Context : The observed abundance peculiarities of many chemical species relative to the expected cluster metallicity in blue horizontal-branch ( BHB ) stars presumably appear as a result of atomic diffusion in the photosphere .
The slow rotation ( typically v \sin { i } < 10 km s ^ { -1 } ) of BHB stars with effective temperatures T _ { eff } > 11,500 K supports this idea since the diffusion mechanism is only effective in a stable stellar atmosphere .

Aims : In this work we search for observational evidence of vertical chemical stratification in the atmospheres of six hot BHB stars : B84 , B267 and B279 in M15 and WF2-2541 , WF4-3085 and WF4-3485 in M13 .

Methods : We undertake an abundance stratification analysis of the stellar atmospheres of the aforementioned stars , based on acquired Keck HIRES spectra .

Results : We have found from our numerical simulations that three stars ( B267 , B279 and WF2-2541 ) show clear signatures of the vertical stratification of iron whose abundance increases toward the lower atmosphere , while the other two stars ( B84 and WF4-3485 ) do not .
For WF4-3085 the iron stratification results are inconclusive .
B267 also shows a signature of titanium stratification .
Our estimates for radial velocity , v \sin { i } and overall iron , titanium and phosphorus abundances agree with previously published data for these stars after taking the measurement errors into account .
The results support the hypothesis regarding the efficiency of atomic diffusion in the stellar atmospheres of BHB stars with T _ { eff } > 11,500 K .

Conclusions :