How do I use the Swift sampler to play a tone then pause before playing the next?

I have code to take a sequence of letters in a string and interpret them as notes. The code will then play the notes. The problem is that they all play at the same time. How do I play them each as a quarter note, essentially to play a note, wait for it to end, and then play the next note?

@IBAction func playButton(sender: AnyObject) {
    fractalEngine.output = "adgadefe"
    var notes = Array(fractalEngine.output.characters)

    var counter = 0
    while counter < notes.count {

            var note = notes[counter]
            if note == "a" {
                play(58)
            }
            else if note == "b" {
                play(59)
            }
            else if note == "c" {
                play(60)
            }
            else if note == "d" {
                play(61)
            }
            else if note == "e" {
                play(62)
            }
            else if note == "f" {
                play(63)
            }
            else {
                play(64)
            }

            counter += 1
    }


    //self.sampler.startNote(60, withVelocity: 64, onChannel: 0)
}

func play(note: UInt8) {
    sampler.startNote(note, withVelocity: 64, onChannel: 0)
}

func stop(note: UInt8) {
    sampler.stopNote(note, onChannel: 0)

}

Here's the code that initiates the sampler:

func initAudio(){

     engine = AVAudioEngine()
     self.sampler = AVAudioUnitSampler()
     engine.attachNode(self.sampler)
     engine.connect(self.sampler, to: engine.outputNode, format: nil)

     guard let soundbank = NSBundle.mainBundle().URLForResource("gs_instruments", withExtension: "dls") else {

     print("Could not initalize soundbank.")
     return
     }

     let melodicBank:UInt8 = UInt8(kAUSampler_DefaultMelodicBankMSB)
     let gmHarpsichord:UInt8 = 6
     do {
     try engine.start()
     try self.sampler.loadSoundBankInstrumentAtURL(soundbank, program: gmHarpsichord, bankMSB: melodicBank, bankLSB: 0)

     }catch {
     print("An error occurred (error)")
     return
     }

    /*
    self.musicSequence = createMusicSequence()
    createAVMIDIPlayer(self.musicSequence)
    createAVMIDIPlayerFromMIDIFIle()
    self.musicPlayer = createMusicPlayer(musicSequence)
    */

}