Patrick Smith

3/10/2011

Chem. 475

Excedrin UV-Vis analysis

Introduction:

In this lab we were tasked with finding the amount of aspirin, acetaminophen, and

caffeine in Excedrin tabs, Mr. Goody’s, and Anacin. The lab encompassed over 3 weeks

of method development, and data gathering to reach the end result. As such, with the

method of choice being that of double beam UV-Vis spectrometry, a definition of the

methods must be discussed.

First, it must be defined what a double beam UV-Vis spectrometer does. Unlike

the single beam, the double beam has a much more complicated layout. It uses rotating

mirrors to allow light to absorb both into and out of the blank. Along with this blank in

one of the cells the next cell will contain the analyte of interest. This allows more

statistical measurements and certainty to be taken, because at each wavelength the

analyte is being measured against a blank.

The other important aspect of what allows this spectrometer to work is its

monochromater. Without one, it would be impossible to get any noticeable data. This

piece is one of the keystones of a spectrometer as it allows only single wavelength

through its selector from a polychromatic light source. This works by having the

polychromatic light entering in through an entrance slit and then using a collimating

mirror to focus the light onto a diffraction grating. This grating when set at a certain

angle will disperse this polychromatic light and give off the desired wavelength to the

focusing mirror which reforms the image that entered. This is then passed onto the exit

slit, which is then emitted to our detector giving us our reading. Further filters can be

added as well, to ensure that even after the monochromater has done its job no other

wavelengths are allowed to reach the detector other then the desired one.

This ability to select for only one wavelength at a time, and to increase our

sensitivity by being able to measure against a blank at every said wavelength, was very

valuable in our attempt to quantify the components of our 3 medicines. The reason being

is that each individual compound would absorb at different wavelengths, and by using

contrived solutions and standards of the 3 medicines we would be able to find a way to

identify through our linest algorithm the quantity of each.

The linest program allowed us the ability to analyze, and find 2 of our 3 known

compounds in our tablets, and then use the algorithm to find the remaining one. To do

this we first had to spend the first week making up our known solutions of aspirin,

acetaminophen, and caffeine in order to get baseline data for them. These extinction

coefficients were essential as they allow us to derive single equations at each wavelength.

These constants at each absorbance help us to identify, and prove that through Beer’s law

of A=bCE that the only unknowns we have if we know the constants of extinction

coefficients, are the concentrations of our compounds. With this idea in place it is then

possible to analyze through the Linest function, and know the answer that is given is our

concentration of the components we seek.

Problems arose during this process though, as the results given indicated

contradictory evidence of what theory should have given us. Most specifically the

acetaminophen in theory was supposed to be the highest absorber above aspirin at around

the 220-240-wavelength area. However, due to a dilution mistake on the acetaminophen,

it appeared the exact opposite of this with aspiring far outstripping the acetaminophen in

absorbance at this maximum thus contradicting the prediction. This problem was not

noticed until the Excedrin tabs, Mr. Goody’s, and the contrived solutions of known

combined compounds were taken the second week. It illustrated the importance of the

extinction coefficients because with them invalidated by this mistake the results in the

linest function were disastrous. As a result of the misfortune however of this mistake we

were better able to see that these constant values in beers law really are necessary to get

statistically sound results and data.

Thus, in the third week we proceeded with making a set of new contrived

solutions with mixtures of different concentrations of the known compounds, and also

mixtures of just pure compound as well. We then threw out the extinction coefficients for

the previous weeks, and decided to use new ones to analyze our tablets. As well, it was

decided that this time that along with Excedrin, Anacin would be used instead of Mr.

Goody’s. The reasoning was that Anacin only contained caffeine and aspirin, and thus

with only two species in it, it would provide an excellent test of our method as there

would be no overlapping interference from the two absorbance’s of aspirin and

acetaminophen. As well we decided to take other precautionary steps to ensure that the

data had no chance of being skewed. .1 M HCl was blanked against this time rather then

water before due to the fact that even though not much would be different in absorbing

there could be a chance that it was noticeable enough to add error to that already present.

