CHEM*3440
Instrumental Analysis
MidTerm Examination
Fall 2004
Duration: 2 hours
[Pd] ppm
0
80
205
325
Instrument
Signal
14.1
27.7
48.4
68.6
1. (10 points)
An atomic absorption experiment found the following results for a
series of standard solutions for dissolved palladium metal ion. The data were plotted
in Excel and from the LINEST function, the following values of the parameters were
obtained:
m = 0.1674
b = 14.17
s
m
= 0.00051
s
b
= 0.101
s
r
= 0.1267
(a)
Determine the concentration (in ppm) of an unknown Pd sample whose average signal was 56.5.
(b) Calculate the standard deviation of this result, given that the above average was the result of 5 measurements.
(c)
What is the 95% confidence interval for this result?
2. (10 points) The following data arose out
of a determination of Cr in water.
The
protocol specified to run a blank, the sample
and the sample with a series of added
standards. Assume that the added standards
did not substantively affect the solution
volume.
Using a graphical approach, calculate the
concentration of Cr in the unknown sample.
Solution
Blank
Unknown
Addition 1
Addition 2
Addition 3
Added Cr
(ppm)
0.000
0.000
2.000
4.000
6.000
Run #1
0.005
0.537
0.666
0.790
0.912
Run #2
0.009
0.548
0.659
0.784
0.901
Run #3
0.006
0.531
0.667
0.805
0.935
3. (5 points) A signal is sampled at a frequency of 15.000 kHz.
The signal actually consists of three sinusoidal
signals at 6.301 kHz, 11.244 kHz, and 32.863 kHz.
If no filtering is applied, at what frequencies would the
signals appear due to signal aliasing?
4. (7 points) Below is a voltage divider network used in a photomultiplier tube.
The resistors shown give an
increasingly large potential step to the next dynode in the chain, assuring that the large volume of electrons are
successfully accelerated to it.
Calculate the potential at each step (B  H) as indicated, given that the potential
at step A is that of the potential source: 1000 V.
A
B
C
D
E
F
G
H
1k
Ω
2k
Ω
3k
Ω
4k
Ω
5k
Ω
6k
Ω
7k
Ω
8k
Ω
1000 V
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(8 points) A photometer with a linear response to light gave a reading of 685 mV with a blank in the light
path and 179 mV when the blank was replaced by an absorbing solution.
Calculate
(a)
the percent transmittance of the sample solution.
(b)
the absorbance of the sample solution.
(c)
the percent transmittance expected if the solution concentration was 1/2 of the original concentration.
(d)
the signal (in mV) expected if the light path were doubled.
7.
(9 points) Explain and relate the following collection of terms:
(a)
Reactance, resistance, and impedance.
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 Fall '06
 DanThomas
 Atom, Operational Amplifier, calibration curve, ppm, percent transmittance, thermal noise

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