2) SAMPLING
Introduction
To produce meaningful information, an analysis must be performed on a sample that has the same
composition as the bulk of material from which it was taken.
When the bulk is large and heterogeneous, great effort is required to get a representative sample.
Example: Consider a trailer containing 25 tons of silver ore. The buyer and seller of the ore must
agree on a price, which will be based primarily on the silver content of the shipment. The ore
itself is inherently heterogeneous, consisting of many lumps that vary in size as well as silver
content.
The assay of this shipment will be performed on a sample that weighs about one gram. For the
analysis to have significance, this small sample must have a composition that is representative of
the 25 tons (or approximately 22,700,000 g) of ore in the shipment.
Isolation of one gram of material that accurately represents the average composition of the nearly
23,000,000 g of bulk sample is a difficult undertaking that requires a careful, systematic
manipulation of the entire shipment.
Sampling is the process of collecting a small mass of a material whose composition accurately
represents the bulk of the material being sampled.
The collection of specimens from biological sources represents a second type of sampling
problem. Sampling of human blood for the determination of blood gases illustrates the difficulty
of acquiring a representative sample from a complex biological system. The concentration of
oxygen and carbon dioxide in blood depends on a variety of physiological and environmental
variables.
For example, inappropriate application of a tourniquet or hand flexing by the patient may cause
blood oxygen concentration to fluctuate. Because physicians make life- and-death decisions based
on results of blood gas determinations, strict procedures have been developed for sampling and
transporting specimens to the clinical laboratory.
These procedures ensure that the sample is representative of the patient at the time it is collected
and that its integrity is preserved until the sample can be analyzed.
Many sampling problems are easier to solve than the two just described. Whether sampling is
simple or complex, however, the analyst must be sure that the laboratory sample is representative
of the whole before proceeding with an analysis.
Sampling is frequently the most difficult step and the source of greatest error. The reliability of
the final results of analysis will never be any greater than the reliability of the sampling step.
Acquiring a sample
Sampling is one of the most important operations in a chemical analysis. Chemical analyses use
only a small fraction of the available sample.
For instance, the fractions of the sandy and loam soil samples that are collected, say from various
agricultural farms, for analyses must be representative of the bulk materials.
Knowing how much sample to collect and how to further subdivide the collected sample to obtain
a laboratory sample is vital in the analytical process.
1
, Sampling, standardization, and calibration are major focal points of Analytical Chemistry. All
three steps require a knowledge of statistics
The specific analytical procedure chosen depends on how much sample is available and, in a
broad sense, how much analyte is present.
ANALYTICAL SAMPLES AND METHODS
Types of Samples and Methods
Analytical methods can be classified in many different ways. Often a method of identifying
chemical species, a qualitative analysis is distinguished from one that detects the amount of a
constituent, a quantitative analysis.
Quantitative methods are classified as gravimetric, volumetric, or instrumental.
Another way to distinguish methods is based on the size of the sample and the level of the
constituents.
(i) Sample Size
Macro analysis - samples of mass more than 0.1 g are analysed.
Semi-micro analysis - performed on a sample in the range of 0.0 I - 0.l g,
Micro analysis – samples analyzed are in the range 10 -1 – 10 - 2 g.
Ultramicro analysis - samples whose mass is lower than 10 -4 g are analysed
NOTE: It is clear that analysis of a:
- I-g sample of soil for a suspected pollutant would be called a macro analysis.
- 5-mg sample of a powder suspected to be an illicit drug would be a micro analysis.
A typical analytical laboratory handles samples ranging from the macro size to the micro and
even ultramicro size.
Techniques for handling very small samples are quite different from those for treating macro
samples.
(ii) Constituent Types
2
Introduction
To produce meaningful information, an analysis must be performed on a sample that has the same
composition as the bulk of material from which it was taken.
When the bulk is large and heterogeneous, great effort is required to get a representative sample.
Example: Consider a trailer containing 25 tons of silver ore. The buyer and seller of the ore must
agree on a price, which will be based primarily on the silver content of the shipment. The ore
itself is inherently heterogeneous, consisting of many lumps that vary in size as well as silver
content.
The assay of this shipment will be performed on a sample that weighs about one gram. For the
analysis to have significance, this small sample must have a composition that is representative of
the 25 tons (or approximately 22,700,000 g) of ore in the shipment.
Isolation of one gram of material that accurately represents the average composition of the nearly
23,000,000 g of bulk sample is a difficult undertaking that requires a careful, systematic
manipulation of the entire shipment.
Sampling is the process of collecting a small mass of a material whose composition accurately
represents the bulk of the material being sampled.
The collection of specimens from biological sources represents a second type of sampling
problem. Sampling of human blood for the determination of blood gases illustrates the difficulty
of acquiring a representative sample from a complex biological system. The concentration of
oxygen and carbon dioxide in blood depends on a variety of physiological and environmental
variables.
For example, inappropriate application of a tourniquet or hand flexing by the patient may cause
blood oxygen concentration to fluctuate. Because physicians make life- and-death decisions based
on results of blood gas determinations, strict procedures have been developed for sampling and
transporting specimens to the clinical laboratory.
These procedures ensure that the sample is representative of the patient at the time it is collected
and that its integrity is preserved until the sample can be analyzed.
Many sampling problems are easier to solve than the two just described. Whether sampling is
simple or complex, however, the analyst must be sure that the laboratory sample is representative
of the whole before proceeding with an analysis.
Sampling is frequently the most difficult step and the source of greatest error. The reliability of
the final results of analysis will never be any greater than the reliability of the sampling step.
Acquiring a sample
Sampling is one of the most important operations in a chemical analysis. Chemical analyses use
only a small fraction of the available sample.
For instance, the fractions of the sandy and loam soil samples that are collected, say from various
agricultural farms, for analyses must be representative of the bulk materials.
Knowing how much sample to collect and how to further subdivide the collected sample to obtain
a laboratory sample is vital in the analytical process.
1
, Sampling, standardization, and calibration are major focal points of Analytical Chemistry. All
three steps require a knowledge of statistics
The specific analytical procedure chosen depends on how much sample is available and, in a
broad sense, how much analyte is present.
ANALYTICAL SAMPLES AND METHODS
Types of Samples and Methods
Analytical methods can be classified in many different ways. Often a method of identifying
chemical species, a qualitative analysis is distinguished from one that detects the amount of a
constituent, a quantitative analysis.
Quantitative methods are classified as gravimetric, volumetric, or instrumental.
Another way to distinguish methods is based on the size of the sample and the level of the
constituents.
(i) Sample Size
Macro analysis - samples of mass more than 0.1 g are analysed.
Semi-micro analysis - performed on a sample in the range of 0.0 I - 0.l g,
Micro analysis – samples analyzed are in the range 10 -1 – 10 - 2 g.
Ultramicro analysis - samples whose mass is lower than 10 -4 g are analysed
NOTE: It is clear that analysis of a:
- I-g sample of soil for a suspected pollutant would be called a macro analysis.
- 5-mg sample of a powder suspected to be an illicit drug would be a micro analysis.
A typical analytical laboratory handles samples ranging from the macro size to the micro and
even ultramicro size.
Techniques for handling very small samples are quite different from those for treating macro
samples.
(ii) Constituent Types
2