As well for the Excedrin and Anacin solutions they were filtered through a .45-

micrommeter syringe filter to remove any of the filler-binding agents in the pills. As well

they were diluted even more then the first time by a factor of 10 to ensure that it was as

dilute as possible eliminating any binding agents whatsoever.

Data and Results

205.0 215.0 225.0 235.0 245.0 255.0 265.0 275.0 285.0 295.00.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Caffeine

Acetaminophen

Aspirin

( )l nm

Ext

inct

ion

Co

effi

cien

ts (

pp

m-1

cm

-1

)

Figure 1: Absorbance data of pure compound solutions.

205.0215.0225.0235.0245.0255.0265.0275.0285.0295.0305.00

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Series2Series4Series6Series8

Figure 2: Sample Spectra of Excedrin compounds.

205.0215.0225.0235.0245.0255.0265.0275.0285.0295.0305.00

0.2

0.4

0.6

0.8

1

1.2

1.4

Series2Series4Series6Series8

Figure 3: Sample Spectra of Anacin tabs.

205.0215.0225.0235.0245.0255.0265.0275.0285.0295.0305.00

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Series2Series4Series6Series8Series10Series12Series14Series16Series18Series20Series22Series24

Figure 4: Extinction coefficient data from contrived solutions.

Excedrin Data:

Group 1Std. Dev. Group 2

Std. Dev. Group 3

Std. Dev. Group 4

Std. Dev.

Aspirin 258.3 ±0.756 262 ±1.04 254.8 ±0.813 245 ±1.09

Acetaminophen 221 ±0.6301 259.5 ±0.864 243.9 ±0.676 251.2 ±0.908

Caffeine 56.1 ±0.751 61 ±1.029 57.5 ±0.805 55.3 ±1.08

Anacin Data:

Group 1Std. Dev. Group 2

Std. Dev. Group 3

Std. Dev. Group 4

Std. Dev.

Aspirin 358.8 ±0.644 369.1 ±0.581 408.9 ±0.744 398.52 ±0.725

Caffeine 30.2 ±0.638 33.6 ±0.573 36.2 ±0.737 33.29 ±0.72

Excedrin Group averages with propagated error:

Drug Compound Average

Std. Deviation

Acetaminophen 243 ±1.56

Aspirin 255 ±1.87

Caffeine 56 ±1.85

Anacin Group averages with propagated error:Drug Compound Average

Std. Deviation

Aspirin 384 ±1.35

Caffeine 33 ±1.34

Discussion of Results and Data:

This data that was collected provided many conclusions that are valuable for

understanding UV-Vis spectroscopic practices. As well, it also showed us how our

method that was developed was verified. Finally, the data obtained was necessary in

determining the reproducibility and accuracy of our results and the spectroscopic method.

First, the most important thing that needed to be verified was that both our

contrived solutions, and the pure compound solution data matched the same absorbance

pathways. As seen in figure 1, the three absorbances of both Aspirin (green)

Acetaminophen (red) and Caffeine (blue) follow a set pattern shown. Caffeine absorbs

almost exclusively in the 275.0-295.0 nm range. No other species absorb at this

wavelength so in the contrived solutions with caffeine by itself or with combinations of

caffeine and other drugs the method put forth should in theory show the same absorbance

peak for caffeine. The solutions without it should show no absorbance at all.

Acetaminophen and aspirin both have their peaks at 235.0-245.0 nm with Acetaminophen

showing the stronger absorbance. Again a similar peak should be shown in our contrived

solution as predicted by our method set forth. Figure 4, which contains our contrived

solutions shows that this prediction holds up with the given spectra following the same

absorbance lines as in figure 1 with our pure solutions.

This proof of what we had gathered in theory before conducting the experiment

was very important because it showed that we could produce accurate extinction

coefficients. This then showed that Beer’s Law truly is proven where the only variable

we have is the concentration of analyte, which then dictates our extinction coefficients

and absorbance’s. It is this relationship that even makes it possible for us to identify

different species in our two products.

