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.
In the US, subscribers can now choose to pay monthly for the service too. Hopefully, a similar scheme will also be introduced to the UK. US Amazon Prime members can now opt to pay $10.99 a month for the service. Previously, the only choice was to pay $100 annually. Naturally, paying monthly will be more than $32 a year extra, but it enables members to spread the cost. What's more, they can cancel at any time, so aren't committed to a 12-month contract.