chapter 3 sampling

Basic Requirements for Analysis

1.1 Sampling and evaluation of experimental data

CHEMICAL ANALYSISTypes of analysis :1. Measurements of physical and chemical

properties.2. Qualitative analysis3. Quantitative analysis4. Diagnostic analysis

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SAMPLING AND EVALUATION OF EXPERIMENTAL DATA

Population - The group of things, items or units under investigation

Sample - Obtained by collecting information only about some members of a "population“

Sampling – Act of collecting sample to produce meaningful information.


SAMPLING

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Sampling is the process to get a representative and homogeneous sample.Representative means that content of analytical sample reflects content of bulk sample.Homogeneous means that the analytical sample has the same content throughout.

What is the purpose of sampling?

SAMPLING Deciding how to obtain a sample for analysis depend on:

i)The size of the bulk to be sampled.ii)The physical state of the fraction to be analyzed (solid, liquid, gas)

iii)The chemistry of the material to be assayed.

(Nothing can be done that would destroy or alter the identity or quantity of the analyte)

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In a chemical analysis :A chemical analysis is usually performed on only small portion of the material collected to be characterized.

If the amount of material is very small and it is not needed for further use, then the entire samples may be used for analysis.

WHAT ARE THE MATERIALS THAT CAN BE SAMPLED?????????


SAMPLING


Air sampling

liquid sampling

solid sampling

SAMPLING-WATER SAMPLING


Water pollution

Soil sampling Soil erosion


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Obtaining a representative sample is the first step of an analysis. The gross sample is several small portions of the sample. This is reduced to provide a laboratory sample. An aliquot of this sample is taken for the analysis sample.

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Steps involved in sampling bulk material

AIR SAMPLINGGrab sampling

An actual sample of air is taken in a flask, bottle, bag or other

suitable container. Done over a period of few seconds or up To

1-2 minutes.


ANALYSIS

CONTINUOUS OR INTEGRATED SAMPLINGGases or vapours are removed from the air over a

measured time-period and concentrated by passage through a solid or liquid sorbent.


A sample is collected by opening a tube, connecting it to a sample pump, and pulling air through the tube with the pump.•Airborne chemicals are trapped onto the surface of the sorbent. The tube is then sealed with push-on caps, and sent to a laboratory for analysis.

SAMPLING GASESTend to be homogeneous.Large volume of samples is required because of their low density.

Air analysis: Use a `Hi-Vol’ sampler that is containing filters to collect particulates.

Liquid displacement method: The sample must has little solubility in the liquid and does not react with the liquid

Breath sample: The subject could blow into evacuated bag.

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SAMPLING SOLIDInhomogeneity of the material, make sampling of solids more difficult.

The easiest way to sample a material is grab sample – the sample taken at random and assumed to be representative.

For reliable results, it is best to take 1/50 to 1/100 of the total bulk. The larger the particle size, the larger the gross sample should be.

The gross sample must be reduced in size to obtain a laboratory sample.

Solid samples may need drying15

PURPOSE OF DRYING SOLID SAMPLES :To ensure that the exact weight is obtained during the QUANTITATIVE chemical analysis.

How it is done :Solid samples dried in oven at 105 – 110oC for 1- 2 hours. Plant and tissue samples dried by heating

Problems associated with drying of samples :

1.Samples might decompose at high temperature2.Some samples are sensitive to heat, therefore drying can be carried out in a desiccators


CONING AND QUARTERING

This process is continued until the gross sample is small enough to be transported to the laboratory.

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Sampling Solids

CONE AND QUARTER

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3. There is no specific technique that can be used for taking the samples. Using an example, explain how to sample either a solid, liquid, or gas sample. (5 marks)

i)Sampling solid Using the method cone and quarter. Divide a pile of material into quarter. Take a sample from each quarter of the pile and crush these sample and form into a smaller conical pile.

Flatten the conical pile and cut into equal quarters.

Two opposite quarters are chosen at random. Crush the quarter further. The whole steps are repeated until a laboratory samples obtain.

SAMPLING LIQUIDS

Liquid samples are homogeneous and are much easier to sample.

The gross sample can be relatively small.If liquid samples are not homogeneous, and have only small quantity, they can be shaken and sampled immediately.

Sampling depends on the types of liquids:i) large volume of liquids (impossible to mix)ii) large stationary liquids (lakes, rivers)iii) biological fluids

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• If water sample is taken from the river, then the water samples is collected at the SURFACE, MIDDLE and at the BOTTOM of the river bed.

SAMPLE STORAGE AND PRESERVATION

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Samples storage purpose:There is a time gap between when the sample is taken and the actual analysis is being carried out.For liquids samples, make sure that it is kept in bottles with stoppers.

Acidic liquid samples can be stored in glass container whereas basic liquid samples in plastic container.Solid samples is easier to keep and have less chance to be adulterated by foreign matters.Sometimes it can also get absorbed or adsorbed to the wall of the container.

What are the problems encounter during storage of samples?The sample can be adulterated / contaminated by foreign matter .

There is a lost of analyte during storage.Decomposition of sample.Side reactions can occur. Example in the air pollution studies the

content of SO2 in air is not stable due to the following reaction

2SO2 + O2 2SO3

To avoid the above reaction the sample is cooled to 4oCThe sample should not react with the wall of the container or get adulterated.

During storage of samples eg liquid samples, sometimes there is a lost of analyte if it is volatile .[so the container should be closed tightly]


SAMPLE STORAGE AND PRESERVATION

Preparing a laboratory sampleConverting the sample to a useful form:Solids are usually ground to a suitable particulate size to get a homogeneous sample.

