Instrumental Analysis ACS Final Prep

what are the difference between calibration sensitivity and analytical sensitivity

calibration sensitivity is the slope of a calibration curve at the concentration of interest while analytical sensitivity is a response to the noise ratio

how to calculate calibration sensitivity

y=mx+b (m is the calibration sensitivity)

graph concentration vs mean analytical signal

how to find the analytical sensitivity

As (concentation) = calibration sensitivity/SD [ ]

how to find the coefficient of variation for the mean of each of the replicate sets

CV = SD/Average x 100

how to find the detection limit for method?

3.3*SD(blank)/calibration sensitivity

voltage in a series

Vt = Vc + Vb + Va

Voltage in a parallel circuit

1/Veff = 1/Va + 1/Vb

Capacitance in a series

1/Ceff = 1/Ca + 1/Cb

capacitance equals

Q/V

if a spectrum is the result of a single scan while another is the result of multiple scans, how man individual spectra were summed to increase S/N from 4.2 to 43?

a) formula?

b) answer

a) (S/N)a = sqrt(n) (S/N)b

b) 105

what type of noise can be reduced by decreasing the temperature of a measurement

thermal noise

what type of noise can be reduced by decreasing the frequency used for the measurement

flicker noise

what type of noise can be reduced by decreasing the bandwidth of the measurement

environmental noise

What would be an appropriate source and detector to determine the emission lines for lithium from 200 to 780 nm?

flame and photo multiplier tube (PMT)

you've been asked to measure the UV absorbance of a sample containing poly cyclic aromatic hydrocarbons, a byproduct of the incomplete combustion of organic matter. What would be an appropriate material for the sample container?

fused silica or quartz

which of the following changes will result in a decreased effective bandwidth?

a) increasing the number of blazes in the monochromatic grating

b) switching from a short-pass filter to a long-pass filter

c) increasing sample concentration

d) decreasing the amount of stray light

a

what would be the distance between blazes for a grating where the first order diffraction line for 600 nm light is reflected at 10 degrees and the angle of incidence light is 45 degrees?

a) formula

b) answer

a) n(wavelength) = distance*(sin(i)+sin(r))

b) 680 nm

which of the following would be considered a discontinuous wavelength selector?

a) quartz prism

b) Czerny-Turner Grating (monochormator)

c)interference filter

d)glass prism

c

which characteristics are desirable for a radiation transducer?

a) high sensitivity to short wavelength light

b) low S/N ratio

c) consistent high output when not illuminated

d) constant response over large wavelength range

e) all of the above

d

what is the appropriate process for atomization for AAS and AES? (3 steps)

desolvation, vaporization, atomization

the doppler effect is one reason for line broadening in atomic spectroscopy. What is the effect on a measured signal?

movement of excited atoms toward the detector causes a shortening in the wavelength of the emitted light

it is better to select an excitation source for AAS with ____bandwidth

smaller

what is the general layout for AES?

sample --> source --> wavelength selector --> detector

a)name the electrical signals that are considered analog?

b) how is the information encoded in an analog signal?

a) current, voltage, charge, and power

b) an analog signal is a signal where the amplitude of the signal changes with time, so the information is encoded either through the amplitude or frequency of the signal (and the signal can be wavelength, magnetic field strength, T,etc)

what is a figure of merit?

it is a numerical value characterizing the quantitative instrument performance and include precision, bias, sensitivity, detection limit, dynamic range and selectivity

How long would it take to discharge a 0.015 microF capacitor to 1% of its full charge through a resistance of 10 mega Ohms?

a) formula

b) answer

a) vc= Vc e^(-t/RC)

b) t = 0.69 seconds

select the type of ionization technique used in MS that would result in significant fragmentation

a) MALDI

b) electron impact

c) high performance

d) chemical ionization

c

which is true for chemical ionization?

a) considered a "hard ionization" technique

b) analyte reacts with electron beam

c) compared to other ionization techniques, less fragmentation occurs

d)compared to other ionization techniques, results in the most repeatable small fragments

c

true or false

formation of ions in electrospray occurs by spraying the sample through a capillary, where it is exposed to a high voltage source, then charged sample undergoes desolvation. Once the Rayleigh limit is achieved, coulombic repulsions overcome any surface tension of the liquid resulting in small, multiply charged species

true

which of the following detectors would be appropriate for measuring the Raman Spectra for samples irradiated with 455 nm light?

a)thermocouple

b)bolometer

c)photographic plate

d)charge-coupled device array (CCD)

