of Pixel at 196 nm aAll values dete

of Pixel at 196 nm
aAll values determined with HGA-500 graphite furnace atomizer (Perkin-Elmer, Norwalk, CT, USA). bCharacteristic mass.21 cComputed from
m as described in ref. 22. dSpectraspan III echelle (Spectrametrics) with 256 pixel LPDA and 25 mm pixel width.22 eSpectraspan III echelle
´ ´
0
(Spectrametrics) with 128 pixel LPDA and 50 mm pixel width.22 fH-20 monochromator with 128 pixel LPDA and 50 mm pixel width.23 gOptima
echelle (Perkin-Elmer) with 256 pixel LPDA and 25 mm pixel width.13 hPhysical width of pixel.
´

portional to the detection limit. The previous section demon- Table 4 CS-AAS detection limits
strated that the normalized, integrated absorbance was
Element Wave- HGA-500 furnace THGA furnace
independent of the source and detector characteristics.
length/
Consequently, the detection limit is directly proportional to nm LSa CS-LPDAb CS-SCDc LSd CS-DEMONe
the absorbance noise.
Eqn. (2) shows that the absorbance noise will grow smaller As 193.7 20 28 12 6 4
as the intensity increases or the read noise decreases. This Se 196.0 30 50 16 9 13
equation also indicates that the absorbance noise will decrease Zn 213.9 1 2 0.1 0.4 0.2
Pb 217.0 10 6 4 4 –
with increasing spectral bandwidth. For example, if the Sb 217.6 15 – 8 4 2.5
entrance slit width is doubled, the intensity will double, the Bi 223.1 6 – 5 – –
number of pixels necessary to cover the profile will double Sn 224.6 20 26 – 10 –
and sA will decrease by √2. The LPDA used previously with Cd 228.8 0.4 0.4 0.07 0.1 0.1
CS-AAS12 had a read noise of about 3000 e −. As a result, Ni 232.0 10 11 – 8 –
read noise was dominant at all intensity levels. The best Be 234.9 1 – – 0.1 0.08
Co 240.6 2 4 – 4 –
detection limits (Table 4) were obtained with the largest Fe 248.3 2 2 – 0.8 0.6
entrance slit width of the echelle, 500 mm. At the time, these
´
Si 251.6 40 – – 15 6
detection limits were the best ever achieved for CS-AAS, but Tl 276.8 10 – 1 9 3
they precluded operation in the high-resolution mode. Mn 279.5 21 0.5 0.2 0.6 0.3
The photon shot noise limited case, the ideal case, is achieved Pb 283.3 5 0.9 0.4 4 1
if the fluctuation noise is eliminated and the read noise is low. Al 309.3 4 – – 3 0.8
Mo 313.3 4 – – 1 2
A high quality CCD will typically have a read noise of less Cu 324.7 1.0 0.6 – 4 1
than 25 e−. Consequently, all but the lowest intensities Ag 328.1 0.5 – – 0.4 0.2
(625 e−) will be shot noise limited. The absorbance noise Cr 357.9 1 – – 0.4 0.8
for the shot noise limited case is aModel 5000 (Perkin-Elmer).21 bCS with linear photodiode array
0.43√I √n 1 0.43√n 1 (LPDA) detector.22cCS with segmented charge coupled array detector
sA (3) (SCD) of Optima (Perkin-Elmer).13 dSIMAA 6000 (Perkin-Elmer).25
I √I eCS with double echelle monochromator (DEMON).26
´

since s √I. This equation shows that, for the shot noise
I
limited case, the absorbance noise is independent of the spectral
bandwidth. Consider again the case where the entrance slit more intense sources, with little success. More signifcant
width is doubled. As stated previously, both n and I will also improvements have been made in increasing the luminosity of
double. With eqn. (2), sA is reduced by √2. With eqn. (3), the spectrometer and quantum eciency of the detectors.
however, sA remains the same. Consequently, the use of a Table 2 shows that the two-dimensional array is thinned and
narrow slit width to maximize spectral resolution does not back illuminated. The result is a quantum eciency of 50%
degrade the detection limit. at 200 nm.
Eqn. (2) shows that if I, the level of radiation striking each Table 4 presents detection limits for some of the most recent
pixel, increases without opening the entrance slit width, the CS-AAS instruments. A comparison of eqns. (2) and (3)
absorbance noise will decrease. Three means of increasing I shows why better detection limits are achieved in the far UV
are to increase the source output, increase the transmission with a CCD array. For the LPDA described above (read noise
eciency of the spectrometer and increase the detection about 3000 e −), the detected intensity must be 9 ×106e − in
eciency. Numerous attempts have been made to develop order for the shot noise to equal the read noise. For the same

J. Anal. At. Spectrom., 1999, 14, 137–146 141

ผลลัพธ์ (อังกฤษ) 3:

Of Pixel at 196 nm
aAll values determined with HGA-500 graphite furnace atomizer (,,, Perkin-Elmer Norwalk CT USA). BCharacteristic. Mass.21 cComputed from
m as described in ref. 22. DSpectraspan III echelle (Spectrametrics) with 256 pixel LPDA and 25 mm. Pixel width.22 eSpectraspan III echelle


, 0 pixel pixel (Spectrametrics) with 128 pixel LPDA and 50 mm pixel width.22 fH-20 monochromator with 128 pixel LPDA and 50 mm pixel width.23 gOptima
echelle (Perkin-Elmer) with 256 pixel LPDA. And 25 mm pixel width.13 hPhysical width of pixel.


portional pixel to the detection limit. The previous section demon - Table 4 CS-AAS. Detection limits
strated that, the normalized integrated absorbance was
Element Wave - HGA-500 furnace THGA furnace
.Independent of the source and detector characteristics.
-
, length Consequently the detection limit is directly proportional. To nm LSa CS-LPDAb CS-SCDc LSd CS-DEMONe
the absorbance noise.
Eqn. (2) shows that the absorbance noise will grow smaller. As 193.7 20 28 12 6 4
as the intensity increases or the read noise decreases. This Se 196.0 30 50 16 9 13
.Equation also indicates that the absorbance noise will decrease Zn 213.9 1 2 0.1 0.4 0.2
Pb 217.0 10 6 4 4 -
with increasing. Spectral bandwidth. For example if the, Sb 217.6 15 - 8 4 2.5
entrance slit width is doubled the intensity will double,,, The Bi 223.1 6 - 5 - -
number of pixels necessary to cover the pro fi le will double Sn 224.6 20 26 - 10 -
and sA will decrease. By √ 2.The LPDA used previously with Cd 228.8 0.4 0.4 0.07 0.1 0.1
CS-AAS12 had a read noise of about 3000 e −. As, a result. Ni 232.0 10 11 - 8 -
read noise was dominant at all intensity levels. The best Be 234.9 1 - - 0.1 0.08
Co 240.6 2 4 4 - -
detection limits (Table 4) were obtained with the largest Fe 248.3 2 2 - 0.8 0.6
entrance slit width of, the echelle 500 mm.? At, the timeThese

Si pixel 251.6 40 - - 15 6
detection limits were the best ever achieved for CS-AAS but Tl, 276.8 10 - 1 9 3
they. Precluded operation in the high-resolution mode. Mn 279.5 21 0.5 0.2 0.6 0.3
The photon shot noise limited case the ideal,, Case is achieved, Pb 283.3 5 0.9 0.4 4 1
if the fl uctuation noise is eliminated and the read noise is low. Al 309.3 4 - To 3 0.8
Mo 313.3 4 - - 1 2
A high quality CCD will typically have a read noise of less Cu 324.7 1.0 0.6 - 4 1
than 25 e −. Consequently,, All but the lowest intensities Ag 328.1 0.5 - - 0.4 0.2
(625 e −) will be shot noise limited. The absorbance noise Cr 357.9 1 - To 0.4 0.8
for the shot noise limited case is aModel 5000 (Perkin-Elmer). 21 bCS with linear photodiode array
0.43 √ I √ n 1 0.43 √ n 1 (LPDA) detector.22cCS with segmented charge coupled array detector
sA (3) (SCD) of Optima (Perkin-Elmer). 13 dSIMAA 6000 (Perkin-Elmer). 25.
I √ I eCS with double echelle monochromator (DEMON). 26


since pixel s √ I. This equation, shows that for the shot noise
. I
limited case the absorbance, noise is independent of the spectral
bandwidth.Consider again the case where the entrance slit more, intense sources with little success. More signifcant
width is, doubled. As stated previously both N, and I will also improvements have been made in increasing the luminosity of
double. With, eqn. (2), sA is reduced by √ 2. With eqn. (3), the spectrometer and quantum eciency of the detectors.
however sA remains, the. Consequently same,.The use of a Table 2 shows that the two-dimensional array is thinned and
narrow slit width to maximize spectral resolution. Does not back illuminated. The result is a quantum eciency of 50%
degrade the detection limit. At 200 nm.
Eqn. (2 shows.) That if I the level, of radiation striking each Table 4 presents detection limits for some of the most, recent
pixelIncreases without opening the entrance, slit width the CS-AAS instruments. A comparison of eqns. (2) and (3)
absorbance. Noise will decrease. Three means of increasing I shows why better detection limits are achieved in the far UV
are to increase. The, source output increase the transmission with a CCD array. For the LPDA described above (read noise
.Eciency of the spectrometer and increase the detection about 3000 e −), the detected intensity must be 9 × 106E − in
eciency.? Numerous attempts have been made to develop order for the shot noise to equal the read noise. For the same

J. Anal. At.? ,,, Spectrom. 1999 14 137 - 146 141
.