Download The background emission signal

Measurement of optical signal
All spectrochemical techniques that operate in the Uvvisible and
IR regions of the spectrum employ similar instrumental
components
1
The major instrumental differences between emission,
photoluminescence, and absorption techniques occur in the
arrangement and type of sample introduction system, encoding
system and information selection system.
All techniques depend upon the measurement of radiant power.
The radiant power monitor or optical transducer-signal
processing- readout system is shown in block diagram form
in Figure 2-9.
The specific transducers and signal processing devices used in
various regions of the spectrum in specific spectrochemical
techniques.
In this section we explore how the analytical signal is extracted from
the readout data in spectrochemical methods.
2
3
Analytical Signal
The analytical signal for emission and chemiluminescence
techniques is defined as the signal to be displayed by the readout
device due only to analyte emission (EE, it is assumed that EE , is
directly related to the radiant power of emission
ΦE. ).
Similarly, the analytical signal in photoluminescence techniques,
EL, is the measured signal due only to radiationally produced
emission of the analyte.
In the case of absorption methods, the analytical signal is the
absorbance A due only to absorption of radiation by the analyte
species.
4
Because of the presence of extraneous signals, such as signals from
concomitants, the sample cell, and room light, at least two
measurements are required to obtain the analytical signal.
sources of the background or extraneous signal that registers on the
readout device:
1) dark signal Ed of the radiant power monitor: the signal present
when no radiation is impingent on the transducer.
2) the background signal, EB due to background radiation that
strikes the transducer.
The background radiation is composed of radiation from all sources
other than the desired optical phenomenon from the analyte.
5
The data domains of the different signals in a spectrochemical
instrument prior to the signal processing system are dependent on the
observation point.
the data are present as
an optical radiant
power in watts.
The transducer can convert this optical signal to an
electrical current, voltage, or charge.
Normally, the output of the signal processing system to
be displayed on the readout device is an electrical
voltage.
Hence, in general in this book, analyte and background signals will be written as
voltages E.
6
Emission and Chemiluminescence Spectrometry
The basic instrumental configuration for wavelength resolved
emission spectrochemical methods is shown in Figure 2-10.
FIGURE 2-10
Instrumentation for emission spectrochemical methods.
The excitation source provides the external energy necessary to excite the analyte species.
For example, the excitation source could be a flame, a plasma, a high-voltage spark
discharge, or a chemical reaction. The sample container holds the sample.
The wavelength selector passes a selected wavelength band emitted by the sample to the
radiant power monitor
7
The emission that results from excitation of the analyte species by
a flame, a plasma, or a chemical reaction encodes the
concentration of the analyte as the radiant power of emission ɸE
In some spectrochemical methods the excitation source and
sample container are an integral unit, as in the nebulizer-burner
used in flame emission and the reaction cell used in
chemiluminescence.
8
When the analytical sample is present in the same cell, a total or
composite signal EtE, is obtained.
This total signal is the sum of :
EE:
Analytical signal
Ed:
Dark signal
EbE:
The background emission signal
To extract the analytical signal, a second measurement is
required to obtain the sum of the dark signal and the
background emission signal.
This second measurement usually made
by replacing the analytical sample with
blank that is ideally identical to the
analytical sample except that the analyte
is missing.
9
If desired, the dark signal can be obtained separately by
locking all radiation from reaching the radiant power
monitor.
The background emission signal could then be obtained from
Ebk- Ed.
In many instruments the blank solution is used to adjust the
readout device to read zero by suppression of the blank
signal.
This establishment of the zero position is still, however, a
measurement of the blank signal.
10
Photoluminescence Spectrometry
11
Here an external source electromagnetic radiation excites the analyte
(photoexcitation).
The analyte concentration is optically encoded as the luminescent
radiant power ɸL , which is measured with the radiant power
monitor.
The emission wavelength selector that views the luminescence of
the sample is typically placed to collect radiation at 90◦ with respect
to the excitation axis.
Other geometries, such as front surface and 180o, are used in
special situations. In some cases only one of the wavelength
selectors is necessary.
12
When the analytical sample is placed in the sample
ELt:
Total luminescence signal
EL:
analyte luminescence signal,
EE:
analyte thermal emission signal
Ebk
Blank signal
Ed:
dark signal
EbE:
background emission signal
Esc:
Scattering signal
EbL:
Background luminescence signal
Analyte and background emission in the UV-visible region are
usually significant only in atomic spectroscopy
13
The analyte luminescence signal EL can be obtained with two
measurements only if the analyte emission signal EE is small
compared to EL, which is often the case.
If EE is significant, subtraction of the blank signal gives a
measured analyte luminescence signal EL that differs from EL as
shown in the equation
14
How EL can be obtained when EE is significant?
the excitation source must be turned off.
Then the two measurements are made to obtaine EE according
to :
Subtraction of EE from E’L gives the true analyte luminescence signal.
In some cases it is possible to eliminate the measured contribution
from analyte emission optically or electronically.
For example, if the excitation source is modulated and alternatingcurrent (ac) amplification is used, the ac luminescence signal can be
distinguished from the dc emission signal.
Often the blank measurement is used to set the zero position of the
readout device.
