Chemical Sample Analysis Techniques and Classification

Obtaining the Sample for Analysis

Constituent Concentration Levels

The classification of constituents within a sample is based on their concentration relative to the total sample mass:

• Major Constituent: More than 1% of total.
• Secondary Constituents: 0.1% to 1% of total.
• Trace Level: Less than 0.1% of total.
• Ultratraces: Parts per million (ppm) level.

Classification of Analytical Methods by Sample Size

Sample Weight and Volume Requirements

Analytical methods are categorized based on the required sample weight (mg) and volume (ml):

• Macroanalysis: Higher than 100 mg; requires 10 ml or more.
• Semimicroanalysis: 10–100 mg; requires 1–10 ml.
• Microanalysis: 1–10 mg; requires 0.1–1 ml.
• Ultramicroanalysis: Less than 1 mg; requires under 0.1 ml.

Key Definitions in Chemical Sampling

• Sampling: The act of obtaining a sample.
• Lot: The entire collection from which samples are taken. Example: All the water of Lake Huacachina.
• Gross Sample: A sample obtained from the Lot for analysis or storage. It is selected to be representative, and its choice is critical for a valid analysis.
• Laboratory Sample: A reduced sample derived from the gross sample. It must maintain the exact composition of the bulk sample. Individual **aliquots** (portions) are taken from the laboratory sample for testing.

Effective Sampling Strategies

Homogeneous Sample Strategy

Once a homogeneous sample is obtained, it is divided into portions. More accurate results are achieved if units are mixed and the analysis is performed on the mixed groups, rather than analyzing individual units separately.

Non-Homogeneous Sample Strategy

Several samples are taken from each stratum. Two samples from each stratum are mixed, and this process yields a representative sample for analysis.

Specific Techniques for Material Sampling

1. Solid Sampling

Solids in Fragments or Particles

A random sample batch is taken. If analysis of material of different sizes is required, a representative sample of each batch size must be formed.

Solids in Compact Form

Samples of metals and alloys are obtained by **filings**, **shredding**, or **drilling**.

2. Liquid Sampling

Homogeneous Liquids

Different samples are taken randomly, similar to materials in suspension. Samples can be collected at different depths. If solids are in suspension, the liquid must remain under constant agitation. A specialized probe called a "sample thief" is frequently used; it is plunged to the desired depth and opened to collect the sample.

3. Gas Sampling

Free Gas in Quantity

A tube is filled with gas, displacing the air initially contained within. These containers are closed afterward using valves or by sealing the ends.

Inaccessible Gas Samples

For gas existing in an oven or a pipe, a hole is drilled in the wall of the container, and a tube fitted with a cooling jacket is inserted. This tube conducts the gas to the analysis apparatus.