analytical chemistry 2001 [email protected] dr kresimir rupnik, ph.d. lsu, fall 2015
TRANSCRIPT
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• HW, pre- Exams
• Check for updates and notes!!!!• E-mail if you have questions, e-mail to report
interesting of difficult parts from reading a night before the class
If you want to do more-and get some credit for it…
• Read additional material from the textbook
• Read articles in Analytical Chemistry (Science, Nature , JACS.. also may have relevant research). Check websites
Introduction to The Analytical Process: Analytical Chemistry in
application
• Chapter 0 (read it !)
Chapter 0 contains IMPORTANT take home messagesabout the Analytical Chemistry: What, why, how
Most answers are given in this “representative” example which deals with the analysis of chocolate.
Q; What are we analyzing for?A: Analytes: caffeine and theobromine.
Q: Why are we analyzing?A: We have a well defined goal related to some application such as are quality, CSI , etc……The questions asked here could be: How
does the amount of caffeine compare with the quality of chocolate, say which is better white or dark chocolate? Or : Are the two chocolate samples from the same origin, say the one that a crime suspect is
eating and the one that was found at the crime scene – and both have similar packing?
…and last but not least important question:
Q : How can we (or a CSI lab) find the caffeine and its quantity in chocolate?
A: Find or make the procedure. In chemical analysis that is a process that can be
described in terms of 7 important steps
• The real life analytical chemists knows the procedure, that is knows general steps in a Chemical Analysis
0-2 IMPORTANT THM: 7 General Steps in Chemical Analysis
• Formulating the question• Selecting analytical procedures• Sampling• Sample preparation• Analysis (Chemical or Physical)• Reporting and interpretation• Drawing conclusions
Clicker!!
STEP 1: Formulating the question: I need to know what I am looking for!
Only then I can select an analytical procedure
STEP 1: Formulating the question: I need to know what I am looking for!
Only then I can select an analytical procedure
STEP 2 , selecting analytical procedure
Choices of methods, more on methods throughout the textbook.
Signal analysis , more in the first 5 chapters
Caffeine again: HPLC p. 4High Performance (Pressure) Liquid Chromatography
Instrument, more on HPLC we will learn in “Separations”, Chapter 23 and on
STEP 3: SAMPLING
• Sampling: how many chocolate bars, which chocolates?
• Constructing a representative sample (Box 0-1, p.7): random, homogeneous, inhomogeneous, segregated material, composite sample
• STEP 4: SAMPLE PREPARATION, • Sample preparation (for chemical analysis):
transforming a sample into a state that is suitable for analyis.
• More in chapter 28
STEP 3, STEP 4: terminology SAMPLES: Analytical chemist takes SAMPLES (small parts) of the materials for analysis. ALIQUOT: A small portion of the sample . Aliquotes masses are measured before analysis!ANALYTE: The compound you want to measure.SAMPLE is ANALYZED for a compound or analyte. Elements, ions, molecules, compounds are identified in sample (example: penicillin in antibiotic). A sample is ANALYZED for elements, ions, molecules, compounds (example: for penicillin). If the amount of elements, ions, molecules, compounds in the sample is found, we use the word quantitation (amount of penicillin in the sample is found). An ASSAY for a compound or analyte If we intend to determine what fraction of a sample is some named material, we use the word determination or ASSAY. The named material is called the ANALYTE. We can run an assay for that material.
PROCEDURES, PROTOCOLS, TECHNIQUES:
A protocol is a fixed sequence of actions (events) to be carried out (example: by US EPA). These methods commonly define:
-sample handling requirements
-analytical TECHNIQUES
VALIDATION:
We need a validation of a new method by showing that its results agree with those obtained using older (other), accepted procedures. When validated, we know that the procedure measures what the analyst says it measures on a specific type of sample. .
INTERFERENTS: They are “the enemy”. Interference occurs when a species (other compounds than analyte) contribute the instrument signal intensity or amplitude , by increasing it or decreasing it. So it looks like if there is more or less of the analyte then the real concentration. Interfering species determine what is the best method and protocol. We can MASK them by other chemicals so they do not interfere.
