The doping of di ff erent elements in the silicon can

Info icon This preview shows pages 14–16. Sign up to view the full content.

View Full Document Right Arrow Icon
The doping of di ff erent elements in the silicon can make electron rich & poor domains. As seen in (a) holes and electrons are randomly distributed. Adding an outside power source (b) causes a depletion layer where conductance is zero. When a photon of light strikes the junction (c), holes and electrons form which provide the completion of the circuit. The current is proportional to the radiant power. Photodiode Arrays– PDAs are a series of silicon photo diodes, with each having a storage capacitor, and a switch that are combined in a integrated circuit on a silicon chip. The number of sensors (silicon photodiodes) in a PDA range from 64 to 4096. The slit width of the instrument allows the radiation to be dispersed over the entire array, allowing the spectral information to be accumulated simultaneously. PDAs are not as sensitive nor have the same S/N ratio as the PMT, but one gains the advantage of gathering multi-channel information (all of spectrum collected simultaneously). Advantage of the PDA is recording the entire spectrum in a fraction of the time required for a conventional scanning spectrometer to scan one wavelength at a time. An example of PDA use is in atomic emission spectroscopy (AES), UV-vis spectrophotometry, fluorescence spectrometry, Raman spectrometry. Errors in Spectrophotometry: Choice of the Wavelength and Bandwidth –
Image of page 14

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
For Quantitative Analysis Absorbance should not be too large or small(~0.4-0.9 units) Absorbance should be measured at the lambda max providing no interference from other species at this lambda. Monochromator bandwidth should be as large as possible but small compared to the spectral bandwidth. Slit width to small allows less light to reach the analyte, smaller S/N ratio and precision. Too wide a monochromator band width (slit width) distorts the peak shapes of the spectrum. As shown in the figure above; the instrumental setting with the band width at 0.1 nm and the choice of 609 nm for λ max would be desirable for the spectra shown. At the 2.0 nm setting there is no fine structure shown in this spectrum. Stray Light, Electrical Noise, Cell Positioning- Stray light can cause problems: Stray light arises from two major sources: 1) Misdirected rays coming from the monochromator.
Image of page 15
2) Light coming from outside the instrument such as the sample compartment lid not closed properly. Other concerns; the correct choice of the sample cell (does it require glass or quartz?) the alignment of the sample cell (and/or the sample cell holder) dust & fingerprints on the cell Obtaining Quantitative Information from UV-vis Absorption Spectrophotometry Beer's Law (aka Beer-Lambert Law) A = ε X b X C where c is the concentration (M), b is the path length (cm), and ε is the molar absorptivity (M –1 cm –1 ). ε is wavelength dependent. Beer's Law works for dilute (~ 0.01 M) solutions in which the absorbing species is not participating in a concentration-dependent equilibrium.
Image of page 16
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern