Geoscience Reference
In-Depth Information
Table 5.8.
Emission lines from a low-pressure mercury pen lamp with the common
calibration wavelengths highlighted in bold
Wavelength/nm
Wavenumber cm
-1
Wavelength/nm
Wavenumber cm
-1
253.652
39,424.14
434.749
23,001.76
265.204
37,706.84
435.833
22,944.58
265.368
37,683.53
546.074
18,312.55
296.728
33,700.90
576.960
17,332.23
302.150
33,096.17
579.066
17,269.18
312.567
31,993.16
708.190
14,120.50
313.155
31,933.09
1013.976
9862.17
313.184
31,930.12
1357.021
7369.08
365.015
27,396.11
1367.351
7313.41
365.484
27,361.01
1529.582
6537.73
366.328
27,297.95
1707.279
5857.27
404.656
24,712.33
2325.307
4300.51
433.922
23,045.60
With this approach care must be taken to ensure that the correct wavelength lines have
been identified and used for calibration. A common error is the use of emission lines that
occur in the second order of the grating, for example, thinking there is calibration line at
507.3 nm, which is not a calibration wavelength but results from the emission line occur-
ring at 253.65 nm.
The elemental emission lines from mercury also act as a very useful means to check the
instrument spectral resolution and reproducibility of scanning. For example,
Figure 5.18
shows the optical resolution obtainable from a research quality fluorimeter.
Scanning reproducibility is intrinsically a function of the quality of the mechanical
design, build, and testing. For research instruments, accuracy of wavelength position and
repeatability in scanning should be equal to or exceed 0.2 nm.
Figure 5.19
highlights the
importance of scanning reproducibility.
5.4.12 Bandpass Selection
The spectral bandpass applied to the monochromators will determine the capability of the
fluorimeter to accurately, or not, record the excitation or emission spectrum shape of a
luminescent sample. The bandpass is determined by the slit width used and also by the dif-
fraction grating parameters in the monochromator. A larger (or wider slits) bandpass means
lower spectral resolution whereas a smaller bandpass (i.e., narrower slits) will result in a
higher spectral resolution. At the same time, the light throughput will change with different
bandpass. As a rule of thumb, doubling the bandpass will increase the signal level by a fac-
tor of four. For this reason, many have used the spectral bandpass selection of an instrument
