Systems and methods for automatic preconcentration of fluid samples using alternating dual holding loops are described. A system embodiment includes, but is not limited to, a first sample loop and a second sample loop alternately fluidically coupled with a sample source; a first valve to alternately introduce fluid from the sample source to the first sample loop and the second sample loop; a second valve to alternately receive fluid from the first sample loop and the second sample loop and to alternately provide access to the preconcentration column to fluid received from the first sample loop and the second sample loop; and a pump system configured to alternately introduce sample held in the first sample loop and sample held in the second sample loop to the preconcentration column via the first valve and the second valve.