The visible-UV spectrometer we use permits the quantitative determination of the absorption and...
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Transcript of The visible-UV spectrometer we use permits the quantitative determination of the absorption and...
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CHROMATOR
DETECTOR
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The visible-UV spectrometer we use permits the quantitative determination of the absorption and
transmission of light in the wavelength range 200 nm* - 900 nm by a clear liquid sample. The device consists of simple components.
* We actually encounter a lower wavelength limit of ~350 nm due
to the use of glass sample containers. Glass absorbs
significant amounts of radiation below 350 nm.
sample compartmen
t
wavelength adjustmentzero
adjustment
wavelength switch
100% transmittance
adjustment
mode switch
The The ApparatusApparatus
display
Variations in SpectrometersVariations in SpectrometersSome models of spectrometers include other
switches and capabilities which do not play a do not play a rolerole in our exercises, e.g., mode switches labeled
– “concentration” or “factor”
• Some models of spectrometers do not include the wavelength switchwavelength switch shown in the preceding picture.
• A few of the spectrometers have an analoganalog instead of digitaldigital output.
While the details may differ slightly, the basic procedures
described herein are the same for all the models.
wavelength switch
Step 1 - Set Step 1 - Set WavelengthWavelength
If spectrometer has wavelength switch, be sure is it set properly
Adjust wavelength
using wavelength adjustment
knob
wavelength reading
Initializing the SpectrometerInitializing the Spectrometer
Once the desired wavelength is set, we adjust the electronics at the selected wavelength. We must set (or confirm) that spectrometer displays:
– 0% transmittance when no light reaches the detector
• When sample compartment cover is closed with no sample in chamber, the light beam is interrupted internally.
– 100% transmittance when the sample absorbs no light at the given wavelength
• This latter step requires the preparation of a BLANK,
BLANK: a solution containing all the substances in the solution except the one whose absorption we seek to determine.
Step 2 - Set/Check 0% Step 2 - Set/Check 0% TransmittanceTransmittance
EMPTY SAMPLE
CHAMBERSample
compartment cover
closedAdjust
transmittance to zero
Mode switch set to
transmittancetransmittanc
e reading
Having adjusted the electronics so the detector reads 0 (zero) percent 0 (zero) percent transmittancetransmittance when no lightno light reaches it, We next want to insure that the detector reports 100 percent 100 percent transmittancetransmittance (at the appropriate wavelength) when the blankblank, but no no samplesample, is in the beam.
The setting of 100% is a critical reference point.
It does not mean that the blank does not absorb light at this wavelength, but that we will measure only the absorption due to the samplesample.
Step 3 - Set/Check 100% Step 3 - Set/Check 100% TransmittanceTransmittance
Place blank in sample
compartment. Cover
closed
Adjust transmittanc
e to 100%
Mode switch set to
transmittancetransmittanc
e reading
DO NOT TOUCH THIS
SETTING
Initializing the Spectrometer
• Major changes in the 0% or 100% controls may require repeating both settings to be sure that adjusting one of them has no effect on the other
– I.e., confirm that transmittance is 0% with no sample after setting 100% with blank
• If 0% or 100% have changed, repeat the initialization steps
In principle, the 0% setting should need no adjustment after the spectrophotometer electronics have stabilized. In practice, small adjustments may be required.
Step 3a – Re-Check 0% Step 3a – Re-Check 0% TransmittanceTransmittance
Remove Blank Sample compartment
cover closed
Re-Adjust transmittanc
e to zeroONLY
if necessary
Mode switch set to
transmittancetransmittanc
e reading
After the Spectrometer is InitializedAfter the Spectrometer is InitializedOnce the 0% and 100% settings are stable,
the spectrometer is ready to measure samples at the setat the set wavelength wavelength
the subsequent steps depend on whether the spectrometer will be used:
to make a series of measurements at the same fixed fixed wavelengthwavelength (e.g., in a Beer’s Law
determination)
to change change wavelengthwavelength (e.g., to determine the
absorption spectrum of a sample)
Only periodic checks of the 0 and 100%
settings are necessary
The 100% setting must be readjusted
each time the wavelength is
changed
OROR
Summary1. Prepare Blank2. Turn on spectrophotometer3. Adjust wavelength setting4. Adjust zero setting with empty sample
compartment5. Adjust 100% setting with blank in sample
compartment6. If working at fixed wavelength, check both
zero and 100% settings periodically 7. If working at varying wavelengths, adjust
100% setting whenever wavelength is changed. Also check zero setting periodically.