Choosing the Method of Analysis
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Q1: What general criteria would you consider when selecting an appropriate analysis method?
Students should be given the time to brainstorm and then research criteria that will aid in selecting the appropriate method of analysis. Some of these considerations include: accuracy, precision, sensitivity, selectivity, robustness, ruggedness, scale of operation, analysis time, availability of equipment, and cost. Depending on the level of students, this may be a simple review of concepts already learned in an analytical course or may require a more in depth discussion before proceeding to the next assignment.
The following questions are part of an assignment where students divided in groups are assigned a specific analyte (calcium and magnesium, phosphates or nitrates) and are asked to research the literature and identify an appropriate method of analysis. Therefore, answers to the following questions are specific to the assigned analyte and the method students end up identifying.
Q2: What analytical methods are best suited to provide the information about your analyte?
Here students would have to consider whether the identified method works well with the physical and chemical state of the sample. Most importantly, they would have to consider the possible concentration range of the analyte in the sample and whether the method is sensitive enough to grant detection. Accuracy and precision will also be a factor and whether one would want a rough estimate of the concentration or a very accurate determination.
Q3: Of these, which techniques/instruments are available for the analysis?
This practical consideration is very important if the analysis is actually carried out as part of the laboratory experience. Students may soon realize that they may have to opt for less desirable methods if a certain piece of equipment is not available.
Q4: Are there cost or timing issues that will influence the choice of method?
Again, this practical consideration is very important, particularly if students are expected to perform the analyses. They may want to research cost of reagents and estimate analysis time. They should develop a clear picture of all that is involved before starting any experiments.
Q5: Do the available methods have sufficient selectivity for the type of sample that will be analyzed?
Students may be unfamiliar with concepts of interference. They should be guided to research the selectivity of a given method and identify whether possible interferents in the sample could affect the analysis outcome.
Q6: What sample pretreatment will be required?
In general, water samples for analysis of cations and anions do not require extensive sample pretreatment. However, students should consider some simple, yet necessary strategies such as filtering the water sample, particularly if large amounts of suspended solids or debris are present.
Q7: What sample size (mass or volume) is needed, and can that be feasibly collected?
Usually, published methods of analysis will specify the amount of sample to collect. If students complete the “sampling” unit of this module, and have been asked to develop a sampling plan, these considerations may have already been discussed.
Q8: What is the anticipated range of analyte concentrations?
It may be difficult for students to answer this question if they are unfamiliar with typical concentrations of the specific analyte in the sample. For water monitoring a good starting point are the EPA regulations which state the concentration limits for compliance.
Q9: What is the limit of detection of the method and is its dynamic range appropriate for the range of concentrations of the analyte?
Answering this question may require a review of the limit of detection and dynamic range concepts. Again, the EPA regulations may aid in deciding whether the method is suitable for the expected concentration of the analyte.
Q10: What is the precision of the method?
A published method will indicate the attainable precision. This question may also be used to spur a discussion about the precision needed in the results. Is the analysis being used to develop a rough idea of the analyte concentration or is a high level of precision needed?
Q11: Is there a target concentration that is important for regulatory purposes?
This question ties well with questions 8 and 9. If students reviewed the EPA regulations for that particular analyte, they may be already aware of limits imposed by the agency and have identified a target concentration.
Q12: Is the analyte in a form (solid, liquid, gas) suitable for the analytical method that you have selected?
Liquid samples, as those collected for water monitoring purposes, are amenable to analysis by multiple techniques. However, samples in the solid or gas phase may pose additional challenges that students may be guided to consider.
Q13: How will the method be validated?
The concept of method validation is very important as validation is required by Good Laboratory Practices (GLP). Validation ensures that the method provides accurate and precise results at a given confidence level. Typically, a method will be validated by analyzing a standard that closely matches the analyte of interest. Alternatively, the method can be validated by comparing the results with those provided by a different method whose accuracy is known.
Q14: What type of calibration will be used?
This question may be used to review or introduce different calibration approaches, including external standard and standard addition methods.
Q15: Do any of the reagents used need to be standardized?
Standardization is paramount in the success of an analysis. Students may be asked to research how to obtain a standard such as buying it from a commercial supplier or preparing it in lab. A discussion of primary vs. secondary standards may be appropriate.
Q16: Can a reference standard be used to ensure accuracy?
Appropriate reference standards are indeed what is used to check the accuracy of a given method.