Disclosed are apparatus, kits, methods, and systems that include a radiation source configured to direct radiation to a sample; a detector configured to measure radiation from the sample; an electronic processor configured to determine information about the sample based on the measured radiation; a housing enclosing the source, the detector, and the electronic processor, the housing having a hand-held form factor; an arm configured to maintain a separation between the sample and the housing, the arm including a first end configured to connect to the housing and a second end configured to contact the sample; and a layer positioned on the second end of the arm, the layer being configured to contact the sample and to transmit at least a portion of the radiation from the sample to the detector.