How a Simple UV-visible Spectrophotometer Works

00:06:48
https://www.youtube.com/watch?v=wxrAELeXlek

Resumo

TLDRProfessor Davis provides an introduction to UV visible spectroscopy and the Beer-Lambert law, detailing the components and operation of a spectrophotometer. He explains how light intensity is measured through a sample and reference cell, demonstrating the effect of sample concentration on light transmittance. The relationship between transmittance and concentration is shown to be exponential, leading to the use of absorbance for easier data analysis. The video concludes with an invitation to check out an upcoming organic chemistry course.

Conclusões

  • 🔬 UV visible spectroscopy measures light absorption.
  • 📏 Beer-Lambert law relates absorbance to concentration.
  • 💡 Spectrophotometer components include light source and detectors.
  • 📉 Adding a sample decreases light intensity.
  • 📊 Transmittance and concentration relationship is exponential.
  • 📈 Absorbance provides a linear relationship for easier analysis.
  • 🔍 Monochromator separates light into wavelengths.
  • ⚡ Detectors convert light intensity to electrical current.
  • 🧪 Logarithm of transmittance simplifies data interpretation.
  • 🌐 More courses available at chem survival.com.

Linha do tempo

  • 00:00:00 - 00:06:48

    Professor Davis introduces UV visible spectroscopy and the Beer-Lambert law, explaining the basic components of a UV visible spectrophotometer, including the source lamp, monochromator, beam splitter, sample compartment, and detectors. He describes how light is processed through these components, leading to the measurement of light intensity before and after passing through a sample. The initial setup shows 100% transmittance with no sample, and as samples are added, the transmittance decreases, illustrating the non-linear relationship between transmittance and concentration. Davis emphasizes the importance of converting this relationship into a linear format using absorbance, as defined by the Beer-Lambert law, making data interpretation simpler and more predictable. He concludes by inviting viewers to check out his upcoming organic chemistry course.

Mapa mental

Vídeo de perguntas e respostas

  • What is UV visible spectroscopy?

    UV visible spectroscopy is a technique used to measure the absorbance of light by a sample in the ultraviolet and visible regions of the electromagnetic spectrum.

  • What is the Beer-Lambert law?

    The Beer-Lambert law relates the absorbance of light to the concentration of the absorbing species in a sample.

  • What are the main components of a spectrophotometer?

    A spectrophotometer typically includes a light source, monochromator, beam splitter, sample compartment, and detectors.

  • How does the addition of a sample affect light intensity?

    Adding a sample that absorbs light decreases the intensity of light exiting the sample cell.

  • Why do scientists prefer to use absorbance instead of transmittance?

    Absorbance provides a linear relationship with concentration, making data analysis and predictions easier.

  • What is the relationship between transmittance and concentration?

    The relationship is exponential; as concentration increases, transmittance decreases exponentially.

  • What is the purpose of the monochromator in a spectrophotometer?

    The monochromator separates light into different wavelengths, allowing only one wavelength to pass through to the sample.

  • What happens to the electrical current generated by the detectors?

    The current generated is proportional to the intensity of light detected, which changes with the concentration of the sample.

  • What is the significance of the logarithm in the Beer-Lambert law?

    Taking the logarithm of transmittance converts the exponential relationship into a linear one, simplifying data analysis.

  • Where can I find more information about Professor Davis's courses?

    More information can be found on the website chem survival.com.

