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Analytical Chemistry > Sampling and Sample Preparation > Flashcards

Sampling and Sample Preparation Flashcards

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1
Q

Process of obtaining a portion of a material
that will represent the subject under study

A

Proper sampling

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2
Q

Differentiate gross, laboratory, and aliquot sample

A

Gross sample - consists of several portions of the material to be tested

Laboratory sample - small portion of the sample that is actually analyzed

Aliquot - measured portion of the volume of a liquid sample taken for the analysis

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3
Q

Differentiate representative and composite sample

A

Representative sample - sample that is typical of the parent material for the characteristics under inspection

Composite sample - consists of two or more portions of material (collected at the same time) selected so as to represent the material being investigated

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4
Q

Differentiate in situ and grab sample

A

In Situ sample - allows monitoring the target population without removing individual grab samples

Grab sample - portion of the target population is collected at a specific time or location, providing a “snapshot” of the target population

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5
Q

composition of the parent material is permanent with respect to position in space and stable in
time

A

Static system

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6
Q

the parent material is changing with respect to time; removal of a portion at any instant represents only a snapshot of that moment in time and particular location

A

Dynamic condition

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7
Q

Define random sampling

A

selected in a way that any portion of the material has an equal and known chance of being chosen; least biased approach to sampling

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8
Q

Define selective or judgmental sampling

A

a sample which is deliberately chosen by using a sampling plan that screens out materials with certain characteristics and/or selects only material with other relevant characteristics (directed/focused sampling)

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9
Q

Define convenience sampling

A

a sampling plan in which samples are collected because they are easily obtained

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10
Q

Define systematic sampling

A

the sample is taken according to a previously arranged regular intervals in time or space

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11
Q

Define systematic-judgmental sampling

A

a prior knowledge about a system to guide a
systematic sampling plan

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12
Q

Define stratified or judgmental-random sampling

A

population being sampled is divided into segments (or strata) and a simple random sample is selected from each

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13
Q

Describe the steps in sampling process

A
  1. Sampling > Sample Preservation > Sample Preparation > Analysis
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14
Q

Describe the steps in sample collection

A

Identify the population > collect sample > reduce gross sample to lab sample > analysis sample

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15
Q

Describe sampling plan and type of sample for homogeneous solution.

A

Grab sample is sufficient and sampled by siphoning, decanting, or by using pipet or syringe

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16
Q

How to ensure homogeneity of liquid?

A

Manual shaking (done before sampling if natural diffusion is slow)

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17
Q

Large stationary liquid can be sampled
using _____

A

thief sampler

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18
Q

Describe sampling plan for biological fluids (e.g. blood)

A

Sampling after 12h of fasting and collected (grab sample) by syringes for blood

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19
Q

Differentiate serum and plasma

A

Serum - fluid separated from clotted
blood, serum does not clot
Plasma - fluid separated from unclotted blood, contains fibrinogens

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20
Q

How to store a blood sample?

A

Use preservatives such as NaF (glucose) or anticoagulant (heparin) and store in a vacutainer

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21
Q

Chemical composition of surface waters, such as streams, rivers, lakes, estuaries, and oceans, is influenced by ____ and ____

A

flow rate and depth

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22
Q

How to get grab sample of natural waters?

A
  1. Submerging a capped bottle below the surface and removing the cap (palm under the bottle, hand not upstream)
  2. After the sample bottle is filled, the cap is replaced, and the bottle is removed
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23
Q

The _____, which may be enriched with heavy metals or contaminated with oil, is avoided when collecting the sample

A

air-water interface

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24
Q

Wells used for collecting groundwater samples must be ___ until the water’s temperature, pH, or specific conductance are constant

A

purged

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25
Q

What type of containers must be used when collecting pesticides, oil and grease, and organics?

A

Glass containers

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26
Q

Why plastic bottles are preferred when collecting sample of trace metals?

