Wednesday, October 23, 2019
Instrumental analysis chemistry Study guide
IR sources Ã¢â¬â consist of inert solid and heated electrically to very high temperatures 1 500-KICK to get a continuum of radiation. Unrest Glower- composed of rare earth oxides (Zero, YEA, and Error) current is passed thru the device so that the device can reach the high temp of 1200-KICK, spectral out in the IR region Global source- silicon carbide rod heated to 1300-KICK Disadvantage to using this is water cooling is required to prevent electrical arching.Incandescent wire sources- tightly wound spiral monochrome wire heated by electrical current to kick IR absorption requirement 1 source of continuous IR radiation 2 IR transducer that's sensitive Mercury Arc Source (Far Ã¢â¬ËR) only for Far IR region only has enough radiant power In this region. Quartz Jacketed tube containing Hag vapor pressurized greater than 1 ATM, electricity passed thru this Hag vapor (pressure emits a continuum of radiation In the far IR region. ) Disadvantage and the advantage to this source Is that It only is seed for the Far IR region only and can't be used for any other IR region.Tungsten Filament lamp (Near IR region)- ordinary filament lamp, convenient source for the near IR of 4000-12800 corn-l , Inexpensive but restricted to Near IR region) CO laser source- laser produces a band of radiation in 900-1100 cam -1, can get 100 closely spaced discreet lines, can tune the laser to a specific line. (More expensive) Advantage- radiant power available In each line (gets several orders of magnitude increase) Thermal transducer-response depends on the heating effect of radiation, radiation is absorbed by a small black body and resultant temp rise is monitored, detects temperature changes that is produced.Heat capacity of the absorbing element must be as small as possible if detectable temp change is to be produced, radiant power of the spectrophotometer beam is IOWA-7 to IOWA-9 watts ( only observes a few thousandths of K change in temp) Has to be in a vacuum and shielded from ther mal radiation emitted by nearby objects Beam from the source is chopped at a specific frequency different from extraneous noise outside of it. Try to minimize size and thickness of the absorbing element to concentrate the entire beam of IR radiation on he surface of the absorbing element.Thermocouples- consist of a pair of Junctions for when 2 pieces of metal (Bismuth and Antimony are fused together BTW metals a potential develops is temp dependent at the Junction potential is temp dependent and varies with the change in temp. Junction is often blackened to improve absorbing capacity, also sealed in a vacuum chamber with a window that's transparent to the IR radiation. Can respond to temp difference, and enhance sensitivity by connecting several thermal couples together= thermopile.Ferroelectric transducers- insulators with special thermal and electric properties instructed from single crystalline wafers. Electric field is applied across material and electric popularization takes pl ace. When there is temp change by radiating it w/ IR radiation then the charge distribution is altered, can be detected as current , used on most Fast response times and track in the time domain) FITS Most commonly used region is mid- IR region (670-4000 CM A-I) can use near IR for low met hydrocarbons Far IR region- used inorganic structures and metal organic species.There is no good solvent that is transparent all the way throughout the IR region of possible interest. Water and alcohol are seldom used BC they strongly absorb and attack the alkali metal halides that make up the cell windows. Liar cells tend to be more narrow in width, than those used in I-JPL's Chromatography 2 primary components Mobile phase- transports signal through the column (can be a gas or a liquid or a supercritical liquid) Stationary Phase- immiscible solid that's fixed in place in a column or on a solid surface.The components in a mixture distribute themselves BTW the pm and the SP to have separation to o ccur the species have to separate themselves BTW mobile and stationary. Stronger species interact more with the SP and more slowly with the mobile phase. -differences in mobility cause the sample components to separate into distinct bands or zones Classification of chromo methods 1) Physical means- when the PM and SP are brought into contact with one another (column chromatography SP held in a tube, pm is pushed through w/ pressure. ) Planar Chromatography- SP supported on a flat plate or piece of paper. PM moves thru the SP by capillary action or gravity. Elution of Bands/ Zones on a Chromatography column One of the component B is moving slower than component A BC it is interacting with he SP. Limitations of linear response BTW analyze concentration and TTS 1) Need distinct stretch or bend 2) Linear response 3) Solvent transmittance is constant.Data collected with Chromatography with peaks that correspond to solute that contain 1) Retention time 2) Peak area Retention time Ã¢â¬â time it takes for the analyze peak to reach the detector (Try) (Tm)- corresponds to the time it takes for the pm to move to the detector. (kill a)- Retention factor used to describe the migration rated of solutes on columns Zone broadening and Column Efficiency- column is more efficient if you have less reddening of peaks Gaussian shape of an ideal chromatography zone; attributed to the random motion of the solute molecules in the chromatographic zones.The size of peaks increases as it moves down the column b/c more time allows spreading to occur. The size of the peak is directly related to the Ã¢â¬Å"residence timeÃ¢â¬ ; the time spent in the column and indirectly related to the velocity of the mobile phase. 2 methods of describing column efficiency 1) Plate height (h) 2) Plate count (N) Efficiency of chromatography columns increases with increased plate count and decreasing plate height.Variables affecting Zone broadening ) effect of pm flow rate in general the magnitude of eff ect on column efficiency depends on the length of time the pm is in contact with the SP (depends on flow rate of the pm) 2) Van Demeter plot Ã¢â¬â efficiency studies by determining the H as a function of PM velocity The smaller the plate height the better the column efficiency (represented on the graph the lowest point on the curve) 3) Linear flow effects zone broadening Theoretical Plate height and Column Variables A) Linear velocity of PM-distance per unit time traveled by the PM ( linear flow rate) B) Diffusion coefficient in PM Ã¢â¬â rate at which the molecules diffuse across a plane of nit area and the concentration gradient in the direction of diffusion in the PM C) Diffusion coefficient in SP -rate at which the molecules diffuse across a plane of unit area and the concentration gradient in the direction of diffusion in the SP D) of the solutes) E) Retention factor Ã¢â¬â time spent by the solute in the SP to the time spent in the pm.