Organic compounds in sludges 217
associate and to be adsorbed firmly on the column require esterification prior to gas
chromatographic determination. The presence of water interferes in esterification so
that complex drying techniques and isolation of the acids by extraction, liquid solid
chromatography, distillation, and even ion exchangers had to be used [19–22].
The introduction of the more sensitive hydrogen flame ionisation detector has
made possible the analysis of dilute aqueous solutions of organic acids by gas liquid chromatography. Problems, such as ‘ghosting’ at high acid concentrations and an
excessive tailing effect of the water in dilute solutions, masking the components, have
been reported for aqueous solutions [23]. Subsequently phosphoric [24] or metaphosphoric acids [25] were added to the liquid phase, resulting in more reproducible column
performance and reduced ‘ghosting’. Addition of formic acid to the carrier gas was
recommended by Chochrane [26] to overcome all the problems normally associated
with analysing free fatty acids by gas chromatography.
Baker et al [27] used FFAP column for direct injection of dilute aqueous solutions of acids (FFAP is a reaction of polyethylene glycol 20 000 and 2-nitrophthalic
acid developed by Varian Aerograph). The acetic acid peak was not clear and the
ability of this column to separate normal and iso fatty acids was not reported. Van
Huyssteen et al [18] successfully used a Chromosorb 101 column coated with 3% FFAP
for separation of volatile acids by direct injection of synthetic aqueous solutions and
anaerobic digestion samples which were first centrifuged and acidified with hydrochloric acid. He injected 1 µl at acid concentration greater than 50 g L
−1 and 2 µl below
50 mg L
−1 . Ghosting was observed upon injecting 2 µl 25 mg L
−1 C 2 –C 6 acid solutions.
Van Huyssteen et al [18] did not try to inject volumes greater than 2 µl. His column
affected complete separation of the C 2 –C 6 straight and branched short chain fatty
acids from synthetic aqueous solutions, but less sharpened peaks were obtained from
anaerobic digester samples. The response with acetic acid approximated that of the
other acids.
An official gas chromatographic method [28] is available from the determination
of volatile fatty acids in sewage sludge. This method is based on gas liquid chromatographic estimation with a flame ionisation detector, and is applicable up to 2000 mg
total volatile fatty acids per litre, while the concentrations of the individual fatty acids
can also be determined. Where this method is not practicable an empirical method
based on the spectrophotometric determination of ferric hydroxamates can be used,
giving a value for total fatty acids expressed as acetic acid. For control purposes a
rapid test is described in which the volatile fatty acids are determined by electrometric
titimetry on the neutralised sludge obtained from the determination of alkalinity.
Narkis and Hendfield-Furie et al [29] have described a direct method for the identification and determination of volatile water soluble C 1 −C 2 acids in municipal waste
water and raw sewage. The method involves direct injection of the sewage into a gas
chromatograph equipped with a Carbowax 20 M on acid-washed Chromosorb W column and a flame ionisation detector. Preliminary preparation of the sample is limited
to the addition of solid metaphosphoric acid to the sewage and removal of precipitated
proteins and suspended solids by centrifuging.
Table 10.1 summarises the individual volatile acid concentrations in raw sewage
determined by the direct injection procedure of Narkis and Henfield-Furie et al [29]
and that of Standard Methods [30]. The results were also expressed as acetic acid
for comparison with the collective total amount of organic acids determined by the
associate and to be adsorbed firmly on the column require esterification prior to gas
chromatographic determination. The presence of water interferes in esterification so
that complex drying techniques and isolation of the acids by extraction, liquid solid
chromatography, distillation, and even ion exchangers had to be used [19–22].
The introduction of the more sensitive hydrogen flame ionisation detector has
made possible the analysis of dilute aqueous solutions of organic acids by gas liquid chromatography. Problems, such as ‘ghosting’ at high acid concentrations and an
excessive tailing effect of the water in dilute solutions, masking the components, have
been reported for aqueous solutions [23]. Subsequently phosphoric [24] or metaphosphoric acids [25] were added to the liquid phase, resulting in more reproducible column
performance and reduced ‘ghosting’. Addition of formic acid to the carrier gas was
recommended by Chochrane [26] to overcome all the problems normally associated
with analysing free fatty acids by gas chromatography.
Baker et al [27] used FFAP column for direct injection of dilute aqueous solutions of acids (FFAP is a reaction of polyethylene glycol 20 000 and 2-nitrophthalic
acid developed by Varian Aerograph). The acetic acid peak was not clear and the
ability of this column to separate normal and iso fatty acids was not reported. Van
Huyssteen et al [18] successfully used a Chromosorb 101 column coated with 3% FFAP
for separation of volatile acids by direct injection of synthetic aqueous solutions and
anaerobic digestion samples which were first centrifuged and acidified with hydrochloric acid. He injected 1 µl at acid concentration greater than 50 g L
−1 and 2 µl below
50 mg L
−1 . Ghosting was observed upon injecting 2 µl 25 mg L
−1 C 2 –C 6 acid solutions.
Van Huyssteen et al [18] did not try to inject volumes greater than 2 µl. His column
affected complete separation of the C 2 –C 6 straight and branched short chain fatty
acids from synthetic aqueous solutions, but less sharpened peaks were obtained from
anaerobic digester samples. The response with acetic acid approximated that of the
other acids.
An official gas chromatographic method [28] is available from the determination
of volatile fatty acids in sewage sludge. This method is based on gas liquid chromatographic estimation with a flame ionisation detector, and is applicable up to 2000 mg
total volatile fatty acids per litre, while the concentrations of the individual fatty acids
can also be determined. Where this method is not practicable an empirical method
based on the spectrophotometric determination of ferric hydroxamates can be used,
giving a value for total fatty acids expressed as acetic acid. For control purposes a
rapid test is described in which the volatile fatty acids are determined by electrometric
titimetry on the neutralised sludge obtained from the determination of alkalinity.
Narkis and Hendfield-Furie et al [29] have described a direct method for the identification and determination of volatile water soluble C 1 −C 2 acids in municipal waste
water and raw sewage. The method involves direct injection of the sewage into a gas
chromatograph equipped with a Carbowax 20 M on acid-washed Chromosorb W column and a flame ionisation detector. Preliminary preparation of the sample is limited
to the addition of solid metaphosphoric acid to the sewage and removal of precipitated
proteins and suspended solids by centrifuging.
Table 10.1 summarises the individual volatile acid concentrations in raw sewage
determined by the direct injection procedure of Narkis and Henfield-Furie et al [29]
and that of Standard Methods [30]. The results were also expressed as acetic acid
for comparison with the collective total amount of organic acids determined by the