Exam 1 Flashcards
A hollow tube that is filled with particles coated with stationary phase material.
Packed Column
It describes the velocity of solvent as it travels through the column.
Linear Flow Rate
It is the elapsed time between sample injection and detection.
Retention Time
It is the fluid that enters the column.
eluent
The separation medium that is bonded to the surface of particles packed in a column.
Stationary Phase
It is a liquid or gas solvent that carries the sample through the separation column.
Mobile Phase
A hollow capillary tube with inner walls that are coated with stationary phase material.
Open Tubular Column
It is the fluid that exits the column.
eluate
A chromatography column having which of the following plate heights would be most efficient?
The smallest
Is longitudinal diffusion of the solute a more serious problem in gas or in liquid chromatography?
What is the major reason for the difference?
Gas Chromatography
Diffusion coefficients of gases are much greater than those of liquids.
Which of the following choices correctly describes the effect of increasing the particle size of the column packing in an HPLC column has on the terms of the van Deemter equation?
B is unchanged; A and C are increased.
H
Plate height
Ux
Flow rate
A
Multiple path (eddy diffusion)
C
Resistance to mass transfer
B
Longitudinal diffusion
For an open tubular column (not capillary electrophoresis), which term(s) in the van Deemter equation does(do) not affect the plate height?
Eddy Diffusion
Consider the van Deemter equation. Which of the following methods can be used to minimize the effects of the resistance to mass transfer term on plate height? Choose all that apply.
Decreasing column radius
Decreasing stationary phase thickness
Consider the van Deemter equation. Why does increasing the flow rate reduce the contribution of longitudinal diffusion to total plate height?
Increasing the flow rate will decreases the time the solute resides in the column, resulting in less diffusional broadening.
It is used to atomize the sample.
Flame
It is typically a photomultiplier tube that generates an electric signal when struck by photons.
Detector
It is composed of the same element that is being analyzed in the sample.
Hollow-Cathode Lamp
It is used to select one line of radiation from the source and remove as much emission for the atomizer as possible.
Monochormator
Atomic absorption and atomic emission spectroscopy each require a flame to atomize samples. Which of the following statements explains which technique requires a more stable flame temperature and why?
atomic emission, because a small population of the excited state can vary significantly as the flame temperature is changed
Hollow-Cathode Lamp
A source that emits narrow atomic lines
Zeeman Effect
The splitting of atomic lines in the presence of a parallel magnetic field
Releasing agent
An additive that reduces chemical interference
atomization
The conversion of sample components into individual atoms
Spectral Interferance
Signal from species not under study that overlaps with the analyte signal
Neublization
The conversion of a liquid sample into very small droplets
Matrix Modifier
An additive that prevents the loss of analyte during charring
Boltzmann Distribution
Describes the population ratio of different states at thermal equilibrium
Pressure broadening
An increase in the atomic linewidth due to collisions between atoms
Atomic Emmisions
Atoms are thermally promoted to the excited state through collisions in the plasma. As they return to a lower energy state, they emit photons. No excitation source outside of the plasma is required.
Atomic Flouresenca
The sample is atomized in the flame. A laser is used to promote the atoms in the flame to an excited state. As the atoms return to the ground state they fluoresce.
Atomic Absorption
The atomized sample in the flame absorbs a portion of the radiation emitted from the source, which is composed of the same element as that which is being analyzed. The amount of radiation that passes through the flame unabsorbed by the analyte is measured.
Glass bead
The spray is directed against it and breaks the droplets into smaller particles.
Baffles
It promotes further mixing of the mist, oxidant, and fuel, and prevents large droplets from reaching the flame.
Nebulizer
It uses the rapid flow of the oxidant to break the sample into a fine mist.
Spray Chamber
It is where the fuel, oxidant, and aerosol are mixed before entering the flame.
Which of the following are advantages of a furnace over a flame in atomic absorption?