NOTE : Analytical chemistry has its own language-learn it (see the “Terms to Understand at the end of the chapter). You can more about these concepts in chapters 0, 5, 28
STEP 5: Chemical (or Physical) Analysis
• Qualitative analysis: which substance
• Quantitative analysis: how much of it
Note: Analytical Chemistry is a Science that helps improve methods of analysis. Novel directions in analysis usually also include smaller, faster, simpler cheaper methods.
Our choice was HPLC –how it works?
Column
~25cm
long
where the
separation
takes place
HPLC Chromatography, p. 4 Fig 0-4, the column
Results of the Caffeine Analysis: Chromatogram, it shows time on the x-axis and detector response or intensity of the signal on the y-axis. Note the three analytes in this example
arrive at different (retention) times!!!
CHAPTER 00: Unnumbered Figure 0-3
CHAPTER 00: Unnumbered Figure 0-4
CHAPTER 00: Figure 0-7
CHAPTER 00: Figure 0-9
CHAPTER 00: Figure 0-10
HANDOUTS
Textbook p 5 and6
Figures 0-5,0-6 and 0-7
Can you tell what is going on here?
CHAPTER 00: Figure 0-11
CHAPTER 00: Figure 0-12
CHAPTER 00: Figure 0-13
Analytical “two-step”
• 1st step: Calibration with known standards. Use standards, pure compound same as analyte with known concentrations and record the reading on the instrument. Form a new diagram (Figure 0-7) to get the calibration curve – you can use Excel fitting.
• 2nd step: Measurement of analyte and quantitative analysis from that known calibration curve.
STEP 6: Data Analysis and Reporting
• This is what we will do in chapters 3, 4 and 5. We will discuss the so called statistical analysis which will give us uncertainty (error) limits to our data presentation and interpretation of our results. It will provide us with the limits of confidence and consequently the limits for possible answers to our question.
• An example: After such analysis was done we can answer the question about caffeine in our chocolate: Table 0-2.
SIGNALS
Total analysis technique & Concentration techniques
SA(signal due to analyte) =k CA (concentration of analyte)
CALIBRATIONS
-Accuracy, precision, sensitivity, selectivity, detection limits, noise
-Robust and rugged method
-Scale of operation (analytes classification )
STEP 6: terminology
There are methods how to calculate the best fit: linear regression of straight-line calibration curves
Calibration curve shows the response of the instruments signal to different concentrations (quantities) of the analyte. It is done by the measurement of standards before the measurement of analyte!! Standards are prepared carefully, not from sample!!
STEP 6:Reporting and STEP 7: Interpreting results
• Analysis of dark and light chocolate, grams of analyte per 100g of chocolate
Analyte Dark chocolate
White chocolate
Theobromine
0.392+-0.002 0.010+-0.007
Caffeine 0.050+-0.003 0.0009+-0.0014
** uncertainties
=standard
deviation
CHAPTER 00: Table 0-1
CHAPTER 00: Table 0-2
STEP 7 : Conclusion
Note on the methods• Not all instruments are “ BIG” lab
equipment. Read about biosensors, p 1
of your textbook.
Note on analytical Chemistry Applications• Industry (almost all need QC and analysis- food,
materials, …)• Bio-Medicine (the basis of diagnosis, individual
susceptibilities, changes..)• Environment (QC, monitoring, regulations)• Law (CSI, ..)• Others
SEE ALSO: Analytical Chemistry Division at ACS and NSF. Check the pending changes in classifications! For new instrumentation see also DOE. For new protocols and standards NIST. For medical applications see NIH.
1. Measurements-Basic Tools
-qualitative aspects (chemistry, physics)-quantitative aspects
(numbers, units, uncertainty)A SHORT REVIEW
Reading: Chapter 1
CHAPTER 01: Opener A
CHAPTER 01: Opener B
CHAPTER 01: Opener C
1-1Units of Measurement1-1Units of MeasurementSI Units (from 1960) = Metric systemSI Units (from 1960) = Metric system• There are two types of units for measured
physical quantities (=number and unit): (1) 7 fundamental (or base) units:
IMPORTANTIMPORTANT
and (2)(many) derived units obtained from base units (example velocity).
Mole = unit for the number of objects
• 6.02 10 ^23 objects is in a mole.
• 6.02 10 ^23 of oranges is one mole of oranges.
• 6.02 10 ^23 = Avogadro’s number = 1 mole of objects
Q: Who found or measured Avogadros’ number first?
