Importance of Proper Sample Collection and Handling
One of the most important steps in the analytical process is ensuring that samples are properly collected, handled, and prepared before being analyzed in the lab. How a sample is collected and stored can significantly impact the reliability and reproducibility of results. Contamination or degradation of samples must be avoided during transportation and storage. Samples need to be representative of the material or substance being tested and protocols need to be followed to maintain sample integrity from point of origin to analytical measurement. Improper sampling techniques can lead to wasted time, effort and resources if analysis needs to be redone due to compromised samples.
Sample Preparation Techniques
There are a variety of techniques used for Sample Preparation depending on the type of material and desired analysis. Some common preparation methods include:
Drying - Remove moisture from samples prior to weighing or further preparation. This is often done by oven drying or freeze drying.
Grinding and Homogenization - Reduce particle size and achieve a consistent composition throughout the sample. This can be done with motors, mills orgrinding beads to break up aggregates and particles.
Digestion and Extraction - Use chemical or physical means to extract analytes from complex sample matrices into a form more suitable for instrumental analysis. Acid or alkali digestion is commonly used to decompose solids. Extractions isolate targeted compounds using solvents.
Filtration and Separation - Separate solids from liquids or remove unwanted particulate matter through techniques like vacuum filtration, centrifugation or chromatography.
Dilution - Adjust concentration to appropriate levels required by analytical methods and instrumentation through serial dilution.
Cleanup and Derivatization - Remove contaminants or modify compounds to improve detection or separation. Solid phase extraction is one cleanup method used. Derivatization chemically modifies analytes for GC or GC-MS analysis.
Standard Preparation - Weigh or aliquot pure analytes to make stock and calibration standard solutions of known concentration for quantitative analysis. Quality control standards are also prepared.
Microbial Sample Prep
For microbial testing, proper aseptic technique and controls are critical to avoid contamination during sample prep. Collection methods aim to keep microbes viable until analysis. Specific protocols apply to different sample matrices like surfaces, fluids, foods or clinical specimens. Prep can involve membrane filtration, serial dilution plating, enrichment in microbial growth media and incubation. Prepared plates are examined microscopically or using automated methods to quantify and sometimes identify microbes present. Molecular methods like PCR preparation involve extracting and purifying nucleic acids from samples.
Low Level Analyte Preparation
Preparing samples containing very low analyte levels, at the ppb (parts-per-billion) or ppq (parts-per-quadrillion) range, poses unique challenges. Specialized cleanroom facilities, contamination controls and sensitive instrumentation are often required. Multiple enrichment or preconcentration techniques are employed to increase analyte concentrations above instrument detection limits. Isotope dilution may also be used for ultra-trace analysis. Techniques like solid phase extraction utilize selective sorbents to isolate and concentrate target compounds from large sample volumes prior to instrumental measurement.
Quality Assurance Practices
Proper documentation of all sample receipt, handling, storage and preparation details is essential for demonstrating quality and defensibility of analytical results. Standard operating procedures ensure consistency is maintained. Periodic instrument calibration and quality control samples are analyzed along with experimental samples to validate system performance. Some laboratories participate in PT (proficiency testing) programs involving interlaboratory sample exchanges to evaluate performance compared to peer results. Adhering to good analytical practices helps demonstrate data integrity to customers.
In sample preparation is a key early step in the analytical process that can make or break the validity of results. Using appropriate, documented techniques suited to analyte properties and desired quantification levels helps ensure labs generate reliable, reproducible data their clients can trust. Attention to proper protocols for collection, transportation, storage and all preparation stages from start to finish is important for quality outcomes.
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