D

which of the following detectors would be appropriate for measuring the IR spectra?

a) CCD array

b) photographic plate

c) photomultiplier tube

d) pyroelectric detector

d

which of the following sources would be appropriate excitation source for collecting IR spectra?

a) hollow cathode tube

b) nernst glower

c) Xenon lamp

d) Nd: YAG laser

C

define the term effective bandwidth

the wavelength range where the intensity is half the maximal value. This parameter will be reported for an instrument to inform the user regarding the resolution of data collected using the instrument

for grating, how many lines per millimeter would be required for the first-order diffraction line for 400 nm to be observed at a reflection angle of 7 degrees when the angle of incidences is 45?

a) formula?

b) answer

a) n(wavelength) = distance (sin(i) +sin (r))

1/d = lines/mm

b) 2072 lines/mm

consider an IR grating with 90 lines per mm and 15 mm of illuminated area. calculate the first-order resolution of this grating

a) formula

b) answer

a) wavelength/change wavelength = n* N

b)1350

what source, wavelength selector, container, and transducer should be used in these scenarios:

a) the investigation of the fine structure of absorption bands in the region 450 to 750 nm

b)obtaining absorption spectra in the far IR (20 to 50 micro meters)

c) a portable device for determining the iron content of natural water based on the absorption of the radiation by the read Fe(SCN) complex

d) the routine determination of nitrobenzene in air samples based on its absorption peak at 11.8 micrometers

e)determining the wavelengths of flame emission lines for metallic elements in the region from 200 to 780 nm

f) spectroscopic studies of the VUV region

g)spectroscopic studies in the near IR

a) W lamp, grating, glass window, PMT

b) globar, interferometer, KBr window, pyroelectric

c) W lamp, filter, T1Br or T1I window, Thermocouple

d) flame, grating, flame, PMT

e) Ar lamp, grating, KBr window, PMT

f) W lamp, grating, glass window, photoconductivity

why is specrtofluometry potentially more sensitive than spectrophotometry?

spectrophotometric measurements rely on a difference in signal whereas specrtofluometric measurements are absolute

explain why the wavelength of light fluoresced by a molecule is loner than the exciting light, but light fluoresced by an atom is the same wavelength as the exciting light

in AE, the atoms are typically in the gas phase and will not interact with one other. Therefore, there is no vibration or rotational relaxation observed when an atom fluoresces. However, molecules can vibrate and rotate, and will interact with other molecules, dissipating a little energy with each interaction. Light emitted will be lower energy than light absorbed

Formulas for calculating Stokes shift

convert wavelength to wave number

subtract individual peak from combined peak

absorbance from transmittance

A= -log(T)

What is the difference between H and D discharge lamps as a source for UV detection

H and D lamps differ only in the gases that are used in the discharge. D lamps generally produce higher intensity radiation

What is the difference between filters and monochromators as wavelength selectors

filters provide low resolution wavelength selection often suitable for quantitative work, but not for qualitative analysis or structural studies. Mononchromators produce high resolution (narrow bandwidths) for both qualitative and quantitative work. Photodiodes and PMTS.

What is the difference between photodiodes and PMTs

a photodiode consists of a photo-sensitive pn-junction diode that is normally reverse-biased. An incident beam of photons causes a photocurrent proportional to the photon flux. A PMT is a vacuum tube is a vacuum tue consisiting of a photoemissive cathode, a series of intermediate electrodes called dynodes and a collection anode. Each photoelectron emitted by the photocathode is accelerated in the E field to the first positively charged dynode where it can produce several secondary electrons. Results in a cascade multiplication of 10^6 or more electrons emitted per emitted photoelectron. Photomultipliers are more sensitive than photodiodes, but require a high voltage power supply compared to the low voltage supplies required by photodiodes. Photomultipliers are larger and require extensive shielding. Photodiodes are better suited for small, portable instruments because of their size and ruggedness.