The spectroscopic relationships do not end there though. Figure 4 comes into play

when trying to understand figures 2 and 3 of the Excedrin and Anacin data. The lines in

figure 4 verify that with the contrived solutions when you combine solutions with

acetaminophen and caffeine rather then getting 2 separate identifiable peaks you instead

are given one peak that is the sum of the two. This is critical in identifying our unknown

compounds as the spectral data in either of the figures 2 or 3 show only one absorbance

line for each test done by a group. By knowing that these two peaks of acetaminophen

and aspirin add together, it allows us to use our contrived solution data where we

gathered extinction coefficients for when there was only one of the two species present

and use it to identify either the acetaminophen or aspirin using the linest program. Along

with one of these, the caffeine can easily be indentified because it absorbs at minimum

absorbance of the other two. With two out of the three found we can then use the lines

program to extrapolate what the third value must be in order to add up to the given

absorbance.

The overall data gathered in this lab was relatively accurate as shown in graphs on

figures 2,3,4 visually, and by the standard deviations given by the linest program. In

particular though there were a few problems that lead to some systematic error in our

procedure. Some of these were fixed, but others were unable to be addressed in the span

of the experiment.

First, during the two weeks that we did our experiment we discovered that

significant systematic error had been added in by using water as our blank rather then

the .1 M HCL that we used as our solvent in solution preparation. It was believed that

because HCL and water nearly absorb in the same way it would not have an impact, but

the mistake highlighted the basic need of any uv-vis spectroscopic procedure, which is

that you must blank against what your solvent is no matter what. This error was then

rectified the third week and results became more streamlined between groups.

Second, were the filler portions of the Excedrin and Anacin. This caused

problems during the first two weeks as when making the dilute solutions the filler

components could not be completely filtered out. As a result when put into the

spectrometer some of the light passing through the sample hit these fillers that were in the

product solutions, and caused abnormal readings then what was expected, and increased

systematic error. The third week this was rectified again by using a .15-micrometer filter

rather then a .45-micrometer filter to allow only the soluble parts of the solutions to pass

through. This again helped to lower our systematic error as the filler was almost

completely removed.

Third, there was the problem of a non-streamlined process in the solution making

of both the contrived solutions, and the unknown samples. To change this, it was

necessary to force each group to make the contrived solutions exactly the same way in

the same procedure, and to make the dilution for the unknown solutions the same as well.

By doing this there was a decreased chance of dilution error being present, and again

systematic error was removed.

Finally, the one main error that was unavoidable that was systematic of our

method was that of Caffeine and its consistently low readings from what was expected in

theory. This can easily be explained by the fact that acetaminophen, and aspirin are in

much higher concentrations then caffeine in each pill. As a result, the caffeine becomes

so dilute during the dilution of the unknown solution that the caffeine approaches the

detection limit more so then the other two. This results in a little more uncertainty, but it

is one that is hard to overcome as it is necessary to be so dilute in order to effectively

read the absorbance’s of the acetaminophen and aspirin as to not over load the solution.

This lab proves the value beyond measure spectroscopy in the modern world. In

the realm of antibiotics and medicine it is vitally important to know that each pill does

contain what its package says. This method we used allows us to verify it in a quantities

method that is both reproducible, and prone to little error. When people’s lives are in the

balance this is a must, and shows the true value of chemistry in our daily lives.

Not only is this method valuable due to its sheer ease of use, and reproducibility,

but it also is verifiable by other methods as well. This same experimental data could be

confirmed by using Standard additions at each concentration to verify the same data we

obtained using the double-beam spectrometer. However, while this is perhaps the best

way to verify it, it shouldn’t be used due to the amount of length in time that it would

take to complete. Another method that could be used to verify this in a different practice

is by using HPLC. By using a form of chromatography we can separate the three

components of the unknown solutions, and then quantify them using an appropriate

method. This way to interchange chemical laboratory methods to obtain the same result

truly shows why spectrometry is such a core backbone of chemistry.