Module 5 Flashcards
5 steps to designing a sampling plan
- Where writhing target population should sample be collected
2.what type of sample should be collected - what’s the minimum amount of sample needed for analysis
- How many samples should be analyzed
- How can overall variance be memorized
When do sampling errors occur
When sample composition is not identical to the population it’s drawn from
What is a homogenous sample
Analyte of interest evenly distributed
Samples can be taken without regard to sampling errors
Determinate sampling errors are insignifigant
What is a heterogeneous sample
Analyte of interest unevenly distributed in matrix
Analyte scan vary in time or space
Determinate errors can be significant
4 types of sampling
- Random
- Judgemental
- Systematic
- Convenience
Random sampling
Samples collected at random
Makes no judgement about the population
Require more times and expense because a greater number of samples are needed to be representative
Judgemental sampling
Collected from population using information about analyte distribution within population
Selective
More biased than random
Systematic sampling
Falls between random and judgemental
Samples collected from target population at regular intervals in time and space
Convenience sampling
Samples collected from population because they are easily obtained
Cost, experience and accessibility are the primary factors
3 methods to obtain samples
Grab sampling
Composite sampling
In situ sampling
Grab sampling
Single sample grabbed from target population at a given time and space
Represents a snapshot of target population
Good to use if population is homogenous
Composite sample
A series of grab samples combined to form one sample
Analyzing a single sample saves time and cost
In situ sample
Taken within population without physically removing the sample
Analytical sensor placed directly in population
Allows for continuous monitoring without removing samples
How to minimize variance
Collecting s’more samples of proper size
3 steps of sampling
- Physicality removing samples from target population
- Preserving the sample
- Preparing the sample for analysis
Why do samples need to be preserved
Samples can undergo chemical and physical changes if not properly preserved and would result in it not being representative of the population
What is an interfering component
Any chemical that contributes to the measured signal of the analyte
What is selectivity
If the method responds primarily to the analyte of interest and is little affected by other substances
How is a methods sensitivity determined
The relative difference in its sensitivity toward the analyte of interest
What’s the goal of analytical separation
To remove the analyte or interfere the from the sample matrix
What must be present to bring about a separation
Significant difference in either the chemical or physical properties
What are separations based on size done by
A porous medium through only target analyte or interference can pass
What is size exclusion chromatography
A physical method of separation that distributes components into two phases, a mobile and stationary phase where one can be extracted from
What is a size exclusion chromatography column packed of
A column packed with a stationary phase of small porous particles of cross linked dextrin or polyacrylamide
What is the stationary phase in SEC controlled by
The degree of cross
Greater cross linking= smaller pore size
How does SEC work
Sample added to head of column and pass with gravity or pump
Chemicals with a molecular size too large to go through pores flow through column unrestricted
Chemicals small i enough to fit in pores of stationary phase will do so and take longer to pass through the length of the column
What is done to separate when there is a difference in mass or density
Separation through centrifugation
Partials experience a greater centrifugal force have faster sedimentation rated and pulled to bottom of centrifuge tube
How is masking done
Transforming the interference into a form that’s not detected
Done by binding interference to a masking agent
Why is masking not a separation technique
Because target analyte and interferent are never physically separated from each other
What is a separation based on partitioning betwwwn two phases
Selective partitioning of the target analyte or interferent between two immiscible phases
What is extraction
When a solute is transferred from one phase to a new one
The sample is extracted one or more times with portions of the second phase
Liquid solid extraction or solid phase extractions
Liquid sample passed through a column containing solid particulates that serve as adsorbent material
How is choice of solid adsorbent
Based on properties of target analyte and the matrix it’s in