A

glass surfaces easily absorb metal ions

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27
Q

How to preserve natural waters sample? (3)

A
  1. Control pH and temperature
  2. Limit exposure to light and atmosphere
  3. Add chemical preservatives
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28
Q

Describe the sample collection for gases (workplace gas)

A
  1. Gas is drawn onto a collection medium using a specially designed sampling pump (will pull the gas into the container)
  2. Flush the container for a predetermined time
  3. Seal the container
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29
Q

Disadvantage of gas sampling

A

Tendency for some gases to adsorb the container’s wall (analytes at concentrations too low to detect may change the chemical composition of sample)

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30
Q

Usual sampling method for gases is ____

A

displacement of liquids (not reacting with or solubilizing the sample)

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31
Q

Sampling of gases which changes the sample from gaseous to a liquid state

A

cryogenic cooling

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32
Q

Describe sampling of gases that utilizes chemical reaction and its type of sample

A

A bottle is used in which a reagent is dissolved in an appropriate media (pretreated with reagent).

In situ sample

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33
Q

Differentiate sampling with and without enrichment

A

Sampling with enrichment - gas is adsorbed into a solid collection phase or adsorbed into a solution

Sampling without enrichment - used to determine organic gases that occur permanently in the air of the workplace

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34
Q

Sample container that is generally used for direct sampling of gas (3)

A
  1. gas storage vessels
  2. gas-tight syringes
  3. gas sample bags pumped at 1-3 L/min flow rate
35
Q

special glass tubes designed to collect airborne
contaminants by bubbling the sample air at a high flow rate into a specific absorbing liquid inside

A

impingers (bubblers)

36
Q

In the sampling process using impingers, air drawn into the impinger is formed through a ____, which is covered by a
liquid such as _____. The pollutant dissolved in the liquid is subsequently analyzed, usually by
____

A

nozzle; high purity water; colorimetric technique

37
Q

The chemical composition of a gas sample is usually stable when it is collected using a ____ (3)

A
  1. solid sorbent
  2. a filter
  3. by cryogenic cooling
38
Q

O2 or CO2 dissolved in liquid (e.g. blood) is considered as ____sample

A

liquid

39
Q

Typical examples of gaseous sample

A
  1. automobile exhaust
  2. emissions from industrial smokestacks
  3. atmospheric gases
  4. compressed gases
  5. solid-aerosol particulates
40
Q

Why solid samples are often the most difficult to sample?

A

Due to inhomogeneity, variation of particle size, and variation within particles

41
Q

Large particulate solids, such as coals and
ores, can be sampled by randomly collecting samples with ___ or by ___

A

a shovel or by riffling

42
Q

Sediments from the bottom of streams, rivers, lakes, estuaries, and oceans are collected with a ___ or ___

A

bottom grab sampler or with a corer

43
Q

equipped with a pair of “jaws” that close when they contact the sediment, scooping up sediment in the process

A

Grab samplers

44
Q

Disadvantage of using grab sampler in collection of sediments

A
  1. tendency to lose finer grained sediment particles as water flows out of the sampler
  2. loss of spatial information both laterally and with depth due to mixing of the sample
45
Q

Describe a cylindrical coring device and its advatange/disadvantage

A

Dropped into the sediment, collecting a column of sediment and the water in contact with sediment.

Advantage: maintain vertical profile (changes in the sediment’s composition with depth are preserved)

Disadvantage: only small surface area is sampled

46
Q

a thin-walled steel tube which retains a core sample when it is pushed into the soil and
removed

A

soil punch

47
Q

Soil sample collected at depths of up to 30
cm is easily collected with ___ or ___ but ____

A

scoops or shovels but the sampling variance is generally high

48
Q

How to collect soil samples at greater depth?

A

Digging and collecting samples

49
Q

How to use soil sampler?

A
  1. Push the sampler into the product
  2. Rotate the handle
  3. Remove sampler
  4. Discharge rod to push sample out
50
Q

Two types of sampling of powder.

A

Static Sampling
1. Scoop
2. Thieving
3. Cone and quartering

Dynamic sampling
1. Table sampling
2. Chute splitting
3. Spin riffling

51
Q

widely used and consists of plunging a scoop into a heaped batch

A

scoop sampling

52
Q

Describe thieving method

A
  1. consists of plunging a capture device (comprising one or more separate sample chambers) into the bulk material to retrieve several small aliquots of the powder
  2. the sample chamber(s) can be opened and closed by an operator via controls at the top of the device
53
Q

sample thief allows material to be collected
______ from _____

A

simultaneously from several locations

54
Q

accomplished by a combination of crushing and grinding the gross sample

A

coning and quartering

55
Q

Discuss the process of coning and quartering

A
  1. gross sample piled into a cone
  2. flattened
  3. divided into quarters
  4. separation of quarters
  5. discarding opposite quarters
  6. repeated
56
Q

superior methods of powder sampling are obtained by using procedures where the sample is removed from a moving powder bulk

A

dynamic sampling

57
Q

Discuss table sampling method

A
  1. involves powder flow along an inclined table in which there are a series of holes
  2. some powder fall through the holes and is discarded
  3. the powder remaining on the incline place passes onto the next row of prism with another holes and more is removed
  4. repeated until the powder at the end of table is sampled
58
Q

Describe chute splitter and chute sampling.

