Solid phase extraction using the Oasis MAX µElution Plate was performed as follows: 50 µL urine samples were pretreated 1:1 with 5% strong ammonia, then loaded onto the SPE plate, washed with 5:95 MeOH:H2O, eluted with 50 µL of 75:25 MeOH:ACN containing 2% formic acid, and diluted with 200 µL of water. Recovery was efficient for all compounds. Figure 2 shows the average recovery from 12 unique lots of urine. Amiloride, clopamide, and canrenone were grouped with the beta-blockers, as they are bases that ionize by positive ESI. Mean recovery for beta-blockers (and basic diuretics) was 85%, with all but one at 80% or higher; %RSDs were all <20%. Diuretic recovery ranged from 65–94%; all %RSDs <20% with the exception of acetazolamide. The use of the Oasis MAX µElution Plate substantially reduced matrix effects associated with simple sample dilution. As Figure 3 shows, matrix effects, particularly ion suppression, increased from negligible levels to up to over 60% for many of the beta blockers and from 20–40% up to 60‒90% for many of the diuretics, even at a 1:10 dilution. Single-factor Anova analysis revealed that matrix effects were significantly increased for all compounds except for acebutolol, metolazone, bumetanide, probenecid, and ethacrynic acid.
