Analytical Chemistry. The Analytical Process Sample Preparation. Dr. A. Jesorka, 6112,

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1 Analytical Chemistry The Analytical Process Sample Preparation Dr. A. Jesorka, 6112,

2 Lecture Summary 1. The analytical process. More than measurement! Several steps from problem to report. Method! 2. Analysis is not only measurement! Sampling and preparation are very important steps. Sampling and preparation also adds to uncertainty 3. Sampling 4. Sample preparation and preservation, post preparation 5. Contamination & Blanks Recommended reading: Sample Preparation Techniques in Analytical Chemistry, J. D. WINEFORDNER, Wiley 2003

3 The Analytical Process Analytical Problem Step 1. Obtain a representative bulk sample. Step 2. Extract from the bulk sample a smaller, homogeneous laboratory sample. Step 3. Convert the laboratory sample into a form suitable for analysis, a process that usually involves dissolving the sample. Step 4. Remove or mask species that will interfere with the chemical analysis. Step 5. Measure the concentration of the analyte in several aliquots. Step 6. Interpret your results and draw conclusions. Solution

4 Precision: How close the same measurements are to one another. The degree of mutual agreement among data that have been obtained in the same way. Precision provides a measure of the random or indeterminate error of an analysis. Accuracy: How close the measurement approaches the real value. Bias: Bias provides a measure of the systematic, or determinate error of an analytical method. - - bias = x - x t, where, x is the population mean and x t is the true value 4

5 SAMPLING AND SAMPLE PREPARATION 6

6 SAMPLING Each step of an analysis contributes random error that affects the over-all standard deviation. Method accounts for only 10% of the overall variance when s samp = 3 s meth 7

7 Sampling plan! 1. From where within the target population should we collect samples? 2. What type of samples should we collect? 3. What is the minimum amount of sample for each analysis? 4. How many samples should we analyze? 5. How can we minimize the overall variance for the analysis? Buger, J. et al. Do Scientists and Fishermen Collect the Same Size Fish? Possible Implications for Exposure Assessment, Environ. Res. 2006, 101,

8 Where to Sample the Target Population? Homogeneous: composition of target population is the same regardless of where we sample, when we sample, or the size of our sample. Heterogeneous: the composition is not the same at different locations, at different times, or for different sample sizes. Sampling Random Judgemental Systematic Random: unbiased estimate of the target population properties A truly random sample is difficult to collect. Haphazard sampling (missing sampling plan): not random and may reflect an analyst s unintentional biases. Good method: First, we divide the target population into equal units and assign a unique number to each unit. Then, we use a random number table to select the units to sample. 9

Systematic Sampling in between the extremes. We target population at regular intervals in space or time.

9 Sampling Random Judgemental Systematic Stratified Covenience Judgmental (Selective) Sampling More biased than random sampling, bur requires fewer samples. Useful if we wish to limit the number of independent variables influencing our results. Systematic Sampling in between the extremes. We target population at regular intervals in space or time. Other Sampling Stratified: preselect distinct units (strata) Convenience: use convenient sampling sites (e.g. wells rather than drilled holes) random systematic 10

10 Uncertainty of Measurement - Sampling Uncertainty: Parameter associated with the result of a measurement, that characterizes the dispersion of the measured values that could reasonably be attributed to the analyte (measurant). 1. Sampling Process - Homogeneity. - Effects of specific sampling strategy (e.g. random, stratified random, proportional etc.) - Effects of movement of bulk medium (particularly density selection) - Physical state of bulk (solid, liquid, gas) - Temperature and pressure effects. - Does sampling process affect composition? E.g. differential adsorption in sampling system. 2. Sample preparation - Homogenisation and/or sub-sampling effects. - Drying. - Milling. - Dissolution. - Extraction. - Contamination. - Derivatisation (chemical effects) - Dilution errors. - (Pre-)Concentration. - Control of speciation effects.

11 Implementing the Sampling Plan 1. Physically removing the sample from its target population, gas liquid 2. Preserving the sample 3. Preparing the sample for analysis. solid Sample preparation is frequently a major time-consuming step in most analyses because it is rarely possible to analyze a neat sample. 13

12 Sample preparation (SP) is frequently necessary for most samples and still remains one of the major time-consuming steps in most analyses. Possible sample preparation steps Measurement process steps 14

13 Sample Preservation Literature: Standard Methods for the Examination of Water and Wastewater, 18th Edition 15

14 Sample Preservation 16

15 Bringing solid samples into solution Dissolve in a suitable solvent Distilled water: the solvent of choice for inorganic salts organic solvents, such as methanol, chloroform, and toluene: organic materials. If not possible, digest with acids/bases! Digestion methods: 17

16 Bringing solid samples into solution: microwave assisted digestion Microwave digestion unit Teflon digestion vessel, which is encased in a thermal sleeve. + higher temperatures ( ºC) and pres-sures ( bar) Short digestion time - Safety concerns high cost Small sample size 18

Bringing solid samples into solution: Fluxes Inorganic samples that resist decomposition by digesting with acids or bases often can be brought into solution by

17 Bringing solid samples into solution: Fluxes Inorganic samples that resist decomposition by digesting with acids or bases often can be brought into solution by fusing with a large excess of an alkali metal salt, called a flux. + Melt dissolves in water - contamination from the flux and the crucible loss of volatile materials 19

18 Sample preparation Survey of most comon methods 1. Removal of insoluble components (filtration or centrifugation) 2. Precipitation of undesirable components followed by filtration/centrifugation 3. Liquid solid extraction followed by filtration or centrifugation 4. Liquid liquid extraction 5. Ultrafiltration 6. Ion-pair extraction 7. Derivatization 8. Complex formation 9. Freeze-drying 10. Solid-phase extraction 11. Ion exchange 12. Use of specialized columns (gel permeation, biobeads, etc.) 13. Preconcentration 14. Precolumn switching 15. Miscellaneous cleanup procedures 20

19 Analyte Separation Main Goal: Separating the analyte from interferents Classification of separation techniques 21

20 Separation vs. Pre-concentration Two common analytical problems: 1. matrix components that interfere with an analyte s analysis; 2. an analyte with a concentration that is too small to analyze accurately. Can often be achieved simultaneously: In the gas chromatographic analysis for organophosphorous pesticides in environmental waters, the analytes in a 1000-mL sample may be separated from their aqueous matrix by extraction using 15 ml of ethyl acetate. concentration increase! Aguilar, C.; Borrul, F.; Marcé, R. M. LC GC 1996, 14,

21 Common instrumental methods and necessary sample preparation steps 23

22 Common sample treatment methods Typically 1-4 steps - Minimize number of steps! - Determine sample loss! Avoid contamination! 24

23 Typical sources of contamination Use blanks to determine contamination 25

24 26

25 Appendix 27

26 How do you know that your analytical method is valid?! 28

HPLC Winter Webinars Part 2: Sample Preparation for HPLC

HPLC Winter Webinars Part 2: Sample Preparation for HPLC Jon Bardsley, Application Chemist Thermo Fisher Scientific, Runcorn/UK The world leader in serving science What am I Going to Talk About? What do