5.6 Instrumental Qualitative Analysis
239
conditions (e.g. temperature, pressure, flow-rate), which have crucial effects
on the measured parameter (retention time), is minimized.
(c) By adding a standard of the analyte to the sample in order to facilitate its
identification. Two chromatographic runs are performed, using the sample
in one and the sample plus standard in the other. If any peak increases in
height as a result, then the analyte will be the species contained in the added
standard. Box 5.18 shows a typical example.
(d) By using one of the techniques in Group 3 (e.g. IR spectroscopy, mass
spectrometry), which provide whole spectra for each chromatographic peak
and facilitate highly reliable identification (as can be seen from Fig. 5.11, the
qualitative information produced by these techniques is of a high level).
Box 5. 18
Chromatographic identification of a sample component by addition of a
standard of the analyte
Reversed-phase liquid chromatography (viz. with water as the mobile phase and a non-polar
material such as C 18 bonded silica as the stationary phase) enables the discrimination of
va rious ingredients of soft drinks (e. g. ascorbic acid, saccharine and caffeine). If the chromatogram obtained by inserting a sample aliquot into the chromatograph (Box 5.16) contains a
peak for an unknown substance, then one can suspect the presence of an additive. Because
sodium benzoate is a frequent occurrence in this context, a new sample aliquot is supplied
with an appropriate amount of the pure additive to make a standard for injection into the
chromatographic system. If the previous, unknown peak grows as a result, then the binary
answer to the question "is sodium benzoate present in the drink?" is YES. On the other hand,
if the peak appears in a different zone (i. e. at a different time) in the chromatogram, then a
different standard should be tested.
Chromatogram of sample
2
o
9min
Chromatogram of sample
spiked with standard
4
3
o
9 min
1: ascorbic aCid: 2:saccharin; 3: caffeine; 4: benzoic acid (confirmed)
239
conditions (e.g. temperature, pressure, flow-rate), which have crucial effects
on the measured parameter (retention time), is minimized.
(c) By adding a standard of the analyte to the sample in order to facilitate its
identification. Two chromatographic runs are performed, using the sample
in one and the sample plus standard in the other. If any peak increases in
height as a result, then the analyte will be the species contained in the added
standard. Box 5.18 shows a typical example.
(d) By using one of the techniques in Group 3 (e.g. IR spectroscopy, mass
spectrometry), which provide whole spectra for each chromatographic peak
and facilitate highly reliable identification (as can be seen from Fig. 5.11, the
qualitative information produced by these techniques is of a high level).
Box 5. 18
Chromatographic identification of a sample component by addition of a
standard of the analyte
Reversed-phase liquid chromatography (viz. with water as the mobile phase and a non-polar
material such as C 18 bonded silica as the stationary phase) enables the discrimination of
va rious ingredients of soft drinks (e. g. ascorbic acid, saccharine and caffeine). If the chromatogram obtained by inserting a sample aliquot into the chromatograph (Box 5.16) contains a
peak for an unknown substance, then one can suspect the presence of an additive. Because
sodium benzoate is a frequent occurrence in this context, a new sample aliquot is supplied
with an appropriate amount of the pure additive to make a standard for injection into the
chromatographic system. If the previous, unknown peak grows as a result, then the binary
answer to the question "is sodium benzoate present in the drink?" is YES. On the other hand,
if the peak appears in a different zone (i. e. at a different time) in the chromatogram, then a
different standard should be tested.
Chromatogram of sample
2
o
9min
Chromatogram of sample
spiked with standard
4
3
o
9 min
1: ascorbic aCid: 2:saccharin; 3: caffeine; 4: benzoic acid (confirmed)