Dry the samples to get rid of absorption water. 24

An important aspect of the sampling processSamples are preserved to prevent from:Decomposition Precipitation of metals from water samplesLoss of water from hygroscopic materialLoss of volatile analytes from water samples

DEFINING REPLICATE SAMPLESReplicate samples are always performed unless the quantity of the analyte, expense or other factors prohibit.

Replicate samples are portion of a material of approximately the same size that is carried through an analytical procedure at the same time and the same way.

Obtaining replicate data on samples improves the quality of the results and provides a measure of their reliability.

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PREPARING SOLUTIONS OF THE SAMPLE

A solvent is chosen that dissolves the whole sample without decomposing the analyte.

Sources of error :i) Incomplete dissolution of the analyte.ii) Losses of analyte by the volatilization.iii) Introduction of analyte as a solvent contamination.iv) Contamination from the reaction of the solvent with vessel walls.

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SAMPLE PREPARATION AND DISSOLUTIONSample dissolution is the digestion or mineralization of a

sample to render it soluble and to destroy organic matter that may interfere with the recovery of the analyte.

Sample dissolution procedures i) Dry ashing – The sample is slowly combusted at a high temperature (400 – 700oC) in a muffle furnace. Atmospheric O2 serves as the oxidant, that is organic matter is burned off, leaving behind inorganic residue that is soluble in dilute acid. Oxidizing aids may be employed.ii) Wet ashing – A method for the decomposition of an organic material, such as resins or fibers, into an ash by treatment with a boiling oxidizing acid or mixture of acids. The acids oxidize organic matter to CO2, H2O and other volatile products, which are driven off, leaving behind salts or acids of the inorganic constituents.

iii) microwave iv) fusion

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ADVANTAGES OF ASHINGThe ability to decompose large sample sizes.Little or no reagents is required.The technique is relatively safe.The ability to prepare samples containing volatile combustion elements such as sulfur, fluorine and chlorine (the Schöniger oxygen flask combustion technique is very popular in this case).

The technique lends itself to mass production.


DISADVANTAGES OF ASHINGLosses due to retention to the ashing container.Losses due to volatilization.Contamination from the ashing container.Contamination from the muffle furnace.Physical loss of 'low density' ashes when the muffle door is opened (air currents).

Difficulty in dissolving certain metal oxides.Formation of toxic gases in poorly ventilated areas. (Note that all charring should take place in a hood and the muffle furnace must have a hood canopy for proper ventilation).


ASHING

DRY ASHINGSimple dry ashing - Performed by weighed sample in crucible, heated in muffle furnace then the residue is dissolve in suitable acid.

- Typical ashing temperatures are 450 to 550 °C. Magnesium nitrate is commonly used as an ashing aid.

- Charring the sample prior to muffling is preferred. Charring is accomplished using an open flame.

- Care must be taken to ensure that non of the volatile elements (Hg, Arsenic, Pb) from escaping during ashing.

- •Dry ashing often used to remove organic substances from interfering with the analyte.

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DRY ASHINGIf the sample are liquids and wet tissues:

- The sample are dried on a stream bath or by gentle heat before they are placed in a muffle furnace.

- The heat from the furnace should be applied gradually up to full temperature to prevent rapid combustion and foaming.

- After dry ashing is complete, the residue is usually leached from the vessel with 1 or 2 mL concentrated or 6 M HCl and transfer to a flask or beaker for further treatment.

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WET ASHING

THE PRINCIPLE OF WET ASHINGUsually use combination of acids to achieve a complete dissolution.

Mixture of HNO3 and HCl is often used (aqua regia)A small amount (5 mL) of H2SO4 is used with larger volumes of HNO3 (20 to 30 mL).

Usually performed in a Kjeldahl flask.HNO3 destroys the bulk of organic matter, but it does not get hot enough to destroy the last traces.

It is boiled off during the digestion process until only H2SO4 remains and dense, white SO3 fumes are evolved and begin to reflux in the flask.

At this point, the solution gets very hot, H2SO4 acts on the remaining organic material. 34

WET DIGESTION

Hydrochloric acid, nitric acid or aqua regia (3:1 HCl:HNO3) dissolve many inorganic substances.

HF acid decompose silicates.Perchloric acid is used to break up organic complexes.

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WET DIGESTION ADVANTAGESSuperior in term of rapidityLow level of temperature maintained Freedom from loss by retention

DISADVANTAGESIntroduction of impurities from the reagent necessary for the reaction.

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MICROWAVE

In some cases the dissolution of sample can be done by using microwave oven to accelerate the dissolution process (at microwaves T=100 – 250oC).

The sample is sealed in specially designed microwave digestion vessel with a mixture of appropriate acids.

Microwave ovens can be used for rapid and efficient drying and acid decomposition of samples.

Advantages of microwave digestions include reduction in times from hours to minutes and low blank levels due to reduced amounts of reagents required.FREE POWERPOINT TEMPLATE: WWW.BRAINYBETTY.COM 37

FUSION A weighed sample is mixed with a flux (sodium peroxide) in a metal (zirconium) or graphite crucible. The mixture is heated over a flame, or in furnace and the resulting fused material is leached with either water or appropriate acid (dilute mineral acid) or alkali.

These techniques are required for sample types that are inorganic in nature and unreactive toward acid decomposition


FUSIONS FOR TRACE ANALYSTSFusions are considered to be more of a 'last resort' by trace analysts because:

They are expensive and often not available (high purity fluxes).

They yield high solids solutions that can salt out in the nebulizer.

Large dilutions of the sample are a necessityThey often require expensive equipment.Spectral interferences from the flux and/or crucible construction material must be considered.

Contamination of the sample with the crucible construction element and impurities must be considered

They are labor intensive.


chapter 3 sampling

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