The ninhydrin reaction, one of the most important method of detecting amino acids, both technically and historically, has been conventionally used to detect their microgram amounts. When amino acids with a free alpha amino groups are treated with an excess of ninhydrin, they yield a purple colored product.
Update January 19: Amazon has increased the monthly price of Amazon Prime from $10.99 to $12.99 for those who opt for the month-to-month plan. However, the annual price remains untouched at $99 per year, which makes it the better option for avid Amazon shoppers.
Your monthly Prime membership and Fresh Add-on are billed separately. Amazon Prime is billed at a cost of $10.99 per month and the Fresh Add-on is billed at a cost of $14.99 per month; a total cost of $25.98 per month. You can sign up here. As an existing eligible Amazon Prime member, you can sign up for a 30-day free trial of the Fresh Add-on.