- higher sensitivity because the atomized sample is in the optical path longer
- a smaller sample amount is required
In atomic spectroscopy there are three main methods of atomization?
combustion flames, graphite furnaces, and inductively coupled plasmas
combustion flames
- Requires at least a 10mL Sample
- Most common fuel-oxidant combination is acetylene air
graphite furnaces
- MAX temp Rec 2550 C
- req. only tens of microliters sample
- Must dry char then automize
inductively coupled plasmas
- Temp reaches 10,000 K
- Very expensive
- can be coupled with max spectrometry
Rank the following atomic analysis methods in order of increasing sensitivity.
- Flame atomic absorption
- Furnace atomic absorption
- plasma emissions
- inductively coupled plasma mass spec.
Anode
It is made of tungsten.
Transparent window
It is made of glass or quartz and allows radiation to pass through uninterrupted.
Ne or Ar Gas
It is ionized when a potential of 500 V is applied between the electrodes.
Hollow Cathode
It is cylindrical and made of the element whose emission lines are desired.
What are the different forms of interference?
Spectral
Chemical
Isobaric
Ionization
Spectral
interference from the overlap of analyte signal with signals from other atoms or molecules, the flame, or the furnace
Chemical
interference from any component of the sample that reduces atomization of the analyte
Isobaric
interference by ions with a similar mass-to-charge ratio
Ionization
interference of analyte atoms which decreases the concentration of neutral atoms
Which of the following describe(s) the beam chopping method of background correction? Check all that apply.
- uses the difference in signals when the lamp is blocked and when the lamp is not blocked to give the desired analytical signal
- corrects for flame emission
Which of the following statements correctly describes the deuterium lamp background correction method?
Deuterium background correction is used to reduce both background scatter and absorbance when using a flame as an atomization technique for atomic absorption spectroscopy. The emission from the broadband deuterium source and the hollow-cathode lamp are alternately passed through the flame. The deuterium source is only absorbed by the background, while the hollow-cathode lamp emission is absorbed by both the analyte and the background. The difference between the deuterium lamp absorbance and the hollow-cathode lamp absorbance is the background corrected signal.
Which atomic absorption atomization method is commonly used in conjunction with Zeeman background correction?
Graphite Furnance
Choose the statement that correctly describes background correction based on the Zeeman effect.
The atomic line output from a hollow-cathode lamp is split into three individual wavelength components when a parallel magnetic field is applied. The three components consist of an unshifted wavelength and two shifted to higher and lower wavelengths. The analyte and background absorption are measured when the field is off, and the background only is measured with the field turned on. The unshifted portion has orthogonal polarization to the shifted portions, and is therefore not observed by the detectors. The magnetic field is alternated and the background signal is subtracted from the total signal.
Choose the correct pair of advantages and disadvantages for Zeeman background correction.
Advantages: highly accurate, measured at the analyte wavelength, rapid Disadvantages: increased spectrometer complexity, high cost, decreased sensitivity
a blank sample containing all components except for analyte that has not been subjected to the steps of the chemical analysis
Reagent blank
concentration interval over which there is measurable response to a change in analyte concentration.
dynamic range
quantities reported upon completion of statistical analysis of the data
results
a blank sample exposed to the environment at the sample collection site and transported in the same manner as other samples back to the lab
field blank
individual measurements
raw data
concentration interval over which the change in detector response is proportional to the analyte quantity
linear range
concentration interval over which linearity, accuracy and precision meet specifications
range
a blank sample containing all components except analyte that has been carried through all steps of the chemical analysis, including sample preparation
method blank
amounts or concentrations determined by using a calibration method
treated data
What are the 3 parts of quality assurance
Use Objectives, Specifications, Assessment
The reproducability of a result
precision
Also called injection precision, reproducability of an instrument reading when the same amount of one sample is introduced
Instrument Precision
reproduce of a measurement on a uniform material by one person on the same day with the same equipment
intra assey precision
reproduce measurement different people different day, different equipment SAME Lab
intermediate percision
SAME sample different people different lab
inter-laboratory precision
Five aliquots of the same sample are injected for a gas-chromatographic analysis.
