Document Title

Miniature Optical Spectrometers

Funded in part by the NSF, ILI Projects, 1996-1999 Chem 407-8

edited: January 21, 2001
comments? Paul F. Endres
file name: nsf/nsf4_spect.htm
(note: preliminary version-- missing images, sample spectra, references and details)

In the mid 1990's a small company called Ocean Optics developed and began selling a miniature diode array spectrometer. Several companies now make similar units. We own several units made by Ocean Optics and by CVI. Several of these were obtained as part of a NSF-ILI grant to invesitgate low cost, high speed instrumentation for introductory teaching laboratories.)

These spectrometers have some important characteristics

they are quite small (One unit is about the size of a small match box)
they are relatively inexpensive ($1000-2500)
light generally enters from an optical fiber
detection is via a diode array or CCD device
sensitivity is adjustable by varying the integration time
spectra can typically be recorded in under a second
the device is controlled and data is obtained via a personal computer
the device has no moving parts
most units overate in the visible and near UV region, but devices are available with other spectra ranges.

The optical design is relatively conventional

light enters via a fiber optic probe
- the CVI units can also accept light directly
the entering light either passes through a slit or the fiber acts as a slit
the light strikes a concave mirror which collimates the beam
- it is important that the light be collimated (parallel beam) when it reaches the grating
the light is directed on a fixed interference grating
the dispersed light is focused by another concave mirror
the light falls on an optical sensor array
- most are CCD device
  - CCD = charge coupled devices
- similar to a slice from a video cam's light sensor
- a typical array has 1000-2000 discrete light sensors
the signal from the detector array is read, typically every 5-50 milliseconds
- detector sensitivity is increased by using a longer integration time
- each detector element records the intensity at a specific wavelength

Basic Applications

Optical Spectrometer to Measure Light Sources

The free end of the optical fiber can be directed towards a lamp or other light source
fluorescent lamps in a room show two characteristic features
- discrete emission lines corresponding to a mercury vapor discharge
- several broad peaks, each corresponding to the emission by one of the phosphors
- most fluorescent lamps use several phosphors to provide a fairly normal approximation of white light
It is also informative to look at other light sources
- gaseous discharge lamps show characteristic line spectra
- LED's show relatively narrow bands
  - See article _____ on LED spectrum vs.. temperature
A solar spectrum can be obtained by directing the optical fiber towards sunlight
- probably best to aim at a white card rather than directly into the sun.
With care, the spectrum from a heated source can be used to optically estimate temperature
It is useful to compare the spectrum from conventional incandescent lamps, photofloods, photographic flash lamps and tungsten halide bulbs.
A dimmer on an incandescent lamp will reveal color shift at lower power and lower filament temperature.

As a Spectrophotometer

The Ocean Optic Instructional Package (spectrometer module) includes a miniature tungsten halide lamp as the light source. The power supply is regulated to maintain constant illumination.
It also includes an assembly that holds a standard 1 cm square cuvet
Three reading are required:
- a dark signal
- a reagent blank
- the sample
  - the dark current is subtracted from all signals
  - at each wavelength, the sample spectra is divided by the value for the blank
  - this yields %T
  - easily converted into Absorbance
- software then produces a plot of absorbance vs.. wavelength
- software also allows one to do kinetics
  - recording the absorbance at one wavelength vs.. time

As a Fluorescence Spectrometer

A blue LED (light Emitting Diode) serves as the excitation source
- A 9V battery can provide stable power for hours of use.
The LED path is 90^o from the optical fiber
The recorded spectrum is the fluorescence emission spectrum
- since fluorescence is generally a weak signal, one typically needs integration times of several hundred milliseconds.
A calibration curve allows one to do fluorimetric concentration measurements
There are some practical limitations in this setup
- In the absence of collection optics the fluorescence intensity needs to be relatively high
- the sample must absorb at the wavelength of the LED
- LED's generally do not provide light below 450 nm some some UV absorbers are not accessible; it may be useful to add a green LED to excit a wider range of samples.
- samples are limited to species with high fluorescence yields
- samples generally need to be more concentrated than for conventional fluorimeters
- at these concentrations linear response is limited due to strong absorption within the sample.
Of course, other light sources could be used but the overall design is more complex
- battery powered fluorescent lamps are inexpensive and they can be equipped with UV lamps intended for bug-zappers.
Easy samples include fluorescein, eosin, Ru(bpy)₃^2_, rhodamine, quinine sulfate and riboflavin.
The fluorescence of chlorophyll can be monitored directly in plant leafs (reference)

