ANA CHEM 3 > Chapter 4. Guide to Method Sampling > Flashcards

Chapter 4. Guide to Method Sampling Flashcards

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1
Q

Define the problem

A

Require a solid understanding of
analytical techniques available

Problem-solving skills

Experience

Intuition, logic and common sense

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2
Q

Define the problem

A

Intent of the measurement

Considerations in sampling and sample preparation

Best technique/ method for doing the analysis

Evaluation / analysis of data

Reporting of results
Resources needed to accomplish the analysis

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3
Q

choosing you test method: the sample and the analyte

A
  • What is the nature and background of
    the problem?
  • What is known about the history of the
    sample?
  • What analyte is important in the
    sample?
  • What is the concentration range of the
    analyte?
  • What degree of accuracy and
    precision is demanded?
  • What other components are present in
    the sample?
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4
Q

Choosing your test method: the analyte

A
  • Have prior, similar efforts been documented in the
    literature?
  • What instruments and equipment are available for
    the determination?
  • How much time is needed to perform the work?
  • How soon does the work need to
    be done?
  • How much money is available to
    accomplish the work?
  • How many samples must one
    measure?
  • Are there limitations to the amount
    of sample that can be used?
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5
Q

What factors to consider?

A

✓ What type of information does the method provide?

✓ What are the advantages or disadvantages of the technique
versus other methods?

✓ How reproducible and accurate is the technique?

✓ How much or how little sample is required?

✓ How much or how little analyte can be detected?

✓ What types of samples can the method be used with?

✓ Will other components of the sample cause interference?

✓ Other factors: speed, convenience, cost, availability, skill
required.

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6
Q

Choosing your test method (for water analysis)

A
  1. Is an approved/ regulatory method based on
    AWWA/ APHA available for use?
  2. What will the results be used for?
    (Regulatory compliance or Process Control)
  3. Is the LOQ achievable?
    (at least 1/10th of the regulatory value)
  4. Use a standard method/ test kit/ or developed
    “in-house” method?
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7
Q

QUALITATIVE INFORMATION ON THE ANALYTE

A

elemental composition

oxidation states

structural information

isotopic distribution of the elements in the sample

polyatomic atoms, functional groups, specific molecules, molecular species

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8
Q

study quantitative information - rough concentration

A
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9
Q
  1. PROPERTIES OF THE SAMPLE/ ANALYTE:
A

Sample
* Phase: solid,
liquid, gas,
dissolved,
suspended
* Amount
available for
analysis
*Homogeneity

Analyte
* Chemical and
physical
properties

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10
Q
  1. FACTORS IN SAMPLE
    PREPARATION
A

✓Phase of the sample- solid, liquid, gas

✓Properties of the analyte:
❑Organic or inorganic
❑Pure substance or mixture
❑Homogeneous or heterogeneous

✓Instrumentation- elemental or molecular

✓Decomposition or dissolution of a solid sample

✓Dilutions made prior to measurement

✓Approaches taken to prevent analyte losses or contamination

✓Separation of interferences from the matrix- element or compound that respond directly to measurement; gives a false
signal; signal may be enhanced or suppressed

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11
Q
  1. ANTICIPATED CONCENTRATION
    OF THE ANALYTE
A

linear dynamic range
limit of linearity
limit of detection

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11
Q

upper and lower
boundary of applicability and may not be
linear over all concentrations

A

limit of dynamic range

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12
Q

calibration curve

A

limit of linearity

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13
Q

most important part in method selection

A

limit of detection

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14
Q

The lowest concentration that can be measured with
reasonable statistical certainty (AOAC)

The lowest concentration of analyte in a sample that
can be detected, but not necessarily quantified,
under the stated conditions of the test (NATA Tech,
Note #3)

The smallest concentration that
can be determined that is
statistically different from a
blank at a specified level of
confidence (typically 95%) This
corresponds to the critical
level.(Currie, 1988, Am. Chem.
Soc.)

A
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15
Q

The true net concentration or amount of the
analyte in the material to be analyzed to the
conclusion that the concentration of the analyte in the material is larger than that of the blank matrix (ISO DIS 118431)

The output signal or value above which it can be affirmed with a stated level of confidence (e.g.,
95%) that a sample is different from a blank
sample containing no analyte of interest (ISO 13530:2009)

A

detection limit

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16
Q

three types of detection limit

A
  • instrument detection limit (IDL)
  • method detection limit (MDL)
  • limit of quantitation (LOQ)/ practical reporting limit (PRL)
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17
Q

▪ the smallest signal above the
background noise that can be
detected reliably

▪ Typically 3 X signal/noise ratio

A

Instrument detection limit

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18
Q

random variation in signal or background

A

noise

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19
Q

net response recorded by a method for a sample

A

signal

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20
Q

a value of S/N = 2 or better is considered to be the minimum ratio needed for the reliable detection of a true signal coming from a sample.

