New insights into the source of the boron memory effect and implications for accurate boron isotope measurements

Abstract

Rationale:The boron (B) memory effect is a concern for B isotope analysis in inductively-coupled plasma mass spectrometry and a potential cause of poor data comparability between laboratories. It is widely assumed that the memory resides in water droplets on the surface of the spray chamber. However, even without the use of the spray chamber, background subtractions are still required to generate accurate data, therefore additional causes for the memory effect exist, which are investigated here.Methods: Different parts of the mass spectrometer were examined to pinpoint the source of a particularly high B background. After identifying the torch as the source of the background, different parts of the torch were soaked in dilute nitric acid, which was analyzed for B over time.Results: B was leached out of the tip of the outer quartz tube of the torch in a fashion similar to borosilicate glass, which suggests the incorporation of B into the silica structure of the torch at high temperatures. Running 3% nitric acid washes effectively reduces the background. B background compositions change based on the solutions run beforehand, therefore different blank subtraction methods generate systematic differences. A new background subtraction method that utilizes B isotope ratios improved the precision by up to 0.14‰. The addition of a water wash step before sample elution led to smaller eluent volumes and improved matrix matching without causing a B breakthrough.Conclusions: An important part of the B memory derives from the torch glass, which incorporates B from sample solutions at high temperatures. Multiple nitric acid washes, matrix matching, blank subtraction, and standard sample bracketing generated accurate B isotope analyses with background/signal ratios as high as 10%, without the need for hazardous chemicals as washes. B isotope values of two sediment standards that represent average post-Archean continental crust were reported. | INTRODUCTIONBoron (B) is highly soluble in water. Its two isotopes, 10 B and 11 B, partition differently between borate and boric acid, the two dominant species of boron in aqueous solutions. [1][2][3][4][5] Because the proportion of borate and boric acid changes according to the pH of the environment, the 10 B/ 11 B ratio is widely used to investigate processes that involve water, including the weathering process, 6-8 ocean pH changes, 9-11 tectonic processes, 12-14 and ore formation. [15][16][17] Multicollector inductively-coupled plasma mass spectrometry (MC-ICP-MS) permits high throughput of B isotope measurements. However, relatively elevated and persistent instrumental B background between samples, or the B memory effect, remains a common concern. This memory effect is generally attributed to B(OH) 3 vapor from previously nebulized solutions that become entrained in droplets coating the walls of the spray chambers. [18][19][20] The use of demountable

Improved alkaline fusion method for B isotope and concentration measurements of silicate materials

“…Finally, after neutralizing the resin with 1.5 mL of water, B is eluted from the resin using 0.6 mL of 3% nitric acid in three separate steps. 31 Part of the ion exchange chromatography is carried out using a Longerpump® BT100-1L 24-channel peristaltic pump. All the procedures, except for the use of the muffle furnace, are carried out in an ISO class 5 Ultraclean chemistry laboratory with a B-free PTFE HEPA filter lab at Nanjing Institute of Geology and Palaeontology, Chinese Academy of Science (NIGPAS).…”

“…We calculated the mean 10 B intensities using the measured 11 B intensity and the isotopic ratio through the equation 10 B = 11 Bx 10 B/ 11 B to take advantage of the higher precision of 11 B/ 10 B isotope ratio measurements. 31…”

Atomic spectrometry update: review of advances in atomic spectrometry and related techniques

“…Cai et al 157 investigated the significant issue of memory effects between samples in measuring B isotopes using ICP-MS. The effect was identified as stemming from the ICP torch, in particular the tip which was found to incorporate B from sample solutions into its silica structure at high temperatures.…”