Units of Measurement: Units of Measurement: SI Units SI Units Powers of ten and exponential (scientific) notation are used for convenience with smaller or larger units in the SI system.
IMPORTANTIMPORTANT
There is more….
Length and MassLength and Mass: meter (m) kilogram (kg).(1 kg
= 2.2046 lb.)
SI Units of MeasurementSI Units of Measurement
Temperature:Temperature: Kelvin Scale
Lowest temperature possible (absolute zero) is zero Kelvin.
Celsius Scale has the same temperature increment (ok. in science, but be careful !)
Absolute zero: 0 K = -273.15oC.
Water freezes at 0oC and boils at 100oC.
To convert: K = oC + 273.15.
IMPORTANTIMPORTANT
Examples: (1) Using two or more conversion factors:
# m -------- # in.
# m * 100cm/m * 1in./2.4cm = # in.
(2) Conversions involving volume and density
# g of 2.00in.3 of gold if density = 19.3g/cm3
2.54cm=1.in. and 1cm3 = 19.3g gold
2.00in.3 *( 2.54cm/1.in. )3 * 19.3g/ 1cm3 =633g
Solving problems in chemistry requires careful manipulation of numbers and their associated units, a method known as dimensional analysis.
Given units can be multiplied and divided to give the desired units.
•Use known conversion factors in the form:(Desired unit / Given unit )
•Desired unit = Given unit(Desired unit / Given unit)
Dimensional AnalysisDimensional Analysis IMPORTANTIMPORTANT
•Example: 8.50 inches in cm?
•Number cm = 8.50in x (2.54cm/1 in) = 21.6cm
CHEM2001-2 Review: Moles, concentrations
etc.# moles for element “E" = # grams of “E" /atomic mass of “E" # moles for molecule “MO" = # grams of “MO"/molecular weight of MO # moles for compound “COm" = # grams of “COm"/formula weight of
“COm"*YOU CAN USE CORRECT NOTATION: MOLAR MASS (MM)
Example: How many moles are present in 60.0 g H2SO4 ? # moles of H2SO4 = 60.0 g H2SO4 * (1 mole H2SO4/98.0 g H2SO4)
= 0.612 moles
Check the numbers!!
1 Molecule
6.02*1023 Molecules
..
Interconverting Masses, Moles, and Numbers of Interconverting Masses, Moles, and Numbers of ParticlesParticles
The MoleThe Mole IMPORTANT
6.02*1023
Molecules
(g/mol)
Number Of molecules
Concentrations of solutionsConcentrations of solutions
• CONCENTRATION: the amount of solute in the dissolved in a given quantity (volume or mass) of solvent or solution.
Molarity = Moles of solute / Liters of solution
Molarity = (n / V) molL-1
What if not molecular compounds? M and F (textbook)
Formality or Formal Concentration: F The molarity of strong electrolites is called formal concentration.
Formula mass (FM) The molecular mass of strong electrolyte is called FM.Note that the concept of “formal” indicates that the compound is not the compound as written, it has been dissociated or we do not know what it is exactly, that is how many are in a unit or how many ions etc.
M== F , same units moles/LMM== FM (FW) same units g/mole
* We will not use amu unit here.
When one mole NaCl is dissolved in H2O, Na+ & Cl- ions are present and no intact NaCl molecules! Term from freshman chemistry "formula weight" ( salts of strong acids and strong bases should dissolve completely).
WHEN one mole CH3COOH acetic acid is placed into water not only does it dissociates into CH3COO- & H+3O ions, but also some CH3COOH remains not dissociated. Thus; all three are present in the aqueous solution. Formality again can be used to describe the solution.
However, one mole of sucrose in one liter of water would be a one molar solution and not a formal one.
Note: Chemists sometimes use formality to describe the initial concentrations of substances that ionize in solution, whereas their equilibrium concentrations are described in terms of molarity. Molarity describes both the initial and equilibrium concentrations of non-ionic substances.
.250L of 1.00M solution of CuSO4
39.9g of CuSO4 (FW= 159.6amu)
Molarity: Moles of solute per liter of solution: How to do it??
