A handheld fluorometer, comprising: a handheld controller module comprising an elongated controller module housing comprising a bottom surface, the controller module housing having a proximal end and a distal end, between which extends a longitudinal axis of the sensor head, a controller that determines a concentration of a substance in a water sample based on a detected fluorescent emission, a display coupled to the controller for displaying the concentration, and an input interface for entering data for use by the controller; an immersible sensor head comprising a light source coupled to the controller, an emission detector coupled to the controller, and a sensor head housing comprising a proximal end and a distal end between which extends a longitudinal axis of the sensor head, the sensor head being connected to the bottom surface of the controller module at the proximal end of the sensor head housing, the sensor head housing further comprising a light source window that transmits excitation light from the light source inside the sensor head housing to an analytical area outside the sensor head housing, and an emission detector window that transmits fluorescent emissions from the analytical area to the emission detector inside the sensor head housing; a sample cup for containing the water sample; and a fastener that removably fastens the sample cup about the sensor head such that at least the light source window and the emission detector window can be immersed in the water sample contained in the sample cup, wherein the water sample occupies the analytical area, characterized in that the sensor head is connected to the controller module such that the longitudinal axis of the sensor head forms an angle with the longitudinal axis of the controller module, wherein the angle is 60 degrees, such that the sensor head is in an upright position with its longitudinal axis substantially perpendicular to a flat support surface when the fluorometer is placed on the support surface with the sensor head accommodated in the sample cup, wherein a bottom portion of the sample cup provides a first footing upon the support surface and the proximal end of the controller module housing provides a second footing. a handheld controller module comprising an elongated controller module housing comprising a bottom surface, the controller module housing having a proximal end and a distal end, between which extends a longitudinal axis of the sensor head, a controller that determines a concentration of a substance in a water sample based on a detected fluorescent emission, a display coupled to the controller for displaying the concentration, and an input interface for entering data for use by the controller; an immersible sensor head comprising a light source coupled to the controller, an emission detector coupled to the controller, and a sensor head housing comprising a proximal end and a distal end between which extends a longitudinal axis of the sensor head, the sensor head being connected to the bottom surface of the controller module at the proximal end of the sensor head housing, the sensor head housing further comprising a light source window that transmits excitation light from the light source inside the sensor head housing to an analytical area outside the sensor head housing, and an emission detector window that transmits fluorescent emissions from the analytical area to the emission detector inside the sensor head housing; a sample cup for containing the water sample; and a fastener that removably fastens the sample cup about the sensor head such that at least the light source window and the emission detector window can be immersed in the water sample contained in the sample cup, wherein the water sample occupies the analytical area, characterized in that the sensor head is connected to the controller module such that the longitudinal axis of the sensor head forms an angle with the longitudinal axis of the controller module, wherein the angle is 60 degrees, such that the sensor head is in an upright position with its longitudinal axis substantially perpendicular to a flat support surface when the fluorometer is placed on the support surface with the sensor head accommodated in the sample cup, wherein a bottom portion of the sample cup provides a first footing upon the support surface and the proximal end of the controller module housing provides a second footing.