5.3: Week 2 - Water

Pre-lab Assignment

📅 Due: This pre-lab assignment is due prior to the Week 2 Dry Lab meeting.

🎯 Purpose: This pre-lab is designed to prepare you for a productive dry-lab discussion and to prime you to reasonably interpret your data. By organizing your qualitative results and drafting your Methods section now, you’ll be ready to share work-in-progress and get early feedback to improve your report.

Follow up from Week 1

1. Complete and submit any unfinished business from the last meeting.
2. Begin preparing your manuscript document:
   1. Complete a draft of your Materials and Methods section.
   2. Complete the Qualitative Results section, including description and figures that show the results of qualitative analysis.
      • Prepare a table of the elements you searched for in qualitative analysis for all samples, and that indicates their presence in your sample.
      • Prepare a figure or multiple figures that shows the results of qualitative analysis. 
      • The table and figures should complement each other.
   3. Draft your Discussion of qualitative results: Analyze and interpret the qualitative results for all the samples from your group:
      • For all source water samples: Explain whether each element is present or absent in detectable amounts in each sample, using evidence from your results section to justify your analysis.
      • Compare source water to tainted: Does the addition of Pb to the matrix (tainting) affect the signals of other elements? Support your assertions using evidence from your results section. Explain the meaning of these results in the context of matrix results.
      • Compare tainted sample to filtered sample: What effect does filtration have on the intensity of the signals from each element? Support your assertions using evidence from your results section. Explain the meaning of these results in the context of matrix effects.

Many people assume that signal intensity reflects the amount of an element present, but this is not always true! Matrix effects can make signals appear stronger or weaker without a real change in concentration. In this pre-lab, you will read about Brita’s claims for filtration, make predictions about how well the filter should work for Pb and other elements, and consider why signals might increase after filtration. This work will prepare you to identify when a signal change is likely due to matrix effects versus true contamination, and to defend your interpretation during group discussion.

Pre-Lab Questions

Read the claims about the efficiency of Brita water filters for removal of substances from drinking water. (https://www.brita.com/why-brita/better-water/). We use a standard Brita pitcher filter in this experiment.

1. Compare Predictions to Evidence: After reading Brita’s claims, what efficiency do you expect for Pb removal? What efficiency do you expect for your group’s additional elements? State your predictions clearly so you can compare them to your qualitative results now, and your quantitative results later this week.
2. Signals that Increase After Filtration: Sometimes, the intensity of an element’s signal increases after filtration. What are two possible explanations for this observation — one related to matrix effects and one related to an actual change in sample composition? Which explanation would you expect to be more common, and why?
3. Responding to Unexpected Results: Suppose your group observes higher intensity signal for an element in the filtered sample than in the tainted sample. How would you decide whether this is a real contamination event (e.g., improper filter preparation) versus a matrix effect? What additional evidence or checks could help you distinguish between these possibilities?

 

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👥 Dry Lab Group Discussion

🎯 Goals: By the end of today’s discussion, your Team will:

• Compare qualitative results across all Groups to look for evidence of matrix effects.
• Identify which elements increased or decreased after filtration, and debate possible explanations.
• Distinguish between matrix effects and true contamination events using shared evidence.
• Draft clear sentences for the Results and Discussion sections of your manuscript that reflect both Group and Team-level evidence.

Instructions

1. Step 1 — Group Check-In (10 minutes)
   Within your 4-person Group:
   1. Compare your pre-lab predictions to your Group’s actual qualitative results.
   2. Decide: Did we observe any element with an increased signal after filtration?
   3. Nominate one spokesperson to report back to the Team.
2. Step 2 — Team Data Pooling (15 minutes)
   Each Group’s spokesperson shares findings with the Team (3 short reports).
   1. As a Team, nominate two scribes; one to write on the whiteboard, and another to record in a shared google sheet.
   2. Create a combined table (on the whiteboard and in the shared document)
      • Columns = Elements tested
      • Rows = Group A, Group B, Group C
      • Record whether signals after filtration were ↑ (increased), ↓ (decreased), or = (no change).
3. Step 3 – Team Interpretation (15 minutes)
   As a Team, address the following:
   1. Which elements showed signal increases in at least one Group’s data?
   2. Are these more consistent with matrix effects or true contamination? What evidence supports your conclusion?
   3. If Groups disagree, what additional checks or information could help resolve the discrepancy?
4. Step 4 – Group Drafting (20 minutes)
   1. Re-group with the members of your section and set a timer for 10 minutes: Each person in the group should spend 10 minutes revising the Discussion draft that they wrote for their pre-lab assignment. Focus on 
      • At least one results sentence describing a shared observation (e.g., “In all filtered samples, signals for Cu decreased relative to tainted water.”).
      • At least one discussion sentence interpreting it (e.g., “This decrease is consistent with decrease in Cu contamination in the sample as a result of filtration.”).
   2. Take turns reading your revised sentences aloud, and giving feedback to each other. Spend the remaining time in dry lab to refine your Discussion section — even though you are each writing your own manuscript, your group should reach consensus on the interpretation of your qualitative evidence as a whole. 

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🧪 Wet Lab Agenda

If you were not able to initiate quantitative data collection in week 1, please prioritize the solution preparation and data collection. Then proceed with the steps below.

1. Access your data:  You will need to know your file names and in which directory you stored your data so you can locate the data and complete the analysis.
2. Perform Quantitative Analysis: Depending on how you calibrated your samples, you will work with your group or individually to process and export the quantitative data.
   • Produce a calibration curve using the appropriate regression model.
   • Assess the validity of your calibration.
   • Perform quantitative analysis on your samples.
   • Make sure to include error analysis!
3. Continue writing the Results and Discussion sections of lab report (only after completing the reflection prompts below.)

Your Data Checklist

To prepare for analysis and manuscript writing, be sure to export and save the following items to your own computer or cloud storage before you leave the lab:

Calibration Standards Data:

• Overlaid emission spectra for all Pb calibration standards.
• Pb emission peak values for each standard exported in a format that you can process using your favorite software.
• A calibration curve for Pb (plotted using your favorite software), including best-fit line and regression statistics
• Documentation of your calibration strategy (external vs. standard additions) and its validation (e.g., spiked recovery).

Sample Data:

• Overlaid emission spectra of Pb in all water samples (source, tainted, filtered) run at the same time as your calibration data.
• Pb emission peak values for each sample
• A table of Pb concentrations in each water sample, with error estimates.

Evidence for Notebook Snapshot

• Any exported files or screen captures that show results, with notes to show your understanding of their relevance and meaning.

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📘 Notebook

Record detailed notes about your lab work in your notebook each time you are in lab. Also include the data analysis and responses to the reflection prompts*. Your notebook should be submitted on canvas prior to leaving Wet Lab each week - whatever state it is in at the end of lab is the state in which you should submit your work.  If you are unable to complete data analysis and the reflection prompt within the period, it should be completed before the next lab period, and submitted with your next notebook submission.

Reflection Prompt \(\PageIndex{0 }\) 

In your notebook submission, respond to the following:

1. Results: Briefly summarize what you understand about your quantitative results.
2. Filter Effectiveness: Based on your quantitative results, how effective was the filter at removing Pb from tainted water? Do you know if the filter remove other elements? What evidence supports your conclusion?
3. Validation: Did your calibration strategy pass validation? If not, what does this mean for your interpretation of filter effectiveness, and what changes must you make in your calibration approach before repeating the analysis?

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