15
Absorption spectrometry
FIGURE 2-12 Absorption spectrometer.
A narrow spectral band from the radiation source is passed through the sample.
The transmitted radiant power is measured by the radiant power monitor.
Replacement of the analytical sample by a reference provides a measure of the reference
radiant power.
The ratio of the radiant power transmitted by the sample to that transmitted by the
reference is used to calculate the absorbance A of the sample.
16
The absorption spectrometer is essentially identical to the
luminescence spectrometer except that the source, sample cell, and
transducer are all on the same optical axis.
This permits the measurement of the transmitted radiant power.
The shutter allows the user to block the radiation source in order to
obtain the dark signal. Usually, only one wavelength selector is
required.
Absorption measurements can be made in two ways:
1. Transmittance readout
2. Direct absorbance readout
Both of these readout schemes are used, although direct
absorbance readout is becoming the more common.
17
Transmittance readout
The ideal or true transmittance T is the ratio of radiant power
passed by the analyte to the radiant power passed by an ideal blank.
It could be obtained by:
(1) measuring the signal Es that results from the source radiant
power passing through the analytical sample,
(2) measuring the signal Er, that results from the source radiant
power passing through the ideal blank or reference solution; and
(3) obtaining the transmittance as in
18
In practice, however, the presence of other signals (dark signal,
background emission) necessitates a third measurement. The
measured transmittance T' is defined by the equation
Est: total sample signal obtained with the source shutter open and
the analytical sample in the sample container
E0t is the zero percent transmittance (0% T) signal obtained with
the shutter closed and the blank in the sample container,
Ert is the 100% transmittance (100% T) signal obtained with the
shutter open and the blank (reference) in the sample container.
19
The 0% T signal E0t is composed of any background emission EbE
and dark current Ed as shown by the equation
When the blank is in the sample container and the shutter open, the
measured total reference signal Ert called the 100% T signal, is
composed of :
the reference transmission signal Er
the 0% T signal
and any background luminescence EbL
20
When the analytical sample is in the sample container and the
shutter is open, the measured signal is Est, the total sample signal.
This signal is given by
where Es is the sample transmission signal, EE is the analyte emission
signal, and EL is the analyte luminescence signal.
Er>>EbL
T‘= T only when
ES>>EL+EE+EbL
21
If one or more of the signals EL, EE, or EbL, is significant, several
additional measurements must be made to correct equation 2-31 to
obtain the true transmittance, information selection techniques
criminate against the extraneous or additional must be used signals.
As in a photoluminescence techniques, analyte and background
emission are usually significant only for atomic spectrometry.
The measured transmittance T' is then used to calculate an
absorbance A' = -log T' which is an approximation to the true
absorbance A = -log T.
22
Direct Absorbance Readout
As part of the signal processing electronics, many modern absorption
spectrometers have provision for obtaining and displaying absorbance
directly. The true absorbance A is given by
where EA is the voltage proportional to the analyte absorbance and k'
is a logarithmic conversion factor in volts per absorbance unit.
The voltage EA and hence A are found from two measurements:
1. a reference logarithmic voltage or zero absorbance Elr, is
obtained with the shutter open and the blank in the sample
container;
2. Then a sample logarithmic voltage Els is obtained with the shutter
open and the analytical sample in the sample container
23
The voltage EA is then given by
The voltages Els, and Elr, are logarithmically related to
Es and Er, as follows:
a constant reference voltage
Often Elris set to zero on the readout device so that Els, is read out
directly as EA.
24
Note that in the two-step absorbance measurement scheme, a
measurement is not made with the light source shutter closed (0%
T) since A would be infinity.
Thus (Ed + EbE) must be negligible compared to Es and Er or
electronically or optically set to zero by other means.
Also, EE + EbL + EL must be negligible so that Es = Est and Er = Ert
otherwise, the measured absorbance A' only approximates the true
absorbance A.
25
26
Modulators
Several types of devices are used to amplitude modulate
a radiation source (see Section 4-5).
Modulation is based on mechanical interruption of a light beam or
on electro-optic, magneto-optic, or acousto-optic phenomena.
Mechanical Choppers
Mechlanical modulators or choppers provide a controlled
periodic physical blocking of a radiation beam.
The lnost common mechanical modulators are based on a
rotating disk or wheel with apertures or vanes as shown in
Figure 3-14a and b.
27
28
The chopping frequency is determined by the number of
apertures and the rotation rate of the shaft of the motor. The
maximum modulation frequency is generally in the range 1 to
10kHz.
Vibrating or tuning fork modulators are based on periodic lateral
movement of one or two vanes in and out of the light path as
illustrated in Figure 3-14c.
Because these are resonant devices, a given chopper is
designed to operate at one frequency (10 to 6000 Hz).
The modulation waveform depends on the size, shape, and
configuration of the vanes and can be sinusoidal.
29
Other types of resonant choppers include oscillating choppers based on torsion rod and
taut band designs (Figure 3-14d), which rely on rotation of a vane in and our of the light
path.
Choppers can be used for specialized functions by ounting mirrors, refractor plates,
gratings, or filters to the vanes.
30
Electro-optic and magneto-optic modulators
31
32