps enhance detections limits.

Z-shaped flow cell

How does size exclusion chromatography separate molecules? What elutes first?

Small particles get stuck in stationary phase so larger particles will elute first

How does cation exchange chromatography separate cations? What elutes first?

Negative site son column hold positive analytes so negative analytes elute first

Describe how the electric double layer in capillary electrophoresis causes the buffer to move inside the capillary tube.

Positive charge of electric double layer are pulled toward cathode pulling entire bulk soln towards cathode

How does capillary gel electrophoresis work? What elutes first?

Crosslinking seperates molecules based off size. Small analytes elute first

A 50.00-mL aliquot of unknown Cu(II) solution was exhaustively electrolyzed to deposit all copper onto the cathode. The mass of the cathode increased 1.087 g. Find the molarity of Cu(II) in the unknown solution. The molar mass of Cu is 63.546 amu

0.3421 M

Both peak area and peak height can be used for quantitation in chromatography. A resolution of at least ______ is needed to use peak height and a resolution of at least ______ is needed to use peak area

1.0, 1.5

A photomultiplier tube

I. must be cooled

II. is less sensitive than a photodiode array

III. can obtain an entire absorbance spectrum instantaneously

I because cooler helps eliminate thermal noise

When calibrating a pH meter it is necessary to ___________.

I. calibrate the electrode at the temperature it will be used

II. keep the electrode moist in between readings

III. only use one pH standard solution for calibration

I and II

What is the difference between hard and soft ionization? Which technique is more likely to allow for the detection of the molecular ion? Which one produces the most fragments?

Hard: lots of fragments, electron impact, less likely to see molecular ion
Soft: Fewer fragments, more likely to see molecular ion, chemical ionization, MALDI, ESI

Which of the following are soft ionization techniques: ESI, MALDI, CI, EI

ESI, MALDI, CI

Which of the allow for analysis of large molecules

ESI, MALDI

What is the nominal mass for coumarin, C9H6O2?

146 g/mol

Calculate the (M+1)+/M+ ratio for coumarin, C9H6O2 .

9.78%

State the differences between single- and double-beam spectrophotometers and explain how each measures the transmittance of a sample. What source of error in a single-beam instrument is absent in the double-beam instrument?

Single beam: Light only passes through sample. This introduces error since intensity can change after taking blank
Double beam: Light passes through both sample and blank

Would you use a tungsten or a deuterium lamp as a source of 300-nm radiation?

Deuterium

What are the advantages and disadvantages of decreasing monochromator slit width?

Advantages: Better resolution

Disadvantage: Less light reaches the detector so it worsens the signal

Why is a photomultiplier such a sensitive photodetector?

It has a series of photosensitive cathodes that amplify the signal at each dyode

What are isosbestic points and why are they observed?

Isosbestic points: The point when two species cross at the same absorbance at a certain wavelength. They are observed to see when one species begins to convert to another during a chemical reaction

[X]=4.42×10^-5 M
[Y]=5.96×10^-5 M

Transferrin is the iron-transport protein found in blood. It has a molecular mass of 81 000 and carries two Fe3 ions. Desferrioxamine B (Chapter 13 opening) is a potent iron chelator used to treat patients with iron overload. It has a b pathlength You should be able to use Equations 19-5 to analyze the spectrum of a mixture. Fluorescence intensity (at low concentration) I kP0c I fluorescence intensity k constant P0 radiant power of incident radiation c concentration of fluorescing species c19Spe 430 19 Spectrophotometry: Instruments and Applications molecular mass of about 650 and can bind one Fe3. Desferrioxamine can take iron from many sites within the body and is excreted (with its iron) through the kidneys. The molar absorptivities of these compounds (saturated with iron) at two wavelengths are given in the following table. Both compounds are colorless (no visible absorption) in the absence of iron DONT DO B

[Transferrin]=8.99 mg/mL

[Fe]= 12.4µg/mL

What are the advantages and disadvantages of furnaces compared to flames in atomic absorption spectroscopy.

Advantages: Furnace gives lower detection limit and requires a smaller sample

Disadvantages: Poorer reproducibility with manual sample introduction (can use auto sample introduction to improve percision)

What are the advantages and disadvantages of the inductively coupled plasma compared to flames in atomic spectroscopy

Advantages: Higher temp minimizes chemical interference, and allows emission to be used over absorption so lamps are not required. Temp is also more uniform reducing self-absorbance

Disadvantages: Cost of equipment, operation increase and harder to repair

In which technique, atomic absorption or atomic emission, is flame temperature stability more critical? Why?

AES since need a consistent number of excited particles

Atomic emission spectrometers provide background correction by measuring the signal at the peak emission wavelength and then at wavelengths slightly above and slightly below the peak, where the signal has returned to background. The mean background is subtracted from the peak signal. Why can’t we use the same technique for background correction in atomic absorption?

Because AAS measures a narrow line from the lamp that overlaps with the analyte's wavelength, there are no off-peak wavelengths with lamp intensity to measure background seperately

What is the purpose of a matrix modifier in atomic spectroscopy?

It can decrease evaporation of analyte or increase evaporation rate of matrix to reduce matrix interference

The first excited state of Ca is reached by absorption of 422.7-nm light. h=6.626×10^-34 J×s c=3.0×10^8 m/s k=1.381×10^-23J/K

(a) What is the energy difference (J) between the ground and excited states?

(b) The degeneracies are g*/g0 = 3 for Ca. Find N*/N0 at 2 500 K

a. 4.703×10^-19 J

b. 3.6×10^-6

Suppose that 5.00 mL of blood serum containing an unknown concentration of potassium, [K]i, gave an atomic emission signal of 3.00 mV. After the addition of 1.00 mL of 30.0 mM K and dilution of the mixture to 10.00 mL, the emission signal increased to 4.00 mV.

(a) Find the concentration of added standard, [S]f, in the mixture.

(b) The initial 5.00-mL sample was diluted to 10.00 mL. Therefore the serum potassium concentration decreased from [K]i to 3K1 4f 5 1 2 3K1 4 i. Use Equation 5-6 to find the original K content of the serum, [K]i.

a. 3.00×10^-3

b. 3.60×10^-3

An unknown sample of Cu2 gave an absorbance of 0.262 in an atomic absorption analysis. Then 1.00 mL of solution containing 100.0 ppm (=100.0 µg/mL) Cu2 was mixed with 95.0 mL of unknown, and the mixture was diluted to 100.0 mL in a volumetric flask. The absorbance of the new solution was 0.500. Find the original concentration of Cu2 in the unknown

1.04ppm

What is the difference between eluent and eluate?

The eluent goes into the column (solvent or gas), the eluate is what comes out

Which column gives narrower peaks in a chromatographic separation: plate height 0.1 mm or 1 mm?

0.1mm since smaller plate gives less band spreading

(a) Figure 21-5b shows the concentration profile of a band of material after it has traveled through a column for 2 min and for 26 min. Why is the band broader after 26 min?

(b) Explain why a chromatographic separation normally has an optimum flow rate that gives the best separation. (ie if flow rate is too low or high)

a. Longitudinal diffusion occurs in the direction of the length of the column so the longer we wait, the broader the band diffusion becomes

b. If too low, the band will spread due to longitudinal diffusion. If too high the band will spread due to rate of mass transfer between the mobile and stationary phase is too slow to keep up with the mobile phase

Why does silanization reduce tailing of chromatographic peaks?

It caps the polar hydroxyl groups with less polar groups eliminating hydrogen bonding which reduces tailing

What kind of information does a mass spectrometer detector give in gas chromatography that is useful for qualitative analysis? For quantitative analysis?

Qualitative: Molecular masses of fragments and ion molecular mass

Quantitative: The detector is directly proportional to the quantity of eluate

How is spiking used in qualitative analysis?

Most reliable to identify a compound using chromatography. If the unknown peak is the same, the peak area will increase

a. Calculate the resolution between peaks A and B and between peaks C peak D. Make sure your answers are reasonable.

b. Find the number of plates and plate height for peak D.

c. What are the capacity factors for C and E?

a. AB=0.89; CD=2.5

b. N=1039 plates; H=0.0144

c. kc=12.; ke=25.

Suggest a reason why the optimum linear flow rate is much higher in gas chromatography than in liquid chromatography

Longitudinal diffusion is much faster in gas than in liquid so the flow rate in gas must be faster to prevent band broadening

What is the advantage of a bonded stationary phase in gas chromatography?

The bonded stationary phase does not bleed from column during use

a. explain why plate height increases at very low and very high flow rates for 5-mm and 3.5-mm particles in Figure 22-17.

b. Plate height does not increase for high flow rates for the 1.8-mm particles in Figure 22-17. What does this observation imply about the rate of equilibration of solute between the mobile and stationary phases?

a. Band broadening is synonymous with plate health

b. Solute has adequate time to go through stationary and mobile phase

Why does a thermal conductivity detector respond to all analytes except the carrier gas?

Thermal conductivty measure changes in relative to the carrier gas, so any compound with a different thermal conductivity produces a signal

Why isn’t a flame ionization detector universal?

It only detects compounds that produce ions in a hydrogen-air flame and works for mostly Carbon molecules

Polar solutes were separated by normal-phase chromatography on bare silica, using methyl t-butyl ether and 2-propanol solvent. How would retention times be affected if eluent were changed from 40 vol% to 60 vol% 2-propanol? (2-propanol is more polar than t-butyl ether)

Retention times would decrease with the polar concentration of the solvent being higher

When you try separating an unknown mixture by reversed-phase chromatography with 50% acetonitrile-50% water, the peaks are eluted between 1 and 3 min and they are too close together to be well resolved for quantitative analysis. Should you use a higher or lower percentage of acetonitrile in the next run?

Lower percent of acetonitrile to increase retention times

Suppose that you try separating an unknown mixture by normal-phase chromatography with the solvent mixture 50% hexane-50% methyl t-butyl ether (which is more polar than hexane). The peaks are too close together and are eluted rapidly. Should you use a higher or lower percentage of hexane in the next run?

Higher percent of hexane to increase retention times

How might you increase the resolution without changing solvents or the type of stationary phase?

Slower flow rate, longer column, or smaller particle

Consider a negatively charged protein adsorbed on anion-exchange gel at pH 8.

(a) How will a gradient from pH 8 to some lower pH be useful for eluting the protein?

(b) How would a gradient of increasing NaCl concentration (at constant pH) be useful for eluting the protein?

a. Charge of protein increases so it is less retained

b. Protein will be displaced from the gel from Cl- pushing it off column

A capillary is set up, with the injector end positive and the detector end negative. In what order will cations, anions, and neutral molecules be eluted?

Cations→Neutrals→Anions

What is electroosmosis?

bulk movement of liquid in capillary electrophoresis caused by an electric field pulling the mobile positive ions in the electric double layer toward the cathode, which drags the solution along.

Why is the electroosmotic flow in a silica capillary five times faster at pH 9 than at pH 3?

more silanol groups are deprotonated at high pH, increasing the negative surface charge and strengthening the electric double layer, which enhances bulk fluid movement toward the cathode.