The class “acids” includes various types of compounds with active hydrogen atoms usually having pK_a < 7. The most important group of organic acids is the compounds with carboxyl fragment -COOH. Some other compounds can be classified not only as O-acids [e.g., hydroxamic acids, -CONHOH ⇌ -C(OH)=NOH], but C-H acids [with the presence of structural fragments -CH(NO₂)₂, -CH(CN)₂, etc.]. Well-known substances of this class for GC analysis are semivolatile fatty acids of triglycerides and lipids, numerous nonvolatile polyfunctional biogenic compounds (including such phenol carboxylic acids like gallic, vanillic, and syringic acid), different acidic herbicides (for example, 2,4-D, 2,4,5-T, MCPB, MCPA, fenoprop, haloxyfop, etc.), and many other substances. Strong inorganic acids like volatile hydrogen halides (HHal) and nonvolatile H₂SO₄, H₃PO₄, etc. can be objects of GC analysis too.

The simplest monofunctional carboxylic acids have boiling points at atmospheric pressure without decomposition and, hence, can be analyzed directly by GC. However, owing to the high polarities of carboxyl compounds, a typical problem of their GC analysis with standard nonpolar phases is the nonlinear sorption isotherm. As a result, these compounds yield broad nonsymmetrical peaks, which leads to poor detection limits and unsatisfactory reproducibility of their retention indices. The recommended stationary phases for direct analysis of free carboxylic acids are polar polyethylene glycols (Carbowax 20M, DBWax, SP-1000, FFAP, etc.). However, these phases have lower thermal stability compared with polydimethyl siloxanes (ca. 225-250 vs. 300-350 °C). This means that the upper limit of GC columns with these polar phases in Retention index (RI) units is not more than 3000-3500 IU. High homologs even of monocarboxylic acids cannot be eluted within this RI window (this is confirmed by the absence of RI data for palmitic acid, C₁₅H₃₁COOH, on the mentioned types of polar phases).

Compound	pKa	Tb,°C	RInonpolar	RIpolar
Acetic acid	4.75	118	638 ± 10	1428 ± 30
Palmitic acid	4.9	351.5	1966 ± 7	No data
Benzoic acid	4.2	250	1201 ± 24	2387 ± 5
Phenylacetic acid	4.2	266	1290 ± 44	No data

Some dicarboxylic acids can also be distilled without decomposition under reduced pressures. This is at least the theoretical grounds for the possibility of their direct GC analysis. Few successive attempts have been described, but these analytes require “on-column” injection of samples and extremely high inertness of chromatographic systems. Many types of polyfunctional carboxylic acids (hydroxy-, mercapto-, amino-, etc.) cannot be analyzed in free, underivatized form owing to either nonvolatility and/or absence of thermal stability. These features are the principal reasons for the conversion of carboxylic acids before their GC analysis into less polar derivatives without active hydrogen atoms.