It would indeed be rare that you will analyze a sample as received. In almost all cases some degree of sample preparation is always necessary. Strict compliance to the prescribed sample preparation procedures is essential for achieving the required accuracy and precision.
To begin with you should have a clear understanding of the difference between a matrix and analyte.
Matrix is the sample or specimen received as such for analysis. It contains other elements or compounds in complex combinations besides the species of interest. Such species can interfere in the analysis
Analyte is the component of the matrix whose analysis is required
Sample Preparation Objectives
The main objectives of sample preparation are:
- Release of analyte from sample matrix
- Removal of interfering species
- Pre-concentration in case analyte levels are lower than the detection limits of the analysis technique to be adopted. Alternately dilution is necessary in case the analyte is present at higher concentrations which is beyond the detection range of the analysis system.
- Derivatization in case sensitivity of detection needs improvement
A sample should be homogeneous and representative of the material of interest. It should be available in sufficient quantity for testing of the required parameters. It is important that sample integrity should be preserved between sample preparation, collection and analysis. Ensure minimal loss due to evaporation, contamination or degradation. The sampling container should be clean and rinsed before collection of the sample. Equally important is it should be properly labeled with indelible ink and label should bear essential details regarding the sample
- On receipt sample details should be documented properly
- Sample should be visually inspected for appearance, homogeneity, damage to container, if any, before acceptance
- Sample quantity should be sufficient for testing the required parameters
- Use environmental chamber for highest sensitivity and your own protection from dangerous samples
Several sample extraction techniques are in use presently
- Liquid – liquid extraction by partitioning the species of interest between two immiscible liquid phases
- Solid – liquid extraction such as SPE, DSPE and SPM
- Precipitation by chemical reaction to convert analyte or impurity to an insoluble precipitate which is separated by filtration or centrifugation
- Distillation for separation of miscible liquids having different boiling points
- Enzymatic treatment
Sample preconcentration or Dilutions
Preconcentration is necessary to increase concentration to measurable levels.The commonly used technique is evaporation on the hot plate.
Dilution of a concentrated sample is necessary to match the linear range of the analytical technique. Dilutions can be a single step or a multistep operation. Pipetting errors in multistage dilutions get multiplied in each stage so great caution is required when carrying out serial dilutions. Use micro pipette of appropriate capacity for multistep dilutions for greater accuracy.
Samples may contain solid particles barely visible ( micron to submicron size). Such suspended particles can lead to clogging of the chromatographic columns or orifices in analyzing systems such as in ICP – MS.
Sample is taken in a syringe and filtered through a filter membrane. The filtrate is collected in a clean rinsed vial.
- Start analysis at the earliest after sample preparation or collection
- Take adequate precaution for storage of temperature or light sensitive samples
- Store samples in prescribed containers otherwise samples can get contaminated during storage
It is important to bear in mind that even the most sophisticated instruments will produce erroneous results if proper precautions are not taken in the sample preparation and handling stages.