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Rolagem automática:
  • 00:00:00
    hey everybody professor Davis here from
  • 00:00:03
    chem survival comm and the YouTube
  • 00:00:04
    channel chem survival and today I'm
  • 00:00:06
    going to give you a brief introduction
  • 00:00:07
    to UV visible spectroscopy and the
  • 00:00:10
    beer-lambert law to begin our discussion
  • 00:00:12
    I've drawn a schematic on the bottom
  • 00:00:14
    half of this screen depicting a fairly
  • 00:00:16
    typical setup for a UV visible
  • 00:00:18
    spectrophotometer now there are many
  • 00:00:21
    different ways to build a
  • 00:00:22
    spectrophotometer but this is one of the
  • 00:00:24
    simpler designs so I thought we'd start
  • 00:00:26
    there it consists of a source lamp which
  • 00:00:29
    is something as simple as the headlamp
  • 00:00:31
    from a motor scooter or more complicated
  • 00:00:33
    like a deuterium lamp or xenon arc lamp
  • 00:00:36
    the next device in line is what's known
  • 00:00:39
    as a monochromator which is two slits
  • 00:00:41
    which separate or are separated by a
  • 00:00:44
    prism or a diffraction grating the next
  • 00:00:50
    element of our spectrophotometer is
  • 00:00:51
    what's known as a beam splitter which
  • 00:00:53
    divides a beam of light into two equal
  • 00:00:56
    parallel beams of light next is the
  • 00:01:00
    sample compartment which contains cells
  • 00:01:02
    for both a reference and a sample and
  • 00:01:05
    finally detectors which are devices that
  • 00:01:08
    convert the impact of photons into
  • 00:01:10
    electrical current that can be monitored
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    by a computer so now that we've defined
  • 00:01:16
    each of the smaller pieces within our
  • 00:01:19
    spectrophotometer let's turn it on and
  • 00:01:21
    see what happens I'll ignite the source
  • 00:01:24
    lamp creating a variety of wavelengths
  • 00:01:27
    of different light this light passes
  • 00:01:30
    through the first slit of the
  • 00:01:31
    monochromator ensuring that all of those
  • 00:01:34
    light photons are traveling along
  • 00:01:36
    parallel pathways so that when they
  • 00:01:39
    strike the prism they are refracted into
  • 00:01:42
    a rainbow of colors so each wavelength
  • 00:01:45
    of light is moving to a different place
  • 00:01:47
    in space so only one wavelength of light
  • 00:01:51
    in this situation is going to make it
  • 00:01:53
    through the second slit in my
  • 00:01:55
    monochromator striking the beam splitter
  • 00:01:57
    and becoming two beams of equal
  • 00:02:00
    intensity these two beams of equal
  • 00:02:04
    intensity will traverse a cell a
  • 00:02:07
    different one for each of course one
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    being the reference and one being the
  • 00:02:10
    sample cell
  • 00:02:12
    as the beam exits these cells that
  • 00:02:14
    strikes the detectors which are firing
  • 00:02:16
    away creating an electrical current and
  • 00:02:19
    you'll notice at the moment that the
  • 00:02:21
    intensity of the light exiting both the
  • 00:02:23
    reference and sample cells is identical
  • 00:02:25
    therefore the current generated by each
  • 00:02:28
    detector is identical so in this case
  • 00:02:31
    the intensity detected by each of my two
  • 00:02:34
    detectors are the same so if I were to
  • 00:02:37
    consider the ratio of the intensity
  • 00:02:39
    leaving the sample cell to that leaving
  • 00:02:41
    the reference cell I see that the
  • 00:02:44
    transmittance of my sample cell is 100%
  • 00:02:47
    that of the reference so I'm going to
  • 00:02:49
    plot that here at zero concentration 100
  • 00:02:52
    percent transmittance now let's add a
  • 00:02:56
    little sample to that cell something
  • 00:02:58
    that might absorb this light now notice
  • 00:03:02
    that when I do this the intensity of the
  • 00:03:05
    light exiting the sample cell has
  • 00:03:06
    decreased and therefore the current
  • 00:03:09
