IntroductoryAnalytical Chemistry

59-320Fall 2013 - v. 2.0

Department of Chemistry & BiochemistryUniversity of Windsor

What is analytical chemistry?

Analytical chemistry is the science of chemical measurements which allows us to ask four basic questions about a sample:

• What ? - What is the sample identity?

• Contents ? - Does sample contain X?

• How much ? - How much X is in the sample?

• Arrangement ? - What is the structure, type, form, etc.?

Uses of analytical chemistry

• Basic chemical analysis

• Forensic chemistry

• Bioanalytical chemistry

• Environmental analysis

• Blood chemistry and hematology

• Materials analysis

• Pharmaceuticals analysisThe hit TV show CSI has inspired a new generation of students to try out analytical chemistry! However, the analyses are not usually quite as rapid and glamourous as the CSI musical analytical chemistry montages.

Techniques in Analytical Chemistry

Most modern analytical chemistry is conducted with sophisticated (and expensive) instrumentation. However, to get the proper results using instrumental analysis, one must have a sound grounding in the basic techniques of analytical chemistry:

• Gravimetric analysis

• Volumetric analysis

• Titrations (potentiometric, gravimetric, etc.)

• Separations (sample preparations, chromatography, etc.)

Titrations are done with an important piece of glassware known as a burette. At this point in your education, you may think you know how to titrate properly.... but we shall see!

Areas in analytical chemistry

There are numerous instrumental techniques that can be applied for a wide array of chemical analyses:

• Spectroscopy

• Spectrophotometry

• Mass spectroscopy

• Chromatography and electrophoresis

• Electrochemistry

• Microscopy

Capillary electrophoresis, a basic separation technique in analytical chemistry, is arguably one of the most important scientific discoveries of the 20th century.

http://www.mun.ca/biology/scarr/48-capillary_electrophoresis.jpg

Ch. 0: The analytical process

Outline:

• 0-1 Measurement of atmospheric CO2

• 0-2 The analytical chemist’s job

• 0-3 General steps in chemical analysis

0-1: Atmospheric CO2

Charles David Keeling (1928-2005) took a series of careful measurements of atmospheric CO2 in 1958 that have been called, “the single most important environmental data set taken in the 20th century.”

• He built a vacuum system (to trap CO2 in dried air using liquid N2) and a manometer (to measure the amount of CO2 gas - remember 59-240?)

• Collaborator Sam Epstein measured 13C/12C ratios with their new mass spectrometer (which were found to decrease with higher CO2 concentrations)

• Measurements via manometer were time consuming, so the use of an IR spectrometer was proposed (along with accurately calibrated standards).

Atmospheric CO2, 2

In 1958 and 1959, he found that CO2 was at a constant 310 ppm in the afternoon, but was higher and variable overnight (many regions in the northern hemisphere). Solar heating creates turbulence that mixes air near the ground with air higher in the atmosphere; however, at night, this air cools, and rests near the ground.

The seasonal variation of CO2 concentrations in the N.H. featured a maximum concentration in May (right before leaves come onto the trees) and a minimum in October at the end of the growing season (i.e., CO2 is withdrawn for growth in summer, and released for dormancy in winter).

The Keeling Curve

In Manua Loa, Hawaii, the CO2 levels have been monitored for over 50 years. The seasonal oscillations are clearly visible, but so is the steady rise of CO2 concentrations. The buildup of CO2 in the atmosphere is known to result in the greenhouse effect, in which CO2 absorbs IR radiation released from the earth’s surface. Furthermore, the pH of the ocean is dropping due to the increased formation of carbonic acid (H2CO3).

Global Climate Change150 years of burning fossil fuels has increased the CO2 concentration in dry atmosphere from a historic peak of 280 ppm to 380 ppm today. This is demonstrated by plotting the Keeling curve onto the same graph with atmospheric variations in CO2 over the last 800,000 years. The CO2 content is measured from air bubbles trapped in Arctic ice, and the age of the ice and atmospheric levels are calculated from the measured isotopic compositions of H and O in the ice.

Analytical Chemist’s Job

Chocolate has very high energy content, tastes great, and also has caffeine (a stimulant) and theobromine (a precursor to caffeine). There are a number of steps which must be taken in order to determine the amount of caffeine in a chocolate bar:

• Sampling

• Sample preparation

• Chemical analysis

• Calibration curves

• Interpretation of results

SamplingSampling refers to procuring/obtaining a representative sample.

Consider: The question regarding the amount of caffeine in chocolate is very general. The analytical approach will change appreciably depending on whether one is investigating (i) one type of chocolate bar, (ii) one variety of chocolate type or (iii) chocolate in general.

One must also consider whether the individual samples are homogeneous (composition is pretty much the same everywhere) or heterogeneous (composition varies from place to place. Heterogeneous materials may be classified as random or segregated.

Sampling, 2Consider the three chocolate bars pictured below:

Mars Bar Cadbury’s Dairy Milk Hershey’s Almond Bar

Which best describes each case? Homogeneous, random heterogeneous or segregated heterogeneous?

Sample Preparation

The amount of caffeine is to be analyzed using high performance liquid chromatography (HPLC). The sample must be prepared by extracting the caffeine into a solution of hydrocarbons - but there are several problems with this:

• Chocolate must be finely ground so caffeine may be extracted; however, chocolate is difficult to grind

• The fats in chocolate must be separated from the rest of the mixture, due to interference in the HPLC portion of the experiment

In order to grind a soft substance like chocolate in to a fine powder, the chocolate must be frozen!

Sample Preparation, 2Ground bits of chocolate were placed in a pre-weighed centrifuge tube.

The fat dissolves in the supernatant, and solid packs in the end of the tube. Caffeine and theobromine are insoluble in petroleum ether. The supernatant is then decanted from the centrifuge tube.

Ground bits of chocolate were placed in a pre-weighed centrifuge tube.

Sample Preparation, 3Caffeine and theobromine, the analytes, are soluble in water.

The chocolate particles are boiled, and a portion of this mixture is quantitatively transferred to a centrifuge tube. After centrifuging, the supernatant is quantitatively withdrawn and filtered to remove particles. (This is repeated multiple times!)

Ground bits of chocolate were placed in a pre-weighed centrifuge tube.

Chemical Analysis

Ground bits of chocolate were placed in a pre-weighed centrifuge tube.

Calibration Curves

Ground bits of chocolate were placed in a pre-weighed centrifuge tube.

Since the two species may have different responses to UV excitation and detection, it is necessary to construct calibration curves for samples of known concentrations. The ppm scale on the bottom describes micrograms (μg) of analyte per gram of solution.

Interpretation of Results

Ground bits of chocolate were placed in a pre-weighed centrifuge tube.

The caffeine content in white chocolate appears to be ca. 2% of that in dark chocolate. The standard deviations are also reported, which are measures of the reproducibilities of the results. For caffeine in dark chocolate, the s.d. (0.002) is less than 1% of the average (0.392); therefore, this result is said to be high reproducible. For theobromine in white chocolate, the s.d. (0.007) is almost as large as the average (0.010); therefore, this result is said to be poorly reproducible.

Interpretation of Results, 2

Ground bits of chocolate were placed in a pre-weighed centrifuge tube.

Clearly, the proposed analytical method may be suitable for dark chocolate samples of the size that were used in this particular study; however, these same methods may not be suitable for studying caffeine content in a range of other materials (methods, sample sizes, sampling techniques, etc. must all be adjusted!)

General Steps in Chemical Analysis

Ground bits of chocolate were placed in a pre-weighed centrifuge tube.

• Formulate the question

• Select analytical procedures

• Choose sampling procedure

• Sample preparation

• Analysis

• Reporting and interpretation

• Drawing conclusions

General Steps in Chemical Analysis

Ground bits of chocolate were placed in a pre-weighed centrifuge tube.