Background:

In the past, analysis of liquid samples with DART®-MS has been carried out by dipping a closed-end glass capillary tube directly into the liquid sample and then presenting the sample covered glass tip into the DART® ionization region. This sampling method was originally performed manually and was later automated using a PAL HTC autosampler for which the sample throughput was approximately 40 seconds per sample.

Experimental:

A motorized linear rail with a rod pusher sample stage was positioned on the DART®-ET ion source and a PAL HTC autosampler was utilized as a syringe-based liquid sample handling device. The rod pusher sample stage was fitted with small diameter ceramic rods (1.6 mm OD) to determine the characteristics of the ceramic sampling surface. The linear rail was set to push the rods into the DART® ionization region at a set speed of 0.8 mm/sec controlled through the DART® application on the Apple iPod® touch user interface.

Results:

Figure 2 shows the results for the spatial resolution experiment. The physical spacing between the ink sample spots on the ceramic rod and the speed of the linear rail were optimized and it was determined from the data in Figure 2 that for the 0.8 mm/s linear rail speed that the 8 mm sample spacing was ideal for throughput and spatial resolution. Figure 1 shows the physical set-up for the liquid spotting onto the top of the ceramic rod sampling surface.

The ceramic rods were ideal for the liquid spotting because the coarse surface of the ceramic material held the liquid droplet in a fixed spot, whereas on a smooth surface the liquid would move around as the rod moved through the DART® ionization region. The data in Figure 3 show that the sample throughput for this new automated liquid sampling method is approximately 11 seconds per sample, a roughly 4-fold improvement over the conventional automated dipping method with the glass capillary tubes.

Data: