Deck 21: Spectrophotometers

Question 1

Which of the following statements regarding double beam spectrophotometers is NOT true?

A)The reference beam allows for automatic correction for source intensity fluctuations.
B)Comparison of the power emerging from the sample and reference beams determines absorbance and transmittance.
C)The reference beam allows for automatic correction for detector drift.
D)The reference beam is created using a stationary mirror.
E)The reference beam is created using a rotating mirror.

Answer 1

The reference beam is created using a stationary mirror.

Question 2

Which statement(s)is(are)true for photomultiplier tubes?
I Photomultiplier tubes are very sensitive detectors,amplifying the photoelectron current by approximately 1 million.
II Emitted photoelectrons are accelerated toward positively charged electrodes called dynodes.
III Electrons strike the dynode with a kinetic energy less than the original emitted kinetic energy.
IV For each electron striking a dynode,more than one electron is knocked free.The additional electrons are accelerated toward the next dynode in the detector where amplification is repeated.

A)II
B)I and IV
C)II,III,and IV
D)I,II,and IV
E)II and III

Answer 2

I,II,and IV

Question 3

Which is NOT true for photodiode arrays?

A)Photodiode arrays respond to more than one wavelength.
B)Photodiode arrays require a polychromator to detect more than one wavelength.
C)Photodiode arrays allow for rapid acquisition of an entire light spectrum.
D)Photodiode arrays have higher signal-to-noise ratios.
E)Spectrophotometers using photodiode arrays are dispersive spectrometers.

Answer 3

Spectrophotometers using photodiode arrays are dispersive spectrometers.

Question 4

Which of the following monochromator terms is NOT correctly defined?

A)grating: optical element with closely spaced lines
B)diffraction: bending of light by a grating
C)refraction: scattering of light by a lens or prism
D)polychromatic: multiple wavelengths
E)monochromatic: one wavelength

Question 5

Spectrophotometers employ various sources of light.Which of the following is NOT true for spectrophotometer light sources?

A)Tungsten lamps emit radiation over the visible spectrum and parts of the ultraviolet and infrared spectrums.
B)Dueterium arc lamps use controlled electric discharges to cause D₂ to dissociate and emit visible radiation.
C)Light emitting diodes emit narrow bands of visible and near-infrared radiation.
D)Lasers emit radiation of a single wavelength.
E)A silicon carbide globar emits infrared radiation when heated to near 1500 K.

Question 6

The true absorbance for a 1.0 x 10⁻⁵ M solution is 0.7526.If the percentage stray light for a spectrophotometer is 0.56%,calculate the percentage by which the apparent concentration deviates from the known concentration.

Question 7

Which statements are true for optical fibers?
I Optical fibers carry light by total internal refraction.
II Optical fibers are constructed of a high-refractive-index transparent core enclosed in a lower refractive-index transparent cladding.
III Optical fibers exploit Snells Law,n₁ sin ₁ = n₂ sin ₂,to transmit light along the length of the optical fiber.
IV If n₁/n₂ < 1,there is an allowed range of angles _i in which essentially all light is reflected at the walls of the core,with negligible amounts of light penetrating the cladding.

A)I and IV
B)I,II,and IV
C)II,III,and IV
D)II and III
E)II and IV

Question 8

The monochromator for a new instrument design must successfully resolve 0.005 nm at 631 nm.Calculate the minimum number of grooves per centimeter for the diffraction grating if the grating has a length of 5 cm.Assume first order diffraction.

Question 9

Which is NOT an appropriate detector for a spectrophotometer?

A)photomultiplier tube
B)thermal conductivity
C)photodiode array
D)charged coupled device
E)phototube

Question 10

A____________________ disperses light into its component wavelengths and selects a narrow band of wavelengths to pass on to the sample or detector.

A)polychromator
B)photodiode array
C)monochromator
D)charge couple device
E)photomultiplier tube

Question 11

Calculate the minimum grating length to successfully resolve 616.50 nm from 616.59 nm for first-order diffraction.The available diffraction grating is 10³ grooves per cm.

A)13.7 cm
B)1)00 cm
C)111.1 cm
D)0)15 cm
E)6)85 cm

Question 12

Stray light is a problem for spectrophotometers that are not tightly sealed.What is the impact of stray light on the recorded absorbance?

A)The apparent absorbance is greater than true absorbance and increases as the amount of stray light increases.
B)The apparent absorbance is greater than true absorbance and decreases as the amount of stray light increases.
C)The apparent absorbance is less than the true absorbance and increases as the amount of stray light decreases.
D)The apparent absorbance is less than the true absorbance and decreases as the amount of stray light decreases.
E)Impossible to determine effect of stray light on measured absorbance.

Question 13

Which property of noise does signal averaging exploit to improve the signal-to-noise ratio?

A)Noise is loud.
B)Noise is random.
C)Noise is predictable.
D)Noise drifts.
E)The amplitude of noise is a constant.

Question 14

Ni emits light at 345.85 nm and 346.17 nm in an atomic emission spectrophotometer.Can the spectrophotometer's monochromator successfully resolve 345.85 nm from 346.17 nm? The grating has a length of 5.0 cm in length and there are 5500 grooves per cm.Assume first-order diffraction.

Question 15

Two components of an interferometer are a moveable mirror and a beam splitter.Which of the following are NOT true statements regarding each component in creating an interferogram?
I When the mirror travels a distance of /4,deconstructive interference occurs.
II The beam splitter alternates the incident light between a reference and the sample.
III When the mirror travels a distance of /2,deconstructive interference occurs.
IV The beam splitter splits the incident light between the stationary mirror and the moveable mirror.

A)I and II
B)I and IV
C)II and III
D)II and IV
E)III and IV

Question 16

____________________ response is a function of incident light and produce an electrical current when struck by .For a given radiant power,current is____________________ analyte concentration.

A)Optical ;photons;constant for varying
B)Detector;photons;proportional to
C)Analyte;analyte particles;constant for varying
D)Detector;analyte particles;inversely proportional to
E)Optical ;analyte particles;inversely proportional to

Question 17

The signal-to-noise ratio for a single spectrum is 2.18.How many additional scans are required to increase the signal-to-noise ratio by a factor of 50?

Question 18

Which property of laser light is NOT correctly defined/described?

A)monochromatic: emits one wavelength of light
B)extremely bright: high power at one wavelength
C)collimated: parallel rays of light
D)polarized: electric field oscillates between two perpendicular planes
E)coherent: all waves in phase

Question 19

____________________ are optical sensors based on an optical fiber.

A)Optodes
B)Optides
C)Optectors
D)Optubes
E)Pristrodes

Question 20

There are many types of noise.Of the noise types below,which is NOT correctly defined?

A)line noise - noise that occurs at discrete frequencies
B)drift noise - low frequency noise caused by flickering or drifting of light intensity
C)Gaussian noise - noise amplitude is dependent on frequency
D)Johnson noise - random fluctuations of electrons in electronic devices
E)shot noise - noise that arises from random variation in the number of photons reaching the detector or small number of electrons and holes generated in a semiconductor