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29 Cards in this Set
- Front
- Back
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In Lab 4, pH reading at the end of the titration was subject to three possible sources of error. Describe them.
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Na+ interference at high pH
Electrode not calibrated above pH 7 Drift in the readings over the course of the titration |
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What should one do frequently to avoid problems with slow drift in spectrophotometric
measurements. [Use correct vocabulary in your answer!] |
Zero the spectrometer with a blank.
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In Lab 6, how and why did the elution order change on column A from that on column B?
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The order of elution was just the reverse on column A that it was on column B. Polarity was the primary determining factor in order of elution on each column. Although relative volatility plays a role in elution order, in our situation where the column temperature was above the boiling point of all the compounds, relative polarity was the controlling factor.
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In Lab 5, why was it best to keep absorbance below 1.0, instead of getting the largest signal (highest absorbance) possible?
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the logarithmic relation of concentration to absorbance results in an increasing uncertainty in the determination of concentration at absorbances greater than 1 (and below absorbance of 0.1). Some of you also correctly pointed out that problems with stray light and similar interferences increase as absorbance increases. The onset of deviation from beer's law depends on a variety of concentration-dependent factors; the absorbance corresponding to the concentration where the law begins to fail is either large or small, depending on how large the absorptivity is.
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What would be the advantages of making a calibration curve?
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A calibration curve can be used to determine concentration even if the response is nonlinear. Calibration is faster if many samples are to be tested. A calibration curve gives more precise results; it therefore gives more accurate results if
the response of the analyte in the samples is the same as it is in the standards. |
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Which of the two methods, calibration or standard addition, would give the most precise results?
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A calibration curve would be more precise than standard addition because a calibration involves interpolation, which is always more precise than extrapolation. Standard addition,
requires extrapolation, so it is less precise. |
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When would standard addition be preferable to the calibration curve method?
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If something in the matrix of the unknown changes the response of the analyte. In this case, the signal measured will be much different from that produced by a standard at the same concentration, and so the calibration curve result will be inaccurate. For example, the unknown may contain an interfering species (that the calibration standards don’t) that complexes with the analyte and changes its spectrum and therefore its absorbtivity. Standard addition would overcome this difficulty.
Also, if only one or two samples are to be determined, standard addition is faster than the calibration curve method. |
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Give an example of a matrix effect in spectrophotometry that standard addition cannot compensate for.
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Standard addition cannot compensate for the interference of a background signal produced in addition to that produced by the analyte. An example is another species in solution absorbing at the same wavelength as the analyte.
The calibration method also cannot correct for such an interference. For both methods, some other means must be found to suppress or correct for the background signal. |
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An analyst reasoned that increasing k′ would improve resolution and so decided to slow the flow rate (decrease the linear velocity) of the helium gas mobile phase. Comment on this strategy.
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Decreasing flow rate wouldn’t increase k′ because, even though tr would increase, tm would also increase proportionally. In fact, if the flow rate or linear flow velocity drops below the minimum in the van Deemter curve, H would increase (N would decrease), and the resolution would worsen.
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What is one means of improving the resolution of separation?
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use a longer column.
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Relate R, N, and L in chromatography.
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Resolution, R, is proportional to N^1/2. N is proportional to L, so R is proportional to L^1/2.
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Are there any drawbacks to the strategy of using a longer column to improve resolution?
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Because resolution improves only as N^1/2 and therefore only as L^1/2, but tr increases proportionally to L, the total analysis time goes up rapidly compared to the improvement in resolution one can achieve. In addition, even though the longer column would give separations with better resolution, it would also result in greater peak broadening, making peaks harder to detect.
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In Probability, what is the equation for expected particle number in a sampling?
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expected particles=np (q=1-p)
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How would careful, slow injection of sample affect a gas chromatographic separation?
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Slow injection spreads the sample out over a longer time, and so peaks will be broadened and perhaps also be asymmetric.
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How would increasing the gas flow rate affect a gas chromatographic separation?
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Retention times will decrease, and bands will sharpen, but, if flow rate exceeds the optimum value, resolution will worsen; This is because the C-term will become unfavorable (compounds will not have sufficient time to diffuse to and equilibrate with the stationary phase before the center of the band is swept downstream). Peaks may also show fronting. On the other hand, if the flow rate was too slow to begin with, increasing flow rate will improve resolution by diminishing the effect of the B-term (less time will be available for bands to diffuse longitudinally up and down the column). Of course, because compounds have different diffusion coefficients, the optimum flow rate will be different for different compounds and so the effect of changing flow rate will affect different parts of the chromatogram differently.
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How would injecting a larger volume of sample affect a gas chromatographic separation?
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Peaks will increase in size, making detection easier peaks will also broaden, worsening resolution, because the initial injection plug will fill a longer length of the column.
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Why is He a much more popular choice as a carrier gas for GC than is N2, even though He is more expensive than is N2 (and is furthermore a non-renewable resource)?
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Because He is a light atom, it has little viscosity. Analyte molecules diffuse faster in He than in the much heavier N2. This means that the optimum flow velocity of the mobile
phase is faster for He than N2. So, separations can be achieved with the same resolution much faster in He than in N2. |
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What equation gives limit of detection?
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(given conc. )X( 3N/S)=L.O.D.
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Could the integration time be increased indefinitely to get any S/N one desired? Explain.
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No, the detector will saturate after some time. In addition, non-random noise begins to
become important at some point. |
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Describe the deficiencies in the theory or assumptions you used for calculating pH meter readings.
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EDHE theory is not accurate above μ = 0.1
Ion pairing is not accounted for, so the value for μ may be overestimated. |
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Name some ways to validate an experiment's procedure.
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• Doing an analysis of a control sample of similar larvae known not to have been exposed to Phenobarbital. This would provide an appropriate zero reference.
• Doing an analysis of larvae that had fed on a Phenobarbital contaminated source. This is similar to spiking the sample with the analyte, but doing it this way would be important because it may well be that Phenobarbital is no longer present in the larvae but only its metabolic products. • Attempting to establish quantitative accuracy that would allow determination of whether a lethal dose of Phenobarbital could have been present in the victim at the time of death. Other things to consider include • Doing the analysis by more than one method • Sending off samples to more than one lab |
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What does standard addition help over come?
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Any interference which affects the signal produced by the analyte
itself will affect the added standard to the same extent as the unknown in the original sample (as long as the sample matrix is kept essentially constant) |
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What can standard addition not overcome?
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an interference that produces a signal itself, such as OH− , the signal from the interfering species cannot be distinguished from that of the analyte and cannot be iliminated by standard addition (or, for that matter, by calibration either).
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give the details of Box Car Avg.
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one run needed
S/N~n^1/2 loss of resolution doesn't remove low freq. noise |
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give the details of Signal Avg.
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many runs needed
S/N~n^1/2 (more time needed) maintains resolution reduces noise for all freq.s can be done as signal is collected (for single points) |
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give the details of Signal Integration
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one run needed
S/N~t^1/2 loss in time resolution doesn't remove low freq. noise often done on-target as signal is collected |
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Van Deemter
B/u |
longitudinal diffusion/avg. flow velocity
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Van Deemter
H-term |
HETP height equiv. theo. plate
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Van Deemter
C*u |
mass transfer
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