Skip to main content
Chemistry LibreTexts

6.8: TA Notes

  1. Last updated
  2. Save as PDF
  • Page ID
    460969

    • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    This page is directed toward laboratory teaching assistants, but is deliberately left open for students to read.

    TA Notes

    After completing this module your students will be able to...

    • describe external calibration and the method of standard additions.
    • describe the pros and cons to using different calibration methods.
    • describe how to validate a calibration using a spike.
    • explain what a matrix effect is, and how to recognize it.

    Pre-Semester Preparations

    Do the following during the TA training and orientation week(s)

    1. Check that the materials listed below are available and sufficient for all students to use them. If they are running low, please create more or inform the instructor. 

      • 0.1 M Nitric Acid should be prepared by dissolving 66 mL of 70% nitric acid (15.8 M) in 10 L of ultrapure water.

        Caution

        This should only be done by a chemist trained to safely handel concentrated nitric acid. 70% nitric acid penetrates standard lab gloves and acid-resistant 5 mil neoprene gloves should be worn. 

      • Pb standard solution, 1000 ppm is provided. This solution is much too concentrated for the students to use directly in their calibration. Thus, a more dilute solution should be made first. A solution of 1 ppm = 1000 ppb is more suitable as a working solution. 

        Caution

        Pb is very toxic to humans and other living organizsms. Wear PPE and handel carefully. 

      • Calibration solutions: A five-point calibration (including the blank) is typical, and a calibration curve should range between 0 and 2-3 times the concentration of the analyte in the samples. TAs should carefully prepare excess external calibration solutions as they complete the module, and have them on hand for instrument performance checks during the semester. 

      • 1g/L Na solution should also be prepared or available to run a sodium bullet test. 

    2. Complete instrument training with Dr. Haas.

    3. Perform the entire experiment as if you were a student.

    4. Read this: Guided-Inquiry Experiment for Teaching the Calibration Method of Standard Addition in the Analysis of Lead with Graphite Furnace Atomic Absorption Spectroscopy | Journal of Chemical Education (acs.org)

    5. Write and submit a report as if you were a student.

    6. Practice grading your and your partner's report using the grading rubric.

    Pre-Lab Preparations

    When you are on TA duty, arrive at least 20-30 minutes early to prepare and ready the following materials for students.

    • 0.1 M nitric acid: this should already be prepared using metal-free nitric acid by the lab director, and excess solution should be available in a large carboy in the hood or prep room. As students arrive to lab, remind them that they should rinse their glassware with this solution to remove all residual metal from the glass.
    • 1 L of drinking water collected by students: As your students arrive on the first day of the module, check that they have collected water or ask them to do it immediately.
    • Pb standard solution, 1000 ppm: This is a solution purchased and used as received. If it is running low please inform the instructor.
    • Deionized water: Please check that the ultrapure water is running and there are no leaks in the system under the sink.
    • Brita filter: Please check that the Brita filter is available and begin rinsing the filter with ultrapure water in preparation for the students to use it.
    • Remind students to sign into lab 
    • Check that the student's pre-lab is complete 

    Helping your students

    During the laboratory session, you are expected to be present and available for the entire 4.5 hours. Your job is to help students successfully complete the experiment while upholding expectations and academic standards for the students. 

    In this particular laboratory session, you should let students choose their calibration strategy. You can discuss the pros and cons of different strategies, but it should ultimately be the decision of the student group as to what method they choose to calibrate the instrument. You should also discuss validation with them, but it is their choice as to how they validate their method or whether they do the validation at all.

    This lab module is meant to be open-eneded and also to demonstrate matrix effects. Please discuss matrix effects with the students and make them aware of this as a possible explanation of their results. The students can, and should, use lab time to test any hypotheses they develop to explain their results. For example, if the signal changes after filtration, and the students think there is a matrix effect, that hypothesis can be tested by the method of standard additions or by a simple spike to each of the samples in question. Alternatively, if students beleive the filter is adding metals to their filtered sample, that hypothesis can be tested by measuring the metal signal of ultrapure water before and after exposure to the filter.

    Encourage students to use at least two full lab periods (assuming a 3-period module) to perform lab work. Ideally, data is actively processed during collection and students are able to finanize data collection and begin writing their reports during the third week of lab.

    Shut down/ clean up 

    15 minutes prior to the end of the period, students should stop lab work and clean up. They are to leave the room in clean condition prior to the end of the period (17:55).

    • Remind students when it is time to clean up.
    • Remind students that all glassware should be returned to the storage locations.
    • Any samples that are stored should be labeled clearly with the student's name and section.
    • Remind students to sign out with the time they leave and get your signature.
    • Check the TA checklist for additional items to be completed.

    This page titled 6.8: TA Notes is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Kathryn Haas.