B.2.4.3 Mobile phase absorbance
This section lists the absorbances at several wavelengths for frequently used mobile phases. Choose the mobile phase
carefully to reduce baseline noise.
The best mobile phase for your application is one that is transparent at the chosen detection wavelengths. With such a
mobile phase, ensure that any absorbance is caused only by the sample. Absorbance by the mobile phase also reduces
the linear dynamic range of the detector by the amount of absorbance the Autozero function cancels. Wavelength, pH,
and concentration of the mobile phase affects its absorbance. Examples of several mobile phases are given in the table
below, where the absorbances are based on a 10-mm path length.
Table B–5: Mobile phase absorbance measured against air or water
Absorbance (AU) at specified wavelength (nm)
200
205
210
215
220
230
240
250
260
280
Solvents
Acetonitrile
0.05
0.03
0.02
0.01
0.01
<0.01 —
—
—
—
Methanol (not degassed)
2.06
1.00
0.53
0.37
0.24
0.11
0.05
0.02
<0.01 —
Methanol (degassed)
1.91
0.76
0.35
0.21
0.15
0.06
0.02
<0.01 —
—
Isopropanol
1.80
0.68
0.34
0.24
0.19
0.08
0.04
0.03
0.02
0.02
Unstablized tetrahydrofuran (THF, fresh)
2.44
2.57
2.31
1.80
1.54
0.94
0.42
0.21
0.09
0.05
Unstablized tetrahydrofuran (THF, old)
>2.5
>2.5
>2.5
>2.5
>2.5
>2.5
>2.5
>2.5
2.5
1.45
Acids and bases
Acetic acid, 1%
2.61
2.63
2.61
2.43
2.17
0.87
0.14
0.01
<0.01 —
Hydrochloric acid, 0.1%
0.11
0.02
<0.01 —
—
—
—
—
—
—
Phosphoric acid, 0.1%
<0.01 —
—
—
—
—
—
—
—
—
Trifluoroacetic acid
1.20
0.78
0.54
0.34
0.22
0.06
<0.02 <0.01 —
—
Diammonium phosphate, 50 mM
1.85
0.67
0.15
0.02
<0.01 —
—
—
—
—
June 12, 2015, 715004747 Rev. A
Page 77