Thanks, to all of those who commented on our GC-IRMS question a few weeks back. To help solve the problem (as suggested by several groups) we replaced our Valco 4-way valve (we were actually using a 3-way) with a smaller SGE valve (VSLNU/005 UNION part# 1236300). This is a zero dead volume glass lined union, which we are used the normal SGE two hole ferrules to make a "3-way" union for the He backflush. So far this set-up seems better, and our backgrounds have come down. -Bill Anderson -- William Anderson Assistant Professor Geology Department and Southeast Environmental Research Center Florida International University Mailing address: Geology Dept., PC 344 FIU 11200 S.W. 8th Street Miami, FL 33199 USA office: 305 348 2693, sec. 348 3572 SERC stable isotope lab: 305 348 3044 fax: 305 348 3877 email: [log in to unmask]