A cuvette is a kind of laboratory glassware , usually a small tube of circular or square cross shape , closed at one end and open at the other , made of plastic , glass , or optical grade quartz and made to hold samples for spectroscopic experiments . A cuvette is a small tube of circular or square cross section , sealed at one end , made of plastic , glass , or fused quartz ( for UV light ) and designed to hold samples for spectroscopic experiments .