Prescision
A known amount of analyte is added to an aliquot of the sample and analyzed with the sample.
accuracy
Blood samples are sent to three separate laboratories for analysis using the same method.
Precision
Identical standards are analyzed by two different methods.
accuracy
The signal is equal to the average signal of the blank plus three times the standard deviation of the signal from the blank.
Limit of Detection
It is the minimum amount of analyte that produces a signal which can be measured with reasonable accuracy.
Limit of Quanitation
The concentration is equal to three times the standard deviation of the signal from the blank divided by the slope of the calibration curve.
Limit of Detection
The concentration is equal to 10 times the standard deviation of the signal from the blank divided by the slope of the calibration curve.
Limit of Quanitation
It is the minimum amount of analyte that produces a signal that is significantly different from the blank.
Limit of dectection
Standard Addition
- Known Quantities of analyte are added to unknown
- Method of choice if Matrix of sample will effect Signal
Internal Standards
- Known amount of standard different then analyte is added to unknown
- Method of choice when instrument response or sample amount varies from run to run
External Standards
Standards that are different then unknown
-measures instrument response as a function of known analyte
Quality Assurance Vs Quality Control
Q.A.:All encompassing system that controls data generation
Q.C: Refers to procedures, policies, and practices regarding data
Warning limit
a line at ± 2s/√ n (or 95% confidence level)
If a check is over the warning limit, a second check is
performed.
Action limit
a line at ± 3s/√ n (or 99.7% CI)
¡ If a check is over the action limit, recalibration/removal is
required.
Validation of a calibration curve depends on
- Correlation coefficient (R2)
- Absence of a response to the blank
- Time since last calibration
- Performance on a calibration check sample
Three common types of calibration are
- External standard
- Internal standard
- Standard addition
LDR: linear dynamic range
is the range over which the
equation of the curve fit can be used to calculate
unknown concentrations
Linear regressions are generated by?
-the method of least
squares
-Equation of the line given by y = mx + b
- Correlation coefficient R2 is a measure of “goodness of fit”
Advantages of Atomic Absorption
-Detection limits in low ppm to high ppb range -Narrow bandwidths = little overlap in spectra, allowing for detection of many elements at once
Limitations of Atomic Absorption
-Need one lamp per element; lamps
are expensive
-Need separate optimization for each
element
Atomic Fluorescence
Laser used to excite atoms in flame -The atoms fluoresce to return to ground state - Potentially 1000 X more sensitive than AA - Easier to detect a weak signal above a dark background than a small decrease in a large amount of light - Equipment is not common, so it’s not widely used
Atomic Emssions
- Flame can only excite small portion of atoms
- Difficult to detect elements by flame AES
- Plasma burns several thousand degrees hotter than a flame
- Increases AES sensitivity (down to low ppb)
Advantages of Atomic Emissions
- Can scan or use a grating to measure emission at all wavelengths
- This is truly a multielement technique
- Requires no lamps
- Follows Beer’s law
Disadvantage of Atomic Emissions
-Plasma machines are much more expensive than flame AA
Atomization
Flame
-Original method, used for decades
Furnace
-Electrically heated, usually made of graphite
Plasma
-Very high temperature; stable, inert (Ar) environment
Furnace Automization has 3 processes?
Drying: heating at low T to remove solvent (~ 125 °C for ~20 s)
Charring (pyrolysis): decomposing organic matter (~1500 °C for ~60 s)
Atomization: sample decomposed to atoms (~2100 C for ~10 s)
Inductively Coupled
Plasma (ICP)
High temperature (~ 2X that of flame), stability,
relatively inert (Ar) eliminate much of
interference encountered with flames
Allows for simultaneous, multielement analyses
when combined with AES
ICP-AES now preferred over Flame-AA, but it’s
more expensive to buy and operate
