Introduction
Certain elements that are of particular importance to cannabis testing, such as Hg, exhibit prolonged residence in the sample introduction region of an ICP-MS and, as a result, necessitate extended washout times or modified rinse reagents. There are several potential causes for this problematic issue, one of which is entirely the result of spray chamber design.
There are two common types of spray chambers used: the Scott type and cyclonic type (Figures 1a and 1b), both of which are available as single-pass or double-pass. In the Scott-type, sample is injected axially along an inner glass tube and then turns 180 degrees to an outer tube, where it is guided into the torch, allowing gravity to separate the large and small aerosolized droplets. In a cyclonic spray chamber, sample aerosol is injected tangentially allowing the largest droplets to be flung to the outside of the chamber and drained away. Overall, cyclonic spray chambers have a smaller surface area upon which the aerosolized sample can contact and onto which it can potentially adsorb. The additional unique design features of Glass Expansion’s Twister™ double-pass cyclonic spray chamber are depicted in Figure 1b.
In addition to the design of the spray chamber, the method of securing the nebulizer to the spray chamber is of significant importance for improved washout and decreased memory effects. Most ICP spectrometers rely on an O-ring seal between the nebulizer and the spray chamber. This design may result in extended washout times due to dead volume around the seal. The new Helix CT, now standard on all Glass Expansion cyclonic spray chambers, eliminates dead volume, resulting in faster washout times and higher sample throughput.
The main feature of the Helix CT spray chamber is a new Helix locking screw with built-in torque control mechanism that allows for a consistent seal of the PTFE ferrule against the nebulizer – making it impossible to overtighten or under-tighten while ensuring a gas-tight seal each and every time. A new PressFit PTFE ferrule provides a chemically inert seal around the nebulizer, which is immune to strong acids and organic solvents routinely used in ICP sample preparation. The new Helix CT cyclonic spray chamber by Glass Expansion, therefore, eliminates all the drawbacks of the O-ring nebulizer seal, while improving user safety by preventing broken nebulizers.
The evolution of Glass Expansion cyclonic spray design, specifically the advantages of the Helix nebulizer-spray chamber interface, was summarized in a 2014 article1. In addition to the Helix interface, the benefits of each Glass Expansion cyclonic spray chamber model are detailed, making it easy for ICP analysts to select the optimum spray chamber to suit their application needs.
Several solutions to the problem of longer washout times (independent of spray chamber design) have been proposed and employed, including extending the duration of solvent rinsing between samples, increasing the acidity of the rinse solution, adding simple rinse matrix modifiers, such as a small amount of Au in the case of Hg carryover, concocting a complex rinse solution, often containing compounds such as ammonia, hydrogen peroxide, EDTA, and or surfactants such as Triton X-100; however, there are possible complications and limitations of such techniques. Extending the duration of solvent rinsing reduces sample throughput and increases solvent consumption. Increasing the acidity of the rinse solution increases the consumption of often expensive trace metal grade acids and can degrade standard peristaltic pump tubing more rapidly, along with the sampler and skimmer cones. Matrix modifiers, such as Au, may be effective for some elements over others, and complex rinse solutions are time-consuming to blend and may contain costly reagents. All of these solutions lead to an increase in the operating cost of an ICP-MS and a decrease in sample throughput. For these reasons, it is imperative to include the design of the spray chamber when considering effective washout procedures.
Results
For the Helix CT vs. O-ring washout comparison, an Agilent® 5100 simultaneous dual view ICP-OES was used in combination with the SeaSpray DC nebulizer and Twister cyclonic spray chamber with Helix CT. The Helix CT is the only nebulizer-spray chamber interface that significantly reduces the dead volume around the nebulizer. This unique design minimizes washout time with highly concentrated samples, reducing sample-to-sample carryover and improving sample throughput. Figure 2 compares the time required to wash out a 10 ppm Molybdenum standard with the Helix CT interface and a Brand-X spray chamber with an O-ring interface. The results show that with the Helix nebulizer interface a 10 ppm standard can be washed out in as little as 4 seconds, whereas Brand-X takes 16 seconds. One can expect this time to significantly increase for more troublesome or sticky elements that are more prone to carryover issues.
For the cyclonic vs. Scott washout comparison, a Shimadzu® ICPMS-2030 was used in combination with a MicroMist DC nebulizer and both a Twister cyclonic spray chamber and a Brand-X double-pass Scott spray chamber to acquire time-resolved data for a 100 ppb solution of Hg preserved in 2% HNO3. The results in Figure 3 show that the design of the cyclonic spray chamber results in a 43% faster washout time using a standard 1% HNO3 rinse solution (with 1 ppm Au). For a 100 ppb Hg solution, the washout time for the cyclonic spray chamber was 36 seconds, whereas the washout time for the Scott-style spray chamber was 63 seconds.
In addition to this comparison, the washout time of a Glass Expansion Tracey spray chamber (single-pass) was compared to the same Twister spray chamber (double-pass). The results in Figure 4 show that the washout time is 39% faster utilizing a single-pass cyclonic spray chamber. For a 100 ppb Hg solution, the washout time for the Tracey was 22 seconds, whereas the washout time for the Twister was 36 seconds.
