To quantify the carboxyhemoglobin (CO-Hb) concentration of a forensic biospecimen (commonly blood or tissue sample) by headspace gas chromatography (HS-GC), most laboratory protocols entail bubbling carbon monoxide (CO) and nitrogen (N2) gas through the sample to saturate it and then liberate free CO gas. However, despite safety precautions, such as the use of a fume hood, extended exposure to CO imposes significant safety hazards to analysts. Moreover, analysts working in poorly equipped laboratories must pay increased attention during CO bubbling to prevent sample overflow. Such difficulties slow down the procedure tremendously, particularly when multiple samples must be analyzed simultaneously. In this study, adjustments to the conventional approach were made by introducing a given blood sample into a syringe equipped with a three-way valve, which then withdraws CO gas from an airtight gas sampling bag. After rotating the syringe, free CO in the blood is removed by moving the plunger up and down a few times and repeating the procedure with nitrogen gas. The resulting blood sample is then used to prepare a concentration series of CO-Hb blood. The sample preparation time for six samples was reduced by approximately 74 %, and no significant differences in CO-Hb levels were observed compared with the conventional method. This modified approach is expected to facilitate the inexpensive and efficient CO-Hb concentration determination of multitudinous samples for accurate forensic analysis.