A
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21
Q

Estimate S/N:
1) Multiple determination of
blank samples.
2) Estimation of best-fit to calibration curves

A
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22
Q

▪ minimum concentration reportable to 99%
confidence level that the analyte is > zero.

–Determined from the analysis of a low
sample concentration in a given matrix.

–MDL is the “criterion for detection”

LOD = 3 x SD of low sample
LOD = 5 x of blank solution (not a calibration blank)
(US-EPA definition)

A

Method detection limit

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23
Q
  • It is a multiple of LOD at a concentration of the
    analyte that can reasonably be determined with
    an acceptable level of accuracy and precision
  • Can be calculated using an appropriate
    standard or sample, and may be obtained from the lowest concentration on the calibration
    curve (excluding the blank)
  • LOQ is 10 SD.
A

Limit of quantitation (LOQ) / Practical reporting limit (PRL)

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24
study graph on what these limits mean in practice
25
(within batch, internal)
repeatability
25
4. DESIRED PRECISION
s2method = s2 sample + s2 technique
26
(between batches, variability)
reproducibility
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4. DESIRED PRECISION
* Influenced by changes in analyst, instrument conditions, reagents, etc * Can be assessed by analysis of at least 10X, calculate SD within batch and between batches * Long term assessment can be derived from QC charts
28
5. DESIRED ACCURACY
trueness; Is the closeness of agreement between a test result and the accepted reference or true value of the property being measured
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ACCURACY CAN BE EVALUATED AS “BIAS” BY:
1. use of CRMs as controls 2. use of traceable RM or material prepared "in house" 3. use of standard/reference method with little or no systematic error 4. use of the method when participating in PTs 5. use of spiked samples, based on blank or positive samples 6. from assessment of QC charts
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STANDARDS AND CALIBRATION CURVE
Working curve “Best-fit” relationship between analytical signal and concentration of analyte Background signal Slope Linearity Control of calibration standards- temp, pH, complexing properties of sample and standards Instrumental parameters- amplitude and frequency of input signal, sensitivity of the detector, timing of measurement of sample relative to calibration, “drift” of measured signals
31
The way in which the result or signal of a method varies with the amount of compound or property being measured
response
32
A plot of the result or signal vs. the known amount of a known compound or property (standard) being measured.
calibration curve
33
solve practice problem on notebook
34
6. INTERFERENCES
* Dictates: * instrument to be selected for measurement * Sample preparation * Separation * Example: Determination of Na ions in potato chips using an ISE- interference of K
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7. ROBUSTNESS/ SYSTEM SUITABILITY
The analytical method should not be sensitive to small changes in procedures e.g., flow rates, reagents, etc ✓Identify any variables which may have an effect on the data ✓Set up experiments using known materials to determine effects ✓Set performance criteria for methods to achieve ✓Minimum slope ✓Lowest standard response ✓Highest sensitivity Robust methods (using standard methods) will generally produce similar results for the same sample when used in independent laboratories
36
8. EXISTING METHODS
Standard methods Official methods Published methods in scientific journals Laboratory-developed methods Customer’s methods
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9. SAMPLING
* Most important step- largest source of error * Sampling plan (lake) : topography, temperature variation, depth * “representative sample”- reflects the true value and distribution of analyte in the original material * In large complex samples – “sub-sampling” * Transport, labelling, storage, documented chain of custody * Prevent changes due to: * Volatilization * Absorption of moisture * Contamination * Absorption/desorption process with sample container
37
SAMPLING CONSIDERATIONS
1. Specific information on how to collect the sample 2. Documentation on where and when the sample was collected 3. Sampling equipment used 4. Proof of maintenance and calibration of sampling equipment 5. Type of sample containers 6. Proper storage of samples 7. Criteria for accepting and rejecting samples 8. Methods for excluding or separating foreign objects- soil with plant roots and debris 9. Proper treatment for sampling-drying, mixing, homogenization & handling 10. Procedure for sub-sampling and compositing 11. Record keeping that documents all actions performed, traces the chain of custody and any auxillary information important.