    generated by its detector has also
  • 00:03:11
    decreased so now the ratio of
  • 00:03:14
    intensities is no longer 100% in this
  • 00:03:17
    case let's say that it's down to 50% at
  • 00:03:20
    a given concentration which I'm going to
  • 00:03:22
    call X if I add another equivalent of my
  • 00:03:27
    sample to the sample cell that decreases
  • 00:03:30
    the intensity even more not only that it
  • 00:03:33
    decreases the intensity at the sample
  • 00:03:35
    cell by a known amount exactly one half
  • 00:03:38
    for each equivalent of sample that I add
  • 00:03:42
    so when I compare my new intensities at
  • 00:03:44
    the sample and reference cells I see
  • 00:03:47
    that I'm now at 25 percent transmittance
  • 00:03:50
    and similarly an incremental increase in
  • 00:03:53
    the concentration once more in my sample
  • 00:03:56
    cell leads to another reduction by 50
  • 00:03:59
    percent and therefore a percent
  • 00:04:02
    transmittance of 12 and a half now we
  • 00:04:07
    have enough data points to see something
  • 00:04:09
    very interesting here the relationship
  • 00:04:11
    between the percent transmittance and
  • 00:04:13
    the concentration of a sample is not
  • 00:04:15
    linear instead it's exponential and
  • 00:04:20
    while it's very useful to have this
  • 00:04:22
    information scientists and spectroscopy
  • 00:04:24
    prefer
  • 00:04:25
    if they can to discuss linear
  • 00:04:28
    relationships within their data because
  • 00:04:30
    this makes for a much simpler discussion
  • 00:04:32
    and it's much easier to predict how
  • 00:04:35
    things will behave if we have a simple
  • 00:04:36
    linear plot to compare this is where the
  • 00:04:44
    beer-lambert law comes in you'll notice
  • 00:04:47
    that right now I have an exponential
  • 00:04:49
    relationship between my percent
  • 00:04:50
    transmittance and my concentration we
  • 00:04:54
    can see that in the equation here where
  • 00:04:56
    the concentration is a term up here in
  • 00:04:58
    the exponent but what August beer did
  • 00:05:03
    was to convert that percent
  • 00:05:05
    transmittance into a new unit called
  • 00:05:07
    absorbance and he did so by taking the
  • 00:05:11
    logarithm or in really the negative
  • 00:05:12
    logarithm of the transmittance taking
  • 00:05:16
    the logarithm of an exponential function
  • 00:05:18
    creates a linear function and so the
  • 00:05:21
    data when plotted as absorbance rather
  • 00:05:23
    than transmittance is actually
  • 00:05:26
    considerably easier to look at not to
  • 00:05:29
    mention it's much easier to predict the
  • 00:05:31
    exact absorbance by either extrapolating
  • 00:05:34
    or interpolating within the data set
  • 00:05:36
    that I've collected this is the utility
  • 00:05:39
    of the beer-lambert law and the reason
  • 00:05:41
    why we convert percent transmittance
  • 00:05:43
    into absorbance so often when conducting
  • 00:05:46
    UV visible spectroscopy that's all for
  • 00:05:51
    now everyone I'm professor Davis from
  • 00:05:52
    chem survival comm and the YouTube
  • 00:05:54
    channel chem Survival I'll see you on my
  • 00:05:56
    next video I'd like to thank everyone
  • 00:06:02
    for making chem survival calm and my
  • 00:06:04
    youtube channel chem survival such a
  • 00:06:05
    success and I'd also like to invite you
  • 00:06:08
    to check out a new project I've been
  • 00:06:10
    working on coming in October 2014 it's a
  • 00:06:13
    36 part organic chemistry course
  • 00:06:15
    developed in collaboration with the
  • 00:06:17
    great courses to get more information
  • 00:06:19
    about my course go to WWE em survival
  • 00:06:23
    com that's wwm survival com thanks again
  • 00:06:30
    for watching everyone and as always I'll
  • 00:06:32
    see you in my next video
  • 00:06:46
    Oh
Etiquetas
  • UV visible spectroscopy
  • Beer-Lambert law
  • spectrophotometer
  • absorbance
  • transmittance
  • light intensity
  • monochromator
  • sample concentration
  • data analysis
  • organic chemistry