“…A CRM was prepared according to Adamberg et al (2005), with minor adjustments as described by Moe et al,(2012). In short, a CRM was made with 12.0 g/L peptone from casein (Merck, Darmstadt, Germany), 1.0 g/L Tween 80 (Koch-Light Laboratories Ltd., Bucks, UK), 0.6 g/L MgSO 4 ·7H 2 O (Merck), 0.3 g/L MnSO 4 ·5H 2 O (Merck), 0.06 g/L FeSO 4 ·7H 2 O (Merck), 0.05 mg/L biotin (Sigma, Steinheim, Germany), 0.1 mg/L folic acid (Sigma), 0.1 mg/L niacinamide (Sigma), 0.1 mg/L pyridoxal-HCl (Sigma), 0.1 mg/L riboflavin (Sigma), 0.1 mg/L thiamine-HCl (Sigma), and 0.2 mg/L panthothenate (Sigma).…”