38
SAMPLING
Grab samples - samples taken at a single point in time Composite samples -samples taken over a period of time or from different locations
39
GAS SAMPLES
▪ Generally considered homogeneous ▪ Samples are mixed before portions are taken for analysis ▪ May be filtered if solid materials are present
40
➢Scrubbing- trapping an analyte out of the gas phase * passing air thru activated charcoal to adsorb organic vapors * Bubbling gas samples thru a solution to absorb analyte ➢Use of gas-tight syringes
41
LIQUID SAMPLES
May be collected as grab or composite Adequate stirring is necessary, but is some cases, stirring may not be desired- analysis of oily layer in water Undesired solid materials are removed by filtration or centrifugation Layer of immiscible solids may be separated by separatory funnel
42
SOLID SAMPLES
Most difficult, not as homogeneous as gas and liquid Large amounts are difficult to mix Must undergo size reduction (milling, drilling, crushing, etc Adsorbed water is often removed by oven-drying
43
SAMPLE PREPARATION
* Type of sample preparation depends on: ➢Nature of sample ➢Technique/ method chosen ➢Analyte to be measured ➢The problem to be solved * Samples may be: ➢Dissolved in water or other solvents ➢Pressed into pellets ➢Cast into thin filmssurface analysis
44
SAMPLE PREP METHODS: DISSOLUTION
➢Homogeneously distributing the analyte in a solvent * Aqueous * Non-aqueous: for organic compounds and polymers * Acid-water mixture: HCl, HNO3, H2SO4 (HF, HClO4 with special care & supervision) * Complexing agents-water : ligands aid solubility
45
SAMPLE PREP METHODS: DECOMPOSITION
➢Chemically converting the sample into a form that can be dissolved in a solvent
46
inorganic analyte into soluble form with concentrated mineral acids (HCl, HNO3, H2SO4 (HF, HClO4 with special care & supervision)
acid decomposition
47
fused with acidic or basic salt (K2CO3 or K2S2O7) at high T, cooled melt is dissolved
fusion
48
sealed oxidation with O2, followed by absorption of product in a solvent
combustion
49
organic sample in hot oxidizing reagent, then elemental analysis
wet ashing
50
heating an organic sample in flame or furnace, followed by dissolution of the ash
dry ashing
51
high pressure in sealed vessel (automated)
microwave assisted
52
SAMPLE PREP METHODS: FILTRATION
➢Removal of a solid substance from solution by exclusion process Paper and glass fiber Membrane filters- polymeric structure with fine pores (0.3 um) Hollow fiber membrane- use of pressure to force a solution through a membrane, ultrafiltration and reverse osmosis (0.25 um)
53
SAMPLE PREP METHODS: EXTRACTION
➢Selective removal of an analyte from a mixture by partitioning between two immiscible phases
54
distribution into two immiscible liquid phases
liquid-liquid extraction (LLE)
55
analyte retention on solid sorbent; followed by elution
solid phase extraction (SPE)
56
analyte sorbed on a thin layer of sorbent (solid, liquid) coated on the outer surface of a fiber exposed to a liquid mixture, followed by redissolution or volatilization of analyte
solid phase microextraction (SPME)
57
a supercritical fluid is created using a gas (like CO2) above the critical temperature; the resulting supercritical fluid is typically used to extract organic analytes from a solid sample followed by its collection by depressurization, on a sorbent or in a solvent
supercritical fluid extraction (SFE)
58
microwave-accelerated extraction of organic analytes from a solid sample with a liquid
microwave assisted extraction (MAE)
59
a sample is placed in contact with a membrane that allows the sample to selectively permeate into a new gas or liquid
membrane extraction
60
volatile organic compounds are allowed to diffuse from a liquid into a headspace above the liquid; analyte-containing gas in the headspace is sampled
headspace extraction
61
SAMPLE PREP METHODS: DISTILLATION
➢Removal or enrichment of a volatile substance based on differences in boiling point batch azeotropic fractional vacuum
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determine the mass of the analyte or some compound chemically related to it
gravimetric methods
63
measure the volume of a solution containing sufficient reagent to react completely with the analyte
volumetric methods
64
involve the measurement of such electrical properties as voltage, current, resistance, and quantity of electrical charge
electroanalytical methods
65
are based on the measurement of the interaction between electromagnetic radiation and analyte atoms or molecules, or the production of such radiation by analytes
spectroscopic methods
66
include the measurement of such quantities as mass-to-charge ratio, rate of radioactive decay, heat of reaction, rate of reaction, sample thermal conductivity, optical activity, and refractive index
miscellaneous methods
67
study techniques to charts
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