The growing quality of smartphone-based Global Navigation Satellite Systems (GNSS) chipsets opens a new frontier for scientific research in positioning, navigation and timing ap- plications. The portability and affordability of these instruments could enhance the current GNSS receiver global network for atmospheric monitoring purposes. However, the quality of the measurements gathered from smartphones have not yet been fully assessed. In this paper, an analysis of the quality of smartphone-based Total Electron Content (TEC) mea- surements is performed. The primary focus of this work is to provide a general analysis on the potential of using smartphone observations for ionospheric sciences. Dual-frequency phase observations are used to measure the relative TEC. For this experiment, GPS L1/L5 and Galileo E1/E5a observations acquired with the Xiaomi Mi8 and Huawei Mate20 X smartphones were considered. Both devices are equipped with the Broadcom BCM47755 chipset, which enables GNSS dual-frequency measurements. More than 100 hours of phase observations at mid-latitude during a low solar activity period were gathered. Three differ- ent setup configurations were defined to assess the effects multipath or signal strength may have in the quality of the phase observations. In addition, to detect and discard unrealistic fluctuating phase observations, a quality-check was performed. In the results, good agree- ment between the slant TEC (sTEC) measurements from the smartphone and the sTEC obtained from a co-located geodetic receiver is presented. Furthermore, the amount and quality of observations discarded by the quality-check are reported, which emphasises the use of the signal strength to indicate the quality of phase observations. The results indicate that the C/N0 and multipath are important —when gathering the data from a geodetic antenna, around 80% of the collected data passed a quality threshold. However, collecting data with the addition of an attenuator, or directly from the smartphone antenna, reduced the valid data to below 50%. However, given the ease of use of a smartphone for data collection, even at 50% of data being usable, this shows potential as a useful course of TEC for ionospheric observations.