What is the difference between single-beam and double-beam instruments for absorbance measurements.

ingle-beam and double-beam instruments for absorbance measurements.A single-beam spectrophotometer employs one beam of radiation that irradiates one cell. To obtain the absorbance, the reference cell is replaced with the sample cell containing the analyte. With a double-beam instrument, the reference cell and sample cell are irradiated simultaneously or nearly so. Double-beam instruments have the advantages that fluctuations in source intensity are cancelled as is drift in electronic components. The double-beam instrument is readily adapted for spectral scanning. Single-beam instruments have the advantages of simplicity and lower cost. Computerized versions are useful for spectral scanning

what is the difference between conventional and multi-channel spectrophotometers

Multichannel spectrophotometers detect the entire spectral range essentially simultaneously and can produce an entire spectrum in one second or less. They do not use mechanical means to obtain a spectrum. Conventional spectrophotometers use mechanical methods (rotation of a grating) to scan the spectrum. An entire spectrum requires several minutes to procure. Multichannel instruments have the advantage of speed and long-term reliability. Conventional spectrophotometers can be of higher resolution and have lower stray light characteristics

Provide 5 characterisics of absorption measurements that make it amenable to quantitative measurements in nearly every field

easy sample prep

selective

linear response to concentration

sensitive

non-destructive

what are shims in NMR spectrometer, and what are their purposes

In NMR,it is critical to have a uniform magnetic field (the more uniform the magnetic field, the higher probability that all proton spins will have the same Larmor frequency). Shims are metal coils carrying electric current. This current can be adjusted to modify the magnetic field

how are ions created in:

a) electron ionization

b)chemical ionization

a) a 70 eV electron strikes the compound creating a M+

b) the compound reacts with a proton donor to make M+

How are ions seperated in a:

a) single quadropole

b) triple quadrupole

a) Two pairs of parallel rods make the quadrupole. Ions are scanned by varying the direct current voltage (Vdc) and radio frequency voltage (Vrf).Similar tothe ion trap, quadrupoles "trap"ions of a specific mass (only ions with the appropriate mass will be able to travel along the rods). The main differenceis the shape of the electrodes (rods vs ring for quadrupole vs ion trap)

b)three-quadrupole systemthat results in higher ion selectivity.Within the 2ndquadrupole, analyte ions are fragmented via collisions with a neutral gas (collision induced dissociation

what is the detection range for:

a)ion trap

b) single quadrupole

c) triple quadrupole

d) TOF

a) less than 20,000 g/mol

b)1500 to 3000 g/mol

c) 1500 to 3000 g/mol

d)50000 g/mol

describe how increasing the rod length would affect the ion separation for a quadrupole

increases the resolution. The longer the rod, the more time to allow ions with a close but not exact target to get off track

if broadening is mainly due to ____, how should flow rate be changed to improve resolution?

a)longitudinal diffusion

b)finite equilibrium time

c)multiple flow paths

a) increase flow rate

b)decrease flow rate

c) no effect

what are the relative advantages and disadvantages of packed and open tubular columns in GC

packed columns have a high sample capacity. Open tubular columns have a better separation efficiency (smaller plate height), shorter analysis time and increased sensitivity

why do open tubular columns provide greater resolution than packed columns in GC

there is no band broadening due to the multiple path (A) term in the van Deemeter equation

When would you use ___ injection in GC?

a) split

b) splitless

c) on-column

a) is good for high concentration samples, scans where you want high resolution, or if you have a dirty sample

b) good for trace analysis

c) good for quantitative analysis or thermally sensitive samples

determining resolution from t and W

resolution = 2(Tr1-Tr2)/(WA +WB)

list the sources of error associated with pH measurements with a glass electrode

uncertainty of buffer solutions used, junction potential errors, junction potential drifts, alkaline or acid errors at extreme pH values, equilibration time, hydration of glass, temperature difference of calibration solution and analyte solutions, dirty electrode

explain the principle operation of an ion-selective electrode

analyte ions equilabrate between the outer solution and the membrane ligand, creating a slight charge imbalance. Changes in the analyte ion concentration can change the potential difference across the solution-membrane interface, causing a junction potential

Which of the following would decrease retention in ion chromatography:

a) increasing the column length

b) increasing flow rate of mobile phase

c) decrease the sample size

d) increase ionic strength of mobile phase

d

which of the following situations correctly describes the effect of increased flow rate of the mobile phase?

a) increased broadening due to the A constant in the van Deemeter equation

b)increased broadening due to the B constant in the van Deemeter equation

c) increased broadening due to the C constant in the van Deemeter equation

b

A constant = path traveled by analyte

B constant = diffusion of analyte via longitudinal and eddy diffusion

C constant = equilibration time of analyte

the use of a larger diameter stationary phase particle for HPLC would result in which of the following:

a) increased retention time

b) increased back pressure

c) increased plate height

d) increased resolution

c

for a size-exclusion chromatography:

a) a smaller stationary phase particle decreases resolution

b) a larger stationary phase particle decreases resolution

c) a smaller pore size of the stationary phase particles increases resolution

d) a larger pore size of the stationary phase particles increases resolution

c