Important:
Example: molarity of a solution
23.4 g Na2SO4 * (1 mol/142gNa2SO4 ) = .165 mol Na2SO4
V=125mL solution (In H2O)= 0.125L
Molarity = .165 mol Na2SO4/ 0.125L = 1.32M
Molarity: Expressing the concentration of electrolytes What are the molar concentrations of each of the ions present in a dissolved electrolyte?
Example: given: 0.025M of calcium nitrate
Ca(NO3)2 0.025M in Ca 2+
what is ion concentration of (NO3)- ( 2 per molecule !!)
2*0.025M = 0.050M in (NO3)-
Molarity: Interconverting Molarity , Moles and Volume
M = n/V
n= V*M = .400 mol = 2.0 L * 0.200M
V=n/M
Molarity (.200M of HNO3 for example ) is a conversion factor between volume of solution (2.L, for example) and moles of solute (0.400mol HNO3, for example).
Molarity: Dilution, from initial concentration to the final with the SAME number of molecules
(moles) only the volume changes n(initial) = n(final)
Example:
M initial (= 3.0M H2SO4) * Vinitial( ? )
= Mfinal (=0.10M H2SO4) * Vfinal (=0.450L H2SO4)
More Ways of Expressing ConcentrationMore Ways of Expressing Concentration
100soln of mass total
soln incomponent of masscomponent of % Mass
610soln of mass total
soln incomponent of masscomponent of ppm
•All methods involve quantifying amount of solute per amount of solvent (or solution).
•Generally amounts are measures are masses, moles or liters.
•Qualitatively solutions are dilute or concentrated.
•Definitions:
Ways of Expressing ConcentrationWays of Expressing Concentration
• Parts per million (ppm) can be expressed as 1 mg of solute per kilogram of solution. – If the density of the solution is 1g/mL, then 1 ppm
= 1 mg solute per liter of solution.
• Parts per billion (ppb) are 1 g of solute per kilogram of solution.
More Ways of Expressing ConcentrationMore Ways of Expressing Concentration
Mole FractionMole FractionRecall mass can be converted to moles using
the molar mass.
l
components all of moles totalcomponent of moles
component of fraction Mole
Molality
Molality = (moles of solute / kilograms of solvent) molkg-
1 »Converting between molarity (M) and molality (m) requires density.
Ways of Expressing ConcentrationWays of Expressing ConcentrationMole Fraction, Molarity, and MolalityMole Fraction, Molarity, and Molality
FYI: Normality (old unit) = (# equivalents / Liters of solution)= " N "
An equivalent =“the amount of one chemical species reacting stoichiometrically with other chemical species DEPENDS ON CHEMICAL REACTION!!!
An equivalent = (# g compound /MW compound) * (# H+ions, if acid) (# OH-ions, if base) (# e- lost or gained if
oxidized or reduced species)So that : EW = FW/nSo that: N= n x M
Examples:
a 1M HCl soln. would be ---> 1N HCl soln.and a 1M H2SO4 would be ---> 2N H2SO4 1M FeCl3 soln where iron is +3, would be ----> 3N Fe+3 soln
pH=-log[H+], What is pCl- ?
1-4 Real chemical reaction: the chemical change is a transformation where atoms (physical objects) are conserved: mass, charge, protons, atoms, electrons
• Reactants and products:
2H2 + O2 2H2O
• Stoichiometric coefficients in front of chemical formulas give the number of reactants and number of products.
Chemical Equations: Description of Chemical Reactions in terms of
Atoms and their quantities
IMPORTANT
Quantitative Information from Quantitative Information from Balanced Equations (ABalanced Equations (AB)B)
Titration (textbook example)
The Equilibrium Constant (Review)
For the reaction aA + bB cC + dD, the equilibrium constant, K, is
K = [C]c[D]d
[A]a[B]b
Reaction is f avored whenever K > 1 Evaluating the equilibrium constant 1) The concentration of solutes should be expressed as moles per liter 2) The concentration of gases should be expressed in atmospheres
3) The concentrations of pure solids, pure liquids and solvents are omitted because they are unity Manipulating Equilibrium Constants
HA H+ + A– K1 = [H+][A–]
[HA]
Reverse Reaction,
H+ + A– HA K1' = [HA]
[H+][A–] =
1K1
Add Two Reactions HA H+ + A– K1 H+ + C CH+ K2
HW and practice
Complete Problem Set 1 , HW 1 and pre-exam.