Immuno‐magnetic Bead Mass Transport and Capture Efficiency at Low Target Cell Densities in Phosphate‐buffered Saline1

Anti‐salmonella immuno‐magnetic bead (IMB) capture efficiency (E) was determined by varying both IMB levels ([IMB]) and mixing time (TMIX) and enumerating captured Salmonella enteritidis cells. We observed that E varied with TMIX as a Pseudo‐first order process with a rate constant (K) of 0.028 ± 0.001 min‐1 and an infinite TMIX asymptote of 0.97 (97% capture). Thus, even at low target cell densities ([S. E.] ± 70 CFU mL‐1), nearly 100% of the bacteria were captured as TMIX approached 2 h. We hypothesize that k is the product of both IMB mass transport (γ) and IMB concentration ([IMB]) terms. Thus, γ represents the total volume which each IMB samples per unit time of mixing. This idea appears reasonable as γ(3.2 ± 0.2 × 10‐9mL min‐1 IMB‐1), determined from E‐based observations at various [IMB] levels (˜106−108 IMB mL‐1) and fixed TMIX (30 min), was nearly identical to γ(3.5 ± 0.1 × 10‐9 mL min‐1 IMB‐1) derived kinetically (TMIX= 0–120 min; [IMB] = 8 × 106 IMBs mL‐1). Estimating a mass transport term (γCALC) founded on classical dynamics we obtained a value 5.3 × 10‐10 mL min‐1 IMB‐1 whereupon γCALC and γ would have completely agreed were the IMB radius (rIMB= 1.4 μm) 0.8 μm larger. The apparent discrepancy in rIMB might be explained by a greater effective (hydrodynamic) rIMB which could result from some combination of IMB swelling and brownian motion. These results argue that IMB‐based target cell capture is modulated by IMB mass transport and collision probability.