A.
Suppose we make a new compound containing of C,H, and N
1. We probably know what it was we were trying to make
2. That doesn’t mean we succeeded
3. We must have evidence that the compound we made is what we wanted
The PE 2400 is a conventional elemental analyzer. It
operates by flash combustion of the specimen
encapsulated in tin cups or disks whereby the specimen is
completely combusted at a temperature of 925 °C. The
resulting gases are chemically scrubbed of the halogens
(and of sulfur in the CHN mode) and are separated in a GC
column. Detection is conducted by a thermal conductivity
detector (TCD). Total C, H, and N are calculated with a
precision of 0.1µg. Element ratios are more significant
since they minimize instrument error. Molar element ratios
may be reliably reported for up to three decimal places.
•
•
•
•
•
•
•
•
Measuring Principle
The sample under test is weighed in using a tin capsule. The required
amount is 1.5 to 2.5 mg of organic material and can exceed 10 mg, if
inorganic matter with little carbon content is investigated. After folding the
capsule (looking rather like wrapped tin foil) the sample is placed in the
auto-sampler.
The tin capsule enclosing the sample falls into the reactor chamber where
excess oxygen is introduced. At about 925 °C the material is "mineralized".
Formation of carbon monoxide is probable at this temperature even under
these conditions of excess oxygen.
The complete oxidation is reached at a tungsten trioxide catalyst which is
passed by the gaseous reaction products. The resulting mixture should thus
consist of CO2, H2O und NOx. But also some excess O2 passes the
catalyst.
The product gas mixture flows through a silica tube packed with copper
granules. In this zone held at about 640 °C remaining oxygen is bound and
nitric/nitrous oxides are reduced. The leaving gas stream includes the
analytically important species CO2, H2O und N2. Eventually included SO2 or
hydrohalogenides are absorbed at appropriate traps.
High purity helium (99.999%) is used as carrier gas. Finally the gas mixture
is brought to a defined pressure/volume state and is passed to a gas
chromatographic system. Separation of the species is done by so called
zone chromatography. In this technique a staircase type signal is registered.
Step height is proportional to the substance amount in the mixture.
Blank values are taken from empty tin capsules
Calibration is done by elemental analysis of standard substances
(acetanilide) supplied by the instrument's manufacturer for this purpose.
Interpreting Elemental Analysis Results
Formula
Mass
calc/found %C %H %N
http://www.chemcalc.org/ has a free calculator for elemental and mass spec
First, calculate the theoretical values for the complex you expected to make.
CrC12H26N4Cl2
349.267
Calculated 41.27 7.50 16.04
Next, compare to the found values. Each element needs to be within 0.40% to prove purity.
42.08 8.30 16.28
Found
This is not a match. They found significantly more C and H than expected. How do we explain
this? We must have less non-Bcyclen mass than we thought. Adding H2O (always a possibility
because many complexes are hygroscopic) would increase H, but would decrease C. A previous
Cr crystal structure had a bound hydroxo in place of a chloro. I will try that based on experience.
CrC12H26N4Cl(OH)
330.822
Calculated 43.57 8.23 16.94
This goes too far. H is ok, but this gives too much C and N. If the complex absorbed water while
being handled, that would add H (which would be ok) and reduce C and N (which we need to do).
348.837
Calculated 41.32 8.38 16.06
CrC12H26N4Cl(OH) • 1H2O
We made progress, but now C is low again. Try adding less water. It is possible to have
fractions of water molecules. I’ll try half a water molecule per complex. To do this on the
website, put the whole complex in parentheses followed by a 2, then H2O in parentheses with a 1.
(CrC12H26N4Cl(OH))2(H2O)1 on website
679.658/2 Calculated 42.41 8.30 16.48
= 339.829
= CrC12H26N4Cl(OH) • 1/2H2O
Now we have a calculated value within 0.40% for all three elements. Other evidence to show the
hydroxo ligand will be needed (as well as an explanation for how it got there), but based on past
experience, this is a reasonable structure. With that other evidence, it would also be acceptable
for publication.
•
Mass Spectrometry = analysis to find out the mass of your compound
– Organic molecules: structural information by looking at fragments
– Metal complexes: most interested in showing that the metal is bound to the ligand
– Looking for the Molecular Peak or Cation Peak
•
How does a Mass Spectrometer Work?
– The sample is heated to vaporize under vacuum, and then ionized
– Electron Impact: high energy beam of electrons knocks out electrons from the molecule
– Electrospray: High voltage applied as a liquid is pushed through a capillary results in
highly charged droplets. Evaporation of the solvent results in ionized solute molecules.
– Charged plates accelerate the ion into the mass analyzer
– Magnetic fields cause the path of the ion to curve based on mass
– The detector is tuned to one mass at a time to generate the spectrum
Average Atomic Masses and Isotope Splitting
1. Why isn’t the mass of Carbon listed at exactly 12 on the periodic table?
2. Natural carbon: 12C(98.89%), 13C(1.11%), and 14C (negligible)
a. Avg. Mass = (0.9889)(12 amu) + (0.0111)(13.0034 amu) = 12.01 amu
b. No individual atoms have this mass
c. On average, all natural carbon atoms have this mass
d. 12.01 amu is the correct value to use for calculations involving carbon
3. What is avg. mass of Cu?
4. 69.09% 63Cu (62.93 amu) and 30.91% 65Cu (64.93 amu)
5. Copper complexes will never show up as simple peaks, but complex ones