A

Chute splitter - consists of a V-shaped trough along the bottom of which is a series of chutes alternately feeding two trays placed on either side of the trough

Chute sampling - sample is poured into the chute splitter and repeatedly halved until a sample of desired size is obtained

59
Q

Describe spin riffling

A

Uses a device that uses mechanical (vibrational) energy to provide a constant flow of material from its holder

the steady flow passes through a divider head that rotates at a constant speed, thus minimizing segregation

60
Q

(Powder) samples stored at _____ are less prone to biodegradation and the loss of volatile material

A

reduced temperature

61
Q

How to prevent loss of volatile material in (powder) sample?

A

The sample completely fills its container without leaving a headspace where gases can collect

62
Q

_____ allows the same number of particles to be sampled with a smaller, more manageable combined mass

A

Reducing the sample’s average particle size

63
Q

The majority of analytical techniques, particularly those used for a quantitative analysis, require that the analyte be ____

A

in solution

64
Q

Metals/alloys can be obtained by ___ (3)

A
  1. Sawing
  2. Milling
  3. Drilling
65
Q

With some metal materials, a representative sample can be obtained by ____ across the piece at ___

A

sawing; random intervals

66
Q

When sampling a metal in the form of a sheet, random samples can be collected with ____

A

a metal punch

67
Q

Samples can be obtained from a metal wire by _____

A

randomly cutting off pieces of an appropriate length

68
Q

Larger pieces of metal, such as bars or
bricks, are best sampled by ____ and collecting __

A

sawing through the metal at randomly selected points and collecting the “sawdust”

69
Q

How to sample biological tissue? (2)

A
  1. If entire organ is removed: homogenized before smaller portions are taken for analysis
  2. If not: small potions maybe combined to form a composite sample, and the composite sample is homogenized before analysis
70
Q

most commonly used for taking samples from bags because they are relatively cheap and sample quickly

A

Simple bag sampling spears (sack samples)

71
Q

To obtain a good cross section of sample, the sampling spear should be ___ in length

A

40-45 cm

72
Q

_____ type of sampling spears penetrates bag easily

A

tapered type

73
Q

Differentiate the solvent used in sample dissolution based on the material.

A

distilled water - inorganic salts
methanol, chloroform, toluene - organic materials

74
Q

If the samples are difficult to dissolve, the first approach is to ____

A

try digesting the sample with acid or base

75
Q

Many digestions are now carried out in closed containers using _____ as a source of energy for heating the solution

A

microwave radiation

76
Q

Advantages of microwave digestion

A
  1. Higher temperature and pressure range
  2. May be accomplished in less than 30 minutes 3. Closed container prevents the loss of volatile gases
77
Q

Inorganic samples that resist decomposition by digestion with acid or bases often can be brought into solution by fusing with a large excess of an alkali metal salt, called a ____

A

flux

78
Q

Discuss dry ashing digestion method and goal

A
  1. sample is placed in a suitable crucible and heated over a flame or in a furnace
  2. C -> CO2; H -> H2O; S -> SO2; N -> N2
  3. the goal is the removal of organic material, leaving behind an inorganic residue, or ash, that can be further analyzed
79
Q

To achieve a separation there must be at least ____ of the analyte and interferent

A

one significant difference between the chemical or physical properties

80
Q

The goal of an analytical separation is to ___

A

remove either the analyte or the interferent from the sample matrix

81
Q

More often than not, the accuracy and precision of an analysis is limited by ___ rather than the measurement step

A

the sampling

82
Q

If the sampling imprecision is relatively large, it is better to ___ and ___

A

use a rapid, lower precision method and analyze more samples

83
Q

We can improve variance due to sampling by

A
  1. collecting more samples of proper size
  2. increasing the number of times we analyze each sample improves the variance

Analytical Chemistry (13 decks)

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