Martins' first goal came at the end of the 19th minute. Eddie Johnson connected with Brad Evans on a give-and-go down the middle of the field, and Evans sent a long through ball toward the top of the penalty area restraining arc. Martins got behind United defender Ethan White, caught up with the ball, and sent a shot past goalkeeper Joe Willis to the back left corner.
His second came midway through second-half stoppage time, a 10-yarder from straight in front of the net.
Gspurning's shutout was his sixth of the season for the Sounders (7-5-3), who have at least one game in hand on every team in the MLS West as they try to move up the conference standings with a stretch of three games in 10 days. They have lost just once in seven home games this season.
United (2-13-3) remained winless in eight road games this season (0-6-2), and was blanked for the 11th time. D.C. has not scored from the run of play in seven matches since May 19, with its only two goals since then coming on penalty kicks.
Martins nearly had another goal midway through the 63rd minute when he and Eddie Johnson worked a give-and-go with Johnson having a 10-yard shot straight in front of the net. But Willis came sliding out to block it.
That turned into a counterattack which led to D.C.'s most dangerous opportunity of the night. Perry Kitchen launched one from 25 yards that was about to dip under the crossbar on the left side when Gspurning leaped to one-hand it over the goal line.