As a Reflectance Spectrometer

White light is allowed to fall on a sample
- Often, an integrating sphere is used to collect light scattered by the surface
Care is taken to avoid direct reflectance of the sample
A blank is recorded as a reference (a MgO coated plate is an easy choice-- burn Mg ribbon near a heat resistant surface)
- The reflectance spectrum is computed as the ratio of sample to reference
One application is the use of test strips that produce a color response to a specific substance

As a Flame Emission Spectrometer

The source can be constructed from
- a conventional propane torch
- an aspirator (we stole ours from old AA Spectrometers)
  - we are looking at medical nebulizers as an alternative; these are relatively low cost (intended as disposable units.)
- a small section of copper plumbing to direct aspirated sample into the flame's air supply
Since the intensity is low, we use a simple sampling lens
- actually a 10X microscope objective
We also use relatively long integration times (500-1000 msec is not uncommon)
The spectrum typically shows each elemental line as a narrow peak, 1-3 pixels wide
A calibration curve, peak height vs.. concentration, can be used for analysis of samples
The systems works well for Li, Na, K, Ca, Sr
- Barium is not a good candidate, but is a useful lesson.
- The spectrum is caused mainly by oxide emission and therefore many weak lines are present

As a way to monitor kinetics

by spectrophotometry
Runs with a half life of about 1 minute are particularly informative
- One can view the spectrum in real time, updated every 1-2 seconds
- The photochromic reaction of mercury(II) thionate is a good example
- the sample is irradiated with a photographic flash lamp
  - or, less dramatically, by holding the cuvet in an intense light beam such as that from a slide projector
- the sample is then quickly placed into the spectrophotometer assembly
- In this case, the blue form shows a strong peak around xxx nm
- As the compound isomerizes to the original form
- the xxx peaks decreases
- a new peak at xxx begins to appear and it increases
- the software can also simultaneously plot absorbance (at one wavelength) vs.. time.
It is also possible to monitor chemiluminescent reactions
- simple luminol based solutions
- monitoring intensity of light sticks vs.. temperature

As a Spectrometer for Electrochemically Generated Species

see Ottle (opticlly transmitting....electrode)

Commercially available accessories include

Oxygen Probe (based on O₂ quenched fluorescence in Ru complexes)
Dip probes for absorption spectrometry
Bifurcated fibers for fluorescence studies
Raman spectrometers are available based on these instruments

As a rough approximation, the devices have relatively uniform sensitivity over the 300-800 nm region. Likewise, the fibers used have relatively uniform transmittance. However, there are some wavelength dependent characteristics, including occasional features due to interference in a fiber.

It is often convenient to use a light source with a purple (green attenuating) filter. This can provide more unifrom light intensity, compensating for the higher green intensity in a typical lamp.

Absolute intensities are more difficult to measure than relatively measurements.

Spectrophotometry, Fluorimetry, Atomic emission spectra, and reflectance spectra basically compare spectrum to known reference.
Characterizing the emission of a lamp produces a response curve, not an absolute...

Software

Generally, the software provided with the instruments has some serious limitations.

Fortunately the manufacturers provide libraries that can be incorporated into user programs

Remarks on the Detector Arrays

A simple diode array consists of a uniformly spaced row of electrical diodes

When the diode absorbs light it becomes electrically conducting
- Ideally, one photon causes one electron to flow across the diode junction
- The electrical current is therefore propotional to light intensity
- This behavior is relatively wavelength independent
In the device, each diode is connected to a small charged capacitor
- The capacitor discharges through the diode
- in the dark, the charge remains
- when the diode is illuminated, the capacitor voltage drops
- After a fixed period of time (the integration time) the capacitor charges are measured.
  - It is not realistic to have 2000 separate wires to the 2000 didoes and capacitors
  - Instead, a series of pulses transfers the signal from one cell to the next.
  - After 2000 timing pulses, the output of each diode has been available for reading
- There is a small leakage even in the dark
  - therefore, the background signal will change with the integration time.
A CCD device behavves similarly.

Some specifications
Ocean Optics PL2000
- Detector is a 2000 element CCD device
- Wavelength (standard) is 350-800 nm
- Each pixel then represents 0.25 nm
- Measured bandwith is approximately 0.5 nm.
- Spectrometer is actually mounted on an electronics board within a PC
- The board provides the signal to control the spectrometer
- The board also provides an ADC to read the detector signals
- The standard fiber is a ___ micron unit
- On earlier models the fiber acts as an entrance slit
CVI-xxx (Sorry, I haven't loaded model numer or specifications yet)
CVI is better known as a laser manufacturer
The spectrometer is external to the PC
It measures approximately 1.5 cm high x 15 cm long x 10 cm wide
The unit has a conventional entrance slit
It can be used with or without optical fiber input
The CCD detector has ____ pixels
Wavelength region is ___-___ nm
Each pixel has a with of __ nm.
Measured bandwidth is about __ nm.

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